Location of particle acceleration

3.4 Location of particle acceleration

Accelerated particles precipitate from the acceleration site or are temporarily trapped. On their way spiraling along the magnetic field, they radiate various radio emissions at frequencies depending on the local plasma. As the corona is transparent to radio emission above the plasma frequency, the accelerated particles outline the geometry of the environment of acceleration and/or the actual site of acceleration.

Gyrosynchrotron radiation is emitted incoherently by relativistic electrons over the whole loop (review by Bastian et al., 1998 , Figure 17 ). The spectrum at high frequencies is close to a power-law, shaped by the initial power-law energy distribution of accelerated electrons. The loop-top radio spectrum falls off far more steeply at high frequencies than does the footpoint spectrum. Thus the centimeter radio emission confirms the differences between loop-top and footpoints found in X-ray sources. In addition to gyrosynchrotron emission, Wang et al. (1994 ); Silva et al. (1996 ) report thermal loop-top emission at a temperature of about 30 MK, in rough agreement with the hot thermal component of the coronal soft X-ray source.

Figure 17:

gif-Movie (265 KB) Thermal emission and a flare observed with the Nobeyama interferometer at 17 GHz. The event has the classic signatures of an event associated with a coronal mass ejection: motion is seen first in the prominence material, then the flare goes off, leaving long-duration soft-X-ray emitting loops around for several hours. Note that the prominence material is at a temperature of less than 10,000 degrees K (shown at top), whereas the flare loops come from material at 10 MK: both cool and hot material show up at this radio frequency. The left panel shows total intensity, and the right panel shows circularly-polarized radio emission. Polarization is only detected during the early phase of the flare, when very energetic electrons fill up the loop and emit intense synchrotron radiation. Courtesy of Stephen White.

The most intense flare radio emission at meter and decimeter wavelengths originates not from single particles, but from waves in the plasma, i.e., from coherent radiation processes. Fast drift radio bursts, or type III bursts, were among the first types of meter wave emissions discovered in the 1940s. The drift of the radiation to lower frequencies with time was interpreted by Wild (1950 ) as the signature of an electron beam propagating upward through the corona at a speed of 0.2 – 0.6 c. Later, occasional reverse-drift bursts were discovered (downward-directed beams). Summaries of the type III observations can be found in Krüger (1979 ), Suzuki and Dulk (1985 ), and Pick and van den Oord (1990 ). Imaging observations have shown that the type III sources are often not single, but emerge simultaneously into different directions. Paesold et al. (2001 ) found double type III sources to diverge from a common source of narrowband spikes around 300 MHz (see Figure 18 ). Figure 19 shows a three-dimensional reconstruction assuming an exponential (constant temperature) model for the density. The spikes observed to be close to the point of divergence suggest the location of the acceleration at an altitude around 90,000 km. Krucker et al. (1995 , 1997a ) located spike sources at about 5 $×$ 10⁵ km altitude. Klein et al. (1997 ) found evidence for acceleration of type III electron beams at the height of one solar radius.

Figure 18:

Spectrogram of type III radio bursts (drifting structures in the upper right) and meter wave type of narrowband spikes (center) (from Benz et al., 1996

Metric spikes have been found to be associated in some cases with impulsive electron events in the interplanetary medium (Benz et al., 2001 ). The low energy cut-off of the interplanetary electron distribution defines an upper limit of the density in the acceleration region (Lin et al., 1996 ). The derived electron density is of the order of 3 $×$ 10⁸ cm^–3, consistent with the density in the source of metric spikes, assuming second harmonic plasma emission. The difference between acceleration height in hard X-rays, particle events and coherent radio waves suggests different acceleration processes.

Figure 19:

Reconstructed trajectories of two radio events involving type III bursts and spikes, both of the kind often observed at meter wavelength. Upper right: Positions with error bars observed by the Nancay Radio Heliograph at three frequencies. The symbols represent the observed frequencies: $×$ for 327.0 MHz, $□$ for 236.6 MHz, and $△$ for 164.0 MHz. Bottom left: Position of the radio emissions on the Sun. The small square indicates the size of the image presented in the upper right image. Upper left: Projection of the sources on the meridian plane (as seen by an observer West of the source as seen from Earth). Lower right: The radio sources projected on the equatorial plane, showing the view of an observer North of the Sun). In both graphs the trajectories have been 3-dimensionally spline interpolated to outline the trajectory (Paesold et al., 2001 ).