I think this is the first paper on them: Observations of Running Penumbral Waves (1972)
While reviewing Hα films of the preceding sunspot of Mount Wilson 18090, taken 1970 September 1 at Big Bear, one of us (A. S.) discovered a series of concentric intensity waves propagating outward through the penumbra of the sunspot (Stein and Zirin 1972). Examination of other films revealed that such waves were visible on other occassions; with subsequent improvements in our instrumental resolution, we have seen them under reasonably good seeing in every sizable sunspot with a stable, regular penumbra.
The running penumbral waves (henceforth referred to as RP waves; the term “Stein waves” is also colloquially used) are seen in figure 1 (plate L4) to consist of successive bright and dark bands expanding outward from the umbra-penumbra boundary. They start out as circular arcs, and propagate more or less uniformly outward (depending on the irregularity of the penumbra).
There's a lot of newer ones on arxiv.
It's awesome that 50 years later we can easily catch them in excellent quality with a <100 mm amateur-grade Hα scope.