Association between Geotail plasma flows and auroral poleward boundary intensifications observed by CANOPUS photometers

Abstract

Abstract. Poleward boundary intensifications are nightside geomagnetic disturbances that have an auroral signature that moves equatorward from the poleward boundary of the auroral zone. They occur repetitively, so that many individual disturbances can occur during time intervals of-1 hour, and they appear to be the most intense auroral disturbance at times other than the expansion phase of substorms. We have used data from three nightside conjunctions of the Geotail spacecraft in the magnetotail with the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) ground-based array in central Canada to investigate the relation between the poleward boundary intensifications and bursty plasma sheet flows and to characterize the bursty flows associated with the disturbances. We have found a distinct difference in plasma sheet dynamics between periods with, and periods without, poleward boundary intensifications. During periods with identifiable poleward boundary intensifications, the plasma sheet has considerable structure and bursty flow activity. During periods without such poleward boundary intensifications, the plasma sheet was found to be far more stable with fewer and weaker bursty flows. This is consistent with the intensifications being the result of the mapping to the ionosphere of the electric fields that give rise to bursty flows within the plasma sheet. Two different types of plasma sheet disturbance have been found to be associated with the poleward boundary intensifications. The first consists of plasma sheet flows that appear to be the result of Speiser motion of particles in a localized region of thin current sheet. The second, seen primarily in our nearest-to-the-Earth example, consists of energy-dispersed ion structures that culminate in bursts of low-energy ions and isotropic low-energy electrons and are associated with minima in magnetic field and temperature and maxima in ion density and pressure. Both types of plasma sheet disturbance are associated with localized regions of enhanced dawn-to-dusk electric fields and appear to be associated with localized enhanced reconnection. Our analysis has shown that poleward boundary intensifications are an important aspect of geomagnetic activity that is distinct from substorms. In addition to their very distinct auroral signature, we have found them to be associated with a prolonged series of ground magnetic Pi 2 pulsations and ground X component perturbations, which peak at latitudes near the ionospheric mapping of the magnetic separatrix, and with a series of magnetic B z oscillations near synchronous orbit. Like substorms, the tail dynamics associated with the poleward boundary intensifications can apparently extend throughout the entire radial extent of the plasma sheet. Color versions of figures are available at http ://www' atmøs'ucla'edu/-larry/geøtail'html'

A joint Cluster and ground-based instruments study of two magnetospheric substorm events on 1 September 2002

“…beam 14) show intensification of velocity towards the radar from 18:30 UT (velocities 400 m/s −1 to 600 m/s −1 ). These are seen in all beams and are similar to the signatures seen by Sandholt et al (2002), who related them to poleward boundary intensifications (Lyons et al, 1999). The directions of these line-of-sight velocities are consistent with westward ion flow.…”

Development of parallel electric fields at the plasma sheet boundary layer

“…Observations from radar combined with optical ground data show that the PBI are associated with localized flows crossing the boundary of the plasma sheet [de la Beaujardière et al, 1994;Lyons et al, 1999Lyons et al, , 2002Zesta et al, 2002Zesta et al, , 2006. Such localized flows can generate field-aligned currents due to the vorticity at the boundaries of the flow.…”

“…It is tempting to relate these observations to observations of Poleward Boundary Intensifications (PBI) that are often seen in the optical aurora [de la Beaujardière et al, 1994;Lyons et al, 1999Lyons et al, , 2002Zesta et al, 2002Zesta et al, , 2006. Indeed, Hull et al [2010] have recently used Cluster observations to indicate that a PBI was directly related to the evolution from an Alfvénic arc to a quasisteady arc.…”

Creation of the substorm current wedge through the perturbation of the directly driven current system: a new model for substorm expansion

“…Disturbances due to lobe field line reconnection take the form of poleward border intensifications (cf. Lyons et al, 1999) and are not part of the substorm current wedge formation. The model presented in this paper does not deal with the poleward border intensifications, and is relevant only to pseudo-breakups and to the onset and subsequent activations that occur in closed field line geometry during the initial stages of an expansive phase.…”