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We analyzed the measured thermospheric response of an extreme solar X17.2 flare that irradiated the Earth's upper atmosphere during the so-called Halloween events in late October/early November 2003. We suggest that such events can serve as proxies for the intense electromagnetic and corpuscular radiation environment of the Sun or other stars during their early phases of evolution. We applied and compared empirical thermosphere models with satellite drag measurements from the GRACE satellites and found that the Jacchia-Bowman 2008 model can reproduce the drag measurements very well during undisturbed solar conditions but gets worse during extreme solar events. By analyzing the peak of the X17.2 flare spectra and comparing it with spectra of young solar proxies, our results indicate that the peak flare radiation flux corresponds to a hypothetical Sun-like star or the Sun at the age of approximately 2.3 Gyr. This implies that the peak extreme ultraviolet (EUV) radiation is enhanced by a factor of about 2.5 times compared to today's Sun. On the assumption that the Sun emitted an EUV flux of that magnitude and by modifying the activity indices in the Jacchia-Bowman 2008 model, we obtain an average exobase temperature of 1950 K, which corresponds with previous theoretical studies related to thermospheric heating and expansion caused by the solar EUV flux

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