Magnetohydrodynamic Waves 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.

Wave modes and instabilities in gravitating magnetized polytropic quantum plasmas including viscosity tensor and FLR corrections

ABSTRACT The present analytical study extends the problems of pressure anisotropy-driven instabilities and gravitational instability in space plasmas to mixed quantum polytropic gas in the interior of dense stars accounting for the effects of viscosity, finite Larmor radius (FLR) and self-gravitational effects. The generalized polytrope pressure laws are considered as adiabatic equations in which the pressure components depend upon the plasma density, magnitude of the magnetic field, and the polytrope indices. The modified properties of waves and instabilities in gravitating quantum plasmas have been analysed using the quantum magnetohydrodynamic (QMHD) fluid description in the magnetohydrodynamic (MHD) and Chew–Goldberger–Low (CGL) limits. In the parallel propagation, the Jeans instability modified by quantum diffraction parameters and firehose mode modified by FLR parameter is obtained separately. The Jeans instability condition depends upon the quantum diffraction term and polytrope index β, and it remains unaffected due to viscosity and ion Larmor frequency. The growth rate of the Jeans instability decreases due to viscosity and quantum diffraction parameters, while the growth rate of the firehose instability increases due to FLR corrections. In the transverse mode, a similar nature is observed in the growth rates; however, the instability region decreases significantly due to polytrope indices and different dispersion properties of MHD and CGL viscous quantum plasmas. The analytical results have been applied in dense degenerate stars to measure the characteristic parameters and understand the MHD wave propagation, pressure anisotropy-driven, and gravitationally driven instabilities.

Геомагнитные эффекты взрывов сибирских метеоров

The geomagnetic effects of falling meteors have been known for a long time, but so far they have not received a complete theoretical explanation. The paper considers the response of the geomagnetic field to the destruction of meteors in Yakutia (March 4, 2014, 21:29 UT) and Khakassia (December 6, 2016) according to the Irkutsk magnetic observatory data of the Institute of Solar-Terrestrial Physics SB RAS. The analysis of variations in the geomagnetic field components showed that the passages and explosions of meteors were accompanied by the appearance of several groups of geomagnetic disturbances in the area of the Irkutsk magnetic observatory. Their appearance can be caused by the propagation of shock, internal gravity and magnetohydrodynamic waves formed as a result of the passages and destructions of meteors. The periods of these wave disturbances and the velocities of their propagation from the generation regions are estimated. It is noted that noticeable wave manifestations in the geomagnetic field give waves with a period of more than 10 minutes, i.e. having gravity modes.

THE ROLE OF MHD WAVES IN HEATING OF THE SOLAR CORONA

Observations in the Solar North Polar Coronal Hole (NPCH) show that this region has high temperature values (10^6-10^8 K). At this temperature, the coronal plasma loses energy to the transition region below through heat conduction and optically thin emission. It is still a matter of debate how to replace this lost energy in the solar corona and how to maintain the observed temperature values. In this study, we aimed to study the wave theory, which is an important model proposed for the heating problem of the solar corona in NPCH. We assumed a model based on Alfvén/ion cyclotron resonance process with O VI ions by using quasi-linear approximation in NPCH and solve the magnetohydrodynamic (MHD) equations for O VI assuming that the non-thermal contribution to the temperature. Using a Matlab code, we performed 3D MHD numerical solutions of Alfvén waves. Our results show that the damping length scales (0.2-1.8 R) and energy flux densities (10^5-10^7 erg/cm^2 s) of Alfvén waves are similar for both plumes and interplumes in NPCH. As a result of our study, we present the contribution of MHD waves that will cause the acceleration of the solar wind and the heating of the solar corona.

Spectropolarimetric investigation of magnetohydrodynamic wave modes in the photosphere: First results from PHI on board Solar Orbiter

Context.In November 2021, Solar Orbiter started its nominal mission phase. The remote-sensing instruments on board the spacecraft acquired scientific data during three observing windows surrounding the perihelion of the first orbit of this phase.Aims.The aim of the analysis is the detection of magnetohydrodynamic (MHD) wave modes in an active region by exploiting the capabilities of spectropolarimetric measurements.Mthods.The High Resolution Telescope (HRT) of the Polarimetric and Helioseismic Imager (SO/PHI) on board the Solar Orbiter acquired a high-cadence data set of an active region. This is studied in the paper. B-ωand phase-difference analyses are applied on line-of-sight velocity and circular polarization maps and other averaged quantities.Results.We find that several MHD modes at different frequencies are excited in all analysed structures. The leading sunspot shows a linear dependence of the phase lag on the angle between the magnetic field and the line of sight of the observer in its penumbra. The magnetic pore exhibits global resonances at several frequencies, which are also excited by different wave modes.Conclusions.The SO/PHI measurements clearly confirm the presence of magnetic and velocity oscillations that are compatible with one or more MHD wave modes in pores and a sunspot. Improvements in modelling are still necessary to interpret the relation between the fluctuations of different diagnostics.

An evolution of hyperbolic magneto-hydrodynamic (HMHD) wave propagating through radiative quantum plasma including cosmic ray pressure

The gravitational (Jeans) instability of radiative quantum plasma including cosmic ray (CR) pressure and diffusion is theoretically investigated using a generalized hyperbolic magneto-hydrodynamic model. It concurrently includes the impacts of the finite electrical resistivity, the Hall parameter, and the Coriolis force. The application of normal mode technique yields a unique form of a generalized dispersion relation. This dispersion relation is further discussed in the different modes of propagation with the different axis of rotation along the direction of the magnetic field. It has been noted that all the considered parameters affect the system's growth rate in both directions, but the Hall parameter does not affect it in the transverse direction. We also explored that together with the CRs, the Hall parameter, resistivity, rotation, and quantum parameter suppressed the Jeans instability's growth rate. Thus, these parameters act as stabilizing agents to the instability. The study identified radiative instability and analyzed the impact of an arbitrary heat-loss function on the system. The current findings provide new theoretical support to the existing various astronomic observations on the cosmic plasma and in the development of unique galactic formations of distinct scale lengths.

Five-minute oscillations of photospheric and chromospheric swirls

Context. Swirls are ubiquitous in the solar atmosphere. They are thought to be related to the excitation of different modes of magnetohydrodynamic waves and pulses, as well as spicules. However, statistical studies of their collective behaviour are rare. Aims. We aim to study the collective as well as the individual behaviour of photospheric and chromospheric swirls detected by the automated swirl detection algorithm (ASDA) from observations obtained by the Swedish 1-m Solar Telescope and the Hinode satellite. Methods. We performed a detailed analysis of six different parameters of photospheric and chromospheric swirls with the wavelet analysis. Two clusters with periods with significant wavelet power, one from 3 − 8 min and the other from 10 − 14 min, were found. The former coincides with the dominant period of the global p-mode spectrum. The wavelet and fast Fourier transform analysis of example swirls also revealed similar periods. Results. These results suggest that global p-modes might be important in triggering photospheric and thus chromospheric swirls. A novel scenario of global p-modes providing energy and mass fluxes to the upper solar atmosphere via generating swirls, Alfvén pulses, and spicules is then proposed.

Gravitational versus Magnetohydrodynamic Waves in Curved Spacetime in the Presence of Large-Scale Magnetic Fields

The general-relativistic (GR) magnetohydrodynamic (MHD) equations for a conductive plasma fluid are derived and discussed in the curved spacetime described by Thorne’s metric tensor, i.e., a family of cosmological models with inherent anisotropy due to the existence of an ambient, large-scale magnetic field. In this framework, it is examined whether the magnetized plasma fluid that drives the evolution of such a model can be subsequently excited by a transient, plane-polarized gravitational wave (GW) or not. To do so, we consider the associated set of the perturbed equations of motion and integrate them numerically in order to study the evolution of instabilities triggered by the GW propagation. In particular, we examine to what extend perturbations of the electric and/or the magnetic field can be amplified due to a potential energy transfer from the GW to the electromagnetic (EM) degrees of freedom. The evolution of the perturbed quantities depends on four free parameters, namely, the conductivity of the fluid, σ; the speed of sound square, 13<Csc2≡γ<1, which in this model may serve also as a measure of the inherent anisotropy; the GW frequency, ωg; and the associated angle of propagation with respect to the direction of the magnetic field, θ. We find that GW propagation in the anisotropic magnetized medium under consideration does excite several MHD modes; in other words, there is energy transfer from the gravitational to the EM degrees of freedom that can result in the acceleration of charged particles at the spot and in the subsequent damping of the GW.

Stability of an hypersonic magnetohydrodynamic shock wave

The shape of the shock wave formed by an hypersonic conducting flow around a plane or axisymmetric obstacle is determined by the velocity at infinity and the Lorentz force generated by a magnetic field generated within the body. This shape is described by a nonlinear differential equation. When linearized near the trivial shock wave (the boundary of the obstacle), it becomes amenable to a stability analysis. The study is conveniently separated in the regions where the slope of the body is not very small and the flatter sections. In both cases the main parameter is the sign of the Lorentz force; stability generally occurs when it presses the fluid inwards, but the precise criteria depend also on several other parameters.

Jupiter’s cloud-level variability triggered by torsional oscillations in the interior

Jupiter’s weather layer exhibits long-term and quasi-periodic cycles of meteorological activity that can completely change the appearance of its belts and zones. There are cycles with intervals from 4 to 9 years, dependent on the latitude, which were detected in 5-μm radiation, which provides a window into the cloud-forming regions of the troposphere; however, the origin of these cycles has been a mystery. Here we propose that magnetic torsional oscillations arising from the dynamo region could modulate the heat transport and hence be ultimately responsible for the variability of the tropospheric banding. These axisymmetric torsional oscillations are magnetohydrodynamic waves influenced by rapid rotation, which have been detected in Earth’s core, and have been recently suggested to exist in Jupiter by the observation of magnetic secular variations by Juno. Using the magnetic field model JRM33, together with a density distribution model, we compute the expected speed of these waves. For the waves excited by variations in the zonal jet flows, their wavelength can be estimated from the width of the alternating jets, yielding waves with a half-period of 3.2–4.7 years at 14–23° N, consistent with the intervals in the cycles of variability of Jupiter’s North Equatorial Belt and North Temperate Belt identified in the visible and infrared observations. The nature of these waves, including the wave speed and the wavelength, is revealed by a data-driven technique, dynamic mode decomposition, applied to the spatiotemporal data for 5-μm emission. Our results indicate that exploration of these magnetohydrodynamic waves may provide a new window to the origins of quasi-periodic patterns in Jupiter’s tropospheric clouds and to the internal dynamics and the dynamo of Jupiter. Torsional waves extend into the deep interior of Jupiter where they can modulate the outgoing heat flux and couple with Jupiter’s weather layer to generate the observed quasi-periodic oscillations in the cloud deck. Such waves can be used to explore the interior structure of gas giants.

CO enhancement by magnetohydrodynamic waves

Context. The formation of molecular gas in interstellar clouds is a slow process, but can be enhanced by gas compression. Magneto-hydrodynamic (MHD) waves can create compressed quasi-periodic linear structures, referred to as striations. Striations are observed at the column densities at which the transition from atomic to molecular gas takes place. Aims. We explore the role of MHD waves in the CO chemistry in regions with striations within molecular clouds. Methods. We targeted a region with striations in the Polaris Flare cloud. We conducted a CO J = 2−1 survey in order to probe the molecular gas properties. We used archival starlight polarization data and dust emission maps in order to probe the magnetic field properties and compare against the CO morphological and kinematic properties. We assessed the interaction of compressible MHD wave modes with CO chemistry by comparing their characteristic timescales. Results. The estimated magnetic field is 38–76 µG. In the CO integrated intensity map, we observe a dominant quasiperiodic intensity structure that tends to be parallel to the magnetic field orientation and has a wavelength of approximately one parsec. The periodicity axis is ~17° off from the mean magnetic field orientation and is also observed in the dust intensity map. The contrast in the CO integrated intensity map is ~2.4 times higher than the contrast of the column density map, indicating that CO formation is enhanced locally. We suggest that a dominant slow magnetosonic mode with an estimated period of 2.1–3.4 Myr and a propagation speed of 0.30–0.45 km s−1 is likely to have enhanced the formation of CO, hence created the observed periodic pattern. We also suggest that within uncertainties, a fast magnetosonic mode with a period of 0.48 Myr and a velocity of 2.0 km s−1 could have played some role in increasing the CO abundance. Conclusions. Quasiperiodic CO structures observed in striation regions may be the imprint of MHD wave modes. The Alfvénic speed sets the dynamical timescales of the compressible MHD modes and determines which wave modes are involved in the CO chemistry.

The Influence of Oxygen Ions on the Formation of a Thin Current Sheet in the Magnetotail

A thin current sheet in the Earth’s magnetotail with characteristic thickness of one to several proton gyroradii is often observed during magnetospheric disturbances named substorms, when a relatively thick current configuration in the magnetotail is narrowed to an extremely small thickness and then can spontaneously be destroyed. The current-sheet destruction is usually accompanied by such active processes as plasma acceleration and heating, as well as generation of an induced electric field and magnetohydrodynamic waves. In this paper, we developed and investigated a model of formation of a thin current sheet in which, along with protons, we have taken into account single-charged oxygen ions coming from the ionosphere into the magnetotail current sheet during magnetically active periods. The aim of this simulation is to study the peculiarities of thin current-sheet formation in two-component plasma and investigate its structure. It is shown that equilibrium configuration can have some characteristic properties. In particular, if the system consists only of protons or heavy ions, single-scale current equilibrium supported by quasi-adiabatic particles is formed. When a current sheet is formed in plasma that consists of a mixture of protons and oxygen ions in comparable concentrations, a current sheet can be formed with heavy ions as current carriers and chaotic proton trajectories that make a negative contribution to the current, due to which the current-density profile becomes bifurcated with the minimum at the center and maxima at the periphery of the sheet. The results may be useful for the interpretation of observational data in the Earth’s magnetotail.

Nonlinear Damping and Field-aligned Flows of Propagating Shear Alfvén Waves with Braginskii Viscosity

Braginskii magnetohydrodynamics (MHD) provides a more accurate description of many plasma environments than classical MHD since it actively treats the stress tensor using a closure derived from physical principles. Stress tensor effects nonetheless remain relatively unexplored for solar MHD phenomena, especially in nonlinear regimes. This paper analytically examines nonlinear damping and longitudinal flows of propagating shear Alfvén waves. Most previous studies of MHD waves in Braginskii MHD have considered the strict linear limit of vanishing wave perturbations. We show that those former linear results only apply to Alfvén wave amplitudes in the corona that are so small as to be of little interest, typically a wave energy less than 10−11 times the energy of the background magnetic field. For observed wave amplitudes, the Braginskii viscous dissipation of coronal Alfvén waves is nonlinear and a factor around 109 stronger than predicted by the linear theory. Furthermore, the dominant damping occurs through the parallel viscosity coefficient η 0, rather than the perpendicular viscosity coefficient η 2 in the linearized solution. This paper develops the nonlinear theory, showing that the wave energy density decays with an envelope . The damping length L d exhibits an optimal damping solution, beyond which greater viscosity leads to lower dissipation as the viscous forces self-organize the longitudinal flow to suppress damping. Although the nonlinear damping greatly exceeds the linear damping, it remains negligible for many coronal applications.

Cosmic ray-driven magnetohydrodynamic waves in magnetized self-gravitating dusty molecular clouds

ABSTRACT The impact of galactic cosmic rays (CRs) in terms of CR pressure and parallel CR diffusion has been investigated on the low-frequency magnetohydrodynamic (MHD) waves and linear gravitational instability in the typical dusty plasma environment of molecular clouds (MCs). The dusty fluid model is formulated by combining the equations of the magnetized electrons/ions and dust particles, including the CR effects. The interactions between CR fluid and gravitating magnetized dusty plasma have been studied with the help of modified dispersion properties of the MHD waves and instabilities using the hydrodynamic fluid–fluid (CR–plasma) approach. CR diffusion affects the coupling of CR pressure-driven mode with dust-Alfvén MHD mode and causes damping in the MHD waves. It persists in its effect along the direction of the magnetic field and is diminished across the magnetic field. The phase-speed diagram shows that for super-Alfvénic wave, the slow mode becomes the intermediate Alfvén mode. The fundamental Jeans instability criterion remains unaffected due to CR effects, but in the absence of CR diffusion, the effects of dust-acoustic speed and CR pressure-driven wave speed are observed in the instability criterion. It is found that CR pressure stabilizes while CR diffusion destabilizes the growth rates of Jeans instability and significantly affects the gravitational collapse of dusty MCs. The charged dust grains play a dominant role in the sub-Alfvénic and super-Alfvénic MHD waves and the collapse of MCs, triggering gravitational instability. The consequences have been discussed to understand the gravitational instability in the dense photodissociation regions of dusty MCs.

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.

Neutrino magnetohydrodynamic instabilities in presence of two-flavor oscillations

The influence of neutrino flavor oscillations on the propagation of magnetohydrodynamic (MHD) waves and instabilities is studied in neutrino-beam driven magnetoplasmas. Using the neutrino MHD model, a general dispersion relation is derived which manifests the resonant interactions of MHD waves, not only with the neutrino beam, but also with the neutrino flavor oscillations. It is found that the latter contribute to the wave dispersion and enhance the magnitude of the instability of oblique magnetosonic waves. However, the shear-Alfvén wave remains unaffected by the neutrino beam and neutrino flavor oscillations. Such an enhancement of the magnitude of the instability of magnetosonic waves can be significant for relatively long-wavelength perturbations in the regimes of high neutrino number density and/or strong magnetic field, giving a convincing mechanism for type-II core-collapse supernova explosion.

Fine Structures of Type-IV Solar Radio Bursts Associated with Stationary and Moving Sources

Various types of fine structure in the continuum emission of type-IV radio bursts are considered asapplied to different types of radiation sources, both stationary and moving. In the case of stationary sources,the origin of the fine structure is associated both with processes in individual magnetic loops (quasi-periodicacceleration and magnetohydrodynamic waves), and with large-scale processes associated with the propagationof magnetohydrodynamic disturbances, the formation of loop arcades, and processes of discrete accelerationof particles synchronous with them, causing the pulsating nature of radio emissions. For the case of amoving source, the generation mechanism largely depends on the magnetic structure of the source (anexpanding magnetic arc or an isolated plasma cloud). In this case, the connection with coronal mass ejectionsand shock waves is also important. Secondary pulsations are explained by a magnetohydrodynamic fluctuationsource in the form of a magnetic loop or cloud. The absence of other fine structures in the continuum ofmoving type-IV bursts may be due to the critical angle of the loss cone for the excitation of whistlers

The Sun at millimeter wavelengths

Aims. We used solar observations of a plage-enhanced network with the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Band 3 and Band 6, together with synthetic continuum maps from numerical simulations with Bifrost in the same bands, to carry out a detailed study of bright small-scale magnetic features. Methods. We made use of an algorithm to automatically identify and trace bright features within the field of view (FoV) of the ALMA observations and the simulation. In particular, the algorithm recovers information of the time evolution of the shape, motion of the centre of gravity, temperature, and size for each feature. These quantities are used to determine the oscillatory properties of each feature utilising wavelets analysis. Results. We found 193 and 293 features in the Bands 3 and 6 observations, respectively. In the degraded simulation, the total number of features were 24 for Band 3 and 204 for Band 6. In the original simulation, the total number of features were 36 for Band 3 and 392 for Band 6. Based on the simulation, we confirm the magnetic nature of the features. We have obtained average oscillation periods of 30–99 s for the temperature, 37–92 s for size, and 37–78 s for horizontal velocity. There are indications for the possible presence of transverse (kink) waves with average amplitude velocities of 2.1–5.0 km s−1. We find a predominant anti-phase behaviour between temperature and size oscillations suggesting that the variations of the bright features are caused by compressible fast-sausage magnetohydrodynamics (MHD) modes. For the first time to our knowledge, we estimated the flux of energy of the fast-sausage waves at the chromospheric heights sampled by ALMA as 453–1838 W m−2 for Band 3 and 3640–5485 W m−2 for Band 6. Conclusions. We have identified MHD waves, both transverse (kink) and compressible sausage modes, in small-scale (magnetic) structures, independently, in both ALMA Band 3 and Band 6 observations, along with their corresponding synthetic images from simulations. The decrease of wave energy-flux with height (from Band 6 to Band 3) could possibly suggest energy dissipation at chromospheric heights, namely, wave heating, with the assumptions that the identified small-scale waves are typical at each band and they propagate upward through the chromosphere.

Modeling Features of Field Line Resonance Observable by a Single Spacecraft at Saturn

AbstractThe observations of Southwood et al. (2021, https://doi.org/10.1029/2020JA028473), using data from the Cassini magnetometer from the final (proximal) orbits of the mission at Saturn, show large scale azimuthally polarized magnetic signals are always present near periapsis. The signals were attributed to standing Alfvén waves excited on the magnetic shells planetward of the Saturn D‐ring. The apparent absence of any systematic variation in frequency as the spacecraft crossed magnetic shells, implied that the signals were not simply locally excited standing Alfvén modes, but were pumped by coupling to global compressional eigenmodes excited in a cavity formed in the dayside magnetosphere. In this study, we use a numerical magnetohydrodynamic (MHD) model to test such theoretical explanations for the observations, by examining in detail the MHD wave coupling and large scale spatial structure of the signals. The modeling not only shows good agreement with the data but further provides new insight into features previously overlooked in the data. In particular, we show how the apparent frequency of a single spacecraft observation is affected by the phase variation present in a local field line resonance.

Interpretations of the cosmic ray secondary-to-primary ratios measured by DAMPE

Precise measurements of the boron-to-carbon and boron-to-oxygen ratios by DAMPE show clear hardenings around $100$ GeV/n, which provide important implications on the production, propagation, and interaction of Galactic cosmic rays. In this work we investigate a number of models proposed in literature in light of the DAMPE findings. These models can roughly be classified into two classes, driven by propagation effects or by source ones. Among these models discussed, we find that the re-acceleration of cosmic rays, during their propagation, by random magnetohydrodynamic waves may not reproduce sufficient hardenings of B/C and B/O, and an additional spectral break of the diffusion coefficient is required. The other models can properly explain the hardenings of the ratios. However, depending on simplifications assumed, the models differ in their quality in reproducing the data in a wide energy range. The models with significant re-acceleration effect will under-predict low-energy antiprotons but over-predict low-energy positrons, and the models with secondary production at sources over-predict high-energy antiprotons. For all models high-energy positron excess exists.