1. Introduction
Simultaneous observations of meteors using optical and radio instruments are widespread. Mainly, these investigations deal with the study of meteor fluxes, their dynamic characteristics, and the detection of meteoroid masses and structure. Recently, a comprehensive study devoted to comparison of stereoscopic and radio methods for meteor observations was performed. The authors collected a huge volume of statistics on meteor images obtained using fast cameras with image intensifiers together with data from the CMOR radar [
1,
2,
3]. There, the authors estimated the efficiency of recording meteor parameters using both methods and settled on the ratio between the ionization coefficient and luminous efficiency. The study was performed for meteoroids with different chemical compositions and velocities. Similar research was performed in [
4] using the MAARSY radar. In [
5], the authors performed a multi-instrumental optical and radio study of non-underdense meteor trails, the ionization of which is significantly higher than the background level at the initial moment. However, the ionization does not cause specular reflection of the sounding radio signal from the meteor trail. In addition to a videocam, a 1-min time resolution ionosonde and SkiYMET meteor radar were also used. According to the authors, different rates of radio echo dissipation from non-underdense meteor trails are explained by the presence of nanoscale meteor dust. Subsequent study [
6] was performed using significantly more observational data from SkiYMET (11 years of observations). It confirmed the key role of aerosols from meteoroids in the formation of long-lasting meteor echoes. The meteoroid mass threshold value needed to form a long-lasting meteor echo was evaluated as 0.02–0.03 kg in that work.
Radio echoes of ionized trails from meteors in the upper atmosphere are also used to determine horizontal wind speed and vertical wind shear in the mesopause region. Mostly, small meteors are used for this, with relatively weak ionization trails of about 1 s lifetime or less. Powerful meteors causing strong ionization with longer lifetimes (10 s or more) cannot be used to reconstruct the wind velocity vector using radar. In this case, the ionized area is non-isotropic and spatial dimensions are quite large. It is considered that the dynamics of such an area under vertical wind shear leads to complex radio echo interference, which is difficult to interpret. Nevertheless, there are works considering complex spectral compositions of radio echo. In reference [
7], the authors demonstrate the radio echoes received by SESCAT radar from both non-underdense and specular long-lasting meteor trails with Doppler shift split into two components. The most probable reason for such bifurcation is the vertical wind shear dragging the trails in opposite directions, as the authors suggest. The study [
8] based on data from the Sanya VHF radar, also indicates distortion of the radio echo from a long-lasting meteor trail as a consequence of neutral wind. Additionally, the authors suggest the existence of meteor dust, which can produce longer trail lifetimes. Based on data from the Gadanki MST radar, the authors of reference [
9] speculate that apart from the reasons above, distortions in radio echoes from specular or non-underdense trails can be induced by meteoroid fragmentation.
The theory of long-lived meteor trail glowing and ionization dynamics was developed in a series of works by W. J. Baggaley based on naked-eye observations and the chemistry of atmospheric and meteor constituents. In reference [
10], evaluations of ionization duration for elevations of 70–100 km were performed using metal oxide ion chemistry. Detailed study of the chemiluminescent processes of basic atmospheric constituents under meteor impact in reference [
11] showed a negligible role for such types of processes in enduring meteor trails. Studies of the chemical process of two-step ozone dissociation with meteor body metals as catalysts in references [
12,
13] led to the understanding that only Na and K atoms can produce long-lasting meteor trail glows in yellow and infrared. Further, reference [
14] connected the brightness and duration of long-lived meteor trails based on sodium canalization of atmospheric ozone dissociation. Additionally, it should be noted that reference [
15] studied the possibility of meteor trails generating red-orange emissions with atomic oxygen.
Optical instrumental observations of long-lasting meteor trails are rare because specific optical observations are basically performed within a small region of the sky and are limited due to weather conditions. Observational campaigns with the participation of amateur photographers facilitated detection of the morphological characteristics of long-lasting meteor trails [
16,
17]. The observed elevations where long-lasting meteor trails develop (75–110 km) and the large-scale spiral shapes emphasize the wind shear mechanism of trail formation. The lifetimes of long-lasting trails described in these studies are mostly of several minutes’ (up to 10) duration. Nevertheless, long-lasting, self-glowing meteor trails (of up to 1 h) have been observed with special instruments and their characteristics, physics, and chemistry described. In reference [
18], a naked-eye observation of a long-lasting meteor trail was described. The meteor’s trail elevation (94 km) and length (30 km) were evaluated with triangulation by witnesses from different points of observation. In addition to direct observational data for this event, the authors obtained photos with a wide-angle (fish-eye) camera equipped with a narrowband optical filter with maximum transmission at the upper atmosphere sodium airglow wavelength. Further, the authors demonstrated sodium height profiles obtained with resonance sodium LIDAR, coinciding with data from eyewitnesses. Evaluation of the intensity of this event showed that for naked-eye observation, sodium only was not sufficient. In reference [
19], the photochemical structure of long-lasting meteor trails was discussed on the basis of multi-spectral images. The authors concluded that in addition to sodium airglow, a significant component of the total intensity may include different bands of hydroxyl emissions, whereas molecular oxygen was unlikely as a probable reason for the meteor trail glow in the near infrared range.
In our work, we consider a unique, long-lasting meteor trail. The trail was observed simultaneously with two spaced wide-angle cameras, meteor radar, and ionosonde. Positions of the instruments are given in
Figure 1. Earlier, characteristics of this meteor trail were studied in references [
20,
21]. In reference [
20], the authors determined spatial and kinematic characteristics of the long-lasting trail and revealed features of its evolution using the Astrometry software [
22]. The heights of meteor trail initiation and finalization were within 75–100 km; brightness was about −7.3 m apparent magnitude. It was shown that all parts of the long-lasting meteor trail propagated in a plane at a height of about 90 km and at a rate of 320 m/s. It was hypothesized that such a movement could not be explained by the wind. In [
21], parameters of the long-lasting meteor trail were compared with ionosonde observations of the trail. Here, it was supposed that the neutral and ionized components of the long-lasting meteor trail moved with different velocities. In our work, we carried out a detailed analysis of the long-lasting meteor trail images while accounting for aberrations in wide-angle optical systems. Stereoscopic analysis was performed, and the data obtained were compared with radio echoes using meteor radar and ionosonde.
3. Discussion
Development of the long-lasting meteor trail started at 22:24:30 UT. The trail dissipated amidst stars at 22:55:41. The trail was moving at heights in the range of 88–106 km. The trail glow could not be explained by solar radiation scattering on dust particles from the meteoroid fragmentation. This is because the solar angle with respect to the horizon required to illuminate the object in coordinates given in
Table 2 within the specified time interval must have been at least −10°. For these time points in the given coordinates, the Sun was at a height of about −20° and about −16° with respect to the horizon.
Primitive spectroscopic analysis demonstrate that the spectrum of the long-lasting meteor glow was mainly in the green, yellow, and red regions. In these regions, near the height of trail development defined by stereoscopy, the airglow comprised atomic oxygen (557.7 nm, 97 km), sodium (589 nm, 90 km), and hydroxyl (basically red and infrared ranges) emissions from photochemical reactions. In reference [
20], sodium was considered to be the most probable reason for the glow. As mentioned above, the hypothesis regarding sodium emissions was also made in reference [
18]. However, Clemesha [
19] extended the obtained result and showed that hydroxyl emissions contributed significantly to the glow of the long-lasting meteor trail. The KEO Sentinel wide-angle camera is equipped with a narrowband light filter with a transmission band that is still wide enough to record one of the lines of hydroxyl emission (9–3), 630.36 nm. It took the camera quite long to record the meteor trail, and this may indirectly confirm the presence of the hydroxyl component in the trail’s glow.
According to the MK-31 meteor radar data, the range to the meteor was 231–237 km. The radar to meteor distance calculated from the coordinates obtained by stereoscopy yielded a value of ~220 km. Unsuspected systemic errors may be a possible reason because the method for detecting directions from all-sky camera data had not been checked using different datasets (snapshots of the same objects by both cameras).
Using data from
Table 2, one can evaluate the velocity of the trail’s horizontal expansion. The trail expanded eastward and westward at an average velocity of 25–30 m/s, and southward at ~70 m/s. Unfortunately, we could not determine northward velocity reliably due to a lack of data. It should be noted that the velocities obtained from stereoscopy were similar to the velocities of ionized trail obtained in reference [
21] and were close to the horizontal wind velocities obtained with the Fabry-Perot interferometer and meteor radar in our study (
Figure 7).
We could not precisely define the vertical wind shear in this work because the interferometer and radar obtained wind velocities averaged in height within about a 10-km range near the airglow maximum or meteor ionization. Nevertheless, the presented trends in variation of horizontal wind velocities for three height ranges indicated significant wind shear in that range. The meridional velocities for the 87 and 97 km heights were the opposite, and the zonal wind velocities at 90 and 97 km were opposite, also. Moreover, we could say that at 87 km, the wind velocity vector was mainly directed southward. At 90 km, it was directed northwest, and at 97 km, the wind velocity vector was directed northeast. This is consistent with the mechanism of meteor trail formation due to wind shear according to data in
Table 2. A schematic view of the spatial distribution of the long-lived meteor trail with wind shear is shown in
Figure 8.
It is difficult to compare data from the MK-31 radar and wide-angle cameras in this work. First, we were not certain that the ionized and glowing parts of the long-lasting meteor trail were the same object at the initial stage of its development. This could also be partially indicated by the difference in ranges determined with the radar and stereoscopy discussed above. Second, the duration of the joint recording of the event joint did not exceed 2 min. During this time, the Allsky-340 was in a stand-by mode waiting for the next exposure, and the KEO Sentinel took only three snapshots. Resolution of the long-lasting meteor trail on the snapshots was very low and stereoscopic analysis was not possible; hence, we could make only rough evaluations.
Evaluation of the trail instantaneous velocity at the initial stage of development for projection onto the Earth surface from an average height of ~90 km shows that the trail moved in all directions with deceleration. Moreover, the trail propagated along the north-south direction with a velocity approximately 2 times higher than along the east-west direction. The averaged radial velocity dropped from an initial 100–200 m/s to the values obtained above using data in
Table 2. The radial velocity approximately corresponded to the velocity of one of the Doppler shift components in the radar echo. Apparently, this component corresponded to the part of the long-lasting trail propagating along the line of sight toward the radar. The second Doppler component crossed the zero line. Probably this Doppler component corresponded to the trail part crossing the line of sight.
Figure 9 shows the projection of the long-lasting meteor trail plotted from KEO Sentinel data just after the radar echo faded out from the trail. The southern part of the trail has shifted from its initial position by about 15° if we look from the radar. Projection of the 65 m/s velocity at this angle to the line of sight is about 250 m/s, which is close to the above radial velocities. Obviously, in our case, a more thorough analysis of the situation is required, which is only possible when there is new information about the event.
The presence of dust particles explains the lasting duration of the ionized trail component dissipation recorded with ionosonde [
5]. Dust particles reduce the diffusion rate, which leads to a longer duration of radio echoes from meteor trails. The other process responsible to the long ionization could be the chemical dynamics of meteoroid metal ions in the atmosphere. Reference [
10] studied dynamics of ionization and found that at 90 km height, the lifetime of electrons responsible for HF reflections can be as long as several hundreds of seconds. Moreover, both processes can explain the lasting glow of the meteor trail itself.
The glow is induced by photochemical reactions of the meteor dust components with the air of the upper atmosphere. If the meteor substance moves as a sublimating dust containing water and sodium, this substance distributed in the upper atmosphere may provide both the rapidly disappearing, sporadic Es observed with the ionosonde, and the spectral composition of the long-lasting meteor trail glow as described above.
Excitation of sodium glow results from reactions [
12]:
causing the yellow airglow registered in Alsky-340 described in this work. The duration of the observed long-lived meteor trail was consistent with [
14], which studied temporal dynamics of (6) with dependence on meteor brightness. In our case, the apparent magnitude of −7.3 m as pointed to in reference [
20] and accordingly with reference [
14], the intensity of the trail from such a meteor should decrease to 0.1 from its initial value during about 10
3 s.
Also the meteoroid could contain K, and as [
12] show, the K also may participate in reactions such as (6):
The wavelength of the K line in this case was 766.5 nm, rather far from the band obtained by KEO Sentinel (±1 nm near 630 nm). Hence, the strong signal registered by KEO Sentinel as described in this work apparently did not arise from K airglow. As the authors of reference [
12] claim: “Excitations of transitions of other meteoric elements are not possible during oxide reactions”; thus, we also ruled out the possibility of Fe, Al, Ca, Mg, or Si generating red airglow in our case.
Theoretically, the red airglow after a strong impact by a meteor can be produced from the molecular states of the main atmospheric constituents—nitrogen and oxygen and their combinations. A detailed study performed in reference [
11] showed a very small contribution from such photochemical processes in long-lasting meteor trails. The possible red airglow in meteor trails was discussed later in reference [
15], but the results of reference [
19] ruled out that possibility and pointed instead to a strong hydroxyl component in observed long-lived meteor trails.
Excitation of the hydroxyl emission may occur due to the following reaction:
The energy balance of (8):
shows that reactions are exothermic. The values
or
and
or
are sufficient to excite the Meinel band of hydroxyl emission (9–3) through (8). A certain amount of excited oxygen atoms
and
must be present at the heights of meteor trail glows because this state results from the Barth mechanism and subsequent transition of atomic oxygen, generating airglow at 557.7 nm
near this height. Therefore, the 630.36 nm line can be registered by KEO Sentinel in long-lived meteor trails containing water. However, more detailed study of this process is needed.
Today, the water in comets, the ancestors of meteors, is considered to be one of the essential sources for the origin of the Earth’s oceans. In [
31], an effort was made to detect water in the constituents of comet 55P/Tempel–Tuttle from the spectral composition of the glow of a meteor from the Leonid flux. However, the authors analyzed the spectral composition of the meteor. Works describing the complex spectral composition of meteor trail glow, such as [
32], considered that trails formed just after meteors with durations not exceeding several minutes. In this case, the complex spectra of the glow would be provided by the chemical composition of ablated meteor material heated by friction in the Earth’s atmosphere. Therefore, reaction (8) would apparently be negligible for these conditions due to water dissociation. However, if meteor material containing relatively weakly bound water moved slowly through the upper atmosphere, as in the case of meteor dust propagation, the possibility of water emerging from the meteor material would occur, allowing reaction (8) to proceed.
4. Conclusions
We considered the dynamics of a unique, long-lasting meteor trail that survived for about half an hour using snapshots from wide-angle cameras and radar data obtained with meteor radar and ionosonde. Stereoscopic analysis allowed us to obtain the spatial dynamics of the trail. Further, we performed primitive spectroscopic analysis of the trail glow. The results were compared to the wind dynamics obtained with the Fabry-Perot interferometer and meteor radar at the height of the long-lived meteor trail development and its direct radar echo data. The results of earlier studies of such events were widely used, as well. Our conclusions are as follows.
The meteor shape (thickening and sharp thinning near the end with heterogeneity present) indicated that it was probably an explosion that caused meteoroid fragmentation while it was moving. Apparently, the explosion formed a cloud of solid aerosol particles (meteor dust). The shock wave and the wind field induced the cloud motion, during which the cloud trail took the shape of a spiral twisted around the normal to the Earth’s surface. Aerosol formed by explosive fragmentation was concentrated at the heights of 88–106 km. Initially, during the first minutes after the explosion, the aerosol was moving in all directions at about 100–200 m/s. After a few minutes, the movement slowed down to typical wind velocities at the heights mentioned. The trail spiral shape was induced by the vertical wind shear existing at these heights during observation of the long-lasting meteor trail. This was confirmed by the background dynamics of the neutral atmosphere at the point where the long-lasting trail was observed with the Fabry-Perot interferometer and meteor radar.
The glowing meteor trail apparently consisted of sodium and hydroxyl components. The source of both components was the meteor body containing sodium and weakly bound water. The meteoroid fragmentation and production of meteor dust resulted in a 10 min ionization trail in the ionograms, and emanation of water from the meteor body reacted with excited oxygen atoms to produce excited hydroxyl states. The sodium airglow was generated as the result of a well-known process of dissociation of mesosphere ozone by Na atoms as a catalyst.
The obtained characteristics of the long-lasting meteor trail, especially the velocities of its expansion, differ from characteristics obtained earlier for this event in [
20,
21]. Therefore, the mechanisms of long-lasting meteor trail formation discussed in these papers require correction.
In our study, we described a method for detecting the spatial structure of a glowing formation using stereo images from all-sky wide-angle cameras. This method is similar to those applied by other research groups involved in the study of characteristics of gravity waves in the upper atmosphere, e.g., [
33]. In addition, similar algorithms were used to determine the SAR-arc height when paired with satellite data and a network of cameras [
34]. Combined with lidar, radar, and satellite data of the National Heliogeophysical Complex [
35,
36], this method will allow a more precise reconstruction of spatial characteristics of such events as wavelike structures, auroral arcs, meteor trails, artificial formations, etc. It is fair to assume that the accumulated statistical data on the events, together with the development of machine vision methods and neural network image processing algorithms, will allow further automation of the process as in [
23,
37] for other applications.