Pulsating aurora: A possible threat to ozone

Top image: Morning side patchy pulsating aurora taken from the International Space Station. Credit: European Space Agency (ESA).

Pulsating aurora is a low emission aurora mainly observed after the well-known fast-moving aurora displays we see around midnight. It is caused by high energy electrons which can reach deep into the atmosphere. The electrons are so energetic that they affect the chemistry of the atmosphere by depleting ozone and perhaps influence climate variability.

12 March 2021
Text: Fasil Tesema, PhD candidate in Middle Atmospheric Physics

The Sun is essential to sustain life on Earth by providing the obvious light and heat. But the Sun also emits harmful radiation and particles that affect the Earth’s environment. We are mostly protected from this radiation and particles by the atmospheric gas and the Earth’s magnetic field. However, some specific locations and processes let the solar and magnetospheric particles and radiation pass through and strike the Earth’s atmosphere. When the solar wind particles collide with the atmospheric gases, we see a colorful aurora over the sky. Auroral displays can be divided into two broad categories: Discrete and diffuse types. Discrete aurora has high brightness and distinctive structures like spirals, curls, and folds, which we usually observe around midnight. The diffuse aurora has low intensity and less structure, which cover a wide area of the sky mainly after midnight.

The aurora color and structure differ because of the origin and energy of the electrons which collide with the most abundant atoms in the Earth’s atmosphere, oxygen and nitrogen.  The most common colors of aurora are red, green, and blue. Red and green aurora are produced when the electrons collide with oxygen, while the blue aurora is a result from collisions between electrons and nitrogen. This indirectly tells us how deep the electrons hit the Earth’s atmosphere. Electrons collide with oxygen at higher altitudes to create red aurora and if electrons are energetic enough, they can reach down to lower altitudes around 100 km to create red and blue aurora. After an auroral display of spirals, curls, curtains, and fast-moving forms, we often observe a more diffuse green aurora emerging. Then, within the diffuse aurora, we start to see relatively structured arcs and patches blinking on and off.

This type of diffuse aurora is called pulsating aurora and is related to the high energy electrons. Pulsating aurora is one of the most common types of diffuse aurora and is often observed after local midnight. It can persist for hours, making it relatively easy to continuously monitor the energetic electrons’ effects in the atmosphere and study where the electrons originate from.

Sources and effects of pulsating aurora electrons

Pulsating aurora is caused by high-energy electrons hitting the Earth’s atmosphere. The electrons originate from the inner parts of the Earth’s magnetosphere (the Earth’s magnetic bubble) and are accelerated and heated by different processes.

Magnetosphere: A magnetic bubble that the Earth’s magnetic field creates to protect the Earth from solar wind particles.

 

Electrons that cause pulsating aurora have enough energy to reach below 100 km, sometimes down to even 70 km from the surface of the Earth. This makes the pulsating aurora electrons vital in the dynamics and chemistry of mesosphere (between 60 and 110 km altitude above the surface of the Earth). These energetic particles produce odd nitrogen and odd hydrogen gases called NO_x and HO_x (NO_x=N, NO, NO₂, and HO_x= H, OH, HO₂), which can participate in a catalytic reaction and deplete ozone. The lifetime of HO_x gases is very short, and they survive for less than a day. However, NO_x gases can persist for months in the atmospheric system depending on the season. During the day and in the summer, the NO_x gases will be removed by the sunlight, while during the night and in wintertime, there will be an accumulation of NO_x gases around 90 km. This accumulation will eventually be transported further down to the lower region (around 50 km) and continue depleting ozone. Ozone is a vital component of the Earth’s atmosphere due to absorption of UV light from the Sun. It regulates the heating and cooling of the atmosphere above 30 km altitude. Reduction of ozone due to the NO_x and HO_x chemical reactions affect the heating and cooling balance in the atmosphere.

This, in turn, affects the wind circulation and eventually contributes to climate variability.

In our recent publications, we studied the chemical impact of pulsating aurora electrons and their energy distribution between different pulsating aurora types. We used satellite measurements of electrons hitting the atmosphere and a localized ion chemistry model (https://www.oulu.fi/sgoenglish/node/18752) to study the magnitude of ozone depletion caused by pulsating aurora. At an altitude around 80 km the immediate effect was up to 70% reduction of the ozone. This effect was still at nearly 20% after two days. This significant loss of ozone suggests that pulsating aurora may also alter the wind circulation and dynamics in the atmosphere, which could also be visible in models that include atmospheric dynamics at a global scale.

We showed that different pulsating aurora types (shown in the all-sky camera images above) are associated with different electron energies. The patchy pulsating aurora, which usually covers a large area of the sky and is common in the morning (see middle all-sky camera image above), has strongest effects below 100 km. The electrons energy associated with patchy pulsating aurora is higher than that of the other two pulsating aurora types: Amorphous and patchy. This makes the patchy pulsating aurora a dominant type in depleting the ozone. The patchy pulsating aurora over the sky is easy to recognize, as shown in the picture taken from the International Space Station.  Therefore, we can visually see that the energetic electrons are bombarding the atmosphere harder during pulsating aurora than other aurora types.

Patchy pulsating aurora: A pulsating aurora with persistent aurora patch pulsating mostly over the entire sky. Amorphous pulsating aurora: Short-lived pulsating aurora which pulsates randomly and sometimes over the entire sky. Patchy aurora: An aurora with limited pulsation at the edge of the long-lived patch.

 

The sources and processes driving the different types and structures of the pulsating aurora shown in the all-sky cameras are still open questions.

Reference
Tesema, F., Partamies, N., Nesse Tyssøy, H., and McKay, D.: Observations of precipitation energies during different types of pulsating aurora, Ann. Geophys., 38, 1191–1202, https://doi.org/10.5194/angeo-38-1191-2020

Tesema, F., Partamies, N., Tyssøy, H.N., Kero, A., and Smith-Johnsen, C. (2020), Observations of electron precipitation during pulsating aurora and its chemical impact, J. Geophys. Res.: Space Physics, https://doi.org/10.1029/2019JA027713