Faraday rotation from API echos

The enhanced echos produced by the API effect can also be used to measure Faraday rotation, i.e., the propagation time difference between the two characteristic modes of radio wave propagation, which is dependent on several plasma parameters, but most importantly electron density. The artificial periodic irregularities are useful here, as they allow us to get strong enhanced backscatter from this region, which would otherwise give such a strong return.

Here is our first Faraday-rotation measurement from API echos. This is still a fairly raw data product, which must be further inverted into an electron density profile.

Faraday rotation measured using enhanced ionospheric backscatter from heating induced artificial periodic irregularities. This allows us to measure small electron densities, which are otherwise difficult to measure. The delay between O and X modes of propagation is given in radians, as a fraction of a radar probing wave cycle.

More post to come, as we have one more week of measurements to go.

Mesospheric velocities with API

One advantage of the API method is that it allows high resolution vertical velocities to be measured between 50-120 km. Here is an example plot from our 9.12.2011 measurement. Around noon (7000 seconds in to the measurement) there are also some tropospheric echos.

Vertical velocities in the 60-120 km region measured using the API method with the EISCAT heater. 

API

This week me and Antti Kero are up in Tromsø doing artificial periodic irregularity (API) experiments with the EISCAT heater, which has recently been upgraded to include a radar receiver.

The idea with the experiments is to first produce a standing wave in the ionosphere, which heats up the plasma, and creates small enhancements of electron density at lambda/2 intervals. These irregularities are then probed with short radar pulses, which are used to probe the decay time of the irregularities. From this, it is possible to determine several ionospheric parameters: ionospheric chemistry, electron density, winds, and neutral density.

5.423 MHz X-mode API echo. There is also a meteor head echo accompanied with a decaying trail echo in this measurement.

We managed to actually get an API echo on our first try! Here is a picture from our second day of measurements. This is an API echo with 5.423 MHz at X-mode heating and probing polarization. We were also lucky to observe a meteor head echo and associated trail echo.