Observing Jupiter By Radio

Jupiter.gif (24543 bytes)If you have been an amateur radio operator or shortwave listener for some time, you probably have already heard it; an unmistakable rush of soft static that sounds amazingly like waves crashing on a seashore. You probably did not know however, that this "interference" was not of earthly origin, rather it originated at least 500,000,000 miles away from us with the planet Jupiter.

Jupiter is the largest and closest to the sun of the "gas giant" planets in our solar system. Like the sun, Jupiter is composed primarily of hydrogen. If Jupiter had been several magnitudes larger during its formation, the core of the planet would have been under sufficient pressure to induce nuclear fusion and our solar system would have had two stars instead of one. As it is, the hydrogen gas within the deeper reaches of the planet (there is no solid surface) is compressed into a "metallic" state where electrons become freely shared by the proton nuclei. Above this inner region lies an "atmosphere" of hydrogen and other gases. Clouds of methane, ammonia, ammonium, hydrosulfide, and water form complex stormy bands which encircle the planet. The Great Red Spot is the most famous feature of Jupiter. It is actually a long duration storm, which because of its tremendous size, has a life expectancy of hundreds of years. From our vantage point, even through a 2" telescope, we see Jupiter as a banded sphere flanked by as many as four tiny but bright moons. The innermost moon, Io, is of interest to us as radio observers of the planet.

Early observations of Jupiter at the microwave wavelength of 3 cm corresponded to a blackbody (broadband thermal emission) of 150 degrees Kelvin. Indeed, that is the approximate temperature of Jupiter's cloud tops. Subsequent observations at lower frequencies began to point to extraordinarily high energies which could not be explained as thermal in nature. These high energy emissions, which occur below 4O.5 Mhz, are the result of a phenomena called synchrotron radiation. This type of radio emission occurs when charged particles, usually electrons, are accelerated to extremely high velocities in a magnetic field. The electrons thus accelerated shed excess energy in the form of radio and sometimes even light frequency waves. In Jupiter's case the magnetic field is provided by the planet itself. The noise storms occur when the inner moon ,Io , passes through major flux lines of magnetic field in such a way that the emissions are essentially beamed in our direction.

The Voyager space probe missions revealed Io to be an extremely active satellite, with large volcanic plumes rising above the icy surface. It is possible that these volcanic eruptions play a role in the radio storm phenomena. In any case, the storms are predictable in nature based on rotation of the magnetic field, the position of Io in its orbit, and the relative position of the Earth. Any experimenter wishing to provide a useful service to other amateur Jupiter observers could develop a computer algorithm to predict these storms.

For casual observing, all that is necessary in the form of equipment is a shortwave receiver of good sensitivity capable of receiving in the 18 to 30 Mhz range. The 21 Mhz ham band is an excellent place to listen for Jupiter. Some older shortwave receivers fall off in sensitivity at about this frequency. In such a case, a pre selective amplifier may be included between the antenna and the receiver. These preamps are available commercially or may be constructed from plans available in amateur radio publications. The antenna need not be anything special; a simple dipole will do. In fact, directional antennas may be a hinderance if they cannot be tracked as Jupiter changes position in the sky. A somewhat better antenna system would include two dipoles, switchable from the operating position. one dipole would oriented north-south and the other east-west. Suspending the dipoles approximately 1/4 wave above a wire poultry netting ground plane may help in reception when Jupiter is near the zenith. If a directional antenna such as a 3 or 4 element yagi is used, then it may be helpful to tilt the antenna upward, perhaps 3O degrees or so, to achieve a compromise in reception when Jupiter lies at higher elevations. Lowering the antenna to a few feet above ground can also increase the angle of reception.

Consult an astronomy magazine such as Sky & Telescope or Astronomy to determine when Jupiter is in view , (remember, it need not be a night time observation). Several ephemeris programs are available for a variety of computer models. Many of these programs are public domain software. These programs provide sky coordinates (right ascension and declination), as well as the altitude and elevation of the planets for any time, date, or location.

Other factors which must be considered are the placement of Jupiter and the Earth in their orbits around the sun and the reflectivity of the ionosphere. The orbital placement may bear somewhat on the strength of the received signal, but perhaps not to the exclusion of hearing the storms. The earth's ionospheric conditions are on the other hand very important. If the frequency at which you are listening seems alive with terrestrial signals from distant points on the globe, then there will be little chance of hearing Jupiter as the ionosphere is so reflective that it will prevent the penetration of signals from space. In this case you can try listening on frequencies closer to 3O Mhz where the ionosphere may still be transparent. If this fails, then you are probably out of luck for the present.

When you finally catch Jupiter, and you will if you are persistent, there are two types "noise" to listen for; the ocean wave type described earlier, which is called an L burst (L for long), and a short burst type static called an S burst. The S bursts often have a "rapid fire" characteristic and tend to drift upward in frequency. You can record these events on audio tape or on a strip chart recorder.

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SBurst Radio Noise from Jupiter

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LBurst Radio Noise From Jupiter

 

Radio Equipment Required to Monitor the
Decametric Synchrotron Emissions

Jovian Receiving System

The equipment required to receive Jovian originated electromagnetic storms is quite reasonable to assemble.

Antenna

Dipole Antenna

The antenna required to observe Jupiter may be as simple as a half wave dipole antenna. The gain from this antenna will be quite low there for requiring a RF preamp to be used.

A half wave dipole antenna can be constructed with a two pieces of wire, 11 feet, 8.4 inches in length connected to a 50 ohm coax cable. One length of wire is connected to the inner conductor, and the second piece of wire is connected to the coax shield. The antenna is laid out on a East-West line.

The antenna should be raised above the ground by poles or some other means to a height of at least seven feet.

DDRR Antenna

Construction plans

The Directional Discontinuity Ring Radiator (DDRR) antenna is a good compromise between the 1/2 wave dipole and a large beam antenna. DDRR is a loop antenna made from soft aluminum or copper tubing, 1/2 inch in diameter and is cut to 125.5 inches (21MHz). A reflector made of metallic window screen and mounted on a wood, metal or PVC tube frame which is placed 5 inches behind the loop antenna. The loop is supported by a minimum of 4 insulating wood or PVC stand-offs attached to the reflectors frame. The coax cable inner conductor is connected to the antenna element and the outer conductor is connected to the wire screen reflector. A good pre-amp should be located very close to the loop antenna element.

The antenna assembly is then located on a East/West line and will be used in a drift scan mode.

Preamp

If the receiver and/or the antenna system lack the necessary sensitivity to detect Jovian noise then an antenna pre-amp will be required. Radio Shack offers a lO db gain pre-amp which can be located at the antenna. They also offer a tuned pre-amp which can be placed next to the receiver. The external pre-amp is preferred. Several other manufactures produce pre-amps in the range of 18 to 23 MHz. Ham radio magazines offer several pre-amps in their advertisements.

Receiver

DX-394 specifications

Any good quality communications receiver capable of receiving in the 18 MHz to 23 MHz range will work. The receivers selectivity is very important in reducing the effect of near by radio emissions. The frequencies that the Jovian noise is detected on is also used by many services. Since there is no protected frequency for the reception of Jovian radio emissions, care must be taken in finding a clear channel at your location.

Note: If at all possible the receiver should have the ability to shut off the AGC. This may reduce the sensitively of the receiver, however it will increase the ability of the receiver to detect the slight signals changes emanating from a Jovian storm.

Receiver Modifications

Modifying the receiver to defeat the AGC will aid in the detection of Jovian storms. The AGC tries to keep the volume constant by biasing the RF or IF amplifiers in such a way as to hold the audio output at a constant level.

Tape Recorder

A tape recorder capable of turning on from a signal level increase (voice actuated) or can be controlled by the communications receiver is necessary to verify the received noise is from Jupiter.

An excellent way to monitor Jovian noise is with a stereo cassette tape deck. The left channel is connected to the audio output of the SW receiver, while the right channel is connected to another receiver monitoring WWV or the Canadian time station. This setup will allow you to time stamp Jovian storms. The time stamp will allow the observer to accurately determine when a Jovian storm has occurred. Once the time is known than the Jovian predictive data can be utilized to determine the type of storm.

Another possibility is to use the audio channel of a VCR. The VCR will allow recordings up to six hours. Time may be determined by the knowing the inches per minute of the mechanical counter.

If you use a VCR to record you will need to provide the TV sync pulses to control the tape. Several VCR users have reported that using pre-recorded tape will provide sufficient sync signal for playback.

Square Law Detector

The Square Law Detector is a interface from the receiver to the recording device. A easy to build detector is shown below. Component placement is not critical. All resistors are 1/4 watt.

Recording Devices

Rustrak 288 Strip Recorder or equivalent
Computers


Observing Meteors Using a FM Radio

Detecting meteors via radio waves is easy!  All you need is a simple antenna and an FM receiver.  The brief signals you'll receive are transmissions from distant radio stations bouncing off ionized meteor trails.

Why Radio?

Much of the interest in amateur radio astronomy, and detection of meteors specifically, centers on the ability to observe when it's overcast or raining.  You can use radio in broad daylight or when the full Moon makes visual observing difficult.

There are other compelling reasons to get involved with radio astronomy.  First, while the visible-light region of the electromagnetic spectrum falls between 4000 and 7500 angstroms the radio spectrum is much wider. 

Second, unlike visual observing you don't need to do your radio work at the time of the event.  You can connect any recording device (a tape recorder, strip chart recorder, or computer) to your receiver and check the data later.

Third, its easy to get started.  All you need is the FM receiver in your living room and an antenna.  If you don't have an outdoor FM antenna, you can purchase a simple, inexpensive one for indoor use at your local Radio Shack store.

Radio Meteors

Meteors themselves do not generate the signals you hear.  As a meteoroid enters Earth's atmosphere and vaporizes, it produces not only a streak of light but also a trail of ionized gas.  Because the trail is ionized it reflects radio waves.  Normally the signal from as FM station radiates away into space, but if it encounters a meteoroid's ionized trail, part of it get reflected back to Earth.   Like a visual meteor, the reflected radio signal is short-lived.  The signal you hear may last from a friction of a second to several seconds.

More signals can be heard using an outdoor, rotatable FM (or FM/TV) antenna.  When using an outdoor antenna, particularly one intended only for TV reception, check for an FM trap (a small box) connecting the antenna feed line to the receiver.  If one is there you must remove it.

Observing

FM frequencies are assigned at 200 kilohertz intervals between 88.1 and 107.9 megahertz. A receiver with a digital readout will help you tune more accurately. If you cannot find a frequency not being used, select one on which the station is weak.

When the receiver is properly tuned, most of the time you'll hear nothing but a steady hiss. When a meteoroid produce an ionized trail in the right part of the sky, you'll briefly hear the signal from the distant FM station. The signal may be so short that you will hear a brief "ping". Or you may hear several words or a few motes of music. Often the reflected signal will be as strong as a local station. These radio signal types you'll hear on the FM broadcast band, but their fast appearance and short duration make them easy to identify. Signals you can eliminate include ones that slowly rise out of the noise and others that rise quickly but remain strong for minutes or hours.

You also need to know when to listen. As with visual observing, you have two options. You can listen for either sporadic meteors or Meteor Showers. No matter how you observe them, sporadic meteors follow the same pattern. The maximum hourly rate occurs at approximately 6 a.m. local time each day, the minimum at 6 p.m. Keep in mind that the rate does not drop to zero a 6 p.m.

While the number of signals you hear will depend on your equipment, one of the advantages of listening to meteors is you can hear ones too faint to see. You can also use your equipment to monitor meteor rates during daylight hours.