Interactions of the heliospheric current and plasma sheets with the bow shock: Cluster and Polar observations in the magnetosheath

Abstract

[1] On 12 March 2001, the Polar and Cluster spacecraft were at subsolar and cusp latitudes in the dayside magnetosheath, respectively, where they monitored the passage by Earth of a large-scale planar structure containing the high-density heliospheric plasma sheet (HPS) and the embedded current sheet. Over significant intervals, as the magnetic hole of the HPS passed Cluster and Polar, magnetic field strengths |B| were much smaller than expected for the shocked interplanetary magnetic field. For short periods, |B| even fell below values measured by ACE in the upstream solar wind. Within the magnetic hole the ratio of plasma thermal and magnetic pressures (plasma b) was consistently >100 and exceeded 1000. A temporary increase in lag times for identifiable features in B components to propagate from the location of ACE to those of Cluster and Polar was associated with the expansion (and subsequent compression) of the magnetic field and observed low |B|. Triangulation of the propagation velocity of these features across the four Cluster spacecraft configuration showed consistency with the measured component of ion velocity normal to the large-scale planar structure. B experienced large-amplitude wave activity, including fast magnetosonic waves. Within the low |B| region, guiding center behavior was disrupted and ions were subject to hydrodynamic rather than magnetohydrodynamic forcing. Under the reported conditions, a significant portion of the interplanetary coupling to the magnetosphere should proceed through interaction with the low-latitude boundary layer. Data acquired during a nearly simultaneous high-latitude pass of a Defense Meteorological Satellites Program satellite are consistent with this conjecture.

Cluster observations of the dusk flank magnetopause near the sash: Ion dynamics and flow‐through reconnection

“…For conditions with δ i,e ≪ 1, guiding center theory describes the particle motion, and the MHD approximation to the generalized Ohm's law applies. A particle species is said to be demagnetized if δ i,e > 1 [ Macmahon , 1965; Vasyliunas , 1975; Scudder et al , 2008; Maynard et al , 2011]. Based on field‐line‐resonance‐ordering considerations, Scudder et al [2008] also demonstrated that δ is more accurately connected to the ratio of the perpendicular electric to magnetic forces experienced by a thermal ion or electron in the plasma's rest frame.…”

“…Early satellite measurements indicate that the LLBL accounts for <10 kV of the ionospheric cross polar cap potential [ Smiddy et al , 1980; Mozer , 1984]. However, LLBL contributions to the polar‐cap potential may become significantly larger during periods of weak or northward IMF and/or high solar wind velocity [e.g., Sonnerup et al , 2001; Engebretson et al , 1998; Maynard et al , 2011]. Along the distant magnetopause where plasma flow is nearly parallel to the boundary, magnetosheath thermal pressure contributes significantly to force balance.…”

Cluster observations of fast magnetosonic waves in the heliosphere current sheet

“…Magnetosonic waves have long been identified in the solar wind [ Burlaga and Turner , ]. In the HCS, fast‐mode waves have been seen during the HCS/HPS passage in the magnetosheath [ Maynard et al , ]. In our event, a fast‐mode wave packet is found at a current layer associated with a steeped edge of a large‐scale transverse Alfvén wave.…”

“…The waves are associated with an electric field fluctuation as large as 4 mV/m and a magnetic field fluctuation as large as 10 nT. The fluctuation amplitude relative to background field ( δ | B |/| B |) is comparable with fast‐mode waves in Maynard et al []. The correlated fluctuations in magnetic field strength and plasma density are shown in Figure e.…”

“…The fluctuation amplitude relative to background DAI ET AL. field ( |B|∕|B|) is comparable with fast-mode waves in Maynard et al [2011]. The correlated fluctuations in magnetic field strength and plasma density are shown in Figure 3e.…”