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Astron. Astrophys. 357, 1115-1122 (2000)

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7. Results

7.1. [FORMULA] and K as function of time

Table 2 reveals the main reason, why these 22 meteoroids with precise data derived from photographic records did not yield gross-fragmentation solutions with constant [FORMULA] and K, and with residuals independent of time. In 20 cases from these 22, K is enormously large during the initial part of the trajectory. The gross-fragmentation model assumes K constant, and makes thus the residuals time dependent. We can generalize: if the gross-fragmentation solution proves to be time dependent, we may be almost sure that the reason lies in very large values of K at the trajectory beginning. The highest value of K we found 54[FORMULA] for PN 39122, 42[FORMULA] for PN 39154, 19[FORMULA] for O 27471, and 16[FORMULA] for PN 39424B.

On the other hand [FORMULA] behaves differently. About half of the cases possess high values of [FORMULA] at center of the trajectory and half of the cases at the trajectory end. In this sense we can use change of [FORMULA] as an additional criterion for meteoroid classification into types (I, II, IIIA, IIIB), dividing these types into cases with high continuous fragmentation at the center of the trajectory, and into cases with high continuous fragmentation at the terminal parts of the trajectory. It is well to note that the 2 exceptional cases, when K at the beginning is not high, exhibit the high continuous fragmentation at the terminal part (high [FORMULA] value at the end of trajectory). In these 2 exceptional cases this seems to be the main reason for not obtaining the gross-fragmentation solutions with time independent residuals.

7.2. Spectral clues

A high resolution spectrum of the bolide O 27471 has been photographed. The spectrum was described by Ceplecha & Padevt (1969). For the purpose of this paper we re-measured and re-analyzed the spectrum by the new method of Borovika (1993). The spectrum covers the heights from 84 to 55 km, corresponding to the time from -0.43 to 1.03 s. However, the only visible line at the beginning is the sodium doublet at 5890 and 5896 Å. We were able to analyze the spectrum in detail only between 0.37 and 1.03 s, after the meteor brightened enough to show a sufficient number of spectral lines on the photographic plate. In this interval, K varied nearly by a factor of three, between 0.5 to 1.5 (see Fig. 6).

The spectrum shows no obvious anomalies and no dramatic changes. The lines of Na I , Mg I , Si I , Ca I , Ca II , Cr I , Mn I and Fe I are present. The excitation temperature of the radiating gas was found to be 4800 [FORMULA] 200 K along the studied part of the trajectory. The line of Si I is rather strong in this spectrum in comparison to the spectra of other meteors. Also Mg I is relatively strong, while Na I is somewhat weaker than in other spectra, though still the brightest line in this spectrum. These facts suggest that the meteor was produced by a silicate rich stony meteoroid.

The spectrum shows two minor changes along the trajectory. They can be seen in Fig. 11. Firstly, the low-excitation inter-combination lines, in particular Fe I multiplet 2, are enhanced at 0.50 s, at the brightest point of the meteor. Inter-combination lines are commonly seen to be bright in meteor spectra, especially in the meteor wake. O 27471 does not show significant wake and those lines probably originate in the outer parts of the radiating region which are not in thermal equilibrium.

[FIGURE] Fig. 11. Blue part of the spectrum of meteor O 27471 at different times. Individual spectra have been shifted vertically for clarity. The lines mentioned in the text are identified. The features marked by asterisk are due to an interfering star trail.

The second change is the increasing strength of calcium lines relatively to other lines toward lower heights. Calcium is underabundant in the radiating gas due to incomplete evaporation but the evaporation efficiency increases at lower heights. Also this effect is common in meteors of similar velocity (Borovika 1993; Ceplecha et al. 1998).

In summary, in this spectrum we did not find any evidences which could explain the changes of the shape-density coefficient K. The changes of K are not represented in the radiation of the meteor, at least in the visual range and above our sensitivity limit.

7.3. Large values of K at the start of luminous trajectories

The large K at the start of almost all examined cases with precise observational data calls for explanation. There are several possibilities.

  1. All the effect is from changing bulk density (outer layers composed of low density material.

  2. Changing head cross-section e.g. due to rotation.

  3. The air density is widely and systematically different from the used model (CIRA 72)

  4. Eq. (1) is not valid and needs some large additional term at the trajectory start

Explanation 1 should be recognizable in spectral records. Even if we are not definitive with our limited spectral analysis, we feel this explanation of so large values of K very improbable. Explanation 2 may be well right, but some of the K values are so large that only an extremely flat shape could explain them, and we are not much inclined to assume that these cases correspond to meteoroids thin as a sheet of paper. Something which is in favor of explanation 2: periodic changes of decelerations and of K at the early parts of the trajectories may well represent rotation of the body. We found periods between 2 and 4 rotations per second for different events. Explanation 3 seems very improbable. One needs changes of the air density against the CIRA 72 model by a factor of the order of 10 on a height differences of the order of 10 km. All these explanations 1 to 3 may act together. But if we take into account that large values of K are typical explanation of almost all differences from the assumption of constant K and [FORMULA], and if we draw our attention to anomalous cases with positive values of [FORMULA] during an extensive part of the trajectory (e.g. Fig. 10), we are inclined to accept explanation 4 as the most probable.

Revision of the basic differential equations is not in the scope of this paper, but we feel that some hints on what is omitted in Eq. (1) are necessary. It should be a rather large additional term, having occasionally about the same value as the existing term at the beginning of trajectories. Omitted gravity term is insignificant in this sense for all examined cases. In this respect some authors in the past mentioned reverse rocket effect (Levin 1961; Bronshten 1983). However, there is another possibility: a meteoroid penetrating through the ionospheric layers is electrically charged (in addition to its original interplanetary charge) and then interferes with much larger volumes of the atmosphere than it would be in case of only aerodynamic drag, and interferes also with the atmospheric electrical charges alternatively decelerating or accelerating the meteoric body.

This problem adds more uncertainty to results on individual meteoroids. Many times in the past we mentioned that meteoroids in the atmosphere behave very individually. It has no sense to speak about an average meteoroid (inclusive meteor showers). Now we are adding another "individualism", the state of the ionospheric layers and electric charge of the meteoroid, which could change atmospheric meteoroid trajectory so much like do the differences among them. In any case we want to devote more attention to this problem in some of our future studies. Very precise trajectories observed, immediate state of the entire atmosphere from all aspects, and good luck for anomalous events to be recorded, this is all we need to proceed to some more general insight into problems of meteoroid interaction with the atmosphere.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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