Spectroscopic and statistical investigations on MBPs

The dynamics of the solar atmosphere are governed and influenced by magnetic fields. These dynamical processes are the source for the space weather which is getting more and more important for humankind (telecommunications, satellite navigation, manned space missions). In this - for a Schrödinger Fellowship - proposed FWF project - spectroscopic and statistical investigations on MBPs - small scale solar magnetic fields should be described and their temporal evolution will be measured. The magnetic field structures to be investigated have come to scientific interest in the recent years when the possibilities and resolution limits were achieved. A better understanding of small scale magnetic fields will help us to understand and solve the following problems in the future: a) the heating of the upper solar atmospheric layers, b) dynamo theories (local versus global), c) long time variability of the solar irradiance (important for climate research). The project includes in detail the following i) analysis of the best up to date available solar data sets and ii) new methods which should be applied on these data sets. The methods have up to now not been applied and/or used in Austria. The used data sets stem from the recent successful Sunrise mission. This mission consisted of a 1 m aperture telescope flown on a balloon gondola in Earth`s stratosphere. Through flying the telescope that high up in the atmosphere the seeing (which reduces and limits the true resolution limits) is minimized and practically vanishes. The applied methods consist of spectropolarimetric inversion techniques. Such methods enable the reproduction of a model solar atmosphere through the measured polarized solar spectrum. The model atmosphere consists of physical parameters like temperature, gas pressure, magnetic field or line of sight velocity component. Up to now the Austrian solar physics community had neither access to these high resolution solar telescope data nor to the spectropolarimetric inversion techniques or experience in handling and applying of such methods. The host institute and Dr. Jose Carlos del Toro Iniesta have been participating in both, the development of the instrument as well as the inversion methods, and are internationally highly recognized in the field of solar physics. By approving this proposal the applicant and also the Austrian science community would gain: Access to one of the leading solar research institutes and an outstanding mission. Work with highly sophisticated data analysis and inversion methods. The gained knowledge and methods would be transferred to Austria. A deeper insight into the physical processes regarding solar magnetic fields and participate in finding a solution to the before mentioned problems.

In the funded Austrian Science Funds (FWF) project spectroscopic and statistical investigations on MBPs the characterization, interpretation, and understanding of small-scale solar magnetic fields was sought to be enhanced. Magnetic bright points (MBPs) belong to the class of small-scale solar magnetic fields (sizes in the range of a few hundreds of km diameter) showing strong magnetic field strengths up to 2 kG (about 1000 times as strong as the magnetic field of the Earth). They play an important role for the understanding of the heating of the upper atmospheric layers of the Sun (chromosphere and corona). These layers are clearly hotter (up to millions of degree Kelvin) than the normally visible photosphere (~5700 K). Moreover, they are of fundamental interest for the balance of the solar magnetic fields, i.e. due to the larger number of small-scale magnetic field concentrations compared to large and extended magnetic fields like sunspots and active regions, the small-scales contain in total even more magnetic flux than the large ones do.Within this project the magnetic field strength distribution of the magnetic bright points got measured for the first time ever for the quiet Sun (which is important for the flux balance as discussed before) as well as their size and brightness in dependence on their solar longitude and latitude. These are two crucial parameters for the estimation of the total solar irradiance variation (TSI variation) variation of the solar spectral composition as well as the total solar irradiance. The TSI variation itself is of vital importance for climatic models of the Earth, as the total solar irradiance and its composition is the primary input parameter in such models. Hence it is of great interest to know how this parameter varies over time and what causes such variations. After such characterizing statistical studies we dedicated our time on the investigation of the detailed temporal evolution of MBPs. For this part of the study we used data from the mostly german-spanish balloon borne mission Sunrise. An automated image segmentation, feature identification, and tracking algorithm was applied to the data coming from the IMaX (Imaging Magnetograph eXperiment) instrument. This was the first time that the evolution of MBPs was followed in highly temporally as well as spatially resolved spectropolarimetric data (comprising spectral as well as polarimetric information) giving new insights into the evolution of MBPs. The generally accepted standard theory for the creation of MBPs, developed in the 70s of the last century, states that a strong magnetic field prohibits the energy transport to the solar surface via convection. Therefore the plasma within the magnetic field concentration starts to cool down and becomes denser. Due to this higher local density the plasma starts to sink down and evacuates by this process the magnetic field concentration. The higher surrounding gas plasma leads now to a collapse of the magnetic field causing a huge increase in the magnetic field strength. This theory is known as convective collapse.In our investigation we were able to identify this mechanism for a few cases in practice. Interestingly we were also able to observe mechanisms and processes different to this classical one. Hence it would be highly interesting to investigate such processes in more detail in the future. E.g. in about 10% of the cases we witnessed quite strong upflows close to the center of the MBP forming downflows. Such spatially and temporally co-located upflows have not been reported before, neither is there a theoretical description for this phenomenon. To shed more light on this, it is necessary to revisit this phenomenon and repeat an in depth analysis.