Meteor Observations Research Articles

Meteor observations with an MF radar

We conducted meteor echo observations using the Buckland Park MF radar (35°S, 138°E) at 00:40–05:45 LT on October 22, 1997. In addition to the usual full correlation analysis (FCA) technique to measure horizontal wind velocities from 60 to 100 km MF radars have a potential to detect meteor echoes and infer winds through their Doppler frequency shifts. Because of the relatively low radio frequency employed MF radars have a great advantage of providing meteor wind well above 100 km altitude, where very few techniques can measure wind velocities. There is a limitation which should be noted as well. The observations are possible only during night time when the electron density of E-region is low enough for the radio wave to penetrate into the upper region. We detected 233 underdense meteor echoes from 80 km to 120 km with a mean height of 104.4 km. Although the transmitting antenna beams were steered toward off-zenith angles of 25°, almost all the echoes were received outside of the main lobe, indicating that conventional MF radar systems with a broad transmitting beam can work well for meteor observations. Bi-hourly wind profiles were obtained from 94 to 114 km altitudes. The profiles revealed a clear wave structure with a downward phase progression with time. FCA winds from 80 to 100 km were also estimated, and a continuous wind structure was obtained from FCA to meteor heights. Note that the present observations happened to be conducted during a major meteor shower activity. However, a majority of the underdense echoes were from non-shower meteors, and observations during non-shower periods will also yield enough information.

Simultaneous measurements of dynamical structure in the mesopause region with lidars and MU radar

In order to clarify the horizontal structure of the wavelike oscillation frequently observed in the night time sodium density profile with a small Gaussian half-width in the middle of the night and a broad distribution at dusk, simultaneous observations with two lidars at Shigaraki (34.9°N, 136.1°E) and Hachioji (35.6°N, 139.4°E) and the MU radar at Shigaraki has been carried out. In the campaign of 35 nights, simultaneous observation was successful in four nights. On December 27–28, 1995, a large scale wave motion was observed by two lidars and the MU radar with meteor observation mode. Phase velocities were almost the same at the two sites and there were only slight differences in phase between the two sites. The wave motion was inconsistent with the component of the tidal wave and similar with the results of hodograph analyses. It is possible that the wave observed by lidars on December 27–28, 1995 was a gravity wave. The results of analyses suggest the possibility of gravity waves which were observed at fixed local time.

Preliminary activity of Leonid meteor storm observed with a video camera in 1997

Optical meteor observations were carried out in Mauna Kea, Hawaii, in November 1997, in order to investigate the activity of Leonid shower under strong moon shine. We report a spectacular event of a meteor burst, with 100–150 meteors in only two seconds, observed at the time of descending node of the comet at 13:31 UT on November 17, 1997. From a preliminary analysis, the spread of the meteors in the orbit are estimated to be approximately 100 km along the trajectory and 50 km in the lateral direction. This observed meteor rate is comparable to the maximum hourly rate reported in 1966. The video camera observation shown here can be very useful for observations during the forthcoming strong Leonid meteor storm.

Simultaneous meteor echo observations by large‐aperture VHF and UHF radars

We report simultaneous meteor echo observations using the Arecibo 430‐MHz and 46.8‐MHz radars. Using identical data‐taking and meteor selection criteria, 1868 and 367 meteors were found in the 430‐MHz and 46.8‐MHz beams, respectively, while 145 were found in both beams during the 7 hours of observation. Of the 367 VHF echoes, there were only 10 trail echoes, while the rest were head echoes, which was quite contrary to expectation. The smaller number of meteors detected by the VHF system and its wider beam width show that UHF meteors are far smaller than the VHF meteors. We estimate that VHF head echoes have a typical effective scattering cross section of the order of 10−3 m2, while the accompanying UHF echoes have an effective scattering cross section of the order of 10−6 m2. The paucity of VHF trail echoes observed leads us to suggest that the ratio of head echo power to the trail echo power increases with decreasing meteor size. When a meteor is too small, a radar can observe the head echo but not the trail echo. Of the 145 meteors observed by both radars, the powers received by the two systems were not correlated. Although antenna beam pattern contributes to the lack of correlation, it is also possible that UHF and VHF echoes may be enhanced by different scattering mechanisms.

The nodal, the new and the old maximum of the Perseid meteor stream

Visual observations of Perseid meteors carried out in 1953–83 at the Onsala Space Observatory are analyzed to study the fine structure of the Perseid meteor stream maximum. Observed hourly rates are corrected for weather conditions and to a zenithal value. The Perseid activity curve is studied (1) for all observed Perseids, (2) for Perseids of app. magnitude 2.0 or brighter. The activity curve for both data sets shows two main peaks, one at solar long. 138.65° (1950.0) , very near to the node of the parent comet (Swift–Tuttle) , and a second peak at 139.42°, corresponding to the traditional maximum of the shower. A third, less conspicuous peak at 138.96° is also evident in the data. This peak could possibly be an early indication of the so-called new maximum of the Perseid stream. The detection of a well-defined peak in rates very near to the node of the parent comet is important since the position of the Perseid visual maximum usually quoted in the literature deviates from the cometary node by almost 1° in solar longitude.

Meteor observations in Japan: new implications for a Taurid meteoroid swarm

Observational evidence is sought that the long-term (104 yr) action of a mean motion resonance with Jupiter can produce structure in a meteoroid stream, concentrating meteoroids in a dense swarm. More specifically, predictions tabulated by Asher & Clube of enhanced meteor and fireball activity from a Taurid Complex swarm in the 7:2 resonance are compared with observational data collected in Japan over several decades. The swarm model was proposed for reasons independent of the observations analysed here, and these newly considered data are shown to be consistent with it. This allows increased confidence in the Taurid swarm theory, and more generally could mean that resonant trapping is a dynamical mechanism affecting a significant amount of meteoroidal material in the inner Solar system.

RADAR Observations of Leonids in Japan

We are observing meteor showers using the MU radar located at Shigaraki, Japan. This is a Mesosphere, Stratosphere and Troposphere radar, belonging to the Kyoto University, with a frequency and a peak power of 46.5M.ffz and 1MW, respectively. This system is characterized by the versatility of the antenna array. By using four antennas out of 475 Yagi arrays together with four receiver channels, we can observe more than several hundred meteor echoes per hour. The beam applied at the meteor observation is a static “doughnut beam”, which is the rotational symmetrical pattern to the vertical line. This beam is effective enough to detect meteor echoes of which the zenith angle is smaller than about 50°, which characterizes the MU radar as an all sky monitoring radar of meteor echoes. For the low velocity meteors, most of the echoes satisfy a condition of the perpendicular reflection, and we can derive the position of the radiant point statistically, and succeeded its application for the Geminids (Watanabe et al. 1992a). For the high velocity meteors, most of the echoes are in the overdense condition. However, we can discriminate the echoes of a specific meteor shower out of the data by checking the echo duration, the height, the strength or the combination of them. We have been monitoring the Perseids from 1990, and succeeded to reveal the unusual activity in 1991 (Watanabe et al. 1992b). For the Leonids, we are continuing the observation from 1991. The drastic increase of the activity has been detected around 23h UT on November 17, 1994. The number of the observed long duration echoes, which came from possibly bright Leonids meteors, were 80 per hour. This should be the beginning of the Leonids activity toward the 1998-1999. We are planning to continue this observation until the end of the activity in the 2000’s.

Interstellar Dust in the Solar System

A fraction of the dust in the interstellar medium (ISM) enters the heliosphere and is measured in situ from spacecraft. This review surveys the in situ measurements and discusses a hence derived model of dust in the local interstellar cloud (LIC). The LIC dust model bears similarities to pristine cometary dust and is characteristic of the warm ISM clouds that fill a part of the ISM in the vicinity of the Sun. Recent and future dust in situ measurements provide a basis for closely studying physical processes in the ISM surrounding the Solar System. The LIC dust is the only dust component measurable in the Solar System that was not previously incorporated in larger Solar System objects. Issues for future LIC dust studies are measuring dust fluxes at the outer heliosphere, measuring the mass distribution with meteor observations and observations from spacecraft, and measuring the LIC dust composition in situ in space.

Observations in Central and Southern Europe

Since September 1996 there has been in operation a new forward-scatter system for meteor observation transmitting signal simultaneously along two mutually almost rectangular baselines with the transmitter at Budrio (ϕB = 44.6° λB = 11.5° E) near Bologna, Italy and the receivers at Modra (ϕL = 48.3° N; λL = 17.3° E), Slovakia and Lecce (ϕL = 40.3° N; λL = 18.2° E) in Southern Italy. The equipment utilizes a continuous wave transmitting frequency at 42.7 MHz, with a mean power of 0.25 kW.

Meteor observations by the Arecibo 430 MHz incoherent scatter radar. II. Results from the time-resolved observations

Meteor observations by the Arecibo 430 MHz incoherent scatter radar. II. Results from the time-resolved observations

Meteor shower radiant positions and structures as determined from single station video observations

Single station video observations of meteors can be used for the precise determination of radiant positions of major and minor meteor showers. Video systems combine large quantities of recorded events with high accuracies, and are therefore applied in the investigation of fine structures within the radiant. In the past four years, amateurs from the Archenhold-Observatory Berlin have been operating the wide angle video system MOVIE. The maxima of several major meteor showers could be recorded. The video tapes were digitized and analysed on PCs. Using the Radiant software developed within the International Meteor Organization, the position was determined of the Quadrantid (α = 229.4° ± 1.5°, δ = +49.7° ± 1.5°), Lyrid (α = 271.6° ± 1.5°, δ = +32.9° ± 1°), Perseid (α = 46.0° ± 2°, δ = +57.7° ± 2°), Orionid (α = 93.6° ± 1°, δ = +14.9° ± 1°) and Leonid (α = 154.5° ± 2°, δ = +21.4° ± 1°) meteor shower radiants and possible radiant fine structures searched for. The obtained radiant positions are in good agreement with the common values in the literature; no significant fine structures could be found. From the video records of the 1995 α-Monocerotid outburst, a radiant at α = 117° ± 3° and δ = +1° ± 2° was derived, which proves older results to be in error by several degrees.

Development of an external interferometer for meteor wind observation attached to the MU radar

An external meteor detection system for the middle and upper atmosphere (MU) radar, Shigaraki, Japan, has been developed in order to monitor the wind velocities at 80–100 km. The characteristics of the MU radar system and the observations are closely inspected, and we design a three‐channel interferometer system using three independent single antennas. The data‐taking system was carefully designed, considering fast data rate, short interpulse period, wide data bits (19 bit), and fast sampling rate (1 million samples/s), and all the logics are assembled by programmable logic devices. This external interferometer was operated since March 1995 during the Doppler beam swinging observation by the MU radar, and the first results are presented. Meteor echo rate was around 2000/day, 1000 of which were usable for postprocessing, i.e., wind measurement. This echo rate is greater than that of the meteor radar system in Jakarta. Wind velocities were determined with time‐height resolutions of 1 hour × 2 km, which showed reasonable agreement with the wind velocities determined from the turbulent echoes in the mesosphere. We present the capability of an external interferometry system attached to the mesosphere‐stratosphere‐troposphere/stratosphere‐troposphere radar as a meteor wind observation system.

Meteor observations by the Arecibo 430 MHz incoherent scatter radar. II. Results from time-resolved observations

Abstract The paper discusses an incoherent scatter radar (ISR) power profile interference detection and meteor filtering technique and presents the meteor interference statistics derived from the Arecibo 430 MHz ISR. The statistics obtained from the Arecibo ISR are comparable with the meteor statistics obtained by other radar and optical techniques. Using the hourly rate of meteor flux, the average visual magnitude of the meteors i s estimated to be as faint as + 14, probably, the faintest ever reported. For low echo meteors, the mean ablation height exhibits a logarithmic relationship with the returned power below 105 km. However, once the echo power exceeds a certain threshold, the mean height remains constant at 107 km for all meteors irrespective of their returned power. The unique aspect of the meteor trails reported here is that they are observed in the radial direction of the radar beam. The lack of aspect sensitivity and the high meteor rate detected by the Arecibo 430 MHz system seem to suggest that the scattering mechanism in the UHF range of frequencies may be different from the mechanism operating at VHF. Some future prospects of exploiting; meteor trails to study the neutral wind and other aspects of the meteor region are also proposed.

A note on the meteoric head echo occurrence

Meteoric head echo statistics based on the Perseid observations carried out by the high-power meteor radar at the Springhill Meteor Observatory in the period 1957–1982 are dealt with. A total of 1945 head echoes was recorded during 271 h covering the central part of the Perseid activity in the interval of solar longitudes 139.5° ⩽ L ⊙ ⩽ 140.10 (eq. J2000.0). The rate of their occurrence shows an irregular diurnal variation with a minimum near upper culmination of the shower radiant. The ratio of the head echo rate to overdense echo rate having durations T ⩾ 1 s varies from 2.1% at 17 h 30 m LT to about 5.0% at 22 h LT. Attention was paid also to the delay of a secondary echo appearing after the head echo. The maximum delay appears at 5 h 30 m LT and coincides with the lower culmination of the Perseid radiant when the mean delay represents some 2.5 s while its minimum value of 0.05 s coincides with the upper culmination at 17 h 30 m LT.

Visual observations of the Perseid meteor shower 1988–1994

Visual meteor observations are used to determine the flux profile of the Perseid stream from 1988 to 1994. The position and peak flux of recently detected outburst maxima are found for each year as is the location and magnitude of the regular maximum. The Perseid stream is found to consist of three primary components; a long-lived and relatively weak background component, a core component which is active for 1–2 days near the main peak of the shower and an outburst component which is active for only a few hours. The outburst peaks show a systematic variation in terms of the solar longitudes of the peak. The position of the outburst shifted backwards in solar longitude (J2000.0) from λ ⊙ = 139°.78 in 1988 to λ ⊙ = 139°.48 in 1992, when it occurred closest to the node of 109P/Swift-Tuttle (139°.44). In subsequent years the peak position shifted forward again to λ ⊙ = 139°.59 in 1994. The magnitude of the peak flux shows a significant difference between returns before 1991 as compared to those after this year. This may indicate an ejection origin other than 1862 for outburst material encountered before 1991. The particle composition of the outburst maximum and core maximum are indistinguishable. A double maximum in the population index associated with these activity maxima has been detected in the average Perseid profile. High temporal resolution study of the flux profile of the ascending branches of the outburst profiles in 1993 and 1994 suggest that this component may consist of sub-components, possibly of differing ages.

The Orbits of Meteorites from Natural Thermoluminescence

The natural thermoluminescence (TL) of meteorites reflects their irradiation and thermal histories. Virtually all ordinary chondrites have been irradiated long enough to have reach saturation natural TL levels, and thus natural TL levels in these meteorites are determined largely by thermal history. The primary heat source for most meteorites is the Sun, and thus natural TL levels are determined primarily by the closest approach to the Sun, i.e., perihelion. By converting natural TL levels to perihelia, using an assumed albedo typical of meteoroid bodies, it is found that most ordinary chondrites had perihelia of 0.85 to 1.0 AU prior to reaching Earth. This range is similar to that calculated from meteor and fireball observations. All common classes of ordinary chondrites exhibit similar perihelia distributions; however, H and LL chondrites that fell in the local morning differ in their natural TL distribution from those that fell in the local afternoon and evening. This is consistent with earlier suggestions that time of fall reflects orbital distribution. The data also suggest that the orbits of some of the H chondrites cluster and may have come from a debris “stream” of meteoroids. If meteorites can exist in “orbital groups,” significant changes in the types and number of meteorites reaching Earth could occur on the <105-year time scale.

The meteoroid environment near Earth

An upgraded model of the interplanetary meteoroid environment, which is based on the ‘Five populations of interplanetary meteoroids’ model of Divine (1993), has been developed to synthesize data from in-situ detection by spacecraft, meteor observations, groundbased zodiacal light, and analysis of lunar microcraters. New model populations have been defined taking into account particle impact velocities and impact directions measured with GALILEO and ULYSSES dust detectors. Both, elliptical and hyperbolic meteoroid populations have been included considering the radiation pressure of the Sun. Particle fluxes for different mass thresholds have been calculated for surface elements of satellites retrieved from space (LDEF, EuReCa and HST solar array) taking into account both gravitational focusing and planetary shielding of the Earth.

Studies of the MLT region using the MU radar and simultaneous observations with OH spectrometer and other optical instruments

The MU radar (middle and upper atmosphere radar) has been used since 1984 to study the dynamics of the mesosphere and lower thermosphere (MLT) as well as the dynamics of the lower atmosphere and the ionosphere. Gravity waves in the mesosphere have been studied extensively over various time and vertical scales during the daylight hours using backscatter from turbulent irregularities. Scatter from meteor trails can be used during day or night to study the dynamics of the MLT, and techniques to apply the MU radar for meteor scatter observations have been developed and recently improved. The meteor observation mode of the MU radar can detect about 15,000–20,000 echoes/day and 10,000 echoes/day are from underdense trails with arrival angle determination. This high meteor echo rate enables us to determine the wind velocities and molecular diffusion coefficient with the time/height resolutions of 30 min × 1 km at 80–100 km for 24 hours a day. From the horizontal distribution of the meteor echoes, horizontal gradients of the wind fields can also be detected, as well as vertical shears of horizontal winds. These high resolution meteor echo observations were made simultaneously with OH spectrometer observations. These combined observations yielded information on both the horizontal and vertical structure of the gravity waves. Comparisons were made between the temperature variations determined by OH airglow and determined by the diffusion coefficient of the meteor echoes and excellent agreement was found. Cooperative observations with sodium lidars, FPI, airglow imagers, and an MF radar are also being carried out.

Implications for meteoroid chemistry from the height distribution of radar meteors

We have gathered substantial evidence from novel radar meteor observations that most meteoroids start to ablate at altitudes considerably in excess of 100 km (at heights up to ∼140 km). This beginning height h b is a strong function of the melting point of the meteoroid. Theoretical modelling indicates that for stony meteoroids observed as faint radio meteors these heights range from h b = 116 km (for entry speeds v ∞ = 70 km s −1) to 90 km (for v ∞ = 15 km s −1). Meteoroids that begin ablating above these heights must have lower temperature melting points. We interpret the observations in terms of a population of the meteoroids largely constituted of heavy organic compounds that are susceptible to gross-fragmentation as they heat and encounter the Earth at speeds greater than ∼30 km s −1.

The Perseid Meteoroid Stream: Characterization of Recent Activity from Visual Observations

The flux profile of the Perseid stream from 1988 to 1994 is analyzed using visual observations of meteors. The position and peak flux of recently detected outburst maxima are found for each year, as are the location and magnitude of the regular maximum. Details of the reduction procedure for these visual data are also presented. The Perseid stream is found to consist of three primary components: a long-lived and relatively weak background component, a core component which is active for 1–2 days near the main peak of the shower, and an outburst component which is active for only a few hours. The latter component changes position and strength dramatically from year to year, while the core maximum remains at a near constant location of λ⊙= 139.°96 ± 0.°05 and shows a variance of less than 30% from a mean peak flux ofS6.5 max= 96 ± 16 meteoroids per 109km3during 1988–1994. The outburst peak locations show no systematic variation from year to year, though the magnitude of the peak flux shows a significant difference for returns before 1991 compared to those after 1991. This may indicate an ejection origin other than 1862 for outburst material encountered before 1991. The particle composition of the outburst maximum and that of the core maximum are indistinguishable. A double maximum in the population index associated with these activity maxima has been detected in the average Perseid profile. High temporal resolution study of the flux profile of the ascending branches of the outburst profiles in 1993 and 1994 suggests that this component may consist of sub-components of differing ages. The mean visual flux of the shower is in excellent agreement with past radar results. The most important features of the stream gleaned from this study which must be explained by any model of the Perseids are also summarized.