ALBERT

Description: We investigate the magnetospheric domain responsible for the generation of ionospheric travelling convection vortices (TCV) by comparing the location of the TCV to the locations of the low-altitude particle-precipitation boundaries deduced from the DMSP satellite measurements. For three very well documented TCV events we are able to identify suitable satellite passes, in the sense that for each event we can identify two to three passes occurring close to the TCV observation in both time and space. In all three cases the comparisons place the TCV centres at or equatorward of the central plasma sheet/boundary plasma sheet precipitation boundary. Thus our results indicate that the field-aligned currents related to the TCV originate in the plasma sheet rather than at the magnetopause or in the low-latitude boundary layer, as previous studies suggest.

Description: Thirteen events of high-latitude ionospheric travelling convection vortices during very quiet conditions were identified in the Greenland magnetometer data during 1990 and 1991. The latitudes of the vortex centres for these events are compared to the energetic electron trapping boundaries as identified by the particle measurements of the NOAA 10 satellite. In addition, for all events at least one close DMSP overpass was available. All but one of the 13 cases agree to an exceptional degree that: the TCV centres are located within the region of trapped, high energy electrons close to the trapping boundary for the population of electrons with energy greater than 〉100 keV. Correspondingly, from the DMSP data they are located within the region of plasmasheet-type precipitation close to the CPS/BPS precipitation boundary. That is, the TCV centres map to deep inside the magnetosphere and not to the magnetopause.Key Words. Ionosphere (Electric fields and currents; Particle precipitation) · Magnetospheric physics (Magnetosphere-ionosphere interaction)

Description: Three geomagnetic satellite missions currently provide high precision magnetic field measurements from low altitude polar orbiting spacecraft. We demonstrate how these data can be used to determine the intensity and location of the horizontal currents that flow in the ionosphere, predominantly in the auroral electrojets. First, we examine the results during a recent geomagnetic storm. The currents derived from two satellites at different altitudes are in very good agreement, which verifies good stability of the method. Further, a very high degree of correlation (correlation coefficients of 0.8–0.9) is observed between the amplitudes of the derived currents and the commonly used auroral electrojet indices based on magnetic measurements at ground. This points to the potential of defining an auroral activity index based on the satellite observations, which could be useful for space weather monitoring. A specific advantage of the satellite observations over the ground-based magnetic measurements is their coverage of the Southern Hemisphere, as well as the Northern. We utilize this in an investigation of the ionospheric currents observed in both polar regions during a period of unusually steady interplanetary magnetic field with a large negative Y-component. A pronounced asymmetry is found between the currents in the two hemispheres, which indicates real inter-hemispheric differences beyond the mirror-asymmetry between hemispheres that earlier studies have revealed. The method is also applied to another event for which the combined measurements of the three satellites provide a comprehensive view of the current systems. The analysis hereof reveals some surprising results concerning the connection between solar wind driver and the resulting ionospheric currents. Specifically, preconditioning of the magnetosphere (history of the interplanetary magnetic field) is seen to play an important role, and in the winther hemisphere, it seems to be harder to drive currents on the nightside than on the dayside.Key words. Ionosphere (electric fields and currents) – Magnetospheric physics (current systems; magnetosphere-ionosphere interactions)

Description: In this study, we perform a statistical investigation of magnetic impulse events identified in the Greenland magnetometer stations through the years 1995–2001. We focus on occurrence statistics that can be determined reliably with an automatic event identification procedure. Durin the first two years we observed almost 40% more events than in the following years. Season is not a significant factor in event occurrence. Event occurrence peaks near 12:00 UT, corresponding to approximately 10:00 magnetic local time (MLT) at the west coast of Greenland. More events occur prior to local noon than after. Event days are not distributed evenly. Large amplitude events, particularly, tend to appear on consecutive days. Events are observed at lower latitudes at earlier local times in a way consistent with the projection of the outer magnetospheric boundary into the ionosphere. Event latitude depends on dipole tilt angle in a manner similar to that reported for the cusp. Events occur during intervals of enhanced Kp. The main reason for this is that the events themselves contribute to the Kp index. Events exhibit a preference for high solar wind velocity. In particular, the large amplitude events occur during high-speed streams. A slight preference for lower density and more radial interplanetary magnetic fields, as compared to the nominal solar wind distribution, is also observed. However, both the nominal solar wind and event distribution exhibit large differences from year to year, indicating that events occur under a broad range of conditions. Key words. Ionosphere (ionospheric disturbances) Magnetospheric physics (magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)

Description: A coordinated ground-based observational campaign using the IMAGE magnetometer network, EISCAT radars and optical instruments on Svalbard has made possible detailed studies of a travelling convection vortices (TCV) event on 6 January 1992. Combining the data from these facilities allows us to draw a very detailed picture of the features and dynamics of this TCV event. On the way from the noon to the drawn meridian, the vortices went through a remarkable development. The propagation velocity in the ionosphere increased from 2.5 to 7.4 km s–1, and the orientation of the major axes of the vortices rotated from being almost parallel to the magnetic meridian near noon to essentially perpendicular at dawn. By combining electric fields obtained by EISCAT and ionospheric currents deduced from magnetic field recordings, conductivities associated with the vortices could be estimated. Contrary to expectations we found higher conductivities below the downward field aligned current (FAC) filament than below the upward directed. Unexpected results also emerged from the optical observations. For most of the time there were no discrete aurora at 557.7 nm associated with the TCVs. Only once did a discrete form appear at the foot of the upward FAC. This aurora subsequently expanded eastward and westward leaving its centre at the same longitude while the TCV continued to travel westward. Also we try to identify the source regions of TCVs in the magnetosphere and discuss possible generation mechanisms.

Description: An event of strong flux pile-up and plasma depletion at the high latitude magnetopause tailward of the cusp has been analyzed based on observations by the suite of Cluster spacecraft. The multi-satellite analysis facilitates the separation of temporal and spatial features and provides a direct estimate for the strength of the plasma depletion layer for this event. A doubling of the magnetic field strength and a forty percent reduction of the density are found. Our analysis shows that roughly half of the total magnetic field increase occurs within 0.6 RE of the magnetopause and another quarter within a distance of 1.2 RE. In addition, the plasma depletion signatures exhibit temporal variations which we relate to magnetopause dynamics. Keywords. Magnetospheric physics (Magnetopause, Cusp and boundary layers; Magnetosheath; Solar windmagnetosphere interactions)