Deriving plasma parameters of large coronal magnetic loops using J-burst observation from LOFAR

<p>Solar type J radio bursts are the signatures of electron beams travelling along closed magnetic loops in the solar corona. Type J bursts provide diagnostics for observing and understanding coronal loops geometry and electron beams dynamics. Due to the observational limitations, large loops around 1 solar radius in height are ill-defined. Whilst J-bursts at meter-wavelengths are well suited for the analysis of coronal loops at these solar altitudes, applying standard empirical solar plasma density distributions have limitations as they are designed for flux tubes extending into the solar wind and do not capture the curvature of such coronal loops.</p><p>We analysed over 20 type J bursts observed by the LOw-Frequency ARray (LOFAR) on the 10th of April 2019. Using a reference height, we derived the ambient plasma density models that varied along the ascending leg of coronal loops, and also with solar altitude. By estimating the density scale height, we inferred physical parameters of large coronal magnetic loops, roughly 0.7 to 1.5 solar radii above the photosphere. These coronal loops had temperatures around 2 MK and pressures around  5 dyn cm<sup>-2</sup> . We then inferred the minimum magnetic field strength of these closed loops to be around 0.3 G. These large coronal loops' plasma conditions are significantly different to smaller coronal loops and loops that extend out into the solar wind.</p>

Excitation of decayless kink oscillations by random motion

We study kink oscillations of a straight magnetic tube with a transitional region at its boundary. The tube is homogeneous in the axial direction. The plasma density monotonically decreases in the transitional region from its value inside the tube to that in the surrounding plasma. The plasma motion is described by the linear magnetohydrodynamic equations in the cold plasma approximation. We use the ideal equations inside the tube and in the surrounding plasma, but take viscosity into account in the transitional region. We also use the thin tube and thin transitional or boundary layer (TTTB) approximation. Kink oscillations are assumed to be driven by a driver at the tube footpoint. We derive the equation describing the displacement in the fundamental mode and overtones. We use this equation to study kink oscillations both in the case of harmonic as well as random driving. In the case of random driving we assume that the driver is described by a stationary random function. The displacements in the fundamental mode and overtones are also described by stationary random functions. We derive the relation between the power spectra of the fundamental mode and all overtones and the power spectrum of the driver. We suggest a new method of obtaining information on the internal structure of coronal magnetic loops based on the shape of graphs of the power spectrum of the fundamental mode.

Resonant damping of kink oscillations of thin cooling and expanding coronal magnetic loops

We have considered resonant damping of kink oscillations of cooling and expanding coronal magnetic loops. We derived an evolutionary equation describing the dependence of the oscillation amplitude on time. When there is no resonant damping, this equation reduces to the condition of conservation of a previously derived adiabatic invariant. We used the evolutionary equation describing the amplitude to study the competition between damping due to resonant absorption and amplification due to cooling. Our main aim is to investigate the effect of loop expansion on this process. We show that the loop expansion acts in favour of amplification. We found that, when there is no resonant damping, the larger the loop expansion the faster the amplitude growths. When the oscillation amplitude decays due to resonant damping, the loop expansion reduces the damping rate. For some values of parameters the loop expansion can fully counterbalance the amplitude decay and turn the amplitude evolution into amplification.