Space weather is a topic of growing interest both to space scientists and researchers whose focus is more Earth bound. Solar physicists have long known that the Sun regularly sends powerful magnetic clouds our way.
When they strike, the damage can be extensive. The magnetic fields induce currents in railway tracks and power lines which can then bring down entire power networks.
But the influence of space weather on the terrestrial weather is less well understood. The solar wind, and the magnetic clouds that sometimes accompany it, contain huge amounts of energy. Much of this is absorbed or deflected by the Earth’s magnetic field.
But from time to time, magnetic clouds can temporarily lower our defences. During these so-called Forbush Decreases, more particles from the solar wind can get through and dump their load in the upper atmosphere.
It stands to reason that such an injection of energy could change the weather. And there’s certainly evidence that this can happen at high latitudes near the poles. But the rest of the planet seems better protected.
Today, Il-Hyun Cho and buddies at the Korea Astronomy and Space Science Institute in Daejeon, say they have the first evidence that the solar wind can influence the sea-level atmospheric pressure at mid latitudes.
These guys searched through space weather records from 1983 to the present looking for times when the solar wind speed exceeded 800 kilometres per second. That’s a stiff breeze that occurs very rarely, less than 0.1 per cent of the time. They found twelve of these high speed events, nine of which were accompanied by a Forbush decrease..
Cho and co then used records from 76 meteorological stations around South Korea to study how the atmospheric pressure at sea level changed during these events.
Sure enough, they found, on average, a small increase in pressure just after each high speed solar wind event. They reckon a fast solar wind increases the pressure by 2.5 hectoPascals. To put this in context, atmospheric pressure at sea level is about 1000 hPa. So that’s a tiny increase
Despite the small number of events and the small change in pressure , these guys say their results are statistically significant.
So what on Earth could be going on? How can the solar wind influence the atmospheric pressure at sea level?
Let’s get some background info out of the way first. Atmospheric physicists have long known that the solar wind injects charged particles into the outer regions of the Earth’s atmosphere, the thermosphere, that stretches out to about 600km.
This has a heating effect which causes the thermosphere to expand and contract. Since many satellites orbit at this height, including the International Space Station, these kinds of effects are important for determining how orbits degrade.
But what of lower, denser parts of the atmosphere? The thinking is that during a Forbush decrease, these charged particles can penetrate further into the atmosphere. Here, the heating effect causes the atmosphere to expand. It is this that influences atmospheric pressure.
Clearly the most extreme events will occur when a Forbush decrease occurs at the same time that the solar wind is at its most powerful. Which is exactly what the Korean team seem to have observed.
To be sure, the effect is small at sea level but that won’t stop people speculating about the effect it could have over much broader areas than the Korean Peninsula. Charged particles are also thought to have a big impact on the rate of cloud formation, since their ionising effect can trigger water droplet formation
Cho and co say they are already looking for bigger datasets concerning Forbush decreases and their effects over wider areas of the planet. The evidence that space weather can affect our is growing. It’ll be interesting to see what they find.
Ref: arxiv.org/abs/1107.1841: Changes in Sea-Level Pressure over South Korea Associated with High-Speed Solar Wind Events
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