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Register a new userThe effect of magnetic field on the damping of slow waves in the solar corona
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Timothy Duckenfield
Publication date
2020
fields
Physics
and research's language is
English
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Slow magnetoacoustic waves are routinely observed in astrophysical plasma systems such as the solar corona. As a slow wave propagates through a plasma, it modifies the equilibrium quantities of density, temperature, and magnetic field. In the corona and other plasma systems, the thermal equilibrium is comprised of a balance between continuous heating and cooling processes, the magnitudes of which vary with density, temperature and magnetic field. Thus the wave may induce a misbalance between these competing processes. Its back reaction on the wave has been shown to lead to dispersion, and amplification or damping, of the wave. In this work the importance of the effect of magnetic field in the rapid damping of slow waves in the solar corona by heating/cooling misbalance is evaluated and compared to the effects of thermal conduction. The two timescales characterising the effect of misbalance are derived and calculated for plasma systems with a range of typical coronal conditions. The predicted damping times of slow waves from thermal misbalance in the solar corona are found to be of the order of 10-100 minutes, coinciding with the wave periods and damping times observed. Moreover the slow wave damping by thermal misbalance is found to be comparable to the damping by field-aligned thermal conduction. We show that in the infinite field limit, the wave dynamics is insensitive to the dependence of the heating function on the magnetic field, and this approximation is found to be valid in the corona so long as the magnetic field strength is greater than 10G for quiescent loops and plumes and 100G for hot and dense loops. In summary thermal misbalance may damp slow magnetoacoustic waves rapidly in much of the corona, and its inclusion in our understanding of slow mode damping may resolve discrepancies between observations and theory relying on compressive viscosity and thermal conduction alone.
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Magnetic reconnection, a fundamentally important process in many aspects of astrophysics, is believed to be initiated by the tearing instability of an electric current sheet, a region where magnetic field abruptly changes direction and electric currents build up. Recent studies have suggested that the amount of magnetic shear in these structures is a critical parameter for the switch-on nature of magnetic reconnection in the solar atmosphere, at fluid spatial scales much larger than kinetic scales. We present results of simulations of reconnection in 3D current sheets with conditions appropriate to the solar corona. Using high-fidelity simulations, we follow the evolution of the linear and non-linear 3D tearing instability, leading to reconnection. We find that, depending on the parameter space, magnetic shear can play a vital role in the onset of significant energy release and heating via non-linear tearing. Two regimes in our study exist, dependent on whether the current sheet is longer or shorter than the wavelength of the fastest growing parallel mode (in the corresponding infinite system), thus determining whether sub-harmonics are present in the actual system. In one regime, where the fastest growing parallel mode has sub-harmonics, the non-linear interaction of these sub-harmonics and the coalescence of 3D plasmoids dominates the non-linear evolution, with magnetic shear playing only a weak role in the amount of energy released. In the second regime, where the fastest growing parallel mode has no-sub-harmonics, then only strongly sheared current sheets, where oblique mode are strong enough to compete with the dominant parallel mode, show any significant energy release. We expect both regimes to exist on the Sun, and so our results have important consequences for the the question of reconnection onset in different solar physics applications.
In the present work we study evolution of magnetic helicity in the solar corona. We compare the rate of change of a quantity related to the magnetic helicity in the corona to the flux of magnetic helicity through the photosphere and find that the two rates are similar. This gives observational evidence that helicity flux across the photosphere is indeed what drives helicity changes in solar corona during emergence. For the purposes of estimating coronal helicity we neither assume a strictly linear force-free field, nor attempt to construct a non-linear force-free field. For each coronal loop evident in Extreme Ultraviolet (EUV) we find a best-matching line of a linear force-free field and allow the twist parameter alpha to be different for each line. This method was introduced and its applicability was discussed in Malanushenko et. al. (2009). The object of the study is emerging and rapidly rotating AR 9004 over about 80 hours. As a proxy for coronal helicity we use the quantity <alpha_i*L_i/2> averaged over many reconstructed lines of magnetic field. We argue that it is approximately proportional to flux-normalized helicity H/Phi^2, where H is helicity and Phi is total enclosed magnetic flux of the active region. The time rate of change of such quantity in the corona is found to be about 0.021 rad/hr, which is compatible with the estimates for the same region obtained using other methods Longcope et. al. (2007), who estimated the flux of normalized helicity of about 0.016 rad/hr.
Drift-pair bursts are an unusual type of solar low-frequency radio emission, which appear in the dynamic spectra as two parallel drifting bright stripes separated in time. Recent imaging spectroscopy observations allowed for the quantitative characterization of the drifting pairs in terms of source size, position, and evolution. Here, the drift-pair parameters are qualitatively analyzed and compared with the newly-developed Monte Carlo ray-tracing technique simulating radio-wave propagation in the inhomogeneous anisotropic turbulent solar corona. The results suggest that the drift-pair bursts can be formed due to a combination of the refraction and scattering processes, with the trailing component being the result of turbulent reflection (turbulent radio echo). The formation of drift-pair bursts requires an anisotropic scattering with the level of plasma density fluctuations comparable to that in type III bursts, but with a stronger anisotropy at the inner turbulence scale. The anisotropic radio-wave scattering model can quantitatively reproduce the key properties of drift-pair bursts: the apparent source size and its increase with time at a given frequency, the parallel motion of the source centroid positions, and the delay between the burst components. The trailing component is found to be virtually co-spatial and following the main component. The simulations suggest that the drift-pair bursts are likely to be observed closer to the disk center and below 100 MHz due to the effects of free-free absorption and scattering. The exciter of drift-pairs is consistent with propagating packets of whistlers, allowing for a fascinating way to diagnose the plasma turbulence and the radio emission mechanism.
We study the effect of photospheric footpoint motions on magnetic field structures containing magnetic nulls. The footpoint motions are prescribed on the photospheric boundary as a velocity field which entangles the magnetic field. We investigate the propagation of the injected energy, the conversion of energy, emergence of current layers and other consequences of the non-trivial magnetic field topology in this situation. These boundary motions lead initially to an increase in magnetic and kinetic energy. Following this, the energy input from the photosphere is partially dissipated and partially transported out of the domain through the Poynting flux. The presence of separatrix layers and magnetic null-points fundamentally alters the propagation behavior of disturbances from the photosphere into the corona. Depending on the field line topology close to the photosphere, the energy is either trapped or free to propagate into the corona.
Extensive particle-in-cell simulations of fast electron beams injected in a background magnetised plasma with a decreasing density profile were carried out. These simulations were intended to further shed light on a newly proposed mechanism for the generation of electromagnetic waves in type III solar radio bursts [D. Tsiklauri, Phys. Plasmas, 18, 052903 (2011)]. The numerical simulations were carried out using different density profiles and fast electron distribution functions. It is shown that electromagnetic L and R modes are excited by the transverse current, initially imposed on the system. In the course of the simulations no further interaction of the electron beam with the background plasma could be observed.
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