The sunspot called AR 2529 came into view on April 7, 2016. The photograph shows his status on April 9. Sunspots are regions of very high magnetic activity. In this case, the stain is of enormous dimensions, larger than the diameter of the earth. The dark zone measures more than 20,000 km.
Photo on 4/9/2016 from Majadahonda, Madrid (Spain)
Sunspots can cause solar flares, more or less likely depending on the shape of the magnetic field. Stain AR 2529 promised little chance of explosion, but finally erupted on April 18 causing a solar flare of class M6.7 (classes are A, B, C, M and X from lowest to highest activity) which implies an intensity of 6.2x10e-5 W / m2.
NASA's Solar Dynamics Observatory recorded the extreme ultraviolet flash of the flare. A pulse of UV radiation from the flare ionized the upper part of Earth's atmosphere and interrupted shortwave radio communications on the light side of our planet.
This is a picture of the giant spot the day before the solar storm:
Photo taken on 4/17/2016 from Majadahonda, Madrid (Spain)