21 November 2012
In our Sun’s most active years, enormous snake-like bubbles of plasma emerge overhead on Earth at nightfall. You can’t see them, but these bubbles can bend and disperse radio waves, interfering with communications networks. Now, a satellite soaring low in Earth’s orbit has observed the continuous birth of these evening-time bubbles for the first time, and scientists have started to chart their evolution.
Plasma bubbles are stretches of the ionosphere that are far less dense than their surroundings, hanging at least 300 kilometers above the Earth. That’s technically in space, which begins about 100 kilometers above sea level. The bubbles span the equator, stretching from north to south and reaching lengths of 2,000 to 3,000 kilometers – about 1/6th the total distance between the poles. Chaosong Huang, a plasma physicist researching the bubbles for the Air Force Research Laboratory in New Mexico, described them as being shaped like a paper towel tube that has been squished a bit, somewhat elliptical but also somewhat rectangular.
Understanding how the bubbles live, survive and die is useful, he said, because the bubbles distort radio waves, such as those that carry GPS signals. “Imagine that the light from a flashlight passes through a piece of glass. If the glass is smooth and uniform, the light will pass through the glass and remain straight on the other side of the glass,” said Huang. “However, if the smooth and uniform glass is replaced by a layer of broken glass, with very tiny pieces, the light from the flashlight will be refracted and diffracted.”
Huang’s study, published today in Journal of Geophysical Research – Space Physics, used a satellite to investigate the life cycle of the bubbles. He and his colleagues found that the evolution of the bubbles depends on the solar cycle. The solar cycle is the 11-year change in the sun’s radiation level. At solar maximum, the sun’s radiation is most intense. The solar cycle started to increase in 2010 and the next maximum is expected in 2013.
The satellite observed the bubbles’ birth in October 2011. Huang found that the large bubbles continuously emerge along the point at which day becomes night, called the sunset line, from Asia to the American continents. The plasma bubbles can stick around in the ionosphere for about three hours during these years of increased solar activity. That’s very different from how they behave during the solar minimum, during which they often appear at about midnight and can last as long as seven hours.
The difference is due to the motion of the ionosphere, Huang said, since plasma bubbles are bigger and most robust when the ionosphere is high, where ions are plenty and neutral atoms are rare, filling the air with positive and negative charge. When the Sun is near its maximum, the ionosphere tends to rise at sunset, then quickly descend overnight. And the bubbles follow suit – emerging at sunset, and dissolving as the night progresses, when the ionosphere descends into the neutral portion of the atmosphere.
Huang, C., de La Beaujardiere, O., Roddy, P., Hunton, D., Ballenthin, J., & Hairston, M. (2012). Generation and characteristics of equatorial plasma bubbles detected by the C/NOFS satellite near the sunset terminator Journal of Geophysical Research, 117 (A11) DOI: 10.1029/2012JA018163
-Sean Treacy, AGU science writing intern