Tailward Flow Research Articles

Magnetotail plasma flow during substorms: A survey with Imp 6 and Imp 8 satellites

This paper presents the results of a statistical analysis of substorm‐associated plasma flow in the sector of the magnetotail within (−18 RE >XSM > −35 RE;|YSM| ≤ 12 RE; |dZSM| ≤2.5 RE). This ‘substorm analysis’ used data from the Los Alamos Scientific Laboratory plasma instruments on the Imp 6 and Imp 8 spacecraft and was keyed to substorms identified primarily in magnetograms from individual auroral zone stations. We find that a sudden establishment or enhancement of flow in a persisting direction is observed at the onset of about one half of occurring substorms. About 80 percent of these sudden changes produce plasma flow in the tailward direction and only 20 percent produce earthward flow. These results seem more consistent with a neutral line model of substorms (e.g., the tearing theory) that predicts tailward plasma flow tailward of a near‐earth ‘substorm neutral line’ than with a rarefaction wave model that does not envision neutral line formation and that predicts solely earthward plasma flow in the plasma sheet during substorms. We also repeated the ‘thinning analysis’ previously used by other authors, that is keyed to occurrence of plasma sheet thinning during periods when the AE index is enhanced, and for which there is no restriction on dZSM. This analysis did not reveal the preponderance of earthward flow during plasma sheet thinning that the other authors reported. Rather, it showed nearly equal occurrence of earthward and tailward flows, with some preference for tailward flows within |dZSM| < 2.5 RE.

Measurements of plasma flow at the dawn magnetopause by Voyager 1

Measurements are presented showing strong tailward flow of ions along the dawn magnetopause as the Voyager 1 spacecraft crossed the earth's magnetosphere boundary following launch on September 5, 1977. With one exception all of the observed flows occur outside the magnetopause. The particle flux measurements at E ≳30 keV, together with the observed magnetic field signatures of the boundary crossing, are consistent with a minimum tailward ion energy flow of ∼(2–7) × 1017 ergs/s at the time of observation. High‐time resolution particle data indicate that the ion flow can vary on a time scale of ∼400 ms. These results, together with recent results from several other spacecraft, show conclusively that a source of energetic particles exists sunward of the dawn‐dusk meridian.

Magnetic fields in flowing magnetotail plasmas and their significance for magnetic reconnection

Results of a statistical analysis of 324 intervals of intense bulk plasma flows in the plasma sheet of the geomagnetic tail, measured by the Imp 6 spacecraft, are presented. Measurements of the magnetic field in the plasma sheet during these flows reveal pronounced dependences of the north‐south magnetic field component Bz(GSM) and the earth‐sun component Bx(GSM) on the flow direction; no such dependence of the By(GSM) component is discerned. Specifically, the fraction of field samples that are oriented southward is significantly greater during intervals of tailward flowing plasma than it is during intervals of earthward flowing plasma. Furthermore, this difference increases markedly as the absolute value of the angle of the magnetic field to the equatorial plane steepens. The total fraction of southward field, even during tailward flows, however, is about one third. As for the Bx component of the field, the distribution of samples in earthward flowing plasma is clearly peaked near zero, but the distribution in tailward flowing plasma is flatter and contains proportionately greater numbers of large positive and negative values of Bx. A moderate increase in ground activity, as determined by the AE index, was observed to be associated with the intervals of plasma flows. These results are reviewed in the context of models of magnetic reconnection in the magnetotail and the geometry of the tail plasma sheet.