Starting at about 80 kilometers above Earth, the bombardment of solar ultraviolet light and X-rays strips atoms and molecules of their electrons and creates a layer of charged particles called the ionosphere. This layer reflects radio waves back to Earth and creates spectacular auroras. This zone is also the locus of a strange phenomenon called the Weddell Sea Anomaly, which can affect communications vital to security and transportation.
Typically, the density of electrons is highest in the upper layer of the ionosphere, where X-rays and ultraviolet rays are most intense. Normally, this upper layer also tends to be most electron dense during the day when the sunlight is most intense. But in the Weddell Sea Anomaly, a region near the tip of South America in the southeast Pacific Ocean, the electron density is highest not at midday but at midnight. The odd reversal was discovered in the 1950s by a team of scientists in Antarctica who sent high-frequency radio signals into the ionosphere and recorded the return signals, a measure called an ionogram.
Researchers have since attributed the location of the Weddell Sea Anomaly to the changing angle of the Earth’s magnetic fields, which enhances the effectiveness of the neutral winds in that region. But Richards et al. doubted that was the full explanation, in part because the changes in the magnetic fields’ angle are relatively small and often cancel each other out.
To look for alternative explanations, the team combined more than 50 years of ionosonde recordings with satellite data from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), a joint Taiwan-U.S. mission consisting of six microsatellites. They fed these data and other data into the field line interhemispheric model, which simulates how the Earth’s magnetic field affects the physics and chemistry of the ionosphere.
Rather than changes in the angle of magnetic field, the team found that the anomaly is confined to this region through its proximity to the auroral energy input, which influences the longitudinal changes in neutral winds and neutral densities. The findings could help scientists better understand other anomalies that occur during events such as solar flares, which can cause major interruptions to worldwide radio and satellite signals. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1002/2016JA023565, 2017)
—Emily Underwood, Freelance Writer