Recent Research on the Magnetospheric Plasmapause

The plasmapause is a three-dimensional field aligned boundary that divides the closed field-line portion of the earth's magnetosphere into two physically distinct regions. The boundary is asymmetric, usually exhibiting a minimum geocentric range near dawn and a maximum near dusk under conditions of moderate but steady geomagnetic agitation (Kp = 2 − 4). The mean equatorial radius of the plasmapause is typically about 4RE, but it may vary from about 5.5RE during periods of extreme quiet to the range 2 − 3RE during great storms. The approximately corotating thermal plasma within the boundary exhibits two types of radial drift motions. These may be visualized as: (a) slow “breathing“ motions that follow the radial variations in a fixed, asymmetric boundary; (b) more rapid, transient (1 − 2 hr) motions that occur when the boundary position varies, as is the case during a polar substorm. The plasmapause involves an abrupt change in electron density, in tube electron content above 1000 km, and possibly in plasma bulk velocity and mean thermal energy. To the ionosphere, the protonosphere inside the plasmapause appears as a large reservoir of thermal protons, while the region outside appears virtually empty. At the plasmapause, equatorial values of electron density change by a factor of 10 to 100 within less than 0.15 earth radii. Satellite VLF experiments suggest that the change may be far more abrupt than this, possibly on a scale of a few kilometers. Studies of the distribution of electron density along the field line in the plasmapause have shown that earlier empirical models of the type N ∝ R−3 are not, in fact, compatible with recent satellite data on topside electron concentrations. Instead, the theoretically palatable diffusive equilibrium model has been found to be an appropriate description for most of the plasmasphere, while a more rapidly varying model appears necessary to describe the tenuous outer region. Details of the latter distribution may vary in important ways as a function of local time. Many wave propagation phenomena of conjugate interest are strongly affected by the presence of the plasmapause. For example, satellites moving poleward through the boundary observe a cutoff in whistlers propagating from the conjugate hemisphere; a decrease in the intensity of fixed-frequency VLF signals propagating upward, and dramatic changes in VLF noise such as the lower hybrid resonance (LHR) noise. In ground recordings made at Eights (L ∼ 4) and Byrd (L ∼ 7) in the austral winter, four distinct magnetospheric regions of propagation may be identified: (I) the outer part of the plasmasphere; (II) the outer “surface” of the plasmapause; (Ill) a belt-like region extending 1−2 RE outward from the near vicinity of the plasmapause; (IV) a region beginning ∼ 1.5 RE beyond the plasmapause and extending several earth radii outward. Each region exhibits special properties with respect to the occurrence and spectral behavior of VLF noise, and, in particular, noise triggered by whistler components. The occurrence of one-hop whistlers propagating in the outer regions III and IV is relatively low, and is particularly low near midnight and in region III at all times except in the midafternoon. Whistlers propagating in regions I, III, and IV exhibit an abrupt half-gyrofrequency upper cutoff, whereas whistlers propagating in region II do not. The ground data on the occurrence of whistlers and noise are broadly consistent with recent surveys of magnetospheric noise carried out on satellites.