A remarkable aurora at the beginning of May this year demonstrated the power that solar storms can emit as radiation, but occasionally the Sun does something much more destructive.
These bursts of protons directly from the Sun’s surface, known as “solar particle events,” can shoot into space like a searchlight.
Records show that roughly every thousand years, Earth is hit by an extreme solar particle event that could cause severe damage to the ozone layer and increase the level of ultraviolet (UV) radiation at the surface.
We analyzed what happens during such an extreme event in an article published today. We also show that during times when the Earth’s magnetic field is weak, these events could have a dramatic effect on life across the planet.
Earth’s critical magnetic shield
Earth’s magnetic field provides life with a vital protective cocoon that deflects electrically charged radiation from the Sun.
In its normal state, it acts like a gigantic bar magnet, with lines of force rising from one pole, looping and falling back down at the other pole, in a pattern sometimes described as an “inverted grapefruit”.
The vertical orientation at the poles allows some ionizing cosmic rays to penetrate down into the upper atmosphere, where they interact with gas molecules to produce the glow we know as the aurora.
However, the field changes a lot over time. In the last century, the north magnetic pole traveled across northern Canada at a speed of about 40 kilometers per year, and the field weakened by more than 6%.
The geological record shows that there were periods of centuries or millennia when the geomagnetic field was very weak or even completely absent.
What would happen without Earth’s magnetic field can be seen by looking at Mars, which lost its global magnetic field and consequently most of its atmosphere in the distant past.
In May, not long after the aurora borealis, a powerful solar particle hit Mars. It disrupted the operation of the Mars Odyssey spacecraft and caused radiation levels on the surface of Mars to be about 30 times higher than what you would receive from a chest X-ray.
Proton force
The Sun’s outer atmosphere emits a constant fluctuating stream of electrons and protons known as the “solar wind”. However, the solar surface also sporadically emits bursts of energy, mostly protons, in solar particle events – which are often associated with solar flares.
Protons are much heavier than electrons and carry more energy, so they reach lower altitudes in Earth’s atmosphere and excite gas molecules in the air. However, these excited molecules only emit X-rays, which are invisible to the naked eye.
Hundreds of weak solar particle events occur in each solar cycle (roughly 11 years), but scientists have found traces of much stronger events throughout Earth’s history. Some of the most extreme were thousands of times stronger than anything recorded by modern instruments.
Extreme solar particle events
These extreme solar particle events occur roughly every few millennia. The most recent occurred around 993 AD and was used to show that Viking structures in Canada used timber harvested in 1021 AD
Less ozone, more radiation
In addition to their immediate effect, solar particle events can also set off a chain of chemical reactions in the upper atmosphere that can damage ozone. Ozone absorbs the sun’s harmful UV rays, which can damage eyesight and DNA (increasing the risk of skin cancer), as well as affect the climate.
In our new study, we used large-scale computer models of global atmospheric chemistry to investigate the effects of an extreme solar particle event.
We found that such an event could deplete ozone levels for a year or so, increase surface UV radiation levels, and increase DNA damage.
However, if the solar proton event came during a period when Earth’s magnetic field was very weak, ozone damage would last for six years, increasing UV radiation levels by 25% and increasing the rate of DNA damage caused by the sun by up to 50%.
Particle explosions from the past
How likely is this deadly combination of a weak magnetic field and extreme solar protons? Given how often each occurs, it seems likely that they occur together relatively often.
In fact, this combination of events can explain several mysterious events in Earth’s past.
The last period of weak magnetic field – including a temporary switch between the north and south poles – began 42,000 years ago and lasted about 1,000 years. Several significant evolutionary events occurred during this time, such as the disappearance of the last Neanderthals in Europe and the extinction of marsupial megafauna including giant wombats and kangaroos in Australia.
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An even larger evolutionary event has also been linked to the Earth’s geomagnetic field. The origin of multicellular animals at the end of the Ediacaran period (565 million years ago), recorded in fossils in the Flinders Mountains of South Australia, occurred after a period of 26 million years of weak or absent magnetic field.
Similarly, the rapid evolution of various groups of animals in the Cambrian explosion (about 539 million years ago) is also related to geomagnetism and high levels of UV radiation.
The contemporary evolution of eyes and hard body shells in many unrelated groups has been described as the best means of detecting and preventing harmful incoming UV radiation while “running away from the light”.
We are still only beginning to explore the role of solar activity and Earth’s magnetic field in the history of life.
Alan Cooper, Distinguished Professor, Charles Sturt University and Pavle Arsenovic, Principal Scientist, University of Natural Resources and Life Sciences (BOKU)
This article is republished from The Conversation under a Creative Commons license. Read the original article.