Masaki N. Nishino

Origin of temperature anisotropies in the cold plasma sheet: Geotail observations around the Kelvin-Helmholtz vortices

To further our understanding of the solar wind entry across the magnetopause under northward IMF, we perform a case study of a duskside Kelvin-Helmholtz (KH) vortex event on 24 March 1995. We have found that the protons consist of two separate (cold and hot) components in the magnetosphere-like region inside the KH vortical structure. The cold proton component occasionally consisted of counter-streaming beams near the current layer in the KH vortical structure. Low-energy bidirectional electron beams or flat-topped electron distribution functions in the direction along the local magnetic field were apparent on the magnetosphere side of the current layer. We discuss that the bidirectionality of the electrons and the cold proton component implies magnetic reconnection inside the KH vortical structure. In addition, we suggest selective heating of electrons inside the vortical structure via wave-particle interactions. Comparing temperatures in the magnetosphere-like region inside the vortical structure with those in the cold plasma sheet, we show that further heating of both the electrons and the cold proton component is taking place in the cold plasma sheet or on the way from the vortices to the cold plasma sheet.

Increase of the tail plasma content during the northward interplanetary magnetic field intervals: Case studies

[1]xa0We have analyzed the structure of the magnetotail current sheet during the northward interplanetary magnetic field (IMF) intervals, using the method devised by Sergeev et al. [1998]. Case studies suggest that systematic variations of the current sheet thickness are seen not only during substorm activities but also during a building up phase of the cold dense plasma sheet when the IMF is northward and the magnetosphere is geomagnetically quiet. It is further found that during such northward IMF intervals the plasma “vertical content,” namely the product N0λ with the plasma density N0 at the plasma sheet center and the characteristic thickness λ, showed an order-of-magnitude increase within several hours. We estimate plasma transport rate during the northward IMF to be as much as 1026/s, which cannot be neglected in comparison with that during the southward IMF (∼1027 s−1).

Geotail Observations of the Cold Plasma Sheet on the Duskside Magnetotail

We have studied signatures of the cold plasma sheet in the duskside near-earth magnetotail with the Geotail data. In the duskside plasma sheet, cold plasmas are found 3-4 hours after the northward turning of the interplanetary magnetic field. The cold plasma sheet with two-temperature ions is often found on the duskside, while cold plasma sheet with one-temperature are also found on the duskside when northward interplanetary magnetic field continues for a very long interval (more than several hours). We discuss the development and evolution of the cold plasma sheet on the duskside.

Observational Signatures of Plasma Transport Across the Low‐Latitude Boundary Layer

It has been recognized that during extended periods of the northward interplanetary magnetic field the tail plasma sheet becomes cold and dense, showing a positive density correlation with the solar wind plasma. Recently it has been also recognized that the plasma density integrated along the Z (north-south) direction across the plasma sheet becomes also high during the northward IMF periods, which suggests a fairly high plasma supply rate of ∼ 10 26 protons/sec amounting nearly 10% of the enhanced supply rate during the southward interplanetary magnetic field periods. While the latter rate is considered to be caused by the efficient dayside magnetopause reconnection, it is not yet known how the plasma transport occurs during the northward interplanetary magnetic field periods. Since the highly evolved LLBL is also observed during such periods, we expect some causal relation between the plasma transport to the plasma sheet and the evolution of the LLBL. We review the key observations and discuss possible physical mechanisms of the plasma transportation.