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  • Communications Effects of Space Weather
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  • Ionosphere Activities due to Solar radio emissions

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.

- Space Weather Effects on Communications -