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

Abstract

Abstract. We investigate energy fluxes and small, kilometrescale Birkeland currents in the magnetospheric cusp at a 1-3 Earth radii altitude and in the ionosphere using satellites when they were, according to the Tsyganenko model, in magnetic conjunction within 50-60 km and up to 15 min apart. We use Cluster and CHAMP satellites, and study three conjunction events that occurred in 2008 and 2009, when the Cluster spacecraft were crossing the cusps at only a few Earth radii altitude. Our goal is to understand better the influence of processes in the magnetospheric cusp on the upper thermosphere and its upwelling which was usually observed by the CHAMP satellite passing the cusp. Three studied events occurred under relatively quiet and steady magnetospheric and ionospheric conditions, which explains why observed thermospheric density enhancements were rather low. Our findings point out that for each studied event soft electron precipitation influences thermospheric density enhancements in a way that stronger electron precipitation produces stronger thermospheric upwelling. Therefore, in the case of these weak events, soft electron precipitation seems to be more important cause of the observed, thermospheric density enhancements than is the Joule heating.

Two‐Dimensional Local Modeling of Thermospheric Heating and Neutral Mass Density Enhancement Driven by Alfvén Waves

“…The following conclusions are obtained. A fractional mass density enhancement becomes evident at altitudes centered between 350 and 400 km roughly 1 hr after the Alfvén resonator modes are set up, although a very modest enhancement appears before that time. The cell of the mass density enhancement continues to exist stably for 2 more hours. The Alfvén resonator mode having a field‐aligned current of 20 $$\mu \mathrm{A}\,{\mathrm{m}}^{-2}$$, which corresponds to Alfvénic Poynting flux of ∼20 mW m −2 , can create a fractional mass density enhancement of slightly more than 30% at 400 km altitude, which is consistent with the result obtained from satellite observations (Kervalishvili & Lühr, 2013; Živković et al., 2015). Comparison between the 300–450 km altitude and 450–650 km altitude mass density enhancements at the time of 3 hours after the Alfvén resonator modes are set up shows that the lower‐altitude one becomes stronger than the higher‐altitude one when the specific heating rate at 300 km altitude exceeds roughly 300 W kg −1 .…”

“…𝐴𝐴 𝐴𝐴A m −2 , which corresponds to Alfvénic Poynting flux of ∼20 mW m −2 , can create a fractional mass density enhancement of slightly more than 30% at 400 km altitude, which is consistent with the result obtained from satellite observations (Kervalishvili & Lühr, 2013;Živković et al, 2015). 3.…”

“…Živković et al. (2015) have analyzed three conjugate observation events by CHAMP and Cluster during geomagnetically quiet conditions in the cusp. The measured mass density enhancements, Poynting flux, and FACs were 20%, $$6.50\,\mathrm{m}\mathrm{W}\,{\mathrm{m}}^{-2}$$, and $$18\,\mu \mathrm{A}\,{\mathrm{m}}^{-2}$$ in the first event, 17%, $$12.25\,\mathrm{m}\,\mathrm{W}{\mathrm{m}}^{-2}$$, and $$5.48\,\mu \mathrm{A}\,{\mathrm{m}}^{-2}$$ in the second event, and 11%, $$9.8\,\mathrm{m}\mathrm{W}\,{\mathrm{m}}^{-2}$$, and $$8.85\,\mu \,\mathrm{A}{\mathrm{m}}^{-2}$$ in the last event, respectively.…”

Alfvénic Thermospheric Upwelling in a Global Geospace Model

“…About 40% of CHAMP satellite orbits exhibit such anomalies (Kervalishvili & Lühr, 2014). Soft electron precipitation, also referred to as broadband precipitation with energies of 100s eV (Živković et al, 2015), and intense, small‐scale field‐aligned currents (SSFACs) (Kervalishvili & Lühr, 2013; Lühr et al, 2004) are well correlated with the anomalies in the cusp region. Enhanced Joule heating by quasistatic electric fields may contribute to the thermospheric upwelling that produces density anomalies, but observations (Schlegel et al, 2005) and models (Deng et al, 2013; Zhang et al, 2012) indicate that this mechanism alone is insufficient.…”