Posted by: Kash Farooq | March 4, 2011

Exploring Sunspots and Coronal Loops with Helioviewer

Having already taken a general look at Helioviewer, I thought I’d use it demonstrate a well known solar feature: sunspots. The Sun is very active at the moment and there are some impressive sunspots to be seen:

Sunspot, 2nd March 2011 11:00 4500 ångströms

Sunspot, 2nd March 2011 11:00 4500 ångströms

The sunspots appear dark because they are cooler than the surrounding material. The photosphere of the Sun (i.e. the area of the Sun that we see –the yellow globe) has a temperature of 6000 °C. Sunspots have a temperature of between 2700 – 4300 °C. Sunspots can be huge and could easily swallow the Earth; they can be up to 80,000 km across.

Here is the same sunspot a few hours later, but zoomed in:

Zooming into the same sunspot

Zooming into the same sunspot

The above images were taken at a wavelength of 4500 ångströms (4.5 × 10-7 metres). This wavelength falls within the visible light part of the electromagnetic spectrum. By looking at the same area with a different wavelength, you can see features that produce radiation at those different wavelengths.

So, by adjusting the Helioviewer date/time to go back in time until the Sunspot is at the edge (the “limb”) of the Sun and then selecting a different wavelength, I see the following:

Coronal Loop, 25 February 2011, 17:00 171 ångströms

Coronal loop, 25 February 2011, 17:00. This image was taken at 171 ångströms, which is around the border of the X-ray and the Ultraviolet parts of the electromagnetic spectrum.

The feature we can see in this image is a coronal loop. These closed loops contain plasma at extremely high temperatures; the temperature of the plasma can exceed 106 K. Plasmas interact very strongly with magnetic fields and that is what is happening here. There are magnetic field lines between the two sunspots and plasma from the Sun has been dragged along the field lines to form a loop.

And this brings us to another feature of sunspots. They often occur in pairs and magnetic field lines connect the two sunspots. Even more remarkable is the magnetic polarity of each sunspot. In any given period of time, the polarities of all “leading” sunspots in the same hemisphere are the same. In other words, as the Sun rotates, if you find that the front sunspot in a pair has a magnetic north pole, then all leading sunspots in each pair in that hemisphere of the Sun will also have north poles.

And, in that same period of time in the other hemisphere of the Sun, the opposite will be true. The leading sunspots will all have south magnetic poles.

Incidentally, magnetic field lines always travel from magnetic north to magnetic south, and hence this is the direction in which the plasma in the coronal loop is dragged.

The “given period of time” that I have been referring to is known as the solar activity cycle. For 11 years of the cycle all leading sunspots in the northern hemisphere of the Sun will have, say, a north magnetic pole. Then, for the next 11 years, all leading sunspots in the northern hemisphere of the Sun will have a south magnetic pole. The activity cycle has peaks and troughs; right now (in 2011) we are at a peak known as the solar maximum. During these periods there is increased magnetic field activity, and this coincides with the appearance of more sunspots and associated features than during a solar minimum.

I’ll end the post with a close up of a sunspot. This image was not captured with Helioviewer:

Sunspot close-up, with granules

Sunspot close-up, with granules. Each granule is a convection cell about 1000 km across.

The granules you see are tops of convection cells and provide an ever changing pattern. Hot material is rising from deep within the upper layers of the Sun, cooling and then falling back down at the darker edges.

The size of each cell gives you an idea of the size of the Sun; each cell is about 1000 km across.

The Sun is huge!

Yet, it’s quite small when we look at other stars out there in the Universe.


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