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 ΔL...
Abstract. We present the first conjugate observations of subauroral ion drifts (SAID) in the magnetosphere (•9000 km altitude) and ionosphere and coincident measurements by four ionospheric satellites. The parameters measured include ion drifts, energetic precipitating electrons and ions, and the magnetic field perturbations associated with field-aligned currents. Observations indicate that SAID are very coherent features that occur simultaneously over a large magnetic local time (MLT) range, from at least 1600 to 2400 MLT. The equatorward extent of SAID, the ion precipitation, and the region 2 field-aligned currents (FAC) flowing into the ionosphere are all shown to be coincident at all MLT locations where SAID are observed. They also appear to be closely related to the conductivity distribution in the subauroral ionosphere and the midlatitude trough. This is interpreted as an indication that their latitudinal distribution is a consequence of the subauroral conductivity structure and the movement of the plasma sheet ion and electron boundaries. Conjugate measurements at diverse altitudes when mapped along field lines are nearly identical, indicating the absence of significant field-aligned potential drops. Temporally separated SAID measurements in similar MLT regions show a reduction with time in the field-aligned current densities with little reduction in the potential drop across the SAID. We interpret the results as an indication that the magnetosphere acts as a current generator in which large FAC are initially required to support the electric field gradient in a SAID event. Subsequent evolution in the E and F regions produces large conductivity gradients that are in the fight sense to remove the intense FAC requirement but maintain the large subauroral electric fields. The reported potential drops in the subauroral region can be a significant fraction of the total, up to 60 kV or more, and must be taken into account when deriving any magnetospheric convection pattern.
The position of the knee in the density of magnetospheric ionization was measured on a high time‐resolution basis using whistlers recorded during July and part of August 1963. (The knee is an abrupt decrease in magnetospheric ionization density, frequently observed at field lines with an equatorial radius of about 4 RE.) The data were obtained at Eights (64°S dipole latitude) and Byrd (70°S dipole latitude) in the Antarctic. The whistler results and results from other experiments confirm that the knee is a regular feature of the magnetosphere. For conditions of steady, moderate geomagnetic agitation (Kp = 2–4), the diurnal variation in geocentric equatorial range to the knee is remarkably repeatable. It is characterized by (1) a slow inward movement of the knee on the nightside, covering about 1.5 RE in 10 hours; (2) a slight outward movement on the dayside covering about 0.5 RE and (3) a rapid outward shift in the late afternoon covering about 1 RE in 1 hour. During periods of changing magnetic activity, the knee position changes with at most a few hours' delay, moving inward with increasing magnetic activity. The results from Eights and Byrd may be generalized to describe a three‐dimensional model of thermal ionization in the magnetosphere involving a dense (∼100 el/cm3) inner region and a tenuous (∼1 el/cm3) outer region separated by a sharp field‐aligned boundary, the plasmapause. During the postmidnight hours, the inward motion of the knee involves a corresponding inward motion of the ionization just inside the plasmapause. The rapid outward shift of the knee near 1800 LT does not involve an outward plasma motion, but instead involves the presence of a region of ‘new’ high‐density (∼100 el/cm3) plasma in the equatorial range of about 4–5 RE. Preliminary evidence shows that, at least in the period 0000–1700 LT, the ionization inside the plasmapause rotates at approximately the angular velocity of the earth.
Study of a new whistler phenomenon shows that the magnetospheric ionization profile often exhibits a 'knee,' that is, a region at several earth radii in which the ionization density drops rapidly from a relatively normal level to a substantially depressed one. The new whistler phenomenon (called, for convenience, the 'knee whistler') is compared with ordinary whistlers and is illustrated by a number of examples recorded at middle-and high-latitude stations. It is suggested that the knee exists at all times in the magnetosphere, and that its position varies, moving inward with increasing magnetic activity. There are indications that conditions of whistler-mode propagation may be unusually favorable on the low-latitude side of the knee and that the region on the high-latitude side may be favorable for the production of triggered ionospheric noise. It is pointed out that knee whistlers account for a substantial number of the observations of deep density depressions during magnetic storms. Several questions of interpretation are raised, and the direction of future investigations is indicated. II•TRODUCTION Recent research on a new whistler phenomenon suggests that the distribution of ionization
Recent research on the structure and dynamics of the magnetospheric thermal plasma indicates that the vast region above an altitude of ∼1000 km rivals the underlying ionosphere in complexity and that it is coupled to the lower region in complicated, physically important ways. An example involves the relation of the electron content of magnetospheric tubes of ionization to the electron content of the regular ionosphere. Tube volume between ∼1000 km and the magnetic equator varies rapidly over a relatively small range of tube end point latitudes, which gives rise to correspondingly rapid variations with latitude in coupled effects that involve interchange of ionization between the upper and lower regions. In the past, some correlative studies involving the plasmapause have been hindered by lack of information concerning (1) the unsteady nature of the process by which the disturbed‐time plasmapause profile is established and (2) the fact that at most times and at most locations the plasmasphere‐plasmapause system is in a state of recovery. A series of equatorial density profiles is shown to illustrate the reduction of plasmapause radius during brief periods of increased disturbance and the recovery of the plasmasphere by various processes, particularly by filling from the underlying ionosphere. A number of research results are presented as part of a ‘quick‐reference guide’ to the plasmasphere. To the ionospheric observer, the plasmapause should appear to have a complex but generally predictable geometry as well as characteristic motions. A crude predictor of plasmapause L value (Lpp) in the post‐midnight period as a function of magnetic disturbance is the formula Lpp = 5.7‐0.47Kp, where Kp is the maximum 3‐hour Kp value in the preceding 12 hours. A ground station at L ∼ 3.7 is recommended as optimum for observation of plasmapause‐associated effects directly overhead. The plasmasphere is regularly perturbed by substorm‐associated convection electric fields, and these apparently have important effects on the nightside ionosphere at middle latitudes. Other known departures of the plasmasphere from corotation are expected to have their counterparts in the ionosphere.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.