Abstract. We present a classification of auroral forms in the dayside highlatitude ionosphere, based on ground observations from Svalbard. Having sorted the different auroral forms by magnetic local time (MLT) and morphological and optical spectral characteristics, we then study them as a function of the orientation of the interplanetary magnetic field (IMF). We find that the IMF clock angle 0 is a good parameter with which to order the different dayside auroras. This is illustrated by two case examples covering the whole dayside: (1) the 4-hour-long passage of the sheath region of the January l0 -ll, 1997, magnetic cloud and (2)

Continuous ground-based observations of the dayside aurora provide important information, complementary to the in situ measurements from satellites, on plasma transport and electromagnetic coupling between the magnetosheath and the magnetosphere. In this study, observations of the polar cusp/dayside oval aurora from Svalbard and simultaneous observations of the nightside aurora from Poker Flat, Alaska, and the interplanetary magnetic field from satellites are used to identify the ionospheric signatures of plasma transfer from the solar wind to the magnetosphere. The characteristics of motion, spatial scale, time of duration, and repetition frequency of certain dayside auroral forms which occur at the time of large-scale oval expansions (interplanetary magnetic field Bz < 0) are observed to be consistent with the expected optical signatures of plasma transfer through the dayside magnetopause boundary layer, associated with flux transfer events. Similarly, more large-scale (time and space) events are tentatively explained by the quasi steady state reconnection process. 1. 10,063 10,064 SANDHOLT ET AL.: MAGNETOPAUSE PLASMA TRANSFER AND DAYSIDE AURORA geomagnetic coordinates of these stations, Ny ,•lesund (NY•) and Longyearbyen (LYR) are 75.4 ø, 131.4 ø (NY•) and 74.4 ø, 130.9 ø (LYR). By this technique the dayside auroras can be observed within the range •69ø-80 ø geomagnetic latitude at midwinter. Local magnetic noon and solar noon at the recording sites occur at •0830 and • 1100 UT, respectively. An all-sky imaging photometer is operated at Ny fklesund. This instrument has a 155 ø field of view (spanning 1200 km for F-region emissions) and a threshold sensitivity of •30 R at 630 nm [cf. Carlson, 1984]. This instrument and an all-sky camera at LYR [Deehr et al., 1980] provided important supplementary information relative to the meridian profiles recorded by the scanning photometers. Dayside geomagnetic disturbances were recorded by standard magnetometers at the three Svalbard stations: Ny •lesund, Hornsund (73.5 ø geomagnetic latitude), and BjOrnOya (71.

Combined observations by meridian‐scanning photometers, all‐sky auroral TV camera and the EISCAT radar permitted a detailed analysis of the temporal and spatial development of the midday auroral breakup phenomenon and the related ionospheric ion flow pattern within the 71°–75° invariant latitude radar field of view. The radar data revealed dominating northward and westward ion drifts, of magnitudes close to the corresponding velocities of the discrete, transient auroral forms, during the two different events reported here, characterized by IMF |BY/BZ| < 1 and > 2, respectively (IMF BZ between −8 and −3 nT and BY > 0). The spatial scales of the discrete optical events were ∼50 km in latitude by ∼500 km in longitude, and their lifetimes were less than 10 min. Electric potential enhancements with peak values in the 30–50 kV range are inferred along the discrete arc in the IMF |BY/BZ| < 1 case from the optical data and across the latitudinal extent of the radar field of view in the |BY/BZ| > 2 case. Joule heat dissipation rates in the maximum phase of the discrete structures of ∼ 100 ergs cm−2 s−1 (0.1 W m−2) are estimated from the photometer intensities and the ion drift data. These observations combined with the additional characteristics of the events, documented here and in several recent studies (i.e., their quasi‐periodic nature, their motion pattern relative to the persistent cusp or cleft auroral arc, the strong relationship with the interplanetary magnetic field and the associated ion drift/E field events and ground magnetic signatures), are considered to be strong evidence in favour of a transient, intermittent reconnection process at the dayside magnetopause and associated energy and momentum transfer to the ionosphere in the polar cusp and cleft regions. The filamentary spatial structure and the spectral characteristics of the optical signature indicate associated localized ˜1‐kV potential drops between the magnetopause and the ionosphere during the most intense auroral events. The duration of the events compares well with the predicted characteristic times of momentum transfer to the ionosphere associated with the flux transfer event‐related current tubes. It is suggested that, after this 2–10 min interval, the sheath particles can no longer reach the ionosphere down the open flux tube, due to the subsequent super‐Alfvénic flow along the magnetopause, conductivities are lower and much less momentum is extracted from the solar wind by the ionosphere. The recurrence time (3–15 min) and the local time distribution (∼0900–1500 MLT) of the dayside auroral breakup events, combined with the above information, indicate the important roles of transient magnetopause reconnection and the polar cusp and cleft regions in the transfer of momentum and energy between the solar wind and the magnetosphere.

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Abstract. The 2D morphology of coherent HF radar and optical cusp aurora has been studied for conditions of predominantly southward IMF conditions, which favours low-latitude boundary layer reconnection. Despite the variability in shape of radar cusp Doppler spectra, the spectral width criterion of > 220 m s–1 proves to be a robust cusp discriminator. For extended periods of well-developed radar backscatter echoes, the equatorward boundary of the > 220 m s–1 spectral width enhancement lines up remarkably well with the equatorward boundary of the optical cusp aurora. The spectral width boundary is however poorly determined during development and fading of radar cusp backscatter. Closer inspection of radar Doppler profile characteristics suggests that a combination of spectral width and shape may advance boundary layer identification by HF radar. For the two December days studied the onset of radar cusp backscatter occurred within pre-existing 630.0 nm cusp auroral activity and appear to be initiated by sunrise, i.e. favourable radio wave propagation conditions had to develop. Better methods are put forward for analysing optical data, and for physical interpretation of HF radar data, and for combining these data, as applied to detection, tracking, and better understanding of dayside aurora. The broader motivation of this work is to develop wider use by the scientific community, of results of these techniques, to accelerate understanding of dynamic high-latitude boundary-processes. The contributions in this work are: (1) improved techniques of analysis of observational data, yielding meaningfully enhanced accuracy for deduced cusp locations; (2) a correspondingly more pronounced validation of correlation of boundary locations derived from the observational data set; and (3) a firmer physical rationale as to why the good correlation observed should theoretically be expected.Key words: Ionosphere (ionospheric irregularities; polar ionosphere)

The dynamics of the cusp region and postnoon sector for an interval of predominantly IMF B y , B z < 0 nT are studied with the CUTLASS Finland coherent HF radar, a meridian-scanning photometer located at Ny A Ê lesund, Svalbard, and a meridional network of magnetometers. The scanning mode of the radar is such that one beam is sampled every 14 s, and a 30°azimuthal sweep is completed every 2 minutes, all at 15 km range resolution. Both the radar backscatter and red line (630 nm) optical observations are closely colocated, especially at their equatorward boundary. The optical and radar aurora reveal three dierent behaviours which can interchange on the scale of minutes, and which are believed to be related to the dynamic nature of energy and momentum transfer from the solar wind to the magnetosphere through transient dayside reconnection. Two interpretations of the observations are presented, based upon the assumed location of the open/closed ®eld line boundary (OCFLB). In the ®rst, the OCFLB is co-located with equatorward boundary of the optical and radar aurora, placing most of the observations on open ®eld lines. In the second, the observed aurora are interpreted as the ionospheric footprint of the region 1 current system, and the OCFLB is placed near the poleward edge of the radar backscatter and visible aurora; in this interpretation, most of the observations are placed on closed ®eld lines, though transient brightenings of the optical aurora occur on open ®eld lines. The observations reveal several transient features, including poleward and equatorward steps in the observed boundaries,``braiding'' of the backscatter power, and 2 minute quasi-periodic enhancements of the plasma drift and optical intensity, predominantly on closed ®eld lines.

The generation of flow and current vortices in the dayside auroral ionosphere has been predicted for two processes ocurring at the dayside magnetopause. The first of, these mechanisms is time-dependent •mgnetic reconnection, in "flux transfer events" (]•Es); the second is the action of solar wind dynamic pressure changes. The ionospheric flow signature of an I•E should be a twin vortex, with the mean flow velocity in the central region of the pattern equal to the velocity of the pattern as a whole. On * the other hand, a pulse of enhanced or reduced dynamic pressure is also expected to produce a twin vortex, but with the central plasma flow being generally different in speed from, and almost orthogonal to, the motion of the whole pattern. In this paper, we make use of this distinction to discuss recent observations of vortical flow patterns in the dayside auroral ionosphere in terms of one or other of the proposed mechanisms. We conclude that some of the observations reported are consistent only with the predicted signature of FTEs. We then evaluate the dimensions of the open flux tubes required to explain some recent simultaneous radar and auroral observations and infer that they are typically 300 km in north-south extent but up to 2000 km in longitudinal extent (i.e., roughly 5 hours of MLT). Hence these observations suggest that recent theories of l•Es which invoke .time--varying reconnection at an elongated neutral line may be correct. We also present some simultaneous observations of the interplanetary magnetic field (IMF) and solar wind dynamic pressure (observed using the IMP8 satellite) and the ionospheric flow (observed using the EISCAT radar) which are also only consistent with the [TE model. We estimate that for continuously southward IMF ( • 5 nT) these l•Es contribute about 30 kV to the mean total transpolar voltage (•30•). Os!o, Norway. [1988]). In particular, the ionospheric signatures of two transient magnetopause processes have been predicted, namely time-dependent magnetic reconnection ("flux transfer events") and magnetopause ;notions induced by dynamic pressure changes in the magnetosheath. Characteristic signatures in the magnetic field and particle populations near the magnetopause have been interpreted in terms of flux transfer events (1;TEs) for over a decade now [Russell and Elphic, 1979; Paschmarm et al., 1982; Berchem and Russell, 1984; Rijnbeek et al., 1984; Saunders et al., 1984; Southwood et al., 1986; Farrugia et al., 1988]. This interpretation calls for the dayside auroral ionosphere to exhibit flow and current signatures [ Southwood, 1985, 1987; Cowley, 1984a, 1986; Lee, 1986]: in fact, were such signatures proven to be absent, this would cast doubt upon the accepted model of the magnetopause I•E signatures invoking magnetic reconnection. In addition, if detected and understood, ionospheric signatures of l•Es offer a unique opportunity to quantify the full voltage which they apply to the coupled magnetosphere-ionosphere system. Two types of signature have been predicted, base...

Abstract.In this event study, we have compared electric field measurements acquired near magnetic noon during a rocket flight from the SvalRak range with solar wind and interplanetary magnetic field (IMF) observations. The cusp is spatially bifurcated relative to its source regions. The data indicate that many effects observed at northern high latitudes were driven by dayside merging in the Southern Hemisphere, probably near the dawn side of the cusp.