Solar Wind Conditions Research Articles

A Variational Approach to Data Assimilation in the Solar Wind

AbstractVariational data assimilation (DA) has enabled huge improvements in the skill of operational weather forecasting. In this study, we use a simple solar‐wind propagation model to develop the first solar‐wind variational DA scheme. This scheme enables solar‐wind observations far from the Sun, such as at 1 AU, to update and improve the inner‐boundary conditions of the solar wind model (at 30 solar radii). In this way, observational information can be used to improve estimates of the near‐Earth solar wind, even when the observations are not directly downstream of the Earth. Using controlled experiments with synthetic observations, we demonstrate this method's potential to improve solar wind forecasts, though the best results are achieved in conjunction with accurate initial estimates of the solar wind. The variational DA scheme is also applied to Solar‐Terrestrial Relations Observatory (STEREO) in situ observations using initial solar wind conditions supplied by a coronal model of the observed photospheric magnetic field. We consider the period October 2010 to October 2011, when the STEREO spacecraft were approximately 80∘ ahead/behind Earth in its orbit. For 12 of 13 Carrington Rotations, assimilation of STEREO data improves the near‐Earth solar wind estimate over the nonassimilated state, with a 18.4% reduction in the root mean square error. The largest gains are made by the DA during times when the steady‐state assumption of the coronal models breaks down. While applying this pure variational approach to complex solar‐wind models is technically challenging, we discuss hybrid DA approaches which are simpler to implement and may retain many of the advantages demonstrated here.

Development of ground pipeline system for high-level scientific data products of the Hisaki satellite mission and its application to planetary space weather

The Hisaki satellite is the first-ever space telescope mission dedicated to planetary sciences. Atmospheres and magnetospheres of our solar system planets are continuously monitored by the extreme ultraviolet (EUV) spectrometer onboard Hisaki. This paper describes a data pipeline system developed for processing high-level scientific and ancillary data products from the Hisaki mission. The telemetry data downlinked from the satellite are stored in a ground telemetry database, processed in the pipeline to imaging spectral data with a 1-min temporal resolution and ancillary data products, and then archived in a public database. The imaging spectra can be further reduced to higher-level data products for practical scientific use. For example, light curves of the power emitted from Jupiter’s aurora and plasma torus with a temporal resolution of 10-min can be reduced from the imaging spectral data; the reduced light curves reveal the transport processes of energy and mass in Jupiter’s magnetosphere and associated interplanetary solar wind conditions. Continuous monitoring with Hisaki will contribute considerably to our understanding of space weather relating to planets in our solar system.

Numerical simulations of ICME–ICME interactions

We present hydrodynamical simulations of the interaction of Coronal Mass Ejections (CME) in the Interplanetary Medium (IPM). In these events, two consecutive CMEs are launched from the Sun in similar directions within an interval of time of a few hours. In our numerical model, we assume that the ambient solar wind is characterized by its velocity and mass-loss rate. Then, the CMEs are generated when the flow velocity and mass-loss rate suddenly change, with respect to the ambient solar wind conditions during two intervals of time, which correspond to the duration of each CME. After their interaction, a merged region is formed and evolve as a single structure into the IPM. In this work, we are interested in the general morphology of this merged region, which depends on the initial parameters of the ambient solar wind and the CMEs involved. In order to understand this morphology, we have performed a parametric study in which we characterize the effects of the initial parameters variations on the density and velocity profiles at 1 AU, using as reference the well-documented event of July 25th, 2004. Based on this parametrization we were able to reproduce the main features of the observed profiles ensuring the travel time and the speed and density magnitudes. Then, we apply the parametrization results to the interaction events of May 23, 2010; August 1, 2010; and November 9, 2012. With this approach and varying the values of the input parameters within the CME observational errors, our simulated profiles reproduce the main features observed at 1 AU. Even though we do not take into account the magnetic field, our models give a physical insight into the propagation and interaction of ICMEs.

Solar perturbations transits in Mercury exosphere

The link existing between the dayside Na exospheric patterns of Mercury and the solar wind-magnetosphere-surface interactions is investigated thanks to the synergy of Earth-based observations with the THEMIS solar telescope and the in-situ measurements of the Interplanetary Magnetic Field (IMF) and proton fluxes at the magnetic cusp regions by MESSENGER. Frequently, two-peak patterns of variable intensity are observed, located at high latitudes in both hemispheres. Occasionally, Na signal is instead diffused above the sub-solar region. In a special case, the Na signal is diffused above the subsolar region, when the MESSENGER data detect the transit of two Coronal Mass Ejections (CMEs). Na emission patterns results to be clearly related to the solar wind conditions at Mercury. Hence, the Na exospheric patterns, observed from ground, could be considered as a natural monitor of solar disturbances when transiting near Mercury.

The response of high latitude ionosphere to the 2015 June 22 storm

This work investigates physical mechanisms triggering phase scintillations on L-band signals under strong stormy conditions. Thanks to selected ground-based Global Navigation Satellite Systems (GNSS) receivers, located both in Antarctica and in the Arctic, an interhemispheric comparison between high latitude ionospheric observations in response to the peculiar solar wind conditions occurred on June 22, 2015 is here shown. To trace back the observed phase scintillations to the physical mechanisms driving it, we combine measurements from GNSS receivers with in-situ and ground-based observations. Our study highlights the ionospheric scenario in which irregularities causing scintillation form and move, leveraging on a multi-observation approach. Such approach allows deducing that scintillations are caused by the presence of fast-moving electron density gradients originated by particle precipitation induced by solar wind variations. In addition, we show how the numerous and fast oscillations of the north-south component of the interplanetary magnetic field (Bz,IMF) result to be less effective in producing moderate/intense scintillation events than during period of long lasting negative values. Finally, we also demonstrate how the in-situ electron density data can be used to reconstruct the evolution of the ionospheric dynamics, both locally and globally.

PC index as a proxy of the solar wind energy that entered into the magnetosphere: 4. Relationship between the solar wind dynamic pressure (PSW) impulses and PC, AL indices

The solar wind dynamic pressure (PSW) impulses impacting on the magnetosphere were separated on the basis of solar wind parameters measured in the point of libration L1 in 1998–2017. Comparison of the “estimated” PSW impulses reduced to the Earth's magnetopause with the appropriate sudden impulses (SI), revealed in the ground-based 1-min SymH index, showed that features of the really observed sudden impulses (moment of beginning, time evolution, intensity) can be significantly different from characteristics of the estimated PSW impulses. Because of this the preference was given to the real SI events over the estimated PSW impulses. Only the events with sudden impulses starting against the background of steady quiet pressure levels were taken for the analysis (N = 143). Relationship between the positive (leaps) and negative (drops) SI events, on the one hand, and PC and AL indices, on the other hand, were examined under the various solar wind conditions, such as: growing and steady solar wind speed at negative and positive values of the IMF BZ component, fluctuating BZ and solar wind speed, steady solar wind speed and BZ = 0. Analysis of SI events showed that the solar wind dynamic pressure impulses themselves are not promote (or insignificantly promote) the solar wind energy input into the magnetosphere, which is controlled by the interplanetary electric field EKL = Vsw*(BY2+BZ2)1/2 and is displayed by the polar cap magnetic activity PC index. When the PSW impulses (leaps or drops) are accompanied by the corresponding changes in EKL field, the PC index correlates (increases or decreases) with the PSW changes. Inconsistency between the PSW and PC behavior becomes evident as soon as the EKL and PSW courses start to diverge. Development of magnetic disturbances (AL index) is governed only by the EKL field and PC index increase irrespective of the PSW impulses magnitude and time evolution.

Forecasting Periods of Strong Southward Magnetic Field Following Interplanetary Shocks

AbstractLong periods of strong southward magnetic fields are known to be the primary cause of intense geomagnetic storms. The majority of such events are caused by the passage over Earth of a magnetic ejecta. Irrespective of the interplanetary cause, fast‐forward shocks often precede such strong southward Bz periods. Here we first look at all long periods of strong southward magnetic fields as well as fast‐forward shocks measured by the Wind spacecraft in a 22.4‐year span. We find that 76% of strong southward Bz periods are preceded within 48 hr by at least a fast‐forward shock, but only about 23% of all shocks are followed within 48 hr by strong southward Bz periods. Then, we devise a threshold‐based probabilistic forecasting method based on the shock properties and the pre‐shock near‐Earth solar wind plasma and interplanetary magnetic field characteristics adopting a superposed epoch analysis‐like approach. Our analysis shows that the solar wind conditions in the 30‐min interval around the arrival of fast‐forward shocks have a significant contribution to the prediction of long‐duration southward Bz periods. This probabilistic model may provide on average a 14‐hr warning time for an intense and long‐duration southward Bz period. Evaluating the forecast capability of the model through a statistical and skill score‐based approach reveals that it outperforms a coin‐flipping forecast. By using the information provided by the arrival of a fast‐forward shock at L1, this model represents a marked improvement over similar forecasting methods. We outline a number of future potential improvements.

Magnetotail boundary crossings at lunar distances: ARTEMIS observations

We compare results from a preliminary analysis of two years of ARTEMIS magnetopause boundary crossings at lunar distances with available empirical models. We remove the effects of variable solar wind flow directions and aberration angles to study the magnetotail cross-section as a function of solar wind conditions. The average magnetopause distance from the central axis is 26 RE, but this distance ranges from 10 RE for high solar wind dynamic pressures and strong northward IMF orientations to 39 RE for low solar wind dynamic pressures and weak southward IMF orientations compared to the nominal solar wind conditions. The time-independent Howe and Binsack (1972) model describes the average location of the crossings very well. For high solar wind dynamic pressures, the Lu et al., (2011) model performs best, while for low pressures the Petrinec and Russell (1996) model gives the closest prediction. As predicted by theory and seen in past studies, the magnetotail cross-section is suggestive of prolate during intervals of strong IMF By, but oblate during intervals of strong IMF Bz. Any asymmetric variation of the tail boundary with respect to the sign of IMF By was not observed. The decreasing size of boundary with the increasing dynamic pressure was found when dynamic pressures are smaller than 2 nPa. Although the scatter is larger, the tail size for pressures larger than 2 nPa suggests a constant radius. The tail boundary size decreases as the strength of IMF Bz increases regardless of its polarity. However, it was also observed that an even stronger southward IMF Bz can cause larger magnetopause size in the presence of large dynamic pressures.

Investigating Solar Wind‐Driven Electric Field Influence on Long‐Term Dynamics of Jovian Synchrotron Radiation

AbstractSince the first discovery of Jovian Synchrotron Radiation (JSR) from Jovian radiation belt in late 1950s, origin of its time variations has been one of the main subjects of the Jovian radiation belt study. JSR is reported to be strongly correlated with solar wind ram pressure with a possible time lag, which remains an unsolved issue. In our study, the influence of dawn‐to‐dusk electric field modulated with solar wind ram pressure on JSR variations is investigated. Alongside the original diffusion coefficient (DLL(UV)), a new diffusion coefficient (DLL(Elec)) which is dependent on solar wind conditions is applied to a one‐dimensional lossy radial diffusion model, and we reproduce long‐term variations of JSR between 1971 and early 2018. For the specific choice of DLL(UV) = 3.0 × 10−10L3[s−1], the correlation coefficient is found to be 0.6 between the simulated JSR and the ground observation data prior to 2005, and the intermittent observation after 2005 supports our simulation as well. We suggest that while DLL(UV) is the primary mode of diffusion that determines the steady profile of electron population and JSR, DLL(Elec) serves as a secondary mode, which controls long‐term variations of JSR.

A Proxy for the Upstream IMF Clock Angle Using MAVEN Magnetic Field Data

AbstractWithout an upstream monitor at Mars to provide a contemporaneous measurement of solar wind conditions, it is useful to have techniques of inferring the upstream solar wind conditions using downstream data. We develop a method to estimate the upstream interplanetary magnetic field (IMF) clock angle, defined as the orientation of the IMF vector in the plane perpendicular to the solar wind velocity, at Mars using magnetometer data from the Mars Atmosphere and Volatile Evolution (MAVEN) mission. The technique fits MAVEN magnetometer data from within the Martian magnetosheath to a model of the draped magnetic field direction in a coordinate system aligned with the motional electric field. The results provide a proxy for the clock angle that agrees with upstream measurements and reproduces the observed distribution of clock angles. Even if upstream observations are available for a given orbit, the proxy provides an estimate of the clock angle at the time when the spacecraft is in the magnetosheath, which may correct for inherent temporal variability, because the solar wind varies on timescales shorter than the 4.5‐hr MAVEN orbit. The clock angle proxy can be applied for any orientation of the MAVEN orbit, even when MAVEN does not traverse into the upstream solar wind.

Solar wind and interplanetary shock parameters near Saturn's orbit (∼10 AU)

In this work, in-situ 1-h averaged plasma and magnetic field spacecraft data are used to characterize solar wind conditions near Saturn's orbit. Further, high resolution solar wind interplanetary data are used to identify and to calculate interplanetary shock parameters. When only plasma or magnetic field data are available, identified discontinuities were classified as possible shocks. Solar wind parameters at 10 AU decrease as expected from the inner heliosphere averages. For instance, solar wind dynamic pressure is ∼2% and ∼40% of its value at 1 and 5 AU, respectively. Wave magnetosonic speed is similar to 5 AU, but it is only about 60% of the 1 AU value. From analysis of high resolution plasma and magnetic field data, only a small number of shocks (13) were identified, because of scarce data availability. On the other hand, a larger number of possible shocks (60) were identified. Most of the confirmed shocks (∼75%) were fast forward shocks, while ∼25% were reverse shocks. Mach numbers ranged from 1.2 to 3.5 (average 2.2). Shock normal angle was on average 68° with oblique to perpendicular propagation to the magnetic field. From solar wind and interplanetary shock parameters analyzed in this work, and comparison with results from other works at different radial distances, it is concluded that conditions for shock occurrence and shock strength seem to increase from 1 to 5 AU and then level off or decrease near ∼10 AU, with shock strength at 10 AU comparable to shock strength at 1 AU. These results are of interest for interplanetary dynamics and for solar wind-planetary magnetosphere coupling.

Energetic Particle Showers Over Mars from Comet C/2013 A1 Siding Spring

AbstractThis paper is a phenomenological description of multispacecraft observations of energetic particles caused by the close flyby of comet C/2013 A1 Siding Spring with Mars on 19 October 2014. This is the first time that cometary energetic particles have been observed at Mars. The Mars Atmosphere and Volatile EvolutioN (MAVEN)‐solar energetic particle (SEP) and the Mars Odyssey‐High Energy Neutron Detector (HEND) instruments recorded evidence of precipitating particles, which are likely O+ pickup ions, during the ~10 hr that Mars was within the region of the comet's coma. O+ pickup ions were also detected several hours after, although whether their origin is the comet or space weather is not conclusive. We discuss the possible origin of those particles and also the cause of an additional shower of energetic particles that HEND observed between 22 and 35 hr after the comet's closest approach, which may be related to dust impacts from the comet's dust tail. An O+ pickup ion energy flux simulation is performed with representative solar wind and cometary conditions, together with a simulation of their energy deposition profile in the atmosphere of Mars. Results indicate that the O+ pickup ion fluxes observed by SEP were deposited in the ionosphere around 105 to 120 km altitude, and they are compared with precomet flyby estimations of cometary pickup ions. The comet's flyby deposited a significant fluence of energetic particles into Mars' upper atmosphere, at a similar level to a large space weather event.

Effects of a Weak Intrinsic Magnetic Field on Atmospheric Escape From Mars

AbstractThe magnetization of a planet significantly changes the nature of atmospheric escape, and the question of whether a planetary field shields atmosphere from erosion by the solar wind or not remains open. Ion escape processes from Mars are investigated under two hypothetical conditions, namely, no intrinsic magnetic field and weak intrinsic dipole field cases under present solar wind conditions based on multispecies magnetohydrodynamics simulations. The existence of a weak dipole field results in an enhancement of the tailward flux of planetary ions through the four escape channels. Two channels are associated with the open fields at the poles, and the others are caused by magnetic reconnection between the planetary and solar wind magnetic fields at the flank magnetopause. The ion escape rate with the weak dipole is greater than in the no‐dipole case. The enhancement is significant for O2+ and CO2+, suggesting ion escape from the low‐altitude ionosphere.

Effects of Geometries and Substructures of ICMEs on Geomagnetic Storms

To better understand geomagnetic storm generations by ICMEs, we consider the effect of substructures (magnetic cloud, MC, and sheath) and geometries (impact location of flux-rope at the Earth) of the ICMEs. We apply the toroidal magnetic flux-rope model to 59 CDAW CME–ICME pairs to identify their substructures and geometries, and select 20 MC-associated and five sheath-associated storm events. We investigate the relationship between the storm strength indicated by minimum Dst index $(\mathrm{Dst}_{\mathrm{min}})$ and solar wind conditions related to a southward magnetic field. We find that all slopes of linear regression lines for sheath-storm events are steeper ( ${\geq}\,1.4$ ) than those of the MC-storm events in the relationship between $\mathrm{Dst}_{\mathrm{min}}$ and solar wind conditions, implying that the efficiency of sheath for the process of geomagnetic storm generations is higher than that of MC. These results suggest that different general solar wind conditions (sheaths have a higher density, dynamic and thermal pressures with a higher fluctuation of the parameters and higher magnetic fields than MCs) have different impact on storm generation. Regarding the geometric encounter of ICMEs, 100% (2/2) of major storms ( $\mathrm{Dst}_{\mathrm{min}} \leq -100~\mbox{nT}$ ) occur in the regions at negative $P_{Y}$ (relative position of the Earth trajectory from the ICME axis in the $Y$ component of the GSE coordinate) when the eastern flanks of ICMEs encounter the Earth. We find similar statistical trends in solar wind conditions, suggesting that the dependence of geomagnetic storms on 3D ICME–Earth impact geometries is caused by asymmetric distributions of the geoeffective solar wind conditions. For western flank events, 80% (4/5) of the major storms occur in positive $P_{Y}$ regions, while intense geoeffective solar wind conditions are not located in the positive $P_{Y}$ . These results suggest that the strength of geomagnetic storms depends on ICME–Earth impact geometries as they determine the solar wind conditions at Earth.

Autogenous and efficient acceleration of energetic ions upstream of Earth's bow shock.

Earth and its magnetosphere are immersed in the supersonic flow of the solar-wind plasma that fills interplanetary space. As the solar wind slows and deflects to flow around Earth, or any other obstacle, a 'bow shock' forms within the flow. Under almost all solar-wind conditions, planetary bow shocks such as Earth's are collisionless, supercritical shocks, meaning that they reflect and accelerate a fraction of the incident solar-wind ions as an energy dissipation mechanism1,2, which results in the formation of a region called the ion foreshock3. In the foreshock, large-scale, transient phenomena can develop, such as 'hot flow anomalies'4-9, which are concentrations of shock-reflected, suprathermal ions that are channelled and accumulated along certain structures in the upstream magnetic field. Hot flow anomalies evolve explosively, often resulting in the formation of new shocks along their upstream edges5,10, and potentially contribute to particle acceleration11-13, but there have hitherto been no observations to constrain this acceleration or to confirm the underlying mechanism. Here we report observations of a hot flow anomaly accelerating solar-wind ions from roughly 1-10 kiloelectronvolts up to almost 1,000 kiloelectronvolts. The acceleration mechanism depends on the mass and charge state of the ions and is consistent with first-order Fermi acceleration14,15. The acceleration that we observe results from only the interaction of Earth's bow shock with the solar wind, but produces a much, much larger number of energetic particles compared to what would typically be produced in the foreshock from acceleration at the bow shock. Such autogenous and efficient acceleration at quasi-parallel bow shocks (the normal direction of which are within about 45 degrees of the interplanetary magnetic field direction) provides a potential solution to Fermi's 'injection problem', which requires an as-yet-unexplained seed population of energetic particles, and implies that foreshock transients may be important in the generation of cosmic rays at astrophysical shocks throughout the cosmos.

Exploration of a Composite Index to Describe Magnetospheric Activity: Reduction of the Magnetospheric State Vector to a Single Scalar

AbstractGeomagnetic activity is usually gauged by a single time‐dependent geomagnetic index. One drawback is that an individual geomagnetic index measures only one aspect of the activity in the Earth's magnetosphere. Here we construct a time‐dependent 11‐element magnetospheric state vector E for Earth that consists of measures of high‐latitude currents, polar cap current, magnetospheric convection, plasma pressure, ion and electron precipitation rates, the intensity of substorm‐injected electrons, and the elapsed time since the last substorm onset. At the same time we construct an eight‐element time‐dependent solar‐wind state vector S from the measured properties of the solar wind at Earth. In this S → E picture a time‐dependent “magnetospheric‐activity index” E(1) is created from a weighted sum of the 11 variables in the magnetospheric state vector, with the weights chosen by the vector correlation properties between the magnetospheric state vector E and the solar wind state vector S. The properties and advantages of this E(1) index are examined for the years 1991–2007. Comparing fits made on different subsets of the data demonstrates the robustness of the definition of E(1); tests using out‐of‐sample data demonstrate the accuracy of solar wind predictions of E(1). The advantages of the composite index E(1) include a more direct association with the solar wind, linearity, high predictability, and versatility with respect to (a) storm‐versus‐quiet intervals, (b) solar maximum versus solar minimum, and (c) the various types of solar wind plasma. Defining magnetospheric storms with the composite index E(1) is explored, and categorizing magnetospheric storms using the magnetospheric state vector E is considered.

A Statistical Survey of Radiation Belt Dropouts Observed by Van Allen Probes

AbstractA statistical analysis on the radiation belt dropouts is performed based on 4 years of electron phase space density data from the Van Allen Probes. The μ, K, and L* dependence of dropouts and their driving mechanisms and geomagnetic and solar wind conditions are investigated using electron phase space density data sets for the first time. Our results suggest that electronmagnetic ion cyclotron (EMIC) wave scattering is the dominant dropout mechanism at low L* region, which requires the most active geomagnetic and solar wind conditions. In contrast, dropouts at high L* have a higher occurrence and are due to a combination of EMIC wave scattering and outward radial diffusion associated with magnetopause shadowing. In addition, outward radial diffusion at high L* is found to cause larger dropouts than EMIC wave scattering and is accompanied with active geomagnetic and solar wind drivers.

A More Viscous‐Like Solar Wind Interaction With All the Giant Planets

AbstractIdentifying and quantifying the different drivers of energy flow through a planetary magnetosphere is crucial for understanding how each planetary system works. The magnetosphere of our own planet is primarily driven externally by the solar wind through global magnetic reconnection, while a viscous‐like interaction with the solar wind involving growth of the Kelvin‐Helmholtz (K‐H) instability is a secondary effect. Here we consider the solar wind‐magnetosphere interaction at all magnetized planets, exploring the implications of diverse solar wind conditions. We show that with increasing distance from the Sun the electric fields arising from reconnection at the magnetopause boundary of a planetary magnetosphere become weaker, whereas the boundaries become increasingly K‐H unstable. Our results support the possibility of a predominantly viscous‐like interaction between the solar wind and every one of the giant planet magnetospheres, as proposed by previous authors and in contrast with the solar wind‐magnetosphere interaction at Earth.

The Impact and Solar Wind Proxy of the 2017 September ICME Event at Mars

AbstractWe study a large interplanetary coronal mass ejection event impacting Mars in mid‐September 2017 numerically. During this time period, MAVEN remained inside the Martian bow shock and therefore could not measure the solar wind directly. We first simulate the event using three steady state cases with estimated solar wind conditions and find that these cases were able to reproduce the general features observed by MAVEN. However, these time‐stationary runs cannot capture the response of the system to large variations in the solar wind associated with the event. To address this problem, we derive a solar wind proxy based on MAVEN observations in the sheath region and their comparison with steady state magnetohydrodynamic model results. The derived solar wind proxy is then used to drive a time‐dependent magnetohydrodynamic model, and we find that the data‐model comparison is greatly improved, especially in the magnetosheath. We are able to reproduce some detailed structures observed by MAVEN during the period, despite the lack of a direct measurement of the solar wind, indicating that the derived solar wind conditions are reliable. Finally, we examine in detail the impact of the event on the Martian system: including variations of the three typical plasma boundaries and the ion loss rates. Our results show that these plasma boundary locations varied drastically during the event, and the total ion loss rate was enhanced by more than an order of magnitude.

Space weathering on the Moon: Farside-nearside solar wind precipitation asymmetry

The lunar surface is continuously under the impact of solar wind plasma, which breaks the chemical bonds of the surface material resulting in weathering of the surface and a modified chemical composition. Ion impact also sputters the surface material, affecting the composition of the lunar exosphere, and it controls the electrical properties of both the lunar surface and the near surface space.We have studied the lunar farside-nearside (FN) hemispherical asymmetry of the solar wind proton impact on the lunar surface along the orbit of the Moon during fast solar wind conditions. The analysis is based on a 3D hybrid model where ions are accelerated by the macroscopic j × B and pressure gradient forces.The derived proton impact surface map shows that the highest cumulative solar wind proton addition on the lunar surface is located on the farside while the most energetic protons precipitate on the nearside. The total ion impact rate was found to be smallest when the Moon is deep in the magnetotail. The total ion impact rate on the lunar surface varies while the Moon orbits the Earth and these longitudinal variations are caused by the magnetosphere and lunar tidal locking.