UC Berkeley leads auroral imaging instrument for first-of-its-kind multi-satellite mission
Every winter, thousands of tourists travel to high-latitude regions like Scandinavia, Canada and Alaska hoping to see the Aurora Borealis or Northern Lights. Vincent Ledvina, an aurora guide and PhD student in space physics at the University of Alaska Fairbanks, estimates he leads 1000 people on aurora tours each year. Some ask Ledvina how the dazzling curtains of light are created, and he tells them that auroras occur when high-energy particles from space are funneled by the Earth’s magnetosphere into the polar atmosphere and collide with different molecules in the air. Ledvina says that more specific questions can be difficult to answer.
“Sometimes people ask what causes different shapes in the aurora, and all I can say is, ‘who knows?’” says Ledvina.
Much of what we see as auroras near the northern and southern magnetic poles begins as plasma currents in the Earth’s magnetotail—the part of the magnetosphere that is stretched behind the Earth like a windsock by the solar wind. Thoroughly understanding how processes that occur hundreds of thousands of miles away create specific auroral formations in the atmosphere will require measurements of particles and electric currents flowing in from the magnetotail combined with orbital auroral imaging. NASA’s Cross-scale Investigation of Earth’s Magnetotail and Aurora (CINEMA) mission aims to do just that.
“Explosive magnetospheric phenomena can have major impacts on our technological systems,” says Robyn Millan, a professor of physics and astronomy at Dartmouth College and principal investigator on CINEMA. “We fundamentally don’t understand when the magnetotail is going to release magnetic energy and how much impact it might have.”Millan, who began her career as a graduate student researcher at UC Berkeley’s Space Sciences Laboratory (SSL) where she studied X-ray bursts emitted by electrons entering Earth’s atmosphere, has served as principal investigator for multiple missions, but CINEMA will be her most complex mission to date. The NASA mission—comprised of a constellation of nine small satellites—will be the first to focus on remote sensing of the magnetotail. Millan believes that the measurements and images that CINEMA gathers will lead to a better understanding of the conditions that initiate substorms and how they create specific auroral forms.
CIMEMA’s camera builds on decades of auroral imaging refinements
When energetic particles from the solar wind strike the sunward-facing bow shock of the magnetosphere, they are captured by the magnetotail where some are cycled back toward the Earth creating a consistent plasma convection pattern known as the Dungey cycle. Energy accumulated by that cycle is frequently disrupted by explosive magnetic events known as substorms, which cause plasma flows to surge toward the Earth, generating intense auroras. Our conception of substorms began in the 1950s and 60s, driven largely by measurement of ground based all-sky imaging of auroras and particle measurements taken by satellites.
All-sky cameras use very wide-angle lenses to capture almost 180-degrees of the sky from horizon to horizon. Early models, which took long, monochromatic exposures on film, were only sensitive enough to observe broad trends. Stephen Mende, a research physicist at SSL and the deputy instrument lead and senior mentor of CINEMA’s auroral imager team, helped develop some of the first all-sky cameras used to image specific wavelengths of light. The observed color of an aurora is determined by the type of atmospheric gas that the bombarding particles collide with. Mende—who then worked at Lockheed Martin—helped design a camera that compressed the light from a wide-angle lens, split it to create two separate images and filtered each image to observe a different color. This allowed the team to deduce the energy level of the particles that created the photographed auroras.
Mende later designed a digital all-sky imaging system that was used by NASA’s Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. THEMIS combined magnetic and particle measurements gathered by 5 satellites with auroral imagery captured by 21 ground-based cameras. THEMIS revealed, very broadly, how substorms work, but it was still limited by ground-based cameras, which can’t take images through clouds.
“People have known about substorms for over 50 years, and we still don’t understand them,” says Mende. “It’s important that we learn how our own Earth systems work.”
Mende, along with a team of scientists and engineers at SSL, hoped to overcome previous limitations when they designed the camera that will be included on each of the nine CINEMA spacecraft. CINEMA’s camera will use a similar sensor as a modern digital camera, but capturing dim auroras from a satellite moving at some 7 kilometers per second is a challenging prospect. CINEMA’s camera will address this problem by taking multiple short exposures, slightly shifting them to compensate for spacecraft motion and then combining them into a single image.
The ubiquitous airglow of Earth’s atmosphere presented another challenge to the imaging team. One of the wavelengths emitted by airglow is 557.7 nanometers (nm), which is also the wavelength of green auroras produced by excited atomic oxygen. If the cameras were attuned to this wavelength they would be overwhelmed by background noise, so the SSL team designed an imaging instrument that would be attuned to 391.4 nm, a violet/ultraviolet glow observed in aurora produced by ionized nitrogen. These two auroral wavelengths often occur simultaneously and so each wavelength can be used to observe the same particle interactions.
“Developing an imager simulation that realistically accounted for optical distortion, spacecraft motion and noise was a big challenge,” says Claire Gasque, an assistant researcher at SSL and a member of the auroral imaging team. “It was a relief when auroras emerged clearly in our simulated images. That gave us confidence that the imager will successfully capture the specific forms we’re targeting.”
CINEMA will measure the magnetotail from afar
The CINEMA team will focus on three different auroral forms and attempt to determine how they are linked to the structure of magnetotail. A poleward boundary intensification (PBI) is a sudden brightening of the poleward edge of an aurora. It is thought to be the first sign of a magnetic reconnection event in the magnetotail—events which occur when magnetic field lines in the magnetosphere encounter oppositely magnetized plasma from the solar wind. The second auroral form CINEMA will study is the auroral streamer, which is a long auroral arc that is oriented north-to-south as opposed to the more common east-to-west formation. The final form is auroral beads, which appear as a stippled line of auroral motes, somewhat like beads on a necklace.
These three auroral forms, which can be seen with the naked eye, may be signatures of invisible plasma flows in the magnetotail. To find out, CINEMA will employ a remote-sensing technique to monitor the magnetic structure of the most distant reaches of the magnetotail. CINEMA’s magnetometers will measure electric currents flowing between the magnetosphere and the ionosphere, and at the same time, particle detectors will measure electrons and ions flowing in from the magnetotail. The angle of these particle flows, compared with the orientation of Earth’s magnetic field, will reveal the structure of the magnetotail along the same magnetic field line, which will allow the CINEMA team to study how the magnetotail changes before explosive events like substorms.
“I hope we’ll learn whether there’s a particular frequency in the plasma flow that guarantees that a streamer will emerge,” says Yen-Jung Wu, an assistant research physicist at SSL and instrument lead for CINEMA’s auroral imager. “Of course, we need to consider that the ionosphere might play a role as well.”
It may turn out that localized magnetic fields in the ionosphere play a larger role in shaping auroral forms than magnetospheric plasma flows—there is still disagreement among auroral physicists over this point. CINEMA’s proximity of the ionosphere-magnetosphere plasma system will allow for detailed study of the region and should clarify which magnetic structures are most important to auroral formation.
CINEMA will include more snapshots of auroras than ever before
A person standing on Earth’s equator at midnight might be directly beneath an intense plasma flow, but the associated aurora wouldn’t emerge at the equator. The energy would follow a magnetic field line to the northern or southern pole where all the magnetic field lines are bottlenecked into a zone called the auroral oval. Each CINEMA spacecraft will pass through the nightside auroral oval about 30 times per day. Though THEMIS took measurements from within the magnetotail itself, the region was too vast for five spacecraft to get a complete picture of the plasma flows there. From low-Earth orbit, CINEMA’s remote sensing may produce findings on the magnetotail that surpass those of the groundbreaking THEMIS mission.
In the first phase of the science mission, the nine spacecraft, following one another in a line, will allow observation of the evolution of the plasma flows and aurora over a 45-minute period. The series of images will serve as a short video with associated magnetic measurements for each frame. In the next phase, the spacecraft will form a 3-by-3 grid formation, which will allow for broader spatial measurements and images.
“We’re going to be able to provide unprecedented measurements that enable the entire heliophysics community to do all different kinds of science,” says Millan.
Ledvina is a member of that broader community who stands to benefit from CINEMA’s datasets—he studies auroral beads and substorms. He’s also a consultant for the citizen science portion of the CINEMA mission, which NASA will evaluate for inclusion during the mission’s next key decision point. The Multi- platform Observations from Volunteers: Ionospheric Experiments (MOVIE) project will work with aurora chasers to record auroras from the ground as CINEMA spacecraft are passing overhead. In addition to getting another angle on specific aurora events, these volunteer videos will be recorded on consumer-grade cameras that will not be limited to a narrow wavelength band and will provide an alternate record of the entire spectrum of visible light. Ledvina believes that such collaborations between scientists and aurora chasers can create a virtuous spiral that will benefit everyone.
“As an aurora chaser, I’m always trying to figure out how to predict substorms based on solar wind data,” says Ledvina. “Hopefully CINEMA will help, because when the auroras go crazy, that’s the best part of the show.”
CINEMA is expected to launch no earlier than 2030. Additional mission partners include Space Dynamics Laboratory, which will build the auroral imaging instrument, and Blue Canyon Technologies, which will build the spacecraft.