A solar flare (or solar plasma eruption) is an explosion in a star's atmosphere, that accelerates plasma into space at great speeds.

After being transported back in time to 1947 Earth, Rom theorized that solar flares could be interfering with their universal translators. (DS9: "Little Green Men")

In 2353, a Kohl planet was rendered temporarily uninhabitable by a solar flare. (VOY: "The Thaw"). In 2371, Grand Nagus Zek talked to a representative of planet which had been affected by solar flares, damaging its agriculture industry. The Ferengi leader agreed to provide the planet with Kohlanese barley. (DS9: "Prophet Motive"). In 2372, intense solar flare activity caused electrodynamic turbulence throughout the Drayan star system. (VOY: "Innocence")

During a scan of Aldean DNA, Doctor Beverly Crusher was able to confirm that solar flare variations were not responsible for the Aldeans' sterility. (TNG: "When The Bough Breaks"). In 2365, the Bringloidi civilization was threatened by solar flares in the Ficus sector (TNG: "Up The Long Ladder").

By the 24th century, various techniques existed to trigger solar flares, including particle beams and electromagnetic pulses. During a battle in the Klingon civil war, Kurn destroyed two pursuing Birds-of-Prey by triggering a solar flare. (TNG: "Redemption II"). In early 2370, the USS Enterprise-D triggered a solar flare and was successful in destroying a Borg ship. (TNG: "Descent"). In 2375, during the Dominion War, the Klingon Bird-of-Prey, Rotarran, triggered a solar flare in the Monac sun. The flare destroyed the Monac shipyards. (DS9: "Shadows and Symbols").

A solar flare is actually a sudden and dramatic release of energy through a break in the Sun's chromosphere in the region of a sunspot, which may last from a few minutes to a few hours. Following an intense solar flare (electron density 1011 compared with 108 in solar quiet times) the ionization in Earth's atmosphere may increase by several orders of magnitude leading to effects such as bright aurorae, magnetic storms, and radio interference.

Solar flares are classified on a scale of importance ranging from 3+ (largest area) to 1- (smallest area). Flares release energy in many forms – electromagnetic (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. The largest solar flares eject a mass of about 10 billion tons at a speed of roughly 1,500 km/s.

Solar flares do have quite dramatic impacts on the Sun's planets, causing in particular geomagnetic storms. The largest recorded geomagnetic storm occurred in 1859, when telegraph wires in both the United States and Europe shorted out, some even causing fires. Auroras were seen as far south as Hawaii, Mexico, Cuba, and Italy-phenomena that are usually only seen near the poles. On 13 March 1989 a severe geomagnetic storm caused the collapse of the Hydro-Québec power grid in a matter of seconds as equipment protection relays tripped in a cascading sequence of events. Six million people were left without power for nine hours, with significant economic loss. The storm even caused auroras as far south as Texas. In August that same year, another storm affected microcomputer chips, leading to a halt of all trading on Toronto's stock market.

Intense solar flares release very-high-energy particles that can be as injurious to humans as the low-energy radiation from nuclear blasts. Earth's atmosphere and magnetosphere allow adequate protection at ground level, but astronauts in space elevated radiation and aircraft flying at high altitudes are subject to potentially lethal doses of radiation. The penetration of high-energy particles into living cells can cause chromosome damage, cancer, and a host of other health problems. Ionospheric storms can affect radio communication, navigation systems, satellites and electric power networks.