Interactions of solar wind plasma with dust grains: Effects of strong plasma anisotropy

In this paper we re-examined the fundamental physics of charging of a dust particle in the moon environment by tenuous anisotropic solar wind plasma. The majority of work on dusty (complex) plasmas is largely concerns with laboratory plasmas, in which charging process of dust grains is very fast, thus making practical the working concept of dynamically equilibrium floating potential and grain charge. However, solar wind plasma parameters are considerably different at the moon orbit, and we found the characteristic charging time of lunar dust grains to be considerably longer, ranging from 3 to 4.6 min for micron size particles, and up to 7.6 h for 10-nm grains, depending on the value of plasma streaming velocity. These findings make it clear that the transient stage of charging process is important in the moon environment, and equilibrium floating potential and grain charge could be considered as long time asymptotic values. For this reason we re-formulated the moon dust charging process as an inherently time-dependent problem and derived the time-dependent charging equation for the grain potential for general case of anisotropic solar wind plasma. Using the results of our kinetics analysis we found that the distribution of charge density over grain surface submerged into solar wind plasma is highly anisotropic, thus making the OML model, which is based on the assumption of isotropic distribution of surface charge density, not applicable to the grain charging problem by the solar wind plasma.     

Acceleration of charged particles by fluctuating and steady electric fields in a X-type magnetic field

Release of stored magnetic energy via particle acceleration is a characteristic feature of astrophysical plasmas. Magnetic reconnection is one of the mechanisms for releasing energy from magnetized plasmas. Collisionless magnetic reconnection could provide both the energy release mechanism and the particle accelerator in space plasmas. Here we studied particle acceleration when fluctuating (in-time) electric fields are superposed on an static X-type magnetic field in collisionless hot solar plasma. This system is chosen to mimic the reconnective dissipation of a linear MHD disturbance. Our results are compared to particle acceleration from constant electric field superposed on an X-type magnetic field. The constant electric field configuration represents the effects of steady state magnetic reconnection. Time evolution of ion and electron distributions are obtained by numerically integrating particle trajectories. The frequencies of the electric field represent a turbulent range of waves. Depending on the frequency and amplitude of the electric field, electrons and ions are accelerated to different degrees and have energy distributions of bimodal form consisting of a lower energy part and a high energy tail. For frequencies (ω in dimensioless units) in the range 0.5 ? ω ? 1.0 a substantial fraction (20%–30%) of the proton distribution is accelerated to gamma-ray producing energies. For frequencies in the range 1 ? ω ? 100.0 the bulk of the electron distribution is accelerated to hard X-ray producing energies. The acceleration mechanism is important for solar flares and solar noise storms but it could be applicable to all collisionless astrophysical plasmas.     

Theoretical modelling of X-ray fluorescence signals for different lunar compositions and dependence on solar activity

We present a forward modelling technique for calculating the surface X-ray spectra for a variety of lunar terrains. Our calculations considered variations in solar fluxes from solar quiescent condition to large flare activity (M1 flare), and expected elemental concentrations in the target, as well as yield, instrumental, and viewing geometry parameters for X-ray induced fluorescence from the lunar surface. Additionally, we present estimates of anticipated XRF signals from prominent K_α lines observable by a collimated 14 cm² X-ray detector from a 100 km lunar orbit with ∼20 km spatial resolution. Our results show that Mg, Al and Si characteristic K_α lines can be observed for all solar conditions. The Ca K_α lines line can be differentiated from a fixed background during more energetic solar conditions such as C1 and M1 flares, whereas Ti and Fe lines are identifiable only during C1 and M1 solar flare conditions for Apollo 12 site composition. Both the K_α X-ray intensity ratios of Mg/Si and Al/Si correlate well with concentration ratios of Mg/Si and Al/Si, respectively, for B1 and M1 solar conditions. The K_α X-ray intensity ratios of Fe/Si and Ca/Si correlates with concentration ratios of Fe/Si and Ca/Si, respectively, for M1 solar condition. In principle, the modelling technique outlined here can be used to determine absolute elemental abundances (Mg, Al, Si, Ca, Ti and Fe) from X-ray spectra measured during recent and future lunar missions.     

Plasma electron temperature variability in lunar surface potential and in electric field under average solar wind conditions

The Moon is immersed in plasma environment. The most interesting challenge of the lunar plasma– field environment is that it is alternatively dominated by the extended but variable outer atmosphere of the Earth – the magnetosphere – and by the extended but highly variable solar atmosphere – the solar wind. Understanding the plasma environment and its interaction with the lunar surface will be beneficial to both manned and robotic surface exploration activities and to scientific investigations. Presented is a preliminary map of variations of lunar surface electric potential over the day side and night side using probe equations and a discussion on dust dynamics in this E-field structure using the data from Electron Reflectometer in Lunar Prospector spacecraft during 1998–1999. On the day side, potential is around 5 V and on the night side it reaches up to −82 V. On the night side region, only highly energetic electrons can overcome this large negative potential. The variation at electron temperature (T_e) strongly reflects in the surface potential. The potential reaches to a value of −82 V for T_e = 58 eV. Surface charging causes the electrostatic transport of charged dust grains. Dust grain size of 0.1 μm shows a levitation height of 4.92 m on lunar day side, 748 m on terminator region and 3.7 km on the night side. The radius of maximum sized grain to be lofted, R_max, peaks at the terminator region (R_max = 0.83 μm). At the transition region dust levitation is almost absent. This region is most suited for exploration activities as the region is free from hazards caused by lunar dust.     

Propagation of upstream protons in the near-earth solar wind

The propagation of energetic protons (35–1600 keV) from the Earth's magnetosphere to the ISEE-3 spacecraft located about 240 earth radii (R_E) upstream in the solar wind is used as a tool to study the interaction between these protons and the solar wind. In this preliminary study we present proton pitch angle distributions seen at different times during the development of upstream events that occur in relatively quiet interplanetary conditions. In general a highly anisotropic sunward flow is seen at the beginning of the events. During the course of the events pitch angle distributions may vary between streaming along the field lines (peaked around 0° pitch angle), a uniform intensity between 0° and 90°, and a peaked distribution around a preferred pitch angle that is often near 90°.     

Contribution of geometry of interaction between interplanetary and terrestrial magnetic fields into global magnetospheric state and geomagnetic activity

The paper presents results of our study of dependence of geomagnetic activity from geoeffective parameters taking into account mutual orientation of the interplanetary magnetic field, electric field of the solar wind and geomagnetic moment. We attract a reconnection model elaborated by us made allowance for changes of geometry of the solar wind–magnetosphere interaction during annual and diurnal motions of the Earth. We take as our data base the interplanetary magnetic field and solar wind velocity measured at 1 a.u. at ecliptic plane for the period of 1963–2005 and K_p, D_st, a_m indices. Taken as a whole a geoeffective parameter suggested by us explains 95% of observed variations of the indices. Changes of the geometric factor determined by mutual orientation of the solar wind electric field and geomagnetic moment explain larger than 75% of observed statistical variations of D_st and a_m indices. Based on our results we suggest a new explanation of semi-annual and UT variation of geomagnetic activity.     

Improved interpretation of solar wind ion measurements via high-resolution magnetic field data

The Wind   spacecraft’s Faraday cups (FC) continue to produce high-quality, in situ observations of thermal protons (i.e., ionized hydrogen) and α$\alpha$-particles (i.e., fully ionized helium) in the solar wind. By fitting a Wind/FC ion spectrum with a model velocity distribution function (VDF) for each particle species, values for density, bulk velocity, and temperature can be inferred. Incorporating measurements of the background magnetic field from the Wind Magnetic Field Investigation (MFI) allows perpendicular and parallel temperature components to be separated. Prior implementations of this analysis averaged the higher-cadence Wind/MFI measurements to match that of the Wind/FC ion spectra. However, this article summarizes recent and extensive revisions to the analysis software that, among other things, eliminate such averaging and thereby account for variations in the direction of the magnetic field over the time taken to measure the ions. A statistical comparison reveals that the old version consistently underestimates the temperature anisotropy of ion VDF’s: averaging over fluctuations in the magnetic field essentially blurs the perpendicular and parallel temperature components, which makes the plasma seem artificially more isotropic. The new version not only provides a more accurate dataset of ion parameters (which is well suited to the study of microkinetic phenomena), it also demonstrates a novel technique for jointly processing particle and field data. Such methods are crucial to heliophysics as wave-particle interactions are increasingly seen as playing an important role in the dynamics of the solar wind and similar space plasmas.     

Changes in sea-level pressure over South Korea associated with high-speed solar wind events

We explore a possibility that the daily sea-level pressure (SLP) over South Korea responds to the high-speed solar wind event. This is of interest in two aspects: first, if there is a statistical association this can be another piece of evidence showing that various meteorological observables indeed respond to variations in the interplanetary environment. Second, this can be a very crucial observational constraint since most models proposed so far are expected to preferentially work in higher latitude regions than the low latitude region studied here. We have examined daily solar wind speed V, daily SLP difference ΔSLP, and daily log(BV²) using the superposed epoch analysis in which the key date is set such that the daily solar wind speed exceeds 800 km s⁻¹. We find that the daily ΔSLP averaged out of 12 events reaches its peak at day +1 and gradually decreases back to its normal level. The amount of positive deviation of ΔSLP is +2.5 hPa. The duration of deviation is a few days. We also find that ΔSLP is well correlated with both the speed of solar wind and log(BV²). The obtained linear correlation coefficients and chance probabilities with one-day lag for two cases are r ? 0.81 with P > 99.9%, and r ? 0.84 with P > 99.9%, respectively. We conclude by briefly discussing future direction to pursue.     

Simulation of ASTROD I test mass charging due to solar energetic particles and interplanetary electrons

As ASTROD I travels through space, its test mass will accrue charge due to exposure of the spacecraft to high-energy particles. This test mass charge will result in Coulomb forces between the test mass and the surrounding electrodes. In earlier work, we have used the GEANT 4 toolkit to simulate charging of the ASTROD test mass due to cosmic-ray protons of energies between 0.1 and 1000 GeV at solar maximum and at solar minimum. Here we use GEANT 4 to simulate the charging process due to solar energetic particle events and interplanetary electrons. We then estimate the test mass acceleration noise due to these fluxes. The predicted charging rates range from 2247 e⁺/s to 47,055 e⁺/s, at peak intensity, for the four largest SEP events in September and October 1989. Although the noise due to charging exceeds the ASTROD I budget for the two larger events, it can be suppressed through continuous discharging. The acceleration noise during the two small events is well below the design target. The charging rate of the ASTROD I test mass due to interplanetary electrons in this simulation is about −11% of the cosmic-ray protons at solar minimum, and over −37% at solar maximum. In addition to the Monte Carlo uncertainty, an error of ±30% in the net charging rates should be added to account for uncertainties in the spectra, physics models and geometry implementations.     

Energy conversion of the flare due to direct electric fields from the sheared reconnection

In this paper we present a new mechanism of the main energy conversion of the solar flare. Since a flare inducing prominence (flux tube) rises V_z ? 300 km s⁻¹, the plasmas below it cannot continuously eject with Alfvén speeds of V_A = 3000 km s⁻¹ but probably with V_z ≈ ±100 km s⁻¹. Plasma up and downflows with V_A will within a short duration be blocked between the chromosphere where reconnected flux tubes are piling up, and the slowly rising flux rope. Hence the Petschek slow shock mechanism is difficult to be realized as a major energy converting mechanism.     

The investigation of powerful solar flares characteristics by analysis of excited states of C and various neutrons capture lines

Accelerated energetic particles in solar flares produced nuclear γ-lines in interactions with ambient solar atmosphere. Analysis of intensity of ratios between various γ-lines allows us to make estimations of abundance of elements, parameters of surrounding media and other solar characteristics. In this article we discuss the flux ratio between two lines from excited states of ¹²C (f_15.11/f_4.44) and our results of preliminary calculation of intensity ratio between two neutron capture lines at ³He and ¹H (f_20.58/f_2.223). In particular we consider the opportunity to obtain n(³He)/n(¹H) ratio during solar flares and using high-energy gamma-emission studying, based on the satellite data. Possible interpretation of spectral features observed during the January 20, 2005 solar flare is discussed. Preliminary analysis of energy spectrum in the band of 2–21 MeV gives n(³He)/n(¹H) ∼ 8 × 10⁻⁴ for January 20, 2005 solar flare.     

Spectral analysis of time-integrated Konus–Wind GRBs: Implication on radiative mechanisms

We have analysed a sample of 328 time-integrated GRB prompt emission spectra taken via the Konus instrument on board the US GGS-Wind spacecraft between 2002 and 2004 using a couple of two-components models, Cut-off Power Law (CPL) + Power Law (PL) and blackbody (BB) + PL. The spectra show clear deviation from the Band function. The PL term is interpreted as the low energy tail of a nonthermal emission mechanism. The distributions of corresponding index β give values β < −2/3 consistent with synchrotron and synchrotron self-Compton mechanisms. The distribution of low energy index α associated with the CPL term shows clear discordance with synchrotron models for 31.4% of the analysed GRBs with values exceeding that for the line of death, α = −2/3. Then, a set of nonthermal radiation mechanisms producing harder slopes, i.e., α > −2/3, are presented and discussed. For the remaining majority (68.6%) of GRBs with CPL index α < −2/3, we show that optically thin synchrotron produced by a power law electron distribution of type, N(γ) ∼ γ^−p, γ₁ < γ < γ₂, for finite energy range (γ₂ ≠ ∞) is a likely emission mechanism with α ∼−(p + 1)/2 in the frequency range ν₁ ? ν ? ν₂ (where ν₂ = η²ν₁ with η = γ₂/γ₁), such that for p > 1/3, one gets α < −2/3. We also show that corresponding spectra in terms of F_ν and νF_ν functions are peaked around frequency ν₂ instead of ν₁, respectively for p < 1 and p < 3. Besides, thermal emission is examined taking a single Planck function for fitting the low energy range. It can be interpreted as an early emission from the GRB fireball photosphere with observed mean temperature, kT′ ∼ 16.8 keV. Furthermore, we have performed a statistical comparison between the CPL + PL and BB + PL models finding comparable χ²-values for an important fraction of GRBs, which makes it difficult to distinguish which model and specific radiation mechanism (possible thermal or nonthermal γ-ray emissions) are best suitable for describing the reported data. Therefore, additional information for those bursts, such as γ-ray polarization, would be highly desirable in future determinations of GRBs observational data.     

Some features of the interplanetary disturbances in the post-solar maximum year period

Features of strong interplanetary disturbances (including 14 shock waves) are considered by the solar wind plasma measurements onboard the PROGNOZ-8 satellite. Examination of large-scale structure of the plasma fluxes enabled us to discover extreme values of proton temperature (～10⁶K) and density (～10²cm^?3) in some cases.The energy transferred by the interplanetary shock waves (10³¹–10³² erg) and their deceleration are estimated. Determination of the plasma parameter jumps for protons and α-particles at the shock front made it possible to estimate the potential barrier (40–400V) depending on magnetosonic Mach number.     

Viewing radiation signatures of solar energetic particles in interplanetary space

A current serious limitation on the studies of solar energetic particle (SEP) events is that their properties in the inner heliosphere are studied only through in situ spacecraft observations. Our understanding of spatial distributions and temporal variations of SEP events has come through statistical studies of many such events over several solar cycles. In contrast, flare SEPs in the solar corona can be imaged through their radiative and collisional interactions with solar fields and particles. We suggest that the heliospheric SEPs may also interact with heliospheric particles and fields to produce signatures which can be remotely observed and imaged. A challenge with any such candidate signature is to separate it from that of flare SEPs. The optimum case for imaging high-energy (E > 100 MeV) heliospheric protons may be the emission of π⁰-decay γ-rays following proton collisions with solar wind (SW) ions. In the case of E > 1 MeV electrons, gyrosynchrotron radio emission may be the most readily detectible remote signal. In both cases we may already have observed one or two such events. Another radiative signature from nonthermal particles may be resonant transition radiation, which has likely already been observed from solar flare electrons. We discuss energetic neutrons as another possible remote signature, but we rule out γ-ray line and 0.511 MeV positron annihilation emission as observable signatures of heliospheric energetic ions. We are already acquiring global signatures of large inner-heliospheric SW density features and of heliosheath interactions between the SW and interstellar neutral ions. By finding an appropriate observable signature of remote heliospheric SEPs, we could supplement the in situ observations with global maps of energetic SEP events to provide a comprehensive view of SEP events.     

Study of the solar Slow Sonic,Alfvén and Fast Magnetosonic transition surfaces

In this paper we study the shape, extend and time variations of the solar wind transition surfaces using the Lima and Priest (1993) hydrodynamic model adequately adapted for the case of the solar wind flow. The transition surfaces, namely the Slow (Sonic), the Alfvén, and the Fast Magnetosonic surface, are important boundaries around the Sun and play a crucial role in the development of the solar wind and the structure of the inner heliosphere. We determine the shape and dimension of these surfaces as a function of heliographic latitude using measurements from Ulysses spacecraft, and we also study their temporal variation using data from spacecrafts at 1 AU (OMNI database). Furthermore, we establish their dependence with the solar activity, demonstrating their shape and location for the last two solar cycles. From this we noticed that the temporal variation of all transition surfaces follows the 11-year solar cycle. Finally, from the OMNI database, we have studied the temporal variation over the past 40 years of the plasma β parameter, the kinetic to magnetic and the kinetic to thermal energy ratios, at a distance of 1 AU from the Sun.     

Heating heavy ions in the polar corona by collisionless shocks: A one-dimensional simulation

Recently a new model for explaining the observations of preferential heating of heavy ions in the polar solar corona was proposed ( and ). In that model the ion energization mechanism is the ion reflection off supercritical quasi-perpendicular collisionless shocks in the corona and the subsequent acceleration by the motional electric field E = −V × B/c. The mechanism of heavy ion reflection is based on ion gyration in the magnetic overshoot of the shock. The acceleration due to the motional electric field is perpendicular to the magnetic field, giving rise to large temperature anisotropy with T_⊥ ? T_∥, in agreement with SoHO observations. Such a model is tested here by means of a one dimensional test particle simulation where ions are launched toward electric and magnetic profiles representing the shock transition. We study the dynamics of O⁵⁺, as representative of coronal heavy ions for Alfvénic Mach numbers of 2–4, as appropriate to solar corona. It is found that O⁵⁺ ions are easily reflected and gain more than mass proportional energy with respect to protons.     

Study of the 27-day variations of the galactic cosmic ray intensity and anisotropy

We demonstrate that the general features of the radial and azimuthal components of the anisotropy of galactic cosmic rays can be studied by the harmonic analysis method using data from an individual neutron monitor with cut off rigidity <5 GV. In particular, we study the characteristics of the 27-day (solar rotation period) variations of the galactic cosmic ray intensity and anisotropy, solar wind velocity, interplanetary magnetic field strength and sunspot number. The amplitudes of the 27-day variations of the galactic cosmic ray anisotropy are greater, and the phases more clearly established, in A > 0 polarity periods than in A < 0 polarity periods at times of minimum solar activity. The phases of the 27-day variations of the galactic cosmic rays intensity and anisotropy are opposite with respect to the similar changes of the solar wind velocity in A > 0 polarity periods. No significant dependence of the amplitude of the 27-day variation of the galactic cosmic ray anisotropy on the tilt angle of the heliospheric neutral sheet is found. Daily epicyclegrams obtained by Chree’s method show that the 27-day variations of the galactic cosmic ray anisotropy during A > 0 polarity periods follow elliptical paths with the major axes oriented approximately along the interplanetary magnetic field. The paths are more irregular during A < 0 polarity periods.     

He-like Ar xvii triplet observed by RESIK

We present the observations of He-like Ar triplet lines obtained by RESIK spectrometer aboard CORONAS-F. Interpretation of intensity ratios between triplet lines of lower Z elements is known to provide useful diagnostics of plasma conditions within the emitting source. Here, we investigate whether triplet line ratios are useful for interpretation of higher Z element spectra. A high sensitivity, low background and precise absolute calibration of RESIK allow to consider in addition also the continuum contribution. This provides a way to determine the Ar absolute abundance from the observed triplet component ratios. The method is presented and the results are shown for two selected flares. Derived values of Ar absolute abundance for these flares are found to be similar: 2.6 × 10⁻⁶ and 2.9 × 10⁻⁶. They fall in the range between presently accepted Ar photospheric and coronal abundances.     

Magnetosphere response to the 2005 and 2006 extreme solar events as observed by the Cluster and Double Star spacecraft

The four identical Cluster spacecraft, launched in 2000, orbit the Earth in a tetrahedral configuration and on a highly eccentric polar orbit (4–19.6 R_E). This allows the crossing of critical layers that develop as a result of the interaction between the solar wind and the Earth’s magnetosphere. Since 2004 the Chinese Double Star TC-1 and TC-2 spacecraft, whose payload comprise also backup models of instruments developed by European scientists for Cluster, provided two additional points of measurement, on a larger scale: the Cluster and Double Star orbits are such that the spacecraft are almost in the same meridian, allowing conjugate studies. The Cluster and Double Star observations during the 2005 and 2006 extreme solar events are presented, showing uncommon plasma parameters values in the near-Earth solar wind and in the magnetosheath. These include solar wind velocities up to ∼900 km s⁻¹ during an ICME shock arrival, accompanied by a sudden increase in the density by a factor of ∼5 and followed by an enrichment in He⁺⁺ in the secondary front of the ICME. In the magnetosheath ion density values as high as 130 cm⁻³ were observed, and the plasma flow velocity there reached values even higher than the typical solar wind velocity. These resulted in unusual dayside magnetosphere compression, detection of penetrating high-energy particles in the magnetotail, and ring current development following several successive injections of energetic particles in the inner magnetosphere, which “washed out” the previously formed nose-like ion structures.     

Kinetic theory of sheath formation in solar wind plasma

We present a general self-consistent kinetic theory for plasma sheath formation in solar wind plasma. The theory could be applied to anisotropic, as well as to isotropic collisionless plasma without resorting to any simplifications, limitations, or assumptions, such as the necessary existence of a ‘pre-sheath’ region of ions acceleration to ensure the Bohm criterion. The kinetic framework is first applied to sheath formation around an arbitrary oriented planar absorbing surface, charged by solar wind anisotropic plasma, under the condition of negligible photoelectric effect. We then make use of our kinetic approach for the plane geometry in isotropic collisionless plasma, as a particular case of a planar electrode orientation parallel to plasma streaming velocity, also analyzing the sheath structure around spherical and cylindrical absorbing electrodes submerged in isotropic collisionless plasma. Obtained results demonstrate principal differences in spatial charge distributions in sheath regions between spherical or cylindrical electrodes of large size and an unbound planar surface submerged in isotropic plasma. In the case of a planar electrode, we directly compare results obtained in our kinetic and hydrodynamic theories and conventional hydrodynamic theory of plasma sheath formation. The outcome from the present study have direct implications to the analysis of plasma sheath structure and associated distribution in space of charged dust grains, which is relevant to the moon exploration near the optical terminator region or in shadowed craters in the moon.