This weekly newsletter delves into a different captivating topic from the world of geoscience, exploring Earth’s fascinating phenomena in bite-sized, easy-to-understand segments each week.
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Monday 19th of February, this is the second part of the two stage newsletter about solar sun flares. I hope you enjoyed learning about solar flares but this topic is too vast and interesting to be covered in just one newsletter; I will assume that you have read part 1 and so if there are elements you don’t understand please re-read part 1. Many of you may have seen the aurora in real life, the aurora, often called the aurora borealis in the Northern Hemisphere and aurora australis in the Southern Hemisphere is a natural light display which is described as an breath-taking and awe-inspiring spectacle. I too have seen the aurora in Iceland and it is amazing. I really recommend if you ever get the chance to see them — do it. So what is the aurora?
The Types of Auroras and Plasma
As you know from the previous newsletter, auroras are caused by magnetic storms triggered by solar activity such as solar flares (explosions on the sun) and coronal mass ejections (CMEs). The energetic particles from these events are carried by solar wind in the form of a solar storm. It’s the interaction of these energetic particles in the atmosphere and magnetosphere which is responsible for the lights. There are many different shapes and types of auroras for example: Corona, Diffuse auroras, ARCS, Dunes and Rays.
Plasma is often called the fourth state of matter and effectively it is superheated matter which is so hot that the electrons are ripped away from the atoms forming ionised gas. It comprises 99% of the visible universe.
Magnetotail
Aurora’s are commonly found in a band at higher latitudes called the auroral oval around the polar caps. Aurora’s form around the magnetospheres tail known as the magnetotail. The magnetotail is composed of many elements, one of them are tail lobes. Most of the volume of the magnetotail is taken up by two clusters of nearly parallel magnetic field lines called tail lobes. The cluster located north of the equator directs towards Earth and encompasses a roughly circular area that includes the northern magnetic pole. Conversely, the southern cluster points away from Earth and connects to the southern polar region. These tail lobes extend far away from Earth because the lobes are penetrated by solar wind plasma as they are extremely low density meaning they can easily flow away along lobe field lines. Separating the two lobes is the plasma sheet which is a weaker layer of magnetic field and denser plasma which is centred on the equator. A diminished magnetic field implies that the plasma experiences less confinement in this area compared to closer to Earth. Meaning occasionally, it may slosh or undulate. Technically, it’s not the solar wind electrons themselves that directly generate auroras. Instead, when the solar wind is strong and the embedded Interplanetary Magnetic Field (IMF) aligns opposite Earth’s magnetic field (southward), more solar wind energy is transferred into the magnetosphere. This process accelerates more electrons from the magnetosphere down along Earth’s magnetic field lines. Consequently, the auroral electrons originate from within Earth’s magnetosphere. The most vivid auroras, often observed in the middle of the night, are actually generated by electrons originating from the tail of the magnetosphere, downstream (away from the sun). So why am I mentioning all of this, well we’re going to be briefly looking at a process called the diffuse aurora to help explain why this phenomena is formed.
The Diffuse Aurora
The diffuse aurora is formed due to the feeble field within the plasma sheet. The ions and electrons within it remain in constant motion, with some of them, particularly electrons, consistently escaping from the ends of their magnetic field lines causing them to approach Earth. As these electrons approach Earth, the majority rebound due to the convergence of field lines, but a portion penetrate the atmosphere and dissipate, resulting in the creation of a diffuse aurora. A diffuse aurora is a form of aurora characterized by a faint, luminous haze extending across a wide expanse of the sky. Unlike the clearly defined shapes of curtains or arcs seen in other types of auroras, diffuse auroras lack distinct boundaries. They often present as a gentle, soft glow that blankets a considerable portion of the sky. For example in the Isle of Skye, a diffuse aurora is recognised as appearing as a pink/red band slowly rising about 10 degrees above the horizon on the Isle of Skye. At the Isle of Skye a diffuse aurora is typically seen around 1–2 hours before the growth phase. What is a growth phase? Well an aurora goes through three distinct stages: growth, expansion and recovery. Effectively, the expansion phase is where the hues and rays of the aurora are brightest. If you were to measure the expansion phase on a magnetometer (a magnetometer is a device that measures magnetic field or magnetic dipole moment) the reading declines very sharply. This occurs because during the expansion phase there is a decrease in the intensity of the cross-tail current, causing the inner edge of the plasma sheet and the cross-tail current to move away from the Earth. This movement results in a significant release of energy from the inner portion of the plasma sheet. The growth stage begins 1–2 hours prior to the expansion phase. During this stage, the plasma sheet shifts towards Earth, and there is a rise in particle energisation, resulting in an escalation of the cross-tail current. This intensification is most pronounced in the portion of the plasma sheet directed towards Earth. Finally, during the recovery phase, the heightened activity experienced during the expansion phase diminishes, and magnetometers are reverting to their background levels if you to were to measure the lifecycle of the aurora on a magnetometer.
The Ionosphere
So let’s bring all our knowledge together and finally understand the aurora. So as mentioned, the solar wind reaches Earth, it encounters the Earth’s magnetic field. The solar wind is comprised of CMEs which I talk about in detail in part 1. This magnetosphere is crucial as it shields Earth’s fragile atmosphere from being blown away by the solar wind, thus preserving conditions essential for life. Most of the solar wind is blocked by the magnetosphere but some of the ions and energetic particles (proton storms) are briefly trapped in ring-shaped holding areas around the planet called the ionosphere which is centered around the geomagnetic poles. Within the ionosphere, ions from the solar wind interact with oxygen and nitrogen atoms in Earth’s atmosphere. The energy released from these collisions creates a vibrant glowing halo around the poles, known as an aurora. Meaning, the most active auroras happen when the solar wind is strongest and the the geomagnetic poles pull these particles towards the dipole ends of the Earth responsible for creating vibrant auroras in the night sky.
The Physics Behind the Colours of The Aurora
The sun emits all visible colours, giving sunlight its white appearance. However, the spectrum of visible light associated with the aurora is more limited. The aurora occurs when charged particles in the solar wind collide with atmospheric atoms and ions. These collisions excite the electrons of the atmospheric atoms. As these electrons return to their original energy levels, the atoms emit visible light at specific wavelengths, producing the colourful display we observe. The hue of the aurora is dictated by the wavelength of emitted light, influenced by the particular atmospheric gas and its electrical condition, as well as the energy of the impacting particle. With the atmosphere primarily composed of nitrogen and oxygen, each emits distinct colours based on their unique line spectra. Atomic oxygen yields the predominant green (at 557.7 nm) and red (at 630.0 nm) hues, while nitrogen contributes to blue and deep red tones.
The intense green light emanates from altitudes ranging between 120 to 180 km. Higher altitudes host the occurrence of red Northern Lights, while blue and violet are mostly visible below 120 km. During periods of heightened solar activity, red hues manifest at altitudes between 90 to 100 km. Occasionally, entirely red Northern Lights are observable, especially at lower latitudes.
Conclusion
In conclusion, auroras are a spectacle to behold and one of the most amazing natural phenom that Earth has to offer. Exploring auroras not only deepens our comprehension of space weather but also provides a captivating insight into the interconnectedness of our planet with the broader solar system. I hope you enjoyed learning about auroras today and gained a deeper appreciation for the remarkable processes that shape our planet. If you want to learn more about geosciences come back every Monday for a new topic which is just as interesting.
Thank You
Thank you for reading, all information is taken from reputable sources and are linked below. All images are free to use under copyright laws.
Webexhibits ‘ National Oceanic and Atmospheric Administration ‘ NASA