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.

[1] The objective of this study is to understand better the propagation of Pi 2 waves in the nighttime region. We examined Pi 2 oscillations that showed high correlation between high-and low-latitude Magnetic Data Acquisition System/Circum Pan-Pacific Magnetometer Network stations (correlation coefficient: jgj ! 0.75). For each horizontal component (H and D) we examined the magnetic local time (MLT) dependence of the delay time of high-latitude Pi 2 oscillations that corresponds to the highest correlation with the low-latitude Pi 2 oscillation. We found the delay time of the high-latitude H showed remarkable MLT dependence, especially in the premidnight sector: we found that in the premidnight sector the high-latitude H oscillation tends to delay from the low-latitude oscillation (<100 s). On the other hand, the delay time of the high-latitude D oscillation was not significant ($±10 s) in the entire nighttime sector. We propose a Pi 2 propagation model to explain the observed delay time of high-correlation highlatitude H. The model quantitatively explains the trend of the event distribution. We also examined the spatial distribution of high-correlation Pi 2 events relative to the center of auroral breakups. It was found that the high-correlation Pi 2 events tend to occur away from the center of auroral breakups by more than 1.5 MLT. The present result suggests that the high-correlation H component Pi 2 oscillations at high latitude are a manifestation of forced Alfvén waves excited by fast magnetosonic waves.

[1] This paper uses the plasma data from Cluster and TC-1 and geomagnetic data to study the geomagnetic signatures of the current wedge produced by fast-flow braking in the plasma sheet. The three fast flows studied here occurred in a very quiet background and were accompanied by no (or weak) particle injections, thus avoiding the influences from other disturbances. All the geomagnetic signatures of a substorm current wedge can be found in the geomagnetic signatures of a current system produced by the braking of fast flows, indicating that the fast flows can produce a complete current wedge which contains postmidnight downward and premidnight upward field-aligned currents, as well as a westward electrojet. The Pi2 precursors exist not only at high latitudes but also at midlatitudes. The starting times of midlatitude Pi2 precursors can be identified more precisely than those of high-latitude Pi2 precursors, providing a possible method to determine the starting time of fast flows in their source regions. The AL drop that a bursty bulk flow produces is proportional to its velocity and duration. In three cases, the AL drops are <100 nT. Because the AE increase of a typical substorm is >200 nT, whether a substorm can be triggered depends mainly on the conditions of the braking regions before fast flows. The observations of solar wind before the three fast flows suggest that it is difficult for the fast flows to trigger a substorm when the interplanetary magnetic field B z of solar wind is weakly southward.Citation: Cao, J. -B., et al. (2010), Geomagnetic signatures of current wedge produced by fast flows in a plasma sheet,

Abstract. To investigate the generation and propagation mechanisms of Pi 2 magnetic pulsations, we have analyzed magnetic field data t?om the 210 ø magnetic meridian (MM) stations. We used 50 Pi 2 events that were simultaneously observed at seven stations along the 210 ø MM during January 1995, and tbcused our analysis on associated magnetic energy, ((/X/--D2+(AD)2)/kt 0. The times when the amplitude of the magnetic energy attained the maximum (T,,,,x) were compared among these stations. We tbund that T,,,,,. has a latitudinal dependence. especially at highel' latitudes. which has not been previously reported. At Kotel'nyy (L=8.50) on the poleward side of the auroral region. T,,,,.,. occurred an average of 21 seconds earlier than T,,,,,. at Guam (L=I.01). and often as much as one minute earlier. The existence of latitudinal variation has implications tbr interpretation of issues related to timing of substorm onset; it is necessaD to consider the global t•atures of Pi 2 events in the study of auroral and magnetospheric substorms. We present ncx• 11ndings on the latitudinal dependence o1' Pi 2 energy translbr. especially at highel' latitudes and near the plasmapause. These findings can affect the interpretation of some results concerning substorm onset-timing issues. Statistical AnalysisThe magnetic pressure, or magnetic energy density, is defined as B-/2t, to. The magnetic energy carned by MHD waves is expressed by ((A/Dr+ (AD)2+ (ALD2)//a0 . That is, the variation of the magnetic energy is proportional to the squared amplitude of magnetic field variation. For Pi 2 events used in this paper, the contribution to the magnetic energy fi'om the Z component is small and negligible, and when the Z component has constant and significant contribution to the magnetic energy, the induction effect caused by ground conditions is considered. rather than the external source. Thus.•xe use ((_XH) 2 + (,_XD)2)//a0 as the estimate of variation in We note that the ZYK data had a timing error during Jan. -Mar. 1995. According to Kikuchi and Araki [1979]. the variation in the electric field accompanying a DP-2-type sudden commencement (sc) is transmitted instantaneously fi'om high latitude to the equator by Earth-ionosphere waveguide mode. Thus. by using three sc events that occurred in the interval, we have adjusted the timing of the ZYK data 1619

Abstract.Peculiarities of daytime and nighttime Pi 2 pulsations at the dip equator are examined by using multipoint measurements from the 210 ø magnetic meridian (MM) magnetometer network. We found that during daytime the amplitude of Pi 2 pulsations at the dip equator is enhanced, at the lower latitudes. Because the zonal ionospheric conductivity at the dip equator is much higher than that at the off-dip equator region, Pi 2 signals are expected to be distorted more effectively at the dip equator. The observations imply that the daytime and nighttime Pi 2 pulsations in the equatorial and low-latitude regions can be explained by invoking an instantaneous penetration of electric field variations from the nightside polar ionosphere to the dayside equatorial ionosphere, and a direct incidence of compressional oscillations from the nightside inner magnetosphere, respectively.