It’s been 124 days since the Parker Solar Probe was launched, and several weeks since it made the closest approach any spacecraft has ever made to a star. Now, scientists are getting their hands on the data from the close approach. Four researchers at the recent meeting of the American Geophysical Union in Washington, D.C. shared what they hope they can learn from the probe. They hope that data from the Parker Solar Probe will help them answer decades-old question about the Sun, its corona, and the solar wind.

Scientists who study the Sun have been anticipating this for a long time, and the waiting has been worth it.

“Heliophysicists have been waiting more than 60 years for a mission like this to be possible. The solar mysteries we want to solve are waiting in the corona.” – Nicola Fox, director of the Heliophysics Division at NASA Headquarters.

The excitement is all around the PSP’s first solar encounter phase. From Oct. 31 to Nov. 11, 2018, Parker Solar Probe completed the first solar encounter phase, speeding through the Sun’s outer atmosphere — the corona — and collecting unprecedented data with four suites of cutting-edge instruments. The PSP will orbit the Sun 24 times, for 24 solar encounter phases. During the mission, the probe will use 7 Venus gravity-assist flybys to incrementally shrink its orbit around the Sun.

Each solar encounter phase occurs when the probe is within .25 AU of the Sun, and during those times the science instruments will collect data. The probe will be exposed to extreme heat and radiation during that time, and can’t communicate. Only once it exits each phase can it send its data back to Earth for heliophysicists to ponder.

“Parker Solar Probe is providing us with the measurements essential to understanding solar phenomena that have been puzzling us for decades.” – Nour Raouafi, PSP project scientist, JHU/APL.

The first solar encounter phase is complete, and though the mission has a lot of work yet to do, Parker scientists shared some of what they hope to learn from the mission at the American Geophysical Union in Washington DC.

When the PSP mission was designed, scientists wanted to address three important questions regarding heliophysics:

• How is the Sun’s outer atmosphere, the corona, heated to temperatures about 300 times higher than the visible surface below?
• How is the solar wind accelerated so quickly to the high speeds we observe?
• How do some of the Sun’s most energetic particles rocket away from the Sun at more than half the speed of light?

“Parker Solar Probe is providing us with the measurements essential to understanding solar phenomena that have been puzzling us for decades,” said Nour Raouafi, Parker Solar Probe project scientist at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland. “To close the link, local sampling of the solar corona and the young solar wind is needed and Parker Solar Probe is doing just that.”

No spacecraft has ever been as close to the Sun as the PSP has, so scientists don’t exactly know what to expect from the data. They know what they hope to learn, but can’t be sure.

“We don’t know what to expect so close to the Sun until we get the data, and we’ll probably see some new phenomena,” said Raouafi. “Parker is an exploration mission — the potential for new discoveries is huge.”

Reports from the PSP suggest that the first science phase captured quality data. That’s partly because of the fly-by of Venus, when the probe was able to take some measurements of the planet, verifying that instruments were functioning. Some data from science phase one has been downloaded, but heliophysicists will have to wait to get their hands on all of it. Because of the challenges in the mission profile, some of the science data from this encounter will not downlink until after the mission’s second solar encounter in April 2019.

The Parker Solar Probe is not the only spacecraft studying the Sun. Other craft include the SOHO (Solar Heliospheric Observatory), the SDO (Solar Dynamics Observatory), and the STEREO-A (Solar and Terrestrial Relations Observatory Ahead) spacecraft. But none of those three have come near as close to the Sun as the PSP, even though they’re doing important science of their own.

“Parker Solar Probe is going to a region we’ve never visited before,” said Terry Kucera, a solar physicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Meanwhile, from a distance, we can observe the Sun’s corona, which is driving the complex environment around Parker Solar Probe.”

The gif below shows actual data from NASA’s Solar and Terrestrial Relations Observatory Ahead (STEREO-A) spacecraft, along with the location of Parker Solar Probe as it flies through the Sun’s outer atmosphere during its first solar encounter phase in November 2018. These images provide key context for understanding Parker Solar Probe’s observations. (Image Credit: NASA/STEREO)

Each of the spacecraft studying the Sun provide a different context and viewpoint for what the others see. The PSP will travel to within .25 AU, while STEREO orbits the Sun at about 1 AU. The SDO is in a geo-synchronous Earth orbit, and SOHO is in a halo orbit around the Sun–Earth LaGrange 1 point.

“The STEREO mission is all about observing the heliosphere from different locations and Parker is a part of that – making measurements from a perspective we’ve never had before,” said Kucera.

The Parker Solar Probe Will Build or Break Our Models

Science is incremental, and scientists with the PSP like to point out that incremental improvement in models of how the Sun works is a part of the PSP’s job, even if we don’t get hands-down answers to our questions.

Models are a good way to test theories about the underlying physics of the Sun. By creating a simulation that relies on a certain mechanism to explain coronal heating — for instance, a certain kind of plasma wave called an Alfvén wave — scientists can check the model’s prediction against actual data from Parker Solar Probe to see if they line up. If they do, that means the underlying theory may be what’s actually happening. If they don’t, then it’s back to the drawing board.

“We’ve had a lot of success predicting the structure of the solar corona during total solar eclipses,” said Riley. “Parker Solar Probe will provide unprecedented measurements that will further constrain the models and the theory that’s embedded within them.”

The record-breaking speed of the PSP is crucial to its work.

The Sun rotates about once every 27 days as we see it from Earth, and the solar structures that drive much of its activity move along with it. That creates a problem for scientists, because they can’t be certain if the variability they see is driven by actual changes to the region producing the activity – temporal variation – or is caused by simply receiving solar material from a new source region — spatial variation. PSP’s speed means it can outrun that problem.

The gif below s from a model showing how the solar wind flows out from the Sun, with the perspective of Parker Solar Probe’s WISPR instrument overlaid.
Credits: Predictive Science Inc.

At certain points, Parker Solar Probe is traveling fast enough to almost exactly match the Sun’s rotational speed, meaning that Parker “hovers” over one area of the Sun for a short amount of time. Scientists can be certain that changes in data during this period are caused by actual changes on the Sun, rather than the Sun’s rotation.

The Parker Solar Probe is part of NASA’s Living with a Star program to explore aspects of the Sun-Earth system that directly affect life and society.

Sources:

• NASA Press Release: Preparing for Discovery With NASA’s Parker Solar Probe
• NASA: AGU 2018 – Expected Data and Scientific Discovery from NASA’s Parker Solar Probe
• NASA Press Release: Parker Solar Probe Reports Good Status After Close Solar Approach
• NASA Press Release: Parker Solar Probe Breaks Record, Becomes Closest Spacecraft to Sun
• NASA: Parker Solar Probe
• NASA Living With a Star Program