Interhemispheric asymmetry of the high‐latitude ionospheric convection pattern

Abstract

The assimilative mapping of ionospheric electrodynamics technique has been used to derive the large-scale high-latitude ionospheric convection patterns simultaneously in both northern and southern hemispheres during the period of January 27-29, 1992. When the interplanetary magnetic field (IMF) B• component is negative, the convection patterns in the southern hemisphere are basically the mirror images of those in the northern hemisphere. The total cross-polar-cap potential drops in the two hemispheres are similar. When B• is positive and IB•I > B•, the convection configurations are mainly determined by B• and they may appear as normal "two-cell" patterns in both hemispheres much as one would expect under southward IMF conditions. However, there is a significant difference in the cross-polar-cap potential drop between the two hemispheres, with the potential drop in the southern (summer) hemisphere over 50% larger than that in the northern (winter) hemisphere. As the ratio of decreases (less thn one), the convection configuration in the two hemispheres may be significantly different, with reverse convection in the southern hemisphere and weak but disturbed convection in the northern hemisphere. By comparing the convection patterns with the corresponding spectrograms of precipitating particles, we interpret the convection patterns in terms of the concept of merging cells, lobe cells, and viscous cells. Estimates of the "merging cell" potential drops, that is, the potential ascribed to the opening of the dayside field lines, are usually comparable between the two hemispheres, as they should be. The "lobe cell" provides a potential between 8.5 and 26 kV and can differ greatly between hemispheres, as predicted. Lobe cells can be significant even for southward IMF, if IBl > IBI. To estimate the potential drop of the "viscous cells," we assume that the low-latitude boundary layer is on closed field lines. We find that this potential drop varies from case to case, with a typical value of 10 kV. If the source of these cells is truly a viscous interaction at the flank of the magnetopause, the process is likely spatially and temporally varying rather than steady state. New Zealand. 6491 6492 LU ET AL.: HIGH-LATITUDE IONOSPHERIC CONVECTION PATTERN Pedersen and Hall conductance models are obtained by combining the auroral conductance model of Fuller-Rowell and Evans [1987] with an empirical model of conductance produced by solar extreme ultraviolet radiation based on Chatanika radar observations. The statistical electric potential model is based on Millstone Hill radar observations [Foster et al., 1986]. Both conductance and potential models are parameterized by the hemispheric power index (HPI) [Foster e! al., 1986]. A very important feature of AMIE is its ability to give quantitative information about the uncertainty in the resultant patterns, so that features mapped reliably can LU ET AL.' HIGH-LATITUDE

A classification of dayside auroral forms and activities as a function of interplanetary magnetic field orientation

“…Burch [1985] and confirmed by the observations of [Lu et al, 1994]. The detailed relationship between the two auroral forms and the merging and lobe convection cells is illustrated by Sandholt et al [1998a].…”

Multi-site observations of the association between aurora and plasma convection in the cusp/polar cap during a southeastward(<i>B<sub>y</sub></i> <u>~</u> |<i>B<sub>z</sub></i>|) IMF orientation

“…Lobe cell convection during |B y | > |B z | IMF conditions is characterized by sunward (zonal) polar cap convection in the postnoon (prenoon) sector for B y > 0(< 0) (Reiff and Burch, 1985;Lu et al, 1994;Eriksson et al, 2002). According to these studies, lobe cell convection is present for both positive and negative B z polarity so long as |B y | ≥ |B z |.…”

“…There is now increasing evidence supporting the view that plasma convection at dayside high latitudes under conditions of a dominant B y component consists of a composite pattern of so-called merging and lobe convection cells (Reiff and Burch, 1985;Knipp et al, 1993;Lu et al, 1994;Weiss et al, 1995;Crooker et al, 1998;Weimer, 2001;Eriksson et al, 2002). While the merging cells are characterized by the transfer of magnetic flux across the open-closed field line boundary (OCFLB) in the cusp region, the lobe cells circulate plasma in the polar cap region, entirely poleward of the OCFLB.…”

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

“…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.…”

“…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.…”