Cluster observations of the substructure of a flux transfer event: analysis of high-time-resolution particle data

Abstract

Abstract. Flux transfer events (FTEs) are signatures of transient reconnection at the dayside magnetopause, transporting flux from the dayside of the magnetosphere into the magnetotail lobes. They have previously been observed to contain a combination of magnetosheath and magnetospheric plasma. On 12 February 2007, the four Cluster spacecraft were widely separated across the magnetopause and observed a crater-like FTE as they crossed the Earth's dayside magnetopause through its low-latitude boundary layer. The particle instruments on the Cluster spacecraft were in burst mode and returning data providing 3-D velocity distribution functions (VDFs) at 4 s resolution during the observation of this FTE. Moreover, the magnetic field observed during the event remained closely aligned with the spacecraft spin axis and thus we have been able to use these 3-D data to reconstruct nearly full pitch angle distributions of electrons and ions at high time resolution (up to 32 times faster than available from the normal mode data stream). These observations within the boundary layer and inside the core of the FTE show that both the interior and the surrounding structure of the FTE consist of multiple individual layers of plasma, in greater number than previously identified. Our observations show a cold plasma inside the core, a thin layer of antiparallel-moving electrons at the edge of FTE itself, and field-aligned ions with Alfvénic speeds at the trailing edge of the FTE. We discuss the plasma characteristics in these FTE layers, their possible relevance to the magnetopause reconnection processes and attempt to distinguish which of the various different FTE models may be relevant in this case. These data are particularly relevant given the impending launch of NASA's MMS mission, for which similar observations are expected to be more routine.

Substructures within a dipolarization front revealed by high‐temporal resolution Cluster observations

“…In this event, for a few seconds, the magnetic field became closely aligned to the spin axis of the Cluster satellites. As a result, observations of the particle distribution at each spin azimuth correspond to a 0°–180° PAD, which provides us with a unique opportunity to study the dynamic plasma features on the DF with continuous 1/4 s resolution [ Khotyaintsev et al , ; Schwartz et al , ; Varsani et al , ]. For the first time, we report the substructures of ion and electron populations on the DF and show details that are not in the spin‐resolution observations.…”

The substructure of a flux transfer event observed by the MMS spacecraft

“…Heated isotropic magnetosheath electrons populate the core flux ropes ( Figure 3Dg) and lead to a local minimum in density and flux, similar to previous observations [Varsani et al, 2014]. Figure 3Dh shows a disturbed distribution in coincidence with the electric field burst described in section 3 (black arrows in Figures 1e and 1f).…”

“…Owen et al [2001] used Cluster observations to define the magnetic field connectivity inferred from the magnetic field and electron signatures. Varsani et al [2014] also investigated the multilayer interior and surrounding structures of a crater FTE based on the electron pitch angles using 125 ms observations of Cluster-PEACE measurements assuming that the electrons were gyrotropic. Owen et al [2008] identified the boundary layer of crater FTEs that features accelerated magnetosheath electrons and injected magnetosheath ions and related the layer to a separatrix region of reconnected flux tubes.…”

A sequence of flux transfer events potentially generated by different generation mechanisms

“…The detailed analysis of the characteristics of the plasma inside the FTEs can help to distinguish among the various generation mechanisms [Lockwood and Hapgood, 1998;Varsani et al, 2014]. Moreover, also the relation of the FTEs with reconnection jets can give important indications to understand the origin of the FTEs.…”