Observations of the solar wind during the flight of Imp 1

Some of the principal results of an experiment designed to investigate the properties of the solar wind and its interaction with the geomagnetic field are discussed. The main objective of the experiment was to measure the energy per unit charge and gross flow characteristics of the positive ion component of the incident plasma as a function of radial distance from the earth. This experiment was carried out on the Imp 1 (Explorer 18) spacecraft using a curved plate electrostatic analyzer. Plasma observations indicate that three distinct regions of space were traversed by the satellite. The first was the geomagnetic cavity where no plasma was observed within the dynamic range of the instrument. The second was the region of turbulent plasma flow associated with the interaction between the solar wind and the geomagnetic field. The third was interplanetary space where the free streaming solar wind was observed. The results presented cover the first 2 1/2 months of satellite life. The plasma data obtained in interplanetary space revealed that the solar wind density rarely exceeded 3 ions cm−3. The free-stream velocity exhibited values from less than 300 km/sec during quiet times to more than 700 km/sec at the time of geomagnetic disturbances. Observations in the transition region consistently showed a large increase in the disorder of the plasma as compared to interplanetary space. Careful examination of the change in the plasma characteristics as the spacecraft passed through this region has allowed the extremities of the transition region boundaries to be extensively mapped. The data revealed that toward the subsolar point the outer boundary of the transition region was located some 4 to 5 earth radii upstream from the magnetospheric boundary and showed increasing standoff distance with increasing angle from the earth-sun line. The geomagnetic cavity was seen to flare out from the subsolar point running approximately parallel to the earth-sun line for angles greater than about 130° to 140° from the subsolar point. No indication of closure of the tail of the magnetosphere was observed. The transition region observations are discussed in light of present theory.