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Solar Radio Burst Effects on Wireless Systems

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Solar Radio Burst Effects on Wireless Systems 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 BURSTS AND 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.
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