Rosetta photoelectron emission and solar ultraviolet flux at comet 67P

TL;DR

This study uses Rosetta's Langmuir probe data to monitor solar ultraviolet flux at comet 67P, revealing variations and potential dust-related scattering effects affecting photoelectron emission.

Contribution

It introduces three methods to extract photoelectron saturation current from Langmuir probe data, providing a new index for solar UV flux at the comet.

Findings

Good correlation with MAVEN/EUVM and TIMED/SEE data at large heliocentric distances

Up to 50% decrease in photoelectron current near perihelion

Possible scattering and absorption by nanograins affecting measurements

Abstract

The Langmuir Probe instrument on Rosetta monitored the photoelectron emission cur- rent of the probes during the Rosetta mission at comet 67P/Churyumov-Gerasimenko, in essence acting as a photodiode monitoring the solar ultraviolet radiation at wave- lengths below 250 nm. We have used three methods of extracting the photoelectron saturation current from the Langmuir probe measurements. The resulting dataset can be used as an index of the solar far and extreme ultraviolet at the Rosetta spacecraft position, including flares, in wavelengths that are important for photoionisation of the cometary neutral gas. Comparing the photoemission current to data measurements by MAVEN/EUVM and TIMED/SEE, we find good correlation when 67P was at large heliocentric distances early and late in the mission, but up to 50 percent decrease of the expected photoelectron current at perihelion. We discuss possible reasons for the photoemission decrease, including scattering and absorption by nanograins created by disintegration of cometary dust far away from the nucleus.