[Prev][Next][Index][Thread]
(meteorobs) Leonid "radiant glow" (long)
Observing the Leonid stream in Interplanetary space.
[SUMMARY: The glow from the particles in the Leonid orbit may be
observed around the time of the nodal crossing (1998 Nov 17.8 UT)
at around
R.A.(2000)Dec.
10h 54m +25.6d near 54 Leo
22h 08m -30.2d between tau PsA and lambda PsA
At peak intensity it is unlikely to be obvious.]
After a cursory check on the web for information on the "radiant glow"
from the Leonids and finding nothing (see Postscript), I have made some
calculations myself.
The radiant glow may have been reported in previous meteor storms, but
I am unaware of adequate evidence to demonstrate this conclusively.
As I understand it, photographs in 1966 failed to show it and the region
appears on some published photographs. However, better instruments and
emulsions and CCDs brought to the task this year may give success.
The radiant glow I am referring to is the reflection of light off the
Leonid particles in space. Around the direction of the *true* radiant,
and close to the time of crossing the plane of the orbit, we are looking
along the sheet of particles within the orbit. This geometry
concentrates the Sun's reflection from the dust. Note that the direction
is the *true* radiant. This is from the tangent to the orbit at the node,
which is the same direction as the velocity vector of the Leonid
particles. It is not the direction of the *apparent* radiant which
includes the motion of the Earth to produce the meteors we see. (A good
analogy would be a thin column of rain falling straight down. You see
the greatest thickness of raindrops standing under it and looking up,
or down. In a car driving through it, the rain hits the front
windshield, not the back, as the car's motion is added to the motion of
raindrops. This give an "apparant radiant" for the rain in front of and
above the car. The "true radiant" of the rain is directly up.)
The direction of the *true* radiant and anti-radiant for the Leonid
stream on 1998 Nov 17.8 UT are:
R.A.(1950)Dec.
radiant 10h 30m +28 (direction particles come from)
anti-radiant 22h 30m -28 (direction particles go towards)
However this doesn't necessarily correspond to the region of greatest
intensity, especially as the main region of the Leonid swarm is passing
inside the Earth's orbit. I calculated an approximate surface brightness
as follows:
1) Define the path of the orbit in the sky (line of variation) at the
time of crossing the comet's orbital plane using the orbit of
55P/Tempel-Tuttle of epoch 1998 March 8.
2) Determine the brightness of an arbitrary "standard" particle for
various points around the orbit, correcting for the distance from the
Sun and Earth.
3) Assume the thickness and flux density of the stream is constant
over a few months either side of the region we encounter this year.
4) Determine the surface area presented to the Earth at each point in the
orbit, for a standard unit of the stream (+/- 1 day arc from that
point)
5) Calculate the intensity per unit surface area (in arbitrary units).
An assumption involved in 2) is that the particle brightness can be
represented by the standard asteroid brightness formula. This is not
true and may affect the results slightly, but the surface brightness
varies rapidly over a short arc suggesting the fine detail is correct.
Another assumption in 3) that the thickness of the stream is constant
will be a reasonable approximation, but the stream will be slightly
narrower towards perihelion. This is unlikely to affect the results,
as the predicted brightest point of the approaching stream is,
coincidently, very close to perihelion.
Using a two day arc of the orbit gives a common number of particles
for each calculated mid point. The apparent angular thickness of the
stream at this point is inversely proportional to the geocentric distance.
The apparent angular length of this two day arc of the orbit gives the
other dimension to calculate an arbitrary surface area. The intensity
(on an arbitrary scale) is determined by the standard asteroid formula
for the mid point for an arbitrary "standard" particle. Note that as the
orbit approaches the Earth, the "two-day arc" becomes grossly non-linear
and the initial point in each table will have the surface intensity
slightly underestimated.
The Surface Intensity only applies to the time of maximum brightness and
this will presumably occur at the peak of the shower. Thus, at the
two times given below the surface intensity is likely to be less than at
Nov 18.8 UT. However, the positions for the two times will give an
indication of the motion of the glow over the course of a day.
RESULTS
Near the TRUE RADIANT
1998 November 17.3 1998 November 18.3
(1950) Surface (1950) Surface
R.A. Dec. Intensity R.A. Dec. Intensity
h m o ' h m o '
10 54.1 +23 14 28 11 47.3 +24 19 3
10 47.7 +25 12 167 10 59.1 +26 44 59
10 48.5 +25 32 216 ~perihelion 10 54.2 +26 38 314 ~perihelion
10 51.2 +25 26 115 10 54.8 +26 18 169
10 54.5 +25 11 89 10 57.0 +25 54 111
10 58.1 +24 52 75 11 00.0 +25 27 83
11 01.8 +24 29 65 11 03.4 +25 00 76
11 05.6 +24 04 63 11 06.9 +24 32 64
11 09.4 +23 39 56 11 10.5 +24 04 59
11 13.1 +23 13 53 11 14.1 +23 35 55
11 26.1 +21 36 48 11 26.8 +21 53 50
11 36.8 +20 10 44 11 37.4 +20 23 46
11 46.5 +18 49 41 11 47.0 +19 00 40
11 55.1 +17 33 37 11 55.7 +17 42 38
12 02.9 +16 23 34 12 03.6 +16 30 36
12 09.8 +15 18 32 12 10.4 +15 24 34
Near the TRUE ANTI-RADIANT
1998 November 17.3 1998 November 18.3
(1950) Surface (1950) Surface
R.A. Dec. Intensity R.A. Dec. Intensity
h m o ' h m o '
21 18.4 -39 33 4 21 50.3 -29 23 32
22 04.4 -32 15 25 22 04.0 -29 28 156
22 07.1 -31 19 216 22 05.2 -29 45 366
22 05.4 -31 09 212 22 03.5 -30 04 156
22 02.5 -31 14 135 22 00.7 -30 24 110
21 59.0 -31 25 104 21 57.4 -30 44 93
21 55.3 -31 38 86 21 53.9 -31 04 77
21 51.5 -31 52 78 21 50.2 -31 22 73
21 47.6 -32 07 70 21 46.4 -31 40 66
21 43.7 -32 21 65 21 42.6 -31 57 64
21 39.9 -32 35 63 21 38.9 -32 14 60
21 36.1 -32 49 57 21 35.2 -32 29 55
21 32.3 -33 02 57 21 30.3 -32 48 53
21 18.2 -33 49 46 21 17.6 -33 34 49
21 07.3 -34 21 42 21 06.9 -34 08 42
20 57.3 -34 47 40 20 57.1 -34 35 37
20 48.2 -35 08 39 20 48.1 -34 57 35
20 39.7 -35 25 35 20 39.7 -35 15 35
My apologies for use of 1950 coordinates. I did these calculations for
the U.K. Schmidt Telescope, which still uses 1950 for the input
field centres.
POSTSCRIPT
Subsequent to writing this, I came across the web page of a Japanese
research group who are trying to observe the "Leonid Meteoric Cloud" with
a wide field CCD camera. The main page is at
http://komadori.planet.kobe-u.ac.jp/~fujii/leonid.htm
and a map of the northern cloud is at
http://komadori.planet.kobe-u.ac.jp/~fujii/location.htm
Although it is not stated in the (english) text, the diagram does indicate
the contraction of the stream towards perihelion. The main page also has
excellent animations of the Earth's passage through the stream, looking
towards both Leo and PsA. There is no indication of relative surface
brightness in these pages however.
Robert H. McNaught
rmn@aaocbn.aaodot gov.au
Follow-Ups:
References: