[1] We have made a direct comparison of a pulsating aurora observed simultaneously from the ground at Syowa in Antarctica and onboard the FAST satellite ($3100 km altitude). The auroral form appeared as east-west-aligned bands consisting of two different types: a poleward moving pulsation and a standing mode pulsation, each with a period of $5 sec. The aurora occurs within the region of an inverted-V structure of lower energy (0.1 -1 keV) electron precipitation. The two different types of pulsating aurora are separated in space by a narrow gap in the inverted-V potential structure. Spatial and temporal variations of the down-going high-energy (>5 keV) electron flux show a one-to-one correspondence with the optical pulsating aurora. The down-going high-energy (1 -10 keV) ion flux modulation is out of phase (anti-correlated) with the highenergy electron flux modulation. These features suggest that the precipitating high-energy electrons, which produce the pulsating aurora, are modulated by the oscillation of the field-aligned electric field located above FAST.

Abstract.A geomagnetic negative sudden impulse (SI') occurred on 3 August 1997 in association with a sudden decrease of solar wind dynamic pressure. The discrete auroras observed by an all-sky TV imager at Zhongshan in Antarctica (magnetic latitude is-•74.5øS) suddenly enhanced their luminosity and moved poleward at the time of the SI'. Following the primary enhancement, three cycles of quasi-periodic luminosity pulsations were observed with periods of about 10-14 min and their average position drifted equatorward. The luminosity pulsations showed one-to-one correspondence with the magnetic pulsations, with the characteristic features of a field-line resonance. The low-altitude DMSP-F13 satellite was almost directly over Zhongshan and observed accelerated electron precipitation with an inverted 'V' structure and an upward field-aligned current. These signatures suggest that the solar wind negative pressure impulse triggered a field-line resonance of the geomagnetic field. The resonance caused the upward and downward field-aligned current sheets, and the field-aligned electron acceleration.

Abstract. Simultaneous all-sky camera and HF radar observations of the visual and E-region radar aurora in the westward electrojet suggest a close relationship between a pair of parallel east-west-aligned auroral arcs, separated by ∼30 km, and a region of strong radar backscatter. Poleward of this a broader region of radar backscatter is observed, though the spectral characteristics of the echoes in these two regions differ considerably. We suggest that the visual aurorae and their radar counterparts are produced in a region of upward fieldaligned current (FAC), whereas the backscatter poleward of this is associated with downward FAC. Relatively low electric fields (∼10 mV m −1 ) are observed in the vicinity of the arc system, suggesting that in this case, two-stream waves are not directly generated through the electrodynamics of the arc. Rather, the generation of irregularities is most probably associated with the gradient drift instability operating within horizontal electron density gradients produced by the filamentary nature of the arc FAC system. The observation of high Doppler shift echoes superimposed on slow background flow within the region of backscatter poleward of the visual aurora is argued to be consistent with previous suggestions that the ion-acoustic instability threshold is reduced in the presence of upwelling thermal electrons carrying downward FAC.

[1] Recent studies have shown that the Low Energy Neutral Atom (LENA) imager on the IMAGE spacecraft in the magnetosphere detects neutral atoms coming from the direction of the high-altitude cusp. The primary source of the neutral atoms has been reported to be the entry of the ions in the cusp, but the evidence for this remains limited. Therefore we herein report the correlative variations of the neutral atom signal in the direction of the high-altitude cusp and the fast anti-sunward convection in the ionospheric cusp from an event of simultaneous observations from IMAGE/LENA and SuperDARN radar. During a period of IMF B Z of À40 to À10 nT on 11 April 2001, LENA on IMAGE at (X GSM , Y GSM , Z GSM ) $ (4 R E , 0 R E , 4 R E ) observed significant enhancements of neutral atom signals in the direction of the high-altitude cusp. In concurrence with these enhancements, SuperDARN radars observed an increase in anti-sunward flow accompanied by equatorward shift of the polar cap boundary. Detailed analyses of the SuperDARN velocity data reveal that the fast anti-sunward flow region widens and shrinks longitudinally in correlation with the increase and decrease of the LENA signal, respectively. This correlation shows that the neutral atom emission in the direction of the high-altitude cusp for southward IMF is due to the ion entry caused by enhanced reconnection on the dayside magnetopause. Hence the present study provides evidence of the cusp ion injection being the primary source of the LENA high-altitude cusp signal for the southward IMF.

Abstract.Observations of a pair of auroral arc features by two imagers, one ground-and one space-based, allows the associated field-aligned current (FAC) and electric field structure to be inferred. Simultaneous observations of HF radar echoes provide an insight into the irregularitygenerating mechanisms. This is especially interesting for the E-region echoes observed, which form the focus of our analysis, and from which several conclusions can be drawn, summarized as follows. Latitudinal variations in echo characteristics are governed by the FAC and electric field background. Particularly sharp boundaries are found at the edges of auroral arcs. Within regions of auroral luminosity, echoes have Doppler shifts below the ion-acoustic speed and are proportional to the electric field, suggesting scatter from gradient drift waves. Regions of downward FAC are associated with mixed high and low Doppler shift echoes. The high Doppler shift component is greatly in excess of the ionacoustic speed, but seems to be commensurate with the driving electric field. The low Doppler shift component appears to be much depressed below expectations.