Solar Flares

One of the things that our department is urging us to do is to write descriptions of our current research more.  This is so people can figure out what our department does easier.  Also, web pages tend to get pretty stale with no new updates.  I figured that one of the best times for me to do this type of thing is when annual reports are due.  I have many different projects (which is another post), so I should be able to do this many times a year.  Sadly, it seems like all of me grants start at one of two times – sometime around the start of summer or sometime near the end of summer.  I therefore have to write a lot of annual reports around then.  I just turned in a report about solar flares and what we have been working on with respect to them.

First, I should discuss what a solar flare is.  Since I am not a solar physicist, I will probably get this all wrong, but since you are not a solar physicist either, you probably don’t care.  If you were to look at the sun and measure its brightness, you would see that it is pretty much constant.  The amount of energy coming from the sun (in terms of overall illumination) doesn’t really change at all.  But, if you look in specific wavelengths, specifically X-Rays and Extreme Ultraviolet wavelengths, the sun can change pretty dramatically.  This is because the X-Rays and EUV radiation is emitted from a pretty hot part of the sun that is changing pretty over the 11-year solar cycle.  The solar corona has lot of magnetic fields, and when those magnetic fields get all twisted up, they can store a lot of energy.  When the magnetic field start to rapidly release that energy, a flare can result.  The brightness of the sun in X-Rays and EUV can increase by orders of magnitude.

Second, I should discuss why you should give a crap.  You probably have heard of sunscreen, right?  Well, that has nothing to do with solar flares.  But, you really should wear sunscreen when going outside in the sun. Sun screen protects you from UV wavelengths. The wavelengths that I am talking about are much shorter.  In fact, the brightness of the sun in these wavelengths can’t even be measured from the ground, because they are all absorbed in the upper atmosphere.  Why should you care, then?  Well, because they are absorbed in the upper atmosphere, we have an ionosphere and a thermosphere.  This region of the atmosphere (above about 100 km, or 60 miles) is very hot because of the absorption of this solar energy.  Solar flares add a lot of extra energy into the atmosphere very quickly.  This causes the (upper) atmosphere to heat up, which can increase drag on satellites, and can cause the ionosphere to go crazy for a few hours to a day.  The ionosphere is very important for people because pilots use the ionosphere to talk to people over the horizon!  High frequency radio signals bounce off the ionosphere, so they can travel significantly further than (for example) a cell phone signal. When you are an airplane over the ocean, and you can’t see the shore, there is a chance that the pilots are relying on the ionosphere so they can talk to someone on land.  Solar flares can screw this up.

Now, what do I have to do with this?  That is an excellent question! We started this grant a couple of years ago, just before one of my students graduated.  One of the main topics of his thesis was solar flares.  He figured out a few very cool things about flares – one of which is that the flare heats up the dayside a lot and this heating causes a pressure bulge which propagates from the dayside to the nightside.  Imagine dropping a stone in a tub of water.  The stone makes a big splash, which creates a wave that propagates away from the rock.  It hits the edge of the tub and reflects back towards the rock.  If there were not very much viscosity, you would see that there would be a big bulge where the waves all come back together.  This happens on the Earth also.  On the nightside, the waves all come back together (around midnight near the equator), and you get a big bulge.  They then pass through each other and continue on back towards the dayside.  On the way, they fade a fair bit and you almost don’t see much of a disturbance on the dayside. 

My new student is starting to learn how to run our model. She has run a certain flare (February 15, 2011) about 30 times.  Each time, she has changed something in the model to see what effect it has on the reaction of the thermosphere and ionosphere to the flare.  Eventually, she turned just about everything that might cause interesting stuff off.  For example, the magnetic field of the Earth is not very uniform and it is not aligned with the North-South axis of the Earth.  She put in a simple model of the magnetic field instead.  She then caused the aurora to be constant with time and the high latitude electric fields to be constant.  She turned off tides and the low latitude electric fields.  Once she did all of these things, we noticed that the model was misbehaving.  Every day at exactly midnight, the entire atmosphere went nuts.  I helped her to try to investigate why this was occurring, but after a few weeks, it became clear that this was something that I needed to look at.  What I found was a very simple error that wouldn’t really be called a bug at all.  The model that we are using is driven at the lower boundary by a statistical model (the lower thermosphere at around 100 km doesn’t change much on a day to day basis, so this is ok to do).  What I figured out is that this model has a day of year dependence, but the day of year is an integer (obviously, right?)  So, when the day of year changes by 1, the sin or cos of that changes by just a bit (but VERY suddenly).  This means that the temperature and density can change just a bit right at the start of the day.  Since the model allows acoustic waves (sound waves), any sudden change in the density and/or temperature can cause very dramatic wave activity in the atmosphere. What I ended up doing is changing the day of year variable from an integer to a float.  Now, when it changes from day 1 to day 2, it actually goes from something like 1.9999 to 2.0000, which is a very gradual change.

Once we got the model to work properly, my student started looking at how the thermosphere and ionosphere reacts to the flare.  She is looking at the dayside and nightside.  She turned off a bunch of stuff and added them back one by one.  She sees that the reaction to the flare on the dayside doesn’t really depend on anything but the flare.  On the nightside, there are huge differences between the different runs.  Around the polar regions, the reaction seems to strongly controlled only by the flare, but it does change depending on the other drivers.  This is one of the interesting regions of study that we are working on.  More runs!  More flares!

That is what we have been working on over the last many months.

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About aaronridley

Professor at the University of Michigan, Department of Climate and Space Science and Engineering.
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One Response to Solar Flares

  1. Pingback: Solar Flares « Aaron Ridley's Blog | Solar Flare 2012

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