Solar Wind Plasma Research Articles

Quantitative Investigation Nonequilibrium Inhomogeneous Plasma of the Heliosphere With Runaway Electrons

We prove that a nonequilibrium inhomogeneous giant gas discharge is realized in the heliosphere with huge values of the parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E/N$ </tex-math></inline-formula> , which determines the temperature of electrons. This quasi-stationary discharge determines the main parameters of the weak solar wind (SW) in the heliosphere. In connection with the development of space technologies and the human spacewalk, the problem of the nature of the SW is acute. The study of the interference of gravitational and electrical potentials at the earth’s surface began with the work of William Gilbert 1600. Such polarization effects–the interference of Coulomb and gravitational forces–have not been studied well enough even in the heliosphere. Our article is devoted to this problem. The Pannekoek–Rosselan–Eddington model does not consider the important role of highly energetic running (away from the Sun) electrons and, accordingly, the duality of electron fluxes in the heliosphere (from the Sun and to the Sun) According to an alternative model formulated by us, highly energetic (escaping from the positively charged Sun) electrons leave the Sun and the heliosphere, and weakly energetic ones, unable to leave the Coulomb potential well (hole–the positively charged Sun and the heliosphere–return to the positively charged Sun. The weak difference between the opposite currents of highly energetic (escaping from the Sun) electrons and weakly energetic (returning to the Sun) electrons is compensated by the current of positive ions and protons from the Su–SW. These dynamic processes maintain a quasi-constant effective dynamic charge of the Sun and the entire heliosphere all the way to the earth’s orbit. At the same time, quasi-neutrality (see discussion in the following) in the Sun and heliosphere is well performed up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−36</sup> . According to experiments and analytical calculations based on our model: 1) the plasma in the corona is nonequilibrium; 2) the maximum electron temperature is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{e}$ </tex-math></inline-formula> 1–2 million degree; 3) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{e}$ </tex-math></inline-formula> grows from 1000 km away from the Sun (see discussion in the following); and 4) the role of highly energetic electrons escaping from the plasma leads to a significant increase in the effective: solar charge and electric fields in the heliosphere in relation to the Pannekoek–Rosselan–Eddington model. This is due to the absence of a compensation layer that screens the effective charge of the Sun It is not formed due to the escape of highly energetic electrons (as in a conventional gas discharge) from the entire heliosphere with high temperatures exceeding the temperature of the Sun’s surface. Thus, the process of escape of highly energetic electrons forms the internal EMF of the entire heliosphere. Interference of gravitational and Coulomb potentials in the entire heliosphere is considered, and it is being manifested in generation of two opposite flows of particles: 1) that are neutral or with a small charge (to the Sun) and 2) in the form of high-energy electrons (escaping from the positively charged Sun) and an SW with positively charged ions with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z/M</i> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\ge0.107$ </tex-math></inline-formula> (from the positively charged Sun). The calculated values of the registered ion parameters in the SW were compared with experimental observations. Reasons for generating the ring current in inhomogeneous heliosphere and inapplicability of the Debye theory in describing processes in the SW (plasma with current) are considered.

Studying wearing resistance of graphene-based materials for the project of the first in the world probe microscope – Earth satellite by solar wind plasma emulation

Abstract In situ monitoring the surface and structure stability of the constructional material in the conditions of outer space solar wind is an interesting scientific and engineering problem. For this purpose, as a part of the project of the first in the world probe microscope – space satellite, multilayered graphenes were plasma treated in conditions imitating solar wind and studied by scanning probe microscope in laboratory conditions. Accelerating factor of laboratory plasma treatment was estimated in order to compare the result with further solar wind exposure in outer space. Multilayered graphenes were selected for the study because if their promising properties as light shields – for equipment protection, as well as for material for solar sailing. It was found that plasma treatment affects surface morphology, and these changes correlate with the duration of the treatment. Thus, the applied method can be applied to simulate the erosional effect of the solar wind on structural materials in open space.

Density Turbulence and the Angular Broadening of Outer Heliospheric Radio Sources at High Latitudes and in the Ecliptic Plane

Abstract Density irregularities are responsible for the scattering of radio waves in the solar wind and astrophysical plasmas. These irregularities significantly affect the inferred physical properties of radio sources, such as size, direction, and intensity. We present here a theory of angular broadening due to the scattering of radio waves by density irregularities that improves the existing formalism used to investigate radio wave scattering in the outer heliosphere and the very local interstellar medium. The model includes an inner scale and both latitudinal and radial dependencies for the density fluctuation spectra and propagation paths for the radiation both near and out of the ecliptic plane. Based on the pickup-ion-mediated solar wind model (PUI model) of Zank et al., we estimate the turbulence and solar wind quantities for the high-latitude fast solar wind. The predictions include the density variance, inner/dissipation scale, velocity correlation length, mean magnetic field, and proton temperature. The density turbulence amplitude is estimated in two ways. First, a simple scaling technique is used to extend the theoretical predictions of the PUI model for the high-latitude wind beyond the heliospheric termination shock. Second, the solar wind and turbulence quantities are calculated near the ecliptic plane using plasma and magnetometer data from the Voyager 2 spacecraft over the period 1977–2018. Based on the turbulence models and observations, we calculate the scattering angle of the radio sources in the high-latitude and near-ecliptic wind. Finally, we compare the numerical results with the analytic predictions from Cairns and Armstrong et al. and the observed source sizes.

The solar wind plasma upstream of Mars observed by Tianwen-1: Comparison with Mars Express and MAVEN

The solar wind plasma upstream of Mars observed by Tianwen-1: Comparison with Mars Express and MAVEN

An In-depth Numerical Study of Exact Laws for Compressible Hall Magnetohydrodynamic Turbulence

Abstract Various exact laws governing compressible magnetohydrodynamic (MHD) and compressible Hall-MHD (CHMHD) turbulence have been derived in recent years. Other than their fundamental theoretical interest, these laws are generally used to estimate the energy dissipation rate from spacecraft observations in order to address diverse problems related, e.g., to heating of the solar wind and magnetospheric plasmas. Here we use various 10243 direct numerical simulation data of free-decay isothermal CHMHD turbulence obtained with the GHOST code (Geophysical High-Order Suite for Turbulence) to analyze two of the recently derived exact laws. The simulations reflect different intensities of the initial Mach number and the background magnetic field. The analysis demonstrates the equivalence of the two laws in the inertial range and relates the strength of the Hall effect to the amplitude of the cascade rate at sub-ion scales. When taken in their general form (i.e., not limited to the inertial range), some subtleties regarding the validity of the stationarity assumption or the absence of the forcing in the simulations are discussed. We show that the free-decay nature of the turbulence induces a shift from a large-scale forcing toward the presence of a scale-dependent reservoir of energy fueling the cascade or dissipation. The reduced form of the exact laws (valid in the inertial range) ultimately holds even if the stationarity assumption is not fully verified.

The Solar Wind at (16) Psyche: Predictions for a Metal World

Abstract NASA’s Psyche spacecraft will carry a magnetometer to main-belt asteroid (16) Psyche to search for remanent magnetic fields, and to attempt electromagnetic sounding of the interior. However, the Psyche spacecraft does not carry an instrument to measure solar wind plasmas. We thus combine data from five missions that have measured the solar wind at the orbit of asteroid (16) Psyche. We characterize these upstream conditions for future modeling and reference. We consider the implications of these ambient conditions for the interaction between (16) Psyche and the solar wind, outlining four possible resulting magnetospheres. Any magnetosphere would be dominated by ion-scale (Hall) physics and exotic electron-scale physics, requiring sophisticated physical modeling to describe. Under these different regimes, plasma generates additional electromagnetic fields, resulting in complex magnetism which may complicate the magnetic environment near asteroid (16) Psyche. Future missions to asteroids would benefit from combined magnetic field and plasma measurements.

How the area of solar coronal holes affects the properties of high-speed solar wind streams near Earth: An analytical model

Since the 1970s it has been empirically known that the area of solar coronal holes affects the properties of high-speed solar wind streams (HSSs) at Earth. We derive a simple analytical model for the propagation of HSSs from the Sun to Earth and thereby show how the area of coronal holes and the size of their boundary regions affect the HSS velocity, temperature, and density near Earth. We assume that velocity, temperature, and density profiles form across the HSS cross section close to the Sun and that these spatial profiles translate into corresponding temporal profiles in a given radial direction due to the solar rotation. These temporal distributions drive the stream interface to the preceding slow solar wind plasma and disperse with distance from the Sun. The HSS properties at 1 AU are then given by all HSS plasma parcels launched from the Sun that did not run into the stream interface at Earth distance. We show that the velocity plateau region of HSSs as seen at 1 AU, if apparent, originates from the center region of the HSS close to the Sun, whereas the velocity tail at 1 AU originates from the trailing boundary region. Small HSSs can be described to entirely consist of boundary region plasma, which intrinsically results in smaller peak velocities. The peak velocity of HSSs at Earth further depends on the longitudinal width of the HSS close to the Sun. The shorter the longitudinal width of an HSS close to the Sun, the more of its “fastest” HSS plasma parcels from the HSS core and trailing boundary region have impinged upon the stream interface with the preceding slow solar wind, and the smaller is the peak velocity of the HSS at Earth. As the longitudinal width is statistically correlated to the area of coronal holes, this also explains the well-known empirical relationship between coronal hole areas and HSS peak velocities. Further, the temperature and density of HSS plasma parcels at Earth depend on their radial expansion from the Sun to Earth. The radial expansion is determined by the velocity gradient across the HSS boundary region close to the Sun and gives the velocity-temperature and density-temperature relationships at Earth their specific shape. When considering a large number of HSSs, the assumed correlation between the HSS velocities and temperatures close to the Sun degrades only slightly up to 1 AU, but the correlation between the velocities and densities is strongly disrupted up to 1 AU due to the radial expansion. Finally, we show how the number of particles of the piled-up slow solar wind in the stream interaction region depends on the velocities and densities of the HSS and preceding slow solar wind plasma.

Modeling Ion Beams, Kinetic Instabilities, and Waves Observed by the Parker Solar Probe near Perihelia

Abstract Recent in situ observations from the Parker Solar Probe (PSP) mission in the inner heliosphere near perihelia show evidence of ion beams, temperature anisotropies, and kinetic wave activity, which are likely associated with kinetic heating and acceleration processes of the solar wind. In particular, the proton beams were detected by PSP/Solar Probe Analyzers-Ion (SPAN-I) and related magnetic fluctuation spectra associated with ion-scale waves were observed by the FIELDS instrument. We present the ion velocity distribution functions (VDFs) from SPAN-I and the results of 2.5D and 3D hybrid-particle-in-cell models of proton and α particle super-Alfvénic beams that drive ion kinetic instabilities and waves in the inner heliospheric solar wind. We model the evolution of the ion VDFs with beams, and obtain the ion relative drifts speeds, and ion temperature anisotropies for solar wind conditions near PSP perihelia. We calculate the partition of energies between the particles (ions) along and perpendicular and parallel to the magnetic field, as well as the evolution of magnetic energy, and compare to observationally deduced values. We conclude that the ion beam driven kinetic instabilities in the solar wind plasma near perihelia are important components in the cascade of energy from fluid to kinetic scales, an important component in the solar wind plasma heating process.

The “Singular” Behavior of the Solar Wind Scaling Features during Parker Solar Probe–BepiColombo Radial Alignment

Abstract At the end of 2020 September, the Parker Solar Probe (PSP) and BepiColombo were radially aligned: PSP was orbiting near 0.17 au and BepiColombo near 0.6 au. This geometry is of particular interest for investigating the evolution of solar wind properties at different heliocentric distances by observing the same solar wind plasma parcels. In this work, we use the magnetic field observations from both spacecraft to characterize both the topology of the magnetic field at different heliocentric distances (scalings, high-order statistics, and multifractal features) and its evolution when moving from near-Sun to far-Sun locations. We observe a breakdown of the statistical self-similar nature of the solar wind plasma with an increase in the efficiency of the nonlinear energy cascade mechanism when moving away from the Sun. We find a complex organization of large field gradients to dissipate the excess of kinetic energy across the inertial range near the Sun, whereas the topological organization of small fluctuations is still primarily responsible for the energy transfer rate at 0.6 au. These results provide, for the first time, evidence of the different roles of dissipation mechanisms near and far away from the Sun.

Improving the Alfvén Wave Solar Atmosphere Model Based on Parker Solar Probe Data

Abstract In van der Holst et al. (2019), we modeled the solar corona and inner heliosphere of the first encounter of NASA’s Parker Solar Probe (PSP) using the Alfvén Wave Solar atmosphere Model (AWSoM) with Air Force Data Assimilative Photospheric flux Transport–Global Oscillation Network Group magnetograms, and made predictions of the state of the solar wind plasma for the first encounter. AWSoM uses low-frequency Alfvén wave turbulence to address the coronal heating and acceleration. Here, we revise our simulations, by introducing improvements in the energy partitioning of the wave dissipation to the electron and anisotropic proton heating and using a better grid design. We compare the new AWSoM results with the PSP data and find improved agreement with the magnetic field, turbulence level, and parallel proton plasma beta. To deduce the sources of the solar wind observed by PSP, we use the AWSoM model to determine the field line connectivity between PSP locations near the perihelion at 2018 November 6 UT 03:27 and the solar surface. Close to the perihelion, the field lines trace back to a negative-polarity region about the equator.

Charge State Calculation for Global Solar Wind Modeling

Abstract The charge state composition of the solar wind carries information about the electron temperature, density, and velocity of plasma in the solar corona that cannot always be measured with remote sensing techniques, due to limitations in instrumental sensitivity and field of view as well as line-of-sight integration issues. However, in situ measurements of the wind charge state distribution only provide the end result of the solar wind evolution from the source region to the freeze-in point. By using 3D global modeling it is possible to follow solar wind plasma parcels of different origin along the path of their journey and study the evolution of their charge states as well as the driving physical processes. For this purpose, we implemented nonequilibrium ionization calculations within the Space Weather Modeling Framework’s solar corona and inner heliosphere modules, to the Alfvén Wave Solar Model (AWSoM). The charge state calculations are carried out parallel to the AWSoM calculations, including all the elements and ions whose ionization-recombination rates are included in the CHIANTI database, namely, from H to Zn. In this work, we describe the implementation of the charge state calculation, and compare simulation results to in situ measurements from the Advanced Composition Explorer and Ulysses spacecraft, and study charge state evolution of plasma parcels along different wind trajectories and wind types.

Diffusive Plasma Transport by the Magnetopause Kelvin-Helmholtz Instability During Southward IMF

At the Earth’s low-latitude magnetopause, the Kelvin-Helmholtz (KH) waves, which are driven by the super-Alfvénic velocity shear across the magnetopause, have been frequently observed during periods of northward interplanetary-magnetic-field (IMF) and believed to contribute to efficiently transporting the solar wind plasmas into the magnetosphere. On the other hand, during southward IMF periods, the signatures of the KH waves are much less frequently observed and how the KH waves contribute to the solar wind transport has not been well explored. Recently, the Magnetospheric Multiscale (MMS) mission successfully detected signatures of the KH waves near the dusk-flank of the magnetopause during southward IMF. In this study, we analyzed a series of two- and three-dimensional fully kinetic simulations modeling this MMS event. The results show that a turbulent evolution of the lower-hybrid drift instability (LHDI) near the low-density (magnetospheric) side of the edge layer of the KH waves rapidly disturbs the structure of the layer and causes an effective transport of plasmas across the layer. The obtained transport rate is comparable to or even larger than that predicted for the northward IMF. These results indicate that the diffusive solar wind transport induced by the KH waves may be active at the flank-to-tail magnetopause during southward IMF.

Fundamental Electromagnetic Emissions by a Weak Electron Beam in Solar Wind Plasmas with Density Fluctuations

Abstract The generation of Langmuir wave turbulence by a weak electron beam in a randomly inhomogeneous plasma and its subsequent electromagnetic radiation are studied owing to two-dimensional particle-in-cell simulations in conditions relevant to type III solar radio bursts. The essential impact of random density fluctuations of average levels of a few percents of the background plasma on the characteristics of the electromagnetic radiation at the fundamental plasma frequency ω p is shown. Not only wave nonlinear interactions but also processes of Langmuir waves’ transformations on the density fluctuations contribute to the generation of such emissions. During the beam relaxation, the amount of electromagnetic energy radiated at ω p in a plasma with density fluctuations strongly exceeds that observed when the plasma is homogeneous. The fraction of Langmuir wave energy involved in the generation of electromagnetic emissions at ω p saturates around 10−4, i.e., one order of magnitude above that reached when the plasma is uniform. Moreover, whereas harmonic emission at 2ω p dominates over fundamental emission during the time evolution in a homogeneous plasma, fundamental emission is strongly dominant when the plasma contains density fluctuations, at least during several thousands of plasma periods before being overcome by harmonic emission when the total electromagnetic energy begins to saturate.

Spontaneous Magnetic Fluctuations and Collisionless Regulation of Turbulence in the Earth’s Magnetotail

Abstract Among the fundamental and most challenging problems of laboratory, space, and astrophysical plasma physics is to understand the relaxation processes of nearly collisionless plasmas toward quasi-stationary states and the resultant states of electromagnetic plasma turbulence. Recently, it has been argued that solar wind plasma β and temperature anisotropy observations may be regulated by kinetic instabilities such as the ion cyclotron, mirror, electron cyclotron, and firehose instabilities; and it has been argued that magnetic fluctuation observations are consistent with the predictions of the fluctuation–dissipation theorem, even far below the kinetic instability thresholds. Here, using in situ magnetic field and plasma measurements by the THEMIS satellite mission, we show that such regulation seems to occur also in the Earth’s magnetotail plasma sheet at the ion and electron scales. Regardless of the clear differences between the solar wind and the magnetotail environments, our results indicate that spontaneous fluctuations and their collisionless regulation are fundamental features of space and astrophysical plasmas, thereby suggesting the processes is universal.

Transport Path of Cold‐Dense Plasmas in the Dusk Magnetotail Plasma Sheet: MMS Observations

AbstractThe near‐Earth plasma sheet becomes cold and dense under northward interplanetary magnetic field (IMF) condition, which suggests efficient solar wind plasma entry into the magnetosphere across the magnetopause for northward IMF and a possible contribution of ionospheric oxygen ion outflow. The cold and dense characteristics of the plasma sheet are more evident in the magnetotail flank regions that are the interface between cold solar wind plasma and hot magnetospheric plasma. Several physical mechanisms have been proposed to explain the solar wind plasma entry across the magnetopause and resultant formation of the cold‐dense plasma sheet (CDPS) in the tail flank regions. However, the transport path of the cold‐dense plasma inside the magnetotail has not been understood yet. Here, we present a case study of the CDPS in the dusk magnetotail by magnetospheric multiscale (MMS) spacecraft under strongly northward IMF and high‐density solar wind conditions. The ion distribution function consists of high‐ and low‐energy components, and the low‐energy one intermittently shows energy dispersion in the directions parallel and antiparallel to the local magnetic field. The time‐of‐flight analysis of the energy‐dispersed low‐energy ions suggests that these ions originate in the region farther down the tail, move along the magnetic field toward the ionosphere and then come back to the magnetotail by the mirror reflection. The pitch‐angle dispersion analysis gives consistent results on the traveling time and path length of the energy‐dispersed ions. Based on these observations, we discuss possible generation mechanisms of the energy‐dispersed structure of the low‐energy ions during the northward IMF.

Alpha/proton Instability in the Presence of Proton and Alpha Temperature Anisotropy and its Application to the Deceleration of Alpha Particles in the Solar Wind

Observations of solar wind plasma find that the drift velocity between alpha beams and protons, v α , is reduced with the heliocentric distance, but the ratio of v α to v A (the local Alfvén velocity) does not vary obviously and has a typical velocity ratio of v α /v A ≤ 1. The alpha beam instability is believed to be responsible for the deceleration of alpha beams in the solar wind. However, the role of the temperature anisotropy of ions (protons and alpha particles) on the alpha beam instability has not been examined. Based on the kinetic theory of the PDRK solver, this study systemically investigates the alpha beam instability in the presence of proton and alpha temperature anisotropies and considers the effects of these temperature anisotropies on the instability at an arbitrary propagation angle relative to the ambient magnetic field. It is found that the real frequencies, growth rates and threshold conditions of the excited waves sensitively depend on the proton temperature anisotropy T i⊥/T i∥ and the alpha temperature anisotropy T α⊥/T α∥, as well as the parallel electron beta β e∥. In particular, for both alpha beam and proton temperature anisotropy driven cases, the parallel magnetosonic/ whistler (PM/W), backward magnetosonic/ whistler (BM/W) and oblique Alfvén/ion cyclotron (OA/IC) waves arise when T i⊥/T i∥ < 1, and the PM/W, OA/IC, parallel Alfvén/ion cyclotron (PA/IC) and mirror waves occur when T i⊥/T i∥ > 1. The presence of anisotropic protons leads to a lower velocity threshold for the OA/IC and BM/W waves at T i⊥/T i∥ < 1 and for the OA/IC, PA/IC and mirror waves at T i⊥/T i∥ > 1. For both alpha beam and alpha temperature anisotropy driven cases, the PM/W and OA/IC waves are unstable when T α⊥/T α∥ < 1, and the PM/W, OA/IC, PA/IC, oblique magnetosonic/ whistler (OM/W) and mirror waves are likely to grow as T α⊥/T α∥ > 1. The presence of anisotropic alpha particles results in a lower threshold velocity for the PM/W wave at T α⊥/T α∥ < 1 and for the PA/IC, OM/W and mirror waves at T α⊥/T α∥ > 1. On account of the influences of the anisotropic proton and alpha, these alpha beam instabilities can effectively constrain the alpha-beam drift velocity to less than or equal to the local Alfvén velocity. These results may shed light on the evolution and deceleration mechanism of alpha particles in the solar wind.

Laboratory Simulation of Solar Wind Interaction With Lunar Magnetic Anomalies

AbstractMagnetic anomalies on the surface of the Moon interact with the solar wind plasma flow, resulting in both magnetic and electrostatic deflection/reflection of charged particles. Consequently, surface charging in these regions differs from regions without magnetic fields. Using the Colorado Solar Wind Experiment facility, this interaction is investigated with high‐energy flowing plasmas (100–800 eV beam ions) that are incident upon a magnetic dipole embedded beneath an insulating surface. The dipole moment is perpendicular to the surface. The plasma potential distribution is measured above the surface using an emissive probe. In the dipole lobe regions the surface is charged to significantly higher positive potentials by the un‐magnetized ion beam impinging on the surface while the electrons remain excluded by the magnetic field. At low ion beam energies the results agree with these expectations as the surface potential follows the ion beam energy. However, at high beam energies, the surface potentials in the electron‐shielded lobe regions remain significantly lower than the expected magnitude. Surprisingly, electrons are detected in the shielded regions by a Langmuir probe. A test particle simulation indicates that secondary electrons induced by the high energy ion beams impinging on the surface can enter the shielded regions, thus lowering the surface potential.

Erratum: Background for gravitational wave signal at LISA from refractive index of solar wind plasma

Erratum: Background for gravitational wave signal at LISA from refractive index of solar wind plasma

Three-dimensional distribution of ions and electrons in the lunar ionosphere originated from the photochemical reactions

ABSTRACT Using a fluid-based time-dependent numerical photochemical model, the three-dimensional distribution of ions and electrons in the lunar ionosphere, originated purely from photochemical reactions, is investigated. The photochemical model includes the production and recombination of 16 ions, namely CO$_2^+$, H2O+, H3O+, OH+, O$_2^+$, O+, Ar+, Ne+, He+, H+, H$_2^+$, CH$_3^+$, CH$_4^+$, and CH$_5^+$. The model also includes the interaction of solar wind with lunar plasma and calculates electron density profiles from the surface to 200 km altitude for the entire latitudes and longitudes. Model runs suggest that the surface electron density at the Moon could be as high as 1.2 × 105 cm−3 over the mid-latitudes if dynamical interaction between the solar wind and lunar plasma is not accounted for. The dominant ions, in this case, would be Ar+, Ne+, and He+. The absence of any intrinsic magnetic field however leads the ionosphere at the Moon to interact continuously with the solar wind and result in the removal of positive ions. This, in turn, leads to a negligible presence of plasma in the lunar ionosphere with a maximum electron density of ∼1600 cm−3. The electron density is maximum during the midnight and post-midnight periods at all the latitudes, and the maximum is centred around the polar region. Though solar wind acts as a strong removal agent, the electron density distribution is controlled by photochemistry, and ions are molecular in origin.

Relationship between geomagnetic storm development and the solar wind parameter ß

We have analyzed the dynamics of solar wind and interplanetary magnetic field (IMF) parameters during the development of 933 isolated geomagnetic storms, observed over the period from 1964 to 2010. The analysis was carried out using the epoch superposition method at intervals of 48 hrs before and 168 hrs after the moment of Dst minimum. The geomagnetic storms were selected by the type of storm commencement (sudden or gradual) and by intensity (weak, moderate, and strong). The dynamics of the solar wind and IMF parameters was compared with that of the Dst index, which is an indicator of the development of geomagnetic storms. The largest number of storms in the solar activity cycle is shown to occur in the years of minimum average values (close in magnitude to 1) of the solar wind parameter β (β is the ratio of plasma pressure to magnetic pressure). We have revealed that the dynamics of the Dst index is similar to that of the β parameter. The duration of the storm recovery phase follows the characteristic recovery time of the β parameter. We have found out that during the storm main phase the β parameter is close to 1, which reflects the maximum turbulence of solar wind plasma fluctuations. In the recovery phase, β returns to background values β~2‒3.5. We assume that the solar wind plasma turbulence, characterized by the β parameter, can play a significant role in the development of geomagnetic storms.