Forbush Decreases and <2 Day GCR Flux Non-recurrent Variations Studied with LISA Pathfinder

Abstract

Non-recurrent short term variations of the galactic cosmic-ray (GCR) flux above 70 MeV n −1 were observed between 2016 February 18 and 2017 July 3 aboard the European Space Agency LISA Pathfinder (LPF) mission orbiting around the Lagrange point L1 at 1.5×10 6 km from Earth. The energy dependence of three Forbush decreases (FDs) is studied and reported here. A comparison of these observations with others carried out in space down to the energy of a few tens of MeV n −1 shows that the same GCR flux parameterization applies to events of different intensity during the main phase. FD observations in L1 with LPF and geomagnetic storm occurrence is also presented.Finally, the characteristics of GCR flux non-recurrent variations (peaks and depressions) of duration < 2 days and their association with interplanetary structures are investigated. It is found that, most likely, plasma compression regions between subsequent corotating high-speed streams cause peaks, while heliospheric current sheet crossing cause the majority of the depressions.

Spacecraft and interplanetary contributions to the magnetic environment on-board LISA Pathfinder

“…Moreover, these missions contain a suite of instruments that allow a complete characterisation of the plasma, tracking parameters such as the solar wind speed or the number density of the plasma that we will refer in the following. Nonetheless, it is worth noticing that, although not designed for that, the radiation monitor on-board LISA Pathfinder (Canizares et al 2011;Armano et al 2018a) allowed for the detection of the passage of large scale interplanetary structures such as high-speed solar wind streams and interplanetary counterparts of coronal mass ejections generating recurrent and non recurrent depressions of the galactic cosmic-ray flux (Armano et al 2018c(Armano et al , 2019c.…”

Study of Galactic Cosmic-Ray Flux Modulation by Interplanetary Plasma Structures for the Evaluation of Space Instrument Performance and Space Weather Science Investigations

“…The increasing trend of the data is due to the decreasing solar modulation during the LPF mission [30]. Observations are also characterized by the presence of 45 recurrent variations >2 days, 3 FDs and 23 non-recurrent variations <2 days [12,13]. The commencement of each recurrent depression was set at the beginning of each continuous decrease of the count rate observed for more than 12 h. Depressions lasting more than 1 day and amplitude >1.5% were considered.…”

“…In Figures 6-8 it can be observed that polar NM observations vary by less than 2% revealing the energy dependence of GCR short-term variations and making simple particle detectors in space optimum instruments to follow in situ the dynamics of individual short-term variations (see also [13]). The AMS-02 experiment proton energy differential flux data averaged over each BR present experimental errors on each data point ranging from 5% at hundreds of MeV through 2% above 100 GeV.…”

“…These findings will be taken into account for future mission performance studies. Three weak FDs were observed on board LPF on 20 July 2016, 2 August 2016 and 27 May 2017 [12,13] at the passage of ICMEs. The first two events are shown in Figure 10, while the last one appears in Figure 11.…”

“…The LPF observations [12,13] are used here to study the role of GCR short-term variations with respect to proton differential flux measurements averaged during different Bartels rotation (BR), i.e., 27-day Sun periods since 1832 February 8. This approach was adopted to reduce the influence of the solar modulation decrease during the mission elapsed time.…”