Scientists tune in to the Sun to monitor melting ice sheets
Glaciologists and electrical engineers from Stanford University have demonstrated that radio signals emitted by the Sun can be harnessed as a passive radar system for measuring the depth of ice sheets.
The Sun and other stars are colossal sources of electromagnetic radiation across the spectrum. In this chaos of signals, Stanford researchers have identified the potential for monitoring ice and polar changes on Earth and across the solar system.
At present, information about the polar subsurface is collected by flying aeroplanes over ice sheets, transmitting an 'active' radar signal from a system on board (airborne ice-penetrating radar). This is a resource-intensive technique which only provides information about conditions at the time of flight and contributions to carbon emissions with every trip.
However, the Stanford researchers have demonstrated a battery-powered receiver with an antenna placed on the ice; this detects solar radio waves as they reach Earth and pass through the ice sheet to the subsurface. This system effectively harnesses natural radio signals rather than produce its own with an airborne transmitter.
The concept was originally proposed by co-author and Nasa scientist Andrew Romero-Wolf as a way of investigating Jupiter’s ice-covered moons. Romero-Wolf found that radio waves emerging from Jupiter interfered with their active ice-penetrating radar systems and decided to exploit these signals – to investigate the subsurface of the moons – rather than work around them.
The radio waves emitted from the Sun span a wide and random spectrum of frequencies. The scientists used this randomness to their advantage, recording a snippet of this activity and listening for that unique signature in the echo created when the solar radio waves are reflected from the bottom of an ice sheet. Measuring the delay between the original recording and the echo allows them to calculate the distance between the surface receiver and the floor of the ice sheet and hence its thickness.
The Stanford researchers tested the technique on Store Glacier in West Greenland. They calculated an echo delay of around 11 microseconds, corresponding to an ice thickness of around 910m - a figure which matches measurements taken from ground-based and airborne radar.
If this system could be miniaturised and deployed in sensor networks, it would offer an unprecedented look at the evolution of Earth’s polar regions.
“Our goal is to chart a course for the development of low-resource sensor networks that can monitor subsurface conditions on a really wide scale,” said lead author Sean Peters. “That could be challenging with active sensors, but this passive technique gives us the opportunity to really take advantage of low-resource implementations.”
While the system only works when the Sun is above the horizon, the proof-of-concept opens up the possibility of adapting to other natural and artificial radio sources in the future. The researchers are also still pursuing their original idea of applying this technique to space missions by harnessing radioactivity emitted by other astronomical objects like Jupiter.
“Pushing the frontiers of sensing technology for planetary research has enabled us to push the frontiers of sensing technology for climate change,” said Professor Dustin Schroeder, a geophysics expert at Stanford University. “Monitoring ice sheets under climate change and exploring icy moons at the outer planets are both extremely low-resource environments where you really need to design elegant sensors that don't require a lot of power.”
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