Solar-interplanetary phenomena in March 1991 and related geophysical and technological consequences

Radiation Measurements, Vol. 26, No. 3, pp. 427-432, 1996

Pergamon

Copyright © 1996 Publishedby ElsevierScienceLid Printed in Great Britain, All rights reserved PII: S1350-4487(96)00060-1 135o-4487/96 $15.oo+ o.oo

SOLAR-INTERPLANETARY PHENOMENA IN MARCH 1991 A N D RELATED GEOPHYSICAL A N D TECHNOLOGICAL CONSEQUENCES* M. A. SHEA and D. F. SMART Space Physics Division (GPSG), Geophysics Directorate/PL 29 Randolph Road, Hanscom AFB, Bedford, MA 01731-3010, U.S.A, Abstract--A short period of intense solar activity in March 1991 initiated a sequence of major perturbations to the interplanetary and terrestrial environment. The cosmic ray intensity underwent rapid and extreme modulations, and a third radiation belt was created. In addition these perturbations resulted in a wake of disruption to a variety of technological systems designed to operate in a more benign spatial and geophysical environment. The various spacecraft and ground-based anomalies which occurred are associated with the geophysical environment at the time of the occurrence. Copyright © 1996 Published by Elsevier Science Ltd

1. INTRODUCTION Although intense periods of solar activity giving rise to major magnetospheric and geophysical effects usually occur each solar cycle, the 22nd solar cycle has been exceptional among the past three cycles. Six periods of intense solar activity have occurred with each episode having a > 10 MeV solar proton fluence, as measured at the Earth, exceeding 10 ~° protons cm -2. These activity episodes have all resulted in major perturbations of the near-Earth space environment that have adversely affected many space and ground-based systems. This article concentrates on the solar, interplanetary, and related geophysical phenomena in March 1991 which resulted in major perturbations in the cosmic radiation environment and also produced many adverse technological effects. 2. SOLAR ACTIVITY The very dynamic geophysical environment in late March 1991 was the result of a series of solar flares and interplanetary magnetic shock structures. A short duration impulsive solar flare with soft X-ray onset at 2243 UT occurred on 22 March. Located at $26, E28 in N O A A region 6555, this 3B flare had an X-ray magnitude of X9.4 (i.e. equivalent to 9.4 x 10 ±4 W m -e) (Coffey, 1991a,b). The flare was accompanied by strong solar gamma ray and radio emission; it was also the source of energetic solar neutrons detected by the Haleakala, Hawaii neutron monitor (Pyle and Simpson, 1991). The identification of a ground-level response to solar neutrons impacting at the top of the atmosphere is indicative of an extremely powerful solar event. Approximately 3 h later an overlapping sequence of

six optically large solar flares combined to give a composite long duration soft X-ray event of magnitude M6.8. This sequence of solar activity was not associated with strong radio emission nor did any of the individual solar flares have the X-ray emission magnitude of the X9.4 flare of the previous day. The onset of the solar proton event which continued for eight days occurred a few hours later. Although solar protons > 100 MeV were measured at satellite altitudes, no significant increases were reported by ground-based neutron monitors. The magnitude of this entire disturbance--from the particle event to the geomagnetic field perturb a t i o n s - w a s not predictable by present methods. Although these spatial and geophysical disturbances were the result of solar activity primarily from N O A A region number 6555, major solar activity from regions in close proximity to region 6555 makes it difficult to uniquely associate specific solar activity with specific interplanetary and terrestrial phenomena. It was extremely unfortunate that there were no coronal mass ejection measurements or interplanetary magnetic field and plasma data from Earthorbiting spacecraft during the critical times of this event as these data would have greatly assisted in analyses of these major perturbations.

3. NEAR-EARTH ENVIRONMENT The combination of solar events on 22-23 March generated a complex interplanetary shock structure that traversed the Sun-Earth distance with an average velocity of over 1400 km s-~. When this interplanetary shock interacted with the Earth's magnetic field at 0342UT on 24 March, the

* Due to circumstances beyond the Publisher's control, this article appears in print without author corrections. 427

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sphere which resulted in the creation of a long lasting (i.e. months) 'third radiation belt' (Mullen et al., 1991). Figure 1 illustrates the solar activity and the near-Earth space environment on 22-23 March 1991. The 1-8 A soft solar X-ray flux as measured by the GOES-7 synchronous orbit spacecraft is shown on the top of the figure. The major solar flares, together ENVIRONMENT

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Fig. 2. Illustration of the solar and terrestrial environment on 23--24 March 1991. Top: the 1-8 ~, soft solar X-ray flux as measured by the GOES-7 synchronous orbit spacecraft. Associated solar flares are indicated immediately below. Four selected integral proton flux energies from the GOES-7 spacecraft arc shown in the center panel. The CsOES-7 magnetometer is shown next followed by the Earth's magnetic field variations obs¢rved at Fredericksburg, Virginia. Below this is the cosmic radiation intensity, as measured by the neutron monitor at Deep River, Canada. A summary of the geophysical phenomena together with spacecraft and other operational anomalies arc listed at the bottom with the arrows indicating the times of the effects.

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with their optical importance, are indicated immediately below. The integral proton flux for four energies, also measured on GOES-7, is illustrated in the center of the figure. The solar proton event with onset between 7 and 8 UT on 23 March is clearly visible. The status of the geomagnetic field at the GOES-7 satellite is shown in the next panel; the smoothly varying magnetic field is typical of the daily variation observed at synchronous orbit. The cosmic radiation intensity, as measured by the neutron monitor at Deep River, Canada, is illustrated in the bottom panel. This detector responds to protons with energy greater than ~ 450 MeV incident at the top of the atmosphere. Figure 2 illustrates the solar, near-Earth space and terrestrial environments on 23 and 24 March. For continuity, the top four panels for 23 March are repeated in the same sequence as shown in Fig. 1. The solar particle flux measured on GOES-7 shows a

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Fig. 3. Illustration of the remarkable cosmic ray modulation present during this solar activity episode. The relative changes in the hourly averaged cosmic ray intensity on 24-25 March 1991 as recorded by the underground cosmic ray telescope at Embudo, New Mexico (response threshold 19 GeV), the Inuvik, Canada cosmic ray muon monitor (response threshold 5 CseV),and the Inuvik, Canada cosmic ray neutron monitor (response threshold ~ 450 MeV). The intensity between 02-03 UT on 24 March was taken as the background level.

steady increase from the onset on 23 March until early 24 March when there is an abrupt and rapid enhancement. This increased intensity around 0400 UT on 24 March is associated with the arrival of a rapidly moving interplanetary shock structure generated by the solar flare activity on 22/23 March.

4. COSMIC RAY FLUCTUATIONS During this episode of geomagnetic activity the cosmic radiation intensity, as measured at the Earth, was extensively modulated for a nine-day period commencing with the onset of the geomagnetic disturbance. The cosmic rays travel through the interplanetary space in the solar system and their modulation is a reflection of the turbulence in the heliosphere. The cosmic ray measurements indicate a very complex spatial structure near the Earth that was capable of modulating galactic cosmic rays greater than 20 GeV. This is shown by the 4% decrease in the hourly value of the cosmic ray intensity measured by the underground cosmic ray telescope at Embudo, New Mexico (energy threshold 19 GeV) as illustrated in Fig. 3. Also shown in this figure is the modulation at other energy thresholds, specifically the 8% decrease for galactic cosmic rays above 5 GeV (indicated by the Inuvik, Canada cosmic ray muon monitor) and the ~ 20% decrease for galactic cosmic rays above ~450 MeV as indicated by the Inuvik, Canada cosmic ray neutron monitor. The apparent phase of these extraordinary modulations was dependent on the viewing direction of each cosmic ray neutron monitor. Stations 'viewing' in the Sunward direction around 1200 UT exhibited a remarkable recovery in the cosmic ray intensity; those 'viewing' in the anti-Sunward direction exhibited a more typical major Forbush decrease. As the Earth revolved and the viewing directions of the various stations changed, the observed cosmic ray intensity also changed. Although asymmetries during Forbush decreases have been recorded previously, the magnitude and shape of these variations is extraordinary. Some of the unexpected rapid and asymmetric cosmic ray variations are illustrated in Fig. 4. Shortly after the passage of the interplanetary shock, the cosmic ray intensity recorded by both the Deep River and Inuvik neutron monitors rapidly decreased; however, the intensity-time profiles recorded at these two stations were vastly different. (The response threshold of both the Deep River and Inuvik, Canada neutron monitors is limited by the atmospheric mass. Both of these stations respond to the galactic cosmic ray flux above approximately 1 GeV.) The cosmic ray intensity at Inuvik steadily decreased to 21.3% below the pre-event background by 1205 UT. However, the intensity at Deep River, which had decreased 16.7% by 0955 UT and remained essentially constant for

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about 2 hours, experienced a rapid increase from a value of 15.5% below the pre-event background at 1205 UT to only 4.8% below the background by 1405 UT. At this same time, the cosmic ray intensity at Inuvik was 20.3% below the pre-event background. Once again the cosmic ray intensity recorded at Deep River plummeted reaching its lowest value of 16.9% below the pre-event background by 1815 UT. However, during this period when the cosmic ray intensity at Deep River was decreasing for the second time, the intensity recorded at Inuvik was increasing rapidly. 5. TECHNOLOGICAL EFFECTS The combination of phenomena in this solar activity episode--an enhanced flux of solar protons in the interplanetary medium together with a very powerful interplanetary shock sequence--gave rise to an extremely hostile space environment and resulted in major geophysical perturbations and a plethora of spacecraft operational anomalies. This sequence of activity produced the second largest proton fluence above 10 MeV that has been measured at the Earth thus far this solar cycle. Various geophysical effects together with spacecraft and other anomalies associated with the solar-terrestrial phenomena on 22-24 March 1991 are listed at the bottom of Fig. 2. These effects are shown in time association with the near-Earth space and terrestrial environment.

During the powerful solar X-ray event, the sunlit ionosphere was strongly ionized and the enhanced electron density resulted in degradation of short wave radio frequency communication. With the arrival of copious solar protons above 10 MeV, a disruption in high latitude point-to-point HF communication occurred with the increased ionization in the polar ionosphere. With increased particle flux, solar panel degradation was evident on the GOES-6 and 7 spacecraft; the GOES-7 power degradation translated to a decrease of 2-3 years in expected satellite lifetime. The presence of high energy solar particles increased the frequency of single event upsets (SEU) on satellite electronics with 37 SEUs reported on six geostationary satellites during the major part of the solar proton event. With the arrival of the interplanetary shock and the ensuing geomagnetic disturbance, the polar auroral oval expanded equatorward with reports of auroral sightings as far south as the state of Georgia and as far north as the Blue Mountain region of New South Wales, Australia. This significant lowering of the auroral boundary toward the equator resulted in low-latitude communication disruption as the polar ionospheric conditions extended to mid and low latitudes. During the major portion of the geomagnetic storm, electrical power relay systems were tripped, damage was reported to electrical distribution transformers and equipment in the Eastern United States and Canada, and Hydro-Quebec experienced several power surges in its power grid. A

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M . A . SHEA and D. F. SMART

mineral survey in northern Queensland, Australia experienced operational delays because of the magnetic storm, and large induced voltages were noted in a pipeline in Central Australia. In addition to the effects mentioned previously, there was loss of automatic attitude control on NOAA-11, and an increased satellite drag requiring a massive updating of the NORAD catalog of orbiting objects. Of more serious consequences was the complete failure of the geosynchronous orbiting MARECS-1 spacecraft early on 25 March. This early model unhardened maritime communications satellite had a history of space environmental induced anomalies, was not in optimal operating condition, and was being employed as a backup communication vehicle. 6, SUMMARY The solar-terrestrial phenomena that occurred in March 1991 was unusual as evidenced by the large fluctuations in the cosmic ray intensity. The second largest proton fluence of this solar cycle, to date, was reported between 22-26 March. Although this was a large event, many of the technological effects that occurred during March 1991 have also been reported during other periods of large solar-terrestrial phenomena such as August 1972. However, the current technology is considerably advanced from the technology used 20 or more years ago. With increased miniaturization of electronic components used in space and on Earth, and the increasing susceptibility

of these components to charged particles and current systems, it is imperative that we understand the solar and interplanetary phenomena that create such a hostile environment.

Acknowledgements--We wish to thank M. D. Wilson of the National Research Council of Canada for the Canadian neutron monitor data and D. B. Swinson for the Embudo muon data. The assistance of J. H. Allen and D. C. Wilkinson of the National Geophysical Data Center, NOAA, Boulder in the graphics and identification of some of the operational anomalies is gratefully appreciated.

REFERENCES Coffey H. E. (Ed.) (1991a) Solar-geophysical Data, No. 565, Part II, pp. 26-27. National Geophysical Data Center, NOAA, Boulder, Colorado, U.S.A. Coffey H. E. (Ed.) (1991b) Solar-geophysical Data, No. 565, Part II, p. 158. National Geophysical Data Center, NOAA, Boulder, Colorado, U.S.A. Elphic R. C., Thomsen M. F., Moore K. R. McComas D. J. and Barne S. J. (1991) Geosynchronous observations of the magnetopause and related phenomena on March 24 and 25, 1991, (Abstract). EOS, 72, No. O/Supplement, 377. Mullen E. G., Gussenhoven M. S., Ray K. and Violet M. (1991) A double-peaked inner radiation belt: cause and effect as seen on CRRES. IEEE Trans. Nucl. Sci. 38, 1713-1720. Pyle K. R. and Simpson J. A. (1991) Observation of a direct solar neutron event on 22 March 1991 with the Haleakala, Hawaii, neutron monitor. In: 22nd International Cosmic Ray Conference Contributed Papers, pp. 53-56. The Dublin Institute for Advanced Studies, Dublin.