On January 4, 2019, at 4:37 EST, the CAPER-2 mission was launched from the Andoya Space Center in Andenes, Norway, on a four-stage Black Brant XII sound rocket. Reaching a height of 480 miles before falling into the Arctic Sea, the rocket flew through the active northern lights, or northern lights, to study the waves that accelerate the electrons in our atmosphere.
CAPER-2, short for Cusp Alfvén and Plasma Electrodynamics Rocket-2, is a sounding rocket mission – a type of spacecraft that transports scientific instruments on short trips to space before returning to Earth. In addition to their relatively low prices and fast development time, sounding rockets are ideal for launch at transient events – such as the sudden formation of the aurora borealis or aurora borealis.
For the CAPER-2 scientists, flying through an aurora provides a peek into a process as fundamental as it is complex: how do particles accelerate in space? NASA studies this phenomenon in an effort to better understand not only the space environment surrounding the Earth – and thus protect our technology in the space of radiation – but also to help understand the very nature of stars and atmospheres throughout the solar system and beyond.
"Throughout the universe you have accelerated charged particles – in the atmosphere of the sun, in the solar wind, in the atmospheres of other planets, and in astrophysical objects," said Jim LaBelle, Dartmouth College space physicist in Hanover, New Hampshire, and principal investigator of the CAPER-2 mission. "An aurora introduces us to a local laboratory where we can observe these processes of acceleration by hand."
Technically, the CAPER-2 team is interested in what happens just before a dawn begins to shine. The electrons, flowing into the atmosphere of space, collide with the atmospheric gases and trigger the brightness of the aurora. Somehow, they gain speed along the way.
"When they reach our atmosphere, these electrons travel 10 times faster than before," said Doug Rowland, a space physicist at NASA's Goddard Space Flight Center in Greenbelt, Md., Who also studies particle acceleration. "We still do not understand the fundamental physics of how this happens."
The CAPER-2 team focused on a special type of aurora that forms during the day. Unlike the aurora nocturna, the aurora diurna is driven by electrons that flow directly from the Sun – and we know much less about them.
"There has been a lot of research done at regular daybreak, but daylight is much less studied," said Craig Kletzing, a space physicist at the University of Iowa in Iowa City and co-investigator of the mission. "There are good indications that there are some similarities and also some differences."
The team is focusing on how electrons that create daytime auroras are pushed by waves, in ways that may or may not differ from nocturnal auroras. Two types of waves are of special interest and have opposite effects. The Alfvén waves, named after Swedish Nobel Prize winner Hannes Alfvén, who predicted its existence in 1942, supposedly accelerate the electrons. These huge waves – measuring tens to hundreds of kilometers in length from peak to peak – propagate along the lines of Earth's magnetic field, whipping electrons back and forth.
On the other side are the Langmuir waves, which are generated by the electrons themselves – a process that steals some of the energy from the electrons and slows them down. CAPER-2 will load a high-resolution wave-particle correlator to measure them, the first probing rocket mission to do so for daylight.
"This is very data intensive," LaBelle said. "It's unique to sound the rockets so you can look at this mechanism at this level of detail."
For the launch, the CAPER-2 team traveled to northern Norway, one of the few places that can put a rocket within range of daylight. Every day, northern Norway rotates under an opening in the Earth's magnetic field, known as the northern polar cusp, where particles of the Sun can seep into our upper atmosphere.
Finding the aurora exactly where it forms is the best way to understand physical processes that are too large to replicate in a laboratory.
"It's kind of a natural laboratory," LaBelle added. "We take our experience to two different environments, where the variables are different, and then we test the theory and answer the questions."
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