Alpha Transmitter Signals Observed by the Van Allen Probes: Ducted Versus Nonducted Propagation. Issue 12 (15th June 2022)

• Record Type: Journal Article
• Authors: Němec, F.; Santolík, O.; Hospodarsky, G. B.; Kurth, W. S.
• Is Part Of: Geophysical research letters. Volume 49:Issue 12(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-06-15
• Subjects: Alpha transmitters -- ducted -- nonducted -- wave analysis; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2022GL098328
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: The interaction of very low frequency transmitter signals with radiation belt electrons depends ultimately on their wave normal angles. In the equatorial interaction region, these can be either low (ducted propagation) or comparatively large (nonducted propagation). Experimentally distinguishing the two modes is complicated, as multicomponent spacecraft data typically do not extend to high enough frequencies with a sufficient frequency resolution. One exception that we exploit are 11.9 kHz signals from Alpha transmitters detectable by the Van Allen Probes spacecraft. We use multicomponent burst mode measurements to distinguish between the ducted and nonducted modes of propagation and to evaluate their relative importance. While the ducted waves are detected less often, they tend to have larger Poynting fluxes. The total power propagating in the two modes is thus comparable. Magnetic local time and in‐situ density fluctuations are main parameters controlling the relative fraction of ducted waves. Plain Language Summary: Powerful military very low frequency transmitters operating at frequencies of a few tens of kHz are important manmade sources of electromagnetic waves propagating in the Earth's inner magnetosphere. They can, in particular, interact with energetic electrons trapped in the radiation belts and result in their precipitation. The efficiency of such interactions depends, among other parameters, crucially on the wave normal angles (WNAs) of propagating transmitter signals. Unfortunately, these are not well known and are expected to depend significantly on the mode of propagation. If the wave propagation is governed by density ducts following Earth's magnetic field lines, the respective WNAs are very low. In the absence of such density ducts, the WNAs are expected to be comparatively large. We use electromagnetic wave measurements performed by the Van Allen Probes spacecraft to determine and systematically analyze WNAs of propagating transmitter signals. This allows us to experimentally distinguish the two propagation types and to evaluate their relative significance. Key Points: Multicomponent Van Allen Probes data are used to distinguish between ducted and nonducted propagation of Alpha transmitter signals Nonducted waves are more frequent, but they tend to have lower Poynting fluxes, so that the total power of the two modes is comparable Relative fraction of ducted waves peaks in the afternoon. In‐situ density fluctuations are higher at ducted than at nonducted times