Very low frequency

About: Very low frequency is a research topic. Over the lifetime, 1540 publications have been published within this topic receiving 24233 citations. The topic is also known as: VLF.

VLF bursts in the night ionosphere of Venus: Estimates of the Poynting flux

Abstract: Even though the Pioneer Venus plasma wave instrument returns a measure of only one component of the electric field due to waves, for the 100 Hz channel it is possible to calcualte an approximate Poynting flux of the waves by making several assumptions. When this is done the Poynting flux at lowest altitudes in a 30 Hz band centered on 100 Hz is found to be about 10 to the -7th W/sq m, independent of ionospheric conditions as indicated by the strength of the magnetic field. The local time variation of the approximate Poynting flux shows a maximum from about 2000 to 2200 LT in accord with inferences from the higher frequency channels. The observed wave energy flux is consistent with that expected from lightning given rates and flash strengths on Venus that are similar to or greater than that of the earth.

Theoretical study of lower ionospheric response to solar flares: sluggishness of D-region and peak time delay

Abstract: The rates of ion production and loss processes in the lower ionosphere during solar and other astronomical ionizing events vary with height. This variations influence the time lags of the response in different ionospheric layers. Very Low Frequency (VLF) signals reflected from any of these layers follow this time lag or delay during a transient cosmic events. One of the easiest ways to study this property is to observe the shift in the peak of VLF signal amplitude with respect to the peak of solar flares. We numerically model to find ion densities and resulting VLF signal perturbations during some solar flares. We clearly find from the model that the delay in the peak of the electron densities (with respect to peak of the ionizing event) in the lower ionosphere varies from height to height. The result also depends on the properties of events, such as peak intensity and sharpness. We investigate analytically how the delay of electron density peak should depend on height varying chemical rate parameters as well as the nature of transient events. Our capability is demonstrated using three classes (namely, X, M and C) of solar flares. The work is a step forward in our goal to employ ionosphere as a natural detector for astronomical observations.

Attenuation of electromagnetic waves at the frequency ~1.7 kHz in the upper ionosphere observed by the DEMETER satellite in the vicinity of earthquakes

Abstract: The DEMETER satellite was the first satellite specifically dedicated to the recording of electromagnetic phenomena connected with seismic activity. Almost 6.5 years of measurements provide good opportunities to analyze a unique dataset with global Earth coverage. We present the results of a statistical study of the intensity of very low frequency electromagnetic waves recorded in the upper ionosphere. Robust two-step data processing has been used. The expected unperturbed distribution of the power spectral densities of electromagnetic emissions was calculated first. Then, the power spectral densities measured in the vicinities of earthquakes are compared with the unperturbed distribution and are examined for the presence of uncommon effects related to seismic activity. The statistical significance of the observed effects is evaluated. We confirm the previously reported results of a very small, but statistically significant, decrease in wave intensities a few hours before times of main shocks using this much larger dataset. The wave intensity decrease at a frequency of about 1.7 kHz is observed only during the night and only for shallow earthquakes. This can potentially be explained by increases in the cut-off frequency of the Earth ionosphere waveguide caused by imminent earthquakes.