The databases used by VOACAP (and REC533) provide predictions by hour and month, and therefore, in order to facilitate a comparison between the experimental measurements and the predicted parameters, the statistical behavior of the observations are also presented in this way. The VOACAP (version 05.0119) predictions have been produced using method 25 (the “all modes table” in which the results from each of the potential propagation modes were output individually) in order to obtain information on each of the potential propagation modes. All VOACAP analysis has been done using the CCIR (International Radio Consultative Committee) ionospheric coefficients, the international smoothed sunspot number (SSN) appropriate to the month and year, and a minimum take off angle of 3°. Weak modes with a carrier to noise density ratio (CNR - in effect the SNR in a 1 Hz bandwidth) below a given threshold (this was receive site and frequency dependent, although the choice of threshold does not significantly affect the results) and those for which the predicted probability of occurrence was zero (i.e., the parameter MUFday = 0 %) were excluded from the analysis. The sporadic E model that is available in VOACAP was not utilized since there is some doubt as to whether it can provide an accurate prediction of signal strength [Lane, 2001], hence the sporadic E layers observed experimentally are not reproduced in the prediction results. An examination of the sporadic E layers observed on the Uppsala–Leicester path will be the subject of a future paper.
 Although the Uppsala–Leicester observations were collected several years after the Halifax–Leitrim data, they will be discussed first since the propagation mechanisms are more straightforward than for the earlier experiments.
5.1. Trough Path Near Sunspot Maximum (Uppsala-Leicester)
 The monthly-by-hour occurrence statistics for measured and predicted TOF as a function of frequency (for winter) and season (for 11.1 MHz) are given in Figures 2 and 3, respectively. Measurements were made on each frequency every three minutes with the result that up to 600 observations were included in each month-hour. In Figure 2, the propagation observed during the day is largely as predicted. At the lowest frequency (4.6 MHz) the signal is not detected because of D region absorption, while at higher frequencies 1F, 2F, and, up to 11.1 MHz, 3F modes are observed close to the times given by VOACAP. It should be noted that the weak daytime 3F mode observed at 14.4 MHz and the 2F mode at 18.4 MHz are not predicted because there is insufficient electron density in the model to support propagation. However, the 4F modes observed at 10 and 11 MHz are not predicted because VOACAP is limited to calculating the propagation for the lowest three modes that reflect from a given ionospheric layer (i.e., in this case, 1F2, 2F2, and 3F2). At night, VOACAP does not perform well since the propagation at long TOF (7 to 11 ms) that is present in the observations from about 2000 UT through to 0600 UT, particularly for 7–11 MHz, is not reproduced. These propagation modes have been found to be consistent with the scattering of the signal from field-aligned irregularities located either in the north wall of the trough or in the auroral oval [Siddle et al., 2004b]. As noted earlier, the sporadic E observed at a TOF of just less than 5 ms will not be present in the predictions since this part of the model has been disabled.
Figure 2. Absolute time of flight versus time of day for Uppsala–Leicester in December 2001 (SSN = 115) at frequencies of (top to bottom) 4.6, 7.0, 10.4, 11.1, 14.4, and 18.4 MHz. The left-hand panels are derived from experimental data with the color scale indicating the number of observations (i.e., a measure of the probability of occurrence). The right-hand panels are derived from VOACAP predictions, the color scale giving the percentage of days for which the mode MUF is above the operating frequency. The monthly mean times when the path midpoint at 200 km altitude was sunlit are indicated by the solid bar at the top of each panel.
Download figure to PowerPoint
Figure 3. Time of flight versus time of day for the 11 MHz signal transmitted from Uppsala and received in Leicester in (top to bottom) March, June, September, and December (SSN = 105, 110, 114, and 115, respectively). The left-hand panels are experimental data, and the right-hand panels are derived from VOACAP. The color scales and the solid bar at the top of each panel are as described in Figure 2.
Download figure to PowerPoint
 The seasonal variation of the observed and predicted TOF for a frequency of 11.1 MHz is presented in Figure 3. During the day at all times, and at night in summer the observations are generally in quite close agreement with the predictions. However, the resolution of the measurements and transmitted pulse width (0.05 and 0.6 ms, respectively) precludes resolving the 1E and 1F1 modes in the summer daytime, and the delay for the summer night-time mode is slightly longer than that predicted. In the nonsummer months when the propagation modes at long TOF are commonly observed at night, these are not predicted by VOACAP. Furthermore, the F region mode predicted just before midnight in March, and to an extent in September, is not present in the observations. It is likely that the reduction in the on-GCP MUF due to the presence of the trough is not reproduced in the VOACAP model and consequently leads to prediction of modes that are not actually present.
 The bearing measurements are presented in two different, but related forms. In the first, the hourly lower-decile, median and upper decile azimuthal (A) values (i.e., the values not exceeded by 10, 50, and 90% of the observations, respectively) observed for 7.0 MHz and 11.1 MHz are presented in Figure 4. In the second, the median and upper-decile values of the absolute deviations in measured azimuth from the GC (ΔA=∣A − AGCP∣) for a four-hour interval around the path-midpoint local midnight are given in Table 1. For all seasons the direction of arrival measured during the day is close to the GCP direction of about 46°, while the median values of ΔA during the day (not shown here) were small and tend to decrease with increasing frequency, ranging from 3° at 4.6 MHz to 1° at 18.4 MHz. At night, the measured azimuth and ΔA are highly dependent on season and frequency. For the midsummer months (June and July), the nighttime median measured azimuth tends to be close to GC and the upper-decile values of ΔA less than 10° for all frequencies. In the winter months, the behavior depends on the frequency. For intermediate frequencies (7.0–11.1 MHz), the signal arrives with a median azimuth about 30–40° below the GC azimuth (i.e., to the north), while the upper-decile, owing to the presence of sporadic E layers, lies close to the GC direction. The median and upper-decile values of ΔA are more than 30° and 40°, respectively. The off-GCP propagation is associated with the modes with long TOF and usually occurs when the operating frequency is above the median MUF predicted by VOACAP, i.e., when on-GC propagation is not usually supported. For 4.6 MHz, much of the propagation is on-GC (median ΔA of 3°), and while there are some instances of off-GC propagation these tend not be as pronounced (upper-decile ΔA = 19°) or as common as for the intermediate frequencies. For the higher frequencies (14.4 MHz and 18.4 MHz – results not shown), propagation at night is, as predicted, rare but when it does occur is generally off-GC by a few tens of degrees or, if on GC, via a sporadic E layer. It is important to note that VOACAP would always predict a fixed value of 46° (i.e., the GC direction) since it does not account for propagation which takes place away from the GCP.
Figure 4. Plots of the observed median (middle line) and upper (top line) and lower decile (bottom line) values of azimuth as a function of time of day for Uppsala–Leicester in (top to bottom) March, June, September, and December. The left-hand panels are for 7.0 MHz, and the right-hand panels are for 11.1 MHz. The solid bar at the top of each panel is described in Figure 2.
Download figure to PowerPoint
Table 1. Median (50) and Upper-Decile (90) Azimuth Deviations From GC, ΔA Observed for Uppsala–Leicester Around Local Midnight (2300–0300 UT) in 2001a
| ||4.6 MHz||7.0 MHz||10.4 MHz|
 The median and upper and lower deciles of the signal strength observed experimentally and derived from VOACAP for 11.1 MHz are presented in Figure 5. In calculating the median and deciles for the signal strength, the instances where the signal was too weak to be detected have been tagged as ‘not received'. If the median or deciles have this ‘undetected signal' value then they have been omitted from the figure (hence the gaps). In calculating the predicted values, the transmitted power of 80 W employed in the experiment has been used, although this has been reduced by 2 dB in order to account for cable losses [Lane, 2001]. Antenna radiation patterns corresponding to ITU-R705 vertical monopoles [ITU-R, 1995] of height of 4 m, with no ground plane have been adopted for both transmitter and receiver. While the chosen antenna represents a reasonable approximation for the receiver site, it is likely to be less accurate for the transmitter. Because of these approximations, it is expected that the absolute measured signal strength may differ from the prediction. However, in order to compare the results, the experimental values have been adjusted so that the median values at dawn in the winter (since there is a good agreement between the observed and predicted propagation mode and minimal D region absorption at that time) are close in value to the predicted ones. The same adjustment (which is frequency dependent) is then used for all hours and months. Examination of Figure 5 reveals that the daytime median values in all seasons are predicted with reasonable accuracy by VOACAP. However, the observed D region absorption is slightly more pronounced than that predicted, although the opposite is the case for lower frequencies (not shown here – although see Halifax–Leitrim results). The predicted lower decile is further from the median than the measured one, particularly before noon in winter and at all times for other seasons. At night, the predictions in summer conform reasonably well to the observations, but this is not the case for other seasons. In winter, the observed signal strength is at times 10 dB or more greater than that predicted by VOACAP (e.g., 0000–0200 UT for the median and 0200–0600 UT for the upper decile). This arises because of the off-GC propagation at long-TOF that is observed, but not predicted. In the spring and autumn, the predicted signal strength is also somewhat higher than that observed. It is possible that this happens because VOACAP overestimates the MUF on the path since the trough is not explicitly included and therefore predicts an on-GCP reflection where this does not occur.
Figure 5. Signal strength versus time of day for an 11.1 MHz signal transmitted from Uppsala to Leicester in (top to bottom) March, June, September, and December 2001 (SSN = 105, 110, 114, and 115, respectively). Measurements are on the left, and VOACAP predictions are on the right. The solid lines are (from the top) the upper decile, median and lower decile. Note that on this scale, the experimental system is incapable of observing signals below about −150 dBr. The solid bar at the top of each panel is described in Figure 2.
Download figure to PowerPoint
5.2. Trough Path Near Sunspot Minimum (Halifax-Leitrim)
 The seasonal variation of median and upper and lower deciles of the azimuthal angle of arrival together with the probability of the MUF exceeding the operating frequency predicted by VOACAP (known as MUFday or mode probability) for 5.1 MHz on the Halifax–Leitrim path is presented in Figure 6. During the day, particularly when MUFday is around 100%, the 5.1 MHz signal is received on-GC. At night, the observations are more likely to be off-GC as MUFday decreases. This behavior is consistent with signal reflection via the walls of the trough becoming the dominant mechanism once the electron density on the great circle is insufficient to support propagation. It is interesting to note that while the deviation from the great circle was predominantly to the north in the Uppsala-Leicester measurements, deviations in either (or sometimes both) direction (e.g., top panel of Figure 6) were found on the Halifax–Leitrim path. Ray-tracing studies [Siddle et al., 2004b] have suggested that the different gradients found in the equatorward wall of the trough at solar minimum and solar maximum may be the cause of this change in behavior. Furthermore, while the propagation at night in the summer was almost invariably on-GC for Uppsala–Leicester, for Halifax–Leitrim, off-GC propagation does occur, albeit less often than in the winter months and with smaller deviations from the GC direction. No observations for the two higher frequencies (10.9 MHz and 15.9 MHz) are presented because signal identification at night was problematic, whilst during the day the signal tended to be within a few degrees of the GC direction.
Figure 6. (left axis) Lower decile (bottom line), median (middle line), and upper-decile (top line) of azimuth measured on Halifax-Leitrim path for 5.1 MHz in March, June, and September 1994 and December 1993 (SSN = 34, 31, 27, and 38, respectively). The dashed line represents the great-circle direction. (right axis) Histogram of the probability of the operating frequency being below the MUF predicted by VOACAP. The solid bar at the top of each panel is described in Figure 2.
Download figure to PowerPoint
 The median and upper-decile values of the absolute deviation in azimuth from the GC direction (ΔA) for a four-hour interval centered on corrected geomagnetic local midnight are given in Table 2. The observations during the day (1400–1800 UT) are not presented since the propagation was very close to on-GCP with the median and upper-decile values not exceeding 2° and 5°, respectively. At night, the median is no more than 6° because the signal is commonly received along the great circle since this frequency is often below the MUF (see Figure 6). The off-GC propagation observed on this path means that the upper-decile values of ΔA range from over 70° in the nonsummer months to around 40° in the summer months. That these values are much higher than those found for the Uppsala–Leicester path (Table 1) for all seasons and frequencies may partly be explained by the differences in the geometries between the two paths (Figure 1). Assuming the reflection comes from a given distance along a line perpendicular to the GC path (e.g., from the northern wall of the trough), then clearly the azimuthal deviation will be greater for the shorter, Halifax–Leitrim, path than for the longer Uppsala–Leicester path. In addition, the gradients of the trough walls are different at sunspot maximum and minimum and this may also contribute to the difference in azimuth deviation since the reflection (or scattering) may take place at a different location within the trough wall.
Table 2. As Table 1, Except for Halifax–Leitrim Around Corrected Geomagnetic Midnight (0200–0600 UT)a
 The observed and predicted median and deciles for signal strengths for 5.1 MHz (Figure 7) have been derived following a similar procedure to that described above, for the Uppsala–Leicester path. At night, the measured and predicted signal strengths are in reasonable agreement, although the measured lower decile is further from the median than the predicted one (in contrast to the Uppsala–Leicester results) and the drop in signal strength between 5–10 UT is more severe in the observations than given by VOACAP. This may occur because the effect of the trough on the on-GC MUF is not considered by VOACAP and therefore, it predicts that propagation is more likely than is actually the case. During the day, the effect of the D region absorption expected by VOACAP is more severe than that actually observed.
Figure 7. Seasonal variation in signal strength on the Halifax-Leitrim path at 5.1 MHz in March, June, and September 1994 and December 1993 (SSN = 34, 31, 27, and 38, respectively). The observations are in the left-hand panels, and the VOACAP predictions are in the right. The solid bar at the top of each panel is described in Figure 2.
Download figure to PowerPoint
5.3. Trans-Auroral Oval Path (Halifax-Alert)
 The observed probability of occurrence (as a percentage of the total number of transmissions) and the median and deciles of the azimuth of arrival have been plotted together with the predicted signal strength and proportion of days in a month for which the operating frequency is below the MUF (MUFday) in Figures 8 and 9. The effect of smoothed sunspot number (SSN) as it varies from 35 in February 1994, to 23 in 1995, and to 10 in 1996 can be observed in Figure 8. The daytime (∼10–23 UT) propagation appears largely unaffected by the change in SSN, with the observed azimuth being largely on-GC, although the signal does arrive from east of the GC around dawn and west of the GC at sunset, while the probability of occurrence is well correlated with the signal strength predicted by VOACAP. The propagation at night does change with SSN with VOACAP predicting that the signal strength (and MUFday) is higher as SSN increases, while the probability of occurrence also increases (the signal strength threshold for detection is about −170 to −180 dBW on this scale). In the short interval around 0900–1200 UT in 1994 (and to a lesser extent in 1995), the probability of occurrence does not decrease as expected from the predictions. However, where the signal does propagate at night, it often arrives from off-GC directions, e.g., the upper-decile azimuth is up to 60° to the west of the GC in 1994, behavior that is not predicted by VOACAP and this may account for this discrepancy.
Figure 8. Halifax-Alert, 10.9 MHz. (left) Histogram of probability of occurrence (left axis) and lower decile, median, and upper decile (bottom to top line, respectively) of predicted signal strength (right axis) for February 1994, 1995, and 1996 (SSN = 35, 23, and 10, respectively). (right) Observed lower decile, median, and upper decile of azimuth (left axis) and histogram of the probability predicted by VOACAP of the operating frequency being below the MUF (right axis). The dashed line is the azimuth corresponding to the great circle path. The solid bar at the top of each panel is described in Figure 2.
Download figure to PowerPoint
 The effect of changing frequency can be observed by comparing the top panels of Figure 8 with those in Figure 9 and by examination of Table 3, where values of ΔA are presented. For all three frequencies, the probability of occurrence is reasonable well correlated with the predicted signal strength. However, for 5.1 MHz the recovery of the signal from the midday (∼1600 UT) D region absorption occurs later than predicted, and for 15.9 MHz, the propagation that occurs between 0000 and 0300 UT is present in neither the VOACAP signal strength nor MUFday results. The nature of the azimuth of arrival behavior changes with frequency, at 5.1 MHz the signal can arrive from off-GC at any time of the day (except for period within a few hours of 0000 UT), while for 10.9 MHz off-GC propagation tends to occur at night when MUFday is low and at dawn and dusk, and for 15.9 MHz the signal also tends arrive from the east of the GC (i.e., azimuths below the GC direction of 181°) around sunrise and to the west of the GC at sunset and for a few hours afterwards. The marked difference in the timing of when the signal arrives from off-GC may be explained by differences in mode of propagation as the frequency is changed. VOACAP predicts that the lowest frequency will propagate principally via 2F2 and 3F2 modes, while for the higher frequencies propagation is normally via a 2F2 mode. The first reflection point of the 3F2 mode is more likely to be in or close to the auroral zone than that of the 2F2 mode (perhaps leading to an increase in ΔA), whilst the second and third reflection points are in the polar cap where the azimuth is more likely to be affected by the large-scale ionospheric features found there. It is noteworthy that the off-GC propagation observed during the day for 5.1 MHz is only found in 1994, when SSN was higher, and not in the subsequent two years (see Table 3). The value of ΔA observed at midnight also tends to vary a little as the SSN decreases (at least for 5.1 MHz and 10.9 MHz) and some seasonal dependence is also present with the deviations being higher in the winter than the summer.
Table 3. As for Table 2, Except for Halifax–Alert (Noon Indicates 1300–1700 UT, and Midnight 0100–0500 UT) and Only the Upper-Decile Values Are Presented
| || ||5.1 MHz||10.9 MHz||15.9 MHz|
5.4. Polar Cap Path (Iqaluit-Alert)
 Observations and VOACAP predictions for 9.3 MHz on the shorter, polar cap, path from Iqaluit to Alert are presented as a function of season in Figure 10. It should be noted that while in February, the sunrise and sunset in the F region are at 1000 and 2330 UT, respectively for April and June the F region is sunlit at all times of day (although, of course, the solar zenith angle does vary with time). For all three months, the probability of occurrence is largely constant with time of day although there is a slight reduction in February that corresponds reasonably well with a decrease in the predicted signal strength. The predicted sharp loss in signal strength accompanied by a significant drop in the mode probability between 1500 and 1700 UT in April and at 1000 UT in June is not present in the observations. Examination of the VOACAP results indicates that for these intervals a weak 2E mode is predicted that prevents propagation by the usual 1F1 or 1F2 mode and that the most reliable mode becomes a weak, less probable 2F mode. By running VOACAP with a minimum takeoff angle of 0.1° (rather than 3°), a 1E mode is predicted at an elevation angle of 0.8° that removes the strong reduction in MUFday and lessens the drop in signal strength. However, since the isotropic transmit antenna we have used in VOACAP is likely to perform significantly better at this elevation than the actual system, it suggests that the VOACAP predictions are inaccurate for this situation. The probability of off-GC propagation is strongly dependent on the season. During the winter, off-GC propagation to both east and west of the great circle is common at night when the predicted probability of propagation is lower, in spring it is mainly deviated to the west, while by summer no significant off-GC is observed. This seasonal variation can also been found in the ΔA data presented in Table 4. There is some evidence to suggest a weak dependence of the azimuth deviation on sunspot number. For example, for 9.3 MHz in February 1994 the azimuth deviation is about 20° higher than that found in February 1996. Of course, the data set is limited and the observations do not cover a broad range of SSN values (from 35 to 10), so it may be difficult to draw firm conclusions from this without further measurements. Finally, the nighttime azimuth deviations are higher for 9.3 MHz than for 5.8 MHz probably because the on-GC mode probability predicted by VOACAP for the lower frequency is usually close to 100%, whilst for the higher frequency it is around 50% at night, in winter. Off-GC modes are usually weaker than those propagating on-GC and therefore will only be observed very infrequently when an on-GC mode is present.
Table 4. As for Table 3, Except for Iqaluit–Alert
| || ||5.8 MHz||9.3 MHz|