Substorm currents: Growth phase and onset

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 R E 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 R E . 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.