A new method for studying the thermospheric density variability derived from CHAMP/STAR accelerometer data for magnetically active conditions

Abstract

Abstract. Thermospheric densities deduced from STAR accelerometer measurements onboard the CHAMP satellite are used to characterize the thermosphere and its response to space weather events. The STAR thermospheric density estimates are analysed using a Singular Value Decomposition (SVD) approach allowing one to decouple large scale spatial and temporal variations from fast and local transients. Because SVD achieves such decomposition by using the reproducibility of orbital variations, it provides more meaningful results than any method based upon data smoothing or filtering.SVD analysis enables us to propose a new thermosphere proxy, based on the projection coefficient of the CHAMP densities on the first singular vector. The large scale spatial variations in the density, mostly related to altitude/latitude variations are captured by the first singular vector; time variations are captured by the associated projection coefficient.The study presented here is focused on time dependent global scale variations in the thermospheric density between 50 N and 50 S geographic latitudes. We show that the time variations in the projection coefficient do in fact represent those in the global density that are associated with magnetic activity as well as with solar EUV radiations. We also show that the NRLMSISE-00 empirical model better accounts for the density forcing by Solar radiations when tuned using Mg II indices. Using the so modified model with an additional geomagnetic parameterization corresponding to quiet geomagnetic situation enables one to define time reference values which are then used to evaluate the impact of geomagnetic activity. The ratio of CHAMP density projection coefficient to the quiet model projection coefficient is a global Correspondence to: M. Menvielle (michel.menvielle@cetp.ipsl.fr) quantity, independent of altitude and latitude, which quantifies the thermospheric density response to auroral energy deposition. It will serve as a proxy of the response of thermospheric density to geomagnetic activity forcing.

Thermospheric mass density: A review

“…See also the review by Prölss (2011). Menvielle et al (2007) decomposed CHAMP density measurements into large scale spatial and temporal variations along the orbit (related mainly to repeatable latitude/altitude variations) versus fast and local transients. They proposed that the latter component could be used as a proxy for the response of the thermosphere to geomagnetic activity forcing.…”

Climatology of the cusp-related thermospheric mass density anomaly, as derived from CHAMP observations

“…It gained attention primary during recent years (e.g. Bruinsma et al, 2004;Liu et al, 2005;Sutton et al, 2005;Menvielle et al, 2007).…”

A statistical study of the observed and modeled global thermosphere response to magnetic activity at middle and low latitudes

“…The method proposed by Menvielle et al [2007] is based on a Singular Value Decomposition analysis (SVD) of CHAMP data on the one hand, and on a specific run of the NRLMSISE‐00 thermosphere model [ Picone et al , 2002] on the other hand. In this method, the analysis is limited to middle and low latitudes between 50°N and 50°S; the error in the observed densities due to residual winds (HWM93 [ Hedin et al , 1996] was used in the density derivation) is less than 10% and varies little with magnetic activity [ Sutton et al , 2007].…”

“…These features strongly suggest that Singular Value Decomposition (SVD) is well suited to extract the altitude/latitude/LT reference profile from 24 h of data. Indeed, Menvielle et al [2007] showed that such SVD analysis of orbit segments between 50°N and 50°S makes it possible to account for most of the density variation observed along one orbit segment in terms of a normalized profile (the first singular vector) multiplied by one projection coefficient, which has the dimension of a density. The latitude/altitude/LT dependence of large‐scale density variations is taken into account by the normalized profile while the UT time dependence of the global thermosphere density behavior is taken into account by the projection coefficient.…”

“… Menvielle et al [2007] also showed that the NRLMSISE‐00 thermosphere model can be used for obtaining a reference density when run with MgII indices [ Viereck et al , 2004], rescaled to F10.7 units, as a proxy of solar EUV radiations and a quiet magnetic activity diurnal Ap index equal to 4. They used this reference density to derive a thermosphere density disturbance coefficient, defined as the ratio of the CHAMP data projection coefficient to the projection coefficient calculated from the NRLMSISE‐00 reference model.…”