Characteristics and Energy Dependence of Recurrent Galactic Cosmic-Ray Flux Depressions and of a Forbush Decrease with LISA Pathfinder

Galactic cosmic-ray (GCR) flux observations in the heliosphere present long-term (>1 year) and short-term (≤27 days) modulations. Both were extensively studied in the last 60 years on Earth with neutron monitors and in space (Forbush 1954, 1958; Storini et al. 1992; Beer 2000; Clem & Evenson 2004; Ferreira et al. 2004; Grimani 2004, 2007; Hajadas et al. 2004; Grimani et al. 2007; Shikaze et al. 2007; Sabbah & Kudela 2011; Usoskin et al. 2011, 2017; Laurenza et al. 2012, 2014).

Long-term variations are associated with the 11-year solar cycle and the 22-year solar polarity reversal. At solar maximum, GCR energy spectra appear depressed by approximately one order of magnitude at 100 MeV n⁻¹ with respect to similar observations gathered at solar minimum (see for instance Papini et al. 1996, and references therein). Moreover, at solar minimum and during negative (positive) solar polarity periods, defined by the solar magnetic field directed inward (outward) at the Sun north pole, positively (negatively) charged particle fluxes are depressed by a maximum of 40% at 100 MeV n⁻¹ with respect to measurements performed during opposite periodicities (Boella et al. 2001; Gil & Alania 2016). Positively charged particles propagate mainly sunward in the ecliptic along the heliospheric current sheet (HCS) during negative solar polarity periods and over the poles during positive polarity epochs. The opposite holds for negatively charged particles (Potgieter & Langner 2004; Ferreira 2005). Particles propagating along the HCS lose more energy than those coming from the poles (Strauss et al. 2011).

The most intense short-term GCR flux drops occur during classical, non-recurrent, Forbush decreases (Forbush 1937; Cane 2000). These depressions are characterized by maximum GCR flux decreases of 30% at 100 MeV n⁻¹ and are associated with the passage of interplanetary counterparts of coronal mass ejections. Recurrent depressions are caused by corotating high-speed solar wind streams (see Iucci et al. 1979, for instance). Quasi-periodicities of 27 days, 13.5 days, and 9 days, correlated with the Sun rotation period (27.28 days for an Earth observer) and higher harmonics, are observed in the cosmic-ray flux, in the solar wind plasma, in the interplanetary magnetic field (IMF), and in the geomagnetic activity indices (Čalogović et al. 2009; Emery et al. 2011). These investigations are typically carried out with neutron monitors that allow for long-term studies of the role of interplanetary structures in modulating the GCR flux (see for instance Simpson 1954; Gil & Alania 2010; Sabbah & Kudela 2011; Badruddin & Kumar 2016). A correlation of the GCR flux short-term variations with the BV product of the IMF intensity (B) and the solar wind speed (V) was investigated by Sabbah (2000). This approach takes into account both cosmic-ray diffusion from IMF and convection in the solar wind. From the point of view of geomagnetic indices, a good correlation of A_p and K_p with both BV and BV², was found by Sabbah (2007). Depressions of the cosmic-ray flux were studied in space since the sixties (see for instance McCracken et al. 1966). Richardson et al. (1996) carried out an extensive campaign of observations of GCR flux short-term variations above a few tens of MeV aboard the Helios 1, Helios 2, and IMP-8 spacecraft. These observations indicated that the effects of corotating interaction regions (CIRs), generated when high-speed solar wind streams, associated with stable, low-latitude extensions of polar coronal holes, overtake leading slow solar wind from the equatorial regions of the Sun, are at the origin of short-term GCR flux modulations (see also Richardson 2004).

A high counting rate particle detector (PD; Cañizares et al. 2011) hosted aboard the European Space Agency (ESA) LISA Pathfinder (LPF) mission (Antonucci et al. 2011, 2012; Armano et al. 2016), allowed for the monitoring of the integral proton and helium nucleus fluxes above 70 MeV n⁻¹ (Araújo et al. 2005; Mateos et al. 2012) with statistical uncertainty at percent level on 1 hr binned data between 2016 February and 2017 July.

The energy dependence of GCR short-term depressions can be studied by exploiting the contemporaneous measurements of cosmic rays in space above a few tens of MeV with missions carrying PDs and on Earth with neutron monitors located at different geographic latitudes. GCR counting rates observed with neutron monitors vary proportionally to the cosmic-ray flux, thus providing a direct measurement of the same, at energies larger than the effective energy (Gil et al. 2017), which ranges between 11–12 GeV and above 20 GeV for near-polar and equatorial stations, respectively.

This paper reports on the characteristics of GCR flux periodicities and depressions observed during the Bartels rotations (BRs) 2490–2508 (from 2016 February 18 through 2017 July 3) after properly taking into account the effects of long-term variations. It is recalled here that the BR number corresponds to the number of 27-day rotations of the Sun since 1832 February 8. The years 2016–2017 were characterized by the presence of near-equatorial coronal holes and equatorward extensions of polar coronal holes, resulting in a very favorable period to carry out the study illustrated here. The energy dependence of recurrent and non-recurrent GCR depressions is also investigated. In particular, it is reported on the characteristics of a classical Forbush decrease, a sudden depression of the GCR flux observed with LPF on 2016 August 2. This occurrence was associated with an increase of the IMF intensity due to the passage of an interplanetary coronal mass ejection (ICME; http://www.srl.caltech.edu/ACE/ASC/DATA/level3/icmetable2.htm and Richardson & Cane 2010) that caused a geomagnetic disturbance of modest intensity started at 21.30 UT of the same day. GCR proton energy spectra in 2016 August before and at the deep of the depression are estimated and presented in this work. These observations indicate the value of interplanetary missions carrying PDs that, while primarily devoted to mission performance purposes, can also provide valuable measurements for space science and space weather studies (see also Hajadas et al. 2004; Lilensten 2007, and references therein).

This manuscript is organized as it follows: Section 2 describes the characteristics of the LPF mission. Section 3 presents the parameterizations of the proton and helium energy spectra during the LPF mission. Sections 4 and 5 report the characteristics and the energy dependence of the observed GCR flux short-term variations, respectively. Finally, Section 6 discusses the capability of the LPF PD to monitor the passage of ICMEs.

LPF was the technology demonstrator mission for LISA, the first space interferometer devoted to gravitational wave detection in the frequency range 10⁻⁴ Hz–10⁻¹ Hz (Amaro-Seoane et al. 2017). The LPF spacecraft was launched from the Kourou base in French Guiana on 2015 December 3 aboard a Vega rocket. It reached its final orbit (which took approximately six months to complete) around the Earth–Sun Lagrangian point L1 at 1.5 million km from Earth at the end of 2016 January. The LPF orbit was inclined at about 45° with respect to the ecliptic plane. Orbit minor and major axes were approximately of 0.5 million km and 0.8 million km, respectively. The satellite spun on its own axis in six months. The LPF satellite carried two, 2 kg cubic platinum-gold free-floating test masses that play the role of mirrors of the interferometer. Protons and ions of galactic or solar origin with energies larger than 100 MeV n⁻¹ penetrated or interacted in about 13 g cm⁻² of spacecraft and instrument materials charging the LPF test masses. This charging process results in spurious noise forces on both test masses (Shaul et al. 2006; Armano et al. 2017). A PD (Cañizares et al. 2011) was placed aboard LPF for in situ monitoring of GCR and solar particle overall flux. The LPF PD was mounted behind the spacecraft solar panels with its viewing axis along the Sun–Earth direction. It consisted of two ∼300 μm thick silicon wafers of 1.40 × 1.05 cm² area, placed in a telescopic arrangement at a distance of 2 cm. For particle energies >100 MeV n⁻¹, the instrument geometrical factor was found to be energy independent and equal to 9 cm² sr for particle isotropic incidence on each silicon layer. When particles traversed both silicon wafers within 525 ns of each other (coincidence mode), the geometrical factor was about one-tenth of this value. A shielding copper box of 6.4 mm thickness surrounded the silicon wafers. The shielding material stopped particles with energies smaller than 70 MeV n⁻¹. The PD allowed for the counting of protons and helium nuclei traversing each silicon layer (single counts) and for the measurement of ionization energy losses of particles in coincidence mode. The single counts were gathered with a sampling time of 15 s and ionization energy losses of events in coincidence mode were stored in the form of histograms over periods of 600 s and then sent to the on-board computer. The maximum allowed detector counting rate was 6500 counts s⁻¹ on both silicon wafers, corresponding to an event integrated proton fluence of 10⁸ protons cm⁻² at energies >100 MeV. In coincidence mode, 5000 energy deposits per second was the saturation limit. The occurrence of SEP events with fluences larger than the saturation limit was estimated to be less than one per year for the period the LPF spacecraft remained in orbit around L1 (Nymmik 1999a, 1999b; Grimani et al. 2012). As a matter of fact, no SEP events characterized by a proton differential flux above a few tens of MeV n⁻¹ overcoming that of galactic origin were observed during the period of the LPF mission operations considered for this analysis.

The LPF 15 s proton (p) and helium (He) single counts gathered between 2016 mid-February and 2017 July 3 were hourly averaged in order to limit the statistical uncertainty on the measurements to 1% (see Figure 1). Observations were interrupted only for brief, planned system resets. The GCR count rate appears modulated on timescales of several days and presents an increasing trend over the mission lifetime due to a decreasing level of the solar activity. It is worthwhile to recall that LPF was sent into orbit during the descending phase of the solar cycle ${\rm{N}}^\circ$ 24 under a positive polarity period. In Grimani et al. (2007), it was shown that during positive polarity periods, the energy spectra, $J(r,E,t)$, of cosmic rays at a distance r from the Sun and at a time t, are well represented by the symmetric model in the force field approximation by Gleeson and Axford (G&A; Gleeson & Axford (1968)) assuming time-independent interstellar intensities $J(\infty ,E+{\rm{\Phi }})$ and an energy loss parameter Φ:

$$\begin{eqnarray}&&\displaystyle \frac{J(r,E,t)}{{E}^{2}-{E}_{0}^{2}}=\displaystyle \frac{J(\infty ,E+{\rm{\Phi }})}{{\left(E+{\rm{\Phi }}\right)}^{2}-{E}_{0}^{2}}\end{eqnarray} \tag{ 1 }$$

where E and E₀ represent the particle total energy and rest mass, respectively. For Z = 1 particles with rigidity (particle momentum per unit charge) larger than 100 MV, the effect of the solar activity is completely defined by the solar modulation parameter ϕ that, at these energies, is equal to Φ (see also Grimani et al. 2009).

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The solar modulation parameter for the first year of the LPF mission (2015–2016 December) was taken from http://cosmicrays.oulu.fi/phi/Phi_mon.txt (see also Usoskin et al. 2006). For the same period, the GCR single counts per sampling time of 15 s, averaged over each BR (GCR_{15 s}), were calculated. A linear correlation was found between the solar modulation parameter ϕ and GCR_{15 s}:

$$\begin{eqnarray}&&{\mathrm{GCR}}_{15{\rm{s}}}=-0.23272\ \phi (\mathrm{MV})+230.73\end{eqnarray} \tag{ 2 }$$

as it is shown in Figure 2 at the right side of the dashed line, indicated by DATA. This observation suggests that the LPF PD did not present any detectable loss of efficiency during the first year of the mission lifetime. Therefore, the same was reasonably assumed for the last six months of mission operations. Projections of the solar modulation parameter for the year 2017 (for which estimates are not available in http://cosmicrays.oulu.fi/phi/Phi_mon.txt) were carried out by extrapolating the same trend shown by GCR_{15 s} and ϕ in 2016 (see Figure 2 at the left of the dashed line indicated by PROJECTIONS). The observed PD single count rate increased by more than 20% during the LPF mission due to a decreasing solar activity. The monthly sunspot number (http://www.sidc.be/silso/home) was observed to decrease smoothly from 58 to 18.5 during the first year of the LPF mission while from 2017 January through the beginning of 2017 July the sunspot number did not change appreciably varying from 26.1 to 19.4. Therefore, the value of ϕ assumed here at the end of the LPF mission can be considered a lower limit.

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The proton and helium energy differential fluxes at the beginning (2015 December–2016 January; ϕ = 550 MV) and at the end (2017 July; ϕ = 320 MV) of the LPF mission were estimated with the model by G&A by using the proton and helium energy spectra at the interstellar medium obtained from a series of balloon flights of the BESS and BESS-Polar experiments (see Shikaze et al. 2007; Abe et al. 2016, for details). The BESS, BESS-Polar, and other balloon-borne experiment data gathered during different periods of solar activity and solar polarity, are reported in Figure 3. In this figure, open (solid) symbols indicate data gathered during positive (negative) polarity periods. In Grimani et al. (2004) and references therein, it was shown that contemporaneous observations of GCR fluxes in the inner heliosphere show variations of ∼3% au⁻¹ and 0.33% per latitude degree off the ecliptic. It can be concluded that particle spectra at the interstellar medium obtained with data gathered near Earth can also be used for LPF, which orbited at just 0.01 au from Earth. Finally, the interstellar spectra by BESS–BESS-Polar were privileged in this work as they were inferred from proton and helium observations gathered during conditions of solar activity and solar polarity similar to those of LPF.

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The energy spectra, F(E), obtained with the G&A model for LPF were interpolated with the function appearing in Equation (3), which is well representative of the GCR observations trend in the inner heliosphere between a few tens of MeV and hundreds of GeV within experimental errors (see for details Papini et al. 1996):

$$\begin{eqnarray}&&F(E)=A\ {\left(E+b\right)}^{-\alpha }\ {E}^{\beta }\quad \mathrm{particles}\ {\left({{\rm{m}}}^{2}\mathrm{sr}{\rm{s}}\mathrm{GeV}{{\rm{n}}}^{-1}\right)}^{-1},\end{eqnarray} \tag{ 3 }$$

where E is the particle kinetic energy per nucleon. The parameters A, b, α, and β were estimated for proton and helium nucleus energy spectra at the beginning and at the end of the LPF mission and reported in Table 1. The energy spectra obtained here for the beginning of the LPF mission (dotted–dashed curve in Figure 3; ϕ = 550 MV) lie, as expected, between the BESS97 (ϕ = 491 MV) and the BESS98 (ϕ = 591 MV) data, gathered during a positive polarity period. In the same figure, maximum projections of proton and helium energy spectra at the end of the LPF mission appear as continuous lines. The same value of ϕ was used for both proton and helium energy spectra estimates.

The power spectral density from the Lomb–Scargle (LS; Lomb 1976; Scargle 1982) periodogram analysis of the whole LPF GCR data sample adopted for this study is shown in Figure 4. The LS periodogram technique is used here to retrieve the periodicities of GCR modulation. Figure 4 shows that periodicities of 9, 13.5, and 27 days correlated to the Sun rotation and higher harmonics periodicities are present in the whole GCR PD data. In order to assess the time variability of these dominant periodicities, the period of observations was divided in three sub-intervals, each encompassing about four and a half months.

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The corresponding LS power spectral densities are displayed in Figure 5. The 9- and 13.5-day periodicities are strongly modulated in time and progressively damped, with the former being the first to disappear. The periodicity related to the Sun rotation is present during the whole observational period, though its value slightly changes in time from about 27 days (middle panel) to 31 days (third panel).

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In order to study the occurrence and the characteristics of individual GCR flux short-term depressions, the LPF PD observations during each BR were compared to IMF, solar wind plasma parameters, and to neutron monitor measurements. Moreover, data gathered during each BR were compared by eye to those observed during previous and subsequent BRs in order to detect the presence of recurring and non-recurring patterns in the variation of GCR data and solar wind parameters.

The effects of the decrease of the solar activity over the mission were reduced by considering the fractional variations of the cosmic-ray flux with respect to the average value during each BR. The same approach was considered in Wiedenbeck et al. (2009) for the ACE experiment in L1. Forty-four recurrent depressions and one classical Forbush decrease were observed. The commencement of individual depressions was set at the beginning of each continuous decrease of the GCR flux observed for more than 12 hr. GCR flux depressions with duration >1 day and amplitude >1.5% were considered for this analysis. Small increases and depressions (<1.5% in amplitude) lasting less than one day were at the limit of statistical significance and therefore were disregarded by interpolating the data trend. In the top panel of Figure 6, the cosmic-ray flux fractional variations during the BR 2491 (from 2016 March 4 through 2016 March 30) present four depressions (according to the definition reported above) starting on March 5, 12, 23, and 29, respectively. The small deeps on March 11–12 and 19–20 along with the small increase on March 16–17 are neglected. The solar wind plasma speed appears in the second panel of Figure 6. The IMF radial component is shown in the third panel and the IMF intensity in the fourth panel. In the third panel of Figure 6, the HCS crossing (taken from http://omniweb.sci.gsfc.nasa.gov./html/polarity/polarity_tab.html) is also indicated. Solar wind plasma and IMF data are taken from the ACE experiment. The GCR flux depressions appear associated with those periods of time during which the solar wind plasma speed (V) is >400 km s⁻¹ and/or the IMF intensity (B) is >10 nT (second and fourth panels in Figure 6). This scenario basically corresponds to the passage of high-speed solar wind streams and/or CIRs (Harang 1968; Storini 1990; Cane et al. 1995; Simpson 1998; McKibben et al. 1999; Bazilevskaya 2000). When GCR short-term variations are correlated with the BV parameter, the role of the magnetic field trend is privileged with respect to that of the solar wind speed as the IMF variations are larger than those of the solar wind speed. The GCR depressions observed with LPF are associated with solar wind speed changes smaller than 30% while the magnetic field is observed to increase up to a factor of five. Therefore, a separate analysis of B and V increases helps in better understanding the dynamics of individual depressions resulting from the interplay of several interplanetary structures that affect the role of different periodicities during the mission as observed in Figures 4 and 5. From the point of view of time profiles of individual depressions, those presenting similar durations for decrease and recovery phases are called ${\text{}}{symmetric}$. All the other depressions are called ${\text{}}{asymmetric}$ (Badruddin & Singh 2006). The symmetric variations are V- or U-shaped. Thirty-nine out of forty-five depressions were found to be asymmetric. Only six appeared symmetric, and out of these, five were found U-shaped and only one V-shaped. The period during which the PD counting rate remained at minimum values between decrease and recovery phases is called here ${\text{}}{plateau}$. A plateau is observed during both U-shaped symmetric and asymmetric depressions and appears correlated with the period the solar wind velocity remains above 400 km s⁻¹. A typical asymmetric depression is that appearing in the top panel of Figure 6 starting on 2016 March 5 with 2 day decrease, ∼1.5 day plateau and 3.5 day recovery periods. In the same figure, a symmetric, U-cup-shaped depression starts on the 23rd of March with decrease, plateau and recovery phases lasting about 2 days each. Decrease, plateau, and recovery phases for each depression during the BR 2491 are shown in colors in the top panel of Figure 6, as an example. Occurrence, characteristics, and association with interplanetary structures of all depressions are summarized in Table 2. The GCR flux depressions that commence at the interaction regions of slow and fast solar wind are associated to CIR in Table 2. Depressions observed to commence during different phases of corotating high-speed solar wind stream passage are indicated by CHSS. HCS crossing (HCSC) and ICMEs are observed to play a minor role in modulating the GCR flux with respect to corotating high-speed solar wind streams during the LPF mission. In Table 2 MFE (magnetic field enhancement) indicates a magnetic structure present in the slow solar wind. The association among magnetic structures and GCR flux depressions was carried out on the basis of contemporaneous IMF and solar wind parameter observations from the ACE experiment.

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Average durations of decrease, plateau, and recovery periods for the 45 GCR depressions observed with LPF are reported in Table 3.

The average GCR flux depression amplitude of 5.1 ± 2.5% appears consistent, within statistical errors, with that reported by Richardson (2004) of 3.2 ± 0.1% for particle nominal energies larger than 60 MeV. The cut-off energy of particles observed with Helios I, Helios 2, and IMP8 was poorly estimated (Richardson 2004) while for the LPF PD observations the same was set with both Monte Carlo simulation and beam test experiment (Araújo et al. 2005; Mateos et al. 2012). However, as the majority of cosmic-ray particles lie in the energy range of hundreds of MeV, a slightly difference in the detection capability of low-energy particles is not expected to make a relevant difference for the above comparison.

The full evolution of one classical, two-step Forbush decrease (Cane 2000) was detected aboard LPF on 2016 August 2, as it is shown in Figure 7. In this figure, all panels are the same as those in Figure 6. The sharp decrease of the GCR flux on August 2 lasted about 10 hr after 12.00 UT, no plateau was observed, and the recovery period was modulated by an incoming high-speed solar wind stream. The GCR depression appears correlated with a contemporaneous increase of the IMF intensity up to 24 nT, while the solar wind speed barely passed 400 km s⁻¹.

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Some clues are reported in the literature about the energy dependence of GCR flux short-term depressions. The energy dependence of 27-day GCR flux variations, for instance, is discussed in Grimani et al. (2015) and references therein. The shielding effect of the atmosphere and the geomagnetic cut-off prevent neutron monitors to carry out a direct measurement of cosmic-ray energy spectra below effective energies of several GeV, although they can be obtained by using models combined with neutron monitors observations (Beer 2000; Hofer & Flückiger 2000; Usoskin et al. 2011, 2017). Interesting attempts to investigate the energy dependence of short-term depressions of cosmic-ray fluxes above a few tens of MeV, through direct measurements with magnetic spectrometers, were carried out by the balloon-borne experiment BESS-Polar I (Thakur et al. 2011) and the satellite experiment PAMELA (Adriani et al. 2011). BESS-Polar I flew from Williams Field near Mc Murdo Station from 2004 December 13 through 2004 December 21. At the beginning of the flight, this balloon-borne experiment observed a recovering proton flux from a previous decrease. The recovery intensity appeared to be of 8%–9% below 0.86 GeV and of 3% above 6 GeV. The authors claimed that this occurrence was due to the transit of a CIR interface or a magnetic cloud (Burlaga et al. 1981) or a combination of the two. This experiment detected a new GCR proton flux depression after the passage of a high-speed solar wind stream on 2004 December 17. The PAMELA experiment carried out the first measurement of proton and helium nucleus differential fluxes in space during a Forbush decrease on 2006 December 14 (16.50 UT–22.35 UT) after two SEP events dated 2006 December 13 and 14. Unfortunately, balloon-borne magnetic spectrometer experiments, like BESS-Polar I, have short duration, and space-borne instruments, like PAMELA, have small geometrical factors; therefore, data must be integrated over periods longer than the typical one-hour data binning required to study recurrent GCR depressions.

The GCR flux fractional variations observed with the LPF PD have been compared to contemporaneous similar measurements carried out with neutron monitors placed at different geographic latitudes. Location, vertical cut-off rigidities, and effective energies for all neutron monitor stations considered in this work are reported in Table 4. Both LPF PD and neutron monitor data were hourly averaged and appear in Figures 8 and 9 for the BRs 2491 and 2496, respectively, as an example. This comparison indicates that while the maximum and average GCR fractional variations observed with LPF above 70 MeV n⁻¹ are of more than 11% and of about 5%, respectively, the same goes down to a maximum of 3% above 11–12 GeV in near-polar stations and to a maximum of 2% above 15 GeV at increasing latitudes.

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During the Forbush decrease observed on 2016 August 2 on LPF, a 3σ decrease of the GCR flux occurred between 12.00 UT and 16.00 UT. The GCR flux reached its minimum at 22.40 UT: data indicated a GCR flux fractional decrease of 9% in L1 in ∼10 hr. The GCR flux depression recovered soon after the deep. The trend of this GCR flux depression appears different from recurrent GCR flux variations observed to be of 2%–3% day⁻¹ and 1%–2% day⁻¹ during the decrease and recovery periods, respectively. In Figure 9, it can be noticed that the amplitude of the same depression is found to be of 3% in near-polar Terre Adelie and Oulu stations, while it is of just 2% and 1% in Rome and Mexico stations, respectively. In order to determine the GCR proton energy differential flux at the deep of the depression at 22.40 UT on LPF, the proton energy differential flux for the month of 2016 August (ϕ = 438 MV) was estimated first above 70 MeV and parameterized following Equation (3), as described in Section 3. The proton integral flux in 2016 August was then calculated as a integral of this differential flux. The proton integral flux, thus obtained, was properly reduced at 70 MeV as indicated by the LPF PD data and at the effective energy of each neutron monitor (reported in Table 4) on the basis of the neutron monitor decreases. Finally, the differential flux was inferred from the integral flux. The proton differential flux in 2016 August before the Forbush decrease and that estimated at the deep of the depression at 22.40 UT on 2016 August 2 were parameterized as reported in Table 5 and are compared in Figure 10.

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The helium differential flux at the deep of the depression was not estimated, as no accurate proton-helium separation was allowed by the PD aboard LPF, and the data trend is biased by protons, as the He/p ratio in GCRs is about 0.1.

Measurements of the energy dependence of GCR flux recurrent and non-recurrent depressions and the study of their evolution can be used to estimate the test-mass charging aboard future generation LISA-like interferometers (Grimani et al. 2015, and references therein). Despite minor changes in the instrument performance due to GCR short-term variations, future interferometers devoted to gravitational wave detection in space will detect sub-femto-g spurious acceleration at low frequencies (∼10⁻⁵ Hz), and the role of any interplanetary disturbance must be evaluated and quantified.

In this section, we evaluate the capability of space missions like LPF, carrying PDs optimized for GCR detection, in monitoring the passage of ICMEs and in forecasting geomagnetic activity, when these interplanetary structures present intense southward magnetic fields that reconnect with the Earth magnetic field and induce geomagnetic activity.

In Figure 11, the Forbush decrease observed with LPF on 2016 August 2 is compared to the contemporaneous IMF intensity and solar wind speed measured by ACE. The transit of an ICME near Earth from 2016 August 2 at 14.00 UT and 2016 August 3 at 3.00 UT, is indicated in the same figure by dashed lines (http://www.srl.caltech.edu/ACE/ASC/DATA/level3/icmetable2.htm; (see also Richardson & Cane 2010)). A detailed description of the characteristics of this ICME is reported in http://www.stce.be/esww14/contributions/public/S4-P1/S4-P1-08-BenellaSimone/Poster_ESWW.pdf.

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The decrease phase of the cosmic-ray flux seems to occur in two steps, suggesting that LPF crossed the region of the shock of the ICME and then the ejecta (Cane 2000). A geomagnetic disturbance was observed to start (${K}_{p}\gt 5$ ) after 21.30 UT. In Figure 12, the LPF GCR flux fractional variations are also compared to the SYM-H geomagnetic index, which allows it to follow the evolution of a geomagnetic disturbance at low latitudes. The characteristics of each GCR short-term flux depression are unique, often resulting from the interplaying effects of consecutive structures propagating in the interplanetary medium. However, for the 2016 August 2 Forbush decrease, in case of appropriate baseline communication strategy, an alert issued by LPF at the time a 3σ GCR flux decrease was reached around 16:00:00 UT, the geomagnetic disturbance observed at the Earth would have been forecasted several hours in advance. PDs aboard space missions allow for studying the energy dependence of GCR short-term depressions, their evolution, and association with interplanetary structures better than allowed by the use of neutron monitor measurements solely (see also Cane 2000, and references therein). The ICME tracking in space by Forbush decreases was also recently discussed in Witasse et al. (2017).

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A PD aboard the ESA mission LPF allowed for the study of GCR short-term flux depressions above 70 MeV n⁻¹ during the descending phase of the solar cycle ${\rm{N}}^\circ 24$. The majority of these depressions are recurrent and associated with corotating high-speed solar wind streams. ICMEs and HCS crossing play a minor role. The average duration of GCR flux depressions observed aboard LPF are found of 9.2 ± 5.0 days. Decrease, plateau, and recovery average periods are 2.8 ± 2.0 days, 1.3 ± 1.2 days, and 5.1 ± 3.8 days, respectively. The average depression intensity is 5.1 ± 2.5%.

The proton energy differential flux at the deep of a Forbush decrease observed on 2016 August 2 was obtained from the integral energy spectrum measurements carried out with LPF PD data and from those of neutron monitors placed in sites characterized by an increasing effective energy. Finally, it was shown that LISA-like and other missions in space, even if primarily devoted to difference science investigations, in some cases, may play the role of sentinels in monitoring the passage of magnetic structures that, when characterized by intense southern components of the magnetic field, induce geomagnetic activity.

The authors are grateful to the anonymous reviewer for his/her constructive comments and precious suggestions that helped them to greatly improve the manuscript. D. Telloni is financially supported by the Italian Space Agency (ASI) under contract I/013/12/0. Sunspot number and solar modulation parameter data were gathered from http://www.sidc.be/silso/home and http://cosmicrays.oulu.fi/phi/Phi_mon.txt, respectively. Data from Wind and ACE experiments were obtained from the NASA-CDAWeb website. We acknowledge the NMDB database (http://www.nmdb.eu) funded under the European Union's FP7 programme (contract No. 213007), and the PIs of individual neutron monitors for providing data. HCS crossing was taken from http://omniweb.sci.gsfc.nasa.gov./html/polarity/polarity_tab.html. This work has been made possible by the LISA Pathfinder mission, which is part of the space-science program of the European Space Agency. The French contribution has been supported by the CNES (Accord Specific de projet CNES 1316634/CNRS 103747), the CNRS, the Observatoire de Paris, and the University Paris-Diderot. E.P. and H.I. also acknowledge the financial support of the UnivEarthS Labex program at Sorbonne Paris Cite' (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). The Albert-Einstein-Institut acknowledges the support of the German Space Agency, DLR. The work is supported by the Federal Ministry for Economic Affairs and Energy based on a resolution of the German Bundestag (FKZ 50OQ0501 and FKZ 50OQ1601). The Italian contribution has been supported by Agenzia Spaziale Italiana and Instituto Nazionale di Fisica Nucleare. The Spanish contribution has been supported by Contracts No. AYA2010-15709 (MICINN), No. ESP2013-47637-P, and No. ESP2015-67234-P (MINECO). M.N. acknowledges support from Fundacion General CSIC (Programa'ComFuturo). F.R. acknowledges an FPI contract (MINECO). The Swiss contribution acknowledges the support of the Swiss Space Office (SSO) via the PRODEX Program of ESA. L.F. acknowledges the support of the Swiss National Science Foundation. The United Kingdom groups acknowledge support from the United Kingdom Space Agency (UKSA), the University of Glasgow, the University of Birmingham, Imperial College, and the Scottish Universities Physics Alliance (SUPA). J.I.T. and J.S. acknowledge the support of the U.S. National Aeronautics and Space Administration (NASA). N.K. acknowledges the support of the Newton International Fellowship from the Royal Society.