In this "zone of preferential heating" above the Sun's surface, temperatures rise overall. More bizarre still, individual elements are heated to different temperatures, or preferentially. Some heavier ions are superheated until they're 10 times hotter than the hydrogen that is everywhere in this area -- hotter than the core of the Sun.
Such high temperatures cause the solar atmosphere to swell to many times the diameter of the Sun and they're the reason we see the extended corona during solar eclipses. In that sense, the coronal heating mystery has been visible to astronomers for more than a half millennium, even if the high temperatures were only appreciated within the last century.
This same zone features hydromagnetic "Alfvén waves" moving back and forth between its outermost edge and the sun's surface. At the outermost edge, called the Alfvén point, the solar wind moves faster than the Alfvén speed, and the waves can no longer travel back to the Sun.
So does the Alfvén point mark the outer edge of the heating zone? And what exactly is changing under the Alfvén point that superheats heavy ions? We should know in the next couple of years. The Parker Solar Probe lifted off in August 2018 and had its first rendezvous with the Sun in November 2018 -- already getting closer to the Sun than any other human-made object.
In the coming years, Parker will get even closer with each pass until the probe falls below the Alfvén point. Researchers predict it should enter the zone of preferential heating in 2021 as the boundary expands with increasing solar activity. Then NASA will have information direct from the source to answer all manner of long-standing questions.
Source: Justin C. Kasper, Kristopher G. Klein. Strong Preferential Ion Heating is Limited to within the Solar Alfvén Surface. The Astrophysical Journal Letters, 2019;