The Role of Solar Soft X-rays Irradiance in Thermospheric Structure

TL;DR

This study demonstrates that solar soft X-ray irradiance significantly influences the Earth's thermospheric temperature, with uncertainties in flux measurements leading to notable variations in model predictions, highlighting the importance of accurate solar flux data.

Contribution

The paper introduces a new ACE1D thermospheric model and quantifies the impact of soft X-ray solar flux on thermospheric temperature, emphasizing the role of non-thermal photoelectrons in heat transfer.

Findings

Soft X-ray energies affect thermospheric temperatures up to 300 km altitude.

Non-thermal photoelectrons are the main heat transfer mechanism.

Uncertainties in solar flux lead to increased temperature prediction errors.

Abstract

We use a new Atmospheric Chemistry and Energetics one-dimensional (ACE1D) thermospheric model to show that the energies deposited by the solar soft x-rays in the lower thermosphere at altitudes between 100 -150 km (Bailey et al. 2000), affects the temperature of the entire Earth's thermosphere even at altitudes well above 300 km. By turning off the input solar flux in the different wavelength bins of the model iteratively, we are able to demonstrate that the maximum change in exospheric temperature is due to the changes in the soft x-ray solar bins. We also show, using the thermodynamic heat equation, that the molecular diffusion via non-thermal photoelectrons, is the main source of heat transfer to the upper ionosphere/thermosphere and results in the increase of the temperature of the neutral atmosphere. Moreover, these temperature change and heating effects of the solar soft x-rays are comparable to that of the strong HeII 30.4nm emission. Lastly, we show that the uncertainties in the solar flux irradiance at these soft x-rays wavelengths result in corresponding uncertainties in modeled exospheric temperature and the uncertainties increase with increased solar activity.