The impact of helium on thermosphere mass density response to geomagnetic activity during the recent solar minimum

Abstract

High‐resolution mass density observations inferred from accelerometer measurements on the CHAMP and GRACE satellites are employed to investigate the thermosphere mass density response with latitude and altitude to geomagnetic activity during the recent solar minimum. Coplanar orbital periods in February 2007 and December 2008 revealed the altitude and latitude response in thermosphere mass density for their respective winter hemispheres was influenced by the relative amount of helium and oxygen present. The CHAMP‐to‐GRACE (C/G) mass density ratio depends on two terms; the first proportional to the ratio of the mean molecular weight to temperature and the second proportional to the vertical gradient of the logarithmic mean molecular weight. For the relative levels of helium and oxygen in February 2007, the winter hemisphere C/G mass density response to geomagnetic activity, although similar to the summer hemisphere, was caused predominantly by changes in the vertical gradient of the logarithmic mean molecular weight. In December 2008, the significant presence of helium caused the mean molecular weight changes to exceed temperature changes in the winter hemisphere leading to an increase in the C/G ratio with increasing geomagnetic activity, in opposition to the decrease observed in the summer hemisphere that was caused primarily by temperature changes. The observed behavior is indicative of composition effects influencing the mass density response and the dynamic action of the oxygen to helium transition region in both latitude and altitude will lead to complex behaviors in the mass density at GRACE altitudes throughout the extended solar minimum from 2007 to 2010.

The winter helium bulge revisited

“…The existence of the thermosphere winter helium bulge was later confirmed by directly measuring helium concentrations using satellite-borne mass spectrometers [e.g., Reber et al, 1968;Mauersberger et al, 1976]. The winter helium bulge has been used to explain anomalous behavior in thermospheric mass density [e.g., Keating and Prior, 1968;Thayer et al, 2012;Liu et al, 2014] and as an indicator of thermosphere dynamics [Reber, 1976].…”

“…New estimates of helium from satellite accelerometer data during this past extreme solar minimum [Liu et al, 2014] and helium's impact on thermosphere mass density [Thayer et al, 2012] reveal the importance of properly describing the dynamic change of helium to characterize the mass density state of the upper atmosphere. But the mechanisms responsible for the helium distribution have not been adequately described nor has helium been implemented in modern general circulation models to enable a more complete evaluation.…”

Semi-Empirical, First-Principles, and Hybrid Modeling of the Thermosphere to Enhance Data Assimilation

“…These data sets now comprise the majority of our understanding of thermospheric composition, the empirical basis of which is embodied by the Mass Spectrometer and Incoherent Scatter (MSIS) series of models [13,14,48], successors of the Ogo 6 model. More recently, Thayer et al [74] inferred strong signatures of helium from differences in total mass densities measured at two different altitudes by high-precision accelerometers on board the Challenging Mini-Satellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites [70]. Liu et al [34] extended this work, showing that the response of the mass density vertical profile during a geomagnetic disturbance is quite sensitive to the atomic oxygen/helium transition height.…”

A model of high‐latitude thermospheric density

“…This choice was made as a compromise between including as much data as possible and eliminating measurements in which the CHAMP altitude was too low compared to the target height of 450 km. Besides, studies showed that the MSISE-00 model was not accurate during the extreme solar minimum of 2008-2009 owing to an unexpectedly large amount of helium [Thayer et al, 2012;Liu et al, 2014a], which would affect our height corrections based on MSISE-00.…”

“…Thus, although we do not have composition data for the present study, the change in the composition is expected to add complexity to the thermospheric response to solar wind forcing described in the following section. (See also Thayer et al [2012] for the composition effect on the total mass density at CHAMP and GRACE altitudes. )…”