WIND Solar-Terrestrial Mission
WIND is a NASA/GSFC solar-terrestrial mission within the US GGS (Global Geospace Science) initiative and also part of the ISTP (International Solar-Terrestrial Physics) program. The objective is to study sources, acceleration mechanisms and propagation processes of energetic particles and the solar wind. Investigation of solar wind mass momentum and energy) with input first from the day-side double lunar swingby orbit, and later from a small halo orbit at L1. WIND, together with GEOTAIL (ISAS, Japan, launch 1992), Polar (NASA, launch 1996), SOHO (ESA/NASA, launch 1995), and the Cluster constellation spacecraft (ESA, launch 2000), constitute the cooperative scientific ISTP program. 1)
The science objectives are:
• Provide complete plasma, energetic particle, and magnetic field input for magnetospheric and ionospheric studies
• Determine the magnetospheric output to interplanetary space in the up-stream region
• Investigate basic plasma processes occurring in the near-Earth solar wind
• Provide baseline, 1AU, ecliptic plane observations for inner and outer heliospheric missions.
WIND is a spin-stabilized S/C at 20 rpm with the spin axis normal to the ecliptic plane. The spacecraft shape is a cylinder of size: 2.4 m diameter and 1.8 m in height. Surface-mounted solar arrays provide 370 W of power, including 144 W for payload instruments. The Wind S/C was built by Martin Marietta Astro Space of Princeton, NJ. Total S/C mass at launch = 1250 kg (300 kg of hydrazine propellant, 195 kg science payload). Nominal lifetime = 3 years (min).
The spacecraft exhibits very favorable EMI/EMC (Electromagnetic Interference/Electromagnetic Compatibility) levels. The wire antennas are 100 m and 15 m tip-to-tip, respectively. The axial antennas are about 12 m tip-to-tip. Each boom is 12 m in length.
Figure 2: Line drawing of the WIND spacecraft with instrument locations (image credit: NASA)
RF communications: On-board recording capability of 1.3 Gbit (digital tape recorder). Transmission via DSN (Deep Space Network) for 2 hr nominal daily contact periods. Science data rates: 5.6 kbit/s realtime and 128 kbit/s playback data. The WIND S/C provides on-board interconnection of instrumentation for data communication. Data sharing among the instruments can be triggered by pattern recognition schemes of the on-board computers.
Launch: WIND was launched Nov. 1, 1994 with a Delta II vehicle from Cape Canaveral, FLA.
1) WIND was initially placed in a double-lunar-swingby orbit near the ecliptic plane with an apogee from 80 to 250 RE and a perigee of 5 to 10 RE during first two years (inclination = 19.6º). In this orbit, lunar gravity assists were used to keep the apogee over the day hemisphere of the Earth, and magnetospheric observations were made. 2)
Figure 3: Overview of the original mission orbit (image credit: NASA)
2) WIND extended mission. In Nov. 1996, WIND was inserted into a ”halo” orbit, about the sunward Sun-Earth gravitational equilibrium point (Lagrangian point L1), varying from 235 to 265 RE. In this orbit WIND measures the incoming solar wind, magnetic fields and particles continuously and provides an approximately one-hour warning to the other ISTP spacecraft of changes in the solar wind.
Figure 4: WIND extended mission April 1998-April 1999 (image credit: NASA) 3)
Status of mission
• July 13, 2021: Magnetars are bizarre objects — massive, spinning neutron stars with magnetic fields among the most powerful known, capable of shooting off brief bursts of radio waves so bright they’re visible across the universe. A team of astrophysicists has now found another peculiarity of magnetars: They can emit bursts of low energy gamma rays in a pattern never before seen in any other astronomical object. 4)
- It’s unclear why this should be, but magnetars themselves are poorly understood, with dozens of theories about how they produce radio and gamma ray bursts. The recognition of this unusual pattern of gamma ray activity could help theorists figure out the mechanisms involved.
- “Magnetars, which are connected with fast radio bursts and soft gamma repeaters, have something periodic going on, on top of randomness,” said astrophysicist Bruce Grossan, an astrophysicist at the University of California, Berkeley’s Space Sciences Laboratory (SSL). “This is another mystery on top of the mystery of how the bursts are produced.”
- The researchers — Grossan and theoretical physicist and cosmologist Eric Linder from SSL and the Berkeley Center for Cosmological Physics and postdoctoral fellow Mikhail Denissenya from Nazarbayev University in Kazakhstan — discovered the pattern last year in bursts from a soft gamma repeater, SGR1935+2154, that is a magnetar, a prolific source of soft or lower energy gamma ray bursts and the only known source of fast radio bursts within our Milky Way galaxy. They found that the object emits bursts randomly, but only within regular four-month windows of time, each active window separated by three months of inactivity.
- On March 19, the team uploaded a preprint claiming “periodic windowed behavior” in soft gamma bursts from SGR1935+2154 and predicted that these bursts would start up again after June 1 — following a three month hiatus — and could occur throughout a four-month window ending Oct. 7.
- On June 24, three weeks into the window of activity, the first new burst from SGR1935+2154 was observed after the predicted three month gap, and nearly a dozen more bursts have been observed since, including one on July 6, the day the paper was published online in the journal Physical Review D. 5)
- “These new bursts within this window means that our prediction is dead on,” said Grossan, who studies high energy astronomical transients. “Probably more important is that no bursts were detected between the windows since we first published our preprint.”
- Linder likens the non-detection of bursts in three-month windows to a key clue — the “curious incident” that a guard dog did not bark in the nighttime — that allowed Sherlock Holmes to solve a murder in the short story “The Adventure of Silver Blaze”.
- “Missing or occasional data is a nightmare for any scientist,” noted Denissenya, the first author of the paper and a member of the Energetic Cosmos Laboratory at Nazarbayev University that was founded several years ago by Grossan, Linder and UC Berkeley cosmologist and Nobel laureate George Smoot. “In our case, it was crucial to realize that missing bursts or no bursts at all carry information.”
- The confirmation of their prediction startled and thrilled the researchers, who think this may be a novel example of a phenomenon — periodic windowed behavior — that could characterize emissions from other astronomical objects.
Mining data from 27-year-old satellite
- Within the last year, researchers suggested that the emission of fast radio bursts — which typically last a few thousandths of a second — from distant galaxies might be clustered in a periodic windowed pattern. But the data were intermittent, and the statistical and computational tools to firmly establish such a claim with sparse data were not well developed.
Figure 5: Since 2014, a magnetar in our galaxy (SGR1935+2154) has been emitting bursts of soft gamma rays (black stars). UC Berkeley scientists concluded that they occurred only within certain windows of time (green stripes) but were somehow blocked during intervening windows (red). They used this pattern to predict renewed bursts starting after June 1, 2021 (stripes outlined in blue at right), and since June 24, more than a dozen have been detected (blue stars): right on schedule (Graphic by Mikhail Denissenya)
- Grossan convinced Linder to explore whether advanced techniques and tools could be used to demonstrate that periodically windowed — but random, as well, within an activity window — behavior was present in the soft gamma ray burst data of the SGR1935+2154 magnetar. The Konus instrument aboard the WIND spacecraft, launched in 1994, has recorded soft gamma ray bursts from that object — which also exhibits fast radio bursts — since 2014 and likely never missed a bright one.
- Linder, a member of the Supernova Cosmology Project based at Lawrence Berkeley National Laboratory, had used advanced statistical techniques to study the clustering in space of galaxies in the universe, and he and Denissenya adapted these techniques to analyze the clustering of bursts in time. Their analysis, the first to use such techniques for repeated events, showed an unusual windowed periodicity distinct from the very precise repetition produced by bodies rotating or in orbit, which most astronomers think of when they think of periodic behavior.
- “So far, we have observed bursts over 10 windowed periods since 2014, and the probability is 3 in 10,000 that while we think it is periodic windowed, it is actually random,” he said, meaning there’s a 99.97% chance they’re right. He noted that a Monte Carlo simulation indicated that the chance they’re seeing a pattern that isn’t really there is likely well under 1 in a billion.
- The recent observation of five bursts within their predicted window, seen by WIND and other spacecraft monitoring gamma ray bursts, adds to their confidence. However, a single future burst observed outside the window would disprove the whole theory, or cause them to redo their analysis completely.
- “The most intriguing and fun part for me was to make predictions that could be tested in the sky. We then ran simulations against real and random patterns and found it really did tell us about the bursts,” Denissenya said.
- As for what causes this pattern, Grossan and Linder can only guess. Soft gamma ray bursts from magnetars are thought to involve starquakes, perhaps triggered by interactions between the neutron star’s crust and its intense magnetic field. Magnetars rotate once every few seconds, and if the rotation is accompanied by a precession — a wobble in the rotation — that might make the source of burst emission point to Earth only within a certain window. Another possibility, Grossan said, is that a dense, rotating cloud of obscuring material surrounds the magnetar but has a hole that only periodically allows bursts to come out and reach Earth.
- “At this stage of our knowledge of these sources, we can’t really say which it is,” Grossan said. “This is a rich phenomenon that will likely be studied for some time.”
- Linder agrees and points out that the advances were made by the cross-pollination of techniques from high energy astrophysics observations and theoretical cosmology.
- “UC Berkeley is a great place where diverse scientists can come together,” he said. “They will continue to watch and learn and even ‘listen’ with their instruments for more dogs in the night.”
• January 13, 2021: On April 15, 2020, a brief burst of high-energy light swept through the solar system, triggering instruments on several NASA and European spacecraft. Now, multiple international science teams conclude that the blast came from a supermagnetized stellar remnant known as a magnetar located in a neighboring galaxy. 6)
- This finding confirms long-held suspicions that some gamma-ray bursts (GRBs) – cosmic eruptions detected in the sky almost daily – are in fact powerful flares from magnetars relatively close to home.
Figure 6: A pulse of X-rays and gamma rays lasting just 140 milliseconds swept across the solar system on April 15, 2020. The event was a giant flare from a magnetar, a type of city-sized stellar remnant that boasts the strongest magnetic fields known. Watch to learn more (video credit: NASA’s Goddard Space Flight Center)
- “This has always been regarded as a possibility, and several GRBs observed since 2005 have provided tantalizing evidence,” said Kevin Hurley, a Senior Space Fellow with the Space Sciences Laboratory at the University of California, Berkeley, who joined several scientists to discuss the burst at the virtual 237th meeting of the American Astronomical Society. “The April 15 event is a game changer because we found that the burst almost certainly lies within the disk of the nearby galaxy NGC 253.”
- Papers analyzing different aspects of the event and its implications were published on Jan. 13 in the journals Nature and Nature Astronomy. 7)
- GRBs, the most powerful explosions in the cosmos, can be detected across billions of light-years. Those lasting less than about two seconds, called short GRBs, occur when a pair of orbiting neutron stars – both the crushed remnants of exploded stars – spiral into each other and merge. Astronomers confirmed this scenario for at least some short GRBs in 2017, when a burst followed the arrival of gravitational waves – ripples in space-time – produced when neutron stars merged 130 million light-years away.
- Magnetars are neutron stars with the strongest-known magnetic fields, with up to a thousand times the intensity of typical neutron stars and up to 10 trillion times the strength of a refrigerator magnet. Modest disturbances to the magnetic field can cause magnetars to erupt with sporadic X-ray bursts for weeks or longer.
- Rarely, magnetars produce enormous eruptions called giant flares that produce gamma rays, the highest-energy form of light.
- Most of the 29 magnetars now cataloged in our Milky Way galaxy exhibit occasional X-ray activity, but only two have produced giant flares. The most recent event, detected on Dec. 27, 2004, produced measurable changes in Earth’s upper atmosphere despite erupting from a magnetar located about 28,000 light-years away.
- Shortly before 4:42 a.m. EDT on April 15, 2020, a brief, powerful burst of X-rays and gamma rays swept past Mars, triggering the Russian High Energy Neutron Detector aboard NASA’s Mars Odyssey spacecraft, which has been orbiting the Red Planet since 2001. About 6.6 minutes later, the burst triggered the Russian Konus instrument aboard NASA’s Wind satellite, which orbits a point between Earth and the Sun located about 930,000 miles (1.5 million kilometers) away. After another 4.5 seconds, the radiation passed Earth, triggering instruments on NASA’s Fermi Gamma-ray Space Telescope, as well as on the European Space Agency’s INTEGRAL satellite and Atmosphere-Space Interactions Monitor (ASIM) aboard the International Space Station.
- The eruption occurred beyond the field of view of the Burst Alert Telescope (BAT) on NASA’s Neil Gehrels Swift Observatory, so its onboard computer did not alert astronomers on the ground. However, thanks to a new capability called the Gamma-ray Urgent Archiver for Novel Opportunities (GUANO), the Swift team can beam back BAT data when other satellites trigger on a burst. Analysis of this data provided additional insight into the event.
- The pulse of radiation lasted just 140 milliseconds – as fast as the blink of an eye or a finger snap.
Figure 7: The giant flare, cataloged as GRB 200415A, reached detectors on different NASA spacecraft at different times. Each instrument pair established its possible location in different swaths of the sky, but the bands intersect in the central part of the bright spiral galaxy NGC 253. This is the most precise position yet established for a magnetar located well beyond our galaxy (image credits: NASA's Goddard Space Flight Center and Adam Block/Mount Lemmon SkyCenter/University of Arizona)
- The Fermi, Swift, Wind, Mars Odyssey and INTEGRAL missions all participate in a GRB-locating system called the InterPlanetary Network (IPN). Now funded by the Fermi project, the IPN has operated since the late 1970s using different spacecraft located throughout the solar system. Because the signal reached each detector at different times, any pair of them can help narrow down a burst’s location in the sky. The greater the distances between spacecraft, the better the technique’s precision.
- The IPN placed the April 15 burst, called GRB 200415A, squarely in the central region of NGC 253, a bright spiral galaxy located about 11.4 million light-years away in the constellation Sculptor. This is the most precise sky position yet determined for a magnetar located beyond the Large Magellanic Cloud, a satellite of our galaxy and host to a giant flare in 1979, the first ever detected.
- Giant flares from magnetars in the Milky Way and its satellites evolve in a distinct way, with a rapid rise to peak brightness followed by a more gradual tail of fluctuating emission. These variations result from the magnetar’s rotation, which repeatedly brings the flare location in and out of view from Earth, much like a lighthouse.
- Observing this fluctuating tail is conclusive evidence of a giant flare. Seen from millions of light-years away, though, this emission is too dim to detect with today’s instruments. Because these signatures are missing, giant flares in our galactic neighborhood may be masquerading as much more distant and powerful merger-type GRBs.
- A detailed analysis of data from Fermi’s Gamma-ray Burst Monitor (GBM) and Swift’s BAT provides strong evidence that the April 15 event was unlike any burst associated with mergers, noted Oliver Roberts, an associate scientist at Universities Space Research Association’s Science and Technology Institute in Huntsville, Alabama, who led the study.
- In particular, this was the first giant flare known to occur since Fermi’s 2008 launch, and the GBM’s ability to resolve changes at microsecond timescales proved critical. The observations reveal multiple pulses, with the first one appearing in just 77 microseconds – about 13 times the speed of a camera flash and nearly 100 times faster than the rise of the fastest GRBs produced by mergers. The GBM also detected rapid variations in energy over the course of the flare that have never been observed before.
- “Giant flares within our galaxy are so brilliant that they overwhelm our instruments, leaving them to hang onto their secrets,” Roberts said. “For the first time, GRB 200415A and distant flares like it allow our instruments to capture every feature and explore these powerful eruptions in unparalleled depth.”
- Giant flares are poorly understood, but astronomers think they result from a sudden rearrangement of the magnetic field. One possibility is that the field high above the surface of the magnetar may become too twisted, suddenly releasing energy as it settles into a more stable configuration. Alternatively, a mechanical failure of the magnetar’s crust – a starquake – may trigger the sudden reconfiguration.
- Roberts and his colleagues say the data show some evidence of seismic vibrations during the eruption. The highest-energy X-rays recorded by Fermi’s GBM reached 3 million electron volts (MeV), or about a million times the energy of blue light, itself a record for giant flares. The researchers say this emission arose from a cloud of ejected electrons and positrons moving at about 99% the speed of light. The short duration of the emission and its changing brightness and energy reflect the magnetar’s rotation, ramping up and down like the headlights of a car making a turn. Roberts describes it as starting off as an opaque blob – he pictures it as resembling a photon torpedo from the “Star Trek” franchise – that expands and diffuses as it travels.
- The torpedo also factors into one of the event’s biggest surprises. Fermi’s main instrument, the Large Area Telescope (LAT), also detected three gamma rays, with energies of 480 MeV, 1.3 billion electron volts (GeV), and 1.7 GeV – the highest-energy light ever detected from a magnetar giant flare. What’s surprising is that all of these gamma rays appeared long after the flare had diminished in other instruments.
- Nicola Omodei, a senior research scientist at Stanford University in California, led the LAT team investigating these gamma rays, which arrived between 19 seconds and 4.7 minutes after the main event. The scientists conclude that this signal most likely comes from the magnetar flare. “For the LAT to detect a random short GRB in the same region of the sky and at nearly the same time as the flare, we would have to wait, on average, at least 6 million years,” he explained.
Figure 8: Astronomers explain the observations of GRB 200415A with the sequence of events illustrated here. A sudden reconfiguration of the magnetar's magnetic field produced a quick, powerful pulse of X-rays and gamma rays. The event also ejected a blob of matter, which followed the pulse traveling at about 99% the speed of light. After a few days, they both reached the boundary, called a bow shock, where a steady outflow from the magnetar causes a pile-up of interstellar gas. Light from the flare passed through, followed many seconds later by the ejected cloud. The fast-moving matter interacted with gas at the bow shock, creating shock waves that accelerated particles and produced high-energy gamma rays. This accounts for the delay in the arrival of the most energetic gamma rays detected by NASA's Fermi spacecraft [video credits: NASA’s Goddard Space Flight Center/Chris Smith (USRA/GESTAR)]
- A magnetar produces a steady outflow of fast-moving particles. As it moves through space, this outflow plows into, slows, and diverts interstellar gas. The gas piles up, becomes heated and compressed, and forms a type of shock wave called a bow shock.
- In the model proposed by the LAT team, the flare’s initial pulse of gamma rays travels outward at the speed of light, followed by the cloud of ejected matter, which is moving nearly as fast. After several days, they both reach the bow shock. The gamma rays pass through. Seconds later, the cloud of particles – now expanded into a vast, thin shell – collides with accumulated gas at the bow shock. This interaction creates shock waves that accelerate particles, producing the highest-energy gamma rays after the main burst.
- The April 15 flare proves that these events constitute their own class of GRBs. Eric Burns, an assistant professor of physics and astronomy at Louisiana State University in Baton Rouge, led a study investigating additional suspects using data from numerous missions. The findings will appear in The Astrophysical Journal Letters. Bursts near the galaxy M81 in 2005 and the Andromeda galaxy (M31) in 2007 had already been suggested to be giant flares, and the team additionally identified a flare in M83, also seen in 2007 but newly reported. Add to these the giant flare from 1979 and those observed in our Milky Way in 1998 and 2004.
- “It’s a small sample, but we now have a better idea of their true energies, and how far we can detect them,” Burns said. “A few percent of short GRBs may really be magnetar giant flares. In fact, they may be the most common high-energy outbursts we’ve detected so far beyond our galaxy – about five times more frequent than supernovae.”
• January 28, 2019: According to Wind project scientist Lynn B. Wilson III , the Wind spacecraft is working nominally in its 25th year on orbit (Ref. 10).
- Wind, together with Geotail, Polar, SOHO and Cluster, constitute a cooperative scientific satellite project designated the International Solar Terrestrial Physics (ISTP) program that aims at gaining improved understanding of the physics of solar terrestrial relations.
• May 16, 2017: Solar wind models: The challenge of predicting space weather, which can cause issues with telecommunications and other satellite operations on Earth, requires a detailed understanding of the solar wind (a stream of charged particles released from the sun) and sophisticated computer simulations. Research done at the UNH (University of New Hampshire) has found that when choosing the right model to describe the solar wind, using the one that takes longer to calculate does not make it the most accurate. 8)
- In the study, published in The Astrophysical Journal, Daniel Verscharen, a research assistant professor in physics at UNH's Space Science Center, compared two commonly used theoretical descriptions, kinetic theory versus magnetohydrodynamics (MHD), when measuring the behavior of turbulence in the solar wind. 9)
- Kinetic theory looks at the solar wind as a composition of rapidly moving particles and uses very complicated mathematical methods that require long periods of time when evaluated on sophisticated super computers.
- The second description, MHD, views the solar wind as being a fluid, or more gas-like, and is much less complicated to calculate. Surprisingly, the study showed that it was the MHD, the model that was faster to calculate, that delivered the more precise predictions.
- "Our research found that it makes a huge difference which model is used," said Verscharen. "We found that the much faster computed MHD models may actually capture some of the solar-wind behavior a lot better than expected. This is a very important result for solar-wind modelers because it may justify the application of MHD, based on first principles and observations."
- To prove his theory, Verscharen collected data taken from the WIND spacecraft, which is currently orbiting in the solar wind, from study co-authors Christopher Chen at the Imperial College London and Robert Wicks from University College London.
- After comparing the theory with the actual spacecraft data, the team found that the type of disturbance they were investigating behaved a lot more like a fluid than a kinetic medium with collisionless particles. This was unexpected because they believed that the kinetic theory should work much better in a gas as dilute, or thin, as the solar wind. - The finding could lead to a more efficient way to forecast space weather for institutions that need to continually model the solar wind, like NASA. Severe space weather can cause satellite and communication failures, GPS loss, power outages, and can even have effects on commercial airlines and space flight.
- In order to forecast the effects that solar wind plasma and energetic particles might have on these systems, modelers currently run different computer simulations and compare the results.
- Verscharen and his team believe that their findings could help develop a set of criteria to determine which type of modeling would be most appropriate for their prediction efforts in specific situations. - "If the solar-wind parameters were a certain way, they could use MHD modeling and if not, they might be better to perform simulations based on kinetic theory," said Verscharen. "It would just provide a more efficient way to predict space weather and the solar wind."
- It is still not understood why the solar wind behaves like a fluid. The researchers hope future studies will determine under which conditions the solar wind can be modeled as a fluid with MHD, and when a kinetic model would be necessary.
• In January 2017, the Wind spacecraft is fully operational in its 23rd year on orbit; the mission is planned to continue for the foreseeable future. Every 2 years, all NASA space missions are required to submit a senior review proposal, the next one is due at the end of February. These submissions are required by congress to continue funding of the mission. 10)
The primary science objectives of the Wind mission are: 11)
- Provide complete plasma, energetic particle and magnetic field for magnetospheric and ionospheric studies.
- Investigate basic plasma processes occurring in the near-Earth solar wind.
- Provide baseline, 1 AU, ecliptic plane observations for inner and outer heliospheric missions.
• In 2016, the Wind mission is in its 22nd year on orbit operating nominally.
• July 2015: The Wind mission is approved by NASA to continue planning against the current budget guidelines. Any changes to the guidelines will be handled through the budget formulation process. The Wind mission will be invited to the 2017 Heliophysics Senior Review. 12)
- Wind was launched in November 1994 to the Earth’s L1 Lagrange point where it spent a number of years before performing maneuvers to move to other vantage points. It executed a number of magnetospheric petal orbits to examine high latitudes. Between 2000 and 2002 Wind moved away from the Sun-Earth line reaching 350 RE in the east-west direction. In 2003 it completed an L2 campaign taking it 500 RE downstream of ACE. Since 2004, Wind has been back at L1 where it is currently in an L1 halo orbit upstream of the Earth, spin stabilized with spin axis aligned with the ecliptic south, which is perpendicular to that of ACE. Wind carries 8 instrument suites that measure thermal to energetic solar particles, quasi-static fields to high frequency radio waves, and gamma rays.
• June 2015: The 2015 Heliophysics Senior Review panel undertook a review of 15 missions currently in operation in April 2015. The panel found that all the missions continue to produce science that is highly valuable to the scientific community and that they are an excellent investment by the public that funds them. 13)
- The spacecraft and instruments are in overall good health and Wind continues to be a remarkably productive mission scientifically. By acting as a near-Earth measurement point and reference for comparison with other spacecraft in terms of solar wind and energetic particle data, Wind enables multipoint ICME (Interplanetary Coronal Mass Ejection) analyses and studies of energetic particle acceleration processes. In addition, the mission’s long-term solar wind and field data sets have recently been reanalyzed by the instrument teams, producing groundbreaking discoveries regarding fundamental plasma processes such as instabilities, wave-particle interactions, shocks, and reconnection, which are directly relevant to NASA’s heliospheric research objectives.
- The Wind project’s PSGs (Prioritized Science Goals) for 2016–2020 are: 1) Continuation of studies of the long-term variation of solar wind abundances and energetic particles over two solar cycles; 2) A new investigation of solar wind magnetic and particle structure with the help of ACE and DSCOVR; 3) A new study to determine kinetic signatures of heating and acceleration; and 4) Continued studies of dust particles in interplanetary space. These topic areas and PSGs are all consistent with the goals and objectives of NASA science and heliophysics research.
- WIND Science Strengths: The high quality Wind data continue to be useful in a remarkably wide range of studies and are used by a large community, as evidenced by over 1 million data downloads from CDAWeb (Coordinated Data Analysis Web) in 2013–2014. Of particular note are the high time resolution field measurements and high precision and high cadence measurements of thermal and suprathermal plasma populations. These full 3D distributions of protons and electrons from the bulk population to those at higher energies are the best available measurements for undertaking studies of fundamental space plasma physics processes such as reconnection, shocks, turbulence, and instabilities.
- WIND value to the HSO
(Heliophysics System Observatory): Wind contributes several unique
measurements to the HSO. Its sensitive radio measurements
Located upstream of the Earth, Wind shares with ACE and DSCOVR the duty as a monitor of the near-Earth solar wind. Studies combining Wind data with those from THEMIS and Van Allen Probes, along with MMS, will continue to provide vital information on the effects of interplanetary plasma, fields, and energetic particles on conditions in near-Earth space. — While ACE and Wind are currently very healthy, they have been in operation for many years. If ACE or Wind should fail, the other can supply most of the crucial measurements needed by the other HSO missions and backup DSCOVR well into the future.
- WIND spacecraft / instrument health and status: Seven of the eight Wind instruments, including all of the field and particle suites, remain largely or fully functional. The TGRS γ-ray instrument is the only one that is turned off. The specific degradations in instrument capabilities are the following: the APE and IT detectors of the EPACT instrument, covering the highest energy ranges do not work. The LEMT and STEP telescopes of the same instrument continue to operate normally. On the SMS instrument, the SWICS solar wind composition sensor was turned off in May 2000. The SMS MASS sensor is in a fixed voltage mode that allows reduced science data collection.
During the past few years, the spacecraft experienced a few instrument latch-ups and single bit flight software errors most likely due to high energy particle single event upsets. On October 27, 2014, the command and attitude processor suffered two single event upsets that resulted in a complete loss of data from October 27, 2014, until November 7, 2014, and a partial data loss from all instruments between November 7, 2014, and November 20, 2014, after which full functionality to all instruments was recovered.
• January 2015: As each year draws to a close, it is common for individuals and organizations to look back to see how far they’ve come and to consider where they’ll go next. For NASA’s Wind spacecraft, which was launched near the end of 1994 to collect a data concerning the solar wind, what lies behind it is now 20 years worth of exploration and information gathering, with up to another 60 years to go from here. 14)
- Wind launched on Nov. 1, 1994, about a year and a half before its sister satellite Polar, which ceased operations in 2008. The two spacecraft comprise the Global Geospace Science (GGS) campaign, created to get a core comprehensive understanding of the solar wind — a stream of particles in the form of plasma that is released continuously from the Sun’s upper atmosphere — and its effects on the Earth’s magnetosphere.
- GGS was extended into the International Solar-Terrestrial Physics (ISTP) Science Initiative, which currently still includes in Geotail from JAXA (Japan Aerospace Exploration Agency), Cluster from ESA (European Space Agency) and the SOHO (Solar and Heliospheric Observatory) from NASA and ESA. According to NASA’s website, the goal of ISTP is also “to understand the physical behavior of the solar-terrestrial system in order to predict how the Earth’s magnetosphere and atmosphere will respond to changes in solar wind.” Additionally, these spacecraft are part of a still-larger fleet called the HSO (Heliophysics System Observatory).
- The Wind spacecraft plays the role of “scout and sentry” in the ISTP fleet, tasked with measuring “crucial properties of the solar wind before it impacts the Earth’s magnetic field and alters the Earth’s space environment (which contains charged particles, electric and magnetic fields, electric currents and radiation) and upper atmosphere in a direct manner,” according to NASA.
Figure 9: The Wind spacecraft from NASA's Heliophysics System Observatory has now spent two decades observing particles from the solar wind (image credit: NASA)
- For its initial mission, Wind was sent into an elliptical orbit around the Lagrangian point, designated as L1. Lagrangian points experience balanced gravity from both the Earth and the Sun, meaning that a third object can be placed at any of these points and stay there relative to the other two bodies. There are five such points in the Earth-Sun system, and L1 lies between Earth and the Sun, about 1.5 million km from the Earth.
- When the solar wind monitor called the ACE (Advanced Composition Explorer) was launched in 1997 and also moved into orbit around L1, NASA took the opportunity to move Wind to L2, the Lagrangian point on the other side of the Earth. From there, it was able to provide measurements from deep within the magnetotail, which is the extension of the magnetosphere that trails behind the Earth.
- Between the years 2000 and 2003, Wind was moved “through a variety of positions, including off to the side of the magnetosphere, 1.5 million miles away from Earth, and a return trip to the magnetotail,” according to NASA. In 2004, Wind was brought back to L1 permanently, where it remains operational and in orbit today.
Figure 10: Diagram of the Lagrangian Points associated with the Sun-Earth system (not to scale), image credit: NASA, WMAP Science Team
- Though it had a planned five-year mission, Wind has enough fuel to continue orbiting L1 until 2074, eight decades after its original ascent. It was built with a certain amount of longevity in mind, and is expected to bring in further useful information for a very long time.
- On March 13, 2015, NASA launched the MMS (Magnetospheric Multiscale) mission, to learn more about magnetic reconnection, which is a process that occurs between the Earth and the Sun when their magnetic fields connect and disconnect (Ref. 14).
• In 2014, the WIND spacecraft and its payload are operating 'nominally'. The Wind mission has been extended nominally for ten years, but of course, funding is only provided till the next Senior Review in two years. Also, all is contingent on the continued healthy operation of the spacecraft. 15)
WIND is a venerable spacecraft approaching its 20th
year of operations. Currently in L1 halo upstream of Earth, WIND
carries a comprehensive package of instruments to measure the plasma
and fields around the spacecraft, including energetic particles and
• In 2013, the WIND spacecraft and its payload are operating 'nominally'. Equipped with heavy shielding and double-redundant systems to safeguard against failure, the spacecraft was built to last. The WIND mission has survived almost two complete solar cycles and innumerable solar flares. 17) 18) 19)
Using data from an aging WIND spacecraft of NASA, researchers have found signs of an energy source in the solar wind that has caught the attention of fusion researchers. NASA will be able to test the theory later this decade when it sends a new probe into the sun for a closer look (Ref. 17).
The discovery was made by a group of astronomers trying to solve a decades-old mystery: What heats and accelerates the solar wind?
The solar wind is a hot and fast flow of magnetized gas that streams away from the sun's upper atmosphere. It is made of hydrogen and helium ions with a sprinkling of heavier elements. Researchers liken it to the steam from a pot of water boiling on a stove; the sun is literally boiling itself away.
But the solar wind does something that steam in the kitchen never does. As steam rises from a pot, it slows and cools. As solar wind leaves the sun, it accelerates, tripling in speed as it passes through the corona. Furthermore, something inside the solar wind continues to add heat even as it blows into the cold of space.
Finding that "something" has been a goal of researchers for decades. In the 1970s and 80s, observations by two German/US Helios spacecraft set the stage for early theories, which usually included some mixture of plasma instabilities, magnetohydrodynamic waves, and turbulent heating. Narrowing down the possibilities was a challenge. The answer, it turns out, has been hiding in a dataset from one of NASA's oldest active spacecraft, a solar probe named Wind.
Figure 11: An artist's concept of the Wind spacecraft sampling the solar wind. Justin Kasper's science result is inset (image credit: NASA)
The source of the heating in the solar wind is ion cyclotron waves. Ion cyclotron waves are made of protons that circle in wavelike-rhythms around the sun's magnetic field. According to a theory developed by Phil Isenberg (University of New Hampshire) and expanded by Vitaly Galinsky and Valentin Shevchenko (UC San Diego), ion cyclotron waves emanate from the sun; coursing through the solar wind, they heat the gas to millions of degrees and accelerate its flow to millions of miles per hour. - Justin Kasper of the Harvard-Smithsonian Center for Astrophysics, Massachusetts, and collaborators present a model that demonstrates how certain plasma waves, called ion cyclotron waves, will preferentially heat heavier ions travelling below a threshold velocity. Kasper's findings confirm that ion cyclotron waves are indeed active, at least in the vicinity of Earth where the WIND spacecraft operates. 20) 21)
Plasma carrying a spectrum of counterpropagating field-aligned ion-cyclotron waves, can strongly and preferentially heat ions through a stochastic Fermi mechanism. Such a process has been proposed to explain the extreme temperatures, temperature anisotropies, and speeds of ions in the solar corona and solar wind. The team of researchers quantified, how differential flow between ion species results in a Doppler shift in the wave spectrum, that can prevent this strong heating. Two critical values of differential flow were derived for strong heating of the core and tail of a given ion distribution function. The comparison of these predictions to observations from the Wind spacecraft reveals excellent agreement. Solar wind helium, that meets the condition for strong core heating, is nearly 7 times hotter than hydrogen on average. Ion-cyclotron resonance contributes to heating in the solar wind, and there is a close link between heating, differential flow, and temperature anisotropy (Ref. 21).
• In 2012, the WIND spacecraft and its payload are operating 'nominally' in their 18th year on orbit. 22)
• The WIND spacecraft is operating almost nominally in 2010 (more than 16 years after launch). It passed the last two Senior reviews and is now in orbit around L1, out of phase with ACE. It will probably last until something goes wrong with the tape recorder, which is not solid state. The following arguments represent a rationale for continuing the Wind mission: 23) 24) 25)
- Wind continues to provide unique and robust solar wind measurements
- Wind is a 3rd solar wind vantage point for STEREO providing a backup capability and enhanced science return for 3-point studies
- Wind and ACE together provide a reasonable probability of maintaining near-Earth solar wind monitoring capabilities for NASA into the next decade
- Wind and ACE are complementary not identical. Thus both are needed to continue to provide complete near-Earth, 1 AU baseline observations for current and future NASA deep space missions.
- Wind’s scientific productivity remains high and its observations continue to lead to significant scientific discoveries for all three research objectives of NASA’s SMD (Science Mission Directorate).
• In 2008, the Wind spacecraft continues to operate in very good health. In 2000, the mission team successfully reconfigured the communications system to enhance the telemetry margin. Reliance on a single digital tape recorder since 1997 has never hampered operations, and the team took measures to minimize its use in order to extend tape recorder life as long as possible.
Seven of the eight Wind instruments, and all of the particles and fields instruments, remain largely or fully operational. Specifically, the EPACT, high energy particle and SMS solar wind composition instruments suffered some degradation, but both continue to provide valuable measurements. The SWE electron instrument required some reconfiguration to maintain its capabilities and the TGRS γ-ray detector, well beyond its design life, has been turned off.
Sensor complement: (MFI, WAVES, SWE, EPACT, SMS, PLASMA, TGRS, KONUS)
The sensor complement allows the constant monitoring of the solar wind plasma, energetic particles, magnetic fields, radio and plasma waves found in the interplanetary medium as well as cosmic gamma ray bursts. 26)
MFI (Magnetic Field Investigation):
MFI PI: R. Lepping, NASA/GSFC. Objectives: investigation of the structure and fluctuations of the interplanetary magnetic field (transport of energy and acceleration of particles in the solar wind). Instrument: Magnetometer measures the intensity and direction of magnetic field vector. The MFI instrument consists of dual triaxial fluxgate magnetometers mounted on a 12 m radial boom, and a data processing and control unit within the spacecraft bus. The magnetometer sensors each produce analog signals proportional to the strength of the magnetic field component aligned with the sensor. These signals are then digitized and processed by a microprocessor controlled data system.
Table 1: Summary characteristics of the MFI instrument
Figure 12: Illustration of the MFI instrument (image credit SSL/UCB)
WAVES (Radio and Plasma Wave Experiment):
WAVES PI: J. Bouqeret, Observatoire de Meudon, France. WAVES was built as a joint effort of the Paris-Meudon Observatory, the University of Minnesota, and the Goddard Space Flight Center (GSFC). Objectives: measurement of the radio and plasma wave phenomena over a very wide frequency range which occur in the solar wind. Specific objectives call for:
• Low-frequency electric waves and low-frequency magnetic fields, from DC to 10 kHz
• Electron thermal noise, from 4 kHz to 256 kHz
• Radio waves, from 20 kHz to 14 MHz
• Time domain waveform sampling, to capture short duration events which meet quality criteria set into the WAVES data processing unit (DPU).
1) Three electric dipole antenna systems provided by Fairchild Space (two are coplanar, orthogonal wire dipole antennas in the spin-plane, the other a rigid spin-axis dipole). The longer and shorter spin plane dipoles have lengths of 50 m and 7.5 m for each wire, respectively, while each spin-axis dipole extends 5.28 m from the top and bottom surfaces of the spacecraft.
2) Three magnetic search coils mounted orthogonally (designed and built by the University of Iowa). The triaxial magnetic search coil for measuring bi-frequency magnetic fields is mounted at the outboard end of a 12 m radial boom.
WAVES instrument elements: There are five main receiver systems: a bi-frequency (DC to 10 kHz) Fast Fourier Transform receiver, a broadband (4 kHz to 256 kHz) electron thermal noise receiver, two swept-frequency radio receivers (20 kHz to l MHz, and l MHz to 14 MHz), and a time domain waveform sampler (up to 120,000 samples per second). The DPU controls and acquires data from all operations of the experiment, and can be reprogrammed from the ground. The receiver systems and DPU are housed within the spacecraft body. WAVES has onboard interconnects with 3-D PLASMA and with SWE.
Note: In Table 2 the spin-plane electric antennas are referred to as Ex and Ey, the axial electric antenna as Ez and the 3 search coil magnetic axes as Bx, By and Bz.
SWE (Solar Wind Experiment):
SWE PI: K. Ogilvie, GSFC. The instrument development is a joint effort of GSFC, UNH (University of New Hampshire), and MIT (Massachusets Institute of Technology). Objectives: measurement ions and electrons in the solar wind and the foreshock regions. Rates of once per minute for ions and 20 times per minute for electrons. Deduction of solar wind velocity, density, temperature, and heat flux. Specific measurement objectives are: 29) 30)
- To provide high time-resolution 3-D velocity distributions of the ion component of the solar wind, for ions with energies ranging from 200 eV to 8.0 keV
- High time-resolution 3-D velocity distributions of subsonic plasma flows including electrons in the solar wind and diffuse reflected ions in the foreshock region, with energies ranging from 7 eV to 22 keV
- High angular resolution measurements of the ”strahl” (beam) of electrons in the solar wind, along and opposite the direction of the interplanetary magnetic field, with energies ranging from 5 eV to 5 keV.
The SWE instrument consists of five integrated sensor/electronics boxes and a data processing unit (DPU). The sensor units are mounted on the top and bottom shelves of the spacecraft, extending through the top and bottom surfaces. The 3-D velocity distribution measurements of the ion component in the solar wind are made by a pair of Faraday cup analyzers, which provide a wide field-of-view and the capability for flow characterization within one spin revolution (3 seconds).
• The SWE instrument includes 2 Faraday cup ion detectors provided by MIT. The Faraday cups provide measurements of the solar wind protons and alpha particles at energy/charge up to 8 keV. The Faraday cup sensor contains a series of wire-mesh, planar grids knitted from tungsten wire and one or more collector plates. The velocity distribution function of ions is measured by applying a sequence of voltages to the ”modulator” grid. With voltage V applied to the grid, only particles having energy/charge (E/Q), greater than V will be able to pass through the grid and continue on to strike the collector plate where they produce a measurable current.
• VEIS (Vector Electron Ion Spectrometers). VEIS of the SWE instrument consists of an array of detectors built by NASA/GSFC for characterizing the solar wind electrons. The SWE on-board data system was provided by UNH. The 3-D velocity distribution measurements of ions and electrons in plasmas having Mach numbers < 1 are made using six cylindrical electrostatic deflection analyzers arranged in two triaxial sets.
• The SWE Strahl sensor consists of a toroidal electrostatic analyzer with channel-plate detectors. The objective is to measure the solar wind strahl, the narrow beam of electrons which travel outward from the sun closely aligned with the interplanetary magnetic field. During one 3 s rotation of the spacecraft, the strahl sensor makes high angular resolution measurements of the electron velocity distribution within a field of view 50º x 50º centered on the average magnetic field direction looking toward the sun, and one-half rotation later, measurements are made along the average field direction looking away from the sun. The measurement range is from 5 eV to 5 keV.
Figure 13: Vector electron ion spectrometer of SWE (image credit: NASA)
Figure 14: Faraday cup of SWE (image credit: NASA)
Figure 15: The Strahl detector of SWE (image credit: NASA)
EPACT (Energetic Particles Acceleration, Composition, Transport):
EPACT PI: T. T. von Rosenvinge, NASA/GSFC. Objectives: investigation of the elemental and isotopic abundances of the minor ions making up the solar wind with energies in excess of 20 keV. Measurements at a rate of once per minute for ions and 20 times per minute for electrons. Deduction of solar-wind velocity, density, temperature, and heat flux. 31)
The EPACT assembly consists of multiple telescopes that also provide a level of protection against single-point failures. The LEMT (Low Energy Matrix Telescope) consists of three identical telescopes, whereas ELITE (Electron Isotope Telescope) consists of two APE (Alpha-Proton-Electron) telescopes and an IT (Isotope Telescope). LEMT and ELITE have been designed, built and tested by the Low Energy Cosmic Ray Group and the Electronics Systems Branch of the Laboratory for High Energy Astrophysics at NASA/GSFC. Later, the STEP (Suprathermal Energetic Particle telescope system) was added to EPACT. STEP contains two identical telescopes. STEP was designed and built by the University of Maryland.
The APE and IT instruments are contained in a single package known as the ELITE. These solid state detector telescopes all use the dE/dx by E method of particle identification, except STEP, which obtains particle mass by measuring time-of-flight and energy. An onboard recorder allows continuous observations to be made.
Figure 16: Illustration of the LEMT assembly (image credit: NASA)
Figure 17: Illustration of ELITE (image credit: NASA)
Figure 18: The STEP (Supra Thermal Energetic Particle) telescope (image credit: NASA)
SMS (Solar Wind Ion Composition Study), the Mass Sensor, and Suprathermal Ion Composition Study). SMS is consisting of: SWICS (Solar Wind Ion Composition Spectrometer), STICS (Suprathermal Ion Composition Spectrometer) and MASS (Mass Resolution Spectrometer); PI: G. Gloeckler, University of Maryland. Science objectives: determine the abundance, velocity, spectra, temperature, and thermal speeds of solar-wind ions (plasma investigations in conjunction with EPACT) entering Earth's magnetosphere. The following measurements are obtained: 32)
- Energy, mass and charge composition of major solar wind ions from H to Fe, over the energy range from 0.5 to 30 keV/e (SWICS)
- High mass-resolution elemental and isotopic composition of solar wind ions from He to Ni, having energies from 0.5 to 12 keV/e (MASS)
- Composition, charge state and 3-D distribution functions of suprathermal ions (H to Fe) over the energy range from 8 to 230 keV/e (STICS).
The SMS experiment consists of five separate packages mounted on the spacecraft body. SWICS uses electrostatic deflection, post-acceleration, and a time-of-flight vs. energy measurement to determine the energy and elemental charge state composition of solar wind ions.
MASS uses energy/charge analysis followed by a time of flight measurement, to determine solar-wind ion composition with high mass-resolution (M/delta M > 100), for the first time.
STICS, similar to SWICS but not using post-acceleration, has a large geometric factor and wide angle viewing for studies of suprathermal ions.
Figure 19: Schematic side view of the SWICS instrument (image credit: University of Maryland)
Figure 20: Schematic side view of the MASS instrument (image credit: University of Maryland)
PLASMA (3-D Plasma and Energetic Particles Experiment):
PLASMA PI: R. Lin, SSL/UCB (Space Science Laboratory/University of California, Berkeley). Objectives: measurement of ions and electrons with energies above that of the solar wind and into the energy particle range. Energy range: 0.03 - 30 keV, sampling rate: 20 times per minute; wide angular coverage, good directional sensitivity. Study of particles in the bow shock and in the foreshock regions. 33)
Figure 21: Illustration of the PLASMA instrument (image credit: UCB)
PLASMA consists of three basic elements, each is designed to cover a different part of the suprathermal particle population.
• SST (Semiconductor Telescopes)
• EESA (Electron Electrostatic Analyzers)
• PESA (Ion Electrostatic Analyzers)
In addition, a Fast Particle Correlator (FPC) combines electron data from the electron analyzer with plasma wave data from the WAVES experiment to study wave-particle interactions.
To avoid effects of the spacecraft potential on the low energy particle detection, EESA-L and EESA-H are mounted on the end of a 0.5 m boom, while PESA-L, PESA-H and the SSTs are mounted on the end of an opposing 0.5 m boom.
Table 3: Specification of the PLASMA instrument
The default mode of operation of the WIND 3-D experiment includes the following features:
- PESA and EESA detectors are swept over their energy range 32 or 64 times per spacecraft spin. Moments (density, velocity, pressure tensor, heat flux) computed on-board, three dimensional distributions with various energy and angular resolutions, and pitch angle distributions are telemetered
- SST data are collected 16 times per spin for 16 or 24 energy channels. Spectra and three dimensional distributions are computed and telemetered at rates depending on the energy band (higher rates for the higher-flux lower energies), and pitch angle distributions are computed.
- Another special mode involves use of the FPC which performs three types of correlation: direct wave-particle correlations, auto-correlations, and burst correlations.
TGRS (Transient Gamma Ray Spectrometer):
TGRS PI: B. Teegarden, NASA/GSFC. TGRS is a collaboration between GSFC and CNRS/CESR (CNRS/Centre d'Etude Spatiale des Rayonnements), Toulouse, France. Objectives: observation of transient gamma-ray events, spectroscopic survey of cosmic gamma-ray transients, measurements of gamma-ray lines in solar flares. Specific measurement objectives are: Measurement ranges: 15 keV - 8.2 MeV. 34) 35)
- Spectroscopic measurements of transient gamma-ray events, in the energy range from 15 keV to 10 MeV with an energy resolution of 2.0 keV @ 1.0 MeV (E/delta E = 500)
- Monitoring of the time variability of the 511 keV line emission from the galactic center, on time scales from ~2 days to >1 year.
The TGRS instrument consists of four assemblies: detector cooler assembly, pre-amp, and analog processing unit, all mounted on a tower on the +Z end of the spacecraft, and a digital processing unit mounted in the body of the spacecraft. The detector is a 215 cm3 high purity n-type germanium crystal of dimensions: 6.7 cm (diameter) x 6.1 cm (length), sensitive to energies in the 20-8000 keV band. The detector is kept at its operating temperature of 85 K by a passive radiative cooler (a two-stage cooler surrounds the detector, providing a field of view of 170º). The resolution of TGRS at 500 keV is about 2 keV FWHM (Full Width at Half Maximum). The TGRS detector has no active shielding and is permanently exposed to ~1.8 steradian of the southern Galactic hemisphere which is unobstructed by the cooler. The radiation environment experienced by TGRS is dominated by two components: diffuse cosmic hard X and gamma radiation, and Galactic cosmic rays.
The germanium detector serves as a reaction medium for incoming gamma rays, which, depending on their energy, are either stopped by or passed through the detector crystal. Particle energy and angle of incidence are calculated based on a number of primary and secondary interaction processes, including photoelectric, Compton, pair and bremsstrahlung radiation as well as the ionization energy losses of secondary electrons.
When a burst or flare occurs, the instrument switches to a burst mode, where each event in the detector is pulse-height analyzed and time tagged in a burst memory. Then the instrument switches to a dump mode for reading out the burst memory.
KONUS (Gamma Ray Burst Investigation):
KONUS PIs: E. Mazets/T. Cline, Ioffe Physical Technical Institute, St. Petersburg, Russia. The GSFC instrument is sponsored by Russia. Science objectives: studies of gamma-ray burst and solar flares in the energy range 10 keV to 10 MeV (similar to the TGRS studies). KONUS has a lower resolution than TGRS but a broader coverage to complement TGRS. KONUS also performs event detection and measures time history. The measurement objectives are: 36)
- Monitoring of gamma-ray burst (GRB) energy spectra over the energy range 10 keV to 10 MeV, with energy resolution E/delta E = 15 @ 200 keV
- Measurement of burst time histories in three energy ranges covering 10 to 770 keV
- High-time-resolution measurement (2 ms resolution) for the high-intensity sections of a burst time history
- Continuous measurement of the gamma ray and cosmic ray background, interrupted only to read out bursts.
The Konus instrument consists of two Russian sensors mounted on the top and bottom of the spacecraft aligned with the spin axis, a U.S. interface box, and a Russian electronics package mounted in the spacecraft body. The sensors, copies of ones successfully flown on the Soviet COSMOS, VENERA and MIR missions, are identical and interchangeable Nal scintillation crystal detectors of 200 cm2 area, shielded by Pb/Sn. The design and location of the two sensors ensure practically isotropic angular sensitivity. The relative count rates recorded by the two detectors provide a burst source locus to within a few degrees relative to the spin axis. Onboard analysis of background and burst events is performed by four pulse height analyzers, four time history analyzers, two high resolution time history analyzers and a background measurement system.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).
The co-operative ISTP program: