SPACE PHYSICS

Pager Satellite Failure May Have Been Related to Disturbed Space Environment

A period of particularly bad "space weather" may have played a part in failure of the Galaxy 4 satellite, which silenced about 80% of the United States' pagers last May.

by D. N. Baker, J. H. Allen, S. G. Kanekal, and G. D. Reeves

A very intense flux of electrons, evident in the magnetosphere earlier this year, may have caused a satellite failure (or at least exacerbated the situation) leading to the loss of telephone pager service to 45 million customers, research has shown. The electrons, known as highly relativistic electrons (HREs), were especially numerous in the weeks preceding the failure. Researchers say HREs have triggered spacecraft anomalies in the past when fluxes are elevated. They therefore believe this energetic electron event could have been behind the failure of the attitude control system of the Galaxy 4 spacecraft at 2200 UT on May 19, 1998. A backup system also failed, either at the same time or earlier, so operators were unable to maintain a stable Earth link.

Galaxy 4 is a heavily used communication satellite at geostationary orbit*. Its sudden failure caused not only widespread loss of pager service but also numerous other communication outages. Using a wide array of datasets, our team of scientists analyzed the space environment for the times in question and found evidence of highly disturbed solar, solar wind*, and geomagnetic conditions in late April and early May. The combination of coronal mass ejections*, solar flares*, and high speed solar wind streams led to a powerful sequence of interplanetary disturbances that hit the Earth. These disturbances produced a deep, powerful, and long-lasting enhancement of the HRE population throughout the outer Van Allen radiation zone. The kinds of disturbances witnessed are indicative of the types of events that may commonly occur during the approaching peak in solar activity in the years 2000 and 2001. It will be most important to determine how well space systems can stand up to the multifaceted effects of the space environment over the next several years.

The evidence is strong that HRE fluxes were substantially elevated above average conditions for a period of about 2 weeks before the Galaxy 4 failure. Long-duration HRE enhancements have in the past been convincingly associated with spacecraft operational failure. For example, we know that high fluxes of energetic electrons can lead to a buildup of electric charge deep inside of spacecraft subsystems. In this process, energetic electrons bury themselves in poorly conducting material such as thermal control blankets, electronic boards, coaxial cables, and insulation. Eventually, if the charge builds up more rapidly (because of electrons continuing to hit the spacecraft) than it leaks away (because of low material conductivity), then there can be an electrostatic discharge event. This is much like small, powerful lightning discharge inside of the spacecraft. Such a discharge can damage or destroy a sensitive circuit or subsystem, and the result can be a spacecraft failure.

Scientists involved in the analysis have noted that whether or not the incident on May 19 was caused by "space weather," it nonetheless shows the vulnerability of society to a single spacecraft failure. The vast number of users affected by the loss of just one spacecraft shows how dependent society is on space technology and how fragile communication systems can be. The Galaxy 4 failure had a large impact because the spacecraft was optimally located over the central United States and could best handle digital pager signals. Eighty percent of all pager traffic was being directed through it. Increasingly, phones, televisions, radios, bank transactions, newspapers, credit card systems, and the like depend on satellites for at least part of their links. It seems very inadvisable to have such complex, societally significant systems susceptible to single-spacecraft failures. This seems particularly true as the peak of the 11-year solar activity cycle, in 2000-2001, approaches.

The typical major communication spacecraft has an estimated value of $200-250 million. Over 100 such spacecraft are in operation today and whole new groups of low- to mid-altitude satellites are being placed in orbit. The invested cost of space assets is staggering. Given the recent record of space environmental disturbances, space researchers believe many more highly disruptive spacecraft failures may occur. They suggest that space systems be made immune to the space environment and backup systems be made readily available to cover space system failure whatever the cause.

Energetic Particle Observations

A detailed view of the magnetospheric particle population was obtained from the SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer) spacecraft, which is in a high-inclination (82°), low-altitude (~600 kilometer) orbit. As such, it samples magnetic fields connecting to almost the whole magnetosphere about every 100 minutes. SAMPEX carries sensor systems capable of measuring very energetic ions and electrons of both solar and magnetospheric origin. Figure 1a is a plot of particle fluxes measured by a large array of solid-state detectors in the Heavy-Ion Large Telescope (HILT) onboard SAMPEX.

	Figure 1
1a
1b

In Figure 1a, particle intensity is plotted according to the color bar to the right of the figure. The vertical axis of the figure is called the "L-parameter" which indicates the magnetic field line position: L is the distance in Earth radii (1 R_E=6372 kilometers) that a given magnetic dipole field line would cross in the equatorial plane. The horizontal axis of Figure 1a is day of year (DOY) for 1998. SAMPEX data are plotted from April 1 (DOY 91) to May 31 (DOY 151).

The data in Figure 1a show rather quiet conditions from DOY 91 to DOY 110. A modest, variable flux of electrons with energies greater than 3 million electron Volts (MeV) in the outer radiation zone between L 3.5 and L 6.0 occurred during that interval. The nature of the radiation environment at low altitudes changed dramatically on DOY 110. A solar energetic particle event began that day and continued for a week or so. Very energetic ions (also identified by other SAMPEX sensors) were seen with high intensities and extending throughout the polar cap region. On DOY 115 (April 25) there was an abrupt increase in HRE fluxes deep in the outer zone peaking near L=4.0. The electron fluxes remained high for the next several days. Another sequence of solar energetic particle events then occurred sporadically until at least DOY 130 (May 10).

Perhaps the most striking and notable event in this entire interval was a very large increase of the flux of HREs very deep in the magnetosphere on DOY 124 (May 4). The so-called "slot region" between the inner and outer Van Allen belts was filled up and another radiation belt feature appeared. The relativistic electrons remained high throughout the outer Van Allen zone for at least the next 2 weeks. Electrons filled a broader region from L=3 to beyond L=5 over the next several-day interval. The relativistic electron enhancement seen in Figure 1a was as intense, long-lasting, and high-energy as any event seen in the magnetosphere over the past several years. Another view of the magnetosphere is offered by the Polar spacecraft, which is in a high-inclination, highly eccentric orbit. As such, it cuts through the Earth's outer Van Allen zone at much higher altitudes than does SAMPEX. The Polar payload includes the HIgh-Sensitivity Telescope (HIST), which measures very energetic (E>0.3 MeV) electrons. Polar obtains high-resolution views of the outer electron belt every 17.5 hours (the Polar orbital period) on one inbound and one outbound pass.

Figure 1b shows Polar HIST data for electrons with E>2 MeV. The format is nearly identical to that for Figure 1a: The flux values are plotted as a function of L and time for DOY 91 (April 1) to DOY 151 (May 31). The Polar data show many of the same features that were present in the SAMPEX data. However, Polar does not survey the region below L~2.5, so it cannot be used in the inner part of the magnetosphere. Where SAMPEX and Polar L values do overlap, the data illustrate the global, coherent nature of HRE events and confirm the very intense flux of relativistic electrons in the magnetosphere in May.

Figure 2 provides a broad, recent history of the energetic electron fluxes measured at geostationary orbit* (GEO) by Los Alamos National Laboratory sensors. The fluences* in each energy range clearly peaked on or just before May 19, around the time the Galaxy 4 spacecraft failed. The May 1998 event was the longest duration and highest energy electron event seen in the past 3 years.

Figure 2

Fig. 2. Electron fluences (cm2) from January 1997 through May 1998 measured by Spacecraft 1994-084 at geostationary orbit. Peak values of the three energy ranges shown all occurred in May 1998, around the time of the Galaxy 4 satellite failure.

The GOES series of spacecraft operated by the National Oceanic and Atmospheric Administration (NOAA) measures the space environment continuously at geostationary orbit. Figure 3a shows the daily average flux of E>2 MeV electrons from April 21, 1998, to May 20, 1998. The electron flux was low on April 21, but then rose progressively over the next week or so, reaching a flux maximum on April 29. The electron intensities then were lower for several days (May 1-4). The average electron intensity jumped up by about two orders of magnitude on May 5 and stayed high for the next 10 days. On May 16, the electron flux diminished by a factor of 2-3, but it remained well above 10⁷ until the end of the plotting sequence. It is important to be aware not only of the GEO electron environment, but also of the energetic ions. Figure 3b shows the time history of the flux of protons (E>100 MeV) measured by the GOES sensors. The period covered is identical to that for electrons in Figure 3a. The data in Figure 3b show several discrete proton enhancements, consistent with those detected at high L values by SAMPEX, as shown in Figure 2a, on April 21, May 2, and May 6.

	Figure 3
3a
3b
	Fig. 3. a) GOES daily flux values of electrons with E>2 MeV for the period from April 21, 1998, to May 20, 1998. Dates of various spacecraft operational problems are noted including the Galaxy 4 failure on May 19. b) Similar to a) but for protons with E>100 MeV (courtesy of H. Singer).

Occurrence of Spacecraft Anomalies

The most significant spacecraft operational anomaly of concern during the period under investigation was of course the Galaxy 4 failure on May 19. The time of failure is noted with arrows on Figures 1, 2, and 3. It is evident that the Galaxy failure occurred after a very long period of elevated high-energy electron fluxes but was not associated with any high-energy solar particle events.

Other spacecraft problems of note during the period included a major Polar anomaly on May 6 and the failure of the Equator-S spacecraft on May 1. The Polar event consisted of a loss of about 6 hours of data because of a processor problem onboard the spacecraft. This was almost certainly due to a single-event upset of the type caused by ion impacts on spacecraft memory. The solar particle event on May 6 was characterized not only by E>100 MeV protons, but also by remarkably high fluxes of very energetic iron nuclei as seen by the ACE satellite. The Polar anomaly occurred within the first moments of the appearance of the very energetic solar particles. The Equator-S failure on May 1 consisted of a loss of the spacecraft central processor (Max Planck Web site: http://www.mpe.mpg.de). The failed processor was actually the backup unit since the primary processor had failed in December 1997. The Equator-S situation has not been analyzed in detail yet, but the failure occurred after a week or more of elevated relativistic electron fluxes throughout the outer radiation zone (Figure 1).

Other Space Environmental Factors

What solar and solar wind conditions led to such remarkable magnetospheric particle events? The International Solar Terrestrial Physics group of spacecraft and the related scientific and operational satellites available today give an unprecedented view of this matter. SOHO (Solar and Heliospheric Observatory) showed a large coronal mass ejection on May 2, 1998. SOHO and GOES sensors also observed a series of large solar flares that gave rise to the energetic particles subsequently detected by SAMPEX and other near-Earth spacecraft (see Figures 1-3). The active sun also produced powerful streams of solar wind plasma that were detected upstream of the Earth. Figure 4 shows the solar wind speed (V_IMF), the interplanetary magnetic field (IMF) north-south component (B_z), and the total IMF strength (B_IMF), for the period DOY 110-140. These data are from the WIND spacecraft.

	Figure 4
	Fig. 4. Solar wind speed, interplanetary magnetic field (IMF) north-south (Bz) component, and IMF strength (BIMF) for day 110 (April 21) to day 140 (May 20) of 1998 as measured by the WIND spacecraft (data courtesy of K. Ogilvie and R. Lepping).

From DOY 121 through DOY 140, four separate streams were recorded in which V_IMF reached peak values of 600 kilometers per second. Such streams are very effective at producing subsequent magnetospheric HRE events. A particularly notable solar wind period occurred on DOY 124 when V_IMF went to ~850 kilometers per second. This is the highest solar wind speed that has been measured near 1 AU in the past several years.

When high solar wind speed occurs in combination with large B_IMF, and especially when B_z is strongly negative, powerful geomagnetic activity and electron acceleration can be expected. Indeed, the planetary magnetic index Kp reached 9 on DOY 124 (May 4). The Kp index is a worldwide measuring tool based on ground magnetometer data. Kp is very "nonlinear", something like the "Richter scale" for earthquakes. The Kp index is usually around 1 or 2. A value of Kp=9 is the largest and strongest geomagnetic disturbance there is. The Dst geomagnetic index on that day reached -218 nano teslas (nT), indicating a major geomagnetic storm, and the provisional auroral electrojet index briefly exceeded 2500 nT. All this is indicative of powerful, global geospace disturbances on May 4. The new radiation belt formation as seen at L=2.2 is only rarely observed and happens under extreme solar wind and geomagnetic conditions.

Why a Particular Spacecraft?

In attempting to associate a given spacecraft anomaly with the space environment, it is often asked why only a particular spacecraft had a problem. Why did other spacecraft not also fail at the same time? Moreover, since the typical geostationary orbit spacecraft has experienced numerous other HRE enhancements, why did they not fail earlier during other events?

Given the probabilistic nature of environmental conditions during catastrophic failure events, one cannot answer these questions definitively. It can be argued in the Galaxy 4 case that the relativistic electrons were at a high flux value for a remarkably long time. The energy spectrum was exceptionally hard as shown by the LANL data (Figure 2). However, many GEO spacecraft made it through without apparent trouble during this time. On the other hand, several GEO spacecraft experienced significant operational problems or anomalies. For example, Japan's GMS satellites experienced more than a dozen operational anomalies from May 4 to 7, 1998.

In the final analysis, as in previous cases, it is quite likely that even under the most severe space environmental stresses, only a few susceptible spacecraft will fail during any particular hostile space weather interval. Normally one cannot "prove" that any particular spacecraft was seriously harmed by the space environment. In general, one can only make a plausibility argument. In some instances, such as the Anik E1 failure in January 1994, laboratory testing virtually removed all doubt. In other cases, only circumstantial evidence exists.

Source: Eos, October 6, 1998, p. 477.

GLOSSARY

coronal mass ejection--large clouds of material expelled from the Sun. These often travel away from the Sun at very high speeds (one to two million miles per hour), especially near sunspot activity maximum. They can contain 10¹⁵ grams (or more) of solar material; Fluence--the number of particles that hit a given area (say a square centimeter) of a spacecraft over a given unit of time. For example, we are often interested in how many 1 million electron Volt particles hit a cm² of spacecraft each day in space: this would be the "daily fluence" of >1MeV electrons; geostationary orbit--the special orbit above the Earth's equator that is at an altitude of 35,600 km (or 22,000 mi). A satellite at this altitude appears to move at the same angular speed as the Earth turns. As such, it stays above the same spot on the Earth indefinitely; magnetosphere--the uppermost part of the Earth's atmosphere. This region is populated by a very tenuous gas (1 to 1000 particles per cubic centimeter) made up mostly of electrons, protons, and atomic nuclei (such as charged oxygen that has moved up from the lower atmosphere), all controlled by the Earth's magnetic field; slot region--the gap that exists (normally) between the inner Van Allen belt and outer Van Allen radiation belt; solar flares--the occasional, powerful energy bursts that occur on the Sun's surface. These often occur in the form of X rays, radio emissions, and powerful bursts of very high energy particles; solar wind--the hot, expanding gas of charged particles that flow away from the Sun toward the cold void of interstellar space. The solar wind contains ions, electrons, and magnetic fields.