Magnetoconvective flows and waves in the lower solar atmosphere

Abstract

Sun has complex magnetic features like sunspots, magnetic bright points, plages, network fields, filaments, etc. These complex magnetic features are found to be associated with various observed dynamical phenomenons like jets, flares, coronal mass ejections (CMEs), and other eruptive events. Magnetic fields hence play an important role in the dynamics of solar atmosphere. In this thesis, the interaction of fluid flows and waves with the magnetic field in the quiet-Sun is studied. For the case of flows, the horizontal fluid motions on the solar surface are tracked over large areas covering the quiet-Sun magnetic network from local correlation tracking of convective granules. To derive these motions continuum intensity and Doppler velocity are taken from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). From these the horizontal divergence, vertical component of vorticity, and kinetic helicity of fluid motions are calculated. With these derived physical parameters, the interaction of magnetic field with fluid motions have been addressed. In our analysis, it has been found that the vorticity (kinetic helicity) around small-scale fields exhibits a hemispherical pattern (in sign) similar to that followed by the magnetic helicity of large-scale active regions (containing sunspots). It has been identified that this observed pattern is a result of the Coriolis force acting on supergranular-scale flows (both the outflows and downflows). Further, it has been observed that the magnetic fields cause transfer of vorticity from supergranular downflow regions to outflow regions, and that they tend to suppress the vortical motions around them when magnetic flux densities exceed about 300 G (HMI). These results are speculated to be of importance to local dynamo action if present, and to the dynamical evolution of magnetic helicity at the small-scale. Also when these analyses are carried out for emerging flux regions, it is found that the foot points of emerging loops affect the fluid motions around them in a characteristic way implying possible unwinding of built-in twists (helicity) of emerging flux tubes. For the case of waves, it is found that the p mode power is suppressed in the magnetised photosphere of the Sun and this suppression is calculated in terms of quiet-Sun values (i.e. normalised with respect to the quiet-Sun). These power suppresions are then analysed in relation to the chromospheric emissions in the UV wavelength bands. The phase-shifts of the waves propagating from photosphere (the continuum intensities for photospheric heights are used) to the chromospheric heights (intensities in UV channels of 1600°A and 1700°A) are calculated. Dependences on line-of-sight, total magnetic field strength, and the inclination of magnetic field of p mode power supression, chromospheric emissions in the UV band 1600°A and 1700°A, and of the phase-shifts of the waves propagating from photosphere to lower chromosphere are studied. The energy fluxes are derived from phase-shifts and from cut-off frequencies. These fluxes are compared with one another for consistancy and also are compared with earlier work. At average formation heights 1700°A and 1600°A, the energy fluxes derived from both the methods are found to be ∼ 5 × 106ergscm−2s−1 and ∼ 2 × 106ergscm−2s−1, respectively for 2-5mHz range. The average chromospheric radiative losses are found to be ∼ 2 × 106ergscm−2s−1. Our results hence show that these waves carry sufficient energy to heat the lower solar atmosphere. The following is the arrangement of the Thesis. Chapter 1 gives an overview of the Sun with focus on the areas mainly related to this thesis. The solar interior and atmospheric structure, different magnetic elements seen on the solar surface, convective motions on the Sun, observed patterns of convection on the solar surface, helicity, oscillations observed in the Sun, heating of solar atmosphere, and lastly observatories used for observation for the completion of the thesis work are discussed in this chapter. The thesis is then divided into two parts: interactions between vortex flows and small-scale magnetic field in the solar photosphere (Chapters 2 and 3), and the effects of waves in chromospheric heating (Chapter 4). In Chpater 2, interactions of fluid motion with magnetic fields are discussed. Chapter 3 focuses on how the fluid motions are affected during flux emergence. Chapter 4 deals with interactions of waves with magnetic fields and effects of these waves on chromospheric heating. Chapter 5 then presents a summary of the results from all chapters, highlight the novel aspects of this Thesis with its impact and then future work on the basis of the current status of results.