Annual Meteor Showers Research Articles

Mechanical strength distribution in Geminid meteoroids derived via fireball modeling

Context. Geminids are the most active annual meteor shower observed on Earth. Their parent is an active asteroid, (3200) Phaethon, which is a target of the planned DESTINY+ mission of the Japan Aerospace Exploration Agency (JAXA). The exact physical nature of (3200) Phaethon and Geminids is still debated. Aims. This paper is devoted to fragmentation modeling of bright Geminid fireballs, which should reveal information about the structure of centimeter-sized Geminid meteoroids. These fireballs were observed by the European Fireball Network (EN) over the past few years. We aim to describe their disintegration cascade in the atmosphere and their mechanical properties, and to derive their precise initial masses and velocities. Methods. We used a semi-empirical fragmentation model that employs an automatic procedure based on parallel genetic algorithms to determine the aerodynamic pressures at which a meteoroid and its parts fragment. This serves as a proxy for the mechanical strength of the body and its subsequent fragments. It enabled us to derive the minimum, median, and maximum mechanical strength and the strength distribution inside the meteoroid and reveal its internal structure. Results. We find that the Geminids begin to crumble at pressures 1–100 kPa, with the strongest parts reaching pressures of between 0.4 and 1.55 MPa before fragmenting. Knowing the spectral type of (3200) Phaethon (a B-type asteroid, part of the C complex), we conclude that the Geminids are made of compact and coherent carbonaceous material. We also find that the minimum aerodynamic pressure that causes the fragmentation of Geminids increases with increasing entry mass of Geminids. In contrast, the median aerodynamic pressure decreases as their entry mass increases. The spectra of all the observed Geminid fireballs show normal content and little variation in terms of sodium.

A proposed alternative dynamical history for 2P/Encke that explains the taurid meteoroid complex

ABSTRACT The Taurid Meteoroid Complex (TMC) is a broad stream of meteoroids that produces several annual meteor showers on Earth. If the linkage between these showers and 2P/Encke is at the centre of most TMC models, the small size and low activity of the comet suggest that 2P/Encke is not the unique parent body of the Taurids. Here, we simulate the formation of the TMC from 2P/Encke and several NEAs. In total, we explored more than a hundred stream formation scenarios using clones of 2P/Encke. Each modelled stream was integrated and compared with present-day Taurid observations. As previously reported, we find that even slight variations of 2P/Encke’s orbit modifie considerably the characteristics of the simulated showers. Most of the comet’s clones, including the nominal one, appear to reproduce the radiant structure of the Taurid meteors but do not match the observed time and duration of the showers. However, the radiants and timing of most Taurid showers are well reproduced by a particular clone of the comet. Our analysis thus suggest that with this specific dynamical history, 2P/Encke is the sole parent of the four major TMC showers that have ages from 7 to 21 ka. Our modelling also predicts that the 2022 Taurid Resonant Swarm return will be comparable in strength to the 1998, 2005, and 2015 returns. While purely dynamical models of Encke’s orbit – limited by chaos – may fail to reveal the comet’s origin, its meteor showers may provide the trail of breadcrumbs needed to backtrack our way out of the labyrinth.

The Global Meteor Network – Methodology and first results

ABSTRACT The Global Meteor Network (GMN) utilizes highly sensitive low-cost CMOS video cameras which run open-source meteor detection software on Raspberry Pi computers. Currently, over 450 GMN cameras in 30 countries are deployed. The main goal of the network is to provide long-term characterization of the radiants, flux, and size distribution of annual meteor showers and outbursts in the optical meteor mass range. The rapid 24-h publication cycle the orbital data will enhance the public situational awareness of the near-Earth meteoroid environment. The GMN also aims to increase the number of instrumentally observed meteorite falls and the transparency of data reduction methods. A novel astrometry calibration method is presented which allows decoupling of the camera pointing from the distortion, and is used for frequent pointing calibrations through the night. Using wide-field cameras (88° × 48°) with a limiting stellar magnitude of +6.0 ± 0.5 at 25 frames per second, over 220 000 precise meteoroid orbits were collected since 2018 December until 2021 June. The median radiant precision of all computed trajectories is 0.47°, 0.32° for $\sim 20{{\ \rm per\ cent}}$ of meteors which were observed from 4 + stations, a precision sufficient to measure physical dispersions of meteor showers. All non-daytime annual established meteor showers were observed during that time, including five outbursts. An analysis of a meteorite-dropping fireball is presented which showed visible wake, fragmentation details, and several discernible fragments. It had spatial trajectory fit errors of only ∼40 m, which translated into the estimated radiant and velocity errors of 3 arcmin and tens of meters per second.

Measuring fluxes of meteor showers with the NASA All-Sky Fireball Network

We present an algorithm developed to measure the fluxes of major meteor showers as observed in NASA’s All-Sky Fireball Network cameras. Measurements of fluxes from the All-Sky cameras not only improve the Meteoroid Environment Office’s (MEO’s) ability to provide accurate risk assessments from major showers, but also allows the mass distribution of meteoroids within the shower to be constrained. This algorithm accounts for the shower-specific and event-specific exposure time and collecting area of the sky for nights where sufficiently large samples of shower meteors (~30 or more from the shower) are observed. The fluxes derived from the All-Sky Fireball Network for the 2015 Geminid, 2016 Perseid and Quadrantid, 2017 Orionid, and 2018 Leonid shower peaks are calculated. All five of these shower fluxes show excellent agreement with expectations from independent measurements at different mass and luminosity limits. For four of these five showers, the measured mass indices are significantly shallower than what is currently assumed by the NASA Meteoroid Environment Office’s (MEO’s) annual meteor shower forecast. A direct comparison between forecasted and measured fluxes at limiting masses of ~1g shows good agreement for the three showers for which the observations took place near their peak activity.

Meteor Shower Forecasting in Near-Earth Space

NASA’s Meteoroid Environment Office produces an annual meteor shower forecast in order to help spacecraft operators assess the risk posed by meteoroid streams. Previously, this forecast focused on the International Space Station and therefore reported meteoroid fluxes and enhancement factors at an orbital altitude of 400 km. This paper presents an updated forecast algorithm that has an improved calculation of the flux enhancement produced by showers and can calculate fluxes at any selected Earth or lunar orbital altitude. Finally, forecasted fluxes for the 2018 Draconid meteor shower, which is expected to produce meteoroid flux enhancements near the Sun–Earth L1 and L2 Lagrange points but not at Earth, are discussed and generated.

In Search of Recent Disruption of (3200) Phaethon: Model Implication and Hubble Space Telescope Search

Abstract Near-Earth asteroid (3200) Phaethon is notable for its association with a strong annual meteor shower, the Geminids, indicative of one or more episodes of mass ejection in the past. The mechanism of Phaethon’s past activity is not yet understood. Here, we present a Hubble Space Telescope (HST) search of meter-sized fragments in the vicinity of Phaethon, carried out during Phaethon’s historic approach to Earth in mid-December of 2017. Numerical simulations conducted to guide HST’s pointing also show that the dynamical evolution of Phaethon-originated particles is quick, as ejected materials take no longer than ∼250 years to spread to the entire orbit of Phaethon. Our search was completed down to a 4 m class limit (assuming Phaethon-like albedo) and was expected to detect 0.035% of particles ejected by Phaethon in the past several decades. The negative result of our search capped the total mass loss of Phaethon over the past few dozen orbits to be 1012 kg at the 3σ level, taking the best estimates of size power-law from meteor observations and spacecraft data. Our result also implies a millimeter-sized dust flux of within 0.1 au of Phaethon, suggesting that any Phaethon-bound mission is unlikely to encounter dense dust clouds.

The dynamical evolution and the force model for asteroid (196256) 2003 EH1

This paper is devoted to the dynamics of asteroid (196256) 2003 EH1 which belongs to the Amor group. It is known that asteroid 2003 EH1 is associated with one of the main annual meteor showers – the Quadrantids. In this work we analyze the influence of various perturbing factors on the asteroid motion. The perturbations’ estimation was done by five different methods based on the nominal orbit evolution and the size of the initial confidence region. The most significant influences on the dynamical evolution of 2003 EH1 are gravitational forces from the Sun, major planets and the Moon, and the relativistic effects (RE) of the Sun. The Earth, the Sun and Jupiter oblateness; gravitational perturbations from Pallas, Ceres, Vesta and Pluto; and the RE of planets, the Moon, and Pluto are of less importance.The researches of chaoticity and evolution of asteroid 2003 EH1 were examined by integrating its motion equations along with 500 clones. The time interval (1000–4000 years) has been determined by integration precision estimation. We estimated the mean exponential growth factor of nearby orbits (MEGNO) and found that MEGNO < 2 only in the interval from 1700 year to 2300 year. After 2300 year the MEGNO parameter increases that indicates motion instability. It shows that the orbit may be considered as regular on the time interval of ±300 years from now, and as chaotic outside this interval. We suppose that the reasons are the frequent close approaches of the asteroid with Jupiter and the overlap of apsidal-nodal resonances.

The age and the probable parent body of the daytime arietid meteor shower

The daytime Arietid meteor shower is active from mid-May to late June and is amongst the strongest of the annual meteor showers, comparable in activity and duration to the Perseids and the Geminids. Due to the daytime nature of the shower, the Arietids have mostly been constrained by radar studies. The Arietids exhibit a long-debated discrepancy in the semi-major axis and the eccentricity of meteoroid orbits as measured by radar and optical surveys. Radar studies yield systematically lower values for the semi-major axis and eccentricity, where the origin of these discrepancies remain unclear. The proposed parent bodies of the stream include comet 96P/Machholz [McIntosh, B.A., 1990. Comet P/Machholz and the Quadrantid meteor stream. Icarus 86, 894 299–304. doi:10.1016/0019-1035(90)90219-Y.] and more recently a member of the Marsden group of sun-skirting comets, P/1999 J6 [Sekanina, Z., Chodas, P.W., 2005. Origin of the Marsden and Kracht Groups of Sunskirting 922 Comets. I. Association with Comet 96P/Machholz and Its Interplanetary Complex. ApJS 923 161, 551–586. doi:10.1086/497374.].In this work, we present detailed numerical modelling of the daytime Arietid meteoroid stream, with the goal to identifying the parent body and constraining the age of the stream. We use observational data from an extensive survey of the Arietids by the Canadian Meteor Orbit Radar (CMOR), in the period of 2002–2013, and several optical observations by the SonotaCo meteor network and the Cameras for All-sky Meteor Surveillance (CAMS).We find the most plausible scenario to be that the age and the formation mechanism of the Arietids is consistent with continuous cometary activity of 96P/Machholz over a time interval of ≈12,000 years. The sun-skirting comet P/1999 J6 suggested by [Sekanina, Z., Chodas, P.W., 2005. Origin of the Marsden and Kracht Groups of Sunskirting 922 Comets. I. Association with Comet 96P/Machholz and Its Interplanetary Complex. ApJS 923 161, 551–586. doi:10.1086/497374.] may contribute to the shower, but the comet break up prior to 950 CE they propose does not reproduce all the characteristics of the observed shower.

On the age and formation mechanism of the core of the Quadrantid meteoroid stream

The Quadrantid meteor shower is among the strongest annual meteor showers, and has drawn the attention of scientists for several decades. The stream is unusual, among others, for several reasons: its very short duration around maximum activity (≈12–14h) as detected by visual, photographic and radar observations, its recent onset (around 1835 AD Quetelet, L.A.J. [1839]. Catalogue des principles apparitions d’etoiles filantes) and because it had been the only major stream without an obvious parent body until 2003. Ever since, there have been debates as to the age of the stream and the nature of its proposed parent body, asteroid 2003 EH1.In this work, we present results on the most probable age and formation mechanism of the narrow portion of the Quadrantid meteoroid stream. For the first time we use data on eight high precision photographic Quadrantids, equivalent to gram–kilogram size, to constrain the most likely age of the core of the stream. Out of eight high-precision photographic Quadrantids, five pertain directly to the narrow portion of the stream. In addition, we also use data on five high-precision radar Quadrantids, observed within the peak of the shower.We performed backwards numerical integrations of the equations of motion of a large number of ‘clones’ of both, the eight high-precision photographic and five radar Quadrantid meteors, along with the proposed parent body, 2003 EH1. According to our results, from the backward integrations, the most likely age of the narrow structure of the Quadrantids is between 200 and 300years. These presumed ejection epochs, corresponding to 1700–1800AD, are then used for forward integrations of large numbers of hypothetical meteoroids, ejected from the parent 2003 EH1, until the present epoch. The aim is to constrain whether the core of the Quadrantid meteoroid stream is consistent with a previously proposed relatively young age (≈200years).

DIVISION F COMMISSION 22: METEORS, METEORITES, AND INTERPLANETARY DUST

Commission 22 (Meteors, Meteorites and Interplanetary Dust) was established at the first IAU General Assembly held in Rome in 1922, with William Frederick Denning as its first President. Denning was an accountant by profession, but as an amateur astronomer he contributed extensively to meteor science. Commission 22 thus established a pattern that has continued to this day that non-professional astronomers were welcomed and valued and could play a significant role in its affairs. The field of meteors, meteorites and interplanetary dust has played a disproportional role in the astronomical perception of the general public through the majestic displays of our annual meteor showers. Those in the field deployed many techniques uncommon in other fields of astronomy, studying the “vermin of space”, the small solid bodies that pervade interplanetary space and impact Earth's atmosphere, the surface of the Moon, and that of our satellites in orbit. Over time, the field has tackled a wide array of problems, from predicting the encounter with meteoroid streams, to the origin of our meteorites and the nature of the zodiacal cloud. Commission 22 has played an important role in organizing the field through dedicated meetings, a data centre, and working groups that developed professional-amateur relationships and that organized the nomenclature of meteor showers. The contribution of Commission 22 to the field is perhaps most readily seen in the work of the presidents that followed in the footsteps of Denning.

A permanent, asymmetric dust cloud around the Moon.

Interplanetary dust particles hit the surfaces of airless bodies in the Solar System, generating charged and neutral gas clouds, as well as secondary ejecta dust particles. Gravitationally bound ejecta clouds that form dust exospheres were recognized by in situ dust instruments around the icy moons of Jupiter and Saturn, but have hitherto not been observed near bodies with refractory regolith surfaces. High-altitude Apollo 15 and 17 observations of a 'horizon glow' indicated a putative population of high-density small dust particles near the lunar terminators, although later orbital observations yielded upper limits on the abundance of such particles that were a factor of about 10(4) lower than that necessary to produce the Apollo results. Here we report observations of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds. The density of the lunar ejecta cloud increases during the annual meteor showers, especially the Geminids, because the lunar surface is exposed to the same stream of interplanetary dust particles. We expect all airless planetary objects to be immersed in similar tenuous clouds of dust.

CAMS: Cameras for Allsky Meteor Surveillance to establish minor meteor showers

First results are presented from a newly developed meteoroid orbit survey, called CAMS – Cameras for Allsky Meteor Surveillance, which combines meteor detection algorithms for low-light video observations with traditional video surveillance tools. Sixty video cameras at three stations monitor the sky above 31° elevation. Goal of CAMS is to verify meteor showers in search of their parent comets among newly discovered near-Earth objects. This paper outlines the concept of operations, the hardware, and software methods used during operation and in the data reduction pipeline, and accompanies the data release of the first batch of meteoroid orbits. During the month of November 2010, 2169 precisely reduced meteoroid trajectories from 17 nights have an error in the apparent radiant of the trajectory <2° and error in speed <10%. Median values of the error are 0.31° and 0.53 km/s, respectively, sufficient to resolve the intrinsic dispersion of annual meteor showers and resolve minor showers from the sporadic background. The limiting visual magnitude of the cameras is +5.4, recording meteors of +4 magnitude and brighter, bright enough to stand out from the mostly fainter sporadic meteors detected as under dense radar echoes. CAMS readily detected all established showers (6) active during the clear nights in November. Of the showers that needed confirmation, we confirm the theta Aurigids (THA, IAU#390), the chi Taurids (CTA, IAU#388), and the omicron Eridanids (OER, IAU#338). We conclude that the iota November Aurigids (IAR, IAU#248) are in fact the combined activity of the theta Aurigids and chi Taurids, and this shower should be dismissed from the list. Finally, there is also a clustering consistent with the zeta Cancrids (ZCN, IAU#243), but we cannot exclude that this is lower perihelion dust belonging to the Orionid shower. Data are submitted to the IAU Meteor Data Center on a semi-regular basis, and can be accessed also at http://cams.seti.org.

Meteor showers of comet C/1917 F1 Mellish

December Monocerotids and November Orionids are weak but established annual meteor showers active throughout November and December. Analysis of a high quality orbits subset of the SonotaCo video meteor database shows that the distribution of orbital elements, geocentric velocity and also the orbital evolution of the meteors and potential parent body may imply a common origin for these meteors coming from the parent comet C/1917 F1 Mellish. This is also confirmed by the physical properties and activity of these shower meteors. An assumed release of meteoroids at the perihelion of the comet in the past and the sky-plane radiant distribution reveal that the December Monocerotid stream might be younger than the November Orionids. A meteoroid transversal component of ejection velocity at the perihelion must be larger than 100 m/s. A few authors have also associated December Canis Minorids with the comet C/1917 F1 Mellish. However, we did not find any connection.

Annual meteor showers at Venus and Mars: lessons from the Earth

We have generated a list of cometary bodies as well as known meteoroid streams that we consider to be prime candidates for producing significant meteor activity in the atmospheres of Venus and Mars. To compile this list, we defined a quantitative criterion based on catalogued properties of comets such as their dynamical class, orbital period and absolute brightness as well as the proximity of their orbits to the planetary orbits. This procedure improves over previous work that considered only this latter quantity as the sole criterion for meteor shower parentage at those planets. The list of Martian (Venusian) candidates contains six (eight) Halley-type comets, 11 (six) intermediate long-period comets, eight (nine) showers originating from known meteoroid streams of Encke or Jupiter-family type and one inert object in a comet-like orbit. Based on these findings, we conclude that (i) meteor shower activity at those planets would be variable on a seasonal scale, just as it is at the Earth, (ii) Venusian and/or Martian meteor showers from bright long-period comets, a population with no representatives in the Earth's vicinity, are a possibility, and (iii) numerous opportunities exist for sampling known Encke-type and Jupiter-family showers to probe their spatial structure far from the Earth's orbit. We calculate local observing circumstances of these showers to aid in their future observational confirmation and characterization.

The Quadrantid meteoroid complex

The Quadrantids, one of the more active of the annual meteor showers, is unusual for its strong but brief maximum within a broader background of activity. It is also notable for its recent onset, the first observation having been likely made in 1835. Until recently, no parent with a similar orbit had been observed and previous investigators concluded that the stream was quite old, with the stream's recent appearance and sharp peak attributed to a fortuitous convergence of meteoroid orbits. The discovery of the near-Earth Asteroid 2003 EH1 on an orbit very similar to that of the Quadrantids has probably unveiled the parent body of this stream [Jenniskens and Marsden, 2003. 2003 EH1 and the Quadrantids. IAU Circ. 8252]. From simulations of the orbit of this body and of meteoroids released from it at different intervals in the past, we find that both the sharp peak and recent appearance of the Quadrantids can most easily be explained by a release of meteoroids from 2003 EH1 near 1800 AD. This is supported by three lines of evidence. First, the evolution of the observed solar longitude of the Quadrantids over time is consistent with release from 2003 EH1 approximately 200 years ago. Second, numerical simulations of meteoroids released from this parent body at this time match the basic orbital characteristics of the Quadrantid stream well. Finally, these simulations also reveal that the Quadrantid core is well reproduced by a single outburst at perihelion circa 1800, whereas earlier releases result in the shower's appearance in our skies significantly prior to 1835. These results apply to the concentrated central core of the stream: the extended background was likely produced at earlier times. In fact, we find that 2003 EH1 is in a state of Kozai circulation along with a number of other comets and NEAs which may form a larger Quadrantid complex. Using the current total duration of the broader background Quadrantid activity compared to our simulations, we suggest a minimum age of ∼3500 years for the stream as a whole. This also represents the approximate lower limit for the age of the complex. We have further identified five comets as well as nine additional NEAs which may be part of the aforementioned complex, the latter all having Tisserand parameters less than three, further suggesting that the are extinct comet nuclei.

THE CORE OF THE QUADRANTID METEOROID STREAM IS TWO HUNDRED YEARS OLD

The Quadrantids are one of the most active annual meteor showers and have a number of unusual features. One is a sharp brief maximum, 12–14 h in length. A second is the Quadrantids, relatively recent appearance in our skies, the first observation having likely been made in 1835. Until recently no likely parent with a similar orbit had been observed and previous investigators concluded that the stream was quite old, with the stream’s recent appearance and sharp peak attributed to a recent fortuitous convergence of meteoroid orbits. The recent discovery of the near-Earth asteroid 2003 EH1 on an orbit very similar to that of the Quadrantids has almost certainly uncovered the parent body of this stream. From the simulations of the orbit of this body and of meteoroids released at intervals from it in the past, we find that both the sharp peak and recent appearance of the Quadrantids can most easily be explained assuming meteoroids were ejected in substantial numbers near 1800 AD.

Extreme albedo comets and the impact hazard

Dynamical balance arguments that involve the capture of long-period comets from the Oort cloud imply that there should be ∼ 1000 times more Halley-type objects than are actually observed. If the active comets rapidly become dormant, with albedos comparable to those of known cometary surfaces, hundreds of such bodies should by now have been detected whereas in fact only a few have been found. If, on the other hand, they disintegrate to dust, we show here that the debris would create a bright, near-spherical zodiacal cloud and ∼ 15‐30 strong annual meteor showers, also contrary to observation. Here we demonstrate that the surfaces of inactive comets, if composed of loose, fluffy organic material like cometary meteoroids, develop reflectivities that are vanishingly small in visible light. The near-Earth objects may therefore be dominated by a population of fast, multi-kilometre bodies too dark to be seen with current near-Earth object surveys. Deflection strategies that assume decades or centuries of warning before impact are inapplicable to this hazard.

The Mazapil meteorite: From paradigm to periphery

Abstract— The remarkable fact about the Mazapil meteorite is that it fell on the same night, in 1885, that the Andromedid meteor shower underwent a spectacular outburst. The simultaneity of these two events has driven speculation ever since. From ˜1886 to ˜1950 the circumstances of the Mazapil fall were taken, by a number of researchers, as the paradigm that demonstrated the fact that comets were actually swarms of meteoritic boulders. Beginning ˜1950, however, most researchers began to adopted the stance that the timing of the Mazapil fall was nothing more than pure coincidence. The reason behind this change in interpretation stemmed from, amongst other factors, the fact that none of the prominent annual meteor showers could be clearly shown to deliver meteorites. Also, with the introduction of the icy‐conglomerate model for cometary nuclei, by F. Whipple in the early 1950s, it became increasingly clear that only exceptional circumstances would allow for the presence of large meteoritic bodies in cometary streams. Further, by the mid 1960s it had been shown that meteorites could, in fact, be delivered to the Earth from the main belt asteroid region via gravitational resonances. With the removal of the dynamical “barrier” against the delivery of meteorites from the asteroid region, the idea that the Mazapil meteorite could have been part of the Andromedid stream fell into complete disfavor. This being said, we nonetheless present the results of a study concerning the possible properties of the parent object to the Mazapil meteorite based upon the assumption that it was a member of the Andromedid stream. This study is presented to illustrate the point that while cometary showers do not yield meteorites on the ground, this does not, in fact, substantiate the argument that no meteoritic bodies reside in cometary streams. Indeed, we find no good reason to suppose that an object with the characteristics of the Mazapil meteorite could not have been delivered from the Andromedid stream. However, we argue that upon the basis of the actual reported observations and upon the scientific maxim of minimized hypothesis and least assumption it must be concluded that the timing of the fall of the Mazapil meteorite and the occurrence of the Andromedid outburst were purely coincidental.

Impact of a major meteor storm on Earth's ionosphere: A modeling study

A comprehensive model of the effect of a major meteor storm on Earth's ionosphere is presented. The model includes meteor stream mass distributions based on visual magnitude observations, a differential ablation model of major meteoric metals, Fe and Mg, and state‐of‐the‐art modeling of the chemistry and transport of meteoric metal atoms and ions subsequent to deposition. Particular attention is paid to the possibility of direct ionic deposition of metallic species. The model is validated by calculating the effect of annual meteor showers on the background metal atom and ion abundances. A metallic ion density increase of up to 1 order of magnitude is observed, in agreement with in situ measurements during showers. The model is exercised for a hypothetical Leonid meteor storm of the magnitude reported in 1966. The model predicts the formation of a layer of metal ions in the ionospheric E region that reaches peak densities of around 1 × 105 cm−3, corresponding to a 2 order of magnitude increase of the quiescent nighttime E region density. Although sporadic E layers reaching or exceeding this density are relatively common, the effect is different in that it persists on the order of days and would be observed over nearly one‐half the globe. The model predictions are consistent with the available 1966 Leonid storm data. In particular, the observation of enhanced, predawn sporadic E activity points to efficient collisional ionization of meteoric metals, as assumed in the model.

Changes in the Frequency Distribution Characteristics of Ionosonde Es Parameters During Major Meteor Activity

The probability of occurrence of both 10 E, and Ib Es in excess of ionosonde limiting frequency has been examined for two Southern Hemisphere stations at periods of recurrent annual meteor shower activity and periods of no shower influx. The analyses show that marked changes occur in probabilities for high values of!o Es and Ib Es indicating structural modifications to Es layers.