U. P. Løvhaug

Events of enhanced convection and related dayside auroral activity

In this paper we study the high-latitude plasma flow variations associated with a periodic (∼8 min) sequence of auroral forms moving along the polar cap boundary, which appear to be the most regularly occuring dayside auroral phenomenon under conditions of southward directed interplanetary magnetic field. Satellite data on auroral particle precipitation and ionospheric plasma drifts from DMSP F10 and F11 are combined with ground-based optical and ion flow measurements for January 7, 1992. Ionospheric flow measurements of 10-s resolution over the range of invariant latitudes from 71° to 76° were obtained by operating both the European incoherent scatter (EISCAT) UHF and VHF radars simultaneously. The optical site (Ny Alesund, Svalbard) and the EISCAT radar field of view were located in the postnoon sector during the actual observations. The West Greenland magnetometers provided information about temporal variations of high-latitude convection in the prenoon sector. Satellite observations of polar cap convection in the northern and southern hemispheres show a standard two-cell pattern consistent with a prevailing negative By component of the interplanetary magnetic field. The 630.0 nm auroral forms located poleward of the persistent cleft aurora and the flow reversal boundary in the ∼1440–1540 MLT sector were observed to coincide with magnetosheath-like particle precipitation and a secondary population of higher energy ions, and they propagated eastward/tailward at speeds comparable with the convection velocity. It is shown that these optical events were accompanied by bursts of sunward (return) flow at lower latitudes in both the morning and the afternoon sectors, consistent with a modulation of Dungey cell convection. The background level of convection was low in this case (Kp =2+). The variability of the high-latitude convection may be explained as resulting from time-varying reconnection at the magnetopause. In that case this study indicates that time variations of the reconnection rate effectively modulates ionospheric convection.

VARIABILITY OF DAYSIDE CONVECTION AND MOTIONS OF THE CUSP CLEFT AURORA

We present measurements of the ionospheric plasma flow over the range of invariant latitudes 71–76°, observed at 10-second resolution using both the EISCAT radars, with simultaneous observations of the 630 nm cusp/cleft aurora made by a meridian-scanning photometer at Ny Alesund, Svalbard. A major increase in the trans-auroral voltage from 5 to 40 kV (associated with sunward convection in the early afternoon sector) is found to follow a southward motion of the aurora and coincide with the onset of regular transient auroral breakup events. It is shown that these observations are consistent with recent theoretical work on how ionospheric flows are excited by time-dependent reconnection at the dayside magnetopause.

Variability of dayside high latitude convection associated with a sequence of auroral transients

10 second resolution ionospheric convection data covering the invariant latitude range from 71° to 76°, obtained by using the EISCAT UHF and VHF radars, are combined with optical data from Ny Alesund during a sequence of auroral transients in the post-noon sector (∼ 15 MLT). Satellite observations of polar cap convection patterns suggest negative BZ and BY components of the interplanetary magnetic field. Burst-like enhancements of westward (sunward) post-noon convection were accompanied by eastward moving auroral forms at higher latitudes, above the convection reversal boundary. In this case the background convection was weak, whereas the integrated potential drop across the radar field-of-view associated with the westward flow bursts was typically ∼ 20-35 kV. The auroral phenomenon consists of a series of similar events with a mean repetition period of 8 min. A close correlation between the auroral activity and convection enhancements in the cleft ionosphere is demonstrated.

Causes of plasma flow bursts and dayside auroral transients: An evaluation of two models Invoking reconnection pulses and changes in the Y component of the magnetosheath field

Longitudinal flow bursts observed by the European Incoherent Scatter (EISCAT) radar, in association with dayside auroral transients observed from Svalbard, have been interpreted as resulting from pulses of enhanced reconnection at the dayside magnetopause. However, an alternative model has recently been proposed for a steady rate of magnetopause reconnection, in which the bursts of longitudinal flow are due to increases in the field line curvature force, associated with the By component of the magnetosheath field. We here evaluate these two models, using observations on January 20, 1990, by EISCAT and a 630-nm all-sky camera at Ny Alesund. For both models, we predict the behavior of both the dayside flows and the 630-nm emissions on newly opened field lines. It is shown that the signatures of steady reconnection and magnetosheath By changes could possibly resemble the observed 630-nm auroral events, but only for certain locations of the observing site, relative to the ionospheric projection of the reconnection X line: however, in such cases, the flow bursts would be seen between the 630-nm transients and not within them. On the other hand, the model of reconnection rate pulses predicts that the flows will be enhanced within each 630-nm transient auroral event. The observations on January 20, 1990, are shown to be consistent with the model of enhanced reconnection rate pulses over a background level and inconsistent with the effects of periodic enhancements of the magnitude of the magnetosheath By component. We estimate that the reconnection rate within the pulses would have to be at least an order of magnitude larger than the background level between the pulses.

Model predictions of the occurrence of non-Maxwellian plasmas, and analysis of their effects on EISCAT data

The recent identification of non-thermal plasmas using EISCAT data has been made possible by their occurrence during large, short-lived flow bursts. For steady, yet rapid, ion convection the only available signature is the shape of the spectrum, which is unreliable because it is open to distortion by noise and sampling uncertainty and can be mimicked by other phenomena. Nevertheless, spectral shape does give an indication of the presence of non-thermal plasma, and the characteristic shape has been observed for long periods (of the order of an hour or more) in some experiments. To evaluate this type of event properly one needs to compare it to what would be expected theoretically. Predictions have been made using the coupled thermosphere-ionosphere model developed at University College London and the University of Sheffield to show where and when non-Maxwellian plasmas would be expected in the auroral zone. Geometrical and other factors then govern whether these are detectable by radar. The results are applicable to any incoherent scatter radar in this area, but the work presented here concentrates on predictions with regard to experiments on the EISCAT facility.

Is EISCAT able to determine H+ temperature and velocity? Numerical simulation

Abstract Previous studies of the escape of the light ion H+ using the EISCAT-VHF radar have demonstrated that there is substantial interest in measurements of the temperature and vertical velocity of this ion. In the altitude range of the polar ionosphere probed by the radar, the relative abundance of H+ ions may reach large enough values to allow the determination of H+ temperature and velocity from the effect on the shape of the scattered spectra. In order to prove that it is possible to deduce the temperature and velocity of this minor ion, a two step numerical simulation has been performed. Since it is hopeless to determine seven ionospheric parameters simultaneously from the spectra, the first step was to establish the best sequence of analysis to estimate these parameters. Synthetic spectra have been used, based on realistic relative H+ ions concentrations currently observed throughout the altitude range probed by the radar, to realize this study. For the first step, perfect spectra without any noise were analysed. In a second step, the procedure has been applied to similar synthetic spectra but with realistic signal-to-noise ratios (SNR) representative of present observations. The results of these simulations show that the determination of H+ temperatures and velocities is a very difficult task and that it is only possible in a very limited altitude range where both the H+ concentration and the signal-to-noise ratio are large enough.

An investigation of two different electron heating events in the ionospheric F-layer

Abstract Measurements from the EISCAT UHF-radar, obtained during conditions of enhanced electron temperatures in the auroral ionospheric F-layer, are presented. From these examples two different heating modes are identified. Type I occurred during a period of substantial auroral activity, while type II was observed during a geomagnetically quiet period. Parameters describing the prevailing ionospheric conditions in these instances are discussed. The emphasis is on an empirical presentation of the two events. However, some suggestions as to the possible nature of the mechanisms causing these phenomena are also advanced.

Temporal and spatial variability of auroral forms in the 10–14 MLT sector: Relationship to plasma convection and solar wind-magnetosphere coupling

Ground-based observations of dayside auroral forms and magnetic perturbations in the arctic sectors of Svalbard and Greenland, in combination with the high-resolution measurements of ionospheric ion drift and temperature by the EISCAT radar, are used to study temporal/spatial structures of cusp-type auroral forms in relation to convection. Large-scale patterns of equivalent convection in the dayside polar ionosphere are derived from the magnetic observations in Greenland and Svalbard. This information is used to estimate the ionospheric convection pattern in the vicinity of the cusp/cleft aurora. The reported observations, covering the period 0700–1130 UT, on January 11, 1993, are separated into four intervals according to the observed characteristics of the aurora and ionospheric convection. The morphology and intensity of the aurora are very different in quiet and disturbed intervals. A latitudinally narrow zone of intense and dynamical 630.0 nm emission equatorward of 75° MLAT, was observed during periods of enhanced antisunward convection in the cusp region. This (type 1 cusp aurora) is considered to be the signature of plasma entry via magnetopause reconnection at low magnetopause latitudes, i.e. the low-latitude boundary layer (LLBL). Another zone of weak 630.0 nm emission (type 2 cusp aurora) was observed to extend up to high latitudes (∼79° MLAT) during relatively quiet magnetic conditions, when indications of reverse (sunward) convection was observed in the dayside polar cap. This is postulated to be a signature of merging between a northward directed IMF (BZ > 0) and the geomagnetic field poleward of the cusp. The coexistence of type 1 and 2 auroras was observed under intermediate circumstances. The optical observations from Svalbard and Greenland were also used to determine the temporal and spatial evolution of type 1 auroral forms, i.e. poleward-moving auroral events occurring in the vicinity of a rotational convection reversal in the early post-noon sector. Each event appeared as a local brightening at the equatorward boundary of the pre-existing type 1 cusp aurora, followed by poleward and eastward expansions of luminosity. The auroral events were associated with poleward-moving surges of enhanced ionospheric convection and F-layer ion temperature as observed by the EISCAT radar in Tromsø. The EISCAT ion flow data in combination with the auroral observations show strong evidence for plasma flow across the open/closed field line boundary.

Search for light ion outflow by incoherent scatter radar

Regular observation of the polar wind was one of several measurements considered feasible when the European Incoherent Scatter (EISCAT) auroral zone incoherent scatter facility was first conceived. Whereas most of the other measurements proposed with EISCAT have been successful, direct observations of the polar wind using the standard incoherent scatter radar analysis schemes have proven to be elusive. This paper presents the first direct measurement of the H+ velocity by a ground-based instrument situated at high latitudes, where thermal outflow of light ions occurs. We have avoided the difficulties associated with standard analysis by optimizing the detection method to emphasize only those features of the spectrum sensitive to the drift of the O+ and H+ ions. We have tested the analysis procedure on EISCAT VHF observed spectra. The data analyzed show unmistakable evidence of differences in drift velocities of the two major ion populations. The results indicate considerable variation in the drift velocity of the light ion component. The light ion velocity shows an overall increase with altitude consistent with H+ outflows predicted by thermal polar wind models. The observed H+ velocities and the prospects for using the procedure on a routine basis are discussed.

Non-Maxwellian ion velocity distributions and their effects on the interpretation of the incoherent scatter spectra

Abstract Typical EISCAT (European Incoherent Scatter) radar experiments may frequently involve measurements of plasmas which are not in thermal equilibrium. The errors involved in using the standard analysis procedures in such cases are investigated by computing theoretical incoherent scatter spectra, appropriate to the F-region auroral ionosphere, using a non-Maxwellian ion velocity distribution function. Realistic experimental data are then simulated to match the input requirements of the EISCAT analysis package and the errors in the derived ion and electron temperatures estimated by comparing the fitted values with the simulation inputs.