A Neo-Assyrian table from the Library of Ashurbanipal has provided space physics researchers with what may be one of the earliest descriptions of the aurora borealis.
How many hundreds or thousands of years have reliable records of the aurora been kept? It turns out a lot. We now know, for example, of the existence of an Assyrian record dating back as far as 660 BCE, or more than 2,600 years ago. This is a big deal. Especially in terms of the study of the aurora, as well as ancient solar behavior, and how both relate to current solar behavior.
Documenting aurorae helps space and solar physicists better understand patterns of solar activity. Magnetic storms on the Sun can release giant plumes and jets of materials, some of which fall back into the Sun and some of which are ejected and spewed across the solar system, for example in the solar wind. Particles that make it to Earth can be funneled along magnetic field lines into Earth’s upper atmosphere (magnetosphere0, where they strike atmospheric particles, causing them to glow. Red aurorae, like the ones documented by the ancient Assyrians, are typically caused by low- energy electrons.
Auroras can display as both diffuse and discrete. In the first case the shape is ill-defined and the aurora is believed to be formed from trapped particles originally in the magnetosphere which then propagate into the lower ionosphere via wave-particle interactions. Thus, multiple- colored auroras can be explained by emissions from different atoms in the upper atmosphere, mainly in the region of the magnetic poles. This is also why, of course, they are more often seen in the vicinity of the N, S magnetic poles.
A great analogy has been given by Prof. Syun Akasofu- former Director of the Geophysical Institute in Fairbanks, AK - who compared the generic aurora to an image on a TV screen. In this case the (polar) upper atmosphere corresponds to the screen and the aurora to the image that would be projected on it, say for a cathode ray TV. The electron beam in the TV corresponds the electron beam in the magnetosphere. In the conventional TV motions of the image are generated by the changing impact points of the electron beam on the screen. Similarly, with the aurora, its motions – such as moving sheets or curtains- are produced by moving impact points of the magnetospheric electron beams.
As far as the recovered ancient auroral record, we have to thank Hisashi Hayakawa, lead author of a new study examining the red aurorae. Hayakawa is an astronomer at Osaka University in Japan and the Rutherford Appleton Laboratory in the
United Kingdom. Hayakawa and his colleagues identified the records by examining ancient cuneiform tablets held in the British Museum. These tablets were carved by Assyrian court scribes, whose job was to document important happenings
in the empire. They often included accounts of celestial events, like comets (ṣallummû), meteors (kakkabu rabû), and lunar and solar halos (tarbāṣu), which were thought to be omens of the future.
In the tablets studied, the Hayakawa team found two references from Nineveh (a city near current- day Mosul, Iraq) and one from Babylon (built along Iraq’s Euphrates River) that describe red aurorae, using terms like akukūtu, meaning red glow, or stating, “red covers the sky.” Using the authorship of the tablets,
researchers think the events happened sometime between 680 BCE and 650 BCE, a century earlier than previous records of aurorae.
Because they follow Earth's magnetic field lines, aurorae are most commonly seen near the poles. For example, this image of a green aurora display I took near Chena Hot Springs, AK, in March 2005:
However, it's important to note strong solar events like x-ray flares can make aurorae visible at lower latitudes. Although today it is rare to see an aurora in the Middle East, 2,000 years ago (owing to precession) the magnetic North Pole was
much closer to Mesopotamia, hovering over the Norwegian archipelago of Svalbard
instead of at its current location just 4° south of the geographic North Pole.
An intriguing aspect uncovered is that the newly identified records also match
indirect evidence of solar activity. Since 2012, several studies have found isotope data of carbon- 14 levels recorded in tree rings that indicate a strong burst of solar activity during the same time period. By adding Assyrian observational evidence to these natural archival data, scientists are better able to confirm that the event was truly a space weather event caused by an extreme solar storm.
According to Ilya Usoskin, a space physicist at the University of Oulu in Finland (not involved with the new research:
"Comparing these data from natural archives to real historical records made by contemporary astrologers at the time is very important. From it, we know that we are on the right track, because the two records match each other.”
Let us take a brief digression to note here that astrology and astronomy were inextricably bound in ancient times. Indeed, the technical and detailed observations of the ancient Sumerians could rightly be said to be the birth of astronomy. Only much later did astrology become radically divergent when it tilted into "horoscopes" or personalizing the stars for the benefit of deciphering out-sized egos.
Back to the Hayakawa research, it was found that ancient magnetic substorms occurring at the time had an intensity rivaling that from the great Quebec events in 1989.* On March 13, a monster CME - accompanied by intense aurorae- triggered induced dynamo currents powerful enough to knock out Quebec's power for nearly 9 hours, as well as fuse thick cables. According to Hayakawa:
“It is likely that the [ancient] storms were considerably large. Storms with similar intensity [today] would be harmful to modern technological infrastructures.”
As I pointed out in earlier, related blog posts e.g.
An understanding of the historical frequency of solar storms (e.g. the Carrington event of 1859) and learning how to predict such events are important for mitigating their effects on our tech-oriented society. The historical data can also help solar and space physicists model how often such extreme events occur and better assess the probability of similar extreme events. In the words of space physicist lya Usoskin:
“Direct observations from the last decades are not very useful here because they just cover too short a period of time, Such historical records are very helpful because now we know that during the last, say, 3,000 years, there were three events of that magnitude (equal to the Quebec storm), which means that on average, we may expect such disasters to occur a few times per millennia.”
Hayakawa et al's paper can be found in: Astrophysical Journal Letters and also below:
https://earth-planets-space.springeropen.com/articles/10.1186/s40623-016-0571-5
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* Given the aurora is basically regarded as a magnetic substorm we use what are known as "substorm models". In most current ones some dynamo action is required which sends currents to specific regions to provide a Lorentz force: (J ⊥ X B) where J ⊥ is the vertical current density and B the magnetic induction associated with the system.. At a particular altitude then, these J ⊥ currents can trigger a substorm.)
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