Coordinated measurements of ELF transmission anomalies and the precipitation of energetic particles into the ionosphere

Abstract

Stimulated by previous findings of a positive qualitative correlation between anomalous ELF signal levels and the occurrence of significant fluxes of precipitating energetic particles, coordinated exercises were conducted between satellite measurements of the precipitating particles and ELF transmissions between the US Navy Wisconsin Test Facility and receiving stations in Maryland, Greenland, Norway, and Italy. Significant fluxes of precipitating electrons were often measured with the Lockheed pay load on the polar‐orbiting satellite 1972–076B. In March‐April 1976 the received signal strengths were found generally to increase during periods when the geomagnetic K indices were higher and when the fluxes of precipitating electrons >160 keV were greater on the average. During other coordinated exercises in March‐April 1975 the general correlations were less evident. Based on the fluxes, energy spectra, and pitch‐angle distributions of the particles, ion‐pair production profiles have been calculated and electron density profiles subsequently obtained with application of known effective electron loss rates. From the coordinated data, correlative studies between signal strengths and electron density profiles have been performed with use of an ELF waveguide mode computer program developed at the Naval Ocean Systems Center. The propagation path to each station of interest is segmented into different regions with an averaging perpendicular to the direction of propagation of the waveguide parameters. With the inclusion of data from several stations, the transmission signal strengths could be studied simultaneously under a variety of ionospheric conditions. The waveguide computer program calculations indicated that energetic electron precipitation events may cause a signal enhancement or degradation, depending upon where the disturbance is occurring along the path. The consistent occurrence during electron precipitation events of signal strength anomalies in either direction is supported by the coordinated satellite/ELF station measurements.

Review of electromagnetic coupling between the Earth's atmosphere and the space environment

“…The presence of finite conductivity, which increases with height, makes the lower and middle atmosphere to act as dissipative waveguide. In this height region, energetic electrons are precipitated and complicate the transmitting properties of the medium (Imhof et al, 1978). Electromagnetic waves in the ELF/VLF range generated from a source on or near the Earth's surface such as lightning/man made transmitters, propagate over long distances through the wave-guide by the process of internal reflection (Budden, 1985;Barr et al, 2000).…”

Summary of ELF propagation validation system field strength measurements, 1976 to 1978

“…It is readily apparent from exceptions to this trend occurred during October [16][17][18] and day of month). It is readily apparent from Fig.…”

“…Because some previous ELF field-strength variations appeared to be correlated with geomagnetic activity [14] - [16], it would be of interest to compare the 1976-1978 measurements with geomagnetic activity. Presented in Figs.…”

“…One of the strongest 1978 solar-particle events occurred in February during the transition period from night conditions in December to day-night conditions in March (at local noon, the sun just barely reached the horizon at Thule). A polar cap absorption (F' CA) (6-dB riometer absorption) began at 09 50 GMT on February 13 and the time of maximum [13][14][15][16][17][18][19][20][21][22][23][24][25] million elecironvolts (MEV) proton flux was 06 00 on February 14. This event caused a 28-km drop in the VLF noontime reflection height at Thule, followed by a gradual return to normal over the next 7 days.…”

Summary of extremely low frequency (ELF) field strength measurements made in Connecticut during 1975

“…Because energetic particle precipitation into the D region tends to increase ionization, making the ionosphere more "daylike" by lowering the effective reflecting height and improving excitation, the observed nighttime field strength decreases are in the opposite sense to what would have been expected. Recently, from coordinated satellite and ELF field strength measurements, Imhof et al [1978] have found that direct particle precipitation into the atmosphere can cause ELF transmission anomalies. In these anomalies the signal strengths may be either attenuated or enhanced depending upon the spatial extent and location of the ionization.…”