3.1 The Quiet Sun
The Sun is a relatively strong emitter of electromagnetic waves over a wide range of frequencies from metric, through infra red and optical to ultra violet and X rays. It is undoubtedly the brightest radio object in the sky and can be detected with modest equipment. Many radio astronomers start by observing the Sun, especially when 'Solar storms' are occurring as the emissions produced are many orders of magnitude larger than those from a quiet or 'quiescent' Sun. A typical spectrum of radio emissions under quiet conditions is given in Figure 3.1. We see that signal strength increases with frequency and decreases with wavelength. This is typical of a thermal source where the generation mechanism is due to thermal agitation of the gaseous material of which the Sun is composed.
Figure 3.1 Radio spectrum of 'quiet' Sun
3.2 Solar storms
When significant sunspots occur, the radio output increases with sharp signal 'spikes' as shown in Figure 3.2 (1)
Figure 3.2 Type 1 solar bursts
The spectrum of storms and bursts is shown in Figure 3.3 and clearly suggests that the generation mechanism is non-thermal (the spectrum slopes the opposite way to the thermal emission from a quiet Sun). The waves from bursts are also strongly circularly polarized by the intense magnetic field associated with sun spots.
Figure 3.3 Spectra of Solar emissions
There are other outbursts from the Sun. Type 2 are found to drift from high to low frequencies with a rate of about 1MHz/ second, are randomly polarized and are believed to be produced by plasma oscillations. Type 3 emissions are sometimes called 'fast drift' bursts because the change frequency is at around 20MHz/ second. Occasionally after a large solar flare there is a long burst of wideband radiation from metres to low microwaves - these are called Type 4 emissions. See Figure 3.4 (2).
Figure 3.4 Dynamic behaviour of Solar emissions
Figure 3.5 shows an enormous solar eruption that occurred in October 2003 - note the size of the plasma ejection compared to the Earth.
Figure 3.5 A Solar eruption
There is much that can be studied on the Sun by measuring the radio output at a number of frequencies that can be received with a general purpose communications receiver and a few types of antenna designed for the HF and VHF bands. Amateur radio astronomers can start by setting up such equipment and monitoring the Sun over long periods of time to establish trends and sudden events such as solar storms.
© Dr David Morgan 2011