FLOW-ALIGNED JETS IN THE MAGNETOSPHERIC CUSP - RESULTS FROM THE GEOSPACE ENVIRONMENT MODELING PILOT PROGRAM

The extended flight of the Airborne Ionospheric Observatory during the Geospace Environment Modeling (GEM) Pilot program on January 16, 1990 , allowed continuous all-sky monitoring of the two-dimensional ionosph eric footprint of the northward interplanetary magnetic field (IMF) cu sp in several wavelengths. Especially important in determining the loc us of magnetosheath electron precipitation was the 630.0-nm red line e mission. The most striking morphological change in the images was the transient appearance of zonally elongated regions of enhanced 630.0-nm emission which resembled ''rays'' emanating from the centroid of the precipitation. The appearance of these rays was strongly correlated wi th the Y component of the IMF: when the magnitude of B-y was large com pared to B-z, the rays appeared; otherwise, the distribution was relat ively unstructured. Late in the flight the field of view of the imager included the field of view of flow measurements from the European inc oherent scatter radar (EISCAT). The rays visible in 630.0-nm emission exactly aligned with the position of strong flow jets observed by EISC AT. We attribute this correspondence to the requirement of quasineutra lity; namely, the soft electrons have their largest precipitating flux es where the bulk of the ions precipitate. The ions, in regions of str ong convective flow, are spread out farther along the flow path than i n regions of weaker flow. The occurrence and direction of these flow b ursts ate controlled by the IMF in a manner consistent with newly open ed flux tubes; i.e., when \B-y\ > \B-z\, tension in the reconnected fi eld lines produce east-west flow regions downstream of the ionospheric projection of the x line, We interpret the optical rays (flow bursts) , which typically last between 5 and 15 min, as evidence of periods of enhanced dayside (or lobe) reconnection when \B-y\ > \B-z\. The lengt h of the reconnection pulse is difficult to determine, however, since strong zonal flows would be expected to persist until the tension forc e in the field line has decayed, even if the duration of the enhanced reconnection was relatively short.