Sky Matters: let's hope for clear skies on the summer solstice

The longest day is fast approaching and this year it occurs on Saturday, June 20th. The so-called Summer Solstice can actually take place on 20th, 21st or 22nd June, depending on the year. At the solstice the Sun rises to its highest point above the horizon in the northern hemisphere and this will happen around 13:35 in Ireland. In truth this is not very much different for a couple of weeks preceding and afterwards.

If the skies were completely devoid of clouds we would receive the largest amount of energy from the Sun at 13:35 on 20th June, but given the unreliable nature of our weather, in particular cloud cover, it is impossible to predict when that actually occurs.

It’s not only cloud that’s variable - the Sun itself is not completely constant. It goes through an 11-year cycle of activity, most easily seen in the changing number of dark areas known as sunspots.

These are regions which are about 2000 degrees centigrade cooler than the surrounding surface of the Sun, although in reality they are still very hot at about 4000 degrees centigrade. For the past twelve months or so the Sun has had very few sunspots and is said to be in a deep solar minimum, deeper than for the past fifty years. Ordinarily this might be of interest only to astronomers, but it has an associated effect which might raise all our spirits somewhat during these days of lockdown.

Along with the number of sunspots the total energy output from the Sun drops slightly at solar minimum and that means the total radiation being dumped into the Earth’s atmosphere has reduced compared to the average. The net impact to bring about a deceleration in the rate of acceleration of climate warming. It might be a small effect, but at least it’s in the right direction. Of course as the sun comes out of solar minimum and heads towards solar maximum in 5 years time, the opposite argument will apply.

One other consequence of the solar minimum is an overall weakening of the strength of the Sun’s magnetic field. The Earth, and indeed all the planets, sit within the shield of the Sun’s magnetic field. It protects us from high energy particles originating somewhere in our own galaxy or more likely (and to reference the opening lines of a famous movie series) in a galaxy far far away.

The particles themselves are the offspring of some of the most exotic phenomena in the universe: supernova explosions, colliding neutron stars, jets fed by supermassive black holes. They wind their way across the vastness of space for billions of years, deflected by the magnetic fields of stars like our Sun in a game of cosmic pin-ball. But when the Sun’s magnetic field drops, as it now has done, they can penetrate more deeply into our own solar system and reach the Earth. Most are destroyed in interactions with molecules in our atmosphere and pose no health risk to us on the ground.

Neither do they bring any health benefits such as the destruction of the coronavirus, lest the spread of fake news have us erroneously celebrating solar minimum. But for astronauts on the International Space Station, above the protection of our atmosphere, high energy particles represent a measurable (but small) increase in the high energy radiation dose they receive (most of which is normally from the Sun).

The issue of radiation exposure for human missions outside of the Earth’s atmosphere remains one of the most challenging we face if we are to ever set sail across the cosmic ocean that envelopes us. As our technologies develop it is, however, one that increasingly needs a solution.

The alternative is premature death or the realisation that Earth is the only home we could possibly inhabit and somehow that seems parochial in the extreme.