Modernization of the spectroheliograph of Coimbra

Ricardo Gafeira

Hosted by University of Coimbra, Portugal on October 22, 2020


The details study of the solar activity and variability on long term data series is an important element on the understanding of the solar dynamics and evolution. In addition, their activity influences several aspects of our lives, such as climate, communications, energy, aviation, and many other fields, sustaining and threatening, simultaneously, our entire technologically-based way of life. Hence, it is paramount to secure the continuity of unbroken and self-consistent data series of solar observations and its study. The associated physical processes and structures on the Sun span over a wide range of values regarding their lifetimes, intensities, and spatial scales. Ideally, to study all these different structures in detail, we need facilities that allow us to observe the full solar disk and spectrum with very high spectral, spatial, and temporal resolutions. Unfortunately, due to technical limitations that cannot be done and there are clear trade-offs to deal with. Even today, elements like spatial resolution versus field of view (FoV), spectral coverage versus temporal resolution, observation of spectral lines in local or non-local thermodynamic equilibrium (depending on the science driver) are among the obstacles that scientists need to keep in mind as limitations for their work. State-of-the-art solar telescopes like the US American 4-meter telescope DKIST, the German GREGOR telescope, or the Solar Orbiter, plus the next generation facilities like the balloon borne SUNRISE III mission, or the future European Solar Telescope will observe the Sun with unprecedented spatial and spectral resolutions. Even though they will use the most advanced technology they do not cover all possible modes of observation. Some key aspects like small FoVs, in some cases limited number of spectral lines or the short lifetime of instruments are among those that some other types of instruments can cover. The spectroheliograph of OGAUC is one of the most durable solar instruments still operating. Having been upgraded only twice, one for new optics and another for digital image recording, it keeps daily observations since 1927. Until now this instrument has been used to study structures visible in the solar atmosphere from their intensity images at specific wavelengths, ignoring most of the visible spectral range. One of the main reasons for that is the lack of tools to extract more information from that type of observations that need to be analysed in Non-Local Thermodynamic Equilibrium (NLTE) regime. However, with the new generation of spectropolarimetric inversion codes, we are now able to invert NLTE spectral lines to extract information about the temperature, the velocity and the magnetic field vector. Such analysis is already possible with several of the aforementioned telescopes, but they do not cover all the possible observing modes. In this project, we propose to take advantage of the operating infrastructure of the OGAUC spectroheliograph and to upgrade it, improving its spectral resolution, adding other spectral regions of interest and increasing the spatial sampling and add polarimetric sensitivity. Due to its flexibility, long term run and set of observed spectral lines and polarimetric sensitivity it will be a competitive state of the art instrument competing with the other solar synoptic (full disk) ground-based spectroheliograph of its category in the. This upgrade, in combination with the new NLTE inversion codes and neural network techniques will allow us to probe at chromospheric and photospheric heights the solar temperature, velocity and magnetic field.

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