How hot is our Star

Let’s assume for a moment that we take a trip to the centre of our star, the Sun, and then move on to the outer regions, i.e. the surface and the atmosphere. As we pass through this giant ball of gas, moving along its radius, we would see gigantic temperature differences. In fact, it is well known that the core of our star is so hot that it reaches about 15 million degrees Celsius. Moving away from the core in our imaginary spacecraft, we find a drastically lower temperature on the surface of the Sun (specifically on the photosphere), about 5500 degrees Celsius.

So far, everything seems to follow a logical reasoning, i.e., the energy produced in the nucleus by nuclear reactions diffuses to the outer regions. This radiation of energy, mainly in the form of X-rays and Gamma rays, causes the temperature to drop from the core to the surface. However, as we move further away from the surface we find a unique situation.

In particular, it happens that the outer layers of the solar atmosphere are much hotter than the surface of the star itself. From the chromosphere to the transition zone and then to the corona, temperatures can vary between 1 and 10 million Celsius. These values, hundreds of times higher than surface temperatures, have puzzled scientists for tens of years.

The Enigma and the Questions of the Sun’s Temperatures

This drastic difference between the temperature of the photosphere and the corona is a major enigma for the scientific community. How can our star become hundreds of times hotter when we move a few thousand kilometres away from its core?

A first explanation came in 1942 from Hannes Alfvén (Nobel prize winner in physics 1970) who spoke of plasma waves. The Swedish physicist developed a theory that magnetised plasma waves could carry incredible amounts of energy through the solar magnetic field. Thus, these waves would be able to transfer energy from the nucleus through the photosphere and corona to explode like heat bombs in the outer regions of the solar atmosphere.

Why the Sun’s atmosphere is much hotter than its surface: from early observations to Alfvén’s theory of the Sun

Over the years, man has been able to learn about and study the solar corona thanks to the phenomenon of solar eclipses. In fact, before the arrival of today’s space probes capable of investigating the far reaches of the universe, the only way to observe the corona was to wait for a total eclipse. In that case the Moon’s special position obscures the brightest part of the Sun, making the corona visible.

Coronium, a new physical element?

During one of these observations in 1869, scientists detected a peculiar and unexpected spectral line. Scientists analysed the light from the Sun using a spectrometer to trace the composition of the radiant element. Each emission is characterised by a wavelength, which in turn is determined by the type of element. However, the observation made in 1869 was unique, as that green spectral line had never been observed before. Consequently, the scientists of the 1869 were delighted to think that they had discovered a new element, thus naming it Coronium.

Some 70 years later, in the late 1930s, the joy of this discovery was cancelled out by the work of astrophysicists Bengt Edlén and Walter Grotrian. They discovered that this particular emission was not from any Coronium. Instead, it was highly ionised iron with half the number of electrons present in a normal iron atom. Soon after, much to the amazement of the two astrophysicists, it was calculated that this level of ionisation (causing the emission) required temperatures in the Sun’s corona of more than 1 million degrees Celsius. This is where the enigma concerning the heat of the solar atmosphere began.

There is an explanation of the heat of the Sun’s atmosphere relative to its surface

The discoveries of Edlén and Grotrian started the headache for researchers. So the scientific community began to search for the answer among the Sun’s main properties. In particular, our star is composed of plasma, which is highly ionised gaseous material with a high electrical charge. The movement of plasma in the convective zone results in the generation of very large electric currents and very strong magnetic fields. These magnetic fields are drawn into the Sun by convection, and then manifest themselves on the surface in the form of dark sunspots, which characterise various magnetic structures of significant intensity.

This is where the Alfvén theory comes in. It posits that waves are created within the solar plasma that can carry enormous amounts of energy over vast distances, such as that between the surface and the solar atmosphere. The theory, later awarded the Nobel Prize in 1970, states that heat travels through solar magnetic flux tubes to the corona, where the explosions responsible for the sudden rise in temperature occur.

The observation of Alfvén waves and future developments

The evidence for such Alfvén waves and flow tubes has been an unresolved issue for many years. Thanks to recent technological development and innovations in new telescopic instruments, it has been possible to observe the physics of the Sun with high precision. In this case, a recent research project conducted by a team of scientists from seven research institutes has finally been able to confirm the existence of Alfvén waves. This important revelation was based on observations made using Interferometric Bidimensional Spectropolarimeter, installed on the Dunn Solar Telescope in New Mexico, together with advanced numerical computer simulations.

This is where the Alfvén theory comes in. It posits that waves are created within the solar plasma that can carry enormous amounts of energy over vast distances, such as that between the surface and the solar atmosphere. The theory, later awarded the Nobel Prize in 1970, states that heat travels through solar magnetic flux tubes to the corona, where the explosions responsible for the sudden rise in temperature occur.