Antihelion Sources Research Articles

Signatures of meteor showers and sporadics inferred from the height distribution of meteor echoes

The SKYiMET meteor radar is capable of measuring the height distribution of ionized meteor trails. Observations of the Sodankyla radar (67°N, 23°E) in 2008–2019 are analyzed. A method is applied, based on the median and quartiles of meteor heights, for distinguishing meteor showers from much more intense background sporadic meteors. Since the shower meteors are commonly less dense and enter the Earth’s atmosphere at a larger velocity, they produce ionization up to 3 ​km higher than sporadic meteors. This effect manifests itself in the form of narrow peaks that are clearly visible in the median and quartiles of the meteor height. Seven established showers with zenith hourly rate (ZHR) ​> ​12 were identified. In addition, using both the height parameters and the radiant distribution, a signature of a former meteor stream evolving into sporadic meteors is found. This additional sporadic inflow has an antihelion source and occurs in the January.

RADIANTS AND ORBITAL DISTRIBUTION OF TV FAINT SPORADIC METEORS

Double station meteor observation using TV CCD unintensified techniques was started in 2013 in Nikolaev astronomical observatory (RI NAO). During observational campaign in 2013-2016 catalog containing 1055 meteoroid orbits has been obtained. Registered meteoroid photometric masses (Kruchinenko, 2012) are between 10 -7 and 10 - 2 kg. Distribution of radiants, velocities and elements of heliocentric orbits with accent on meteoroids with low masses are analyzed in the work. Comparison with data from meteoroid orbit catalogs SonotaCo, EDMOND and NFC networks is also given. Two kinds of databases/catalogs are distinguished: “narrow field” and “wide field” depending on which type of camera system were used in observations. For analysis of the different catalogs only sporadic meteors were selected. Comparison of sporadic meteoroid radiants and orbital elements for different databases has shown that “narrow field” observations have next typical characteristics: 1) more relative number of low-mass meteoroids (about 30%) than large databases obtained by wide-angle observations; 2) significant increasing of number of low-velocity ( V g 4.6 predominantly; 3) low-mass meteoroids in “wide-field” databases caused Apex contribution only, notable part of this objects from “narrow-field” data has radiants close to Antihelion source.

CAMS newly detected meteor showers and the sporadic background

The Cameras for Allsky Meteor Surveillance (CAMS) video-based meteoroid orbit survey adds 60 newly identified showers to the IAU Working List of Meteor Showers (numbers 427, 445–446, 506–507, and part of 643–750). 28 of these are also detected in the independent SonotaCo survey. In total, 230 meteor showers and shower components are identified in CAMS data, 177 of which are detected in at least two independent surveys. From the power-law size frequency distribution of detected showers, we extrapolate that 36% of all CAMS-observed meteors originated from ∼700 showers above the N=1 per 110,000 shower limit. 71% of mass falling to Earth from streams arrives on Jupiter-family type orbits. The transient Geminids account for another 15%. All meteoroids not assigned to streams form a sporadic background with highest detected numbers from the apex source, but with 98% of mass falling in from the antihelion source. Even at large ∼7-mm sizes, a Poynting–Robertson drag evolved population is detected, which implies that the Grün et al. collisional lifetimes at these sizes are underestimated by about a factor of 10. While these large grains survive collisions, many fade on a 104-y timescale, possibly because they disintegrate into smaller particles by processes other than collisions, leaving a more resilient population to evolve.The meteors assigned to the various showers are identified in the CAMS Meteoroid Orbit Database 2.0 submitted to the IAU Meteor Data Center, and can be accessed also at http://cams.seti.org.

Structure and sources of the sporadic meteor background from video observations

Abstract We investigate and discuss the structure of the sporadic meteor background population in the near-Earth space based on video meteor orbits from the SonotaCo database (SonotaCo 2009, WGN, 37, 55). The selection of the shower meteors was done by the Southworth–Hawkins streams-search criterion (Southworth & Hawkins 1963, Smithson. Contr. Astrophys., 7, 261). Of a total of 117786 orbits, 69.34% were assigned to sporadic background meteors. Our analysis revealed all the known sporadic sources, such as the dominant apex source which is splitting into the northern and southern branch. Part of a denser ring structure about the apex source connecting the antihelion and north toroidal sources is also evident. We showed that the annual activity of the apex source is similar to the annual variation in activity of the whole sporadic background. The antihelion source exhibits a very broad maximum from July until January and the north toroidal source shows three maxima similar to the radar observations by the Canadian Meteor Orbit Radar (CMOR). Potential parent bodies of the sporadic population were searched for by comparison of the distributions of the orbital elements of sporadic meteors, minor planets and comets.

DYNAMICAL MODEL FOR THE TOROIDAL SPORADIC METEORS

More than a decade of radar operations by the Canadian Meteor Orbit Radar have allowed both young and moderately old streams to be distinguished from the dispersed sporadic background component. The latter has been categorized according to broad radiant regions visible to Earth-based observers into three broad classes: the helion and anti-helion source, the north and south apex sources, and the north and south toroidal sources (and a related arc structure). The first two are populated mainly by dust released from Jupiter-family comets and new comets. Proper modeling of the toroidal sources has not to date been accomplished. Here, we develop a steady-state model for the toroidal source of the sporadic meteoroid complex, compare our model with the available radar measurements, and investigate a contribution of dust particles from our model to the whole population of sporadic meteoroids. We find that the long-term stable part of the toroidal particles is mainly fed by dust released by Halley type (long period) comets (HTCs). Our synthetic model reproduces most of the observed features of the toroidal particles, including the most troublesome low-eccentricity component, which is due to a combination of two effects: particles' ability to decouple from Jupiter and circularize by the Poynting-Robertson effect, and large collision probability for orbits similar to that of the Earth. Our calibrated model also allows us to estimate the total mass of the HTC-released dust in space and check the flux necessary to maintain the cloud in a steady state.

Seasonal variations in the north toroidal sporadic meteor source

Abstract—Determining the origins of the sporadic meteoroid sources helps determine their current properties. We have analyzed four years of orbital radar data, looking at how the rates, radiants, and orbits of meteoroids in the north toroidal sporadic source change throughout the year. Twelve broad radiant concentrations, separated in either time or radiant location, are identified. Six are broad distributions associated with more focused shower activity, and six are not associated with major showers. Four of the six concentrations not associated with showers have been named Toroidal, Toroidal A, Toroidal B, and Toroidal C, because of their constant location at the north toroidal centre. The other two, which appear close to the north toroidal source and drift toward the helion and antihelion sources respectively, have been named the Helion Arc and the Antihelion Arc. The twelve radiant concentrations generally last for more than ten degrees solar longitude, and those which may have a single parent are likely composed of orbitally evolved material.

A dynamical model of the sporadic meteoroid complex

Sporadic meteoroids are the most abundant yet least understood component of the Earth's meteoroid complex. This paper aims to build a physics-based model of this complex calibrated with five years of radar observations. The model of the sporadic meteoroid complex presented here includes the effects of the Sun and all eight planets, radiation forces and collisions. The model uses the observed meteor patrol radar strengths of the sporadic meteors to solve for the dust production rates of the populations of comets modeled, as well as the mass index. The model can explain some of the differences between the meteor velocity distributions seen by transverse versus radial scatter radars. The different ionization limits of the two techniques result in their looking at different populations with different velocity distributions. Radial scatter radars see primarily meteors from 55P/Tempel–Tuttle (or an orbitally similar lost comet), while transverse scatter radars are dominated by larger meteoroids from the Jupiter-family comets. In fact, our results suggest that the sporadic complex is better understood as originating from a small number of comets which transfer material to near-Earth space quite efficiently, rather than as a product of the cometary population as a whole. The model also sheds light on variations in the mass index reported by different radars, revealing it to be a result of their sampling different portions of the meteoroid population. In addition, we find that a mass index of s = 2.34 as observed at Earth requires a shallower index ( s = 2.2 ) at the time of meteoroid production because of size-dependent processes in the evolution of meteoroids. The model also reveals the origin of the 55° radius ring seen centered on the Earth's apex (a result of high-inclination meteoroids undergoing Kozai oscillation) and the central condensations seen in the apex sources, as well as providing insight into the strength asymmetry of the helion and anti-helion sources.

The radiant distribution of AMOR radar meteors

A large data set provided by the highly sensitive Advanced Meteor Orbit Radar (AMOR) facility is used to investigate the structure of the sporadic meteor complex. The helion, antihelion and apex apparent sources are clearly found. Observational bias is then removed to reveal the true source distributions as observed on Earth. A long-standing problem in meteor science has been the difference in observed meteor flux between the helion and antihelion source directions. Consideration of the effects of atmospheric interference and Faraday rotation is found to lead to a closer balance between these. The orbital distributions present within the different regions are also discussed. The apex region is found to have a strong retrograde component and a weaker prograde component that exists at high southerly latitudes and that contains orbits with particularly high inclinations. The retrograde component reduces substantially after inclusion of observational bias corrections. Care should be taken in comparing the results presented here with those from other radar systems: AMOR is sensitive to dust as small in diameter as ∼40 μm, while the limiting sensitivity of most contemporary systems is an order of magnitude larger.

Meteoroid Orbits: Implications for Near‐Earth Object Search Programs

ABSTRACT: The available orbital database on macroscopic potential impactors of our planet (asteroids and comets, collectively near‐Earth objects or NEOs) numbers less than one thousand, whereas there have been some hundreds of thousands of orbits of Earth‐impacting meteoroids determined in various surveys, mostly using meteor radars. If one assumes that NEOs have orbital characteristics broadly similar to meteoroids, then the orbits of the latter can give important indications concerning the conduct of search programs designed to discover large NEOs well ahead of any catastrophic impact, allowing ameliorative action to be taken. For smaller NEOs that cannot be telescopically detected until the day or so before impact, the radiant distribution of observed meteors shows the regions of the sky from which impactors are most likely to emanate. It is shown that the vast majority of meteoroids striking the Earth have geocentric (apparent) radiants within two near‐ecliptic regions a few tens of degrees wide and centered on longitudes ±90° from the apex of the Earth's way (the so‐called helion and antihelion sources). These are bodies with low inclinations, large eccentricities (e= 0.7 – 0.9) and quite small semimajor axes (mostly a= 1.3 – 2.5 AU). After allowing for the terrestrial motion about the Sun (conversion to the true radiant), the longitudes are around ±120° from the apex. For a ground‐based search on the nightside, the best search region is that within ∼20° of the arc joining the geocentric and true radiants (longitudes 90° and 120°). On the dayside, proximity to the solar direction argues for a space‐based surveillance program, if small NEOs are to be found just prior to impact.

Radiant distribution of meteoroids encountering the Earth

The radiant distribution of meteors detected during the Harvard Radio Meteor Project, 1968–1969, Synodic Year Program have been reanalysed to remove selection effects associated with the radar observations. Corrections are made for the observing schedule, antenna beam patterns, the radio diffusion ceiling, speed dependence of ionization production and the flux enhancement due to the Earth's gravity. These give the radiant distribution for meteoroids larger than 10 −4 g encountering the Earth. The dominant concentrations come from the so-called Helion and Antihelion sources, with peak fluxes 6–8 times that of the background. The radiant distribution expected for impact craters larger than a given diameter is provided for comparison with spacecraft observations. For these conditions the Apex and Toroidal sources also becomes significant.

The annual variation in the radiant distribution of sporadic meteors

Meteor data obtained from the Southern Hemisphere confirm that the strengths of the helion, apex and antihelion sources of sporadic meteor activity are in the ratio 2:1:2 when averaged over a whole year. During the year the helion and anti-helion source strengths vary from 0·8 to 4·0 times that of the apex.