Why is the Sun’s corona (atmosphere layer) so much hotter than the Sun’s surface?

Sun, Solar Flares, Star, Sun Corona

The heating of the Sun’s corona is one issue that has remained baffling for solar physicists for a long time as it throws up several interesting questions. One of the most baffling of this has to do with why the temperature of the solar corona is far greater than the one recorded on the surface. The difference here is in the region of millions of Kelvins. Scientists have attempted to give a lasting and logical answer to the problem and it is in the view of this that several theories have been propounded. However, that does not solve the problem of knowing which of these theories are correct at the end of it all. 

The issue was first seen when scientists Walter Grotrian and Bengt Edlen made some discoveries in the solar spectrum. From this, they were able to make even further discoveries relating to the emission lines that were observed during solar eclipses not emanating from an unknown element referred to as coronium but also observable elements that are at really high levels of ionization. 

Doing the comparison between coronal and also the photospheric atmospheric temperatures of 6,000 kelvins brings up the question regarding the maintenance of the coronal temperature which is 200 times higher in hotness. The main issue here is concerning how energy can be transmitted into the corona before its conversion to heat within a relatively short span. 

These high levels of temperature need energy to be transmitted from the inner core to the coronal layer via non-thermal processes. This is as a result of the fact that the laws of physics governing thermodynamics do not allow the flow of heat straight from the solar photosphere (or surface), measured at around 5,800 K, to the corona layers which are a lot hotter. 

The layer that is between the corona and the photosphere, is a relatively thin space or zone in which there is a gradient in the level of energy flow. Even though this layer is described as thin, it can vary from tens to as much as hundreds of kilometers in thickness. The thing with energy is that there can be no transfer from the cooler photosphere to the corona via the normal transfer of heat as that is going to go against the second law of thermodynamics. An example here is going to be having a light bulb increasing the temperature of the air that surrounds it to something higher than the surface of a glass. Therefore, some other kind of energy transfer has to be involved when it comes to the heating of the layers of the corona. 

The degree of power that is needed for the heating of the solar corona can be deduced as the difference that occurs between heating by thermal condition and coronal radiative losses via the chromosphere via the transition region. This is around one kilowatt for each square meter of the surface area of the chromosphere and that is a minute fraction of the light energy that escaped.

Several coronal theories have been put forward but the thing is that a pair of theories are the most outstanding. The first one is the theory of the nanoflares (or also called magnetic reconnection) or the theory of wave heating. Over the past few decades, however, it has to be stated that not either of these two outstanding theories has been able to fully explain the extreme temperatures of the coronal layers. 

Scientists in 2012 were able to do imaging using the High-Resolution Coronal Imager on a sounding rocket and it showed a set of wound braids right in the corona. The explanation is that the unraveling and reconnection of braids work as the principal sources of heating of the active solar corona allowing the temperatures to reach as much as 4 million Kelvin. 

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