Ionosphere-Holes and Radio Propagation

By Robert H. Welsh

For those of us that have been around radio for some time, many of us began our interest in short-wave radio by listening to the various short-wave broadcast stations. These stations have been operated by many different nation-states and commercial enterprises. The larger, nationally funded transmitters used AM modulation, powerful transmitter stations, and directional antennas. They operated in the 60, 27, and 25 meter bands. One can still copy some of these international stations in the high end of the 40 meter band. I began listening to these stations on my grandparents PHILCO AM, FM, SW radio near the New Jersey shore. Occasionally, I would listen to amateur radio operators AM transmissions as well. I found all those transmissions exciting, and I tried to understand how those radio waves could travel such long distances. Later in my life, while stationed with the U.S. Army Security Agency at an electronic intercept site in northern Turkey, I installed a Collins R-390 receiver in the tracking van where I worked. My operator and I regularly listened to the BBC from London and Radio Hilversum from the Netherlands. These transmissions provided us with news and music. Of course, stations such as the Voice of America or Radio Moscow provided interesting sources of propaganda from their respective governments. For amateurs, these international broadcast stations could be used to measure radio propagation from different parts of the globe; but, there have been other uses of international short-wave stations. I will focus on these other uses in this article.

MISSILE EXHAUST PLUMES SHORTWAVE RADIO SIGNALS

During a different assignment at a U.S. Army Security Agency electronic intercept site in northern California, I maintained and operated a short-wave monitoring system. Inside a communications van located outside the main operations building, we connected a large number of Collins R-390 receivers to several rhombic antenna systems that were directed toward Australia and Asia. Signals copied by these different AM broadcast stations were detected and coupled to recording and analysis equipment. During each experiment, the U.S. Navy launched missiles from their site at Point Mugu California. We monitored how the high-temperature exhaust plume from the missile engine changed the ionization of the D, E and F layers of the ionosphere as we copied the various stations. The typical exhaust plume temperature of these missiles is on the order of 2300 Kelvin (almost 3600 degrees Fahrenheit). A radar located at Stanford University provided missile altitude data via an HF radio link just outside the 20 m amateur band. The data gave us knowledge of when the missile passed through different layers of the ionosphere. See Figure 1. We measured the receiver’s AGC voltage as the missile passed through the F-layer of the ionosphere at altitudes of 160 km (100 miles). Note that the F-layer has the highest concentration of free electrons and is considered the most important layer for long-distance HF communications. As the missile passed through the F-layer, we observed that the short-wave broadcast signals varied. The AGC voltage of the receivers indicated a decrease in signal strength. After the missile passed through the F-layer, we observed a slow increase in signal strength until the signal strength returned to the level before the missile passed through that layer. What then is the effect of the exhaust plume on the ability of the ionosphere to maintain the propagation of short-wave radio signals? Experiments have shown that the exhaust plume of a missile chemically induces a change in the ionosphere. Water and hydrogen in the

Figure 1. Potential atmospheric effects due to space-power rocket launch. (Courtesy Advisory Group for Aerospace Research & Development, Conference Proceedings, Seine France, Number 295, April 1981, p. 45)

exhaust plume become molecular ions which react with the ionized oxygen molecules of the ionosphere thus creating a decrease in electron density; this could create a hole in the ionosphere as the missile exhaust penetrates the layers of the ionosphere.1

AMATEUR EXPERIMENTS

The amateur radio community was given an opportunity to be a part of a similar experiment. The September 1979 issue of QST, The Journal of the American Radio Relay League, published an article “The Great Ionosphere-Hole Experiment”. An Atlas-Centaur rocket was to be launched from the Kennedy Space Center.2 According to the launch characteristics, the Centaur stage burns from 209 km (130 miles) to 466 km (290 miles). This places the rocket exhaust plume within the accepted height of the F-layer. Beacon transmitters were set up in Puerto Rico in the 80, 40, 20 and 15 meter bands. The beacon transmitters utilized stepped-power outputs at 10 dB intervals. The results of the Ionospheric-Hole experiment were published in a later issue of QST. 3 About 150 amateurs were involved in receiving and recording the signals from the beacon transmitters set up in Puerto Rico. Amateurs from 35 states and several Canadian provinces provided signal strength reports. Some reports were S-meter readings and some reports were from strip-chart recorders. The 80 m reports indicated no significant change in signal strength; the same was true for the 40 m reports. No reports of signal change were reported from those monitoring the 20 m signal. Again, the 15 m beacon signal yielded no measureable change in signal strength. There were several reports from stations monitoring 10 and 6 meters with no measureable change in signal strength. Another study was performed based on the concept of a solar-powered satellite system. This system would use large satellites in geosynchronous orbit to capture solar energy. The captured solar energy would be converted into microwave signals that would be transmitted to earth stations. The hypothesis considered that the received energy would be supplied to the electrical grid as an additional source of electricity.4 The transmitted microwave energy would be beamed to Earth at a frequency of 2.45 GHz with a power of 2.2 W/cm2 from the center of the satellite antenna. It is suggested that there would be at least 60 antenna arrays spread over the continental United States at a separation distance of 300 km (180 miles). See Figure 2.

MORE EXPERIMENTAL CONFIRMATIONS

The Solar Power Satellite Proceedings issue about rocket plume exhaust suggests the following scenario:

“The lowest layers of the ionosphere (the D- and E-regions) could be affected by both rocket launches and spacecraft reentry. The effluents from these space operations include water vapor, hydrogen gas, and thermal energy during launch, and ablated materials, oxides of nitrogen, and thermal energy during reentry. These effluents would modify the composition and properties of the ionosphere and might influence climate, satellite-based surveillance systems, radio communications, navigation systems, microwave propagation (SPS power=beam stability} and magnetosphere processes. While the likelihood of altering the electron and ion composition seems to be fairly high, the magnitude of the impacts is uncertain.

The effects of nitrogen oxides formed during reentry and the effects of ablated materials do not appear to be important at this time. Calculations have shown that injection of water and carbon dioxide into the F-region of the ionosphere results in both plasma reduction (electronion recombination) and enhanced airglow (visible and

IR emissions from excited molecules). These predictions have been verified both inadvertently during the Skylab launch and deliberately during the Lagopedo experiments.

Plasma reductions can result in interference with radio communications and navigation systems. Enhanced airglow, while not a serious matter at ground level, can contribute to the noise level of satellite-based surveillance systems.” A more recent experiment regarding the effects of a rocket engine burn in the ionosphere occurred during July 2018. During this experiment, observations indicated the exhaust plume amplified VLF radio signals from the U.S. Navy VLF station NML (North Dakota) and VLF station NWC (Australia). The U.S. Naval Research Laboratory monitored these transmissions as the BT-4 rocket engine of the Cygnus spacecraft passed through the ionosphere. No effects were noted on VLF waves due to the launch vehicle engine. A later experiment in May 2020 yielded an amplification of the whistler signals at the top of the ionosphere.5 If one considers the very low frequencies of these effects, there appears to be minimal effect at amateur radio frequencies. It is interesting to note that the 25.2 kHz signal from VLF station NML was amplified by a factor of 1000 times (30 dB) due to the interaction of the exhaust plume and the ionosphere. No equivalent effects appear to have occurred at amateur radio frequencies.

Figure 2. Ionospheric hole creation. (Courtesy Final Proceedings of the Solar Satellite Program Review)

SPACE SHUTTLE LAUNCHES AND IMPLICATIONS FOR STARLINK

Recently, the concept of exhaust plume problems in the ionosphere has arisen again in connection with the StarLink program being developed by SpaceX. This system has already launched 60 satellites with the goal of putting more than 1,400 satellites into low-earth orbit at 550 km (340 miles). SpaceX envisions eventually placing almost 30,000 satellites into orbit for the StarLink program. These 227 kg (500 pounds) satellites are part of a system that will provide a global broadband internet service from space. No direct measurements have currently been made on the ionosphere effects of so many rocket launches. Measurements of Space Shuttle engine exhaust have been studied by the Naval Research Laboratory. These measurements covered 25 years of Shuttle launches. Changes in propagation were observed as the Shuttle Orbital Maneuvering Subsystem burned during passage through the ionosphere, and high-frequency radio propagation was found to have been affected. In addition, measureable changes in GPS signals and scattering of radar signals did occur.6

CONCLUSIONS

What conclusion can be drawn based on the various experimental data stated above? It appears that rocket exhaust plumes do have short-term effects on high-frequency radio propagation. When one considers the variability of high-frequency radio propagation, those of us that use the ionosphere will not notice any changes in the propagation of our signals. If we lose a signal due to a rocket launch, we will probably put the short-time loss to occasional QSB and go on our way working stations around the world. Since the majority of U.S. rocket launches are made from either the Florida or California coasts, many operators will not notice any adverse effects.

ACKNOWLEDGEMENT

I would like to express my sincere thanks to the following individuals for their gracious assistance: Paul A Bernhardt, KF4FOR and Joseph H. Reisert, W1JR. 1. Heki, Kosuke, Ionospheric Hole Behind an Ascending

Rocket; Earth and Space Science (Hokkaido University, 2008). 2. Bernhardt, P.A., et al, Great Ionospheric Hole

Experiment, QST, September 1979, pp. 22-23. 3. Bernhardt, P.A., et al, Results, Great Ionospheric Hole

Experiment, QST, November 1980, pp. 26-31. 4. The Final Proceedings of the Solar Power Satellite

Program Review, April 22-25, 1980, Lincoln Nebraska. 5. Bernhardt, P.A., et al, “Strong Amplification of ELF/

VLF Signals in Space Using Neutral Gas Injections from a Satellite Rocket Engine,” Earth and Space Science

Open Archive, September 2020. 6. Bernhardt, P.A., “25 Years of Ionospheric Modification with Space Shuttle OMS Burns,” 2011 XXXth URSI

General Assembly and Scientific Symposium, August 2011.

ABOUT THE AUTHOR

Robert Welsh (at the Very Large Array, Socorro New Mexico) is now in his 60th year as an amateur radio operator; actively chasing DX, islands, etc. He is an Assistant Professor of Physics and Astronomy at Bucks County Community College in Newtown, Pennsylvania, where he is the faculty sponsor of club station KB3YRR. Prior to teaching, Robert worked in the Defense Electronics industry dealing with radar and RF systems. He served as an Electronic Warfare Equipment technician in the U.S. Army Security Agency at several NSA intercept sites. He occasionally performs microwave radio galactic studies at the National Radio Astronomy Observatory, Green Bank, West Virginia. He was part of the microwave Galactic Plane Survey using a dual-frequency radio telescope operating at 9.7 GHz and 14.3 GHz. While employed at NRAO, he developed a web page for teaching radio astronomy, see http://www.gb.nrao.edu/~glangsto/ lessons/index.html.

Registration will open soon! www.radioclubofamerica.org