The plasmapause revisited

Abstract

Saturation of the dc double-probe instrument on Explorer 45 has been used to identify the plasmapause. Fifteen months of these data resulted in a data base of sufficient size to statistically study the average position of the plasmapause over 14.5 hours of magnetic local time (MLT) under differing magnetic conditions. The afternoon-evening bulge in the L coordinate of the plasmapause versus local time was found to be centered between 20 and 21 hours MLT during magnetically quiet periods and shifted toward dusk as activity increased, but always postdusk. During quiet periods a bulge in the L coordinate near noon was also seen, which disappeared as activity increased. The average local time distribution plasmapause position during high magnetic activity was irregular in the afternoon region, where large-scale convection models predict the creation of plasmatails or detached plasma regions from increases in the solar wind induced convection. The results suggest that solar wind induced convection is partially shielded from the dayside. As the intensity of the convection is increased, it more effectively penetrates the dayside, which shifts the postdusk bulge nearer to dusk and eliminates the quiet time bulge near noon (created by Sq dynamo ionospheric electric fields projecting into the magnetosphere when dayside convection is weak). Correlations of the plasmapause L coordinate with Dst were more ordered than corresponding correlations with Kp or with the maximum Kp over the previous 12 hours.