On August 9, 2011 at 3:48 a.m. EDT, the sun emitted an Earth-directed X6.9 flare, as measured by the NOAA GOES satellite. These gigantic bursts of radiation cannot pass through Earth's atmosphere to harm humans on the ground, however they can disrupt the atmosphere and disrupt GPS and communications signals. In this case, it appears the flare is strong enough to potentially cause some radio communication blackouts. It also produced increased solar energetic proton radiation — enough to affect humans in space if they do not protect themselves.
As of March 2014, this flare is the largest of solar cycle 24.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 18.104.22.168.0