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,

We examine magnetic field data obtained by the Cassini spacecraft on a sequence of high‐latitude orbits in Saturn's magnetosphere spanning October 2006 to May 2007 to determine whether planetary‐period oscillations are present on polar open field lines, such as have been found previously in near‐equatorial magnetic field data. Such oscillations are found generally to be present with amplitudes ∼0.5–1 nT, somewhat smaller than the few nT amplitudes typical of the quasi‐dipolar equatorial region. The polarization characteristics in the northern and southern polar regions are determined and found to differ significantly from those in the equatorial region. The phases of the oscillations in the northern and southern hemispheres are also determined relative to the equatorial oscillations, and hence relative to each other, requiring extension of the equatorial oscillation phase model to the end of 2007, spanning the interval of high‐latitude orbits. The results show that the overall pattern of field oscillations is not consistent with a rotating external current system that mimics a rotating transverse dipole in the outer regions. Rather, we suggest that the overall field perturbations are associated with a rotating partial ring current and its field‐aligned closure currents, the latter favoring the southern ionosphere during the southern summer conditions examined. A physical picture is presented that links together observed planetary‐period modulations in the middle and outer magnetospheric field, plasma, and radio emissions that may be subject to further test and makes predictions as to how these phenomena will evolve during future Saturn equinox and northern summer conditions.

Abstract. This study employs observations from several sources to determine the location of the polar cap boundary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified. These data sources include global auroral images from the Ultraviolet Imager (UVI) instrument on board the Polar spacecraft, SuperDARN HF radar measurements of the convection flow, and low altitude particle measurements from Defense Meteorological Satellite Program (DMSP) and National Oceanographic and Atmospheric Administration (NOAA) satellites, and the Fast Auroral SnapshoT (FAST) spacecraft. Changes in the open flux content of the magnetosphere are related to the rate of magnetic reconnection occurring at the magnetopause and in the magnetotail, allowing us to estimate the day-and nightside reconnection voltages during two substorm cycles. Specifically, increases in the polar cap area are found to be consistent with open flux being created when the IMF is oriented southwards and low-latitude magnetopause reconnection is ongoing, and decreases in area correspond to open flux being destroyed at substorm breakup. The polar cap area can continue to decrease for 100 min following the onset of substorm breakup, continuing even after substorm-associated auroral features have died away. An estimate of the dayside reconnection voltage, determined from plasma drift measurements in the ionosphere, indicates that reconnection can take place at all local times along the dayside portion of the polar cap boundary, and hence presumably across the majority of the dayside magnetopause. The observation of ionospheric signatures of bursty reconnection over a wide extent of local times supports this finding.

[1] Transpolar arcs are auroral features that extend from the nightside auroral oval into the polar cap. It is well established that they occur predominantly when the interplanetary magnetic field (IMF) has a northward component (B z > 0). Results concerning how the magnetic local time at which transpolar arcs form might depend upon the IMF dawn-dusk component (B Y ) are more mixed. Some studies have found a correlation between these two variables, with Northern Hemisphere arcs forming predominantly premidnight when B Y > 0 and postmidnight when B Y < 0 and vice versa in the Southern Hemisphere. However, a more recent statistical study found that there was no significant correlation, and other studies find that the formation of moving arcs is triggered by a change in the sign of the IMF B Y component. In this paper, we investigate the relationship between the magnetic local time at which transpolar arcs form and the IMF B Y component. It is found that there is indeed a correlation between the magnetic local time at which transpolar arcs form and the IMF B Y component, which acts in opposite senses in the Northern and Southern hemispheres. However, this correlation is weak if the IMF is only averaged over the hour before the first emergence of the arc and becomes stronger if the IMF is averaged 3-4 h beforehand. This is consistent with a mechanism where the magnetic local time at which the arc first forms depends on the B Y component in the magnetotail adjacent to the plasma sheet, which is determined by the IMF B Y component during intervals of dayside reconnection in the hours preceding the first emergence of the arc. We do not find evidence for the triggering of arcs by an IMF B Y sign change.