What is a Solar Flare ?

Solar flares are mass ejections from the surface of the Sun caused by the spontaneous reconnection of magnetic field lines. Solar flares are so violent that they would be capable of incinerating entire continents if the Earth were held close to them. Solar flares pose a danger to astronauts due to the energetic particles they release over long distances.
Like some other energetic astronomical events, solar flares release massive amounts of energy across the entire electromagnetic spectrum, from the longest — wavelength radio to the shortest — wavelength gamma rays. Solar flares tend to occur in active regions around sunspots, and their frequency matches the intensity of sunspots at any given time, ranging between once a week to several per day. Solar flares are powerful enough to temporarily disrupt long-range radio communication on Earth. The magnetic reconnection events that power solar flares take place on timescales of minutes to tens of minutes.
Solar flares are related to Coronal Mass Ejections, another type of stellar phenomena whereby large quantities of solar atmosphere are ejected into space at great speeds. In a solar flare, electrons, protons, and heavy ions may be accelerated to speeds close to that of light. For an unfortunate astronaut outside the Earth’s atmosphere and lacking sufficient shielding, this could mean instant death. Therefore, scientists are very concerned about studying solar flares so they might better predict them.

The first solar flares were observed in 1856 as bright flares on the edges of sunspots. Relative to the size of the Sun itself, solar flares are quite small, but relative to the Earth and other planets, they are large. Energetic particles released by solar flares contribute to the creation of the beautiful aurora borealis and aurora australis.
Solar flares cause the release of a large cascade of particles known as a proton storm, which is what can be dangerous to astronauts. A few decades ago, it was believed that proton storms could only travel at approximately 8% the speed of light, theoretically giving astronauts two hours to reach shelter in case of an observed solar flare. But recently, in 2005, a proton storm was observed reaching the vicinity of the Earth only 15 minutes after the initial observation, indicating a speed approximately a third that of light. This increases the solar flare risk for astronauts, and provides a design challenge for engineers designing long-range spacecraft, such as journeys to Mars.