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A spectroscopy pipeline for the Canary island long baseline observatory meteor detection system
Journal Pre-proof A spectroscopy pipeline for the Canary island long baseline observatory meteor detection system Regina Rudawska, Joe Zender, Detlef Koschny, Hans Smit, Stefan Löhle, Fabian Zander, Martin Eberhart, Arne Meindl, Imanol Uriarte Latorre PII:
S0032-0633(18)30218-6
DOI:
https://doi.org/10.1016/j.pss.2019.104773
Reference:
PSS 104773
To appear in:
Planetary and Space Science
Received Date: 11 June 2018 Revised Date:
10 September 2019
Accepted Date: 6 October 2019
Please cite this article as: Rudawska, R., Zender, J., Koschny, D., Smit, H., Löhle, S., Zander, F., Eberhart, M., Meindl, A., Latorre, I.U., A spectroscopy pipeline for the Canary island long baseline observatory meteor detection system, Planetary and Space Science (2019), doi: https://doi.org/10.1016/ j.pss.2019.104773. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
A spectroscopy pipeline for the Canary Island Long Baseline Observatory meteor detection system Regina Rudawskaa , Joe Zendera , Detlef Koschnya,b , Hans Smita , Stefan L¨ohlec , Fabian Zanderc , Martin Eberhartc , Arne Meindlc , Imanol Uriarte Latorred a
Science Support Office European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands b Lehrstuhl f¨ ur Raumfahrttechnik, TU Munich, 85748 Garching, Germany c Universitat Stuttgart, Institut f¨ ur Raumfahrtsysteme, 70569 Stuttgart, Germany d Technical University of Berlin, Berlin, Germany
Abstract We demonstrated the capability of the updated Canary Island Long Baseline Observatory (CILBO) meteor detection system to measure relative elements intensities of meteors. Meteor spectra provide valuable information on the chemical properties of individual meteoroids. In some cases, this may be the only information on the chemical composition of the parent bodies, and on transforming processes that occur during the meteoroid’s journey from its source to Earth. The CILBO spectroscopic program has been created with the intention of carrying out regular systematic spectroscopic observations. At the same time, the meteoroid trajectory and pre-atmospheric orbit are independently measured from data collected by the other cameras in the network. We presented the meteor spectroscopy pipeline developed by the Meteor Research Group of the European Space Agency, and it’s application to the spectroscopic survey of Geminid meteor shower observed by CILBO. Keywords: Meteors, Spectroscopy, Geminids
Preprint submitted to Planetary and Space Science
October 17, 2019
1
1. Introduction
2
Meteor spectra provide valuable information on the chemical properties
3
of individual meteoroids (Trigo-Rodriguez et al., 2003; Boroviˇcka et al., 2005;
4
Jenniskens, 2007; Madiedo et al., 2013b; Voj´aˇcek et al., 2015; Boroviˇcka and
5
Berezhnoy, 2016; Rudawska et al., 2016; Matloviˇc et al., 2017; Bloxam and
6
Campbell-Brown, 2017). In some cases, this may be the only information on
7
the chemical composition of the parent bodies, and on transforming processes
8
that occur during the meteoroid’s journey from its source to Earth.
9
The physical properties of meteoroids within the Geminids – one of the
10
most prominent showers visible from northern hemisphere latitudes – are
11
of particular interest due to their atypical parent body. The Geminid me-
12
teor shower has been dynamically associated with near-Earth asteroid (3200)
13
Phaethon (Whipple, 1983; Fox et al., 1984; Hunt et al., 1985; Kramer and
14
Shestaka, 1992; Williams and Wu, 1993a). Modelling the dynamics of Gemi-
15
nid stream meteoroids can match many features of the observed shower, but
16
properties such as a double-peaked activity profile (Ryabova, 2016) are still
17
not fully understood. This also leads to uncertainties in the age and therefore
18
the dynamical history of the Geminids. The annual periodic appearance of
19
the meteor shower makes the parent body an extraordinary object to observe
20
in itself. In 2009, weak activity of asteroid Phaethon itself was reported (Je-
21
witt and Li, 2010; Jewitt, 2012; Li and Jewitt, 2013). The most plausible
22
cause of this observed brightening is dust production as a result of thermal
23
fracture and decomposition.
24
25
Existing datasets of spectra contain observations of the Geminids (Boroviˇcka et al., 2005; Madiedo et al., 2013a; Rudawska et al., 2013; Voj´aˇcek et al., 2
26
2015; Rudawska et al., 2016). More recently, Voj´aˇcek et al. (2015) show that
27
Geminids within their database of low resolution spectra have sodium-poor
28
spectra, consistent with them being on a near-Sun orbit for a significant pe-
29
riod of time. They also show that there is a significant variation in the level
30
of sodium within the Geminids.
31
The identification of the chemical composition of a meteor spectra is typ-
32
ically extracted from an existing catalogue, as published by e.g. Halliday
33
(1968), Ceplecha (1971), Bronshten (1981), and Borovicka (1994a). The cat-
34
alogue published by Borovicka (1994a,b) is based on thermal equilibrium, a
35
simple geometrical model, and an assumption on the chemical composition
36
(among others), from which the number of free electrons and the ionization
37
state can be computed for a given temperature. Whereas we used exist-
38
ing catalogues for earlier meteor spectra analysis, e.g. Zender et al. (2002);
39
Rudawska et al. (2014), this analysis is based on a radiation model (PA-
40
RADE) that allows the modelling of both chemical composition and temper-
41
ature to the obtained meteor spectra.
42
The Canary Island Long Baseline Observatory (CILBO) performs con-
43
tinuous video observations of meteors from two stations on the islands of
44
La Palma and Tenerife. A camera with an objective grating is located on
45
Tenerife. This has provided us with a database of individual meteor spec-
46
tra observed within the time period of 2014 to 2017. Here we examine the
47
spectra of 14 Geminid meteoroids.
48
In this paper we introduce the meteor spectroscopy pipeline developed
49
by the Meteor Research Group of the European Space Agency, and it’s ap-
50
plication to the spectroscopic survey of Geminid meteor shower observed
3
51
by CILBO. In Section 2 we briefly describe the spectroscopy pipeline itself.
52
Section 3 summarizes the major findings. In Section 4 we present our con-
53
clusions.
54
2. Data acquisition and data reduction
55
The Meteor Research Group (MRG) of the European Space Agency oper-
56
ates the double-station meteor camera system CILBO (Canary Island Long-
57
Baseline Observatory). Currently, five image-intensified video cameras ob-
58
serve the night sky every clear night. Since full operations in 2012 (Koschny
59
et al., 2014a), about 70 000 meteors have been observed. With two of the
60
cameras (ICC7 and ICC9, set up on Tenerife and La Palma, respectively), we
61
have recorded almost 20000 double-station meteors (Koschny et al., 2017).
62
The recently installed large field-of-view cameras (LIC1 and LIC2) typically
63
record between 1300 and 1700 meteors per month. The 3D trajectory and
64
heliocentric orbits of these meteoroids were computed, and stored in the
65
Virtual Meteor Observatory (VMO), which is the long-term archive of the
66
International Meteor Organisation’s video meteor camera network (Koschny
67
et al., 2008, 2014b). Meteor orbits are computed using the MOTS code (Me-
68
teor Orbit and Trajectory Software) (Koschny and Diaz del Rio, 2002). In
69
particular, it contains a record of precise measurements of the Geminid me-
70
teors. In the last years, the system was upgraded to include the recording
71
of meteor spectra (Koschny et al., 2015), operating image-intensified camera
72
with objective grating (ICC8).
4
73
2.1. Meteor Spectroscopy Pipeline
74
ICC8, equipped with an objective grating with 600 lines/mm, is located
75
on the Tenerife station next to the ICC7 camera. It is tilted such that it
76
records the first order spectrum of a meteor whose zero order passes through
77
the centre of ICC7. Using the precise position of the zero order as recorded
78
in ICC7, the wavelength recorded in ICC8 can be determined (Figure 2).
79
The frames of the ICC8 camera are stored on disk by a frame grabbing
80
routine that is triggered by the METREC software, whenever a meteor is
81
detected in the ICC7 camera. The individual frames, stored in the bitmap
82
image file (BMP) format, are bundled into a single flexible image transport
83
system (FITS) format. The FITS header keywords are partly collected from
84
the METREC stored information, e.g. the meteor stream by the VIdeo
85
Data Archiving System (VIDAS, see Zender et al. (2014)). The FITS file
86
contains now all frames of an event. In addition, an additional frame is
87
created containing for each pixel the maximum value of this pixel location
88
over all frames, the so-called total image.
89
VIDAS further provides a user interface to add the values of the FITS
90
keywords for an event by the operator. In the next step, VIDAS is used to
91
perform the radiometric calibration by applying the dark current and flat
92
field correction to each of the frames. Because not each meteor detection in
93
ICC7 results in a visually detectable spectrum in ICC8, a METeor Spectra
94
Selector (MESS) routine is used to pre-select visually detectable spectra.
95
MESS analyses the ICC7 data and pre-selects the brightest events (brighter
96
than magnitude +3), maps these into the ICC8 events, displays individual
97
frames of the event of the ICC8, and allows an operator to select or release
5
98
the event for further processing. The selected events are then spectrally
99
registered by VIDAS: the METREC produced INF file of ICC7 contains for
100
each frame the sky coordinates of the meteor (right ascension, declination).
101
From this (RA, DEC) pair of ICC7, VIDAS computes the (RA, DEC) pair
102
of each frame and each wavelength between 400 nm and 800 nm in steps of
103
0.5 nm. The (RA, DEC, wavelength) triple for each frame is then processed
104
into a (x, y, wavelength) triple in the image coordinates. The spectrum is
105
then computed by collecting for each frame from 400 nm to 800 nm the pixel
106
value indicated by the (x, y, wavelength) triple.
107
VIDAS currently applies this algorithm to the total image. The resulting
108
spectra of an event are stored in a FITS file, together with the wavelength
109
information and the spectral response curve. There are two possibilities
110
to compare synthetic spectra to the observed spectra: either one applies
111
the spectral response correction to the observed spectra and compares to
112
the synthetic spectra, or one applies the spectral response correction to the
113
synthetic spectra and compares to the observed one. In the Model Meteor
114
Spectrum (MMS) routine we implemented the second option thus applying
115
the spectral response function at a given wavelength to the synthetic spectra,
116
following the approach described in Boroviˇcka et al. (2005). That is also the
117
reason for Figures 2, 3, and 5 to show uncalibrated spectra only. Spectra of
118
all meteors presented in this paper are available at https://www.cosmos.
119
esa.int/web/meteor/data-access.
120
The synthetic spectra are produced by ESA’s PlasmA RAdiation DatabasE
121
(PARADE) tool, originally used to simulate variation of probe entries into
122
planetary atmosphere’s (Smith, 2003; Pfeiffer et al., 2003; Liebhart et al.,
6
123
2012). PARADE calculates the energy state transitions in atoms and molecules
124
and provides the emission coefficient j in W/m3 /sr1 /m. We fit the emission
125
coefficient to the line intensity for each chemical element separately. In the
126
next step, we integrate over the line width and derive the relative elemental
127
intensity. The PARADE configuration has been gradually expanded in the
128
past years to include atoms and molecules measured in meteoroids (Loehle
129
et al., 2018). Together with components of the air (O, N, N2 ) the following
130
chemical species are already implemented: Na, Mg, Fe, Ca, Cr, C, K, T, V,
131
Mn, Ni, Co, CH, CN, Li, AlO, and TiO.
132
Generally, the modelling of the radiative emission of gas species is done
133
by calculating the line position (i.e. the centre wavelength) and the line
134
intensity and the line profile (see Herzberg (1945, 1950)). The position of
135
an emission (or absorption) line is defined by the energy gap between the
136
two energy levels, whose values are taken from the NIST database1 . For
137
molecules the vibrational and rotational radiative transmissions are applied
138
using the Born-Oppenheimer approximations. A detailed discussion of the
139
modelling used can be found in Loehle et al. (2018).
140
Meteor spectra is composed of blackbody radiation and line emissions.
141
The MMS allows the selection of a few parameters used by the PARADE
142
tool to simulate a synthetic spectrum. These parameters currently allow
143
to simulate one or two different temperature regimes, and the number den-
144
sity of the chemical elements of interest. The process is iterative: in the
145
first assumption, one or two temperature regimes are selected for the most 1
https://www.nist.gov/pml/atomic-spectra-database
7
146
prominent spectral line in the measured spectrum, often a metallic line, e.g.
147
Mg and Fe, and a reasonable number density of the chemical element. Rea-
148
sonable values for temperature and number density of the single element are
149
applied until the measured and synthetic spectrum fit well. In the next it-
150
eration, more elements are added, keeping the temperature unchanged. The
151
measured meteor spectra is influenced by the instrumental response of the
152
spectrograph and the optical elements, which are also taken into account.
153
The measured values of the corresponding element is obtained as integration
154
over the full width of a peak and retrieve the relative intensity of the chemical
155
element in question.
156
157
The individual steps of the data processing pipeline are sketch-out in Figure 1.
158
Figure 2 shows the visual meteor as observed by the ICC7 camera on the
159
top-left sub-image. The total image of ICC8 is shown on the right sub-image,
160
with the VIDAS identified lines over plotted as white lines. The lower plot
161
contains measured spectra with the identified elements after applying the
162
PARADE simulation using the MMS. The modelled spectrum is compared
163
with an observed spectrum of Geminid meteor integrated along its trajectory
164
in Figure 3. The spectral response curve of ICC8 is shown in Figure 4.
165
3. Results
166
The ICC8 camera has been in operation since 2012. Up until mid-2017,
167
we were able to collect about 20000 double-station meteors, including 1265
168
cases captured with meteor spectra of variable quality (from very faint and
169
hard to reduce to very bright and overexposed). The simultaneous double 8
170
meteor observations carried out by ICC7 and ICC9 allow the determination of
171
the meteor trajectory and its orbital parameters, which were computed using
172
the Meteor Orbit and Trajectory Software (MOTS) (Koschny and Diaz del
173
Rio, 2002). To obtain the error bars, 256 Monte-Carlo runs were performed
174
for each meteor, assuming an astrometric inaccuracy of 1.4’ or 1/3 pixel as
175
determined by Schmidt (2019) for our camera systems. The given error bars
176
are the median absolute deviation of the results. It should be noted that
177
the orbits are rather inaccurate, as all meteors were saturated in our double-
178
station camera systems. This makes it difficult to determine the photometric
179
center. In this paper, we present results for 14 reduced cases of Geminids
180
shown in Figures 3-8, Table 1, and Table 2, all confirmed to be part of the
181
Geminid meteor stream. No Meteor ID
a
e
ω
Ω
i
1
20131214T034441 1.26 ± 0.23 0.88 ± 0.04
324.4 ± 2.5
262.1 ± 0.0
22.2 ± 3.6
2
20131214T040503 1.32 ± 0.34 0.90 ± 0.06
325.4 ± 2.5
262.1 ± 0.0
24.5 ± 7.1
3
20131214T041019 1.24 ± 0.33 0.88 ± 0.06
324.5 ± 2.3
262.1 ± 0.0
22.2 ± 0.0
4
20131214T041338 1.40 ± 0.29 0.90 ± 0.04
324.2 ± 1.4
262.1 ± 0.0
24.0 ± 4.4
5
20131214T041909 1.31 ± 0.34 0.89 ± 0.05
324.8 ± 2.8
262.1 ± 0.0
23.6 ± 6.4
6
20151210T043202 1.25 ± 0.38 0.88 ± 0.07
322.9 ± 8.3
258.6 ± 0.0
22.1 ± 7.4
7
20151213T060110 1.35 ± 0.33 0.89 ± 0.05
323.4 ± 1.6
260.7 ± 0.0
23.2 ± 6.1
8
20151214T042226 1.48 ± 0.33 0.90 ± 0.03
323.2 ± 1.2
262.6 ± 0.0
25.0 ± 4.0
9
20151214T053616 1.44 ± 0.25 0.90 ± 0.02
324.8 ± 0.8
261.7 ± 0.0
25.4 ± 3.4
10
20151215T032955 1.59 ± 0.24 0.90 ± 0.02
321.7 ± 1.0
265.6 ± 0.0
23.7 ± 2.4
11
20151215T042956 1.19 ± 0.29 0.86 ± 0.10
323.2 ± 4.8
262.6 ± 0.0
20.4 ± 7.6
Table 1: Orbital elements of analysed Geminids from the double-station observations. Columns shows: semi-major axis and perihelion distance in AU, eccentricity, argument of perihelium, longitude of ascending node, and inclination in degrees.
9
No Meteor ID
RA
DEC
M
mp
vg
Na/Mg
Fe/Mg
Hb
He
KB
KB class
PE
PE class
1
20131214T034441
114.2
32.2
0.1
0.1
36.4
0.53
0.22
103.1
83.2
6.82
C1
-5.14
II
2
20131214T040503
115.5
31.7
-1.0
4.0
34.0
0.54
0.29
104.4
73.2
6.78
C1
-5.09
II
3
20131214T041019
114.5
32.3
0.8
0.1
35.9
0.43
0.31
104.2
82.3
6.74
C1
-4.98
II
4
20131214T041338
114.0
32.3
-0.8
0.4
35.7
0.35
0.36
104.6
76.0
6.71
C1
-4.87
II
5
20131214T041909
114.5
31.9
0.5
0.1
36.5
0.48
0.41
99.2
79.7
7.13
B
-4.83
II
6
20151210T043202
110.8
33.1
0.0
0.1
38.6
0.26
0.28
101.3
80.1
6.97
C1
-4.94
II
7
20151213T060110
112.8
33.0
0.5
0.0
36.2
0.45
0.14
103.2
80.5
6.89
C1
-4.83
II
8
20151214T042226
114.2
32.7
-0.6
0.3
36.7
0.53
0.31
103.1
72.7
6.83
C1
-4.62
II
9
20151214T053616
114.3
32.5
-1.3
0.7
35.0
0.50
0.30
104.1
75.2
6.78
C1
-4.84
II
10
20151215T032955
112.6
32.5
-0.8
0.3
36.7
0.48
0.24
101.9
79.4
6.92
C1
-5.05
II
11
20151215T042956
115.2
32.1
-1.1
0.4
36.0
0.38
0.24
100.8
77.6
7.01
C1
-4.98
II
12
20131208T213645*
-1.2
0.32
0.10
13
20131213T031836*
-1.6
0.19
0.36
14
20131213T041302*
-1.7
0.73
0.38
Table 2: Right ascension and declination in degrees, absolute magnitude, photometric mass in g, geocentric velocity in km s−1 , ratios of NaI (1)/MgI (2) and FeI (15)/MgI (2), beginning and end height meteors in km. The last columns give material strength parameters and the meteor class according to those parameters as defined in Ceplecha (1988). Additional single-station meteors assigned by METREC software are marked by *.
10
182
3.1. Meteor spectrum and material strengths classification
183
The meteor spectra has been divided into four main classes base on mea-
184
surements of three main elements (Boroviˇcka et al., 2005). The brightest lines
185
in a meteor video spectrum belong usually to not resolved Mg I (2) triplet
186
and Na I (1) doublet. Between them lie Fe I (15) multiplet of partially re-
187
solved lines between 520–550 nm. All of those elements are the most common
188
in the observed meteor spectrum, falling into the range of wavelengths where
189
detectors have their sensitivity maximum.
190
The meteor spectrum classification contains then the following classes:
191
Iron, Na-free, Na-rich, and Mainstream meteoroids. The latter one is addi-
192
tionally sub-divided and includes: Normal, Fe-poor, Na-poor and Enhanced-
193
Na sub-class of meteoroids. We describe here only those (sub-)classes that
194
Geminid meteoroids fall into. For further reading see Boroviˇcka et al. (2005).
195
Na-free meteoroids have high material strength (Boroviˇcka et al., 2005).
196
From the orbital point of view Na-free meteoroids are divided into two dis-
197
tinct populations: Sun-approaching orbits and cometary (Halley) type orbits.
198
In both cases the common feature of meteor spectrum is the lack of the Na I
199
(1) line. For cometary type orbits the reason for depletion of Na is likely to
200
be the long exposure to cosmic rays on the surface of comets during their stay
201
in the Oort cloud. Later on, when such comets enter the inner Solar System,
202
due to graduate disintegration of the refractory crust compact Na-free mete-
203
oroids are produced. Another efficient way of releasing Na is solar heating at
204
low heliocentric distances (6 0.2 AU), which is also responsible making the
205
material strength larger. In this case, frequent approaches to the Sun, i.e.
206
more perihelion passages, exposure meteoroids (in the millimetre-size range)
11
207
to the heat that leads to the loss of Na irrespective of their origin. Therefore,
208
the older meteoroids the more Na is depleted.
209
Na-poor meteoroids represent a transition class of meteoroids between
210
Na-free and Normal. They consist in features similar to both classes. Mete-
211
oroids of this class can have orbits of cometary type or/and with low perihelia.
212
In comparison to Na-free meteoroids, the Na I (1) line is visible but is signif-
213
icantly weaker than expected. Meteoroids of the Normal class are found on
214
both cometary and asteroidal orbits (Boroviˇcka et al., 2005). The position
215
on the ternary graph lays near chondritic values (see Figure 6). Except for
216
iron for which line intensity appear to be often fainter, reaching chondritic
217
values only for more massive meteoroids where bright lines become optically
218
thick.
219
On the other end of the normal class of meteoroids lay Fe-poor class.
220
Members of this class have nearly normal content of sodium with significant
221
depletion of iron that does not let classify them as normal meteoroids. Fe-
222
poor meteoroids also have lower the average material strength and probably
223
originated from comets.
224
225
Examples of Na-poor, Normal and Fe-poor meteoroids observed by CILBO is presented in Figure 5.
226
The graphic representation of the spectral classification is often repre-
227
sented in the form of a ternary graph (see Figure 6 in Boroviˇcka et al. (2005)).
228
However, for better analysis the spectral classification is combined with the
229
orbital and the material strength classifications.
230
The material strengths of meteoroids are estimated using parameters re-
231
lated to the ablation beginning and terminal heights (KB and PE , respec-
12
232
tively) that were introduced by Ceplecha (1968); Ceplecha and McCrosky
233
(1976). On their basis a corresponding meteoroid material strength classi-
234
fication was introduced as well (Ceplecha, 1988). Here, as well as with the
235
meteor spectrum classification, material strength classification distinguish
236
between the different types of material groups (with characteristic typical
237
for strong asteroidal bodies to fragile comets).
238
3.2. CILBO Geminids
239
In Figure 6 we compare the measured relative intensities of the magne-
240
sium, sodium, and iron multiplets obtained by integration along the whole
241
path of a meteor. Also shown are meteors from the Geminid surveys (Boroviˇcka
242
et al. (2005) (diamonds) and Voj´aˇcek et al. (2015) (crosses)). The majority of
243
Geminid meteoroids represent Fe-poor spectral class with similar Na I/Mg I
244
and Fe I/Mg-I ratios, see Table 2. We do observe however that the appar-
245
ent variations in sodium reveals dispersion of its content in individual cases,
246
ranging from Na-free bodies to Normal type composition. The position of
247
our Geminids in the ternary diagram is closer to Geminds in Voj´aˇcek et al.
248
(2015), filling the gap in their sample, and concurrently linking with Gemi-
249
nids from Boroviˇcka et al. (2005).
250
Only five Geminds in our sample were observed from two stations, there-
251
fore, only for them trajectory and orbital elements were calculated (Table
252
1). The beginning of the meteor luminosity path allows to classify the ma-
253
terial strength of meteoroids. Moreover, depending on an entry speed and
254
mass of a meteoroid, the ablation starts at higher heights for meteoroids
255
of cometary origin than the asteroidal one (Ceplecha and McCrosky, 1976;
256
Koten et al., 2004). Figure 7 shows meteor beginning height as a function 13
257
of speed., where our double-station Gemnids from CILBO (blue circles) are
258
compared to those derived by Boroviˇcka et al. (2005). Here grey symbols
259
represent: Normal (filled square), Na-poor (filled circle), Fe-poor (filled di-
260
amond), enhanced Na (filled triangle), irons (cross), Na-free (circle), and
261
Na-rich (triangle). The lines show the empirical mean beginning of average
262
meteoroids strength (solid) and their limits (dashed) (Boroviˇcka et al., 2005).
263
Position of CILBO Geminids once again shows the measured dispersion in
264
sodium. This is also presented in Figure 8, where the observed Mg/Na line
265
intensity ratio in CILBO Geminds as a function of meteor speed. The corre-
266
sponding figures (Figures 7 and 8) for the double-station meteors with known
267
heights have high beginning heights of ablation. Therefore, they have an av-
268
erage material strength that is typically expected for Normal and Fe-poor
269
class (Boroviˇcka et al., 2005).
270
4. Conclusions
271
The Canary Island Long Baseline Observatory (CILBO) spectral cam-
272
era covers limiting magnitudes of faint meteors. Already gathered data and
273
preliminary analysis show that the meteor spectroscopic survey at CILBO
274
will support well other existing meteor spectroscopic observations. Particu-
275
larly those that are carried out of meteor showers campaigns and/or are fo-
276
cused on centimetre-sized meteors (Boroviˇcka et al., 2005; Jenniskens et al.,
277
2014; Rudawska et al., 2013, 2016) and fireballs (Borovicka, 1993; Madiedo
278
et al., 2013b). Therefore, CILBO spectroscopic program will supply existing
279
databases in elemental compositions for meteoroids.
280
In the sample of the Geminids collected by CILBO we see a variation in 14
281
the level of sodium. Our Geminds show Na-free, Na-poor, and Normal spec-
282
tra (Boroviˇcka et al., 2005). The Geminids are known to start ablation at
283
high altitudes. It causes the release of volatile component much quicker, and
284
therefore, the Geminds may not contain this component either. Moreover,
285
the variation of volatiles can be correlated with the age of the meteoroids
286
as well. The Geminid meteoroid stream has small perihelion distance of
287
q = 0.14 AU. Thus, those cases with more Na I represent a sample of rela-
288
tively young meteoroids that experienced shorter time exposure to the Sun,
289
and the one with less sodium are older.
290
291
5. Future work
292
While this paper has demonstrated the potential of the CILBO spectral
293
pipeline, a few opportunities for extending the scope of it remain. Here we
294
present some of planned future works:
295
• To improve effectiveness of meteor spectrum detection in collected data,
296
we plan to modernize METeor Spectra Selector (MESS) applying ma-
297
chine learning techniques.
298
• We are currently extending PARADE database to include more atoms
299
and particularly more molecules common in meteor spectra (CaO,
300
MgO, FeO,...), and preliminary results of AlO and TiO are encour-
301
aging (Loehle et al., 2018).
302
• Moreover, the upgraded version of ESA Meteor Research Group soft-
303
ware is planned to be available as an open-source python package called 15
304
mrg-tool. It will include also the described above part of the meteor
305
spectrum pipeline (mrg spectrum).
306
307
Acknowledgments We acknowledge funding support from the faculty of ESA’s Science Sup-
308
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22
ICC7
ICC8
meteor detection
store frames
BMP
METREC
GRAB
BMP
fits creation VIDAS
FITS
radiometric calibration VIDAS
FITS
FITS
dark current flat field
MESS
event selection
FITS
INF
FITS
spectral registration
spectral response
VIDAS
FITS
plasma radiation
ablation modeling MMS
PARADE
23
Figure 1: Diagram of the meteor spectrum data pipeline.
120
MgI
Relative intensity
100 80
OI
60 NaI
40 20 0 400
CrI FeI FeI
450
FeI
500
FeI FeIOI MgI
OI N2
NI
OI N2
550 600 650 Wavelength / nm
700
N2
750
800
Figure 2: An example of meteor observed by CILBO on 14 December 2013 at 04:13:02 UT. In the top left is the meteor seen by the ICC7 camera on Tenerife. In the right picture the same meteor is shown as recorded by the ICC8 camera. The bottom graph shows the profile of meteor spectra without sensitivity correction with chemical species
.
24
100 Relative intensity
80 60 40 20 0 400
450
500
550 600 650 Wavelength [nm]
700
750
800
Figure 3: Observed spectrum of a meteor (ID: 20131213T041302) integrated along its trajectory (blue) compared to modelled spectrum (orange). Spectrum presented here is without spectral response correction.
Relative intensity
1.0 0.8 0.6 0.4 0.2 0.0 400
450
500
550 600 650 Wavelength [nm]
700
750
800
Figure 4: Sensitivity of the camera obtained by measuring a spectrum of Vega. The relative intensity has been normalized to unity at 455.5 nm.
25
ICC8_20131213T031836_spect
35
Relative intensity
30 25 20 15 10 5 0 400
450
500
550 600 650 Wavelength [nm] ICC8_20131214T041302_spect
700
750
800
450
500
550 600 650 Wavelength [nm] ICC8_20131208T213645_spect
700
750
800
450
500
550 600 650 Wavelength [nm]
700
750
800
100 Relative intensity
80 60 40 20 0 400 25
Relative intensity
20 15 10 5 0 400
26 Figure 5: Meteor spectra of different spectral class (without the spectral response correction). Top: Na-poor (ID: 20131213T031836); middle: Normal (ID: 20131213T041302); bottom: Fe-poor (ID: 20131208T213645).
Fe I
40 |
30 |
20 |
Mg I
15 |
Na I
Figure 6: The measured relative intensities of the magnesium, sodium, and iron multiplets. Line intensities are obtained by integration along the whole path of the meteor. Also shown are meteors from the Geminid surveys: Boroviˇcka et al. (2005) (diamonds) and Voj´aˇcek et al. (2015) (crosses).
27
130 120 110 100 90 70
80
Beginning height [km]
● ●● ● ● ●● ● ●
0
20
40
60
80
Speed [km/s] Figure 7: Meteor beginning height as a function of speed. Geminids from our analysis (blue circles). Grey symbols represent Boroviˇcka et al. (2005): Normal (filled square), Napoor (filled circle), Fe-poor (filled diamond), enhanced Na (filled triangle), irons (cross), Na-free (circle), and Na-rich (triangle).
28
1.0 0.5 0.0 −1.0
−0.5
Na/Mg (log)
2 18 ● ● 9● 5 10 ● ● 7 ● 3 ● 11 ● 4 ● 6 ●
10
20
30
40
50
60
70
Speed [km/s] Figure 8: The observed Mg/Na line intensity ratio as a function of meteor speed. The solid line is an approximate fit of the meteors classified as having normal Mg and Na intensities (Boroviˇcka et al., 2005). Used symbols are identical to the ones in Figure 7.
29
• • •
We demonstrated the capability of the ESA/CILBO meteor detection system. We analysed a sample of meteor spectra collected in December 2013 and 2015. In the sample of the Geminids collected by CILBO we see a variation in the level of sodium.
S0032-0633(18)30218-6
DOI:
https://doi.org/10.1016/j.pss.2019.104773
Reference:
PSS 104773
To appear in:
Planetary and Space Science
Received Date: 11 June 2018 Revised Date:
10 September 2019
Accepted Date: 6 October 2019
Please cite this article as: Rudawska, R., Zender, J., Koschny, D., Smit, H., Löhle, S., Zander, F., Eberhart, M., Meindl, A., Latorre, I.U., A spectroscopy pipeline for the Canary island long baseline observatory meteor detection system, Planetary and Space Science (2019), doi: https://doi.org/10.1016/ j.pss.2019.104773. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
A spectroscopy pipeline for the Canary Island Long Baseline Observatory meteor detection system Regina Rudawskaa , Joe Zendera , Detlef Koschnya,b , Hans Smita , Stefan L¨ohlec , Fabian Zanderc , Martin Eberhartc , Arne Meindlc , Imanol Uriarte Latorred a
Science Support Office European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands b Lehrstuhl f¨ ur Raumfahrttechnik, TU Munich, 85748 Garching, Germany c Universitat Stuttgart, Institut f¨ ur Raumfahrtsysteme, 70569 Stuttgart, Germany d Technical University of Berlin, Berlin, Germany
Abstract We demonstrated the capability of the updated Canary Island Long Baseline Observatory (CILBO) meteor detection system to measure relative elements intensities of meteors. Meteor spectra provide valuable information on the chemical properties of individual meteoroids. In some cases, this may be the only information on the chemical composition of the parent bodies, and on transforming processes that occur during the meteoroid’s journey from its source to Earth. The CILBO spectroscopic program has been created with the intention of carrying out regular systematic spectroscopic observations. At the same time, the meteoroid trajectory and pre-atmospheric orbit are independently measured from data collected by the other cameras in the network. We presented the meteor spectroscopy pipeline developed by the Meteor Research Group of the European Space Agency, and it’s application to the spectroscopic survey of Geminid meteor shower observed by CILBO. Keywords: Meteors, Spectroscopy, Geminids
Preprint submitted to Planetary and Space Science
October 17, 2019
1
1. Introduction
2
Meteor spectra provide valuable information on the chemical properties
3
of individual meteoroids (Trigo-Rodriguez et al., 2003; Boroviˇcka et al., 2005;
4
Jenniskens, 2007; Madiedo et al., 2013b; Voj´aˇcek et al., 2015; Boroviˇcka and
5
Berezhnoy, 2016; Rudawska et al., 2016; Matloviˇc et al., 2017; Bloxam and
6
Campbell-Brown, 2017). In some cases, this may be the only information on
7
the chemical composition of the parent bodies, and on transforming processes
8
that occur during the meteoroid’s journey from its source to Earth.
9
The physical properties of meteoroids within the Geminids – one of the
10
most prominent showers visible from northern hemisphere latitudes – are
11
of particular interest due to their atypical parent body. The Geminid me-
12
teor shower has been dynamically associated with near-Earth asteroid (3200)
13
Phaethon (Whipple, 1983; Fox et al., 1984; Hunt et al., 1985; Kramer and
14
Shestaka, 1992; Williams and Wu, 1993a). Modelling the dynamics of Gemi-
15
nid stream meteoroids can match many features of the observed shower, but
16
properties such as a double-peaked activity profile (Ryabova, 2016) are still
17
not fully understood. This also leads to uncertainties in the age and therefore
18
the dynamical history of the Geminids. The annual periodic appearance of
19
the meteor shower makes the parent body an extraordinary object to observe
20
in itself. In 2009, weak activity of asteroid Phaethon itself was reported (Je-
21
witt and Li, 2010; Jewitt, 2012; Li and Jewitt, 2013). The most plausible
22
cause of this observed brightening is dust production as a result of thermal
23
fracture and decomposition.
24
25
Existing datasets of spectra contain observations of the Geminids (Boroviˇcka et al., 2005; Madiedo et al., 2013a; Rudawska et al., 2013; Voj´aˇcek et al., 2
26
2015; Rudawska et al., 2016). More recently, Voj´aˇcek et al. (2015) show that
27
Geminids within their database of low resolution spectra have sodium-poor
28
spectra, consistent with them being on a near-Sun orbit for a significant pe-
29
riod of time. They also show that there is a significant variation in the level
30
of sodium within the Geminids.
31
The identification of the chemical composition of a meteor spectra is typ-
32
ically extracted from an existing catalogue, as published by e.g. Halliday
33
(1968), Ceplecha (1971), Bronshten (1981), and Borovicka (1994a). The cat-
34
alogue published by Borovicka (1994a,b) is based on thermal equilibrium, a
35
simple geometrical model, and an assumption on the chemical composition
36
(among others), from which the number of free electrons and the ionization
37
state can be computed for a given temperature. Whereas we used exist-
38
ing catalogues for earlier meteor spectra analysis, e.g. Zender et al. (2002);
39
Rudawska et al. (2014), this analysis is based on a radiation model (PA-
40
RADE) that allows the modelling of both chemical composition and temper-
41
ature to the obtained meteor spectra.
42
The Canary Island Long Baseline Observatory (CILBO) performs con-
43
tinuous video observations of meteors from two stations on the islands of
44
La Palma and Tenerife. A camera with an objective grating is located on
45
Tenerife. This has provided us with a database of individual meteor spec-
46
tra observed within the time period of 2014 to 2017. Here we examine the
47
spectra of 14 Geminid meteoroids.
48
In this paper we introduce the meteor spectroscopy pipeline developed
49
by the Meteor Research Group of the European Space Agency, and it’s ap-
50
plication to the spectroscopic survey of Geminid meteor shower observed
3
51
by CILBO. In Section 2 we briefly describe the spectroscopy pipeline itself.
52
Section 3 summarizes the major findings. In Section 4 we present our con-
53
clusions.
54
2. Data acquisition and data reduction
55
The Meteor Research Group (MRG) of the European Space Agency oper-
56
ates the double-station meteor camera system CILBO (Canary Island Long-
57
Baseline Observatory). Currently, five image-intensified video cameras ob-
58
serve the night sky every clear night. Since full operations in 2012 (Koschny
59
et al., 2014a), about 70 000 meteors have been observed. With two of the
60
cameras (ICC7 and ICC9, set up on Tenerife and La Palma, respectively), we
61
have recorded almost 20000 double-station meteors (Koschny et al., 2017).
62
The recently installed large field-of-view cameras (LIC1 and LIC2) typically
63
record between 1300 and 1700 meteors per month. The 3D trajectory and
64
heliocentric orbits of these meteoroids were computed, and stored in the
65
Virtual Meteor Observatory (VMO), which is the long-term archive of the
66
International Meteor Organisation’s video meteor camera network (Koschny
67
et al., 2008, 2014b). Meteor orbits are computed using the MOTS code (Me-
68
teor Orbit and Trajectory Software) (Koschny and Diaz del Rio, 2002). In
69
particular, it contains a record of precise measurements of the Geminid me-
70
teors. In the last years, the system was upgraded to include the recording
71
of meteor spectra (Koschny et al., 2015), operating image-intensified camera
72
with objective grating (ICC8).
4
73
2.1. Meteor Spectroscopy Pipeline
74
ICC8, equipped with an objective grating with 600 lines/mm, is located
75
on the Tenerife station next to the ICC7 camera. It is tilted such that it
76
records the first order spectrum of a meteor whose zero order passes through
77
the centre of ICC7. Using the precise position of the zero order as recorded
78
in ICC7, the wavelength recorded in ICC8 can be determined (Figure 2).
79
The frames of the ICC8 camera are stored on disk by a frame grabbing
80
routine that is triggered by the METREC software, whenever a meteor is
81
detected in the ICC7 camera. The individual frames, stored in the bitmap
82
image file (BMP) format, are bundled into a single flexible image transport
83
system (FITS) format. The FITS header keywords are partly collected from
84
the METREC stored information, e.g. the meteor stream by the VIdeo
85
Data Archiving System (VIDAS, see Zender et al. (2014)). The FITS file
86
contains now all frames of an event. In addition, an additional frame is
87
created containing for each pixel the maximum value of this pixel location
88
over all frames, the so-called total image.
89
VIDAS further provides a user interface to add the values of the FITS
90
keywords for an event by the operator. In the next step, VIDAS is used to
91
perform the radiometric calibration by applying the dark current and flat
92
field correction to each of the frames. Because not each meteor detection in
93
ICC7 results in a visually detectable spectrum in ICC8, a METeor Spectra
94
Selector (MESS) routine is used to pre-select visually detectable spectra.
95
MESS analyses the ICC7 data and pre-selects the brightest events (brighter
96
than magnitude +3), maps these into the ICC8 events, displays individual
97
frames of the event of the ICC8, and allows an operator to select or release
5
98
the event for further processing. The selected events are then spectrally
99
registered by VIDAS: the METREC produced INF file of ICC7 contains for
100
each frame the sky coordinates of the meteor (right ascension, declination).
101
From this (RA, DEC) pair of ICC7, VIDAS computes the (RA, DEC) pair
102
of each frame and each wavelength between 400 nm and 800 nm in steps of
103
0.5 nm. The (RA, DEC, wavelength) triple for each frame is then processed
104
into a (x, y, wavelength) triple in the image coordinates. The spectrum is
105
then computed by collecting for each frame from 400 nm to 800 nm the pixel
106
value indicated by the (x, y, wavelength) triple.
107
VIDAS currently applies this algorithm to the total image. The resulting
108
spectra of an event are stored in a FITS file, together with the wavelength
109
information and the spectral response curve. There are two possibilities
110
to compare synthetic spectra to the observed spectra: either one applies
111
the spectral response correction to the observed spectra and compares to
112
the synthetic spectra, or one applies the spectral response correction to the
113
synthetic spectra and compares to the observed one. In the Model Meteor
114
Spectrum (MMS) routine we implemented the second option thus applying
115
the spectral response function at a given wavelength to the synthetic spectra,
116
following the approach described in Boroviˇcka et al. (2005). That is also the
117
reason for Figures 2, 3, and 5 to show uncalibrated spectra only. Spectra of
118
all meteors presented in this paper are available at https://www.cosmos.
119
esa.int/web/meteor/data-access.
120
The synthetic spectra are produced by ESA’s PlasmA RAdiation DatabasE
121
(PARADE) tool, originally used to simulate variation of probe entries into
122
planetary atmosphere’s (Smith, 2003; Pfeiffer et al., 2003; Liebhart et al.,
6
123
2012). PARADE calculates the energy state transitions in atoms and molecules
124
and provides the emission coefficient j in W/m3 /sr1 /m. We fit the emission
125
coefficient to the line intensity for each chemical element separately. In the
126
next step, we integrate over the line width and derive the relative elemental
127
intensity. The PARADE configuration has been gradually expanded in the
128
past years to include atoms and molecules measured in meteoroids (Loehle
129
et al., 2018). Together with components of the air (O, N, N2 ) the following
130
chemical species are already implemented: Na, Mg, Fe, Ca, Cr, C, K, T, V,
131
Mn, Ni, Co, CH, CN, Li, AlO, and TiO.
132
Generally, the modelling of the radiative emission of gas species is done
133
by calculating the line position (i.e. the centre wavelength) and the line
134
intensity and the line profile (see Herzberg (1945, 1950)). The position of
135
an emission (or absorption) line is defined by the energy gap between the
136
two energy levels, whose values are taken from the NIST database1 . For
137
molecules the vibrational and rotational radiative transmissions are applied
138
using the Born-Oppenheimer approximations. A detailed discussion of the
139
modelling used can be found in Loehle et al. (2018).
140
Meteor spectra is composed of blackbody radiation and line emissions.
141
The MMS allows the selection of a few parameters used by the PARADE
142
tool to simulate a synthetic spectrum. These parameters currently allow
143
to simulate one or two different temperature regimes, and the number den-
144
sity of the chemical elements of interest. The process is iterative: in the
145
first assumption, one or two temperature regimes are selected for the most 1
https://www.nist.gov/pml/atomic-spectra-database
7
146
prominent spectral line in the measured spectrum, often a metallic line, e.g.
147
Mg and Fe, and a reasonable number density of the chemical element. Rea-
148
sonable values for temperature and number density of the single element are
149
applied until the measured and synthetic spectrum fit well. In the next it-
150
eration, more elements are added, keeping the temperature unchanged. The
151
measured meteor spectra is influenced by the instrumental response of the
152
spectrograph and the optical elements, which are also taken into account.
153
The measured values of the corresponding element is obtained as integration
154
over the full width of a peak and retrieve the relative intensity of the chemical
155
element in question.
156
157
The individual steps of the data processing pipeline are sketch-out in Figure 1.
158
Figure 2 shows the visual meteor as observed by the ICC7 camera on the
159
top-left sub-image. The total image of ICC8 is shown on the right sub-image,
160
with the VIDAS identified lines over plotted as white lines. The lower plot
161
contains measured spectra with the identified elements after applying the
162
PARADE simulation using the MMS. The modelled spectrum is compared
163
with an observed spectrum of Geminid meteor integrated along its trajectory
164
in Figure 3. The spectral response curve of ICC8 is shown in Figure 4.
165
3. Results
166
The ICC8 camera has been in operation since 2012. Up until mid-2017,
167
we were able to collect about 20000 double-station meteors, including 1265
168
cases captured with meteor spectra of variable quality (from very faint and
169
hard to reduce to very bright and overexposed). The simultaneous double 8
170
meteor observations carried out by ICC7 and ICC9 allow the determination of
171
the meteor trajectory and its orbital parameters, which were computed using
172
the Meteor Orbit and Trajectory Software (MOTS) (Koschny and Diaz del
173
Rio, 2002). To obtain the error bars, 256 Monte-Carlo runs were performed
174
for each meteor, assuming an astrometric inaccuracy of 1.4’ or 1/3 pixel as
175
determined by Schmidt (2019) for our camera systems. The given error bars
176
are the median absolute deviation of the results. It should be noted that
177
the orbits are rather inaccurate, as all meteors were saturated in our double-
178
station camera systems. This makes it difficult to determine the photometric
179
center. In this paper, we present results for 14 reduced cases of Geminids
180
shown in Figures 3-8, Table 1, and Table 2, all confirmed to be part of the
181
Geminid meteor stream. No Meteor ID
a
e
ω
Ω
i
1
20131214T034441 1.26 ± 0.23 0.88 ± 0.04
324.4 ± 2.5
262.1 ± 0.0
22.2 ± 3.6
2
20131214T040503 1.32 ± 0.34 0.90 ± 0.06
325.4 ± 2.5
262.1 ± 0.0
24.5 ± 7.1
3
20131214T041019 1.24 ± 0.33 0.88 ± 0.06
324.5 ± 2.3
262.1 ± 0.0
22.2 ± 0.0
4
20131214T041338 1.40 ± 0.29 0.90 ± 0.04
324.2 ± 1.4
262.1 ± 0.0
24.0 ± 4.4
5
20131214T041909 1.31 ± 0.34 0.89 ± 0.05
324.8 ± 2.8
262.1 ± 0.0
23.6 ± 6.4
6
20151210T043202 1.25 ± 0.38 0.88 ± 0.07
322.9 ± 8.3
258.6 ± 0.0
22.1 ± 7.4
7
20151213T060110 1.35 ± 0.33 0.89 ± 0.05
323.4 ± 1.6
260.7 ± 0.0
23.2 ± 6.1
8
20151214T042226 1.48 ± 0.33 0.90 ± 0.03
323.2 ± 1.2
262.6 ± 0.0
25.0 ± 4.0
9
20151214T053616 1.44 ± 0.25 0.90 ± 0.02
324.8 ± 0.8
261.7 ± 0.0
25.4 ± 3.4
10
20151215T032955 1.59 ± 0.24 0.90 ± 0.02
321.7 ± 1.0
265.6 ± 0.0
23.7 ± 2.4
11
20151215T042956 1.19 ± 0.29 0.86 ± 0.10
323.2 ± 4.8
262.6 ± 0.0
20.4 ± 7.6
Table 1: Orbital elements of analysed Geminids from the double-station observations. Columns shows: semi-major axis and perihelion distance in AU, eccentricity, argument of perihelium, longitude of ascending node, and inclination in degrees.
9
No Meteor ID
RA
DEC
M
mp
vg
Na/Mg
Fe/Mg
Hb
He
KB
KB class
PE
PE class
1
20131214T034441
114.2
32.2
0.1
0.1
36.4
0.53
0.22
103.1
83.2
6.82
C1
-5.14
II
2
20131214T040503
115.5
31.7
-1.0
4.0
34.0
0.54
0.29
104.4
73.2
6.78
C1
-5.09
II
3
20131214T041019
114.5
32.3
0.8
0.1
35.9
0.43
0.31
104.2
82.3
6.74
C1
-4.98
II
4
20131214T041338
114.0
32.3
-0.8
0.4
35.7
0.35
0.36
104.6
76.0
6.71
C1
-4.87
II
5
20131214T041909
114.5
31.9
0.5
0.1
36.5
0.48
0.41
99.2
79.7
7.13
B
-4.83
II
6
20151210T043202
110.8
33.1
0.0
0.1
38.6
0.26
0.28
101.3
80.1
6.97
C1
-4.94
II
7
20151213T060110
112.8
33.0
0.5
0.0
36.2
0.45
0.14
103.2
80.5
6.89
C1
-4.83
II
8
20151214T042226
114.2
32.7
-0.6
0.3
36.7
0.53
0.31
103.1
72.7
6.83
C1
-4.62
II
9
20151214T053616
114.3
32.5
-1.3
0.7
35.0
0.50
0.30
104.1
75.2
6.78
C1
-4.84
II
10
20151215T032955
112.6
32.5
-0.8
0.3
36.7
0.48
0.24
101.9
79.4
6.92
C1
-5.05
II
11
20151215T042956
115.2
32.1
-1.1
0.4
36.0
0.38
0.24
100.8
77.6
7.01
C1
-4.98
II
12
20131208T213645*
-1.2
0.32
0.10
13
20131213T031836*
-1.6
0.19
0.36
14
20131213T041302*
-1.7
0.73
0.38
Table 2: Right ascension and declination in degrees, absolute magnitude, photometric mass in g, geocentric velocity in km s−1 , ratios of NaI (1)/MgI (2) and FeI (15)/MgI (2), beginning and end height meteors in km. The last columns give material strength parameters and the meteor class according to those parameters as defined in Ceplecha (1988). Additional single-station meteors assigned by METREC software are marked by *.
10
182
3.1. Meteor spectrum and material strengths classification
183
The meteor spectra has been divided into four main classes base on mea-
184
surements of three main elements (Boroviˇcka et al., 2005). The brightest lines
185
in a meteor video spectrum belong usually to not resolved Mg I (2) triplet
186
and Na I (1) doublet. Between them lie Fe I (15) multiplet of partially re-
187
solved lines between 520–550 nm. All of those elements are the most common
188
in the observed meteor spectrum, falling into the range of wavelengths where
189
detectors have their sensitivity maximum.
190
The meteor spectrum classification contains then the following classes:
191
Iron, Na-free, Na-rich, and Mainstream meteoroids. The latter one is addi-
192
tionally sub-divided and includes: Normal, Fe-poor, Na-poor and Enhanced-
193
Na sub-class of meteoroids. We describe here only those (sub-)classes that
194
Geminid meteoroids fall into. For further reading see Boroviˇcka et al. (2005).
195
Na-free meteoroids have high material strength (Boroviˇcka et al., 2005).
196
From the orbital point of view Na-free meteoroids are divided into two dis-
197
tinct populations: Sun-approaching orbits and cometary (Halley) type orbits.
198
In both cases the common feature of meteor spectrum is the lack of the Na I
199
(1) line. For cometary type orbits the reason for depletion of Na is likely to
200
be the long exposure to cosmic rays on the surface of comets during their stay
201
in the Oort cloud. Later on, when such comets enter the inner Solar System,
202
due to graduate disintegration of the refractory crust compact Na-free mete-
203
oroids are produced. Another efficient way of releasing Na is solar heating at
204
low heliocentric distances (6 0.2 AU), which is also responsible making the
205
material strength larger. In this case, frequent approaches to the Sun, i.e.
206
more perihelion passages, exposure meteoroids (in the millimetre-size range)
11
207
to the heat that leads to the loss of Na irrespective of their origin. Therefore,
208
the older meteoroids the more Na is depleted.
209
Na-poor meteoroids represent a transition class of meteoroids between
210
Na-free and Normal. They consist in features similar to both classes. Mete-
211
oroids of this class can have orbits of cometary type or/and with low perihelia.
212
In comparison to Na-free meteoroids, the Na I (1) line is visible but is signif-
213
icantly weaker than expected. Meteoroids of the Normal class are found on
214
both cometary and asteroidal orbits (Boroviˇcka et al., 2005). The position
215
on the ternary graph lays near chondritic values (see Figure 6). Except for
216
iron for which line intensity appear to be often fainter, reaching chondritic
217
values only for more massive meteoroids where bright lines become optically
218
thick.
219
On the other end of the normal class of meteoroids lay Fe-poor class.
220
Members of this class have nearly normal content of sodium with significant
221
depletion of iron that does not let classify them as normal meteoroids. Fe-
222
poor meteoroids also have lower the average material strength and probably
223
originated from comets.
224
225
Examples of Na-poor, Normal and Fe-poor meteoroids observed by CILBO is presented in Figure 5.
226
The graphic representation of the spectral classification is often repre-
227
sented in the form of a ternary graph (see Figure 6 in Boroviˇcka et al. (2005)).
228
However, for better analysis the spectral classification is combined with the
229
orbital and the material strength classifications.
230
The material strengths of meteoroids are estimated using parameters re-
231
lated to the ablation beginning and terminal heights (KB and PE , respec-
12
232
tively) that were introduced by Ceplecha (1968); Ceplecha and McCrosky
233
(1976). On their basis a corresponding meteoroid material strength classi-
234
fication was introduced as well (Ceplecha, 1988). Here, as well as with the
235
meteor spectrum classification, material strength classification distinguish
236
between the different types of material groups (with characteristic typical
237
for strong asteroidal bodies to fragile comets).
238
3.2. CILBO Geminids
239
In Figure 6 we compare the measured relative intensities of the magne-
240
sium, sodium, and iron multiplets obtained by integration along the whole
241
path of a meteor. Also shown are meteors from the Geminid surveys (Boroviˇcka
242
et al. (2005) (diamonds) and Voj´aˇcek et al. (2015) (crosses)). The majority of
243
Geminid meteoroids represent Fe-poor spectral class with similar Na I/Mg I
244
and Fe I/Mg-I ratios, see Table 2. We do observe however that the appar-
245
ent variations in sodium reveals dispersion of its content in individual cases,
246
ranging from Na-free bodies to Normal type composition. The position of
247
our Geminids in the ternary diagram is closer to Geminds in Voj´aˇcek et al.
248
(2015), filling the gap in their sample, and concurrently linking with Gemi-
249
nids from Boroviˇcka et al. (2005).
250
Only five Geminds in our sample were observed from two stations, there-
251
fore, only for them trajectory and orbital elements were calculated (Table
252
1). The beginning of the meteor luminosity path allows to classify the ma-
253
terial strength of meteoroids. Moreover, depending on an entry speed and
254
mass of a meteoroid, the ablation starts at higher heights for meteoroids
255
of cometary origin than the asteroidal one (Ceplecha and McCrosky, 1976;
256
Koten et al., 2004). Figure 7 shows meteor beginning height as a function 13
257
of speed., where our double-station Gemnids from CILBO (blue circles) are
258
compared to those derived by Boroviˇcka et al. (2005). Here grey symbols
259
represent: Normal (filled square), Na-poor (filled circle), Fe-poor (filled di-
260
amond), enhanced Na (filled triangle), irons (cross), Na-free (circle), and
261
Na-rich (triangle). The lines show the empirical mean beginning of average
262
meteoroids strength (solid) and their limits (dashed) (Boroviˇcka et al., 2005).
263
Position of CILBO Geminids once again shows the measured dispersion in
264
sodium. This is also presented in Figure 8, where the observed Mg/Na line
265
intensity ratio in CILBO Geminds as a function of meteor speed. The corre-
266
sponding figures (Figures 7 and 8) for the double-station meteors with known
267
heights have high beginning heights of ablation. Therefore, they have an av-
268
erage material strength that is typically expected for Normal and Fe-poor
269
class (Boroviˇcka et al., 2005).
270
4. Conclusions
271
The Canary Island Long Baseline Observatory (CILBO) spectral cam-
272
era covers limiting magnitudes of faint meteors. Already gathered data and
273
preliminary analysis show that the meteor spectroscopic survey at CILBO
274
will support well other existing meteor spectroscopic observations. Particu-
275
larly those that are carried out of meteor showers campaigns and/or are fo-
276
cused on centimetre-sized meteors (Boroviˇcka et al., 2005; Jenniskens et al.,
277
2014; Rudawska et al., 2013, 2016) and fireballs (Borovicka, 1993; Madiedo
278
et al., 2013b). Therefore, CILBO spectroscopic program will supply existing
279
databases in elemental compositions for meteoroids.
280
In the sample of the Geminids collected by CILBO we see a variation in 14
281
the level of sodium. Our Geminds show Na-free, Na-poor, and Normal spec-
282
tra (Boroviˇcka et al., 2005). The Geminids are known to start ablation at
283
high altitudes. It causes the release of volatile component much quicker, and
284
therefore, the Geminds may not contain this component either. Moreover,
285
the variation of volatiles can be correlated with the age of the meteoroids
286
as well. The Geminid meteoroid stream has small perihelion distance of
287
q = 0.14 AU. Thus, those cases with more Na I represent a sample of rela-
288
tively young meteoroids that experienced shorter time exposure to the Sun,
289
and the one with less sodium are older.
290
291
5. Future work
292
While this paper has demonstrated the potential of the CILBO spectral
293
pipeline, a few opportunities for extending the scope of it remain. Here we
294
present some of planned future works:
295
• To improve effectiveness of meteor spectrum detection in collected data,
296
we plan to modernize METeor Spectra Selector (MESS) applying ma-
297
chine learning techniques.
298
• We are currently extending PARADE database to include more atoms
299
and particularly more molecules common in meteor spectra (CaO,
300
MgO, FeO,...), and preliminary results of AlO and TiO are encour-
301
aging (Loehle et al., 2018).
302
• Moreover, the upgraded version of ESA Meteor Research Group soft-
303
ware is planned to be available as an open-source python package called 15
304
mrg-tool. It will include also the described above part of the meteor
305
spectrum pipeline (mrg spectrum).
306
307
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ICC7
ICC8
meteor detection
store frames
BMP
METREC
GRAB
BMP
fits creation VIDAS
FITS
radiometric calibration VIDAS
FITS
FITS
dark current flat field
MESS
event selection
FITS
INF
FITS
spectral registration
spectral response
VIDAS
FITS
plasma radiation
ablation modeling MMS
PARADE
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Figure 1: Diagram of the meteor spectrum data pipeline.
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MgI
Relative intensity
100 80
OI
60 NaI
40 20 0 400
CrI FeI FeI
450
FeI
500
FeI FeIOI MgI
OI N2
NI
OI N2
550 600 650 Wavelength / nm
700
N2
750
800
Figure 2: An example of meteor observed by CILBO on 14 December 2013 at 04:13:02 UT. In the top left is the meteor seen by the ICC7 camera on Tenerife. In the right picture the same meteor is shown as recorded by the ICC8 camera. The bottom graph shows the profile of meteor spectra without sensitivity correction with chemical species
.
24
100 Relative intensity
80 60 40 20 0 400
450
500
550 600 650 Wavelength [nm]
700
750
800
Figure 3: Observed spectrum of a meteor (ID: 20131213T041302) integrated along its trajectory (blue) compared to modelled spectrum (orange). Spectrum presented here is without spectral response correction.
Relative intensity
1.0 0.8 0.6 0.4 0.2 0.0 400
450
500
550 600 650 Wavelength [nm]
700
750
800
Figure 4: Sensitivity of the camera obtained by measuring a spectrum of Vega. The relative intensity has been normalized to unity at 455.5 nm.
25
ICC8_20131213T031836_spect
35
Relative intensity
30 25 20 15 10 5 0 400
450
500
550 600 650 Wavelength [nm] ICC8_20131214T041302_spect
700
750
800
450
500
550 600 650 Wavelength [nm] ICC8_20131208T213645_spect
700
750
800
450
500
550 600 650 Wavelength [nm]
700
750
800
100 Relative intensity
80 60 40 20 0 400 25
Relative intensity
20 15 10 5 0 400
26 Figure 5: Meteor spectra of different spectral class (without the spectral response correction). Top: Na-poor (ID: 20131213T031836); middle: Normal (ID: 20131213T041302); bottom: Fe-poor (ID: 20131208T213645).
Fe I
40 |
30 |
20 |
Mg I
15 |
Na I
Figure 6: The measured relative intensities of the magnesium, sodium, and iron multiplets. Line intensities are obtained by integration along the whole path of the meteor. Also shown are meteors from the Geminid surveys: Boroviˇcka et al. (2005) (diamonds) and Voj´aˇcek et al. (2015) (crosses).
27
130 120 110 100 90 70
80
Beginning height [km]
● ●● ● ● ●● ● ●
0
20
40
60
80
Speed [km/s] Figure 7: Meteor beginning height as a function of speed. Geminids from our analysis (blue circles). Grey symbols represent Boroviˇcka et al. (2005): Normal (filled square), Napoor (filled circle), Fe-poor (filled diamond), enhanced Na (filled triangle), irons (cross), Na-free (circle), and Na-rich (triangle).
28
1.0 0.5 0.0 −1.0
−0.5
Na/Mg (log)
2 18 ● ● 9● 5 10 ● ● 7 ● 3 ● 11 ● 4 ● 6 ●
10
20
30
40
50
60
70
Speed [km/s] Figure 8: The observed Mg/Na line intensity ratio as a function of meteor speed. The solid line is an approximate fit of the meteors classified as having normal Mg and Na intensities (Boroviˇcka et al., 2005). Used symbols are identical to the ones in Figure 7.
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• • •
We demonstrated the capability of the ESA/CILBO meteor detection system. We analysed a sample of meteor spectra collected in December 2013 and 2015. In the sample of the Geminids collected by CILBO we see a variation in the level of sodium.