*************************** ** THE GEM MESSENGER ** *************************** Volume 5, Number 1 January 5, 1995 ------------------------------------------------------------------ Mini-GEM Workshop Report, December 4, 1994, San Francisco - Part I ------------------------------------------------------------------ Boundary Layer Campaign, Working Group 3: Currents and Mapping Co-chairs: Eigil Friis-Christensen and Nancy Crooker Our working group held two informal sessions to advance the case studies of transient events begun at the summer workshop in Snowmass, to discuss the role of models for interpretation, and to plan working strategies for the next six months. During the first session, Therese Moretto and Eigil Friis-Christensen displayed all available magnetometer data for several events in a new and highly effective format. On a map on a computer screen, they showed how the equivalent convective flow vectors at all available stations varied as a function of time, with 20-sec resolution. With magnetometer coverage over nearly twelve hours of local time, the paths of traveling convection vortices could easily be traced. Earlier attempts to trace TCV paths in local time by overlaying plots of magnetic time variations from longitudinally spaced stations were often unsuccessful. The reason for this became clear in the new display format: TCVs move in nonuniform ways and change shape as they travel. Characteristics of the specific events are: 12 Jan 94, 0800-1200: Identified in optical data, this event is classified as a Poleward Moving Auroral Form (PMAF) with a possible associated TCV. Both phenomena began around 0817, the PMAF north of the persistent arc marking the polar cap boundary and the TCV probably south of it. The PMAF moved poleward, albeit rather slowly, and the TCV westward. There were several PMAFs before and after this event without any signature of a TCV. In the 0817 case, both phenomena may have been generated by the same trigger, but their development appeared to be completely independent. Bjorn Jacobsen, who, unfortunately, could not attend the workshop, is taking the lead in analyzing both this event and a similar event on 14 Jan 94, 0939-1230. 14 Oct 93, 1305: A series of 3-4 classical TCVs were clearly tracked over a large span of local time in the new format. Riometer data in both hemispheres showed correlated but not outstanding features. Interplanetary data are available but not helpful in determining cause owing to large fluctuations in most parameters. 18 Dec 93, 1355: A pair of TCVs was accompanied by a strong riometer signal between them. Because of possible time delays between riometer signals and their causes, it is not clear if the signal was caused by the field-aligned current centered on one of the vortices, even though they did not coincide. The event was associated with a change in the global convection pattern. The second session began with a presentation by Eftyhia Zesta and Jeff Hughes on their candidate TCV events during disturbed periods, 11 Oct 93, 1400-1800, and 25 Oct 93, 1200-1800. Vortices were present but often difficult to distinguish from other features, and their motions and shapes were irregular. The problem of separating vortices from global convection changes becomes even more pronounced in these cases. Global substorm effects were also undoubtedly present. The present goal is to document the complex patterns, pointing out vestiges, if any, of the isolated TCV signatures. Bob Clauer presented an analysis of data from 4 Aug 91, 1200-1500, which has already been submitted for publication. A long series of continuous magnetometer and radar variations (IPCL in pulsation terms) occurred near the dayside convection reversal boundary during a period of strong IMF-By-driven flows across noon. Distortions in the boundary as a function of time were deduced. These oscillations were driven rather than resonant and may be related to the dayside merging rate. Gang Lu showed AMIE results at 5 min intervals during periods of Progressing Polar Convection Disturbances (PPCD) on 2 Aug 91, 1240-1435, and 3 Aug 91, 1300-1520, as reported earlier by Peter Stauning, Bob Clauer, and co-workers. The AMIE convection patterns show a vortex at the tip of the crescent cell that periodically appears and disappears. Anticipated poleward movement of the vortex is not a clear feature of the pattern. One of the outstanding features of these transient events is their large size. This was made especially clear by the presentation methods at the workshop and was repeatedly commented upon by the audience. As Hermann Luehr pointed out at the Snowmass workshop, the total field- aligned current associated with the transients is typically 100 kA and can be as large as 300 kA. The large size of transients and their strong currents are encouraging to modelers. As Gang Lu demonstrated, there was no problem resolving PPCDs with the AMIE technique, and plans were made to apply the technique to a case of TCVs. Discussion ensued as to whether or not the technique should be applied to filtered data in order to separate the vortex patterns from the background convection. John Lyon agreed that the transients are large enough to be seen in his MHD model, and he plans to pursue the topic as funding allows. As discussed at the workshop, his is one of the few models that can test for a solar wind cause of transients. Testing the model was also discussed. Measured solar wind parameters, including the orientation of discontinuities obtained by minimum variance technique, could be used as input to the model, and the resulting signatures in the ionosphere could be compared with observed signatures. The new global view of TCVs afforded by the expanded magnetometer coverage raised the issue of documenting their morphology in a quantitative way. George Siscoe suggested a superposed epoch analysis once enough cases have been analyzed to produce statistically meaningful results. Bengt Sonnerup suggested developing a kinematic model which, when fit to the data, could give TCV parameters such as speed, total field-aligned current strength, location and distribution as a function of travel time. Bob Sitar, who has done some kinematic modeling of TCVs with Mark McHenry's model, offered to extend his efforts to the new global view. The role of ionospheric conductivity in transient formation was discussed at length. Ray Greenwald warned that the transient convection enhancements commonly observed by radar and associated with flux transfer events could yield a false TCV magnetometer signal in the presence of patches of enhanced conductivity. This effect would be minimized in summer by the overall higher conductivity then. Gang Lu pointed out that any information obtained on conductivity from satellite data are always folded into the AMIE technique. Consequently, the group decided that the next case study of TCVs, in which the AMIE technique would be applied, should use summer data near the time of a satellite overpass. Until the next Snowmass workshop in six months, work on the case studies already begun will continue and be guided toward the publication stage by the lead scientist for each, with the cooperation of any who wish to contribute supplementary data. In addition, one new summer case study incorporating the AMIE technique, as described above, will be initiated. The modeling efforts suggested above will proceed as time and funding allow. +-------------------------------------------------------------------------+ |To add name to the mailing list, send a message to: editor at igpp.ucla.edu | |For message to whole GEM mailing list, send to: gem at igpp.ucla.edu | |For message to a specific working group (BL Campaign), send to: | | gem_field at igpp.ucla.edu (WG1); gem_boundary at igpp.ucla.edu (WG2); | | gem_current at igpp.ucla.edu (WG3); gem_data at igpp.ucla.edu (WG4) | | gem_ggcm at igpp.ucla.edu (WG5) | | gem_chair at igpp.ucla.edu (WG chairs, Odile and W. J. Hughes) | | | |URL of GEM Home Page: http://igpp.ucla.edu/gem/Welcome.html | |Please update your e-mail address. | |CAUTION: Do not send messages to gem at igpp.ucla.edu unless you want | | your message to go to everyone in the GEM mailing list! | +-------------------------------------------------------------------------+