Nineteen-Year-Old Data Explains Why Solar Wind Is So Hot

A good spacecraft is the kind that keeps on providing researchers with valuable data year after year. Wind, the spin-stabilized spacecraft NASA launched into a halo orbit around the L1 Lagrange point in 1994, is one such spacecraft. Recently, researchers looking through years old data from Wind discovered a power source lurking in the solar wind. Not only does the discovery explains a lot about the mysteries of the solar wind, it’s got fusion researchers pretty excited.

The Wind spacecraft predates modern data collection methods; it uses magnetic tapes similar to 8-tracks to record and play back its data. It’s also equipped with heavy shielding and double-redundant systems to safeguard everything against failure. It was built to last, so far withstanding almost two full solar cycles and innumerable solar flares. It’s also been in the path of the solar wind for nearly 20 years.

Solar wind, the flow of magnetized gas that streams away from the sun's upper atmosphere, is made of hydrogen and helium ions with some heavier elements sprinkled in. It’s like steam coming off a pot of boiling water–the solar wind is the product of the Sun boiling off its atmosphere. But there’s a key difference between the steam and solar wind. In your kitchen, steam slows and cools as it leaves the pot. The solar wind does the opposite, heating and speeding up as it leaves the Sun and blasts through frigid airless space. The solar wind can triple in speed leaving the Sun’s corona reaching up to a million miles per second.

Since the 1970s, researchers have been trying to figure out how and why this happens. Wind has given them the answer.

Justin Kasper of the Harvard-Smithsonian Center for Astrophysics and his team made the discovery by processing the spacecraft's full 19-year record of solar wind temperatures as well as magnetic field and energy readings. The source of the solar wind’s heating, they found, is ion cyclotron waves. Ion cyclotron waves refer to the movement of ions with respect to plasma. In this case, protons circle around the Sun’s magnetic field in wavelike rhythms falling into resonance as they course through the solar wind. This heats the gas to millions of degrees and accelerating its flow to millions of miles per hour. Looking through Wind’s old data, Kasper confirmed that ion cyclotron waves are active in the solar wind. At least in the L1 Lagrange point where the Wind probe operates.

But that’s not all. In addition to heating and accelerating the solar wind, ion cyclotron waves also account for some of its stranger properties.

The solar wind is not like wind on Earth. On Earth, the atmosphere keeps all the elements of wind, the oxygen, nitrogen, and other particles traveling at the same speed and at a constant temperature. The various chemical elements of the solar wind – hydrogen, helium, and heavier ions – travel at different speeds and have different temperatures depending on the direction of flow. The heavier elements move faster and are generally hotter than the lighter elements.Ion cyclotron waves explain this. Heavy ions achieve a better resonance with ion cyclotron waves compared to their lighter counterparts, which means they gain more energy and heat as they travel.

It’s this behaviour of heavy ions that intrigues fusion researchers.

Heavy ions radiating heat can contaminate the material in fusion reactors, cooling the plasma enough that fusion reactions become impossible. But learning about the ion cyclotron waves that heat the solar wind might be a clue to researchers on how to reverse the fusion-killing reactions. Theoretically, ion cyclotron waves could be used to heat or even remove the contaminating heavy elements from fusion reactors, restoring the necessary thermal balance to the plasma.

The next step in solar wind research is to see if ion cyclotron waves work the same way deep inside the sun's atmosphere where the solar wind begins its journey. To answer this question, NASA is launching a new probe in 2018. Solar Probe Plus will go far enough into the Sun’s atmosphere that its disk will look 23 times bigger than it does in our sky. It’s closest approach will be about 4.4 million miles from the surface, which will heat the probe to more than 2,550 degrees Fahrenheit and expose it to more intense radiation than any spacecraft has ever experienced. Solar Probe Plus has one goal: to sample the sun's plasma and magnetic field at the very source of the solar wind. This next solar probe will outstrip Wind scientifically, but that doesn’t mean the vintage probe will be rendered obsolete. It still has 60 years' worth of fuel left in its tanks, so there’s no telling what other weird solar secrets the nearly 20-year-old probe will uncover during its lifetime.