Dynamics of thin current sheets: Cluster observations
Publication date: 29 Jun 2007
Authors: Baumjohann, W. et al.
Journal: Annales Geophysicae
Copyright: CopernicusThe paper tries to sort out the specific signatures of the Near Earth Neutral Line (NENL) and the Current Disruption (CD) models, and looks for these signatures in Cluster data from two events. For both events transient magnetic signatures are observed, together with fast ion flows. In the simplest form of NENL scenario, with a large-scale two-dimensional reconnection site, quasi-invariance along Y is expected. Thus the magnetic signatures in the S/C frame are interpreted as relative motions, along the X or Z direction, of a quasi-steady X-line, with respect to the S/C. In the simplest form of CD scenario an azimuthal modulation is expected. Hence the signatures in the S/C frame are interpreted as signatures of azimuthally (along Y) moving current system associated with low frequency fluctuations of Jy and the corresponding field-aligned currents (Jx). Event 1 covers a pseudo-breakup, developing only at high latitudes. First, a thin (H~2000 km~2 rhoi, with rhoi the ion gyroradius) Current Sheet (CS) is found to be quiet. A slightly thinner CS (H~1000-2000 km~1-2 rhoi), crossed about 30 min later, is found to be active, with fast earthward ion flow bursts (300-600 km/s) and simultaneous large amplitude fluctuations (deltaB/B~1). In the quiet CS the current density Jy is carried by ions. Conversely, in the active CS ions are moving eastward; the westward current is carried by electrons that move eastward, faster than ions. Similarly, the velocity of earthward flows (300-600 km/s), observed during the active period, maximizes near or at the CS center. - Remainder of abstract truncated -