Solar flares as proxy for the young Sun: satellite observed thermosphere response to an X17.2 flare of Earth's upper atmosphere

摘要

We analyzed the measured thermospheric response of an extremesolar X17.2 flare that irradiated the Earth's upper atmosphere during theso-called Halloween events in late October/early November 2003. We suggestthat such events can serve as proxies for the intense electromagnetic andcorpuscular radiation environment of the Sun or other stars during theirearly phases of evolution. We applied and compared empirical thermospheremodels with satellite drag measurements from the GRACE satellites and foundthat the Jacchia-Bowman 2008 model can reproduce the drag measurements verywell during undisturbed solar conditions but gets worse during extremesolar events. By analyzing the peak of the X17.2 flare spectra and comparingit with spectra of young solar proxies, our results indicate that the peakflare radiation flux corresponds to a hypothetical Sun-like star or the Sunat the age of approximately 2.3 Gyr. This implies that the peak extreme ultraviolet (EUV) radiationis enhanced by a factor of about 2.5 times compared to today's Sun. On theassumption that the Sun emitted an EUV flux of that magnitude and by modifyingthe activity indices in the Jacchia-Bowman 2008 model, we obtain an averageexobase temperature of 1950 K, which corresponds with previous theoreticalstudies related to thermospheric heating and expansion caused by the solarEUV flux.