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.

The auroral electron data obtained during the flight of Polar 3 over an auroral arc (Maynard et al., 1977) were utilized as an input to a computation of the Hall and Pedersen conductivities of the auroral ionosphere produced by the particle precipitation. These conductivities, together with the in situ electric field measurements made on board the rocket, allowed an analysis of the electrodynamics of this auroral arc to be carried out. It was found that the local electric field variations correlated very well with the reciprocal of the height‐integrated Pedersen conductivity, a result suggesting that the auroral ionosphere was, to an extent, electrically isolated from the source of the electric field in the outer magnetosphere. The Joule power dissipation associated with ionospheric current flow was found to decrease abruptly from a value of ∼12 ergs/cm²/s column equatorward of the arc to a very low value within the confines of the arc. However, the sum of the Joule dissipation and the electron energy flux, which is the total energy input to the neutral atmosphere, did not display any abrupt variation across the equatorward boundary of the arc. The auroral electrojet, inferred from J = σ · E by assuming no neutral winds, was not directed parallel to this arc, nor was the current intensified within this arc. Instead, the electrojet flowed in a sheet extending equatorward from this arc for a distance of at least 100 km. The presence of an E region neutral wind will greatly affect the current patterns within this arc but will have only a first‐order effect on the current system equatorward of this arc. These observations are generally consistent with a model of auroral arc in which resistivity along the magnetic field lines linking the auroral ionosphere to the outer magnetosphere isolates these two regions from one another and results in magnetic‐field‐aligned potential differences which may accelerate auroral electrons.

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.

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 phenomeno• 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 ø to 76 ø were obtained by operating both the European incoherent scatter (EISCAT) UHF and VHF radars simultaneously. The optical site (Ny •lesund, 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 B r 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.

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Continuous optical observations of cusp/cleft auroral activities within ≈ 09‐15 MLT and 70‐76° magnetic latitude are studied in relation to changes in solar wind dynamic pressure and interplanetary magnetic field (IMF) variability. The observed latitudinal movements of the cusp/cleft aurora in response to IMF Bz changes may be explained as an effect of a variable magnetic field intensity in the outer dayside magnetosphere associated with the changing intensity of region 1 field‐aligned currents and associated closure currents. Ground magnetic signatures related to such currents were observed in the present case (January 10, 1993). Strong, isolated enhancements in solar wind dynamic pressure (Δp/p ≥ 0.5) gave rise to equatorward shifts of the cusp/cleft aurora, characteristic auroral transients, and distinct ground magnetic signatures of enhanced convection at cleft latitudes. A sequence of auroral events of ≈ 5‐10 min recurrence time, moving eastward along the poleward boundary of the persistent cusp/cleft aurora in the ≈ 10‐14 MLT sector, during negative IMF Bz and By, conditions, were found to be correlated with brief pulses in solar wind dynamic pressure (0.1 < Δp/p < 0.5). Simultaneous photometer observations from Ny Ålesund, Svalbard, and Danmarkshavn, Greenland, show that the events often appeared on the prenoon side (≈ 10‐12 MLT), before moving into the postnoon sector in the case we study here, when IMF By < 0. In other cases, similar auroral event sequences have been observed to move westward in the prenoon sector, during intervals of positive By. Thus a strong prenoon/postnoon asymmetry of event occurence and motion pattern related to the IMF By polarity is observed. We find that this category of auroral event sequence is stimulated bursts of electron precipitation that originate from magnetosheath plasma that has accessed the dayside magnetosphere in the noon or near‐noon sector, possibly at high latitudes, partly governed by the IMF orientation as well as by solar wind dynamic pressure pulses.

Information on the location of microburst source regions is limited. One measurement at L ≈ 8.5 placed the source within 4 RE of the ionosphere. Measurements at 5≲ L ≲6, though less conclusive, suggested that source regions may be located either near the equatorial plane or at higher magnetic latitudes along the field line. This paper reports simultaneous observations of bremsstrahlung X rays and VLF radiowave emissions that reveal a detailed correlation between electron microbursts precipitated in one hemisphere and chorus elements of rising frequency recorded at the conjugate point. The measurements were made at Roberval, Canada, and Siple Station, Antarctica (L ≈ 4.1), during magnetic substorms on July 9 and 15, 1975. The relationship between electron energy (50 ≲ E ≲ 200 keV) and wave frequency ( ≲ f ≲4 kHz), and the measured time difference (0.01 s ≤Δt ≤0.13 s) between detection of the electrons and waves at ionospheric conjugate points are consistent with near‐equatorial cyclotron resonance interactions occurring outside the plasmasphere. In both cases, the observations could be accounted for if a diffusive‐equilibrium distribution of electron density along the field line was assumed. The so‐called ‘collisionless’ (or R−4) model of electron density was not in accord with the observations. Some evidence is found for a separation of the wave growth and electron scattering regions. Evidence is also found indicating that the process of electron scattering requires a finite time, up to ∼80 ms under the conditions of these observations. The present results suggest that microburst generation regions are located within 20° of the equator on subauroral field lines.