Thermospheric density in the Earth's magnetic cusp as observed by the Streak mission

Abstract

[1] Measurements of neutral gas density in the thermosphere at the base of the Earth's south magnetic cusp from the Streak mission are reported and discussed. In contrast to recent reports of enhanced density in the cusp, these measurements show the density to be depleted relative to the surrounding region. The difference is interpreted as an altitude effect. This observation and interpretation lead to new constraints on the physical mechanisms that could explain cusp upwelling and are inconsistent with a Joule heating mechanism. A mechanism based on heating by soft cusp particle precipitation is put forth, and model calculations are used to show how it explains the relative depletions observed by Streak as well as the enhancements at higher altitudes reported previously. Citation: Clemmons,

Investigation of energy transport and thermospheric upwelling during quiet magnetospheric and ionospheric conditions from the studies of low- and middle-altitude cusp

“…The quantitative analysis for all three events presented in Table 1 indicate that stronger soft electron precipitation is accompanied by stronger density enhancements in the thermosphere, which further strengthens the importance of soft electron precipitation in thermospheric upwellings. Further arguments that support this can be found in Clemmons et al (2008) and Sadler et al (2012). Clemmons et al (2008) has explained density enhancements as a result of direct energy transfer from soft electrons to neutrals.…”

“…Further arguments that support this can be found in Clemmons et al (2008) and Sadler et al (2012). Clemmons et al (2008) has explained density enhancements as a result of direct energy transfer from soft electrons to neutrals. On the other hand, Sadler et al (2012) suggested that density enhancements could be explained as a combination of soft electrons which heat electrons in the thermosphere, and cause ion outflow.…”

“…Further, Joule heating rate is roughly 1 order of magnitude smaller than needed to explain strong upwelling events. For those, a combination of Joule heating and soft particle energy flux was important, according to simulations (Crowley et al (1996), Clemmons et al (2008), Deng et al (2013), etc.). In this study, Cluster has observed soft particle energy flux at about 2 Re.…”

“…Rentz and Lühr (2008) had emphasized the role of Joule heating, while Clemmons et al (2008) as well as Sadler et al (2012) discarded it. They rather advocated soft particle precipitation as the cause of upwelling, which would directly heat the neutral atmosphere, and, according to Sadler et al (2012), also trigger ion upflow with an additional drag effect on the neutrals.…”

Polar cap ionosphere and thermosphere during the solar minimum period: EISCAT Svalbard radar observations and GCM simulations

“…During the last decade, the variations, dynamical features, temperature and density structures in the polar cap and/or cusp region of the thermosphere/ionosphere have been clarified from radar and satellite observations (e.g., Innis and Conde, 2001;Maeda et al, 2002;Lühr et al, 2004;Liu et al, 2005;Fujiwara et al, 2007;Clemmons et al, 2008;Maeda et al, 2009, and references therein). In addition to recent works, Moe and Moe (2008) have reviewed historical studies on the high-latitude thermospheric mass density anomaly.…”