The two component dustball model was used in numerical lag computation. Detached grain lag is typically less than 2 km, with expected wakes of a few hundred meters. True wake in television meteors is masked by apparent wake due to the combined effects of image persistence and blooming. To partially circumvent this problem, we modified a dual MCP intensified CID video system by addition of a rotating shutter to reduce the effective exposure time to about 2.0 ms. Preliminary observations showed that only 2 of 27 analyzed meteors displayed statistically significant wake

Hawkes, Robert L.

Robertson, M. C.

English

NASA Technical Reports Server

rAsteroids, Comets, Meteors 1991, pp. 517-520
Lunar and Planetary Institute, Houston, 1992
N9
WAKE IN FAINT TELEVISION METEORS
M.C. Robertson* and R.L. Hawkes
Physics Department, Mount Allison University,
Sackville, New Brunswick, Canada EOA 3C0
5/ o- c/ o
/
The two component dustball model was used in numerical lag computation. Detached grain lag is typically less
than 2 km, with expected wakes of a few hundred meters. True wake in television meteors is masked by apparent
wake due to the combined effects of image persistence and blooming. To partially circumvent this problem, we
modified a dual MCP intensified CID video system by addition of a rotating shutter to reduce the effective
exposure time to about 2.0 ms. Preliminary observations showed that only 2 of 27 analyzed meteors displayed
statistically significant wake.
INTFIODUCTION
Wake is the degree to which an instantaneous meteor image is spatially distributed. The classic wake paper (Mc-
Crosky, 1958) pointed out that a large proportion (28%) of Super-Schmidt meteors showed significant luminosity
in "off" portions of rotating shutter photographs. Since wake was more prevalent in low velocity meteors, he ar-
gued that wake was due to differential grain deceleration (lag), and not the result of atomic excitation and decay
processes. The lag of grains from a fragmenting dustbail was consistent with the emerging view of meteoroids as
low density, structurally weak bodies which fragmented during flight (Jacchia, 1955), which in turn was in agree-
ment with Whipple's "dirty snowball" cometary model (Whipple, 1950, 1951). Studies of meteor wake provide
one method of determinin_g the size distribution of meteoroid constituent grains. McCrosky (1958) suggested that
grains of the order of 10- kg matched observational wake evidence, a value in agreement with flare analysis re-
suits (Smith, 1954; Simonenko, 1968a). Although image intensified video detectors are common in meteor obser-
vation (Hawkes & Jones, 1986), to our knowledge this is the first study of television meteor wake.
NUMERICAL STUDY OF GRAIN lAG
We have previously performed numerical solutions of the coupled differential equations of meteoroid grain flight
(Nicol et al., 1985; Fyfe & Hawkes, 1986). For this work we have added an equation specifying the lag of the
grain, and have computed the luminous intensity. The rate of change of height, h, can be expressed as
dh= -v cos(z) (1)
dt
where v is the instantaneous meteor speed and z the zenith angle. The meteoroid grain deceleration is
d._y_v=_FAv2pa + g (2)
dt m 1/3p2n_3
with F the drag coefficient, A the grain shape factor, p_ the atmospheric density, m the instantaneous meteor
mass, Pm the grain bulk density and g the acceleration due to gravity. The rate of mass loss is given by
dm - Am2/3 [4cle(_b - "l_a)- -_] (3)
dt Lp2m/3
where L is the sum of the latent heats of vaporization and fusion, o the Stefan-Boltzman constant, T b the boiling
point of the meteoroid material, T a the effective atmospheric temperature, e the emissivity and A the heat trans-
fer coefficient. We have considered thermal radiation, but, assuming very small grains, have ignored thermal heat
capacity. If lag, I, is defined as the separation between a grain and an undecelerated parent meteoroid
dl = voo- v (4)
dt
where voo is the initial speed. This definition of lag is reasonable for grains ejected from much larger parent
bodies. If the luminous visual power is proportional to the rate of change of kinetic energy of ablated meteoric
atoms, and the luminous efficiency factor varies linearly with speed, the grain luminosity will be
517
"1"current address Physics Department, Queen's University, Kingston, Ontario, Canada K7L 3N6
https://ntrs.nasa.gov/search.jsp?R=19930010044 2020-03-17T07:27:51+00:00Z
518 Asteroids, Comets, Meteors 1991
M=-2.5log{- v3mt (5)2 dt /
We performed numerical integration of eqtns. (1) through (4), and then applied equation (5), using a fourth-order
Runge-Kutta procedure with adaptive step size control (a slightly modified form of procedure RKQC in Press et
al., 1986). A cubic polynomial fit between height and the logarithm of the atmospheric density (according to the
U.S. Standard Atmosphere) was used. Constant values used were A = 1.21, g = 9.60, F ---1.0, L = 6.0xl06, A =
1.0, e = 1.0, Ta = 280, Tb = 2100 and "Co=1.0xl0 -10 (in all cases SI units). Weperformed 2304 numerical _
runs, corresponding to all combinations of the following values: cos z (0.4, 0.7, 1.0 ); preatmospheric velocity
(15, 41, 60, 66 km s-12; initial grain mass (10 -8, 10-9, 10-10,10-11, 10-12, 10-13,10 -14, 10-15 kg); grain bulk den-
sity (700, 3500 kg m-J); and ejection height _85, 90_95, |00, I_XV20,_I30, 140, i50, 160 km). Space
does not permit presentation of the complete numerical results (see Robertson, 1990). Fig. 1 illustrates typical re-
suits for 41 km s-1 velocity and cos(z) = 0.7. Forthe sm_est grains, even the higher Speeds will not produce
significant ablation, and noiag is ploited. Asexpected_ the lag increases as the grain mass or grain density de-
creases, Since the smaller mass to Surface area ratio results in more rapid deceieration. The iag increases With in-
creasing height of ejection. The principal reason to carry out lag computationsqs not to confirm these predictable
trends, but rather to determine the order of magnitude of the lag. According to the dustball model of Hawkes &
'i r_,,OS(_')"--017..............• velocity = 41 km s"1 .
_,2.01_ density = 700
< 1._
-Jo.
80 100 120 140 160
HEIGHT OF EJECTION (KM)
3.0
Figure 1 2.5.
Plot of lag at maximum luminosity
versus mass of detached grains _" 2.0"
and height of grain ejection. The
above plot Is for a meteoroid den-
sity of 700 kg m"3, while the plot
on the right Is for 3500 kg m"3.
_" 1.51
- 1.01
051
£
Immm| j
mimmmm -?
80 100
O Log(m):-8 _ :_
L6g (m) = :9
A Log (m) : -10
0 Log(m)=-11 :
4. Log (m) = -12
X Log (m) = -13
• Log (m) = -14
• Log (m) =-15
....... k i i i • - -cos (z) = 0.7
velocity = 41 km s-1
density = 3500 kg m-"
12o 14o 160
...... HEi_TOF EJECTiON(KM)
: : z : L z: _- =
Jones (1975) the composite meteoroid has two constituents, grains and a binding material of somewhat lower boil-
ing point. For bright meteors grain release and grain ablation occur almost simultaneously. For faint meteors the
grains will be released prior to grain ablation, and an essentially independent cluster of grains encounters the at-
mosphere. Wake could result from grains of uniform size released at different heights, or from grains of different
i
=
=
=
g
_=_-
N
=
Z
Asteroids, Comets, Meteors 1991 519
sizes ejected at the same height. For example, Fig. 1 suggests that a mixture of grains of size 10-9 to 10-10 kg
would lead to wake of approximately 250 m. If grain release occurs high in the atmosphere, and at a variety of
heights, rather spectacular lags and associated wakes result. Grains of size 10-I 1 kg released over a range from
110 to 150 km would result in about a 2 km wake.
OBSERVATIONAL EQUIPMENT
Interpretation will be most straightforward in the case of meteors small enough to have fragmented into a cluster
of grains prior to the onset of intensive grain evaporation. Beech (1984, 1986) has applied techniques developed
by Hapgood et at. (1982) to estimate the critical size, which he suggests may be -2 for Perseid meteors, -I for
Geminid meteors and +0.5 for Southern-Taurid meteors. However, there is considerable scatter in the data, and
image intensified television systems should be used to achieve the required sensitivity with confidence. The prob-
lem with such systems is that the combined effects of persistence and meteor motion during the frame integration
time results in extensive apparent wake which totally masks real wake. Blooming of bright images provides a
further complication. The persistence problem can be reduced by making the effective image intensifier exposure
short. The optimum way to accomplish this would be to employ an electronically gated image intensifier, but cost
precluded our adoption of this approach. We chose instead to use a mechanical rotating shutter driven by a 30 Hz
synchronous motor (to keep the shutter in phase with the line locked video camera). The shutter had two circular
I MCP I MCP CID
I image image video
I intensifier intensifier camera
video monitor
30 Hz Synch
Rotating
Shutter 00011 video H videorecorder timegenerator
Figure 2 Block Diagram of Observing Equipment
holes of radii 3.0 cm in an overall shutter of radius 15.0 cm, resulting in one exposure per video field (1/60 s),
with an effective exposure time of about 2.0 ms. A block diagram of our observational equipment is shown in
Fig. 2. The first microchannel plate (MCP) image intensifier was a Varo model 3603-1 (luminous gain 30,000).
It was lens coupled to an ITT 4562 camera (a MCP fibre-optically coupled to a GE TN2505 CID video camera).
The sensitivity was about +7.5 M with a 135 mm/f3.5 objective lens (field of view 8.1 ° by 6.1°). The rotating
shutter was clearly effective in reducing apparent wake due to image motion and persistence, with most frames in-
dicating roughly circular meteor images.
ANALYSISANDRESULTS
Data were collected during 5 nights in Aug. and Sep. 1989 from Sackville, N.B., Canada (long. 64° 22' 24" W;
lat. 45* 53' 35" N). A total of 27 meteors were analyzed from a 24h 48m observing period. Each video frame
was digitized using an Oculus-200 video digitizer in an IBM XT {resolution 512x5t2 pixels, 7 bit (128 gray
levels) depth}. If a meteor exhibits wake then the pixels contributing to the meteor's image should be noncircular
with the best fit line through those pixels aligned with the meteor trajectory. After each video frame had been
background corrected a linear regressional analysis was performed with every pixel weighted according to that
pixel's luminous intensity. A meteor was only deemed to have wake if more than one of its images produced a
slope in agreement (within the standard error) with the meteor trajectory. Of the 27 meteors studied, only 2 dem-
onstrated statistically significant wake. To determine the size of the wake we started at the image centroid and
moved backward in the direction of the trajectory until the pixel intensity was less than two standard deviations
above the background noise. This apparent wake was then corrected for blooming and image motion during the
520 Asteroids, Comets, Meteors 1991
2.0 ms effective exposure. The maximum corrected wake was 30 pixels for one meteor, and 7 pixels for the
other. In both cases the maximum wake point was near the end 0fth_ry. A rad[ani-_ippaient speed check
indicated that the two meteors were sporadic, and we can only estimate the statistically probable spatial dimension
of the wake. At the elevation angle of 54 °, assuming an ablation height of 95 km and 0.7 as a correction for the
statistically probable trail orientation, the probable spatial dimensions of the wake at maximum are 780 m and
160 m ....
_DISCUSSION : .....
The failure to find significant lag in most of the meteors is surprising. One explanation would be that grains were
all of approximately the same size, and were ejecied at about the same height, so that each grain would lag by the
same amount. Simonenko (1968b) found that the range of fundamental meteoroid grain sizes was quite limited.
Another possibility would be that the grains were Very large, in which case they would have smali lags which
might not be evident with the spatial resolution of our observing system. The origin of fainter meteors is general-
ly assumed to be cometary, but some authors (Olsson-Steel, 1988) have recently suggested that the asteroid belt
may be an important source for smaller meteoroids. This might mean that the structure of small meteoroids is
more compact, and not a loose collection of grains. The boiling point of the glue might be very similar to that of
the grains in which case there would be little time for lag. One should keep in mind that these results are based on
a small number of meteors, and with an observing system limited in spatial and intensity resolution.
Acknowledgements:
This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Bell Jun-
ior Research Fellowship Program. Stewart Walker provided technical assistance. David Fleming assisted with
image digitization and provided helpful comments on an earlier draft of the pa_r_ - _ :: _-_; =_ _ i
References:
Beech, M. (1984) The structure of meteoroids. Mon. Not. R. astr. Soc. 211,617-620.
Beech, M. (1986) The Draconid meteors. Astron. J. 91, 159-162. = = - _:
Fyfe, J.D.D. & Hawkes, R.L. (1986) Residual mass from ablation of meteoroid grains detached during atmospher-
ic flight Planet. Space Sci. 34, 1201-1212
Hapgood, M., Rothwell, P. & Royvrik, O. (1982) Mon. Not. R. astr. Soc. 201,569-577.
Hawkes, R.L. & Jones, J. (1975) A quantitative model for the ablation of dustball meteors. Mon. Not. R. astr.
Soc. 173, 339-356. : :: ....... :__ _ ..... _ _::::: :
Hawkes, R.L. & Jones, J. (1986) Electro-optical meteorobservation techniques and resuiis. Q.Jl. R. astr. Soc. 27,
569-589.
Jacchia, L.G. (1955) The physical theory of meteors. Viii: Fragmentation as cause of the faint meteor anomaly.
Astrophys J. 121,521-527.
McCrosky, R.E. (1958) The meteor wake. Astron. J. a3, 97-106.
Nicol, EJ., Macfarlane, J. & Hawkes, R.L. (t985) Residual mass from atmospheric ablation of small meteoroids.
Planet. Space Sci. 33, 315-320.
Olss0fi:Steel; D: (i988)-Theoi'eiical meteor radiants-o_ reCentiy_discovered asteroqds and_com_ts, and tffin"
showers of known meteoroid streams. Attst. J. Astr. 2, 93-101.
Press, W.H., Flannery, B.P., Teukolsky, S.A. & Vetterling, W,T. (1986) Numerical Recipes." The art of scientific
computing. Cambridge Univ. Press, New York. 735 pp.
Robertson, M.C. (1990) A theoretical and observational study of meteor wake. B.Sc. Honours Thesis, Mount Al-
lison University, Sackville, N.B. Canada. 124 pp. ..... -: :, ::::-:-:
Simonenko, A.N. (1968a) TheT_havi0ur of small pariicles detached from a meteoroid. Soviet Astron. - AJ 12,
341-348.
Simonenko, A.N_ (_:968b) The separation of small particles from meteor bodies, and its influence on some
parameters of meteors. In Physics and Dynamics of Meteors (L. Kresak and P.M. Millman, eds.), 207-216.
D. Riedel, Dordrecht._::_: _-_ _=-- ................... _: _: _ _ :...... :: .... _ : :
Smith, H.J. (1954) The physical theory of meteors: ViThe masses of meteor-flare fragments. Astrophys. J. 119,
Whipple, F.L. (1950) A comet model: I. The acceleration of Comet Encke. Astrophys. J. 111,375-394.
Whipple, F.L. (1951) A comet model: II. Physical relations for comets and meteors. Astrophys. J. 113, 464-474.
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Wake in faint television meteors

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