A solar flare is the largest explosive event of the sun's repetoire.  It is an intense burst of radiation (x rays, light, etc.) powered by the release of magnetic energy.   [13] They are also associated with the sun's 11-year magnetic maxima cycle.  Like CMEs, they occur near sunspots, usually along the dividing/neutral line between poles.   "Flares release energy in many forms - electro-magnetic (Gamma rays and X-rays), energetic particles (protons and electrons), and mass flows.  Flares are characterized by their brightness in X-rays (X-Ray flux).  The biggest flares are X-class flares.  M-Class flares have a tenth the energy and C-Class flares have a tenth of the X-ray flux seen in M-Class flares." [14] Each letter category has 9 divisions within it, and the X class even more.  Note that their power is measured in watts/meter^2.   The xray strength of solar flares has to be measured from satellites since Earth is protected from such xrays by its magnetosphere.

Radiation (not particles, which are much slower) from the sun arrives here on earth within minutes.  What is somewhat surprising to me is that even though the flares are the most violent thing the sun does, their x-ray and other radiation usually only affect radio and GPS communication on earth.  The radiation can do plenty of damage to our satellites and astronauts, but within the cocoon of earth's magnetosphere, the radiation alone usually only affects radio/GPS.  But before you think of solar flares as benign,  MANY solar flares also launch CMEs and in fact, as the heavy hitter of solar activity, can have the power to launch HUGE CMEs at high speeds, which DO dramatically affect systems on earth.   Usually, you can't SEE solar flares with the naked eye because the background sun is too bright.  In the photo at left, taken about a month ago, the first of FIVE X-class solar flares from a single earthward sunspot is erupting.  It would appear the 2013-2014 solar maximum is not quite over.  This was taken in UV which is usually shown as teal.

NASA SDO image of Oct 19, 2014 X class solar flare http://www.nasa.gov/content/goddard/sdo-observes-an-x-class-solar-flare/#.VHaaB1fF9o8

THE IMPORTANCE OF SUNSPOTS:  FLARES AND CMEs Until this report, I didn't understand just how important it was to keep an eye on sunspots--both flares and CME's usually launch from them.  NOAA even manages a website Space Weather with most of the news coming from sunspots--the entire left column is all about the kinds of solar activity I have described here.  In fact, the largest sunspot seen in 24 years,  AR 12209, has just spun around again earthward.  [15]  Just last month, October 19-27, 2014, FIVE X-class solar flares erupted from a sunspot in one earthward spin. [16] I didn't understand that the complexity of the magnetic fields in sunspots correlates with the likelihood that they will flare or fling CMEs.  They classify sunspots on an alpha-beta-gamma-delta basis referring to the intricacy of the sunspot complex and the pattern of its polarities.  The more complex, the more oppositely-charged particles are thundering by one another, and the more likely that the fields will disconnect and reconnect elsewhere quickly and violently, releasing a flare or CME or both.  [17] Beneath the sunspot, the field can be twisting, causing the upper loop to slip and reconnect.  (see image to right)

NOAA http://www.universetoday.com/51585/solar-flares-can-now-be-predicted-more-accurately/