*************************** ** THE GEM MESSENGER ** *************************** Volume 6, Number 41 July 9, 1996 -------------------------------------------------------------- 1996 Snowmass Workshop Boundary Layer Campaign Working Group 2 Working Group Report -------------------------------------------------------------- Co-chairs: C. T. Russell and Antonius Otto Introduction Working Group 2 met 4 times at the 1996 Snowmass meeting discussing flux transfer events, the low altitude manifestation of FTEs, magnetopause processes and the first results of the POLAR mission in and near the polar cusp. In a fifth session we reviewed the advances made in the areas under the purview of the working group during the last 5 years, stimulated at least in part by the deliberations of the working group. Below we summarize the results of this year's workshop followed by a summary of the successes of the working group. Flux Transfer Events The nature and source of flux transfer events have been topics of intense interest throughout the boundary layer campaign. This session began with a report by H. Kawano on his exhaustive study of FTE's seen by ISEE 1 and 2 as identified by a computer algorithm. In order to test whether the foreshock played a role in FTE generation Kawano examined both spiral and ortho-spiral IMF orientations. In both orientations FTEs were found uniformly at all local times, a finding inconsistent with a foreshock source. FTE's themselves were found to be most numerous closest to the magnetopause, and largest in the magnetosheath. These observations together with the observed control of FTEs by the IMF are consistent with reconnection as a source of FTEs. C. T. Russell presented work of Guan Le on the magnetic and plasma structure of FTEs. Flux transfer events may be divided into two parts regardless of whether they are in the magnetosheath or magnetosphere: a draped field region and a core. In the core the magnetic field is twisted and irregular and the plasma is a mixture of magnetosheath and magnetospheric field. In the draped field region the plasma properties are those of the surrounding medium. David Sibeck showed results of a 2D MHD simulation performed with H-C. Ku. While this simulation showed some of the features exhibited by FTEs, such as plasma cores, the 2D simulation was insufficient to demonstrate the full range of FTE signatures. Antonius Otto, however, used his 3D MHD simulation to demonstrate that in 3 dimensions the various characteristics of FTEs including their time scales could be reproduced. In his model the 3D magnetic flux linkage led to an obstruction of the reconnection flow and the formation of FTEs. The simulation also indicated the random decay of a single reconnection patch in multiple patches without the formation of a long x line. The consensus of the participants was that FTEs formed a class of magnetospheric transients quite distinct and distinguishable from those associated with pressure fluctuations. Low Altitude Manifestations of FTEs? The question of whether temporally varying dayside auroral processes reflect transient reconnection at the magnetopause has long interested the GEM community. W. Lyatsky proposed a model that suggested that poleward moving auroral forms were associated with an instability associated with a gap between the low latitude boundary layer and the plasma sheet. Bjorn Jacobsen provided examples of the auroral signatures that have inspired their interpretation as the auroral manifestation of plasma entry into the magnetosphere, including an example of a simultaneous observation of westward travelling convection vortices and poleward motion of the optical forms. Alan Rodger showed how SuperDarn HF raders could be used to monitor both the cusp and FTEs. Finally, Roger Smith discussed work with J. Minow on the effect of the finite lifetime of auroral emissions on the appearance of poleward moving auroral forms, thereby pointing out problems in determining the location and evolution of PMAFs and explaining many of the otherwise puzzling behavior of these PMAFs. Dissipation at the Magnetopause The magnetopause is a region in which microscale processes may have global influences. Yet to date very little is understood about the role of instabilities in controlling the rate of reconnection. Jim Drake provided details of his simulation of turbulence in the magnetopause current layer showing the broadening of the current layer through whistler turbulence. Jay Johnson presented his work on the interaction of compressional waves in the magnetosheath with the magnetopause and discussed mode conversion to kinetic Alfven waves and the associated plasma transport. Finally, Paul Song showed examples of the ELF waves in the magnetopause current layer. He stressed that the energy in the waves was principally in the magnetic component of the waves. The power in these whistler mode waves increased with the local magnetic shear angle. POLAR First Results The GEM meeting provided one of the first forums for the POLAR investigators to get together to jointly examine their data and to discuss these observations with the public at large. Observations were reported by C. T. Russell (magnetic field), Nelson Maynard (electric fields), Jim Horwitz (core ions), Bill Peterson (energetic ion mass/energy composition), Harlan Spence (very energetic to relativistic particles), and Judy Cumnock (ultraviolet imaging). Terry Onsager (ions and electrons) who was at a parallel session later discussed his data informally. This initial look at the data demonstrated that the polar magnetosphere is an active region with dramatic distortions of the field and strong entry of magnetosheath plasma on many occasions. The POLAR spacecraft cuts through the field aligned current systems in both the north and south hemispheres. Interesting electric field structure and ion distributions are seen in these regions. In order to study these phenomena in more detail, a set of special study intervals were chosen. Those wishing to contribute to these studies are invited to contact the coordinator listed for each event. Interval Event Coordinator May 29, 1996 0300-0800 Sheath-like plasma in lobe W. Peterson June 6, 1996 0400-0600 Energetic particles in lobe H. Spence April 9, 1996 0900-1600 Energetic particles in lobe H. Spence March 25, 1996 0000-0300 Polar Cusp with Southward IMF C. T. Russell April 8, 1996 1300-1700 Polar Cusp with Northward IMF N. Maynard April 28, 1996 0800-1300 Polar Cusp with Northward IMF N. Maynard May 3, 1996 1000-1500 Polar Cusp with Northward IMF N. Maynard May 20, 1996 2253-2304 Ion Conics J. Horwitz April 15, 1996 0233:43-0237:40 Ion Conics J. Horwitz We plan to also examine control events during which the satellite covered similar geomagnetic coordinates but under different solar wind conditions. Success Stories Magnetospheric Transients. The outer regions of the magnetosphere are visited by numerous compressional disturbances. Various postulates have been made for such disturbances from pressure pulses in the solar wind, to pressure modulations due to the foreshock and its variations to transient reconnection. While this topic was the subject of much debate over the last several years, we now recognize that there are two distinct types of transients in the outer magnetosphere. Pressure fluctuations produce large scale variations that reach far into the magnetosphere. Transient reconnection produces smaller scale fluctuations that are largest at the magnetopause and that decay in amplitude with distance from the magnetopause. These phenomena co-exist and are usually distinguishable. Dayside Low Latitude Boundary Layer. The paradigm for the formation of the low latitude boundary layer prior to the GEM Boundary Layer Campaign was that when the IMF was northward, cross-field diffusion and momentum transfer associated wave induced boundary motions produced the boundary layer. When the IMF was southward, plasma entered the magnetosphere through reconnection. However, the Boundary Layer campaign clearly demonstrated that reconnection plays a critical role for both northward and southward IMF conditions with high latitude entry for strongly northward IMF. Cross-field diffusion and wave processes play at most a minimal role in the entry of plasma in the dayside boundary layer, although they may modify the plasma within this region. Most of the dayside LLBL is now believed to be an open field lines for southward IMF. Inner Magnetosheath Boundary Layer. Just as a boundary layer forms on the inside of the magnetopause, another one forms on the outside of the magnetopause between it and the magnetosheath flow. This boundary layer was discovered well before the advent of the GEM campaign and described in terms of the depletion of magnetic flux tubes by a combination of kicking (at the shock) and squeezing the particles along the magnetic field away from the subsolar region. One of the successes of the Boundary Layer campaign was the recasting of this theory in terms of the MHD solution of the interaction of a flow with an obstacle. Thus this layer is now recognized to consist of a slow mode compression of the plasma followed by a slow mode rarefaction as the plasma expands as it flows around the obstacle. When the magnetopause is a reconnecting boundary, however, this slow mode structure does not develop as such. Rather any slow mode structure is enveloped in the structure associated with the magnetopause itself. Dayside Auroral Forms Associated with Transient Reconnection. As the GEM program began interest was strong in identifying the dayside auroral manifestation of transient reconnection on the magnetopause. Early attention focused on twin convection vortices but these phenomena did not seem to exhibit the intimate control by the IMF associated with reconnection induced phenomena. Rather poleward moving auroral forms seemed to be the low altitude manifestation of high altitude reconnection. Nevertheless some controversy still exists on the source of these phenomena and work still needs to be done in this area. Cross-Scale Coupling at the Magnetopause. The role of microprocesses is a major unresolved problem for the dayside magnetopause. With the lack of a well identified dissipation mechanism, macroscale modelers often assume resistivity to provide the necessary dissipation for reconnection. The majority in the community assume that whatever dissipation is required by the magnetic reconnection will ultimately be provided, say by the thinning of current sheets until the current can no longer be carried. Nevertheless an intrepid few have persisted in determining how this dissipation is provided using theoretical techniques, numerical simulations and observations. The working group has fostered and encouraged such efforts but there is still much to be done in this area. +-------------------------------------------------------------------------+ |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 | | | |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! | +-------------------------------------------------------------------------+