NASA set to deliver a knock-out PUNCH to mysteries of the solar wind

The secrets of the solar wind will soon come under renewed scrutiny thanks to the launch of an upcoming NASA mission that will gaze at the sun and the inner solar system.

The mission, called PUNCH, which stands for the Polarimeter to Unify the Corona and Heliosphere, is a constellation of four suitcase-sized satellites that are set to blast off for a sun-synchronous low polar orbit on Sunday (March 2). Skirting the day/night terminator, these satellites will have overlapping views of the sky from overhead to the horizon, meaning they'll be able to watch as the solar wind streams towards us.

"PUNCH is going to provide an unprecedented three-dimensional view of the corona as it turns into the solar wind and fills our solar system," Nicholeen Viall, PUNCH's mission scientist from NASA's Goddard Space Flight Center, told reporters on Feb. 13.

One of the PUNCH satellites possess a narrow-field imager armed with a coronagraph. This is a device that creates an artificial total solar eclipse to block the body of the sun so that we can see its outer atmosphere, the corona.

The other three satellites each have wide-field imagers that can watch as the corona forms the solar wind, and then track that detected solar wind as it travels through space.

PUNCH is able to do this by effectively wearing sunglasses.

The narrow- and wide-field imagers in this mission are polarimeters; they have filters that only allow in light particles, or photons, that oscillate in certain directions. It's just like how polarized sunglasses manage polarized light.

The polarized light that PUNCH will be looking for is being reflected off electrons carried on the solar wind. However, this polarized light signal is feeble. In fact, a staggering 99.9% of the polarized light that PUNCH will detect comes from other things, such as background stars and the Zodiacal light. The 0.1% that PUNCH’s scientists are interested in then has to be teased out of the overall data.

However, as a by-product of PUNCH's entire polarimetric measurements, scientists will manage to make the most comprehensive polarimetric star map ever made, which will be invaluable to astrophysicists studying the properties of stars, such as magnetic activity.

"The raw pictures will look very much like ordinary star fields, [but] the magic of this mission is the data processing on the ground," the mission's principal investigator, Craig DeForest of the Southwest Research Institute, said during the Feb. 13 teleconference.

This data will allow PUNCH to follow the direction in which material in the solar wind is moving. For example, when the sun burps out a cloud of plasma called a coronal mass ejection (CME), PUNCH can determine the CME's exact trajectory and velocity, following the CME all the way through the inner solar system and towards Earth (if it's indeed headed to us).

When a CME buffets our planet's magnetic field, it can trigger geomagnetic storms that can hamper radio communications, endanger satellites and astronauts, and impact power grids on the ground. It can also spark beautiful auroral lights over Earth's polar regions.

"We expect we'll revolutionize how space weather is forecasted," said Deforest. "We're the first mission able to track space-weather events in three dimensions."

PUNCH is also interested in the fine structure of the solar wind. How the solar wind is "blown" from the sun is still unclear, but it's not a smooth transition from the corona into the solar wind. Magnetic switchbacks, where the solar wind appears to zig-zag, as well as eddies and clumps, not to mention the giant CMEs, are all inherent parts of the solar wind.

"What's amazing about PUNCH is the amount of detail that it will get," said Viall. "We will get to see the global structure of those enormous mass ejections, and also the bits and pieces that are small compared to the coronal mass ejections, but which are still large compared to the Earth."

PUNCH isn't the only spacecraft studying the solar corona and the transition into the solar wind. NASA's Parker Solar Probe regularly flies closer to the sun than any other spacecraft has gone before; it has come within 3.9 million miles (6.2 million kilometers) of the sun'ss surface, the photosphere, and, in doing so, has entered the solar corona.

By combining PUNCH's observations with theParker Solar Probe's, it will be possible to see the big picture of the solar wind streaming away from our star's corona, and even glean some close-up details of that transition.

"Together, the two missions will provide complementary datasets showing how the sun gives rise to the solar wind and space weather," said DeForest.

PUNCH will also team up with another mission that launches later this year, called IMAP, the Interstellar Mapping and Acceleration Probe. Whereas PUNCH is looking in-system towards the sun, IMAP will be gazing outwards, towards the edge of the heliosphere, which is the vast magnetic bubble blown by the solar wind and which encapsulates the planets and the Kuiper Belt.

IMAP will study the solar wind as it journeys to the outer boundary of the heliosphere, called the heliopause, located about 120 astronomical units (17.95 billion kilometers, or 11.15 billion miles) from the sun. In doing so, IMAP will look at where the solar wind gives way to the interstellar medium, and with Parker Solar Probe getting up close to the birth of the solar wind, PUNCH will bridge the gap between them.