An artist’s rendition of the Parker Solar Probe approaching the Sun. Astronomers have used data from Parker, along with data from other solar missions, to detect and study Solar stream interactions. Credit: NASA/Johns Hopkins APL/Steve Gribben  When a fast solar wind stream erupts from a coronal hole (a cooler region in the Sun’s atmosphere) and overtakes a slower moving solar wind stream, a stream interaction region (SIR) can form. In the SIR, a density “pileup” of compressed plasma develops upstream of the interface; typically there is a peak in pressure followed by a rarefaction region in the fast solar wind component. As the SIR propagates away from the Sun, to distances of one astronomical unit or beyond, the compression can form a shock that efficiently accelerates charged particles. Thus SIRs are a major source of energetic particles in interplanetary space. Coronal holes, the main sources of the high-speed stream, rotate as the Sun rotates on its axis, and the SIR structure...