Banded electron structures in the plasmasphere

Abstract

The low-energy plasma analyzer on CRRES has detected significant fluxes of 10-eV to 30-keV electrons trapped on plasmaspheric field lines. On energyversus-time spectrograms these electrons appear as banded structures that can span the 2 < L < 6 range of magnetic shells. We present an example of banded electron structures, encountered in the nightside plasmasphere during the magnetically quiet January 30, 1991. Empirical analysis suggests that two clouds of low-energy electrons were injected from the plasma sheet to L < 4 on January 26 and 27 while the convective electric field was elevated. The energies of electrons in the first cloud were greater than those in the second. DMSP F8 measurements show that after the second injection, the polar cap potential rapidly decreased from >50 to <20 kV. Subsequent encounters with the lower-energy cloud on alternating CRRES orbits over the next 2 days showed a progressive, earthward movement of the electrons' inner boundary. Whistler and electron cyclotron harmonic emissions accompanied the most intense manifestations of cloud electrons. The simplest explanation of these measurements is that after initial injection, the Alfv•n boundary moved outward, leaving the cloud electrons on dosed drift paths. Subsequent fluctuations of the convective electric field penetrated the plasmasphere, transporting cloud elements inward. The magnetic shell distribution of electron temperatures in one of the banded structures suggests that radiative energy losses may be comparable in magnitude to gains due to adiabatic compression. Introduction This paper presents an example of "banded structures" of trapped electrons with energies of <30 keV encountered by the low-energy plasma analyzer (LEPA) aboard the Combined Release Radiation Effects Satellite (CRKES) on plasmaspheric magnetic shells (L < 4) during magnetically quiet intervals. We refer to these electron stmctUr es as "banded" because of their appearance on energy-versus-time spectrograms. Although other satellites, such as Explorer 45, Dynamics Explorer 1 (DE 1), and ISEE ! and 2, carried sensors capable of detecting electrons inn the appropriate energy range and passed through this region of space, we believe that this is the first report 0f observing low-keV electrons inside the plasmaspheke. When DE 1 passed through the ß plasmasphere, it Wa s generally traveling along rather than across magneti c shells. Because of their high apogees, the ISEE sateIIit 'es passed very quickly across the 2 < L _< 4 region; often at off-equatorial magnetic latitudes. Explorer 45, also known as the Small Scientific Satellite A (Sa-A), flew in a low-inclination orbit with apogee at a geocentric distance of 5.24 RE [Longanecker and Hoffman, 1973] . The main objective of the S3-A mission was to understand the relationship of ring current acceleration and transport to the dynamics of geomagnetic storms and substorms. Smith and Hoffman [1974] identified a class of 1-to 100-keV protons whose trajectories carry them well inside of the plasmapause during the mai...

Multiple discrete-energy ion features in the inner magnetosphere: 9 February 1998, event

“…Kistler et al (1999) and Angelopoulos et al (2002) showed, on the basis of numerical simulation, that multiple minima of the ion flux can be produced in the sub-keV energy range when a realistic convection electric field model is employed. Multiple bands of electrons have been observed (Burke et al, 1995), and Liemohn et al (1998) attributed them to changes in the large-scale convection electric field.…”

Banded electron structure formation in the inner magnetosphere

“…The observations discussed by Burke et al [1995] are shown as energy-time spectrograms for --90 ø pitch angle as the satellite swept through the nightside magnetosphere (out near geosynchronous orbit). Because apogee at this time period was near local midnight, time is somewhat analogous to radial distance.…”

Electrostatic electron cyclotron harmonic waves observed by the Akebono satellite near the equatorial region of the plasmasphere

“…Furthermore, its occurrence frequency is almost coincident with the f Qn wave frequency at Vg = 0 (group velocity) belonging to the UHR wave branch. The frequency of the f Qn wave is deduced from the plasma wave dispersion analysis, taking into account a typical electron distribution function of the cold and hot component based on the Cassini (Rymer et al, 2001) and CR-RES (Burke et al, 1995) satellite observations. Therefore, from the above discussion on the occurrence frequency of the ESCH waves, it can be concluded that these phenomena are f Qn waves.…”

“…Although we need to be informed of the particle observation in the plasmasphere for the calculation of the dispersion curves, it is unfortunate that the Akebono satellite does not operate the low-energy particle observations obtained from the LEP detector in order to avoid the effect on radiation belt particles. Therefore, instead of using the particle observations by the Akebono satellite in this region, we referred to the Cassini (Rymer et al, 2001) and CRRES (Burke et al, 1995) satellite observations for the background cold and hot plasma distributions in the low L-value (L < 3.0) region of the plasmasphere, respectively. Both satellite observations suggest that the plasma distribution in this region consists of two components of the cold and hot plasmas.…”

“…The Cassini satellite observations supported the assumption that the background cold plasma distribution in the plasmasphere can be regarded as Maxwellian. On the other hand, from the low-energy electrons observations by the CRRES satellite, Burke et al (1995) showed that there exists hot component plasma with energy and a perpendicular temperature to the magnetic field of 3 keV and 750 eV, respectively, in the inner plasmasphere around L = 2.6. Moreover, the pitch angle observations of the keV-range electrons in this region indicated the concentration of the electron fluxes around 80-90…”