The Sun

In the November Newsletter you will have read a couple of items about the violent activity of the Sun at the end of October, and that we may now be entering a relatively quiet period after an angry 100 years. There were also reports on radio and television of the unusual solar event which, at one time, was said to threaten satellite, telephone and other communication systems and even power supplies. Also, those of you who listen regularly on the HF bands may have noticed some very unpredictable conditions extending into the first week of November. In the event, it didn’t prove as serious as predicted but it did provide some spectacular “Northern Lights” in some parts of the world which were seen as far South as Texas. I therefore thought it was worth saying a few words about the source of these events, our Sun.

Our sun is a pretty ordinary star, situated about half way out from the centre of our galaxy, (the Milky Way), on one of the galaxy’s spiral arms. Like all stars it was born by the “collecting together” of space debris from previous now extinct stars, and “virgin hydrogen” from the Big Bang. This collecting together, slow at first, is governed by gravity, whereby every piece of matter in the universe is attracted towards every other piece of matter by a force proportional to the product of their masses and inversely proportional to the square of their distance apart. As this process continues, its rate increases. The central mass gets greater and pulls in more matter which arrives ever faster resulting in a huge hot mass with an enormous central pressure due to all the stuff above it. At a certain very high temperature and pressure, the hydrogen, with atomic weight one (which comprises about 95% of the total mass collected), is crushed into Helium, with atomic weight four, (via a rather complicated process), with the generation of a huge amount of energy. This “fusion process”, whereby 4 hydrogen atoms are fused together into one Helium atom, only takes place in the sun’s central core which occupies the middle quarter of the sun’s diameter. The remaining outer three quarters just acts as a fuel reserve and a sort of molten thermal insulator slowly conveying heat from the core to the surface where it is radiated. The mechanism of conveying heat is by huge convection currents of hot material welling up from near the hotter core to the surface where it cools and then returns to near the core again to be re-heated. This gives the appearance on the surface of the sun of boiling porridge. The visible surface of the sun is known as the “Photosphere”, though it actually emits more than just light and other wavelengths of electro-magnetic radiation. It also emits ionised gases which radiate out at great speed, known as the solar wind. The “equator” of the photosphere rotates about once every 27 days, but the sun is not solid like the earth and different regions rotate at slightly different speeds.

I referred to the material at, and just below the surface, as a liquid, but actually it is so hot that the electrons of each atom are not bound in orbits around their respective nuclei but are completely detached and free to roam. Such a state of matter is called a plasma. Because the electrons are not bound in orbits around their respective nuclei, and because they are more mobile than the nuclei, they can get separated from them by the convection process, resulting in electric currents and associated magnetic fields around the convection currents within the plasma. The number of positive charges on all the nuclei and the number of negative charges on all the electrons however remain exactly equal. The magnetic fields, when strong enough, can affect the appearance of both the photosphere and the thinner, but still very hot gases above it, known as the “Chromosphere”. This is because the charged particles, both positive and negative can only flow freely along the lines of a magnetic field. Any movement of electric charge across a magnetic field results in a force on the charge which tends to move it at right angles both to its original motion and to the direction of the magnetic field. (Fleming’s left hand rule of “O” Level physics). Localised regions of high magnetic field near the photosphere disrupt the gases in the Chromosphere and the convection in the photosphere, and lead to patches of the surface being a few tens to a few hundreds of degrees cooler than the neighbouring regions. These cooler patches therefore appear darker and are known as “Sunspots”. The average temperature of the Photosphere is about 6000 Celsius.

The Photosphere emits a huge range of radiation, ranging from radio noise through microwave, infra-red, visible, ultraviolet, (UVA and UVB), X-rays to gamma-rays. The ionising portions of these radiations, (UV, X and gamma rays) are particularly intense from the sunspot regions. All electro-magnetic radiations travel at the speed of light and reach the earth in about 8 minutes, where they interact with our atmosphere. The ionising components produce the familiar ionised layers, D, E, F1 and F2 of the ionosphere which enable “over the horizon” and “round the world” propagation of HF radio waves.

The bursting bubbles of “boiling porridge” on the sun’s surface sometimes eject significant amounts of matter which are visible from earth as “filaments” or “prominences”. These burst through the Chromosphere and into the Corona, but they nearly always return to the sun’s surface via strange and sometimes tortuous paths guided by local magnetic fields. However, a small amount of matter in the form of ionised gas, (i.e. atoms or molecules which have gained or lost one or more electrons and are therefore electrically charged), which is emitted by the photosphere always penetrates the Chromosphere and Corona. This escapes into space in all directions, forming the solar wind. Sometimes localised regions of the Corona become thin, when they are known as Coronal Holes. These allow significant amounts of matter to be ejected at high velocity which leave the sun altogether and head off into space with the more constant and thinner solar wind. These significant amounts of matter are known as Coronal Mass Ejections and although their density is low in terms of Earth standards, (a few tens of particles per cubic centimetre), their velocity is high, (hundreds to a couple of thousand km per second). Even at these speeds, they take days rather than minutes to reach the earth. If they head in our direction, they can ionise huge depths of the earth’s atmosphere, (not just the usual layers), and cause spectacular aurora and magnetic storms. This is what is believed to have happened at the end of October. The way in which the incoming matter and radiation interact with our atmosphere and our Earth’s magnetic field, and enable or disable HF communication is another and complicated story for another occasion.


        John  G0NVZ
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