Our Violent Sun

Overview

The relatively calm solar atmosphere can be torn asunder by sudden, brief and intense outbursts called solar flares. They are the most powerful explosions in the solar system, releasing energies of up to 20 million 100 megaton terrestrial nuclear bombs (amounting to 10²⁵ Joule or 10³² erg) in 100 to 1,000 seconds.

Solar flares generate high-speed electrons that emit intense radiation at invisible radio and X-ray wavelengths.

Hare-associated protons and heavier ions can be beamed down into the lower solar atmosphere, producing nuclear reactions with the emission of gamma-ray spectral lines, meson decay gamma rays and neutrons that move nearly at the speed of light.

Another type of explosive solar activity, called coronal mass ejections, or CMEs for short, is observed using white-light coronagraphs aboard spacecraft, including the Large Angle Spectrometric COronagraph (LASCO) aboard the SOlar and Heliospheric Observatory (SOHO). CMEs expand away from the Sun at speeds of hundreds of thousands of meters per second, becoming larger than the Sun and removing up to fifty billion tons (5 x 10¹³ kilograms) of coronal material. The depleted regions of the corona are detected as reductions in the soft X-ray emission observed by the Yohkoh spacecraft.

Explosive solar flares and CMEs can be ignited when current-carrying magnetic loops come together and coalesce in a process called magnetic re-connection. During this coronal merging process, the stressed magnetic fields partially annihilate each other, release energy stored in them, and reconnect into less-energetic, more stable configurations. The Soft X-ray Telescope, or SXT, aboard Yohkoh finds a soft X-ray, cusp-type geometry in the low corona, seen edge-on at the apex of long-lived, gradual (hours) flaring loops that can be associated with coronal mass ejections. This morphology was predicted by magnetic reconnection theory. Yohkoh’s Hard X-ray Telescope, or HXT, has discovered non-thermal, loop-top hard X-ray sources located just above short-lived, impulsive (minutes) flaring loops detected by the SXT, suggesting that very energetic electrons can also be accelerated near the reconnection site.

Impulsive solar flares eject protons and electrons into interplanetary space with energies that are thousands and even millions of times greater than those usually present in the solar wind. These particles can be directed by the interplanetary magnetic field to collision with the Earth. Fast coronal mass ejections generate strong interplanetary shock waves that can also hit the Earth; they are associated with long-duration soft X-ray flares and intense long-lived proton and electron events.