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Signatures of Meteor Showers and Sporadics Inferred from the Height Distribution Of 1 2 Signatures of meteor showers and sporadics inferred from the height distribution of 3 meteor echoes 4 5 Renata Lukianova1,2, Alexander Kozlovsky3, Mark Lester4 6 7 1Space Research Institute, Moscow, Russia 8 2 Saint-Petersburg State University, Russia 9 3Sodankylä Geophysical Observatory, Sodankylä, Finland 10 4Department of Physics and Astronomy, University of Leicester, Leicester, UK 11 12 Corresponding author: Renata Lukianova, e-mail: [email protected] 13 Address: 84/32 Profsoyuznaya Str., Moscow Russia 117997. Phone #: +7(495)333-52-12 14 15 Abstract 16 17 The SKYiMET meteor radar is capable of measuring the height distribution of ionized 18 meteor trails. Observations of the Sodankyla radar (67°N, 23°E) in 2008-2019 are 19 analyzed. A method is applied, based on the median and quartiles of meteor heights, for 20 distinguishing meteor showers from much more intense background sporadic meteors. 21 Since the shower meteors are commonly less dense and enter the Earth’s atmosphere at a 22 larger velocity, they produce ionization up to 3 km higher than sporadic meteors. This 23 effect manifests itself in the form of narrow peaks that are clearly visible in the median and 24 quartiles of the meteor height. Seven established showers with zenith hourly rate (ZHR) > 25 12 were identified. In addition, using both the height parameters and the radiant 1 26 distribution, a signature of a former meteor stream evolving into sporadic meteors is found. 27 This additional sporadic inflow has an antihelion source and occurs in the January. 28 29 Key words: meteor radar, meteor shower, sporadic meteors, height profile of meteor 30 echoes, radiant distribution of meteors 31 32 Highlights 33 34 • Parameters of the vertical profile of daily meteor echoes from the Sodankyla SKYiMET 35 meteor radar (67N) are calculated. 36 37 • Peaks in the median and quartiles of the meteor height distribution during major 38 (ZHR>12) meteor showers are observed. 39 40 • The shower meteoroids produce ionization trails at considerably higher altitudes than the 41 sporadic background. 42 43 • Simple and robust method for the recognition of the occurrence of shower meteors on the 44 much more intense sporadic background 45 46 • The radiant distribution reveals a signature of a dispersed meteor stream evolving into 47 sporadics. 48 49 • This additional quasi-sporadic inflow occurring in the end of January, likely has a north 50 toroidal source. 51 2 52 53 1 Introduction 54 55 Sporadic meteoroids constantly enter the atmosphere, while showers of an unusually high 56 rate of meteors arriving from the same radiant occur within a relatively short (over a 57 period of days) time interval at the same dates each year. Sporadic meteors are mostly dust 58 particles from asteroids and long-decayed comets. It was found that Helion and Antihelion 59 sporadic sources are mainly caused by Jupiter-Family and long period comets, with about 60 10% contribution from asteroidal sources (Campbell-Brown, 2008; Nesvorny et al., 2011; 61 Pokorny et al 2014). Comet Encke (the parent, or at least relative, of the Taurid complex 62 meteor showers) and Tempel-Tuttle may account for much of the sporadic background 63 (Weigert et al., 2009). 64 65 Meteor streams are products of the disintegration of cometary nuclei, which primarily 66 consist of ice mixed with loose, easily decaying particles of relatively low density. 67 Typically, for streams, the parent comet crossed the Earth’s orbit in the recent past. Meteor 68 showers feed the sporadics because the different forces in the solar system (e.g. gravitation, 69 radiation) disperse the meteoroid stream, which then become a part of the sporadic 70 background. A smaller fraction of meteor showers originates from asteroids, possibly from 71 dust released during collisions, thermal decomposition and rotational spin-up. One such 72 example is the Phaeton, which moves in an elliptical orbit around the Sun, and the thermal 73 decomposition of surface minerals near the perihelion leads to the formation of dust (Li 74 and Jewitt, 2013). Usually the streams are encountered at the same solar longitude each 75 year. Some streams (like the Perseids) have very long orbital periods, but are steady annual 76 streams. The Leonids tend to increase approximately every 33 years as the parent comet 77 passes perihelion, but many perihelion passes have been entirely unremarkable. 3 78 79 It has long been a goal of various types of observations to identify and quantify the meteor 80 showers. Starting with initial individual observations with the naked eye, which made it 81 possible to determine the very approximate intensity and radiant of a shower, the 82 techniques for detecting meteors have become quite comprehensive. To date, more than 83 one hundred showers of different strength and duration have been documented 84 (Jenniskens, 2006). The main modern facilities include the rapid-run optical instruments 85 (e.g. Kelley et al., 2000; Clemesha et al., 2001) and various meteor radars. 86 87 Meteoroids bombarding the Earth’s atmosphere collide with the atmospheric gases and 88 produce luminous ionization at the heights of 70-120 km. The localized region around the 89 head of meteor is highly ionized (approximately 1015 el/m3), though small in scale (Close 90 et al., 2002). As a meteor moves through relatively dense air, the cylinder-shaped 91 ionization trail expands radially and decays due to ambipolar diffusion in the background 92 atmosphere. The trails and head scatter radio waves efficiently and thus various radar 93 techniques are used for the meteor detection (e.g., Elford, 2004; Holdsworth et al., 2004; 94 Reid et al., 2006; Younger et al., 2009). One of the most common types of meteor radars is 95 the monostatic All-Sky Interferometric Meteor Radar (SKiYMET), which illuminates a 96 broad expanse of sky using a small number of antennas. SKiYMET, including its hardware 97 and software systems, is a dedicated meteor detection instrument capable of achieving 98 meteor count rates of over 20000 per day. The main end-products include the mesospheric 99 neutral winds and temperature, which are routinely calculated after analysis of the acquired 100 radio echoes from the meteor trails. While SKiYMETs are mainly used to determine 101 atmospheric parameters and not to detect individual meteors, the raw data contains some 102 useful information that can be used to characterize properties of meteor showers. In this 103 regard, SKiYMETs complement the existing series of other, more specific radars which 4 104 allow the measurement of individual meteor echoes and orbits. One of the most powerful 105 facilities is the Canadian Meteor Orbit Radar, CMOR (Blaauw et al., 2011; Campell- 106 Brown 2008), which is the SKiYMET-type instruments. Calculating trajectories (and 107 therefore radiants) with CMOR is possible because there are several additional receiving 108 station nearby to have several specular detections of the same meteor. Another facility is 109 the Southern Argentina Agile Meteor Radar, SAAMER (Janches et al., 2013). The main 110 focus for SAAMER is meteor trails, although several transmitting antennas can be 111 combined to increase the gain for a meteor head echo modus. The Norwegian Middle 112 Atmosphere Alomar Radar System, MAARSY (Schult et al., 2018), the Europoean 113 Incoherent Scatter Radar, EISCAT 930 MHz (Kero et al., 2008) and the Arecibo 430 MHz 114 (Mathews et al., 2014) are the narrow beam radars capable of a detailed study of meteoroid 115 deceleration, fragmentation and orbits. The Advanced Meteor Orbit Radar, AMOR 116 (Baggaley et al., 1994) had a fan-shaped beam sawing mainly trail echoes. As CMOR, 117 AMOR and SAAMER have additional outlying receivers at distances of ~10 km from the 118 transmitter and in multi-station mode, they allow speeds and orbits of the individual 119 sporadic and shower meteors to be determined. High-power, large-aperture (HPLA) 120 Jicamarca 50 MHz radar (Chao and Woodman, 2003) and the Shigaraki MU radar (Kero et 121 al., 2012) enable echoes from a meteor’s head. The radars operating as individual meteor 122 detection systems are often only available for such use on a restricted scheduling basis, 123 while a number of cost-effective, compact and designed to run unattended SKiYMETs are 124 constantly growing over the world, as they are routinely used to monitor the mesosphere. 125 126 Despite powerful search techniques having been developed together with extended 127 datasets, questions still remain about the identification of the occurrence and properties of 128 the sporadic meteors and showers, especially the minor ones (Asher et al., 2019; Williams 129 et al., 2019). For example, it is still not easy to distinguish the shower members from the 5 130 much more intense sporadic background. The velocity, size, mass and chemical 131 composition of the meteors belonging to a particular shower are not always established in 132 detail. The atmospheric effects (primarily relates to the altitude at which different types of 133 meteors ablate) of the shower meteors may differ from those of the sporadic meteors. 134 135 Recognizing the complexity of these problems Lukianova et al. (2018) proposed a new 136 simple and robust method for meteor shower recognition based on the heights of the 137 ionized trails. We utilize the routine observations of the Sodankyla SKYiMET meteor 138 radar (MR), which has operated continuously since 2008, and provides an enormous data 139 set of the meteor count and related parameters. In this paper, we extend the analysis to the 140 entire period of observations up to 2019. We show that the heights, at which the meteor 141 echoes are detected with the SKYiMET, serve as a good indicator of the meteor shower 142 occurrence. Since the ionization efficiency in collisions between the atmospheric 143 molecules and ablating meteoritic material depends on the inherent properties of the 144 meteoroid, and the properties of the shower members and sporadic meteors are different, 145 therefore, the height distribution of the echoes makes it possible to distinguish showers 146 from the sporadic background.
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