Low-latitude boundary layer (LLBL)

Introduction

Characteristics

• low latitude part of the dayside boundary layer extending also to the evening local times
• thickness increases with increasing distance from the subsolar point, being about 0.5-1 Re at the dawn-dusk meridian
• may be partially on open field lines, especially during southward IMF Bz, when it is not as thick as during northward IMF
• mixture of plasma of magnetospheric and magnetosheath origin; the magnetosheath plasma may be leaking diffusively through the magnetopause into this region
• region of general tailward plasma flow
  • there is some evidence that, in the magnetospheric side of the LLBL, there exists a stagnation region with LLBL-like plasma with variable flow (both in direction and magnitude); this could be due to Kelvin-Helmholtz instability
    • at low altitudes this means that the convection reversal boundary may not coincide exactly with the LLBL/BPS particle boundary, but occurs withing the LLBL
• site of the dayside region 1 current system
  • most intense upward current close to 1400-1600 MLT, and downward 0800-1000 MLT
• partially thermalized with respect to magnetosheath plasma, with lower flow velocities and higher temperatures (with considerable variability and spatial inhomogeneity, however)
  • densities: 0.5 - 10 cm^-3
  • temperatures: from less than 100 eV to 1000-2000 eV
  • flow: generally tailward from about 100 km/s up to magnetosheath values (or higher, like 800 km/s)
• region of bidirectional streaming electron populations (50-200 eV)
• there is a systematic relationship between the electron number density and temperature, i.e., the transition parameter (Hapgood and Bryant, 1990, 1992; Lockwood and Hapgood, 1997).
• The so called "halo" region Earthward of the dayside LLBL may relate to the LLBL itself (Sckopke et al, 1981) or the cold plasma sheet (Phan et al., ICS-4 meeting, 1998).

References

• Eastman, T. E., E. W. Hones Jr., S. J. Bame, and J. R. Asbridge, The magnetospheric boundary layer: Site of plasma, momentum, and energy transfer from the magnetosheath into the magnetosphere, Geophys. Res. Lett., 3, 685, 1976.
• Fujimoto, M., T. Mukai, H. Kawano, M. Nakamura, A. Nishida, Y. Saito, T. Yamamoto, and S. Kokubun, Structure of the low-latitude boundary layer: A case study with Geotail data, J. Geophys. Res., 103, 2297-2308, 1998.
• Hapgood, M. A., and D. A. Bryant, Re-ordered electron data in the low-latitude boundary layer, Geophys. Res. Lett., 17, 2043-2046, 1990.
• Hapgood, M. A., and D. A. Bryant, Exploring the magnetospheric boundary layer, Planet. Space Sci., 40, 1431-1459, 1992.
• Lockwood, M., and M. A. Hapgood, How the magnetopause transition parameter works, Geophys. Res. Lett., 24, 373-376, 1997.
• Sckopke et al., Structure of the low-latitude boundary layer, J. Geophys. Res., 86, 2099-, 1981.

• Space Physics Text Book of Oulu