Smooth electron density transition from plasmasphere to the subauroral region

Abstract

[1] Upper hybrid resonance (UHR) noise band and the nonthermal continuum (NTC) radiation were routinely observed by the radio plasma imager (RPI) onboard the IMAGE satellite when it transited the plasmasphere and subauroral regions. The lower cutoff frequencies of the UHR band and the NTC radiation provide an estimate of the electron plasma frequencies along the satellite orbit. A steep electron density gradient, which defines the plasmapause, was commonly observed when IMAGE transited from the plasmasphere to the subauroral region or vice versa. It appears however that, on many occasions, the electron density transition from the plasmasphere to the subauroral region is smooth without a clear signature of the plasmapause. Such smooth transitions can occur at various magnetic local times. The events presented in this study were observed after geomagnetic activities had been quiet for 2 or more days with Kp primarily less than 3. The smooth density transitions from the plasmasphere to the subauroral region may be an observational evidence of the suggested plasmaspheric wind.

Refilling process in the plasmasphere: a 3-D statistical characterization based on Cluster density observations

“…The density transition at the PBL is not always observed (Tu et al, 2007;Darrouzet et al, 2009b). Functional forms developed in the past infer the PBL location at the equator only and have the disadvantage of not taking into account the individual history of the plasmasphere (Carpenter and Anderson, 1992;Larsen et al, 2007).…”

“…Such a smooth transition is possible if the co-rotation dominates over the convection at large radial distance. Tu et al (2007) presented some cases of extended plasmasphere having smooth density transition for weak geomagnetic activity (K p < 3) during two or more days. They suggest that this is indirect observational evidence of the plasmaspheric wind.…”

“…Conversely, knowledge of the electron density n e in the plasmasphere and its outer edge is necessary to study how dense cold plasma can affect whistler-mode waves that scatter radiation belt electrons (3 < L < 7). Many studies demonstrated that the presence of ElectroMagnetic Ion-Cyclotron (EMIC) waves (Summers and Thorne, 2003;Spasojević et al, 2004;Borovsky and Steinberg, 2006;Usanova et al, 2010), plasmaspheric hiss (Lam et al, 2007) and whistler-mode chorus (Summers et al, 2004) at the PBL and within the drainage plumes can pitch-angle scatter relativistic electrons during storms (at different time scales, depending on the waves). Borovsky and Denton (2009) show clear evidence that buildup of the outer plasmasphere plays an important role in the decay of the radiation belt electrons (∼ 150 keV) during intervals of extreme geomagnetic calm.…”

“…Little attention has been paid to model such a smooth density transition at the PBL. Actually, ignoring the density distribution along depleted flux tubes is a real difficulty when one wishes to undertake and validate simulations of plasmasphere filling/refilling (Tu et al, , 2006(Tu et al, , 2007.…”

Detection of a plasmaspheric wind in the Earth's magnetosphere by the Cluster spacecraft

“…Indirect evidence suggesting the presence of a plasmaspheric wind has been provided in the past from the plasma refilling timing, indicating a continuous plasma leak from the plasmasphere (Lemaire and Shunk, 1992; al., 2003), and from the smooth density transitions from the plasmasphere to the subauroral region, observed during quiet conditions and at various magnetic local times (Tu et al, 2007). Direct detection of this wind has, however, eluded observation in the past.…”