Conjugate observations of the day-side reconnection electric ®eld: A GEM boundary layer campaign

Data from HF-radars are used to make the ®rst simultaneous conjugate measurements of the dayside reconnection electric ®eld. A period of 4 h around local magnetic noon are studied during a geospace environment modeling (GEM) boundary layer campaign. The interplanetary magnetic ®eld (IMF) was southward whilst the eastward component (By) was variable. The ̄ow patterns derived from the radar data show the expected conjugate asymmetries associated with IMF jByj > 0. High-time resolution data (50 and 100 s) enable the ̄ow of plasma across the open/closed ®eld line boundary (the separatrix) to be studied in greater detail than in previous work. The latitude of the separatrix follows the same general trend in both hemispheres but shows a hemispherical di€erence of 4°, with the summer cusp at higher latitude, as expected from dipole tilt considerations. However, the short-time scale motion of the separatrix cannot be satisfactorily resolved within the best resolution (300 m s) of the experiment. The orientation of the separatrix with respect to magnetic latitude is found to follow the same trend in both hemispheres and qualitatively ®ts that predicted by a model auroral oval. It shows no correlation with IMF By. However, the degree of tilt in the Northern (summer) Hemisphere is found to be signi®cantly greater than that given by the model oval. The convection pattern data show that the meridian at which throat ̄ow occurs is di€erent in the two hemispheres and is controlled by IMF By, in agreement with empirically derived convection patterns and theoretical models. The day-side reconnection electric ®eld values are largest when the radar's meridian is in the throat ̄ow or early afternoon ̄ow regions. In the morning or afternoon convection cells, the reconnection electric ®eld tends to zero away from the throat ̄ow region. The reconnection electric ®eld observed in the throat ̄ow region is bursty in nature.