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“The Universe Beyond the Plasma Frequency” by Kerry Hensley


After centuries of watching the night sky with increasingly sophisticated tools, there’s still plenty we don’t know about the universe. Part of the challenge is that we’re stuck on a planet that’s surrounded by an atmosphere that blocks certain types of radiation from reaching the ground. It’s hard to complain too much, since if X-rays could make it to Earth’s surface humans would either be dead or a very different kind of organism, but it has certainly slowed our progress. We’ve managed to launch telescopes into orbit around Earth to observe most wavelengths of light, but there’s a huge chunk of extraterrestrial radiation that we currently have no means to observe. The culprit? A layer of plasma in Earth’s atmosphere called the ionosphere.

A Brief Overview of the Ionosphere

In 1901, Guglielmo Marconi made a discovery that would change the way the world communicates: he received a radio transmission from over 2,000 miles away, demolishing the idea that radio signals can travel only as far as the visible horizon. Marconi was wrong about how the radio waves made it that far (he thought they would naturally follow Earth’s curvature rather than traveling in straight lines), but the discovery prompted Oliver Heaviside and Arthur Kennelly to propose that the waves had actually been reflected by plasma—atoms and molecules split into electrons and positively charged ions—in the atmosphere. Two decades later, scientists were able to prove that such a layer of plasma existed and dubbed it the ionosphere. (Fans of Andrew Lloyd Webber musicals might remember a mention of the Heaviside Layer in Cats; the Heaviside Layer is another name for a part of the ionosphere. So while cats may or may not go to heaven, they do make it as far as the ionosphere. Now you know.)


Auroral emission, seen here from the International Space Station, is generated within the ionosphere by particles funneled into Earth’s atmosphere by its magnetic field. Credit: NASA

Despite their importance to radio communication, ionospheres are rarely referenced directly in SF. One exception that I can think of is Kim Stanley Robinson’s 1992 novel Red Mars, in which Mars’s small radius and tenuous ionosphere (about ten times less dense than Earth’s) are noted to make surface-to-surface radio communications more challenging. In Fritz Leiber’s 1951 short story, “A Pail of Air,” it’s the complete lack of an ionosphere that’s important; after Earth is knocked out of orbit, it ends up far enough from the Sun that its whole atmosphere falls to the ground as snow. As a result, the survivors lose the ability to communicate long-distance via radio. The ionosphere is also indirectly implicated in any SF story where a solar storm impacts radio communications or GPS on Earth; other than the electronics themselves being fried by high-energy particles and radiation, what’s being affected during a solar storm that causes communications to go haywire is the ionosphere.

The ionosphere is formed when ultraviolet and X-ray photons from the Sun, as well as high-velocity particles from faraway sources like supernovae, collide with the atoms and molecules in Earth’s atmosphere, stripping off electrons and generating a layer of plasma. Earth’s ionosphere ranges from about 60 km (37 miles) to over 1,000 km (621 miles) above the surface, encompassing many satellites in low-Earth orbit as well as the International Space Station. At its densest, the ionosphere reaches a few million electrons in a single cubic centimeter. The ionospheric plasma is highly variable, changing with the time of day, season, phase of the solar activity cycle, and in response to any transient solar events like flares or coronal mass ejections.

As Marconi discovered over a century ago, we can use the ionosphere to help us communicate over long distances. As radio waves pass through a plasma, the electrons respond to the electric field of the wave and begin to wiggle around at the same frequency as the wave. (The ions in the plasma respond too, but since they’re so much more massive than the electrons they do it much more slowly.) Depending on the frequency of the wave, the electrons will either keep pace with the wave and reflect it back toward the ground or be unable to keep up and allow the wave to pass through.


This cartoon shows how radio waves travel large distances around Earth by bouncing off of plasma in the atmosphere. Whether or not a radio wave can escape the atmosphere depends on its frequency as well as the angle at which it’s transmitted. Credit: NASA/GSFC

The highest frequency that the electrons can match is known as the plasma frequency. Let’s say you have a blob of plasma and you disturb it in some way. Maybe you push some electrons together so that one area has more negative charge than the rest. Because the like charges repel each other, the electrons spring apart and inevitably overshoot their original positions and end up sloshing back and forth, forming plasma waves. The frequency of these waves depends on how dense the plasma is; denser plasma has a higher intrinsic plasma frequency. For a density of one million electrons per cubic centimeter (typical of Earth’s ionosphere), the plasma frequency comes out to about 9 MHz.

This is glossing over a lot of the details of how radio waves interact with a plasma, but the bottom line is that high-frequency radio waves will be reflected at higher altitudes (where the plasma density is greater) than low-frequency radio waves, if they are reflected at all. This is true for radio waves that originate outside Earth’s atmosphere as well, so while the ionosphere enables radio communication, it also prevents us from observing the universe or receiving any potential SETI signals at any frequency below the plasma frequency.


This cartoon shows the wavelengths that are blocked by Earth’s atmosphere. For any opaque wavelengths shorter than ~5 centimeters, absorption by atoms and molecules in Earth’s atmosphere is responsible for the opacity. Above ~20 meters, Earth’s ionosphere reflects the waves back to space. Credit: NASA


What We Might Be Missing

So we’re unable to observe a huge chunk of the electromagnetic spectrum. How big of a deal is that really? It’s impossible to say exactly what lies hidden at those extremely long radio wavelengths, but here are a few examples of what we might expect to find.


The First Stars and the Epoch of Reionization

Long ago, the universe was a dark place. A few hundreds of millions of years after the Big Bang, the first stars ignited and their starlight ionized the surrounding gas and allowed photons to spread outward into the universe. The period in which the first stars began to shine is known as the Epoch of Reionization.

There’s a lot we still don’t know about this time period. One of the best ways to study it is by observing the 21-cm (1420 MHz) emission line of hydrogen, which is usually visible from Earth—but not in this case. Since we’re searching for signals from far, far back in cosmic time, the 1420 MHz emission has been stretched out to far lower frequencies—low enough that they’re hidden from view by the ionosphere.


A timeline from the Big Bang to the present. The Epoch of Reionization is expected to have lasted hundreds of millions of years. Credit: NASA, ESA, A. Fields (STScI)

Observations of the universe as far back as the cosmic dark ages would have a huge impact on cosmology, allowing us to test our theories of how the universe has inflated and how quickly stars and galaxies formed. Ground-based observatories like the Low-Frequency Array (LOFAR)i have already begun the search, but the search area (in terms of frequency) is limited by the ionosphere. We do have one detection of the first starsii, but there is still a lot we don’t know. The lower the frequency, the farther back in cosmic time we can probe.


Exoplanet Magnetic Fields

Planets around other stars, otherwise known as exoplanets, are one of the hottest topics in astronomy right now, and they don’t show any sign of losing popularity. As we amass data from missions like the Kepler Space Telescope (may it rest in peace) and the Transiting Exoplanet Survey Satellite (TESS)iii, we’re starting to be able to draw conclusions about exoplanets as a whole as well as individual planets.

One of the many lingering questions is how to determine whether or not an exoplanet has a magnetic field. Many scientists think that Earth’s magnetic field was important for the development of life, so we’d like to be able to tell from a distance whether or not an exoplanet has a magnetic field. Radio waves are generated when magnetized stellar winds—extensions of stellar atmospheres that flow outward from the star—interact with magnetized planets (as is the case for Earth). Searches for these signals are underway, but so far they’ve been inconclusive. Pushing the search to lower frequencies could help us detect exoplanetary magnetic fields at last, helping us to narrow our focus from the thousands of known exoplanets to those most likely to harbor life.


Let’s Get Speculative: Fast Radio Bursts

I love a good astrophysical mystery just as much as the next person, and there’s no better mystery than fast radio bursts (FRBs). FRBs are bizarre, energetic, millisecond-long radio signals, and astronomers have come up with dozens of theories for what could be causing them. (Neutron stars colliding, supernovae, magnetars releasing stellar flares . . .) As we observe more and more of these extremely powerful radio signals, we’re starting to eliminate some of the possibilities.

At first, it seemed like FRBs were transient, one-off events, which could mean that they were caused by something cataclysmic like a star exploding as a supernova. Then, in 2015, astronomers discovered the first ever repeating FRB, which means that not all FRBs have a cataclysmic source. The discovery of the second-ever repeating FRB by the Canadian Hydrogen Intensity Mapping Experiment (CHIME)iv was announced in early 2019—expect this field to get really exciting as CHIME discovers more of these weird signals. So far, the FRBs detected have had frequencies in the 400-1,400 MHz range. The detection of FRBs at really, really low radio frequencies would negate some existing theories or help us come up with new ones altogether.


Extraterrestrial Communication

In Les Johnson’s novel Mission to Methone, the Guardian is thrilled when humanity finally gains the ability to communicate by radio since it can eavesdrop far more easily that way. If you were an alien species spying on Earth and didn’t want your own communications to be detected, it would be simple to tune your transmitters to frequencies that we can’t detect from the ground. This goes for beings spying on other planets as well, since any planet with an atmosphere that orbits a star will have an ionosphere.

Many SETI searches focus on radio wavelengths that are seen essentially everywhere in the universe, like emissions from neutral hydrogen and hydroxyl (OH) molecules, which fall between 1,420 and 1,720 MHz. Since so many astrophysical sources emit at these frequencies, anyone or anything that develops the ability to detect radio waves will observe at these frequencies, making them a smart choice if you’re hoping to be found by other civilizations.

Do the very low frequencies that we’re missing out on offer a good or better chance for detecting other civilizations? It’s not yet clear. One potential issue is that the Milky Way itself emits a lot of radiation at very low frequencies, providing a constant background of noise. Despite this, it’s almost certainly worth taking a peek once we get the chance; these wavelengths represent a huge chunk of the Cosmic Haystackv—a multidimensional volume of space, time, and frequency in which we might detect extraterrestrial signals—that we know nothing about.

It’s hard to know what we might find at these wavelengths, but we can look to the past for evidence of what could happen in the future: each time we’ve opened up a window into a part of the electromagnetic spectrum that we couldn’t access previously, we’ve made unexpected discoveries. There’s no reason for this part to be any different.


What to Do About It

So we’re stuck observing the universe at frequencies higher than the plasma frequency, and there’s plenty of interesting stuff that we’re missing out on—including phenomena we haven’t thought of yet. What can we do about it?

The good news is that we’ve known about this problem for decades, and scientists have already come up with some ideas. In the 1980s, astronomers devised an experiment using the Challenger shuttle to make observations at frequencies below the plasma frequency. In the Plasma Depletion Experimentvi, Challenger astronauts fired the shuttle’s orbital maneuvering subsystem engines as the spacecraft passed over radar and radio observatories, releasing 244 kilograms of carbon dioxide, water, and hydrogen exhaust. The exhaust caused the positive ions and electrons to combine and reform neutral atoms and molecules, creating a depletion in the ionospheric plasma. Decreasing the plasma density means longer-wavelength radio waves can pass through the atmosphere and reach the ground.

In this experiment, the exhaust release was timed so that the absolute minimum plasma density could be reached: at night, in the winter, during the least active time of the solar cycle, at a location where the plasma density is unusually low under normal conditions—the “mid-latitude trough” over Australiavii. Through the “ionospheric hole” created in the experiment, they were able to observe radio waves from the Milky Way which are normally blocked from view. Just a peek, though—the hole disappeared after a few hours.


After the engine burn at about 3:00 AM, the galactic radio emissions at 1.704 MHz increased. The emissions at the other frequencies weren’t affected as much since the lower frequencies still couldn’t make it through the ionosphere and the higher frequencies were able to make it through before the experiment. Credit: Ellis et al. (1988)

This is an interesting short-term solution, but it certainly isn’t feasible to send up a rocket every time you want to look out into the universe at these frequencies. The only long-term solution is to set up a radio telescope outside Earth’s atmosphere. This has been done before for small radio telescopesviii; Japan’s now-defunct Highly Advanced Laboratory for Communications and Astronomy (HALCA) satellite carried an 8-meter (26-foot) diameter radio telescope, which was designed to observe frequencies as low as 1.6 GHz (18-cm wavelength). Russia’s 10-meter (33-foot) Spektr-R satellite, which can go as low as 0.32 GHz (92-cm wavelength), lost contact with the ground earlier in 2019 and may not be salvageable.

Even if either of those telescopes were still operational, they wouldn’t be able to observe the wavelengths we care about because they’re just too small. We’re interested in wavelengths of tens of meters, if not longer. For a given resolution (the ability of a telescope to distinguish objects of a certain angular size), the size of the telescope scales with the wavelength you want to observe. That’s why radio telescopes are so much larger than optical telescopes; currently, the largest optical telescope is 10.4 meters (34 feet) in diameter, while the largest single radio dish is a whopping 500 meters (1,640 feet) acrossix.

This means we need to send some awfully big telescopes into space. Unfortunately, big telescopes are very costly, and we’re talking about scales much larger than anything that has been done before. The greater the budget, the greater the risk, and while space agencies across the globe have launched dozens of successful space telescopes, it’s still a risky endeavor (just ask any astronomer what they think about the possibility of the highly anticipated and long overdue James Webb Space Telescope malfunctioning a million miles from Earth with no hope of a repair mission . . . expect some nervous laughter and an immediate change of subject). Not all missions are as fortunate as Hubble, which has been operating more or less seamlessly for nearly three decades; in 2016, Japan’s Hitomi satellite (also known as ASTRO-H) broke apart after less than two months in orbit.

Let’s say we can solve our budgetary struggles and get a radio telescope into orbit. Why stop there? Another (potentially more challenging) option is to construct a radio telescope on the Moon. The Moon offers the same main advantage as Earth orbit: no atmospheric interference. (Technically, the Moon does have a sort of thin, wimpy atmosphere, but it’s nothing more than a little sodium, argon, and potassium released from the lunar surface as it’s bombarded by the solar wind.) Although lunar gravity isn’t as low as the microgravity environment of Earth orbit, it’s still low enough to mitigate an issue that plagues huge, steerable radio telescopes like the 100-m (328-ft) Green Bank Telescope in the National Radio Quiet Zone in Green Bank, WV. The telescope is so massive that it sags and deforms under its own weight, requiring tiny motors to push the panels of the huge dish to compensate.


The Green Bank Telescope, while not the largest radio telescope in the world, is the largest one capable of being rotated and tilted at different angles. It weighs nearly 17 million pounds and, at 485 feet tall, is just barely taller than the world’s tallest roller coaster. Credit: NRAO/GBO


Other the other end of the electromagnetic spectrum is the Chandra X-ray Observatory. It doesn’t look much like its larger cousin, and it works in a very different way. X-ray photons are guided to the detector by glancing off the mirrors, which are arranged cylindrically. Credit: Eastman Kodak; Marshall Space Flight Center

The biggest reason to put a radio telescope on the Moon is that the far side, which never faces Earth, is without a doubt the best place to make radio observations at any frequency. The Earth is practically shouting out radio signals, but on the far side of the Moon you wouldn’t hear a peep. The Sun is rather noisy in the radio as well, but would be hidden from view during the long lunar night, allowing for observations with incredibly low interference.

Even making the big assumption that you could ensure timely downlink of data from a telescope that exclusively points away from Earth, there are still, of course, huge challenges to overcome.

How do you make sure that your telescope stays at a safe, operational temperature during both the 14-day lunar day and the 14-day lunar night? Electronics don’t tolerate large temperature changes well, and the Moon swings between -298 and 224°F (-183 and 107°C). One solution is to place the telescope so that it’s either permanently in sunshine or shadow, so you only have to deal with one temperature extreme. Cold is a better choice; space telescopes operate at even lower temperatures. You could build the telescope on a Moon-encircling track to keep it permanently in shadow, but a better solution is to construct the telescope in the permanent darkness of a polar lunar crater, paired with some nearby solar panels or another energy source. (The polar craters could also be a good location for a lunar outpost since they have water ice.)

How are we going to get all the material needed to build a telescope (or better yet, an entire array of telescopes) up there in the first place?! Hauling tons of material to the Moon isn’t impossible, it’s just expensive—a lunar radio telescope will cost several billion dollars. Without interest from one or more national space agencies (or extremely enthusiastic billionaires with an army of rocket scientists on speed dial), it’s not going to happen. A way to get some of the benefits of a larger telescope without the huge price tag is to combine the signals from an array of many smaller telescopes. The array will have the same resolution as a single telescope with a diameter equal to the largest distance between two telescopes in the array. You won’t be able to see objects as faint as a larger telescope can, but the trade-off may be worth it.

With the recent announcement that NASA will attempt to send humans to the Moon by 2024, it’s high time to start thinking about answers to these questions and what our return to the Moon could mean for the future of radio astronomy and radio SETI searches. (It’s good to be skeptical of that deadline, which will almost certainly be pushed back as it has in the past. But it’s better to make plans now and have them ready when the time does come.) While there are difficulties to this accelerated timeline, it’s exciting to imagine what comes next.

NASA is far from the only space agency with Moon on the brain, thoughx; China’s Chang’e-4 mission is already paving the way to a lunar-orbiting radio observatory. The mission consists of a lander and rover, which touched down on the far side of the Moon in January 2019, and a relay satellite, which preceded the lander in May 2018. A future goal of the mission, to be achieved in the 2030s, is to establish an array of radio antennae in orbit around the Moonxi.


Schematic (not to scale) of the Chang’e-4 mission, demonstrating how the relay satellite helps the lander and rover communicate with receivers on Earth. Credit: Loren Roberts for The Planetary Society

It’s clear that we can expect increased interest in lunar radio astronomy as we prepare to return to the Moon and hopefully establish permanent outposts there. I’m excited to see how we solve the many technical challenges that a lunar observatory entails. Once we do, we’ll get to crack open the final hidden part of the electromagnetic spectrum and see the universe in an entirely new light.



Footnotes

i http://www.lofar.org/

ii https://www.nature.com/articles/d41586-018-02616-8

iii https://www.nasa.gov/tess-transiting-exoplanet-survey-satellite/

iv Learn more about CHIME here: https://chime-experiment.ca/

v You can read more about the Cosmic Haystack in this article: https://aasnova.org/2018/11/30/searching-for-alien-needles-in-the-cosmic-haystack/

vi Mendillo, M. et al. (1987) Spacelab-2 Plasma Depletion Experiments for Ionospheric and Radio Astronomical Studies. Science, 238 (4831). pp. 1260-1264.

vii Ellis, G. R. A. et al. (1988) Radioastronomy Through an Artificial Ionospheric Window: Spacelab 2 Observations. Advances in Space Research, 8 (1). pp 63-66.

viii Space history buffs will notice that I’m omitting spacecraft with radio antennae capable of observing the wavelengths we care about. There have been a few (e.g. Explorer 49), but their existence doesn’t preclude the need for large telescopes or telescope arrays in Earth or lunar orbit.

ix There’s a cool article about this massive telescope, which was recently constructed in China, here: https://www.sciencemag.org/news/2016/09/world-s-largest-radio-telescope-will-search-dark-matter-listen-aliens

x There are many, many planned and proposed missions for Earth-orbiting, Moon-orbiting, and lunar-surface radio observatories—too many to discuss here! I’m focusing on the Chang’e-4 mission because of its recent concrete steps toward a permanent lunar observatory, but some others that you might be interested in are the Dark Ages Radio Explorer (DARE; http://lunar.colorado.edu/dare/) and the International Lunar Observatory (ILO; would initially consist of a visible-light telescope but a small radio telescope would follow; https://www.iloa.org/).

xi https://www.isispace.nl/projects/ncle-the-netherlands-china-low-frequency-explorer/



Copyright © 2019 Kerry Hensley


Kerry Hensley is a native of West Virginia and currently lives in Boston, Massachusetts, where she is a Ph.D. candidate in astronomy at Boston University. She is a writer for the award-winning research news website AAS Nova as well as Astrobites, a graduate-student-run research blog. Her essay recounting her time at the National Radio Astronomy Observatory, "Quiet Zone," received awards for both Nonfiction and Emerging Writers Prose in the West Virginia Writers Annual Writing Contest. After graduating from Williams College in 2014 with a BA in Astrophysics and Chinese, she was a planetary science intern at NASA’s Jet Propulsion Lab and a Fulbright English Teaching Assistant in Nan’ao, Taiwan, where she attempted to convince 200 indignant elementary schoolers that Pluto is not a planet.