Thursday, June 1, 2017

Understanding Anthropogenic Effects on Space Weather

Artist's depiction of NASA's Van Allen Probes, with the Van Allen radiation belts rendered in false color for visibility  Image: NASA

Effects of human behavior are not limited to Earth's climate or atmosphere; they are also seen in the natural space weather surrounding our planet. "Space weather" in this context includes conditions in the space surrounding Earth, including the magnetosphere, ionosphere, and thermosphere.

A recent survey by a team of scientists including Phil Erickson, assistant director of MIT Haystack Observatory, has resulted in an article in the journal Space Science Reviews. The study provides a comprehensive review of anthropogenic, or human-caused, space weather impacts, including some recent findings using NASA's Van Allen Probes twin spacecraft.

As space scientist James Van Allen discovered in the 1950s and 1960s, two radiation belts surround Earth with a slot between them. The inner edge of the outer Van Allen radiation belt is particularly interesting, as it is composed of high-energy "killer" electrons that have the potential to permanently damage spacecraft. Tracking the inner edge of the radiation belt is important for GPS navigation, communication, and other satellite-based systems to help protect them from this naturally occurring radiation. 

Until recently, it was thought that the inner edge of the outer belt was under nearly all conditions located at the plasmapause, the outer boundary of cold, dense plasma surrounding Earth that is produced daily by the sun's extreme ultraviolet rays. During geomagnetic storms, extra energy from solar flares and coronal mass ejections interact with and compress the plasmasphere. Scientists originally thought that under these conditions, the inner edge of the outer Van Allen belt would contract with the compression of the plasmasphere and move closer to Earth.

Research using the Van Allen Probes has discovered instead that during particularly intense geomagnetic storms, the inner edge of the outer belt does not follow suit but instead keeps its distance from the Earth, holding off the inner extent of "killer electrons" possessing damage potential. This inner limit to high-energy electrons occurs at the edge of strong human-origin radio transmissions created for a very different purpose.

Strong very low frequency (VLF) radio waves have been used for nearly a century to communicate with submarines, as they penetrate seawater well. But in addition to traveling through the ocean, the VLF waves also propagate upward along magnetic field lines and form a "bubble" of VLF transmissions, reaching to about the same spot that the ultra-relativistic electrons seem to stop during superstorms. The communications signals can interact with and remove some of these high-energy particles through loss to our atmosphere. This new understanding implies that human-origin systems can have an unexpected effect on high-energy space weather around our planet during these unusual, intense storms in space.

The Space Science Reviews survey also explores a more direct effect caused by humans on the near-Earth space environment. High-altitude nuclear detonation tests during the Cold War also affected the near-Earth environment by creating long-lasting artificial radiation belts that disrupted power grids and satellite transmissions. Such tests are now banned: In particular, the 1963 Partial Test Ban Treaty — signed by all nuclear powers at the time — specifically prohibits nuclear weapons testing in the atmosphere. However, a large body of information on the effects of these atmospheric tests exists, and the article examines these historical nuclear explosions to further study of anthropogenic effects on space weather. 

Understanding human-origin space weather under these extreme conditions allows us to greatly enhance our knowledge of natural effects and allows essential engineering and scientific work aimed at protecting the planet's ground-based and satellite technology. “Nuclear atmospheric tests were a human-generated and extreme example of some of the space weather effects frequently caused by the sun,” says Erickson. “If we understand what happened in the somewhat controlled and definitely extreme conditions caused by one of these man-made events, and combine it with studies into longer term effects such as the VLF communications 'bubble,' we can more readily advance our knowledge and prediction of natural variations in the near-space environment.”

The work highlights the importance of continuing research into space weather — both naturally occurring effects and those influenced by human behavior — as an essential part of society's advance toward a more complex, spacefaring society.


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