In addition to viewing the sun's luminous outer atmosphere (corona), which under normal circumstances is lost in the glare of the rays, they will be using the eclipse to calculate the star's current diameter, which changes as the sun expends energy.
The number of channel molecules present means that, in normal circumstances, about 70% of the free-floating outer channels are lined up with tethered inner ones at any given time, with each dual channel letting through about 1m sodium ions per second.