*************************** ** THE GEM MESSENGER ** *************************** Volume 4, Number 15 August 31, 1994 --------------------------------------------------------------------- REPORT ON 1994 SNOWMASS WORKSHOP - BOUNDARY LAYER CAMPAIGN -- Part II --------------------------------------------------------------------- WP 3: CURRENT SYSTEMS AND MAPPING co-chairs: Eigil Friis-Christensen and Nancy Crooker For the 1994 Snowmass Workshop, WG 3 expanded their ongoing studies of traveling convection vortices (TCVs) in three ways: a) by including other transient ionospheric signatures of magnetopause processes, such as poleward moving magnetic disturbances and auroral forms, and any relationships between them, b) by choosing specific events for coordinated studies with complementary data from multiple sources, especially multiple magnetometer chains, with special attention to active periods for space weather studies, and c) by incorporating at the outset efforts to model the background polar cap size, boundary location, and convection pattern specific to the solar wind conditions for each event. To help implement the expanded program, Hermann Luehr agreed to serve as coordinator of the event studies. His comprehensive tutorial lecture to the entire workshop audience set the tone for the global approach in the working group sessions. All four sessions included both observational and theoretical presentations and discussion. Below we summarize first the modeling results and then the observational results. MODELS Five models were discussed: 1) the Schulz, Lyons et al. model, 2) the Toffoletto-Hill model, 3) the IZMIRAN Electrodynamic Model (IZMEM), 4) the Fedder-Lyon MHD model, and 5) the Lysak pressure-pulse model. Models 1 and 2 are sophisticated versions of superposed field models that map the solar wind electric field from the magnetopause to the ionosphere. Model 3 gives empirical convection patterns parameterized by the IMF. Model 4 is a computer simulation of the solar wind interaction with Earth's dipole. Model 5 addresses the formation of TCVs in a dipole geometry. To implement (c) above, output from Models 1-3 was solicited in advance for the four events with available solar wind data. Determining background solar wind conditions from the data plots for input to the models became a challenging task in view of the large amplitude of the fluctuations present in most of the parameters for these events. The task raised serious questions about the validity of the concept of a "background convection pattern", since in the models the pattern depends primarily upon the y and z components of the IMF, the averages of which were much smaller than the fluctuation amplitudes and, hence, highly uncertain. Given these questionable solar wind parameters, Models 1-3 produced similar background convection patterns of the standard two-cell type. Margaret Chen described Model 1 and presented its results. Model 1 has recently been successful in matching certain aspects of polar cap boundary position with observations, such as the interhemispherical difference in cusp latitude as a function of dipole tilt angle. As predictions for the events, however, neither Model 1 nor Model 2 can as yet specify absolute polar cap boundary location, shape and size based on solar wind parameters alone. An additional problem was identified in Model 1: The convection pattern becomes unrealistic on the nightside owing to convergence of the equipotentials at a null point. Several ways to solve this problem were discussed, and Larry Lyons and Margaret Chen agreed to look into them. Model 2 results were given by Tom Hill and Ching Deng. They have already solved the null point convergence problem by adding the equivalent of an expansion fan to the tail. Adding the low-latitude region of return flow to complete the convection cell pattern was discussed as a next step for improving their ability to compare with observations. The difference in polar cap shape predicted by Models 1 and 2 was briefly mentioned (also in WG 2). Compared to observations from space, Model 2 produces an elongated shape that becomes more pronounced as the IMF turns northward, consistent with only the darkest regions of the polar cap being open, while Model 1 produces a round polar cap, consistent with shape of the auroral border. The Model 3 results were forwarded by Volodya Papitashvili, who could not attend the meeting. They showed the average empirical patterns for the event IMF orientations and seasons, including the low-latitude return flow. These patterns are readily available by electronic mail. Presentations on Models 4 and 5 addressed transient phenomena directly. Joel Fedder demonstrated Model 4's impressive ability to reproduce the pattern of auroral brightening observed by the Viking spacecraft in response to a solar wind pressure pulse accompanied by a change in IMF orientation. The only drawback is that at present the simulations can be matched to data only if the transient occurs after a lengthy of period of northward IMF, which matches the model's baseline setting. John Lyon used Model 4 to show the pronounced effects on the polar cap geometry and convection pattern in response to a major shock wave accompanied by a change in IMF and solar wind flow directions. Bob Lysak presented the most recent results from his increasingly sophisticated numerical modeling scheme for calculating field-aligned currents generated at the polar cap boundary by solar wind pressure pulses. His total current strength of 60 kA compares reasonably well with the values of 100-300 kA obtained from observations by Hermann Luehr. Although the workshop stimulated interactions between modelers and observationalists within their own groups, in general the anticipated cross-interactions did not occur. In the case of Models 1-3, the observationalists are not yet ready to consider their signatures in relation to the global convection pattern, and Models 1 and 2 are not yet ready to match several aspects of observed patterns. The workshop did serve to stimulate progress in these directions. In the case of Model 4, the baseline problem must be solved before comparisons with event data can be made. Model 5 will be used for quantitative comparison with TCV currents. OBSERVATIONS The aim of WG 3 was to select event categories/phenomena covering: a) Poleward Moving Auroral Forms, PMAFs b) Progressing Polar Convection Disturbances, PPCDs c) Convection Reversal Boundaries, CRBs d) Traveling Convection Vortices, TCVs, divided into isolated events during relatively quiet conditions and events during disturbed conditions. For the TCVs, which had been discussed at the earlier GEM workshops, this workshop provided the possibility of using a more extensive data set than previously because of the recent establishment of the MACCS magnetometer array in Canada and the possibility of incorporating Svalbard data. Some specific questions about TCVs addressed at the workshop are: 1. What is the spatial distribution of TCVs? What does a given TCV look like at different longitudes? Does it change its form during motion? 2. Are the motions inferred from different locations consistent with a model of a (primarily) tailward moving system of vortices? 3. Is a TCV seen at one location also a TCV seen from a different location? 4. Are there any systematic relationships between TCVs and optical features? Or absorption events? 5. Which models are most relevant for the TCV: pressure pulse, K-H instability, field-line resonance? 6. How do vortices form? Do they form around noon? The selection of events was very restricted because the MACCS data were not fully available before the summer of 1993, and the Svalbard data had only been processed for the "dark" period, fall of 1993 and winter 1993/1994. In addition to these constraints, the general geomagnetic activity during these intervals was not very favorable regarding, for instance, the occurrence of PPCDs. The chosen events are briefly described below. a) PMAFs 12 Jan 1994: A large PMAF was associated with large magnetic signatures and possibly with a simultaneously forming TCV at 0820 UT. 14 Jan 1994: A "broad" oval disappeared and a "new" oval appeared to the South (and coming from the noon sector) associated with a westward current. b) PPCDs 27 Oct 1994: Only one "decent" and not even remarkable event was detected during a thorough search through the Oct 1993 to Jan 1994 interval. The event was atypical because normally the PPCDs are associated with IMF By variations during a nearly constant negative Bz. For this event the IMF showed large Bz variations, positive and negative, simultaneous with the By variations. 29 March 1992: This event was included as a supplement, even though the MACCS data are not available this early. c) CRBs 4 Aug 1991: This event can be traced from the East Greenland data to the West Coast by means of the MAGIC (Greenland Ice Cap) stations. d) TCVs - isolated events 14 Oct 1993, around 1300 UT: Near local noon in Greenland a twin vortex moved westward (tailward) on the West Coast and eastward (also tailward) on the East Coast. This is consistent with the westward motion observed at MACCS and the eastward motion at Svalbard, fitting the form of a classic TCV. 18 Dec 1993, one around 1350 UT and another around 1510 UT: For these events all Greenland stations showed an eastward motion while MACCS data indicated westward motion. No motion could be firmly identified in the Svalbard data. For the second event clear absorption was associated with the leading FAC. TCVs - disturbed conditions 11 Oct 1993 1600-1625 UT: MACCS showed eastward (sunward) motion in the prenoon sector (unusual). Greenland data showed eastward (tailward) motion, but a very distorted pattern. 25 Oct 1993 around 1400 UT: Greenland data showed eastward motion while MACCS data showed westward motion. 25 Oct 1993 at 1635 UT: MACCS data showed eastward motion while the Greenland data indicate that this event is probably not a TCV because it is seen nearly simultaneously at all stations. The following principal investigators were tentatively selected to examine the events from their perspective and discuss with the event coordinator, Hermann Luehr, the possibility of preparing a paper based on combined data sets. Event Date Principal investigator (possible co-authors) Jan 12, 1992 Fasel Jan 12, 1994 Jacobsen, Moretto, Friis-Christensen Jan 14, 1994 Jacobsen Oct 27, 1993 Limited interest in pursuing this event Mar 29, 1992 Clauer, Rosenberg Aug 04, 1991 Rosenberg, Clauer Oct 14, 1993, 1300 Moretto, Friis-Christensen, Luehr Dec 18, 1993, 1350 Moretto, Friis-Christensen, Luehr, Rosenberg 1510 Rosenberg, Oct 11, 1993, 1600 Zesta, Engebrettson, Hughes Oct 25, 1993, 1400 Luehr, Zesta, Engebrettson, Hughes Oct 25, 1993, 1635 Pinnock +-------------------------------------------------------------------------+ |To add name to the mailing list, send a message to: guan 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. Lotko) | |GEM Bulletin Board: telnet terra.igpp.ucla.edu; user name: gem or GEM | | contents: GEM Messengers, magnetic indices, IMF data | |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! | +-------------------------------------------------------------------------+