An ISEE/whistler model of equatorial electron density in the magnetosphere

Abstract

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 ∼00–15 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 15‐24 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 near‐equatorial 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 logne = Σxi, where x1 = −0.3145L+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) Lppi is specified, with some qualifications, as Lppi = 5.6 ‐ 0.46Kpmax, where Kpmax is the maximum Kp value in the preceding 24 hours. The plasmapause density profile is described as logne =logne(Lppi) − (L ‐ Lppi)/Δpp, where Δpp 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, Δpp is suggested to be ∼0.1 (∼600 km) at night and to increase across the dayside, but values no greater than ∼ Δpp=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 ne = ne(Lppo) × (L/Lppo)−4.5, where Lppo is the outer limit of the plasmapause segment. The model includes the effects of a factor‐of‐order ∼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 5‐10 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 (dusk/dawn difference ΔLppi 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 Earth's rotation. Structured regions of dense plasma of plasmaspheric origin are known to appear in the afternoon‐evening sector at radii larger than those of the “main” plasmasphere. These are believed to be the more extended and/or outlying features of the plasmasphere bulge that have previously been reported; they are not represented by the present model.