The dark region seen on the face of the sun at the end of March 2013 is a coronal hole (just above and to the right of the middle of the picture), which is a source of fast solar wind leaving the sun. As it traveled through the solar system, this high-speed stream of plasma pushed up against slower solar wind ahead of it, and eventually formed a high-pressure region. This high-pressure region crashed into Saturn's magnetic bubble several weeks later in May 2013, causing bright auroral displays.
This image was obtained by the atmospheric imaging assembly on NASA's Solar Dynamic Observatory on March 28, 2013.
Image credit: NASA/SDO/AIA