Sun Quakes

Over the past couple of weeks I’ve been doing a series of posts about my joint favourite star, our sun. First up was a general overview, second was a post on nuclear fusion, the process that powers the sun and that we hope to harness here on earth in the coming decades. This time I’m going to write a post about helioseismology.
I have been reading as much as I can about the now well established field of helioseismology and the first thing I had to learn was what it even is. In my innocence I thought that there might be quake type events happening in the sun but that turns out not to be the case. If you think about it for a while, though, you can see why this would be wrong. The earth has many different components to it, the most relevant being the lithosphere which is made up of the tectonic plates of the surface and the top most part of the upper mantle. Here, where two continents meet, you might have one subducting under another or perhaps they will be grating against each other in opposite directions. In this situation it’s possible that they might get a bit stuck and every now and then they will suddenly slip causing an earthquake. Asides from the damage we’re all too used to seeing on the surface this will also cause sound waves to propagate through the earth. These sound waves will move through different substances in different ways dependent upon their properties and so you can use this to study the internal structure of, in this case, the earth. This is a key part of what seismology is here on the third rock but we can use the same principals with the sun; there, however, the waves aren’t caused by solid rock but by shifting plasma densities.
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Here you can see the ripples of a sun quake radiating out from its source. In just one hour the ripple travels a distance equivalent to 10 earth diameters. Image courtesy of NASA.
Plasma, which is basically what the sun is, is a gas that has been stripped of electrons. The sun is not a boring, uniform, unchanging sphere of plasma but a highly active and dynamic one. It is hotter at its centre than it is nearer the surface and this can result in hotter, less dense plasma rising higher amongst cooler, more dense plasma. My best interpretation of what I’ve read, and I could easily be wrong, is that this causes a sound wave to be produced, perhaps through friction but don’t quote me on that. Either way a sound wave is created. This wave then propagates and can be detected by instruments aboard the Solar Dynamic Observatory (SDO) and the Solar Heliospheric Observatory (SOHO), both satellites orbiting the sun. The waves tend to have frequencies of 1-5 microHertz, or around five minutes and amplitudes of hundreds of kilometres. Given this we can know at what sort of speed they should move and where there are deviations from this we can infer the density of the medium through which they’re moving. This has given us a great level of detail about the internal structure of the sun.
Like all good theories though, it needs to be tested for accuracy and this has been done quite exquisitely. A few years ago scientists were able to detect these sound waves some 60,000km below the surface of the sun, they then predicted that this would result in the production of some sunspots at a given location. Two days later, sure enough, the sunspots appeared. You can see a video of it happening here:
This is super awesome. Not just because it represents an advance in our understanding of an obscure branch of solar science but because it has practical implications for us here on earth. How so? Well, the magnetic field of the sun is particularly intense where sunspots are formed and at the end of the video, as the rotation of the sun begins to take the activity from view, you can see that vast arcs of plasma, greater than the size of the earth itself, are formed along the lines of the magnetic field. The sunspots are at the bases of the arcs where they are anchored to the surface, but if they are strong enough then these arcs can snap free and hurl matter out into the solar system. Small events are referred to as solar flares and larger ones are called coronal mass ejections (CME). These can be highly significant as they can fling billions of tons of matter into space at a time. If the earth happens to get in the way of this then you can expect a pretty impressive display by the aurorae at both poles as the matter interacts with our own magnetic field. Nothing wrong with that. If it is an especially large CME, though, then this could play havoc with electrical equipment all over the planet.
In late summer of 1859 the largest CME on record was observed. Known as the Carrington Event, it caused the aurorae to be seen as far south as the Caribbean and as far north as New Zealand. It also, however, caused widespread electrical damage. Telegraph stations across the globe went down, some operators were electrocuted, pylons sparked and some devices that were turned off began to operate. This was at a time when we were just at the beginning of the electrical age, if this happened to us today then it could be potentially devastating. It has been estimated that the cost to the United States alone would be in the region of 0.6-2.6 trillion dollars. That is a solar storm we do not want to happen, the problem is that, eventually, it will. Estimates say that there is a 12% chance that there will be another by the year 2022 and that it is all but guaranteed by the end of the century. So it’s not a matter of if but when.
Here is where the research comes in. That CME took just 17 hours to reach us from the sun. That is not much time to enact emergency procedures to protect our electrical systems, not that we actually have any such procedures as of yet. With this research though it would be possible to have at least a couple of days notice which should be plenty of time to prepare the world saving weeks of disruption and trillions of dollars – just as soon as we know how to prepare.
Perhaps the most important message to take home from this is that you never know where the benefits of scientific research might lead. There are certain groups of people cough politicians cough cough who want scientists to solve certain specific problems and will only fund research aimed at doing so. Sometimes this can work, but sometimes it won’t, and what should never be discounted is good old fashioned, curiosity driven, blue sky research where you fund a scientist simply to figure something out because it’s interesting. This has led to many of the most significant breakthroughs in human history, perhaps most notably in the creation of the internet. In this case a bunch of guys sat around staring at the sun, so to speak, quietly developing our notion of space weather and now we have a way of potentially saving our society as we know it. This is just one of the reasons I love science.
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