Filamentary structures in the magnetotail lobes

Abstract

Filamentary structures consisting of energetic plasma have been observed in the magnetotail lobes. Using plasma, energetic particle, composition, and magnetic field data from the ISEE 1 and 2 spacecraft, we have established that these structures may be related to polar cap auroras. The filaments consist of energetic (several keV) plasma with a quasi‐isotropic or field‐aligned velocity distribution similar to that in the plasma sheet boundary layer. Composition analysis reveals that the origin of the plasma is the plasma sheet or plasma sheet boundary layer. The gradients in the energetic particle fluxes show that the structures are approximately aligned in the XZ plane, so that the possibility of simple plasma sheet flapping can be ruled out. Field‐aligned currents are associated with the filamentary structures. The current densities when mapped along the magnetic field lines are comparable with low‐altitude measurements over the polar cap. While the occurrence of the filaments is not correlated with interplanetary conditions, the convection pattern within the filaments appears to be related to the interplanetary magnetic field direction. When the IMF BZ is positive, the E × B convection is in the sense of a dusk‐to‐dawn electric field. Our observations of field‐aligned currents and plasma convection are consistent with those made by polar‐orbiting satellites.

Polar/Toroidal Imaging Mass‐Angle Spectrograph survey of earthward field‐aligned proton flows from the near‐midnight tail

“…In any case, there is the larger issue of whether a fluid description is appropriate in the first place. As pointed out above, but discussed only briefly, the TIMAS data show evidence that spatial scales as small as proton gyroradii are involved [see also Spjeldvik and Fritz, 1981;Williams, 1981;Huang et al, 1987], and this does bring into question the validity of MHD in this context.…”

Small-scale characteristics of extremely high latitude aurora

“…In the summer hemisphere maximum emissions in the transpolar arc (theta aurora) can range from 500 to 2000 R, in the winter hemisphere maximum emissions are less than 500 R, as measured by Polar UVI. Transpolar arcs have similar particle characteristics as the auroral oval, which suggests an origin in the plasma sheet or plasma sheet boundary layer (see also Meng, 1981;Peterson and Shelley, 1984;Frank et al, 1986;Frank and Craven, 1988;Huang et al, 1987Huang et al, , 1989. Obara et al (1996) presented the probability of average energy of precipitating electrons producing small-scale Sun-aligned arcs in the night side and day side polar cap, based on a few hundred such satellite measurements.…”

The connection between transpolar arcs and magnetotail rotation

“…Different topologies of the magnetotail have been proposed for TPA events. The plasma sheet has been suggested to be bifurcated [Frank and Craven, 1988], to have filamentary extensions into the lobes [Huang et al, 1987] or to be asymmetrically expanded [Meng, 1981].…”