I have written about some interesting highlights from the annual National Radio Sciences Meeting of URSI in Boulder, CO, and would like to finish that off with a topic near and dear to my brain: radio astronomy through the ionosphere.
Low frequency radio astronomy has enjoyed a resurgence in the last few years, partly driven the search for the signal of the epoch of reionization, or EoR. By low frequency, I mean technically around what is called the VHF or “very high frequency” radio band (30MHz – 300MHz). However, most of radio astronomy has centered on the GHz bands, with most recent pushes out to both higher AND lower frequencies to work on new science.
Low frequency radio astronomy is a bit harder than the radio astronomy of the past in part because of the Earth’s ionosphere. This is the ionized part of the atmosphere, meaning that it consists of negatively and positively charged particles. These charged particles can interact with light, aka electromagnetic radiation, and the problem becomes apparent as you move down to tens and hundreds of megahertz. (If you are not used to working with frequencies, just remember that the FM radio band is in this range, and that the wavelength of the light is on the scale of meters! GHz frequencies are measured in centimeters.)
The ionosphere refracts the incoming light, but irregularities and turbulence make it refract differently across a large field of view. In a way, it is analogous to the problem that optical astronomy has had to deal with all along. Stars seem to “twinkle” because of the turbulent troposphere (low, water-bearing layer of the atmosphere.) The latest generations of optical telescopes have been fitted with adaptive and even active optics, where the mirrors are slightly deformed in sync with the changing atmosphere in order to compensate for these effects and get really clear pictures.
The situation is a bit harder for radio astronomy. In order to get the spatial resolution on the sky, as well as the sensitivity needed, we build interferometers, or whole arrays of individual radio telescopes (antennas, elements, stations, etc.) linked together to make one BIG telescope. These interferometers don’t just take an image of the sky, as an optical telescope would, but measure spatial frequencies on the sky. Or, very, very roughly, how much brightness is in big things versus small things. This is then mathematically transformed (imperfectly) into an image. In this way, an interferometer needs to preserve the phase information of light, not just its amplitude, in order to make a proper sky map. And phase is just what the ionosphere likes to mess with.
So what do we do? Well, as I sometimes say, “one astronomer’s noise is another astronomer’s data.” As we are trying to remove the effects of the ionosphere from our data, there are lots of ionospheric scientists who want to the study the ionosphere itself! So, URSI has been doing a joint-session of ionospheric physics and radio astronomy in the annual US meeting. The astronomers can make measurements of the phase changes in their data and try to interpret them in terms of the ionosphere, whereas the ionospheric people know a lot about how it behaves, and can share data taken with their instruments, such as GPS arrays and ionosondes. They, in turn, can use the telescope data as well to probe new size and time scales. Amazingly, there is still much to know!
In particular, I’m one of those radio astronomers trying to quantify the impact that the ionosphere has on our instrument, PAPER. In order to see the very, very faint EoR signal, we need to make accurate maps and clear out all the foreground “trash” first. I’ve been tracking the short-timescale, small movements of bright sources in the sky as they move through our telescope’s field of view, and determining empirically what ionospheric effects we need to be careful of. Then, I can make a model of our future, large dataset, and apply a realistic ionosphere model to see just how bad the problem will be. (Or not bad… don’t want to be a Debbie Downer!)
The two science communities are just now learning to speak to each other, in terms of jargon and familiarity with each others data. As I learned over dinner and drinks later that evening, we astronomers still have a lot of learn from the ionosphere guys and gals, and there is great opportunity for collaboration! I’m excited for this, as it can only help my push to finish my thesis and get this ionosphere characterized for the ever-important EoR detection to come!