Abstract. Highlights are presented from studies of the electric field data from various regions along the CLUS-TER orbit. They all point towards a very high coherence for phenomena recorded on four spacecraft that are separated by a few hundred kilometers for structures over the whole range of apparent frequencies from 1 mHz to 9 kHz. This presents completely new opportunities to study spatialtemporal plasma phenomena from the magnetosphere out to the solar wind. A new probe environment was constructed for the CLUSTER electric field experiment that now produces data of unprecedented quality. Determination of plasma flow in the solar wind is an example of the capability of the instrument.

The electric-field and wave experiment (EFW) on Cluster is designed to measure the electric-field and density fluctuations with sampling rates up to 36000 samples s -I. Langmuir probe sweeps can also be made to determine the electron density and temperature. The instrument has several important capabilities. These include (I) measurements of quasi-static electric fields of amplitudes up to 700 m V m -I with high amplitude and time resolution, (2) measurements over short periods of time of up to five simualtaneous waveforms (two electric signals and three magnetic signals from the seach coil magnetometer sensors) of a bandwidth of 4 kHz with high time resolution, (3) measurements of density fluctuations in four points with high time resolution. Among the more interesting scientific objectives of the experiment are studies of nonlinear wave phenomena that result in acceleration of plasma as well as large-and small-scale interferometric measurements. By using four spacecraft for large-scale differential measurements and several Langmuir probes on one spacecraft for small-scale interferometry, it will be possible to study motion and shape of plasma structures on a wide range of spatial and temporal scales. This paper describes the primary scientific objectives of the EFW experiment and the technical capabilities of the instrument.

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.

[1] Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This

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 Rosetta dual Langmuir probe instrument, LAP, utilizes the multiple powers of a pair of spherical Langmuir probes for measurements of basic plasma parameters with the aim of providing detailed knowledge of the outgassing, ionization, and subsequent plasma processes around the Rosetta target comet. The fundamental plasma properties to be studied are the plasma density, the electron temperature, and the plasma flow velocity. However, study of electric fields up to 8 kHz, plasma density fluctuations, spacecraft potential, integrated UV flux, and dust impacts is also possible. LAP is fully integrated in the Rosetta Plasma Consortium (RPC), the instruments of which together provide a comprehensive characterization of the cometary plasma.

Abstract. For accurate measurements of electric fields, spherical double probes are electronically controlled to be at a positive potential of approximately 1 V relative to the ambient magnetospheric plasma. The spacecraft will acquire a potential which balances the photoelectrons escaping to the plasma and the electron flux collected from the plasma. The probe-to-plasma potential difference can be measured with a time resolution of a fraction of a second, and provides information on the electron density over a wide range of electron densities from the lobes (∼ 0.01 cm −3 ) to the magnetosheath (> 10 cm −3 ) and the plasmasphere (> 100 cm −3 ). This technique has been perfected and calibrated against other density measurements on GEOS, ISEE-1, CRRES, GEOTAIL and POLAR. The Cluster spacecraft potential measurements opens the way for new approaches, particularly near boundaries and gradients where four-point measurements will provide information never obtained before. Another interesting point is that onboard data storage of this simple parameter can be done for complete orbits and thereby will provide background information for the shorter full data collection periods on Cluster. Preliminary calibrations against other density measurements on Cluster will be reported.