The Super Dual Auroral Radar Network (SuperDARN) has been operating as an international co-operative organization for over 10 years. The network has now grown so that the fields of view of its 18 radars cover the majority of the northern and southern hemisphere polar ionospheres. SuperDARN has been successful in addressing a wide range of scientific questions concerning processes in the magnetosphere, ionosphere,

[1] We trace the formation and subsequent motion of a transpolar arc in response to dayside and nightside reconnection. Both high-and low-latitude dayside reconnection are observed, as well as periods of substorm and nonsubstorm nightside reconnection, during the 7-hour interval of interest on 19 January 2002. We speculate that the arc is formed by a burst of nonsubstorm nightside reconnection and that its subsequent motion is controlled predominantly by the rate of dayside high-latitude reconnection, siphoning open flux from the dusk sector polar cap to the dawn sector. The observations allow us to quantify the rates of reconnection: on the nightside, 35 and 100 kV during nonsubstorm-and substorm-related bursts, respectively; on the dayside, 30 and 100 kV for high-and low-latitude reconnection. The latter values give effective merging line lengths of 1 and 5.5 R E for northward and southward interplanetary magnetic field, respectively. We suggest that transpolar arc motion will be controlled not only by the B y component of the IMF but also by the relative magnitude of the B z component, when jB y j > B z motion will be dawnward for B y < 0 nT and duskward for B y > 0 nT; however, when B z > jB y j, we expect that the arc will move toward the noon-midnight meridian of the polar cap.Citation: Milan, S. E., B. Hubert, and A. Grocott (2005), Formation and motion of a transpolar arc in response to dayside and nightside reconnection,

[1] The spectrographic imager at 121.8 nm (SI12) of the far ultraviolet (FUV) experiment onboard the IMAGE spacecraft produces global images of the Doppler-shifted Lyman a emission of the proton aurora. This emission is solely due to proton precipitation and is not contaminated by dayglow, allowing us to monitor the auroral oval on the dayside as well as on the nightside. Remote sensing of the polar aurora can be advantageously supplemented by use of ground-based data from the Super Dual Auroral Radar Network (SuperDARN) that monitors the ionospheric convective flow pattern in the polar region. In the present study, the SI12 images are used to determine the location of the open/closed field line boundary and to monitor its movement. The SuperDARN data are then used to compute the ionospheric electric field at the location of the open/closed boundary. The total electric field is then computed along the boundary accounting for its movement via Faraday's law so that the dayside and nightside reconnection voltages can be derived. This procedure is applied to several substorm intervals observed simultaneously with IMAGE FUV and SuperDARN. The dayside reconnection voltage feeds the magnetosphere with open flux, which is later closed by nightside reconnection. The calculated dayside reconnection rate is consistent with the solar wind properties measured by the Geotail, Wind, and ACE satellites. We identify the presence of nightside reconnection due to pseudobreakups taking place during the growth phase. In several cases, we establish that the nightside reconnection rate is maximum at the time of the substorm expansion phase onset or shortly after, reaching $120 kV, and then slowly returns to undisturbed values of $30 kV. The flux closure rate can also start intensifying prior to expansion phase onset, producing pseudobreakups.

Abstract. We perform two superposed epoch analyses of the auroral evolution during substorms using the FUV instrument on the Imager for Magnetopause-to-Aurora Global Explorer (IMAGE) spacecraft. The larger of the two studies includes nearly 2000 substorms. We subdivide the substorms by onset latitude, a measure of the open magnetic flux in the magnetosphere, and determine average auroral images before and after substorm onset, for both electron and proton aurora. Our results indicate that substorms are more intense in terms of auroral brightness when the open flux content of the magnetosphere is larger, and that magnetic flux closure is more significant. The increase in auroral brightness at onset is larger for electrons than protons. We also show that there is a dawn-dusk offset in the location of the electron and proton aurora that mirrors the relative locations of the region 1 and region 2 current systems. Superposed epoch analyses of the solar wind, interplanetary magnetic field, and geomagnetic indices for the substorms under study indicate that dayside reconnection is expected to occur at a faster rate prior to low latitude onsets, but also that the ring current is enhanced for these events.

Abstract. We present SuperDARN radar observations of the nightside high-latitude ionospheric flow during two 6-hour intervals of quasi-steady northward interplanetary magnetic field (IMF). During both intervals (01:30-07:30 UT on 2 December and 21:00-03:00 UT on 14/15 December 1999), the solar wind and IMF remained relatively steady with B z positive and B y negative, such that the IMF clock angle was ∼−50 • to −60 • . Throughout both intervals the radar data clearly indicate the presence of a highly distorted B ydominated twin cell flow pattern, indicative of an open magnetosphere, which is confirmed by DMSP and auroral data. Estimates of the changes in open flux present during each interval indicate approximately balanced dayside and nightside reconnection at rates of ∼30-35 kV over the full 6 h. However, strong bursts of flow with speeds of over ∼1000 m s −1 are observed near magnetic midnight on time scales of ∼1 h, which are associated with increases in the transpolar voltage. These are indicative of the net closure of open flux by reconnection in the tail. During one large flow burst, the nightside reconnection rate is estimated to have been ∼1.5 times the dayside rate, i.e. ∼45-60 kV compared with ∼30-40 kV. Magnetic bays, which would indicate the formation of a substorm current wedge, are not observed in association with these bursts. In addition, no low-latitude Pi2s or geostationary particle injections were observed, although some local, small amplitude Pi2-band (5-50 mHz) activity does accompany the bursts. Coincident measurements of the flow and of the low amplitude magnetic perturbations reveal nightside ionospheric conductances of no more than a few mho, indicative of little associated precipitation. Therefore, we suggest that the flow bursts are the ionospheric manifestation of bursty reconnection events occurring in the more distant geomagnetic tail. The main implication of these findings is that, under the circumstances examined here, the convection cycle is not equivalent to the usual substorm cycle that occurs for southward IMF.