The Sun in Hydrogen-alpha light

This is an image of the solar chromosphere in H-α light. To explain what we are seeing, we need to know a little about the structure of the Sun’s atmosphere.

The bright ‘surface’ of the Sun which gives us our light is called the photosphere. It is a layer about 300 miles deep and has a temperature of about 6000°C. If we use neutral-density filters to reduce the amount of light by about 99.999%, we can see dark sunspots, bright faculae and a general granulation all over the surface.

Above the photosphere is the chromosphere, a more tenuous layer where the temperature rises to about 20,000°C. This temperature gives the particles enough energy to enable the hydrogen gas to ionize and recombine, and it’s the recombination that emits photons of light at the particular wavelength we call the H-α spectral line, the wavelength being 656.28nm. This is a deep red colour and the chromosphere would look red if we could see it without the bright photosphere below it. In fact the rim of the chromosphere can be seen during a total solar eclipse when the Moon briefly hides the photosphere.

By using a special filter that blocks almost all light except that of the H-α wavelength, a lot more detail and activity can be seen in the chromosphere, and this image is an example of what can be seen.

The uppermost layer of the Sun’s atmosphere is the corona, which extends thousands of miles into space. It too can be seen only during a total eclipse, and its temperature is measured in the millions of degrees, but being extremely tenuous the amount of heat it carries is much less than the lower layers.

The physics of how the corona becomes so hot is still not well understood.

Prominences

These spectacular structures erupt above the surface of the chromosphere and can be seen to develop and fall back or dissipate on a timescale of minutes.

Remembering that the Sun is about a hundred times the size of the Earth, it follows that the prominences in these images are much bigger than our planet. Like sunspots, they are created by lines of magnetic flux emerging from the Sun’s surface layers.

These images of the Sun were taken on 6th July 2013 through a Meade 5000ED 80mm refractor using a Lunt LS60 H-α filter with a 0.7Å bandpass (the bandpass – the range of wavelengths passed by the filter – is typically expressed in Ångstrom units, these being 1/10th of a nanometer). The camera used was a Flea3 FL3-U3-13S2M taking approx 120 frames per second. Each image is a stack of about 200 frames processed using Registax.