A basic overview of ionospheric phenomenology is provided. Some of the modern electronic systems of interest and the extent to which the ionosphere may place limits on design and operation are discussed. The effects of importance to electronic systems are defined, and the effects are discussed by frequency regime: the ELF ( >

Ionospheric effects on modern electronic systems

Several natural and man-made sources of electromagnetic energy exists near the ground (e.g., thunderstorms and lightning, earthquakes, power line radiation) as well as in the upper atmosphere (e.g., ionospheric dynamo, magnetospheric dynamo). The lower and middle atmosphere in between behaves like a transmission line for electromagnetic waves possessing an inhomogeneous and anisotropic electric conductivity. The transmission line distorts the low-frequency band of electromagnetic pulses which propagate up or down the line. The electric conductivity in this line can vary considerably in time and space, in particular during solar cosmic ray events. In this paper, our present knowledge about sources of electromagnetic energy and the influence on the transmission line "lower and middle atmosphere" on the propagation of low-frequency electromagnetic waves and DC electric fields is outlined, and practical and theoretical implications are discussed.

http://iopscience.iop.org/article/10.1088/0031-8949/1987/T18/029/pdf

Electromagnetic Coupling Between Lower and Upper Atmosphere

A demonstration of an aerostat-supported extremely low frequency/very low frequency (ELF/VLF) transmitting antenna was performed. At ELF the vertical electric dipole (VED) antenna radiated at least 100,000 times more power than would a horizontal electric dipole having the same moment. That efficiency was achieved with an altitude of 12,500 feet (3810 m). Calculations show that the radiated power will increase as the fourth power of aerostat altitude. The tether antenna exhibited a corona onset voltage of 180 kV and was resistant to the degrading effects of ELF corona. Prolonged in-corona operation is therefore possible. The antenna was continuously tuned, despite changes in height and capacitance caused by the aerostat flight dynamics. The huge 300-H ELF tuning inductor posed no problem. Enhanced VED moments were achieved at ELF by operation at voltages up to 260 kV, 40% above the corona onset voltage. At VLF the antenna emulated a monopole that had a radiation efficiency greater than 90%. The measured bandwidths were large: 1.5 kHz at 23 kHz and 3.5 kHz at 34 kHz. The antenna height exceeded one-quarter wavelength at VLF, so the antenna could be tuned capacitively and required relatively low base voltages. At both VLF and ELF the measured fields agreed closely with predictions.

An aerostat-supported ELF/VLF transmitter

Headache is one of the most commonly encountered neurologically related complaints in HIV patients. The authors review the relationship between HIV infection and headache and discuss the role of neuroimaging in this select patient group.

Headache in the HIV patient: a review with special attention to the role of imaging.

This paper presents measurements of both transverse electric (TE) and transverse magnetic (TM) LF atmospheric noise made aboard a free-floating balloon at altitudes up to 20 km. Data are given for rms noise level, TE/TM noise ratio, amplitude probability distribution, and impulsiveness parameter VD. The TE/TM noise ratio was greater than reported by other authors and implies major contributions from horizontally polarized sources. Calculations of the fields generated by lightning discharges as a function of altitude and orientation show that horizontal discharges are weak TE radiators at VLF/LF unless their altitude exceeds a few kilometers. Above several kilometers, however, horizontal discharges can radiate TE noise almost as efficiently as vertical ones radiate TM noise. Most horizontal lightning channels do, in fact, occur above 3 km and are more frequent than vertical channels. Moreover, the horizontal component of lightning is often two or three times greater in extent than the vertical component, even for cloud-to-ground discharges. It therefore appears that TE noise might well be caused primarily by direct radiation from horizontal lightning rather than by geomagnetically converted radiation from vertical lightning.

Low‐frequency transverse electric atmospheric noise: Measurement and theory

Airborne VLF antennas radiate energy that propagates via both transverse electric (TE) and transverse magnetic (TM) modes in the earth-ionosphere waveguide. In order to compare the structure of such signals, measurements were made using rocket probes launched from Wallops Island, Virginia. The probes measured TE and TM fields at all altitudes between the ground and the base of the ionosphere. The nearly horizontal airborne transmitting antenna radiated a TE signal that was stronger than its TM signal at altitudes above about 10 km. The signals comprised one or more well-defined TE or TM waveguide modes. Calculated height profiles agree well with the measured ones and correctly reproduce details of profile structure caused by interaction between two or more modes.

R. P. Harrison

Papers

Comparison of calculated and measured height profiles of transverse electric VLF signals across the daytime earth‐ionosphere waveguide