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.

Remote sensing of the magnetospheric plasma by means of whistler mode signals

Abstract: Early in the past decade of U.S. Antarctic research, the whistler method of measuring equatorial electron density was found to agree with in situ satellite electron density measurements by a radio technique. Furthermore, the whistler method of measuring the east-west component of the convection electric field in the outer plasmasphere was found to agree, under conditions of mapping in a dipole magnetic field, with simultaneous results from incoherent scatter radar. A global model of the east-west convection electric field in the outer plasmasphere during substorms was developed. The detection of whistlers and their use for magnetospheric diagnostics have been important elements in recent studies of burst precipitation into the ionosphere induced by whistlers and by other transient whistler mode waves propagating in the magnetosphere. Whistlers have also been used to obtain data on the L values and equatorial electron densities associated with the propagation paths of signals from the Siple VLF transmitter. The process of untrapping of downcoming wave energy from ducts in the upper ionosphere and the upward repropagation of portions of the energy following reflection in the lower ionosphere lead to the excitation of adjacent ducts as well as to upward propagation in the nonducted mode. Efficient interduct coupling has been found to occur over north-south ionospheric distances of >1000 km. Studies of the outer limits of observed ducting revealed dayside path radii in the range 6–8 RE and nightside limits of ∼5.5 RE. Ducted propagation beyond the plasmapause was found to occur regularly in the 0000–1800 MLT time range, but with variable rates and at various locations with respect to the plasmapause position. The special features of this propagation are believed to be related to conditions of lightning excitation, ionospheric penetration, and wave-particle interactions that are special to the region beyond the plasmasphere. New aspects of Siple wave injection experiments were demonstrated by the application of a new phase measurement method to Siple signals that did not exhibit fast temporal growth during passage through the magnetosphere. This method, a refinement of techniques developed previously by New Zealand workers, is capable of detecting fluctuations in phase path with period of ∼10 s and greater and thus can be used to study magnetospheric convection and coupling fluxes along field lines of propagation as well as pulsations associated with ultralow-frequency perturbations of the geomagnetic field. Additional topics discussed include results from direction-finding experiments and evidence of the dependence of whistlers upon magnetospheric wave amplification.

Perturbations to the lower ionosphere by tropical cyclone Evan in the South Pacific region

Abstract: Very Low Frequency (VLF) electromagnetic signals from navigational transmitters propagate worldwide in the earth-ionosphere waveguide formed by the earth and the electrically conducting lower ionosphere. Changes in the signal properties are signatures of variations in the conductivity of the reflecting boundary of the lower ionosphere which is located in the mesosphere and lower thermosphere, and their analysis is, therefore, a way to study processes in these remote regions. Here we present a study on amplitude perturbations of local origin on the VLF transmitter signals (NPM, NLK, NAA and JJI) observed during tropical cyclone (TC) Evan, 9-16 December 2012 when TC was in the proximity of the transmitter-receiver links. We observed a maximum amplitude perturbation of 5.7dB on JJI transmitter during 16 December event. From Long Wave Propagation Capability model applied to three selected events we estimate a maximum decrease in the nighttime D-region reference height (H') by ~5.2 km (13 December, NPM) and maximum increase in the daytime D-region H' by 6.1 km and 7.5 km (14 &16 December, JJI).The results suggest that the TC caused the neutral densities of the mesosphere and lower thermosphere to lift and sink (bringing the lower ionosphere with it), an effect that may be mediated by gravity waves generated by the TC. The perturbations were observed before the storm was classified as a TC, at a time when it was a tropical depression, suggesting the broader conclusion that severe convective storms, in general, perturb the mesosphere and the stratosphere through which the perturbations propagate.