Photoelectron fluxes observed by FAST compared with model predictions incorporating SNOE observations of the solar soft X-ray irradiance

Details Photoelectron fluxes observed by FAST compared with model predictions incorporating SNOE observations of the solar soft X-ray irradiance Photoelectron fluxes observed by FAST compared with model predictions incorporating SNOE observations of the solar soft X-ray irradiance

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsa32a0675b&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #SA32A-0675

Physics

0342 Middle Atmosphere: Energy Deposition, 0358 Thermosphere: Energy Deposition, 2427 Ionosphere/Atmosphere Interactions (0335), 7538 Solar Irradiance

Scientific paper

Photoelectrons are those electrons produced when atoms or molecules in the upper atmosphere are photoionized. These electrons carry the excess energy of the photon remaining from the ionization and can have energies up to and greater than 1 keV. Photoelectrons are important in that they play a significant role in the energetics of the upper atmosphere, resulting in ionization, dissociation, and excitation of atoms and molecules. There have been long standing issues with regard to understanding the magnitude of the terrestrial photoelectron flux as models have not been able to reproduce the observations without scaling the solar soft X-ray irradiance by factors of two to four. The Fast Auroral Snapshot (FAST) spacecraft was launched in August of 1996. While its primary goals focus on the study of auroral energetic particles, in January of 1999 it began making low-latitude observations. From measurements by the FAST energetic electron sensor, upward flowing photoelectron fluxes in the energy range of 50 eV to 1 keV have been obtained. These measurements are in agreement with earlier measurements of the terrestrial photoelectron flux. The Student Nitric Oxide Explorer (SNOE) spacecraft was launched in February of 1998. Since then it has been making daily observations of the solar soft X-ray irradiance in bandpasses of 2 - 7, 6 - 19, and 17 - 20 nm. SNOE observes larger values of the solar soft X-ray irradiance than reported by earlier observations or predicted by empirical models; however, the SNOE observations are in agreement with many suggestions of the solar soft X-ray irradiance obtained from geophysical observations such as airglow and electron densities. These irradiance measurements are used in a photoelectron model that includes transport. Observations of photoelectron fluxes for the first solar rotation of 1999 are modeled. The model photoelectron spectra are in good agreement with the observed photoelectron spectra over most of the 50 eV to 1 keV energy range. The agreement is within the combined uncertainties of the SNOE and FAST instruments. It is concluded that the SNOE observations of the solar soft X-ray irradiance provide a resolution to the longstanding issue of understanding the magnitude of the terrestrial photoelectron flux.

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