Radio Astronomy SA

Some recent examples of Solar Radio Burst types detected by our Murchison Widefield Array antenna based Callisto Radio Spectrometers at Sunnydale.  The equipment we use limits our effective frequency range to between 108MHz and 370MHz, the lower 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.

After large flares narrow bands of intense radiation drift sometimes slowly, and often irregularly, towards lower frequencies.  Type II bursts typically occur at around the time of the soft X–ray peak in a solar flare.  The spectra occasionally show the presence of a second harmonic in frequency, but are often so complex as to preclude such identifications.  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 superalvenic shocks originating in the flare or coronal mass ejection (CME) and moving through the corona.  This solar flare was 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 (75-700MHz/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.  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 continuum radiation is a steady enhancement of the background level over a wide band of the spectrum, and is often followed by a noise storm (type I).  At times, however, an extremely intense form of continuum radiation is observed covering a frequency band of more than 300 MHz.  It often occurs during and after great outbursts, and may last 10-300 minutes.  The wavy, periodic intensity variations in frequency are not caused by standing waves in the receiving system.  They are not properties of the type IV burst.  This can be confirmed by comparing the same flare from observations at different locations (e-Callisto network).

Type V bursts are always associated with type III bursts and follow them for a few tens of seconds.  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.  Duration is typically less than 1 minute.  The exciter is the electron beam that is partially scattered at high altitude.