*************************** ** THE GEM MESSENGER ** *************************** Volume 5, Number 20 September 13, 1995 ----------------------------------------------------------- Report on GEM Snowmass Meeting, June 26-30, 1995 - Part III ----------------------------------------------------------- BOUNDARY LAYER CAMPAIGN WG 2: PARTICLE ENTRY, BOUNDARY STRUCTURE AND TRANSPORT Co-chairs: Chris Russell and Lou Lee Throughout the GEM Boundary Layer Campaign there has been steady progress in understanding the magnetospheric boundary layers and this year was no exception. In the past most progress was made with observations at high altitudes, near the equatorial plane and near the nose of the magnetosphere. Here it became clear that reconnection was the dominant physical mechanism both when the IMF was southward and when it was northward. The evidence for this was multifold. There was no evidence for diffusion being important since the plasma boundaries remained sharp. Boundary layers appeared to be rather uniform in properties as if the entire layer was a mixture of similar starting plasmas and the boundary layers contained the occasion impulsive injection with field aligned flows. This year additional work on this problem by Guan Le suggested that when the IMF was strongly northward the LLBL consisted of two regions: one on open field lines of most recently reconnected field connected to the Earth at one end and one on closed field lines that had undergone reconnection at high latitudes in both the north and the south tail lobes. Until this year controversy had remained over the interpretation of similar data at low altitudes. However, this year the existence of a semi-quantitative model by Terry Onsager allowed Pat Newell to change his opposition to a reconnection source for dispersive ion features in the LLBL and cusp regions. Pat Newell explained clearly his new interpretation in a tutorial presentation. This was followed by a detailed examination of this mechanism and its consequences by Mike Lockwood. While there is still more to be understood, there is more agreement in the boundary layer community than there ever has been in the past. While it is presumptuous to claim that this accord was solely the result of the GEM workshops, it is true that the denouement of this new understanding took place at the GEM meetings. There was much new to be learned in addition to the resolution of old controversies. The GEOTAIL mission was just completing its third year of operation in the equatorial region and despite its optimization for tail studies has proven itself to be a powerful probe of processes at the dayside magnetopause. The latest data on the GEOTAIL studies at the magnetopause and in the low latitude boundary layer were reported by M. Nakamura and M. Fujimoto. In November 1994 after a series of unfortunate delays, the WIND spacecraft was finally launched into an orbit almost the mirror image of that of GEOTAIL. Where GEOTAIL skimmed the magnetopause and goes far down the tail, WIND passes through the near tail region and goes deep into the solar wind. While not optimized for magnetospheric studies, the instruments on the WIND spacecraft still are a powerful advance over the instruments of the late 70's and early 80's on which most of our present understanding is based. Preliminary results of low latitude electrons by T. Onsager, on energetic magnetosheath particles by R. Skaug, and on energetic particle composition measurements by H. Paquette. All results were very intriguing but clearly not yet at the stage of sophisticated analysis that allows the testing of models. The magnetosphere has boundary layers outside the current layer as well as inside. The magnetized plasma of the magnetosheath has to be deflected around the obstacle so that both the plasma and the magnetic field are carried around the obstacle and this process must obey the laws of physics for which MHD is usually a good approximation. MHD tells us that there are three waves, each of which has an othogonal role to play in the interaction. Under most circumstances the slowest mode is the wave that compresses the density and rarefies the magnetic field or vice versa (in an expansion fan). This mode is generally referred to as the slow mode. The mode should stand in front of the magnetopause where the normal component of the magnetosheath flow matches the slow mode velocity. Controversy has arisen in the past both over the existence and the proper physical explanation for such structures. Paul Song has been the main advocate of the standing slow mode wave. Lou Lee's group have in turn presented an alternate explanation in terms of transient phenomena. This year was no exception. Paul Song presented improved evidence for the phenomenon through the comparison of observations with space forecaster's models. This presentation, thus, made use of two of the present thrusts of the GEM campaign; the comparison of in situ data with theory; and the use of space forecast models to predict magnetospheric behavior. The space forecast models, as expected, did not exhibit the variations seen in the observations as they were only gasdynamic in nature but they clearly proved the phenomenon was spatial and not temporal in nature. The opposite viewpoint was presented by M. Yan and Y. Lin who discussed models of the interaction of rotational discontinuities with the bow shock and the generation of pressure pulses in the magnetosheath. The remaining presentations engendered far less controversy. There were discussions of time varying reconnection and flux transfer events by Lockwood, the location of magnetopause reconnection by Onsager, magnetic reconnection by A. Otto, H. Cai and J. Drake, wave interaction with the magnetopause current layer by J. Johnson and the low latitude boundary layer by C-Q Wei. Overall there was much more to be discussed than there was time to do so but nevertheless much was accomplished. Chris Russell +-------------------------------------------------------------------------+ |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. 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