Definition Of Shower Structure

A meteoroid orbit is defined by the five classical orbital elements (q,e,i,w,Q)\ it is equivalently given by four directly observable parameters: the radiant position, geocentric speed (Vg), and time of detection. In the current study the latter method of orbit definition is adopted; the time of detection is measured in terms of the mean solar longitude (A0) and the radiant position is measured in terms of the ecliptic longitude with respect to the Sun (A - A0) and ecliptic latitude (/?). (All angular parameters are here referred to the J2000.0 equinox.)

A meteor shower is defined as an over-density in the (A —A0, /?, Vq, A0) parameter space. Visual inspection of the non-shower orbit characteristics reveals the most obvious large-scale features present in this space to be the antihelion, helion and apex sporadic sources (e.g. [4])—any shower present within the data set must necessarily stand out against this background. The 2-D radiant position sub-space is searched in the current study using wavelet enhancement techniques to extract structure. While the search is inherently a 4-D problem, it is found that geocentric speed is strongly dependent on radiant position: only large-scale partitioning in Vq is applied. The overlapping partitions used are defined by 20 kms-1 wide bins about centres of 20, 30 and 40 kms-1 for prograde meteoroid orbits; these centres are changed to 50, 60 and 70 kms-1 for retrograde orbits. In each case, in addition to this partitioning, a search of unpartitioned data is also carried out. The time dimension is dealt with by sliding windows in A0; both 2° and 6° widths are used to step in 1° increments between vernal equinoxes.

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