An extended study of the low‐latitude boundary layer on the dawn and dusk flanks of the magnetosphere

Abstract

We present a study of the low‐latitude boundary layer (LLBL) using ISEE 1 energetic particle, plasma, and magnetic field data obtained during numerous traversals of the LLBL that occurred on 66 ISEE 1 passes through the magnetospheric flank LLBL region. We use energetic particle distributions to determine dawn and dusk LLBL behavior and topology for varying orientations of the magnetosheath and/or interplanetary magnetic field (M/IMF), for different local times, and for changing levels of geomagnetic activity (Kp). This study corroborates and extends the earlier work of Williams et al. (1985) who presented a detailed study of two (dusk and dawn) ISEE 1 passes through the LLBL region for the case of northward M/IMF. We find that the dawn and dusk LLBL are on closed geomagnetic field lines for northward M/IMF but are on a combination of closed and open field lines for a southward M/IMF. The energetic particle distributions show that cases of reverse‐draped field lines in the LLBL are consistent with an open field line topology. In addition, we find that the LLBL is thicker (thinner) for northward (southward) M/IMF and becomes thicker with increasing distance from the subsolar point. LLBL electric fields nominally are in the few (3–5) millivolts per meter range and display an apparent maximum value of ∼10 mV/m. These electric fields capture magnetospherically drifting particles as they approach the LLBL and propel them tailward. In this way, the plasma sheet is the dominant source of energetic (≳10 keV) particles in the LLBL while the magnetosheath appears to be the dominant source for lower‐energy (≲10 keV) LLBL particles.

Simultaneous Geotail and Wind observations of reconnection at the subsolar and tail flank magnetopause

“…Thick flank LLBLs that have been reported to-date tend to be associated with northward IMF conditions [Mitchell et al, 1987;Phan et al, 1997Phan et al, , 2005Fairfield et al, 2000;Hasegawa et al, 2004]. Diffusive entry or nonlinear Kelvin-Helmholtz Instability (KHI) have been suggested as the entry mechanisms in those studies.…”

“…Diffusive entry or nonlinear Kelvin-Helmholtz Instability (KHI) have been suggested as the entry mechanisms in those studies. For southward IMF, the flank LLBL tends to be significantly thinner [Mitchell et al, 1987] and in one event, Gosling et al [1986] showed that there was no plasma boundary layer beyond the reconnecting magnetopause current layer. Newell and Meng [1998] and Phan et al [2005] suggested that for southward IMF, the reconnection rate at the low-latitude magnetopause may exceed the rate of plasma entry from nonreconnection processes, to the extent that reconnection may destroy any pre-existing boundary layer.…”

The permeability of the magnetopause to a multispecies substorm injection of energetic particles

“…In particular, energetic particles (greater than tens of keV) are often observed outside of the magnetopause boundary that is observed as more abrupt in lower energy particles and magnetic fields. The interaction of energetic particles with the Earth's magnetopause has been an active area of research [e.g., Williams et al, 1979;Krimigis et al, 1986;Sibeck et al, 1987;Mitchell et al, 1987;Kudela et al, 1992], and it has been established that energetic particles escape the magnetosphere into the magnetosheath via a variety of processes: (1) acceleration at the dayside reconnection region and escape across a rotational discontinuity along reconnected magnetic field lines [Speiser et al, 1981;Cowley, 1982]; (2) outward streaming along interconnected magnetosheath and magnetospheric magnetic field lines without acceleration [Scholer et al, 1981]; (3) finite-gyroradius "leakage" as the particles cross a tangential discontinuity of the magnetopause [Sibeck et al, 1987;Sibeck and McEntire, 1988;Paschalidis et al, 1994]; or (4) escape into the magnetosheath caused by particle scattering in the magnetopause current sheet during enhanced solar wind dynamic pressure [Zong and Wilken, 1999].…”

Kelvin-Helmholtz vortices and secondary instabilities in super-magnetosonic regimes

“…In this case we would not expect a large transport of plasma and energy or, in general, a strong interaction between the solar wind and the magnetosphere. However, during the period in which the IMF points northwards, a strong mixing layer is observed (Lennartsson and Shelley, 1986;Mitchell et al, 1987;Fujimoto et al, 1998) at low latitudes. In order to account for this observed exchange of mass and energy it was proposed (Belmont and Chanteur, 1989;Otto and Fairfield, 2000;Nakamura and Fujimoto, 2005) that the velocity shear between the magnetosphere and the magnetosheath leads to the onset of the Kelvin-Helmholtz (KH) instability.…”