Abstract
Abstract. We study a series of transient entries into the low-latitude boundary layer (LLBL) of all four Cluster spacecraft during an outbound pass through the mid-afternoon magnetopause ( [ XGSM, YGSM, ZGSM ] ≈ [ 2, 7, 9 ] RE). The events take place during an interval of northward IMF, as seen in the data from the ACE satellite and lagged by a propagation delay of 75 min that is welldefined by two separate studies: (1) the magnetospheric variations prior to the northward turning (Lockwood et al., 2001, this issue) and (2) the field clock angle seen by Cluster after it had emerged into the magnetosheath (Opgenoorth et al., 2001, this issue). With an additional lag of 16.5 min, the transient LLBL events correlate well with swings of the IMF clock angle (in GSM) to near 90°. Most of this additional lag is explained by ground-based observations, which reveal signatures of transient reconnection in the pre-noon sector that then take 10–15 min to propagate eastward to 15 MLT, where they are observed by Cluster. The eastward phase speed of these signatures agrees very well with the motion deduced by the cross-correlation of the signatures seen on the four Cluster spacecraft. The evidence that these events are reconnection pulses includes: transient erosion of the noon 630 nm (cusp/cleft) aurora to lower latitudes; transient and travelling enhancements of the flow into the polar cap, imaged by the AMIE technique; and poleward-moving events moving into the polar cap, seen by the EISCAT Svalbard Radar (ESR). A pass of the DMSP-F15 satellite reveals that the open field lines near noon have been opened for some time: the more recently opened field lines were found closer to dusk where the flow transient and the poleward-moving event intersected the satellite pass. The events at Cluster have ion and electron characteristics predicted and observed by Lockwood and Hapgood (1998) for a Flux Transfer Event (FTE), with allowance for magnetospheric ion reflection at Alfvénic disturbances in the magnetopause reconnection layer. Like FTEs, the events are about 1 RE in their direction of motion and show a rise in the magnetic field strength, but unlike FTEs, in general, they show no pressure excess in their core and hence, no characteristic bipolar signature in the boundary-normal component. However, most of the events were observed when the magnetic field was southward, i.e. on the edge of the interior magnetic cusp, or when the field was parallel to the magnetic equatorial plane. Only when the satellite begins to emerge from the exterior boundary (when the field was northward), do the events start to show a pressure excess in their core and the consequent bipolar signature. We identify the events as the first observations of FTEs at middle altitudes.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)
Highlights
The low-latitude boundary layer (LLBL) is characterised by the presence of both magnetosheath and magnetospheric plasma inside the main magnetopause current sheet (Hones et al, 1972; Akasofu et al, 1973; Eastman et al, 1976; Haerendel et al, 1978; Eastman and Hones, 1979; Sonnerup, 1980; Sckopke et al, 1981; Mitchell et al, 1987; Hapgood and Bryant, 1990; Gosling et al, 1990a, b, c; Song et al, 1990; Sckopke, 1991; Traver et al, 1991; Fuselier et al, 1992; Woch and Lundin, 1993; Woch et al, 1993; Saunders, 1983; Hapgood and Lockwood, 1993, 1995; Phan et al, 1997; Savin et al, 1997; Fujimoto et al, 1998). The origin of this layer is one of the major unanswered questions in magnetospheric physics and a key unknown in this regard is the topology of the LLBL field lines: it is interesting to note that roughly half of the papers cited above interpret the LLBL in terms of closed field lines, and the other half in terms of open field lines
Observations confirm the existence of an “open LLBL” at the magnetopause
When approaching the magnetopause on an outbound pass on 14 January 2001, Cluster underwent a series of brief intersections with the Low-Latitude Boundary Layer (LLBL), both before and after it intersected the cusp at about 13:30 (Opgenoorth et al, 2001, this issue)
Summary
The low-latitude boundary layer (LLBL) is characterised by the presence of both magnetosheath and magnetospheric plasma inside the main magnetopause current sheet (Hones et al, 1972; Akasofu et al, 1973; Eastman et al, 1976; Haerendel et al, 1978; Eastman and Hones, 1979; Sonnerup, 1980; Sckopke et al, 1981; Mitchell et al, 1987; Hapgood and Bryant, 1990; Gosling et al, 1990a, b, c; Song et al, 1990; Sckopke, 1991; Traver et al, 1991; Fuselier et al, 1992; Woch and Lundin, 1993; Woch et al, 1993; Saunders, 1983; Hapgood and Lockwood, 1993, 1995; Phan et al, 1997; Savin et al, 1997; Fujimoto et al, 1998). In both (2) and (3), gradient and curvature drift across the open-closed boundary may sometimes help to replenish magnetospheric plasma that has been lost when it flowed across the magnetopause along open field lines
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