The SSRT monitors solar storms by using radio waves to sense sudden disturbances in ionosphere

The ionosphere is an ionized layer in the atmosphere roughly 50-600 km above the Earth’s surface.   Its ionization is caused by incoming UV and X-ray radiation from the sun.  The degree of ionization increases with the amount of solar radiation received, and therefore tends to depend on the latitude, the season, and the time of day (see image opposite, credit NASA).  Ionization is also dramatically affected by exceptional events such as solar flares and coronal mass ejections.

The ionosphere is excellent propagator of radio waves.  Short wave communications such the BBC World Service are broadcast across the globe thanks to the ability of the ionosphere to carry radio waves beyond the transmitter’s line of sight.  The strength of the propagation by the ionosphere depends on the degree of ionization in quite complex ways, but the essential point is that short terms changes in the degree of ionization can be detected by monitoring the changing power of a distant radio signal that is being carried through the ionosphere, thus indicating the occurrence of solar storms.

The kind of radio signal that we would like to monitor is ideally available all over the world, receivable at long range, and transmitted at constant power.  Fortunately such a system exists.  It is the Very Low Frequency (VLF) submarine communications network.  The VLF band at 3-60 kHz is used for submarine communications because only such low frequencies can penetrate through sea water to be picked up by submerged submarines.  There are several dozen naval transmitters in use around the world.  One of the most powerful is the 24 kHz transmitter at Cultler, Maine, USA. 

The SSRT chart is a diurnal pattern that can be interpreted to reveal solar storms

Let’s take a quick tour of some typical traces produced by the SSRT and see how they alert us to solar storms. This image shows a typical signal received from an SSRT on a quiet day.  The local time at the site of the radio telescope in Tokyo, Japan is on the x-axis.  The power of the received signal from the VLF transmitter at Ebino, 900 km away to the south-west is shown on the y-axis.  The scale is in decibels, which is simply a logarithmic scale of power relative to some arbitrary level. The distinctive outline of the trace is caused by the daily cycle of day and night.  During the daytime UV radiation from the sun increases the ionization of the ionosphere, but after the sun has set, electrons and ions start to recombine and ionization decreases.  Perhaps counter-intuitively, the effect of the increased daytime radiation and ionization is to reduce the signal strength.  This is to do with the layered structure of the ionosphere.

Now let’s look at a trace on a day with a distinct solar flare. Solar flares are categorized according to their strength, as measured in Log Watts / Sq. m. The strongest flares are X-class, and have a power of -4 or greater on this scale. 

C-class solar flares caputred by the SSRT

The charts below give an example of some C-class solar flares captured with the SSRT, the most prominent example being observed just after 15:00 local time on the day in question

This atricle is an extract from an e-book that is available with further information about the Solar Storm Radio Telescope and how to build your own radio telescope and join the network if you are interested. The SSRT is not hard to build and the parts should cost no more than about US$200.