Abstract
Abstract. Radio wave absorption in the ionosphere is a function of electron density, collision frequency, radio wave polarisation, magnetic field and radio wave frequency. Several studies have used multi-frequency measurements of cosmic radio noise absorption to determine electron density profiles. Using the framework of statistical inverse problems, we investigated if an electron density altitude profile can be determined by using multi-frequency, dual-polarisation measurements. It was found that the altitude profile cannot be uniquely determined from a “complete” measurement of radio wave absorption for all frequencies and two polarisation modes. This implies that accurate electron density profile measurements cannot be ascertained using multi-frequency riometer data alone and that the reconstruction requires a strong additional a priori assumption of the electron density profile, such as a parameterised model for the ionisation source. Nevertheless, the spectral index of the absorption could be used to determine if there is a significant component of hard precipitation that ionises the lower part of the D region, but it is not possible to infer the altitude distribution uniquely with this technique alone.
Highlights
Jansky (1933) determined that certain detected radio noise was of cosmic origin and was associated with our galaxy, founding a new branch of science – radio astronomy.Shain (1951) observed the absorption of this cosmic radio noise by the ionosphere
One later example built by Little and Leinbach (1959) – the RIOMETER – gave the name “riometer” to this generic class of instrument and the term “riometry” to the measurement of cosmic noise absorption (CNA) by the ionosphere
The comparison with the nearby incoherent scatter radar indicated that multi-frequency riometry could determine comparable electron density profiles based on the a posteriori probability distribution of two free parameters, which is in turn based on the least-squares fit between the measured absorption spectrum and the parameterised model
Summary
Jansky (1933) determined that certain detected radio noise was of cosmic origin and was associated with our galaxy, founding a new branch of science – radio astronomy. Measurements using a continuous spectrum of radio frequencies (spectral riometry) have been compared to an alternate method (incoherent scatter radar) to validate their measurement of electron density enhancement (Kero et al, 2014). The comparison with the nearby incoherent scatter radar indicated that multi-frequency riometry could determine comparable electron density profiles based on the a posteriori probability distribution of two free parameters (electron precipitation energy and flux), which is in turn based on the least-squares fit between the measured absorption spectrum and the parameterised model. We utilise the framework of statistical inverse problems (Kaipio and Somersalo, 2006) to study how well the electron density profile of the lower ionosphere can be determined using a multi-frequency riometer. This work is a mathematical approach, which explains the difficulties encountered in earlier studies in an objective manner
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