Physics

Research

17 pages, 2436 KiB

Open AccessArticle

Numerical Simulations of the Decaying Transverse Oscillations in the Cool Jet

by Abhishek K. Srivastava and Balveer Singh

Physics 2023, 5(3), 655-671; https://doi.org/10.3390/physics5030043 - 25 Jun 2023

Cited by 1 | Viewed by 1616

In the present paper, we describe a 2.5D (two-and-a-half-dimensional) magnetohydrodynamic (MHD) simulation that provides a detailed picture of the evolution of cool jets triggered by initial vertical velocity perturbations in the solar chromosphere. We implement random multiple velocity,

V_{y}

, pulses of [...] Read more.

In the present paper, we describe a 2.5D (two-and-a-half-dimensional) magnetohydrodynamic (MHD) simulation that provides a detailed picture of the evolution of cool jets triggered by initial vertical velocity perturbations in the solar chromosphere. We implement random multiple velocity,

V_{y}

, pulses of amplitude 20–50 km s

^{- 1}

between 1 Mm and 1.5 Mm in the Sun’s atmosphere below its transition region (TR). These pulses also consist of different switch-off periods between 50 s and 300 s. The applied vertical velocity pulses create a series of magnetoacoustic shocks steepening above the TR. These shocks interact with each other in the inner corona, leading to complex localized velocity fields. The upward propagation of such perturbations creates low-pressure regions behind them, which propel a variety of cool jets and plasma flows in the localized corona. The localized complex velocity fields generate transverse oscillations in some of these jets during their evolution. We study the transverse oscillations of a representative cool jet J

_{1}

, which moves up to the height of 6.2 Mm above the TR from its origin point. During its evolution, the plasma flows make the spine of jet J

_{1}

radially inhomogeneous, which is visible in the density and Alfvén speed smoothly varying across the jet. The highly dense J

_{1}

, which is triggered along the significantly curved magnetic field lines, supports the propagating transverse wave of period of approximately 195 s with a phase speed of about 125 km s⁻¹. In the distance–time map of density, it is manifested as a transverse kink wave. However, the careful investigation of the distance–time maps of the x- and z-components of velocity reveals that these transverse waves are actually of mixed Alfvénic modes. The transverse wave shows evidence of damping in the jet. We conclude that the cross-field structuring of the density and characteristic Alfvén speed within J

_{1}

causes the onset of the resonant conversion and leakage of the wave energy outward to dissipate these transverse oscillations via resonant absorption. The wave energy flux is estimated as approximately of 1.0 × 10

^{6}

ergs cm

^{- 2}

s

^{- 1}

. This energy, if it dissipates through the resonant absorption into the corona where the jet is propagated, is sufficient energy for the localized coronal heating. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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22 pages, 12530 KiB

Open AccessArticle

p-Mode Oscillations in Highly Gravitationally Stratified Magnetic Solar Atmospheres

by Michael Griffiths, Norbert Gyenge, Ruisheng Zheng, Marianna Korsós and Robertus Erdélyi

Physics 2023, 5(2), 461-482; https://doi.org/10.3390/physics5020032 - 18 Apr 2023

Cited by 2 | Viewed by 2170

Abstract

The aim of the study reported in this paper is to gain understanding of solar global oscillations and the propagation characteristics of p-mode oscillations in the highly gravitationally stratified magnetic solar atmosphere. The paper presents the results of 3D (3-dimensional) numerical magnetohydrodynamic [...] Read more.

The aim of the study reported in this paper is to gain understanding of solar global oscillations and the propagation characteristics of p-mode oscillations in the highly gravitationally stratified magnetic solar atmosphere. The paper presents the results of 3D (3-dimensional) numerical magnetohydrodynamic (MHD) simulations of a model solar atmosphere with a uniform, vertical and cylindrically symmetric magnetic field. We use simulation drivers which result in oscillations mimicking the behaviour of p-mode oscillations. The paper reports the variation of the energy flux and oscillation frequency of the magnetosonic modes and examines their dependence on the magnetic field strength. We report results for the temporal analysis of observational data for the quiet Sun and for a region containing a small sunspot (solar pore). We compare the temporal analysis of results from observations of these ubiquitous intensity oscillations with numerical simulations of potential signatures of global oscillations of the solar atmosphere. We conclude that magnetic regions of the solar atmosphere are favourable regions for the propagation of a small leakage of energy by slow magnetosonic modes. The results also exhibit a variation in the frequency of the oscillations at different heights in the low-to-mid solar atmosphere and for different values of the magnetic field. The numerically obtained periodic behaviour and variation in frequency, even in this simplified model atmosphere, is consistent with the observational data. We find frequencies and frequency variations that are similar to measurements obtained from the intensity time series of images taken by the Solar Dynamics Observatory. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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21 pages, 2810 KiB

Open AccessArticle

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

by José Luis Ballester

Physics 2023, 5(2), 331-351; https://doi.org/10.3390/physics5020025 - 30 Mar 2023

Cited by 4 | Viewed by 1880

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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12 pages, 2201 KiB

Open AccessArticle

Resonant Fast-Alfvén Wave Coupling in a 3D Coronal Arcade

by Andrew Wright and Thomas Elsden

Physics 2023, 5(1), 310-321; https://doi.org/10.3390/physics5010023 - 17 Mar 2023

Cited by 2 | Viewed by 1502

Abstract

The resonant excitation of Alfvén waves using the fast magnetosonic mode is important in space plasmas. In this paper, we consider a simple model of a three-dimensional (3D) coronal arcade. A numerical approach is used to produce a driven normal mode. We find [...] Read more.

The resonant excitation of Alfvén waves using the fast magnetosonic mode is important in space plasmas. In this paper, we consider a simple model of a three-dimensional (3D) coronal arcade. A numerical approach is used to produce a driven normal mode. We find that resonant coupling can occur in 3D, but there are new features that are absent in 2D. In particular, the polarisation of the Alfvén waves can vary with position throughout the Resonant Zone. Moreover, there are an infinite number of possible paths the resonant waves can exist on. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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22 pages, 584 KiB

Open AccessArticle

The Interplay between Coronal Holes and Solar Active Regions from Magnetohydrostatic Models

by Jaume Terradas

Physics 2023, 5(1), 276-297; https://doi.org/10.3390/physics5010021 - 28 Feb 2023

Cited by 2 | Viewed by 1677

Abstract

Coronal holes (CHs) and active regions (ARs) are typical magnetic structures found in the solar corona. The interaction of these two structures was investigated mainly from the observational point of view, but a basic theoretical understanding of how they are connected is missing. [...] Read more.

Coronal holes (CHs) and active regions (ARs) are typical magnetic structures found in the solar corona. The interaction of these two structures was investigated mainly from the observational point of view, but a basic theoretical understanding of how they are connected is missing. To address this problem, in this paper, magnetohydrostatic models are constructed by numerically solving a Grad–Shafranov equation in two dimensions. A common functional form for the pressure and temperature in the CH and in the AR are assumed throught the study. Keeping the parameters of the CH constant and modifying the parameters of the nearby bipolar AR, one finds essentially three types of solutions depending on the magnitude and sign of the magnetic field at the closest foot of the AR to the CH. Two of the three solutions match well with the observation, but the third solution predicts the existence of closed magnetic field lines with quite low density and temperature with opposite characteristics to those in typical ARs. Simple analytical expressions are obtained for the pressure, temperature and density at the core of the AR and their dependence upon several major physical parameters are studied. The results obtained in this paper need to be contrasted with observations. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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15 pages, 3307 KiB

Open AccessArticle

Influence of the Magnetic Field Topology in the Evolution of Small-Scale Two-Fluid Jets in the Solar Atmosphere

by Elton Everardo Díaz-Figueroa, Gonzalo Ares de Parga and José Juan González-Avilés

Physics 2023, 5(1), 261-275; https://doi.org/10.3390/physics5010020 - 27 Feb 2023

Viewed by 1946

Abstract

In this paper, a series of numerical simulations is performed to recreate small-scale two-fluid jets using the JOANNA code, considering the magnetohydrodynamics of two fluids (ions plus electrons and neutral particles). First, the jets are excited in a uniform magnetic field by using [...] Read more.

In this paper, a series of numerical simulations is performed to recreate small-scale two-fluid jets using the JOANNA code, considering the magnetohydrodynamics of two fluids (ions plus electrons and neutral particles). First, the jets are excited in a uniform magnetic field by using velocity pulse perturbations located at

y_{0} =

1.3, 1.5, and 1.8 Mm, considering the base of the photosphere at

y = 0

. Then, the excitation of the jets is repeated in a magnetic field that mimics a flux tube. Mainly, the jets excited at the upper chromosphere (

y \sim 1.8

Mm) reach lower heights than those excited at the lower chromosphere (

y \sim 1.3

Mm); this is due to the higher initial vertical location because of the lesser amount of plasma dragging. In both scenarios, the dynamics of the neutral particles and ions show similar behavior, however, one can still identify some differences in the velocity drift, which in the simulations here is of the order of

10^{- 3}

km/s at the tips of the jets once they reached their maximum heights. In addition, the heat due to the friction between ions and neutrals (

Q_{i, n}^{i n}

) is estimated to be of the order of 0.002–0.06 W/m

^{3}

. However, it hardly contributes to the heating of the surroundings of the solar corona. The jets in the two magnetic environments do not show substantial differences other than a slight variation in the maximum heights reached, particularly in the uniform magnetic field scenario. Finally, the maximum heights reached by the three different jets are found in the range of some morphological parameters corresponding to macrospicules, Type I spicules, and Type II spicules. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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14 pages, 1048 KiB

Open AccessArticle

Damping and Dispersion of Non-Adiabatic Acoustic Waves in a High-Temperature Plasma: A Radiative-Loss Function

by Sergei Derteev, Nikolai Shividov, Dzhirgal Bembitov and Badma Mikhalyaev

Physics 2023, 5(1), 215-228; https://doi.org/10.3390/physics5010017 - 15 Feb 2023

Cited by 5 | Viewed by 2111

Abstract

The behavior of acoustic waves in a rarefied high-temperature plasma is studied; as an example, the plasma of the solar corona is considered. Effects of thermal conductivity and a heating/radiative loss are taken into account; data on a temperature distribution of a radiation [...] Read more.

The behavior of acoustic waves in a rarefied high-temperature plasma is studied; as an example, the plasma of the solar corona is considered. Effects of thermal conductivity and a heating/radiative loss are taken into account; data on a temperature distribution of a radiation intensity obtained from the CHIANTI 10 code are used. The classical Spitzer expression for a full-ionized plasma is used for the thermal conductivity. Based on the found values of the radiation-loss function, the cubic spline method is used to construct an approximate analytical expression necessary for studying linear waves. A dispersion relation is obtained, and a frequency, a phase speed, and a damping coefficient are found. Dispersion and damping properties are considered for a temperature of about

10^{6}

K and a particle density of about

10^{15}

m^{- 3}

, which are typical for the coronal plasma. In sum, superiority in the dispersion and damping of the thermal conduction is shown; the heating and radiation loss manifest themselves at large wavelengths. In accordance with general results by Field, a condition was found under which the acoustic oscillations become unstable. It is shown that at certain values of the temperature and density, the wave damping is dominated by the heating/radiative loss misbalance. Thus, the earlier results on mechanisms of damping of observed acoustic waves in the solar corona are refined here. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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12 pages, 3127 KiB

Open AccessArticle

Stability of Slow Magnetoacoustic and Entropy Waves in the Solar Coronal Plasma with Thermal Misbalance

by Dmitrii Y. Kolotkov, Valery M. Nakariakov and Joseph B. Fihosy

Physics 2023, 5(1), 193-204; https://doi.org/10.3390/physics5010015 - 8 Feb 2023

Cited by 10 | Viewed by 1950

Abstract

The back-reaction of the perturbed thermal equilibrium in the solar corona on compressive perturbations, also known as the effect of wave-induced thermal misbalance, is known to result in thermal instabilities chiefly responsible for the formation of fine thermal structuring of the corona. We [...] Read more.

The back-reaction of the perturbed thermal equilibrium in the solar corona on compressive perturbations, also known as the effect of wave-induced thermal misbalance, is known to result in thermal instabilities chiefly responsible for the formation of fine thermal structuring of the corona. We study the role of the magnetic field and field-aligned thermal conduction in triggering instabilities of slow magnetoacoustic and entropy waves in quiescent and hot active region loops, caused by thermal misbalance. Effects of the magnetic field are accounted for by including it in the parametrization of a guessed coronal heating function, and the finite plasma parameter

β

, in terms of the first-order thin flux tube approximation. Thermal conduction tends to stabilize both slow and entropy modes, broadening the interval of plausible coronal heating functions allowing for the existence of a thermodynamically stable corona. This effect is most pronounced for hot loops. In contrast to entropy waves, the stability of which is found to be insensitive to the possible dependence of the coronal heating function on the magnetic field, slow waves remain stable only for certain functional forms of this dependence, opening up perspectives for its seismological diagnostics in future. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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7 pages, 500 KiB

Open AccessCommunication

Modeling the Magnetic Field of the Inner Corona in a Radially Expanding Solar Wind

by Andrey G. Tlatov and Ivan Berezin

Physics 2023, 5(1), 161-167; https://doi.org/10.3390/physics5010012 - 29 Jan 2023

Cited by 3 | Viewed by 1523

Abstract

The magnetic field in the interplanetary medium is formed by the action of magnetic field sources on the photosphere of the Sun and currents in the expanding atmosphere of the Sun and the solar wind. In turn, the high-speed plasma flow changes the [...] Read more.

The magnetic field in the interplanetary medium is formed by the action of magnetic field sources on the photosphere of the Sun and currents in the expanding atmosphere of the Sun and the solar wind. In turn, the high-speed plasma flow changes the configuration of the magnetic field lines. The problem of determining the parameters of the magnetic field near the Sun is thus a three-dimensional problem of the interaction of the magnetic field and the plasma of the solar wind. We present analytical expressions for calculating the total magnetic field vector

\vec{B} (r, θ, ϕ)

(in spherical coordinates) for a radially expanding solar wind flow of finite conductivity. The parameters of the solar wind are given in the form of a dimensionless magnetic Reynolds number given as an arbitrary function of the radius, r:

R_{m}

=

r σ μ v = ξ (r)

, where

σ

,

μ

, and v denote, respectively, the conductivity, magnetic permeability, and velocity of the solar wind. The solution for the magnetic field components is obtained in the form of a decomposition in spherical functions and a radial part depending on the distance from the Sun. Examples of calculations of the configuration of magnetic fields and structures of the solar corona for the solar eclipse of 21 August 2017 are given. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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21 pages, 6355 KiB

Open AccessArticle

Heating and Cooling in Transversely Oscillating Coronal Loops Powered by Broadband, Multi-Directional Wave Drivers

by Thomas Howson and Ineke De Moortel

Physics 2023, 5(1), 140-160; https://doi.org/10.3390/physics5010011 - 29 Jan 2023

Cited by 5 | Viewed by 1760

Abstract

Recent studies have identified the potential for coronal wave heating to balance radiative losses in a transversely oscillating low-density loop undergoing resonant absorption, phase mixing and the Kelvin–Helmholtz instability. This result relied on a continuous, resonant oscillatory driver acting on one of the [...] Read more.

Recent studies have identified the potential for coronal wave heating to balance radiative losses in a transversely oscillating low-density loop undergoing resonant absorption, phase mixing and the Kelvin–Helmholtz instability. This result relied on a continuous, resonant oscillatory driver acting on one of the loop footpoints and similar setups with non-resonant driving produce insufficient heating. Here, we consider broadband and multi-directional drivers with power in both resonant and non-resonant frequencies. Using three-dimensional magnetohydrodynamic simulations, we impose transverse, continuous velocity drivers at the footpoints of a coronal loop, which is dense in comparison to the background plasma. We include the effects of optically thin radiation and a uniform background heating term that maintains the temperature of the external plasma but is insufficient to balance energy losses within the loop. For both broadband and multi-directional drivers, we find that the energy dissipation rates are sufficient to balance the average energy losses throughout the simulation volume. Resonant components of the wave driver efficiently inject energy into the system and these frequencies dominate the energetics. Although the mean radiative losses are balanced, the loop core cools in all cases as the wave heating rates are locally insufficient, despite the relatively low density considered here. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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15 pages, 11646 KiB

Open AccessArticle

The Impact of Radio Frequency Waves on the Plasma Density in the Tokamak Edge

by Dirk Van Eester and Nil Tournay

Physics 2023, 5(1), 116-130; https://doi.org/10.3390/physics5010009 - 28 Jan 2023

Viewed by 3127

Abstract

A simple model is presented to describe how the radio frequency electromagnetic field modifies the plasma density the antenna faces in tokamaks. Aside from “off-the-shelf” equations standardly used to describe wave-plasma interaction relying on the quasilinear approach, it invokes the ponderomotive force in [...] Read more.

A simple model is presented to describe how the radio frequency electromagnetic field modifies the plasma density the antenna faces in tokamaks. Aside from “off-the-shelf” equations standardly used to describe wave-plasma interaction relying on the quasilinear approach, it invokes the ponderomotive force in presence of the confining static magnetic field. The focus is on dynamics perpendicular to the

B_{o}

magnetic field. Stronger fields result in density being pushed further away from the launcher and in stronger density asymmetry along the antenna. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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18 pages, 2733 KiB

Open AccessArticle

Advances in the Implementation of the Exactly Energy Conserving Semi-Implicit (ECsim) Particle-in-Cell Method

by Giovanni Lapenta

Physics 2023, 5(1), 72-89; https://doi.org/10.3390/physics5010007 - 18 Jan 2023

Cited by 8 | Viewed by 2333

Abstract

The energy-conserving semi-implicit (ECsim) method presented by the author in 2017, is a particle-in-cell (PIC) algorithm for the simulation of plasmas. Energy conservation is achieved within a semi-implicit formulation that does not require any non-linear solver. A mass matrix is introduced to linearly [...] Read more.

The energy-conserving semi-implicit (ECsim) method presented by the author in 2017, is a particle-in-cell (PIC) algorithm for the simulation of plasmas. Energy conservation is achieved within a semi-implicit formulation that does not require any non-linear solver. A mass matrix is introduced to linearly express the particle-field coupling. With the mass matrix, the algorithm preserves energy conservation to machine precision. The construction of the mass matrix is the central nature of the method and also the main cost of the computational cycle. Here, three methods that modify the construction of the mass matrix are analyzed. First, the paper considers how the sub-cycling of the particle motion modifies the mass matrix. Second, a form of smoothing that reduces the noise while retaining exact energy conservation is introduced. Finally, an approximation of the mass matrix is discussed that transforms the ECsim scheme to the implicit moment method. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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13 pages, 1643 KiB

Open AccessArticle

Probability Distribution Functions of Solar and Stellar Flares

by Takashi Sakurai

Physics 2023, 5(1), 11-23; https://doi.org/10.3390/physics5010002 - 28 Dec 2022

Cited by 7 | Viewed by 2332

Abstract

The paper studies the soft X-ray data of solar flares and found that the distribution functions of flare fluence are successfully modeled by tapered power law or gamma function distributions whose power exponent is slightly smaller than 2, indicating that the total energy [...] Read more.

The paper studies the soft X-ray data of solar flares and found that the distribution functions of flare fluence are successfully modeled by tapered power law or gamma function distributions whose power exponent is slightly smaller than 2, indicating that the total energy of the flare populations is mostly due to a small number of large flares. The largest possible solar flares in 1000 years are predicted to be around X70 (a peak flux of 70 × 10⁻⁴ W m⁻²) in terms of the GOES (Geostationary Operational Environmental Satellites) flare class. The paper also studies superflares (more energetic than solar flares) from solar-type stars and found that their power exponent in the fitting of the gamma function distribution is around 1.05, which is much flatter than solar flares. The distribution function of stellar flare energy extrapolated downward does not connect to the distribution function of solar flare energy. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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12 pages, 1235 KiB

Open AccessArticle

Exploring the Ideal MHD Quasi-Modes of a Plasma Interface with a Thick Nonuniform Transition

by Roberto Soler

Physics 2022, 4(4), 1359-1370; https://doi.org/10.3390/physics4040087 - 8 Nov 2022

Viewed by 1549

Abstract

Nonuniform plasma across an imposed magnetic field, such as those present in the solar atmosphere, can support collective Alfvénic oscillations with a characteristic damping time. The damped transverse oscillations of coronal loops are an example of this process. In ideal magnetohydrodynamics (MHD), these [...] Read more.

Nonuniform plasma across an imposed magnetic field, such as those present in the solar atmosphere, can support collective Alfvénic oscillations with a characteristic damping time. The damped transverse oscillations of coronal loops are an example of this process. In ideal magnetohydrodynamics (MHD), these transient collective motions are associated with quasi-modes resonant in the Alfvén continuum. Quasi-modes live in a non-principal Riemann sheet of the dispersion relation, and so they are not true ideal MHD eigenmodes. The present study considers the illustrative case of incompressible surface MHD waves propagating on a nonuniform interface between two uniform plasmas with a straight magnetic field parallel to the interface. It is explored how the ideal quasi-modes of this configuration change when the width of the nonuniform transition increases. It is found that interfaces with wide enough transitions are not able to support truly collective oscillations. A quasi-mode that can be related with a resonantly damped surface MHD wave can only be found in interfaces with sufficiently thin transitions. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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17 pages, 12054 KiB

Open AccessArticle

Alfvén Wave Conversion and Reflection in the Solar Chromosphere and Transition Region

by Paul Cally

Physics 2022, 4(3), 1050-1066; https://doi.org/10.3390/physics4030069 - 8 Sep 2022

Cited by 3 | Viewed by 2111

Abstract

Series solutions are used to explore the mode conversion of slow, Alfvén and fast magnetohydrodynamic waves injected at the base of a two-isothermal-layer stratified atmosphere with a uniform magnetic field, crudely representing the solar chromosphere and corona with intervening discontinuous transition region. This [...] Read more.

Series solutions are used to explore the mode conversion of slow, Alfvén and fast magnetohydrodynamic waves injected at the base of a two-isothermal-layer stratified atmosphere with a uniform magnetic field, crudely representing the solar chromosphere and corona with intervening discontinuous transition region. This sets a baseline for understanding the ubiquitous Alfvénic waves observed in the corona, which are implicated in coronal heating and solar wind acceleration. It is found that all three injected wave types can partially transmit as coronal Alfvén waves in varying proportions dependent on frequency, magnetic field inclination, wave orientation, and distance between the Alfvén/acoustic equipartition level and the transition region. However, net Alfvénic transmission is limited for plausible parameters, and additional magnetic field structuring may be required to provide sufficient wave energy flux. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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8 pages, 427 KiB

Open AccessArticle

Existence of Purely Alvén Waves in Magnetic Flux Tubes with Arbitrary Cross-Sections

by Michael S. Ruderman and Nikolai S. Petrukhin

Physics 2022, 4(3), 865-872; https://doi.org/10.3390/physics4030055 - 29 Jul 2022

Cited by 2 | Viewed by 1725

Abstract

We study linear torsional Alfvén waves in a magnetic flux tube with an arbitrary cross-section. We assume that the equilibrium magnetic field is propagating in the z-direction in Cartesian coordinates

x, y

, and z. The tube cross-section is bounded [...] Read more.

We study linear torsional Alfvén waves in a magnetic flux tube with an arbitrary cross-section. We assume that the equilibrium magnetic field is propagating in the z-direction in Cartesian coordinates

x, y

, and z. The tube cross-section is bounded by a smooth closed curve. Both plasma and magnetic field are homogeneous outside this curve. The magnetic field magnitude is a function of x and y, while the density is a product of two functions: one dependent on z and the other dependent on x and y. As a result, the Alfvén speed is also equal to

V_{0} (x, y)

times a function of z. We define Alfvén waves as waves that do not disturb plasma density. We show that these waves can exist only when the magnetic field magnitude is a function of

V_{0}

. When the condition of existence of Alfvén waves is satisfied, the waves are polarised in the directions tangent to the level lines of

V_{0} (x, y)

and orthogonal to the equilibrium magnetic field. We found that the Alfvén wave amplitude has a specific form that depends on a particular coordinate system. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Marcel Goossens on the Occasion of His 75th Birthday)

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