3.9 Metric type II radio bursts

Many strong flares are accompanied by so-called type II radio bursts (for an early review see Nelson and Melrose, 1985). They appear as stripes of enhanced radio emission slowly drifting from high to low frequencies in dynamic radio spectra. The frequency of this characteristic radiation drops from several hundred MHz to about 20 MHz or even less within some tens of minutes. The equivalent wavelength of this radiation is in the meter range, and thus these bursts are often called metric type II radio bursts. An impressive example from Cane and Erickson (2005Jump To The Next Citation Point) is shown in Figure 18View Image. In this particular case the first harmonic was also observed, as is often the case. It is generally thought that type II radio bursts are due to electrons accelerated at an outward propagating coronal shock front. The decrease in frequency corresponds to motion of the shock through the radial plasma density profile. For a known (or assumed) density profile, one can deduce the propagation speed of the driving shock wave.
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Figure 18: The radio type II burst of November 1, 2003, from 22:00 UT on. This is a clear example of a strong metric type II burst that extends from 300 to 10 MHz. It was observed from 3 independent instruments: from the WAVES instrument on the WIND spacecraft and the ground-based helio-spectrogtraphs. The WAVES data cover the frequency range below 14 MHz, the BIRS data run from 14 to 57 MHz, and the Culgoora data run from 57 to 570 MHz. Fundamental and harmonic bands of the type II are easily seen, along with splitting of each band. After the type II bands, some broadband type IV is seen in the Culgoora and BIRS data. A short, type II like feature is seen in the BIRS data between 23:06 and 23:14 UT. The light area in the BIRS data between 22:34 and 22:48 UT results from ionospheric absorption of the galactic background due to solar UV. From Cane and Erickson (2005Jump To The Next Citation Point).

Sometimes the main trace of the radio-signature in the frequency-time plot is accompanied by a second component from apparently much faster propagating features. It appears as rapidly drifting (some 10 MHz/s) stripes of enhanced radio emission shooting up from the “backbone” (i.e., the main type II trace in the frequency-time plot) towards both high and low frequencies (see Figure 19View Image from Mann and Klassen, 2005Jump To The Next Citation Point). These “herringbones,” as Roberts (1959) called them, are regarded as signatures of upward and downward pointed electron beams, which are accelerated by the shock waves associated with the “backbone”. Mann and Klassen (2002) found some morphological differences between these herringbone electrons and the type III electrons: the latter ones are faster by about a factor of 2 and apparently result from a different acceleration process near the flare site.

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Figure 19: Dynamic radio spectrum (A) of the solar type II burst during the event on June 30, 1995. The “backbone” is slowly drifting from 60 to 42 MHz. The “herringbones” are nicely visible during the whole event. The radio intensity is coded by grey scale. The bottom frames (B, C) present the intensity time derivative of the individual “herringbones.” From Mann and Klassen (2005Jump To The Next Citation Point).

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