Solar Wind Flux Research Articles

Reconstruction of an evolving magnetic flux rope in the solar wind: Decomposing spatial and temporal variations from single‐spacecraft data

AbstractWe present a novel single‐spacecraft method for decomposing spatial and temporal variations of physical quantities at points along the path of a spacecraft in space‐time. The method is designed for use in the reconstruction of evolving two‐dimensional, approximately magnetohydrostatic structures in a space plasma. It is an extension of the one developed by Sonnerup and Hasegawa (2010) and Hasegawa et al. (2010), in which it was assumed that variations in the time series of data, recorded as the structures move past the spacecraft, are all due to spatial effects. In reality, some of the observed variations are usually caused by temporal evolution of the structure during the time it moves past the observing spacecraft; the information in the data about the spatial structure is aliased by temporal effects. The purpose here is to remove this time aliasing from the reconstructed maps of field and plasma properties. Benchmark tests are performed by use of synthetic data taken by a virtual spacecraft as it traverses, at a constant velocity, a magnetic flux rope growing sufficiently slowly (relative to the Alfvén speed) in a two‐dimensional magnetohydrodynamic simulation of magnetic reconnection. These tests show that the new method can better recover the space‐time behavior of the flux rope than does the original version, in which time‐aliasing effects had not been removed. An application of the new method to a solar wind flux rope, observed by the ACE spacecraft, suggests that the cross‐sectional shape of the core part of the flux rope was varying.

The Solar Wind Ion Analyzer for MAVEN

The Solar Wind Ion Analyzer (SWIA) on the MAVEN mission will measure the solar wind ion flows around Mars, both in the upstream solar wind and in the magneto-sheath and tail regions inside the bow shock. The solar wind flux provides one of the key energy inputs that can drive atmospheric escape from the Martian system, as well as in part controlling the structure of the magnetosphere through which non-thermal ion escape must take place. SWIA measurements contribute to the top level MAVEN goals of characterizing the upper atmosphere and the processes that operate there, and parameterizing the escape of atmospheric gases to extrapolate the total loss to space throughout Mars’ history. To accomplish these goals, SWIA utilizes a toroidal energy analyzer with electrostatic deflectors to provide a broad 360∘×90∘ field of view on a 3-axis spacecraft, with a mechanical attenuator to enable a very high dynamic range. SWIA provides high cadence measurements of ion velocity distributions with high energy resolution (14.5 %) and angular resolution (3.75∘×4.5∘ in the sunward direction, 22.5∘×22.5∘ elsewhere), and a broad energy range of 5 eV to 25 keV. Onboard computation of bulk moments and energy spectra enable measurements of the basic properties of the solar wind at 0.25 Hz.

THE SOLAR WIND AS A POSSIBLE SOURCE OF FAST TEMPORAL VARIATIONS OF THE HELIOSPHERIC RIBBON

We present a possible source of pickup ions (PUIs) the ribbon observed by the Interstellar Boundary EXplorer (IBEX). We suggest that a gyrating solar wind and PUIs in the ramp and in the near downstream region of the termination shock (TS) could provide a significant source of energetic neutral atoms (ENAs) in the ribbon. A fraction of the solar wind and PUIs are reflected and energized during the first contact with the TS. Some of the solar wind may be reflected propagating toward the Sun but most of the solar wind ions form a gyrating beam-like distribution that persists until it is fully thermalized further downstream. Depending on the strength of the shock, these gyrating distributions can exist for many gyration periods until they are scattered/thermalized due to wave-particle interactions at the TS and downstream in the heliosheath. During this time, ENAs can be produced by charge exchange of interstellar neutral atoms with the gyrating ions. In order to determine the flux of energetic ions, we estimate the solar wind flux at the TS using pressure estimates inferred from in situ measurements. Assuming an average path length in the radial direction of the order of a few AU before the distribution of gyrating ions is thermalized, one can explain a significant fraction of the intensity of ENAs in the ribbon observed by IBEX. With a localized source and such a short integration path, this model would also allow fast time variations of the ENA flux.

The coupling between the solar wind and proton fluxes at GEO

Abstract. The relationship between the solar wind and the proton flux at geosynchronous Earth orbit (GEO) is investigated using the error reduction ratio (ERR) analysis. The ERR analysis is able to search for the most appropriate inputs that control the evolution of the system. This approach is a black box method and is able to derive a mathematical model of a system from input-output data. This method is used to analyse eight energy ranges of the proton flux at GEO from 80 keV to 14.5 MeV. The inputs to the algorithm were solar wind velocity, density and pressure; the Dst index; the solar energetic proton (SEP) flux; and a function of the interplanetary magnetic field (IMF) tangential magnitude and clock angle. The results show that for lowest five energy channels (80 to 800 keV) the GEO proton fluxes are controlled by the solar wind velocity with a lag of two to three days. However, above 350 keV, the SEP fluxes, accounts for a significant portion of the GEO proton flux variance. For the highest three energy channels (0.74 to 14.5 MeV), the SEPs account for the majority of the ERR. The results also show an anisotropy of protons with gyrocenters inside GEO and outside GEO, where the protons inside GEO are controlled partly by the Dst index and also an IMF-clock angle function.

A statistical study of proton precipitation onto the Martian upper atmosphere: Mars Express observations

Due to the small size of the Martian magnetic pile‐up region, especially at the subsolar point, heated protons with high enough energy can penetrate the induced magnetosphere boundary without being backscattered, i.e., they precipitate. We present a statistical study of the downgoing ~ keV proton fluxes measured in the Martian ionosphere by the Analyzer of Space Plasma and Energetic Atoms experiment onboard the Mars Express spacecraft. We find that on the dayside, the events of proton penetration occur during 3% of the observation time; the precipitation is an intermittent phenomenon. The proton events carry on average ~0.2% of the incident solar wind flux. Therefore, the induced magnetosphere is an effective shield against the magnetosheath protons. The events are more frequent during fast solar wind conditions than during slow solar wind conditions. The sporadic proton penetration is thought to be caused by transient increases in the magnetosheath temperature. The precipitating flux is higher on the dayside than on the nightside, and its spatial deposition is controlled by the solar wind convective electric field. The largest crustal magnetic anomalies tend to decrease the proton precipitation in the southern hemisphere. The particle and energy fluxes vary in the range 104–106 cm–2 s–1 and 107–109 eVcm–2 s–1, respectively. The corresponding heating for the dayside atmosphere is on average negligible compared to the solar extreme ultraviolet heating, although the intermittent penetration may cause local ionization. The net precipitating proton particle flux input to the dayside ionosphere is estimated as 1.2 · 1021 s–1.

Chandra ACIS-S imaging spectroscopy of anomalously faint X-ray emission from Comet 103P/Hartley 2 during the EPOXI encounter

We present results from the Chandra X-ray Observatory’s characterization of the X-ray emission from Comet 103P/Hartley 2, in support of NASA’s Deep Impact Extended close flyby of the comet on 04 November 2010. The comet was observed 4 times for a total on target time of ∼60ks between the 17th of October and 16th of November 2010, with two of the visits occurring during the EPOXI close approach on 04 November and 05 November 2010. X-ray emission from 103P was qualitatively similar to that observed for collisionally thin Comets 2P/Encke (Lisse, C.M. et al. [2005]. Astrophys. J. 635, 1329–1347) and 9P/Tempel 1 (Lisse, C.M. et al. [2007]. Icarus 190, 391–405). Emission morphology offset sunward but asymmetrical from the nucleus and emission lines produced by charge exchange between highly stripped C, N, and O solar wind minor ions and coma neutral gas species were found. The comet was very under-luminous in the X-ray at all times, representing the 3rd faintest comet ever detected (LX=1.1±0.3×1014ergs−1). The coma was collisionally thin to the solar wind at all times, allowing solar wind ions to flow into the inner coma and interact with the densest neutral coma gas. Localization of the X-ray emission in the regions of the major rotating gas jets was observed, consistent with the major source of cometary neutral gas species being icy coma dust particles. Variable spectral features due to changing solar wind flux densities and charge states were also seen. Modeling of the Chandra observations from the first three visits using observed gas production rates and ACE solar wind ion fluxes with a charge exchange mechanism for the emission is consistent with the temporal and spectral behavior expected for a slow, hot wind typical of low latitude emission from the solar corona interacting with the comet’s neutral coma. The X-ray emission during the 4th visit on 16 November 2010 is similar to the unusual behavior seen for Comet 17P/Holmes in 2007 (Christian, D.J. et al. [2010]. Astrophys. J. Suppl. 187, 447–459) as the solar wind became dominated by a less ionized and faster plasma, more typical of outflow from polar coronal hole regions. We postulate that the overall faintness of the comet seen during all visits is due to the unusually well mixed dust and gas content of this hyperactive comet’s coma producing Auger electrons rather than X-rays via charge exchange with the solar wind. An alternative possible explanation for the faintness of the comet’s X-ray emission, and its unusual high CV and unusually low CVI emission, is that the impinging solar wind was drastically slowed in the inner coma, below 150kms−1, before charge exchanging with cometary neutrals.

Hydroxyl (OH) production on airless planetary bodies: Evidence from H+/D+ ion-beam experiments

The hypothesis that bombardment of lunar soil with solar-wind protons might form hydroxyl (OH) and perhaps HOH has been tested by experiments with Apollo 16 (highlands) and Apollo 17 (mare) soils. Pre-dried soils (500°C) were bombarded with 1.1keV protons and deuterons and provide unambiguous evidence for the formation of OH or OD in both samples. This hypothesis further predicts the formation of hydroxyl (OH) on other airless planetary/asteroidal bodies, with a sufficient solar-wind flux. Deuteron implantation of unaltered lunar soils and a heat-treated plagioclase specimen cause simultaneous OH depletion and OD formation. Ion bombardment of lunar soils simulates the dynamic process of hydroxyl formation and may also deplete intrinsic OH, thereby effectively contributing to the day/night, diurnal variability of OH reported by Sunshine et al. (2009). Our results emphasize the need to use lunar soils with space-weathered exteriors in laboratory simulations of the solar wind. Infrared spectra of hydrogen ion-beam implanted soils are similar to spectra obtained at RELAB (Brown Univ.) and to those observed by remote sensing confirming the solar-wind hypothesis.

Plasma penetration of the dayside magnetopause

Data from the Cluster spacecraft during their magnetopause crossing on 25 January 2002 are presented. The magnetopause was in a state of slow non-oscillatory motion during the observational period. Coherent structures of magnetosheath plasma, here typified as plasmoids, were seen on closed magnetic field lines on the inside of the magnetopause. Using simultaneous measurements on two spacecraft, the inward motion of the plasmoids is followed from one spacecraft to the next, and it is found to be in agreement with the measured ion velocity. The plasma characteristics and the direction of motion of the plasmoids show that they have penetrated the magnetopause, and the observations are consistent with the concept of impulsive penetration, as it is known from theory, simulations, and laboratory experiments. The mean flux across the magnetopause observed was 0.2%–0.5% of the solar wind flux at the time, and the peak values of the flux inside the plasmoids reached approximately 20% of the solar wind flux.

Heliolatitude and Time Variations of Solar Wind Structure from in situ Measurements and Interplanetary Scintillation Observations

The 3D structure of the solar wind and its evolution in time are needed for heliospheric modeling and interpretation of energetic neutral atoms observations. We present a model to retrieve the solar wind structure in heliolatitude and time using all available and complementary data sources. We determine the heliolatitude structure of solar wind speed on a yearly time grid over the past 1.5 solar cycles based on remote-sensing observations of interplanetary scintillations, in situ out-of-ecliptic measurements from Ulysses, and in situ in-ecliptic measurements from the OMNI 2 database. Since in situ out-of-ecliptic information on the solar wind density structure is not available apart from the Ulysses data, we derive correlation formulae between the solar wind speed and density and use the information on the solar wind speed from interplanetary scintillation observations to retrieve the 3D structure of the solar wind density. With the variations of solar wind density and speed in time and heliolatitude available, we calculate variations in solar wind flux, dynamic pressure, and charge-exchange rate in the approximation of stationary H atoms.

SimultaneousSwiftX-ray and UV views of comet C/2007 N3 (Lulin)

We present an analysis of simultaneous X-Ray and UV observations ofcomet C/2007 N3 (Lulin) taken on three days between January 2009 and March 2009 using the Swift observatory. For our X-ray observations, we used basic transforms to account for the movement of the comet to allow the combination of all available data to produce an exposure-corrected image. We fit a simple model to the extracted spectrum and measured an X-ray flux of 4.3+/-1.3 * 10^-13 ergs cm-2 s-1 in the 0.3 to 1.0 keV band. In the UV, we acquired large-aperture photometry and used a coma model to derive water production rates given assumptions regarding the distribution of water and its dissociation into OH molecules about the comet's nucleus. We compare and discuss the X-ray and UV morphology of the comet. We show that the peak of the cometary X-ray emission is offset sunward of the UV peak emission, assumed to be the nucleus, by approximately 35,000 km. The offset observed, the shape of X-ray emission and the decrease of the X-ray emission comet-side of the peak, suggested that the comet was indeed collisionally thick to charge exchange, as expected from our measurements of the comet's water production rate (6--8 10^28 mol. s-1). The X-ray spectrum is consistent with solar wind charge exchange emission, and the comet most likely interacted with a solar wind depleted of very highly ionised oxygen. We show that the measured X-ray lightcurve can be very well explained by variations in the comet's gas production rates, the observing geometry and variations in the solar wind flux.

Empirical energy spectra of neutralized solar wind protons from the lunar regolith

We present an empirical model of the energy spectra for hydrogen energetic neutral atoms (ENA) backscattered from the lunar surface based on Chandrayaan‐1 Energetic Neutral Atom (CENA) observations. The observed energy spectra of the backscattered ENAs are well reproduced by Maxwell‐Boltzmann distribution functions. The backscatter fraction is constant and independent of any solar wind parameters and the impinging solar wind angle. The calculated backscatter fraction is 0.19, and the 25% and 75% percentiles are 0.16 and 0.21. The empirical parameters of the Maxwell‐Boltzman distribution derived from the CENA imager have no correlations with the upstream solar wind parameters, except for a good correlation between the solar wind velocity and the temperature of the backscattered ENAs. These results suggest that the reflected ENAs have experienced several collisions during the interaction with the loose lunar grains, and are then released into space. The mathematical model of the energy spectra of the backscattered ENAs is expressed by a function of the solar wind flux and velocity, which can be used for future investigations of regolith‐solar wind interaction.

Two spacecraft observations of magnetic discontinuities in the solar wind with STEREO

Directional discontinuities in the solar wind magnetic field are often explored by either single‐spacecraft studies with tens of thousands of events or multiple‐spacecraft studies with between ten and a few hundred events. The unique orbits of the twin STEREO spacecraft afford the opportunity for a multiple‐spacecraft study of tens of thousands of directional discontinuities at variable spacecraft separation. Automated algorithms are developed both for identifying magnetic directional discontinuities at a single spacecraft and for matching discontinuities between spacecraft. Four months of data yield tens of thousands of discontinuities per spacecraft and several thousand discontinuities identified at both spacecraft. Rates of occurrence, minimum lengths, and aspect ratios are determined for discontinuities identified at both spacecraft. Distributions of directional discontinuity properties, including thickness, normal vector orientation, waiting times between successive discontinuities, and the angle through which the magnetic field rotates while crossing the discontinuity (shear angle), are determined for directional discontinuities observed at both spacecraft as well as those observed at only a single spacecraft. It is found that the distributions of many of these properties are different between directional discontinuities observed by both spacecraft and those observed by only a single spacecraft. Based on the nature of these differences, the population of discontinuities observed by both spacecraft is most consistently interpreted as the walls of solar wind flux tubes.

Enhancement of Terrestrial Diffuse X-Ray Emission Associated with Coronal Mass Ejection and Geomagnetic Storm

Abstract We present an analysis of a Suzaku observations taken during the geomagnetic storm of 2005 August 23–24. We found a time variation of diffuse soft X-ray emission when a coronal mass ejection hit Earth and caused a geomagnetic storm. The diffuse emission consisted of fluorescent scattering of solar X-rays and an exospheric solar wind charge exchange. The former is characterized by a neutral oxygen emission line due to strong heating of the upper atmosphere during the storm time, while the latter is dominated by the sum of C V, C VI, N VI, N VII, O VII, and O VIII emission lines due to an enhanced solar wind flux in the vicinity of the exosphere. Using the solar-wind data taken with the ACE and WIND satellites, a time correlation between the solar wind and the strong O VII line flux was investigated. We estimated necessary column densities for the solar X-ray scattering and exospheric SWCX. From these results, we argue that a part of the solar-wind ions enters inside the magnetosphere and causes the SWCX reaction.

Does the space environment affect the ecosphere?

The Sun is now emerging from a deep and protracted solar minimum, when the power, pressure, flux, and magnetic flux of solar wind were at their lowest levels [McComas et al., 2008; Schwadron and McComas, 2008;Connick et al., 2011]. Because of an anomalously weak heliospheric magnetic field and low solar wind pressure, galactic cosmic rays (GCRs)—protons, electrons, and ionized nuclei of elements accelerated to high energies—achieved the highest fluxes observed in the space age (Figure 1) [Mewaldt et al., 2010]. Related observations have shown remarkably rapid changes in the fluxes of energetic neutral atoms (ENAs) used by NASA's Interstellar Boundary Explorer mission to image the global heliosphere surrounding the solar system [McComas et al., 2010]. These changes in ENAs are caused by decreasing solar wind pressure. Does the recent anomalous deep solar minimum hint at larger changes in store? And how do changing GCR fluxes and conditions on the Sun influence Earth's ecosphere? Given the fact that GCR radiation can damage living tissue, causing cellular mutagenesis, the changing state of the Sun may have serious implications for life on the planet.

Heavy ion escape from Mars, influence from solar wind conditions and crustal magnetic fields

We have used more than 4 years of Mars Express ion data to estimate the escape of heavy ions ( O + , O 2 + and CO 2 + ) from Mars. To take the limited field of view of the instrument into account, the data has been binned into spatial bins and angular bins to create average distribution functions for different positions in the near Mars space. The net escape flux for the studied low solar activity period, between May 2007 and May 2011, is 2.0 ± 0.2 × 10 24 s −1. The escape has been calculated independently for four different quadrants in the Y MSO − Z MSO plane, south, dusk, north and dawn. Escape is highest from the northern and dusk quadrants, 0.6 ± 0.1 × 10 24 s −1, and smallest from the south and dawn quadrants, 0.4 ± 0.1 × 10 24 s −1. The flux ratio of molecular ( O 2 + and CO 2 + ) to O + ions is 0.9 ± 0.1, averaged over all quadrants. The flux difference between the north and south quadrants is statistically significant, and is presumed to be due to the presence of significant crustal magnetic fields in the southern hemisphere, reducing the outflow. The difference between the dawn and dusk quadrants is likely due to the magnetic tension associated with the nominal Parker angle spiral, which should lead to higher average magnetic tension on the dusk side. The escape increases during periods of high solar wind flux and during times when co-rotating interaction regions (CIR) affect Mars. In the latter case the increase is a factor 2.4–2.9 as compared to average conditions.

Detection of m/q = 2 pickup ions in the plasma environment of the Moon: The trace of exospheric H2+

[1] The Solar Wind Ion Detectors (SWIDs) on the Chang'E-1 spacecraft, while orbiting the Moon, occasionally observed two continuous flux peaks with energies not exceeding 8 and 4 times that of the prevailing solar wind proton energy. These form parallel curves (PCs) with an energy ratio of 2 in the energy-time spectrogram. The fluxes of the two curves are comparable, around 10−5 ∼ 10− 4 of the solar wind flux. The pitch angle distribution of PC particles is concentrated around 90°. The velocity space distribution of PC particles shows distinct double-ring feature, suggesting the existence of a pickup ion species with m/q = 2. Pickup ions from local interstellar medium, the inner sources and the lunar exosphere are investigated. We conclude that this observation may be the first in situ evidence for H2+ ions in the lunar exosphere, thus providing new insights on the evolution and fate of solar wind hydrogen in the solar system.

Interplanetary medium condition effects in the South Atlantic Magnetic Anomaly: A case study

One way to investigate the magnetosphere–ionosphere coupling is through the simultaneous observation of different parameters measured at different locations of the geospace environment and try to determine some relationships among them. The main objective of this work is to examine how the solar energetic particles and the interplanetary medium conditions may affect the space and time configuration of the ring current at low-latitudes and also to get a better understanding on how these particles interfere with the lower ionosphere in the South Atlantic Magnetic Anomaly region ( SAMA). To accomplish this, the cosmic noise absorption ( CNA) and the horizontal component of the Earth's magnetic field data measured from sites located in the SAMA region were compared with the proton and electron fluxes, interplanetary medium conditions (solar wind and the north–south component of the interplanetary magnetic field measured on board satellites), the SYM-H index and magnetometer data from Kakioka (KAK-Japan), located significantly outside the SAMA region. The time series analyzed correspond to the geomagnetic disturbance that occurred on August 25–30, 1998. The analysis was performed by implementing wavelet techniques, with particular attention to singularities detection, which highlights the presence of transient signals. The results are discussed in terms of the first three wavelet decomposition levels of the parameters. The magnitude of wavelet coefficients of the solar wind and proton flux at the two energy ranges analyzed is timely well correlated, indicating that these two signals are energetically linked. The larger wavelet coefficient amplitude of KAK and VSS magnetograms shows time delays that are compatible with an asymmetric configuration of the ring current, considering that at the storm time, VSS was at the dawn sector of the magnetosphere and KAK at the dusk side. The wavelet analysis of CNA signals reveals that the signal may be sensitive to the ionization produced by energetic electrons and protons as well. The time delays observed in wavelet coefficients may give an indication of the different accelerating process to which the particles are submitted when traveling along the magnetic field lines, from higher to lower latitudes, and the likely contribution of these particles to the ionization measured as an absorption of the cosmic noise in the lower ionosphere.

Global inventory of Helium-3 in lunar regoliths estimated by a multi-channel microwave radiometer on the Chang-E 1 lunar satellite

Helium-3 (3He) implanted by solar wind in the lunar regolith is a valuable resource because of its potential as a fusion fuel. On the basis of the Apollo regolith samples, a linear relationship between 3He abundance and solar wind flux, optical maturity and TiO2 content has been presented. China successfully launched its first lunar exploration satellite Chang-E 1 (CE-1) on October 24, 2007. A multi-channeled microwave radiometer was aboard the satellite with the purpose of measuring microwave thermal emission from the lunar surface layer. From the multi-channel brightness temperature (T b) observed by CE-1, the global distribution of the regolith thickness was inverted from the multi-channel T b, and was used to evaluate the total amount of 3He per unit area in the lunar regolith. The global inventory of 3He was estimated as being 6.6×108 kg; 3.7×108 kg for the lunar nearside and 2.9×108 kg for the lunar farside.

Non-thermal water loss of the early Mars: 3D multi-ion hybrid simulations

In this study we analyze the non-thermal loss rates of O +, O 2 + and CO 2 + ions over the last 4.5 billion years (Gyr) in the Martian history by using a 3D hybrid model. For this reason we derived the past solar wind conditions in detail. We take into account the intensified particle flux of the early Sun as well as an Martian atmosphere, which was exposed to a sun's extreme ultraviolet (EUV) radiation flux 4.5 Gyr ago that was 100 times stronger than today. Furthermore, we model the evolution of the interplanetary magnetic field by a Weber & Davis solar wind model. The ‘external’ influences of the Sun's radiation flux and solar wind flux lead to the formation of an ionospheric obstacle by photoionization, charge exchange and electron impact. For the early Martian conditions we could show that charge exchange was the dominant ionization mechanism. Several hybrid simulations for different stages in the evolution of the Martian atmosphere, at 1, 2, 5, 10, 30 and 100 EUV, were performed to analyze the non-thermal escape processes by ion pick-up, momentum transfer from the solar wind to the ionosphere and detached ionospheric plasma clouds. Our results show a non-linear evolution of the loss rates. Using mean solar wind parameters the simulations result in an oxygen loss equivalent to the depth of a global Martian ocean of about 2.6 m over the last 4.5 Gyr. The induced magnetic field strength could be increased up to about 2000 nT. A simulation run with high solar wind density results in an oxygen loss of a Martian ocean up to 205 m depth during 150 million years after the sun reached the zero age mean sequence (ZAMS).

Simulations of the photoelectron sheath and dust levitation on the lunar surface

The lunar surface represents a complex plasma environment due to the presence of solar ultraviolet (UV) radiation, the incoming solar wind flux and charged, levitated micron‐ and sub‐micron sized dust particles. Photoemission due to solar UV radiation dominates the charging environment, creating a photoelectron sheath above the lunar surface. To further investigate the dusty plasma environment on the surface of the Moon, a one‐dimensional particle‐in‐cell (PIC) code has been designed specifically for the lunar surface. The code has been validated against analytic solutions for photoelectron sheaths with basic photoelectron energy distributions. Simulations have focused on the role of the emitted photoelectron energy distribution and solar UV variability in determining the sheath profile. Additionally, the charging and levitation of test dust particles in the photoelectron sheath are studied. Limits on the maximum size and height of levitated dust grains are also presented.