VLF propagation in disturbed environments
Abstract
Long-range VLF/LF propagation of transverse magnetic (TM) and transverse electric (TE) signals at frequencies from 10 to 50 kHz is considered. Model ionospheres corresponding to solar proton events or spread-debris nuclear environments are used in full wave calculations. Such disturbances constrict the Earth ionosphere waveguide, increasing excitation factors and attenuation rates. The signals are therefore usually degraded beyond a few megameters, although either enhancement or degradation can occur at shorter distances. Ohmic heating of heavy ions in the lower ionosphere is the main loss mechanism for intense disturbances. Only TM modes are efficiently radiated by ground based transmitters. However, TE signals can be important for air-to-air links with terminal elevations of 20 kft or more, and nearly horizontal trailing wire antennas. Air-to-air TE signals propagate better than TM modes over poorly conducting ground such as exists throughout Greenland and much of Canada, and can fill nulls in the TM signal. For paths over highly conducting ground, TM modes suffer less degradation than TE signals during intense disturbances; but, for ground conductivities less than about 0.0001mhos/m, TM signals are more adversely affected than TE signals.
- Publication:
-
In AGARD Medium
- Pub Date:
- February 1982
- Bibcode:
- 1982mlvl.agarS....F
- Keywords:
-
- Electric Pulses;
- Ionospheric Propagation;
- Magnetic Signals;
- Radio Waves;
- Transverse Waves;
- Very Low Frequencies;
- Electric Field Strength;
- Mathematical Models;
- Microwave Attenuation;
- Ohmic Dissipation;
- Transmission Efficiency;
- Communications and Radar