Alfvénic Thermospheric Upwelling in a Global Geospace Model

Abstract

Motivated by low-altitude cusp observations of small-scale (~1 km) field-aligned currents (SSFACs) interpreted as ionospheric Alfvén resonator modes, we have investigated the effects of Alfvén wave energy deposition on thermospheric upwelling and the formation of air density enhancements in and near the cusp. Such density enhancements were commonly observed near 400 km altitude by the CHAMP satellite. They are not predicted by empirical thermosphere models, and they are well correlated with the observed SSFACs. A parameterized model for the altitude dependence of the Alfvén wave electric field, constrained by CHAMP data, has been developed and embedded in the Joule heating module of the National Center for Atmospheric Research (NCAR) Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model. The CMIT model was then used to simulate the geospace response to an interplanetary stream interaction region (SIR) that swept past Earth on 26-27 March 2003. CMIT diagnostics for the thermospheric mass density at 400 km altitude show (1) CMIT without Alfvénic Joule heating usually underestimates CHAMP's orbit-average density; inclusion of Alfvénic heating modestly improves CMIT's orbit-average prediction of the density (by a few %), especially during the more active periods of the SIR event. (2) The improvement in CMIT's instantaneous density prediction with Alfvénic heating included is more significant (up to 15%) in the vicinity of the cusp heating region, a feature that the MSIS empirical thermosphere model misses for this event. Thermospheric density changes of 20-30% caused by the cusp-region Alfvénic heating sporadically populate the polar region through the action of corotation and neutral winds.

High‐Resolution Poynting Flux Statistics From the Swarm Mission: How Much Is Being Underestimated at Larger Scales?

“…We thus infer that the unfiltered Swarm measurements at 16 Hz are observing small‐scale fluctuations due to Alfvén waves, driving small‐scale or “AC” Poynting flux (Knudsen et al., 1992). Poynting flux on this scale is further associated with neutral mass density enhancements in the thermospheric cusp region (Billett et al., 2021; Hogan et al., 2020; Lotko & Zhang, 2018).…”

ASHLEY: A New Empirical Model for the High‐Latitude Electron Precipitation and Electric Field

“…The impacts of soft electron precipitations on the I‐T system will be more comprehensively investigated in the future by coupling the ASHLEY model to a GCM. There are also other mechanisms altering the altitudinal Joule heating, such as Alfvén waves incident from the magnetosphere (e.g., Hogan et al., 2020; Lotko & Zhang, 2018; Verkhoglyadova et al., 2018). The relative significance of two different mechanisms to the I‐T system under different conditions will also be an interesting topic that deserves future explorations.…”

Thermospheric Density Perturbations Produced by Traveling Atmospheric Disturbances During August 2005 Storm

“…The neutral density enhancements that are underestimated or missed by MAGE in the high‐latitudes are most likely related to the cusp‐region heating sources that are not yet included in MAGE. These include other sources of precipitation, such as direct entry cusp precipitation and broadband electron precipitation (Zhang et al., 2015), and Alfven wave heating (Hogan et al., 2020), that will be implemented in the MAGE model in due course.…”

“…Extensive modeling work has been performed to improve the description of storm‐time thermospheric heating resulting from the interaction with the magnetosphere, including improved empirical specifications (Weimer, 2005), data assimilation models (Lu et al., 2016), or coupling to physics‐based magnetosphere models (e.g., Connor et al., 2016; Wang et al., 2004). Other attempts to reproduce neutral density perturbations have added additional physics to coupled MIT models such as soft precipitation (Deng et al., 2013; Zhang et al., 2012), and Alfven wave heating (Hogan et al., 2020). These have all demonstrated some success in capturing statistical properties (Deng et al., 2013; Zhang et al., 2012) or longer duration orbit averages (Hogan et al., 2020; Lei et al., 2010) of neutral density enhancements observed by CHAMP.…”

“…Other attempts to reproduce neutral density perturbations have added additional physics to coupled MIT models such as soft precipitation (Deng et al., 2013; Zhang et al., 2012), and Alfven wave heating (Hogan et al., 2020). These have all demonstrated some success in capturing statistical properties (Deng et al., 2013; Zhang et al., 2012) or longer duration orbit averages (Hogan et al., 2020; Lei et al., 2010) of neutral density enhancements observed by CHAMP. While coupled models produce neutral density enhancements more self‐consistently, the transient nature and timescale of TADs and associated perturbations require sufficient model resolution and an accurate representation of the relevant physical processes to resolve the localized generation and global propagation of TADs.…”