Structure of the auroral precipitation region in the dawn sector: relationship to convection reversal boundaries and field-aligned currents

Simultaneous DMSP F7 and Viking satellite measurements of the dawnside high -latitude auroral energy electron and ion precipitation show that the regio n of the low and middle altitude auroral precipitation consists of three ch aracteristic plasma regimes. The recommendation of the IAGA Working Group I IF/III4 at the IAGA Assembly in Boulder, July 1995 to decouple the nomencla ture 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 generate d by the plasma injection to the inner magnetosphere in the nightside and t he subsequent drift plasma to the dawnside around the Earth. Precipitating par tides 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 anisotr opic. with the peak near 90 degrees. The next part is the Auroral Oval (AO) , a structured electron regime which closely resembles the poleward portion of the nightside auroral oval. The typical electron energy is several keV, and the ion energy is up to 10 keV. Ion distributions are predominantly is otropic. In some cases, this plasma regime may be absent in the prenoon sec tor. Poleward of the Auroral Oval, there is the Soft Small Scale Luminosity (SSSL) regime. It is caused by structured electron and ion precipitation w ith 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 di stant magnetosphere is discussed. For mapping of the plasma regimes to the equatorial plane of the magnetosphere, the empirical model by Tsyganenko (1 995) 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 electron s and isotropic ion precipitation (AO) is mapped to the dawn periphery of t he Central Plasma Sheet (CPS); the soft small scale structured precipitatio n (SSSL) is mapped to the outer magnetosphere close to the magnetopause, i. e. the Low Latitude Boundary Layer (LLBL). In the near-noon sector, earthwa rd fluxes of soft electrons, which cause the Diffuse Red Aurora (DRA), are observed. The ion energies decrease with increasing latitude, The plasma sp ectra 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 t he ionosphere at the SSSL latitudes (Region 1) and flow out at the AO or DA Z latitudes (Region 2). In the dawn and dusk sectors, the large-scale Regio n 1 and Region 2 FAC generation occurs in different plasma domains of the d istant magnetosphere. The dawn and dusk FAC connection to the traditional R egion 1 and Region 2 has only formal character, as FAC generating in variou s magnetospheric plasma domains integrate in the same region (Region 1 or R egion 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 con trolled by the azimuthal IMF component (By) It is suggested to extend the n otation of the plasma pattern boundaries, as proposed by Newell et al. (199 6), for the nightside sector of the auroral oval to the dawn sector.