An ISEEwhistler model of equatorial electron density in the magnetosphere

An empirical model of equatorial electron density in the magnetosphere has been developed, covering the range 2.25 L 8. Although the model is primarily intended for application to the local time interval 0015 MLT and to situations in which global magnetic conditions have been slowly varying or relatively steady in the preceding 20 hours, a way to extend the model to the 1524 MLT period is also described. The principal data sources for the model were 1 electron density profiles deduced from sweep frequency receiver SFR radio measurements made along nearequatorial ISEE 1 satellite orbits and 2 previously published results from whistlers. The model describes, in piecewise fashion, the saturated plasmasphere, the region of steep plasmapause gradients, and the plasma trough. Within the plasmasphere the model profile can be expressed as log n e x i , where x 1 0.3145 L 3.9043 is the principal or reference term, and additional terms account for 1 a solar cycle variation with a peak at solar maximum, 2 an annual variation with a December maximum, and 3 a semiannual variation with equinoctial maxima. The location of the inner edge of the plasmapause outer limit of the plasmasphere L ppi is specified, with some qualifications, as L ppi 5.6 0.46Kp max , where Kp max is the maximum Kp value in the preceding 24 hours. The plasmapause density profile is described as log n e log n e L ppi L L ppi pp , where pp is the scale width of the plasmapause, or distance in L value over which the density drops by an order of magnitude. For modeling purposes, pp is suggested to be 0.1 600 km at night and to increase across the dayside, but values no greater than pp 0.025 150 km, the limiting spatial resolution of the ISEE SFR, have been observed. The inner part of the plasma trough, prior to significant refilling, is described as n e n e L ppo LL ppo 4.5 , where L ppo is the outer limit of the plasmapause segment. The model includes the effects of a factoroforder 5 diurnal variation in electron density in the plasma trough region, as well as a relatively abrupt transition near dusk from day to night trough levels. It also includes an approach at large L values to a limiting low density of 1 el cm 3 . It is possible that the trough levels in the model are a factor of 510 higher than trough levels in some nightside regions during the early phases of substorms. ISEE data indicate that for those profiles on which one or more plasmapause decreases can be identified, the mean radius of the innermost plasmapause varies only slightly with magnetic local time, exhibiting a slight bulge near 18 MLT duskdawn difference L ppi of order 0.5. This is apparently due to the strong influence of the nightside plasmapause formation process, the effects of which are felt over much of the dayside following delays associated with the Earths rotation. Structured regions of dense plasma of plasmaspheric origin are known to appear in the afternoonevening sector at radii larger than those of the main plasmasphere. These are believed to be the more extended andor outlying features of the plasmasphere bulge that have previously been reported they are not represented by the present model.