Scientists always love a thoroughly mysterious signal. Although one has never been traced back to alien life, terrific planet-wide thunderstorms on alien worlds far from our own have generated detectable radio bursts. Other such signals have been emitted from the heart of our own galaxy, perhaps due to the destruction of dark matter.
A new study published in the journal Geophysical Research Letters has claimed to solve the enigma of another of these signals. The so-called “150-kilometer echoes” have been baffling scientists for half a century, but it turns out that every single person on the planet has been looking at the culprit their whole lives.
Every single day at dawn and dusk, the phenomenon rears its head: Radar signals, initially being sent into space by scientists, have been reflecting back towards the ground as echoes. Something at a height of 150 kilometers (90 miles) is acting as a mirror for these signals, and towards noon, the “depth” of the mirror extends downwards to a height of 20 to 30 kilometers (12 to 19 miles).
The mirror then rises again towards sunset, where it disappears at night. Probes sent into the atmosphere during sunrise and sunset have failed to identify any potential source of the mirroring effect.
Researchers at Boston University decided to try and identify this mirror once and for all. “When you actually figure out something that nobody’s ever figured out before, that’s when it’s really worthwhile,” Meers Oppenheim, an astronomer at Boston University and joint co-author of the study, said in a statement.
The echoes, tracked throughout the day, at varying altitudes. Oppenheim and Dimant/Jorge Chau/GRL
They noted that the source of the echoes grew more powerful during solar flares and became considerably weaker during eclipses. The researchers also pointed out that the increasing depth of the signal during the day matched the angle of incoming solar radiation. This circumstantial evidence seemed to point to our local star as being potentially culpable, but the signal was unlikely to be coming from the Sun itself.
Ultraviolet light from the Sun is known to tear off the electrons of oxygen and nitrogen molecules in the upper atmosphere, which sends these electrons off at high speeds and with considerable energies. The interactions between these electrons, the electron-stripped molecules (ions) and other particles, are likely to cause the ions to vibrate in batches.
As the study suggests, this mass vibration could generate energetic waves that may be strong enough to reflect radar signals coming from the ground. Using computer simulations to replicate this mechanism, the researchers did indeed show that the energy from the incoming solar radiation is enough to ultimately produce radar-reflecting batches of vibrating ions.
As the Sun rises and moves across the sky during the day, the angle of incoming solar radiation increases, and the vibrating ion batches appear lower in the sky. Towards sunset, the angle decreases, and the “mirror” moves back upwards again. During energetic solar flares, more vibrating ions can be produced, and the mirror becomes more powerful.
Far from just solving a long-held mystery, the team note that this atmospheric mirror, which is easily located, can be used to track the complex movements of atmospheric currents and tides. In any case, this once-mysterious signal is definitely not coming from an alien civilization; perhaps more importantly, it’s not just coming from someone using their microwave at the wrong time of day.