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Magnetosphere & Solar Protons

The solar wind

The Sun is releasing vast quantities of particles into space, the bulk of which are protons and electrons, with a few percent of alpha particles and some heavier nuclei. Those particles with a relatively low energy (with less than 100 keV energy) moves into the interplanetary medium as a supersonic (400 km ) stream known as the solar wind. The flux of these particles reaching Earth can be as high as particles , measured just outside of the Earth's magnetosphere.

The solar wind extends far beyond the Earth's orbit out into the solar system. The boundary where solar influence gives way to the interstellar medium lies beyond the orbit of Neptune and has yet to be detected by spacecraft leaving the solar system.

The solar-wind, being electrically charged, cannot easily penetrate into the Earth's magnetic field, and thus the solar-wind particles are rapidly decelerated and deflected when they meet the geomagnetic field. This leads to the formation of a supersonic shock-wave on the sunward side of the Earth known as the bow-shock. This interaction has the effect of compressing the geomagnetic field to approximately 10 Earth-radii (Re, where 1 Re = 6378 km) distance over the midday meridian, whilst, conversely, stretching it for over 1,000 Re into a long tail (known as the magnetotail) over the midnight meridian. In this way the solar-wind distorts the geomagnetic field into a vast comet-shaped volume, around which the majority of the solar-wind particles are deflected.

The strength of the solar wind fluctuates depending on the activity of the Sun and periods of high activity can produce auroral activity on Earth. When the solar wind interacts with the geomagnetic field, some of its energy is stored in the magnetotail in the form of magnetic potential energy. This energy build-up is released periodically when magnetic re-connection takes place in the region of the magnetotail around 15 RE. This results in energised plasmas being injected towards the Earth, causing a magnetic sub-storm and ultimately resulting in auroral activity as the plasma precipitates into the upper atmosphere. The prediction and observation of auroras can give us a fascinating insight into current solar activity - you can find out more about this on the Aurora Watch website on our Web-links page.

These plasmas can extend into the high-altitude geostationary orbits causing the surfaces of spacecraft to become highly charged and the sudden discharge of these surfaces by electrical arcing may cause spacecraft to incorrectly function, for example creating glitches in computer systems. This sort of effect is not a problem for small low-altitude satellites such as the Surrey spacecraft, where the potential differences that can be induced by the plasma across the small spacecraft surfaces are too small to cause any damage. However, it is interesting for research physicists to use sensors on the satellites to measure the plasma and radiation intensity. In this way low orbit satellites that cover the whole surface of the Earth can be used to build up accurate maps of the geomagnetic field and of the distribution of cosmic radiation in the magnetosphere. This idea is developed later in this topic where you can analyse real data of proton events measured by Surrey's KitSat during a particularly intense solar flare.

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