The larger auroral oval relative to the modern is the result of a weaker dipole magnetic field and stronger solar wind dynamic pressure. The auroral intensity is brighter due to solar wind densities many times greater than those today, and the dominant color reflects greater energies of the precipitating particles and the mildly reducing Paleoarchean atmosphere.
Credit: J. Tarduno and R. Cottrell/University of Rochester
SYDNEY: A weak magnetic field and powerful solar wind stripped water from the early Earth's atmosphere 3.5 billions years ago and created stunning auroras, scientists said.
Scientists have long thought that Mars' small magnetic field left it vulnerable to the solar wind, a stream of charged particles emitted by the Sun that interacts with Earth's magnetic field and atmosphere, and forms the auroras on Earth.
Because Mars is small, its atmosphere may have been stripped away entirely by the solar wind billions of years ago. Earth's size and strong magnetic field protected its atmosphere.
Ancient magnetic field much weaker
But the oldest measurement made of the ancient Earth's magnetic field, published in Science, shows 3.5 billion years ago the field was much weaker and the solar wind much stronger, enabling it to rip water molecules from the Earth's atmosphere.
"With a weak magnetosphere and a rapid-rotating young Sun, the Earth was likely receiving as many solar protons on an average day as we get today during a severe solar storm," says geophysicist John Tarduno from the University of Rochester, New York state, lead author of the study.
"That means the particles streaming out of the Sun were much more likely to reach Earth. It's very likely the solar wind was removing volatile molecules, like hydrogen, from the atmosphere at a much greater rate than we're losing them today."
Rock crystals point to magnetic field
Since hydrogen was probably locked up in water molecules, the result means that the early Earth probably had much less water, says Tarduno.
Tarduno and colleagues from the University of KwaZulu-Natal in South Africa measured the magnetic field using ancient silicate crystals found in volcanic rocks in South Africa.
These crystals, which contain a record of conditions of the early Earth, are extremely rare. Not only must they have remained unaltered by heat and pressure during billions of years of Earth's history, they need to also contain the original record of the magnetic field. In some crystals, a new magnetic field is superimposed on the rocks because of the interactions of the solar wind and the Earth's atmosphere.
Ancient solar wind much stronger
The researchers used very sensitive magnetometers to measure the record of the magnetic field in nanometre-sized inclusions within the crystals. They then compared this with estimates of the strength of the solar wind 3.5 billion years ago, made by astronomer and physicist Eric Mamajek from the University of Rochester.
Mamajek found the ancient solar wind was "a couple of orders of magnitude stronger" than it is today. The results mean the boundary where the Earth's magnetic field throws off the solar wind (called the magnetopause) would have been much nearer Earth than it is today, the researchers say. This left the Earth's atmosphere more vulnerable to penetration by the solar wind.
Space physicist Brian Fraser, director of the Centre for Space Physics at the University of Newcastle in Australia, said it "made sense" and that it would have led to impressive auroras back then.
"The auroral zones would have headed to the equator, which is what happens [now] when you have large magnetic storms," he says.
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