Solar Flares

Every so often a small region of the Sun erupts in a flash of radiation and energetic particles. We call these eruptions solar flares. The main driving force behind solar flares is the energy contained in the Sun's magnetic field. Like an elastic band, magnetic field stores energy when it is twisted: on the Sun the motions of the solar surface from which the field emanates does the twisting. This stores energy in the field (or elastic band) in the form of additional tension and pressure. A solar flare occurs because the magnetic field can only store a limited amount of energy. When the energy in the field gets too large it is suddenly released into the solar atmosphere. Try twisting an elastic band as much as you can. What eventually happens?

The sudden onset of a solar flare can be seen in data from the Transition Region and Coronal Explorer (TRACE). Click on the image to see a movie of a flare which occurred on May 19 1999. [The movie is 1.3 Mbytes!].

Radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves, through optical emission to x-rays and gamma rays. The amount of energy released is the equivalent of millions of 100-megaton hydrogen bombs exploding at the same time! The first solar flare ever recorded was by two amateur astronomers in England on September 1, 1859. Richard C. Carrington and Richard Hodgson, were independently observing sunspots when a remarkable and unexpected phenomenon occurred:

Excerpt from Carrington's paper on the first observation of a solar flare Carrington's drawing of what he saw
While engaged in the forenoon of Thursday, September 1, in taking my customary observation of the forms and positions of the solar spots, an appearance was witnessed which I believe to be exceedingly rare. The image of the sun's disk was, as usual with me, projected on to a plate of glass coated with distemper of a pale straw color, and at a distance and under a power which presented a picture of about 11 inches diameter. I had secured diagrams of all the groups and detached spots, and was engaged at the time in counting from the chronometer and recording the contacts of the spots with the cross-wires used in the observation, when within the area of the great north group (the size of which had previously excited great remark), two patches of intensely bright and white light broke out ...

... seeing the outburst to be very rapidly on the increase, and being somewhat flurried by the surprise, I hastily ran to call some one to witness the exhibition with me, and on returning within 60 seconds, was mortified to find that it was already much changed and enfeebled. Very shortly afterwards the last trace was gone. In this lapse of 5 minutes, the two patches of light traversed a space of about 35,000 miles.

Carrington's flare was a rare type of flare known as a white light flare because it produced a dramatic enhancement in optical emission which requires an extremely large event. A more modern observation of such a flare yields images like this one taken by the Big Bear Solar Observatory:

The energy released in a flare is ten million times greater than the energy released from a volcanic explosion, yet it is less than one-tenth of the total energy emitted by the Sun every second. In addition to the radiation, electrons, protons and heavy atomic nuclei are ejected with very large velocities. These charged particles can play havoc with the magnetic field around the Earth leading to satellite disruptions, enhanced aurora and perhaps even power outages.

The intense radiation from a solar flare travels to Earth in eight minutes. As a result:

  • The Earth's upper atmosphere becomes more ionized and expands.

  • Long distance radio signals can be disrupted by the resulting change in the Earth's ionosphere.

  • A satellite's orbit around the Earth can be disturbed by the enhanced drag on the satellite from the expanded atmosphere.

  • Satellites' electronic components can be damaged.

  • Energetic particles from solar flares can, on occasion reach the Earth and be particularly dangerous to astronauts and to electronic instruments in space.

  • Astronaut's on their way to Mars (or the moon) are particularly vulnerable to the damaging effects of solar radiation.
  • In this way solar flares can be similar in effect to coronal mass ejections which are regarded as being the primary source of disturbances at the Earth.

    The relationship between a solar flare and a coronal mass ejection is a complicated one. But sometimes they are directly connected as in the example shown here from the Yohkoh/SXT. The first two frames show a "candle-flame" structure which is the aftermath of a mid-size solar flare occurring on 16 August 1998. Two days later, this same region of the solar corona undergoes a sudden change, ejecting plasma into space, leaving behind a hole which slowly fills in again. This latter behavior is similar to that observed in coronal mass ejections.

    Solar flares are generally associated with regions of strong magnetic fields and occur predominantly near sunspots in active regions. During a solar flare the corona can increase in temperature from a few million degrees to over 20 million degrees and sometimes as high as 100 million degrees!

    A solar flare can be sufficiently strong that it disrupts the magnetic field of the surrounding active region setting up oscillations in neighboring loops. These dynamical signatures help scientists determine the physics of what is going on during a solar flare.

    The frequency of flares coincides with the Sun's eleven year cycle. When the solar cycle is at a minimum, active regions are small and rare and few solar flares are detected. These increase in number as the Sun approaches the maximum part of its cycle. The Sun recently passed the maximum phase of the most recent cycle which showed a double peake (in July, 2000 and September, 2001 see The Sunspot Cyle).

    The Active Sun