Spectral diagnosis of solar prominences has been performed almost routinely for decades. Thanks to constantly improved high-resolution observations associated with more realistic digital models, these objects are always better described. However, because these analyzes are essentially based on the shapes and intensities of the spectral lines, it is very difficult to distinguish the importance of the physical processes which sign on the shape of these lines. For example, the intensity of the spectral line depends on the kinetic temperature of the plasma particles, but also on the density, the intensity of illumination of the prominence of the photosphere, and other factors. The thermal expansion of the line is also difficult to distinguish from the expansion of said microturbulences. Thus, in conventional models, the plasma kinetic temperature is usually determined indirectly, often based on analogies or physically justified assumptions.
Petr Heinzel from ASU has already discussed the issue of direct temperature measurement using radio radiation with his colleagues. Indeed, in the protrusions, the radio radiation is predominantly optically thick and the determined luminance temperature therefore corresponds directly to the kinetic temperature of the plasma. The optical thickness is a factor that can be determined by observing the same prominence at the same time in the hydrogen spectral line Hα. The ASU staff therefore designed an observation campaign with simultaneous observation of the Sun in the Hα line and of the ALMA interferometer, but without success. It’s so interesting that another team has had similar success, which managed to observe a clearly visible prominence on April 19, 2018, both with the help of the ALMA millimeter-wave radio interferometer, and in same time with the MSDP Imaging Spectrograph in Bialkov, Poland. ). After a period of exclusivity for the authors of the observation proposal, the ALMA data was published and our team was able to get to work.
The radio data was very carefully reduced using the corresponding tasks of the CASA reduction package, to which the Czech authors under the guidance of M. Bárta also contribute very intensively. In the same way, the data cubes acquired by Bialkov’s large coronagraph were processed with appropriate tools, and the two types of observations were translated on top of each other so that they corresponded spatially. It should be noted here that the spatial resolution in the optical region of the Polish observatory coincidentally matches very well the spatial resolution of the ALMA interferometer in radio waves, although in the future ALMA should provide much higher resolution. raised on the Sun.
Further data analysis went hand in hand with numerical modelling. From the images of the Hα line, it is not possible to directly determine the required optical thickness at 3 mm, but only the so-called emission rate. However, the calibration relationship between these two quantities had to be found using numerical modeling of the radiation in the protrusions. The authors set up a network of more than 100,000 models on which they found the calibration relationship essential.
They then used it to convert the brightness temperature, which is proportional to the intensity of radio radiation measured by the ALMA interferometer, into kinetic temperature. Unfortunately, the experience of the numerical model has shown that a clear recalculation of these two quantities is only possible for clear parts of the protrusions. The less clear areas then did not allow a clear solution and were not taken into account in the next step.
The author team mentions another unknown in the book, and that is the fill factor. That is, the percentage of the elementary image box (pixel) that is actually occupied by the prominence structures. This fill factor is unknown and could only be determined if observations at extreme spatial resolution were available, which is currently not possible. Thus, the authors presented a solution to their problem for several realistic values of the fill factor and showed that the kinetic temperatures in this protuberance typically oscillate between 6,000 and 12,000 degrees. The consequence of the unknown fill factor is, among other things, that the temperatures determined in the protrusion decrease towards its edge. However, this can only be the apparent effect of “blurring” the edge of the protrusion, as the fill factor decreases to the edge. In reality, there should be warmer but finer structures.
The work presented is truly avant-garde. It shows that an appropriate combination of observations can be used to determine in principle the physical quantities which it has hitherto been more or less necessary to estimate. At the same time, the authors point out that if they had current observations from multiple ALMA channels, the analysis would be even easier and would eliminate some of the issues mentioned here. Observations with higher spatial resolution, which should be possible with the ALMA interferometer, would also help to obtain unambiguous results.
Note: ALMA (Atacama Large Millimeter Array) is a large radio interferometer in the Atacama Desert in Chile, an international project of ESO, USA and Japan.
P. Heinzel et al., ALMA as a thermometer of prominence: first observationsAstrophysical Journal Letters 927 (2022) id.L29, arXiv preprint:2202.12761
teacher. RNDr. Petr Heinzel, DrSc.
ASCR Astronomical Institute Solar Department
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