Solar Wind Plasma Research Articles

A preliminary investigation of the empirical relationship between small-amplitude Forbush Decreases and solar wind disturbances

ABSTRACTHigh-magnitude (amplitude $\le -3{{\ \rm per\ cent}}$) Forbush decreases (FDs) are generally employed by researchers investigating the solar-terrestrial connection mechanisms. Although it has been observed that small-amplitude FDs are relatively important as they may be the outcome of the response of cosmic ray time-intensity variations to solar ejections that generate interplanetary and solar wind plasma disturbances, empirical relations between weak FDs and solar-terrestrial parameters are rarely tested. In an attempt to analyse the suggested strong connections between weak FDs and solar-terrestrial phenomena, we employed some comparatively more efficient, accurate, and highly sensitive versions of the recently developed computer FD event selection software. Large catalogues of low-amplitude (FD(per cent) ≥−3) Forbush events were selected from Apatity, Moscow, Newark, and Oulu NMs. These catalogues allow us to test, for the first time, the empirical relations between small-amplitude FDs and solar wind data. We find significant negative correlations between solar wind speed (SWS) and the small FDs at OULU, NWRK, and MOSC stations. While the relation at OULU is strong and statistically significant at 95 per cent confidence level, the weak correlation at NWRK and MOSC is only significant at a 90 per cent level. The negative correlation between the small events at OULU and the interplanetary magnetic field (IMF) is also significant at the a 95 per cent level. The relation between SWS and IMF and high-amplitude FDs were also tested, and the correlation coefficients were negative, strong, and statistically significant at a 99.9 per cent level of significance.

Optimal Interplanetary Transfer of Solar Wind Ion Focusing Thruster-Based Spacecraft

The Solar Wind Ion Focusing Thruster (SWIFT) is a highly-innovative propellantless propulsion concept, recently proposed by Gemmer and Mazzoleni. In its nominal configuration, a SWIFT consists of a conically-shaped mesh of positively-charged conducting tethers, with its vertex linked to the spacecraft and its axis oriented towards the Sun. The SWIFT collects and filters the solar wind plasma and suitably directs the positive ions, which are then accelerated by an ion thruster. Such a device is theoretically able to generate a deep-space propulsive acceleration that comes, in part, from the solar wind dynamic pressure impinging on the conical grid and, in part, from the positive ion beam. In particular, the orientation of the ion beam may be chosen in such a way as to set the resultant propulsive acceleration and steer the spacecraft. The aim of this paper is to analyze the performance of a SWIFT-propelled spacecraft in an orbit-to-orbit two-dimensional interplanetary transfer. To that end, some mission scenarios are studied, in an optimal framework, by minimizing the total flight time necessary for the spacecraft to complete the transfer as a function of the propulsion system performance parameters. Numerical simulations are used to compare the optimal flight times calculated in simplified Earth–Venus and Earth–Mars transfers with those obtained by considering other propellantless propulsion systems.

The strong effect of electron thermal conduction on the global structure of the heliosphere

ABSTRACTVoyager 1 and 2 crossed the heliopause at ∼122 au in 2012 and ∼119 au in 2018, respectively. It was quite a surprise because the thickness of the inner heliosheath obtained at that time by the existing time models of the global heliosphere was significantly larger (by 20–40 au). Until now, the problem of the heliosheath thickness has not been fully resolved. Earlier, in the frame of an oversimplified toy model of nearly isothermal solar wind plasma, it has been shown that the effect of electron thermal conduction may significantly reduce the thickness of the inner heliosheath. In this paper, we present the first results of our 3D kinetic-magnetohydrodynamic (MHD) model of the global heliosphere, where the effect of thermal electron conduction has been considered rigorously. The thermal conduction acts mainly along the magnetic field lines. Classical and saturated thermal fluxes are employed when appropriate. It is shown that the effects of thermal conduction are significant. The thickness of the inner heliospheric is reduced. It is a desired effect since it helps to reconcile the thickness obtained in the model with Voyager data. The other effects are the strong depletion of the heliosheath plasma temperature towards the heliopause and the increase of the plasma temperature in the supersonic solar wind upstream of the termination shock.

Solar Radio Emissions and Ultralight Dark Matter

Ultralight axions and dark photons are well-motivated dark matter candidates. Inside the plasma, once the mass of ultralight dark matter candidates equals the plasma frequency, they can resonantly convert into electromagnetic waves, due to the coupling between the ultralight dark matter particles and the standard model photons. The converted electromagnetic waves are monochromatic. In this article, we review the development of using radio detectors to search for ultralight dark matter conversions in the solar corona and solar wind plasma.

Imbalanced Turbulence Modified by Large-scale Velocity Shears in the Solar Wind

We have investigated how the degree of imbalance in solar wind turbulence is modified by large-scale velocity shears in the solar wind plasma. The balance between counterpropagating Alfvénic fluctuations, which interact nonlinearly to generate the turbulence, has been quantified by the cross helicity and Elsasser ratio. Velocity shears at a 30 minute timescale were identified, with the shear amplitude defined in terms of the linear Kelvin–Helmholtz (KH) instability threshold. The shears were associated with 74 interplanetary coronal mass ejection (ICME) sheaths observed by the Wind spacecraft at 1 au between 1997 and 2018. Typically weaker shears upstream of the sheaths and downstream in the ICME ejecta were also analyzed. In shears below the KH threshold, imbalance was approximately invariant or weakly rising with shear amplitude. Above the KH threshold, fluctuations tended toward a balanced state with increasing shear amplitude. Magnetic compressibility was also found to increase above the KH threshold. These findings are consistent with velocity shears being local sources of sunward fluctuations that act to reduce net imbalances in the antisunward direction, and suggest that the KH instability plays a role in this process.

Effects of Force in the Martian Plasma Environment With Solar Wind Dynamic Pressure Enhancement

AbstractDisturbed solar wind dynamic pressure is one of the important external drivers which could cause significant impacts on Martian plasma environment. In this study, a 3D multifluid multispecies numerical model is established to simulate the interaction between solar wind and Mars. Functions of electromagnetic forces applied on different ion species were analyzed. We found that the total electromagnetic force peaks near bow shock (BS) and magnetic pileup boundary (MPB) with clear asymmetry features, acting to decelerate solar wind plasma across boundary layers, and compresses heavy ions toward the planet inside the MPB. For solar wind protons, electron pressure gradient force dominates near BS and Hall electric field force dominates near MPB, controlling the location of plasma boundary. Furthermore, the morphology of motional electric field force shows clear north‐south asymmetry, leading to the formation of asymmetric structures and plasma flow in Martian space environment. The response of BS, MPB to a solar wind dynamic pressure enhancement event, as well as the effects of electromagnetic forces in this process are also investigated. After the arrival of solar wind pulse, the magnitudes of electromagnetic forces increased simultaneously to balance the enhanced solar wind dynamic pressure, while the peaks of forces moved inward with BS and MPB. The magnitudes and peaks of ion velocity in subsolar region show similar variations as well, with the greater enhancement of forces leading to the greater increase of ion velocities, indicating that the changes of forces influence boundary layers through the variation of plasma speed.

Solar wind plasma variations with interplanetary magnetic field during solar cycles 22–24

Solar wind plasma variations with interplanetary magnetic field during solar cycles 22–24

Quantifying the global solar wind-magnetosphere interaction with the Solar-Terrestrial Observer for the Response of the Magnetosphere (STORM) mission concept

Much of what we know about the solar wind’s interaction with the Earth’s magnetosphere has been gained from isolated in-situ measurements by single or multiple spacecraft. Based on their observations, we know that reconnection, whether on the dayside magnetopause or deep within the Earth’s magnetotail, controls the bulk flow of solar wind energy into and through the global system and that nightside activity provides the energized particles that power geomagnetic storms. But by their very nature these isolated in-situ measurements cannot provide an instantaneous global view of the entire system or its cross-scale dynamics. To fully quantify the dynamics of the coupled solar wind-magnetosphere requires comprehensive end-to-end global imaging of the key plasma structures that comprise the magnetosphere which have spatial resolutions that exceeds anything possible with multi-point or constellation situ measurements. Global, end-to-end, imaging provides the pathway to understanding the system as a whole, its constituent parts, and its cross-scale processes on a continuous basis, as needed to quantify the flow of solar wind energy through the global magnetospheric system. This paper describes how a comprehensively-instrumented single spacecraft in a high-altitude, high-inclination orbit coupled with ground-based instruments provides the essential observations needed to track and quantify the flow of solar wind energy through the magnetosphere. This includes observations of the solar wind plasma and magnetic field input, the magnetopause location in soft X-rays, the auroral oval in far ultraviolet, the ring current in energetic neutrals, the plasmasphere in extreme ultraviolet, the exosphere in Lyman-α, and the microstructure of the nightside auroral oval from ground-based all sky cameras.

Unusual shrinkage and reshaping of Earth’s magnetosphere under a strong northward interplanetary magnetic field

The Earth’s magnetosphere is the region of space where plasma behavior is dominated by the geomagnetic field. It has a long tail typically extending hundreds of Earth radii (RE) with plentiful open magnetic fluxes threading the magnetopause associated with magnetic reconnection and momentum transfer from the solar wind. The open-flux is greatly reduced when the interplanetary magnetic field points northward, but the extent of the magnetotail remains unknown. Here we report direct observations of an almost complete disappearance of the open-flux polar cap characterized by merging poleward edges of a conjugate horse-collar aurora (HCA) in both hemispheres’ polar ionosphere. The conjugate HCA is generated by particle precipitation due to Kelvin-Helmholtz instability in the dawn and dusk cold dense plasma sheets (CDPS). These CDPS are consist of solar wind plasma captured by a continuous dual-lobe magnetic reconnections, which is further squeezed into the central magnetotail, resulting in a short “calabash-shaped” magnetotail.

Quasilinear approach to bump-on-tail instability using non-Maxwellian distributions

Abstract Spacecraft observations provide evidence about the existence of various waves and kinetic instabilities in solar wind plasmas. These instabilities and waves play an important role in the dynamics of the solar wind. In the present work, bump-on-tail (BOT) instability is revisited by employing the κ and then the modified (r, κ) distributions using a quasilinear approach. The evolution of bump-on-tail instability is studied analytically and numerically by using the solar wind data. The growth rate of BOT instability for varying ratios of temperature and number density is also observed. It is observed that the maximum growth rate increases with increasing number density and decreases with increasing electron temperature in the bump. The effect of spectral indices κ in the kappa distribution and r and κ in the modified (r, κ) distribution functions ultimately contributes towards the shoulder and tail of the distribution functions and the results appear to have been modified. Stimulation of wave energy density shows rapid variation in the quasilinear evolution of bump-on-tail instability as compared to the simple linear model.

Unconventional Surface Charging Within Deep Cavities on Airless Planetary Bodies: Particle‐In‐Cell Plasma Simulations

AbstractSurface charging properties of a nonconducting surface that has a deep cavity and is in contact with the solar wind plasma are investigated by means of the particle‐in‐cell plasma simulations. The modeled topography is an abstract representation of deep depressions found on the irregular surfaces of solid planetary bodies. The simulations have revealed unconventional charging features in that the cavity bottom is charged up to positive values even without any electron emission processes such as photoemission, provided that the surface location is accessible to a portion of incoming solar wind ions. The major driver of the positive charging is identified as drifting ions of the solar wind plasma, and an uncommon current ordering where ion currents exceed electron currents is established at the innermost part of the deep cavity. This also implies that the deep cavity bottom may have a positive potential of several hundred volts, which is similar in magnitude to the kinetic energy of an ion drift motion. The present study also clarifies the role of photoelectrons in developing the distinctive charging environment inside the cavity. The photoemitted electrons can no longer trigger positive charging at the cavity bottom, but rather exhibit the effect of relaxing positive potentials caused by the solar wind ions. The identified charging process, which is primarily due to the solar wind ions, is localized at the depths of the cavity and may be one possible scenario for generating intense electric fields inside the cavity.

Exploring the Solar Wind-Planetary Interaction at Mars: Implication for Magnetic Reconnection.

The Martian crustal magnetic anomalies present a varied, asymmetric obstacle to the imposing draped interplanetary magnetic field (IMF) and solar wind plasma. Magnetic reconnection, a ubiquitous plasma phenomenon responsible for transferring energy and changing magnetic field topology, has been observed throughout the Martian magnetosphere. More specifically, reconnection can occur as a result of the interaction between crustal fields and the IMF, however, the global implications and changes to the overall magnetospheric structure of Mars have yet to be fully understood. Here, we present an analysis to determine these global implications by investigating external conditions that favor reconnection with the underlying crustal anomalies at Mars. To do so, we plot a map of the crustal anomalies' strength and orientation compiled from magnetic field data collected throughout the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Then, we create "shear maps" which calculate and plot the angle of shear between the crustal fields and a chosen external field orientation. From there we define a "shear index" to quantify the susceptibility of a region to undergo reconnection based on a given overlaid, external field orientation and the resulting shear map for that region. We demonstrate that the shear analysis technique augments analysis of local reconnection events and suggests southward IMF conditions should favor dayside magnetic reconnection on a more global scale at Mars.

Electron density revealing the boundaries of Mercury’s magnetosphere via serendipitous measurements by SORBET during BepiColombo first and second Mercury swing-bys

Aims. We aim to establish the boundaries of Mercury’s magnetosphere through a comparison of the drops and rises of the electron density revealed by PWI/SORBET, during BepiColombo’s first and second swing-bys of Mercury carried out on 1 October 2021 and 23 June 2022, with global 3D magnetohydrodynamic simulations. Methods. SORBET was switched on during both swing-bys and its radio spectra were re-analysed using a new method based on the theory of the quasi-thermal-noise spectroscopy and adapted to measurements registered with a non-deployed antenna (as planned for the entire cruise phase). In parallel, magnetohydrodynamical (MHD) global simulations of Mercury’s magnetosphere were run under different solar wind conditions. Profiles of the electron density obtained from SORBET data were compared with three MHD simulations, using different values for solar wind sonic Mach numbers and plasma β. Results. Three drops and rises of electron density are clearly identified with the boundaries of the magnetosphere (bow shock, magnetopause, and boundary of a region dominated by closed magnetic field lines) on the inbound part of the first Mercury swing-by. Conclusions. On the inbound part of the first swing-by, a good match is found between the SORBET data and the MHD simulations, revealing the quick reorganisation of the Mercury’s magnetosphere in a variable solar wind. This study also highlights the essential role of the electron density in the future detection of Mercury’s magnetosphere boundaries once BepiColombo will orbit the planet from December 2025.

Magnetospheric Multiscale Observations of Markov Turbulence on Kinetic Scales

In our previous studies we have examined solar wind and magnetospheric plasma turbulence, including Markovian character on large inertial magnetohydrodynamic scales. Here we present the results of the statistical analysis of magnetic field fluctuations in the Earth’s magnetosheath, based on the Magnetospheric Multiscale mission at much smaller kinetic scales. Following our results on spectral analysis with very large slopes of about −16/3, we apply a Markov-process approach to turbulence in this kinetic regime. It is shown that the Chapman–Kolmogorov equation is satisfied and that the lowest-order Kramers–Moyal coefficients describing drift and diffusion with a power-law dependence are consistent with a generalized Ornstein–Uhlenbeck process. The solutions of the Fokker–Planck equation agree with experimental probability density functions, which exhibit a universal global scale invariance through the kinetic domain. In particular, for moderate scales we have the kappa distribution described by various peaked shapes with heavy tails, which, with large values of the kappa parameter, are reduced to the Gaussian distribution for large inertial scales. This shows that the turbulence cascade can be described by the Markov processes also on very small scales. The obtained results on kinetic scales may be useful for a better understanding of the physical mechanisms governing turbulence.

Does Turbulence along the Coronal Current Sheet Drive Ion Cyclotron Waves?

Evidence for the presence of ion cyclotron waves (ICWs), driven by turbulence, at the boundaries of the current sheet is reported in this paper. By exploiting the full potential of the joint observations performed by Parker Solar Probe and the Metis coronagraph on board Solar Orbiter, local measurements of the solar wind can be linked with the large-scale structures of the solar corona. The results suggest that the dynamics of the current sheet layers generates turbulence, which in turn creates a sufficiently strong temperature anisotropy to make the solar-wind plasma unstable to anisotropy-driven instabilities such as the Alfvén ion cyclotron, mirror-mode, and firehose instabilities. The study of the polarization state of high-frequency magnetic fluctuations reveals that ICWs are indeed present along the current sheet, thus linking the magnetic topology of the remotely imaged coronal source regions with the wave bursts observed in situ. The present results may allow improvement of state-of-the-art models based on the ion cyclotron mechanism, providing new insights into the processes involved in coronal heating.

Parker Solar Probe Encounters the Leg of a Coronal Mass Ejection at 14 Solar Radii

We use Parker Solar Probe (PSP) observations to report the first direct measurements of the particle and field environments while crossing the leg of a coronal mass ejection (CME) very close to the Sun (∼14 Rs). An analysis that combines imaging from 1 au and PSP with a CME model, predicts an encounter time and duration that correspond to an unusual, complete dropout in low-energy solar energetic ions from H–Fe, observed by the Integrated Science Investigation of the Sun (IS⊙IS). The surrounding regions are populated with low-intensity protons and heavy ions from 10s to 100 keV, typical of some quiet times close in to the Sun. In contrast, the magnetic field and solar wind plasma show no similarly abrupt changes at the boundaries of the dropout. Together, the IS⊙IS energetic particle observations, combined with remote sensing of the CME and a dearth of other “typical” CME signatures, indicate that this CME leg is significantly different from the magnetic and plasma structure normally assumed for CMEs near the Sun and observed in interplanetary CMEs farther out in the solar wind. The dropout in low-energy energetic ions may be due to the cooling of suprathermal ions at the base of the CME leg flux tube, owing to the rapid outward expansion during the release of the CME.

Solar wind interaction with Mars: electric field morphology and source terms

ABSTRACTThe correlation between space environment conditions and the properties of escaping ions is a central topic of Mars research. Although empirical correlations have been visible in the data, a physics-based interpretation, rather than statistics-based pictures, has not been established yet. As a first effort, we investigate the electric field, the direct contributor to ion acceleration, in the Mars plasma environment from a hybrid plasma model (particle ions and fluid electrons). We use Amitis, a hybrid model combined with an observation-based ionospheric model, to simulate the Mars–solar wind interaction under nominal solar wind plasma conditions for perpendicular and Parker spiral directions of the interplanetary magnetic field (IMF). The simulations show following results: (1) the electric field morphology is structured by the IMF direction and the different plasma domains in the solar wind–Mars interaction; (2) asymmetry of the electric field between the hemispheres where the convective electric field points inward and outward, respectively, due to the mass loading and asymmetric draping of the magnetic field lines; (3) the motional electric field dominates in most regions, especially in the dayside magnetosheath; and (4) the Hall term is an order of magnitude weaker and significant in the magnetotail and plasma boundaries for a perpendicular IMF case. The Hall term is relatively stronger for the Parker spiral case. (5) The ambipolar electric field, in principle, agrees with Mars Atmosphere and Volatile Evolution measurements in the magnetosheath.

Plasma sheath around sunlit moon: monotonic and non-monotonic structures

ABSTRACTAn analytical investigation of the sheath formation over sunlit moon under the influence of observed solar ultraviolet/extreme ultraviolet radiation and solar wind plasma has been presented. Poisson’s equation is coupled with the latitude-dependent Fermionic photoelectrons, non-Maxwellian solar wind electrons and cold ions to derive the electric potential, electric field, and population density profiles within the photoelectron sheath. We notice that the high-energy photons of the solar spectra in addition to Lyman- α, significantly contribute in determining the surface charge and subsequent sheath structure. A traditional Debye (Type C) sheath forms around the terminator due to dominant plasma electron accretion and marginal photoemission. While moving towards the equator, the photoelectron contribution increases, yielding monotonic (Type B) and non-monotonic (Type A) sheath solutions over sunlit locations. The calculations show that the non-monotonic potential structures are more stable near the terminator region, while both types of potential structures are probable near the equator region for the nominal solar wind plasma. A vertical sheath extension of ∼60 m and ∼12 m is predicted for typical solar wind plasma at the equator corresponding to Type A and Type B sheath, respectively. Under exotic plasma conditions (e.g. Earth’s magnetospheric tail lobe, plasma sheet), non-monotonic solutions become stable for the photoemission-dominated region, and the entire sunlit lunar surface may acquire negative potential.

Fe Ion Composition in Solar-Wind Streams in the Solar Corona and Heliosphere

Analysis of the distributions of Fe ions over the stages of ionization in solar-wind (SW) plasma provides valuable information on the formation of SW streams and plasma heating processes, as well as for identifying SW sources on the Sun. When passing SW plasma through the corona, its ion composition evolves and finally “freezes” at distances of the order of several solar radii from the solar surface, remaining further practically unchanged in the heliosphere. This makes it possible to obtain information about the physical conditions in its source and the solar corona from the SW charge state. Average charge QFe, which is usually used to characterize the distributions of Fe ions, does not take into account all the features of the evolution of the ion composition, which does not allow one to extract more detailed information about the state of SW plasma. In this study, to describe the charge state of Fe ions, three parameters q4, q8, and q12 are introduced, which characterize the relative fractions of ions with charges Z = 0–7, 8–11, and 12–20, respectively, and conditionally correspond to “cold,” “medium,” and “hot” SW plasma components. According to the measurements of the Fe ion composition in 2010 on the STEREO-A spacecraft, the characteristic values of these parameters for different types of SW streams are given. The problem of modeling ion distributions in SW plasma based on diagnostic data on the parameters of coronal sources is considered. Using the example of the event associated with coronal-mass ejection on August 18, 2010, it is shown that the parameters of the charge state of Fe ions calculated from the model distributions are in good agreement within errors with the measurement data.

Project Sun Gazers

An actual task of modern solar physics is the prediction of geoeffective phenomena, which include the fast solar wind, flares, and coronal plasma ejections that cause perturbations of the geomagnetic field in near-Earth space. Some of these disturbances are well known as magnetic storms. The goal of the Sun Gazers project is to create a database, three-dimensional visualization and forecast of the events described above. This database will allow us to match initial and boundary conditions for MHD-modeling. Visualization is necessary for visual comparison and analysis of the events of the above database.