Recurrent galactic cosmic-ray flux modulation in L1 and geomagnetic activity during the declining phase of the solar cycle 24

TL;DR

This study analyzes short-term galactic cosmic-ray flux variations during the declining phase of solar cycle 24, using data from the ESA LISA Pathfinder spacecraft, and explores their relation to solar wind and geomagnetic activity.

Contribution

It provides new insights into the characteristics of recurrent GCR flux modulations at L1 and their dependence on interplanetary plasma parameters during solar cycle 24.

Findings

Recurrent GCR flux variations correlate with high-speed solar wind streams.

GCR flux modulations depend on interplanetary magnetic field and plasma conditions.

Recurrent GCR variations serve as proxies for geomagnetic activity.

Abstract

Galactic cosmic-ray (GCR) flux short-term variations ($<$1 month) in the inner heliosphere are mainly associated with the passage of high-speed solar wind streams (HSS) and interplanetary (IP) counterparts of coronal mass ejections (ICMEs). Data gathered with a particle detector flown on board the ESA LISA Pathfinder (LPF) spacecraft, during the declining part of the solar cycle 24 (February 2016 - July 2017) around the Lagrange point L1, have allowed to study the characteristics of recurrent cosmic-ray flux modulations above 70 MeV n$^{-1}$. %These modulations are observed when the solar wind speed is $>$ 400 km s$^{-1}$ and/or the IP magnetic field intensity $>$ 10 nT. It is shown that the amplitude and evolution of individual modulations depend in a unique way on both IP plasma parameters and particle flux intensity before HSS and ICMEs transit. By comparing the LPF data with those gathered contemporaneously with the magnetic spectrometer experiment AMS-02 on board the International Space Station and with those of Earth polar neutron monitors, the GCR flux modulation was studied at different energies during recurrent short-term variations. It is also aimed to set the near real-time particle observation requirements to disentangle the role of long and short-term variations of the GCR flux to evaluate the performance of high-sensitivity instruments in space such as the future interferometers for gravitational wave detection. Finally, the association between recurrent GCR flux variation observations in L1 and weak to moderate geomagnetic activity in 2016-2017 is discussed. Short-term recurrent GCR flux variations are good proxies of recurrent geomagnetic activity when the B$_z$ component of the IP magnetic field is directed northern.