What Is Discrete Aurora? (It’s Not Discreet!)

Bright discrete, and dim diffuse aurora

Photo by Donna Lach showing discrete and diffuse aurora together, taken facing east-northeast at about 6:30 am on November 4, 2021 near Plumas, Manitoba. Note that the discrete aurora on the left of the photo shows vertical, fine-line “filament” structures and tall rays, while a dimmer, cloudlike, diffuse aurora is visible on the right of the photo.

While “discreet” means something that is a little bit secretive or unobtrusive, “discrete” auroras are distinct, bright, narrow bands—most commonly, photos of auroras are of this type. They typically have a definite lower border and can stretch high into the sky, like curtains, when viewed from the side. From below they are very narrow. They can wave slowly or race across the sky.. They are broadest, brightest, and/or most active around midnight local time. (By contrast, “diffuse” aurora usually have little motion, are quite dim, and might even be confused with clouds.)

In this post, we’ll talk from a Western science perspective (which is one of many) about some of the characteristics of these showy, dancing ribbons in the sky. 

How do we know about discrete aurora?

Want to learn more? Check out our Journal Club reading video of the classic article “Observed Characteristics of Auroral Forms” by T. Neil Davis

People have observed the aurora and developed knowledge about it for millennia. Relatively recently, videography and satellite observations have added additional perspectives. In the 1970s, around the same time that scientists were beginning to be able to film aurora from the ground, the satellites Isis II and DMSP were some of the first to be able to take photos from space of auroras shining over wide areas. Looking at images from Isis 2, scientists noticed bright, ribbonlike auroras toward the poles, and dimmer, foggy, cloudlike auroras toward the equator. They named the bright ribbons “discrete auroras,” and the cloudlike lights “diffuse auroras”. Since then, other rockets, planes, and spacecraft have taken to the skies, with different instruments that measure aurora in different ways. These measurements can be paired with ground-based photographs to analyze auroral structures—but this is extremely difficult to do because with an ever-changing aurora, the timing needs to be very exact.

Satellite image of Hudson Bay area showing aurora

Satellite image by the Suomi NPP satellite showing bright discrete aurora with fainter diffuse aurora to the south. Note that the discrete auroras are thin across the arc and long along a continental scale.

What are some characteristics of discrete aurora?

The distinctive appearance of discrete aurora generally graces the most impressive aurora photos. Important characteristics include:

  • They are optically much brighter than other kinds of auroras, and the more energy is poured into an aurora (via high energy particles from space), the brighter the aurora will be. The difference in brightness between discrete and diffuse auroras was obvious in early photos from space and was one of the primary characteristics that led scientists to categorize the different types.

  • Arcs of discrete aurora appear as narrow layers of thin vertical sheets, like a skyborne flaky pastry. These can be more than 1000 kilometers long, but only 50 meters to 10 kilometers wide. When you look at them from the side, you see the flat side of the sheets. But if you are lucky enough to be directly below these narrow structures, photos can reveal many narrow layers. The arcs themselves can also occur in multiple sets, but these are more widely spaced. 
  • Arcs move sideways along their length. Sometimes arcs moving in opposite directions can curve into relatively small curls or large spirals. The mechanics for how curls and spirals are made are different, and they rotate in opposite directions. They are, however, a popular subject for photography—spirals have been compared by citizen scientists to cinnamon rolls. Have you seen these features before?
Illustration from “Observed Characteristics of Auroral Forms” by T. Neil Davis

Illustration from “Observed Characteristics of Auroral Forms” by T. Neil Davis

Surfing Through Space: Alfvén Waves

This section summarizes content from a 2022 presentation by Dr. Jim Schroeder and Laura Hollister to the Aurorasaurus Ambassadors. 

Let’s take a deeper dive into some of the science behind aurora, starting with plasma, the fourth state of matter. You may also hear plasma called “ionized gas.” When a gas is superheated, its atoms split apart into electrons (negatively charged particles) and ions (positively charged particles). The charged particles of plasma move on their own, dancing to magnetic fields in space. While in our daily lives we might encounter it in fire, lightning, or electric sparks, it actually makes up the vast majority of the universe. 

There is a type of electromagnetic wave that exists in plasma called Alfvén waves. While we are used to thinking about sound waves or ocean waves, Alfvén waves are a little different. Plasmas have the special ability to respond to magnetic and electric fields, which can create waves. Imagine plucking a string on a guitar and watching waves travel down the string. Similarly, magnetic field lines can be disturbed, creating waves and accelerating some electrons that cause aurora. There is a cone-shaped acceleration region, flaring out about 1-3 earth radii from above the north and south auroral regions. Like tiny surfers, electrons in these spaces can ride Alfvén waves toward Earth. 

Animated gif showing particles surfing waves

Graphic by Dr. Jim Schroeder

To observe an invisible wave you need instruments to measure electric and magnetic waves, both their strength and direction. Do Alfvén waves contribute to the visible, discrete aurora? Signs of a display of the Lights that could be Alfvénic aurora might include filamentation and tall rays (see photo by Donna Lach above). 

The Power of Citizen Science

Since the first satellite photos of aurora in the 1970s, it has proven challenging to connect simultaneous optical photos from the ground with satellite instrument imaging from above. Quite simply, it’s hard to time pictures just right so that scientists can confirm multiple views of the same auroral structure. This is one place citizen science can make a concrete difference in the study of aurora: providing different, simultaneous viewpoints from which aurora can be observed, and submitting them to citizen science projects like Aurorasaurus. Want to take part? Doublecheck that your camera’s clock is set as accurately as possible–this helps scientists line up your photo with others–then photograph the aurora and submit your observations to aurorasaurus.org. Chase safely, and enjoy!

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