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 First observations of intense GPS L1 amplitude scintillation activity in the midlatitude ionosphere at latitudes corresponding to the northeastern United States have been made. Moderate to severe, these scintillations result from space weather effects due to a disturbed ionosphere. Moderate to severe scintillations can degrade or even disrupt communication and navigation systems relying upon transionospheric radio wave propagation. A modified GPS receiver was used to record GPS satellite signal strength at Cornell University (53.2° magnetic latitude) during a magnetospheric disturbance on September 25–26, 2001 from 0000–0400 UTC. This disturbance (Kp = 6, minimum Dst = −110 nT) prompted the ionospheric trough to move equatorward over the northeastern U.S. and produced large plasma densities and gradients attributed to storm-time effects. This disturbance resulted in intense L-band amplitude scintillations (≥20 dB, S4 ≈ 0.8) which are highly uncharacteristic at this magnetic latitude. Concurrent measurements of TEC showed steep density gradients (∼30 TEC/deg) and evidence of irregularity structuring.
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 Radio wave scintillation studies of the ionosphere are one method of sensing ionospheric irregularities, primarily in the E and F regions [Yeh and Liu, 1982]. Scintillations occurring at many different frequencies have been studied in equatorial regions [Aarons et al., 1983], where a Rayleigh-Taylor instability is the primary instability source, and auroral regions [Basu et al., 1983; Coker et al., 1995; Aarons and Lin, 1995], where structured F-region density gradients are the primary instability source [Basu et al., 1993]. Contrary to these extremes, the midlatitude ionosphere is generally regarded as a less active scintillation environment and, at L-band frequencies, is considered to be absent of scintillations, lacking the necessary mechanisms commonly required to produce irregularities. Generally, both large ionospheric densities and the presence of irregularities are required to produce scintillations, with lower frequencies being more severely affected. For example, VHF signals are more vulnerable to ionospheric scintillations than L-band signals. On the other hand, it is known that the midlatitude region, at magnetic latitudes corresponding to the northeastern United States, is subject to severe F-region electron density structuring due to the space weather effects of magnetospheric disturbance electric fields [Foster, 1995, and references therein]. Recent studies indicate that the midlatitude region is even more complicated than previously thought, with many different scales of structuring possible [Kelley et al., 2000; Makela et al., 2001]. This paper provides evidence of moderate to severe scintillations of Global Positioning System (GPS) L1 signals occurring at midlatitude and shows that the occurrence of such scintillations is associated with the presence of the ionospheric trough and storm-enhanced plasma density.
 The term “scintillation” is used in a variety of contexts to refer to different physical phenomena producing a variety of effects on transionospheric signals. In this paper scintillation refers to diffraction which produces amplitude fluctuations on the ground. For a wave to experience diffractive effects, it must passthrough structures on the order of the Fresnel radius, , where λ is the wavelength and z is the minimum distance from the structures to the observation point. In the case of ionospheric scintillations, irregularities of a particular size (∼250 m at 1.57 GHz) are required to cause a transionospheric wave's amplitude to fluctuate at some observation point on the ground. With a GPS satellite signal as the source, these fluctuations are viewed as temporally and spatially varying diffraction patterns, both meridionally and zonally correlated, that traverse across the ground at some velocity related to the ionospheric drift velocity and the signal ionospheric intersection point velocity [Beach, 1998; Kintner et al., 2001].
 The nighttime F-region trough is well known to move equatorward during times of geomagnetic activity and to be a source of ionospheric irregularities, producing scintillations on both radio stars and satellite signals at HF and VHF frequencies [see, for example, Kersley et al., 1972; Nichol, 1973; Kersley et al., 1975; Schunk et al., 1976]. Subsequent studies demonstrated that VHF and UHF scintillations were frequently associated with the edges of the ionospheric trough where density gradients were the largest [Bowman, 1991; Bishop et al., 1993]. The most intense midlatitude events typically occurred during geomagnetic storms where detailed studies indicated complex structures with rapid temporal variations [Bust et al., 1997]. Satellite studies suggest that the complexity is produced by a combination of so-called penetrating electric fields associated with an asymmetric ring current and particle precipitation [Burke and Maynard, 2000]. These studies suggest both that the midlatitude ionosphere is more active than currently appreciated and that the ionospheric/magnetospheric processes producing the midlatitude structure are not understood. This paper will contribute to the former assertion and motivate more attention to the latter assertion.
 In a recent comprehensive survey using incoherent scatter radar during geomagnetic storms, Vo and Foster  showed that the ionospheric trough moves over the northeastern United States, while storm-enhanced density (SED) abutting the equatorward edge of the trough can increase TEC by a factor of 10. Associated with the trough are large electron density gradients (up to 50 TEC/deg), also occurring at midlatitudes during magnetospheric disturbances. Electron density gradients in midlatitude regions are enhanced by two main factors. First, southward movement of the ionospheric trough toward the higher density midlatitude region occurs during large magnetic Kp. Second, abutment of SED against the equatorward edge of the trough yields large, latitudinally aligned electron density gradients in that region during the post-sunset sector. These gradients and the large velocity shears associated with subauroral ion drifts (SAID) may be susceptible to instabilities that produce irregularities.
 Scintillations can cause disruptions in communication and navigation systems that rely upon satellite communications. Effects on these systems range from sporadic availability of satellite phone access [de Paula et al., 1999] to GPS satellite loss of lock [Kintner et al., 2001]. These disruptions, occurring in regions close to large metropolitan areas, have potentially profound effects on the actual and perceived performance and reliability of these systems.
 A modified single frequency (L1) GPS receiver, developed at Cornell University [Beach, 1998] and ported to RTLinux [Ledvina, 2000], was used to measure and record the received instantaneous power (at 50 Hz) and phase (at 10 Hz) of GPS satellite signals. The signal power is the sum of the squares of the in-phase and quadrature components, while the phase is the angle subtended by the in-phase and quadrature components. Scintillations in power and phase appear to a stationary receiver as a time-varying diffraction pattern traversing the ground. With regard to any discrepancies between the terms “amplitude scintillations” and “power scintillations,” in this paper they both refer to observations of fluctuations in the measured signal power. From this point on, all references to fluctuations in observed signal power will be denoted as amplitude scintillations. Also measured, using a dual frequency (L1/L2) GPS receiver, was the path-integrated electron density, or total electron content (TEC). TEC measurements are determined by computing the relative phase and code delays of the received L1 and L2 signals inflicted by the electron plasma [e.g., Lanyi and Roth, 1988]. Typical satellite geometry allows for 6–8 satellites in view at any given time. All measurements were taken at Cornell University at 42.4°N latitude and 283.5°E longitude (53.2°N magnetic latitude and 359.5°E magnetic longitude) during the night spanning September 25–26, 2001. The single frequency GPS receiver recorded scintillations and a Novatel dual frequency GPS receiver measured TEC.
 One indicator of the presence and intensity of scintillations is the S4 scintillation index, which is defined as the normalized (to unity) standard deviation of the received signal power. On September 25–26, S4 indices reached a peak value of approximately 0.8, indicating strong to severe scintillation activity. The top panels in Figure 1 show the S4 show index for four different satellites in view during this evening. These four satellites were chosen because they exhibited the largest S4 indices. The satellites were in the field of view at different times and travelled in opposite magnetic directions: satellites 3 and 31 travelled magnetically northward, while 8 and 27 travelled magnetically southward. The lower panels show the TEC values for the same four satellites. The dashed lines show the quiet-time values for both S4 and TEC. Clearly, this was a period of enhanced TEC (3–10 times larger), steep density gradients, and strong amplitude scintillations on multiple satellite signals.
 TEC measurements can provide insight into the presence of irregularities associated with scintillations [Beach, 1998]. Previous work shows that highly varying TEC profiles are associated with structures that can cause scintillations at the L band in equatorial regions [Bhattarcharyya, 2000]. The bottom parts of each plot in Figure 1 show the independently measured TEC for each satellite. It is apparent from these plots that the largest scintillations are collocated with steep gradients in TEC. To emphasize this relationship, Figure 2 shows the signal power from satellite 31 in the middle panel and TEC in the bottom panel. The maximum amplitude of power variation is nearly 20 dB, which results in occasional loss of lock of the GPS signal when it falls below the noise floor. Notice that the onset of scintillations corresponds exactly to the steep TEC gradient.
 In order to provide perspective as to the geographic location of these electron density gradients and scintillations, Figures 3 and 4 show ionospheric puncture point tracks at an altitude of 350 km where the gray-scale swaths indicate the S4 index and TEC, respectively. In Figures 3 and 4 each “X” and the adjacent number represent the location of a GPS satellite and the time in UTC hours, respectively. The four satellites corresponding to the tracks each have the label “SV” and the satellite number. The label “ITH” represents the location of Cornell University. From the plots, these scintillations obviously affected a wide area over the northeastern U.S., including major metropolitan areas.
Figure 4 depicts a trough-like structure with enhanced electron density at the equatorward edge, which is consistent with previous observations of the ionospheric trough at midlatitudes [Evans et al., 1983]. Indeed, our case looks similar to work by Vo and Foster , where the ionospheric trough relocated southward with adjacent SED at its equatorward edge.
 The top panel of Figure 2 shows Dst during the entire two-day span of September 25–26. The shaded region on the top panel represents the time span of the plots in the middle and bottom panels which show signal power and TEC, respectively. The minimum Dst at −110 nT indicates are latively moderate geomagnetic storm. Initiation of the scintillation event coincides with the main phase of the geomagnetic storm near the minimum in Dst and after local sunset, suggesting a possible relationship between the scintillation activity and the timing of the storm. In a study of the effects of two geomagnetic storms at midlatitude, Basu et al.  noted similar coordination between post-local sunset storm activity and scintillations, albeit at frequencies lower than L-band.
 Most of the scintillation activity, and certainly the strongest cases, exist on or near steep electron density gradients. In order for scintillations to occur, irregularities with a scale size on the order of the Fresnel radius (250 m) are required. The role of the aforementioned magnetospheric disturbance in generating and sustaining these structures on or near the gradients is not obvious. Furthermore, it is not known if these scintillations exist during daytime hours, since our data spans only from dusk to dawn. Another, more general, study is in order. It is likely that midlatitude scintillations are more common than previously thought, noting that Doherty et al.  recently demonstrated measurements of weak L-band midlatitude scintillations. However, the necessary conditions for developing intense scintillations may be so exacting that only rare occurrences are possible. These questions, along with a more thorough analysis of our data, including the amalgamation of other data, will be tackled in future work.
 We have observed moderate to severe GPS L1 amplitude scintillations in the midlatitude region where they were previously thought to be nonexistent. The combination of the equatorward movement of the ionospheric trough, along with a storm time-enhanced density (SED) disturbance, plays a major role in the formation of irregularities that cause scintillations. Due to the presence of scintillations, communication and navigation systems relying upon transionospheric propagation may experience varying degrees of degradation in performance and reliability.
 This research was supported by ONR grant number N00014-92-J-1822. JJM is funded in part by a National Science Foundation Graduate Research Fellowship.