At the start of a cycle, large flares and CMEs on the sun are usually non-existent. Sun’s activity changes with an approximate period of 11 years called as the solar cycle. Over the years, many studies of the FIP effect have shown that the amount of FIP effect, which basically tells by how much the (low FIP) elements in the Corona are more compared to the Photosphere, is different in different structures of the Corona. Thus studying one problem might give insights about the other and vice versa! The theories that are trying to explain the FIP effect predict that the FIP effect and the Coronal Heating Problem are intimately connected with each other. So it was expected that the elemental abundances in the Corona will reflect that of the Photosphere. Flares and CMEs are huge explosions that supply material and energy from the Photosphere to the Corona. So, why can’t it have different abundances of elements than the Photosphere, right? This is because all the material that is present in the Corona comes from the lower atmospheric layers through flares and Coronal mass ejections (CMEs).
![sun corona temp sun corona temp](https://www.qrg.northwestern.edu/projects/vss/docs/media/space-environment/corona.gif)
Now you might ask why is this surprising? After all, Corona is a different layer with a very different temperature. Using spectroscopy, that is, by studying light from the Corona, he discovered that the absolute abundances (i.e., the abundances with respect hydrogen) of certain elements the low FIP elements like magnesium (Mg), silicon (Si) and iron (Fe) were mode abundant (3-4 times higher!) in the Corona than in the Photosphere. Pottasch in 1963 found something very weird concerning the elements present in the Corona. Soon after the launch of the first satellites with instruments to study the Sun in UV and X rays, Stuart R. This effect deals with the abundances of elements in the Corona. But since the ozone layer blocks UV and X rays (thankfully, otherwise we would not exist as we do now!), Coronal studies began only with advent of the space age and satellites in the 1960s.Īnother phenomenon that is related and similarly astonishing to the ‘Coronal Heating Problem’ that occurs in the transition region and the Corona is called as the First Ionization Potential (FIP) Effect. This means that telescopes on the Earth are enough to observe and study the Photosphere. As we have seen, since the Corona is at a million degree kelvin temperature, it emits mainly in the high energy part of the spectrum, that is, in UV and X-rays, while the Photosphere emits mainly in optical wavelengths that we can perceive with our naked eyes. The innermost regions of the fire will be bluish compared to the outer parts, which will be red, orange and yellow. You can see this in daily life in the flames of a stove. This is because the hotter a material is, the light coming from it will be of higher energy. The Corona is mainly visible in UV and X rays. You can read more about this in the article: Using machine learning to infer Solar Coronal heating The reason for this temperature increase in the solar atmosphere is one of the long standing problems of solar physics referred to as the Coronal Heating Problem.
![sun corona temp sun corona temp](https://i.ebayimg.com/images/g/ilQAAOSwyVBgMSU7/s-l500.jpg)
More mystifyingly, things get extremely hot (literally) as we move into the next layer, the transition region, which is just a 100 km in height but the temperature drastically increases from a few thousand kelvin to a few million kelvin! Temperature stays at a million degree kelvin in the Corona (which extends throughout the solar system) as well, which is the uppermost atmospheric layer of the Sun. As the numbers show, in the Chromosphere, as we move upwards it gets hotter and not cooler as common sense suggests! The temperature in the Chromosphere varies between about 4000 K at the bottom and 8000 K at the top. The next layer is the Chromosphere, that is in between a height of about 400 - 2100 km. This is expected because as we move away from a source of energy and heat, the temperature drops. The temperature here decreases from 6000K, at the bottom of the layer, to 4500K at the top of the layer.
![sun corona temp sun corona temp](https://media.nature.com/w700/magazine-assets/d41586-017-04397-y/d41586-017-04397-y_15118458.jpg)
The visible orange-yellow surface of the Sun that we can see even with the naked eye is called the Photosphere. This article explains how these measurements were made. By studying the XSM spectra for the days when the Sun was very quiet, researchers have found that the abundances of all Mg, Al, Si is around two times higher than its abundances in the photosphere! They also calculated the temperature of the quiet Sun corona to be around 2 MK. XSM onboard Chandrayaan-2 observed the Sun during the deepest solar minimum of the past century from in 2019-2020.