The neutral sheet in the geomagnetic tail: Its motion, equivalent currents, and field line connection through it

Abstract

From March 22 to May 26, 1964, during orbits 30 through 47, the IMP 1 satellite was located within the earth's magnetic tail, and measurements of the imbedded neutral sheet were possible. Forty‐two observations of magnetic field reversals accompanied by a decrease in the field magnitude were detected. Of these, 38 are identified as crossings of a well‐developed magnetic neutral sheet or current sheet. The relatively thin neutral sheet lies within a broad region of magnetic field depression and plasma enhancement, or ‘plasma sheet.’ The neutral sheet frequently appears to be moving relative to the satellite with a maximum velocity of a few kilometers per second, and on many orbits multiple crossings occur. The formation of the sheet appears to begin near the magnetic equatorial plane at a geocentric distance of 10±3 RE. The sheet thickness appears to be larger close to the earth and the dawn side of the tail and smaller farther away and toward the noon‐midnight meridian. At its thinnest region the sheet appears to be about 500 km thick and about 5000 km at the thickest part. The normal component of magnetic field within the sheet appears from these measurements to be 1–4 gammas toward the dawn side of the tail and less than 1 gamma near the noon‐midnight meridian plane. Thus the IMP 1 results favor those models of the tail field topology in which merging of field lines occurs across the imbedded neutral sheet.

Mapping of the Earth's bow shock and magnetic tail by Explorer 33

“…An average tail field magnitude of 16 gammas was found. These measurements further provided the first experimental detection of the neutral sheet in the geomagnetic tail (Ness, 1965;Speiser and Ness, 1967). In a second inspection of earlier Explorer 14 measurements in the magnetic tail, Cahill (1966) also found examples of field reversal undoubtedly associated with the neutral sheet.…”

“…The observed diurnal wobble of the sheet about its axis due to the daily 11.7" nutation of the earth's magnetic axis and the observations of multiple sheet traversals due to 'rflappingf' of the sheet both establish that the sheet is always in motion. The results of an analysis of 42 IMP 1 neutral sheet traversals suggest that the sheet is probably less than 1 R E thick, though evidence for an increase in thickness toward the dawn magnetopause was found (Speiser and Ness, 1967). Thus the neutral sheet is a relatively thin region of field reversal embedded in a plasma sheet that is often 4-6 times as thick (Bame et al, 1967).…”

Double Star/Cluster observation of neutral sheet oscillations on 5 August 2004

“…Oscillations of the neutral sheet have been identified by the multiple crossings of the neutral sheet by spacecraft. These oscillations are due to flapping motions of the neutral sheet in the north-south direction (Speiser and Ness, 1967), waves traveling along the axis of the magnetotail (Speiser, 1973) or wave motion along the dawn-dusk direction (Lui et al, 1978).…”

Survey of large-amplitude flapping motions in the midtail current sheet

“…One dynamic phenomenon, the flapping motions of the tail current sheet, manifests in observations as dramatic variations of the main magnetic field component (tail-aligned, B x ) in the tail, often with the change in the B x sign, indicating a crossing of tail current central surface. Being known since first spacecraft measurements in the tail (Speiser and Ness, 1967), it is frequently observed in the active plasma sheet and is interesting both as a spectacular phenomenon (origin, properties, possibility to use as a diagnostic tool of active processes), as well as a tool to probe the sheet structure by crossing quickly across considerable portions of the current sheet (CS) thickness. There was a number of past studies which noticed their association with interplanetary shocks (McComas et al, 1986), with substorms and/or fast flows (Toichi and Miyazaki, 1976, Bauer et al, 1995, although counter-examples were also provided.…”