Off‐equatorial current‐driven instabilities ahead of approaching dipolarization fronts

Abstract

AbstractRecent kinetic simulations have revealed that electromagnetic instabilities near the ion gyrofrequency and slightly away from the equatorial plane can be driven by a current parallel to the magnetic field prior to the arrival of dipolarization fronts. Such instabilities are important because of their potential contribution to global electromagnetic energy conversion near dipolarization fronts. Of the several instabilities that may be consistent with such waves, the most notable are the current‐driven electromagnetic ion cyclotron instability and the current‐driven kink‐like instability. To confirm the existence and characteristics of these instabilities, we used observations by two Time History of Events and Macroscale Interactions during Substorms satellites, one near the neutral sheet observing dipolarization fronts and the other at the boundary layer observing precursor waves and currents. We found that such instabilities with monochromatic signatures are rare, but one of the few cases was selected for further study. Two different instabilities, one at about 0.3 Hz and the other at a much lower frequency, 0.02 Hz, were seen in the data from the off‐equatorial spacecraft. A parallel current attributed to an electron beam coexisted with the waves. Our instability analysis attributes the higher‐frequency instability to a current‐driven ion cyclotron instability and the lower frequency instability to a kink‐like instability. The current‐driven kink‐like instability we observed is consistent with the instabilities observed in the simulation. We suggest that the currents needed to excite these low‐frequency instabilities are so intense that the associated electron beams are easily thermalized and hence difficult to observe.