Magnetic field models are used to study electric currents that flow during growth phases and onsets of magnetospheric substorms. Large cross‐tail currents between altitudes of about 7 and 10 RE are required near midnight during growth phase in order to produce the observed magnetic field perturbations at synchronous altitude. Considerations of the particle fluxes needed to carry growth phase currents showed that the required current can be carried by the drift of a particle population whose energy density is about 20 keV/cm³. This energy density frequently is present at synchronous altitude after injection events. However, the model calculations require establishment of the large currents beyond synchronous altitude during growth phase. No observations of substantial flux increases during growth phase within the region of interest could be found. As a partial explanation of this problem, we found that a modest increase in particle energy density can produce a substantial increase in cross‐tail current. The increased currents carried by unaccelerated preexisting particles in the changing growth phase magnetic field can play a significant role in altering the magnetic field at synchronous altitude. This process involves a positive feedback effect, with preexisting particles carrying more cross‐tail current as soon as any perturbation begins to stretch tail field lines. It is concluded that more extensive observations of the changes in particle fluxes and pitch angle distributions during growth phase are needed in the equatorial region near 8 RE. Such observations also will help determine whether the energy of the plasma that is injected near synchronous altitude during substorms primarily is introduced slowly as fluxes build up during growth phase or primarily is introduced suddenly by the local conversion of magnetic field energy to particle energy at onset. In the first case, an injection event primarily would represent the inward motion of a population which already exists at substorm onset. In the latter case, strong impulsive acceleration would be an intrinsic part of the injection process. For our calculations, substorm onset is modeled by diverting current to the ionosphere in a wedge near midnight. It is found that field lines within the wedge collapse dramatically even if only a portion of the cross‐tail current is diverted. At the same time, a satellite outside the current wedge sees field lines become more taillike. It is suggested that diversion of only the electron cross‐tail current to the ionosphere is enough to initiate a substorm. Ion drift is reduced substantially within the wedge as field lines become more dipolar even if the ion energy density remains large. Finally, it is noted that very strong drift shell splitting effects should be seen if cross‐tail current is diverted only in a wedge near midnight.
We find that slow magnetoacoustic waves produce the magnetic field perturbations of the largest amplitude in the 0.01‐ to 0.1‐Hz frequency range within the ‘highly disturbed’ magnetosheath. The frequencies quoted are frequencies seen by the satellite. Rotational Alfvén waves with periods of several minutes and longer are also detected on most orbits. The power spectrum of rotational waves usually rises much more steeply below 0.01 Hz than the power spectrum of magnetoacoustic waves. We conclude that the magnetoacoustic waves are produced or strongly amplified at the earth's bow shock or in the outer magnetosheath. The observed rotational waves may be produced beyond the bow shock and carried into the magnetosheath by the solar wind.
Abstract. Two years of Geotail data in the (-30 < x < -8, lyl < 15) R E region first were sorted into (x, y, [3) boxes. Direct measurements of the average electron and ion current densities, symmetry assumptions, and the momentum equation were used to get three different estimates of the electric current in each box. The momentum equation method gave the most consistent results, while the other two methods provided complementary information about particle drifts. The average common drift of electrons and ions was found to be comparable to the average differential drift of ions with respect to electrons. These two components of the ion drift velocity tended to cancel on the dawnside, resulting in currents that were primarily carried by electrons moving at the common drift speed. The two ion drifts added on the duskside where ions carried most of the cross-tail current.The particle and magnetic field measurements were used to estimate the z thickness of each [3 box.A concentration of the long-term-averaged cross-tail current was seen near the neutral sheet. The region of nonadiabatic orbital motion had an average characteristic length scale of-•0.4 RE.The principal plasma sheet extended to -2.5 R E from the neutral sheet at midnight and to -5 R E in the .flanks. The final result is a method to create models in (x, y, z) coordinates of the long-term-averaged values of any of the measured fluid parameters or fields. The isotropic portion of the pressure tensor was used as an example of one parameter that can be modeled. These pressure plots showed that the x component of the long-term-averaged magnetic field line tension force is important everywhere, that the z component is small everywhere, and that the y component is significant in the flanks. IntroductionThis paper develops methods needed to make three-dimensional (3-D) measurement-based long-term-averaged models of particles and fields in the plasma sheet. The goal is to use these 3-D models to study the average current sheet structure and the transport of quantities such as energy, momentum, and magnetic flux. The hardest step is estimating how far a satellite is from the neutral sheet, which is defined as the sheet on which B x = O. Trajectory information in GSM or GSE coordinates gives an adequate measure of the satellite x and y locations because typical scale lengths in these directions are of the order of 10 R E . In contrast, the characteristic z-scale length is near 1 R E . In addition,
[1] Eight years of Geotail particle and magnetic field measurements were separated into 12 data sets on the basis of the ion flow speed. The same measurements were separated into 12 other data sets using a magnetic flux transport sorting parameter. Magnetic field lines in the three-dimensional models created using these two sorting methods were dipolar when the flow or transport was fast and stretched into a taillike configuration when the flow or transport was slow. The magnitude of B x measured in the outer central plasma sheet decreased weakly and B z at the neutral sheet increased strongly as the magnetic flux transport rate increased. These observations showed that fast flow flux tubes typically were located near but earthward of the primary region in which localized region 1 sense currents were diverted to the ionosphere. The plasma density was low and the temperature was high when the flow was fast. The particle pressure depended only weakly on flow speed. The average entropy was higher at z = 0 during fast flow events than it was anywhere in the region that could be studied when flows were slow or moderate. The average entropy also decreased as jzj increased. These observations suggest that the plasma was irreversibly heated by the process that produced the fast flows. Ions and electrons were found, on average, to be remarkably isotropic at the neutral sheet. Scattering through 90°each minute during slow and moderate flow conditions and as rapidly as every 10 s during the fastest flows was needed to maintain this average degree of isotropy. The temperature anisotropy increased away from the neutral sheet, reaching 1.1-1.3 at some point along most field lines. This variation along field lines was attributed primarily to a parallel electric field needed to maintain charge neutrality. The average ion to electron temperature ratio was as low as 5 and as high as 10 in certain regions and under specific flow conditions. These observations showed that electrons and ions were heated or cooled at different rates depending on their locations and bulk flow speeds.Citation: Kaufmann, R. L., W. R. Paterson, and L. A. Frank (2005), Relationships between the ion flow speed, magnetic flux transport rate, and other plasma sheet parameters,
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