Over the last few weeks a lot of media attention has been paid to “solar storms” and their possible consequences for us. We also know that as solar activity rises towards the next solar maximum, such events will become more frequent. What are being described as “solar storms” are really events called “coronal mass ejections” or “CME’s”. What are they and why do they worry us?
Inside the Sun, the flows of solar material generate strong magnetic fields. These erupt through the surface and become visible as sunspots, regions of magnetic activity, and loops. Imagine that one of these has just emerged, and is growing. It consists of a magnetic field loaded with million-degree, ionized gas, or plasma. More magnetic flux emerges and the loop gets bigger, eventually reaching a height of hundreds of thousands of kilometres above the solar surface. As it grows, it gets more floppy, and distorted by hitting other loops and increasingly buffeted by the solar wind. In addition, movement and twisting of its foot points also distort the loop. These distortions cause energy to be stored in the loop and it becomes increasingly stressed. As the stress increases, it is easier for something small to trigger a catastrophic failure. Eventually it happens: the loop snaps from its foot points and most of it, typically about 1.5 billion tonnes of very hot gas and magnetic fields are catapulted into space at a speed ranging from 500 to 3,000 kilometres a second.
There is a point 1.5 million kilometres sunward from Earth, where the gravitational fields of the Sun and Earth combine to form a stable place were we can park satellites permanently. This location, called a Lagrange Point, therefore is a good place to keep a continuous eye on the Sun, so that is where we put our key solar monitoring spacecraft.
We can see the CME lift off from the Sun, and get an idea as to whether it is heading for us. Then it vanishes. Depending on its speed it takes between one and three days to cover the 150 million kilometres between the Sun and Earth. During that period we know it’s coming, but not exactly when it will arrive. Because we need to prepare, it would be nice to know exactly when it will hit us. However, with the CME being invisible for almost its entire trip to Earth, that is currently very difficult.
We next detect the CME when it hits the satellites at the Lagrange Point. Depending upon the speed of the CME, we have between 10 and 30 minutes notice before it hits us. The consequence is a magnetic storm. These can cause power outages and other problems with our infrastructure. High-energy particles are funnelled down into the Polar Regions, triggering auroral displays and possible radiation hazards for air travellers on polar routes.
We need to know the arrival time more accurately, and with more advance notice than a few minutes. Big efforts are being made to improve our ability to track CME’s over the part of their trip to us where we cannot see them. Both ground-based and space-based options are being looked at.
Venus and Jupiter are conspicuous in the west after sunset, although Jupiter is getting lower in the sky. Mars is high in the Southeast; Saturn rises about 8 p.m. The Moon will reach last quarter on the 13th.
Ken Tapping is an astronomer with the National Research Council, and is based at the Dominion Radio Astrophysical Observatory, Penticton.