Solar Wind Flux Research Articles

Iron content determines how space weathering flux variations affect lunar soils

Previous work has established that the solar wind and micrometeoroids produce spectral changes on airless silicate bodies. However, the relative importance of these two weathering agents, the timescales over which they operate, and how their effects depend on composition have not yet been well determined. To help address these questions we make use of the fact that solar wind and micrometeoroid fluxes vary with latitude on the Moon. Previous work has shown that this latitudinally varying flux leads to systematic latitudinal variations in the spectral properties of lunar soils. Here we find that the way in which a lunar soil's spectral properties vary with latitude is a function of its iron content, when we consider soils with 14–22 wt% FeO. In particular, a 50% reduction in flux corresponds to a significant increase in reflectance for 14 wt% FeO soils, while the same flux reduction on 21 wt% FeO soils is smaller by a factor of ~5, suggesting that this brightening effect saturates for high FeO soils. We propose that lower iron soils may not approach saturation because grains are destroyed or refreshed before sufficient nano- and micro-phase iron can accumulate on their rims. We compare our results to the spectral variations observed across the Reiner Gamma swirl, which lies on a high‑iron surface, and find it has anomalous brightness compared to our predictions. Swirls in Mare Marginis, which lie on a low iron surface, exhibit brightness differences that suggest reductions in solar wind flux between 20 and 40%. Our inferences suffer from the limited latitudinal extent of the maria and the convolution of micrometeoroid flux and solar wind flux changes with latitude. Superior constraints on how space weathering operates throughout the inner solar system would come from in situ measurements of the solar wind flux at lunar swirls.

Reiner Gamma: A Magnetized Elliptical Disk on the Moon

AbstractThe geologic origin of the Moon's crustal magnetic anomalies is unknown. Reiner Gamma is one of the most studied anomalies, and it is correlated with symmetric bright albedo markings known as swirls. Here we propose that its source magnetization arises from a uniformly magnetized elliptical disk, resulting from the melt sheet or floor deposits of an oblique impact crater. The magnetization was likely acquired in a dynamo field and may be as high as ~70 A/m, perhaps due to incorporation of impactor materials. The disk produces vertical fields at its edges that may channel solar wind flux to the surface, producing an elliptical dark region, while neighboring regions remain shielded and bright. Interestingly, the disk appears to be magnetized along its semiminor axis. Measurements of the low altitude magnetic field at Reiner Gamma would test these predictions and help answer additional questions about its interaction with the solar wind.

Global Distribution of the Solar Wind Flux and Velocity From SOHO/SWAN During SC‐23 and SC‐24

AbstractWe analyze SOHO (SOlar Heliospheric Observatory)/SWAN (Solar Wind ANisotropy) hydrogen Lyman‐α data collected between 1996 and 2018 to derive the solar wind latitudinal distribution over time. Full‐sky interplanetary Lyman‐α maps are inverted to derive the total hydrogen ionization rate latitude profiles, normalized to proton charge‐exchange and photoionization. Using Interplanetary Scintillation velocities to calculate the velocity‐dependent charge‐exchange cross‐sections, we produce the solar wind flux latitudinal profiles. Finally, we compute solar wind velocity latitude profiles, based on the dynamic pressure and energy flux conservation (calculated from OMNI data) over latitude. SWAN reproduces the Interplanetary Scintillation velocity profiles up to at least ±60°, and also agrees with Ulysses in situ measurements for solar minimum periods in 1996–1997 and 2007. During solar maximum, discrepancies are more frequent because in situ data reflect local solar wind conditions, while SWAN data reflect global conditions in the heliosphere.

Soft X‐ray Imaging of the Magnetosheath and Cusps Under Different Solar Wind Conditions: MHD Simulations

AbstractThe soft X‐ray emissions from the Earth's magnetosheath and cusp regions are simulated under different solar wind conditions, based on the PPMLR‐MHD code. The X‐ray images observed by a hypothetical telescope are presented, and the basic responses of the magnetopause and cusp regions are discernable in these images. From certain viewing geometries, the magnetopause position in the equatorial plane, as well as the latitudinal scales and azimuthal extent of cusp can be directly extracted from the X‐ray images. With these reconstructed positions, the issues we are able to analyze include but are not limited to the compression of magnetopause and widening of the cusp after an enhancement of solar wind flux, as well as the erosion of the magnetopause and equatorward motion of cusp after the southward turning of the interplanetary magnetic field. Hence, the X‐ray imaging is an appropriate technique to study the large‐scale motion of magnetopause and cusps in response to solar wind variations.

Effects of Space Weathering and Porosity on the Far‐UV Reflectance of Amundsen Crater

AbstractThe lunar South Pole crater Amundsen is a prime location to study the effects of space weathering in the far ultraviolet. Amundsen's equator‐facing terrace walls are highly illuminated while the northern side of the crater has permanently shaded regions (PSRs). Using data from the Lunar Reconnaissance Orbiter Lyman Alpha Mapping Project, we investigate signatures of space weathering in different regions of Amundsen. We find that regions of the surface that receive large amounts of solar illumination and solar wind flux (e.g., the southern terrace walls) display high Lyman‐α albedos and blue spectral slopes in the 175–190‐nm region, indicative of increased regolith maturity due to solar wind weathering and thermal cycling. Amundsen's PSRs, however, receive no direct solar illumination and very little solar wind flux and have a lower albedo across Lyman Alpha Mapping Project's entire band pass (57–197 nm) than illuminated regions of the crater. We conclude that the low PSR albedos correspond to high regolith porosity in the PSRs. These PSRs are extremely cold regions with very minor thermal cycling. Thermal cycling might be a process that reduces porosity in regions of the crater exposed to a wide range of temperatures, thus increasing their albedos across the entire wavelength range. However, our analysis of the present data set was unable to uniquely identify its role. The low albedos in the PSRs may also result from extreme charging effects inside the PSRs, causing lofting and redeposition of dust, as well as dielectric breakdown, which would act to increase regolith porosity.

The Magnetic Field Geometry of Small Solar Wind Flux Ropes Inferred from Their Twist Distribution

This work extends recent efforts on the force-free modeling of large flux rope-type structures (magnetic clouds, MCs) to much smaller spatial scales. We first select small flux ropes (SFRs) by eye whose duration is unambiguous and which were observed by the Solar Terrestrial Relations Observatory (STEREO) or Wind spacecraft during solar maximum years. We inquire into which analytical technique is physically most appropriate. We consider three models: (i) linear force-free field ($\bigtriangledown\times$ B = $\alpha (r) $ B) with a specific, prescribed constant $\alpha$ (Lundquist solution), and (ii) with $\alpha$ as a free constant parameter (Lundquist-alpha solution), (iii) uniform twist field (Gold-Hoyle solution). We retain only those cases where the impact parameter is less than one-half the FR radius, $R$, so the results should be robust (29 cases). The SFR radii lie in the range [$\sim$ 0.003, 0.059] AU. Comparing results, we find that the Lundquist-alpha and uniform twist solutions yielded comparable and small normalized $\chi^2$ values in most cases. We then use Grad-Shafranov (GS) reconstruction to analyze these events further. We then considered the twist per unit length, $\tau$, both its profile through the FR and its absolute value. We find $\tau$ to lie in the range [5.6, 34] turns/AU. The GH model-derived $\tau$ values are comparable to those obtained from GS reconstruction. We find that twist unit length ($L$) is inversely proportional to $R$, as $\tau \sim 0.17/R$. We combine MC and SFR results on $\tau (R)$ and give a relation which is approximately valid for both sets. The axial and azimuthal fluxes, $F_z$ and $F_\phi$, vary as $\approx 2.1 B_0 R^2 \times10^{21}$ Mx and $F_{\phi}/L \approx 0.36 B_0 R \times10^{21}$Mx/AU. The relative helicity per unit length, $H/L \approx 0.75 B_0^2 R^3$$\times 10^{42}$ Mx$^2$/AU.

Improved space weathering maps of the lunar surface through radiative transfer modeling of Kaguya multiband imager data

Space weathering is a continuous process operating on the surface of the Moon producing nanophase and microphase iron on the rims of mineral grains and within agglutinates. The spectral effects due to these nanophase and microphase iron particles in the lunar regolith are well characterized in the visible and near-infrared wavelengths. In this work, we used the Hapke radiative transfer technique to model multispectral data (415–1550 nm) from the Kaguya Multiband Imager to map the nanophase and microphase iron abundances across the lunar surface. In order to produce these maps, we developed a reflectance model for silicate host particles as a function of FeO content using magnetic and Lunar Soil Characterization Consortium (LSCC) data. We tested the accuracy of our radiative transfer technique and found that the model can predict the nanophase and microphase iron abundances within 0.1 wt% Fe and 0.6 wt% Fe, respectively. By using a FeO and ilmenite map and our relationship between host particle reflectance and FeO abundance, we produced a nanophase, microphase, and submicroscopic iron abundance map. We found the approximate saturation limit for nanophase and microphase iron with respect to FeO. In general, the nanophase and microphase iron saturation limit increases with FeO content, except in soils that have FeO content of 6–15 wt%, the nanophase iron saturation is nearly constant. We also observed that the microphase iron abundance saturates before the nanophase iron abundance. These maps could be used to determine relative age and maturity, but with limitations. Our analyses of these maps showed that (1) swirls contain low abundances of nanophase iron, but no anomalous microphase iron abundances, (2) nanophase and microphase iron abundances are lower at higher latitudes, which suggests lower solar wind and micrometeoroid impact flux at these latitudes, (3) decreased nanophase iron abundances in the nearside suggest a lower solar wind flux due to Earth's magnetotail, and (4) in contrast to Mercury, the microphase to nanophase iron abundance ratio is lower on the Moon due to lower surface temperatures and less intense space weathering.

Precipitation of Hydrogen Energetic Neutral Atoms at the Upper Atmosphere of Mars

AbstractWe study the properties of neutral hydrogen atoms precipitating onto the upper atmosphere of Mars. Energetic neutral atoms (ENAs) are produced by the charge exchange process between protons of solar wind (both upstream and shocked) as well as planetary origins and the Martian exospheric neutrals. Using a global hybrid plasma model for Mars‐solar wind interaction combined with an up‐to‐date exosphere model of Mars, we calculate the fluxes, spatial distributions, energy spectra, and direction distributions of hydrogen ENAs (H‐ENAs) at the Martian exobase for each source proton population. H‐ENAs originating from the upstream solar wind region and the magnetosheath dominate the precipitation. Two percent of the solar wind flux penetrates through the magnetic barrier as H‐ENAs under solar minimum conditions. The precipitating solar wind H‐ENA flux is axially symmetric about Sun‐Mars line, while the magnetosheath and planetary H‐ENAs have higher fluxes and a more‐extended precipitation area in the hemisphere where the convection electric field is pointing away from the planet, causing a significant precipitation beyond the terminator. The observed asymmetry is consistently explained by an asymmetric plasma flow in the dayside magnetosheath. The solar wind dynamic pressure increases the solar wind H‐ENA precipitation normalized by the upstream proton flux, due to a closer bow shock position and thus a higher exospheric column density for charge exchange. The spatial distribution of the magnetosheath solar wind and planetary H‐ENAs becomes more axially symmetric with increased dynamic pressure. The solar wind interaction with Mars exhibits more gas‐dynamic‐like signatures for higher dynamic pressure.

Assessment of the Shielding Effect of the Earth's Magnetic Field on Lunar OH/H2O

AbstractWhen the Moon passes through Earth's magnetosphere, it is believed that the amount of OH/H2O that would typically be created is reduced because the incident solar wind flux on the lunar surface decreases. To confirm this, we analyzed changes in OH/H2O absorption feature in the infrared region with or without the shielding effect of the magnetosphere using Moon Mineralogy Mapper (M3) data in low‐latitude regions. We report that the strength of absorption feature near 3 μm for the region from the equator up to the 10° latitude decreased when the Moon is inside Earth's magnetosphere relative to when it was outside of it. This indicates that the amount of OH/H2O on the shielded surface is less than on the unshielded surface. We confirmed that Earth's magnetosphere affects the OH/H2O distribution at low‐latitude regions, so the OH/H2O on the surface is originated by solar wind implantation. However, this study includes certain level of uncertainties in thermal corrections, and thus, sophisticated additional attempts are needed.

Modeling Martian Atmospheric Losses over Time: Implications for Exoplanetary Climate Evolution and Habitability

Abstract In this Letter, we make use of sophisticated 3D numerical simulations to assess the extent of atmospheric ion and photochemical losses from Mars over time. We demonstrate that the atmospheric ion escape rates were significantly higher (by more than two orders of magnitude) in the past at ∼4 Ga compared to the present-day value owing to the stronger solar wind and higher ultraviolet fluxes from the young Sun. We found that the photochemical loss of atomic hot oxygen dominates over the total ion loss at the current epoch, while the atmospheric ion loss is likely much more important at ancient times. We briefly discuss the ensuing implications of high atmospheric ion escape rates in the context of ancient Mars, and exoplanets with similar atmospheric compositions around young solar-type stars and M-dwarfs.

Martian ionosphere observed by MAVEN. 3. Influence of solar wind and IMF on upper ionosphere

In two previous papers (Dubinin et al., 2016, 2017a) we studied the influence of the crustal magnetic field and solar irradiance on the upper ionosphere of Mars based on the observations made on Mars Express. Here we discuss effects of solar wind and the interplanetary magnetic field using the MAVEN observations. It is shown that the upper ionosphere at solar zenith angles ≥∼40o and altitudes above ∼350−400 km is asymmetrical along the direction of the motional electric field. In the hemisphere, in which the motional electric field is directed toward the planet, the ionosphere is denser and expands to higher altitudes as compared to the ionosphere in the opposite hemisphere. The difference in the density of O+ and O2+ ions in both hemispheres achieves almost one order. Such a difference arises due to a different ion convection. Ion convection in both ionospheres occurs in the same direction as the motion of the shocked solar wind in the adjacent magnetosheath although with very different velocities. The upper ionosphere is also sensitive to the variations in the solar wind dynamic pressure, solar wind flux and the value of the motional electric field with a distinct depletion at altitudes above ∼450−500 km with increase of these solar wind characteristics. The depletion is accompanied by an increase in the horizontal ion velocities. As a result, the ion fluxes to the tail only insignificantly vary with the modest variations of the solar wind characteristics.

An analytical investigation: Effect of solar wind on lunar photoelectron sheath

The formation of a photoelectron sheath over the lunar surface and subsequent dust levitation, under the influence of solar wind plasma and continuous solar radiation, has been analytically investigated. The photoelectron sheath characteristics have been evaluated using the Poisson equation configured with population density contributions from half Fermi-Dirac distribution of the photoemitted electrons and simplified Maxwellian statistics of solar wind plasma; as a consequence, altitude profiles for electric potential, electric field, and population density within the photoelectron sheath have been derived. The expression for the accretion rate of sheath electrons over the levitated spherical particles using anisotropic photoelectron flux has been derived, which has been further utilized to characterize the charging of levitating fine particles in the lunar sheath along with other constituent photoemission and solar wind fluxes. This estimate of particle charge has been further manifested with lunar sheath characteristics to evaluate the altitude profile of the particle size exhibiting levitation. The inclusion of solar wind flux into analysis is noticed to reduce the sheath span and altitude of the particle levitation; the dependence of the sheath structure and particle levitation on the solar wind plasma parameters has been discussed and graphically presented.

Asymmetric Space Weathering on Lunar Crater Walls

AbstractUsing new topography‐corrected spectral data from the SELENE spacecraft, here we report a new lunar crater property produced by space weathering. We find that the optical properties of north, south, east, and west walls vary systematically across the Moon; pole‐facing walls are brighter and less red (i.e., less mature) than their equator‐facing counterparts as latitude increases, which we explain by reduced solar wind flux in pole‐facing slopes. On the nearside, we find that east‐west differences in crater wall brightness and redness vary with longitude, which we explain by solar wind shielding as the Moon passes through the Earth's magnetosphere. Because micrometeoroids are largely unaffected by magnetosphere passage, the longitudinal effect is used to discriminate between micrometeoroid and solar wind effects. Thus, for the first time we quantify how surface optical properties vary with solar wind flux.

Mars Under Primordial Solar Wind Conditions: Mars Express Observations of the Strongest CME Detected at Mars Under Solar Cycle #24 and its Impact on Atmospheric Ion Escape

AbstractAn extremely strong Coronal Mass Ejection (CME) impacted Mars on 12 July 2011, while the Mars Express spacecraft was present inside the nightside ionosphere. Estimated solar wind density and speed during the event are 39 particles cm−3 and 730 km/s, corresponding to nominal solar wind flux at Mars when the solar system was ∼1.1 Ga old. Comparing with expected average atmospheric heavy ion fluxes under similar XUV conditions, the CME impact is found to have no significant effect on the escape rate 3.3 × 1024s−1, with an upper limit at 1025s−1 if the observed tail contraction is not taken into account. On the subsequent orbit, 7 h later after magnetosphere response, fluxes were only 2.4% of average. As such, even under primordial solar wind conditions we are unable to find support for a strong solar wind‐driven ion escape, rather the main effect appears to be acceleration of the escaping ions by ×10–×20 typical characteristic energy.

The Effect of Solar Wind Variations on the Escape of Oxygen Ions From Mars Through Different Channels: MAVEN Observations

AbstractWe present multi‐instrument observations of the effects of solar wind on ion escape fluxes on Mars based on the Mars Atmosphere and Volatile EvolutioN (MAVEN) data from 1 November 2014 to 15 May 2016. Losses of oxygen ions through different channels (plasma sheet, magnetic lobes, boundary layer, and ion plume) as a function of the solar wind and the interplanetary magnetic field variations were studied. We have utilized the modified Mars Solar Electric (MSE) coordinate system for separation of the different escape routes. Fluxes of the low‐energy (≤30 eV) and high‐energy (≥30 eV) ions reveal different trends with changes in the solar wind dynamic pressure, the solar wind flux, and the motional electric field. Major oxygen fluxes occur through the tail of the induced magnetosphere. The solar wind motional electric field produces an asymmetry in the ion fluxes and leads to different relations between ion fluxes supplying the tail from the different hemispheres and the solar wind dynamic pressure (or flux) and the motional electric field. The main driver for escape of the high‐energy oxygen ions is the solar wind flux (or dynamic pressure). On the other hand, the low‐energy ion component shows the opposite trend: ion flux decreases with increasing solar wind flux. As a result, the averaged total oxygen ion fluxes reveal a low variability with the solar wind strength. The large standard deviations from the averages values of the escape fluxes indicate the existence of mechanisms which can enhance or suppress the efficiency of the ion escape. It is shown that the Martian magnetosphere possesses the properties of a combined magnetosphere which contains different classes of field lines. The existence of the closed magnetic field lines in the near‐Mars tail might be responsible for suppression of the ion escape fluxes.

Initial analysis of ion fluxes in the magnetotail of Mars based on simultaneous measurements on Mars Express and Maven

Simultaneous operation of two Mars satellites, equipped with instruments for the study of the plasma environment close to Mars, the European satellite Mars Express and American satellite MAVEN, allows one to investigate the influence of the interplanetary environment on the Martian magnetosphere and atmospheric losses, induced by the solar wind, for the first time, with a sufficient degree of confidence. In this paper, the data from measurements on the Mars Express satellite (MEX) of heavy ion losses are analyzed in comparison with the solar wind and magnetic field measurements on the MAVEN satellite. The main issue is the spatial structure of the escaping ion flux and the influence of the nonstationarity of the solar wind flux on the escape rate.

Surface charging of a crater near lunar terminator

Past lunar missions have shown the presence of dust particles in the lunar exosphere. These particles originate from lunar surface and are due to the charging of lunar surface by the solar wind and solar UV flux. Near the lunar terminator region, the low conductivity of the surface and small scale variations in surface topology could cause the surface to charge to different surface potentials. This paper simulates the variation of surface potential for a crater located in the lunar terminator regions using Spacecraft Plasma Interaction Software (SPIS). SPIS employs particle in cell method to simulate the motion of solar wind particles and photoelectrons. Lunar crater has been found to create mini-wake which affects both electron and ion density and causes small scale potential differences. Simulation results show potential difference of 300 V between sunlit area and shadowed area which creates suitable condition for dust levitation to occur.

ARTEMIS observations of the solar wind proton scattering function from lunar crustal magnetic anomalies.

Despite their small scales, lunar crustal magnetic fields are routinely associated with observations of reflected and/or backstreaming populations of solar wind protons. Solar wind proton reflection locally reduces the rate of space weathering of the lunar regolith, depresses local sputtering rates of neutrals into the lunar exosphere, and can trigger electromagnetic waves and small-scale collisionless shocks in the near-lunar space plasma environment. Thus, knowledge of both the magnitude and scattering function of solar wind protons from magnetic anomalies is crucial in understanding a wide variety of planetary phenomena at the Moon. We have compiled 5.5 years of ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) observations of reflected protons at the Moon and used a Liouville tracing method to ascertain each proton's reflection location and scattering angles. We find that solar wind proton reflection is largely correlated with crustal magnetic field strength, with anomalies such as South Pole/Aitken Basin (SPA), Mare Marginis, and Gerasimovich reflecting on average 5-12% of the solar wind flux while the unmagnetized surface reflects between 0.1 and 1% in charged form. We present the scattering function of solar wind protons off of the SPA anomaly, showing that the scattering transitions from isotropic at low solar zenith angles to strongly forward scattering at solar zenith angles near 90°. Such scattering is consistent with simulations that have suggested electrostatic fields as the primary mechanism for solar wind proton reflection from crustal magnetic anomalies.

Evidence for detection of energetic neutral atoms by LADEE

The integrated current of the Lunar Dust Experiment (LDEX) instrument aboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) includes sources of current in addition to small (<0.3μm) dust grains. LDEX current correlates strongly with solar wind density and flux, though solar wind ions should be excluded from the signal. Energetic neutral atoms (ENAs), produced by solar wind that is neutralized and reflected from the moon's surface, might contribute to LDEX current. To investigate this possibility, we developed a geometrical model for ENA detection. We find high correlation between predicted ENA flux and LDEX current. Based on the inverse dependence of ENA detection sensitivity on solar wind speed, we exclude sputtering from the spherical surface inside the instrument as the mechanism for ENA detection. Additionally, we observe a drop in LDEX current that is consistent with a drop in the predicted ENA flux associated with a lunar magnetic anomaly (LMA) centered at -178°E and 6°N selenographic longitude and latitude, respectively. This provides additional confirmation that ENAs are the main contributor to the LDEX current rather than ions, because the ENA flux decreases in LMA regions and reflected hydrogen ion flux increases in LMA regions.

Comparison of cyclic variations in the flux of the solar wind and the global temperature of the Earth

The Earth’s global temperature is treated as indicator of climate changes. Relatively reliable assessments of temperature variations are obtained on the basis of results from instrumental observations performed only since the middle of the 19th century. It is significant that the temperature series are nonstationary, a consequence of global climate changes. Empirical mode decomposition (EMD) is used for temperature series pretreatment, allowing us to eliminate a long-term component and a linear trend. Along with expansion in the EMD series, analysis of the amplitudes of significance is used to eliminate noise. The resulting refined quasistationary series of data on the Earth’s surface temperature is studied via spectral analysis to identify the cyclic component.