ComSciCon and Magnetic Fields

I can’t believe it is almost three months since my last post. Between the holidays, finishing up a project at work, and January-term classes and workshops, it’s been rather busy here. One of the events during this time was the Communicating Science Workshop, local edition. We met with an incredible array of experts in various fields, including (but not limited to) Scot Osterweil, creator of the Zoombinis games and a personal hero of mine; Thomas Levenson, MIT Professor, science writer, and documentary film-maker; and David Aguilar who may actually be the real “most interesting man in the world!”

Besides these star-studded panels, we also had to complete a short writing piece about our field aimed for a general audience. After peer review and a very helpful set of edits from Michael Fisher (Harvard University Press), I wanted to share my writing from the ComSciCon Write-A-Thon. My target publication was the Harvard GSAS Alumni magazine, whose deadline was unexpectedly changed before I was able to submit the final version. I view this as an opportunity to share it with a wider audience (you, dear readers), but I wanted to convey that it was written with a specific prompt in mind: “What surprising, innovative, or unexpected contribution will your field make to explaining, shaping, or solving a problem faced by society this century?” (Word limit was 500 words, which I hit perfectly!) So here it is:

The Sun is surrounded by a network of invisible magnetic forces that help to form dangerous storms in space. However, the Sun appears inactive in the sky, just as a bar magnet sitting on a table seems inert. We can map out the magnetic field around a bar magnet by simply scattering iron filings around it and watch as they line up in a pattern of lines that trace the hidden forces at work. This experiment is easy enough to do at home, but how can we learn about fields on grander scales?

During the least active part of its cycle, the Sun’s magnetic field acts like that of a bar magnet, but often the field structure is not as simple. The magnetic field of the Sun affects the material around it, altering the flow of particles away from it (the solar wind) and creating beautiful loops that we can view in short-wavelength images. However, the magnetic field also accelerates material that can slam against Earth’s protective shell called the magnetosphere, affecting communications satellites and causing power grid failure. Mapping the magnetic field structure near the Sun is essential to the modeling and prediction of when the Sun will emit these potentially dangerous events, including flares, coronal mass ejections, or high-speed solar wind streams towards Earth.

Since there aren’t enough iron filings on our planet for a map of the magnetic field between the Sun and the Earth, scientists are developing satellites that can fly closer and closer to the Sun. Helios 2 set the current record in April 1976, when it travelled over two thirds of the 93 million miles between the Earth and the Sun. Scheduled to launch in 2018, a satellite called Solar Probe Plus (SPP) will smash Helios 2’s record. SPP includes instruments being built at the Harvard-Smithsonian Center for Astrophysics for a NASA mission to make a detailed map of the Sun’s magnetic environment. Six years after launch, SPP will reach its final stable orbit that puts it 96% of the distance to the Sun (it will be only 3.7 million miles from the Sun). As it carefully maps out the magnetic fields within the Sun’s upper atmosphere, the spacecraft will withstand temperatures in excess of 2,500 degrees Fahrenheit.

Maps from SPP of the Sun’s magnetic field will be crucial to prediction models like the one I have been working on for my Ph.D in Harvard’s astronomy department. I have written a program to decipher how different magnetic field structures at the Sun affect the properties of the solar wind that flows past the Earth. The only input my code requires is the magnetic field strength at different heights above the Sun’s surface. Accurate magnetic field measurements are essential for successful wind speed predictions. Computer modeling programs like mine will use SPP data to move beyond approximations and extrapolations of magnetic fields and will use real-time measurements to provide advanced warning necessary for protection against the dangerous events produced by the Sun.

“Laughter is the sun that drives winter from the human face.”
Victor Hugo

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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.

For more information, check out:

[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

Sphere of Influence

It’s an exciting time right now, when humans have finally made it to interstellar space. If you haven’t heard, Voyager 1 has exited the heliosphere. And this time we mean it!

http://xkcd.com/1189/

Seriously, though, the news has been all over the internet lauding this most recent confirmation that Voyager 1 has made it out (see JPL Press Release). The real question, however, is out of what? Put simply, the discovery being presented is that the plasma densities that Voyager 1 is measuring have increased significantly, suggesting it has moved out of the sphere of influence created by the constant outflow from the Sun we call the Solar Wind. This outflow creates a cozy little bubble called the heliosphere, but this is only one way to measure the extent of the Sun’s influence.

http://news.discovery.com/space/voyager-1-flys-into-a-mystery-magnetic-highway-121203.htm

Artist’s conception of the location of Voyager 1 and 2 from December 2012

When the Solar wind slows down because of the increased pressure of the interstellar medium, it sets up a termination shock as it decelerates past the speed of sound (think of this as a sonic boom, backwards). There is then a region of the outer heliosphere called the heliosheath, and the idea right now is that Voyager 1 has made it out of this region.

“But wait!” you exclaim. “That’s just a boundary based on pressure and density! What about gravity?” That’s a very astute observation. To put everything into perspective, I’ll call on the graphic design skills of NASA/JPL-Caltech professionals…

http://www.nasa.gov/mission_pages/voyager/voyager20130912f.html#.UjNSuaUUyMV

A schematic of the key components of the solar system. Please note: the scale on this graphic is logarithmic, not linear

You’re likely familiar with the eight (not nine!) planets of the Solar System and the fact that they orbit the Sun because they are gravitationally bound (if this second point is troublesome, perhaps review this wikipedia article) What you may be less aware of are the other main sources of material in our little neighborhood.

  • The asteroid belt, containing dwarf planet Ceres, lies between Mars and Jupiter.
  • The Kuiper belt, containing among other things dwarf planets Pluto and Eris, lies just beyond the orbit of Neptune.
  • The Oort Cloud best represents the extent of the Sun’s gravitational influence, as it is a spherical cloud extending from a few thousand AU from the Sun to a few hundred thousand AU.

The Oort Cloud is the source of most of the long-period comets we see coming through the inner solar system. Voyager 1 and 2 will be long dead before their trajectories bring them anywhere close to this “edge of the Solar System,” so it isn’t surprising that as a species we’d prefer to take the smaller victory of making it out of the heliosphere.

Finally, if we consider the Sun’s furthest possible influence, we would have to travel five billion light-years away, where the first rays of light emitted by our newborn star are currently streaming outward at the speed of light. Five billion light-years is equivalent to 2.9×10^22 miles i.e. 29,000,000,000,000,000,000,000 miles (for the curious, this would be 29 sextillion miles in America or 47 trilliard kilometers in France). This certainly puts our accomplishments with the Voyagers into perspective. To track these satellites real-time, check out http://voyager.jpl.nasa.gov/where/.

…and knowing what it had to tell of the night passing swiftly on
toward further darknesses, but moving also toward a new sun.
Fahrenheit 451, Ray Bradbury

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Hello world!

As I write this, I am still working out the likelihood I will be able to post regular updates. This is mostly due to the fact that I have never blogged before (we will conveniently ignore the xanga page I had in middle school), and the only pen-and-paper journal I have been able to keep for longer than two weeks is one of the five-year one-sentence-per-day versions.

However, I do know that this is a reasonable place to collect my thoughts and gather yours. If you are an astronomer, I’d like to know if there is any specific aspect of solar physics that has helped you in your research or provided progress in your field. If you are not an astronomer, rather more probable, than I am more than happy to answer any questions you have about any topic in astronomy; just leave a comment on this post or use the following contact form to send me an email.

There is more day to dawn. The sun is but a morning star.
Walden, Henry David Thoreau