A curve fitting procedure for mathematically representing the auroral oval is described. It is shown that the Feldstein ovals and quiet auroral ovals of the night side in DMSP photographs can be approximated by an offset circle, plus a small (≲ 1° magnitude) Fourier component in corrected geomagnetic coordinates. Parameters are introduced, Θ and (θo, ϕo), that indicate the dynamic motion of the auroral oval size and center location, respectively. Using the parameter Θ, an analysis of several DMSP photographs shows a strong correlation between the southward interplanetary magnetic field (−Bz) and the size of the auroral oval.

Abstract. We present a comprehensive observational study of the magnetospheric response to an interplanetary magnetic field (IMF) tangential discontinuity, which first struck the postnoon bow shock and magnetopause and then swept past the prenoon bow shock and magnetopause on July 24, 1996. Although unaccompanied by any significant plasma variation, the discontinuity interacted with the bow shock to form a hot flow anomaly (HFA), which was observed by Interball-1 just upstream from the prenoon bow shock. Pressures within and Earthward of the HFA were depressed by an order of magnitude, which allowed the magnetopause to briefly (-7 min) move outward some 5 R E beyond its nominal position and engulf Interball-1.A timing study employing nearby Interball-1 and Magion-4 observations demonstrates that this motion corresponded to an antisunward and northward moving wave on the magnetopause. The same wave then engulfed Geotail, which was nominally located downstream in the outer dawn magnetosheath. Despite its large amplitude, the wave produced only minor effects in GOES-8 geosynchronous observations near local dawn. Polar Ultraviolet Imager (UVI) observed a sudden brightening of the afternoon aurora, followed by an even more intense transient brightening of the morning aurora. Consistent with this asymmetry, the discontinuity produced only weak near-simultaneous perturbations in highlatitude postnoon ground magnetometers but a transient convection vortex in the prenoon Greenland ground magnetograms. The results of this study indicate that the solar wind interaction with the bow shock is far more dynamic than previously imagined and far more significant to the solar wind-magnetosphere interaction.

In an attempt to place short-lived, high-speed magnetotail flows termed bursty bulk flow events (BBFs) in the context of substorm phenomenology we analyze one such event that took place on April 11, 1985, using data from several spacecraft and many ground stations. The substorm onset, which took place at 0127 UT, had a meridian 2 hours of local time east of AMPTE/IRM. The satellite did not detect high-speed flows at that time. A high-latitude (--•70 ø corrected geomagnetic) substorm intensification took place at 0202 UT centered --•0.5 hour of local time west of the AMPTE/IRM meridian. The ISEE 2 satellite at the magnetotail lobe and the LANL 019 satellite at geosynchronous altitude were both at the same meridian as AMPTE/IRM at the time. The 0202 UT substorm intensification was associated with (1) a dipolarization at the ISEE 2 satellite at 0200:30 UT, (2) a BBF onset at AMPTE/IRM at 0202 UT accompanied by an intense dipolarization consistent with current wedge formation, (3) an energetic particle injection at geosynchronous altitude that took place at 0204 UT. The plasma acceleration region associated with this substorm intensification was estimated to be --• 8 R E tailward of AMPTE/IRM. Thus, during this activity the BBF event was due to an observed tail collapse Earthward of X --• -26 RE. The Earthward energy transport measured at AMPTE/IRM can account for the expected magnetospheric power consumption if the BBF has a cross-sectional area of only 1-2 R2e in the Y-Z direction. Similarly, the Earthward magnetic flux transport rate measured at AMPTE/IRM during the BBF event can result in a potential drop comparable to the expected transpolar cap potential if the BBF event has a size of 1-2 R E in the Y direction. The large amounts of flux transport measured past the satellite necessitate the existence of lobe flux reconnection tailward of AMPTE/IRM. The above results assume the validity of the frozen-in condition over the --•10-min duration of the BBF event.Although activity continued in the ionosphere and the ring current for well over 1.5 hours after the 0202 UT substorm intensification, most of the earthward energy and magnetic flux transport past IRM had ceased --•10 min after the BBF onset. We propose that the fast flows transport and pile up magnetic flux through a very narrow (a few R E in Y extent) flow channel in the midtail to the edge of an expanding dipolarization front in the near-Earth region. After the plasma sheet dipolarizes at a given location enhanced flux transport ceases, resulting in an apparent short (10-min timescale) duration of the fast flows. Unlike the near-Earth plasma sheet, which dipolarizes across many hours of local time, the midtail plasma sheet may exhibit longitudinally localized dipolarization. This may explain the often observed lack of one-to-one correlation between midtail activity and substorms.

[1] Thermospheric O/N 2 column density ratios referenced at a N 2 column density of 10 17 cm À2 are obtained using the IMAGE SI-13 and TIMED/GUVI far-ultraviolet (FUV) dayglow data, AURIC simulation results, and MSIS86 model. Each of the magnetic storms occurring during a 4-day period (1-4 October 2002) caused significant O/N 2 depletion that was detected by both of the IMAGE SI-13 and GUVI instruments. The depletion extended down to latitudes of 10°and À5°in the Northern and Southern Hemispheres, respectively. Simultaneous measurements show an excellent agreement between the SI-13 and GUVI O/N 2 on both global and local scales. The IMAGE SI-13 O/N 2 data provide direct optical evidence that the O/N 2 depletion corotates with the Earth. The GUVI O/N 2 indicate the depletion in both of the hemispheres is not symmetric owing to the seasonal effect and differences in heating and convection induced winds. Both the IMAGE SI-13 and GUVI O/N 2 maps also provide a good opportunity for future modeling efforts.

[1] We compare the timing and spatial extent of dipolarization with auroral breakup by using magnetic field data (1-min time resolution) from GOES 8 and GOES 9 and global auroral images (37-s frame rate) from the Polar ultraviolet imager. We survey the entire year of 1997 data and obtain 32 unambiguous dipolarization events that had ionospheric footprints within 2 hours in magnetic local time (MLT) from their associated auroral breakups. It is found that the auroral breakup preceded the arrival of dipolarization at GOES by an average of 1.7 ± 2.7 min. (1.0 ± 2.1 min for events ±1 hour from the auroral breakup). The delay of geosynchronous dipolarization represents a propagation time, as the substorms rarely initiated at the GOES location. Propagation velocities derived from the propagation time are $60 km s À1 eastward, $75 km s À1 westward, and $270 km s À1 earthward at geosynchronous orbit. We also show for the first time that auroral bulge and dipolarization region closely map; the bulge is the region of currents out of the ionosphere. The eastern edge of the dipolarization region, where field-aligned currents are flowing into the ionosphere, is, based on the T89 magnetic field model, located at $0.5 hour in MLT eastward of the eastern edge of the auroral bulge. Within the initiation site meridian the dipolarization onset was observed $1 min prior to the auroral breakup at the GOES altitude. This time is associated with Alfvén transit time and therefore depends on the radial location of the onset. The azimuthal scale of the initiation site is found to be $0.5 hour in MLT in the ionosphere or less than $1 Earth's radius at 6.6 R E . These results support current diversion into the ionosphere as a critical part of substorms. Finally, a simple timing analysis of specific substorm onset time features suggests that near-Earth reconnection cannot occur before current disruption and therefore cannot be the cause of substorm.