European Space Weather Week 2025

Validating solar soft x-ray irradiance spectra with photoelectron measurements using MAVEN at Mars

Not scheduled

20m

Mon 27/10, Tue 28/10, Wed 29/10: Idun; Thu 30/10: Tonsalen

Poster SWR2 - Interdisciplinary Insights into Space Weather Events of Solar Cycle 25: From Solar Origins to Planetary Impacts SWR2 – Interdisciplinary Insights into Space Weather Events of Solar Cycle 25: From Solar Origins to Planetary Impacts

Speaker

Ed Thiemann (LASP, University of Colorado)

Description

Solar soft x-ray irradiance has been historically difficult to measure and “accepted” irradiance levels have been revised at least twice since initial values were published by Hinteregger et al. [1981] using Atmospheric Explorer E (AE-E) data. The first major revision was proposed by Solomon et al. [2001], who using soft x-ray irradiance measurements from the Student Nitric Oxide Explorer (SNOE), concluded that soft x-ray irradiance must be increased by a factor of four above the Hinteregger et al. [1981] values. A key element of the analysis of Solomon et al. [2001] was ruling out absolute calibration differences by comparing measured photoelectron spectra with that from a model using both the existing and proposed levels of soft x-ray irradiance. That situation largely remains the same today for Earth but a significant number of photoelectron measurements have been made at Mars over the past two decades.
We present initial results of an ongoing study that uses relatively recent measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter at Mars, which is instrumented to measure both solar soft x-ray irradiance, with the Extreme Ultraviolet Monitor (EUVM), and atmospheric photoelectrons, with the Solar Wind Electron Analyzer (SWEA). MAVEN EUVM broadband measurements at 0-7 nm and 17-22 nm are used to constrain the Synthetic Reference Spectrum (SynRef) Model physics-based spectral irradiance model, which is input to the Super-Thermal Electron Transport (STET) model of photoelectron flux. The STET modeled photoelectrons are compared with SWEA photoelectron measurements to better constrain the absolute soft x-ray irradiance levels. The analysis includes a range of solar activity, spanning the declining phase of the last solar cycle, from late 2014 through late 2018. While this analysis uses measurements made at Mars, the resulting improved understanding of absolute soft x-ray irradiance fluxes is applicable to space weather at Earth and throughout the solar system.

Hinteregger, H. E., Fukui, K., & Gilson, B. R. (1981). Observational, reference and model data on solar EUV, from measurements on AE‐E. Geophysical Research Letters, 8(11), 1147-1150.
Solomon, S. C., Bailey, S. M., & Woods, T. N. (2001). Effect of solar soft X‐rays on the lower ionosphere. Geophysical research letters, 28(11), 2149-2152.