Morphology and diagnostic potential of the ionospheric Alfvén resonator

The layering of the ionosphere leads to the formation of resonators and waveguides of various kinds. One of the most well-known is the ionospheric Alfvén resonator (IAR) whose radiation can be observed both on Earth’s surface and in space in the form of a fan-shaped set of discrete spectral bands (DSB), the frequency of which changes smoothly during the day. The bands are formed by Alfvén waves trapped between the lower part of the ionosphere and the altitude profile bending of Alfvén velocity in the transition region between the ionosphere and the magnetosphere. Thus, IAR is one of the important mechanisms of the ionosphere-magnetosphere interaction. The emission frequency lies in the range from tenths of hertz to about 8 Hz — the frequency of the first harmonic of the Schumann resonance. The review describes in detail the morphology of the phenomenon. It is emphasized that the IAR emission is a permanent phenomenon; the probability of observing it is primarily determined by the sensitivity of the equipment and the absence of interference of natural and artificial origin. The daily duration of the DSB observation almost completely depends on the illumination conditions of the lower ionosphere: the bands are clearly visible only when the D layer is shaded. Numerous theoretical IAR models have been systematized. All of them are based on the analysis of the excitation and propagation of Alfvén waves in inhomogeneous ionospheric plasma and differ mainly in sources of oscillation generation and methods of accounting for various factors such as interaction of wave modes, dipole geometry of the magnetic field, frequency dispersion of waves. Predicted by all models of the cavity and repeatedly confirmed experimentally, the close relationship between DSB frequency variations and critical frequency foF2 variations serves as the basis for searching ways of determining in real time the electron density of the ionosphere from IAR emission frequency measurements. It is also possible to estimate the profile of the ion composition over the ionosphere from the data on the IAR emission frequency structure. The review also focuses on other results from a wide range of IAR studies, specifically on the results that revealed the influence of the interplanetary magnetic field orien tation on oscillations of the resonator, and on the facts of the influence of seismic disturbances on IAR.

Morphology and diagnostic potential of the ionospheric Alfvén resonator

The layering of the ionosphere leads to the formation of resonators and waveguides of various kinds. One of the most well-known is the ionospheric Alfvén resonator (IAR) whose radiation can be observed both on Earth’s surface and in space in the form of a fan-shaped set of discrete spectral bands (DSB), the frequency of which changes smoothly during the day. The bands are formed by Alfvén waves trapped between the lower part of the ionosphere and the altitude profile bending of Alfvén velocity in the transition region between the ionosphere and the magnetosphere. Thus, IAR is one of the important mechanisms of the ionosphere-magnetosphere interaction. The emission frequency lies in the range from tenths of hertz to about 8 Hz — the frequency of the first harmonic of the Schumann resonance. The review describes in detail the morphology of the phenomenon. It is emphasized that the IAR emission is a permanent phenomenon; the probability of observing it is primarily determined by the sensitivity of the equipment and the absence of interference of natural and artificial origin. The daily duration of the DSB observation almost completely depends on the illumination conditions of the lower ionosphere: the bands are clearly visible only when the D layer is shaded. Numerous theoretical IAR models have been systematized. All of them are based on the analysis of the excitation and propagation of Alfvén waves in inhomogeneous ionospheric plasma and differ mainly in sources of oscillation generation and methods of accounting for various factors such as interaction of wave modes, dipole geometry of the magnetic field, frequency dispersion of waves. Predicted by all models of the cavity and repeatedly confirmed experimentally, the close relationship between DSB frequency variations and critical frequency foF2 variations serves as the basis for searching ways of determining in real time the electron density of the ionosphere from IAR emission frequency measurements. It is also possible to estimate the profile of the ion composition over the ionosphere from the data on the IAR emission frequency structure. The review also focuses on other results from a wide range of IAR studies, specifically on the results that revealed the influence of the interplanetary magnetic field orien tation on oscillations of the resonator, and on the facts of the influence of seismic disturbances on IAR.

Research into connection between spectral resonance structures and harmonics of ionospheric Alfvén resonator

We present the results of modeling of the ionospheric Alfvén resonator (IAR) and compare the results of electromagnetic waves passing through IAR to Earth's surface with the dynamic spectrum of simultaneous observations of spectral resonance structures (SRS). IAR is simulated using ionospheric parameters obtained from measurements made with the CP-1 program of the Scandinavian EISCAT radar. The IAR model is employed to calculate coefficients of reflection RC(f) and transmission TC(f) of electromagnetic waves in the frequency range 0–5 Hz. The observed dynamic SRS spectrograms consist of spectral lines, in which frequencies, time variations of frequencies, and distances between adjacent resonant lines are confidently determined. The calculated frequencies of maxima of the signal transmission coefficient TC to Earth's surface correspond to the observed frequencies of the dynamic spectrum of SRS.

Research into connection between spectral resonance structures and harmonics of ionospheric Alfvén resonator

We present the results of modeling of the ionospheric Alfvén resonator (IAR) and compare the results of electromagnetic waves passing through IAR to Earth's surface with the dynamic spectrum of simultaneous observations of spectral resonance structures (SRS). IAR is simulated using ionospheric parameters obtained from measurements made with the CP-1 program of the Scandinavian EISCAT radar. The IAR model is employed to calculate coefficients of reflection RC(f) and transmission TC(f) of electromagnetic waves in the frequency range 0–5 Hz. The observed dynamic SRS spectrograms consist of spectral lines, in which frequencies, time variations of frequencies, and distanc-es between adjacent resonant lines are confidently de-termined. The calculated frequencies of maxima of the signal transmission coefficient TC to Earth's surface correspond to the observed frequencies of the dynamic spectrum of SRS.