Deciphering the solar small-scale magnetic field

• Applicant: Dr. Catherine Fischer
• Subject Area: Astrophysics and Astronomy
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 273324322

• Final Report Year: 2019

Final Report Abstract

The small-scale magnetic field on the Sun plays a major role in the structure of the solar atmosphere. It is thought to be the cause of the temperature rise in the outer solar atmosphere as well as being the one maintaining the hot solar corona. However, we know little about its dynamic evolution and its methods of coupling the solar atmosphere. Observations connecting magnetic field and velocity dynamics in the photosphere with the energetic response in the chromosphere in the quiet solar atmosphere are actually rare. The difficulty in such observations is in obtaining and aligning multi-wavelength (and therefore height-dependent) data and unambiguously assigning a relation between the rapidly evolving layers. Yet, these observations are crucial for our understanding of how the energy made available by the interaction of the omnipresent granular motions with the embedded magnetic field is transported to the higher atmospheric layers. Our aim was to study the enigmatic small-scale magnetic field and investigate the coupling between the photosphere and chromosphere. To this end, we studied events such as, for example, convective collapse and flux cancellation – the intensification and removal of magnetic field at the solar surface, respectively. By combining spectropolarimetric, spectroscopic, and imaging data covering various wavelength ranges we were able to further strengthen our understanding of the intricate and very dynamic behaviour of smallscale magnetic field. The major findings were: (1) We analysed flux cancellation events and found evidence of magnetic loop reconnection in the form of substantial heating of the lower atmosphere. In one event, using spectropolarimetric data from two different formation heights in the solar atmosphere, we were able to trace an emerging magnetic field colliding with pre-existing magnetic field. In the aftermath, we detected a bright H-alpha loop which we interpret as a post-reconnection loop. This is one of the few flux cancellation events reported in which we have evidence of the trajectory of the involved magnetic loops. (2) During the rapid expansion of an exploding granule, we witnessed a magnetic flux tube being squeezed by the opposing horizontal fields of the granules resulting in an elongation of the magnetic element. This triggered an oscillatory impulse within the magnetic element detected in the chromospheric diagnostics. Utilizing a wavelet analysis, we identify an upward propagating shock front traveling through the solar atmosphere. The energy is deposited in the mid-chromosphere. We might have observed the signature of direct energy transfer from granulation flows to chromospheric heating. (3) We studied magnetic elements undergoing an intensification of their magnetic field. Observations had been confined so far to the photosphere/low chromosphere. We found that the magnetic elements undergoing convective collapse experience an excess down flow even up to the mid chromospheric layers. This indicates that there is a chromospheric response to the convective collapse events which needs to be incorporated into the models of these events. (4) We reported the first clear observational evidence of a magnetic flux-sheet emergence in the quietsun and characterised its development. This type of emergence had been detected in simulations and is different to the more widely known magnetic loop emergence in that the emerging magnetic field is formed by the growing granule into a flux sheet covering the entire granule. The magnetic flux tube embedded in the convection zone that was dragged to the surface delivered magnetic flux with one to two orders of magnitude larger to the solar atmosphere than what was reported for magnetic loop emergence. This event is, therefore, capable of carrying a large amount of magnetic flux within several minutes to the solar surface and could play an important role in the determination of the magnetic flux budget. With these findings we have contributed in deciphering the quiet-sun small-scale magnetism by studying its appearance and by revealing connections between the events taking place in the photosphere and their chromospheric response. In 2020, the new 4-m solar telescope DKIST will open its dome and will obtain first light. The small-scale magnetic field will be observed with the highest resolution ever, expanding our knowledge of this essential ingredient in the solar make-up and evolution.

Publications

• Quiet Sun Magnetic Field Evolution Observed with Hinode SOT and IRIS (2016), ASPCS, Vol. 504, 19
  C.E. Fischer, N. Bello González, and R. Rezaei
  
• Quiet sun magnetic fields (2016), NSPM23
  C.E. Fischer
  
• Chromospheric impact of an exploding solar granule (2017), A&A, 602, L12
  C.E. Fischer, N. Bello González, and R. Rezaei
  
• Magnetic flux emergence followed by magnetic flux cancellation in the quiet sun observed with the Interferometric BIdimensional Spectrometer (IBIS) (2018), NSPM24
  C.E. Fischer and R. Rezaei
  
• Observations of solar small-scale magnetic flux-sheet emergence. (2019), A&A Letters
  C.E. Fischer, J.M. Borrero, N. Bello Gonzalez, and A.J. Kaithakkal