As the days faithfully follow each other, there are occasions when nature will present different facets of its presence, which could be described as the ‘light shows of nature’.
Although known from the very early days of human existence, the deeper understanding of many of these observations is only now coming to the fore. This is very much the case with the ‘Northern Lights’, also known as the ‘Aurora Borealis’.
The Northern Lights
The extensive efforts to understand the nature of the Northern Lights, stems in one sense from scientific curiosity but in another from a need to understand how to withstand the influence of the solar wind from the Sun on the Earth’s ever-developing technology infrastructure, which includes satellites and power grids.
In addition, intense solar wind bombardment can also cause higher levels of harmful radiation for passenger aircraft. There are also some subtle effects of how the human eye detects aspects of the light of the Northern Lights.
It was only with the launching of a series of satellites dedicated to solar observation that an improved understanding was obtained of the Sun’s solar wind, which streams out to the Earth.
The joint Nasa/ESA Ulysses probe was launched in 1990 and completed three eccentric orbits round the Sun over an 18-year period with its closest distance of approach of 200 million km.
Figure 1: Image of sun’s surface taken by the Solar Orbiter craft in October 2023, indicating rising activity on the sun’s surface
Image: ESA & Nasa/Solar Orbiter/EUI Team
Soho continuously points at the Sun with a range of scientific instruments and is still a viable working system that has provided key information about the ejection of matter from the Sun’s surface. One of the key presentations on Nasa’s Soho web pages is a time lapsed continuous video of 20 years of the Sun’s coronal activity presented in less than an hour.
More recently the Solar Orbiter, launched in February 2020, has been able to take images of the Sun’s surface from as close as 42 million km, which has allowed observation of the Sun’s surface in unprecedented detail.
These highly successful missions are in fact part of a more extensive research programme1 to further investigate the effect of the Sun on the entire solar system.
In viewing time lapse images of the ejected solar wind, the Sun has the appearance of a gigantic ‘catherine wheel’ spinning out strands of ejected material from the Sun’s outer coronal layer. This ‘spinning image’ is on account of the rotation of the Sun, where equatorial regions rotate fastest in a period of around 25 days, while at the poles the period of circulation is more like 30 days.
On occasion major events termed a coronal mass ejection (CME) take place where instability on the Sun’s surface results in billions of tons of matter (mainly hydrogen) being ejected into space. It is these events that result in the highest levels of solar wind interaction on Earth. The speed of travel of such CMEs can range from around 250 kilometres per second (km/s) to as fast as 3,000km/s and where the fastest can reach Earth’s orbit in less than a day.
Figure 2 (below) indicates how the solar wind ‘pinches’ the Earth’s magnetic field on the side nearest the Sun and elongates it on the opposite side.
One of the adverse effects of intense displays of the Northern Lights is that low Earth orbit satellites are exposed to increased atmospheric drag and consequential lowering of their orbit height by several kilometres. While definitions vary, low Earth orbit is often described as between 300km and 1,600km.
As the lines of charged particles travel along the lines of the Earth’s magnetic field towards the poles, they generate the Northern Lights in northern latitudes and the Aurora Australis in southern latitudes as the particles make contact with the Earth’s atmosphere.
Extensive research2 over a considerable period has provided an insight into the highly complex mechanisms of how the Earth’s magnetic field essentially protects the Earth from the ravages of the solar wind and to some extent from the effects of cosmic rays.
Compared to the Earth, Venus and Mars have practically no magnetic field. It is considered that over time any atmosphere that Mars once retained has been effectively driven off by the solar wind.
One feature that has so far not been extensively reported in the media is the so-called South Atlantic Anomaly,3 which is described as a ‘dent’ in the Earth magnetic field, identified as a weak spot in the overall protective shield of the magnetosphere, presenting a hazard to low Earth orbit satellites passing through this region.
Images taken from the International Space Station (ISS) can often reveal the large extent of the auroras within the Earth’s atmosphere. Figure 3 indicates the northern lights observed over Canada from the vantage point of the ISS some 400km above the Earth’s surface, which is an amazing spectacle.
The main components of the Earth’s atmosphere are nitrogen, oxygen, argon and carbon dioxide with concentrations of 78%, 21%, 0.93% and 0.05% respectively.
In addition, there are trace amounts of neon, helium, methane, krypton, hydrogen and water vapour. The most commonly observed aurora colour is green and is typically produced with interaction with oxygen molecules in the Earth’s atmosphere at altitudes
of around 100 to 300km.
Where interaction with oxygen takes place at higher altitudes between around 300 to 400km during intense solar activity, a red aurora colour can be observed, and it is typical that the highest aurora colour observed is red. The colours of blue and purple are associated with interactions with nitrogen at levels below around 100km.
Figure 3, above, shows the view of the Northern Lights above Canada (image credit: Nasa)
Colour perception at low light levels and high altitude
As the level of light on the retina reduces, the retinal detection mode switches from that of cone colour vision to that of rod vision.4, 5 Most species have not developed colour vision in low light levels, one exception being the hawkmoth, which has a colour sensitivity over a thousand times greater than humans.
In gazing up at the sky underneath a plume of the Northern Lights, the perception personally observed was that of a plume of white colourless illumination. When, however, observed using a smartphone or digital camera, it would likely have revealed the inherent colours within the display.
Where also there is independent control of sensitivity (ISO value) and exposure time in a smartphone, a high ISO value and an exposure of several seconds would be appropriate for image capture.
For a conventional digital camera, the focus mode should be set to manual to pick out, for example, a feature in the foreground, with selection of a high ISO value and exposure time of several seconds.
With auto focus selected, there is unlikely to be enough light for this mode to function appropriately. Immobilising the camera/smartphone by, for example, using a tripod will also improve the image quality.
Short exposure times will pick up specific variations within the aurora and longer exposures will tend to integrate and smooth out such features.
Human physiology at high altitude6 has become an established research topic with potential application to critical care medicine. One parameter observed within such challenging expeditions has been the response of colour vision to increasing altitude, although the number or reported studies is limited.
One study recorded a reduction in FM-100 Hue test scores between 1,660m and 4,200m, which was not significant.7 There is a general perception, however, that at extreme high altitude, colour vision is significantly impaired, and which can present an additional hazard to, for example, map reading and navigation.
Noctilucent clouds
Noctilucent clouds8 or ‘night shining’ clouds are clouds which form at extremely low temperatures at a height of between 50 and 90km. They are considered to consist of minute ice crystals that have formed from the remnants of meteors burning up in the atmosphere.
Rising levels of methane due to global warming, which reacts with sunlight and oxygen to form water vapour, is also considered a possible cause of increased observation of such clouds. There are, therefore, possible links with global warming.
Noctilucent clouds have a wispy appearance when they present during the summer months when the upper atmosphere is at its coldest. Such clouds can often be visible after sunset due to their reflection of light at high altitudes and are increasingly observed at more southern latitudes in the northern hemisphere. The often rapidly changing features of such clouds provides information of the physical dynamics of the upper atmosphere.
Figure 5, pictured above, shows some noctilucent clouds over Germany (image credit: Matthias Süßen).
Nasa’s AIM mission, which was operational between 2007 and 2023, allowed observation of the life cycle of such clouds and where valuable information was gained regarding linkage of weather patterns due to atmospheric mixing between different latitudes. When viewed from the ISS, noctilucent clouds can be seen at different stratified levels in the high atmosphere, resembling the layers of an onion.
Rainbows
It is generally considered that the first documented description of the phenomenon of the rainbow was due to Al-Farisi (1260 to 1320) a mathematician from Persia who also studied the phenomenon of light.
Al-Farisi, also known as Kamal al-Din, was able to demonstrate the refraction/reflection/refraction of light in a hollow glass sphere representing a raindrop. Al-Farisi was familiar with the earlier works of Ibn al-Haytham who had previously investigated the refraction 
of light.
Figure 6 (left) indicates the classic representation of an incident ray of sunlight being refracted/reflected/refracted by a raindrop for a specific height, h, above the midline of a raindrop, creating a primary rainbow.
The actual angles relate to a characteristic angle associated with a maximum of light of a specific wavelength. This behaviour of light incident on the raindrop at different values of h is more clearly illustrated in figure 7, where light will appear to be concentrated in a band of colours from light incident near the top of the raindrop.
Figure 7: Simulation of angle of refracted red and blue light as a function of height (h) of incident light above the centre of the raindrop (0 = at centre; 1 = at top) (see figure 6)
The sky under the arc of a rainbow (figure 8) typically appears lighter than the sky above the arc. This is due to light being scattered towards the observer without obvious colour. Light incident in the lower half of a raindrop in the mode of the primary rainbow is refracted/reflected/refracted upwards and does not contribute to the primary arc of colour observed at ground level, but would be observed by someone looking down from an aircraft.
Usually, rainbows present with a secondary rainbow some way above the primary one (figure 9). This is caused by light undergoing another element of reflection with observation of the reverse of colours as observed in the primary rainbow. High sensitivity imaging systems can in fact observe light patterns produced by additional modes of reflection/refraction inside raindrops.
Where a rainbow can form at the position of the first rays of light from a sunrise, a circular rainbow can form. The observed light of a rainbow is also highly polarised, so, if viewed through a rotating polarising filter, the rainbow can be made to appear and disappear.
Lightning
The phenomenon of lightning10,11 is a research field of interest for Nasa, where key information relating to lightning forecasts is of major importance during the fuelling and launch stage of spacecraft.
Through convection currents in cloud systems, friction between water droplets or ice crystals can result in the separation of charge corresponding to high voltage differences. Lightning acts as the mechanism to allow current to flow to lower such potential differences and which can take place within a cloud system (sheet lightning) or between the cloud system and the ground (fork lighting).
Early investigation of lightning spectra12 was undertaken using a Meinel spectrogram where the emission lines in wavelength range 310nm to 960nm was recorded on emulsion film plates. This indicted a multitude of spectral emission lines involving principally molecules/atoms/ions of nitrogen and oxygen with contributions also from argon.
It had been considered that the maximum voltage typically present in natural lightning was around 150kV, through the detection of muon particles by a team in India13 during thunderstorms raised this value to 1.3 GV. The thunderstorms were observed to be acting as particle accelerators.
Summary
Many aspects of nature involve its appreciation through good visual function. Often the observed effects of ‘the light shows of nature’ are the result of subtle effects of the physics of light and where a relevant motto would be ‘the more you look, the more you see’.
References
- 1 2009 Heliophysics Roadmap Team, Heliophysics: The Solar And Space Physics of A New Era, NASA, Available at:- https://explorers.larc.nasa.gov/EX/PDF_FILES/Heliophysics_Roadmap_2009_tagged-quads.pdf
- Buis, Alan, Earth’s Magnetosphere: Protecting Our Planet from Harmful Space Energy available at:- https://science.nasa.gov/science-research/Earth-science/Earths-magnetosphere-protecting-our-planet-from-harmful-space-energy/
- Soni PK, Kakad B, Kakad A, Deepening of radiation belt particles in South Atlantic Anomaly Region: A scenario over past 120 years, Advances in Space Research, 69 (5), 2022, 2090-2101
- Kelber A, Yovanovich C, Olsson P. Thresholds and noise limitations of colour vision in dim light. Philos Trans R Soc Lond B Biol Sci. 2017 Apr 5;372(1717):20160065.
- Kelber A, Lind O. Limits of colour vision in dim light. Ophthalmic Physiol Opt. 2010 Sep;30(5):454-9
- Davies AJ, Morris DS, Kalson NS, Wright AD, Imray CH, Hogg CR; Birmingham Medical Research Expeditionary Society. Changes to colour vision on exposure to high altitude. JR Army Med Corps. 2011 Mar;157(1):107-9
- Mazýcýoðlu M, Karaküçük S, Öner A, Ulusal H, Mirza GE, Effects of high altitude on colour vision of climbers during an expedition to Mt. Aðri, Ecriyes Medical Journal, 2006, 28(3), 134-137
- Dubietis A, Dalin P, Balčiūnas R, Černis K, Pertsev N, Sukhodoev V et al. Noctilucent clouds: modern ground-based photographic observations by a digital camera network. Appl Opt. 2011 Oct 1;50(28):F72-9
- MacCannell, Daniel, Rainbows: Nature and Culture, 2018, Reaktion Books, London.
- MacGorman DR and Rust WD (1998). The electrical nature of storms. Oxford University Press.
- Rakov VA, Uman M A. Lightning: physics and effects. Cambridge University Press, 2003.
- Wallace L, The Spectrum of Lightning. The Spectrum of Lightning, Astrophysical Journal, 1964, 139; 986-993
- Hariharan, B, Chandra, A, Dugad, SR, Gupta, SK, Jagadeesan, P, Jain, A et al, Measurement of the Electrical Properties of a Thundercloud Through Muon Imaging by the GRAPES-3 Experiment, American Physical Society, Phys. Rev. Lett., 122, 105101, 2019, DOI: https://doi.org/10.1103/PhysRevLett.122.105101
