Sunspots and Active Regions

I wanted to write a short note on the use of the terms “sunspots” and “active regions” when discussing structure on the Sun’s surface. To do that, however, requires an aside as to the nature of the Sun’s surface itself. The Sun has a core where hydrogen is built into helium at a temperature of millions of degrees. As we move further away from the core, the temperature drops (as one would expect). At some point, we reach a point where the density has dropped to a point where photons can escape and stream outwards. This radius, at 695,500 km (or 1 solar radius), is deemed the location of the Sun’s surface, which we call the photosphere.

The photosphere is 5800 K in temperature (nearly 10,000 degrees Fahrenheit!), and above it lies the lowest part of the solar atmosphere, called the chromosphere. The chromosphere is at similar temperatures to the photosphere. However, between the chromosphere and the outer solar atmosphere, called the corona, there is a region of intense temperature increase, and the corona reaches temperatures of over one million Kelvin.

It’s important to take a moment to realize the absurdity of this: the region further from the Sun is hotter than the surface of the Sun. Everything we know about hot objects relies on the simple fact that they feel cooler the further we are from them. If you put your hand near an open flame, it is heated. As you pull your hand away, it cools. The light grey curve in the plot below shows what we expect for the temperature of the Sun as we move away from its surface at r = 1 solar radius. However, the maroon line shows on a log-log scale what the previous graph was showing us: there is a thin transition region where temperatures skyrocket to millions of degrees. Although there is a whole field of research looking into the physical processes to explain this, scientists have not yet definitively determined the causes. Crazy stuff!tempstructureBack to the matter at hand, however. I’ve introduced the terms for the Sun’s surface (the photosphere) and the Sun’s atmosphere (the chromosphere and corona). This distinction is key to understanding the relation between sunspots and active regions on the Sun.

sunspot_horseshoe_magnet_sm

Sunspots are cooler patches of plasma at the photosphere. They are at a lower temperature because magnetic field lines are bursting out of the Sun’s interior and restricting the flow of hot plasma from reaching those parts of the surface. Sunspots come in pairs, since these emerging field lines form loops with the two opposite-polarity footpoints defining the locations of the sunspot pairs. To take observations of the photosphere of the Sun, we need to use visible wavelengths, such as the 4500 Angstrom observation in the middle of the three-image figure below. If we instead look at shorter wavelengths, corresponding to ultraviolet light, we are seeing plasma that has been trapped along these magnetic field loops and has been transported into the corona. Big, bright active regions in ultraviolet observations are usually found directly above the sunspots seen in visible-light observations. Scientists find the shorter wavelength observations much more useful because they reveal so much detail. See for yourself! Note that some active regions lie above sunspots that are small enough (or warm enough) not to appear in the visible-light images.
Made using Helioviewer.org

All in all, the short answer of what the difference is between sunspots and active regions is that there isn’t one in a sense. Both are specific indicators of strong magnetic fields above the surface of the Sun, the kinds of magnetic fields that can release flares and coronal mass ejections. It kind of makes you think twice when you look up at the rather uniform bright yellow spot in the sky.

“Some painters transform the sun into a yellow spot,
others transform a yellow spot into the sun.”
Pablo Picasso

The Carrington Event

In the middle of a winter night in central Alaska, a family of four gazes up at the dazzling, dancing ribbons of green and purple light in the sky. These Northern lights, also called aurorae, are one of the most benign effects of a major event in space weather known as a geomagnetic storm.

In New York City, dozens of transformers have blown, plunging the city into darkness. The aurorae are now visible, even at latitudes so far from the poles. The power lines, such long lengths of metal wire, have overloaded with current as the protective magnetic shell around the Earth, called the magnetosphere, ripples and wobbles in response to a massive attack by our normally friendly Sun.

Although most people check the weather forecasts at least once a week (“Is it going to rain tomorrow?” “Do I need a sweater today?”), few have ever checked or even heard of Space Weather. Space weather forecasting can tell us if and when the harmful effects of events on the Sun’s surface or in the solar atmosphere will reach the Earth, causing the events described above. Coronal mass ejections (CMEs) are tracked from the Sun to the Earth, where they can be very destructive when they reach the Earth’s magnetosphere.

The Sun-Earth Connection: NOT TO SCALE!

Flares are given categories based on the peak X-ray flux measured by the GOES satellite. X-class flares are the most energetic (and therefore dangerous). M-class flares are sort of in the “middle” (that’s how I remember the order!) energy range, and C-class flares are a hundred times less energetic than X-class flares. Scientists also assign numbers: an M6 flare has six times the peak X-ray flux of an M1 flare and twice the peak flux of an M3 flare. An X2 flare has ten times the peak flux of an M2 and one hundred times the peak flux of a C2 flare. Here are example observations from GOES.

Typically, harmful effects from solar flares and CMEs go unnoticed by the everyday person. Sure, satellite electronics are often interrupted by these solar storms, but since most communications satellites operate as part of a network, there are seldom dropped calls that can be blamed directly on our star. However, there have been massive space weather events in the past that, were they to happen today, could seriously mess with our comfortable, technology-rich lives.

Now before there’s any panicking, I want to clarify that the images in your head from Hollywood disaster movies are not what I’m talking about (Note: spoilers in the following two sentences). Neutrinos from the Sun will not start mutating and cause Bible-level floods (2012). Nor will a massive solar flare actually reach the Earth and kill off everyone (Knowing).

However, let us consider a real, historical example of the actual havoc the Sun can wreak here on Earth. The Carrington Event was a massive space weather event from 1859, caused by a series of flares and coronal mass ejections (CMEs) from a region of intense magnetic fields on the Sun’s surface called an active region. Current solar observatories typically look at these regions in extreme ultra-violet radiation where the heated plasma in the Sun’s atmosphere can be measured. Back in the mid-1800s, however, we had no way of observing the Sun in wavelengths other than visible light. This meant that Richard Carrington was looking at sunspots, darker parts of the Sun’s surface that suggest the presence of strong magnetic fields (a topic for a future post). Here’s what he saw:

Carrington observed bright flashes at the points labelled A-D on the diagram above on September 1st, 1859. The following day, the effects on the Earth started to become apparent. Aurorae are caused when energetic particles from the Sun are able to flow along the magnetic field of the Earth’s magnetosphere and interact with electrons in the Earth’s atmosphere. Typically, aurorae are seen only at the poles of the Earth, where the outermost magnetic field lines of the magnetosphere connect back to the Earth’s surface. The Carrington Event, however, produced large quantities of energetic particles traveling at uncommonly high speeds and caused aurorae to be seen as far south as the Caribbean.

Aurorae themselves are not harmful, but they are a warning that more dangerous effects of a geomagnetic storm are in store. On September 2nd, 1859, telegraph lines around the world failed, shooting off sparks and starting fires. When the CME hit the Earth’s magnetosphere, it deformed it; any change to a magnetic field will produce a current, and these currents overloaded the long wires of the telegraph system. As the storm subsided, these currents diminished, but remained strong enough to run the telegraph system without any other power source.

Consider, then, what would happen if this same storm occurred now. Satellite electronics would be knocked out, taking down our network of communication and GPS. The currents that overloaded the telegraph lines of the 1800s would completely blow out the transformers that our power grids rely on. The storm itself could take out hundreds of giant transformers and cause a chain reaction of failures. Replacing this expensive equipment would take a long time, leaving cities without power for weeks or months. It certainly gives one something to think about every time the media reports on a space weather event.

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[Edit note: the first two paragraphs are the result of a writing exercise led by Tom Levenson during the Communicating Science Conference (2014 local). The third paragraph was thus amended to fit together with these new pieces.]

“If you want a place in the sun, you have to expect a few blisters.”

Loretta Young