Dynamics of thin current sheets: Cluster observations

Abstract

Abstract. The 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 J y and the corresponding field-aligned currents (J x ). Event 1 covers a pseudo-breakup, developing only at high latitudes. First, a thin (H ≈2000 km≈2ρ i , with ρ i the ion gyroradius) Current Sheet (CS) is found to be quiet. A slightly thinner CS (H ≈1000-2000 km≈1-2ρ i ), 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 (δB/B∼1). In the quiet CS the current density J y 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. During the active phase of Event 1 no signature of the crossing of an X-line is identified, but an X-line located beyond Cluster could account for the observed ion flows, provided that it is active for at least 20 min. Ion flow bursts can also be Correspondence to: W. Baumjohann (baumjohann@oeaw.ac.at) due to CD and to the corresponding dipolarizations which are associated with changes in the current density. Yet their durations are shorter than the duration of the active period. While the overall ∂B z /∂t is too weak to accelerate ions up to the observed velocities, short duration ∂B z /∂t can produce the azimuthal electric field requested to account for the observed ion flow bursts. The corresponding large amplitude perturbations are shown to move eastward, which suggests that the reduction in the tail current could be achieved via a series of eastward traveling partial dipolarisations/CD. The second event is much more active than the first one. The observed flapping of the CS corresponds to an azimuthally propagating wave. A reversal in the proton flow velocity, from −1000 to +1000 km/s, is measured by CODIF. The overall flow reversal, the associated change in the sign of B z and the relationship between B x and B y suggest that the spacecraft are moving with respect to an X-line and its associated Hall-structure. Yet, a simple tailward retreat of a largescale X-line cannot account for all the observations, since several flow reversals are observed. These ...

Formation of the high-energy ion population in the earth's magnetotail: spacecraft observations and theoretical models

“…Such transient processes do not require the magnetotail reconfiguration and can operate within spatially localized regions with scales ∼ 1000 km. There are several candidates which play the role of these transients (see reviews by Baumjohann et al, 2007;Sharma et al, 2008). Here, we would like to mention dipolarization fronts (Runov et al, 2009(Runov et al, , 2011, small-scale reconnections (Wygant et al, 2005;Retinò et al, 2008), and plasma jet braking (Nakamura et al, 2009(Nakamura et al, , 2013.…”

Thin embedded current sheets: Cluster observations of ion kinetic structure and analytical models

“…An important result of CLUSTER measurements in the magnetotail is abundance of thin CS (TCS), with the thickness of the order of several ion larmor radii and with very diverse properties (Asano et al, 2005;Petrukovich et al, 2006Petrukovich et al, , 2007Runov et al, 2006;Nakamura et al, 2006;Baumjohann et al, 2007). Though TCS were observed also earlier by ISEE-1,2 spacecraft (Mitchell et al, 1990;Pulkkinen et al, 1993;Sergeev et al, 1993), their occurrence was not quite clear thus far.…”

Current sheet flapping in the near-Earth magnetotail: peculiarities of propagation and parallel currents

“…However, other manifestations of CS dynamics (e.g. earthward plasma flows with dipolarisation fronts, formation of plasmoids) are also observed (Sharma et al, 2008;Baumjohann et al, 2007). One can distinguish two kinds of flapping motion: the vertical oscillations of the quasi-horizontal CSs (when the normal N to the CS plane is basically along the z axis, |N z | > 0.8, and kink-like waves do not propagate) and formation of tilted CSs (when the normal is significantly tilted, |N z | < 0.8, and kink-like waves can propagate along the dusk-dawn direction) (see details of this classification in Rong et al, 2015).…”