By Michelle Tebolt, summer intern 2017
Above our heads, the aurora provides one of the biggest and best light shows on Earth. The light moves about, flashes across the sky, similar to some of the types of man-made lights we are familiar with. However, this light show isn’t to set the mood for a party. It holds information about the physics surrounding Earth. By studying pulsating and flickering auroral morphology, scientists can gain valuable knowledge about the magnetosphere and electromagnetic waves surrounding the planet.
As a summer intern at NASA Goddard Space Flight Center, I arrived not really knowing anything about the features of the aurora. However, as I spent the summer searching for instances of flickering and pulsating aurora over millions of images of the sky, I began to understand the science behind the famous natural light show and the features that it presents. Although there are aspects of these types of aurora that some experts still don’t understand, the overarching concepts are as simple as knowing the difference between groovy lava lamps and energetic strobe lights.
What is “Morphology”?
When studying auroral morphology, we are examining how the aurora changes physically over space and time. Since the aurora is dynamic, its shape is constantly evolving over various time scales. It appears to stretch, condense, and sometimes fluctuate in brightness or intensity, hence the common description of the aurora “dancing across the sky.” Some of this dynamic behavior can be categorized into different types of aurora, such as flickering and pulsating aurora. Although both of these terms seem to be describing the same sort of activity, they differ in both origin and appearance.
Pulsating Aurora – The Lava Lamp
As described in another Aurorasaurus blog post from 2015, the term “pulsating aurora” is used to describe aurora that periodically fluctuates in brightness.
It occurs when particles travel over magnetic field lines in space, bouncing between the North and South pole rather like a game of ping pong. When they are closer to the poles, the particles speed up and change direction in a process called “mirroring.” This continues until other electromagnetic waves become involved, called very low frequency waves or VLF waves. These waves interact with the particles, changing the mirroring points so that they are closer to Earth’s surface, and lowering the hypothetical ping pong paddles down to the level of the atmosphere. Once these particles reach this height, they interact with Earth’s atmosphere and are lost from the space ping pong game. Instead, they collide with particles in the atmosphere and create the pulsating features seen in the aurora. The very low frequency waves are unique to this type of aurora and to the characteristic on/off behavior.
The resulting light show can be compared to the glowing behavior of a lava lamp. Distinct areas and patches of the pulsating aurora periodically get brighter and dimmer. This is similar to how the colorful fluid inside a lava lamp shifts and glows in obvious clumps, shining brightly for a bit before dispersing. The pulsating patches can vary in size, just like the light dollops inside a lava lamp, although the aurora patches are on the scale of 10-100 km (just a little too big to fit on a bedside table). Also, the relaxing and sluggish movement of material in the lamp reflects the timescale of the pulsating aurora. These fluctuations usually occur over the scale of one to ten seconds. This gradual change in brightness makes the aurora act as Earth’s very own lava lamp, stretching across the night sky and providing a natural, groovy, glow.
Flickering Aurora – The Strobe Light
Flickering aurora is created under different circumstances. There is a constant stream of electrons passing into Earth’s atmosphere and making the aurora. This stream of electrons can be disturbed by electromagnetic ion cyclotron waves, or EMIC waves. The electrons descend into the atmosphere, ready to collide with other particles and form the aurora. Suddenly, EMIC waves come screaming in, knocking some the electrons off course. This disturbance leaves small gaps in the electron stream and causes the features seen in flickering aurora.
The relaxing mood lighting provided by the pulsating aurora lava lamp is gone, only to be replaced by the flickering aurora’s energetic fluctuations, similar to a vigorous and sometimes disorienting strobe light. The intensity variation is much quicker, flashing on and off about five to ten times per second. It also covers an area of the aurora much smaller than the pulsating aurora, on the scale of only a few kilometers. It’s also believed that this quick flickering originates much closer to the surface of the Earth than the origins of the pulsating aurora, only 3 to 5 thousand kilometers off the ground. More research is currently being done on the flickering aurora, trying to capture it in data as it shutters bright and dim over the course of fractions of a second, creating the ambiance for nature’s own concert and magic show.
The Educational Light Show
At the conclusion of my internship, I had found about 7 instances of flickering aurora and 2 hours worth of pulsating aurora. Once I found these features, I was able to analyze the camera images of them to measure their intensity, position, and velocity. Eventually these measurements will be used to compare the flickering and pulsating aurora from this location in
Greenland to auroral features present at lower latitudes such as Alaska and Montana. The images I analyzed were captured with a ground-based camera. It sat night after night in Greenland, pointing up at the night sky and continuously taking photos. It was a lot of data to sift through to find nights that were not cloudy and showed bright aurora, and even more work was needed to search these times to find the flickering and pulsating features. The timescale of these features make them a challenge for citizen scientists to capture by camera, but they can be noticed by eye if one knows what to look for.
Spending time studying and researching these auroral features helps bring about a greater understanding of the magnetosphere, and allows scientists to create more accurate magnetic field models. Since all of the dynamic processes that occur within the magnetotail are mapped through the aurora at the polar caps, understanding every single feature, on time scales ranging from of fractions of seconds to hours, gives us a blueprint of the physics occurring within this region of magnetic field lines which protect us from some of the sun’s electromagnetic radiation.
Michelle Tebolt was a summer intern at NASA Goddard Space Flight Center in 2017. She is an astrogeophysics major at Colgate University going into the third year of her undergraduate degree. She is enthusiastic about all things astronomy, geology, and physics related, including the aurora and its features. Contrary to what this article suggests, she does not own a lava lamp.