Are There Any Structural Changes In Deeper Layers Of The Sun? (Planetary Science)

Sarbani Das in her recent paper examined whether or not there are any changes in the deeper layers of the sun by using helioseismic data for two solar cycles: solar cycle 23 and 24. She found that there are significant changes in the solar convection zone and that the sound speed in the solar convection zone decreases compared to sound speed below it as the sun becomes more active. Findings of her study recently appeared in Arxiv.

The solar interior is divided into four regions by the different processes that occur there. The innermost region is the “core”, where energy is generated. This energy diffuses outward by radiation through the radiative zone and by convective fluid which flows through the convection zone. The thin interface layer called the “tachocline”, present between the radiative zone and the convection zone is where the Sun’s magnetic field is thought to be generated. Several studies demonstrated that, there are changes in solar dynamics and it is quite easy to detect. However, no studies demonstrated changes in solar structure, as it is very hard to detect. Thus, this inspired Sarbani Das to examined whether there are any changes in the deeper layers of the sun or not.

By using helioseismic data obtained over two solar cycles, and sacrificing resolution in favour of lower uncertainties, she showed that there are significant changes in the solar convection zone.

“Using MDI data, we find a relative squared sound-speed difference of 2.56×10¯5 at the convection-zone base between the maximum of solar Cycle~23 and the minimum between Cycles~23 and 24. The squared sound-speed difference for the maximum of Cycle~24 obtained with HMI data is 1.95×10¯5. GONG data support these results.”

— Sarbani Das, Astrophysicist and Professor in the Department of Astronomy, Yale University

She also found that, as solar activity increases, the sound speed in the region above the base of the convection zone, i.e., the tachocline region, decreases compared with that below the convection-zone base. This result is consistent with the assumption that the sound-speed changes are a result of magnetic fields.

Inversion results for the difference in sound speed below and above the convection-zone base. © Sarbani Das

Moreover, she showed that there is a change in the gradient with the solar cycle. This change is most significant at base of the convection zone. This implies a change in the position of the convection zone base. However, she couldn’t able to confirm this change with current results or available frequencies.

“The results are intriguing enough to examine the results further. Given that the current data sets are not ideal, the analyses need to be repeated with frequencies obtained from time series that are longer; while longer time series will smooth out some of the solar-cycle related changes, overlapping sets will still reveal the changes. Two-year data sets at the solar maxima and two years at the minima and a few two-year sets in between will help.”

— she concluded

Reference: Sarbani Das, “Evidence of solar-cycle related structural changes in the solar convection zone”, Arxiv, pp. 1-11, 2021. arXiv:2106.08383

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