Pages 1075-1080

ISTP: RELATIVISTIC PARTICLE ACCELERATION AND GLOBAL ENERGY TRANSPORT

D.N. Baker1, H.E. Spence2, J.B. Blake3

1Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA,
 E-mail: baker@lynx.colorado.edu
2Center for Space Physics, Boston University, Boston, MA 02215, USA
3Aerospace Corporation, Los Angeles, CA 90009, USA

ABSTRACT

With the International Solar Terrestrial Physics (ISTP) constellation of spacecraft, plus key measurements from other scientific and operational spacecraft, it is possible to study the flow of energy from the sun through the interplanetary medium into the magnetosphere and upper atmosphere of the Earth. We use Yohkoh soft x-ray images to identify large solar coronal holes. These solar regions give rise to high-speed solar wind streams which are detected and characterized by measurements from the WIND spacecraft. The high speed streams drive the Earth's magnetosphere quite substantially giving rise to strong relativistic electron acceleration throughout the outer radiation zone. The SAMPEX spacecraft in low Earth orbit and the MCP package onboard a high-inclination, elliptical-orbit satellite map the electron belts as they wax and wane under solar wind stream influence. New measurements from the CEPPAD investigation of the POLAR spacecraft are able to examine the spectral and angular distribution properties of accelerated magnetospheric electrons with unprecedented resolution. These data show that the Earth's magnetosphere is a strong accelerator of high energy electrons. Thus, the ISTP program may be able to contribute substantially to an important aspect of plasma astrophysics.

INTRODUCTION

Long-term observations in the outer magnetosphere (L~6.6) demonstrate that energetic electron fluxes are strongly modulated by solar wind streams (Baker et al., 1986; 1994). Lower energy ( 300 keV) particle fluxes track the solar wind variations quite closely and appear to be the direct product of magnetospheric substorm activity (which is obviously controlled by the solar wind and the IMF). Higher energy (>300 keV) particle fluxes are also modulated by the solar wind streams, but are not directly related to substorm acceleration mechanisms. In fact, the highest energy electrons show strong recurrence tendencies at the 27-day rotation period of the sun (Williams, 1966; Paulikas and Blake, 1979). These appear to closely parallel "recurrent" geomagnetic storms, so it is important from a physical point of view to understand these phenomena.

Given the intense interest in understanding the acceleration and transport of high-energy magnetospheric particles and their dependence on solar and magnetospheric conditions, we use data in the early part of 1996 from several scientific and operational spacecraft to examine global solar-terrestrial connections. These data are employed to provide a broad context for simultaneously observing the changes occurring at the sun, in the interplanetary medium, and in the geospace environment under solar minimum conditions. The ISTP spacecraft armada and coordinated ground-based measurements yield a comprehensive view of the complex processes that modulate energetic charged particles in the magnetosphere; the importance of these particles is underscored in this study through the possible association of a major satellite failure with the radiation environment within which it was operating.

OBSERVATIONS

In the present era since the launch of the WIND spacecraft in November 1994 we are fortunate to have nearly continuous solar wind data. Figure 1a shows the solar wind speed (VSW) measured by the Solar Wind Experiment (SWE) onboard WIND (Ogilvie et al., 1995). Data are shown as hourly averages for Day 1 (January 1) to Day 100 (9 April) of 1996. It is evident from Figure 1a that there were several strong solar wind stream events throughout early 1996. Notable examples of solar wind speed peaks include those occurring on Days ~15, ~28, ~42, ~57, ~72, and ~82. Obviously, there were "recurrent" streams that were associated with the 27-day solar rotation period: The interleaving of two sets of such recurrent streams produced an ~13-day stream enhancement periodicity.

Fig. 1. (a) The solar wind speed measured by WIND is shown for the first 100 days of 1996. (b) Daily-average fluxes of 2-6 MeV electrons at L=5 from the PET sensor on SAMPEX (bottom panel).

Figure 1b shows 2-6 MeV electron intensities measured by the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) spacecraft at low-Earth, polar orbit (see Baker et al., 1993). A subset of the Proton-Electron Telescope (PET, see Cook et al., 1993) data averaged each day for L=5 (± 0.1) is plotted for the period Day 1 to Day 100 of 1996. The SAMPEX data show relatively brief flux peaks associated with solar wind streams in January and February, but the larger and longer lasting flux enhancement of late-March is quite clear. The occurrence of relativistic electron enhancements associated with solar wind streams (but delayed by some days) has been known for some time (e.g., Paulikas and Blake, 1979).

Given the well-known result that the largest solar wind streams originate from solar coronal holes (e.g., Feldman et al., 1978), we have examined available solar soft X-ray (Yohkoh) and extreme ultraviolet (SOHO) data to identify solar wind stream source regions. Figure 2, for example, shows a Yohkoh soft X-ray image of the sun taken at 0533:26 UT on 7 February. In general, the solar corona was seen in Yohkoh data to be extremely quiet and unstructured during the early part of 1996. However, a large, trans-equatorial coronal hole is evident near central meridian on 7 February. This hole undoubtedly gave rise to the solar wind stream which the Earth encountered on 10 February (see Figure 1a). This same coronal hole was seen in Yohkoh images earlier in January and again in early March of 1996.

Fig. 2. A soft x-ray image of the Sun (courtesy of L. Acton and L. Bargatze) taken by the Yohkoh spacecraft at 0533:26 UT on 7 February 1996. A coronal hole is seen near central meridian extending from the south polar region across the equatorial plane.

A key to producing enhanced geomagnetic activity and subsequent relativistic electron events within the magnetosphere is a combination of high-speed solar wind and strongly southward IMF. Figure 3 shows in the bottom panel the solar wind speed from Day 60 through Day 100. In the upper panel are shown hourly averages of the IMF north-south component (Bz) for Days 75-86 in GSM coordinates. It is seen that the IMF was rather strongly and persistently southward from Day 77 (17 March) to near the end of Day 81 (21 March). This negative Bz was "piled up" at the leading edge of the high-speed solar wind stream which began on ~Day 80 (20 March). The southward IMF and high value of VSW produced a notable increase in the global magnetic index, Ap. The Ap value peaked on 20 March when the IMF was most negative.

Fig. 3. Bottom panel: A detail of solar wind speed for Days 60-100 of 1996. Top panel: Hourly averages of the interplanetary magnetic field Bz component for Days 75-86.

In Figure 4, we show data from the geostationary orbit GOES-8 spacecraft for electrons with E>2MeV. Note that the GOES data are shown as daily fluence averages. Looking at the low frequency temporal patterns, it is seen from Figure 4 that each solar wind stream was associated with a large and relatively brief enhancement of >2 MeV electron fluxes (e.g., in mid-January, in February, and in early March). On the other hand, the strong and complex solar wind variations of late March produced a long-lasting (>2 week) enhancement of relativistic electrons at geostationary orbit beginning on ~12 March; this persisted until essentially the beginning of April.

A more extensive examination of the SAMPEX data at low-Earth orbit fully supports the idea that relativistic electrons were substantially enhanced throughout the outer trapping zone for nearly two weeks in late March. Figure 5a shows data for electrons with E>0.4 MeV for the period 1 January to 28 April (Day 1-150). The data are plotted for several L-values with the flux shown versus time. The data are shown as daily averages. It is seen that at each L-value, the electron fluxes dropped to very low values around Day 70 (~10 March) and then the fluxes increased rapidly thereafter. There was a slight drop on ~Day 81 with a persistent climb in flux values after that time until ~Day 90. Several strong flux enhancement events were seen in the early part of 1996 and each of these is related to a solar wind stream event.

Figure 5b shows data from a dosimeter onboard a spacecraft at ~60° inclination and having a highly elliptical orbit extending out to ~6.6 RE. We show several selected energy channels as labeled and the data have been averaged for each day. The period covered is the same (Day 1-150) as in Figure 5a. We see that the dosimeter shows exactly the same features as the SAMPEX measurements: Each individual brief flux enhancement in January and February looks the same in both data sets. Moreover, the intense and long-lasting flux increases of late-March and April of 1996 are also clear. Thus, the magnetospheric electron increases are a truly coherent phenomenon.

A final view of the electron environment in the outer zone is provided by exciting new data from the Comprehensive Energetic Particle Pitch Angle Distribution (CEPPAD) investigation onboard the recently launched POLAR spacecraft (Blake et al., 1995). The High-Sensitivity Telescope (HIST) sensor on POLAR measures 0.4 to >10 MeV electrons. In Figure 6, we show electron fluxes versus L for two similar passes through the outer zone. Data for 10 March 1996 ( Day 70) were taken when the radiation belts were rather weak. It is seen that HIST Channel 11 (covering 3.5 MeV   E  5 MeV) reached a peak count-ing rate of only 500 counts/sec. Later, when the radiation belts were much more intense, the flux of electrons measured by HIST was much greater. For example, a similar pass through the radiation belts on 26 March 1996 at L values and local times comparable to the pass on 10 March shows that the peak HIST/Ch. 11 counting rate was ~7 x 103 c/s and the entire outer zone profile was much broader in time. This indicates a much more substantial outer zone electron population on 26 March.

DISCUSSION

Observations presented in this paper reveal that relativistic electrons increase in absolute intensity, often by a factor of 10 or more, throughout much of the outer magnetosphere on a time scale of order 1 day. These enhancements occur even during quite weak recurrent geomagnetic storm activity. Abrupt flux enhancements might be expected to occur in the very outer magnetosphere, but it is more remarkable that low L-shells, deep within the magnetospheric cavity, are able to respond so prominently. As noted above, available solar wind data show that the large electron intensity increases are associated with high-speed solar wind streams impinging upon the magnetosphere.

Given the large, solar wind speed enhancements observed in conjunction with the electron flux increases examined here, large dynamic pressure (V2) enhancements are expected. These pressure increases would compress the magnetospheric field substantially, and in so doing would cause time-dependent (and spatially-varying) electric fields. However, the high-speed stream effects would certainly not be as strong as a major shock (and associated storm), nor would the compression occur as quickly. Any substantial inductive electric field associated with the changing B-field would cause electrons from relatively high L-shells to be driven inward by the solar wind stream compression and in the process the relativistic magnetic moment invariant (µr = P2/2moB, where P is the perpendicular momentum and mo is the electron rest mass) would be preserved. Thus, the perpendicular relativistic energy increases in proportion to the square root of the local magnetic field strength (B) at the final particle location. However, it would be surprising in this scenario to achieve very high energization due to the relatively slow compressions associated with solar wind streams. We look forward to a future evaluation of this acceleration mechanism.

While the relativistic electrons that are the topic of this paper may still have an unclear acceleration mechanism, their impact on space systems is well known. Scientific satellites (e.g., SCATHA and CRRES) have simultaneously measured the energetic electron environment and monitored the deleterious effects they can cause on spacecraft electronics. One serious effect is deep dielectric charging which occurs when energetic electrons penetrate and charge dielectric materials. When the fluxes are high enough for a sufficiently long time, the accumulated charge can cause the dielectric to break down, resulting in an electric pulse that can couple into and profoundly disturb critical components. The association between an elevated fluence of relativistic electrons and coincident increases in space system "anomalies" is well established (see Vampola, 1987; Baker et al., 1994).

The practical consequence of relativistic electron modulation in the outer zone is emphasized in the context of the Sun-Earth connection outlined in our study. It is of interest to note that the Telesat Canada Anik E1 communication satellite located at 111°W at geostationary orbit suffered a severe operational problem on 26 March 1996 (Danylchuk, 1996). The Anik satellite lost all power from its south solar panel array when the array was effectively disconnected from the satellite payload at 2047 UT. The 50% power loss reduced the spacecraft’s capacity from 24 C-band channels to nine channels and it reduced the Ku-band capacity from 32 channels to 10 (Baker et al., 1996). It is not expected that the lost solar panel can be recovered, thus this was a permanent degradation of communication capability for Telesat Canada. It affects a broad range of video, voice, and data services throughout North America. Service to Telesat Canada customers was restored after about six hours by link switches to other spacecraft and by using backup systems such as fiber optics ground links (Roberts, 1996).

In summary, a remarkable array of scientific and operational spacecraft, as shown here, allows us to demonstrate that the high-energy electron environment was quite elevated throughout late-March 1996. The satellite and ground-based data suggest that the space environment could have caused, or at least exacerbated, the conditions onboard Anik E1 that led to the power failure that crippled the spacecraft (Baker et al., 1996) through the mechanism of deep-dielectric charging. Moreover, data from the vast ISTP complex allows us to study relativistic electron acceleration – a problem of great astrophysical interest – with unprecedented completeness.

Fig. 4. GOES-8 measurements of electrons with E>2 MeV for January to early April of 1996 (after Baker et al., 1996). Fig. 5. (a) L-sorted SAMPEX electron data for Days 1-150 of 1996 showing data for the E>0.4 MeV electron channel.
Fig. 5. (b) Data from a dosimeter on a spacecraft in a highly elliptical, 60°-inclination orbit with apogee at 6.6 RE. Fig. 6. POLAR spacecraft data for high energy electrons measured by the CEPPAD/HIST detector comparing 10 March 1996 data with 26 March 1996 data. The spacecraft cuts in L and local time (LT) were similar for these passes. The fluxes are shown versus L value for two channels and clearly demonstrate the much higher fluxes and greater radiation belt width on 26 March (after Baker et al., 1996).

ACKNOWLEDGMENTS

The authors thank the numerous people who have unselfishly shared their knowledge and data. Special thanks are given to L. Bargatze and L. Acton for Yohkoh data and to J. Gurman and J.-P. Delaboudiniere for SOHO data. We also thank members of the SAMPEX, POLAR, WIND, GOES, and various other operational spacecraft teams for unflagging support. This work was supported by NASA. Ground-based data from the NOAA-NGDC and from CANOPUS have been especially appreciated. This paper is based upon a more extensive report in the ISTP Newsletter (Baker et al., 1996).

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