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Solar Probe Plus, and More
Space Science Center researchers land a role in NASA’s long-awaited mission to the Sun, and reestablish UNH’s long participation in the agency’s Heliospheric Theory Program

THE IDEA was first launched in 1958 at the dawn of the Space Age and, if all goes according to plan, in 2018 a rocket will finally send a heat-hardened spacecraft into the Sun’s atmosphere, or corona, in an effort to unravel twin mysteries that have dogged space physicists for decades: Why is the corona hotter than the surface of the Sun, and how is the solar wind accelerated?

It took NASA’s Solar Probe Plus mission 60 years to go from concept to reality because of what’s being asked of the spacecraft – to orbit in a region of space “just” 4 million miles away where temperatures of low density plasma approach 1 million degrees Centigrade. And that’s positively chilly compared to regions closer in.

 
The Solar Probe Plus spacecraft approaches
the Sun in this artist's conception

Photo courtesy of JHU/APL.

   

Counterintuitively, just above the Sun’s surface – which is a mere 5,000 degrees – the corona ramps up to 2 million degrees. Generally, if heat flows from object A (Sun’s surface) to object B (corona), B is cooler than A. Not so with our star.

After the spacecraft is launched it will make seven fly-bys of Venus using the planet’s gravitational pull to orbit ever closer to the Sun. By 2024 the probe will rendezvous with the Sun reaching a perihelion (closest approach) of 9.5 solar radii (about 4 million miles). At this manageable distance, and traveling at approximately 450,000 miles per hour at perihelion, the car-sized Solar Probe Plus spacecraft will make measurements with sensitive instruments mostly hidden behind a revolutionary carbon-composite shield. In this hot zone the shield will heat up to 2,600 degrees Fahrenheit while the instruments will conduct their work at room temperature.

“Understanding why this heating of the corona occurs is a longstanding puzzle in astrophysics,” notes Ben Chandran, Space Science Center rofessor of theoretical plasma physics and astrophysics. Chandran, professor Marty Lee, and associate professor Nathan Schwadron were recently selected to provide theoretical science support and data analysis on three of the mission’s instrument suites."

What’s more, Chandran and others from the SSC were also selected in a separate but topically related NASA grant for $1.3 million to conduct basic research on the other big mystery – the origin of the solar wind. The work will provide some theoretical foundations needed to understand the mission’s observations and measurements. With the selection, UNH once again has a role in NASA’s Heliosphysics Theory Program (formerly the Solar-Terrestrial Theory Program), from which until recently it received funding for two decades straight.

Of the related grants Chandran says, “The two fit hand in glove, and in many ways this is a good time to be doing theoretical investigations into the origin of the solar wind because of the buildup to the Solar Probe Plus mission.”For twenty years, SSC solar theorists (including Joe Hollweg, Marty Lee, Terry Forbes, Phil Isenberg, Bernie Vasquez, and Sergei Markovskii) probed different aspects of the solar wind and heating of the solar corona, looking at plasma (hot ionized gases) physics questions and the ways that waves and particles interact in these plasmas.

Under the current grant reestablishing work under the Heliosphysics Theory Program, the SSC group, which now also includes Chuck Smith, Toni Galvin, and Jean Perez, will continue looking at the origins of the solar wind and, notes Chandran, will now bring in "some additional ideas about turbulence – the random, disordered fluctuations that span a wide range of wavelengths in a system” – the system in this case being the solar wind.

ben chandran  
Ben Chandran
Photo by K.Donahue, UNH-EOS.

One of the ways the Sun can superheat its atmosphere is by launching energy in the form of a plasma wave, particularly the Alfvén wave, which is akin to a wave on a ‘string’ where the string is really the magnetic field lines threading the whole system.

Just like turbulent eddies in a stream of water, turbulence in plasma breaks up into smaller eddies creating waves in the solar wind that can interact with one another and break up into even smaller waves. Eventually their energy cascades to very small length scales, small wavelengths, and then dissipates.

“We’re interested in a lot of things that pertain to this basic paradigm – for example, how the waves are launched, what happens to them as they propagate, and how they become turbulent,” Chandran says.

The cascade and dissipation processes are key to unraveling the puzzles because, ultimately, the way the waves “damp out” by interacting with the plasma may be the key to understanding how the energy gets from the Sun to particles of the solar wind, thereby heating them and driving them further out into the corona.

The solar wind: defining Earth’s space environment
Proof of the supersonic solar wind’s existence was uncovered in the early 1960s, but despite decades of investigation and theoretical origin remains unsolved. And this is due, at least in part, to the fact that the simple physics explanations simply don’t work. Things well understood by physicists – thermal conduction and pressure forces – fail to explain either why the corona is so hot or why the wind flows away from the Sun as fast as it does; it shoots past the Earth at up to 1.8 million miles per hour.

“So clearly there’s some extra energy source at work other than thermal conduction, and this is likely to be these turbulent waves or something related to them, some flow of energy out of the Sun that gets converted into heat and then manages to drive material away from the Sun very rapidly,” Chandran says.

These complex interactions are at the forefront of current plasma physics research and at the heart of the Solar Probe Plus mission.

Beyond the need to better understand the universe we live in, nailing down these complex physical processes is of practical importance: the solar wind defines the Earth’s space environment and creates “space weather.” Interactions between our magnetosphere and the solar wind can trigger violent magnetic storms that damage satellites and ground-based power stations, pose radiation hazards to astronauts, and disrupt GPS positioning on aircraft flying over the poles.

Says Chandran, “Ideally we’d like to build predictive models of the solar wind and the eruptions from the Sun that propagate through the solar wind so we can tell when these things are coming, how fast they’ll propagate, and how they’ll move within the solar wind. This type of forecasting would be of great practical interest.”

But understanding all this has implications much farther from home as well, for the same processes that drive the Sun apply to other stars throughout the universe, and what is learned by Solar Probe Plus and other solar-focused missions will tell us a lot about astrophysics in general.

“Many systems much farther out there, whether it’s the disks of plasma swirling around black holes or clusters of galaxies that are filled with immense volumes of plasma, are also turbulent,” Chandran notes. “Unlocking the secrets of the solar wind will tell us tremendous amounts about systems throughout the universe that we can only observe remotely.”

The Solar Probe Plus mission will provide a bounty of new data and a new frontier of astrophysics that will lead to fundamentally new discoveries “that challenge the theories we develop up to that point and probably force us to uncover new theories and new physical understandings of these processes,” says Chandran.

He adds, “The more sophisticated our theoretical understanding of this system, the richer the returns will be from the mission. So there’s this nice synergy that UNH is part of – the theoretical research but also the future experimental efforts as well.”

by David Sims, Science Writer, Institute for the Study of Earth, Oceans, and Space. Published in Summer 2011 issue of EOS Spheres.