*************************** ** THE GEM MESSENGER ** *************************** Volume 12, Number 36 September 9, 2002 -------------------------------------------------------------------- GEM 2002 Workshop Report: The Steady Magnetospheric Convection (SMC) Working Group Sessions -------------------------------------------------------------------- From: Ennio Sanchez Joe Borovsky 2002 GEM Workshop, Telluride Report from the GEM-SMC Working Group The Steady Magnetospheric Convection (SMC) working group hosted three sessions at the 23-28 June GEM Telluride Workshop: 1. IMF Conditions for SMCs 2. The role of the ionosphere in regulating SMCs 3. Comparative studies of SMCs P. O'Brien presented results of a statistical analysis of IMF conditions leading to SMC conditions. Starting with a definition of SMC as a period of sustained stable AE >200 nT, it becomes apparent that SMCs are more likely for moderate solar wind velocity and relatively weak southward Bz (~ -3 nT). It is also apparent that SMC duration is sub-exponential (the longer steady convection lasts, the lower the probability of onset) and that very steady Bz is not necessary for SMC occurrence. J. Borovsky's statistical analysis of solar wind data compared the contribution of two drivers of convection: reconnection and viscosity and established that for mild Bz south conditions both drivers are comparable and posed the question of whether SMCs are shutting off the viscous driver. A crucial element in determining how the magnetosphere responds to solar wind input is the configuration of the magnetotail. Joachim Birn is exploring the role of thin current sheets in the magnetotail as mediators between solar wind driver, tail activity and auroral activity. Also of critical importance is the determination of the magnetopause in the time-dependent case. Parameters that can be influential for the establishment of equilibrium in the magnetotail include total magnetic flux change, maximum strength and duration of the electric field and the relationship between tail flaring and total pressure in the magnetotail. Mass and entropy conservation arguments can be used to isolate constraints on magnetospheric convection. A. Otto applied these concepts to suggest that average entropy of newly closed flux tubes could be larger for SMCs relative to substorms. He also suggested that the average reconnection rate might be smaller for SMCs and pointed out that observations of plasmasphere erosion should establish a measure of the strength of convection that could be used to identify differences with respect to substorm convection. IMAGE EUV observations of the plasmapshere, presented by M. Spasojevic and J. Goldstein, show the formation of a plume and a steady, smooth plasmasphere during SMCs. By comparison, non-SMC conditions usually show a highly structured plasmasphere. Furthermore, the end of Bz south conditions trigger a rotation of the plasmasphere tail. A systematic analysis of EUV measurements for SMCs is planned to quantify fundamental plasmaspheric parameters, such as the rate of erosion and plume formation, for SMCs and compare them to their values for non-SMC periods. In discussing the role of the ionosphere as a regulator of SMCs, B. Strangeway noted that an initial overview of FAST observations suggests little qualitative difference in SMC particle precipitation relative to non-SMC situations. SuperDARN measurements (N. Fox) suggest a stable two-cell convection pattern as a distinguishing characteristic of SMCs. Temporally and spatially stable aurora, as seen by POLAR VIS (J. Sigwarth), appears to be a concurrent characteristic of SMCs. It was suggested (B. Strangeway) that steady ionospheric convection conditions characteristic of SMCs are likely to produce strong neutral winds that can have significant impact on M-I coupling by reducing electric current demand from the ionosphere. Another parameter of significance for M-I coupling is the thickness of the ionosphere because it introduces, for instance, dependence of the electric current vector direction with altitude. Altitude-dependence of M-I coupling is apparent also in tall F-region arcs in the polar cap boundary intensifications that form after long periods of southward IMF (J. Semeter.) S. Ohtani, B. Anderson and C. Goodrich presented intercalibrations of IRIDIUM measurements of magnetic field deflection with field-aligned current (f.a.c.) distributions from the Fedder-Lyon-Mobarry MHD computer simulation during long periods of steady southward IMF conditions. Three examples representing different magnetospheric responses were discussed: one response was characterized by sawtooth geosynchronous dipolarizations (11 August, 2000). SMC response was apparent at other times (23 November, 1999) and periods of SMC-like response alternated with isolated injections was observed in other cases (e.g., 13 August, 1999). As the full depth of this rich data-simulation continues to be mined, several fundamental questions regarding SMCs should find an answer. Sawtooth oscillations with a 2-hour periodicity were resolved by AMIE during a southward IMF period on 17-21 April, 2002 in which ionospheric convection was largely stable (G. Lu). Sawtooth oscillations were also observed in the magnetic field of the magnetotail for the same period by GEOTAIL (D. Fairfield and T. Moore). Sawtooth oscillations represent themselves a separate class of response of a strongly driven magnetosphere and comparative studies with SMC periods will help answer the question of what kind of threshold separates the different responses of the magnetosphere to solar wind forcing. One response commonly found in SMCs consists of large amplitude ULF pulsations (~30 min period) that can be measured simultaneously by ground-based magnetometers and by field and particle detectors at geosynchronous and mid- to far tail altitudes (L. Lyons). This property suggests a large-scale coherence of the magnetosphere-ionosphere system. G. Erickson posed that, although it has been established that SMCs represent a state distinct from quiet times, storms or substorms, it has not been established whether there are other conditions that define SMCs uniquely. Erickson also pointed out certain key questions about the tail configuration that allows SMCs. For instance: -Is it dominated by flow channels of Pontius-Wolf bubbles? Or, -How steady-state is the tail configuration? -Is there a middling X-line? (unless the X-line is near the inner edge of the sunward convection region, there will still be a PV^gamma crisis.) Relating to this problem, an important question is: Does convection not get close to Earth? Relevant to tail configuration Erickson the following posed question: Given that SMCs do appear to dissipate energy at substorm levels but in a steady rather than impulsive fashion, can the rate of dissipation be large enough to solve the PV^gamma problem? This question opens up related questions such as: Do thin current sheets play a particularly important role in creating magnetic-field-parallel electric field potentials and ionospheric dissipation as well as non-frozen plasma transport? Erickson pointed out how SMCs provide important constraints for the substorm problem by answering, for instance, the question of whether pseudobreakups occur along the inner edge of the oval during SMCs. If the answer is no, then the NGO model is viable and then one must ask why there is no ballooning: Is it because convection doesn't generate a significant pressure gradient and there are no drift waves present or is it because convection is too strong to allow drift wave reversal of the electric field? If pseudobreakups do occur, then one must ask why ballooning doesn't trigger substorms: Is it that ballooning was never going to trigger a substorm in the first place or that the tail is not susceptible to develop any instability because of the configuration it has settled into? These questions constitute the heart of the research thrust envisioned for the GEM-SMC working group in the year to come. An electronic workshop has been created to post and archive these questions and their answers, as they become available. The electronic workshop also contains survey plots of IMF and POLAR UVI and VIS data for all periods of steady southward IMF between January 1996 and December 2000, to facilitate comparative and statistical studies. The GEM-SMC working group electronic workshop is accessible at (http://isr.sri.com/iono/SMC/HomePageForWorktools.html). +-------------------------------------------------------------------------+ |To add name to the mailing list or for a message to the GEM community | | please contact: editor at igpp.ucla.edu | | | |URL of GEM Home Page: http://www-ssc.igpp.ucla.edu/gem/Welcome.html | |Please update your e-mail address. | +-------------------------------------------------------------------------+