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Recent work has shown that solar wind dynamic pressure changes can have a dramatic effect on the particle precipitation in the high-latitude ionosphere. It has also been noted that the preexisting interplanetary magnetic field (IMF) orientation can significantly affect the resulting changes in the size, location, and intensity of the auroral oval. Here we focus on the effect of pressure pulses on the size of the auroral oval. We use particle precipitation data from up to four Defense Meteorological Satellite Program (DMSP) spacecraft and simultaneous POLAR Ultra-Violet Imager (UVI) images to examine three events of solar wind pressure fronts impacting the magnetosphere under two IMF orientations, IMF strongly southward and IMF Bz nearly zero before the pressure jump. We show that the amount of change in the oval and polar cap sizes and the local time extent of the change depends strongly on IMF conditions prior to the pressure enhancement. Under steady southward IMF, a remarkable poleward widening of the oval at all magnetic local times and shrinking of the polar cap are observed after the increase in solar wind pressure. When the IMF Bz is nearly zero before the pressure pulse, a poleward widening of the oval is observed mostly on the nightside while the dayside remains unchanged. We interpret these differences in terms of enhanced magnetospheric reconnection and convection induced by the pressure change. When the IMF is southward for a long time before the pressure jump, open magnetic flux is accumulated in the tail and strong convection exists in the magnetosphere. The compression results in a great enhancement of reconnection across the tail which, coupled with an increase of magnetospheric convection, leads to a dramatic poleward expansion of the oval at all MLTs (dayside and nightside). For near-zero IMF Bz before the pulse the open flux in the tail, available for closing through reconnection, is smaller. This, in combination with the weaker magnetospheric convection, leads to a more limited poleward expansion of the oval, mostly on the nightside. Key words. Magnetospheric physics (solar windmagnetosphere interactions; magnetospheric configuration and dynamics; auroral phenomena)

Ion precipitation data from two co-orbiting Defense Meteorological Satellite Program satellites (F6 and F8) are used to investigate magnetopause reconnection models. We examine differential fluxes between 30 eV and 30 keV, from a Southern Hemisphere, prenoon pass during the morning of January 10, 1990. Data from the first satellite to pass through the region (F6) show two distinct ion energy dispersions ∼1° of latitude apart, between 76° and 79° magnetic latitude. The electron data exhibit similar features at around the Same region but with no or little energy dispersion, consistent with their high velocities. We suggest that the two energy dispersions can be explained by two separate injections resulting from two bursts of magnetopause reconnection. Data from the second satellite (F8), which moved through the same region 1 min later, reveal the same energy-dispersed structures, only further poleward and with less overall flux. This temporal evolution is consistent with two recently reconnected flux tubes releasing their plasma as they move antisunward away from dayside merging sites. However, an observed overlap between the two ion energy dispersions suggests a more complex reconnection geometry than usual models can accommodate. We propose a generalized reconnection scenario that unifies the Bursty Single X-Line and the Multiple X-Line Reconnection models. A simple time-of-flight particle precipitation model is constructed to reproduce the ion dispersions and their overlap. The modeling results suggest that for time-dependent reconnection the dispersion overlap is observed clearly at low altitudes only for a short period compared with the evolution timescale of the ion precipitation.

[1] Over the past few years, the prominent role of solar wind dynamic pressure in enhancing dayside and nightside reconnection and driving-enhanced ionospheric convection has been documented by both ground and spaceborne instruments. For a previous case study of an abrupt increase in solar wind dynamic pressure, Super Dual Auroral Radar Network (SuperDARN) measurements of plasma convection within the dayside polar ionosphere revealed an immediate enhancement of plasma convection. The convection enhancement variation closely follows the variation in solar wind pressure. The dayside enhancement was followed by a nightside convection increase about 40 min later, which has similar variation characteristics as seen on the dayside. We now use SuperDARN flow measurements during a large number of solar wind pressure enhancements to conduct a superposed epoch analysis of the effects of solar wind pressure fronts on the dayside and nightside ionospheric convection. The results for the dayside show an increase of convection for nearly all interplanetary magnetic field (IMF) Bz values. The response is more pronounced and immediate (within minutes) for southward IMF, with a duration of 20–30 min. The response time scales increase to 5–10 min for northward IMF, and the enhanced flows last for 30–50 min. We also find a significant enhancement of nightside convection, particularly for small values of IMF By, that follows about 10–15 min after the dayside response and can last for 40–50 min.

[1] Recent studies showed that, regardless of the orientation of the Interplanetary Magnetic Field (IMF), ULF wave activity in the solar wind can substantially enhance the convection in the high latitude ionosphere, suggesting that ULF fluctuations may also be an important contributor to the coupling of the solar wind to the magnetosphere‐ionosphere system. We conduct a statistical study to understand the effect of ULF power in the IMF on the cross polar cap potential, primarily focusing on northward IMF. We have analyzed the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) calculations of the polar cap potential, a IMF ULF index that is defined as the logarithm of Pc5 ULF power in IMF, and solar wind velocity and dynamic pressure for 249 days in 2003. We find that, separated from the effects of solar wind speed and dynamic pressure, the average cross polar cap potentials show a roughly linear dependence on the ULF index, with a partial correlation coefficient of 0.19. Highly structured convection flow patterns with a number of localized vortices are often observed under fluctuating northward IMF. For such a convection configuration, it is hard to estimate properly the cross polar cap potential drop, as the enhanced flows around the vortices that may be associated with IMF fluctuations do not necessarily yield a large potential drop. Thus, despite the relatively small correlation coefficient, the linear trend we found gives support to the significant role of IMF ULF fluctuations on the coupling of the solar wind to the magnetosphere‐ionosphere system.

[1] Over the past few years the prominent role of solar wind dynamic pressure in enhancing dayside and nightside reconnection and driving enhanced ionospheric convection has been documented by both ground and spaceborn instruments. Super Dual Auroral Radar Network (SuperDARN) observations show that solar wind pressure fronts induce significantly enhanced ionospheric convection in the dayside ionosphere. In parallel, Defense Meteorological Satellite Program (DMSP) precipitating particle measurements and POLAR Ultra-Violet Imager (UVI) images have demonstrated that sudden solar wind pressure increases also significantly reduce the size of the polar cap, especially on the nightside, suggesting an enhancement of magnetotail reconnection. MHD models of the interaction of the magnetosphere with solar wind pressure fronts have reproduced the enhancement of dayside reconnection but have failed so far to account for the observed closing of the polar cap on the nightside and the suggested magnetotail reconnection increase. We use SuperDARN measurements of ionospheric convection within the nightside polar ionosphere, including near the magnetic separatrix, to evaluate the strength of the observed nightside reconnection enhancement after an abrupt increase in solar wind dynamic pressure on 6 November 2000 and compare it with similar observations on the dayside. We show that an enhancement of nightside convection occurs after a sudden increase in solar wind pressure, delayed by about 40 min compared with the observed dayside convection enhancement. The nightside enhanced flows are observed crossing the open-closed boundary determined by POLAR UVI data, indicating an enhancement of tail reconnection that is possibly due to the pressure increase and is, in addition to the tail reconnection, associated with the more immediate closing of the polar cap.

[1] Knowledge of the dominant temporal and spatial scales of auroral features is instrumental in understanding the various mechanisms responsible for auroral particle precipitation. Single spacecraft data always suffer from temporal/spatial ambiguity. In an effort to separate the temporal and spatial variations of the aurora, we use electron and ion precipitation data from two co-orbiting satellites, F6 and F8 of the Defense Meteorological Satellite Program (DMSP). The two spacecraft have almost identical polar orbits with a small difference in period. As a result the time difference between the two measurements varies with time. We use two statistical tools in order to determine the most probable lifetimes and spatial dimensions of the prevalent auroral features. The first tool is cross-correlation analysis between the magnetic latitude series of electron and ion, number and energy fluxes measured by the two DMSP spacecraft. As one spacecraft overtakes the other, the variable time lag between the two measurements results in different cross-correlation of the two series. We explore the dependence of this variation on the time lag between the satellites. We find that the electron precipitation exhibits a decreasing correlation between the two spacecraft with increasing time lag, whereas there is only a small similar effect for the ion precipitation data. The second statistical tool is cross-spectral analysis, for which we compute the so-called coherence function as a function of frequency (or inverse wavelength) and hence size of the auroral features. The coherence function is a measure of the stability of auroral features of different sizes. We investigate its variation as a function of the time separation between the two measurements. We show that the coherence function of both electrons and ions remains high for up to 1.5 min spacecraft separations for all features larger than about 100 km in width. For smaller features the coherence is lower even for time lags of a few seconds. The results are discussed in the context of characteristic temporal and spatial auroral scales deduced from complementary studies and expected from theory.

[1] Fourier transforms have traditionally been used in the past for time-frequency analysis of numerous physical problems. Recently, a new time-frequency analysis technique using temporally confined base functions, called wavelets, has been applied to many problems. The advantage of the wavelets over the conventional sinusoidal functions is their time localization property, providing information not only about the frequency or scale size of the features present but also about their location in the time series. Here we compare the performance of the two techniques on the time-frequency analysis of ground magnetometer data and especially their ability to pick up the field line resonance (FLR) frequency of a resonating magnetic field line using automated FLR determination techniques. We find that the automated techniques work better with the Fourier transforms giving less variable FLR frequency. The high temporal resolution of the wavelet transforms can be useful for detailed analyses of short time periods, but it becomes a disadvantage when it comes to automated techniques for the entire dayside magnetosphere, yielding a highly variable FLR frequency.

[1] It is well known that a persistent southward Interplanetary Magnetic Field (IMF) produces increased geomagnetic activity. It has recently been shown that a sudden increase in solar wind pressure results in poleward expansion of the auroral oval and closing of the polar cap over a wide range of MLTs, and this effect is more pronounced under southward IMF orientation. We show that southward IMF conditions combined with high solar wind dynamic pressure immediately after a pressure front impact lead to enhanced coupling between the solar wind and the terrestrial magnetosphere, significantly increasing the geoeffectiveness of the solar wind. We evaluate geoeffectiveness by the coupling efficiency, defined as the ratio of the cross-polar-cap potential measured by Defense Meteorological Satellite Program (DMSP) spacecraft to the cross-magnetospheric potential calculated using solar wind parameters. We examine changes in the size of the polar cap and the coupling efficiency for a number of solar wind pressure enhancements under southward IMF configuration. We confirm the previously observed closing of the polar cap and show that there is a simultaneous increase of the coupling efficiency. This increase is measured for all cases, despite the fact that the magnetosphere is greatly compressed, and the increase is measured even when the solar wind electric field is reduced.

[1] It has recently been found that solar wind dynamic pressure changes can dramatically affect the precipitation of magnetospheric particles on the high-latitude ionosphere. We have examined the effect of large solar wind dynamic pressure increases on the location, size, and intensity of the auroral oval using particle precipitation data from Defense Meteorological Satellite Program (DMSP) spacecraft. Three events have been selected for study during the time period after 1997 when four DMSP spacecraft (F11–F14) were simultaneously operational. Interplanetary magnetic field (IMF) orientation is different from event to event. For each event, we determine equatorward and poleward boundaries of the auroral oval before and after an increase in solar wind pressure. Also, using measured integral fluxes, we construct precipitating particle energy input maps for the auroral oval. All cases studied show a significant change of the auroral oval location, size, and intensity in response to the solar wind pressure pulse. Most prominent are an increase of the auroral zone width and a decrease of the polar cap size when the solar wind dynamic pressure increases under steady southward IMF conditions. An increase in total precipitating particle energy flux is also observed. A smaller response is seen when the IMF Bz has a simultaneous northward turning and when it is nearly zero before the pressure enhancement. Our results also point to significant differences between the auroral precipitation response to solar wind pressure changes and its response to isolated substorms, the former inducing a global auroral reaction while the latter is related to more localized premidnight disturbances. Auroral UV observations from the Polar spacecraft during our events are found to give results consistent with the results we get from the precipitating particle observations.