Radio Astronomy SA

Some recent examples of Solar Radio Burst types detected by our Long Wavelength Array antenna based Callisto Radio Spectrometers at Sunnydale.  Plasma emissions dominate most of the observed bursts below 100MHz and down to the ionospheric cutoff frequency around 10MHz.  The equipment we use limits our effective frequency range to between 15MHz and 88MHz, the upper limit capped to avoid the FM radio broadcast band.  Each image is a 15 minute scan of radio emissions from the Sun.  The scan frequencies are indicated by the scale to the left of each image and the relative SRB intensity is indicated by the scale to the right of each image.

A noise storm consists of long series of short and narrow-band bursts continuing over hours or days.  Polarization is always high (circular) and the sign of the polarization can change within one day.  The bursts are superimposed on a background of slowly varying enhanced radiation which has been described as a ”continuum”, although it is possible that the background may itself be composed of a large number of overlapping bursts.  Noise storms are normally spread over a large frequency band but are rarely seen above 350MHz.  On many occasions the bursts have bandwidths of a few MHz and lifetimes extending from 0.1 seconds to 1 second.  Chains of bursts last nearly 1 minute .  At frequencies below 40MHz drifting bursts of bandwidth up to 30 MHz and lifetimes less than a second may predominate.

Type II bursts typically occur at around the time of the soft X-ray peak in a solar flare and are identified by; a slow drift to lower frequencies with time in dynamic spectra, the frequent presence of both fundamental and second-harmonic bands (with a frequency ratio of 2); and splitting of each of these bands into two traces but are often so complex as to preclude such identifications.  The duration of the burst may be between 3 and 30 minutes.  In rare cases Type II bursts show a characteristic called ’herring bones’.  The velocity of the solar disturbance giving rise to these slow-drift bursts may be deduced from their rate of change of frequency.  This velocity is of the order of 1000 km/s and corresponds to superalfvenic shocks originating in the flare or coronal mass ejection (CME) and moving through the corona.  Type II bursts are always seen in conjunction with flares, even though some of those flares are very small events.  In this image, the solar flare was a class C6.7 and relatively small.

Type III bursts are brief radio bursts that drift very rapidly in frequency versus time.  They can occur singularly; with typical burst durations of 1 to 3 seconds, in groups; with group durations of up to 5 minutes, or storms/underlying continuum; which may run for hours.  As the frequency of each burst decreases, the burst duration increases while the drift rate (1-10MHz/s) decreases.  In groups, Type III bursts cease almost exactly at the time of the flare's soft X-ray peak, regarded as the time at which energy release in the flare ends.  The radio emission is caused by flare accelerated electron beams propagating through the corona at up to half the speed of light,  typically along open magnetic field lines.  Type III bursts are a very common phenomenon and typically occur in groups of 3 to 10 with a total duration of less than 60 seconds.

The defining characteristic of a Type V burst is an extended phase following a Type III emission at low frequencies, lasting for up to several minutes.  Type V bursts are often difficult to identify, particularly if there are other bursts present at the same time.  They appear like a flag attached at low frequency to the Type III burst.

The emission is quasi-continuous with a bandwidth of less than the Type III burst.  The exciter is the electron beam that is partially scattered at high altitude.

Series of Type III bursts over a period of 10 minutes or more, with no period longer than 30 minutes without activity.

The U-burst, first identified by Maxwell and Swarup and Haddock (Maxwell, 1958), is a type of solar event lasting up to about 10 seconds in which the frequency or the emission first drifts rapidly downwards, then increases again.  On the dynamic spectrum the burst has appearance of an inverted letter U.  The frequency drift rate at the sides of the ’U’ is of the same order as that observed in ordinary type III bursts;  the U-burst is now generally regarded as sub-class of type III, and is believed to be generated by plasma waves set up by fast electron streams in the solar corona following a loop-shaped magnetic field that turns back to the chromosphere.