ERAU Prescott Observatory

HF Solar Noise Monitor (Callisto)

The high magnetic fields in the sunspot-producing active regions also give rise to explosions known as solar flares. When the twisted field lines cross and reconnect, energy explodes outward with a force exceeding that of millions of hydrogen bombs.

Temperatures in the outer layer of the sun, known as the corona, typically fall around a few million kelvins. As solar flares push through the corona, they heat its gas to anywhere from 10 to 20 million K, occasionally reaching as high as a hundred million.

Because solar flares form in the same active regions as sunspots, they are connected to these smaller, less violent events. Flares tend to follow the same 11-year cycle. At the peak of the cycle, several flares may occur each day, with an average lifetime of only 10 minutes.
Classification of Solar Flares - NASA/Goddard Space Flight Center
Solar Storms and HF Solar Noise

HF effects from solar flares are generally only for the duration of the flare event (20-60 minutes) and effect frequencies >10 MHz.  The most damaging effects of a solar flare is actually the arrival of the shockwave 2-3 days later, triggering a geomagnetic storm.

The shockwave travels through the sun’s magnetic field lines, electric currents and eratic radio emissions are generated by the dynamo effect, called a Type II storm. The sun’s plasma frequency becomes lower at greater distances.  Therefore, as the shockwave travels away from the sun, the bursts are heard at lower and lower frequencies. This is important to astronomers. By measuring the time is takes for the bursts to drift from one frequency to a lower one, the velocity of the shockwave can be determined.
Solar Spectrum

The radio observatory operates a HF receiver capable of monitoring for solar activity. In the past the observatory has used a narrowband single channel noise spectrometer called the Radio Jove Receiver. The single audio output was recorded on a computer and displayed continuously on a simulated strip chart recorder. Even though when calibrated this receiver was capable of detecting and allowing measurement of solar events that effected the earth, its single channel did not provide enough dynamic range over the HF spectrum.

The Radio Jove Receiver was replaced with the Callisto Receiver. The Callisto receiver is a frequency agile radio Spectrograph capable of covering from 45 MHz to 850 MHz. The receiver can be programed to cover any portion of this frequency range in up to 400 increments.

Callisto ReceiverThe receiver instrument itself is called Callisto. Callisto can be used as a basic system consisting of the receiver, a linear polarized antenna system and control/logging software.  A more advanced system includes a tower-mounted preamplifier or low noise amplifier and a focal plane unit (FPU) with antenna polarization switching and noise calibration capabilities.

The main applications are observation of solar radio bursts and rfi-monitoring for astronomical science, education and outreach.

The observatory operates its Callisto receiver on the Radio JOVE dual dipole antenna. Various bands can be monitored but it is usually kept in the 20 to 45 MHz range using an upconverter.