Radiation from High Speed Flare Electrons

tte electrons accelerated during solar flares emit invisible radiation across the electromagnetic spectrum, from the shortest X-rays and gamma rays to the longest radio

FIG. 7.4 Synchrotron radiation High-speed electrons moving at velocities near that of light emit a narrow beam of synchrotron radiation as they spiral around a magnetic field. This emission is sometimes called non-thermal radiation because the electron speeds are much greater than those of thermal motion at any plausible temperature. The name "synchrotron" refers to the man-made, ring-shaped synchrotron particle accelerator where this type of radiation was first observed; a synchronous mechanism keeps the particles in step with the acceleration as they circulate in the ring.

FIG. 7.4 Synchrotron radiation High-speed electrons moving at velocities near that of light emit a narrow beam of synchrotron radiation as they spiral around a magnetic field. This emission is sometimes called non-thermal radiation because the electron speeds are much greater than those of thermal motion at any plausible temperature. The name "synchrotron" refers to the man-made, ring-shaped synchrotron particle accelerator where this type of radiation was first observed; a synchronous mechanism keeps the particles in step with the acceleration as they circulate in the ring.

waves, but the protons are too massive to emit intense radio and X-ray radiation, tte radio waves provide information about the location, size, magnetic fields and temperature during a solar flare, ttey additionally demonstrate that flare electrons have been accelerated to very high speeds. Energetic electrons that have been accelerated during solar flares are also hurled out into interplanetary space, emitting intense radio emission in the process.

As the high-speed electrons spiral down along magnetic channels, they generate intense radio emission; sometimes-called radio bursts to emphasize their brief, energetic and eruptive characteristics. tte magnetic fields bend the paths of the electrons into a circle, resulting in the emission of a narrow beam of synchrotron radiation (Fig. 7.4), named for the man-made synchrotron particle accelerator where it was first observed.

Most of the energy radiated during a solar flare is emitted as hard and soft X-rays, ttey provide detailed information about the flare process, including why and where they occur, tte X-ray radiation is named bremsstrahlung, the German word for "braking radiation." It is produced when a free electron, unattached to an atom, passes near an ion (Fig. 7.5). tteir electrical interaction bends the path of the electrons, which radiate soft X-rays in the process, ttere is thermal bremsstrahlung, which depends on the random thermal motion of the hot electrons; it is emitted when electrons have been heated during the flare to temperatures of about 10 million kelvin. Electrons are also briefly accelerated to energies much higher than the mean energies of thermal plasma at any plausible temperature, and these very high-speed electrons give rise to non-thermal bremsstrahlung (Fig. 7.6).

FIG. 7.5 Bremsstrahlung When a hot electron moves rapidly and freely outside an atom, it inevitably moves near a proton in the ambient gas. There is an electrical attraction between the electron and proton because they have equal and opposite charge, and this pulls the electron toward the proton, bending the electron's trajectory and altering its speed. The electron emits electromagnetic radiation in the process. This radiation is known as bremsstrahlung from the German word for "braking radiation".

FIG. 7.5 Bremsstrahlung When a hot electron moves rapidly and freely outside an atom, it inevitably moves near a proton in the ambient gas. There is an electrical attraction between the electron and proton because they have equal and opposite charge, and this pulls the electron toward the proton, bending the electron's trajectory and altering its speed. The electron emits electromagnetic radiation in the process. This radiation is known as bremsstrahlung from the German word for "braking radiation".

X-ray astronomers distinguish between soft X-rays and hard X-rays, with the softer variety having less energy than the hard type, tte energy that X-rays carry is a measure of the energy of the electrons that produce it, often specified in units of kilo-electron volts, abbreviated keV. Soft X-rays have energies between 1 and 10 keV, and hard X-rays lie between 10 and 100 keV. In contrast, gamma rays can have energies greater than one Mev, or a million electron volts. And radiation at different X-ray wavelengths describes different aspects of the flare time profile (Fig. 7.7). So what you see during a solar flare depends on how you look at it.

At the impulsive stage of a solar flare, electrons are accelerated rapidly, in a second or less, to energies that can exceed one MeV. tte high-energy electrons emit hard X-rays and gamma rays that mark the flare onset, ttese energetic, high-speed, non-thermal electrons are believed to be accelerated above the tops of coronal loops, and to radiate energy by non-thermal bremsstrahlung and synchrotron radiation as they are beamed down along the looping magnetic channels into the low corona and chromosphere.

During the decay, or post-impulsive, phase, energy is gradually released on longer time-scales of tens of minutes. Soft X-rays slowly build up in strength and usually reach peak intensity during the post-impulsive decay phase. Initially cool material in the chromosphere is heated by down flowing flare electrons that emit the hard X-rays, tte heated gas expands upward into the low-density corona along magnetic loops that shine brightly in soft X-rays.

Major solar flares are often accompanied by the disruption of a filament that was held up by the magnetic loops in the active region, ttese closed magnetic fields are blown open during the impulsive phase; but the newly opened magnetic field lines reconnect, forming an arcade of post-impulsive flare loops, aligned like the bones in your rib cage or the arched trestle in a rose garden (Fig. 7.8). ttey are subsequently filled by plasma flowing up from the chromosphere at their footpoints.

FIG. 7.6 Energy spectrum of flare radiation The spatially integrated energy spectrum of the radiation photons during a 14-second time interval at the peak of the solar flare on 20

FIG. 7.6 Energy spectrum of flare radiation The spatially integrated energy spectrum of the radiation photons during a 14-second time interval at the peak of the solar flare on 20

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