Energy conversion regions as observed by Cluster in the plasma sheet

Abstract

[1] In this article we present a review of recent studies of observations of localized energy conversion regions (ECRs) observed by Cluster in the plasma sheet at altitudes of 15-20R E . By examining variations in the power density, E · J, where E is the electric field and J is the current density, we show that the plasma sheet exhibits a high level of fine structure. Approximately three times as many concentrated load regions (CLRs) (E · J > 0) as concentrated generator regions (CGRs) (E · J < 0) are identified, confirming the average load character of the plasma sheet. Some ECRs are found to relate to auroral activity. While ECRs are relevant for the energy conversion between the electromagnetic field and the particles, bursty bulk flows (BBFs) play a central role for the energy transfer in the plasma sheet. We show that ECRs and BBFs are likely to be related, although details of this relationship are yet to be explored. The plasma sheet energy conversion increases rather simultaneously with increasing geomagnetic activity in both CLRs and CGRs. Consistent with large-scale magnetotail simulations, most of the observed ECRs appear to be rather stationary in space but varying in time. We estimate that the ECR lifetime and scale size are a few minutes and a few R E , respectively. It is conceivable that ECRs rise and vanish locally in significant regions of the plasma sheet, possibly oscillating between load and generator character, while some energy is transmitted as Poynting flux to the ionosphere.

Secondary reconnection sites in reconnection-generated flux ropes and reconnection fronts

“…As mentioned above, there is already strong evidence of electronscale dissipations in reconnection exhausts 20,42 . However, the scale of the reconnection sites confined within the structure formed by the interchange instability remain within the domain of the electron-scale processes.…”

Energy exchanges in reconnection outflows

“…As can be observed the exchanges are bidirectional with magnetic energy being both created and destroyed (panel d) and work being done both by the particles and by the electric field (panel c). This bidirectional flow of energy has been reported also in satellite data observations taken in the outflow region of reconnection events [36]. While we are interested in overall net effects and in whether the generator and load regions (using the nomenclature in [36]) produce an net effect, the large energy exchange is important in its own right even if it were to even out: local observations can and have measured it.…”

“…This bidirectional flow of energy has been reported also in satellite data observations taken in the outflow region of reconnection events [36]. While we are interested in overall net effects and in whether the generator and load regions (using the nomenclature in [36]) produce an net effect, the large energy exchange is important in its own right even if it were to even out: local observations can and have measured it. In Section IV, we will return to show that energy is indeed on average transferred from the magnetic field to the particles.…”

Where should MMS look for electron diffusion regions?

“…Following the ripples of the front, one can notice how energy exchange alternates from positive to negative alternating generator regions (where the particle energy is transferred to the fields) to load regions (where electromagnetic energy goes to the particles). Recently, observational evidence has accumulated supporting such strong and at the same time oscillating energy exchanges at the fronts [21,22]. Based on observations from the Cluster [21] and THEMIS [22] missions, J · E has been estimated locally, observing large values in the fronts, with a tendency for load regions to be found near the reconnection site and for a more equal distribution of load and generator regions further downstream from reconnection [21].…”

“…Recently, observational evidence has accumulated supporting such strong and at the same time oscillating energy exchanges at the fronts [21,22]. Based on observations from the Cluster [21] and THEMIS [22] missions, J · E has been estimated locally, observing large values in the fronts, with a tendency for load regions to be found near the reconnection site and for a more equal distribution of load and generator regions further downstream from reconnection [21]. This evidence is obviously in agreement with an interpretation based on the drift-interchange mode discussed here [23].…”