Doppler Studies of Near-Antapex UHF Radar Micrometeors

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1 Icarus 143, (2000) doi: /icar , available online at on Doppler Studies of Near-Antapex UHF Radar Micrometeors D. Janches and J. D. Mathews Communication and Space Sciences Laboratory, 316 EE East, Penn State University, University Park, Pennsylvania D. D. Meisel Department of Physics and Astronomy, SUNY Geneseo, 1 College Circle, Geneseo, New York ; and Communication and Space Sciences Laboratory, 316 EE East, Penn State University, University Park, Pennsylvania V. S. Getman Communication and Space Sciences Laboratory, 316 EE East, Penn State University, University Park, Pennsylvania and Q.-H. Zhou Arecibo Observatory, Box 995, Arecibo, Puerto Rico Received November 12, 1998; revised September 1, 1999 A radar micrometeor is the radar-scattering signature from the free electrons in the plasma generated by entry of a dust-sized meteoroid into the atmosphere. We report the first direct Doppler measurements, made using the Arecibo Observatory 430-MHz radar, of the so-called meteor head echo. Our observations demonstrate that this region is moving with the speed of the meteoroid as determined from the meteor head-echo altitude time trajectory and that this radar return is distinct spatially and in velocity from the much more commonly observed trail echo. We also report the first observations of near-antapex micrometeors which are characterized by the very slow atmospheric speeds expected from low-eclipticinclination objects entering the atmosphere from behind Earth s orbital path. Of the 32 meteors observed during four early evening hours of observations on 10 January 1997, velocities were determined for 18 of the meteors of which 7 were at or just below Earth escape velocity (11.2 km/s). We give heliocentric orbits for the 11 meteor events with speeds greater than the escape velocity and present a detailed analysis of these orbital parameters and their possible origins. One particle was determined to be interstellar: a preliminary analysis indicates that the ecliptic coordinates of the radiant relative to the local standard of rest (LSR) (with the solar motion relative to the nearby stars removed) are λ = 43.02, β = 43.28, V = km/s or, in system II galactic coordinates l II = 219.8, b II = 52.4, V = 25.1 km/s. c 2000 Academic Press Key Words: meteors; meteoroids; radar; orbits. 1. INTRODUCTION Radar meteor science is an interdisciplinary area with the precise determination of the extra atmospheric speeds of micrometeors, determination of the radar-meteor scattering mechanism(s), and determination of the meteoric mass flux into the upper atmosphere as just three of its major problems. Meteor observations at Arecibo Observatory (AO) grew from incoherent scatter radar observations of the ionosphere where meteors were simply treated as measurement noise (see discussion surrounding Figs. 1 and 2, Mathews et al. 1997b) in a distinctly different scientific focus. Data-taking and analysis capabilities reached the point that meteors could be observed with the current 150-m and 1-ms range and time resolutions, respectively, at the 430-MHz radar frequency. As discussed below, these UHF radar observations are very distinct from the classic HF/VHF radar observations in that the head echo (radar scattering from the region immediately surrounding the meteoroid) is always observed leading uniquely to very accurate speed determinations. Accurate meteoroid speed along with accurate pointing information characteristic of the narrow-beam AO radar is required for determination of the meteoroid orbit, which in turn is needed to predict the meteoroid origin whether a member of a stream, a sporadic object, or interstellar (Taylor et al. 1996). Questions of how these particles interact with the Earth s atmosphere on entry can also be addressed if their velocities are obtained accurately and deceleration is detected (Evans 1965, Verniani 1966, 1973, Love and Brownlee 1991). The radar meteor return derives not directly from scattering from the meteoroid but from scattering by the free electrons present in the plasma that is formed by interaction of the meteoroid with the atmosphere (Mathews et al. 1997a). This is clear in that meteors are not seen much above 100-km altitude where atmospheric density is not sufficient to generate /00 $35.00 Copyright c 2000 by Academic Press All rights of reproduction in any form reserved.

2 348 JANCHES ET AL. significant ionization. The meteor radar return has long been classified as trail and head echoes (McKinley 1955). While the trail echo is most widely used with HF and VHF radars to determine meteor parameters, some controversy has surrounded the origins of the head echo, which appears to move with the velocity of the meteroid (McIntosh 1963, Jones and Webster 1991, Simek 1997). There were some attempts in the past to use the return radio signal from the head echo to calculate the meteoroid s speed (McKinley 1961). However, these observations were typically done using low-power HF/ VHF meteor radars and the head echo was usually not present. The lack of a well-resolved head echo leads to the use of the developing trail-echo diffraction pattern to obtain velocities and so the problem of the head-echo origin and velocity has remained largely unresolved (Taylor et al. 1994, 1996; Baggaley et al. 1997; Cervera et al. 1997). As noted in Mathews et al. (1997a), the AO UHF radar always sees head echoes with only a possible diffuse return from the trail instead of the classic, long-lived trail echo observed with lowpower, lower-frequency radars. We have developed a Doppler technique that permits the first direct, instantaneous measurement of meteor speeds from the highly resolved head echo. This approach contrasts considerably with the classic approach that depends in some manner on the rate of development of the Fresnel-scattering trail return for single- or multiple-receive-site radars (McKinley 1961, Cervera et al. 1997). While following the basic technique introduced by Mathews et al. (1997a), the approach reported here uses a double (coherent radar) pulse scheme that allows measurement of the pulse-to-pulse Doppler phase shift, thus yielding the line-of-sight speed while retaining range resolution. In this scheme the pulse duration is 1 µs with a separation of 10 µs repeated every 1 ms, with in-phase and in-quadrature voltages recorded every 1 µs (150-m-range resolution) over the altitude region km. This approach yields Doppler estimates every 1 ms from the leading edge of the meteor return from each pulse. The system bandwidth employed in this scheme was 2 MHz which yielded totally independent range samples. The pulse-to-pulse Doppler phase shift is given by ( ) I φ = tan 1, (1) R where R and I are the real and imaginary components of the pulse-to-pulse correlation function (Hagen and Farley 1973) expressed as (Mathews 1976) R = S(t)S(t + t) + C(t)C(t + t), I = S(t)C(t + t) C(t)S(t + t), (2a) (2b) where C and S are the in-phase and in-quadrature components of the voltage of the returned signal and t is the pulse separation. The instantaneous line-of-sight Doppler meteor velocity can be determined from the phase shift as V = λ φ 4π t, (3) with λ the radar wavelength (69.7 cm). The ambiguity velocity due to the 2π phase aliasing for the given radar wavelength and pulse separation is 34.8 km/s. The average meteor velocity given by the range-rate trajectory is needed to solve this ambiguity. Traditional meteor radars rarely detect meteors near the antapex because their velocities (and, thus, rate of plasma production) and rate of occurrence are low. Because we see the head echo with our technique, the measurements of low velocities present no instrumental difficulties. 2. RESULTS The observations reported here occupied 4 h of the early evening ( Atlantic Standard Time = UTC 4 h)on 10 January In this interval 32 meteor events were detected, and of these, 18 events were of sufficient duration and magnitude to determine velocity. Figure 1 shows an example meteor event for which velocity was obtained. The Fig. 1 event, as in most extended events we observe, exhibits a well-defined leading edge for each pulse return. This leading edge is the head echo while the short, diffuse trail-like echo is also usually observed in the return of both pulses and has now been identified as largely an artifact of the relative slow decay of the transmitter pulse at the 1% level. Figure 2 gives the observation trajectory (in ecliptic longitude and latitude) of the tilted (zenith distance = 9.89, AZ = 124 ), unsteered 1/6 beamwidth antenna across the sky as Earth rotated. In this paper we present the radial velocity and orbital results for the 18 events for which velocities were obtained. The errors in (leading edge or head echo) Doppler velocity determination ranged over 3 10% and compared very well with the velocities derived from the range-rate trajectory of the leading edge of the meteor return (head echo). The results of both approaches to velocity determination are given in Table I, which also summarizes decelerations discussed later. The strong agreement between the rate at which the meteor return moves radially (the trajectory-based velocity) and the Doppler speed determination provides the first unambiguous evidence that the classic head echo originates from plasma traveling at the speed of the meteoroid. These results thus suggest that the radar head echo (the region of plasma production) is always present and that previous observations using classic HF/VHF radars have simply failed to consistently detect it. The likely cause of this failure is radar sensitivity where, in simple terms, the radar scattering cross section of the trail oriented perpendicular to the beam in the classic meteor radar case (McKinley 1961) would typically be much larger (order of m 2 ) than the point-scattering head-echo radar scattering cross section as evidenced by the scattering cross sections we observe (order of 10 8 m 2 ) even when scaled to lower frequencies. Details of the head-echo scattering mechanism remain unclear. From Doppler velocities (>11.2 km/s) and assumed downthe-beam trajectories (i.e., no tangential velocity), we calculated true geocentric velocities and radiants by rigorously correcting

3 DOPPLER STUDIES OF NEAR-ANTAPEX UHF RADAR MICROMETEORS 349 FIG. 1. Radar return signal-to-noise ratio (SNR) on logarithmic scale (grayscale intensity) versus altitude and time with resolutions of 150 m and 1 ms, respectively. A radar micrometeor (fifth entry in Table I) appears as two parallel SNR enhancements due to use of two radar pulses to facilitate Doppler measurements. The leading edge in decreasing altitude of each return is the head echo while the diffuse return above the leading edge is the trail echo due largely to the relatively slow decay of the transmitted power. FIG. 2. Observation trajectory across the sky in ecliptic longitude/latitude as seen in the locus of meteor events for which velocities were (asterisks) and were not (diamonds) determined. The antapex is located at 0 latitude (β) and longitude (λ). The true radiants for the meteors whose orbits were obtained are represented by the triangles. Note the profound effects of beam motion and zenith attraction on the true geocentric radiant due to the extremely low atmospheric velocities in this sample.

4 350 JANCHES ET AL. TABLE I Micrometeor Trajectories and Doppler Velocities Local time SNR Range V trajectory V Doppler Deceleration Fig. 3 (AST) (db) (km) (km/s) (km/s) (km/s 2 ) No. 19:19: ± ± :57: ± ± ± :10: ± ± :18: ± ± :38: ± ± :43: ± ± 1 20:46: ± ± :57: ± ± ± :59: ± ± :01: ± ± ± :02: ± ± :04: ± ± :05: ± ± ± :09: ± ± :16: ± ± :21: ± ± :21: ± ± ± :37: ± ± for Earth rotation and zenith attraction as is essential for such slow-moving meteors. The resultant heliocentric orbital parameters for those events having apparent atmospheric speeds higher than Earth s escape velocity (11.2 km/s) are given in Table II. Table III gives the corrected geocentric radiants as well as the heliocentric radiants and aphelion position for those meteors for which orbits were calculated. The errors of the calculated quantities in Tables II and III are mean estimates based on the assumption of observed velocity extremes in the orbit calculation program. Seven of the Table I events have radial velocities just a bit less than 11.2 km/s, while four events are well below this value; thus the calculation of their heliocentric orbits was not possible. These events seem most likely to be highly elliptical geocentric orbits of aerocaptured particles (Hunten 1997) or possibly space debris. Rigorous dynamical studies of the origins of these geocentric particles remain to be done, but they would need to include lunar perturbations. A detailed study of the geocentric particles is beyond the scope of the present investigation and is not discussed further. The orbits of the 11 heliocentric particles are given in Fig. 3. The numbering of the Fig. 3 orbits, in order of ascending eccentricity, is given in the last column of Table I and first column of Tables II and III. One case (particle 11) gives a well-determined eccentricity greater than unity, suggesting the possibility of interstellar origin. A second (particle 10) also is hyperbolic within the upper error level, but this is not certain and so we do not present an analysis of it here. Assuming negligible planetary perturbations as appears to be at least approximately correct for a preliminary analysis, we find the true interstellar radiant with solar influence/motion removed and in ecliptic coordinates of particle 11 is λ = 43.02, β = 43.28, V = km/s where the radial velocity is inbound (thus the minus sign). The system II galactic coordinates are l II = 219.8, TABLE II Micrometeor Orbital Parameters Fig. 3 i a ω q q t No. (deg) (AU) (deg) (deg) e (AU) (AU) (days) ± ± ± ± ± ± 0.5 (20.4 ± 4.5) 2 0.9± ± ± ± ± ± 0.8 (13.2 ± 0.2) ± ± ± ± ± ± 2.3 (22.07 ± 0.9) ± ± ± ± ± ± 0.4 (22.12 ± 0.2) ± ± ± ± ± ± 0.3 (21.13 ± 0.1) ± ± ± ± ± ± 0.7 (20.46 ± 0.2) ± ± ± ± ± ± 4.2 (18.73 ± 0.5) ± ± ± ± ± ± 2.4 (22.23 ± 0.4) ± ± ± ± ± ± 32.1 (21.91 ± 0.7) ± ± ± ± ± ± (14.76 ± 0.1) ± 0.2 (4.42 ± 1.9) ± ± ± (9.7 ± 3.9) (23.02 ± 0.3)

5 DOPPLER STUDIES OF NEAR-ANTAPEX UHF RADAR MICROMETEORS 351 TABLE III Radiants Geocentric radiant positions Heliocentric radiants Heliocentric positions of aphelion (corrected for aberration and zenith attraction) (in ecliptical coordinates) (in ecliptical coordinates) Fig. 3 λ β V Corrected λ 0 β 0 V Heliocentric λ apparent β apparent No. (deg) (deg) (km/s) (deg) (deg) (km/s) (deg) (deg) ± 0.46 (13.74 ± 1.93) 1.10 ± ± 4.2 (0.48 ± 1.1) 31.0 ± ± 7.3 (0.18 ± 0.6) ± 0.40 (8.10 ± 0.81) 3.86 ± ± 1.8 (0.93 ± 0.3) 33.6 ± ± 1.3 (0.24 ± 0.1) ± 0.51 (10.63 ± 0.81) 6.06 ± ± 4.3 (1.84 ± 0.9) 34.7 ± ± 2.0 (0.83 ± 0.5) ± 0.2 (10.5 ± 0.4) 6.47 ± ± 1.4 (1.94 ± 0.3) 35.0 ± ± 0.8 (0.88 ± 0.2) ± 0.07 (9.88 ± 0.09) 9.25 ± ± 0.6 (2.44 ± 0.1) 37.3 ± ± 0.2 (1.15 ± 0.1) ± 0.1 (9.68 ± 0.1) 9.41 ± ± 0.8 (2.42 ± 0.2) 37.5 ± ± 0.4 (1.11 ± 0.1) ± 0.3 (9.08 ± 0.4) ± ± 2.3 (2.39 ± 0.4) 38.4 ± ± 1.0 (1.03 ± 0.2) ± 0.1 (10.26 ± 0.1) ± ± 1.2 (3.23 ± 0.2) 39.3 ± ± 0.4 (1.80 ± 0.2) ± 0.2 (9.97 ± 0.1) ± ± 1.8 (3.52 ± 0.3) 41.2 ± ± 0.7 (1.96 ± 0.2) ± 0.07 (7.41 ± 0.06) ± ± 0.5 (2.32 ± 0.1) 41.6 ± ± 0.2 (0.86 ± 0.05) ± 0.05 (10.38 ± 0.02) ± ± 0.7 (4.61 ± 0.1) 44.7 ± ± 0.2 a (4.67 ± 0.1) a a Hyperbolic origin point with velocity = 14 km/s. b II = 52.4, V = 25.1 km/s. The velocity agrees well with the spacecraft inferences regarding hypervelocity dust (Grün et al. 1994) and is well below the interstellar meteor velocities claimed by Taylor et al. (1996), thus underscoring the velocity sensitivity of our radar method. 3. DISCUSSION AND CONCLUSIONS We have reported the first observations of near-antapex micrometeors which are characterized by low apparent atmospheric speeds. In addition, we clearly associate, via the first direct FIG. 3. Orbits seen from above the solar system of the 11 heliocentric particles listed in Table I. All orbits lie nearly in the ecliptic and particle 11 is apparently interstellar (hyperbolic orbit).

6 352 JANCHES ET AL. Doppler measurements, the meteor head echo with a region of plasma moving at the speed of the meteoroid. The Doppler technique also yields particle deceleration along the trajectory. In particular we have checked for deceleration of all the Table I particles. Deceleration of the Table I heliocentric orbit particles is between 40 and 100 km/s 2 with 50% error in five cases; the other cases yielded no deceleration within reasonable error limits. These results are the first deceleration measurements in faint micrometeors. Preliminary dynamical analysis indicates that sputtering of single atoms/molecules is the dominant micrometeoroid mass-loss mechanism and that the particle does not undergo complete ablation during the period of radar visibility. Previous observations by Mathews et al. (1997a) indicate that UHF-radar detected particles at AO seem to be unusually durable. The AO micrometeors thus seem to be physically more like the Geminid-stream particles than those of showers produced by cometary dust whose orbital parameters might be similar. If these particles are really associated with comets then they must be considerably different from the average particles of most cometary streams. Although the present sample is small (32 meteors events) because they were made near-antapex, physically they show no difference in ablation or scattering properties compared with other UHF micrometeors seen by the AO radar (Zhou et al. 1995, 1998, Mathews et al. 1997a, Zhou and Kelley 1997). The orbit sampling circumstances of the antapex period reported here are quite different from those reported by Mathews et al. (1997a), but still one orbital similarity is evident. Physically in the atmosphere the antapex meteors do not seem to be different from those previously reported (Mathews et al. 1997a) except for their low in-atmosphere velocities. In terms of mean eccentricity, they also differ little [ e =0.5 ±0.3 for antapex and e =0.5±0.2 for Mathews et al. (1997a)]. Coincidentally these values agree quite well with the eccentricity average of the Jackson and Zook (1992) 30-µm-size particle models for short-period comets and with North Apex sporadic meteors e =0.6±0.3 (Jones and Brown 1993). Mean a for antapex, omitting the hyperbolic one, is 3.7 ± 3.4, while for Mathews et al. (1997a) a =1.4±0.2 AU. The antapex value is poor because of the small sample, but once again the Mathews et al. (1997a) value agrees with the North Apex sporadic meteor a =1.3±0.9 AU (Jones and Brown 1993). Jackson and Zook do not give a values so no comparison with their work is possible. The highest a for sporadic meteors is for the antihelion source of Jones and Brown (1993) at a =1.9±0.8. When comparing inclinations, our antapex meteors have i =2 ±1 while i =124 ± 1 for Mathews et al. (1997a). Once again the Jones and Brown (1993) data on the North Apex source agree with Mathews et al. (1997a). For a comparison of the antapex i with other values one must keep in mind that the antapex dynamical situation is strongly biased against sampling any retrograde orbits. Thus we find that neither the Jackson and Zook (1992) models (short-period comets, i =17 ± 3 ; asteroids, 7 ± 2 ) nor the Jones and Brown (1993) antihelion sporadic meteor source ( i =16 ± 8 ) agrees very well with our results. Certainly the Mathews et al. (1997a) objects are closely related to sporadic meteors, while the antapex meteors appear to be less so. But without a detailed statistical study of the sampling biases and an enlarged sample size one cannot really be sure. The agreement shown between the e values may be fortuitous when one considers that physically our particles seem to most resemble the Geminid meteors. Geminid particles are produced by 3200 Phaethon, recently described as an asteroid (Luu 1993) although the possibility that it is an extinct comet nucleus has been suggested (Gustafson 1989). While Phaethon s semimajor axis value (=1.41 AU) falls in the range of the antapex particles, its eccentricity is large (0.9) and therefore is more like those of the Mathews et al. (1997a) particles seen in the UTC January 18, 1995 period ( a =1.66 AU, e =0.8). Thus it is not easy to assign a progenitor object to these particles: in eccentricity, they appear to be produced by short-period comets, in inclination by asteroids, and in semimajor axes and physical characteristics, by Phaethon-type objects. Finally, we detected an apparent interstellar particle (No. 11) and traced its orbit back to 50 AU, showing no significant encounters with any planets. A preliminary analysis gives a general direction of origin and the incoming speed but the source object is not obvious. Durable particles are needed to survive interstellar passage and the apparent strength of the particles we observe would seem to fit the requirements better than fluffy cometarytype meteoroids. Even with solar motion removed, our ISP still has motion with respect to the local standard of rest. This is quite unlike the ISPs detected by spacecraft (Grün et al. 1994) which are largely entrained in the interstellar gas flow. Calculation of the trajectory backward in time shows two recent stellar encounters : SAO83442 (spectral type K4 Ve, 0.6-parsec miss distance 161,000 years ago) and HD (spectral type G0; 0.52-parsec miss distance 241,000 years ago), but neither of these are considered to be likely sources of this particle. We simply note that a number of objects (e.g., novae, supernovae, protostars, red giants) seem capable of producing such particles at the observed LSR velocity. Thus more observations are needed to ellucidate possible sources for Arecibo ISPs. 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