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(meteorobs) Sporadic Flux Variations (long)
Hi Lewis, et al,
This is a late entry in answer to your question about sporadic meteor flux
variations. I have been up to my neck in schoolwork this week, and have
only now had the chance to answer.
In considering meteor flux variations on a very basic level, there are two
major factors to consider: the "apex effect" of the Earth's motion as it
orbits the Sun, and actual variations in the meteoroid distribution itself
around the Earth's orbit. Let's first discuss the apex effect, and assume a
nice homogeneous meteoroid distribution about the Earth's orbit.
The "apex of the Earth's way" is a point on the ecliptic about 90 deg behind
the Sun, and is the point toward which the Earth appears to be traveling at
any given moment. Like the Sun, the apex point moves about 0.99 deg along
the ecliptic each day. There is also an antapexpoint on the ecliptic, which
is 180 deg from the apex point. As a general thumbrule, The higher the apex
point gets in a particular observer's sky, the higher the number of sporadic
meteors encountred. Conversely, the higher the antapex point gets in a
particular observer's sky, the fewer the number of sporadic meteors
encountred. Note that the apex point is not a shower radiant, but
corrosponds to the portion of the Earth's atmosphere which has the highest
mathematical probability of interesecting (or capturing) meteoroids in a
variety of orbits.
The most obvious effect of this phenomena is the diurnal variation in
sporadic rates, with the highest sporadic rates occurring in early morning,
and lowest rates occurring in early evening. Due to the tilt of the Earth's
axis (23.5 deg), a seasonal variation is also observed in the sporadic rates.
For an observer at latitudes greater than 23.5 deg North, an annual peak in
sporadic activity will be observed around the Autumnal Equinox (about
September 21). At this time, the apex point will be above the observer's
horizon for the longest number of hours each day. A corrosponding trough in
sporadic rates will occur around the Vernal Equinox (March 21) each year,
with the antapex point above the horizon for the longest number of hours.
Bringing our observer down from the north and into the tropics, an
intresting effect is observed. Moving south from the Tropic of Cancer (23.5
deg North), the annual peak will begin to shift towards a point earlier in
the year. Also, a secondary peak will appear as our observer approaches the
equator, six months from the first peak. At the equator, two peaks will
occur each year, corrosponding to the Summer and Winter Solstices (June and
December 21). Two troughs will also be present, corrosponding to the
Autumnal and Vernal Equinoxes. If the observer continues south, a dominant
peak will again appear, shifting towards the Vernal Equinox as he/she
approaches the Tropic of Capricorn.
Still in our homogeneous meteor flux, observers below 23.5 deg South would
have an annual peak corrosponding to the Vernal Equinox, and a trough
corrosponding to the Autumnal Equinox, the opposite of the northern hemisphere.
To round out this discussion, equatorial observers will have the highest
degree of difference from high to low in their diurnal flux variation, but
also have the lowest decree of difference in their annual flux variation.
Observers at latitudes far from the equator will have the lowest degree of
diurnal variation (from morning to evening), but will also have the highest
degree of annual variation during the year.
Whew!
OK. As we all know, the real meteoroid distribution is not homogeneous, and
both shower and non-shower meteors alike appear to have a noticeably higher
density in the later half of the year, and a lower density in the first half
of the year. Even with the apex effect counter-acting this unequal
distribution, southern hemisphere observers also see higher rates in the
fall of the year, and lower rates in the spring (McKinley, 1961).
In the northern hemisphere, the natural distribution and apex effects are
additive, creating a wider variation from fall to spring. In the southern
hemisphere, the two effects are subtractive, creating a much smaller annual
variation.
If both shower and non-shower meteors are combined, then the annual peak
will be placed even earlier in the summer for northern observers. At
Jodrell Banks, Lovell (1954) reported an annual peak in June for the
back-scatter radars, while my own forward-scatter system shows an annual
peak about July-August. My trough is about February-March, with daily rates
already on the rise as the Lyrids approach.
I hope this has helped to answer your question. There is still a lot to
learn in this area, and questions such as to what degree do either shower or
non-shower meteors contribute to the "sporadic" flux are difficult to
address. Over time, we can hopefully discover the answers.
Take care,
Jim Richardson
Graceville, FL
Richardson@DigitalExpdot com
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