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The colour of the light corresponds to the release of discrete chunks of energy by the atmospheric particles, and is also an indicator of how much energy was absorbed in the initial collision.
The frequency and intensity of auroral displays is related to activity on the Sun, which follows an 11-year cycle. Currently, we are approaching the next maximum, which is expected in 2025.
Fox Fires, a short film inspired by the Finnish folk tale of the aurora borealis.
Connections to the Sun
Such displays have long been documented by peoples throughout North America, Europe, Asia and Australia.
In the 17th century, scientific explanations for what caused the aurora began to surface. Possible explanations included air from Earth’s atmosphere rising out of Earth’s shadow to become sunlit (Galileo in 1619) and light reflections from high-altitude ice crystals (Rene Descartes and others).
In 1716, English astronomer Edmund Halley was the first to suggest a possible connection with Earth’s magnetic field. In 1731, a French philosopher named Jean-Jacques d'Ortous de Mairan noted a coincidence between the number of sunspots and aurora. He proposed that the aurora was connected with the Sun’s atmosphere.
It was here that the connection between activity on the Sun was linked with auroras here on Earth, giving rise to the areas of science now called “heliophysics” and “space weather”.
Earth’s magnetic field as a particle trap
The most common source of aurora is particles traveling within Earth’s magnetosphere, the region of space occupied by Earth’s natural magnetic field.
Images of Earth’s magnetosphere typically show how the magnetic field “bubble” protects Earth from space radiation and repels most disturbances in the solar wind. However, what is not normally highlighted is the fact that Earth’s magnetic field contains its own population of electrically charged particles (or “plasma”).
Model representation of Earth’s magnetic field interacting with the solar wind.
The magnetosphere is composed of charged particles that have escaped from Earth’s upper atmosphere and charged particles that have entered from the solar wind. Both types of particles are trapped in Earth’s magnetic field.
The motions of electrically charged particles are controlled by electric and magnetic fields. Charged particles gyrate around magnetic field lines, so when viewed at large scales magnetic field lines act as “pipelines” for charged particles in a plasma.
The Earth’s magnetic field is similar to a standard “dipole” magnetic field, with field lines bunching together near the poles. This bunching up of field lines actually alters the particle trajectories, effectively turning them around to go back the way they came, in a process called “magnetic mirroring”.
‘Magnetic mirroring’ makes charged particles bounce back and forth between the poles.
Earth’s magnetosphere in a turbulent solar wind
During quiet and stable conditions, most particles in the magnetosphere stay trapped, happily bouncing between the south and north magnetic poles out in space. However, if a disturbance in the solar wind (such as a coronal mass ejection) gives the magnetosphere a “whack”, it becomes disturbed.
The trapped particles are accelerated and the magnetic field “pipelines” suddenly change. Particles that were happily bouncing between north and south now have their bouncing location moved to lower altitudes, where Earth’s atmosphere becomes more dense.
As a result, the charged particles are now likely to collide with atmospheric particles as they reach the polar regions. This is called “particle precipitation”. Then, when each collision occurs, energy is transferred to the atmospheric particles, exciting them. Once they relax, they emit the light that forms the beautiful aurora we see.
Curtains, colors and cameras
The amazing displays of aurora dancing across the sky are the result of the complex interactions between the solar wind and the magnetosphere.
Aurora appearing, disappearing, brightening and forming structures like curtains, swirls, picket fences and traveling waves are all visual representations of the invisible, ever-changing dynamics in Earth’s magnetosphere as it interacts with the solar wind.
As these videos show, aurora comes in all sorts of colours.
The most common are the greens and reds, which are both emitted by oxygen in the upper atmosphere. Green auroras correspond to altitudes close to 100 km, whereas the red auroras are higher up, above 200 km.
Blue colors are emitted by nitrogen – which can also emit some reds. A range of pinks, purples and even white light are also possible due to a mixture of these emissions.
The aurora is more brilliant in photographs because camera sensors are more sensitive than the human eye. Specifically, our eyes are less sensitive to color at night. However, if the aurora is bright enough it can be quite a sight for the naked eye.
Where and when?
Catching aurora in the southern hemisphere.
Even under quiet space weather conditions, aurora can be very prominent at high latitudes, such as in Alaska, Canada, Scandinavia and Antarctica. When a space weather disturbance takes place, auroras can migrate to much lower latitudes to become visible across the continental United States, central Europe and even southern and mainland Australia.
The severity of the space weather event typically controls the range of locations where the aurora is visible. The strongest events are the most rare.
So, if you’re interested in hunting auroras, keep an eye on your local space weather forecasts (US, Australia, UK, South Africa and Europe). There are also numerous space weather experts on social media and even aurora-hunting citizen science projects (such as Aurorasaurus) that you can contribute towards!
A rare sighting of the aurora australis from central Australia, with Uluru in the foreground.
Get outside and witness one of nature’s true natural beauties – aurora, Earth’s gateway to the heavens.
Brett Carter, Associate Professor, RMIT University and Elizabeth A. MacDonald, Space Physicist, NASA
This article is republished from The Conversation under a Creative Commons license. Read the original article.
By the turn of the second millennium, Swahili people embraced Islam, and some of their grand mosques still stand at the UNESCO World Heritage sites of Lamu in Kenya and Kilwa in Tanzania.
Self-governance ended following Portuguese colonization in the 1500s, with control later shifting to the Omanis (1730-1964), Germans in Tanganyika (1884-1918) and British in Kenya and Uganda (1884-1963). Following independence, coastal peoples were absorbed into the modern nation-states of Somalia, Kenya, Tanzania, Mozambique and Madagascar.
The Swahili island settlement of Kilwa, in present-day Tanzania, grew over centuries to be a major coastal city and trading center. Pictures From History/Universal Images Group via Getty Images
So who were the Swahili people, and where did their ancestors originally come from?
Ironically, the story of Swahili origins has been molded almost entirely by non-Swahili people, a challenge shared with many other marginalized and colonized peoples who are the modern descendants of cultures of the past with extraordinary achievements.
Working with a team of 42 colleagues, including 17 African scholars and multiple members of the Swahili community, we’ve now published the first ancient DNA sequences from peoples of the Swahili civilization. Our results do not provide simple validation for the narratives previously advanced in archaeological, historical or political circles. Instead, they contradict and complicate all of them.
Colonization affected how the story was told
Western archaeologists in the mid-20th century emphasized the connections of the medieval Swahili to Persia and Arabia, sometimes suggesting that their impressive achievements could not have been attained by Africans.
Post-colonial scholars, including one of us (Kusimba), pushed back against that view. Earlier researchers had inflated the importance of non-African influences by focusing on imported objects at Swahili sites. They minimized the vast majority of locally made materials and what they revealed about African industry and innovation.
But viewing Swahili heritage as primarily African or non-African is too simplistic; In fact, both perspectives are byproducts of colonialist biases.
The truth is that colonization of the East African coast did not end with the departure of the British in the middle of the 20th century. Many colonial institutions were inherited and perpetuated by Africans. As modern nation-states formed, with governments controlled by inland peoples, Swahili people continued to be undermined politically and economically, in some cases as much as they had been under foreign rule.
Decades of archaeological research in consultation with local people aimed to address the marginalization of communities of Swahili descent. Our team consulted oral traditions and used ethnoarchaeology and systematic surveys, along with targeted excavations of residential, industrial and cemetery locations. Working with local scholars and elders, we unearthed materials such as pottery, metal and beads; food, house and industrial remains; and imported objects such as porcelain, glass, glass beads and more. Together they revealed the complexity of Swahili everyday life and the peoples’ cosmopolitan Indian Ocean heritage.
For generations, Swahilis have maintained matrilineal family burial gardens such as this one in Faza town, Lamu County. Chapurukha Kusimba, 2012, CC BY-ND
Ancient DNA analysis was always one of the most exciting prospects. It offered the hope of using scientific methods to obtain answers to the question of how medieval people are related to earlier groups and to people today, providing a counterweight to narratives imposed from outside. Until a few years ago, this kind of analysis was a dream. But because of a technological revolution in 2010, the number of ancient humans with published genome-scale data has risen from nothing to more than 10,000 today.
Surprises in the ancient DNA
We worked with local communities to determine the best practices for treating human remains in line with traditional Muslim religious sensitivities. Cemetery excavations, sampling and reburial of human remains were carried out in one season, rather than dragging on indefinitely.
A detailed line drawing captures the way one person’s remains were discovered during cemetery excavation at Mtwapa in 1996. Eric Wert, 2001, CC BY-ND
Our team generated data from more than 80 people, mostly elite individuals buried in the rich centers of the stone towns. We will need to wait for future work to understand whether their genetic inheritance differed from people without their high status.
Contradicting what we had expected, the ancestry of the people we analyzed was not largely African or Asian. Instead, these backgrounds were intertwined, each contributing about half of the DNA of the people we analyzed.
We found that Asian ancestry in the medieval individuals came largely from Persia (modern-day Iran), and that Asians and African ancestors began mixing at least 1,000 years ago. This picture is almost a perfect match to the Kilwa Chronicle, the oldest narrative told by the Swahili people themselves, and one almost all earlier scholars had dismissed as a kind of fairy tale.
Another surprise was that, mixed in with the Persians, Indians were a significant proportion of the earliest migrants. Patterns in the DNA also suggest that, after the transition to Omani control in the 18th century, Asian immigrants became increasingly Arabian. Later, there was intermarriage with people whose DNA was similar to others in Africa. As a result, some modern people who identify as Swahili have inherited relatively little DNA from medieval peoples like those we analyzed, while others have more.
One of the most revealing patterns our genetic analysis identified was that the overwhelming majority of male-line ancestors came from Asia, while female-line ancestors came from Africa. This finding must reflect a history of Persian males traveling to the coast and having children with local women.
One of us (Reich) initially hypothesized that these patterns might reflect Asian men forcibly marrying African women because similar genetic signatures in other populations are known to reflect such violent histories. But this theory does not account for what is known about the culture, and there is a more likely explanation.
Even under colonial rule and through modern times, traditional Swahili culture has retained its matriarchal nature. Pictures from History/Universal Images Group via Getty Images
Traditional Swahili society is similar to many other East African Bantu cultures in being substantially matriarchal – it places much economic and social power in the hands of women. In traditional Swahili societies even today, ownership of stone houses often passes down the female line. And there is a long recorded history of female rulers, beginning with Mwana Mkisi, ruler of Mombasa, as recorded by the Portuguese as early as the 1500s, down to Sabani binti Ngumi, ruler of Mikindani in Tanzania as late as 1886.
Our best guess is that Persian men allied with and married into elite families and adopted local customs to enable them to be more successful traders. The fact that their children passed down the language of their mothers, and that encounters with traditionally patriarchal Persians and Arabians and conversion to Islam did not change the coast’s African matriarchal traditions, confirms that this was not a simple history of African women being exploited. African women retained critical aspects of their culture and passed it down for many generations.
How do these results gleaned from ancient DNA restore heritage for the Swahili? Objective knowledge about the past has great potential to help marginalized peoples. By making it possible to challenge and overturn narratives imposed from the outside for political or economic ends, scientific research provides a meaningful and underappreciated tool for righting colonial wrongs.
Chapurukha Kusimba, Professor of Anthropology, University of South Florida and David Reich, Professor of Genetics and of Human Evolutionary Biology, Harvard University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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