Authors: H. A. Farooki, M. E. Cuesta, R. Bandyopadhyay, L. Y. Khoo , G. Livadiotis, D. J. McComas, J. S. Rankin, M. M. Shen, and J. R. Szalay

Solar energetic particles (SEPs) have been demonstrated to follow a kappa distribution distinct from the solar wind. These findings are based on a method that tests whether there is a kappa distribution behind observed SEP power law spectra and yields thermodynamic properties (density, temperature, and kappa) of the SEP population. Previously, this method was applied to a single SEP event using only measurements from IS⊙IS/HET on board the Parker Solar Probe (PSP). In this study, we examined the thermodynamic properties derived through this method applied to PSP data from 2018–2024. A surprising result is that despite both having small local fluctuations, the temperature is almost always close to ∼1 MeV, whereas the density spans several orders of magnitude. To validate this finding, we use an alternative method by directly fitting kappa distributions to measurements over energies ranging from 3–50 MeV using both LET and HET. This allows us to use energies sufficiently close to the typical temperature of ∼1 MeV. We also applied both methods to measurements from STEREO-A. All resulted in the same conclusion: the temperature of the SEP population is almost always close to ∼1 MeV.