Occasionally, though, an event happens that can jostle them out of place, sending electrons raining down into Earth's atmosphere. These high-energy particles in space make up what are known as the Van Allen radiation belts, and their discovery was one of the first of the space age. A new study from my research team has found that electromagnetic waves generated by lightning can trigger these electron showers.
After Explorer 1 launched, though, they noticed that their instrument was detecting significantly higher levels of radiation than expected. Rather than measuring a distant source of radiation beyond our solar system, they appeared to be measuring a local and extremely intense source.
This measurement led to the discovery of the Van Allen radiation belts, two doughnut-shaped regions of high-energy electrons and ions encircling the planet.
Scientists believe that the inner radiation belt, peaking about 621 miles (1000 kilometers) from Earth, is composed of electrons and high-energy protons and is relatively stable over time.
The outer radiation belt, about three times farther away, is made up of high-energy electrons. This belt can be highly dynamic. Its location, density and energy content may vary significantly by the hour in response to solar activity.
The discovery of these high-radiation regions is not only an interesting story about the early days of the space race; it also serves as a reminder that many scientific discoveries have come about by happy accident.
It is a lesson for experimental scientists, myself included, to keep an open mind when analyzing and evaluating data. If the data doesn't match our theories or expectations, those theories may need to be revisited.
In a study led by Max Feinland, an undergraduate student in my research group, we stumbled upon some of our own unexpected observations of Earth's radiation belts. Our findings have made us rethink our understanding of Earth's inner radiation belt and the processes affecting it.
Originally, we set out to look for very rapid - sub-second - bursts of high-energy electrons entering the atmosphere from the outer radiation belt, where they are typically observed.
Many scientists believe that a type of electromagnetic wave known as "chorus" can knock these electrons out of position and send them toward the atmosphere. They're called chorus waves due to their distinct chirping sound when listened to on a radio receiver.
Feinland developed an algorithm to search for these events in decades of measurements from the SAMPEX satellite. When he showed me a plot with the location of all the events he'd detected, we noticed a number of them were not where we expected. Some events mapped to the inner radiation belt rather than the outer belt.
This finding was curious for two reasons. For one, chorus waves aren't prevalent in this region, so something else had to be shaking these electrons loose.
The other surprise was finding electrons this energetic in the inner radiation belt at all. Measurements from NASA's Van Allen Probes mission prompted renewed interest in the inner radiation belt. Observations from the Van Allen Probes suggested that high-energy electrons are often not present in this inner radiation belt, at least not during the first few years of that mission, from 2012 to 2014.
Our observations now showed that, in fact, there are times that the inner belt contains high-energy electrons. How often this is true and under what conditions remain open questions to explore. These high-energy particles can damage spacecraft and harm humans in space, so researchers need to know when and where in space they are present to better design spacecraft.
Lightning, the large electromagnetic discharges that light up the sky during thunderstorms, can actually generate electromagnetic waves known as lightning-generated whistlers.
These waves can then travel through the atmosphere out into space, where they interact with electrons in the inner radiation belt - much as chorus waves interact with electrons in the outer radiation belt.
To test whether lightning was behind our inner radiation belt detections, we looked back at the electron bursts and compared them with thunderstorm data. Some lightning activity seemed correlated with our electron events, but much of it was not.
Specifically, only lightning that occurred right after so-called geomagnetic storms resulted in the bursts of electrons we detected.
Geomagnetic storms are disturbances in the near-Earth space environment often caused by large eruptions on the Sun's surface. This solar activity, if directed toward Earth, can produce what researchers term space weather. Space weather can result in stunning auroras, but it can also disrupt satellite and power grid operations.
We discovered that a combination of weather on Earth and weather in space produces the unique electron signatures we observed in our study. The solar activity disturbs Earth's radiation belts and populates the inner belt with very high-energy electrons, then the lightning interacts with these electrons and creates the rapid bursts that we observed.
These results provide a nice reminder of the interconnected nature of Earth and space. They were also a welcome reminder to me of the often nonlinear process of scientific discovery.
Research Report:Lightning-induced relativistic electron precipitation from the inner radiation belt
Related Links
Atmospheric and Space Physics, University of Colorado Boulder
Solar Science News at SpaceDaily
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