Low-latitude boundary layer (LLBL)
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The low-latitude boundary layer, first discussed by Eastman et al. (1976), is part of
the magnetospheric boundary layer.
Large fraction of all the plasma, momentum, and energy transfer from the
magnetosheath into the magnetosphere occurs through it.
- low latitude part of the dayside boundary layer extending also to the evening
- 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).
- 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,
- 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.