Wednesday, 9 January 2013

Inversion of electron density from multi-frequency absorption measurements

Riometers were discussed a while ago on this blog. In essence, these are instruments that measure how much radio waves are absorbed in the D-region of the ionosphere (50-100 km). The more electron density in this region, the more absorption is observed. The absorption is also dependent on frequency of the radio wave. The higher the frequency, the lower the absorption. The amount of absorption is determined by comparing cosmic radio noise power to typical values for "normal" quiet day, where we would expect to know the amount of absorption based on ionospheric models. 

A few months ago we managed to capture an auroral precipitation event that caused clear absorption. This aurora was also captured at the Sodankylä Geophysical Observatory by the all-sky camera. 
Aurora at approximately 4:50 UT on 14.11.2012 captured by the UCL all-sky camera located at the Sodankylä Geophysical Observatory. Note: Image Copyright by University College London, UK. All rights reserved.
KAIRA was running in wide band riometer mode during this time with 244 subbands observing the vertical direction. You can see the measured values of absorption in the following figure. 
Measured absorption on the vertical beam of the LBA, starting from 12 UT on 13.11.2012.  Notice that while there is some man-made radio interference, most of the LBA band is pretty clean. 
Since then, we have been developing software to translate these kinds of measurements into physical quantities, namely electron density profiles. A few days ago we got the first results, and even though this is still work in progress, we are proud to show them to you now! The following image shows the resulting fit to the measurement.
The best model fit the the measurements. 
 And the following image shows the electron density profiles that our fitting routine gives for the data.
The resulting electron density (N_e/cm^3) profile that was fitted. 


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