Elsevier

Icarus

Volume 188, Issue 1, May 2007, Pages 162-174
Icarus

Sporadic meteor sources as observed by the Jicamarca high-power large-aperture VHF radar

https://doi.org/10.1016/j.icarus.2006.11.006Get rights and content

Abstract

We present, for the first time, the main sources of sporadic meteors as inferred from meteor-head echoes obtained by a high-power large-aperture radar (HPLAR). Such results have been obtained at the Jicamarca HPLAR (11.95° S, 76.87° W, 1° dip angle). Observations are based on close to 170,000 meteors detected in less than 90 h spread over 14 days, between November 2001 and February 2006. Meteors with solar orbits are observed to come from basically six previously known sources, i.e., North and South Apex, Helion, Anti-Helion, and North and South Toroidal, representing 91% of the observations. The other 9% represents meteors with observed velocities greater than the Sun's escape velocity at 1 AU, most of them of extra-solar origin. Results are given before and after removing the Earth's velocity and the sources are modeled with two-dimensional Gaussian distributions. In general, our results are in very good agreement with previously known sources reported by Jones and Brown [Jones, J., Brown, P.G., 1993. Mon. Not. R. Astron. Soc. 265, 524–532] using mainly specular meteor radar (SMR) data gathered over many years and different sites. However, we find slightly different locations and widths, that could be explained on the basis of different sensitivities of the two techniques and/or corrections needed to our results. For example, we find that the North and South Apex sources are well defined and composed each of them of two collocated Gaussian distributions, one almost isotropic with ∼10° width and the other very narrow in ecliptic longitude and wide in ecliptic latitude. This is the first time these narrow-width sources are reported. A careful quantitative analysis is needed to be able to compare the strengths of meteor sources as observed with different techniques. We also present speed and initial altitude distributions for selected sources. Using a simple angular sensitivity function of the combined Earth–atmosphere–radar instrument, and an altitude selection criteria, the resulting meteor sources are in better qualitative agreement with the results obtained with SMRs.

Introduction

Since the early 1940's specular meteor radars (referred to as SMR hereafter) have been the main source of meteor observations entering the Earth's atmosphere (e.g., Jones and Brown, 1993, Galligan and Baggaley, 2005; and references therein). These systems use wavelengths between 3 and 15 m, on few occasions systems with larger wavelengths have been used (e.g., Steel and Elford, 1987). The combined results from these systems gathered over many years of observations have allowed the identification of the currently well accepted six sources of sporadic meteors, i.e., North Apex (NA), South Apex (SA), Helion, Anti-Helion (AH), North Toroidal (NT), and South Toroidal (ST) (e.g., Jones and Brown, 1993, Taylor and Elford, 1998, Galligan and Baggaley, 2005).

In this work we present for the first time the sources of sporadic meteors as inferred from the meteor-head echoes detected by the Jicamarca High-Power Large-Aperture Radar (HPLAR). This type of radar (e.g., ALTAIR, Arecibo, EISCAT, Jicamarca, Millstone Hill, MU, and Sondrestorm) has been used for meteor-head studies since mid 1990's originally motivated by predictions of several returns of the Leonids meteor showers (e.g., Janches et al., 2000b, Close et al., 2002, Sato et al., 2000, Chau and Woodman, 2004). The radar frequencies from these radars start around 50 MHz (at Jicamarca and MU) and go as high as 1.29 GHz (at Sondrestorm). These radars are mainly sensitive to the plasma surrounding the meteoroid as it enters the Earth's atmosphere (between 70 and 140 km). Faster geocentric meteoroids will ionize at higher altitude and will have more chance to be observed by the HPLARs (e.g., Janches and ReVelle, 2005). More details on the scattering mechanism of meteor-head echoes are given by Close et al. (2004) and Mathews (2004).

We first present the experimental setup and meteor parameters either directly measured or derived. The expected statistical errors for some derived parameters are presented in Appendix A. In Section 3 we show the meteor sources as seen by the Jicamarca radar and the speed and initial altitude distributions of selected meteors sources, including the not well defined Antapex (or Apex prograde) source. Finally results are interpreted and discussed, special emphasis is given to the comparison with results obtained by SMRs.

Section snippets

Experimental setup and meteor parameters

Meteor-head observations have been performed using the large Jicamarca array (300m×300m) for transmission and at least three quarter sections for reception (75m×75m). The same linear polarization (north–east) has been used in both modes. The antennas were phased to point on-axis (1.46° from zenith towards the south–west). Complex voltages (raw data) from at least three quarters were recorded. Note that we only need the information from three non-collinear antennas in order to locate the

Meteor sources

Before determining the radiant, i.e., where meteors are coming from (derived from the meteor vector velocities), we have performed Earth zenith attraction and Earth rotation corrections, to get the undisturbed meteor velocity with respect to the Earth (instantaneous) inertial frame of reference (also called geocentric velocities in the literature). This correction is mainly important for meteors with slow geocentric velocities (the Earth's escape velocity is 11.2km/s). Note that we have not

Speed and initial altitude distributions

Now that we have identified different meteor populations, we could proceed to present statistics for any meteor parameter or pair of parameters. However in this work we only present the speed and initial altitude distributions of selected sources. In Fig. 5 we show the geocentric (in black) and heliocentric (in red) speed distributions for the following meteor sources: (a) All retrograde Apex sources, (b) narrow SA, (c) Antapex (Apex-prograde), (d) AH, (e) ST, and (f) extra-solar. Although not

Discussion

As shown in previous works (e.g., Janches et al., 2003, Close et al., 2002, Chau and Woodman, 2004), the HPLARs provide meteor measurements with very high accuracy in velocity, initial altitude, and deceleration, among other parameters. Using interferometry, Jicamarca's results show that the location (inferred from the velocities) of the meteors on a modified (non-inertial) ecliptic coordinate system are also excellent, being able to observe pretty much all the known sporadic meteors sources

Concluding remarks

Using the Jicamarca (11.95° S, 76.87° W, 1° dip angle) high-power large-aperture radar (HPLAR), we have detected 170,000 meteor heads in less than 90 h of observations spread in 14 days, between November 2001 and February 2006. From these observations 91% are of from the Solar System and the remaining appear to be extra-solar.

The solar meteors are mainly clustered in six main sources, i.e., South and North Apex, Helion, Anti-Helion, and South and North Toroidal. These sources have been

Acknowledgements

We thank P. Brown, D. Holdsworth, D. Janches, and M. Oppenheim for their comments, suggestions, and encouragement during different stages of this work. The Jicamarca Radio Observatory is a facility of the Instituto Geofísico del Perú and is operated with support from the NSF Cooperative Agreement ATM-0432565 through Cornell University.

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