A polar aurora, called "boreal" -
© Egil Sjøholt / Pexels
Studying the polar lights allows us to better understand the disturbances on our technological systems, according to a study published by our partner The Conversation.
The Earth is not the only planet to be the seat of polar aurora since it also occurs on Jupiter, Saturn, Uranus and Neptune.
The analysis of this phenomenon was carried out by Mathieu Barthelemy, professor, director of the University Space Center at Grenoble Alpes University.
Whether you are a researcher or not, the first answer that comes to mind is that you study the Northern Lights “because it is beautiful”.
If this often represents a deep and primary motivation for researchers working on the subject, there are other reasons which lead us to observe the auroras which occur on Earth… or on other planets.
Because the Earth is not the only planet to be the seat of the polar aurora.
As we will see, to produce polar auroras, you need an atmosphere, a magnetic field and solar wind: conditions met on Jupiter, Saturn, Uranus and Neptune where auroras have been detected.
On Mercury, whose atmosphere is very thin, light emissions resembling auroras are detected.
On Mars, where the magnetic field is "frozen" in the Martian crust (the trace of a past magnetic field), light emissions in areas of strong magnetic field were observed by a space mission in 2005. Venus does not present magnetic field, but there are diffuse emissions in the upper atmosphere - are these auroras?
The debate is not settled.
Aurora seen from Skibotn, Norway, in March 2017 © Thierry Sequies (CSUG / UGA)
In all cases, these different light emissions are rich in information on the atmospheres of the planets that they illuminate, for example on their chemical composition or their dynamics, but also on the flow of particles which come from the sun.
Auroras are linked to the activity of the Sun
On Earth, we often speak in common parlance of the northern lights, which only refers to those that occur in the North, and we therefore forget the southern lights of the southern hemisphere - the generic term is actually "polar lights" .
The polar lights are caused by charged particles from the Sun, a flow called the "solar wind".
These particles, mainly electrons and protons, are “picked up” by the Earth's magnetic field and plunge into the atmosphere around the magnetic poles.
Aurora Borealis over Snæfellsnes Glacier in Iceland © Diana Robinson / Flickr CC BY-NC-SA
They are concentrated along ovals centered around the magnetic pole, at about 20 ° latitude, and collide with atmospheric gas, which causes an "excitation" of the atoms and molecules of this gas.
The light emission that we see in a polar aurora comes from the relaxation of the atmospheric gas after this excitation, which is called "fluorescence".
At what altitude are these multicolored auroras?
When observing a polar aurora from the ground, it is difficult to know what altitude it is at - just as it is difficult to know the altitude of the clouds.
It was a Norwegian physicist, Kristian Birkeland, who at the start of the 20th century, in addition to understanding the phenomenon and its links with the Sun, estimated the altitude of these emissions - after a very imprecise first estimate by Lord Henry Cavendish in Eighteenth century.
Carl Størmer and Bernt Johannes Birkeland, his assistant, photograph an aurora borealis in Alta (Norway) in 1910. Anders Beer Wilse (1865 - 1949) © Wikipedia CC BY-NC-SA
He first experimented in the mountains to see if he was in the dawn, which was not the case.
He will then measure the angle at which we see an aurora from several points, by "triangulation" and, in collaboration with Carl Størmer, another Norwegian physicist, estimated the altitude at about 100 km.
In addition to green, we can also observe purple and red.
These colors depend essentially on the gas which is excited and relaxes: green and red are due to oxygen while purple is due to a nitrogen ion (N2⁺).
Purple is emitted around 90 kilometers, green around 120 kilometers and red around 220 kilometers.
Other emissions exist in particular in the ultraviolet, but they are not visible from the Earth, because they are filtered by the atmosphere.
What do the polar lights tell us today?
Studying the polar aurora allows us to better understand the upper atmosphere, but also the disturbances that the solar wind causes when entering the atmosphere on our technological systems: radio or wired communications, electrical networks, positioning systems, aviation, satellites.
In March 1989 in Quebec, a solar storm caused a power cut of several hours and another caused many disturbances on the satellites in 2003.
Auroras are a way to get information about how the energy of these particles is deposited in the atmosphere.
How to use the polar auroras to study the flow of particles from the Sun?
Researchers use the aurora to observe the solar wind, from a distance and over the long term.
Just as it is impossible to do meteorology by measuring temperatures or winds only once a year for a few minutes, to predict the polar lights and electromagnetic disturbances linked to the solar wind, continuous and long-term data are indispensable - a discipline called "space meteorology".
This remains complex since summer is unsuitable for observing the polar auroras (it is little or no night in the polar regions, in summer) and the aurorae can be masked by clouds.
It is therefore difficult to ensure continuous surveillance of the aurora when you stay with your feet on the ground.
It is unfortunately also very difficult to make
in situ
measurements
of the polar lights over the long term, because installing instruments that are stationed at an altitude of 100 or 200 km is complex: stratospheric balloons only go up to 50 km. altitude, satellites can hardly stay in orbit for long below 300 kilometers, and a rocket that passes through these regions would reside there for only a very short time.
Instruments on board aircraft suffer from very short observation times, and LIDAR is in development for these observations, but does not yet reach layers beyond 100 kilometers.
Last night we managed to shoot sodium light into the sky!
#mesosphericmagnetometry #geomagnetism #ALOMAR # tromsøgeophysicalobservatory System is not yet stable enough, but we are working on it!
😎 pic.twitter.com/aiPO93trx6
- Magnar G. Johnsen (@magnargj) September 28, 2019
One solution is to place the observation instrument
above the aurora
.
With a satellite in orbit at 500 km, we can make 15 to 16 orbits per day, without the obstacle of clouds, and therefore have an almost continuous monitoring of these light emissions and observe wavelengths that are filtered. by the atmosphere (in the ultraviolet for example).
Finally, it is possible to observe the polar aurora from space when it is daylight, by "aiming at the limb", that is to say without the line of sight intercepting the terrestrial globe, in grazing incidence. .
Observation by satellite naturally encounters other challenges: the spatial conditions are complex to control (vacuum, extreme temperatures, radiation, etc.) and the satellite is in rapid movement, which complicates the taking of an image (approximately 7 kilometers per second for an orbit at 500 kilometers).
Observe the polar lights from space
Numerous polar aurora observation experiments have therefore been sent into space to detect ultraviolet emissions or to overcome clouds and daylight, for example the REIMEI satellite of the Japanese space agency.
Our Grenoble team launched the project in 2017 to build a nanosatellite, weighing a little over 2 kg, equipped with an imager specially built for the observation of these auroras in visible wavelengths.
After many adventures and great collaborations, in particular with the University of Moscow and the amateur radio community, the AMICal Sat satellite took off from the Kourou base on September 3, 2020 aboard the Vega rocket.
We received its first aurora image in early November 2020, showing that it is possible to observe these aurorae from space using a tiny satellite built by students.
From the images produced by the on-board camera, it will be possible to determine the intensity of the aurorae in the different colors captured by the detector's red, blue and green filters.
At the same time, we have designed simulations to calculate these emissions as a function of the flux and energy of particles entering the atmosphere and their energies.
By “reversing” these simulations, we will reconstitute these parameters at the times when the photos were taken.
Our “Aurora borealis” dossier
First, we will focus mainly on electrons with relatively low energies (less than 30 kiloelectronvolts).
These are the ones which, via the currents they create, can disrupt the electrical networks on the ground and pose big problems for the satellites, because they tend to accumulate on the surface of the satellites and cause discharges within them which can damage components, especially electronics, and therefore cause them to lose control.
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This analysis was written by Mathieu Barthelemy, professor, director of the University Space Center at Grenoble Alpes University.
The original article was published on The Conversation website.
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