Abstract. Simultaneous DMSP F7 and
Viking satellite measurements of the dawnside high-latitude auroral energy
electron and ion precipitation show that the region of the low and middle
altitude auroral precipitation consists of three characteristic plasma regimes.
The recommendation of the IAGA Working Group IIF/III4 at the IAGA Assembly in
Boulder, July 1995 to decouple the nomenclature of ionospheric populations from
magnetospheric population is used for their notation. The most equatorial regime
is the Diffuse Auroral Zone (DAZ) of diffuse spatially unstructured
precipitating electrons. It is generated by the plasma injection to the inner
magnetosphere in the nightside and the subsequent drift plasma to the dawnside
around the Earth. Precipitating particles have a hard spectrum with typical
energies of electrons and ions of more than 3 keV. In the DAZ, the ion
pitch-angle distribution is anisotropic, with the peak near 90°. The next part
is the Auroral Oval (AO), a structured electron regime which closely resembles
the poleward portion of the night-side auroral oval. The typical electron energy
is several keV, and the ion energy is up to 10 keV. Ion distributions are
pre-dominantly isotropic. In some cases, this plasma regime may be absent in the
pre-noon sector. Poleward of the Auroral Oval, there is the Soft Small Scale
Luminosity (SSSL) regime. It is caused by structured electron and ion
precipitation with typical electron energy of about 0.3 keV and ion energy of
about 1 keV. The connection of these low-altitude regimes with plasma domains of
the distant magnetosphere is discussed. For mapping of the plasma regimes to the
equatorial plane of the magnetosphere, the empirical model by Tsyganenko (1995)
and the conceptual model by Alexeev et al. (1996) are used. The DAZ is mapped
along the magnetic field lines to the Remnant Layer (RL), which is located in
the outer radiation belt region; the zone of structured electrons and isotropic
ion precipitation (AO) is mapped to the dawn periphery of the Central Plasma
Sheet (CPS); the soft small scale structured precipitation (SSSL) is mapped to
the outer magnetosphere close to the magnetopause, i.e. the Low Latitude
Boundary Layer (LLBL). In the near-noon sector, earthward fluxes of soft
electrons, which cause the Diffuse Red Aurora (DRA), are observed. The ion
energies decrease with increasing latitude. The plasma spectra of the DRA regime
are analogous to the spectra of the Plasma Mantle (PM). In the dawn sector, the
large-scale field-aligned currents flow into the ionosphere at the SSSL
latitudes (Region 1) and flow out at the AO or DAZ latitudes (Region 2). In the
dawn and dusk sectors, the large-scale Region 1 and Region 2 FAC generation
occurs in different plasma domains of the distant magnetosphere. The dawn and
dusk FAC connection to the traditional Region 1 and Region 2 has only formal
character, as FAC generating in various magnetospheric plasma domains integrate
in the same region (Region 1 or Region 2). In the SSSL, there is anti-sunward
convection; in the DAZ and the AO, there is the sunward convection. At PM
latitudes, the convection is controlled by the azimuthal IMF component (By ). It
is suggested to extend the notation of the plasma pattern boundaries, as
proposed by Newell et al. (1996), for the nightside sector of the auroral oval
to the dawn sector.
Key words. Magnetospheric physics
(current systems; magnetospheric configuration and dynamics; plasma convection) |