Magneto-thermal convection in solar prominences

Plasma bubbles in the solar corona Recent observations of solar prominences with the optical telescope aboard the Hinode satellite have revealed dark, low-density bubbles that undergo Rayleigh–Taylor instabilities and evolve into dark plumes within coronal cavities — large, low-density regions formed by hemispheric-scale magnetic flux ropes in the outer solar atmosphere. New optical and extreme-ultraviolet data from Hinode and the recently launched NASA Solar Dynamics Observatory show that these prominence cavity structures are heated to temperatures of at least 250,000 K and perhaps as high as 10 6 K, which is 25–100 times hotter than the overlying prominence. These findings identify a source of buoyancy for these plasma bubbles and point to a previously unrecognized form of magneto-thermal convection in the outer solar atmosphere. Coronal cavities are large low-density regions formed by hemispheric-scale magnetic flux ropes suspended in the Sun’s outer atmosphere 1 . They evolve over time, eventually erupting as the dark cores of coronal mass ejections 2 , 3 . Although coronal mass ejections are common and can significantly affect planetary magnetospheres, the mechanisms by which cavities evolve to an eruptive state remain poorly understood. Recent optical observations 4 of high-latitude ‘polar crown’ prominences within coronal cavities reveal dark, low-density 5 ‘bubbles’ that undergo Rayleigh–Taylor instabilities 6 , 7 to form dark plumes rising into overlying coronal cavities. These observations offered a possible mechanism for coronal cavity evolution, although the nature of the bubbles, particularly their buoyancy, was hitherto unclear. Here we report simultaneous optical and extreme-ultraviolet observations of polar crown prominences that show that these bubbles contain plasma at temperatures in the range (2.5–12) × 10 5 kelvin, which is 25–120 times hotter than the overlying prominence. This identifies a source of the buoyancy, and suggests that the coronal cavity–prominence system supports a novel form of magneto-thermal convection in the solar atmosphere, challenging current hydromagnetic concepts of prominences and their relation to coronal cavities.