MHD waves in the solar north polar coronal hole (original) (raw)
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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.
Low-Frequency Waves in Space Plasmas, 2016
Coronal holes are the dark patches in the solar corona associated with relatively cool, less dense plasma and unipolar fields. The fast component of the solar wind emanates from these regions. Several observations reveal the presence of magnetohydrodynamic (MHD) waves in coronal holes which are believed to play a key role in the acceleration of fast solar wind. The recent advent of high-resolution instruments had brought us many new insights on the properties of MHD waves in coronal holes which are reviewed in this article. The advances made in the identification of compressive slow MHD waves in both polar and equatorial coronal holes, their possible connection with the recently discovered highspeed quasi-periodic upflows, their dissipation, and the detection of damping in Alfvén waves from the spectral line width variation are discussed in particular.
A new model for heating of the Solar North Polar Coronal Hole
This article presents a new model of the North Polar Coronal Hole (NPCH) with the aim of revealing the dissipative/propagative characteristics of magnetohydrodynamic (MHD) waves. We investigate the effects of isotropic viscosity and anisotropic heat conduction on the propagation characteristics of MHD waves in the NPCH. We first model the NPCH by considering differences in the radial direction as well as in the direction perpendicular to the line of sight (los) in temperature, particle number density and non-thermal velocities between plumes and interplume lanes, for the specific case of O VI ions. This model includes parallel and perpendicular (to the magnetic field) heat conduction and viscous dissipation. Next, we derive the dispersion relations for MHD waves in cases of the absence and presence of parallel heat conduction. In the case of the absence of parallel heat conduction, we find that MHD wave dissipation depends strongly on viscosity for modified acoustic and Alfvén waves. The energy flux densities of acoustic waves vary between 10 4.7 and 10 7 erg cm −2 s −1 , while the energy flux densities of Alfvén waves turn out to be between 10 6 and 10 8.6 erg cm −2 s −1. When there is parallel heat conduction, we calculate the damping length-scales and the energy flux densities of magnetoacoustic waves. Our results suggest that modified magnetoacoustic waves may provide a significant source for the observed preferential acceleration and heating of O VI ions, thus coronal plasma heating, and an extra accelerating agent for the fast solar wind in the NPCH, depending on the values of the transport coefficients.
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.
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.
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.
The Astrophysical Journal, 2022
Previous studies have suggested that the Kelvin–Helmholtz instability (KHI) and magnetohydrodynamic (MHD) wave emissions via the KHI along various shear flow boundaries in a solar–terrestrial environment may be possible. We expand upon these previous studies to investigate the linear and nonlinear evolution of the KHI and emission of MHD waves along the boundaries of coronal mass ejections (CMEs). Our results demonstrate that the KHI and MHD wave emission due to the KHI are possible along the CME boundaries during the KHI development. We found that magnetic field orientation in the region outside of the CME has strong effects on the strength of MHD wave emission. While a smaller parallel component of the magnetic field resulted in larger growth rates in the KHI development, a larger parallel component of the magnetic field resulted in stronger MHD wave emissions. For all cases we investigated, we identified emitted waves to be fast MHD waves. We suggest that these emitted MHD waves ...
MHD turbulence and heating of the open field-line solar corona
Nonlinear Processes in Geophysics, 2003
This paper discusses the possibility that heating of the solar corona in open field-line regions emanating from coronal holes is due to a nonlinear cascade, driven by low-frequency or quasi-static magnetohydrodynamic fluctuations. Reflection from coronal inhomogeneities plays an important role in sustaining the cascade. Physical and observational constraints are discussed. Kinetic processes that convert cascaded energy into heat must occur in regions of turbulent small-scale reconnection, and may be similar in some respects to ion heating due to intense electron beams observed in the aurora.
MHD oscillations in solar and stellar coronae: Current results and perspectives
Advances in Space Research, 2007
Wave and oscillatory activity is observed with modern imaging and spectral instruments in the visible light, EUV, X-ray and radio bands in all parts of the solar corona. Magnetohydrodynamic (MHD) wave theory gives satisfactory interpretation of these phenomena in terms of MHD modes of coronal structures. The paper reviews the current trends in the observational study of coronal oscillations, recent development of theoretical modelling of MHD wave interaction with plasma structures, and implementation of the theoretical results for the mode identification. Also the use of MHD waves for remote diagnostics of coronal plasmas is discussed. In particular, the applicability of this method to the estimation of the coronal magnetic field is demonstrated.
Three-Dimensional Propagation of Magnetohydrodynamic Waves in Solar Coronal Arcades
Astrophysical Journal, 2010
We numerically investigate the excitation and temporal evolution of oscillations in a two-dimensional coronal arcade by including the three-dimensional propagation of perturbations. The time evolution of impulsively generated perturbations is studied by solving the linear, ideal magnetohydrodynamic (MHD) equations in the zero-β approximation. As we neglect gas pressure the slow mode is absent and therefore only coupled MHD fast and Alfvén modes remain. Two types of numerical experiments are performed. First, the resonant wave energy transfer between a fast normal mode of the system and local Alfvén waves is analyzed. It is seen how, because of resonant coupling, the fast wave with global character transfers its energy to Alfvénic oscillations localized around a particular magnetic surface within the arcade, thus producing the damping of the initial fast MHD mode. Second, the time evolution of a localized impulsive excitation, trying to mimic a nearby coronal disturbance, is considered. In this case, the generated fast wavefront leaves its energy on several magnetic surfaces within the arcade. The system is therefore able to trap energy in the form of Alfvénic oscillations, even in the absence of a density enhancement such as that of a coronal loop. These local oscillations are subsequently phase-mixed to smaller spatial scales. The amount of wave energy trapped by the system via wave energy conversion strongly depends on the wavelength of perturbations in the perpendicular direction, but is almost independent from the ratio of the magnetic to density scale heights.