A new space-based telescope aims to address a key solar mystery.
A new experiment will explore a region of the Sun that’s tough to see from the surface of the Earth. The solar corona—the elusive, pearly white region of the solar atmosphere seen briefly during a total solar eclipse—is generally swamped out by the dazzling Sun. Now, the Coronal Diagnostic Experiment (CODEX) will use a coronagraph to create an ‘artificial eclipse’ in order to explore the poorly understood middle corona region of the solar atmosphere.
CODEX launched as part of the cargo manifest on SpaceX’s Cargo Dragon this week, on mission CRS-31. CRS-31 arrived at the ISS and docked at the Harmony forward port of the station on November 5th.
CODEX is a partnership between NASA’s Goddard Spaceflight Center, Italy’s National Institute for Astrophysics (INAE) and KASI (Korea Astronomy and Space Science Institute). Technical expertise for the project was provided by the U.S. Naval Research Laboratory (NRL).
CODEX will be mounted on the EXPRESS (Expedite the Processing of Experiments to the Space Station) Logistics Carrier Site 3 (ELC-3) on the ISS.
Why Use Coronagraphs
Coronagraphs work by blocking out the Sun with an occulting disk. The disk used in CODEX is about as wide as an orange. Though coronagraphs can work on Earth, placing them in space is an easy way to eliminate unwanted light due to atmospheric scattering.
Targeting the middle region of the corona is crucial, as it’s thought to be the source of the solar wind. But what heats this region to temperatures actually hotter than the surface below? This rise is in the order of a million degrees, versus 6000 degrees Celsius for the solar photosphere. The same unknown process accelerates particles to tremendous speeds of over a million kilometers an hour.
CODEX seeks to address this dilemma, and will measure Doppler shifts in charged particles at four filtered wavelengths. The instrument will need to center and track the Sun from its perch on the exterior of the ISS. To this end, this must occur while speeding around the Earth once every 90 minutes. CODEX will be able to see the Sun roughly half of the time, though seasons near either solstice will allow for near-continuous views.
CODEX will work with NASA’s Parker Solar Probe and ESA’s Solar Orbiter (SolO) in studying this coronal heating dilemma. In addition, it will also join the Solar Heliospheric (SOHO’s) LASCO C2 and C3 coronagraph in space. Another new coronagraph instrument in space is the National Oceanic Atmospheric Administration’s CCOR-1 (Compact Coronagraph) aboard the GOES-19 satellite in geosynchronous orbit.
A Solar Wind Riddle
“CODEX measures the plasma’s temperature, speed and density around the whole corona between 3 and 10 solar radii, and will measure how those parameters evolve in time, providing new constraints on all theories of coronal heating,” Niicholeen Viall (GFSC-Solar Physics Laboratory) told Universe Today. “Parker Solar Probe measures these plasma parameters in the upper corona (getting as close as 10 solar radii) in great detail, but it makes those measurements in situ (from one one location in space and time) and only briefly that close to the Sun.”
The goal of CODEX is to provide a holistic view of solar wind activity. “In contrast, CODEX provides a global view and context of these plasma parameters and their evolution,” says Viall. “Additionally, CODEX extends the measurements much closer to the Sun than Parker Solar Probe (PSP), linking the detailed measurements made at PSP at 10 solar radii through the middle corona, down to ~3 solar radii, closer to their source. This is important because most of the coronal heating has already taken place by 10 solar radii, where PSP measures.”
A Dual Mystery
Two theories vie to explain the solar heating mystery. A first says that tangled magnetic fields are converted into thermal power. These are in turn fed into the corona as bursts of energy. Another says that oscillations known as Alfvén waves inject energy in a sort of feedback loop in the lower corona.
“Solar Orbiter has (an) EUV (Extreme ultraviolet) and white light imager that could be used to connect the CODEX measurements to their sources on the Sun,” says Viall.
Understanding this region and the source of the solar wind is crucial to predicting space weather. This is especially vital when the Sun sends powerful corona mass ejections our way. Not only can these spark low latitude aurorae, but these can also impact communications and pose a hazard to satellites and astronauts in space.
“CODEX is similar to all coronagraphs, in that they block light out from the photosphere to see the much fainter corona.” Says Viall. CODEX’s field of view has overlap with, but is different than SOHO’s coronagraphs and CCOR. The largest difference though, is that CODEX has special filters that can provide the temperature and speed of the solar wind, in addition to the density measurements that white light coronagraphs always make.”
The Past (and Future) of Coronagraphs in Space
Furthermore, there’s also a history of coronagraphs aboard space stations. This goes all the way back to the white-light coronagraph aboard Skylab in the early 1970s.
Looking to the future, more coronagraphs are headed space-ward. ESA’s solar-observing Proba-3 launches at the end of November. Proba-3 will feature the first free-flying occulting disk as part of the mission. PUNCH (the Polarimeter to UNify the Corona and Heliosphere) will feature four micro-sat orbiters. The mission will rideshare launch with NASA’s SPHEREx mission early next year.
“PUNCH is a white light coronagraph and set of heliospheric imagers that together image from six solar radii out through the inner heliosphere.” Says Viall. PUNCH will be able to watch the structures that CODEX identifies as they as they evolve and are modulated father out in the heliosphere.”
Fianlly, astronomers can also use coronagraph-style instruments to image exoplanets directly. The Nancy Grace Roman Space telescope (set to launch in 2027) will feature one such instrument.
It will be exciting to see CODEX in action, as it probes the mysteries of the solar wind.
