You can’t buy it at a farmer’s market, but it’s all around us. Last week I discussed I discussed the electromagnetic radiation spectrum in a fairly broad, general sense (it’s a big spectrum). Early on, I made a passing reference to natural forms of electromagnetic radiation, especially natural radio waves, x-rays and gamma rays. As I also discussed, visible light, as well as infrared and UV, are also electromagnetic radiation, but since they’re visible, I think there’s less need to really explain their natural sources.
Let’s start with conventional radio waves. Recall that these waves are typically categorized as those with frequencies ranging from about 3kHz to 300GHz. Another thing worth mentioning is that just like visible light, all electromagnetic radiation travels at the speed of light. Now, on to natural radio waves. Artificial radio waves were first discovered in the early days of electricity experimentation. As anyone who’s experimented with electricity will tell you, one of the inevitable effects is creating a spark. These early experimenters learned that these sparks actually created radio waves that could be picked up by a receiver device. Early radios used this phenomenon to function and they were often called spark-gap transmitters. They quickly fell out of use though because they tend to transmit over a very broad band, which makes interference inevitable. Earth naturally creates its own spark gaps as well during lightning storms. Just like the tiny spark gaps in early radio transmitters, a bolt of lightning will also create a radio wave, albeit a broad band and chaotic one.
The other primary sources of naturally occurring radio waves are astronomical objects. These need not even be local objects, since radio waves travel at the speed of light. The universe is pretty old, and while our neck of the woods has been relatively quiet in the past few billion years, the rest of the universe has been very busy. Our greatest local source of astronomical radio waves is the sun. In fact, all stars will give off radio waves, being the hot balls of nuclear fusion that they are. Other sources include supernova remnants, pulsars, star-forming regions as well as echoes from the Big Bang itself.
All of that stuff is just radio waves though—pretty harmless. We do study and listen to them though, to learn more about the universe beyond what we can see with telescopes. From an astronomical perspective, most of the things that generate radio waves also generate natural x-rays. Another minor source of local naturally-occurring x-rays are cosmic rays. This is where things get a bit confusing, so let me clarify some of this. X-rays as we’ve already learned are electromagnetic radiation. As such, they are not composed of matter. They are essentially the propagation of an electromagnetic field. They require no specific medium for transport (air, water, vacuum) and in the case of higher energies, can pass straight through solid materials. Cosmic rays on they other hand are not rays in the same sense. Cosmic rays are actually composed of charged particles (protons and high-energy atomic nuclei) which are physical.
Cosmic rays interact with the earth’s atmosphere and magnetosphere in a variety of ways. This is difficult to describe without diagrams, but I’ll try my best. Because cosmic rays are charged particles, they are subject to the influence of electromagnetic fields, including the giant magnetic field that surrounds the entire earth. The shape of this field (with poles situated at roughly the north and south geographic poles) channels and directs most of these particles away from the earth’s atmosphere and surface. However, particles are able to trickle down to lower altitudes at the poles. When these particles interact with molecules in the atmosphere, many things can happen, including generation of x-rays, gamma rays and other secondary particles (muons, alpha particles, electrons and neutrons). This is also what causes the polar luminescence known as the “northern lights”.
So that’s one way gamma rays can be generated naturally. Almost all astronomical sources of gamma rays (like x-rays) are screened by our atmosphere and magnetosphere. If this weren’t the case, we wouldn’t be alive as a species. Gamma rays are also generated via gamma decay of naturally occurring radioisotopes. There are even rare conditions that occur on earth that can create gamma rays including lightning strikes and so-called “terrestrial gamma ray flashes”. Both of these are the result of specific conditions in the atmosphere resulting in the availability of natural high-energy voltages. These events are particularly rare though, and are primarily of interest from a theoretical perspective. The actual amount of danger presented by these natural terrestrial gamma rays is very low. Artificial sources of gamma rays such as nuclear fission are far more dangerous because of the amounts of radiation involved.
High-energy electromagnetic radiation is not only dangerous to biological material but it can also wreak havoc on electronic systems. Semiconductor junctions are particularly susceptible to ionizing radiation (which includes high energy gamma rays). The mechanisms at work are far too complex to explain here, but on a macro level, the issue becomes that if components in digital circuitry (semiconductors) cannot be relied upon to maintain their bits (0s and 1s) the entire circuit becomes useless. In April of 2010, the Voyager 2 space probe (well out of our solar system at that point) experienced some data formatting issues. Those with more active imaginations postulated that the probe had been “attacked” by aliens, but engineers traced the problem to a single bit that had been flipped by ionizing radiation.
I think I might try to come up with a simpler topic next week, because this article even has my head spinning. Hopefully I’ve been able to at least illuminate a bit more about electromagnetic radiation for you.