Characterization of the IMF B_y‐dependent field‐aligned currents in the cleft region based on DE 2 observations

Abstract

The interplanetary magnetic field (IMF) By‐dependent distribution of field‐aligned currents in the cleft region is studied, using the magnetic field and plasma data from 47 passes of Dynamics Explorer (DE) 2. These orbits were chosen on the conditions that cusp/cleft particles are detected and that at the same time the IMF By and Bz components satisfy the criteria |By|≥5 nT and |Bz|≤5 nT during the satellite's crossing of the relevant field‐aligned current region. When By is positive (negative) in addition to satisfying these conditions, there is a strong eastward (westward) magnetic perturbation caused by a pair of field‐aligned current sheets, consisting of an equatorward sheet with downward (upward) current and a poleward sheet having upward (downward) current. These By‐dependent field‐aligned currents in the equatorward and the poleward sheets are referred to as the low‐latitude cleft current (LCC) and the high‐latitude cleft current (HCC), respectively. The cusp/cleft electron precipitation region and the LCC region overlap with each other to a varying degree irrespective of the sign of By. For positive (negative) By, LCC has the same direction as the morning (afternoon) region 1 current or the afternoon (morning) region 2 current. Thus an interpretation has been given in the past that the LCC region is an extension of the region 1 or region 2 current system. However, in this paper we present an alternative view that the LCC region is not an extension of the region 1 or region 2 current system and that a pair of LCC and HCC constitutes the cleft field‐aligned current regime. The proposed pair of cleft field‐aligned currents is explained with a qualitative model in which this pair of currents is generated on the open field lines that have just been reconnected on the dayside magnetopause. The model assumes a quasi‐steady reconnection operating within certain longitudinal width extending to both sides of the stagnation point on the dayside magnetopause. The reconnected flux tubes move under the influences of the field tension and the magnetosheath flow. When the magnetosheath By is positive, the northern hemisphere field lines reconnected on the eastward side of the stagnation point are pulled toward higher latitudes, and the field lines reconnected on the westward side of the stagnation point are pulled along the dawnside magnetopause flank. The electric fields associated with these motions are present immediately inside the magnetopause (rotational discontinuity). This is the source region of LCC and HCC. The electric fields are transmitted along the field lines to the ionosphere, creating a poleward electric field and a pair of field‐aligned currents when By is positive; the pair of field‐aligned currents consists of a downward current at lower latitudes (LCC) and an upward current at higher latitudes (HCC). In the By negative case, the model explains the reversal of the field‐aligned current direction in the LCC and HCC regions.

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

“…It has been previously documented that the fading phase of PMAFs occurs within the regime of mantle precipitation (Sandholt et al, 1993). Steady-state aspects of the high-latitude dynamo process for the actual IMF orientation (IMF B y > 0) are discussed by, for example, Taguchi et al (1993). Our aurora-convection results are in good agreement with the recent study of Provan et al (2002), showing that the high-latitude pulsed ionospheric flows are observed within the regime of mantle precipitation and can be considered as "fossilized signatures of dayside reconnection, observed as the field lines are being pulled antisunward by the magnetosheath flow.…”

“…One view is that reconnection in such cases can occur where there are antiparallel components in the external magnetosheath magnetic field and the Earth's field (Gonzalez and Mozer, 1974;Cowley, 1981;Sonnerup, 1984;Taguchi et al, 1993;Kawano and Russell, 1997;Siscoe et al, 2001). According to this view, reconnection can take place over large areas of the dayside magnetopause, for example, along a tilted reconnection line traversing the subsolar region and extending to high latitudes in both hemispheres.…”

Pulsed flows at the high-altitude cusp poleward boundary, and associated ionospheric convection and particle signatures, during a Cluster - FAST - SuperDARN- Søndrestrøm conjunction under a southwest IMF

“…Siscoe and Cummings, 1969;Vasyliunas et al, 1982;Stern, 1984;Taguchi et al, 1993;Siscoe et al, 1991;Farrugia et al, 2003). However, the role of this region as a site of momentum transfer in association with pulsed magnetopause reconnection/FTEs has not been discussed much in the literature, mainly for reasons discussed above.…”

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

“…The corresponding magnetic disturbance in the cusp region is estimated approximately as 600 nT. Such a disturbance was observed in the cusp region when B y component of IMF was equal A9 nT (Taguchi et al, 1993). It means that the modelled situation corresponds to the case when B y changes from 0 to A9 nT and back to 0 during an hour.…”

“…To investigate the e ects of the disturbed ®eld-aligned current variations in the cusp for IMF B y < 0 we have used the following model input variations of the ®eld-aligned currents based on the data by Rei and Burch (1985), Taguchi et al (1993), Yamauchi et al (1993), and Ohtani et al (1995). We have added the ®eld-aligned current¯owing into (out of) the ionosphere at the Northern (Southern) Hemisphere along the 80°g eomagnetic latitude at the 1130±1400 MLT sector and owing out at the 1000±1130 MLT sector (right top panel in Fig.…”