Vishal Upendran
Vishal Upendran Vishal is a 4th year research scholar at the Inter University Centre for Astronomy and Astrophysics, Pune. He studies various aspects of the solar atmosphere and near-Earth spaceweather. He does so through space and ground based astronomy, theoretical simulations and through machine learning.

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Anyway the plasma moves - it does really matter!

Light when seen correctly provides rich information on what happens in the cosmos. In this article, we showcase our work on identifying signatures of the solar wind in the solar chromosphere – far lower in the solar atmosphere than what has been expected. We take the reader through our analysis, interpretations and speculations on our observations, and present a unified model of solar wind emergence and the heating of quiet corona.

NOTE: This article is a follow-up to Understanding the solar chromosphere through spectral lines (which I shall henceforth call A1) written by yours truly. While I have presented a summary of the previous article here too, possibly you would enjoy reading them together better!

Light, as we have seen in #A1, encodes in it information regarding the world. It is especially of great help to astronomers in figuring out what-where-why something happens. For those of us folks who study the Sun, we are bathed – or rather, burnt crisp with light. And so, light or photons carry with them information regarding various aspects of different regions on the Sun. Now, we have seen in #A1 that each element or ion has a characteristic fingerprint in the form of “spectral lines”. The properties of these spectral lines depend on the plasma properties where the particular ion is present. Critically, it depends on the local temperature, density, velocity, and any other turbulent flows present in the plasma. These show up as effects on the line intensity, the line “shift” and the line width. And so by “reading out” these spectral lines, we can get an estimate of how much plasma is present, how fast it is moving, and in what direction it is moving – at the very least!. The solar atmosphere is composed of the photosphere (at ~5500 K), the chromosphere (at ~10,000 to 80,000 K), Transition region (TR; ~1,00,000 K) and the corona (>10,00,000 K). The rise in temperature with the height of the atmosphere is the rather much-debated “Coronal heating problem”, which presents more questions than answers1. The corona then extends outwards as the streaming solar wind, with plasma moving at great speeds outwards from the Sun2. The exact reason(s) behind the large temperature of the corona, and the height (or the layer of the atmosphere) from where the solar wind starts are not known well, and are an unsolved problem in solar physics. In #A1, we attempted to understand the underlying cause of these two problems in tandem by studying two regions called the Coronal Holes (CHs) and Quiet Sun (QS). The CHs are regions seen as dark in the corona against the background QS - see Fig. 1, first panel.

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