Predicting global average thermospheric temperature changes resulting from auroral heating

Abstract

[1] The total Poynting flux flowing into both polar hemispheres as a function of time, computed with an empirical model, is compared with measurements of neutral densities in the thermosphere at two altitudes obtained from accelerometers on the CHAMP and GRACE satellites. The Jacchia-Bowman 2008 empirical thermospheric density model (JB2008) is used to facilitate the comparison. This model calculates a background level for the "global nighttime minimum exospheric temperature," T c , from solar indices. Corrections to this background level due to auroral heating, DT c , are presently computed from the Dst index. A proxy measurement of this temperature difference, DT c , is obtained by matching the CHAMP and GRACE density measurements with the JB2008 model. Through the use of a differential equation, the DT c correction can be predicted from IMF values. The resulting calculations correlate very well with the orbit-averaged measurements of DT c , and correlate better than the values derived from Dst. Results indicate that the thermosphere cools faster following time periods with greater ionospheric heating. The enhanced cooling is likely due to nitric oxide (NO) that is produced at a higher rate in proportion to the ionospheric heating, and this effect is simulated in the differential equations. As the DT c temperature correction from this model can be used as a direct substitute for the Dst-derived correction that is now used in JB200, it could be possible to predict DT c with greater accuracy and lead time.

Solar cycle dependence of solar wind energy coupling to the thermosphere

“…Higher concentrations of NO in turn lead to faster relaxation times (t T ) during solar maximum storms than during solar minimum storms. In the present study t T was assigned a constant value of 6.5 h, which is close to the solar maximum value determined by Weimer et al [2011] through least squares analyses. This choice did not have the unintended consequence of creating a compensating, solar cycle dependence in a T .…”

“…[30] The present analysis and that of Weimer et al [2011] should be regarded as complementary reflections on the application of equation (2) toward operational modeling of the storm time thermosphere. Where this study concentrated on the solar cycle dependence of driving terms, Weimer and coworkers focused on the relaxation terms.…”

“…[32] A next logical step, that lies beyond the scope of this report, is to synthesize the approaches developed by Weimer et al [2011] and here to quantify energy transferred from the interplanetary medium to the magnetosphere-ionospherethermosphere system. Such a synthesis must go beyond the simple fix of parametrizing changes in t T and possibly t D over the solar cycle.…”

“…Supporting evidence is presented by applying the solar cycle corrected a T /a D term in equation (4) to estimate T ∞ SW during the Halloween Storm of 2003 and by comparing results with GRACE data. Section 4 (1) summarizes our empirically based conclusions, (2) considers the significance of the reported F 10.7a dependence on the efficiency of solar wind energy coupling to the storm time thermosphere and ring current, and (3) comments on similarities/differences with results of Weimer et al [2011].…”

“…[9] Recently, Weimer et al [2011] specified the storm time variability of T C and its relation to the total electro-magnetic power (H(t)) into the ionosphere-thermosphere by analyzing 4 years (2003)(2004)(2005)(2006) of obit-averaged values inferred from measurements of accelerometers on the CHAMP and GRACE satellites. H(t) was obtained by integrating Poynting fluxes, inferred from ACE measurements, over northern and southern high latitudes [Weimer, 2005].…”