Constraining the Dynamics of Active Region Flux Tubes in the Solar Convection Zone

Panda, Suman (2018) Constraining the Dynamics of Active Region Flux Tubes in the Solar Convection Zone. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The Sun is the star closest to us. It is magnetically active and is the source of solar wind, electromagnetic radiation and energetic particles which affect the space weather around the earth. The magnetic field of Sun is responsible for most of the dynamic activity of the Sun. In this study, we have looked at the interrelationship of different active region parameters obtained from vector magnetograms to glean information about flux tube dynamics in the solar convection zone. We have carefully chosen β spots which are least susceptible to kink instability so that we can look at other independent dynamics of the flux tube and different forces acting on it during its buoyant rise through solar convection zone. We verify two well-known phenomena i.e. the Joy’s law and the hemispheric helicity rule. In addition, we look at the tilt twist relationship which supports the predictions of the Σ effect model proposed by Longcope, Fisher & Pevtsov 1998. This model attributes the dispersion in observed value of twist to the turbulent convective buffeting of flux tubes during their buoyant rise. Σ-effects predicts no correlation between tilt and twist, which we establish in our data set. It also predicts that the dispersion in tilt and twist is independent of latitude which we again confirm. This implies that the turbulence in the solar convection zone is independent of latitude and is isotropic. To study the scale of instability governing the rise of flux tubes, we investigate the relationship between separation distance and flux. We recalculate separation distance by subtracting the area contribution from each spot. Their positive correlation indicates that larger active regions are formed through larger instabilities. We suggest that the formation of larger instabilities is possible if convection cells in the solar convective zone are spatially and temporally coherent on the scales which leads to the rise of on AR with large separation distances. But the probability of such an event occurring is less which is apparent from the fact that very few active regions emerge with large separation distance. This sets a constraint on the coherence length of convective turbulence in the solar convection zone. We have also investigated the dependence of tilt angle on separation distance of AR. It is observed that ARs with small separation distance have a large tilt angle with large dispersion. We suggest that this is because smaller ARs which are of the length scale of convective granules are affected more by convective turbulent buffeting. This leads to the larger tilt angle and larger dispersion in tilt as well. In this study, we highlight the importance of studying different parameters and their interrelationship which can give us insights on the subsurface magnetic flux tube dynamics in the Sun.

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