The Radio-Sun

The Sun was one of the first objects studied by early radio astronomers.  It is not as powerful an emitter of radio waves as many other objects, but its close proximity to us makes it appear radio-bright to us here on the third planet. In the year 2000, the Sun is expected to peak in sunspot number and the related solar activity level.  That means there should be lots of solar flares on the Sun's surface and the Earth should receive a number of geomagnetic storms as a result. In other words, this should be an exciting time to begin monitoring the Sun with radio receivers and magnetometers.

When there is a solar flare on the Sun's surface, there is often an accompanying burst of radio energy projected into space. You can monitor these bursts with standard short wave and vhf receivers with modest antennas. The receiver should be able to detect AM (amplitude modulated) signals. FM receivers are not good for this purpose.  A pre-amplifier between the antenna and the receiver will help things greatly at vhf, but on frequencies below 30 MHz, a preamp is probably not necessary. A good and inexpensive candidate for a vhf solar flare receiver might be an aircraft band radio which covers the 120-140 MHz range. Ramsey electronics sells an inexpensive kit.  A small 3 or four element Yagi antenna pointed towards the sun should be adequate if a preamplifier is used.

The solar burst shown above was recorded at 20.1 mHz with very simple equipment on June 10, 2000 by Wes Greenman, engineer of the University of Florida Radio Observatory.  Wes used a Radio Jove receiver and dual dipole antenna to record the solar burst which appears as the prominent hump on the left side of the chart.  The stair-stepped signal on the right side of the chart is a calibration signal.

Solar radio bursts are classified as follows:

Type I  Short, narrow band events that usually occur in great numbers
together with a broader band continuum. May last for hours or days.

Type II  Slow drift from high to low frequencies. Often show fundamental
and second harmonic frequency structure.

Type III  Rapidly drift from high to low frequencies. May exhibit harmonics.
Often accompany the flash phase of large flares.

Type IV  Flare-related broad-band continua.

Type V  Broad-band continua which may appear with III bursts. Last 1 to 2
minutes, with duration increasing as frequency decreases.

Burst Examples

Type II


Type III

09/20/2000 with sound

04/02/2001 Class 20 Xray Flare
04/06/2001 Class 5 Xray Flare
06/08/2003 Radio Spectrograms from WCCRO Type II and X-ray  with sound

The Shark Fin Signature and a False Shark Fin

Detection by Ionospheric Effects

Another way to spot solar flares is by an indirect means. With large solar flares come blasts of x-rays. When the x-rays hit the Earth's ionosphere, (the charged particle layers of our atmosphere), the way the ionosphere reflects radio waves is disturbed. At short wave frequencies, a dip in signal strength of distant stations can often be observed.  At VLF frequencies, below 150 kHz, the opposite effect is observed and the signal strength of distant station will jump suddenly and slowly decline.  Using a VLF receiver permanently tuned to a distant station or even to static from distant tropical rain storms is considered a quite reliable way to detect x-ray solar flares. Most radio receivers will not tune low enough in frequency to be used for VLF solar flare detection. An up-converter which allows you to listen at these low frequencies using a standard short wave receiver can be found on this website.

Magnetic Storms

Lastly, solar flares also emit high velocity charged particles. These particles take one or two days to reach Earth, (where as the radio waves and x-rays reach us in about 9 minutes). When the particles arrive they slam into the Earth's magnetic field and distort it. Some of the particles are channeled along the magnetic field lines towards the poles and produce beautiful auroras.  The distortion of the magnetic field produces what we refer to as a geomagnetic storm. These storms can be observed with magnetometers. Severe geomagnetic storms can disrupt large electric power grids and even cause blackouts.

Thermal Emissions

Everything emits electromagnetic waves in proportion to physical temperature.  The Sun is about 6000 degrees at the surface of the photosphere. The "quiet sun", that is, the Sun when it is in a period of low sunspot activity is easily detected at microwave frequencies where its "thermal" emissions are strongest.   It is thus often the first object an amateur radio astronomer will turn their uhf or microwave antenna toward when testing.  If you can't detect the Sun, you probably won't be able to detect anything with your uhf or microwave radiotelescope. The quite sun is a relatively stable signal source and can be used to make a rough calibration of your systems sensitivity.

Solar Links

Free Spectrograph Software allows real-time monitoring of Jupiter and Solar storms from UFRO and WCCRO.

Most Recent Spectrograph Images from the WCCRO

Green Bank Solar Spectrometer

NOAA FTP site with recent radio burst info.

Daily updated ACE data which you can compare to your radio observations.

Marshall Space Flight Center Solar Physics

List of TYPE II/III/IV bursts from WAVES/WIND satellite. 

Bruny Island Radio Spectrometer - 3 to 45 MHz in Tasmania.

IPS Australian solar site.

Hiraiso Solar Terrestrial Research Center,
a Japanese radio site.


Daily images in different wavelengths:
National Solar Observatory / Sacramento, Peak NSO CORONAL DATA.

Thanks to Tom Ashcraft for many of the above solar links!


Last Updated Jan 9, 2009