Remote sensing of weak, small-scale magnetic fields is paramount in solar physics. However, the seeing complicates the required polarisation measurements. One of the methods to overcome this problem is modulating the light much faster than the seeing, so scientists can practically “freeze” the atmosphere during the measurements.
FSP modulator and calibration unit at the German Vacuum Tower Telescope in Teide Observatory (Tenerife, Spain) / Source: F. Iglesias 2016 (PhD thesis)
Remote sensing of weak small-scale solar magnetic fields is of utmost relevance when attempting to respond to a number of important open questions in solar physics, such as the total amount of magnetic flux on the Sun, the structuring and dynamics of the chromospheric magnetic field, or the magnetic processes related to energy transfer through the solar atmosphere, among others.
The most common way to characterise solar magnetic fields is by studying the polarisation of sunlight (polarimetry). This is one of the most delicate measurements in astronomy, needing very high spatial, spectral, and temporal resolution. Unfortunately, since such measurements are photon starved, intrinsic trade-offs between these requirements are inevitable.
Even more, with polarisation being invisible to both the human eye and detectors, the measurement process is tricky. Solar polarimeters resort to polarisation modulators, which encode the polarisation information into intensity variations. Usually, four intensity measurements are taken in rapid succession and then combined to deduce the polarisation of the incoming light. This is called a modulation cycle.
Atmospheric seeing and polarisation measurements
Modulation allows scientists to calculate the original polarisation of the light, but how accurately? As already stated, very high spatial, spectral, and temporal resolutions are needed in order to study small-scale magnetic processes on the Sun.
On top of that, there are two added complications: the changes that the telescope itself can introduce in the solar polarisation (known as instrumental polarisation) and the artificial signals generated by turbulence in the Earth’s atmosphere (atmospheric seeing).
The European Solar Telescope is designed in such a way that instrumental polarisation will cancel out, making it the first polarisation-free telescope by design (still, safeguards are being added along the way).
Seeing-induced polarisation is a different story. Since four sequential intensity measurements need to be combined to obtain the original polarisation, it is important that the solar scene does not vary during the time those measurements are taken. But the seeing distorts the images at very high speed, leading to artificial polarisation signals. Fortunately, adaptive optics systems and image reconstruction techniques significantly decrease the image distortions caused by seeing. They have vastly improved the polarisation measurements made from ground-based telescopes, although still not in a way comparable to those from space telescopes.
Overcoming seeing with a Fast Solar Polarimeter
One of the methods to overcome the effect of atmospheric seeing is modulating much faster than the seeing, so scientists can practically “freeze” the images within a modulation cycle period.
This is one of the approaches that could be used by the European Solar Telescope. Indeed, scientists and engineers at the Max Planck Institute for Solar System Research (MPS, Germany), in close collaboration with the semiconductor laboratory of the Max Planck Society, are developing a Fast Solar Polarimeter prototype to operate in the visible part of the spectrum.
This prototype draws from a seemingly simple idea: with most of the seeing power contained in the 1–100 Hz frequency range, a modulation frequency of the order of 100 Hz would be required to modulate faster than the typical time scales of the seeing, drastically suppressing seeing-induced polarisation.
“This requires the detectors of the Fast Solar Polarimeter to be run at up to 400 frames per second. The high frame rate has another key advantage: it allows for an efficient image reconstruction with minimal impact on the noise level of the data”, explains Alex Feller (MPS). In this way both the image quality and the polarimetric sensitivity are improved.
However, to achieve this high frame rate with low noise in the camera is proving to be challenging: “The higher the frame rate, the less sunlight we can collect in individual frames, and the higher the contribution of camera noise to the total noise level in the polarisation measurement", elucidates Feller. “Still, in the last few years, the speed, size and noise of the sensors have drastically improved, a very fortunate development which our field can benefit a lot from”, he concludes on a hopeful note.
- F. A. Iglesias, A. Feller, K. Nagaraju, S. K. Solanki, 2016, “High-resolution, high-sensitivity, ground-based solar spectropolarimetry with a new fast imaging polarimeter - I. Prototype characterization”, A&A, 590, A89.
- F. A. Iglesias and A. Feller, "Instrumentation for solar spectropolarimetry: state of the art and prospects", 2019, Opt. Eng. 58(8) 082417.
- F. Zeuner, R. Manso Sainz., A. Feller, M. van Noort, S.K. Solanki, F. Iglesias, K. Reardon, and V. Martínez Pillet, 2020, "Solar Disk Center Shows Scattering Polarization in the Sr I 4607 Å line", The Astrophysical Journal, 893, L44.