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Heating the Chromosphere in the Quiet Sun

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chromosphere
Photo of the Sun’s chromosphere taken with NASA’s SOHO observatory. A longstanding puzzle in solar astronomy is how heat gets delivered to the Sun’s upper atmosphere. [NASA/SOHO]

The best-studied star — the Sun — still harbors mysteries for scientists to puzzle over. A new study has now explored the role of tiny magnetic-field hiccups in an effort to explain the strangely high temperatures of the Sun’s upper atmosphere.

solar temperatures

Schematic illustrating the temperatures in different layers of the Sun. [ESA]

Strange Temperature Rise

Since the Sun’s energy is produced in its core, the temperature is hottest here. As expected, the temperature decreases further from the Sun’s core — up until just above its surface, where it oddly begins to rise again. While the Sun’s surface is ~6,000 K, the temperature is higher above this: ~10,000 K in the outer chromosphere.

So how is the chromosphere of the Sun heated? It’s possible that the explanation can be found not amid high solar activity, but in quiet-Sun regions.

In a new study led by Milan Gošić (Lockheed Martin Solar and Astrophysics Laboratory, Bay Area Environmental Research Institute), a team of scientists has examined a process that quietly happens in the background: the cancellation of magnetic field lines in the quiet Sun.

Activity in a Supergranule

IRIS quiet-Sun observations

Top left: SDO AIA image of part of the solar disk. The next three panels are a zoom of the particular quiet-Sun region that the authors studied, all taken with IRIS at varying wavelengths: 1400 Å (top right), 2796 Å (bottom left), and 2832 Å (bottom right). [Gošić et al. 2018]

The Sun is threaded by strong magnetic field lines that divide it into supergranules measuring ~30 million meters across (more than double the diameter of Earth!). Supergranules may seem quiet inside, but looks can be deceiving: the interiors of supergranules contain smaller, transient internetwork fields that move about, often resulting in magnetic elements of opposite polarity encountering and canceling each other.

For those internetwork flux cancellations that occur above the Sun’s surface, a small amount of energy could be released that locally heats the chromosphere. But though each individual event has a small effect, these cancellations are ubiquitous across the Sun.

This raises an interesting possibility: could the total of these internetwork cancellations in the quiet Sun account for the overall chromospheric heating observed?

Simultaneous Observations

To answer this question, Gošić and collaborators explored a quiet-Sun region in the center of a supergranule, making observations with two different telescopes:

  1. The Swedish 1 m Solar Telescope (SST), which provides spectropolarimetry that lets us watch magnetic elements of the Sun as they move and change, and
  2. The Interface Region Imaging Spectrograph (IRIS), a spacecraft that takes spectra in three passbands, allowing us to probe different layers of the solar atmosphere.

Simultaneous observations of the quiet-Sun region with these two telescopes allowed the scientists to piece together a picture of chromospheric heating: as SST observations showed opposite-polarity magnetic-field regions approach each other and then disappear, indicating a field cancellation, IRIS observations often showed brightening in the chromosphere.

Falling Short

SST quiet-Sun observations

SST observations, including the continuum intensity map (upper left), magnetogram showing the magnetic field elements (upper right), and intensity maps in the core of the Ca II 8542 Å line (lower left) and Hα 6563 Å line (lower right). [Gošić et al. 2018]

By careful interpretation of their observations, Gošić and collaborators were able to estimate the total energy contribution from the hundreds of field cancellations they detected. The authors determined that, while the internetwork cancellations can significantly heat the chromosphere locally, the apparent number density of these cancellations falls an order of magnitude short of explaining the overall chromospheric heating observed.

Does this mean quiet-Sun internetwork fields aren’t the cause of the strangely warm temperatures in the chromosphere? Perhaps … or perhaps we don’t yet have the telescope power to detect all of the internetwork field cancellations. If that’s the case, upcoming telescopes like the Daniel K. Inouye Solar Telescope and the European Solar Telescope will let us answer this question more definitively.

Citation

M. Gošić et al 2018 ApJ 857 48. doi:10.3847/1538-4357/aab1f0