Shower Detections

Separate searches have been performed for all 64 combinations of two wavelet probe sizes (3° and 6°), two time window widths (2° and 6°), sporadic source region selec-tion(antihelion, helion, prograde apex and retrograde apex) and geocentric speed (win dows 20 kms-1 wide centred at 20, 30 and 40 kms"1 for prograde and centred at 50, 60 and 70 kms-1 for retrograde). While a number of significant peaks have been identified in these wavelet enhancement based searches, few of these survive close scrutiny. Only the well-established major showers, in addition to a very small number of other showers, are worthy of further comment. The searches in each sporadic source region in which these definite showers appear are discussed below—only the optimal search in each case is shown although there are generally several others in which each shower is also found significantly.

Figure 1. Search in the antihelion region using a 3° wavelet, a 2° sliding Ae window and Vg of 20 ±10 kms"1.

In the antihelion region the Southern 5 Aquarids (sda) and a Capricornids (cap) appear significantly. Although these showers have radiants occurring in a similar region over the same period of the year, it is possible to resolve them due to the speed partitioning— the average Vq of the meteoroids in these streams is separated by ~ 20 kms-1. Figure 1 shows an example of the cap shower detection; a similar example of the sda detection has been already given in Galligan and Baggaley [4] in a profile in which an unidentified shower candidate was also found about A0 = 313°—the latter candidate is labelled "Peak 1" and defined in Table 1. All of these profiles were obtained using a 3° wavelet probe and a 2° wide time window. In contrast, the 6° wavelet probe is found to remove the significance of the cap due to its tendency to focus on the sporadic background: similarly, the wider 6° time window tends to minimise the time over which the shower is significant to the point where weak showers such as the cap are undetectable. Figure 1 presents four profiles: the upper two profiles show the change in radiant position of the maximum ((A — Aq)m, Pm) over the equinoctial year while the third shows the maximum amplitude (WM) and the fourth shows the normalised maximum amplitude (Wm/N$) (N$ is the total number of meteors of all speeds within the whole radiant region under study); for each

Figure 2. Search in the helion region using a 3° wavelet, a 6° sliding A0 window and Vq of 30 ± 10 km s"1.

of the amplitude profiles the lower dotted line represents the 99% confidence level while the upper one corresponds to 99.99%. The cap is shown to be briefly 99.99% significant in normalised amplitude at A0 ~ 120°. The sda has been shown in Galligan and Baggaley [4] to be particularly strong after this point and it is possible that the cap significance has been contaminated by the perturbation on the background rate used in the normalisation. This possibility appears to be supported by the radiant position profiles over the A0 € [120°, 130°] period where a different steady-state compared with the surrounding time-period appears and also by the clear significance of the non-normalised amplitude over this period. However, removal of the selected sda meteors from the data set is not found to increase the normalised profile significance of the cap. The number of meteors detected from the sda per year is an order of magnitude higher that that detected from the cap; the cap is therefore much closer to the "noise" level than is the sda making their reality more difficult to establish.

In the helion region the Daytime Sextantids (dsx) is found and, as shown in Figure 2, this shower appears well above the 99.99% significance level. Due to some equipment outages and sporadic-E interference at the time of the shower over the years covered, the 6° time window is found to be more appropriate than the 2° window. The dsx is found to be significant for all helion region parameter permutations, apart from the 20 ± 10 kms-1 Vq partition which includes some very weak/possible shower peaks. The strongest peaks are found in the 30 ± 10 kms-1 partition in good agreement with the ~ 30 kms-1 mean shower VG expected based on other studies. Another significant peak occurring about A0 ~ 45° is also identified in Figure 2. There are reasons to doubt the reality of this shower candidate. The normalised amplitude profile peak is only significant for a few degrees of solar longitude whereas the peak structure itself lasts for ~ 30° with a gradual rise to, and fall from, the peak over that time. Such motion is suggestive of an Earth observation effect on the sporadic background; the period of observation is also rather

Solar Longitude J2000, deg

Figure 3. Search in the retrograde apex region using a 3° wavelet, a 6° sliding A0 window and meteoroids with geocentric speeds of 60 ± 10 kms-1.

Solar Longitude J2000, deg

Figure 3. Search in the retrograde apex region using a 3° wavelet, a 6° sliding A0 window and meteoroids with geocentric speeds of 60 ± 10 kms-1.

long for a shower (though the Southern 5 Aquarids has been shown to be active for a similar period in Galligan and Baggaley [4]). This peak is labelled "Peak 2" for further study later in this paper. A third significant peak in this figure at A0 ~ 105° is dismissed as an artifact of the low background rate and shower activity as evidenced in the original amplitude profile.

As expected, the retrograde orbit apex region is found to be based almost completely on the sporadic meteors. The well-known bias towards retrograde meteoroid detection on Earth-based platforms is responsible for this. Figure 3 shows the only shower detected by the wavelet enhancement method in this region to be the r? Aquarids (eta) which is detected at the 99.99% significance level for a period centred about A0 ~ 46°. This shower is found to be present in all Vq partitions, apart from 50 ± 10 kms-1, owing to the relatively large geocentric speed measurement uncertainty on eta shower meteors. The motion of the apex region maximum radiant position generally follows a symmetrical curve throughout the year, however, about the time of the eta there is a pronounced change in the ecliptic longitude and to a lesser extent the ecliptic latitude of the radiant maximum—these distinctive changes help to confirm the reality of this shower.

The prograde apex region shows no significant structure. This region is sparsely populated containing only high inclination or low latitude orbits. The dearth of meteors and the uncertainties in the measurements of each meteoroid's orbit make the recording of only background noise in the search profiles inevitable.

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