Aurora - 3 Understand it

The Enigmatic Dance of Aurora Borealis: Unveiling the Science Behind

The Aurora Borealis, with its mesmerizing display of vibrant colors, has captivated humanity for centuries. Also known as the Northern Lights, this celestial phenomenon has sparked endless fascination and curiosity. But what lies behind this breathtaking dance in the sky? Let's delve into the science behind the Aurora Borealis and uncover its secrets.

At the core of this phenomenon lies the intricate interplay between our planet's magnetic field, charged particles from the Sun, and the Earth's atmosphere. The process begins with the Sun, which continuously emits a stream of high-energy particles, mainly electrons and protons, known as the solar wind.
As the solar wind approaches the Earth, it encounters our planet's magnetic field, forming what is called the magnetosphere. The magnetosphere acts as an invisible shield, deflecting the majority of the solar wind and protecting our planet from its harmful effects.
However, some of these charged particles overcome the magnetic field's defenses and get funneled towards the Earth's poles along the lines of the magnetic field. As they approach the atmosphere, these particles collide with atoms and molecules of gases such as oxygen and nitrogen.
When these collisions occur, energy is transferred to the atoms, causing them to become excited. Like a celestial light switch, this excitation process primes the atoms to release this excess energy in the form of light.

Colourful aurora

Each atom releases energy at specific wavelengths, giving rise to the distinctive hues of the auroras: green, pink, red, blue, and purple.
The green glow, the most commonly observed color, is produced when excited oxygen atoms return to their grounded state. Red and purple hues stem from excited nitrogen and oxygen molecules, respectively. And when the solar wind's charge particles interact with ions in the atmosphere, rarer blue and pink shades grace the sky.
Moreover, the altitude at which these collisions occur plays a crucial role in the formation and colors of the auroras. Green auroras usually appear around 100-150 kilometers above the Earth's surface, while red hues radiate from heights above 200 kilometers.
While solar activity is the primary driver of the intensity and frequency of auroras, Earth's own magnetic activity also influences the spectacle. During intense geomagnetic storms, caused by disturbances in the magnetosphere, the auroras can be seen at lower latitudes and with increased brilliance.

Variations in shape

After becoming addicted to this magical light show and seeing many different shapes/types of aurora over the years I wanted to know more. After researching the topic and some good guides later it’s understood the aurora can display many shapes or types. They tend to fall into the following categories - Arcs, Diffuse, Bands or Curtains/Ribbons, Corona and Pillars. Researches are still witnessing new displays which don’t fall into these types. A couple of examples of more recent ones are STEVE and Dune Aurora.

Arcs

Diffuse

Bands

Corona

Pillars

Picket Fence

I will move on and briefly discuss the some of the components that have to align to allow us to witness this magical dance.

Solar Wind

What is solar wind? The solar wind is emitted from the sun in all directions. The source of the solar winds is the corona of the sun. Electrons and photons are moving too fast to be bound by solar gravity and cast out into space. The solar wind then travels through space and comes into contact with earth. Luckily we have a protective shield in the form of the magnetosphere. This protects us from harmful radiation.

As the solar wind hits our magnetosphere is buffeted and causes it to be compressed. This happens on the day-side of the earth that’s facing the sun. On the night-side our shield the magnetosphere is stretched like a teardrop forming the magnetotail.

The fun bit - some of the solar wind doesn’t get deflected and gets through the magnetosphere. These charged particles follow earths magnetic fields lines being accelerated through earths atmosphere and as mentioned above these charged particles collide with gases such as oxygen then the light show will begin.

IMF - Interplanetary magnetic field

How to imagine the IMF? As the solar wind flows towards earth it brings with it part of the suns magnetic field. This can change in strength and direction. Quantifying the IMF

Bt - The strength of the IMF. A guide to the measurement - 10nt moderately strong, 20nt Strong and 30nt very strong. The stronger the Bt the more chance of aurora activity.
Bx, By and Bz - This is given as a vector. The one to watch here is the Bz which measures the direction of the IMF parallel to the geomagnetic field. Earths geomagnetic field is found in a north-south orientation. As with two magnets oppsosites attract. What this means in relation to the Bz is that when the value is negative it denotes a south orientation which makes for a more likely chance of activity.

Solar wind speeds

If you can imagine that the average solar wind speed leaves the sun at around the 400 km/s and slows down to 300 km/s by the time is reaches Earth. This speed is constantly changing and can even exceed 1000 km/s. With charged particles hitting the Earths magnetosphere at greater speeds, the impact increases our chance witnessing aurora.

So what increase the solar wind speed? Solar events such as coronal mass ejections (CME) and coronal high speed streams (CH CHSS) . What are CME and CH HSS?

CME- Coronal mass ejections is when a huge burst of solar plasma is ejected from the suns corona. This ejection includes plasma and magnetic field. These tend to be faster than the background solar wind at 300 - 400 km/s. They can even reach speeds unto 3000 km/s. At this speed they can take less than 24 hrs to reach earth. The sun launches many CME’s but only a few have a direct hit with our magnetic field. The others may just graze it.

The two mains causes of the the suns corona being ejected into space are

Solar flares - Solar flares happen when the magnetic field lines of the sun become tangled and twisted. This puts the corona under extreme stress and the magnetic field lines want to realign into a less tense configuration through magnetic reconnection. The excess energy explodes from the sun as a solar flare. To put this into context the force released is like millions of nuclear bombs going off at once. Solar flares produce the most powerful CME’s but keep in mind is that not all solar flares release CME’s.
Filaments - The second source of CME’s are filaments. These are enormous clouds of ionised gas that extend out from the suns surface. They loop between two areas of polarity, twisting, tangling along the structures of the suns magnetic field. They are cooler and denser than the plasma underneath. Filaments can last for several weeks but becomes unstable and either collapse back toward the Sun or erupt into outer space releasing a CME. If earth bound then it may cause aurora.

Solar wind density

The density of the solar wind is measured in particles per cubic centimetre, p/cm3. The average is around 4 p/cm3 and 20p/cm3 considered moderate, 40 p/cm3 high and 60 p/cm3 as very high. The more particles there are the more influence on auroral activity. The denser the solar wind the more electrons there are to collide with atoms and molecules in our atmosphere and intern the chance of bright and colourful aurora.

Solar cycle

Substorms
Known as magnetospheric substorms, auroral substorm of simply substorms describe disturbances in Earths magnetosphere. High bursts of energy are hurled towards the poles causing auroral displays at high latitudes. Calm aurora will brighten, dance and show more defined structure. Substorms don’t tend to last very long and undergo three phases -
- Growth - The calm before the storm as it were. This stage is called the growth phase. It can be slow or take many hours. As an example of may see a calm aurora arc
- Expansion - When enormous amounts of energy are released from the magnetotail. This energy is carried along the earths magnetosphere down towards the poles. Example - the arc may increase in brightness, width and gains structure. This intern happens due to the arrival of the flow or particles. Aurora can become very strong and dance all over the sky. I like to think of it as an explosion. It can be very short or can last for a period of time.
- Recovery - This phase happens after the expansion phase and is almost like the sky recharging. You can witness pulsating or flickering. There is always a chance that another substorm can happen. You may see another or this recovery phase may lead to the aurora dying out completely.

Geomagnetic storms

Magnetic Storms also termed geomagnetic storms are disturbances of Earths magnetosphere caused by a major solar event. Magnetic storms tend to produce the best and most explosive shows.

Measurements - What resources do we have for forecasting?

There are a lot of terrible data on the web and many apps that try to predict aurora. I recommend looking at professional sources such as NASA, Spaceweatehr.com and National Oceanic and Atmospheric Administration (NOAA). I can also highly recommend our friend ADRIEN MAUDUIT that runs Nightlightsfilms. He produces an excellent guide to aurora chasing.

Obviously there is a lot more depth to the science of aurora but this our introduction and hope it helps with a little be more understanding of one of the most beautiful phenomenons on earth.

Understanding the Aurora