A dark gap in the sun's atmosphere--a "coronal hole"--is spewing solar wind toward Earth. Estimated time of arrival: July 19th. This morning, UV-filtered telescopes onboard NASA's Solar Dynamics Obervatory photographed the opening:
Image
© SDO/AIAA composite of EUV images at three wavelengths: 211 Å, 193 Å, and 171 Å.

Coronal holes are places where the sun's magnetic field opens up and allows hot gas to escape. A million mile-per-hour stream of solar wind flowing from this hole could spark polar geomagnetic storms when it arrives early next week. High-latitude sky watchers should be prepared for auroras.