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

Abstract

Abstract. In a case study we demonstrate the spatiotemporal structure of aurora and plasma convection in the cusp/polar cap when the interplanetary magnetic field (IMF) B z < 0 and B y |B z | (clock angle in GSM Y − Z plane: 135 • ). This IMF orientation elicited a response different from that corresponding to strongly northward and southward IMF. Our study of this "intermediate state" is based on a combination of ground observations of optical auroral emissions and ionospheric plasma convection. Utilizing allsky cameras at NyÅlesund, Svalbard and Heiss Island (Russian arctic), we are able to monitor the high-latitude auroral activity within the ∼10:00-15:00 MLT sector. Information on plasma convection is obtained from the SuperDARN radars, with emphasis placed on line of sight observations from the radar situated in Hankasalmi, Finland (Cutlass). A central feature of the auroral observations in the cusp/polar cap region is a ∼30-min long sequence of four brightening events, some of which consists of latitudinally and longitudinally separated forms, which are found to be associated with pulsed ionospheric flows in merging and lobe convection cells. The auroral/convection events may be separated into different forms/cells and phases, reflecting a spatiotemporal evolution of the reconnection process on the dayside magnetopause. The initial phase consists of a brightening in the postnoon sector (∼12:00-14:00 MLT) at ∼73 • MLAT, accompanied by a pulse of enhanced westward convection in the postnoon merging cell. Thereafter, the event evolution comprises two phenomena which occur almost simultaneously: (1) westward expansion of the auroral brightening (equatorward boundary intensification) across noon, into the ∼10:00-12:00 MLT sector, where the plasma convection subsequently turns almost due north, in the convecCorrespondence to: P. E. Sandholt (p.e.sandholt@fys.uio.no) tion throat, and where classical poleward moving auroral forms (PMAFs) are observed; and (2) auroral brightening at slightly higher latitudes (∼75 • MLAT) in the postnoon lobe cell, with expansion towards noon, giving rise to a clear cusp bifurcation. The fading phase of PMAFs is accompanied by a "patch" of enhanced (∼1 km/s) poleward-directed merging cell convection at high latitudes (75-82 • MLAT), e.g. more than 500 km poleward of the cusp equatorward boundary. The major aurora/convection events are recurring at ∼5-10 min intervals.

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

“…9, FAST is at this time well within the regime of CPS precipitation. The present observation of enhanced flow speed in the regime of mantle precipitation is consistent with PIF characteristics that were previously observed (Provan et al, 2002;Sandholt et al, 2003).…”

“…Individual signatures include: poleward-moving auroral forms (PMAFs; Sandholt et al, 1986Sandholt et al, , 1990Sandholt et al, , 2003, pulsed ionospheric flows/flow channels (PIFs; Pinnock et al, 1993Pinnock et al, , 1995Rodger and Pinnock, 1997;Neudegg et al, 1999;Provan et al, 2002), poleward-moving radar auroral forms (PMRAFs; Wild et al, 2001), an ion energy-latitude dispersion signature in the cusp of the staircase variety ("stepped cusp"; Cowley et al, 1991;Newell et al, 1989;Escoubet et al, 1992). The reliable and unique association of these signatures to transient reconnection requires in general multi-point observations.…”

Dayside flow bursts and high-latitude reconnection when the IMF is strongly northward

“…Moore et al (2002) calculated the shape of the X-line for the full range of clock angles using a generic T89 internal magnetic field model. They found an X line shape that is very similar to that inferred by Sandholt et al (2003), for dominant IMF B y , but also found that subsolar reconnection will weaken and disappear for nearly northward B z (small clock angles). As the clock angle increases from near zero, the X line traverses the subsolar region, producing some weak antisunward flow, but mainly azimuthal flows corresponding to overdraping effects, as we note here.…”

“…Sandholt et al, 1998). Moore et al (2002) and Sandholt et al (2003) found that subsolar reconnection will weaken and disappear for nearly northward B z . There have been a wide range of proposed reconnection topologies that might exist for IMF B z northward (Dungey, 1963;Russell, 1972;Cowley, 1981 and1983;Crooker, 1992;Onsager and Lockwood, 1997;Lockwood and Moen, 1999;Sandholt et al, 2000;Onsager et al, 2001), but there are only two basic types.…”

“…When the IMF is southward, reconnection at the lowlatitude magnetopause between the closed magnetospheric field lines and the IMF field lines in the sheath, which has been referred to as low-latitude reconnection, results in the formation of open flux tubes, anti-sunward flow in the polar cap, twin-cell convection in the polar ionosphere, and a concurrent expansion of the polar cap (Siscoe and Huang, 1985;Cowley and Lockwood, 1992). Since impulsive dayside reconnection was first observed (Haerendel et al, 1978;Van Eyken et al, 1984;Goertz et al, 1985), it has been believed to be the primary mechanism for the transfer of flux from the Earth's magnetosheath to the magnetosphere and episodes of such flux transfer are referred to as flux transfer events (FTEs, Russell and Elphic, 1978).…”