7. The Violent Sun

       
    • There are three types of explosive phenomena resulting from magnetic reconnection in the low solar corona – they are solar flares, eruptive prominences and coronal mass ejections.

    • The relatively calm solar atmosphere can be torn asunder by sudden, brief outbursts called solar flares, the most powerful explosions in the solar system. In 100 to 1,000 seconds, they release energy equivalent to millions of 100-megaton hydrogen bombs exploding at the same time, and raise the temperature of Earth-sized regions in the low corona up to 20 million kelvin.

    • Large looping arches of magnetism, containing relatively cool material with a temperature of about 10,000 kelvin, can suddenly expand out into space; such erupting prominences or filaments are often associated with coronal mass ejections.

    • Coronal mass ejections rip out billions of tons of material from the corona, hurling it into interplanetary space in expanding magnetic bubbles that rapidly rival the Sun in size.

    • Solar flares occur in solar active regions, above sunspots where the magnetic fields are strong.

    • Solar flares are brief, sudden and unpredictable.

    • Solar flares are more frequent and powerful at the maximum of the Sun’s 11-year magnetic activity cycle.

    • Most solar flares cannot be seen in the visible sunlight that we detect with our eyes. Rare visible solar flares, detected with optical telescopes, are called white-light flares.

    • Solar flares can outshine the entire Sun at X-ray and radio wavelengths.

    • Flares are detected in the chromosphere by tuning into the red spectral line of hydrogen, the Balmer alpha transition at 656.3 nanometers. These chromospheric hydrogen-alpha flares often exhibit two parallel ribbons of light, attributed to energetic particles beamed down into the chromosphere along newly linked coronal loops.

    • Flare ribbons have been detected at extreme-ultraviolet wavelengths from TRACE, with high spatial and temporal resolution. It has been used to measure the separation speed of loop-footpoint ribbons and the rate of magnetic reconnection above them.

    • Hard X-rays are detected during the impulsive phase of solar flares, when electrons are accelerated to high velocities and hurled down to the footpoints of coronal loops.

    • Soft X-rays are emitted during the decay phase of solar flares, when material from the chromosphere rises up to fill newly linked coronal loops in a process called chromospheric evaporation.

    • Solar-flare X-rays are emitted by bremsstrahlung, or braking radiation, when electrons encounter protons.

    • The slow, smooth rise of the flare emission at soft X-rays resembles the time integration of a flare’s impulsive hard X-ray radiation. This Neupert effect suggests that the electron beams that give rise to the hard X-rays produce chromospheric evaporation.

    • Hard X-rays are emitted as the bremsstrahlung of high-speed, non-thermal electrons, accelerated in the low corona during the impulsive flare phase and beamed down toward the photosphere. Less energetic thermal electrons give rise to soft X-ray bremsstrahlung.

    • The Bragg Crystal Spectrometer aboard Yohkoh has been used to show that flaring gas is heated in the chromosphere from 10,000 to 20 million kelvin, flowing up into flaring coronal loops that produce the flare soft X-ray emission.

    • Yohkoh’s Hard X-ray Telescope has been used to discover non-thermal, loop-top hard X-ray sources located just above flaring loops detected by the Soft X-ray Telescope, suggesting that very energetic electrons are accelerated above the flare loops.

    • A flare particle acceleration site that is located about 50% higher than the soft X-ray flare loop heights has been inferred from electron time-of-flight measurements with the Compton Gamma Ray Observatory and from ground-based observations of upward and downward moving radio bursts.

    • Rare white-light flares correlate well with the flare hard X-ray emission, in both space and time, suggesting a similar origin.

    • Flare-associated protons and heavier ions can be beamed down into the lower solar atmosphere, producing brief, non-sustainable nuclear reactions with the emission of gamma-ray spectral lines and neutrons that move nearly at the speed of light.

    • Gamma-ray lines emitted during solar flares include the 0.511 MeV electron-positron annihilation line and the 2.223 MeV neutron capture line.

    • The first gamma-ray images of solar flares, in the 2.223 MeV neutron-capture line, have been obtained from an instrument aboard RHESSI, showing that the flaring gamma-ray sources can be spatially displaced from the hard X-ray ones.

    • Solar flares result from magnetic energy stored in the low solar corona.

    • Solar flares originate near the apex of coronal loops.

    • A solar flare is triggered by magnetic interaction and reconnection.

    • Explosive solar flares and CMEs can be ignited when magnetized coronal loops come together and reconnect in the low solar corona. Stored magnetic energy is released rapidly at the place where the magnetic fields touch and merge together.

    • During coronal magnetic reconnection, the stressed coronal loops partially annihilate each other; release magnetic energy stored in them, and reconnect into less-energetic, more stable configurations.

    • Signatures of magnetic reconnection include soft X-ray cusps and hard X-rays emitted in the low solar corona.

    • A filament is a region of relatively cool gas embedded in the corona, seen in absorption on the solar disk and suspended by a long, low-lying core magnetic loop. A prominence is a filament seen in emission at the edge, or limb, of the Sun.

    • An erupting prominence, or filament, can be caused by excessive shear of its low-lying, core magnetic loop, which can rise to open the overlying magnetic arches that straddle it.

    • Coronal Mass Ejections, abbreviated CMEs, are seen edge-on with a white-light coronagraph. It has an occulting disk that blocks the intense glare of the photosphere and permits the corona to be seen.

    • Many thousands of coronal mass ejections have been observed since the 1970s from space-borne coronagraphs. The LASCO instrument aboard SOHO has recorded images of more than ten thousand coronal mass ejections, and incidentally resulted in the discovery of more than a thousand comets.

    • At the minimum in the Sun’s 11-year activity cycle, coronal mass ejections appear near equatorial latitudes with an average rate of 0.5 events detected per day; near cycle maximum they appear at all latitudes, including the poles, with an average rate of 6 events per day.

    • Coronal mass ejections can exhibit a three-part structure – a bright outer front, followed by a darker, underlying cavity, surrounding a brighter core; but not all coronal mass ejections display these nearly circular shapes.

    • CMEs are huge, much bigger than solar flares or active regions; the CMEs rapidly become larger than the Sun.

    • Coronal mass ejections hurl between 1 and 50 billion tons, or 1012 to 5 x 1013 kilograms, of material out from the Sun at apparent speeds near the Sun of up to 3,400 kilometers per second. But most coronal mass ejections exhibit apparent speeds of between 300 and 500 kilometers per second.

    • CMEs generate shock waves that accelerate high-energy particles and may cause intense geomagnetic storms when directed toward Earth.

    • Coronal mass ejections are often associated with, and followed by, eruptive prominences that can reform into an arcade of magnetic loops detected in soft X-rays or at extreme ultraviolet wavelengths.

    • Coronal loops are sent into quivering oscillations in the aftermath of solar flares. Such oscillations can be used to infer the physical properties of the loops, in a technique known as coronal seismology.

    • Coronal mass ejections can generate waves that propagate across the entire Sun, which have been detected with the EIT instrument on SOHO.

    • Coronal mass ejections are associated with depleted regions of the corona, which have been detected as reductions in the soft X-ray emission and a related dimming of the Sun’s extreme-ultraviolet radiation.

    • The rate of occurrence of solar flares, erupting prominences and coronal mass ejections varies in step with the 11-year cycle of solar magnetic activity, becoming more frequent near the cycle maximum and suggesting an origin related to strong magnetic fields.

    • Coronal mass ejections and solar flares may be a manifestation of a similar energy-release process in the solar corona. Exceptionally fast and energetic coronal mass ejections are commonly accompanied by solar flares, and vice versa, but each type of solar outburst can occur without the other one and there appears to be no general cause-effect relation between the two.

    • Coronal loops can remain stable for days; weeks or even months, and then explode, perhaps when the magnetic loops become twisted into an unstable configuration.

    • When large, twisted sigmoid (S or inverted S) shapes appear in soft X-ray images of the Sun, they can give advance notice, by a few days, of mass ejections that might collide with the Earth.

    • Solar flares and coronal mass ejections most likely arise from coronal loops that have been sheared and twisted into non-potential configurations with free magnetic energy available to power the outbursts. This twist can be detected as active-region sigmoid structures in soft X-rays, and in images of both eruptive prominences and coronal mass ejections.

    • A twisted magnetic flux tube that emerges into the corona can become unstable to form an erupting prominence or coronal mass ejections. There are at least two models for the outbursts, and one of them includes the kink instability that sets in when there is too much twist.

    • GONG and SOHO MDI data have been used with the techniques of local helioseismology to detect deep swirling flows that circulate horizontally around solar active regions, and to show that the strength of this twisting flow is correlated with the intensity of X-ray flares emitted from active regions.

    • Helioseismic holography is used with SOHO MDI or GONG data to determine the location and magnitude of active regions anywhere on the backside of the Sun days before they rotate into view on the visible solar disk. Daily images of these unseen active regions are available on the web, and can be used to give advance warning of possible solar flares and coronal mass ejections when the active regions rotate to face the Earth.

Copyright 2010, Professor Kenneth R. Lang, Tufts University