From Supernovae to sparks - how cosmic rays affect lightning rates at Earth

Tuesday 16th August, 2016

We were pleased to welcome back Prof Chris Scott who is Professor of Space and Atmospheric Physics from Reading University. He has visited us in the past but that was 15 years ago when he came to talk to us about spaceweather, which is basically how the Sun's emissions can affect our technological way of life. On this occasion he explained in his talk "From Supernova to Sparks" how cosmic rays from space can influence the weather on Earth.

Prof Scott began by saying that the Earth actually sits within the atmosphere of the Sun, for the Sun is constantly emitting a stream of particles and radiation in all directions and these emissions are known as the solar wind. The speed of this wind can vary between 300 and 800 kilometres per second and the Earth's magnetic field funnels it down to the poles where it collides with atmospheric molecules to produce shimmering curtains of light known as auroras. The solar wind is not constant as the Sun's activity varies on a cycle of roughly 11 years and the auroras usually occur when the Sun is most active and shedding more material into the Solar System.

He then explained that when some of the particles in the solar wind penetrate the Earth's atmosphere the violence of the collision can produce a conductive path that can trigger a bolt of lightning. However, it is not just the Sun that produces cosmic rays but exploding stars called supernovas also emit them. So, whilst some lightning may be triggered by solar activity other cosmic sources may be responsible for lightning strikes.

He added that when the Sun is at its most active it has a stronger magnetic field that shields the Solar System from externally produced cosmic rays and yet due to its heightened activity the Sun itself will be producing more of these particles that can trigger lightning. Conversely, when the Sun is relatively quiet it will let through more cosmic rays from beyond the Solar System.

It is not only the Sun's 11-year cycle of activity that causes variation in the solar wind for the Sun also rotates. On average its rotation rate is 27 days but the equatorial regions rotate faster than the poles. This mismatch can tie the Sun's magnetic field up in knots and give rise to areas where its magnetic field is not looped back on itself but reaches far into the Solar System. These areas, called "coronal holes" are more prevalent when solar activity is low and they allow the solar wind to escape at much higher speeds compared to the rest of the Sun's outer atmosphere. As the Sun is rotating, these fast and slow solar wind regions will spread out into the Solar System somewhat like a rotating lawn sprinkler spreads water.

The density of the solar wind builds up at the boundary between these fast and slow streams as the fast material will try to overtake the slower. So when these denser areas with more solar particles collide with the Earth's atmosphere there is more chance of them triggering lightning. This model of the solar wind spiralling outwards and having a mixture of slow and fast streams has been confirmed in Prof Scott's research which shows an increase in lightning strikes every 27 days which matches the average rotation rate of the Sun..