R. A. Christensen

High‐Z energetic particles at geosynchronous orbit during the Great Solar Proton Event Series of October 1989

Comparatively high levels of 2- to 50-MeV ions of carbon, nitrogen, oxygen, neon, magnesium, silicon, sulphur, and iron have been identified at geosynchronous orbit by the synchronous orbit particle analyzer, the “SOPA” detector, on board the satellite 1989-046, which became operational in September 1989. This detector is described, and time histories of some of the above mentioned ions are given for the solar energetic particle event series of late October 1989.

Substorm injection of relativistic electrons to geosynchronous orbit during the great magnetic storm of March 24, 1991

The great March 1991 magnetic storm and the immediately preceding solar energetic particle event (SEP) were among the largest observed during the past solar cycle, and have been the object of intense study. We investigate here, using data from eight satellites, the very large delayed buildup of relativistic electron flux in the outer zone during a 1.5-day period beginning 2 days after onset of the main phase of this storm. A notable feature of the March storm is the intense substorm activity throughout the period of the relativistic flux buildup, and the good correlation between some temporal features of the lower-energy substorm-injected electron flux and the relativistic electron flux at geosynchronous orbit. Velocity dispersion analysis of these fluxes between geosynchronous satellites near local midnight and local noon shows evidence that both classes of electrons arrive at geosynchronous nearly simultaneously within a few hours of local midnight. From this we conclude that for this storm period the substorm inductive electric field transports not only the usual (50–300 keV) substorm electrons but also the relativistic (0.3 to several MeV) electrons to geosynchronous orbit. A simplified calculation of the electron e × B and gradient/curvature drifts indicates that sufficiently strong substorm dipolarization inductive electric fields (≳ 10 mV/m) could achieve this, provided sufficient relativistic electrons are present in the source region. Consistent with this interpretation, we find that the injected relativistic electrons have a pitch angle distribution that is markedly peaked perpendicular to the magnetic field. Furthermore, the equatorial phase space density at geosynchronous orbit (L = 6.7) is greater than it is at GPS orbit at the equator (L = 4.2) throughout this buildup period, indicating that a source for the relativistic electrons lies outside geosynchronous orbit during this time. Earthward transport of the relativistic electrons by large substorm dipolarization fields, since it is unidirectional, would constitute a strong addition to the transport by radial diffusion and, when it occurs, could result in unusually strong relativistic fluxes, as is reported here for this magnetic storm.

Relativistic electrons in the outer-zone: An 11 year cycle; Their relation to the solar wind

We examine Los Alamos energetic electron data from 1979 through the present to show long term trends in the trapped relativistic electron populations at geosynchronous‐earth‐orbit (GEO). Data is examined from several CPA and SOPA instruments to cover the interval from 1979 through June, 1994. It is shown that the higher energy electrons fluxes (E>300 keV) displayed a cycle of ≊11 years. In agreement with other investigators, we also show that the relativistic electron cycle is out of phase with the sunspot cycle. We compare the occurrences of relativistic electrons and solar wind high speed streams and determine that on the time scale of 15 years the two do not correlate well. The long‐term data set we provide here shows a systematic change of the electron energy spectrum during the course of the solar cycle. This information should be useful to magnetospheric scientists, model designers and space flight planners.

The proton and electron radiation belts at geosynchronous orbit: Statistics and behavior during high-speed stream-driven storms

The outer proton radiation belt (OPRB) and outer electron radiation belt (OERB) at geosynchronous orbit are investigated using a reanalysis of the LANL CPA (Charged Particle Analyzer) 8-satellite 2-solar cycle energetic particle data set from 1976 to 1995. Statistics of the OPRB and the OERB are calculated, including local time and solar cycle trends. The number density of the OPRB is about 10 times higher than the OERB, but the 1 MeV proton flux is about 1000 times less than the 1 MeV electron flux because the proton energy spectrum is softer than the electron spectrum. Using a collection of 94 high-speed stream-driven storms in 1976–1995, the storm time evolutions of the OPRB and OERB are studied via superposed epoch analysis. The evolution of the OERB shows the familiar sequence (1) prestorm decay of density and flux, (2) early-storm dropout of density and flux, (3) sudden recovery of density, and (4) steady storm time heating to high fluxes. The evolution of the OPRB shows a sudden enhancement of density and flux early in the storm. The absence of a proton dropout when there is an electron dropout is noted. The sudden recovery of the density of the OERB and the sudden density enhancement of the OPRB are both associated with the occurrence of a substorm during the early stage of the storm when the superdense plasma sheet produces a “strong stretching phase” of the storm. These storm time substorms are seen to inject electrons to 1 MeV and protons to beyond 1 MeV into geosynchronous orbit, directly producing a suddenly enhanced radiation belt population.

Multi‐satellite characterization of the large energetic electron flux increase at L=4–7, in the five‐day period following the March 24, 1991, solar energetic particle event

Following the giant magnetic storm that started on March 24, 1991, and the immediately‐preceding solar energetic particle (SEP) event, a dramatic increase in the flux of energetic electrons was observed to occur on several satellites (using Los Alamos instruments aboard two geosynchronous satellites and two GPS satellites, plus energetic electron data from the CRRES satellite) sampling the L=4–7 region of the magnetosphere. We find that this flux buildup at the larger L‐values (L=6–7) first appears near the magnetic equator and subsequently spreads to higher magnetic latitudes; the flux buildup near the magnetic equator peaks first at the higher L before it peaks at the lower L analysis of the angular distribution of energetic electrons at geosynchronous orbit shows that the flux buildup begins first with the buildup of energetic electrons (>300 keV) moving perpendicular to the magnetic field.

The energy spectrometer for particles (ESP): Instrument description and orbital performance

The ESP detector is functionally described, along with the pertinent orbital and spin properties of the spacecraft that supports it. The phoswiched plastic/BGO scintillators sensor design, electronic implementation, and resulting data types are recounted, and the ground calibration procedures are reported. Several illustrative examples of data are given, including the solar proton event of 29 September 1989, and the nearly periodic episodes of high relativistic electron flux that are associated with solar coronal holes which have been a dominant feature of the space weather over the past few years. 2 refs., 10 figs., 1 tab.