Evolutionary tracks of quiet-Sun magnetic features

Abstract

This thesis presents a study of quiet-Sun magnetic features in the solar photosphere. Magnetic fields in the quiet Sun organize on small spatial scales, evolve very rapidly, and produce weak polarization signals. With these properties their observation requires high spatial and temporal resolution together with sensitive and accurate polarimetric measurements. It is for these instrumental limitations that the origin and evolution of these features remains elusive. The Imaging Magnetograph eXperiment (IMaX) is an imaging spectropolarimeter that flew over the Artic Circle aboard the SUNRISE balloon-borne stratospheric mission. IMaX was designed to mitigate the above mentioned issues and has provided polarimetric observations with unprecedentedly high spatial resolution of a hundred kilometers. Flying in the stratosphere, it obtained stable, nearly seeing-free time series, and its imaging capabilities allowed to cover large areas of the Sun simultaneously. All these features are indeed crucial when studying the highly dynamic nature of the quiet-Sun magnetism. The thesis gathers empirical evidence of magnetoconvection at the smallest scales ever observed. The evolutionary tracks of several different quiet-Sun magnetic structures in a continuous interaction with photospheric convection are presented. Specifically, we study 1) the formation and evolution of an isolated magnetic element; 2) the dynamics of multicore magnetic structures; and 3) the relation between magnetic features and convectively driven, long living sinks at the junctions of several mesogranular cells. Seen at a scale of one hundred kilometers, we find that the evolution of an isolated quiet- Sun magnetic element is a complex process where many phenomena are involved. The formation starts when a small-scale magnetic loop emerges through the solar surface in a granular upflow. Its footpoints are soon swept to nearby intergranular lanes where some, weak positive polarity patches are already present.

The negative polarity footpoint cancels out with an opposite polarity feature while the positive one and other remaining patches are advected by converging granular flows toward a long-living sink. The magnetic fields agglomerate in the sinkhole and a new element with a magnetic field strength in equipartition with the kinetic energy density of convective motions is formed. The intergranular downflow then begins to increase within the magnetic feature while the surrounding granules compress it until kiloGauss field strengths are reached. During this process, a bright point appears at the edge of the flux concentration almost co-spatial with an upflow plume. The development of the magnetic element does not stop here since we discover that is indeed unstable. The magnetic element displays an oscillatory behavior as the field strength weakens and rises again with time. Focusing on extended magnetic structures that harbor multiple bright points in their interiors, we find that they are resolvable into a series of more elemental inner magnetic cores, each of which appears related with a single bright point. The inner cores are strong and vertical. They all are surrounded by common, weaker, and more inclined fields. We interpret these structures as bundles of flux concentrations in the lower photosphere that expand with height to merge into a common canopy in the upper photospheric layers. The evolution of the individual magnetic cores is completely governed by the local granular convection flows. Through this interaction, they continuously intensify, fragment, and merge in the same way that chains of bright points in photometric observations have been reported to do. This evolutionary behavior results in magnetic field oscillations of the global entity. We conclude that the magnetic field oscillations previously discovered in small quiet-Sun magnetic elements correspond to the forcing by granular motions and not to characteristic oscillatory modes of thin flux tubes. Finally, we analyze the relation between mesogranular flows, localized downdrafts, and quiet-Sun magnetic fields. We study first the statistical properties of the sinks.

Some of them manifest as whirlpools while the others display radially symmetrical converging flows. Their spatial distribution reveals that they are located at the vertices between neighboring mesogranules. We proof quantitatively that the strongest fields tend to concentrate at sinkholes. Meanwhile, the small-scale magnetic loops do not show any preferential distribution at mesogranular scales. We also analyze one of the mesogranules in more detail and observe that magnetic loops appearing inside the mesogranular cell can be advected by horizontal flows toward its vertex. If confirmed by new observations, these results can imply that the formation of magnetic elements through the concentration of loop footpoints in mesogranular vertices is ubiquitous over the solar surface.