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

Data from HF-radars are used to make the first simultaneous conjugate measu rements of the dayside reconnection electric field. A period of 4 h around local magnetic noon are studied during a geospace environment modeling (GEM ) boundary layer campaign. The interplanetary magnetic field (IMF) was sout hward whilst the eastward component (By) was variable. The flow patterns de rived from the radar data show the expected conjugate asymmetries associate d with IMF \By\ > 0. High-time resolution data (50 and 100 s) enable the fl ow of plasma across the open/closed field line boundary (the separatrix) to be studied in greater detail than in previous work. The latitude of the se paratrix follows the same general trend in both hemispheres but shows a hem ispherical difference of 4 degrees, with the summer cusp at higher latitude , as expected from dipole tilt considerations. However, the short-time scal e motion of the separatrix cannot be satisfactorily resolved within the bes t resolution (300 m s(-1)) of the experiment. The orientation of the separa trix with respect to magnetic latitude is found to follow the same trend in both hemispheres and qualitatively fits that predicted by a model auroral oval. It shows no correlation with IMF By. However, the degree of tilt in t he Northern (summer) Hemisphere is found to be significantly greater than t hat given by the model oval. The convection pattern data show that the meri dian at which throat flow occurs is different in the two hemispheres and is controlled by IMF By, in agreement with empirically derived convection pat terns and theoretical models. The day-side reconnection electric field valu es are largest when the radar's meridian is in the throat flow or Parry aft ernoon flow regions. In the morning or afternoon convection cells, the reco nnection electric field tends to zero away from the throat flow region. The reconnection electric field observed in the throat flow region is bursty i n nature.