Draughts of the Stars

In our last post we took a glimpse at auroras in the making – a spectacular light show staged not by a Roman deity sadly, but by the solar wind. Today we will see where this solar wind originates from (the Sun, unsurprisingly) and see how the draughts of the stars yield the lights of the skies.

The Sun, as dazzling and fearsome as it may be, is harder to explore than you’d imagine. One problem is that, despite its overpowering gravitational field, firing something at the Sun doesn’t actually send it into the Sun – without making precise calculations you’d probably end up in orbit around the Sun and in a position where you’re too far away from the surface of the Sun to make any useful measurements.

The other problem is that the entire Solar System lies roughly in one plane (the geometric surface of course, not the airborne vehicle), and there is indeed a reasonable explanation for this. The planets were originally formed from a spinning cloud of dust in which the Sun was at the centre. This isn’t a very stable state, however, because the conservation of angular momentum (the quantity that measures how fast something is spinning) prefers that all the dust be spinning in the same direction (either clockwise or anticlockwise) rather than be swarming around the Sun in three dimensions. Consequently the cloud contracted into a disc, and afterwards the material clumped together to form the planets. The planets don’t lie perfectly in one plane, but they’re very close – the majority of planets are within only a couple of degrees out.

Orbital Plane
The orbital plane of the eight planets of the Solar System (and the asteroid belt and Pluto!). Image: Physics Forums

Why is this a problem though? This wouldn’t be a problem if we wanted to explore the equator of the Sun, but this is a problem if we wanted to explore the poles. Incidentally like in an aeroplane, once you’re in the plane, it’s difficult (but not impossible thankfully) to escape. Here we need to call in the other planets for help. Ulysses for instance, a space probe that made polar orbits of the Sun in the 1990s, swung by Jupiter for a boost to escape the plane.

Our many exploratory missions of the Sun have revealed numerous fascinating phenomena. Many of these phenomena come down to the Sun’s magnetic field, which although is only twice as strong as that of the Earth, is far more powerful by virtue of the Sun’s size. In fact, the Sun’s magnetic field is strong enough to repel the pressure exerted by the interstellar medium, and this ‘bubble’ of space around the Sun within which this magnetic field is dominant is called the heliosphere. The Sun’s magnetic field gives rise to many interesting features, for example sunspots, solar flares and coronal mass ejections to name a few. Here we’ll be focusing on the solar wind.

To say that the surface of the Sun is quite hot is an understatement to say the least. The visible surface of the Sun, called the photosphere, can reach temperatures of around 6000 °C. During a solar eclipse you may also have seen a ‘halo’ around the Sun (hopefully not directly or you wouldn’t be able to read this post!) which can attain temperatures in excess of a million degrees Celsius. This structure is known as the corona, and is the part of the Sun that we are interested in here.

The corona during a total solar eclipse. Image: Luc Viatour

The corona is halo-like not because the Sun is angelic (or maybe it is, but that’s for anthropologists to decide) but because the high temperatures have ionised atoms into a plasma, a little like what happens in a nuclear fusion reactor. Because particles in a plasma are charged, they are compelled to follow the Sun’s magnetic field lines which loop out of and back into the Sun. This plasma swirls around the Sun’s surface, trapped by magnetic field lines, and glows due to the scattering of sunlight and due to the excitation of ions.

But as we know, it’s never impossible to escape (except a black hole from within the event horizon, we think). If the plasma is hot enough, it will have enough energy to escape the magnetic field lines and the gravitational force and blow off into space as what we call the solar wind. It’s not strictly the Sun’s magnetic field which keeps the interstellar medium out of the heliosphere, but rather this escaped plasma.

Unfortunately we can’t really make any measurements of the solar wind on the surface of the Earth as we’re shielded by the Earth’s magnetic field (which is a good thing actually), so where’s a better ‘nearby’ place to measure it? The Moon!

The Solar Wind Composition Experiment was an experiment designed by a Swiss team led by physicist Johannes Geiss to investigate the composition of solar wind on the Moon during the Apollo missions. This was the only non-American experiment to be part of the Apollo missions. The apparatus was rather simplistic, essentially an aluminium foil fixed to a pole. The team even managed to convince NASA to have the foil placed down immediately after landing in order to maximise exposure time to the solar wind.

Solar Wind Composition Experiment
Buzz Aldrin setting up the Solar Wind Composition Experiment. Image: NASA

Here’s where it gets interesting though. Some bright spark had the idea of having the Swiss flag attached to the foil, so Neil Armstrong stepped onto the Moon, said his famous quote ‘one small step for man…’ and all that, Buzz Aldrin set up the experiment, and then the American flag was planted onto the surface of the Moon. Which means that… the first flag planted on the Moon was actually Swiss!

If there’s anything more impressive than planting your own flag on the Moon, it’s manipulating someone else into planting it on the Moon.


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