The spectacular aurora borealis earlier this May showed the power that solar storms can unleash in the form of radiation, but every now and then the sun does something even more devastating. Known as “solar particle events,” these proton bursts coming directly from the sun’s surface can shoot into space like a spotlight.
Records show that the Earth experiences an extreme solar event about once every thousand years that can severely damage the ozone layer and increase ultraviolet (UV) radiation levels at the surface. We analyze what happens during such an extreme event in a paper published on Monday, July 1. We also show that these events can have dramatic effects on life on the planet when the Earth’s magnetic field is weak.
Earth’s critical magnetic shield
The Earth’s magnetic field provides an important protective cocoon for life by deflecting electrical radiation from the sun. In its normal state, it functions like a giant bar magnet, with lines of force rising from one pole, spinning around, and returning to the other pole in a pattern sometimes described as an “inverted grapefruit.” The vertical orientation of the poles allows ionizing cosmic radiation to penetrate into the upper atmosphere, where it interacts with gas molecules to create the glow we know as the aurora borealis.
But over time, the field changes a lot. 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%. The geological record shows centuries, even millennia, when the geomagnetic field was very weak or even non-existent. By looking at Mars, which in the ancient past lost its global magnetic field, we can see what Earth would be like without its magnetic field, and consequently much of its atmosphere. In May, shortly after the aurora borealis, Mars was hit by a powerful solar event. This disrupted the Mars Odyssey spacecraft and caused radiation levels about 30 times higher than those seen during chest X-rays on the Martian surface.
Proton force
The Sun’s outer atmosphere emits a constantly oscillating stream of electrons and protons known as the “solar wind.” However, the Sun’s surface also occasionally emits bursts of energy, mostly protons, in solar particle events often associated with solar flares.
Protons are much heavier than electrons and carry more energy, so they can reach lower altitudes in the 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 flares occur each solar cycle (about 11 years), but scientists have found traces of much more powerful events throughout Earth’s history. Some of the most extreme were thousands of times stronger than those recorded by modern instruments.
Extreme solar particle events
These extreme solar particle events occur approximately every few thousand years. The last one occurred around AD 993 and was used to demonstrate the use of woodcutting in Viking building in Canada in AD 1021.
Less ozone, more radiation
In addition to their direct effects, solar particle events can also set off a chain of chemical reactions in the upper atmosphere that can lead to ozone depletion. Ozone absorbs the sun’s harmful ultraviolet radiation, which can damage vision and DNA (increasing the risk of skin cancer), as well as affecting the climate.
In our new study, we used large-scale computer models of global atmospheric chemistry to investigate the effects of extreme solar particles.
We found that such an event could lead to ozone depletion for about a year, increasing surface UV radiation and DNA damage. However, if the solar proton event occurred at a time when the Earth’s magnetic field was very weak, ozone damage would last for six years, UV radiation levels would increase by 25%, and the rate of DNA damage caused by sunlight would increase by 25% to 50%.
Particle explosions from the past
How likely is this deadly combination of a weak magnetic field and an extreme solar proton event? Given how frequently each of these events occurs, it is likely that they occur together relatively often. In fact, this combination of events could explain many mysterious events in Earth’s past.
The last period of weak magnetic field, which included a temporary transition between the north and south poles, began 42,000 years ago and lasted about 1,000 years. Many important evolutionary events occurred during this period, 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 was 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 from the Flinders Ranges of South Australia, followed a 26 million year period of weak or no magnetic field.
Similarly, the rapid evolution of various animal groups during the Cambrian explosion (about 539 million years ago) was linked to geomagnetism and high levels of ultraviolet radiation. The simultaneous evolution of eyes and hard body shells in several unrelated groups was described as the best way to both detect and avoid harmful incoming UV rays while “running away from the light.”
Source: Port Altele
