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navigation links: current status and future plans
Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1745 – 1754
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Speci'cation and forecasting of scintillations in communication=navigation links: current status and future plans S. Basua; ∗ , K.M. Grovesa , Su. Basub , P.J. Sultana a Air
Force Research Laboratory, VSBI, 29 Randolph Road, Hanscom AFB, MA 01731, USA Sciences Division, National Science Foundation, 4201 Wilson Boulevard, VA 22230, USA
b Atmospheric
Abstract The ionosphere often becomes turbulent and develops electron density irregularities. These irregularities scatter radio waves to cause amplitude and phase scintillation and a1ect satellite communication and GPS navigation systems. The e1ects are most intense in the equatorial region, moderate at high latitudes and minimum at middle latitudes. The thermosphere and the ionosphere seem to internally control the generation of irregularities in the equatorial region and its forcing by solar transients is an additional modulating factor. On the other hand, the irregularity generation mechanisms in the high-latitude ionosphere seem to be driven by magnetospheric processes and, therefore, high-latitude scintillations can be tracked by following the trail of energy from the sun in the form of solar 4ares and coronal mass ejections. The development of a global speci'cation and forecast system for scintillation is needed in view of our increased reliance on space-based communication and navigation systems, which are vulnerable to ionospheric scintillation. Such scintillation speci'cation systems are being developed for the equatorial region. An equatorial satellite equipped with an appropriate suite of sensors, capable of detecting ionospheric irregularities and tracking the drivers that control the formation of ionospheric irregularities, has also been planned for the purpose of specifying and forecasting equatorial scintillations. In the polar region, scintillation speci'cation and forecast systems are yet to emerge although modeling and observations of polar cap plasma structures, their convection and associated irregularities have advanced greatly in recent years. Global scintillation observations made during the S-RAMP Space Weather Month in September 1999 are currently being c 2002 Published by Elsevier analyzed to study the e1ects of magnetic storms on communication and navigation systems. Science Ltd. Keywords: Ionosphere; Irregularities; Polar and equatorial region; Scintillation; Speci'cation and forecasting
1. Introduction The earth’s ionized upper atmosphere often becomes turbulent and develops electron density irregularities. These irregularities scatter radio waves from satellites in the frequency range of 100 MHz–4 GHz (Basu et al., 1988; Aarons, 1993; Aarons and Basu, 1994). In the presence of
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a relative motion between the satellite, the ionosphere and the receiver, the received signal exhibits temporal 4uctuations of intensity and phase, called scintillations. Intensity scintillations cause signals to fade below the average level. When the depth of fading exceeds the fade margin of a receiver, the signal becomes buried in noise and signal loss and cycle slips are encountered. Phase scintillations induce a frequency shift and, when this shift exceeds the phase lock loop bandwidth, the signal is lost and the receiver spends valuable time reacquiring the signal. Overall, in the presence of scintillations, the performance of communication and navigation systems is degraded.
c 2002 Published by Elsevier Science Ltd. 1364-6826/02/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 2 ) 0 0 1 2 4 - 4
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Scintillations are strong at high latitudes, weak at middle latitudes and intense in the equatorial region (Basu et al., 1988). Scintillation at all latitudes attain its maximum value during the solar maximum period when the F-region ionization density increases and the irregularities occur in a background of enhanced ionization density. In addition, high-latitude scintillations are enhanced during magnetic storms (Aarons, 1997; Aarons et al., 2000). As such, scintillation e1ects, expected to be encountered during the upcoming solar maximum period between 2001 and 2004, are of concern to systems engineers. At high latitudes, scintillations are found to be associated with large-scale plasma structures. Experimentally, the two states of the polar ionosphere controlled by the IMF, and their association with high-latitude large-scale plasma structures known as patches, blobs and sun-aligned arcs, have been discovered in the 1980s (Weber et al., 1984, 1986; Tsunoda, 1988; Carlson, 1994; Rodger et al., 1994; Basu and Valladares, 1999). More recently, the association of large-scale plasma structures with intermediate scale irregularities (tens of km to tens of meters), responsible for scintillations, has been examined in the framework of observations and modeling (Keskinen et al., 1988; Aarons and Rodger, 1991; Basu et al., 1995). At low latitudes, at the time of sunset, an enhanced eastward electric 'eld, called the pre-reversal enhancement, generally develops at F-region heights. As a result, the ionosphere moves upward, develops steep density gradients in the bottomside F-region and becomes unstable to the Rayleigh–Taylor instability (Kelley, 1989). Large-scale plasma depletions, called plasma bubbles, form in the bottomside of the F-region and become populated with small-scale irregularities as the bubbles rise to great heights. The small-scale irregularities cause intense scintillations of satellite signals along two belts of high ionization den◦ ◦ sity at 20 N and 20 S dip latitudes, called the equatorial anomaly (Basu et al., 1988). Scintillations at low latitudes, as in other regions, attain a maximum value during the solar maximum period owing to the increased value of the background ionization density. However, low-latitude scintillation is not fully dictated by solar transients, such as magnetic storms. In this region, intense scintillations are observed during magnetically quiet periods as well. When a satellite-based communication or navigation system operates in a scintillating environment and loss of signal occurs, an average user attributes the problem to interference or to failure of equipment either at the transmitting end, the receiving end or the satellite itself. If forecasts of scintillations can be provided, the users will not waste their resources and may instead evolve alternate strategies. The scintillation phenomenon has been studied for several decades and the climatology of scintillations, namely, its variation with latitude, season, local time, magnetic activity and solar cycle, is well documented. A robust climatological model, WBMOD, is also available (Secan et al., 1995, 1997). However, scintillations exhibit extreme variability in
space and time. As such, climatological models are useful for planning purposes only. For the support of space-based communication and navigation systems, we need weather models, i.e., real-time speci'cation and forecast models. This paper provides an overview of scintillations at high and low latitudes and considers issues related to the speci'cation and forecast of scintillation. A scintillation speci'cation system, developed for the South American sector, is outlined. The issues related to the speci'cation and forecast of polar scintillation is discussed. Also an equatorial satellite, called the Communication and Navigation Outage Forecast System (C=NOFS) which has been planned for specifying and forecasting scintillations in the equatorial region, is described. 2. Results and discussions Fig. 1 shows an updated schematic of the global distribution of worst-case scintillation at L-band frequencies during the solar maximum and the minimum period, originally published by Basu et al. (1988). At such high frequencies, scintillations do not readily get saturated and, as such, the variations in scintillation magnitudes can be well represented. The left-hand panel represents the solar maximum condition when scintillation attains its maximum value. The magnetic north and south poles are at the top and the bottom and the magnetic equator is in the middle. The noon meridian is on the left, midnight is on the right and 18 LT is in the middle. It may be noted that L-band scintillation is most intense in the equatorial region, moderate at high latitudes and generally absent at middle latitudes. The diagram represents winter conditions over the Northern Hemisphere and shows that the scintillations are considerably more intense over the winter polar cap when the sun remains below the horizon. The left panel of Fig. 1 shows that at high-latitudes scintillations, in general, occur over the nightside auroral oval and over the polar cap virtually at all local times. In the winter polar cap moderately strong L-band scintillation is observed in association with the so-called polar cap patches (Weber et al., 1984). When the interplanetary magnetic 'eld (IMF) is directed southward, patches with high-ionization density are observed to enter the polar cap from the dayside auroral oval, convect in the anti-sunward direction and eventually exit into the nightside auroral oval. One mechanism by which patch formation is achieved corresponds to changing the plasma convection pattern in response to the IMF By component during periods of southward Bz (Sojka et al., 1994). Among several other formation mechanisms, one mechanism that considers the role of large plasma 4ows in the formation of discrete patches has been experimentally substantiated (Rodger et al., 1994; Valladares et al., 1994). A recent study has shown a fairly remarkable similarity between polar cap scintillation observations caused by mesoscale irregularities (tens of km to tens of meter) and the Utah State University’s TDIM model predictions of
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Fig. 1. Schematic of the global morphology of scintillations at L-band frequencies during the solar maximum (left panel) and solar minimum (right panel) conditions. Reproduced from S. Basu and K.M. Groves, Speci'cation and forecasting of outages on satellite communication and navigation systems, Space Weather, Geophysical Monograph 125, 424–430, 2001. Published 2001 by the American Geophysical Union. Reproduced/modi'ed by permission of American Geophysical Union.
patches (Basu et al., 1995). The co-existence of macroscale and mesoscale plasma structures implies that intermediate scale structures causing scintillations of satellite signals develop at the edges of polar cap patches through the gradient drift instability mechanism and permeates the entire polar cap patch (Ossakow et al., 1978; Guzdar et al., 1998). A statistical study of kilometer scale irregularities, based on DE-2 measurements, shows their signi'cant summer=winter variation and that the strongest irregularity amplitude and velocity turbulence occur in the cusp region (Kivanc and Heelis, 1997). The polar cap ionosphere alternates between two states (Carlson, 1994) depending on whether the IMF is southward (Bz ¡ 0), as discussed above, or northward (Bz ¿ 0). For Bz northward conditions, plasma convection over the polar cap becomes structured and weak, stable sun-aligned arcs are observed. Fig. 1 illustrates in a cartoon form plasma structures over the polar cap that are elongated in the noon– midnight direction but narrow in the dawn–dusk direction and are characterized by their dawn–dusk motion. In general, sun-aligned arcs cause weak scintillations of satellite signals presumably because of the low ionization density in sun-aligned arcs. However, during the solar maximum period enhanced scintillations have been recorded in association with such arcs (Basu et al., 1990). A topical review on polar plasma structures has been published recently in this journal (Basu and Valladares, 1999). It has synthesized theory, observations, and simulations of polar cap plasma structures for Bz southward and northward as well as the transition state from southward to northward conditions when polar cap patches continue to be observed because of their 'nite lifetime.
With our current knowledge of polar cap plasma structures, associated scintillation causing irregularities and their control by the IMF, it seems that we are poised to develop a polar scintillation speci'cation and forecast system. As discussed in previous paragraphs, we have the basic framework for the development of such a system for polar cap patch associated scintillation, which is the most intense variety of polar scintillations. The factors that still need to be resolved relate to determining the trajectories of patches when the convection pattern varies in response to changes in the IMF and the lifetime of patches when the IMF changes from a southward to a northward orientation. Such attempts have recently been made (Crowley et al., 2000; Pedersen et al., 2000). From Fig. 1, it may be noted that most intense scintillations occur in the equatorial region where the onset of scintillation at all frequencies usually occur immediately after sunset. However, the decay time of scintillation varies with frequency. L-band scintillations decay around midnight whereas at lower frequencies, such as at 250 MHz, scintillation continue for several hours after midnight (not shown in Fig. 1). Equatorial scintillations vary with latitude with a minimum at the magnetic equator and are maximum around ◦ ◦ two belts at 20 N and 20 S magnetic latitudes called the crests of the equatorial anomaly. Fig. 2 is a schematic of equatorial electrodynamics. It illustrates that during the daytime, the dynamo electric 'elds at o1-equatorial E-region locations map along the magnetic 'eld to F-region heights at the magnetic equator and the resulting E × B (E being the electric 'eld and B the magnetic 'eld intensity) force lifts the F-region plasma to higher altitudes. Then, by the action of forces parallel to B due to
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Fig. 2. Illustration of the equatorial electrodynamics and the formation of the equatorial anomaly in F2 ionization density.
gravity and plasma pressure gradients, the uplifted plasma di1uses along the magnetic 'eld towards the north and south of the magnetic equator to form two belts of high ionization ◦ ◦ density at magnetic latitudes of 20 N and 20 S, the equatorial ionization anomaly (Hanson and Mo1et, 1966). At the time of sunset, the zonal neutral wind and conductivity gradient caused by the sunset terminator interact to develop an enhanced eastward electric 'eld on the dayside of the terminator and a westward electric 'eld on the nightside (Farley et al., 1986; Kelley, 1989; Haerendel and Eccles, 1992; Crain et al., 1993). The enhanced eastward electric 'eld, called the pre-reversal enhancement, causes the F-region to move upward, steepen the bottomside density gradient and to trigger the Rayleigh–Taylor instability. As a result, the low-density plasma from the bottomside intrudes into the topside ionosphere (Kelley, 1989) to form plasma bubbles that develop mesoscale irregularities and cause scintillation of satellite signals. The pre-reversal enhancement of eastward electric 'eld leads to the development of small-scale irregularities through the operation of a hierarchy of instabilities and seems to be an important source for the destabilization of the ionosphere as well as scintillation of satellite signals (Basu et al., 1996; Fejer et al., 1999). This electric 'eld also leads to a resurgence of the equatorial anomaly in the post-sunset period. As a result, scintillations at frequencies as high as 4 GHz can be encountered near the crests of the equatorial anomaly during the solar maximum since the intermediate scale irregularities occur in an environment of high ionization density in the anomaly region.
Fig. 3, for the equatorial anomaly region during a solar maximum period, shows that the magnitude of scintillation decreases with increasing frequency. Scintillation is quite intense, not only at 250 MHz but at L-band frequencies as well, and remains detectable even at frequencies as high as 4 GHz. Non-stationary signal intensity 4uctuations in the middle panel may arise from radio wave scattering by steep coherent electron density irregularity structures at F region heights. The equatorial irregularities evolve in longitudinally discrete (∼250 km in magnetically east–west direction) plasma bubbles that, on occasions, attain heights exceeding 1000 km at the magnetic equator. These altitudinally extended plasma bubbles map along magnetic 'eld lines to F-region heights at locations to the north and south of the magnetic equator. Near the crests of the equatorial anomaly, the overhead sky becomes interspersed with the footprints of longitudinally discrete and north–south elongated structures. These structures generally move in the eastward direction by the action of vertically oriented polarization electric 'eld (Rishbeth, 1971). The space-time variation of irregularity structures in plasma bubbles has been tracked near the anomaly region by performing simultaneous airglow and multi-point scintillation measurements by using signals from GPS satellites (Weber et al., 1996). Figs. 3 and 5 of that paper show, respectively, the signature of plasma bubbles in multiple 630 nm airglow depletion structures and portray how scintillations on di1erent GPS links vary as the ray paths intersect the depletions. The diagrams illustrate that the width and the altitude extent of plasma
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Fig. 3. Illustration of the variation of scintillation magnitude with frequency obtained from multi-frequency transmissions of the Marisat satellite on February 3 1981. The top panel shows intense intensity scintillations at 250 MHz, the middle panel shows strong scintillations at 1541 MHz and the bottom panel indicates weak activity at 3954 MHz. The data were recorded at Ascension Island, near the crest of the equatorial anomaly, during solar maximum. Note LT = UT − 1 h.
bubbles determine how many satellites su1er scintillation simultaneously. Fig. 4 shows the occurrence statistics of 1:5 GHz scintillation in the equatorial anomaly region during the pre-midnight period (20 –24 LT). The top panel refers to magnetically quiet periods (Kp = 0–3); the middle panel to active periods (Kp = 3+ –9) and the bottom panel show the sunspot number. It may be noted that scintillation occurrence increases from 1987 to 1989 as the sunspot number increases. The diagram indicates that there are no data during June–August 1988 but past data indicated that these months correspond to the seasonal minimum in scintillation occurrence when scintillations are virtually absent over the American-Atlantic longitude sector. It is also interesting to note that the occurrence is much enhanced during magnetically quiet periods (top panel). It seems that during magnetically quiet periods, the turbulence in the nighttime equatorial F region is driven internally and cannot be predicted by following the trail of energy from the sun that cause enhanced magnetic activity on earth. The average occurrence statistics of equatorial scintillation shown in Fig. 4 may be useful for planning purposes, but it is of limited value to the users because of the extreme day-to-day variability of equatorial scintillation. Fig. 5 illustrates the extent of the day-to-day variability of scintillation at 250 MHz recorded at Ancon, Peru, on the magnetic equator. The data show the daily variation of the S4 index, de'ned as the normalized standard deviation of signal intensity 4uctuations (Briggs and Parkin, 1963), during 24 –30 October, 1998, which corresponds to the month of maximum scintillation occurrence in Peru. There was no scintillation on three nights within a total of seven nights. All these days correspond to a magnetically quiet period with
Kp ¡ 20. Such variability of scintillation emphasizes the need for even a scintillation speci'cation over a limited area populated by users. Fig. 6 shows the output of the scintillation speci'cation system, Scintillation Network Decision Aid (SCINDA), which has been developed for the equatorial region in the American sector (Groves et al., 1997). The system combines scintillation magnitude and zonal drift measurements made at two sites, one near the magnetic equator and the other near the anomaly region, by using transmissions from two satellites one to the east and the other to the west of the stations. The data drives the equatorial scintillation model, which uses upwelling and zonal motion to produce three-dimensional scintillation structures. Fig. 6 shows the development of successive scintillation structures, elongated in the north– south direction by about 3000 km, and their movement in the eastward direction. The Internet brings this to the user. The increasing magnitude of scintillations is indicated by the green, yellow and red colors. In view of the eastward drift and few hours lifetime of scintillation structures, the SCINDA system is able to provide short-term forecasts of scintillation to users located east of the observing stations. Fig. 7 shows that DMSP satellites in polar orbit can measure the latitude variation of in situ electron density at 840 km. The satellite crosses the equator at 18 h local time, which is about one and a half hour prior to the onset of scintillation. The left-hand panel shows the latitude variation of electron density. The second and fourth diagrams in the left-hand panel, with 4at-topped latitude variations, indicate an enhanced eastward electric 'eld prior to sunset. This is because on days of enhanced eastward electric 'eld, the latitude variation of electron density exhibits more anomaly-like features at 840 km. The right-hand panel shows that on these
1750 S. Basu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1745 – 1754 Fig. 4. Variation of the occurrence of 1:5 GHz scintillation with the sunspot number observed at Ascension Island, near the crest of the equatorial anomaly, during pre-midnight period. The top panel shows the occurrence statistics for magnetically quiet (Kp = 0–3) conditions; the middle panel for disturbed (Kp = 3+ –9) conditions and the bottom panel shows the variation of the sunspot number. Reproduced from S. Basu and K.M. Groves, Speci'cation and forecasting of outages on satellite communication and navigation systems, Space Weather, Geophysical Monograph 125, 424–430, 2001. Published 2001 by the American Geophysical Union. Reproduced/modi'ed by permission of American Geophysical Union.
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Fig. 5. Daily variation of the intensity scintillation index, S4 , at Ancon, Peru with UT during October 24 –30, 1998. All days correspond to magnetically quiet conditions with Kp ¡ 20. Note LT = UT − 5 h.
particular evenings scintillations were observed 1 h later at the longitude of the DMSP transit. The top and the third diagrams in the left-hand panel did not show 4at-topped electron density variations and the corresponding 'gures in the right-hand panel are not associated with any scintillation activity. On such days because of reduced eastward electric 'eld, the anomaly-like feature remains below the satellite altitude. The DMSP data has been incorporated in the SCINDA system to provide an actual forecast capability. Fig. 8 shows the conceptual schematic of an equatorial satellite called the C=NOFS. This satellite will be launched in an elliptical orbit, perigee 400 km and apogee 700 km, ◦ with an orbital inclination of 13 . The satellite in situ sensors
will include a Langmuir probe, digital ion drift meter, vector electric 'eld and neutral wind measuring instruments. It will also include a tri-frequency (150, 400, 1067 MHz) beacon and a GPS occultation receiver. These sensors will probe the destabilizing and stabilizing forces in the ionosphere, namely, the pre-reversal enhancement of the zonal electric 'eld, driven by the zonal neutral wind, and the meridional wind, respectively. These data will drive an equatorial ionospheric model, which will be able to forecast the onset of plasma instability. The instability will be tracked through its non-linear phase to derive the irregularity amplitude and the irregularity spectrum. When the satellite in situ measurements detect the presence of turbulence, radio wave
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Fig. 6. Illustrates three-dimensional irregularity structures in South America mapped by the Scintillation Network Decision Aid (SCINDA), and based on an irregularity model together with 250 MHz scintillation measurements made at Ancon, Peru and Antofagasta, Chile.
Fig. 7. The left panel shows the latitude variation of the in situ electron density measured by a DMSP satellite on four successive days. The right-hand panel shows the corresponding scintillation index, S4 , observed at Ancon, Peru and Taltal, Chile. The LT is UT − 5 h (approximately).
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digisondes at polar, auroral, sub-auroral and equatorial sites. The results are being analyzed to isolate the ionospheric drivers, study large- and intermediate-scale plasma structures which impact communication and navigation systems. 3. Summary
Fig. 8. Schematic of the C=NOFS, a proposed equatorial satellite with its suite of sensors. Reproduced from S. Basu and K.M. Groves, Speci'cation and forecasting of outages on satellite communication and navigation systems, Space Weather, Geophysical Monograph 125, 424–430, 2001. Published 2001 by the American Geophysical Union. Reproduced/modi'ed by permission of American Geophysical Union.
scattering theory will be used to determine: (i) the magnitude of phase and intensity scintillation of satellite signals propagating through the turbulent medium and (ii) to characterize the temporal structure of the amplitude and phase of the satellite signal received on ground. This will be validated by scintillation measurements performed on the ground using the tri-frequency beacon transmissions. In addition, when the sensor suites intercept plasma bubbles, the irregularity amplitude and spectrum, as well as its dynamics, will be determined. These in situ measurements, coupled with the data-driven background ionospheric model, will allow the system to specify and forecast the occurrence and magnitude of scintillation and determine the motion of scintillation structures. With an orbital period of 90 min, the satellite will be able to track the evolution and motion of each plasma bubble at 90-min intervals. The space-based C=NOFS together with the ground-based SCINDA will be able to specify, forecast and validate scintillation products for the users. Under the auspices of the S-RAMP group of SCOSTEP, a global space-weather campaign was conducted in September 1999 during which an intense magnetic storm occurred. The global community of researchers performed space-based and ground-based measurements. ACE provided real-time observations of the IMF and the solar wind parameters that control the plasma processes at high latitudes. Many of the incoherent scatter radars and the global GPS network of the International Geodynamic Service (IGS) obtained data that can characterize the ionosphere and its structure. The Air Force Research Laboratory performed coordinated measurements with all sky 630 nm imagers, scintillation receivers and
Currently, at high latitudes, the major drivers of plasma processes in the ionosphere, namely, the solar wind and the IMF components can be tracked in real time. During solar maximum conditions, when scintillations attain their maximum values, the development of a scintillation speci'cation and forecast system within the polar cap is most important. At present, it is diLcult to develop such a system for northward IMF conditions in view of the chaotic state of the convection pattern. However, the framework of such a system for IMF southward conditions is available when strong patch-induced scintillations are encountered. In order to succeed, the trajectories of polar cap patches have to be established for varying IMF and the saturation amplitude of irregularities determined from the standpoint of plasma instability theories. At equatorial latitudes, scintillation speci'cation systems are a reality, primarily because the irregularity motion is very ordered, the irregularities are 'eld-aligned and the lifetime of scintillation causing irregularities is long. Although theoretical studies, intensive observations and re'ned modeling have isolated the stabilizing (Maruyama, 1988; Mendillo et al., 1992) and the destabilizing forces (Basu et al., 1996; Sultan, 1996; Fejer et al., 1999) in the equatorial ionosphere, the forecasting of equatorial scintillation remains a challenging task. It is hoped that satellites such as C=NOFS will be able to track simultaneously various stabilizing and destabilizing forces and thereby advance our capability to forecast equatorial scintillations. Acknowledgements The work at the Air Force Research Laboratory was partially supported by the AFOSR Task 2311AS. References Aarons, J., 1993. The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence. Space Science Reviews 63, 209. Aarons, J., 1997. Global Positioning System phase 4uctuations at auroral latitudes. Journal of Geophysical Research 102, 17,219. Aarons, J., Basu, S., 1994. Ionospheric amplitude and phase 4uctuations at the GPS frequencies. In: Proceedings of ION GPS-94, Arlington, VA, p. 1569. Aarons, J., Rodger, A.S., 1991. The e1ects of electric 'eld and ring current energy increases on F layer irregularities at auroral and subauroral latitudes. Radio Science 26, 1115. Aarons, J., Lin, B., Mendillo, M., Liou, K., Codrescu, M., 2000. Global Positioning System phase 4uctuations and ultraviolet
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Kelley, M.C., 1989. The Earth’s Ionosphere. International Geophysical Series, Vol. 43, Academic, San Diego, CA, p. 71. Keskinen, M.J., Mitchell, H.G., Fedder, J.A., Satyanarayana, P., Zalesak, S.T., Huba, J.D., 1988. Nonlinear evolution of the Kelvin–Helmholtz instability in the high latitude ionosphere. Journal of Geophysical Research 93, 137. Kivanc, O., Heelis, R.A., 1997. Structures in ionospheric number density and velocity associated with polar cap ionization patches. Journal of Geophysical Research 102, 307. Maruyama, T., 1988. A diagnostic model for equatorial spread F, 1, Model description and application to electric 'eld and neutral wind e1ects. Journal of Geophysical Research 93, 14,611. Mendillo, M., Baumgardner, J., Pi, X., Sultan, P.J., 1992. Onset conditions for equatorial spread-F. Journal of Geophysical Research 97, 13,865. Ossakow, S.L., Chaturvedi, P.K., Workman, J.B., 1978. High altitude limit of the gradient drift instability. Journal of Geophysical Research 83, 2691. Pedersen, T.R., Fejer, B.G., Doe, R.A., Weber, E.J., 2000. An incoherent scatter radar technique for determining two-dimensional horizontal ionization structure in polar cap F region patches. Journal of Geophysical Research 105, 10,637. Rishbeth, H., 1971. Polarization 'elds produced by winds in the equatorial F-region. Planetary and Space Science 19, 357. Rodger, A.S., Pinnock, M., Dudney, J.R., Baker, K.B., Greenwald, R.A., 1994. A new mechanism for polar patch formation. Journal of Geophysical Research 99, 6425. Secan, J.A., Bussey, R.M., Fremouw, E.J., Basu, S., 1995. An improved model of equatorial scintillation. Radio Science 30, 607. Secan, J.A., Bussey, R.M., Fremouw, E.J., Basu, S., 1997. High latitude upgrade to the Wideband ionospheric scintillation model. Radio Science 32, 1567. Sojka, J.J., Bowline, M.D., Schunk, R.W., 1994. Patches in the polar ionosphere: UT and seasonal dependence Journal of Geophysical Research 99, 14,959. Sultan, P.J., 1996. Linear theory and modeling of the Rayleigh– Taylor instability leading to the occurrence of equatorial spread-F. Journal of Geophysical Research 101, 26,875. Tsunoda, R.T., 1988. High latitude F region irregularities: a review and synthesis Reviews of Geophysics 26, 719. Valladares, C.E., Basu, Su., Buchau, J., Friis-Christensen, E., 1994. Experimental evidence for the formation and entry of patches into the polar cap. Radio Science 29, 167. Weber, E.J., Buchau, J., Moore, J.G., Sharber, J.R., Livingston, R.C., Winningham, J.D., Reinisch, B.W., 1984. F-layer ionization patches in the polar cap. Journal of Geophysical Research 89, 1683. Weber, E.J., Klobuchar, J.A., Buchau, J., Carlson Jr., H.C., Livingston, R.C., de la Beaujardiere, O., McCready, M., Moore, J.G., Bishop, G.J., 1986. Polar cap F layer patches: structure and dynamics Journal of Geophysical Research 91, 12,121. Weber, E.J., Basu, S., Bishop, G., Bullett, T., Kuenzler, H., Ning, P., Sultan, P., Valladares, C.E., Araye, J., 1996. Equatorial ◦ plasma depletion precursor signatures and onset observed at 11 south of the magnetic equator. Journal of Geophysical Research 101, 26,829.
www.elsevier.com/locate/jastp
Speci'cation and forecasting of scintillations in communication=navigation links: current status and future plans S. Basua; ∗ , K.M. Grovesa , Su. Basub , P.J. Sultana a Air
Force Research Laboratory, VSBI, 29 Randolph Road, Hanscom AFB, MA 01731, USA Sciences Division, National Science Foundation, 4201 Wilson Boulevard, VA 22230, USA
b Atmospheric
Abstract The ionosphere often becomes turbulent and develops electron density irregularities. These irregularities scatter radio waves to cause amplitude and phase scintillation and a1ect satellite communication and GPS navigation systems. The e1ects are most intense in the equatorial region, moderate at high latitudes and minimum at middle latitudes. The thermosphere and the ionosphere seem to internally control the generation of irregularities in the equatorial region and its forcing by solar transients is an additional modulating factor. On the other hand, the irregularity generation mechanisms in the high-latitude ionosphere seem to be driven by magnetospheric processes and, therefore, high-latitude scintillations can be tracked by following the trail of energy from the sun in the form of solar 4ares and coronal mass ejections. The development of a global speci'cation and forecast system for scintillation is needed in view of our increased reliance on space-based communication and navigation systems, which are vulnerable to ionospheric scintillation. Such scintillation speci'cation systems are being developed for the equatorial region. An equatorial satellite equipped with an appropriate suite of sensors, capable of detecting ionospheric irregularities and tracking the drivers that control the formation of ionospheric irregularities, has also been planned for the purpose of specifying and forecasting equatorial scintillations. In the polar region, scintillation speci'cation and forecast systems are yet to emerge although modeling and observations of polar cap plasma structures, their convection and associated irregularities have advanced greatly in recent years. Global scintillation observations made during the S-RAMP Space Weather Month in September 1999 are currently being c 2002 Published by Elsevier analyzed to study the e1ects of magnetic storms on communication and navigation systems. Science Ltd. Keywords: Ionosphere; Irregularities; Polar and equatorial region; Scintillation; Speci'cation and forecasting
1. Introduction The earth’s ionized upper atmosphere often becomes turbulent and develops electron density irregularities. These irregularities scatter radio waves from satellites in the frequency range of 100 MHz–4 GHz (Basu et al., 1988; Aarons, 1993; Aarons and Basu, 1994). In the presence of
∗ Corresponding author. Tel.: +1-202-404-4384; fax: +1-202767-0631. E-mail address: [email protected] (S. Basu).
a relative motion between the satellite, the ionosphere and the receiver, the received signal exhibits temporal 4uctuations of intensity and phase, called scintillations. Intensity scintillations cause signals to fade below the average level. When the depth of fading exceeds the fade margin of a receiver, the signal becomes buried in noise and signal loss and cycle slips are encountered. Phase scintillations induce a frequency shift and, when this shift exceeds the phase lock loop bandwidth, the signal is lost and the receiver spends valuable time reacquiring the signal. Overall, in the presence of scintillations, the performance of communication and navigation systems is degraded.
c 2002 Published by Elsevier Science Ltd. 1364-6826/02/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 2 ) 0 0 1 2 4 - 4
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Scintillations are strong at high latitudes, weak at middle latitudes and intense in the equatorial region (Basu et al., 1988). Scintillation at all latitudes attain its maximum value during the solar maximum period when the F-region ionization density increases and the irregularities occur in a background of enhanced ionization density. In addition, high-latitude scintillations are enhanced during magnetic storms (Aarons, 1997; Aarons et al., 2000). As such, scintillation e1ects, expected to be encountered during the upcoming solar maximum period between 2001 and 2004, are of concern to systems engineers. At high latitudes, scintillations are found to be associated with large-scale plasma structures. Experimentally, the two states of the polar ionosphere controlled by the IMF, and their association with high-latitude large-scale plasma structures known as patches, blobs and sun-aligned arcs, have been discovered in the 1980s (Weber et al., 1984, 1986; Tsunoda, 1988; Carlson, 1994; Rodger et al., 1994; Basu and Valladares, 1999). More recently, the association of large-scale plasma structures with intermediate scale irregularities (tens of km to tens of meters), responsible for scintillations, has been examined in the framework of observations and modeling (Keskinen et al., 1988; Aarons and Rodger, 1991; Basu et al., 1995). At low latitudes, at the time of sunset, an enhanced eastward electric 'eld, called the pre-reversal enhancement, generally develops at F-region heights. As a result, the ionosphere moves upward, develops steep density gradients in the bottomside F-region and becomes unstable to the Rayleigh–Taylor instability (Kelley, 1989). Large-scale plasma depletions, called plasma bubbles, form in the bottomside of the F-region and become populated with small-scale irregularities as the bubbles rise to great heights. The small-scale irregularities cause intense scintillations of satellite signals along two belts of high ionization den◦ ◦ sity at 20 N and 20 S dip latitudes, called the equatorial anomaly (Basu et al., 1988). Scintillations at low latitudes, as in other regions, attain a maximum value during the solar maximum period owing to the increased value of the background ionization density. However, low-latitude scintillation is not fully dictated by solar transients, such as magnetic storms. In this region, intense scintillations are observed during magnetically quiet periods as well. When a satellite-based communication or navigation system operates in a scintillating environment and loss of signal occurs, an average user attributes the problem to interference or to failure of equipment either at the transmitting end, the receiving end or the satellite itself. If forecasts of scintillations can be provided, the users will not waste their resources and may instead evolve alternate strategies. The scintillation phenomenon has been studied for several decades and the climatology of scintillations, namely, its variation with latitude, season, local time, magnetic activity and solar cycle, is well documented. A robust climatological model, WBMOD, is also available (Secan et al., 1995, 1997). However, scintillations exhibit extreme variability in
space and time. As such, climatological models are useful for planning purposes only. For the support of space-based communication and navigation systems, we need weather models, i.e., real-time speci'cation and forecast models. This paper provides an overview of scintillations at high and low latitudes and considers issues related to the speci'cation and forecast of scintillation. A scintillation speci'cation system, developed for the South American sector, is outlined. The issues related to the speci'cation and forecast of polar scintillation is discussed. Also an equatorial satellite, called the Communication and Navigation Outage Forecast System (C=NOFS) which has been planned for specifying and forecasting scintillations in the equatorial region, is described. 2. Results and discussions Fig. 1 shows an updated schematic of the global distribution of worst-case scintillation at L-band frequencies during the solar maximum and the minimum period, originally published by Basu et al. (1988). At such high frequencies, scintillations do not readily get saturated and, as such, the variations in scintillation magnitudes can be well represented. The left-hand panel represents the solar maximum condition when scintillation attains its maximum value. The magnetic north and south poles are at the top and the bottom and the magnetic equator is in the middle. The noon meridian is on the left, midnight is on the right and 18 LT is in the middle. It may be noted that L-band scintillation is most intense in the equatorial region, moderate at high latitudes and generally absent at middle latitudes. The diagram represents winter conditions over the Northern Hemisphere and shows that the scintillations are considerably more intense over the winter polar cap when the sun remains below the horizon. The left panel of Fig. 1 shows that at high-latitudes scintillations, in general, occur over the nightside auroral oval and over the polar cap virtually at all local times. In the winter polar cap moderately strong L-band scintillation is observed in association with the so-called polar cap patches (Weber et al., 1984). When the interplanetary magnetic 'eld (IMF) is directed southward, patches with high-ionization density are observed to enter the polar cap from the dayside auroral oval, convect in the anti-sunward direction and eventually exit into the nightside auroral oval. One mechanism by which patch formation is achieved corresponds to changing the plasma convection pattern in response to the IMF By component during periods of southward Bz (Sojka et al., 1994). Among several other formation mechanisms, one mechanism that considers the role of large plasma 4ows in the formation of discrete patches has been experimentally substantiated (Rodger et al., 1994; Valladares et al., 1994). A recent study has shown a fairly remarkable similarity between polar cap scintillation observations caused by mesoscale irregularities (tens of km to tens of meter) and the Utah State University’s TDIM model predictions of
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Fig. 1. Schematic of the global morphology of scintillations at L-band frequencies during the solar maximum (left panel) and solar minimum (right panel) conditions. Reproduced from S. Basu and K.M. Groves, Speci'cation and forecasting of outages on satellite communication and navigation systems, Space Weather, Geophysical Monograph 125, 424–430, 2001. Published 2001 by the American Geophysical Union. Reproduced/modi'ed by permission of American Geophysical Union.
patches (Basu et al., 1995). The co-existence of macroscale and mesoscale plasma structures implies that intermediate scale structures causing scintillations of satellite signals develop at the edges of polar cap patches through the gradient drift instability mechanism and permeates the entire polar cap patch (Ossakow et al., 1978; Guzdar et al., 1998). A statistical study of kilometer scale irregularities, based on DE-2 measurements, shows their signi'cant summer=winter variation and that the strongest irregularity amplitude and velocity turbulence occur in the cusp region (Kivanc and Heelis, 1997). The polar cap ionosphere alternates between two states (Carlson, 1994) depending on whether the IMF is southward (Bz ¡ 0), as discussed above, or northward (Bz ¿ 0). For Bz northward conditions, plasma convection over the polar cap becomes structured and weak, stable sun-aligned arcs are observed. Fig. 1 illustrates in a cartoon form plasma structures over the polar cap that are elongated in the noon– midnight direction but narrow in the dawn–dusk direction and are characterized by their dawn–dusk motion. In general, sun-aligned arcs cause weak scintillations of satellite signals presumably because of the low ionization density in sun-aligned arcs. However, during the solar maximum period enhanced scintillations have been recorded in association with such arcs (Basu et al., 1990). A topical review on polar plasma structures has been published recently in this journal (Basu and Valladares, 1999). It has synthesized theory, observations, and simulations of polar cap plasma structures for Bz southward and northward as well as the transition state from southward to northward conditions when polar cap patches continue to be observed because of their 'nite lifetime.
With our current knowledge of polar cap plasma structures, associated scintillation causing irregularities and their control by the IMF, it seems that we are poised to develop a polar scintillation speci'cation and forecast system. As discussed in previous paragraphs, we have the basic framework for the development of such a system for polar cap patch associated scintillation, which is the most intense variety of polar scintillations. The factors that still need to be resolved relate to determining the trajectories of patches when the convection pattern varies in response to changes in the IMF and the lifetime of patches when the IMF changes from a southward to a northward orientation. Such attempts have recently been made (Crowley et al., 2000; Pedersen et al., 2000). From Fig. 1, it may be noted that most intense scintillations occur in the equatorial region where the onset of scintillation at all frequencies usually occur immediately after sunset. However, the decay time of scintillation varies with frequency. L-band scintillations decay around midnight whereas at lower frequencies, such as at 250 MHz, scintillation continue for several hours after midnight (not shown in Fig. 1). Equatorial scintillations vary with latitude with a minimum at the magnetic equator and are maximum around ◦ ◦ two belts at 20 N and 20 S magnetic latitudes called the crests of the equatorial anomaly. Fig. 2 is a schematic of equatorial electrodynamics. It illustrates that during the daytime, the dynamo electric 'elds at o1-equatorial E-region locations map along the magnetic 'eld to F-region heights at the magnetic equator and the resulting E × B (E being the electric 'eld and B the magnetic 'eld intensity) force lifts the F-region plasma to higher altitudes. Then, by the action of forces parallel to B due to
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Fig. 2. Illustration of the equatorial electrodynamics and the formation of the equatorial anomaly in F2 ionization density.
gravity and plasma pressure gradients, the uplifted plasma di1uses along the magnetic 'eld towards the north and south of the magnetic equator to form two belts of high ionization ◦ ◦ density at magnetic latitudes of 20 N and 20 S, the equatorial ionization anomaly (Hanson and Mo1et, 1966). At the time of sunset, the zonal neutral wind and conductivity gradient caused by the sunset terminator interact to develop an enhanced eastward electric 'eld on the dayside of the terminator and a westward electric 'eld on the nightside (Farley et al., 1986; Kelley, 1989; Haerendel and Eccles, 1992; Crain et al., 1993). The enhanced eastward electric 'eld, called the pre-reversal enhancement, causes the F-region to move upward, steepen the bottomside density gradient and to trigger the Rayleigh–Taylor instability. As a result, the low-density plasma from the bottomside intrudes into the topside ionosphere (Kelley, 1989) to form plasma bubbles that develop mesoscale irregularities and cause scintillation of satellite signals. The pre-reversal enhancement of eastward electric 'eld leads to the development of small-scale irregularities through the operation of a hierarchy of instabilities and seems to be an important source for the destabilization of the ionosphere as well as scintillation of satellite signals (Basu et al., 1996; Fejer et al., 1999). This electric 'eld also leads to a resurgence of the equatorial anomaly in the post-sunset period. As a result, scintillations at frequencies as high as 4 GHz can be encountered near the crests of the equatorial anomaly during the solar maximum since the intermediate scale irregularities occur in an environment of high ionization density in the anomaly region.
Fig. 3, for the equatorial anomaly region during a solar maximum period, shows that the magnitude of scintillation decreases with increasing frequency. Scintillation is quite intense, not only at 250 MHz but at L-band frequencies as well, and remains detectable even at frequencies as high as 4 GHz. Non-stationary signal intensity 4uctuations in the middle panel may arise from radio wave scattering by steep coherent electron density irregularity structures at F region heights. The equatorial irregularities evolve in longitudinally discrete (∼250 km in magnetically east–west direction) plasma bubbles that, on occasions, attain heights exceeding 1000 km at the magnetic equator. These altitudinally extended plasma bubbles map along magnetic 'eld lines to F-region heights at locations to the north and south of the magnetic equator. Near the crests of the equatorial anomaly, the overhead sky becomes interspersed with the footprints of longitudinally discrete and north–south elongated structures. These structures generally move in the eastward direction by the action of vertically oriented polarization electric 'eld (Rishbeth, 1971). The space-time variation of irregularity structures in plasma bubbles has been tracked near the anomaly region by performing simultaneous airglow and multi-point scintillation measurements by using signals from GPS satellites (Weber et al., 1996). Figs. 3 and 5 of that paper show, respectively, the signature of plasma bubbles in multiple 630 nm airglow depletion structures and portray how scintillations on di1erent GPS links vary as the ray paths intersect the depletions. The diagrams illustrate that the width and the altitude extent of plasma
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Fig. 3. Illustration of the variation of scintillation magnitude with frequency obtained from multi-frequency transmissions of the Marisat satellite on February 3 1981. The top panel shows intense intensity scintillations at 250 MHz, the middle panel shows strong scintillations at 1541 MHz and the bottom panel indicates weak activity at 3954 MHz. The data were recorded at Ascension Island, near the crest of the equatorial anomaly, during solar maximum. Note LT = UT − 1 h.
bubbles determine how many satellites su1er scintillation simultaneously. Fig. 4 shows the occurrence statistics of 1:5 GHz scintillation in the equatorial anomaly region during the pre-midnight period (20 –24 LT). The top panel refers to magnetically quiet periods (Kp = 0–3); the middle panel to active periods (Kp = 3+ –9) and the bottom panel show the sunspot number. It may be noted that scintillation occurrence increases from 1987 to 1989 as the sunspot number increases. The diagram indicates that there are no data during June–August 1988 but past data indicated that these months correspond to the seasonal minimum in scintillation occurrence when scintillations are virtually absent over the American-Atlantic longitude sector. It is also interesting to note that the occurrence is much enhanced during magnetically quiet periods (top panel). It seems that during magnetically quiet periods, the turbulence in the nighttime equatorial F region is driven internally and cannot be predicted by following the trail of energy from the sun that cause enhanced magnetic activity on earth. The average occurrence statistics of equatorial scintillation shown in Fig. 4 may be useful for planning purposes, but it is of limited value to the users because of the extreme day-to-day variability of equatorial scintillation. Fig. 5 illustrates the extent of the day-to-day variability of scintillation at 250 MHz recorded at Ancon, Peru, on the magnetic equator. The data show the daily variation of the S4 index, de'ned as the normalized standard deviation of signal intensity 4uctuations (Briggs and Parkin, 1963), during 24 –30 October, 1998, which corresponds to the month of maximum scintillation occurrence in Peru. There was no scintillation on three nights within a total of seven nights. All these days correspond to a magnetically quiet period with
Kp ¡ 20. Such variability of scintillation emphasizes the need for even a scintillation speci'cation over a limited area populated by users. Fig. 6 shows the output of the scintillation speci'cation system, Scintillation Network Decision Aid (SCINDA), which has been developed for the equatorial region in the American sector (Groves et al., 1997). The system combines scintillation magnitude and zonal drift measurements made at two sites, one near the magnetic equator and the other near the anomaly region, by using transmissions from two satellites one to the east and the other to the west of the stations. The data drives the equatorial scintillation model, which uses upwelling and zonal motion to produce three-dimensional scintillation structures. Fig. 6 shows the development of successive scintillation structures, elongated in the north– south direction by about 3000 km, and their movement in the eastward direction. The Internet brings this to the user. The increasing magnitude of scintillations is indicated by the green, yellow and red colors. In view of the eastward drift and few hours lifetime of scintillation structures, the SCINDA system is able to provide short-term forecasts of scintillation to users located east of the observing stations. Fig. 7 shows that DMSP satellites in polar orbit can measure the latitude variation of in situ electron density at 840 km. The satellite crosses the equator at 18 h local time, which is about one and a half hour prior to the onset of scintillation. The left-hand panel shows the latitude variation of electron density. The second and fourth diagrams in the left-hand panel, with 4at-topped latitude variations, indicate an enhanced eastward electric 'eld prior to sunset. This is because on days of enhanced eastward electric 'eld, the latitude variation of electron density exhibits more anomaly-like features at 840 km. The right-hand panel shows that on these
1750 S. Basu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1745 – 1754 Fig. 4. Variation of the occurrence of 1:5 GHz scintillation with the sunspot number observed at Ascension Island, near the crest of the equatorial anomaly, during pre-midnight period. The top panel shows the occurrence statistics for magnetically quiet (Kp = 0–3) conditions; the middle panel for disturbed (Kp = 3+ –9) conditions and the bottom panel shows the variation of the sunspot number. Reproduced from S. Basu and K.M. Groves, Speci'cation and forecasting of outages on satellite communication and navigation systems, Space Weather, Geophysical Monograph 125, 424–430, 2001. Published 2001 by the American Geophysical Union. Reproduced/modi'ed by permission of American Geophysical Union.
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Fig. 5. Daily variation of the intensity scintillation index, S4 , at Ancon, Peru with UT during October 24 –30, 1998. All days correspond to magnetically quiet conditions with Kp ¡ 20. Note LT = UT − 5 h.
particular evenings scintillations were observed 1 h later at the longitude of the DMSP transit. The top and the third diagrams in the left-hand panel did not show 4at-topped electron density variations and the corresponding 'gures in the right-hand panel are not associated with any scintillation activity. On such days because of reduced eastward electric 'eld, the anomaly-like feature remains below the satellite altitude. The DMSP data has been incorporated in the SCINDA system to provide an actual forecast capability. Fig. 8 shows the conceptual schematic of an equatorial satellite called the C=NOFS. This satellite will be launched in an elliptical orbit, perigee 400 km and apogee 700 km, ◦ with an orbital inclination of 13 . The satellite in situ sensors
will include a Langmuir probe, digital ion drift meter, vector electric 'eld and neutral wind measuring instruments. It will also include a tri-frequency (150, 400, 1067 MHz) beacon and a GPS occultation receiver. These sensors will probe the destabilizing and stabilizing forces in the ionosphere, namely, the pre-reversal enhancement of the zonal electric 'eld, driven by the zonal neutral wind, and the meridional wind, respectively. These data will drive an equatorial ionospheric model, which will be able to forecast the onset of plasma instability. The instability will be tracked through its non-linear phase to derive the irregularity amplitude and the irregularity spectrum. When the satellite in situ measurements detect the presence of turbulence, radio wave
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Fig. 6. Illustrates three-dimensional irregularity structures in South America mapped by the Scintillation Network Decision Aid (SCINDA), and based on an irregularity model together with 250 MHz scintillation measurements made at Ancon, Peru and Antofagasta, Chile.
Fig. 7. The left panel shows the latitude variation of the in situ electron density measured by a DMSP satellite on four successive days. The right-hand panel shows the corresponding scintillation index, S4 , observed at Ancon, Peru and Taltal, Chile. The LT is UT − 5 h (approximately).
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digisondes at polar, auroral, sub-auroral and equatorial sites. The results are being analyzed to isolate the ionospheric drivers, study large- and intermediate-scale plasma structures which impact communication and navigation systems. 3. Summary
Fig. 8. Schematic of the C=NOFS, a proposed equatorial satellite with its suite of sensors. Reproduced from S. Basu and K.M. Groves, Speci'cation and forecasting of outages on satellite communication and navigation systems, Space Weather, Geophysical Monograph 125, 424–430, 2001. Published 2001 by the American Geophysical Union. Reproduced/modi'ed by permission of American Geophysical Union.
scattering theory will be used to determine: (i) the magnitude of phase and intensity scintillation of satellite signals propagating through the turbulent medium and (ii) to characterize the temporal structure of the amplitude and phase of the satellite signal received on ground. This will be validated by scintillation measurements performed on the ground using the tri-frequency beacon transmissions. In addition, when the sensor suites intercept plasma bubbles, the irregularity amplitude and spectrum, as well as its dynamics, will be determined. These in situ measurements, coupled with the data-driven background ionospheric model, will allow the system to specify and forecast the occurrence and magnitude of scintillation and determine the motion of scintillation structures. With an orbital period of 90 min, the satellite will be able to track the evolution and motion of each plasma bubble at 90-min intervals. The space-based C=NOFS together with the ground-based SCINDA will be able to specify, forecast and validate scintillation products for the users. Under the auspices of the S-RAMP group of SCOSTEP, a global space-weather campaign was conducted in September 1999 during which an intense magnetic storm occurred. The global community of researchers performed space-based and ground-based measurements. ACE provided real-time observations of the IMF and the solar wind parameters that control the plasma processes at high latitudes. Many of the incoherent scatter radars and the global GPS network of the International Geodynamic Service (IGS) obtained data that can characterize the ionosphere and its structure. The Air Force Research Laboratory performed coordinated measurements with all sky 630 nm imagers, scintillation receivers and
Currently, at high latitudes, the major drivers of plasma processes in the ionosphere, namely, the solar wind and the IMF components can be tracked in real time. During solar maximum conditions, when scintillations attain their maximum values, the development of a scintillation speci'cation and forecast system within the polar cap is most important. At present, it is diLcult to develop such a system for northward IMF conditions in view of the chaotic state of the convection pattern. However, the framework of such a system for IMF southward conditions is available when strong patch-induced scintillations are encountered. In order to succeed, the trajectories of polar cap patches have to be established for varying IMF and the saturation amplitude of irregularities determined from the standpoint of plasma instability theories. At equatorial latitudes, scintillation speci'cation systems are a reality, primarily because the irregularity motion is very ordered, the irregularities are 'eld-aligned and the lifetime of scintillation causing irregularities is long. Although theoretical studies, intensive observations and re'ned modeling have isolated the stabilizing (Maruyama, 1988; Mendillo et al., 1992) and the destabilizing forces (Basu et al., 1996; Sultan, 1996; Fejer et al., 1999) in the equatorial ionosphere, the forecasting of equatorial scintillation remains a challenging task. It is hoped that satellites such as C=NOFS will be able to track simultaneously various stabilizing and destabilizing forces and thereby advance our capability to forecast equatorial scintillations. Acknowledgements The work at the Air Force Research Laboratory was partially supported by the AFOSR Task 2311AS. References Aarons, J., 1993. The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence. Space Science Reviews 63, 209. Aarons, J., 1997. Global Positioning System phase 4uctuations at auroral latitudes. Journal of Geophysical Research 102, 17,219. Aarons, J., Basu, S., 1994. Ionospheric amplitude and phase 4uctuations at the GPS frequencies. In: Proceedings of ION GPS-94, Arlington, VA, p. 1569. Aarons, J., Rodger, A.S., 1991. The e1ects of electric 'eld and ring current energy increases on F layer irregularities at auroral and subauroral latitudes. Radio Science 26, 1115. Aarons, J., Lin, B., Mendillo, M., Liou, K., Codrescu, M., 2000. Global Positioning System phase 4uctuations and ultraviolet
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