Statistical study of transient plasma structures in magnetotail lobes and plasma sheet boundary layer: Interball-1 observations

Abstract. The present paper is devoted to the statistical study of plasma structures observed at the interface between magnetospheric lobes and the plasma sheet at distances of 15–25 RE . The majority of the registered structures are bursty earthward ion streams with energies about 10–30 keV known as "beamlets". The paper describes the results of beamlet statistical analysis for different interplanetary magnetic fields (IMF), clock-angles and various magnetospheric conditions. It is shown that the energy of beamlets increases monotonously with a distance from the neutral sheet (NS) during slightly southward IMF and quiet magnetospheric conditions. Under these conditions, beamlets are observed up to 5–6 RE over the NS. In contrast, the beamlet energy decreases with increasing distance from the NS if the IMF is northward, and the region of beamlet observation becomes much wider up to 12 RE above the NS. The same (inverse) beamlet energy dependence on the distance from the NS is also registered during active magnetospheric periods. Yet, the plasma structures of various duration with quasi-isotropic ion velocity distributions resembling the plasma of the plasma sheet (PS) were observed in the same region. Statistical study performed for various IMF clock-angles demonstrates the clear difference in the spatial distributions of such plasma structures for two extreme IMF directions. When IMF is primarily northward, PS-like plasma fills the lobe region almost entirely and could be associated with the PS expansions. For primarily southward IMF time intervals, the probability of observing PS-like structures of long duration sharply decreases with the increasing distance from the NS, but PS-like structures of short durations (< 500 s) are distributed almost uniformly up to 12 RE above the NS. This feature cannot be described in terms of the PS flapping motions and requires another explanation. Relevance and limitations of the existing models attempting to explain the generation mechanisms of these transient structures are briefly discussed in the concluding part of the paper.

Key words. Magnetospheric physics (magnetotail; magnetotail boundary layers; plasma sheet)