Conveners:
Stanislav Sazykin (sazykin@rice.edu)
Phil Erickson (pje@haystack.mit)
Bob Lysak (bob@aurora.space.umn.edu)
2005 Tuesday 28 June 1000-1200 AM
Bob Spiro of Rice University presented a tutorial on "Sub-Auroral Electric Fields: An Inner Magnetosphere Perspective" (0800-0845 AM) as a lead-in to the joint workshop. This tutorial is on-line at the CEDAR website http://cedarweb.hao.ucar.edu/workshop/tutorials/2005/lu05.pdf and at the GEM website http://www-ssc.igpp.ucla.edu/gem/tutorial/index.html. Bob led off by briefly reviewing how magnetospherically driven sub-auroral electric fields affect Earth's plasmasphere and sub-auroral ionosphere, including discussions of the formation of the mid-latitude ionization trough, of storm-associated changes in the structure of the ionosphere, of electric field penetration effects on the equatorial ionosphere (Spread-F and scintillation), and of the role that inner magnetospheric electric fields play in the formation of the plasmasphere. This was followed by brief descriptions of the morphology of sub-auroral electric field structures as measured by the Millstone Hill ISR and by low-altitude satellites (e.g., DMSP). Our current theoretical understanding of how the inner magnetiosphere affects conditions in the sub-auroral ionsophere was presented in the context of the Rice Convection Model (RCM), including discussions of shielding and over-shielding. Typical RCM results were presented that showed many of the typical electric field features associated with SAPS. From the RCM point-of-view these features occur naturally in the evening local time sector whenever downward Region 2 field-aligned currents associated with shielding the inner magnetosphere are located equatorward of the region of enhanced conductivity within the evening diffuse aurora. A SAPS-like feature typically appears in the RCM as a latitudinally confined region of enhanced poleward-directed electric field needed to close the downward Region 2 field-aligned current across the low-conductivity sub-auroral ionosphere. Within the context of the RCM, SAPS and associated phenomena are not, strictly speaking, penetration events since they occur poleward of the shielding Region 2 currents. True penetration events, in the RCM, are associated with under- and over-shielding equatorward of the shielding currents.
The workshop was an opportunity for both CEDAR and GEM scientists to compare notes and exchange data and ideas on the subject of electrodynamic coupling between the sub-auroral/mid-latitude ionopshere and the inner magnetosphere. Although the "focus" topic was Sub-Auroral Polarization Stream (SAPS) electric field phenomena, the 14 speakers addressed a broader range of issues in this very active research topic from the theoretical, modeling and observational points of view. The workshop was oversubscribed, and although there was no time left for a separate open discussion, many talks were followed by multiple questions and brief discussions.
Opening the workshop, John Foster presented an ionospheric perspective of the SAPS phenomena, based on Millstone Hill incoherent radar measurements of subauroral electron densities and convection velocities combined with GPS Total Electron Contect (TEC) maps. It was emphasized that the magnetosphere drives SAPS but the ionosphere controls SAPS characteristics. John also suggested that inside the (latitudinally) wider SAPS region, narrower and structured regions of highly dynamic large-amplitude electric fields have been observed in DMPS ion-drift data that may be caused by an ionospheric feedback-instability mechanism. Anatoly Streltsov presented his theoretical/modeling work that gives one possible explanation of this effect in terms of Alfven waves resonator effect. Mike Liemohn showed structured inner-magnetospheric electric fields calculated with the self-consistent version of the RAM ring current model. Some of those structures appear to be similar to SAPS. Mike pointed out that such structures are not found in IMAGE HENA particle flux maps of the ring current region, and challenged experimentalists to reconcile model results with HENA observations. Two other presentations by modelers addressed more general aspects of global magnetosphere-ionosphere coupling at subauroral and mid-latitudes. Austrid Maute gave a short update on her work in developing an electrostatic potential solver to be used with the Lyon-Fedder-Mobary (LFM) global MHD code as part of the Center for Integrated Space Modeling (CISM) project. One purpose is to allow for hemispheric asymmetry in the code. The code is still under development. Bob Lysak described his current work in first-principles magnetosphere-ionosphere modeling extending his model to mid-latitude regions of low conductivity. All of these modeling talks indicated that we are still quite far from being able to explain the observations.
The need for modeling was evident as there was an abundance of observations presented at the session. Jerry Goldstein, in his talk, used IMAGE EUV and HENA observations to point out the close relation of the (cold) plasmaspheric and (hot) ring current particle populations. According to him, among the still-unanswered questions are: How do magnetospheric electric fields very spatially, and what is the role of SAPS electric fields on sharpening storm-time plasmaspheric plumes? An emerging role of meter scale (HF and VHF) coherent radar measurements in deducing convection electric fields at subauroral latitudes was evident from three talks by Ray Greenwald (initial observations of SAPS with the newest SuperDARN Wallops Island HF radar), Murray Parkinson (Australian TIGER HF radar observations of SAPS during substorms, talk given by Stan Sazykin), and Melissa Meyer (University of Washingon MRO passive radar VHF coherent scatter SAPS and SAID observations). In the next few years, these new techniques will give us a new way to look at the dynamics of sub-auroral convection. In-situ electric field measurements by CLUSTER spacecarft presented by Pamela Puhl-Quinn indicate the presence of SAPS in the dusk-side inner magnetosphere. While SAPS were shown in case studies, there is also an empirical parameterized model of the convection electric field constructed from CLUSTER observations. Pamela pointed out that when the location of the plasmapause is estimated from spacecraft potential and electron density measurements, it roughly coincides with the SAPS region.
On the subject of observations of broader M-I coupling at mid-latitudes, Attila Komjathy gave an overview of ionospheric electron density global changes during the inital phases of "superstorms" based on TEC maps derived at JPL from GPS receivers on board the CHAMP satellite. Enormous changes in mid-latitude TEC are necessarily related to magnetospheric electric fields, although a quantitative explanation has not been given yet. Ian Mann talked about another global aspect of M-I coupling during superstorms --- observations of intense ULF (Pc5-band) waves at mid-latitudes, and challenged those working in the field to come up with an explanation. Chin Lin described his recently-published work on modeling ionospheric low-latitude effects of storm-time magnetospheric electric fields.
The workshop generated plenty of interest evident in the number of presenters (14) and also in terms of participation. In summary, there is a clear need for continuation of the very productive discussion started at this workshop.