Solar Wind Minor Ions Research Articles

Fractionation of Solar Wind Minor Ion Precipitation by the Lunar Paleomagnetosphere

Abstract The analysis of solar wind material implanted within lunar soil has provided significant insight into the makeup and evolutionary history of the solar wind and, by extension, the Sun and protosolar nebula. These analyses often rely on the tacit assumption that the Moon has served as an unbiased recorder of solar wind composition over its 4.5 billion yr lifetime. Recent work, however, has shown that for a majority of its lifetime, the Moon has possessed a dynamo that generates a global magnetic field with surface field strengths of at least 5 μT. In turn, the presence of such a field has been shown to significantly alter the lunar–solar wind interaction via the formation of a lunar “paleomagnetosphere.” This paleomagnetosphere has implications for the flux of solar wind minor ions to the lunar surface and their subsequent implantation in lunar soil grains. Here we use a three-dimensional hybrid plasma model to investigate the effects of the lunar paleomagnetosphere on the dynamics and precipitation of solar wind minor ions to the lunar surface. The model results show that the lunar paleomagnetosphere can suppress minor ion fluxes to the lunar surface by more than an order of magnitude and strongly fractionates the precipitating solar wind in a complex, nonlinear fashion with respect to both the minor ion charge-to-mass ratio and the surface paleomagnetic field strength. We discuss the implications of these results with respect to both the analysis of trapped material in lunar grains and the semiquantitative 40Ar/36Ar antiquity indicator for lunar soils.

On the Long-term Weathering of Airless Body Surfaces by the Heavy Minor Ions of the Solar Wind: Inputs from Ion Observations and SRIM Simulations

Abstract The importance of solar wind minor ions heavier than alpha particles in weathering airless body surfaces is an open debate. The fundamental question at stake is whether the variety of different minor ion species, their high masses, and their high charge states may overcome their low densities in the solar wind to enable them to significantly contribute to ion weathering processes. Here, long-term effects that develop on geological timescales are investigated. To do so, the long-term averaged energy spectrum of thermal and suprathermal solar wind ions is estimated by compiling and contrasting ion measurements gathered by the Advanced Composition Explorer (ACE), Wind, Solar TErrestrial RElations Observatory (STEREO), ARTEMIS, and Mars Atmosphere and Volatile Evolution (MAVEN) missions. The long-term ion environment is then convolved with Stopping and Range of Ions in Matter simulations. Combining these data and models, we find that solar wind minor ions significantly alter airless body surfaces, as they contribute to 8%–14% of the total sputtering and create 20%–50% of atomic displacements at depths greater than 100 nm. The new approach presented in this article therefore confirms that solar wind minor ions play an important role in the ion weathering of airless surfaces throughout the solar system.

A Comparison of Alpha Particle and Proton Beam Differential Flows in Collisionally Young Solar Wind

Abstract In fast wind or when the local Coulomb collision frequency is low, observations show that solar wind minor ions and ion subpopulations flow with different bulk velocities. Measurements indicate that the drift speed of both alpha particles and proton beams with respect to the bulk or core protons rarely exceeds the local Alfvén speed, suggesting that a magnetic instability or other wave–particle processes limits their maximum drift. We compare simultaneous alpha particle, proton beam, and proton core observations from instruments on the Wind spacecraft spanning over 20 years. In nearly collisionless solar wind, we find that the normalized alpha particle drift speed is slower than the normalized proton beam speed, no correlation between fluctuations in both species’ drifts about their means, and a strong anti-correlation between collisional age and alpha–proton differential flow, but no such correlation with proton beam–core differential flow. Controlling for the collisional dependence, both species’ normalized drifts exhibit similar statistical distributions. In the asymptotic, zero Coulomb collision limit, the youngest measured differential flows most strongly correlate with an approximation of the Alfvén speed that includes proton pressure anisotropy. In this limit and with this most precise representation, alpha particles drift at 67% and proton beam drift is approximately 105% of the local Alfvén speed. We posit that one of two physical explanations is possible. Either (1) an Alfvénic process preferentially accelerates or sustains proton beams and not alphas or (2) alpha particles are more susceptible to either an instability or Coulomb drag than proton beams.

REGULATION OF ION DRIFTS AND ANISOTROPIES BY PARAMETRICALLY UNSTABLE FINITE-AMPLITUDE ALFVÉN-CYCLOTRON WAVES IN THE FAST SOLAR WIND

We study the preferential heating and differential acceleration of minor ions by dissipation of ion-acoustic waves (IAWs) generated by parametric instabilities of a finite-amplitude monochromatic Alfv?n-cyclotron pump wave. We consider the associated kinetic effects of Landau damping and nonlinear pitch-angle scattering of protons and ? particles in the tenuous plasma of coronal holes and the fast solar wind. Various data collected by Wind spacecraft show signatures for a local transverse heating of the minor ions, presumably by Alfv?n-cyclotron wave dissipation, and an unexpected parallel heating by a so far unknown mechanism. Here, we present the results from a set of 1.5 dimensional hybrid simulations in search for a plausible explanation for the observed field-aligned kinetic features in the fast solar wind minor ions. We investigate the origin and regulation of ion relative drifts and temperature anisotropies in low plasma ?, fast solar wind conditions. Depending on their initial drifts, both ion species can heat up not only transversely through cyclotron resonance and non-resonant wave-particle interactions, but also strongly in the parallel direction by Landau damping of the daughter IAWs. We discuss the dependence of the relative ion drifts and temperature anisotropies on the plasma ? of the individual species and we describe the effect of the pump wave amplitude on the ion heating and acceleration.

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.

Solar wind charge exchange X-ray emission from Mars

Aims. We study the soft X-ray emission induced by charge exchange (CX) collisions between solar-wind, highly charged ions and neutral atoms of the Martian exosphere. Methods. A 3D multi species hybrid simulation model with improved spatial resolution (130 km) is used to describe the interaction between the solar wind and the Martian neutrals. We calculated velocity and density distributions of the solar wind plasma in the Martian environment with realistic planetary ions description, using spherically symmetric exospheric H and O profiles. Following that, a 3D test-particle model was developed to compute the X-ray emission produced by CX collisions between neutrals and solar wind minor ions. The model results are compared to XMM-Newton observations of Mars. Results. We calculate projected X-ray emission maps for the XMM-Newton observing conditions and demonstrate how the X-ray emission reflects the Martian electromagnetic structure in accordance with the observed X-ray images. Our maps confirm that X-ray images are a powerful tool for the study of solar wind - planetary interfaces. However, the simulation results reveal several quantitative discrepancies compared to the observations. Typical solar wind and neutral coronae conditions corresponding to the 2003 observation period of Mars cannot reproduce the high luminosity or the corresponding very extended halo observed with XMM-Newton. Potential explanations of these discrepancies are discussed.

Chandra observations of Comet 9P/Tempel 1 during the Deep Impact campaign

We present results from the Chandra X-ray Observatory's extensive campaign studying Comet 9P/Tempel 1 (T1) in support of NASA's Deep Impact (DI) mission. T1 was observed for ∼ 295 ks between 30th June and 24th July 2005, and continuously for ∼ 64 ks on July 4th during the impact event. X-ray emission qualitatively similar to that observed for the collisionally thin Comet 2P/Encke system [Lisse, C.M., Christian, D.J., Dennerl, K., Wolk, S.J., Bodewits, D., Hoekstra, R., Combi, M.R., Mäkinen, T., Dryer, M., Fry, C.D., Weaver, H., 2005b. Astrophys. J. 635 (2005) 1329–1347] was found, with emission morphology centered on the nucleus and emission lines due to C, N, O, and Ne solar wind minor ions. The comet was relatively faint on July 4th, and the total increase in X-ray flux due to the Deep Impact event was small, ∼ 20 % of the immediate pre-impact value, consistent with estimates that the total coma neutral gas release due to the impact was 5 × 10 6 kg ( ∼ 10 h of normal emission). No obvious prompt X-ray flash due to the impact was seen. Extension of the emission in the direction of outflow of the ejecta was observed, suggesting the presence of continued outgassing of this material. Variable spectral features due to changing solar wind flux densities and charge states were clearly seen. Two peaks, much stronger than the man-made increase due to Deep Impact, were found in the observed X-rays on June 30th and July 8th, 2005, and are coincident with increases in the solar wind flux arriving at the comet. Modeling of the Chandra data using observed gas production rates and ACE solar wind ion fluxes with a CXE mechanism for the emission is consistent, overall, 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, with intermittent impulsive events due to solar flares and coronal mass ejections.

Chandra observations of Comet 9P/Tempel 1 during the Deep Impact campaign

We present results from the Chandra X-ray Observatory's extensive campaign studying Comet 9P/Tempel 1 (T1) in support of NASA's Deep Impact (DI) mission. T1 was observed for ∼ 295 ks between 30th June and 24th July 2005, and continuously for ∼ 64 ks on July 4th during the impact event. X-ray emission qualitatively similar to that observed for the collisionally thin Comet 2P/Encke system [Lisse, C.M., Christian, D.J., Dennerl, K., Wolk, S.J., Bodewits, D., Hoekstra, R., Combi, M.R., Mäkinen, T., Dryer, M., Fry, C.D., Weaver, H., 2005b. Astrophys. J. 635 (2005) 1329–1347] was found, with emission morphology centered on the nucleus and emission lines due to C, N, O, and Ne solar wind minor ions. The comet was relatively faint on July 4th, and the total increase in X-ray flux due to the Deep Impact event was small, ∼ 20 % of the immediate pre-impact value, consistent with estimates that the total coma neutral gas release due to the impact was 5 × 10 6 kg ( ∼ 10 h of normal emission). No obvious prompt X-ray flash due to the impact was seen. Extension of the emission in the direction of outflow of the ejecta was observed, suggesting the presence of continued outgassing of this material. Variable spectral features due to changing solar wind flux densities and charge states were clearly seen. Two peaks, much stronger than the man-made increase due to Deep Impact, were found in the observed X-rays on June 30th and July 8th, 2005, and are coincident with increases in the solar wind flux arriving at the comet. Modeling of the Chandra data using observed gas production rates and ACE solar wind ion fluxes with a CXE mechanism for the emission is consistent, overall, 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, with intermittent impulsive events due to solar flares and coronal mass ejections.

Modeling the Energy Budget of Solar Wind Minor Ions: Implications for Temperatures and Abundances

The outflow of oxygen and silicon ions in the solar wind has been studied using a model that extends from the chromosphere into interplanetary space, with emphasis on understanding the energy budget of the minor ions. The model solves coupled gyrotropic transport equations, which account for temperature anisotropies and heat conduction, for all charge states, and includes ionization and recombination. The minor ions are heated with a constant heating rate per particle in the corona. In the transition region the thermal force causes minor ions to flow faster than protons, with an abundance that can be less than half of the chromospheric abundance. The ions quite suddenly decouple from the proton flow in the corona, and above this point the ion flow is independent of the proton flow. For high heating rates the coronal abundance is comparable to the chromospheric abundance, the ion terminal wind speed is high, and most of the deposited energy is lost into the solar wind. Low heating rates lead to very large coronal abundances and a low terminal flow speed, and the main energy loss is then through collisions with protons and electrons in the corona. The heavy ions become much hotter than protons in the corona, even without preferential heating of the ions. However, preferential heating is necessary to prevent a large abundance enhancement in the corona and to achieve flow speeds close to the speeds observed by the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO). The abundance enhancement implies that lowering the heating rate per particle in general leads to an increase in the total energy flux absorbed by the ions.

Deceleration of streaming alpha particles interacting with waves and imbedded rotational discontinuities

Alphas (He42+) and other ions in the interplanetary medium show a tendency to stream near or below the local Alfvén speed (VA) relative to the main component of protons. Because VA decreases with increasing distance from the Sun, forces must exist to slow the heavier ions with increasing distance. We have conducted hybrid simulations in a plasma with particle protons and alphas and with a quasineutralizing electron fluid. Simulation runs with other streaming minor ions were also performed. In our simulations, a group of Alfvén waves steepen and generate imbedded rotational discontinuities (RDs) and compressional waves. We examine cases of almost steady waveforms and RDs and ones with evolving waveforms and RDs due to a significantly nonuniform background. When alphas stream with the waves and imbedded RDs faster than the protons, they decelerate more rapidly from higher speeds and heat. We have concluded that alphas do not remain at one streaming speed due to nonlinear Lorentz forces from the wave compressional component and the presence of collisionless dissipation, which dissipates this component so that bulk alpha kinetic energy is ultimately deposited into alpha thermal energy. Imbedded RDs play no significant role in the overall deceleration of alphas. Protons heat similarly to cases without alphas and are slightly accelerated so that the total ion momentum along B0 is nearly conserved. For small streaming speeds (≲0.5 Alfvén speeds), the deceleration rate can be relatively small because the loss of streaming energy competes with the gain in wave kinetic energy required by large‐amplitude Alfvén waves. Less proton and more alpha heating is also found since alphas can resonate with the left‐handed portion of oblique waves. Starting from rest, alphas and protons can develop a small differential flow in which Lorentz and pressure forces become balanced. The simulation behavior of alphas for streaming speeds near the Alfvén speed is fairly consistent with solar wind observations in high‐speed streams. Turbulent Alfvénic fluctuations do have a small compressional component and so might be responsible for the observed deceleration and heating of alphas. Simulations with other streaming minor ions also gave deceleration, suggesting that the behavior of a wide range of solar wind minor ions might be explained by the same processes that affect alphas.

Charge exchange-induced X-ray emission from comet C/1999 S4 (LINEAR).

Using soft x-ray observations of the bright new comet C/1999 S4 (LINEAR) with the Chandra x-ray observatory, we have detected x-ray line emission created by charge exchange between highly ionized solar wind minor ions and neutral gases in the comet's coma. The emission morphology was symmetrically crescent shaped and extended out to 300,000 kilometers from the nucleus. The emission spectrum contains 6 lines at 320, 400, 490, 560, 600, and 670 electron volts, attributable to electron capture and radiative deexcitation by the solar wind species C(+5), C(+6), N(+7), O(+7), and O(+8). A contemporaneous 7-day soft x-ray light curve obtained using the Extreme Ultraviolet Explorer demonstrates a large increase in the comet's emission coincident with a strong solar flare on 14 and 15 July 2000.

Kinetic properties of solar wind minor ions and protons measured with SOHO/CELIAS

Using observations of the Charge Time‐of‐Flight(CTOF) charge and mass spectrometer of the Charge, Element and Isotope Analysis System (CELIAS), and of CELIAS/proton monitor on board the Solar and Heliospheric Observatory (SOHO), we present an overview of speeds and kinetic temperatures of minor ions and protons in the solar wind near solar minimum, covering the Carrington Rotations 1908 to 1912. In the case of a collision‐dominated solar wind the speed of minor ions is expected to be lower or equal to the speed of the protons, and all species are expected to have equal temperatures. On the other hand, minor ions can be accelerated and heated by wave‐particle interaction. In this case, equal thermal speeds of all species are expected. CTOF data allow the determination of the kinetic parameters of various ions with high accuracy and with high time resolution. The mean O6+ speed of the observed period is 390 km s−1. The speeds of Si7+ and Fe9+ correlate well with O6+, the linear correlation coefficient being 0.96 or higher. Our results also indicate that silicon and iron tend to lag behind oxygen with a speed difference of ∼20 km s−1 at 500 km s−1. At the same time, the kinetic temperature of the ions under investigation exhibit the well‐known mass proportionality, which is attributed to wave‐particle interactions. During the period of low solar activity in consideration, many cases are observed where the kinetic temperature is extraordinarily low (104 K for O6+).

Modeling of cometary X-rays caused by solar wind minor ions.

X-ray emission was discovered in comet Hyakutake (C/1996 B2) by the Röntgen satellite in 1996, and these emissions were attributed to the excitation of high charge state solar wind ions due to electron capture from cometary molecules or atoms. Using the plasma flow in the coma of Hyakutake calculated by a three-dimensional adaptive magnetohydrodynamic model, the density distribution of solar wind ions in the coma and the resulting x-ray emission were computed. The calculated High Resolution Imager count rate of 4.4 per second and the spatial distribution of the x-ray emission agree with the observations. A detailed energy spectrum of cometary x-rays is predicted in the 80 to 2000 electronvolt energy range. Cometary x-rays present a sensitive tool to monitor cometary activity and solar wind ion composition.

Modeling the effects of fast shocks on solar wind minor ions

Observations show that when α particles and other minor ions in the solar wind plasma encounter fast shocks they are heated more than protons and their bulk motion is decelerated less than protons. These effects have been studied using a three‐fluid model, and the model predictions have been compared with observations. The comparison indicates that for supercritical fast shocks the three‐fluid model can explain cross‐shock minor ion heating which is significantly greater than that of protons. When the ratio of specific heats for minor ions, γα, equals 2, both the lesser cross‐shock deceleration and the greater heating of minor ions than of protons can be predicted by the model; thus the minor ion heating through the shock transition region is consistent with the involvement of 2 degrees of freedom. Because the analysis is formulated in the de Hoffmann‐Teller frame of reference, the method is not valid for perpendicular shocks or when the angle is large. These results agree with the few extant observations and might be confirmed by further observations at the Earth's bow shock.

Ion‐cyclotron heating and acceleration of solar wind minor ions

The resonant acceleration and heating of solar wind minor ions via interactions with a spectrum of dispersive or nondispersive undamped ion‐cyclotron waves are investigated. The principal goal is to determine how the dimensionless parameters of the problem affect the heating and acceleration processes. In addition, simple physical interpretations of the heating and acceleration are given. The exact numerical evaluations of the heating and acceleration rates imply that (1) the total heating rate is roughly proportional to the mass of the ion; (2) dispersive waves should lead to νi >να >νp, where i refers to ions heavier than He++; (3) dispersive waves have a slight tendency to yield ∂Ti/∂t >(mi/mα) ∂Tα/∂t, while non‐dispersive waves have a slight tendency to yield ∂Ti/∂t <(mi/mα) ∂Tα/∂t. The predictions of this model are compared with observations and may offer an explanation for some observed properties of minor ions.