Galactic Cosmic Rays at Mars and Venus: Temporal Variations from Hours to Decades Measured as the Background Signal of Onboard Micro-Channel Plates

TL;DR

This study analyzes 17 years of MCP background counts from Mars and Venus orbiters to investigate galactic cosmic ray variations, revealing correlations with solar activity, planetary absorption effects, and insights into GCR characteristics in the inner solar system.

Contribution

It demonstrates that MCP background counts can effectively measure GCR variations over long timescales and reveals planetary absorption effects and a lag in GCR response to solar activity.

Findings

MCP background counts correlate inversely with sunspot numbers.

GCR absorption by Mars extends beyond its solid surface, indicating a larger effective absorption radius.

GCR variations at Mars show a ~9 months lag relative to solar activity.

Abstract

A Micro-Channel Plate (MCP) is a widely used component for counting particles in space. Using the background counts of MCPs on Mars Express and Venus Express orbiters operated over 17 years and 8 years, respectively, we investigate the galactic cosmic ray (GCR) characteristics in the inner solar system. The MCP background counts at Mars and Venus on a solar cycle time scale exhibit clear anti-correlation to the sunspot number. We conclude that the measured MCP background contain the GCR information. The GCR characteristics measured using the MCP background at Mars show features that are consistent with the ground-based measurement in solar cycle 24. The time lag between the sunspot number and the MCP background at Mars is found ~9 months. The shorter-term background data recorded along the orbits (with a time scale of several hours) also show evident depletion of the background counts due to the absorption of the GCR particles by the planets. Thanks to the visible planetary size change along an orbit, the GCR contribution to the MCP background can be separated from the internal contribution due to the \b{eta}-decay. Our statistical analysis of the GCR absorption signatures at Mars implies that the effective absorption size of Mars for the GCR particles have a >100 km larger radius than the solid Martian body.