MHD Waves in Coronal Holes (original) (raw)
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MHD waves in the solar north polar coronal hole
Astronomische Nachrichten, 2010
The effects, hitherto not treated, of the temperature and the number density gradients, both in the parallel and the perpendicular direction to the magnetic field, of O VI ions, on the MHD wave propagation characteristics in the solar North Polar Coronal Hole are investigated. We investigate the magnetosonic wave propagation in a resistive MHD regime where only the thermal conduction is taken into account. Heat conduction across the magnetic field is treated in a non-classical approach wherein the heat is assumed to be conducted by the plasma waves emitted by ions and absorbed at a distance from the source by other ions. Anisotropic temperature and the number density distributions of O VI ions revealed the chaotic nature of MHD standing wave, especially near the plume/interplume lane borders. Attenuation length scales of the fast mode is shown not to be smoothly varying function of the radial distance from the Sun.
The Astrophysical Journal, 1985
It is well established observationally that high-speed solar wind streams originate in coronal hole regions in the solar corona. Models of the solar wind flow based on this observation indicate that heat conduction alone cannot account for the observed properties of the wind and that other sources of heat and/or momentum must be sought. One suggested source for this additional momentum is "wave pressure" generated by magnetohydrodynamic (MHD) waves. Theories of wave-driven winds exist, but they are not consistent with the observed fact that high-speed streams originate in discrete magnetic structures in the solar corona. The waves assumed responsible for acceleration of the high-speed solar wind streams should have periods of approximately a hundred seconds if they are driven by photospheric turbulence. But MHD waves with periods this large have wavelengths X> d, where d is the characteristic transverse size of the coronal hole. Current theories for the acceleration of the solar wind by MHD waves are valid only if the wavelength of the disturbance is much smaller than the characteristic transverse size of the coronal structure. This limit is not appropriate for the propagation of disturbances with periods P ä 100 s in the acceleration region of the solar wind. In this paper the effect of coronal hole magnetic structure on the propagation of MHD waves of all periods is considered. It is found that for the wave-period range discussed above the coronal hole structure acts as a " leaky " MHD waveguide, i.e., wave flux which enters at the base of the coronal hole is only weakly guided by the coronal hole structure. A significant amount of wave energy leaks through the side of the coronal hole into the surrounding corona. Dispersion relations are derived to describe the propagation of these leaky waves from both a geometric optics and an eigenmode approach. These two approaches are shown to yield equivalent results. The dispersion relation is then solved numerically to obtain both the real and the imaginary parts of the propagation constant. An analytic solution valid at high frequencies is also presented. It is found that for some frequencies the solutions of the dispersion relation indicate waves with negative, i.e., downward, group velocity. These waves cannot carry energy up from the photosphere into the corona, and therefore cannot contribute to the acceleration of the wind. Estimates for the periods of these waves give P ~ 100 s. The force on the coronal hole plasma due to the propagation of the leaky wave modes is calculated. It is found that the net force can be viewed as the sum of two terms : (1) magnetic wave pressure and (2) magnetic wave tension. The relative magnitude of these two forces is frequency-dependent. For very long wave periods or very short wave periods the pressure force is dominant. However, whenever the Alfvén wavelength is of the order of the transverse scale size of the coronal hole, the tensile force can completely dominate the wavepressure term, resulting in a net downward force on the plasma within the coronal hole. The calculations presented here show that the wave tensile force can be important for waves with periods of the order of a few hundred seconds. Taken together, these two results indicate that the solar wind plasma does not couple as efficiently to the 300 s oscillations as spherically symmetric theories of MHD wave-driven winds have indicated in the past. Numerical evaluation of this coupling efficiency must await a more detailed model incorporating the source terms at the base of the corona.
Magnetohydrodynamic waves and coronal seismology: an overview of recent results
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012
Recent observations have revealed that magnetohydrodynamic (MHD) waves and oscillations are ubiquitous in the solar atmosphere, with a wide range of periods. We give a brief review of some aspects of MHD waves and coronal seismology that have recently been the focus of intense debate or are newly emerging. In particular, we focus on four topics: (i) the current controversy surrounding propagating intensity perturbations along coronal loops, (ii) the interpretation of propagating transverse loop oscillations, (iii) the ongoing search for coronal (torsional) Alfvén waves, and (iv) the rapidly developing topic of quasi-periodic pulsations in solar flares.
Magnetohydrodynamic waves in coronal polar plumes
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2006
Polar plumes are cool, dense, linear, magnetically open structures that arise from predominantly unipolar magnetic footpoints in the solar polar coronal holes. As the Alfvén speed is decreased in plumes in comparison with the surrounding medium, these structures are natural waveguides for fast and slow magnetoacoustic waves. The simplicity of the geometry of polar plumes makes them an ideal test ground for the study of magnetohydrodynamic (MHD) wave interaction with solar coronal structures. The review covers recent observational findings of compressible and incompressible waves in polar plumes with imaging and spectral instruments, and interpretation of the waves in terms of MHD theory.
EVOLUTION OF MAGNETOHYDRODYNAMIC WAVES IN LOW LAYERS OF A CORONAL HOLE
The Astrophysical Journal, 2014
Although a coronal hole is permeated by a magnetic field with a dominant polarity, magnetograms reveal a more complex magnetic structure in the lowest layers, where several regions of opposite polarity of typical size of the order of 10 4 km are present. This can give rise to magnetic separatrices and neutral lines. MHD fluctuations generated at the base of the coronal hole by motions of the inner layer of the solar atmosphere may interact with such inhomogeneities, leading to the formation of small scales. This phenomenon is studied on a 2D model of a magnetic structure with an X-point, using 2D MHD numerical simulations. This model implements a method of characteristics for boundary conditions in the direction outer-pointing to Sun surface to simulate both wave injection and exit without reflection. Both Alfvénic and magnetosonic perturbations are considered, and they show very different phenomenology. In the former case, an anisotropic power-law spectrum forms with a dominance of perpendicular wavevectors at altitudes ∼10 4 km. Density fluctuations are generated near the X-point by Alfvén wave magnetic pressure and propagate along open fieldlines at a speed comparable to the local Alfvén velocity. An analysis of energy dissipation and heating caused by the formation of small scales for the Alfvénic case is presented. In the magnetosonic case, small scales form only around the X-point, where a phenomenon of oscillating magnetic reconnection is observed to be induced by the periodic deformation of the magnetic structure due to incoming waves.
Solar Physics
Established in the late 1990s, solar MHD seismology has rapidly grown into a unique tool for diagnostics of the solar atmospheric plasma. This Topical Collection presents the current state-of-the-art in the field, addressing both observational and theoretical aspects: from oscillations in coronal loops and filaments to quasi-periodic pulsations in flares, MHD wave dynamics in non-adiabatic coronal conditions, waves in the lower atmosphere, and novel techniques and approaches to theoretical modelling and data analysis.
Magnetohydrodynamic Waves and Coronal Heating: Unifying Empirical and MHD Turbulence Models
The Astrophysical Journal, 2013
We present a new global model of the solar corona, including the low corona, the transition region and the top of chromosphere. The realistic 3D magnetic field is simulated using the data from the photospheric magnetic field measurements. The distinctive feature of the new model is incorporating the MHD Alfven wave turbulence. We assume this turbulence and its non-linear dissipation to be the only momentum and energy source for heating the coronal plasma and driving the solar wind. The difference between the turbulence dissipation efficiency in coronal holes and that in closed field regions is because the non-linear cascade rate degrades in strongly anisotropic (imbalanced) turbulence in coronal holes (no inward propagating wave), thus resulting in colder coronal holes with the bi-modal solar wind originating from them. The detailed presentation of the theoretical model is illustrated with the synthetic images for multi-wavelength EUV emission compared with the observations from SDO AIA and Stereo EUVI instruments for the Carrington rotation 2107.
Astronomy & Astrophysics, 2006
Aims. We study the possible role of magnetohydrodynamic (MHD) waves in the heating of solar corona and magnetic coronal loops. Methods. Taking into account viscosity and thermal conductivity, we obtained a general fifth order dispersion relation for MHD waves propagating in a homogeneous, magnetically structured, compressible low-β plasma. The general fifth order dispersion relation has been solved numerically, and we discuss its application to magnetic coronal loops with the help of data provided by the NIXT mission. Results. The dispersion relation results in three modes, namely slow, fast, and thermal. The damping of both slow-and fast-mode waves depends upon the plasma density, the temperature, the magnetic field strength, and the angle of propagation relative to the background magnetic field. Slow-mode waves contribute to the heating of the solar corona, if one considers that they are generated in the corona by turbulent motions at magnetic reconnection sites. Calculations of wave damping rates determined from the dispersion relation indicate that slow-mode waves with periods of less than 60 s damp sufficiently rapidly and dissipate enough energy to balance the radiative losses, whereas the fast-mode waves with periods of less than 3 s may damp at rates great enough to balance the radiative losses in active regions. In the case of magnetic coronal loops, it is observed that slow-mode waves with frequencies greater than 0.003 Hz and fast mode waves with frequencies greater than 0.28 Hz (high frequency) are needed for coronal heating and to balance the radiative losses in active regions.
The Temporal Evolution of Linear Fast and Alfven MHD Waves in Solar Coronal Arcades
Highlights of Spanish Astrophysics V, 2010
The excitation and temporal evolution of fast and Alfvén magnetohydrodynamic oscillations in a two-dimensional coronal arcade are investigated. The approach is to consider an equilibrium magnetic and plasma structure and then to introduce a perturbation trying to mimic a nearby disturbance, such as a flare or filament eruption. By numerically solving the time-dependent linearised MHD wave equations the properties of the solutions have been studied. First, the properties of uncoupled fast and Alfvén waves are described. Then, longitudinal propagation of perturbations is included, and the properties of coupled waves are determined.
Slow magnetoacoustic waves in coronal loops
2000
A theoretical model interpreting propagating disturbances of EUV emission intensity, recently observed in coronal loops, is constructed in terms of slow magnetoacoustic waves. The model is one-dimensional and incorporates effects of nonlinearity, dissipation due to finite viscosity and thermal conduction, and gravitational stratification of plasma in the loop. It has been found that, for the observationally detected parameters of the waves, the main factors influencing the wave evolution are dissipation and stratification. The upwardly propagating waves of observed periods (5-20 min) are found to decay significantly in the vicinity of the loop apex, explaining the rarity of observational detection of downwardly propagating waves. The model provides a theoretical basis for development of MHD seismology of the coronal loops.