GCR flux 9-day variations with LISA Pathfinder

Galactic cosmic-ray (GCR) energy spectra in the heliosphere vary on the basis of the level of solar activity, the status of solar polarity and interplanetary transient magnetic structures of solar origin. A high counting rate particle detector (PD) aboard LISA Pathfinder (LPF) allows for the measurement of galactic cosmic-ray and solar energetic particle (SEP) integral fluxes at energies > 70 MeV n−1 up to 6500 counts s−1. Data are gathered with a sampling time of 15 s. A study of GCR flux depressions associated with the third harmonic of the Sun rotation period (∼ 9 days) is presented here.

The symmetric model in the force field approximation by Gleeson and Axford [8] allows for the estimate of the energy spectra of cosmic rays at a distance r from the Sun, at a time t during periods of positive polarity, when time-independent interstellar intensities are assumed. An energy loss related to the charge of cosmic-ray particles and a solar modulation parameter, φ, above about 100 MeV are considered. The role played by the GSMF polarity on the drift of positive (negative) particles in the heliosphere during periods of negative (positive) polarity must be also taken properly into account. Protons and nuclei reaching Earth during negative polarity periods come mainly from the ecliptic regions along the Heliospheric Current Sheet (HCS). An opposite situation holds during positive magnetic field polarity periods when positive particles come from the polar region of the Sun. Particles propagating along the HCS lose more energy than those coming from the poles.
Nominal quasiperiodicities of 27, 13.5 and 9 days related to the Sun rotation period and its higher harmonics are also observed in the cosmic-ray flux, the interplanetary magnetic field (IMF), the solar wind plasma and the geomagnetic activity parameters [9]. GCR observations gathered in space above a few tens of MeV by Helios 1, Helios 2 and IMP-8 experiments indicated that the effects of corotating interaction regions, generated by high-speed solar wind streams overtaking preceding slower solar wind, are at the origin of the 9-day GCR modulations [10]. A high counting rate particle detector (PD) aboard LISA Pathfinder (LPF) presents a typical response of more than 100 counts s −1 in 15 s sampling time, allowing for limiting the statistical uncertainty on one-hour binned GCR proton and helium nucleus flux data at percent level above 70 MeV n −1 . The aim of this work is to report on the characteristics of ∼ 9-day GCR short-term variations, compatible with the third harmonic of the solar rotation period, during the first three months of the LPF mission.
2. The Particle Detector aboard LPF LPF [11,12,13] is the ESA (European Space Agency) key technology demonstrator mission for LISA [14], the first interferometer devoted to gravitational wave detection in space. The LPF spacecraft was successfully launched from the Kourou base in French Guiana on December 3rd, 2015 aboard a Vega rocket. The satellite reached its six-months, final orbit around the Earth-Sun Lagrangian point L1 at about 1.5 million km from Earth on January 2016. The LPF orbit is inclined at about 45 degrees on the ecliptic plane. Orbit minor and major axes are approximately of 0.5 million km and 0.8 million km, respectively. LPF carries two 2-kg cubic platinum-gold free-floating test masses that play the role of mirrors of the interferometer. The charging process due to GCRs and SEPs results in spurious force noise on both test masses [15]. The effects of the charging process were properly estimated before the mission launch with Monte Carlo simulations [16,17] carried out on the basis of GCR and SEP occurrence projections for the period orbits about the L1 point [18]. The minimum energy of protons and ions (98% in composition of both GCRs and SEPs [19]) penetrating and charging the LPF test masses is 100 MeV n −1 . The conservative choice of monitoring GCR above 70 MeV n −1 on LPF was made in order not to underestimate the overall particle flux charging the test-masses. The LPF PD is mounted behind the spacecraft solar panels and oriented along the Sun-Earth direction. This device consists of two, 300 µm thick, silicon wafers of 1.4 x 1.05 cm 2 area, separated by 2 cm and placed in a telescopic arrangement [20,21]. For particle energies > 100 MeV n −1 the instrument geometrical factor is found to be energy independent and equal to 9 cm 2 sr for particle isotropic incidence on each silicon wafer. In coincidence mode the detector geometrical factor is about one tenth of this value. A shielding, copper box surrounds the silicon wafers. The box thickness is 6.4 mm. This shielding material stops particles with energies smaller than 70 MeV n −1 . The PD allows for the counting of particles crossing each silicon layer and for the measurement of ionization energy losses of particles passing through both silicon wafers (coincidence mode). The PD data are stored in the form of histograms over periods of 600 s and then sent to the on-board computer. The maximum allowed PD counting rate is 6500 counts s −1 , corresponding to an event integrated proton fluence of 10 8 protons cm −2 at energies > 100 MeV. In coincidence mode, up to 5000 energy deposits per second can be stored in each histogram. The expected occurrence of SEP events with fluence > 10 8 protons cm −2 at energies > 30 MeV, for instance, is less than 1 per year as it follows from the Nymmik model [22]. None of SEP events, characterized by proton differential fluxes above 100 MeV n −1 overcoming that of galactic origin, were observed since the LPF launch and up to the time of writing. This scenario makes the whole period of the LPF mission the most favourable for GCR short-term variation investigation. Data from the PD are available since mid-February 2016.

GCR and interplanetary solar wind plasma parameter short-term variations
The parameterization of the GCR energy spectra adopted here follows from the work by Papini, Grimani & Stephens [19]: The values of the parameters A, b, α and β for proton and helium nucleus energy spectra at the beginning of the LPF mission (corresponding to a solar modulation parameter φ=550 MV c −1 [23]) appear in Table 1. Table 1. Parameterization of proton and helium energy spectra during the first part of the LPF mission.  Table 2. The interval of time during which the flux of GCRs remains depressed between decrease and recovery phases is called here plateau. The depression intensity was observed to be of 6%, on average. Earth neutron monitor observations, being representative of GeV GCR fluxes, present maximum depression intensities of 2%. In general, the dynamics of

Conclusions
The high counting rate PD placed aboard LPF allows for the measurement of GCR proton and helium flux oscillations compatible with the third harmonic of the Sun rotation period above 70  MeV n −1 . One-hour binned data present errors at percent level. GCR depressions under study appear to be correlated with an IMF intensity > 10 nT and/or a solar wind speed > 400 km s −1 . Their average intensity and duration are of (7.6±1.6) days and (6.0±1.7)%, respectively. Low-vertical geomagnetic cutoff neutron monitor observations show that these same depressions appear of just 2% above 5.5 GeV.