Jay R. Cummings

Relativistic electron acceleration and decay time scales in the inner and outer radiation belts: SAMPEX

High-energy electrons have been measured systematically in a low-altitude (520 × 675 km), nearly polar (inclination = 82°) orbit by sensitive instruments onboard the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX). Count rate channels with electron energy thresholds ranging from 0.4 MeV to 3.5 MeV in three different instruments have been used to examine relativistic electron variations as a function of L-shell parameter and time. A long run of essentially continuous data (July 1992–July 1993) shows substantial acceleration of energetic electrons throughout much of the magnetosphere on rapid time scales. This acceleration appears to be due to solar wind velocity enhancements and is surprisingly large in that the radiation belt “slot” region often is filled temporarily and electron fluxes are strongly enhanced even at very low L-values (L ∼ 2). A superposed epoch analysis shows that electron fluxes rise rapidly for 2.5 ≲ L ≲ 5. These increases occur on a time scale of order 1–2 days and are most abrupt for L-values near 3. The temporal decay rate of the fluxes is dependent on energy and L-value and may be described by J = Ke-t/to with to ≈ 5–10 days. Thus, these results suggest that the Earths magnetosphere is a cosmic electron accelerator of substantial strength and efficiency.

PET: a proton/electron telescope for studies of magnetospheric, solar, and galactic particles

The proton/electron telescope (PET) on SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer) is designed to provide measurements of energetic electrons and light nuclei from solar, Galactic, and magnetospheric sources. PET is an all solid-state system that will measure the differential energy spectra of electrons from approximately 1 to approximately 30 MeV and H and He nuclei from approximately 20 to approximately 300 MeV/nucleon, with isotope resolution of H and He extending from approximately 20 to approximately 80 MeV/nucleon. As SAMPEX scans all local times and geomagnetic cutoffs over the course of its near-polar orbit, PET will characterize precipitating relativistic electron events during periods of declining solar activity, and it will examine whether the production rate of odd nitrogen and hydrogen molecules in the middle atmosphere by precipitating electrons is sufficient to affect O/sub 3/ depletion. In addition, PET will complement studies of the elemental and isotopic composition of energetic heavy (Z>2) nuclei on SAMPEX by providing measurements of H, He, and electrons. Finally, PET has limited capability to identify energetic positrons from potential natural and man-made sources. >

Measurements of the Ionic Charge States of Solar Energetic Particles Using the Geomagnetic Field

The mean charge states of C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Fe, and Ni ions with ~15-70 MeV nucleon^(-1) in the two large solar energetic particle (SEP) events of 1992 October 30 and November 2 have been determined using measurements of the invariant latitude of the cosmic-ray cutoffs of these elements from the Mass Spectrometer Telescope on the polar-orbiting SAMPEX satellite. The deduced charge state values are in good agreement with the mean values measured directly in previous SEP events at much lower energies of ~1 MeV nucleon^(-1). The inferred equilibrium source temperatures are confirmed to be typically 2 × 10^6 K, which provides additional evidence that SEPs in gradual-type events are accelerated coronal material.

Evidence for Multiply Charged Anomalous Cosmic Rays

New measurements from the SAMPEX spacecraft show that most anomalous cosmic ray (ACR) oxygen nuclei with energies above 20 MeV nucleon^(-1) are multiply ionized, with ionic charge states of Q = 2, Q = 3, and probably higher. This new result contrasts with lower energies, at which most ACRs are singly charged. The observed abundance of multiply charged ions agrees with estimates of the fraction of singly charged ACRs that undergo electron stripping during acceleration if the timescale for acceleration to 10 MeV nucleon^(-1) is ~1 yr. The existence of multiply charged ACRs helps explain their acceleration to high energies, and it has implications for several other studies.

MAST: a mass spectrometer telescope for studies of the isotopic composition of solar, anomalous, and galactic cosmic ray nuclei

The mass spectrometer telescope (MAST) on SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer) is designed to provide high-resolution measurements of the isotopic composition of energetic nuclei from He to Ni (Z=2 to 28) over the energy range from approximately 10 to several hundred MeV/nucleon. During large solar flares MAST will measure the isotopic abundances of solar energetic particles to determine directly the composition of the solar corona, while during solar quiet times MAST will study the isotopic composition of galactic cosmic rays. In addition, MAST will measure the isotopic composition of both interplanetary and trapped fluxes of anomalous cosmic rays, believed to be a sample of the nearby interstellar medium. >

A 2‐D model simulation of downward transport of NOy into the stratosphere: Effects on the 1994 austral spring O3and NOy

Simulations using SAMPEX and HALOE data suggest that NO_y produced by thermospheric processes and by relativistic electron precipitation in the mesosphere and lower thermosphere have been important to stratospheric NO_y and O_3 during the austral spring in 1994. The relative importance of the two NO_y sources is discussed. The results are supported by an analysis of HALOE NO_x and CH_4 data during October 1994 and are in agreement with ATMOS NO_y observations made in November 1994.

Precipitating electrons: Evidence for effects on mesospheric odd nitrogen

Observations of electron fluxes made by the PET and LICA instruments aboard SAMPEX have been used with NO measurements made by HALOE aboard UARS to provide evidence of mesospheric and lower thermospheric NO formation due to precipitating electrons. Results indicate significant NO increases from 70 to 120 km which are associated with the occurrence of enhanced electron populations in the outer trapping regions of the magnetosphere, 2.5 ≤ L ≤ 7, which precipitate into the atmosphere.

New evidence for geomagnetically trapped anomalous cosmic rays

We report new observations of ≥ 15 MeV/nuc trapped heavy ions with Z ≥ 2, made on the polar-orbiting SAMPEX spacecraft in late 1992 and early 1993. A trapped population that includes He, N, O, and Ne is found to be located at L ≈ 2. We conclude that the observed N, O, and Ne ions are “anomalous” cosmic rays, trapped by the mechanism proposed by Blake and Friesen. While it is not expected that this mechanism would also trap anomalous He, the characteristics of the trapped He population are generally consistent with those of N, O, and Ne.

Geomagnetically trapped anomalous cosmic rays

Since its launch in July 1992, the polar-orbiting satellite SAMPEX has been collecting data on geomagnetically trapped heavy ions, predominantly O, N, and Ne, at energies ≳ 15 MeV/nucleon and in a narrow L shell range near L = 2. Their location, elemental composition, energy spectra, pitch angle distribution, and time variations all support the theory that these particles originated as singly ionized interplanetary anomalous cosmic rays that were stripped of electrons in the Earths upper atmosphere and subsequently trapped. The O are observed primarily at pitch angles outside the atmospheric loss cones, consistent with a trapped population, and their distribution there is nearly isotropic. The abundances relative to O of the N, possibly Ne, and especially C are lower than the corresponding interplanetary values, which may be indicative of the trapping efficiencies. The distributions of trapped N, O, and Ne in energy and L shell suggest that most of the ions observed at the SAMPEX altitude of ∼600 km are not fully stripped when initially trapped. A comparison of the trapped intensity with the much lower interplanetary intensity of anomalous cosmic rays provides model-dependent estimates of the product of the trapping probability and the average trapped particle lifetime against ionization losses in the residual atmosphere for particles that mirror near the SAMPEX altitude.

Observations of the remnants of the ultrarelativistic electrons injected by the strong SSC of 24 March 1991

Data from the SAMPEX spacecraft has shown the presence near L = 1.9 of the remnants of the ultrarelativistic electrons (above 15 MeV) injected into the magnetosphere on 24 March 1991. The decay of the electrons has been tracked since July 1992. The electron e-folding lifetime is in the range of six to twelve months, and the location in L of the peak intensity moved inward by ∼0.05 units in one year. The inferred electron lifetime is similar to that of ∼0.5 MeV electrons, which is in agreement with extrapolation to higher energies of the theoretical estimates of Lyons and Thorne (1972).