Space weather today

Changes in the Sun's output and the solar wind that disturb the space round the Earth: flares, eruptions of magnetised gas, and fast streams of solar wind. When they reach the Earth they stir its magnetic field (measured by Kp), light up the aurora, and can upset radio, satellites and power grids. NOAA rates them on three scales: R for radio blackouts, S for radiation storms and G for geomagnetic storms. (NOAA SWPC)

Sunspots are dark patches on the Sun's visible surface. NOAA explains that strong magnetic fields there make cooler, less dense and darker areas. Clearly visible groups get a four-digit NOAA region number, and NOAA tracks each one as the Sun's rotation carries it across the disc. Rapid changes in their magnetic fields make them the most likely sources of flares. (NOAA SWPC)

Flares are graded by their peak X-ray brightness measured by NOAA's GOES satellites: A, B, C, M and X, each class ten times the one before (an M flare starts at 10 to the minus 5 watts per square metre). They last from minutes to hours, and the strong ones black out HF radio on the sunlit side of the Earth as they happen. (NOAA SWPC)

It is measured at L1, 1.6 million km toward the Sun, upstream of the Earth. A faster wind and a magnetic field pointing south (negative Bz) let more energy into the Earth's magnetic field, which is what NOAA's OVATION model uses to forecast the aurora 30 to 90 minutes ahead. (NOAA SWPC)

From NASA's Solar Dynamics Observatory: HMI photographs the visible surface with its sunspots, and AIA images the hot corona in extreme ultraviolet. We load the latest from Helioviewer.org, which keeps SDO's archive up to date. Each picture shows the time it was taken. (NASA Solar Dynamics Observatory, Helioviewer.org (NASA and ESA))