首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The height distributions, velocity distributions and flux measurements of underdense echoes determined from meteor radar observations are significantly affected by the attenuation associated with the initial radius of meteor trains. Dual-frequency radar observations of a very large set of sporadic radar meteors at 29 and 38 MHz yield estimates of the initial train radius and its dependence on height and meteoroid speed as determined by the time-delay method. We provide empirical formulae that can be used to correct meteoroid fluxes for the effect of initial train radius at other radio frequencies.  相似文献   

2.
Experimental and theoretical work on the transverse dimensions of meteoric plasma trains have not converged to provide generally accepted values especially uncertain is the dependence of the train radii on meteor speeds. The roles of the meteoroid structure, fragmentation and plasma processes such as ion–electron instabilities need establishing. Knowledge of the quantitative spatial distribution of plasma in meteor trains is essential for a correct interpretation of fluxes and orbital characteristics. A current project is described which employs the AMOR 26 MHz radar facility in conjunction with a frequency managed radar operating at longer wavelengths designed to measure the ionization train radii, heights, atmospheric speeds and orbits of individual meteors.  相似文献   

3.
We present a method to calculate the radiation pressure force to gravity ratio on meteoroids from their atmospheric flight. Radiation pressure corrections to meteor orbits are negligible for fireballs; of the order of or less than the measurement errors ( 1%) for photographic meteors; of the order of and in some cases substantially larger than the measurement errors ( 10%) for radar meteors.  相似文献   

4.
Abstract— Two‐station electro‐optical observations of the 1998 Leonid shower are presented. Precise heights and light curves were obtained for 79 Leonid meteors that ranged in brightness (at maximum luminosity) from +0.3 to +6.1 astronomical magnitude. The mean photometric mass of the data sample was 1.4 × 10?6 kg. The dependence of astronomical magnitude at peak luminosity on photometric mass and zenith angle was consistent with earlier studies of faint sporadic meteors. For example, a Leonid meteoroid with a photometric mass of ～1.0 × 10‐7 kg corresponds to a peak meteor luminosity of about +4.5 astronomical magnitudes. The mean beginning height of the Leonid meteors in this sample was 112.6 km and the mean ending height was 95.3 km. The highest beginning height observed was 144.3 km. There is relatively little dependence of either the first or last heights on mass, which is indicative of meteoroids that have clustered into constituent grains prior to the onset of intensive grain ablation. The height distribution, combined with numerical modelling of the ablation of the meteoroids, suggests that silicate‐like materials are not the principal component of Leonid meteoroids and hints at the presence of a more volatile component. Light curves of many Leonid meteors were examined for evidence of the physical structure of the associated meteoroids: similar to the 1997 Leonid meteors, the narrow, nearly symmetric curves imply that the meteoroids are not solid objects. The light curves are consistent with a dustball structure.  相似文献   

5.
A comparative study of meteor ablation in the atmospheres of the Earth and Venus is presented. The classical single body meteor ablation model is extended to incorporate a heat penetration depth estimate allowing the simulation of larger meteoroids, than would an isothermal model. The ablation of icy and rocky meteoroids, with densities of 1.0 and 3.4 g cm−3, respectively, and initial radii of up to for rock and for ice (equivalent to an initial mass of in both cases), was simulated in both atmospheres. In general venusian meteors are brighter than terrestrial equivalents. Large, slow, rocky objects may be up to 0.7 mag brighter on Venus, while small, icy particles with entry speeds in the range 30-60 km s−1, are found to be upwards of 2.7 mag brighter than at the Earth. Venusian meteors reach maximum brightness at greater altitudes than would similar particles at the Earth. Rocky meteoroids have their points of maximum brightness some 15-35 km higher up at Venus, between 90 and 120 km, whereas, for icy particles this altitude difference is about 5-25 km higher up than at the Earth, in the range 100-125 km. These findings agree, for the most part, with recent analytical studies. Venusian meteors, which last from 100 ms to , tend to be shorter-lived than terrestrial meteors, with correspondingly shorter visible trails. Large (), slow () icy particles reach a maximum magnitude of ∼−2 at Venus and remain visible for about one second, with a large section of the smaller faster meteoroids simulated here remaining visible for several hundred milliseconds. In light of recent space-based meteor observations at the Earth [Jenniskens, P., Tedesco, E., Muthry, J., Laux, C.O., Price, S., 2002. Meteorit. Planet. Sci. 37, 1071-1078], such brightness, height and duration estimates as suggested in this work, may be used in developing future observational campaigns to be carried out from Venus orbit.  相似文献   

6.
The meteor radar response function is an important tool for analyzing meteor backscatter observed by radar systems. We extend previous work on the development of the response function to include a non-uniform meteor ionization profile, provided by meteor ablation theory, in contrast to what has been assumed in the past. This has the advantage that the height distribution of meteors expected to be observed by a radar meteor system may be accurately modeled. Such modeling leads to meteor height distributions that have implications for the composition of those meteoroids ablating at high altitudes which may be observed by “non-traditional” meteor radars operating at MF/HF. The response function is then employed to investigate meteor backscatter observed by narrow beam MST radars which in recent years have been used increasingly to observe meteors.  相似文献   

7.
The density of the Galactic meteoroid background formed by the loss of large dust grains escaped from circumstellar disks during the formation of a star is estimated. This density is shown to be substantially higher than the local density of meteoroids that escaped from dusty stellar disks as a result of two-star collisions. The flux of meteors of the Galactic background near the Earth is calculated for a given latitude of the observing site taking into account the velocity distribution of dust particles and the motion of particles in the solar gravity field. The expected rate of the Galactic background meteor events at the AMOR radar latitude is lower by at least a factor of 30 than the reported frequency of interstellar meteor events that are supposedly registered in the AMOR experiment.  相似文献   

8.
Abstract— In this paper, we study the extremely high beginning parts of atmospheric trajectories of seven Leonid meteors recorded by sensitive TV systems equipped with image intensifiers up to apparent magnitude +6.5. For all seven cases, we observed comet‐like diffuse structures with sizes on the order of kilometers that developed quickly during the meteoroids' descent through the atmosphere. For the brightest event with a maximum absolute magnitude of ?12.5, we observed an arc similar to a solar protuberance and producing a jet detectable several kilometers sideways from the brightest parts of the meteor head, and moving with a velocity over 100 km/s. These jets are common features for the seven studied meteors. Precise position in trajectory, velocity, and brightness at each point is available for all seven meteors, because of double‐station records on 85 km base‐line. When these meteoroids reached 130 km height, their diffuse structures of the radiation quickly transformed to the usual meteor appearance resembling moving droplets, and meteor trains started to develop. These meteor phenomena above 130 km were not recognized before our observations, and they cannot be explained by standard ablation theory.  相似文献   

9.
The velocity distribution of meteoroids at the Earth is measured using a time-of-flight measurement technique applied to data collected by the CMOR radar (29.85 MHz). Comparison to earlier velocity measurements from the Harvard Radio Meteor Project suggests that HRMP suffered from biases which underestimated the number of fragmenting meteoroids. This bias results in a systematic underestimation of the numbers of higher velocity meteoroids. Other works (cf. Taylor and Elford, 1998) have also found additional biases in the HRMP which suggest the original HRMP meteoroid velocity analysis may have underestimated the fraction of high velocity meteors by factors up to 104.  相似文献   

10.
New methods of determining meteor speeds using radar are giving results with an accuracy of better that 1%. It is anticipated that this degree of precision will allow determinations of pre-atmospheric speeds of shower meteors as well as estimates of the density of the meteoroids. The next step is to determine under what conditions these new measurements are reliable.Errors in meteoroid speeds determined using a Fresnel transform procedure applied to radar meteor data are investigated. The procedure determines the reflectivity of a meteor trail as a function of position, by application of the Fresnel transform to the time series of a radar reflection from the trail observed at a single detection station. It has previously been shown that this procedure can be used to determine the speed of the meteoroid, by finding the assumed speed that gives a reflectivity image that best meets physical expectations. It has also been shown that speeds determined by this method agree with those from the well established “pre-to phase” method when applied to reflections with a high signal to noise ratio. However, there is a discrepancy between the two methods for weaker reflections. A method to investigate the discrepancy is described and applied, with the finding that the speed determined by using the Fresnel transform procedure is more accurate for weaker reflections than that given by the “pre-to phase” method.  相似文献   

11.
S. Close  P. Brown  M. Oppenheim 《Icarus》2007,186(2):547-556
High-power, large-aperture (HPLA) radars detect the plasma that forms in the vicinity of a meteoroid and moves approximately at its velocity; reflections from these plasmas are called head echoes. For over a decade, HPLA radars have been detecting head echoes with peak velocity distributions >50 km/s. These results have created some controversy within the field of meteor physics because previous data, including spacecraft impact cratering studies, optical and specular meteor data, indicate that the peak of the velocity distribution to a set limiting mass should be <20 km/s [Love, S.G., Brownlee, D.E., 1993. Science 262, 550-553]. Thus the question of whether HPLA radars are preferentially detecting high-velocity meteors arises. In this paper we attempt to address this question by examining both modeled and measured head echo data using the ALTAIR radar, collected during the Leonid 1998 and 1999 showers. These data comprise meteors originating primarily from the North Apex sporadic meteor source. First, we use our scattering theory to convert measured radar-cross-section (RCS) to electron line density and mass, as well as to convert modeled electron line density and mass to RCS. We subsequently compare the dependence between mass, velocity, mean-free-path, RCS and line density using both the measured and modeled data by performing a multiple, linear regression fit. We find a strong correlation between derived mass and velocity and show that line density is approximately proportional to mass times velocity3.1. Next, we determine the cumulative mass index using subsets of our data and use this mass index, along with the results of our regression fit, to weight the velocity distribution. Our results show that while there does indeed exist a bias in the measured head echo velocity distribution, it is smaller than those calculated using traditional specular trail data due to the different scattering mechanism, and also includes a bias against the low-mass, very high-velocity meteoroids.  相似文献   

12.
Particles of mass less than about 1 gm are a minor fraction of the total matter impinging on the Earth averaged over millennia time scales. However, these particles dominate during a single particular year and produce the most obvious evidence of incoming extra-terrestrial matter in the form of ablation trails in the atmosphere which are visible at night as meteors.Observations of meteors give astronomical information on the composition, structure, and cometary associations of the particles. The composition is deduced from optical spectra of meteors, whilst telescopic studies of the trails during formation give information on the physical structure of the particles. Any cometary associations are deduced from measurement of meteor orbits determined photographically, using television, or by radar.Meteors occur in the atmosphere at heights from about 70 to 120 km. Optical observations are restricted to night-time and usually under conditions of low moonlight. A typical television based detector can record +8M meteors with a sporadic rate of 15–20 per hour and velocities accurate to about 3%. The luminosity of the trail is strongly dependent on the velocity of the meteoroid (to about the third power).Radar observations of meteors are unrestricted by weather or time of day, and can readily detect meteors at least two orders of magnitude smaller in mass than those detectable optically. Again the observations are heavily biased toward the higher velocities as the electron line density varies approximately asV 3.5. However, the higher the velocity of the meteoroid the greater the height of the meteor trail, and the reduced probability of radar detection due to rapid diffusion of the trail. Thus radar observations tend to select meteors in the intermediate velocity range 30–40 km s–1.  相似文献   

13.
Recently, meteor head echo detections from high powered large aperture radars (HPLA) have brought new measurements to bear on the study of sporadic interplanetary meteors. These same observations have demonstrated an ability to observe smaller meteoroids without some of the geometrical restrictions of specular radar techniques. Yet incorporating data from various radar reflection types and from different radars into a single consistent model has proven challenging. We believe this arises due to poorly understood radio scattering characteristics of the meteor plasma, especially in light of recent work showing that plasma turbulence and instability greatly influences meteor trail properties at every stage of evolution. In order to overcome some of the unknown relationships between meteoroid characteristics (such as mass and velocity) and the resulting head echo radar cross-sections (RCS), we present our results on meteor plasma simulations of head echo plasmas using particle in cell (PIC) ions, which show that electric fields strongly influence early stage meteor plasma evolution, by accelerating ions away from the meteoroid body at speeds as large as several kilometers per second. We also present the results of finite difference time domain electromagnetic simulations (FDTD), which can calculate the radar cross-section of the simulated meteor plasma electron distributions. These simulations have shown that the radar cross-section depends in a complex manner on a number of parameters. In this paper we demonstrate that for a given head echo plasma the RCS as a function of radar frequency peaks at sqrt (2*peak plasma frequency) and then decays linearly on a dB scale with increasing radar frequency. We also demonstrate that for a fixed radar frequency, the RCS increases linearly on a dB scale with increasing head echo plasma frequency. These simulations and resulting characterization of the head echo radar cross-section will both help relate HPLA radar observations to meteoroid properties and aid in determining a particular radar facility’s ability to observe various meteoroid populations.  相似文献   

14.
Abstract— In this paper, we provide an overview of meteors with high beginning height. During the recent Leonid meteor storms, as well as within the regular double station video observations of other meteor showers, we recorded 164 meteors with a beginning height above 130 km. We found that beginning heights between 130 and 150 km are quite usual, especially for the Leonid meteor shower. Conversely, meteors with beginning heights above 160 km are very rare even among Leonids. From the meteor light curves, we are able to distinguish two different processes that govern radiation of the meteors at different altitudes. Light curves vary greatly above 130 km and exhibit sudden changes in meteor brightness. Sputtering from the meteoroid surface is the dominating process during this phase of the meteor luminous trajectory. Around 130 km, the process switches to ablation and the light curves become similar to the light curves of standard meteors. The sputtering model was successfully applied to explain the difference in the beginning heights of high‐altitude Leonid and Perseid meteors. We show also that this process in connection with high altitude fragmentation could explain the anomalously high beginning heights of several relatively faint meteors.  相似文献   

15.
Radio science and meteor physics issues regarding meteor “head-echo” observations with high power, large aperture (HPLA) radars, include the frequency and latitude dependency of the observed meteor altitude, speed, and deceleration distributions. We address these issues via the first ever use and analysis of meteor observations from the Poker Flat AMISR (PFISR: 449.3 MHz), Sondrestrom (SRF: 1,290 MHz), and Arecibo (AO: 430 MHz) radars. The PFISR and SRF radars are located near the Arctic Circle while AO is in the tropics. The meteors observed at each radar were detected and analyzed using the same automated FFT periodic micrometeor searching algorithm. Meteor parameters (event altitude, velocity, and deceleration distributions) from all three facilities are compared revealing a clearly defined altitude “ceiling effect” in the 1,290 MHz results relative to the 430/449.3 MHz results. This effect is even more striking in that the Arecibo and PFISR distributions are similar even though the two radars are over 2,000 times different in sensitivity and at very different latitudes, thus providing the first statistical evidence that HPLA meteor radar observations are dominated by the incident wavelength, regardless of the other radar parameters. We also offer insights into the meteoroid fragmentation and “terminal” process.  相似文献   

16.
The distribution of meteor signals reflected from a backscatter radar is considered according to their duration. This duration time (T) is used to classify the meteor echoes and to calculate the mass index (S) of different meteoroids of shower plus sporadic background. Observational data on particle size distribution of the Geminid meteor shower are very scarce, particularly at low latitudes. In this paper the observational data from Gadanki radar (13.46°N, 79.18°E) have been used to determine the particle size distribution and the number density of meteoroids inside the stream of the Geminid meteor shower. The mean variation of meteor number density across the stream has been determined for three echo duration classes, T<0.4, T=0.4–1 and T>1 s. We are more interested in the appearance of echoes of various durations and therefore meteors of various masses in order to understand more on the filamentary structure of the stream. It is observed that the faint particle flux peaks earlier than the larger particles. We found a decreasing trend in the mass index values from the day of peak activity to the next observation days. The mass index profile was found to be U-shaped with a minimum value near the time of peak activity. The observed minimum s values are 1.64±0.05 and 1.65±0.04 in the years 2003 and 2005, respectively. The activity of the shower indicates the mass segregation of meteoroids inside the stream. Our results are best comparable with the “scissors” structure model of the meteoroid stream formation of Ryabova [2007. Mathematical modeling of the Geminid meteoroid stream. Mon. Not. R. Astron. Soc. 375, 1371–1380] by considering the asteroid 3200 Phaethon as an extinct comet.  相似文献   

17.
Many meteoroids burn up between about 120 km and 70 km, deposit metals and dust and form ionized trails which are detected by radars. Model studies about the influence of neutral or positively charged background dust on the ambipolar diffusion indicate that significant smaller decay times should be observed for weak meteor echoes compared to strong meteor echoes which can affect the estimation of temperatures. The variation of meteor decay times in dependence on echo strength, height, and season was studied using radar observations at 69° N, 22° S, and 67° S. Significantly reduced decay times were found for weak echoes below about 88 km at low latitudes throughout the year, and at high latitudes with the exception of summer. In summer at high latitudes, decreasing decay times of weak and strong meteors are observed at altitudes below about 85 km during the appearance of noctilucent clouds. The impact of reduced decay times on the estimation of neutral temperatures from decay times is discussed.  相似文献   

18.
The structures of the meteor streams of cometary origin—Draconids, Ursids, Perseids, and Lyrids—and the streams presumably connected with asteroids—Taurids and α-Capricornids—are compared. The comparative analysis was performed by the mass distribution of meteoroids in the stream and the activity profile for the meteors with the maximum recorded stellar magnitude +3 m and brighter. Visual observations of 1987–2008 from the database of the International Meteor Organization (IMO) and earlier sources were considered. It has been shown that the structures of the meteor streams of cometary and, presumably, asteroidal origin differ somewhat by the activity profile and the mass distribution of meteoroids in the cross-section of a stream along the Earth’s orbit.  相似文献   

19.
In the fall of 2005, a dedicated meteor observing campaign was carried out by the Panoramic Camera (Pancam) onboard the Mars Exploration Rover (MER) Spirit to determine the viability of using MER cameras as meteor detectors and to obtain the first experimental estimate of the meteoroid flux at Mars. Our observing targets included both the sporadic meteoroid background and two predicted martian meteor showers: one associated with 1P/Halley and a potential stream associated with 2001/R1 LONEOS. A total of 353 images covering 2.7 h of net exposure time were analyzed with no conclusive meteor detections. From these data, an upper limit to the background meteoroid flux at Mars is estimated to be for meteoroids with mass larger than 4 g. For comparison, the estimated flux to this mass limit at the Earth is [Grün, E., Zook, H.A., Fechtig, H., Giese, R.H., 1985. Icarus 62, 244-272]. This result is qualitatively consistent, within error bounds, with theoretical models predicting martian fluxes of ∼50% that at Earth for meteoroids of mass 10−3-101 g [Adolfsson, L.G., Gustafson, B.A.S., Murray, C.D., 1996. Icarus 119, 144-152]. The MER cameras, even using the most sensitive mode of operation, should expect to see on average only one coincident meteor on of order 40-150 h of total exposure time based on these same theoretical martian flux estimates. To more meaningfully constrain these flux models, a longer total integrated exposure time or more sensitive camera is needed. Our analysis also suggests that the event reported as the first martian meteor [Selsis, F., Lemmon, M.T., Vaubaillon, J., Bell, J.F., 2005. Nature 435, 581] is more likely a grazing cosmic ray impact, which we show to be a major source of confusion with potential meteors in all Pancam images.  相似文献   

20.
The tristatic EISCAT 930-MHz UHF system is used to determine the absolute geocentric velocities of meteors detected with all three receivers simultaneously at 96 km, the height of the common radar volume. The data used in this study were taken between 2002 and 2005, during four 24-h runs at summer/winter solstice and vernal/autumnal equinox to observe the largest seasonal difference. The observed velocities of 410 tristatic meteors are integrated back through the Earth atmosphere to find their atmospheric entry velocities using an ablation model. Orbit calculations are performed by taking zenith attraction, Earth rotation as well as obliquity of the ecliptic into account. The results are presented in the form of different orbital characteristics. None of the observed meteors appears to be of extrasolar or asteroidal origin; comets, particularly short-period (<200 yr) ones, may be the dominant source for the particles observed. About 40 per cent of the radiants can be associated with the north apex sporadic meteor source and 58 per cent of the orbits are retrograde. There is evidence of resonance gaps at semimajor axis values corresponding to commensurabilities with Jupiter, which may be the first convincing evidence of Jupiter's gravitational influence on the population of small sporadic meteoroids surveyed by radar. The geocentric velocity distribution is bimodal with a prograde population centred around 38 km s−1 and a retrograde population peaking at 59 km s−1. The EISCAT radar system is located close to the Arctic Circle, which means that the North Ecliptic Pole (NEP) is near zenith once every 24 h, i.e. during each observational period. In this particular geometry, the local horizon coincides with the ecliptic plane. The meteoroid influx should therefore be directly comparable throughout the year.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号