A Deeper Look at Sunspots

The year was 1769 in the highland country of Scotland that an astronomer first noticed a lowland feature on the solar disk. A cavity or hole in the Sun! What was the depression? It was the black core or umbra of your typical sunspot. Sunspots are in fact optical sinkholes where we see down about a thousand kilometers lower than the surrounding photosphere. The density of matter (H+ ions) within a sunspot is also lower giving credence that it is more then just an optical hole. Why sunspots contain less material and are dark will be explained later but this hollowed aspect of sunspots can indeed be seen using a standard solar filter. The sunspot should have a decent sized umbra and penumbra and be observed as it approaches the edge of the solar disk. The depression is most noticeable at the solar limb where, instead of the umbra peaking above the surrounding penumbra, it’s gone or mitigated. You’re seeing right through it. The disappearance of the umbra near the limb is known as the Wilson Effect named after the Scottish astronomer Alexander Wilson who noted it.

But can you look down the transparent hole? Can you get a better sense there is a depression? If your local astronomy club has access to an H-Alpha scope the answer tilts to possible. In H-Alpha, the contrast and texture of the umbra, penumbra, granular photosphere and surrounding plage can be seen. Combined with oblique views of a good sized spot, this really makes 3D structure pop. It is in fact easy to see plage, bright arcs or “sinuous rivers” of white light, extend all the way down to “canyons” between granules of the photosphere. Sunspots themselves can look like a vortex of dark plasma precisely because they are a vortex of dark plasma. With binoviewers, a good sunspot group in H-Alpha can make you believe the Sun is going to eat itself inside out starting at the spot. (See video at http://sanjoseastronomy.blogspot.com/2012/08/sun.html) Just a few weeks ago I got a look of a really “sick” one. Do you recall 2010 Space Odyssey - the part where the obelisk begins to suck Jupiter’s outer cloud shell in? I suppose I could be exaggerating. But if you ask as Annabelle and Dale of Benicia who have never looked at the Sun, they easily called this dark abyss out for what it was. Without being told a thing they noted it and were repeating “whoa” like there was a team of dogsleds on the run inside the eyepiece... An H-Alpha Sunspot, it’s a magnetically cool phenomenon.

Why Sunspots are dark: The photosphere is in some sense, the true surface of the Sun. Our eye sees it as a bright radiator of all visible light and it’s density is so high we cannot see through. It is opaque. Underneath lies the convection zone where energy (heat) is transported to the surface by the process of Hydrogen plasma (hydrogen atoms stripped of their electrons) churning (see cutout figure of sun - the thumbnails on the left). A churning gas of H ions is a positive current – just like electrons moving from one end of a wire to another is negative current. However a magnetic field can impede this current. Sunspots form when a section of this churning H ion gas is permeated by a magnetic field from the solar interior. The magnetic field strength can reach up to 4000 Gauss, 10,000 times the strength of Earth’s magnetic field. This magnetic force stops ion motion across it’s field lines and essentially quenches convection. Since convection is the main source of energy transport to the surface, less heat and radiant energy reaches the surface through the spot and the region looks significantly dimmer – dark. Sunspots are around 2000K cooler than the surrounding photosphere (5700K) because of the energy loss. And at this cooler temperature, recent studies have shown that H2 molecules can form, align or couple with the magnetic field adding to the stability of the entire sunspot.

Why Sunspots are Semi-transparent: The opacity of the photosphere is due to H-ion density, which depends strongly on temperature. In sunspots since the temperature is lower the opacity is lower and we see deeper. In addition, the magnetic field force produces a pressure of its own inside sunspots. As the magnetic “B” field increases the pressure of the H ions inside the spot must fall to equalize with the surround photosphere (See equation). And if the pressure of the H ions falls, density is reduced again making the sunspot less opaque.

By similar use of the ideal gas law, the above equation (see the third thumbnail on the left) also shows that if you increase the magnetic field strength within the convection zone, sunspots can grow in size/volume to keep the pressure equal to the surround. More on sunspot formation and viewing prominences in an upcoming issue.

Want to be instantly alerted when there is an active solar flare, or see a glossary of terms on the features of the sun? To learn more, check out some useful links here: http://sanjoseastronomy.blogspot.com/2012/08/sun.html.