EISCAT observations of bursts of rapid flow in the high latitude dayside ionosphere Article Published Version Todd, H., Bromage, B. J. I., Cowley, S. W. H., Lockwood, M., van Eyken, A. P. and Willis, D. M. (1986) EISCAT observations of bursts of rapid flow in the high latitude dayside ionosphere. Geophysical Research Letters, 13 (9). pp. 909- 912. ISSN 0094-8276 doi: https://doi.org/10.1029/GL013i009p00909 Available at http://centaur.reading.ac.uk/38911/ It is advisable to refer to the publisher's version if you intend to cite from the work. Published version at: http://dx.doi.org/10.1029/GL013i009p00909 To link to this article DOI: http://dx.doi.org/10.1029/GL013i009p00909 Publisher: American Geophysical Union All outputs in CentAUR are protected by Intellectual Property Rights law, including copyright law. Copyright and IPR is retained by the creators or other copyright holders. Terms and conditions for use of this material are defined in the End User Agreement . www.reading.ac.uk/centaur CentAUR Central Archive at the University of Reading Reading's research outputs online GEOPHYSICAL RESEARCH LETTERS, VOL. 13, NO. 9, PAGES 909-912, SEPTEMBER 1986 ElSCAT OBSERVATIONS OF BURSTS OF RAPID FLOW IN THE HIGH LATITUDE DAYSIDE IONOSPHERE H. Todd] B J I Bromage2 S W.H Cowley] M Lockwood2 A.P vangyken 2,3 andD M Willis Abstract. Observations are presented of more similar to the largest of those described short-lived, highly structured bursts of rapid by Goertz et al. (]985). Here we present data plasma flow observed with the ElSCAT radar in encompassing two well-defined burst events. the high latitude dayside ionosphere. It is We show that the data are consistent with shown that the properties of the bursts are observations of poleward-moving twin vortices consistent with ionospheric perturbations as predicted for FTEs by Southwood (]985). caused by impulsive, localized reconnection at the Earth's magnetopause, i.e. by flux The EISCAT POLAR Experiment transfer events. The main aim of the POLAR experiment is the Introduction measurement of flows in the dayside auroral zone and polar cusp far to the north of the Spacecraft observations near the Earth's transmitter site at Troms•, with good time dayside magnetopause have shown that the resolution [van Eyken et at., ]984; Willis et processes which couple magnetospheric and at., ]986]. At these latitudes the geometry magnetosheath plasmas across the boundary of the tristatic ElSCAT system is unfavourabte, often have a localized and impulsive character, so the beam-swinging technique is employed. particularly when the Bz componentof the The radar is pointed at a low elevation angle interplanetary magnetic field (IMF) is small (2].5 ø ) and swung successively between or negative [Russell and Etphic, ]979; Sckopke azimuths ]2 ø on either side of geographic et at., ]98]; Rijnbeek et at., ]984]. The azimuth 344 ø , approximately the direction of "pulses" of boundary layer plasma which are the L-shell meridian at Troms•. Useful return then observed have been interpreted by Cowley signals are obtained where the beam passes (]982) and Paschmann et at. (]982) as resulting through the high density F-region ionosphere, from localized, impulsive reconnection (i.e. corresponding typically to the first 5 to ]0 flux transfer events (FTEs)), but as the of the range gates employed, depending upon signature of "impulsive penetration" by e.g. geophysical conditions. The centres of the Lundin and Dubinin (]984). Cowley (]984, gates are separated by 75 km along the beam. ]986) and Southwood (]985) have given brief Here we present data from the first 5 range theoretical descriptions of the expected gates whose centres span the invariant ionospheric effects, showing that their latitude range from 70.8 ø to 73.2 ø at steps of size should be •300 km, at least for the 0.6 ø corresponding to heights of 2]] to 346 km. largest magnetopause events. Bursts of flow The dwell time at each of the two azimuth from the dayside auroral zone into the polar settings is 2 min, and the continuously cap have been observed during periods of high integrated data are recorded every ]5 sec. magnetic activity using the STARE data by Rotation to the new azimuth then takes 30 sec, Goertz et at. (]985). The most prominent of so that the full cycle of two dwell periods their events had a spatial scale of •200 km and two movements is 5 min. Here we present and with peak flows •2 km s-] the main data at the full ]5 sec resolution, which is perturbation lasted for • 2 min at any one comparable with the time resolution of the location. These authors related their STARE system. observations to FTEs, and pointed out that "impulsive penetration" would give rise to Observations equatorward-directed bursts, contrary to their observations. The data discussed here are from one of a In initial results from the "POLAR" ElSCAT series of POLAR experiments conducted in (European Incoherent Scatter) radar experiment, coordination with AMPTE-UKS measurements in van Eyken et at. (]984) also reported the solar wind [Willis et at., ]986]. They observations of a poleward-directed flow burst. were obtained on 27 October ]984 during a This event appeared to have a larger spatial period of moderatemagnetic activity (Kp = 2+). scale (more than 400 km), a longer time scale Away from midnight, the flows observed at (•]0 min) and lower flow speeds (•300 m s-I) POLAR latitudes generally correspond to the than those reported by Goertz et at. (]985). auroral zone return flows of the usual twin- Subsequent POLAR experiments (Willis et at., cell convection pattern. On 27 October fast ]986) detected bursts whose parameters are eastward flows on the dawn-side had retreated poleward of the field of view after 03]0 UT (add 2.5 hours for MLT), and fast westward •BlacketrLaboratory, Imperial College flows in the dusk cell were not observed until RutherfordAppleton Laboratory. ]]20 UT, following a sharp southward turning 3Nowat Millstone Hill Radar of the IMF [Rishbeth et at., ]985]. Between these times the POLAR field of view was thus Copyright ]986 by the American Geophysical Union. located generally equatorward of the main flow cells, though sporadic flows were observed, Paper number 6L 6248 which were mainly eastward before 0600 UT and 0094-8276/86/006L-6248503.00 westward thereafter. 9O9 910 Todd et al.' Bursts of Rapid Ionospheric Flow 27 October 19B4 procedure used to obtain plasma parameters i i i i i i from the backscatter signal auto-correlation function failed to converge. Estimates of the 5 probable errors in the 1-o-s velocities are Bx (nT) typically ñ45, 37, 37, 57 and ]02 ms-] for -5 gates ] to 5 respectively. Two similar bursts were observed in the interval 0635 to 0645 UT. They thus occurred in the period of sporadic dayside flows just after the predominant direction changed from eastward to westward. At the beginning of the interval shown in Figure ] 1-o-s speeds are small, typically N]00-200 ms-] or less, and remain so up to the end of the dwell period at azimuth ] at 0635 UT. On turning to azimuth 2, at 0635:30 UT, however, a large discrete 1 ooo burst of flow is observed, which is directed Ga•e 5 73.2A initially poleward, reverses to become equatorward and then turns poleward once more. - The measurements show that a similar pattern 1 ooo occurs in each range gate, but with a - Ga•e 4 72.6A ' successive delay of approximately 20 sec between each gate with increasing latitude (judging from the velocity reversal). The apparent propagation speed of the flow pattern u 1000 along the radar line of sight is therefore o N3.5 km s-t, rather faster than the peak poleward flows of •].5 to 2 km s-] measured •: 0 during the event. The subsequent dwell at "'•1" oo•;i• I azimuth ] starting at 0638 UT then indicates • 1000 I the presence of moderate poleward flows of o •200-300 m s-I, which generally decline to I small values, though there is some indication c 0 that poleward flow begins to pick up again at the end of this dwell period. The following 1 ooo two dwell periods (064] to 0645 UT) then show a repeat of the pattern, though with smaller t Ga•e1 x 70.8A speeds. The initial development of these events 6:30 6:40 appears to have occurred mainly in the 30 sec intervals when the radar was moving towards Universal Time azimuth 2, at •0635 UT and •0640 UT. Fig. ]. AMPTE-UKS IMF data (top) and ElSCAT Examination of the two ]5 sec data dumps POLAR data (bottom) for the interval 0626 to acquired during the first of these intervals 0650 UT on 27 October ]984. The UKS (not shown) indicates that poleward flows magnetometer data are 5 sec averages and started to grow during the first ]5 sec plotted as GSM components. The UKS spacecraft interval while the radar was moving, and was located in the solar wind at GSM (X,Y,Z) reached values similar to those shown at the = (]7.], -7.0, 3.5) RE during the interval. beginning of the azimuth 2 dwell during the The POLAR data consist of ]5 sec averages of remainder of the swing. The development of 1-o-s velocities measured ]2 ø to the west the burst thus appears to have been very rapid (circles) and ]2 ø to the east (crosses) of indeed, an inference substantiated by local magnetic north at the transmitter site examination of other bursts where the onset at TromsM.