C. Y. Huang

Thin current sheets in the magnetotail during substorms: CDAW 6 revisited

The global magnetic field configuration during the growth phase of the CDAW 6 substorm (March 22, 1979, 1054 UT) is modeled using data from two suitably located spacecraft and temporally evolving variations of the Tsyganenko magnetic field model. These results are compared with a local calculation of the current sheet location and thickness carried out by McPherron et al. (1987) and Sanny et al. (this issue). Both models suggest that during the growth phase the current sheet rotated away from its nominal location, and simultaneously thinned strongly. The locations and thicknesses obtained from the two models are in good agreement. The global model suggests that the peak current density is ∼120 nA/m2, and that the cross-tail current almost doubled its intensity during this very strong growth phase. The global model predicts a field configuration that is sufficiently stretched to scatter thermal electrons, which may be conducive to the onset of ion tearing in the tail. The electron plasma data further support this scenario, as the anisotropy present in the low-energy electrons disappears close to the substorm onset. The electron contribution to the intensifying current in this case is of the order of 10% before the isotropization of the distribution.

Nonadiabatic heating of the central plasma sheet at substorm onset

Heating events in the plasma sheet boundary layer and central plasma sheet are found to occur at the onset of expansive phase activity. The main effect is a dramatic increase in plasma temperature, coincident with a partial dipolarization of the magnetic field. Fluxes of energetic particles increase without dispersion during these events which occur at all radial distances up to 23 RE, the apogeee of the ISEE spacecraft. A major difference between these heating events and those observed at geosynchronous distances lies in the heating mechanism which is nonadiabatic beyond 10 RE but may be adiabatic closer to Earth. The energy required to account for the increase in plasma thermal energy is comparable with that required for Joule heating of the ionosphere. The plasma sheet must be considered as a major sink in the energy balance of a substorm. We estimate lobe magnetic pressures during these events. Changes in lobe pressure are generally not correlated with onsets or intensifications of expansive phase activity.

Two encounters with the flank low-latitude boundary layer : further evidence for closed field topology and investigation of the internal structure

We examine the structure of the flank low-latitude boundary layer (LLBL) through differential energy spectra and particle angular anisotropies for traversals of the dawn flank (December 19, 1977) and dusk flank (July 7, 1978) during periods of predominantly northward magnetosheath field orientation. The LLBL during these crossings is known to consist of two regions with distinct energetic particle signatures, known in previous work (Williams et al., 1985) as the stagnation region and the LLBL (referred to in this paper as “outer LLBL”). We present spectra obtained from combined ISEE 1 low-energy proton and electron differential energy analyzer (LEPEDEA) and medium energy particle instrument (MEPI) data extending over the 200 eV/q to 2 MeV energy range for the plasma sheet, stagnation region, outer LLBL and magnetosheath regions. The stagnation region is shown to be a unique region on the basis of abrupt changes observed in energy spectra, as well as the previously identified changes in bulk flow velocity, and flow azimuth upon entry into the region (Williams et al., 1985). This uniqueness is suggested to be the result of different convective histories (different source populations), but not the result of distinctly different physical mechanisms. The stagnation region and the outer LLBL are each a mixture of plasma sheet and magnetosheath populations, but the stagnation region contains a relatively higher fraction of plasma sheet particles, consistent with its placement earthward of the outer LLBL. No obvious ion heating was observed for any of the crossings of the magnetopause or of the LLBL for either event. Evidence for energization of thermal electrons (either magnetosheath or ionospheric in origin) appears during the dusk flank crossing. Bidirectional field-aligned ion distributions are observed with typically 5-to-1 enhancement of the flux along the magnetic field during certain portions of the dusk flank crossing. Bidirectional field-aligned electron distributions appear in the outer LLBL, stagnation region, and adjacent plasma sheet during both dawn and dusk flank crossings with typical ratios between field-aligned and perpendicular flux on the order of 10-to-1. We find a quantitative flux balance in opposite field directions for these electrons in the stagnation region, outer LLBL, and adjacent plasma sheet on both dawn and dusk flanks, supporting the conclusion that the magnetic field is topologically closed during these crossings.

Wave properties near the subsolar magnetopause: Pc 1 waves in the sheath transition layer

We study the waves in the frequency range of Pc 1 in the sheath transition layer of the magnetopause from the ISEE 1 and 2 observations. The waves are enhanced in the sheath transition layer, although they are scattered into the magnetosheath when the outer edge of the sheath transition layer is not sharp. A statistical study of 16 cases shows that the wave frequency is proportional to and equal to about 44% of the ion gyrofrequency. The wave amplitude is much greater when the interplanetary magnetic field (IMF) is southward than when the IMF is northward. The waves are left-handed polarized when the IMF is southward but are linearly polarized for northward IMF. The direction of maximum variation is perpendicular to both the background field and the gradients of the field and density when the IMF is northward. For southward IMF, the waves are more turbulent. The wave generation mechanisms apparently depend on the IMF orientations rather than the shock geometry. To investigate the free energy to generate the waves for northward IMF, a method is developed combining the measurements from the fast plasma experiment and Lepedea to obtain a high time resolution estimate of the temperature anisotropy when the IMF is strongly northward. The estimated ion temperature anisotropy is enhanced, up to a factor of 2, within the sheath transition layer for northward IMF.

Low‐energy particle layer outside of the plasma sheet boundary

The ISEE spacecraft in the geomagnetic tail frequently crossed the high-latitude boundary of the plasma sheet. On a number of these crossings on the morningside (between 15 RE and 22 RE) the ISEE instruments detected an enhanced population of low-energy electrons and ions immediately adjacent to the plasma sheet boundary layer (PSBL). The electrons in this low-energy layer (LEL) have energies less than a few hundred eV, and they are aligned along the magnetic field direction propagating in the tailward direction. The ions have energies less than 100 eV and are also streaming along the magnetic field direction but in the earthward direction. These particles are clearly distinguished from the bulk of the particles in the plasma sheet and the PSBL. These observations may help clarify where the various particle features in the geomagnetic tail map to in the ionosphere. It is suggested that the LEL maps to the soft (<1 keV) electron precipitation region poleward of the plasma sheet boundary.

Magnitude of BZ in the neutral sheet of the magnetotail

Statistical estimates of the average value of B[sub Z] in the magnetotail neutral sheet between 10 and 22 R[sub E] in X, and [minus]10 to 10 R[sub E] in Y (GSM coordinates) are given for different phases of geomagnetic activity. With few exceptions the average value of B[sub Z] is found to be between 5 and 8 nT, but generally near 7 nT, irrespective of the type or phase of activity. This is considerably higher than is predicted in current magnetic field models of the magnetotail. The magnetic field within 2 R[sub E] of the neutral sheet is examined to determine whether B[sub Z] remains approximately constant. The authors do not find this to be true, B[sub Z] decreasing significantly over this distance from [delta]Z = 0. The structure of the cross-tail current and variations in current density with substorm phase appear to be more complicated than is commonly accepted. 14 refs., 6 figs., 8 tabs.

Observations of plasma sheet expansion at substorm onset, R = 15 to 22 Re

We have used a large number of auroral magnetograms to identify four isolated substorms and estimate their onset times. At the onsets, ISEE-1 was in the vicinity of magnetic midnight at radial distances of 15.6 to 21.8 Re and very near the outer boundary of the plasma sheet. We find that, for each event, the plasma sheet expanded, and the magnetic field dipolarized at the inferred onset time. Our most definitive event occurred while ISEE was at a geocentric radial distance of 21.8 Re. This result conflicts with previous understanding, though further verification of the result is required. Our observations show very similar characteristics to those observed at synchronous orbit, and they are consistent with an extension of a portion of the substorm current wedge to the radial distance of the satellite. If this explanation is correct, ISEE must have been within the longitude range of the substorm current wedge at the onsets.

Velocity distributions in the plasma sheet boundary layer

Abstract It is agreed that the plasma sheet boundary layer is a site of highly dynamic plasma behavior, with fast flows, field-aligned currents and intense broadband electrostatic noise commonly detected in this region. Most of the studies in the literature focus on macroscopic plasma parameters, e.g. density, velocity and temperature. These studies, while useful, do not allow for detailed comparisons between mechanisms to produce the ion beams characteristic of the boundary layer. In this presentation three-dimensional ion velocity distributions at the high-latitude edges of the plasma sheet are shown, and the theories which have been suggested to account for the streaming ions are compared with the observations. In addition anisotropic distributions observed in the neutral sheet are presented, and the relation to the plasma sheet boundary layer distributions is discussed.

Global and local estimates of the current sheet thickness: CDAW-6

Abstract The growth phase development of the near-Earth magnetotail configuration during the CDAW-6 substorm (22 March 1979, 1054 UT) is studied using two complementary methods. The global magnetic field configuration is modeled using data from two suitably located spacecraft and temporally evolving variations of the Tsyganenko magnetic field model. These results are compared with a local calculation of the current sheet location and thickness previously carried out by McPherron et al. /1/. The good agreement between the two methods provides a positive test for the accuracy of the global model. Both models indicate the formation of an extremely thin current sheet during the substorm growth phase. The plasma electron data together with the rate of pitch-angle scattering estimated from the global field model suggest that the onset may be initiated by the chaotization of electrons and consequent growth of the tearing instability.