The solar wind comprises charged particles - mostly ionized hydrogen and helium - hurtling toward us at speeds of several hundred kilometers a second (about a million miles per hour). The atmosphere of the Earth is protected from the ravages of the solar wind by the Earth's magnetic field. This was probably very important for the evolution, and survival, of the Earth's atmosphere. The reason Mars is now devoid of surface liquid water seems likely to be because of its lack of a magnetic field for protection against loss of oxygen by solar wind erosion. When it was much younger - only a billion years old or so - the Sun was also much more magnetically active than it is today. Its wind was likely much stronger and more violent too. In order to understand the evolution of the Earth's atmosphere through time, we must first begin to understand what the effect of the early solar wind was, and whether the terrestrial magnetic field was sufficiently strong to resist it.
We studied the solar wind–magnetosphere interaction for the young Sun and Earth corresponding to a time of about 3. Gyr ago - about the time when life first emerged. We performed detailed magnetohydrodynamic simulations for the young Sun by using a surface magnetic field map with enhanced field strength guided by observations of young solar-like stars. The solar wind model was propagate to the Earth’s magnetosphere and we computed how the magnetosphere was affected. We also looked at how this interaction changed with solar rotation rate and sunspot placement, both of which have changed significantly over the intervening 3.5 billion years. The solar rotation rate has halved and sunspots have migrated from appearing more toward the poles to their current low-latitudes. The Earth's magnetic field strength was also perhaps only half its present strength early times. We find that the early solar wind would have penetrated much closer to the Earth than the present day wind, and consequently would have had much greater access to the paleoatmosphere, with potential for erosion. One of the key references parameters for the solar wind effect on the magnetosphere is the so-called "standoff distance", which is the distance from the Earth at which the solar wind is stopped by the magnetic field. The present day standoff distance is about . We find that the paleoearth would likely have suffered a standoff distance of only 5 earth radii (RE) and perhaps less, compared with the present day value of about 10.7 RE. This work formed part of the PhD thesis of Dr. Glenn Sterenborg and was published in Volume 116 of the Journal of Geophysical Research in 2011.
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