For a team of scientists from the University of Hawaii and the National Solar Observatory, Sacramento Peak, there is no more fitting place to observe sunspots than Sunspot, New Mexico. Using a bypass approach, the astronomers measured how much molecular hydrogen (H2) — a form of hydrogen normally associated with cooler places than the Sun’s scorching surface — exists in sunspots’ centers. The team’s detection of H2 inside sunspots, a first in solar astronomy, may help explain the behavior of magnetic fields inside the dark phenomena.
The presence of H2 inside sunspots has eluded scientists for many decades. Since the 1970s scientists have observed H2 in areas above sunspots using spacecraft, but they were never able to observe the molecule inside the spots themselves. Because H2 existed above sunspots it was a pretty good bet that it was also in the spots’ cooler centers, called umbrae, says coauthor Sarah Jaeggli (now at Montana State University).
Between 2001 and 2010 Jaeggli and her colleagues used the NSO’s Dunn Solar Telescope in New Mexico to observe 23 active regions. Although they did not directly observe H2, they inferred its presence using a well-known proxy, hydroxide. It was spectroscopic measurements of hydroxide that allowed the astronomers to determine that significant amounts of molecular hydrogen existed in the spots’ coolest regions, in keeping with current theories. To Jaeggli’s knowledge, this is the first time a ground-based telescope has been used to measure H2 in sunspots.
Sunspots have different physical dynamics than their surroundings, boasting cooler temperatures and stronger magnetic fields than other areas (the cooler temperatures are why the regions appear dark against the solar surface). In fact, it is the magnetic fields that make sunspots so cool. As magnetic fields rise from deep within the Sun to the surface, or photosphere, they “choke” the nearby gas: The cyclical movement of heated material from within the Sun to its surface would normally keep the gas at a scorching 5,000 to 6,500 degrees Kelvin, but this convection cannot effectively reach a region cocooned in magnetic field. As a result, the gas temperature decreases — by roughly 1,500 degrees — and that drop opens a window of opportunity for H2 to form.
In their paper, published in the February 1st issue of The Astrophysical Journal, the scientists report the magnetic field strength, temperature, and amount of H2 compared to the total number of atoms in a spot for seven of the 23 active regions they observed. What they found was that the strongest magnetic fields contained the most H2 molecules — the highest H2 abundance reported was 2.3 percent.
But the astronomers argue that the connection between magnetic fields and molecular hydrogen isn’t just one-way. H2 also encourages “a rapid intensification of the magnetic field,” the scientists report. H2’s formation lowers the gas pressure inside a spot, because the molecule takes up less volume than free-floating hydrogen atoms. With lower pressure the spot shrinks, compressing the magnetic field trapped in the hot, ionized gas. The more concentrated magnetic field further cools the gas, allowing more molecules to form, and so on. The authors suggest this runaway effect is temporary and that the temperature can drop only so much; past a certain point, an increase in the magnetic field won’t affect the spot’s temperature, preventing further H2 formation long before all the spot’s hydrogen combines into molecules.
Measuring the abundance of H2 in sunspots adds an important piece to the puzzle for understanding sunspot evolution, said Matthew Penn (National Solar Observatory, Tucson) who was not involved with the study. Although sunspots only last from days to weeks, their strong magnetic fields cause massive solar flares that can harm satellites around the Earth. Therefore, understanding how magnetic fields evolve in a sunspot over time could prove an effective tool for forecasting hostile space weather.
“Magnetic field measurements are where it’s at,” Penn says. “Being able to understand how the molecules themselves can be used to measure magnetic fields is a key step ... It’s really essential to understand how the magnetic field will evolve to predict a flare.”