MSM Simulation of the Plasmaspheric Shoulder, and Comparison to Data from IMAGE EUV Jerry Goldstein (1) (713 348 3409; jerru@hydra.rice.edu) Robert W. Spiro (1) Patricia H. Reiff (1) Richard A. Wolf (1) Bill R. Sandel (2) John W. Freeman (1) Richard L. Lambour (3) 1 Department of Physics and Astronomy, Rice University, 6100 Main Street, MS 108, Houston, TX 77005, United States 2 University of Arizona, Lunar and Planetary Lab, 1040 East 4th Street, Rm 901, Tucson, AZ 85721, United States 3 MIT Lincoln Lab, Group 91, 244 Wood Street, Lexington, MA 02420, United States Between the hours of 6--10 UT on May 24, 2000, the IMAGE Extreme Ultraviolet (EUV) instrument observed a shoulder-shaped bulge in the morning sector plasmapause. Apparently corotating with the outer plasmasphere, this shoulder moved from its first-observed location near the dawn terminator (at 6 UT), and steepened as it approached noon local time. Simulation results of the data-driven Magnetospheric Specification Model (MSM) have reproduced the formation (at approximately 5 UT) and subsequent time development of the shoulder feature. In the model, the shoulder formation is apparently caused by a dusk-to-dawn penetration electric field, which is triggered by a sudden and strong northward turning of the IMF. The effect of the penetration E-field is to create an antisunward flow of cold plasma in the midnight-dawn quadrant, producing a bulge which steepens and becomes more shoulder-like as it corotates with the plasmasphere. The good agreement between model and observational data of May 24 suggests that evidence for plasmaspheric shoulders may be expected in EUV observations made during other periods of strong, sudden northward IMF turnings. In addition to the May 24 event, we have identified four days (June 10-11, August 2, and August 5, 2000) where sudden northward IMF turnings preceded the formation of shoulder-like features, seen in EUV data. The shoulders of these alternate events were not as prominent as that of May 24, but MSM simulation results suggest that these shoulders could also have been due to penetration E-fields. This study may indicate that a significant role, the production of shoulder-like bulges in the early morning sector plasmasphere, is played by the penetration E-field in global plasmaspheric dynamics. _______________ Submitted to the Spring 2001 AGU Meeting, Boston, Massachusetts