Ionospheric Scintillation Impact Report€¦ · Ionospheric Scintillation Impact Report...

NWRA NorthWest Research

Associates, Inc.University of Arizona Science and Technology Park

9040 S. Rita Road, Suite 2214 • Tucson, AZ 85747 • (520) 663-3570 • FAX (520) 663-3570 • http:/www.nwra-az.com

Ionospheric Scintillation Impact Report

Prepared for the Instituto Argentino de Radioastronomia

NWRA–BELL–06–R326

14 July 2006

Ionospheric Scintillation Impact Report NWRA-BELL-06-R326

This report of the potential impact of ionospheric scintillation on system performance was produced for the Instituto Argentino de Radioastronomia, by Northwest Research Associates, Inc. (NWRA).

If you should have questions regarding this report, please contact the report author, Mr. James Secan, at NWRA's Tucson office (520-319-7773, [email protected]).

System Synopsis This Ionospheric Scintillation Impact Report (ISIR) provides a summary of the potential impact of ionospheric scintillation on a radio astronomy instrument, the Square Kilometer Array (SKA), proposed to be constructed in Argentina (31.705° S, 69.75° W). The summary is provided in the form of 72 day-of-year versus time (DVT) contour plots of the expected worst-case S4 intensity-scintillation index for four frequencies of interest (100, 250, 600, and 1,000 MHz) at nine viewing geometries (overhead, and 30° and 60° elevation at 0°, 90°, 180°, and 270° azimuth from true north). Plots were generated for both solar minimum (defined as a sunspot number of 10) and solar maximum (150) conditions for all 36 frequency-geometry combinations.

General Scintillation Environment The proposed location in Argentina is located just south of a region of potentially severe ionospheric scintillation, the southern equatorial anomaly. In general, the scintillation levels in this region show a strong diurnal variation, a seasonal variation which peaks in intensity near the equinoxes, and increases with the 11 year solar cycle (driven by increases in the solar output of extreme ultraviolet, or EUV, radiation).

The diurnal variation is marked by rapid onset just after local sunset (at the point of the raypath between the ground-station and satellite), typically peaking within an hour or so of onset and then decreasing in intensity until local midnight. This pattern can be disrupted by strong geomagnetic disturbances, which can suppress the scintillation in the post-sunset time sector and trigger onset of scintillation in the post-midnight time sector. In rare cases, scintillation can continue into the post-sunrise time sector, but it typically dies out soon after local sunrise.

The most severe scintillation in this region is caused by plasma turbulence that develops along the walls of large plasma-depletion structures, often referred to as plumes, which can form in the post-sunset equatorial ionosphere. These structures are typically 100 km across in latitude and several thousand in longitude, and extend in altitude from about 180 km to over 800 km. Individual structures are typically separated by several hundred km in latitude, and they move eastward at roughly 100 m/sec. These structures do not form every night, even when all factors (SSN, geomagnetic activity, season) indicate the potential for formation. Scintillation can be present in the absence of these depletion structures, but at much lower levels.

Figure 1 (at the end of this report) illustrates the scintillation setting for the proposed location. This is a contour plot of the percent of time the S4 intensity scintillation index (defined as the RMS variation of signal intensity) is expected to exceed 0.5, which indicates moderate levels of scintillation. The solid star in the figure indicates the location of the proposed SKA facility. The figure was generated for extreme scintillation conditions (based on the selected geophysical

Page: 1 14 July 2006


conditions, season, and time-of-day) for the lowest frequency considered in this analysis (100 MHz). This map was generated by calculating the S4 index for paths from each ground location on the map to an overhead source.

Note that the site lies just south of the region of the most intense scintillation. This region is known to move in latitude from night to night, a phenomenon we do not attempt to include in our scintillation models due to lack of data upon which to base such a model. Over the period of a year, we would expect this feature to be sometimes south of the location shown in the figure and sometimes north, but the average location should be as used in the model.

Geophysical Setting The parameters that specify the geophysical setting for the scintillation model are the sunspot number (SSN) and the planetary index of geomagnetic activity (Kp). The SSN index is used as a surrogate for the solar EUV radiation levels, and sets the general level of solar production of plasma in the ionosphere. The Kp index provides a measure of the general level of geomagnetic activity and is used as an indicator of electrodynamics in the post-sunset equatorial ionosphere.

In the model runs used in this study, we have used a sunspot number of 10 for solar minimum conditions, and a sunspot number of 150 for solar maximum conditions. We have used a fixed value of 1.0 (1o) for Kp. Note that in the equatorial region, low levels of geomagnetic activity result in the highest levels of scintillation in the post-sunset period, where the highest levels of activity are observed. At higher levels of geomagnetic activity, the model reduces the scintillation levels observed in the post-sunset sector, but increases the levels seen later in the evening and into the morning hours.

Description of Model Results The results of the 72 model runs are shown in Figures 2 through 37. The model was run to generate estimates of the 95th percentile in the expected S4 scintillation distribution for the given date, time, and geophysical conditions. As such, these plots can be interpreted as the expected worst-case intensity scintillation levels in terms of the S4 scintillation index. Each page shows two plots for a single frequency/geometry case: the upper plot for solar minimum conditions and the lower plot for solar maximum. The figures are grouped by frequency: Figures 2 through 10 for 100 MHz, 11 through 19 for 250 MHz, 20 through 28 for 600 MHz, and 29 through 37 for 1,000 MHz. Within each frequency, the overhead geometry is first, followed by the 60° elevation case at 0° azimuth, the 30° elevation at 0° azimuth, and so on around the other three azimuth headings.

Note that the ripple effect seen along the edges of the scintillation enhancement in many of the plots is an artifact of the plotting software and not a model effect. The observed, and model, variations are all smoother than is evident in the plots. Also note that in a few cases, the scintillation over the entire span of the plot was below the first contour level of S4 = 0.1. In those cases, the plots are completely blank.



Caveats The general caveat for all products based on SCINTMOM or WBMOD is that these are climatological models designed to provide estimates of the expected levels of ionospheric scintillation for given geophysical conditions. Thus, scintillation maps generated from these models, such as that shown in Figure 1 of this report, should not be viewed as snapshots of the spatial distribution of scintillation on a given night, but rather as contours of the expected levels of scintillation, should it occur. These models provide expected levels of activity over a collection of observations, not individual realizations of the spatial coverage of scintillation for a specific day. A major weakness in the models is that they are based on a small number of stations, with barely adequate coverage of the latitude and longitude variations known to exist in ionospheric scintillation morphology.

A major source of uncertainty for the location of interest in this report is the placement of the peak of scintillation activity in the post-sunset equatorial region. This peak is most likely co-located with a local peak in the maximum ionospheric F2-layer densities, known as the Appleton or equatorial anomaly. This peak in F2 density occurs in a range 15-20° north and south of the geomagnetic equator. It forms during the day, and extends across the sunset terminator. The scintillation that occurs in the post-sunset equatorial region is strongest at the anomaly peak latitude, primarily because there is more plasma available to be structured resulting in larger values for ΔNe and thus larger levels of scintillation. The proposed SKA site is located just south of the latitude where the WBMOD model places this peak. While ionospheric research has shown that this peak moves closer to and away from the geomagnetic equator from day-to-day, the data sets available for the WBMOD climatology have not provided the necessary information to adequately include this movement in the model. Thus, the peak is set at a fixed latitude distance from the geomagnetic equator. In addition, very little data was available for modeling the transition from the peak into the mid-latitude ionosphere, so there is a certain amount of uncertainty in how quickly the scintillation levels drop off from the peak into the mid-latitude ionosphere.

The results of these two issues (location of the peak and transition away from the peak) is that there is some uncertainty in how far the strong scintillation region will extend over the region of sky to be viewed from the proposed SKA location. The results presented here are most likely close to the median location for the peak and of the rate of decrease in scintillation levels into the mid-latitude region, and both the location of the peak and rate of decrease will likely vary from day to day. This is, however, a source of some uncertainty in the results.

Finally, the results at the lowest frequency, 100 MHz, are for a frequency range not sampled by any of the data sets used in producing the model climatology. The bulk of the data used in generating the climatology in the equatorial region was from GPS signals at frequencies of 1,227.60 and 1,575.42 MHz. Earlier climatology development used data from two stations (Kwajalein Island and Ancon, Peru) at frequencies as low as 137.67 MHz, and data from three other stations (Huancayo, Peru; Manila, Phillipines; and Ascension Island) at 250 MHz. The propagation model does explicitly scale with frequency, but care should be taken in interpreting the results below the 137.67 MHz lowest frequency in the data used in generating the climatology.



Figure 1. Scintillation coverage map.



Figure 2. The 95th percentile S4 as a function of GMT and day of the year for 100 MHz, overhead geometry. The top plot is for solar minimum conditions; the lower plot for solar maximum conditions.



Figure 3. The 95th percentile S4 as a function of GMT and day of the year for 100 MHz, 60° elevation angle, 0° azimuth angle. The top plot is for solar minimum conditions; the lower plot for solar maximum conditions.




Page: 10 14 July 2006



Page: 11 14 July 2006



Page: 12 14 July 2006



Page: 13 14 July 2006



Page: 14 14 July 2006



Page: 15 14 July 2006



Page: 16 14 July 2006



Page: 17 14 July 2006



Page: 18 14 July 2006



Page: 19 14 July 2006



Page: 20 14 July 2006



Page: 21 14 July 2006



Page: 22 14 July 2006



Page: 23 14 July 2006



Page: 24 14 July 2006



Page: 25 14 July 2006



Page: 26 14 July 2006



Page: 27 14 July 2006



Page: 28 14 July 2006



Page: 29 14 July 2006



Page: 30 14 July 2006



Page: 31 14 July 2006


Figure 29. The 95th percentile S4 as a function of GMT and day of the year for 1,000 MHz, overhead geometry. The top plot is for solar minimum conditions; the lower plot for solar maximum conditions.

Page: 32 14 July 2006


Figure 30. The 95th percentile S4 as a function of GMT and day of the year for 1,000 MHz, 60° elevation angle, 0° azimuth angle. The top plot is for solar minimum conditions; the lower plot for solar maximum conditions.

Page: 33 14 July 2006



Page: 34 14 July 2006



Page: 35 14 July 2006



Page: 36 14 July 2006



Page: 37 14 July 2006



Page: 38 14 July 2006



Page: 39 14 July 2006



Page: 40 14 July 2006

Ionospheric Scintillation Impact Report€¦ · Ionospheric Scintillation Impact Report...

Documents

Transcript of Ionospheric Scintillation Impact Report€¦ · Ionospheric Scintillation Impact Report...