Types of Aurora

By Allison Jaynes

aurora_magnetosphere

Discrete auroral arcs. Image courtesy: http://webhost.ischool.uw.edu/~drwilson/

The aurora represents a great visual tool for scientists to use in the study of the space environment. That’s one reason why the data that Aurorasaurus citizen scientists provide is so valuable! Surveying auroral emissions is a bit like looking at a giant television screen; the picture can help scientists figure out what is happening with energetic particles, and electromagnetic fields, from just above the Earth to far out in surrounding space. But the processes are so complex and little understood that we still don’t know, for example, exactly what causes discrete auroral arcs such as the one shown above. Disappointing? Not to us science geeks! We love the challenge and the mystery – it’s what keeps life and work interesting. For now, though, let’s talk about the space around our planet and some types of aurora that we can characterize better than the beautiful and dynamic arcs.

The magnetosphere

The Earth resides inside a distorted ‘bubble’ or ‘shell’ of magnetic field that is shaped directly by the constant buffeting of the solar wind. This windsock shape has a tail that points in the direction opposite the sun, and the entire structure is called the Earth’s magnetosphere. While this term may sound very sci-fi, the magnetosphere is extremely valuable to life on Earth: it’s what keeps us protected from the uninterrupted (and sometimes extreme) stream of charged solar wind particles that heads towards our little planet at an average speed of 400 km/s.

cartoon_magnetosphere

Cartoon of Earth’s magnetosphere. The left side of the figure is closest to the sun, with the tail of the ‘windsock’ extending away from the sun. Image courtesy of nasa.gov.

These particles can gain access into our magnetosphere, mostly through the back door, down in the tail region. Once inside, they can be energized and hurled back towards Earth during events called substorms. The visual evidence of this substorm activity is revealed as the dancing lights of the aurora. As mentioned in a previous Aurorasaurus science post, these particles are directed along magnetic field lines and primarily end up causing aurora closer the Earth’s magnetic poles in a region dubbed the auroral oval. When geomagnetic activity is enhanced, however, this oval can expand and become visible to residents at lower latitudes. The picture is further complicated by the introduction of parallel electric fields, which are required to accelerate the auroral particles to the higher energies we observe. The magnetosphere and ionosphere are full of electric fields: some are steady over time but many are transient, making it very difficult to get a global sense of the electric fields that are present at any one time.

When substorms erupt, the sequence can follow a well-defined pattern that aurora-watchers may become familiar with after many nights of observing. This pattern includes formations of discrete arcs (such as is shown in Figure 1), equatorward expansion of arcs (seen as the bands moving south in the sky), and finally, if one is lucky, a rush of the arc in a poleward direction and a beautiful breakup aurora to follow. Breakup aurora can include a dynamic and oftentimes colorful display of rays, curtains, and swirling arcs. These phases can be observed through various means on the ground by looking at allsky cameras and ground magnetometer signals from varying geographic locations. And parts of this sequence can also be observed using satellites orbiting the Earth and taking data within our magnetosphere. The substorm community can be a contentious one, however, since rarely do two substorms look alike and the particle and field measurements taken in space can sometimes be contradictory from one event to the next! In fact, the entire process remains fairly mysterious and the exact mechanisms that contribute to the auroral acceleration region are largely unknown. The world space agencies have maintained a suite of magnetospheric satellites over the years, including the THEMIS, Cluster, Polar, FAST, Akebono and Van Allen Probes missions. These efforts have greatly elevated our understanding of the aurora and it’s magnetospheric signatures, but there is still so much more to learn.

Pulsating aurora

There are types of aurora that we know more about, like where they come from and what exactly causes them. One of those types is pulsating aurora, a special type of diffuse aurora that occurs in patches and turns on and off (or pulsates) periodically. Although the pulsating aurora mechanism had been theorized back in the 1970’s, it has only been confirmed with observational data recently.

Essentially, electromagnetic waves in space of a certain type can affect the particle’s motion along the magnetic field line – if that motion gets changed enough, the particle will be ‘lost’ to the atmosphere when it comes close to Earth near the poles, and become what we see as aurora. This sequence happens over and over again: the waves dump particles into the atmosphere (turning “on” the aurora), the waves lose their energy and stop dumping particles (turning “off” the aurora), and finally the waves gain energy again as new particles drift into the area and the cycle begins again. This cyclical process gives us pulsating aurora, with astonishing effects at times.

The video clip above is of a particularly mesmerizing episode of pulsating aurora (sped up to many times the original rate), taken by a white-light all sky camera, which has a wide view that can see almost from horizon to horizon in all directions. Imagine looking at this overhead as you stand outside in the frozen northern tundra. The whole show is caused by electromagnetic waves affecting electrons that are 26,000 miles (or over 6 Earth radii) away from where you are standing! Next time you spot the aurora, take a second to think about all the processes that happen far away in the Earth’s magnetosphere that come together and create the natural light show that inspires such awe.

**

Allison Jaynes is a scientist in the space physics group at the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder. She’s marveled over a great aurora in Kaktovik, Alaska, the best one she’s ever seen. She analyzes energetic particle data from NASA’s Van Allen Probes mission and has participated in three sounding rocket missions to study the aurora. She is part of the Aurorasaurus Scientist Network.

 

Share this post by clicking an icon below!
Share on Google+Share on TumblrShare on FacebookEmail this to someoneShare on RedditTweet about this on Twitter

Leave a Reply

Your email address will not be published. Required fields are marked *