Date: 01-15-00

Probable Role of Plasma Instabilities in Anomalous

High Altitude Luminosity (AHAL) Observed in Meteors

and SpaceVehicle Re-entries1

Andrei Yu. Olíkhovatov



The problem of meteoroid flight is an interdisciplinary one, involving many branches of science. Despite significant progress in understanding the physics of meteor phenomena, several aspects are still not well understood. Among them, there are problems of the origin of the so-called head echo; luminous meteor trails of very long persistence; and the visibility of meteors at anomalously high altitude. In this paper the latter problem is considered. It concerns luminosity of meteors at heights above about 120 km. The author (here and below Ďthe authorí refers to A. OI'khovatov unless otherwise stated) published several articles on the subject in the early 1990s. Since that time new data have appeared which confirm the general ideas previously put forward.


Observational Data

Meteor data. It has been generally accepted that meteors initially appear a little bit higher than 100 km height, usually no more than about 110 - 115 km. But in the last couple of years several reports have appeared describing anomalously high altitudes of some meteors. For example, Y. Fujiwara and colleagues [1] discovered beginning heights of two Leonid moderate fireballs of 160 km. During the Dutch Leonid meteor expedition to China in 1998, beginning heights of luminous trajectories of bright Leonids were observed up to 200 km by an all-sky video system [2,3]. Dr. Ceplecha wrote [3] to the author that the nature of the high-velocity, high-altitude meteor-radiation is an enigma. With hundred of meters of free mean path for atmospheric gases are excluded Excitation of the air molecules in sufficient degree to explain the radiation through impacts on meteoroid surfaces seems to be excluded, too, in Ceplechaís opinion.

In reality, the phenomenon of anomalous high altitude luminosity (AHAL) of meteors has been known for at least several decades. In the late 1940s Soviet astronomer I. Astapovich saw in the Crimea during some nights what he called "blue meteor trails" at heights up to 160 km. A remarkable meteor spectrum was recorded in 1970 [4]. At first, at 137 km height yellow-red continuum appeared, which was ascribed [4] to the first positive group of nitrogen bands. Down to 116 km the level of brightness was about constant. At 116 km it brightened rapidly from its previous level into a uniform continuum. At 106 km the D-line of neutral sodium appeared and at 105 km all other lines of the typical meteor spectrum also appeared. The continuum remained dominant to 87 km. were the meteor ended abruptly. The meteor speed was about 18.5 km/s and its magnitude (luminosity) was about zero.

There are other reports of meteors with unusually strong nitrogen emissions. For example, there is a recording of 2 meteors within 2.5 hours with especially strong nitrogen bands, which "may indicate an abnormal atmospheric condition at the time"[5]. It is important to note that 2 MHz radar detects meteors up to 140 km height [6].

Space-vehicle re-entry data. Although not widely known, glow phenomena associated with space-vehicle re-entries, which cannot be explained as glow of re-entry plasma, were investigated in the USSR in the early 1960s [7]. They were discovered by A. Lazarev and N. Uspenskii in 1960 during a night experiment with a re-entry vehicle. On July 6, 1960 a radiometer (wavelengths 1.8 - 3.2 microns, i.e. infrared) installed on the re-entry vehicle registered an upsurge of brightness commencing at 160 km height reaching a maximum at 125 km. The level of the brightness was pulsating. Then the brightness dropped sharply and began to increase again from about 100 km to 85 km, where radio transmission "blacked out" due to re-entry plasma. In another experiment on July 24, 1962 on a re-entry vehicle there were several radiometers working in 0.8 - 3.2 micron band. During the approach to Earth at heights of 145 - 105 km the radiometers were saturated with a strong signal. Then, at lower heights, the signal dropped to a minimum at 90 km. and later raised again.

It is well known that re-entry plasma forms at heights below about 100 - 80 km (about 80 km for the re-entry of SOYUZ-sized space vehicles, and about 100 km for the larger Space Shuttle). Radiometers detected the re-entry plasma signal. But what was the source of the "120 km" signal? Investigations conducted at the time were unable to find a comprehensive explanation. So the results were accepted with some suspicion until they were confirmed during manned space vehicle re-entries [7]. On April 25, 1971 there was a night re-entry of the SOYUZ-10 spacecraft. During the re-entry phase the crew was looking out through the windows. About 2 - 3 minutes after the re-entry vehicle separated from its main body a weak, whitish-violet glow was seen through the windows. The glow gradually increased in brightness. Some time later a small deceleration was felt, and small particles suspended in the air began to move to the floor of the vehicle. The brightness of the glow continued to increase, achieving the earth's daylight sky level. The impression was that the vehicle had moved out of the earthís shadow and had flown over its daylight side. The glow was uniform. After the vehicle reached a height about 80 km, the violet color of the glow changed to white, the glow began to pulsate, and pink stripes appeared in the glow. Soon it began to resemble a campfire by its red color and pulsating strips.

After the SOYUZ-10 data, Soviet scientists realized that the 1960s results were correct; there was indeed a real unexplained glow with a maximum at about 120 km. It was again confirmed by the crew of the SOYUZ-23 spacecraft during a night re-entry on October 16, 1976. Light inside the re-entry vehicle was switched on. The glow was first noticed by the crew when their re-entry vehicle was at 120 km height. At that time the glow was in a form of flashes with a frequency of about 1 per second. Then the duration of the flashes and repetition rate increased and the glow transformed into a continuous one. It resembled a white fog and lasted for a rather long time. Below about 100 km reddish re-entry plasma glow was added. The latter glow was very different from the previous one.

Analysis and Discussion

So we can state the reality of the AHAL for meteors and for space-vehicle re-entries. But the question arises: what is a physical mechanism of the luminosity?

Regarding the space-vehicle re-entries, one proposed explanation was that it was caused by a non-equilibrium ionospheric glow excited by flight of the space vehicle. Among the proposed origins (put forward by L. Grechikhin, S. Avakyan and others) were collisions with atmospheric molecules, photoelectrons due to solar flares and electric discharges due to electrification of the space vehicle [7]. Richard Spalding [8] interprets the observed AHAL of meteors to be explainable as due to ions from the ionosphere being (electrostatically) attracted to the incoming body and colliding with it to produce the light. Regarding the author's opinion, at first, it would be reasonable to note that Leonidsí meteoroid ram surface is heated up several hundreds degrees due to gasdynamic drag at 160 km height. And due to the loose, fragile structure of Leonids their behavior is hard to predict at this large height. Moreover, it would be reasonable to note, that atmospheric density can increase as much as 60% during high solar activity at 160 km height, and to 130% at 200 km height. Atmospheric gravity waves also can alter the density up to a few dozen percent. This can shift the "meteor height scale upward by 20-30 km at these heights.

Anyway, as it has been demonstrated above, besides Leonids, there are other examples of AHAL phenomenon that can't be explain by these ways. There are several reasons why the author is inclined to think that plasma instabilities play a major, or at least, a significant role in the AIHAL. These arguments are explained below.

Luminous efficiency. Despite the above-mentioned problems with analysis of Leonids, let's begin with them, as the most well known example of AHAL. Let's consider the Leonid meteoroid moving at 71 km/s, which has produced meteors with visual magnitude of about -4 when entering the dense atmosphere. Unfortunately, it is hard to estimate the luminosity, of the Leonids at 160 km [1] due to a lack of data. If to accept that the threshold magnitude for TV camera observation is normally about +7 [1], then the corresponding luminosity in the visible spectrum is on the order of several watts. Let's compare that with the meteoroid's gas dynamic drag power. The latter for the above-mentioned Leonid meteoroid can be estimated as on the order of a hundred watts. It means that a ratio of transformation of kinetic energy into visible luminosity (i.e. so called "luminous efficiency") is at least on the order of several percent. By comparison, luminous efficiency in the dense atmosphere (about 100 km and below) for a similar sized Leonid is considered to be an order of magnitude less [9]. But a difference in spectral sensitivity of a TV-camera and photographic film possibly can explain the difference. So if the above estimations are correct, it means that either the high altitude "single collisions" mechanism of luminosity is, at least as effective as the "continuum flow" mechanism is at lower heights, or that there are other mechanisms. The first idea looks unlikely, but due to uncertainty of the data a final conclusion regarding Leonids can't be done. Events other than Leonids AHAL point toward the other mechanisms.

Possible AHAL mechanisms In general, there can be two types of possible AHAL mechanisms. The first one is associated with a conversion of the meteoroid's kinetic energy into the luminosity. In the second mechanism the luminosity energy is supplied from the ionosphere. Letís begin with the first possibility.

Ion beam instabilities. At a height of 160 km the mean free path of the ambient neutrals is dozens of meters. So the meteoroid is moving in a free-stream flow field. Ambient neutrals striking the meteoroid can transform some part of their kinetic energy into excitation and ionization, as the ionization level of ambient neutrals corresponds to meteoroid's speeds of about 10-15 km/s. For example, it is thought [9] that about 30% of oxygen atoms striking a meteoroid at 60 km are ionized. Then the neutrals and ionized components are "sprayed" from the meteoroid's surface, and their speed relative to the ambient ion (-) sphere is approximately that of the meteoroidís speed. In other words, there is a beam of ions moving through the ionospheric plasma with a meteoroid's velocity. Such an ion beam is exposed to many types of plasma instabilities leading to generation of plasma waves, electron heating, etc. Here the author will call them simply, the "ion beam instabilities," to prevent going into detailed considerations.

The instabilities can lead to the Alfven critical ionization [l0]. In its typical scenario, neutrals as well as newly created ions travel across the geomagnetic field. The ions slow down as they transfer kinetic energy to the electrons via plasma waves. It is thought [10] that the primary role is played by electrostatic plasma waves, due to lower hybrid and/or modified two stream instabilities. They heat up electrons in the plasma and ionize the neutrals. The ionization occurs when kinetic energy of the neutrals is equal to their ionization potential [l0]. The critical speed is 10.6 km/s for molecular nitrogen and 15.6 km/s for atomic oxygen. To excite luminosity even a slower speed is sufficient. As we saw, the speed of neutrals and ions repelled by the meteoroid can be much higher. Unfortunately, applications of the critical ionization velocity mechanism for the ionospheric environment have not been investigated enough. Experiments with injections from space vehicles give ambiguous results, so maybe the mechanism is not very effective in these cases [10]. On the other hand, the speed of a meteoroid is much higher than that of space-vehicles, and it can make the critical ionization mechanism more effective.

Other Mechanisms. Efficiency of the ion beam instabilities AHAL mechanism declines with a decrease in the speed of a meteoroid. The ion beam instability mechanism alone canít explain the 1970 meteor event as well as the luminosity associated with the SOYUZ re-entries. So there must be other mechanisms. And there are many other facts that hint at them.

One of the facts is the recording of visible waves propagating through the ionosphere during experiments with ionospheric explosions [11]. Spherical "luminous waves" propagate through the ionosphere with a speed 1-3 km/s for about 10 seconds immediately after an injection. Soviet cosmonauts saw "luminous waves" spread after a bolide event, caused by a meteoroid or a space-vehicle re-entry. Similar ionospheric waves have also been detected in the ionosphere by radio methods and by magnetometers [12], a level of localized energy deposition into ionosphere of a hundred megajoules is enough for their global detection by radio methods. Due to their unclear nature, the author prefers to call all these events "magnetoionospheric waves" temporarily, until their nature will be discovered. The waves seem to generate ionospheric plasma irregularities, and the speed of the waves can attain several hundreds kilometers per second, at least. Regarding the AHAL phenomenon, the most important are the waves with speeds of several-dozens of kilometers per second, i.e., comparable with speeds of meteoroids and space vehicles. Despite the nature of the waves being unclear, it is known from a general theory of waves that when the speed of a particle (object) is equal to the speed of a wave propagating through medium, it usually leads to coupling between the particle and the wave. In other words, it is plausible to suggest, that if a meteoroid's (or space-vehicle's) speed is about the speed of a magnetoionospheric wave, the wave can be amplified. One of the main conditions necessary for the coupling is a possibility of a more or less strong interaction of the meteoroid (or space-vehicle) with the ionosphere. The possibility has already been partially demonstrated. Other examples are given below, which also demonstrate other effects closely related with AHAL.

At first, the author would like to attract attention to the fact that sometimes during experiments with rockets, the influence of the rocket (in passive flight) on the ionosphere is registered. Generation of ion-acoustic waves and increase of electron temperature up to 2000 - 2500 deg. K in the wake of the rocket due to rocket-plasma interaction, sometimes associated with ionospheric irregularities, is known [13]. During the Equion experiment [14], high-energy particles were detected (that were) aligned with the rocket's ram direction. So the authors of article [14] speculate that the particles seen were from an as yet unexplained interaction between the rocket and its environment in conditions that happen also to be conducive to ionospheric spread F irregularities. In addition, hints of the triggering of a marginally stable ionosphere by rocket passage were revealed [14]. Here the author can add that there are many other evidences that in some circumstances passive flight of a space-vehicle can disturb the ionosphere as is sometimes even registered from the ground [15]. In the authorís opinion some types of the disturbances are connected with magnetoionospheric waves.

Anyway, there are many indications that even a weak disturbance, produced by a meteoroid or a space-vehicle can trigger ionospheric plasma instabilities (leading to plasma waves, electron heating, etc.), when the plasma is in an unstable condition [15,16]. A role of an ionospheric electric field in a meteor trail's radiolocation also has been revealed [17]. As in these phenomena energy is transferred from the ionosphere. So they are irregular, unlike the more or less regular ion beam instabilities mechanism. A good example of this "unstable ionosphere" factor is the above-mentioned 1970 meteor event [4]. Gas dynamic drag power in this event was just a few watts at 137 km height, so it was too small to allow spectral measurements. Apparently. there are many possible "unstable ionospheric plasma" mechanisms. And it hard to classify them, as the nature of many of them is not well-understood, and evidently much more must be done to investigate it. However, already now the fact of their existence helps us to understand better AHAL and some other hard-to-explain meteor phenomena.

Other applications

Luminous meteor trails. The problem of long-lived luminous meteor trails is not completely resolved. In the author's opinion, besides chemiluminiscent reactions. excitation by electrons due to plasma instabilities may play an important role [18]. It seems that experiments with injection of high-speed plasma into the lower ionosphere supports this idea [19]. They revealed that the high-speed plasma injection can lead to the formation of a large luminous area in the ionosphere, which continues to glow up to 3 minutes, despite that it contains practically pure air. It is important to add here, that cosmonauts sometimes see the "whole atmosphere glow," which, in the author's opinion, can be favorable to AHAL, and to the appearance of long-living luminous meteor trails [15].

A good confirmation that luminous meteor trails can, at least sometimes, be produced by plasma instabilities is an observation of a "jet of luminosity" produced by meteor in the electrified ionospheric D-region [20].

Another interesting phenomenon, which in the author's opinion is an analog of the meteorís luminous trail [18] formed by plasma instabilities, has been seen during a Space Shuttle re-entry [21].

Head echo. An unresolved problem of meteor physics is the meteor head echo [22], i.e., a radar target moving with a meteor velocity. The author has proposed that a head echo is caused by generation of plasma waves in the surrounding ionospheric plasma and in the meteorís ablation products [23]. The ion beam instability could be one of the sources of the plasma waves, since at head echo heights the density of ions "sprayed and repelled" by a meteoroid is rather high, and moreover, the ions are not trapped by the geomagnetic field. Also, maybe some coupling with the magnetoionospheric waves is important also, as the waves are associated with ionospheric irregularities. Evidently, much more must be done to investigate details.

This interpretation of a head echo predicts some shift between a meteoroidís velocity calculated from its trajectory, and from its Doppler radar return, due to the plasma waves [23]. And it seems that such predictions are being confirmed. Radar data indicate some difference between these two velocities [22].

Power events. Very bright bolides are rare events. But some ideas about ionospheric processes associated with them can be obtained through examination of large rocket launches. For example, during the Aug. 30, 1983 night launch of the Space Shuttle, the main engines themselves were producing a much brighter orange flame than expected [24]. It was pulsating almost as if an engine was running unstable, but all the engines were OK [24]. In the author's opinion, the pulsation was caused by plasma instabilities in a shock wave around the engine exhaust at about 100 km height. A re-entry of the Shuttle was accompanied by the above-mentioned luminous phenomenon in its wake 6 days later [21].

The plasma instability factor also can explain some anomalous effects associated with Saturn rocket launches in 1960s. During some launches an anomalous noise-like fluctuation and attenuation affecting radio transmission occurred as the rocket, under power from liquid-hydrogen liquid-oxygen engine, passed through certain altitude regions between 100 and 250 km [25]. An idea was put forward [25] that the effects were caused by an interaction of the engine exhaust with the ionosphere. But attempts to calculate the effects have failed [25]. Here the author would like to say the following. The irregular character of the effects hints on the unstable ionosphere plasma role. Moreover, low-latitude ionospheric irregularities are registered the most often at these altitude regions. And indeed, published ionospheric sounding data [26] during one of the launches reveals appearances of a spread F, and sporadic E layer at some distance from the rocket trajectory a few minutes after the launch. This suggests that they could be caused by the magnetoionospheric waves, generated by the launch. The author's evaluation has shown that the type of spread F irregularities can be responsible for the observed effects [l5]. Is interesting also to mention a luminosity associated with exhaust plume of the Apollo 8 spacecraft [27]. Maybe it was caused by the plasma effects, and not by solar scattering [15]?

Geophysical meteors. In many of the above-mentioned events, a meteroid (or a space-vehicle) was just a trigger of ionospheric plasma instabilities. But sometimes there may be no need for a meteoroid to act as a trigger. A meteor-like phenomena can be produced by ionospheric plasma processes without a meteoroid as a trigger. Mechanisms of such strong self-organization of ionospheric plasma are not clear. As expected, these geophysical meteors are the most often in an auroral zone where plasma normally is in an unstable state. There they are called "auroraI meteors" [28].

In another case, a huge fireball explosion of non-meteoroidal origin was registered by a Soviet cosmonaut on May 5, 1981. It is quite remarkable that similar phenomena are registered in the lower atmosphere also [29-31], where there is no "classic" plasma. The latter events are the hardest to explain.

Concluding remarks

As we can see, plasma effects can play a large role in meteor phenomena. Especially important are the effects in the case of unstable conditions of ionospheric plasma. In the latter case resulting phenomena are difficult to foresee. Currently, our knowledge of plasma processes is not good enough for such predictions. Nevertheless, the author hopes that this work will attract the attention of experts in meteors, and in plasma physics, and will help to foster cooperation between them.


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