------------------------------------------------ Report on GEM Snowmass Meeting, June 26-30, 1995 ------------------------------------------------ 1995 SNOWMASS WORKSHOP REPORT: BOUNDARY LAYER CAMPAIGN WORKING GROUP 3: CURRENT SYSTEMS AND MAPPING LEADERS: Nancy Crooker and Eigil Friis-Christensen I. General Issues Last year at Snowmass WG 3 expanded their ongoing studies of traveling convection vortices (TCVs) to include other transient ionospheric signatures of magnetopause processes, such as poleward moving magnetic disturbances and auroral forms, and any relationships between them. Specific events were chosen for coordinated studies, and efforts to model the background polar cap size, boundary location, and convection pattern specific to the solar wind conditions for each event were begun. This year the observational studies blossomed into multi-faceted projects with new and somewhat surprising results, as described below. The sense of being on the verge of breakthroughs in understanding was palpable. Spirited discussions were held between a large number of participants with a variety of backgrounds. The sense of enthusiasm created by the new results carried over into a closing discussion on whether or not the Boundary Layer Campaign, as the first of six overlapping campaigns originally planned for the GEM program, should be phased out. As Chris Russell pointed out, a positive response from an existing group facing extinction is always inevitable. The quality of the response, however, placed it well above a plea for existence. Bob Clauer verbalized the overwhelmingly obvious fact that the group is far from stale. Partly owing to preparations for the upcoming, much larger-scale ISTP program, the ground experimenters are approaching a state of readiness for studies with unprecedented coverage. Magnetometer arrays covering 180 deg around the northern polar cap boundary are finally in place. The SUPERDARN radar system (see below) and a planned array of automated ground stations are nearly in place. Mark Engebretsen reported that on-line data from the Antarctic Geophysical Observatory Network (AGONET) of ground-based instruments will soon be available. With all of these systems in place to coordiante GEM provides a unique and much-needed forum and motivation for coordinated studies using these vast instrumental resources. Mary Hudson pointed out how ironic it would be to phase out the Boundary Layer Campaign just when the CLUSTER mission was about to be launched, since CLUSTER is specifically designed to analyze the magnetopause. The modeling efforts took a new turn. As reported last year, stimulating discussions took place on how the steady state models could be improved to facilitate comparison with data, and this year Cheng Ding reported that he had "kept his promise" in improving the Toffoletto-Hill model, completing the closure of the convection pattern outside the polar cap in a versatile way that can accommodate cells with unequal potential drops. But for the purpose of providing background conditions for the transient features of interest to WG3, it became clear last year that the interplanetary magnetic field is much too variable during these events to set meaningful boundary conditions for the models. Consequently, WG3 has changed its focus from steady state to time- dependent models. In view of WG3's longstanding emphasis has on time-dependent phenomena, the group proposed to change its name to reflect that emphasis. But opinions expressed at the final plenary session were against change that was directed from the general to the more specific, and the original name has been retained. II. Program Summary Alan Rodger opened the first WG3 session on Monday morning with an invited tutorial on the SUPERDARN radar program. His excellent presentation covered a wide variety of ways in which boundary layer problems are being addressed through international collaborative efforts. The radar network gives a unique perspective on the transient ionospheric signatures of particular interest to WG3. In addition, it may soon provide the community with routine parameters such as cross- polar-cap potential calculated from the merging line footprint. The first session on Monday afternoon addressed auroral signatures of magnetic transients. Bjorn Jacobsen presented intriguing results from ongoing analysis of the designated WG3 events on 12 and 14 Jan 94: a traveling convection vortex (TCV) and a poleward moving auroral form (PMAF) were generated at the same location, presumably by the same driver, but each moved its separate way thereafter. Hermann Luehr briefly reported that WG 3 isolated TCV events on 18 Dec 93 and 14 Oct 93 both appeared to be triggered by directional discontinuities in the solar wind, the first at the start of enhanced convection and the second at the end. In keeping with the theme of the session, he then presented new results from 14 Jan 94 event, where the following sequence was identified: a directional discontinuity imediately followed by three PMAFs with a recurrence rate of 5-10 min, then a new directional discontinuity followed by two PMAFs. The PMAF events were accompanied by poleward-moving field-aligned current pairs in the presence of eastward convection, a pattern considerably different from TCVs. Roger Arnoldy remained unconvinced that magnetic signatures of PMAFs and TCVs differ, since they both give magnetic impulse signatures of field- aligned currents. From rocket data he deduced that PMAFs move poleward with the open-closed magnetic field boundary (and, incidentally discovered an associated, highly-accelerated electron population). In a separate study he found that the Pc1 bursts associated with magnetic impulses occur on closed field lines, separated from the open-closed boundary location of the impulses. In the following discussion, there appeared to be a general consensus that although auroral signatures of TCVs seem indistinguishable from PMAFs, the radar and magnetometer signatures are clearly different. Roger Smith pointed out that whether or not a magnetic signature accompanies a PMAF will depend upon the ionospheric conductivity, and Bob Lysak pointed out that it will also depend upon the scale size of the field-aligned currents. Mike Lockwood reminded the group that the difference between red and green line patterns must be taken into account in any explanation of these phenomena. Gerard Fasel reported that the PMAFs that re-brighten are remarkably strongly confined to four hours of local time centered on noon, another factor to take into account. The second Monday afternoon session addressed new TCV projects, primarily those stimulated by discussions at the mini-workshop in December in San Francisco. Eigil Friis-Christensen reported that his group is addressing the question of whether conductivity gradients can cause or enhance TCVs. He proposed a 10 Jun 94 event for coordinated study, since it occurred in the summer when conductivity becomes more uniform. Comparison with results from the AMIE procedure should be best in the summer, since the AMIE technique depends upon an assumed conductivity distribution. Mark Engebretsen noted that, serendipitously, 10 Jun 94 is also a designated AGONET event. Hermann Luehr reported on results from a superposed epoch analysis of 20 large-amplitude TCVs that occurred during the period Oct 93 - Mar 94. These clearest TCVs invariably arise around 10 LT in the vicinity of the convection throat. They are recurrent phenomena, occurring sometimes on successive days during geomagnetically quiet intervals in the declining phases of storms and then recurring 13 or 25 days later during the quiet interval in the declining phase of the next storm. A question raised by this study concerns whether quiet times were favored only because that is when TCVs are easiest to see. Dave Sibeck noted that he was presenting a poster about the possible influence of ionospheric conductivity on TCV identification during quiet and during disturbed conditions. In mapping the TCVs from the ionosphere to the equatorial plane, Hermann found that they lay well inside the magnetopause along the flanks of the magnetosphere. Bill Lotko commented that they might not be a boundary phenomenon after all, if the mapping can be trusted. Joel Fedder showed a video of the Fedder-Lyon model's response to input of real solar wind data containing pressure changes and directional discontinuities. Although the results were difficult to see, Joel could distinguish different ionospheric effects of the two phenomena. The ionosphere's response to discontinuities appears to be a set of field- aligned currents traveling antisunward on either side of the polar cap boundary in the form of Region 1 current adjustments to the new IMF configuration. Joel plans to subtract the background convection pattern in order to make the resulting vortices easier to see. Therese Moretto presented work done with Peter Stauning correlating magnetometer and riometer data. Coincident with the 18 Dec 94 TCV event, a riometer absorption event was observed with features atypical of events caused by precipitation. Further, the maximum of the absorption event coincided with the presence of a very large convection electric field rather than with the passage of an upward field-aligned current filament of a vortex center. These results suggest that most likely the source of this event was either E-region electron heating or drifting F- region patches. In other events, the absorption appeared to be caused by precipitation, but still no clear correlation was found between the maxima of the absorption events and the passage of vortex centers corresponding to upward field-aligned currents. Many interesting and relevant papers were presented at Monday evening's poster session, but these will not be reviewed here. Some of these papers may be viewed through the GEM page on the World Wide Web. A basic issue that arose during the poster session, however, relates directly to the material described above and deserves mention here. It concerns whether the currents responsible for TCVs are generated at the magnetopause or in the ionosphere. Hermann Luehr, for example, favors the magnetopause source, in which case the ionospheric conductivity would play no role in whether or not the disturbance could be seen by magnetometers, as suggested above by Roger Smith. On the other hand, in the Fedder-Lyon model, for example, the currents are the ionosphere's response to the change in solar wind conditions. In this case conductivity plays a major role. From this point of view, Dave Sibeck noted that the enhanced conductivity region near 1000 LT might be responsible for the clearest TCVs forming there, as suggested in one of the papers. On Tuesday morning, the first session addressed a new area of interest for WG3, the global coherence between dayside and nightside events. Kile Baker showed a video of SUPERDARN radar data from a project headed by Ray Greenwald on the dayside effects of substorms. A strong afternoon vortex appeared prior to a substorm onset and then rapidly dissipated at onset. Steve Mende then reviewed and updated the well- known correlation between the AE index, a measure of substorm activity, and the equatorward extent of dayside aurora. The remainder of the session was devoted to data presentations for the GEM/CEDAR National Space Weather Event Study under the direction of Delores Knipp. The second Tuesday morning session was led by Hermann Luehr, coordinator of last year's selected events. Closure status on these projects was reported by Eftyhia Zesta, Therese Moretto, Bob Clauer, and Gang Lu. Eftyhia Zesta reported on a reaxamination of the Oct 11 and Oct 25, 1993 events, both of which had been selected as potential TCV events during moderate disturbances. Increased doubt was expressed whether it was still fruitful to classify and study these events as TCVs since they are considerably distorted by the (unknown) ambient magnetic disturbances. Therese Moretto presented the Oct 10 and Dec 18, 1993 events based on a comprehensive geomagnetic data set spanning about 180 deg in longitude, including the Canadian, Greenland, and Scandinavian sector. Due to the extensive data coverage, it was possible to use a new technique to present the data. By calculating the divergence of the magnetic disturbance vectors, the distribution of upward and downward field- aligned currents could be estimated for each point in time. These 2-D images of the field-aligned current variations associated with TCVs clearly demonstrated that the simple picture of a constant constellation of localised field-aligned current filaments drifting at constant speed along a roughly constant invariant latitude line, which is compatible with observations from a single line of stations, does not suffice to describe the development of the events during their entire life-time. In particular it was demonstrated that the motion of the TCVs is far from smooth and that the sizes and forms of the current regions are much more diverse and dynamic than one is led to believe from the single chain observations. Bob Clauer discussed the Aug 04, 1991 event showing a"street" of vortices, presumably caused by a KH instability. Simultaneous radar measurements show that the vortices occurred at the convection reversal boundary, which, in turn, was modulated by the vortices. Of particular interest in this event was the existence of two distinct frequencies in the oscillations. A lower one, corresponding to a period of 28 minutes, was attributed to the KH-instability. The higher frequency component, corresponding to a period of 14 minutes and dominant in the more southern part of the Greenland stations, was attributed to field-line resonance on closed field lines. Gang Lu examined the Aug 2, 1991 event that at previous GEM workshops had been discussed as an example of the poleward progressing disturbances associated with a varying By component during periods of southward IMF. The AMIE results, however, could be interpreted as an intensity modulation of a pair of stationary field-aligned currents near the cusp. III. Future Direction Because many questions regarding transient events are still far from fully understood, they will remain an important research topic. On the other hand, the group felt that the specific projects undertaken during the past year, having benefitted significantly from the WG 3 discussions and the coordinated data gathering effort facilitated by the GEM program, could now proceed succesfully to closure without the direct assistance from WG 3. Consequently, new primary topics will be selected for the next workshop. Particular interest was expressed in the following three topics: 1. Coherence of dayside and nightside magnetospheric current systems. 2. Origin, geometry, and dynamics of the region 1 current system. 3. A preparation for and coordination with the satellite community. It was felt that the maturity of the GEM program, demonstrated in the workshops, is a valuable asset for the satellite programs like GGS, ISTP, CLUSTER, Interball, FAST, Equator-S, and Oersted.