Solar Plasma Research Articles

Advanced Γ Method for Small-scale Vortex Detection in the Solar Atmosphere

Ubiquitous vortical structures are considered to act as a natural source of various solar plasma phenomena, for example, a wide range of magnetohydrodynamic waves and jet excitations. This work aims to develop an advanced vortex detection algorithm based on the Γ method and using a separable convolution kernel technique. This method is applied to detect and analyze the photospheric vortices in 3D realistic magnetoconvection numerical and observational data. We present the advanced Γ method (AGM), and our results indicate that the AGM performs with better accuracy in comparison with the original Γ method. The AGM allows us to identify small- and large-scale vortices with no vortex interposition and without requiring the changing of the threshold. In this way, the nondetection issue is mostly prevented. It was found that the Γ method failed to identify the large and longer-lived vortices, which were detected by the AGM. The size of the detected vortical structures tends to vary over time, with most vortices shrinking toward their end. The vorticity at the center is also not constant, presenting a sharp decay as the vortex ceases to exist. Due to its capability of identifying vortices with minimum nondetection, the vortex properties—such as lifetime, geometry, and dynamics—are better captured by the AGM than by the Γ method. In this era of new high-resolution observation, the AGM can be used as a precise technique for identifying and performing statistical analysis of solar atmospheric vortices.

A theoretic analysis of magnetoactive GES-based turbulent solar plasma instability

ABSTRACT A recently reported gravito-electrostatic sheath (GES) model is procedurally applied to study the turbumagnetoactive helioseismic oscillation features in the entire bi-fluidic solar plasma system. The bounded solar interior plasma (SIP, internally self-gravitating), and the unbounded solar wind plasma (SWP, externally point-gravitating) are coupled through the interfacial diffused solar surface boundary (SSB) due to an exact gravito-electrostatic interplay. A numerical platform on the developed theoretic formalism reveals the evolution of both dispersive and non-dispersive features of the modified GES mode fluctuations in new parametric windows. Different colourspectral profiles exhibit important features of the GES-based SIP–SWP perturbations elaborately. It is illustratively shown that the thermostatistical GES stability depends mainly on the radial distance, magnetic field, equilibrium plasma density, and plasma temperature. We see that their dispersive features are more pertinently pronounced in the self-gravitational domains (SIP) than the electrostatic counterparts (SWP). Besides, different characteristic parameters with accelerating (or decelerating) and stabilizing (or destabilizing) effects influencing the entire solar plasma stability are illustratively portrayed. We speculate that, in the SIP, the long-wave (gravitational-like) helioseismic fluctuations become highly dispersive showing more propagatory nature than the shorter ones (acoustic-like). The short waves show more propagatory propensity than the longer ones in the SSB and SWP regime. The reliability of our proposed investigation is bolstered along with the tentative applicability and future scope in light of the current solar observational scenarios, such as SOHO, STEREO, SDO, PSP, and SolO.

Investigating the variations in the composition and heating of interacting ICMEs

Interacting coronal mass ejections (CMEs) have been commonly reported during the STEREO era. With the interaction of CMEs in the heliosphere, it is expected that the participating CMEs will either merge to form a single interplanetary CME (ICME) or will arrive as distinct entities or ICMEs at 1 AU. Previous studies have focused on in situ observations of solar wind, i.e., plasma and magnetic field properties to understand the nature of the CME–CME interaction and its impact. In this study, we examine the observations of composition parameters of those ICMEs that resulted due to the interaction of two CMEs during their propagation between the Sun and the Earth. We report two events of the CME–CME interaction observed in 2012, of which one led to a merged structure after the interaction, as observed at 1 AU. The second interaction event was reported to arrive at L1 as two distinct structures. Our analysis reveals distinct composition signatures in the form of ion charge state enhancements. The results improve our understanding of the signatures of ICMEs and different complex structures formed after the interaction. The study reveals that compression can occur due to the passage of the shock associated with the following CME through the preceding CME and not due to the CME–CME interaction. The results also highlight the importance of the comparison of solar wind proton velocity data with the expected temperature data, in particular, to understand the ICME–ICME interaction processes.

An Inductively Coupled Plasma System for Investigating Spectropolarimetric Responses of Solar Plasmas to Anisotropic Fields

High precision measurements and accurate modeling of atomic polarization under three-dimensional radiation transfer are crucial to understand the structures of magnetized solar plasmas. To develop and validate spectropolarimetric measurements and analyses, we set up an inductively coupled plasma (ICP) generator designed especially for ∼1-eV plasmas interacting with radiation and weak magnetic fields. The device was put in front of the focal plane of the Horizontal Spectrograph of the Domeless Solar Telescope at Hida Observatory of Kyoto University. In helium discharges, the typical electron temperature, electron density, and helium column density of the ICP are comparable values to those of solar prominences, and the direct comparison of spectra shows almost the same opacity at He I 1083 nm. Magnetic and radiation fields were introduced to the ICP, and the system successfully reproduced reasonable spectropolarimetric signals as compared with those from the solar prominences.

Causality in heliophysics: Magnetic fields as a bridge between the Sun’s interior and the Earth’s space environment

Our host star, the Sun, is a middle-aged main sequence G type star whose activity varies. These variations are primarily governed by solar magnetic fields which are produced in the Sun’s interior via a magnetohydrodynamic dynamo mechanism. Solar activity manifests across different timescales, spanning transient phenomena such as flares, energetic particle events and coronal mass ejections to short to long-term modulation of solar irradiance, plasma winds, open flux and cosmic ray flux in the heliosphere. Collectively, these phenomena define space weather and space climate, which impact the state of the near-Earth space environment, the Earth’s magnetosphere, atmosphere and our space-reliant technologies. Understanding physical processes that are at the heart of solar variability and which causally connect the Sun–Earth system is therefore of immense importance to humanity. Such understanding leads to predictions of the impact of solar activity on our planet and provides a window to explore the plasma universe and other star–planet systems, including assessing the habitability of (exo)planets. In this review, based on our research on the solar–terrestrial system and extant scientific literature, we illuminate processes related to the genesis of solar magnetic fields in the Sun’s interior, their emergence and evolution, their manifestation as solar eruptive events, and their eventual impact on the geospace environment mediated via solar winds and storms. We focus on few phenomena that establish causal connections and demonstrate how our current understanding can lead to development of predictive capabilities encompassing the domain of heliophysics.

Models of thermal conduction and non-local transport of relevance to space physics with insights from laser–plasma theory

Models of solar and space plasmas require an accurate model for thermal transport. The simplest such model is to assume that the fluid approach is valid and that local transport models can be used. These local transport coefficients are derived under the assumption that the electron mean-free path is “small” compared to the temperature scale length. When this approximation breaks down, non-local transport models or thermal flux limiters must be used to maintain a physically realistic model. This article will review the background theory of how small is “small” for the mean-free path and what options there are for including non-local transport within the fluid framework. Much of this recent work has been motivated by laser–plasma theory, where mean-free paths can be large and the Spitzer–Harm approach is never used.

Solar Cosmic Ray Acceleration by a Shock Wave in the Lower Solar Corona on May 7, 1978

Based on the theory of diffusive shock acceleration of charged particles, we have theoretically studied the spectra of protons produced by a shock wave driven by a coronal mass ejection (CME) in the lower solar corona with known solar plasma parameters for the solar cosmic ray (SCR) event recorded near the Earth’s orbit on May 7, 1978 (ground level enhancement no. 31, GLE31) using numerical methods. The proton flux data from the CPME instrument installed on the IMP-8 spacecraft and the worldwide network of ground-based neutron monitors combined with the particle measurements by the telescopes on the IMP-7 and IMP-8 satellites have been used to analyze the event. In our calculations, to satisfactorily reproduce the proton spectrum measured at the Earth’s orbit, the CME velocity was assumed to be constant and equal to 600 km s–1. The shock acceleration of SCRs proceeded up to 1.4 R⊙ solar radii for 256 s.

Nonlinear Coupling of Alfvén and Slow Magnetoacoustic Waves in Partially Ionized Solar Plasmas: The Effect of Thermal Misbalance

Solar chromosphere and photosphere, as well as solar atmospheric structures, such as prominences and spicules, are made of partially ionized plasmas. Observations have reported the presence of damped or amplified oscillations in these solar plasmas, which have been interpreted in terms of magnetohydrodynamic (MHD) waves. Slow magnetoacoustic waves could be responsible for these oscillations. The present study investigates the temporal behavior of the field-aligned motions that represent slow magnetoacoustic waves excited in a partially ionized prominence plasma by the ponderomotive force. Starting from single-fluid MHD equations, including radiative losses, a heating mechanism and ambipolar diffusion, and using a regular perturbation method, first- and second-order partial differential equations have been derived. By numerically solving second-order equations describing field-aligned motions, the temporal behavior of the longitudinal velocity perturbations is obtained. The damping or amplification of these perturbations can be explained in terms of heating–cooling misbalance, the damping effect due to ambipolar diffusion and the variation of the first adiabatic exponent with temperature and ionization degree.

Formation of multiphase plasma in galactic haloes and an analogy to solar plasma

Thermal instability (TI) potentially explains the origin of cold gas in the intracluster medium (ICM), which is heated sufficiently by AGN feedback. The Hαfilaments seen in cluster cores provide strong motivation for TI. The hot (∼107K) ICM coronae allow the growth of isobaric TI. The multiphase medium (cold-dense—hot-diffuse) forms once TI saturates. However, gravitational stratification can spatially constrain TI, and thermal conduction is known to stabilize all scales below the field length (λF). In addition, the transport of energy is anisotropic along magnetic fields. Thermal conduction may further trigger gyroscale instabilities and effective reduction ofλF. However, cold gas at small scales(<λF)needs to be verified in observations. The virial temperature in galactic haloes is lower (∼106K) and opens the regime of isochoric TI. In this regime, the cooling time is typically shorter than the sound-crossing time, and large-scale isochoric clouds are rendered unstable. The linear and non-linear isochoric clouds have interesting differences which potentially lead to either fragmentation of the cloud or not. On saturation, TI produces a turbulent medium that helps mix phases and thermalize kinetic energy and thus completes a cycle of condensation and heating. Various aspects of condensation, stratified turbulence, and magnetized transport are physically identical in solar coronae but scaled down to lower luminosity (similar temperatures). We will discuss the recent progress in TI, its connection to observations, and the analogy to solar prominences.

Future high-resolution and high-cadence observations for unraveling small-scale explosive solar features

Solar coronal jets are frequently occurring collimated ejections of solar plasma, originating from magnetically mixed polarity locations on the Sun of size scale comparable to that of a supergranule. Many, if not most, coronal jets are produced by eruptions of small-scale filaments, or minifilaments, whose magnetic field reconnects both with itself and also with surrounding coronal field. There is evidence that minifilament eruptions are a scaled-down version of typical filament eruptions that produce solar flares and coronal mass ejections (CMEs). Moreover, the magnetic processes building up to and triggering minifilament eruptions, which is often flux cancelation, might similarly build up and trigger the larger filaments to erupt. Thus, detailed study of coronal jets will inform us of the physics leading to, triggering, and driving the larger eruptions. Additionally, such studies potentially can inform us of smaller-scale coronal-jet-like features, such as jetlets and perhaps some spicules, that might work the same way as coronal jets. We propose a high-resolution (∼0″. 1 pixels), high-cadence (∼5 s) EUV-solar-imaging mission for the upcoming decades, that would be dedicated to observations of features of the coronal-jet size scale, and smaller-scale solar features produced by similar physics. Such a mission could provide invaluable insight into the operation of larger features such as CMEs that produce significant Space Weather disturbances, and also smaller-scale features that could be important for coronal heating, solar wind acceleration, and heliospheric features such as the magnetic switchbacks that are frequently observed in the solar wind.

The Contribution of The Joule-Thomson Effect To Solar Coronal Heatin

Two of the three gases that display isenthalpic Joule-Thomson (JT) warming under laboratory conditions are hydrogen and helium, the main constituents of the solar plasma, as the inversion point at which expansion results in heating is only ~51 K for helium and ~193 K for hydrogen, but the temperatures that are attained by this route are at most a few hundred K. Increases in ion temperature by several orders of magnitude are claimed for hydrogen plasmas subject to expansion into a vacuum; modest increases are reported for the short lived tests of this effect that have been carried out in space in the wakes of artificial satellites and of the Moon.

A New Global Nonlinear Force-Free Coronal Magnetic-Field Extrapolation Code Implemented on a Yin–Yang Grid

The solar magnetic field dominates and structures the solar coronal plasma. Detailed insights into the coronal magnetic field are important to understand most physical phenomena there. While direct, routine measurements of the coronal magnetic field are not available, field extrapolation of the photospheric vector-field measurements into the corona is the only way to study the structure and dynamics of the coronal field. Here we focus on global coronal structures traditionally modeled using spherical grids and synoptic vector magnetograms as boundary conditions. We developed a new code that performs nonlinear force-free magnetic-field extrapolations in spherical geometry. Our new implementation is based on a well-established optimization principle on a Cartesian grid and a single spherical finite-difference grid. In the present work, for the first time, the algorithm is able to reconstruct the magnetic field in the entire corona, including the polar regions. The finite-difference numerical scheme that was employed in previous spherical-code versions suffered from numerical inefficiencies because of the convergence of those grids on the poles. In our new code, we implement the so-called Yin–Yang overhead grid, the structure of which addresses this difficulty. Consequently, both the speed and accuracy of the optimization algorithm are improved compared to the previous implementations. We tested our new code using the well known semi-analytical model (Low and Lou solution). This is a commonly used benchmark for nonlinear force-free extrapolation codes.

Parametric Evolution of Power-law Energy Spectra of Energetic Electrons in the Coronal Loops

Fast electron beams (FEBs) are one of the main products of various active events and are ubiquitous in solar, space and cosmic plasmas. They reveal themselves in hard X-ray and radio emissions. The observed characteristics of X-ray and radio emissions sensitively depend on the energy distribution of FEBs, which usually have a power-law energy spectrum. As FEBs travel in the solar atmosphere, their energy distribution can considerably vary due to the interaction with ambient plasmas. Tang et al. investigated the evolution of the energy spectrum of the FEBs traveling along a flare loop and discussed the possible effects on associated hard X-ray (HXR) and radio emissions. Considering the ubiquitous coronal loops in active regions, in the present paper, we investigate the parametric evolution of the energy spectra of FEBs when propagating along coronal loops. Here, we take the sunpot atmospheric model as an approximate coronal loop atmosphere model. The results show that the energy loss has an important impact on the cutoff behavior and energy spectra of FEBs when precipitating in a coronal loop with density ratio n b /n e = 0.01. The initially single power-law spectrum with a steepness cutoff can evolve into a more complex double power-law spectrum or two “knees” power-law spectrum with a flattened steepness cutoff behavior or saturation cutoff behavior. Our calculations also demonstrate that the energy spectrum evolution is not obvious if n b /n 0 = 0.001 as Tang et al. asserted. The present results are helpful for a more comprehensive understanding of the dynamic spectra of HXR and radio emissions from FEBs.

Study of electron impact excitation of H-like Si13+ ion in dense plasma environment

The relativistic configuration interaction technique has been employed to study the electron impact excitation collision strengths of H-like Si13+ ion in strongly coupled plasma environment. The analytical plasma screening b-potential has been adopted to determine the physical effects of the screened Coulomb interaction due to the influence of plasma on the atomic structure and dynamics affecting atomic processes. The modified Dirac equations are solved to obtain the bound and continuum state wave functions. We examine the plasma screening effect by considering the plasma conditions at the electron temperatures of 200, 600 and 1000 eV over electron density range of 2.0 × 1023 - 1.0 × 1024 cm−3. The present results highlight that the inclusion of plasma environment induces significant effect on the binding energies, transition energies, radiative transition probabilities, oscillator strengths and collision strengths. It has been observed that the collision strength decreases with increase in plasma electron density for a fixed temperature. Our results will be beneficial for laser produced plasmas, high energy density physics, astrophysical applications and will open the door to modelling and advanced diagnostics of solar and stellar plasmas.

Interrogating solar flare loop models with IRIS observations 2: Plasma properties, energy transport, and future directions

During solar flares a tremendous amount of magnetic energy is released and transported through the Sun’s atmosphere and out into the heliosphere. Despite over a century of study, many unresolved questions surrounding solar flares are still present. Among those are how does the solar plasma respond to flare energy deposition, and what are the important physical processes that transport that energy from the release site in the corona through the transition region and chromosphere? Attacking these questions requires the concert of advanced numerical simulations and high spatial-, temporal-, and spectral-resolution observations. While flares are 3D phenomenon, simulating the NLTE flaring chromosphere in 3D and performing parameter studies of 3D models is largely outwith our current computational capabilities. We instead rely on state-of-the-art 1D field-aligned simulations to study the physical processes that govern flares. Over the last decade, data from the Interface Region Imaging Spectrograph (IRIS) have provided the crucial observations with which we can critically interrogate the predictions of those flare loop models. Here in Paper 2 of a two-part review of IRIS and flare loop models, I discuss how forward modelling flares can help us understand the observations from IRIS, and how IRIS can reveal where our models do well and where we are likely missing important processes, focussing in particular on the plasma properties, energy transport mechanisms, and future directions of flare modelling.

The Temperature Dependence of Hot Prograde Flows in Solar Active Regions

Using simultaneous observations from the EUV Variability Experiment (EVE) and imaging from the Atmospheric Imaging Assembly (AIA), we characterise the temperature dependence of apparent hot flows in solar active regions. The EVE instrument performs Sun-as-a-star spectroscopy and is composed of two spectrographs: MEGS-A and MEGS-B (Multiple EUV Grating Spectrograph-A, -B). It is known that EVE can measure wavelength shifts and thus can observe relative Doppler velocities in solar atmospheric plasmas over an extended temperature range. However, MEGS-A is affected by a known astigmatism effect (Chamberlin: Solar Phys.291, 1665, 2016); inhomogeneities in EUV brightness on the solar surface result in purely instrumental wavelength errors. We validate our methods by independently quantifying this effect and comparing to Chamberlin’s results, and we explore the wavelength dependence as an extension of his formula as derived for He ii 304 Å. MEGS-B is unaffected by this instrumental effect in any case, and this has allowed us to find evidence of hot prograde flows in active regions. Using our image-based models for the astigmatism and flows, we independently confirm our original MEGS-B result. We now extend our knowledge of the temperature dependence of these flows via the additional Fe emission lines available in MEGS-A. We find a monotonic increase of apparent flow speed with temperature up through lines of Fe xvi, nominally formed at about 6.4 MK.

Effects of noise on the accuracy of plasma bulk parameters derived from velocity moments of in-situ observations

We expose and quantify the inaccuracies of plasma bulk parameters derived from the calculation of velocity moments of noisy in-situ plasma observations. First, we simulate typical solar wind proton plasma observations, obtained by a typical top-hat electrostatic analyzer instrument. We add background noise to the simulated observations and analyze them by applying standard methods to derive the plasma density, speed, and temperature. We then compare the analysis results with the parameters we use to simulate the observations in the first place, in order to quantify the inaccuracies in the calculated plasma parameters as functions of the noise level in the observations. We find that even noise levels that are smaller than 1% of the signal peak, lead to significant inaccuracies in some plasma parameters. The plasma temperature suffers the biggest inaccuracies and the plasma speed the smallest. Our results highlight the importance of removing noise from observations when calculating the moments of the constructed plasma distributions. We finally, evaluate one simple method to remove uniform background noise automatically from measurements, which is useful for future on-board analyses.

A monitoring campaign (2013–2020) of ESA’s Mars Express to study interplanetary plasma scintillation

AbstractThe radio signal transmitted by the Mars Express (MEX) spacecraft was observed regularly between the years 2013–2020 at X-band (8.42 GHz) using the European Very Long Baseline Interferometry (EVN) network and University of Tasmania’s telescopes. We present a method to describe the solar wind parameters by quantifying the effects of plasma on our radio signal. In doing so, we identify all the uncompensated effects on the radio signal and see which coronal processes drive them. From a technical standpoint, quantifying the effect of the plasma on the radio signal helps phase referencing for precision spacecraft tracking. The phase fluctuation of the signal was determined for Mars’ orbit for solar elongation angles from 0 to 180 deg. The calculated phase residuals allow determination of the phase power spectrum. The total electron content of the solar plasma along the line of sight is calculated by removing effects from mechanical and ionospheric noises. The spectral index was determined as$-2.43 \pm 0.11$which is in agreement with Kolmogorov’s turbulence. The theoretical models are consistent with observations at lower solar elongations however at higher solar elongation ($>$160 deg) we see the observed values to be higher. This can be caused when the uplink and downlink signals are positively correlated as a result of passing through identical plasma sheets.

Solar plasma characterization in Kappa (κ)-modified polytropic turbomagnetic GES-model perspective

ABSTRACT The gravitoelectrostatic sheath (GES) model for exploring the surface emission mechanism of the solar wind plasma (SWP), transformed from the solar interior plasma (SIP) via the solar surface boundary (SSB), has recently been reformulated in different conditions. It considers the realistic plasma key factors previously remaining unaddressed. This continued study elaborately explores the diverse GES-microphysical features influenced mainly by the plasma non-thermality, bulk polytropicity, magneto-activity, logatropicity, etc. It shows that, in the SIP (SWP), the electric potential is κ-insensitive, the Mach number is κ-sensitive, and the current density is κ-sensitive (insensitive). The Mach number is insensitive and the current density is insensitive (sensitive) to the relative temperature, Te/Ti, in the SIP (SWP). The inhomogeneity-induced GES properties are illuminated with the gradient-scale behaviours of the relevant physical variables on both SIP and SWP scales. The self-organization of the bi-scaled solar plasma system is well confirmed by mapping the electric and gravity field gradients in a defined colour phase space illustratively alongside reliability check-ups. A judicious application of the newly derived model-structuring equations results in a non-thermally modified form of the equivalent κ-Bohm sheath criterion featuring the GES formation alongside electric current-flow analyses. Its reliability is validated with a sensible comparison with the extremal cases in fair corroboration with the previous sheath scenarios extensively reported elsewhere. The relevancy of our investigation on the solar multiparametric variations in the GES fabric is lastly bolstered in light of the recent solar astronomic scenarios, such as SolO and PSP.

Spectroscopy of the interaction of cosmic plasma flows with the Earth's atmospheric aerosols

The article deals with questions about the interaction of solar plasma flows with atmospheric gas containing CDP molecules and particles. Modeling and calculations of existing electric currents and their structure were carried out. Conclusions are drawn about the effect on the structure of the current both from the side of the atmosphere and from cosmic streams of different composition. The equation of motion of a plasma with a condensed dispersed phase (CDP) in the Earth's ionosphere is considered. Conclusions are drawn about the effect of CDP on the thermal balance of plasma. It has been found that in rarefied layers of the ionosphere, plasma flows mixed with CDP are subjected to significant additional heating from CDP particles, and the lower ones serve as a heat-removing agent. Attention is drawn to the most important aspects of the results obtained in the context of their impact on the operation of power supply networks during strong solar flares. Taking into account the nature of the explosive processes that form the plasma flows from which the electric current is formed, the fluctuations of the currents inside the magnetic tubes are determined. The possible influence of these factors on the operation of the power grid line in the mountains, around the polar and equatorial zones is also considered. Such influence depends on the amplitude fluctuations of the physical parameters of the medium that forms the atmospheric currents.