Characteristics of ionospheric convection and field‐aligned current in the dayside cusp region

Abstract

The assimilative mapping of ionospheric electrodynamics (AMIE) technique has been used to estimate global distributions of high‐latitude ionospheric convection and field‐aligned current by combining data obtained nearly simultaneously both from ground and from space. Therefore, unlike the statistical patterns, the “snapshot” distributions derived by AMIE allow us to examine in more detail the distinctions between field‐aligned current systems associated with separate magnetospheric processes, especially in the dayside cusp region. By comparing the field‐aligned current and ionospheric convection patterns with the corresponding spectrograms of precipitating particles, the following signatures have been identified: (1) For the three cases studied, which all had an IMF with negative y and z components, the cusp precipitation was encountered by the DMSP satellites in the postnoon sector in the northern hemisphere and in the prenoon sector in the southern hemisphere. The 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 mantle 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 region 1 currents mainly on closed field lines. (3) Topologically, the cusp/mantle currents appear as an expansion of the region 1 currents from the dawnside and duskside and they overlap near local noon. When By is negative, in the northern hemisphere the downward field‐aligned current is located poleward of the upward current; whereas in the southern hemisphere the upward current is located poleward of the downward current. (4) Under the assumption of quasi‐steady state reconnection, the location of the separatrix in the ionosphere is estimated and the reconnection velocity is calculated to be between 400 and 550 m/s. The dayside separatrix lies equatorward of the dayside convection throat in the two cases examined.

Seasonal effects in the ionosphere-thermosphere response to the precipitation and field-aligned current variations in the cusp region

“…The second variant of the calculations is more realistic because it considers not only the background seasonal variation of the ionospheric and thermospheric parameters but also includes the seasonal e ects in the magnetospheric input parameters such as geometry and FAC intensity in accordance with data of LU et al (1995). As has been shown, these seasonal e ects in the input parameters do not in¯uence noticeably the ionospheric and thermospheric responses to the precipitation and FAC variations in the cusp region.…”

“…It was studied by many authors (McDiarmid et al, 1979;Bythrow et al, 1982;Friis-Christensen et al, 1985;Gussenhoven, 1988;Erlandson et al, 1988;Woch et al, 1993;Yamauchi et al, 1993;de La Beaujardiere et al, 1993;Xu and Kivelson, 1994;Lu et al, 1995). From their results we can conclude that the dayside soft electron precipitation and FAC region consists of the overlapping parts including the cusp proper copopulated with magnetosheath-like electrons, mantle FACs (traditional cusp FACs) shifted polewards from the cusp, near-noon part of region 1 currents,¯owing partly along closed ®eld lines and partly along open ®eld lines, copopulated with cusp particles, and the other dayside region 1 FAC copopulated with low-latitude boundary layer (LLBL) type particles.…”

“…In reality, not only the background state of the ionosphere and thermosphere is di erent in the summer and winter but the precipitating¯uxes and FACs may be di erent in the summer and winter hemispheres as well (Iijima and Potemra, 1976;Bythrow et al, 1982;Fujii and Iijima, 1987;Newell and Meng, 1988;Yamauchi and Araki, 1989;Lu et al, 1994Lu et al, , 1995. This is why we performed the second variant of the calculations where asymmetric input data for the summer and winter precipitating¯uxes and ®eld-aligned currents have been taken from the patterns derived by Lu et al (1995) by combining data obtained from the satellite, radar and ground magnetometer observations for these events. Lu et al (1995) used the assimilative mapping of ionospheric electrodynamics (AMIE) technique, derived by Richmond and Kamide (1988), to estimate global``snapshot'' distributions of high-latitude convection and ®eld-aligned current by combining data obtained nearly simultaneously both ground and from space.…”

“…Now, in the present investigation we have studied mainly the seasonal e ects in the thermospheric and ionospheric responses to the soft electron precipitation and ®eld-aligned current variations, of the order of an hour in duration, in the summer and winter cusp regions simultaneously using the same model as in Namgaladze et al (1996b). It should be expected that seasonal e ects in the thermospheric and ionospheric disturbances may be rather signi®cant due to at least two factors: (1) seasonal variations in the background state of the undisturbed thermosphere and ionosphere, i.e., of the neutral, ion and electron densities and temperatures and electric conductivities (Fuller-Rowell et al, 1988;Sojka and Schunk, 1989;Kirkwood, 1996); and (2) seasonal variations of the``input'' parameters such as the precipitating particle¯uxes and ®eld-aligned current densities (Iijima and Potemra, 1976;Bythrow et al, 1982;Fujii and Iijima, 1987;Newell and Meng, 1988;LU et al, 1994LU et al, , 1995. Correspondingly, two variants of the calculations have been performed both for the IMF B y < 0.…”

“…Geomagnetically asymmetric input data for the summer and winter precipitating¯uxes and ®eld-aligned currents have been taken from the patterns derived by Lu et al (1995) by combining data obtained from the satellite, radar and ground magnetometer observations for these events when precipitations were weaker but the magnetospheric convection was stronger than in the ®rst variant. The results of the model calculations will be compared with those obtained in the ®rst variant and with the observations and the possible physical causes of the predicted seasonal ionospheric and thermospheric e ects in the cusp regions will be discussed.…”