Hotter Than You Think

When you observe a solar eclipse, the main disk of the Sun is covered but its outer atmosphere is still visible. What you can see is the solar corona – an atmosphere that stretches millions of kilometres into surrounding space, and a phenomenon that has baffled scientists for about fifty years.

The corona exhibits a number of unusual properties. The Sun’s core is about 20 million degrees Celsius; its surface is a cool 5000 degrees and, logically, one would expect its temperature to decrease further away. But in reality, temperatures increase dramatically above the Sun’s surface. On average, the corona is about two million degrees Celcius – four hundred times hotter than the Sun’s surface and comparable to parts of its core.

The reason we know it is so hot is because it emits large amounts of X-rays. These are a very energetic form of radiation, which associate with high temperatures. The gases in the atmosphere are heated to such an extent that their electrons get stripped off, causing the atoms to become positively charged “ions”. The resulting soup of positive ions and negative electrons is known as plasma, which emits X-rays when heated.

This discovery was made in 1942 by Swedish physicist Bengt Edlen, who studied the spectral lines created by the corona (for more on spectral lines, read this). In spectroscopy, a certain pattern of spectral lines corresponds to a specific element. However, Edlen observed spectral lines which didn’t appear to associate with any known elements, and dubbed the unknown substance “Coronium”. He later showed that the corona was not composed of any new elements. Rather, it consisted of atoms such as iron and nickel which had lost thirteen or fourteen electrons.

Typically, iron only loses two or three electrons. The more electrons you try to remove, the harder it becomes, because the positive charge of the nucleus binds electrons very strongly. Thus, knocking off thirteen electrons from an atom requires a colossal amount of energy – an energy which, not surprisingly, translates to about 2 million degrees. One explanation for this immense amount of energy is that the Sun’s enormous magnetic field induces electrical currents in the corona. These highly unstable currents can collapse, causing a surge of power which heats the corona to the observed temperatures.

We see the Sun rise and fall every day, and perhaps this numbs us to its staggering importance. The Sun powers life; it gives us light, heat and energy; it keeps our planet in orbit. Without it, we would cease to exist. It is therefore unsurprising that scientists remain so keen to learn more about it.