Researchers have discovered that Earth's magnetosphere carries a reversed electric charge on its morning side, contrary to long-held assumptions. Satellite data and simulations reveal negative charges there instead of positive, with the pattern flipping near the equator. This finding, led by teams from Japanese universities, explains plasma motion's role in shaping space weather.
Earth's magnetosphere, the protective bubble formed by its magnetic field, influences geomagnetic storms that can disrupt satellites and communications. Scientists previously assumed this region was positively charged on the morning side and negatively charged on the evening side, as electric forces flow from positive to negative. However, recent satellite measurements have shown the opposite: the morning side is negatively charged, while the evening side is positive.
A team from Kyoto University, Nagoya University, and Kyushu University investigated this anomaly using large-scale magnetohydrodynamic simulations. These models incorporated a steady stream of high-speed solar wind from the sun. The results confirmed the observations, indicating that the reversed pattern does not apply uniformly. In polar regions, the charge polarity aligns with traditional theory, but near the equator, it flips across a wide area.
The reversal stems from plasma motion. "In conventional theory, the charge polarity in the equatorial plane and above the polar regions should be the same. Why, then, do we see opposite polarities between these regions? This can actually be explained by the motion of plasma," explains corresponding author Yusuke Ebihara of Kyoto University. Solar magnetic energy enters Earth's field, moving clockwise on the dusk side toward the poles. Earth's field lines run upward near the equator and downward near the poles, creating opposing orientations with plasma flow that lead to the charge reversal.
"The electric force and charge distribution are both results, not causes, of plasma motion," Ebihara adds. This insight clarifies plasma convection, which drives space phenomena like radiation belts filled with high-energy particles. The research, published in the Journal of Geophysical Research: Space Physics, also has implications for other planets, such as Jupiter and Saturn, enhancing understanding of magnetospheric dynamics across the solar system.