A new way to measure magnetic fields in the solar chromosphere

The sun’s atmosphere has three layers: the photosphere, the chromosphere, and the corona. The photosphere is the visible surface and the corona is the wispy, tenuous outermost layer visible during solar eclipses. The chromosphere is sandwiched between the two and is incredibly thin, measuring just 1 per cent of the sun’s radius.

Strong magnetic fields gather hot plasma in the chromosphere into a network of super-sized granulation cells that can stretch to over 30,000 km across, about 2.5 earth diameters, and 1,500 km deep. On the edges of the cells sit spicules: spikes of magnetically confined plasma that shoot up into the corona above. The release of magnetic energy is linked to events such as solar flares, which can impact electrical infrastructure on the earth. So, understanding how magnetic fields emerge from, heat, and accelerate chromospheric plasma is important.

One way to measure the magnetic fields is to take advantage of what is called the Zeeman effect. Strong magnetic fields split the lines observed in the sun’s spectrum. Recently, a team of solar physicists led by Philip Judge of the National Center for Atmospheric Research, Colorado, did something altogether new: assess the comparative merits of using all spectral lines—from X-ray to infrared wavelengths—to measure magnetic fields as high as possible in the solar atmosphere.

The team concluded that the best lines for the job are the magnesium h and k lines near 2,800 x 10⁻⁸ cm, or ångstroms (Å). However, they also found that combining them with the numerous chromospheric lines of iron between 2,585 Å and the h line at 2,803 Å would provide a far more discriminating probe of magnetic structure in the chromosphere. All of these wavelengths sit in the ultraviolet part of the spectrum. A modest space-borne telescope focussing on the same part of the spectrum could provide a way to untangle some of the biggest riddles about the chromosphere and the violent eruptions associated with it.