Ionosphere and Wireless

A century after Marconi’s feat, the ionosphere remains both a facilitator and a disturber in numerous communications applications. The military, as well as police and fire emergency agencies in many nations, continue to rely on wireless links that make extensive use of frequencies from kHz to hundreds of MHz and that use the ionosphere as a reflector. Commercial air traffic over the north polar regions continues to grow following the political changes of the late 1980s-early 1990s, and this traffic relies heavily on RF communications.

Changes in the ionosphere that affect RF signal propagation can be produced by many mechanisms including direct solar photon emissions (solar UV and x-ray emissions), solar particles directly impacting polar region ionospheres, and radiation belt particles precipitated from the trapped radiation environment during geomagnetic storms.
At higher (few GHz) frequencies the production of “bubbles” in ionosphere densities in equatorial regions of the Earth can be a prime source of scintillations in satellite-toground signals. Engineers at the COMSAT Corporation discovered these effects after the deployment of the INTELSAT network at geosynchronous orbit [Taur, 1973]. This discovery is an excellent example of the surprises that the solar-terrestrial environment can hold for new technologies and for services that are based upon new technologies.

A major applications satellite program (C/NOFS), scheduled for launch in 2006, has been designed by the U.S. Department of Defense to explicitly study the causes and evolutions of the processes that produce equatorial region bubbles, and to examine means of mitigation. Disturbed ionosphere currents during geomagnetic storms can also be the cause of considerable problems at all geomagnetic latitudes in the use of navigation signals from the Earth-orbiting Global Positioning System (GPS), which provides precise location determination on Earth.

These ionosphere perturbations limit the accuracy of positional determinations, thus presently placing limits on some uses of space-based navigation techniques for applications ranging from air traffic control to ship navigation to many national security considerations. The future European Galileo Navigations Satellite System (GNSS) will also have to take into account ionosphere disturbances in order to ensure its successful operations.

Ionosphere and Earth Currents

The basic physical chain of events behind the production of large potential differences across the Earth’s surfacebegins with greatly increased electrical currents flowing in the magnetosphere and the ionosphere. The temporal and spatial variations of these increased currents then cause large variations in the time rate of change of the magnetic field as seen at Earth’s surface.

The time variations in the field in turn induce potential differences across large areas of the surface that are spanned by cable communications systems (or any other systems that are grounded to Earth, such as power grids and pipelines).

Telecommunications cable systems use the Earth itself as a ground return for their circuits, and these cables thus provide highly conducting paths for concentrating the electrical currents that flow between these newly established, but temporary, Earth “batteries”.

The precise effects of these “anomalous” electrical currents depend upon the technical system to which the long conductors are connected. In the case of long telecommunications lines, the Earth potentials can cause overruns of the compensating voltage swings that are designed into the power supplies [e.g., Anderson et al., 1974] that are used to power the signal repeaters and regenerators (the latter in the case of optical transmissions). Major issues can arise in understanding in detail the effects of enhanced space-induced ground electrical currents on cable systems. At present, the time variations and spatial dependencies of these currents are not well understood or predicable from one geomagnetic storm to the next. This is of considerable importance since the induced Earth potentials are very much dependent upon the conductivity structure of the Earth underlying the affected ionosphere regions. Similar electrical current variations in the space/ionosphere environment can produce vastly different Earth potential drops depending upon the nature and orientation of underground Earth conductivity structures in relationship to the variable overhead currents.

Modeling of these effects is becoming advanced in many cases. This is an area of research that involves a close interplay between space plasma geophysics and solid Earth geophysics, and is one that is not often addressed collaboratively by these two very distinct research communities (except by the somewhat limited group of researchers who pursue electromagnetic investigations of the Earth).

Ionosphere Activities due to Solar Radio Emissions

Solar radio noise and bursts were discovered more than six decades ago by Southworth [1945] and by Hey [1946] during the early research on radar at the time of the Second World War. Solar radio bursts produced unexpected (and initially unrecognized) jamming of this new technology that was under rapid development and deployment for war-time use for warnings of enemy aircraft [Hey, 1973].

Extensive post-war research established that solar radio emissions can exhibit a wide range of spectral shapes and intensity levels [e.g., Kundu, 1965; Castelli et al., 1973; Guidice and Castelli, 1975; Barron et al., 1985], knowledge of which is crucial for determining the nature and severity of solar emissions on specific technologies such as radar, radio, satellite ground communications receivers, or civilian wireless communications.

Research on solar radio phenomena remains an active and productive field of research today [e.g., Bastian et al., 1998; Gary and Keller, 2004]. Some analyses of local noon time solar radio noise levels that are routinely taken by the U.S. Air Force and that are made available by the NOAA World Data Center have been carried out in order to assess the noise in the context of modern communications technologies. These analyses show that in 1991 (during the sunspot maximum interval of the 22nd cycle) the average noon fluxes measured at 1.145 GHz and at 15.4 GHz were 162.5 and –156 dBW/(m2 4kHz), respectively [Lanzerotti et al., 1999]. These values are only about 6 dB and 12 dB above the 273º K (Earth’s surface temperature) thermal noise of -168.2 dBW/(m2 4kHz). Further, these two values are only about 20 dB and 14 dB, respectively, below the maximum flux of –142 dBW/(m2 4kHz) that is allowed for satellite downlinks by the ITU regulation RR2566. Solar radio bursts from solar activity can have much larger intensities.

As an example of an extreme event, that of May 23, 1967, produced a radio flux level (as measured at Earth) of 105 solar flux units (1 SFU = 10-22 W/(m2 Hz)) at 1 GHz, and perhaps much larger [Castelli et al., 1973]. Such a sfu level corresponds to –129 dBW/(m2 4kHz), or 13 dB above the maximum limit of –142 dBW/(m2 4kHz) noted above, and could cause considerable excess noise in any wireless cell site that might be pointed at the Sun at the time of the burst. An example of a portion of a study of solar burst events that is directed towards understanding the distributions of events that might produce severe noise in radio receivers is shown in Figure 8 [Nita et al., 2004]. Plotted here is the cumulative distribution of intensities of 412 solar radio bursts measured in 2001-2002 (during the maximum of the 23rd solar cycle) at a frequency of 1.8 GHz at the NJIT Owens Valley Solar Array.

The exponent of a power law fit to the distribution is shown; the roll-over of the distribution at the lowest flux density is believed to be a result of decreased instrument sensitivities at the very lowest levels.

Using such distributions, and taking into account the time interval over which the data were acquired, the probability of a burst affecting a specific receiver can be estimated. Bala et al. [2002], in an analysis of forty years of solar burst data assembled by the NOAA National Geophysical Data Center, estimated that bursts with amplitudes 103 solar flux units (sfu) at f ~ 1 GHz could cause potential problems in a wireless cell site on average of once every three to four days during solar maximum, and perhaps once every twenty days or less during solar minimum.

Reference : Louis J. Lanzerotti, "Space Weather effects on communications", pp247-250, Springer, 2007