Connecting Solar Physics to Space Weather in Sunspot

NSO's two main telescopes at Sunspot, New Mexico.

This week I'm attending the National Solar Observatory's 26th workshop at the aptly-named town of Sunspot, New Mexico.  The NSO has observing facilities at Big Bear, California, Kitt Peak, Arizona, here in New Mexico, and soon will have the world's most advanced telescope in the Advanced Technology Solar Telescope on Haleakala in Hawaii.  Sunspot facility has been around since the 1950's when the Air Force created it to study the Sun's activity.  The military had realized the usefulness of monitoring solar activity as early as the start of World War 2 when it was realized that solar storms had a negative impact on the effective range of short-wave radios, which were then the only means of long-range wireless communication.  By the 1950's the mechanism for this disturbance had been explained by connecting the x-rays and energetic particles emitted by solar storms to the ionization state of the upper atmosphere.  The purpose of the solar observing facilities established here at Sunspot was two-fold: monitor the Sun and alert the military of conditions that might impact them, as well as to conduct basic research on the Sun.

The NSO facility here at Sunspot long ago transitioned from an Air Force facility to a National Science Foundation lab, but it's two-fold mandate remains the same: predict what the Sun is going to do and explain why.  The conference I'm attending is focused on connecting those two missions.  But more broadly, this gets at an interesting concept in basic science, namely why do we do basic science?

One answer which we generally sell to the public is that we do science in order to produce tangible benefits - cure cancer, make faster computers, reduce pollution, etc.  The other answer is that we are exploring the natural world and this is the one that researchers prefer when talking to other researchers.  As far as I know nobody is opposed to either of those motivations, but there is a, of course, a question of balance.

In solar physics that balance is particularly sensitive.  There is a lot of funding available for space weather monitoring from an operational standpoint.  Commercial and military satellite operators, power grid controllers, those that rely heavily on GPS, and the manned space program need accurate and timely predictions about space weather. As with many complex systems sometimes it's easier and even more accurate to simply fit phenomenological models to the data rather than try to build physics-based models.  In the long run, understanding the physics will provide the most accurate forecasts, but often there's a lot more short-term payoff by simply looking for patterns in the data without trying to understand them.

So this week we're trying to bridge the gap a little bit in solar physics at a place that embodies the balance.  And it doesn't hurt that it's a beautiful place to visit.

View of White Sands National Monument from Sunspot.

Great Videos of Solar Storms

As you may have read the Sun had a bit of a temper tantrum earlier this month.  Here's a great video of the flares (there are actually two of them in rapid succession) from NASA's Solar Dynamics Observatory.


And here is a time-lapse movie of the strong northern lights the storm generated when it hit Earth's magnetosphere.


With the solar cycle just heating up, it may be a stormy couple years.

What is the Ideal Conference Size?

View of the Golden Gate from SSL

I recently attended the Flux Emergence Workshop 2011 at the Space Sciences Laboratory on UC-Berkeley's campus, which was a new experience in my conference-going career.  As the acronym implies, FEW is designed for a very small group (~25 attendees).  Everyone that attends is given a 45 minute time-slot and asked to prepare a 30 minute talk.  The other 15 minutes are designed to be used for questions and discussion both during and after each speaker's presentation.  It is also highly focused on a single topic - transport of magnetic fields from the solar interior to the solar atmosphere.  It was an excellent conference and easily the most productive meeting I have ever attended from a research standpoint.

This has led me to think about the ideal conference size and how to improve the conference going experience.  First, let me divide conference into two groups:  community conferences and topical conferences.  Community conferences are things like the AAS meetings, the APS March, April, and regional meetings, and the AGU.  These meetings are extremely broad and are designed to reinforce the sense of community felt within the broad confines of an academic discipline.  Let me set aside those types of meetings for the rest of this discussion.

The other type of meeting is what I call a topical conference, and I mean that very broadly.  This is any type of meeting that is designed to address some sub-field.  For me that would cover everything from the Solar Physics Division meeting, which covers all of solar physics and has roughly 300 attendees (depending on the location), to the Flux Emergence Workshop. Any conference focused on a given instrument, observatory, or technique would also count. I imagine that most if not all grad students have attended several such meetings.

There are a lot of variable that control the conference experience, but I want to focus on just one, namely number of attendees.  I have attended 2 conference with less than 40 attendees, 2 conferences with between 40 and 80 attendees, 3 conference in the 80 to 150 attendee range, and 2 conferences with more than 150 attendees.  Here is my ranking of conference size based on my experiences, from best to worst:

1. Less than 40:  This size encourages extensive interaction with the entire group.  The group is small enough that everyone knows everyone by the end of the week and there is ample opportunity to discuss the details of each person's research.  This size also forces a strong topical focus.
2. 80 to 150:  This size means that the meeting has some topical focus but is big enough to bring in a large cross-section of the sub-field.  With a group this size you are likely to have a few people whose research is extremely relevant to yours to talk to.
3. More than 150:  This size means that pretty much every major research group in your sub-field will be represented.  The major challenge is finding the people you want to talk to and actually finding time to have a meaningful conversation.
4. 40 to 80:  This size of conference comes in last because these are generally too large for an extremely focused meeting but not large enough to bring in a good cross section of the sub-field.

Of course my experience comes from a relatively small sub-field, so maybe the numbers don't transfer to other fields, but I am interested in the commentariat's views.

The Cosmic Sexiness Ladder

Nobody gets into astrophysics because they want to study the Sun.  Usually we start out wanting to find exoplanets or black holes, and then at some point get hooked into solar physics because there is such a wealth of data on the sun.  It turns out this effect can be quantified by measuring a quantity known as "cosmic sexiness", which is defined as "relative visceral appeal of different fields of astrophysics".  From Jeremy Drake, solar physicist:

He then notes that below the Sun would go atomic physics, followed by the weather.

Solar Physics Jobs In A Recession

Solar physics enjoys the blessing/curse of being a small sub-field of in the physics and astronomy community.  The Solar Physics Division of the American Astronomical Society (AAS) has roughly 600 registered members out of over 7,000 registered members of the AAS.  For comparison, the Division of Condensed Matter of the American Physical Society has over 4,000 members.  My point is the that is if the physics world is Europe, solar physics is like Latvia.

One of the advantages of being a small field is that it is possible to track almost all of the post-docs and potentially permanent positions without too much trouble. I have been doing this for the past few years using the AAS Job Register and positions listed in the Solar News.  Generally I would like to know what sort of job market I'm going to be jumping into in a year or two, but I'm also interested to see the effect of the recent economic difficulties on the job market.  So here are the results:

I've sorted the positions by locations (US or everywhere else) and into post-docs or potentially permanent positions, although the line between the two is often a bit fuzzy.  Essentially post-docs include anything that looked like it was designed for someone coming right out of grad school, while the permanent positions include anything that might become something long term.

In the US there have been roughly equal numbers of post-docs and long-term positions, meaning that on average one should expect to hold one post-doc before getting something long term.  Roughly 60% of the permanent positions, however, are either research positions at national labs or observatories, or support staff (e.g., programming, education/public outreach, etc.).  The situation in the US is also exacerbated by the situation in Europe where there are nearly three times as many post-docs each year as long-term positions.  This leads to a net migration of foreign post-docs into US permanent positions, for which I have only anecdotal evidence.

In looking at the graph, one doesn't see any clear indication of the current economic turmoil aside from the fact that 2010 looks like a less-than-stellar year in all categories.  This may be the result of two factors:  first, it may take several years to see the effect of the poor economy propagate through the state and federal governments and the larger university community before it hits physics departments directly; second, the $865 million Solar Dynamics Observatory was launched in February 2010, so there has been a build up of hiring in related research positions and post-docs over the past couple years, which may partially offset a recession-related dip in hiring.

The bottom line is that solar physics, like all fields of basic science, is a tough career choice.  There are a lot of very smart people vying for few ideal permanent positions.  But trying to get into the field is not the career Russian-roulette that is seen in some fields.

There Really Are Religious Scientists

When scientists make the news for something they say about religion, it often comes across as if all scientists are atheists or at least committed agnostics.  Stephen Hawking made waves when he stated that God isn't needed to explain the universe, and Richard Dawkins seems to constantly be in the news spouting off about the evils of religion and the glories of atheistic science.  From my department's roughly 50 graduate students I have heard maybe a dozen  disdainful tirades against religion but only two people (one of which was myself) openly profess any sort of religious affiliation.  It can seem, at times, that serious research science is a religion-free zone.

That's why I was fascinated to learn about a fellow named Eric Priest (holding the sun to the right).  Dr. Priest is an emeritus professor of mathematics at St. Andrews University in Scotland, a solar physicist, a winner of prizes from the American Astronomical Society and the Institute of Physics, a fellow of the Royal Society, and an honorary lecturer at Harvard, the University of Oslo, and a number of other places around the globe.  He's a serious scientist who has had a long and productive career at the forefront of his field.  He's also quite religious.

From a sermon he delivered at University Chapel at St. Andrews:

So should we trust science or God?  My answer is clearly both – but in different ways.  Science & Religion are much closer in approach than perhaps you realised.  We all need Science to learn more of nature God’s universe and to tackle problems of 21st century.  we are each on a journey of discovery in this life, in company of the communities of which we are part and with the guidance & support right at core of reality of  a God whose Holy Spirit cares for each one of us. 

So let us pray: 
Lord Jesus, we pray that you will continue to guide and inspire us, as we learn more of the nature of your incredible universe, and as we seek to follow you in our journeying all the days of our lives.  Amen

It warmed my heart to read that, not only for the sentiments but for the source.

Turbulence in Wavenumber Space

As Joe has been talking about the CMB in wavenumber or spherical harmonic space, I thought I'd bring up another area where it makes more sense to talk is wavenumber space than physical space:  turbulence.  Richard Feynman famously called turbulence "the last great unsolved problem in classical physics."

There are several reasons turbulence had boggled the brightest minds in physics, math, and engineering for over a century.  Physically turbulence extends over many length scales - think of a waterfall for example.  All of the kinetic energy gained from the fall must go somewhere and it turns out that somewhere is heat (and sound, but mostly heat).  But to turn kinetic energy in a fluid like water into heat, one needs viscosity.  In a waterfall, viscosity is effective at dissipating heat through motions on the order of 1 micron.  So to understand the turbulence in a waterfall that is something like 10 meters high one needs to understand every micron of the way.  On top of that turbulence is chaotic (in the technical sense of the word), meaning that it is essentially random and unpredictable.  As an example, take this visualization of jet of fluid entering a super-sonic flow.

There are other reasons turbulence is a really hard problem, but it turns out that what we call the "range of scales" problem is where thinking in terms of sizes makes more sense than thinking in terms of physical position.  For you math-junkies out there, that means an integral transform to either Fourier space (for things in boxes) or spherical harmonic space (for spheres).  Either way, when you compute the amount of power at each size-scale in the flow, you get a plot that looks like this for the turbulent magnetic field in the solar wind:

...or this for water in tidal channels:

 ... or this for simulations of solar convection:

Here are three different materials, three different temperature and density regimes, and even a collision-less plasma just for fun.  All are doing different things on large scales (small wavenumbers) and the very smallest scales (large wavenumbers), but in between all of them show a fall-off proportional to wavenumber to the negative five-thirds power.  In fact it's nearly universal - energy cascades from large scales to small scales the same way in all turbulent flows.  So a process that is chaotic, random, and unimaginably complex in physical space is really very orderly in wavenumber space.

Spots in Southern California, Part 3

How do you build a sun spot?  Previously one needed a sun, which is very inconvenient in terms of logistics and safety issues.  It turns out that now you can build one with just an amazing computer code and a large supercomputer, which is much more convenient and it runs out pretty darn effective.  Below are a real and a simulation sunspot.  I'll let you figure out which is which.

In a truly groundbreaking simulation, Matthias Rempel of the National Center for Atmospheric Research here in Boulder has created a realistic simulation of a sunspot that appears to correctly reproduce almost all of a the observed features of real sunspots.

This is the kind of numerical model most of us computational scientists dream about at night.

The Awakening Sun Might Keep 2012 Theorists Happy.

I find it interesting that NASA is warning there may be "unprecedented levels of magnetic energy from solar flares after the Sun wakes “from a deep slumber” sometime around 2013" and that furthermore

Scientists believe it could damage everything from emergency services’ systems, hospital equipment, banking systems and air traffic control devices, through to “everyday” items such as home computers, iPods and Sat Navs.

Due to humans’ heavy reliance on electronic devices, which are sensitive to magnetic energy, the storm could leave a multi-billion pound damage bill and “potentially devastating” problems for governments.

 This should keep people concerned about December 23, 2012 (eerily close to 2013) happy.

So Nick, should we be scared?  Is the sun going to awaken and destroy human civilization?  Do we quickly need to make another Inconvenient Truth movie now about how if we all own too many electronic devices the sun might destroy society?

Say Hello to SDO

The Solar Dynamics Observatory was launched on February 11th of this year and is now officially online, taking pretty pictures, and releasing data. It will provide images of the entire sun at 500 mile resolution every 10 seconds and stream all of that data (close to 2 TB per day) down to earth. How to deal with 2 TB of data per day is another issue entirely.

I'm going to be posting more about NASA's latest solar observatory, but for now, say hello to SDO and great images of the sun (click for full resolution).

Getting a Job in a Small Subfield

As most of you know I am an aspiring solar physicist. Solar physics is a small sub-field of physics, especially compared to things like condensed matter or cosmology. To give you an idea of just how small it is there are only 58 junior members of the solar physics division of the AAS, which means that there are at most 100 graduate students in solar physics. I don't know how many grad student cosmologists there are out there but it seems like there are about 58 pre-prints published in cosmology every day.

Small fields like solar physics allow for a more congenial and casual atmosphere in some respects since pretty much anybody that's been around for a while knows pretty much everybody in the field. Due to the small size, solar physics has developed it's own online newsletter - the "Solar News" - that anyone can submit information to, including job openings. That means that pretty much every job that comes up in solar physics from NSF section chief to post-doc at Western Montana A&M goes through the Solar News.

Out of curiosity I went through the Solar News archive and figured out the number of jobs each year in solar physics from 2005 to 2009. To start I only included the ones in the US and I sorted those into three groups: post-docs, research positions (temporary and tenure-track), and tenure-track faculty positions. Behold the graph:
Over those five years, there were an average of 17 post-docs, 11 research positions, and 5 faculty positions available each year. If we further assume that a third of those 58 junior members of the solar physics division graduate each year (this is probably high but let's run with it anyway), that means that on average there are 19 PhD's minted each year and for those 19 people, there will be 17 post-docs available. And when those 17 post-docs are looking for potentially-permanent positions there will be 11 research and 5 faculty positions waiting for them. That means that on average 84% of those that graduate in solar physics will keep doing research in solar physics - and this excludes those that get jobs in industry or in primarily teaching roles. That's not bad at all.

However there is one other factor. Europe and Asia have become major players in solar physics. I am unable to find data on how many PhD's they produce each year, but I do have data on how many post-doc and potentially-permanent positions they advertise each year. Behold graph #2:
Foreign countries produce a lot of post-docs, but a comparably small number of permanent positions. That means that there is a large influx of solar physicists with a post-doc or two under their belts into the market for long-term positions in the US.

Overall, however, the job outlook in solar physics is quite rosy. There are few people competing for few jobs, which tends to work out pretty well.

The Sun Wakes Up

Many of you have heard about the current solar minimum being longer than normal. I've blogged about it here. Well it turns out the sun is not quite dead yet:
Yes after a long winter's nap, the sleepy sun has risen again. In fact if you look at the website for NOAA's Space Weather Prediction Center, you can see that solar cycle 24 is finally getting rolling:

Physics Spotlights Turbulent Convection

If you're not familiar with the American Physical Society's online review Physics, you should take a look right now. What is Physics? From the APS:

"Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. The Physical Review journals alone published over 18,000 papers last year. How can an active researcher stay informed about the most important developments in physics?

Physics highlights exceptional papers from the Physical Review journals. To accomplish this, Physics features expert commentaries written by active researchers who are asked to explain the results to physicists in other subfields. These commissioned articles are edited for clarity and readability across fields and are accompanied by explanatory illustrations."

In other words, Physics is the cliff notes version of the best new research being done across all of physics. It's like a 5 minute version of a colloquium, without the speaker playing with his or her microphone.

And why do I bring this up now? Because the latest issue features a review of current issues in turbulent convection, a topic near and dear to my heart. And they used a great picture of convective cells on the solar surface (at right). Hooray for Physics!

The Research Doldrums

Back when nautical vessels were 100% wind powered, there was a saying that what sailors feared more than storms were calms - times when the wind simply refused to blow, leaving the ship effectively stranded. Near the equator there is a region called the doldrums where strong solar heating of the earth's surface creates strong vertical motions that tend to stifle horizontal winds, leaving ships stuck.

Anyone who has been in basic science research for a while can tell you that research is a little like sailing in a wooden sailboat on the wide oceans. Sometimes you discover the New World, sometimes you get raided by pirates, and sometimes you get stuck in the doldrums, baking under a hot sun, waiting for the winds to pick up and get you moving again. And although modern researchers have more options than becalmed 17th century sailors, sometimes it doesn't seem that way.

I am currently stuck in the doldrums. I am trying to improve our models of the sun by improving the way we model physics at small-scales. This falls under the long-standing problem of how to model turbulence, which has an annoying tendency to take large scale motions, break those motions into increasingly smaller scale motions, and finally dissipate the kinetic energy in small scale motions via viscosity. It's a hard problem and I'm not trying to solve it so much as use what little insight others have gained to improve our models of the sun.

I have two so-called turbulence models that should do this, but when I implement them into our code sadness ensues. One of these models seems to upset the fundamental balances of how energy is transported outwards in the solar convection zone and defies all my attempts to understand why it does so. The other appears to be extremely computationally expensive making it more work than it is worth. I have been working on these two problems for several months now, but every time I make a little progress and I feel a slight breeze and begin to think the wind might be picking up, it dies down again. And so I remain stranded on my little boat of research, praying for some wind.

Ironically, the sun remains in the doldrums as well with almost no magnetic activity. Perhaps it can't get its small-scale turbulence models to work either.

Where Have All the Sunspots Gone?

If you look at the sun today (or let a hundred million dollar satellite look at it for you) you'll see something like this:
In fact if you would have looked at the sun almost anytime in the past 6 months, you would have seen that same thing. Why is that worthy of a blog post? Because there are no spots. In fact as of right now there hasn't been a sunspot for 5 days. If you look at a plot of recent sunspot activity, like the one below, you'll see that there hasn't been much happening on the sun for a while now. It's expected that we won't have a lot of sunspots right now because we are currently in solar minimum (the bottom of the solar cycle).

However, if you compare the number of sun spot-less days in the current solar minimum with the last one in 1996, it is easy to see (if you click of the graph, sorry for the small size) that something odd is going on this time around.For some reason in this solar minimum there are far fewer sun spots than last time. In fact, this is shaping up to be the weakest solar cycle since the 1920's. What would be really exciting, however, would be if the solar cycle actually shut off for a while as it appears to have done in the late 1600s. For those of us in the solar dynamo community, it's fun to have a little bit of variety in our lives even if that variety means nothing is happening.

Solar Simulations Rock New York

For the past year our research group has been working with the folks at the American Museum of Natural History in New York on their latest show Journey to the Stars which looks at stars from the Big Bang to the future end of our solar own sun. Ben Brown, my office-mate, did most of the work on our end and our simulations ended up producing about 3 minutes of the 25 minute show.

Journey to the Stars premiered last week in New York and was reviewed by both Scientific American and the New York Times. Scientific American's review has us particularly excited. They said:

"One highlight is a simulation of the interior of the sun, showing its convection and churning magnetic field. The demo came courtesy of Juri Toomre's group at the University of Colorado at Boulder, and required about 14 million hours of supercomputer time spread across four major U.S. supercomputing centers. Hundreds of billions of bytes of data were processed, all of which went into the visualization of the solar interior."

We'd love to make all of our science this accessible to the public, but even the 25 minutes of this show cost millions of dollars to produce. Hopefully, though, it will make its way to many other planetariums and help the public understand a little more of the wonder of astrophysics and the practical reasons to spend billions of dollars studying the sun.

And if your in New York, stop by and take a look. I hear it is a great show and they go to great pains to use real data/simulations and top-notch science as much as possible. You can watch the trailer or buy tickets here.

Solar Cycles and Global Climate

Ryan previously made reference to an idea I get asked about a lot, so I thought I'd leave a quick post with my two cents on the matter. While I am a solar physicist, I do not work on issues of solar irradiance or even solar changes to space weather, so I am not an expert in this exact field. However, I do work on dynamo models that explore the sun's magnetic behavior over hundreds of year. Basically, I am not the best person to ask about this, but I'm familiar with what is going on in the field.

So here's the question: is the sun causing global warming? And here's the quick answer: no, at least not in the past 50 years. Here's a more detailed response:

The sun's magnetic activity does appear to impact the Earth's climate. Historical records indicate that a lull in solar activity called the Maunder minimum corresponded to a very cold period (especially in Europe) from 1645 to 1715 AD. The exact mechanism linking solar activity and climate is not well understood because the variations in total solar luminosity are extremely small (less than 0.1%), which rules out direct effects. However, there are several indirect effects that may drive climate changes including modifications to upper-atmospheric chemistry and increased cosmic ray fluxes changing cloud formation reates. Whatever the mechanism, the sun appears to drive changes of about +/- 0.5 degrees C. But that cannot account for the rapid change in temperatures in the last 50 years. And here's a figure that shows it:
This figure, compiled by the good people at NASA-Marshall's Solar Physics Group shows some correlation between solar activity and temperature, but the large spike in temperature since 1950 does not have a corresponding increase in solar activity. So while the sun might cause something on the order to a 0.5 degree change in global average temperature, it does not explain the current warming trend. Atmospheric CO2 concentration does a far better job matching the warming trend. So while the sun is a player in our climate, it is not the dominate agent of change right now.

As a side note, these solar forcing effects are just one more set of parameters that get fed into climate models, providing more poorly constrained parameters to fiddle with.

Walking on the Sun... Almost

Today NASA and ESA (the European Space Agency) announced that they are sending a probe to the sun. And to generate excitement they've made a PRESS RELEASE!!!!!, complete with an actual photo taken by a photographer who went along for the ride in the future and then sent his phot back via time travel. Go NASA/ESA!

In some ways it's amazing that humanity has never before sent a mission to the dominant object in our solar system, but on the other hand the sun isn't exactly hard to observe from, say, your window. In many ways the sun actually gets harder to observe the closer you get to it because (a) it's big and (b) it's really hot. However in as little as 5 years, there may be a nice little man-made box orbiting the sun inside the orbit of Mercury. This bold but lonely piece of hardware has been named SOLO (SOLar Orbiter).

So what's the big deal about going to the sun? We can see it quite well from our safe orbit here on Earth, we can probe its interior with helioseismology, and we can even sample it because it's constantly spitting plasma at us in the solar wind. Well it turns out that one of the big reasons to go to the sun is that we really only see part of the sun. We have good observations for only a band around the solar equator extending to about 60 degrees north and south. On the Earth that would mean we miss the poles and as a friend who visited the South Pole Telescope will tell you, the poles are very, very different places than the rest of the planet. In addition our simulations of the solar interior seem to indicate that the polar regions on the sun may exhibit different convective patterns. So I say go to the sun and step on it.

Sunspots

I have had a number of questions about sun spots, so instead of putting them in comments, I thought I'd give a brief overview in a post. Feel free to ask any questions you may still have in comments.

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The life of a sunspot begins deep in the solar convection zone. Here tubes of strong magnetic field are generated by the dynamo processes. Examples of such magnetic structures appear in our simulations (although not yet our simulations of the sun - this one comes from a solar mass star spinning 3 times faster - but we're getting there), as seen here in a 3-D visualization that I made for the San Diego Supercomputing Center's 2008 calender. The blue and yellow-red bands are two loops of magnetic field in the middle of the convection zone. The fact that they even exist in the middle of violently turbulent convection is amazing - but that is another post. For right now, they are remarkable because they are regions of strong (as much as ~2 Tesla) magnetic field.
Magnetic fields produce pressure in the fluid proportional to the square of the magnetic field. This causes the regions of extremely strong field to expand - thereby becoming less dense than the surrounding fluid. This results in the regions of strong field becoming buoyant and rising. Occasionally, one of these tubes of magnetic field lines makes it all the way through the convection zone and manages to rise out of the photosphere - the solar surface.

In plasmas, the fluid and the magnetic field are stuck together. In most of the sun, fluid forces are much greater than magnetic forces, so the fluid ends up dragging the magnetic field around. In areas of strong magnetic field, however, the magnetic forces dominate and so the fluid can no longer push the magnetic fields around. This means that in a sunspot, the fluid in the spot can no longer mix effectively with the fluid outside of the sunspot. This causes the fluid to cool (and become darker) as it is radiating all of its heat out into space without getting much from the hot plasma around or below it. Thus the sunspot is an indirect effect of the magnetic field looping in and out of the photosphere.

A great picture of a sunspot can be found below. This was taken by the Swedish Solar Telescope, which uses adaptive optics to get really amazing pictures. As you can see, the story I'm telling is a very simplified one, but it is essentially true. With convection turned off by the strong magnetic fields, the only way to get heat into a sunspot is via conduction - a much less efficient process near the solar surface than convection. Conduction , in part, causes the smearing that appears near the edge of sunspots. Solar flares and coronal mass ejections occur when the magnetic field sticks too far out of the solar surface in a upside-down U shape. At some point, the bottom ends of the U get too close together and "reconnect". This reconnection leaves a smaller U shape and a closed loop of magnetic field floating above the solar surface. This closed loop quickly decays, pumping all of the energy stored in the magnetic field into heat which essentially causes a massive explosion releasing magnetic energy as kinetic energy. This explosion can shoot huge amounts of x-rays and 10 million degree plasma into space. Occasionally, we happen to be unlucky enough to get in the way.

Sunspots always obey a few rules:
1) They can never appear alone. In the image above, there is an entire group of spots, which is quite common. But for every field line that exits the solar surface there must be a field line entering the solar surface. This means that sunspots dissapear together - even when there are explosive events like reconnection.
2)For reasons we still don't understand, for each 11 year solar cycle, all of the sunspots in northern hemisphere appear with the same leading polarity and all sunspots in the southern hemisphere will have the opposite polarity in the leading spot. For example, in the current solar cycle, all of the leading spots have the magnetic field pointing out of the surface and the trailing spots have the field pointing into the surface. Every 11 years, that polarity switches.
3)Most sunspots do not causes flares or coronal mass ejections - they simply fade away as the magnetic field slowly diffuses outward.

I hope that answers some questions. Please feel free to ask any more in the comments.

The Solar Dynamo

This is part two of my series of posts on the exciting field of solar physics. For part one, click here.

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Despite centuries of study, we fundamentally don't understand how the sun generates its magnetic field and magnetic cycles of activity, but we do have some ideas. Here's what we know about the way the sun (and by extension other stars) produces and varies its magnetic field.

The source of the sun's magnetism is some kind of dynamo process. Dynamos occur when a highly conducting material shears against itself in the presence of a magnetic field. There are several requirements for this to occur. First, the material must be a very good conductor, such as the ionized plasma in the solar interior or the liquid iron in the earth's core. A good conductor means that charge carrying particles are essentially free to move through the material. When a free, charged particle encounters a magnetic field line, it will begin to move along that field line in a helix pattern. The magnetic field forces the fluid to move along it, while the fluid circling the field line creates a current that reinforces the magnetic field. This is referred to as the frozen in condition because the fluid can only flow along magnetic field lines and the magnetic field lines are continually regenerated by the motion of the fluid. In non-superconducting materials, like the solar interior, this is an imperfect process and it is not strictly true, howeverthe frozen in condition is still a good approximation.

In the solar convection zone, magnetic fields exist in the middle of extremely turbulent convection. When the convective motions cause motion of the fluid along a magnetic field line, they stretch the field line, much like taffy stretches when you pull it. This stretching puts energy into the magnetic field, causing it to grow stronger. The solar interior is therefore one gigantic, fusion-powered taffy pull which constantly regenerates the sun's magnetic field.

I should also mention that dynamo processes are inherently non-linear. The "taffy pull" effect requires advection, which mathematically comes in the form of the gradient of the velocity squared. That term (and a couple other non-linearities) cause me to periodically wake up at night in a cold sweat. Because of the non-linear properties, dynamos are both generally chaotic and almost impossible to work with analytically (although some brave people like Matthias Rempel at the National Center for Atmospheric Research try anyway). This means that almost all theoretical work must be done numerically.

So a dynamo seems like a nice theoretical construct for the source of the sun's magnetic field, but is that actually what is going on? And what about those cycles of magnetic activity? Can a dynamo explain that butterfly diagram from the last post? Tune in to my next post and we'll talk about how we investigate what is actually happening inside the sun.