Substorm currents: Growth phase and onset

Abstract

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.

Storm‐like dynamics of Jupiter's inner and middle magnetosphere

“…With Earth magnetospheric injections the most prominent magnetic signatures result from the sudden transition of the magnetic configuration from a stretched, more neutral-sheet-like configuration to a more dipolar configuration [e.g., Kaufmann, 1987]. This transition is called a "dipolarization."…”

“…It was shown by Mauk et al [1996] This situation is very similar to the situation at Earth. Earth magnetospheric injections occur in regions where the plasma beta can be high and in the region of transition between dipolar and neutral sheet geometries [Hoffman, 1973;Williams, 1983;DeForest and Mcllwain, 1971;Kaufmann, 1987]. It is believed that the hot plasma injections are driven by energy that is stored in the form of magnetic fields and associated electric currents.…”

A substorm model with onset location tied to an auroral arc

“…The CCI theory focuses on the inward motion and buildup of the cross-tail current as a result of increased tangential stress at the magnetopause during the substorm growth phase [Kaufmann, 1987] A preliminary quasi-linear calculation suggests that resistivity can be enhanced through wave-particle interaction by 11 to 12 orders of magnitude above its classical Coulomb value.…”

Structure, force balance, and evolution of incompressible cross-tail current sheet thinning

“…[33] In a two-dimensional model of the magnetotail [Kaufmann, 1987], the net dawn to dusk sheet current density integrated over the latitudinal direction I y is given by…”

“…Such a profile has been found to be maintained persistently before the substorm onset [Saito et al, 2010], proving observational support of the idea that B z evolves in a manner that the excess pressure issue is avoided. During the growth phase, intense cross-tail current density is required in the near-Earth tail to account for the observed increasingly stretched field [Kaufmann, 1987]. It is often speculated that increasing plasma pressure due to enhanced convection may account for the increase of the cross-tail current density [e.g., Zaharia and Cheng, 2003].…”