CHARACTERISTICS OF IONOSPHERIC CONVECTION AND FIELD-ALIGNED CURRENT IN THE DAYSIDE CUSP REGION

The assimilative mapping of ionospheric electrodynamics (AMIE) techniq ue has been used to estimate global distributions of high-latitude ion ospheric convection and field-aligned current by combining data obtain ed nearly simultaneously both from ground and from space. Therefore, u nlike the statistical patterns, the ''snapshot'' distributions derived by AMIE allow us to examine in more detail the distinctions between f ield-aligned current systems associated with separate magnetospheric p rocesses, especially in the dayside cusp region. By comparing the fiel d-aligned current and ionospheric convection patterns with the corresp onding spectrograms of precipitating particles, the following signatur es have been identified: (1) For the three cases studied, which all ha d an IMF with negative y and z components, the cusp precipitation was encountered by the DMSP satellites in the postnoon sector in the north ern hemisphere and in the prenoon sector in the southern hemisphere. T he equatorward part of the cusp in both hemispheres is in the sunward flow region and marks the beginning of the flow rotation from sunward to antisunward. (2) The pair of field-aligned currents near local noon , i.e., the cusp/mantle currents, are coincident with the cusp or mant le particle precipitation. In distinction, the field-aligned currents on the dawnside and duskside, i.e., the normal region 1 currents, are usually associated with the plasma sheet particle precipitation. Thus the cusp/mantle currents are generated on open field lines and the reg ion 1 currents mainly on closed field lines. (3) Topologically, the cu sp/mantle currents appear as an expansion of the region 1 currents fro m the dawnside and duskside and they overlap near local noon. When B-y is negative, in the northern hemisphere the downward field-aligned cu rrent is located poleward of the upward current; whereas in the southe rn hemisphere the upward current is:located poleward of the downward c urrent. (4) Under the assumption of quasi-steady state reconnection, t he location of the separatrix in the ionosphere is estimated and the r econnection velocity is calculated to be between 400 and 550 m/s. The dayside separatrix lies equatorward of the dayside convection throat i n the two cases examined.