Field‐aligned currents in the Earth's magnetotail

Abstract

Three field‐aligned current sheets are directly observed with plasma and magnetometer instrumentation on board the ISEE spacecraft located at ∼20 RE in the postmidnight sector of the magnetotail. These current sheets are encountered at the northern plasma sheet boundary as the plasma sheet expanded past the spacecraft positions during the recovery phase of a magnetic substorm. This expansion speed, Vz in solar magnetospheric coordinates, is 14 km/s. The corresponding convection electric fields E⊥ are derived from the three‐dimensional proton velocity distributions as measured with the quadrispherical Lepedea plasma instrumentation. The average current densities within the three field‐aligned current sheets are +3.3 × 10−9, −1.3 × 10−8, and +1.1 × 10−8 A/m², in order of decreasing distance to the plasma sheet. Current densities in the first and third sheets are directed into the ionosphere, and the current carriers are ionospheric electrons drifting away from the ionosphere. The second, or central, current sheet is directed away from the ionosphere; however, the source of the current‐carrying electrons is unclear. This central current sheet is thought to be associated with ionospheric electron precipitation producing discrete auroral arcs via an acceleration mechanism at intermediate altitudes. The thicknesses of the three magnetotail current sheets are 0.36, 0.55, and 0.74 RE, again in order of decreasing distance from the plasma sheet. Current intensities, the products of current densities and sheet thicknesses, of the field‐aligned current sheets are measured simultaneously with magnetometers and are +0.010, −0.028, and +0.018 A/m, respectively. These values agree reasonably well with those derived from the direct plasma measurements. Electron drift velocities VD within the current sheets are in the range 0.05 to 0.1 Ve, where Ve is the electron thermal velocity. The ion sound velocity Cs ≃VD. These plasma parameters, including the fact that the protons are hot with Tp ≃3Te, appear to provide an unfavorable situation for any substantial steady state anomalous resistivity at this location of the magnetotail. Current‐driven ion cyclotron and/or ion cyclotron drift instabilities may be responsible for broadband electrostatic noise previously observed at the boundary of the plasma sheet.

Non-adiabatic Ion Acceleration in the Earth Magnetotail and Its Various Manifestations in the Plasma Sheet Boundary Layer

“…Kivelson and Russell 1995). Early FACs studies, based on observations of magnetic field variations (Aubry et al 1972;Fairfield 1973;Sugiura 1975;Elphic et al 1985;Ohtani et al 1988;Ueno et al 2002), as well as on the analysis of 3D electron velocity distribution functions (Frank et al 1981) in the midtail region, were aimed at revealing their role in establishing the large-scale Region 1 and Region 2 current systems appearing in low-altitude spacecraft observations (Iijima and Potemra 1978). It was also reported that during quiet geomagnetic periods the Region 1 and Region 2 currents become weaker (Rich and Gussenhoven 1987) or even disappear.…”

Particle simulation of the auroral zone showing parallel electric fields, waves, and plasma acceleration

“…Future studies will include different boundary conditions at high altitudes, such as distribution functions in which a net current [Frank et al, 1981] is initially present in the simulation. The best situation would be to use observed ion and electron distributions from a high-altitude satellite above the primary auroral acceleration region.…”

Field‐aligned currents associated with substorms in the vicinity of synchronous orbit: 1. The July 5, 1979, substorm observed by SCATHA, GOES 3, and GOES 2

“…It is supposed that the currents are located near the outer plasma sheet boundary. There is evidence that many sheets of field-aligned currents exist near the outer plasma sheet boundary in the deep magnetotail during substorm expansion phases [Frank et al, 1981;Elphic et al, 1985]. If the currents consist of a double sheet of oppositely flowing currents at the outer plasma sheet boundary, their effects can be confined into the region between the two current sheets.…”