Solar Radio Burst Effects on Wireless Systems

Dale E. Gary Center for Solar-Terrestrial Research Physics Dept., New Jersey Institute of Technology 323 M L King Jr Blvd, Newark, NJ, USA [email protected]

Abstract—The Sun is capable of producing strong radio However, even more extreme events are possible, as shown by emission during times of flaring activity that can directly affect the events of December 2006, two of which are shown in red wireless communication and navigation systems without warn- in Fig. 1 (see [8]). ing. We give some examples of specific effects that have been documented, including increased dropped-call levels on cellular telephone systems and system-wide interference on the Global Positioning System (GPS). To assess the potential and degree of risk presented by radio outbursts from the Sun, we survey what is known about the frequency of occurrence of solar bursts as a function of frequency and time. We show that a firm assessment of risk remains unknown due to lack of complete coverage in the monitoring of solar bursts, but we present some expectations based on current knowledge. We conclude that effects on wireless systems in space are likely to occur, but can be mitigated by considering and accounting for solar burst properties.

I.INTRODUCTION The Sun undergoes a regular activity cycle of 11 years average duration. Its most recent cycle of activity, solar cycle 23, peaked in the year 2000, and hence is the first solar max- imum since wireless systems became prevalent. The current solar cycle 24 was expected to peak in 2011, but the most recent minimum has been extended well beyond the typical duration[1], and the next peak is now expected to be delayed until 2013[2]. Therefore, we can expect an increased level of solar activity for at least the next four years. During solar cycle 23, a number of effects on wireless communication and Fig. 1. Radio flux density of the Sun and galactic background, over 5.5 navigation systems has been documented. These include the decades in frequency. The levels in black and gray are from circa 1985, and relatively well-known effects of solar X-ray and EUV flux on the Radio Solar Telescope Network (RSTN) saturation levels (blue) were set according to these expectations. However, in December 2006, bursts were the Earth’s atmosphere and ionosphere, as well as magnetic observed (red) that exceeded these saturation levels by more than an order of storm effects due to the interaction of Coronal Mass Ejections magnitude. Figure adapted from [3]. (CMEs) with the Earth’s magnetosphere. This paper, though, describes new Space Weather effects discovered during the In contrast to the typical burst events, these extreme events previous solar maximum, the direct radio frequency interfer- can cause widespread outages. The 2006 December 06 event ence on wireless communication and navigation systems by has been documented to have caused an outage of Global solar radio bursts. Positioning System (GPS) navigation services over the entire Figure 1 summarizes the radio flux density (in units of sunlit hemisphere of the Earth for a period of at least 10 W m−2 Hz−1) output by the Sun under various conditions. minutes[9], [8], [10], [11]. In section II, we describe the The Quiet Sun is the strongest natural radio source in the characteristics of the bursts that give rise to radio frequency sky at frequencies above about 300 MHz, yet its non-flaring interference on wireless systems, and establish the flux density output is far below that expected to cause problems with levels that are likely to cause problems. In section III, we give wireless systems. The gray band in Figure 1 (adapted from [3]) an overview of the occurrence rate of solar bursts as a function represents the highest flux density levels achieved during solar of frequency and time (within the solar cycle) as currently bursts, as they were known circa 1985. As first discovered known, and discuss how complete is our knowledge of burst during solar cycle 23, these levels are sufficient to cause occurrence and what means for risk assessment. We conclude occasional radio frequency interference effects on cellular in section IV with some possible ways to mitigate the effects telephone systems ([4], [5], [6]) and navigation systems ([7]). of solar bursts on wireless systems in space and on the ground.

978-1-4577-0811-4/11/$26.00 ©2011 IEEE 661 II.SOLAR RADIO BURSTSAND FLUX DENSITY LIMITS 15 a) RCP Solar radio bursts come in a great variety of strengths (flux 10 densities), durations, time and frequency behavior. For radio 7 5 frequency interference, typically it is the instantaneous flux 3 2 density (spectral power per unit area) that is of concern, with Frequency [GHz] 1.2 18:40 18:50 19:00 19:10 19:20 19:30 19:40 19:50 20:00 the duration a secondary concern. Typical peak flux densities Time [UT] range from a few solar flux units (1 sfu = 10−22 W m−2 Hz−1) to perhaps 30,000 sfu for a large burst. Bursts are also 15 b) LCP due to several different emission mechanisms, the two most 10 7 important of which are gyrosynchrotron (GS) emission (an 5 incoherent mechanism involving gyration of mildly-relativistic 3 2 Frequency [GHz] 1.2 electrons in the solar magnetic field) and plasma emission (a 18:40 18:50 19:00 19:10 19:20 19:30 19:40 19:50 20:00 family of coherent processes in which electrons accelerate Time [UT] coherently due to interactions with waves in the coronal plasma). As we will discuss further in section III, these Fig. 2. OVSA dynamic spectrum of the 2006 Dec 06 event. a) right circular polarization (RCP). b) left circular polarization (LCP). The flux density scale two emission mechanisms tend to dominate on either side in the plot ranges logarithmically from 1 to 104 sfu, so the very bright RCP of a frequency dividing line of around 2-3 GHz, with GS L-band emission (1-2 GHz) is saturated on this scale and appears white. emission at higher frequencies and plasma emission at lower True saturation (see text) causes the data to be flagged as bad data, and appears black in the figure, which is especially noticeable at 1.2 GHz. The frequencies. The extreme burst of 2006 December 06 provides brightest L-band RCP emission occurs between 19:30 and 19:40 UT, and a good example of this dichotomy, and is a good example causes instrumental artifacts at higher frequencies (between 15-18 GHz, and of general behavior for a large burst. Figure 2 shows the at times at other frequencies as well). 1.2-18 GHz dynamic spectrum (frequency-time plot) in the two senses of circular polarization, obtained with the Owens Subsequent studies ([13], [14], [6]) have used the limit Valley Solar Array (OVSA). The red color scale shows the of 1000 sfu for characterizing the typical limit above which flux density scaled logarithmically. The nearly unpolarized GS cellular telephone systems may begin to be affected. Obviously emission is smoothly varying in the range 2-18 GHz, while different systems will have different sensitivities to solar radio the rapidly fluctuating emission below 2 GHz is the coherent bursts. Reference [13] examined 40 years of solar burst records plasma emission (actually due to the electron-cyclotron maser to determine the occurrence rate of bursts above any limiting mechanism[12]). The plasma emission is so strong in right- flux density, and some results of that study will be described circular polarization (RCP) that if the flux density were scaled in more detail in section III. linearly the GS emission would be too weak to see relative to the plasma emission. The fluctuations seen in left-circular Likewise, the potential for solar radio bursts to affect GPS polarization (LCP) are in fact simply cross-talk from the RCP navigation signals was first discussed by [15], who calculated channel, and the coherent emission is consistent with being that for a typical L1 C/A code GPS receiver with a 1 dB gain 100% polarized. We will come back to this point in section antennas, the noise floor is roughly equivalent to 20,000 sfu. IV. The first detection of an effect confirmed to be due to a solar The question remains, what flux density level can cause an burst was given by [7]. In that case, a receiver with a 4 dB C/N effect on wireless systems. A complete answer to this question gain saw a 2.3 dB reduction in carrier-to-noise ( o) ratio requires detailed knowledge of the system in question. How- due to a solar burst with 8,700 sfu of RCP flux density. It is ever, one way to answer is to investigate actual occurrences. important to note that only the RCP flux density is relevant, For cellular telephone systems, [4] first reported a greatly because GPS broadcasts only right-circular polarization. Both enhanced dropped call rate averaged over all base stations for [15] and [7] concluded that there would be a low likelihood the east-facing links in a major US state during local sunrise of serious effects on GPS from solar radio bursts, due to what (the north and west-facing links were normal). The Sun on proved to be an incomplete record of solar burst flux density. the day in question was undergoing flaring activity, and it is Within months of publication [7], the Sun produced the 2006 supposed that the Sun was within the beam of the east-facing December record-setting bursts mentioned earlier. antennas. A simple analysis[5] shows that the receiver noise III.SOLAR BURST OCCURRENCE STATISTICS at a system temperature of T = 300 K for a bandwidth of 30 kHz is To gain an idea of how often bursts of a given flux density kT B ≈ −129 dBm. occur, [13] studied 40 years of solar radio burst data collected from observatories world-wide during 1960-1999, as compiled The equivalent solar flux density, Feq for a typical cellular by the National Oceanic and Atmospheric Administration base station operating at 900 MHz, and a single polarization (NOAA). They obtained powerlaw fits to the number of bursts antenna of gain G = 10 is per unit flux density range (dN/dS in events sfu−1) and the 8πkT corresponding fits to the cumulative number N(> S), where F = ≈ 960 sfu. eq Gλ2 S is the flux density. There were a sufficient number of bursts

662 (518,606 entries covering 155,396 events) that the data could noise floor corresponding to 100 sfu, will be operating during be fit separately within different ranges of frequency, time, and years near solar maximum, and will operate at a frequency of phase of the solar cycle. An example is shown in Figure 3. 1.4 GHz. Table 3 of [13] gives the parameters for this case as N(> 1 sfu) = 77377 events in 4383 days, λ = −1.78, and Cgeo = 1.9. Then the number of bursts per day S > 100 sfu is given by

N(S >100 sfu, ν = 1.4 GHz) (1) (77377) 100−1.78+1 = 1.9 4383 days = 0.923 events day−1.

This would provide an estimate of how often a solar burst might occur, on average, with this flux density. If the 100 sfu limit only applies when a high-gain antenna is pointing at the Sun, then an evaluation of how often that may occur should be made and applied to the above result. In this manner, [13] provides a best guess as to risk of impact on any given system due to solar bursts. Unfortunately, as was mentioned before, the 40-year record Fig. 3. Histograms of cumulative number of bursts above the flux density of bursts appears to undercount the largest bursts, which given on the abscissa, for all frequencies above 2 GHz, shown separately for bursts occurring (a) near solar maximum and (b) near solar minimum. probably carry the largest risk for some systems with low- Also shown is a powerlaw fit with the parameters of intercept (N > 1 sfu) gain antennas (e.g. the GPS system). After the 2006 December and powerlaw index (λ) shown as text in each panel. Cgeo is a geographic bursts occurred, which far exceeded the largest previously correction factor for missing bursts, which, when applied, shifts the fit to that shown by the dotted line in each panel. From [13]. reported burst, it became clear that the problem was with the monitoring stations, in particular the Radio Solar Telescope In [13], evidence was also given that the 40-year record of Network (RSTN) operated by the US Air Force. As shown bursts was incomplete in several respects. One way in which by the blue lines in Fig. 1, the saturation level of RSTN the record is incomplete is in uneven coverage over the various was set according to the canonical expectation of maximum longitudes of Earth. The participating observatories changed flux of bursts, i.e. 100,000 sfu at 1.4 GHz, 50,000 sfu above over time, and some longitudes reported fewer bursts at times 1.4 GHz, and 500,000 sfu below 1.4 GHz. In addition, [14] and than others. This is a relatively minor problem, since we can [6] studied a uniform dataset of radio bursts from the OVSA assume that bursts of all sizes are missed equally so that the array, and further explained the clear differences in statistical distribution of bursts is not affected, hence can be corrected behavior above and below the 2-3 GHz band discussed in by a simple multiplicative geographic correction factor Cgeo section II. In searching further ([16]), it has become clear as shown in Fig. 3. Another minor, unavoidable problem is that the 100,000 sfu saturation limit for RSTN is responsible the undercounting (roll-over) seen at the small-burst (left) end for many of the missing bursts. In addition, for GPS effects of the distributions in Fig. 3. This is due to the difficulty of at least, the data are not complete due to the fact that it is identifying small bursts, and it is obvious from the plots that only the RCP emission that is of concern, whereas RSTN such incompleteness begins to set in below about 20 sfu. A measures only total intensity (Stokes I). For this reason, more serious problem occurs near the large-burst (right) end additional measurements with a better world-wide monitoring of the distributions. In both distributions it is obvious that system are needed in order to assess the true risk to wireless the number of the largest bursts falls below the powerlaw systems. Meanwhile, about the best one can do is use the extrapolation, with a limit of around 105 sfu at solar maximum burst occurrence distributions in [13] and [14], and extrapolate and 20,000 sfu at solar minimum. We will come back to this them at face value. When this is done, bursts the size of the in a moment, but first, we mention some other findings of the record 2006 December 06 event (marked in red in Fig. 1, work of [13]. which exceeded one million sfu) are found to occur only once Note that the powerlaw index, λ ≈ −1.8 is the same for both every 3600 days at solar maximum (i.e. once in 10 years). distributions. It turns out that the slopes of the distributions are Although rare, such extreme bursts can have a very wide remarkably constant regardless of frequency or phase of the impact, as in the case of the 2006 December 06 event that cycle. Nevertheless, [13] tabulated the slopes and intercepts caused loss of navigation for GPS receivers over the entire for 8 frequency ranges and 6 time ranges, to allow engineers sunlit hemisphere of Earth (e.g. [9], [11], [16]). In this case, to use the data for assessing risk to any system they may the outage lasted about 10 minutes. Impacts on other wireless wish to study. As a concrete example, say a system with a systems were not reported, but in many cases they may not

663 be apparent, or can be attributed to other causes. not suffer from saturation, and has the capability of making measurements in both RCP and LCP polarizations. IV. CONCLUSION ACKNOWLEDGMENT As we have seen, solar radio emission from flares occurs This work is supported by NASA grant NNX11AB49G across the entire radio spectrum, and on rare occasions can and NSF grant AST-0908344 to New Jersey Institute of reach extreme flux densities that can and do cause radio fre- Technology. quency interference to wireless communication and navigation systems. Emission above about 2 GHz is generally due to REFERENCES incoherent gyrosynchrotron emission from electrons trapped [1] S. R. Cranmer, J. T. Hoeksema, & J. L. Kohl, Ed., SOHO-23: Under- in magnetic loops, and reaches 30,000-50,000 sfu about once standing a Peculiar Solar Minimum, ser. Astronomical Society of the per year during solar maximum. Emission below 2 GHz can Pacific Conference Series, vol. 428, Jun. 2010. [2] D. Biesecker. (2009, May) Solar Cycle 24 Pre- sometimes be due to coherent plasma processes, and have diction Updated May 2009. [Online]. Available: reached more than 1 million sfu. The rate of occurrence of http://www.swpc.noaa.gov/SolarCycle/SC24/index.html these bursts is unknown due to the saturation limit of the [3] G. A. Dulk, The solar atmosphere, solar magnetism and solar activity. CSIRO, 1985, pp. 19–35. RSTN patrol instruments. [4] L. J. Lanzerotti, D. J. Thomson, and C. G. Maclennan, Engineering Bursts exceeding about 1000 sfu can cause problems for Issues in Space Weather. New York: Wiley/IEEE, 1999, pp. 25–50. cell phone towers whose link antennas are pointed at the Sun [5] B. Bala, L. J. Lanzerotti, D. E. Gary, and D. J. Thomson, “Noise in wireless systems produced by solar radio bursts,” Radio Science, vol. 37, at sunrise and sunset. Although the probability is small for any no. 2, p. 1018, Mar. 2002. given tower, it is always sunrise and sunset somewhere in the [6] G. M. Nita, D. E. Gary, and L. J. Lanzerotti, “Statistics of solar world, and the effects can be widespread. Bursts exceeding microwave radio burst spectra with implications for operations of microwave radio systems,” Space Weather, vol. 2, p. S11005, Nov. 2004. 20,000 sfu have caused measurable effects for GPS receivers, [7] A. P. Cerruti, P. M. Kintner, D. E. Gary, L. J. Lanzerotti, E. R. de Paula, and a burst of > 1 million sfu resulted in a world-wide outage and H. B. Vo, “Observed solar radio burst effects on GPS/Wide Area of GPS navigation. Augmentation System carrier-to-noise ratio,” Space Weather, vol. 4, p. S10006, Oct. 2006. In designing a system, some things can be done to reduce [8] D. E. Gary, “Cause and extent of the extreme radio flux density reached risk. Certainly if a high-gain antenna is involved, care should by the solar flare of 2006 December 06,” in Proc. 12th International be taken that it does not point directly at the Sun. If the fre- Ionospheric Effects Symposium, Alexandria, VA, May 2008, pp. 18–25. [9] A. P. Cerruti, P. M. Kintner, D. E. Gary, A. J. Mannucci, R. F. Meyer, quency is adjustable, a frequency above 2 GHz (and preferably P. Doherty, and A. J. Coster, “Effect of intense December 2006 solar above 3 GHz) should be chosen. Since coherent solar radio radio bursts on GPS receivers,” Space Weather, vol. 6, p. S10D07, Oct. bursts are often highly polarized, a system with switchable 2008. [10] E. L. Afraimovich, V. V. Demyanov, D. E. Gary, A. B. Ishin, and G. Y. polarization may offer some lowered risk. The 2006 December Smolkov, “Failure of GPS functioning caused by extreme solar radio 06 event, for example, was 100% RCP. Systems operating events,” in Proc. 12th International Ionospheric Effects Symposium, on LCP would have continued to work without interference. Alexandria, VA, May 2008, pp. 10–17. [11] C. S. Carrano, C. T. Bridgwood, and K. M. Groves, “Impacts of the Even systems working on linear polarization would have seen December 2006 solar radio bursts on the performance of GPS,” Radio noise a factor of two lower in flux density. However, bursts Science, vol. 44, p. RS0A25, Aug. 2009. dominated by RCP and those dominated by LCP should [12] R. A. Treumann, “The electron-cyclotron maser for astrophysical ap- plication,” Astronomy and Astrophysics Reviews, vol. 13, pp. 229–315, occur with equal probability (it is set by the polarity of the Aug. 2006. magnetic field in the source region of the corona, which occurs [13] G. M. Nita, D. E. Gary, L. J. Lanzerotti, and D. J. Thomson, “The Peak with equal probability on the Sun), so to lower risk using Flux Distribution of Solar Radio Bursts,” Astrophysical Journal, vol. 570, pp. 423–438, May 2002. polarization, one would have to build redundant systems in [14] G. M. Nita, D. E. Gary, and J. Lee, “Statistical Study of Two Years of each polarization, able to switch from one to the other on Solar Flare Radio Spectra Obtained with the Owens Valley Solar Array,” short notice. Astrophysical Journal, vol. 605, pp. 528–545, Apr. 2004. [15] J. A. Klobuchar, J. M. Kunches, and A. J. Van Dierendonck, “Eye on the In conclusion, when considering the use of wireless systems ionosphere: Potential solar radio burst effects on GPS signal to noise,” for mission-critical operations, all sources of noise in the GPS Solutions, vol. 3(2), pp. 69–75, Mar. 1999. space environment should be considered. Solar radio bursts [16] P. M. Kintner, B. O’Hanlon, D. E. Gary, and P. M. S. Kintner, “Global Positioning System and solar radio burst forensics,” Radio Science, constitute one source of noise that, while perhaps rare, can vol. 44, p. RS0A08, Jun. 2009. affect a very wide region of space. To better characterize the occurrence rate of the largest solar radio bursts, a better world- wide solar radio monitoring system is needed—one that does

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