Spatial structures of magnetic depression in the Earth's high‐altitude cusp: Cluster multipoint observations

Abstract

[1] Magnetic depression structures (magnetic holes) of short time duration from seconds to minutes have been studied using Cluster data in the high-latitude cusp. Our multispacecraft analysis revealed that the magnetic depressions are spatial structures traveling across the spacecraft, and this result was further strengthened by the calculation of the boundary normal directions and velocities using various methods. In this article, we show that multiple properties of the magnetic depressions are consistent with those of mirror structures observed in the magnetosheath or solar wind. The plasma in the cusp is rarely unstable with respect to mirror instability. However, as has been shown by previous studies, once a large magnetic hole is created by mirror instability, it becomes relatively stable and can survive for extended periods of time even if surrounding plasma conditions drop well below the mirror threshold. Although local generation of these structures cannot be completely ruled out in some cases, we propose an interpretation of the magnetic depressions observed in the cusp as mirror structures generated upstream and convected to the cusp by plasma flow. Specifically, the magnetic holes could be generated in the magnetosheath and enter the cusp due to the open geometry of the cusp magnetic field.

Cluster and TC-1 observation of magnetic holes in the plasma sheet

“…From the "interlaced" magnetic field profiles, we can tell that the magnetic hole is a spatial structure, rather than a temporal effect (e.g. Shi et al, 2009a). This can be confirmed by considering the angle between the normals of its two boundaries.…”

“…As ions can be heated in the perpendicular direction by the quasi-perpendicular bow shock , mirror mode structures are observed in the corresponding terrestrial magnetosheath (Kaufmann et al, 1970;Crooker et al, 1979;Tsurutani et al, 1982) and in the terrestrial cusp (Shi et al, 2009a). Observations and numerical simulations have shown that magnetic peaks are rarely seen in mirror stable plasma because of their rapid decay, but magnetic holes could survive in a mirror stable plasma.…”

Solar wind entry into the high-latitude terrestrial magnetosphere during geomagnetically quiet times

“…S1c,d,g). In our previous studies 35,36 , we have reviewed a method on how to empirically distinguish the spatial 'pass through' effects from temporal 'enter/retreat' ones based on the temporal sequences of the four spacecraft. In summary, if we find 'interlaced' profiles of parameters observed by different spacecraft, it should be interpreted as a spatial 'pass through' structure.…”

“…Additionally, event selection was made according to the particle energy flux in the lobe: we require that the energy band between 700 ev and 2 keV, and at least one order of magnitude for all energy bands, contain a particle energy flux greater than 8 Â 10 4 keVs À 1 cm À 2 si À 1 keV À 1 . Based on these criteria, only events with magnetosheath or cusp-like 35,36 ions with energy around 1 keV were selected, and the upflowing ions with lower energy 29,34 from the ionosphere were excluded.…”

“…Therefore, we call these regions with incoming solar wind plasmas 'entry regions' at the nightside high-latitude magnetosphere during geomagnetic (nearly) quiet periods. For more than half of the events (63), we could determine spatial regions when multi-point ion or electron density profiles were available onboard Cluster using the technique we mentioned above 35,36 .…”