Eruptive phenomena in the solar atmosphere radiation-mhd modeling and code development

Abstract

A bewildering variety of eruptive and ejective phenomena continually take place in the solar atmosphere on a wide range of space and time scales. Particular attention was devoted in the past decades to those associated with the reconnection of magnetic field lines of separate plasma systems that come into contact in the atmosphere, especially when this is the result of the emergence of magnetic flux from the solar interior. Such events can cause a large disruption of the solar atmosphere, lead to the release of magnetic energy which is turned into kinetic and internal energy of the plasma and radiation, launch impulsive mass ejections, and bring about the reconfiguration of the magnetic domain structure in the chromosphere and corona. Even though observationally known for many decades now, among those dynamic phenomena there is one whose understanding has progressed very slowly: the surges.

Surges are cool, dense and non-collimated ejections typically observed in chromospheric lines, like Hα 6563 Å, with velocities of a few to several tens of km s-1 and lengths of 10-50 Mm. They frequently arise in the solar atmosphere related to other transient events like UV bursts or EUV/X-Ray coronal jets.

Through idealized numerical experiments, the surges have been explained as a by-product of magnetic reconnection taking place between emerging and preexisting magnetic systems in the atmosphere that eventually causes chromospheric plasma to be dragged into higher layers. In spite of their interest, those experiments miss a number of fundamental physical mechanisms at work when the surges are ejected, so they constitute only a preliminary step in this field.

This thesis addresses the surge phenomenon under a fourfold perspective: (1) its basic approach is theoretical, and is carried out by modeling the formation and evolution of the surges using a radiation-magnetohydrodynamics (R-MHD) code that includes a realistic treatment of the material plasma properties and radiation transfer; (2) from an observational point of view, we analyze coordinated high-resolution observations of the chromosphere and transition region (TR), exploring the response of TR lines to a surge ejection and the close relationship of surges to other events like UV bursts; (3) from a forward modeling perspective, synthetic observations are created that permit us to understand some of the peculiar features seen in the actual observations and provide a theoretical basis for them; and (4) from the point of view of code development, we have created a Fortran module that improves the computational efficiency of the ambipolar diffusion term and opens up the possibility of including partial ionization effects on the electrodynamics of the system via the generalized Ohm's law.

The first objective is achieved thanks to the possibilities afforded by the Bifrost code. Via 2.5D R-MHD experiments of magnetic flux emergence from the solar layers beneath the surface up to the corona, we have found that a surge is formed in spite of the prior interaction of the the emerging field with the granular cells at and below the surface. The surge detaches from the emerged region as a consequence of strong shocks that develop following the impact of plasmoids ejected along the current sheet in the reconnection site. The surge plasma elements experience accelerations well in excess of the solar gravity value in the onset phase, while it undergoes free-fall in the central and decay phases. Using detailed Lagrange tracing, we have discerned different populations with distinct evolutionary patterns within the surge, some of them directly related to the heating/cooling processes included as part of the Bifrost code. In fact, we have found that a non-small fraction of the surge could not be obtained in previous and more idealized experiments because of the lack of a proper treatment of the thermodynamics and entropy sources.

The second goal of the thesis has been accomplished through the observation of an episode of simultaneous occurrence of an Hα surge and of a UV burst obtained with the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS), respectively. Although surges are traditionally related to chromospheric lines, we have found that they can exhibit enhanced UV emission in TR lines of Si IV with brighter and broader spectral profiles than the average TR. Furthermore, we have provided additional observational evidence to support the common origin and relationship between surges and UV bursts.

The third objective is achieved by means of forward modeling of numerical experiments that include nonequilibrium ionization of key elements in the TR like silicon and oxygen. The results show that the TR enveloping the surge is strongly affected by nonequilibrium ionization effects, noticeably increasing the number of emitters of the main lines of Si IV and O IV. The departure from statistical equilibrium is due to the short characteristic times of the optically-thin losses and heat conduction during the surge evolution. In addition, we have concluded that line-of-sight effects are important to understand prominent and spatially intermittent Si IV and O IV brightenings within the surges.

Finally, the last goal of this thesis has been accomplished through the creation of a new module in the Bifrost code that implements the Super Time-Stepping (STS) method to speed up the calculation of the ambipolar diffusion term. After carrying out an in-depth analysis of the method, we have found the optimum combination of parameters that ensures stability and efficiency. As a first instance of work to be completed in the postdoctoral phase, in the final chapter we have started to explore the effects of ambipolar diffusion on the magnetic flux emergence process leading to the surge phenomenon and in the surge itself.