Recent Research on the Magnetospheric Plasmapause

Abstract

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 ...

Magnetospheric Electrons

“…During magnetic storms lower energy (<1.6 MeV) electron fluxes are greatly enhanced (23,30,31,77) and move in unison with the storm-time plasmapause to lower L-values (64). This behavior is consistent with impulsive convective injection to the vicinity of the compressed plasmapause (L ^ 3 to 4) (11,15); thus convection provides the outer-zone source for radial diffusive transport into the slot and inner-zone. Also evident in Figure 8 is the rapid depletion of >1.6 MeV electrons at L = 3 to 5 during the early phase of a magnetic storm (28,29,67).…”

The Midlatitude Trough and the Plasmapause in the Nighttime Ionosphere Simultaneously Observed by DEMETER During 2006–2009

“…The guidance that occurs is a form of gradient trapping of VLF wave energy, and guiding is possible at both the inner and outer edges of the plasmapause. Therefore, the plasmapause position P wh is identified by 0 (Carpenter et al, ) or the minimum of C w in the midlatitude ionosphere of the two hemispheres (i.e., the whistler cutoff in Carpenter, ) The selected orbit reveals that P wh appears at 56.76° geomagnetic in the Northern Hemisphere ( NP wh ), which is on the equatorward side of the detected NT N e and NT T e , and however −59.13° geomagnetic in the Southern Hemisphere ( SP wh ) almost the same position of the detected ST N e and ST T e (Figure e). It can be seen in the daytime that the T N e and T T e positions are difficult to identify, while the P wh position is still detectable, which appears at 60.92° and −58.99° in the Northern and Southern Hemispheres, respectively (Figures b, d, and f).…”

Equilibrium structure of equatorially mirroring radiation belt protons

“…Here (p) is the weighted average of tree and bound electrons in the radiation belts and is given by p = Prree + 0.08pbounU [e.g., Cornwall, 1972]. The number density of free thermal electrons Prree is taken from the model Prree = 13(4.1/L) 4'64 L > 4.1 10tree--250(4.1/L) 4'64 L _< 4.1 which is also due to Cornwall [1972] and is based on data given by Carpenter [1968]. The model for the corresponding number density of bound electrons is, of course, the same as the exospheric neutral hydrogen model: lobound = [H].…”