The remarkable aurora in early May of this year showed the power that solar storms can emit as radiation, but every now and then the sun does something much more destructive. These explosions of protons, fired directly from the sun’s surface and shooting out into space like a searchlight, are known as “solar particle events.”
Data shows that Earth is hit by an extreme solar particle explosion about once every thousand years, which can cause severe damage to the ozone layer and increase the amount of ultraviolet (UV) radiation at the Earth’s surface.
We have analyzed what happens during such an extreme event in a paper published today. We also show that at times when the Earth’s magnetic field is weak, these events can have a dramatic effect on life on the planet.
The Earth’s Critical Magnetic Shield
The Earth’s magnetic field provides a crucial protective cocoon for life by deflecting electrically charged radiation from the sun. In its normal state, it functions like a giant bar magnet, with field lines rising from one pole, looping around it, and diving back down at the other pole in a pattern sometimes described as a “reverse grapefruit.” The vertical orientation at the poles allows some of the ionizing cosmic radiation to penetrate into the upper atmosphere, where it interacts with gas molecules to create the glow known as the aurora borealis.
However, the field changes dramatically over time. Over the past century, the north magnetic pole has been moving across northern Canada at a rate of about 40 kilometers per year, and the field has weakened by more than 6%. Geological data show that there have been periods of centuries or millennia when the geomagnetic field was very weak or even absent.
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We can see what would happen without Earth’s magnetic field by looking at Mars, which lost its global magnetic field in the distant past, and most of its atmosphere as a result. In May, not long after the aurora, a strong solar particle event hit Mars. It disrupted the operation of the Mars Odyssey spacecraft and caused radiation levels on the Martian surface that were about 30 times higher than what you would receive during a chest X-ray.
The power of protons
The Sun’s outer atmosphere emits a constant, fluctuating stream of electrons and protons known as the “solar wind.” However, the Sun’s surface also sporadically emits bursts of energy, usually 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 the Earth’s atmosphere, exciting gas molecules in the air. However, these excited molecules emit only X-rays, which are invisible to the naked eye.
Hundreds of weak solar particle events occur every solar cycle (about 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 about every few millennia. The most recent one occurred around 993 AD and was used to show that Viking buildings in Canada used wood that had been cut in 1021 AD.
Less ozone, more radiation
In addition to their direct effect, solar particles can also trigger a series of chemical reactions in the upper atmosphere that can deplete ozone. Ozone absorbs harmful UV radiation from the sun, which can damage vision and also DNA (increasing the risk of skin cancer), and can also affect the climate.
In our new study, we used large computer models of global atmospheric chemistry to investigate the consequences of an extreme solar particle explosion.
We found that such an event could reduce ozone levels for about a year, increasing UV levels at the Earth’s surface and increasing DNA damage. But if a solar proton event occurred during a period when the Earth’s magnetic field was very weak, the ozone damage would last for six years, increasing UV levels by 25% and increasing the rate of solar-induced DNA damage by up to 50%.
Particle explosions from the past
How likely is this deadly combination of weak magnetic field and extreme solar proton events? Given how common each is, it seems likely that they occur together relatively often.
This combination of events may provide an explanation for a number of mysterious events from Earth’s past.
The most recent period of weak magnetic field – including a temporary switching of the north and south poles – began 42,000 years ago and lasted about 1,000 years. Several important evolutionary events occurred around this time, including the disappearance of the last Neanderthals in Europe and the extinction of marsupial megafauna, including giant wombats and kangaroos in Australia.
An even larger evolutionary event is also 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 Ranges in South Australia, occurred after a period of 26 million years of weak or absent magnetic field.
Similarly, the rapid evolution of diverse animal groups in the Cambrian explosion (about 539 million years ago) has also been linked to geomagnetism and high UV levels. The simultaneous evolution of eyes and hard body shells in multiple unrelated groups has been described as the best way to detect and avoid the harmful incoming UV rays, in a “flight from the light”.
We are only at the beginning of our investigation into the role of solar activity and Earth’s magnetic field in the history of life.