Talking about the weather

Being British is a fairly good excuse for spending too much time talking about the weather. Seemingly, we’re famed for it. Though not all the weather that affects us is purely from Earth. There’s one thing that causes weather that most of us don’t even realise exists. And it’s blindingly obvious. Literally.

The Sun powers everything in the solar system. The radiant energy striking Earth is the driving force behind all of Earth’s weather systems, but the Sun does a lot more than just that. Even as you read this, millions of solar cosmic rays (high energy protons) are striking Earth’s upper atmosphere. They’re splitting molecules into ions and fragmenting atomic nuclei. A few of these even make it down to Earth’s surface. Don’t worry, they’re nothing to be afraid of (though they do make a mess of data taken by scientists and astronomers). Every once in a while, an inexplicable speck in a photograph taken on your mobile phone camera will be due to a cosmic ray hitting your camera’s CCDs at just the right moment.

The thing is, all of this is being monitored all of the time. Detectors, both ground based and in orbit, are measuring the flux of protons constantly bombarding Earth – they’re the cause of the fabulous dancing aurorae seen near Earth’s poles. And if you know where to look, you can find out how the space weather is doing currently…

From the NASA instrument Advanced Composition Explorer Solar Wind Electron Proton Alpha Monitor (ACE for short), this is the current flux of protons hitting Earth. ACE is a little spacecraft which has been sitting between Earth and the Sun since 1997. It measures the solar wind and transmits the data back to Earth in (more or less) real time. Every time some event on the Sun such as a flare or a coronal mass ejection occurs, ACE measures how the solar wind changes. The image here is how things read right now. 6 protons per cubic centimetre may not sound like a lot, but think for a second how small that is compared to Earth. There are rather a lot of protons flying towards us.

And there are even larger forces at play. This is the heliospheric current sheet.

This is from NASA’s ENLIL model (I don’t know what that acronym stands for, but Enlil was the name of a Sumerian deity, which makes it a pretty cool name). It’s a time-dependent model of the Sun’s heliosphere, not hugely different to the kind of models meteorologists use to predict weather here on Earth. What you’re looking at is a real-time model of density variations in the solar wind, as it radiates away from the Sun.

The heliospheric current sheet is technically the largest structure in the solar system. It’s the “surface” in the solar system where the Sun’s magnetic field changes from north to south, and because the Sun has such a turbulent magnetic field which is constantly rotating, it makes a twisting 3D spiral shape. This warping in the magnetic field sculpts the solar wind. Charged particles ride it the way surfers ride waves. As a result, the density of the solar wind varies quite a lot – and you can see from the ENLIL image that Earth has just entered a more dense region of solar wind, coinciding with the increased proton density you can see in the ACE image. I love knowing how things work…

So how does all of this affect us? Mostly it’s only a big deal if you happen to be an astronaut or a spacecraft (in orbit, solar flares can cause hazardous amounts of solar wind particles). Since the last time the Sun was particularly active, though, humanity has put a lot of satellites in orbit. Space weather may well start to affect communications satellites, interfering with everything from international business to satellite TV. Solar flares can eject protons at up to a third the speed of light. One of the strongest solar flares ever recorded was in 1859, when it caused aurorae as far south as Hawaii and actually set telegraph systems on Fire! In 1989, while not quite so severe, another large flare affected electrical grids and computer systems.

As technology on Earth is becoming ever more sophisticated and humans, as a species, become ever more reliant on electronic equipment, we need to start realising that we’re no longer influenced only by this planet. It isn’t just Earth that affects us anymore. Perhaps it won’t be too long until we start seeing space weather reports after the evening news!

If nothing else, it’s nice to know when to expect pretty aurorae in the sky…

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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