In the September 2013 issue of Physics Today (page 12), Johanna Miller describes the quest to determine how magnetic reconnection powers solar flares. Plasma structures tantalizingly suggestive of reconnection have been observed from the ground and from space since the 1960s. The new images from the Atmospheric Imaging Assembly that she cites offer higher angular resolution, but they cannot overcome a basic limitation: Studies of coronal morphology and motions can never, in themselves, demonstrate the existence of reconnection.
Rather, such studies leave solar physicists in a position reminiscent of one they faced a century ago, when the vortex-like structures observed around sunspots suggested that they were of magnetic origin. Proof, however, came only when George Ellery Hale used Zeeman splitting to actually detect the spots’ intense magnetic field.
As Miller mentions, a key signature of reconnection seems to be an intense motional electric field.1 Some evidence for such fields has been observed using a relatively simple polarimeter to measure the Stark effect.2 A state-of-the-art electrograph installed on, for example, the Advanced Technology Solar Telescope or flown in space could open the door to more sensitive study of motional electric fields. Comparison of those observations with the recently developed three-dimensional flare models that Miller describes might finally enable us to decipher the role of reconnection in flares.
Only observations can settle whether the potential drops expected with reconnection occur across solar structures that produce detectable emission in Stark-affected hydrogen lines. We need to explore more incisive diagnostics complementary to extreme UV imaging if we want to understand magnetic energy release in astrophysical phenomena.
