Simulation of ASTROD I test mass charging due to solar energetic particles

TL;DR

This paper uses GEANT4 simulations to estimate test mass charging from solar energetic particles for ASTROD I, revealing that large SEP events can cause acceleration noise exceeding mission requirements, suggesting the need for continuous discharge.

Contribution

It provides the first detailed simulation of solar energetic particle charging effects on ASTROD I using GEANT4, highlighting potential noise issues during large SEP events.

Findings

Charging rates during SEP events can reach up to 64,300 e/s.

Large SEP events can cause acceleration noise exceeding ASTROD I's noise budget.

Continuous discharge is recommended to mitigate charging noise.

Abstract

As ASTROD I travels through space, its test mass will accrue charge due to galactic cosmic-rays and solar energetic particles incident on the spacecraft. This test mass charge will result in Coulomb forces between the test mass and the surrounding electrodes. In earlier work using the GEANT4 toolkit, we predicted a net charging rate of nearly 9.0 +e/s from cosmic-ray protons between 0.1 and 1000 GeV at solar maximum, and rising to 26.5 +e/s at solar minimum. Here we use GEANT4 to simulate the charging process due to solar energetic particle events and to estimate the magnitude of acceleration noise due to this charging. The predicted charging rates range from 2840 to 64300 +e/s, at peak intensity, for the 4 largest SEP events in September and October 1989. For the 2 larger events, the acceleration disturbances due to charging exceeds the ASTROD I acceleration noise budget requirement. Continuous discharge should be considered for suppressing this charging noise. The acceleration noise during the 2 small events is well below the design target, although during these events, the net charging rate will be dominated by these solar fluxes.