A new method for measuring the meteor mass index: application to the 2018 Draconid meteor shower outburst

ABSTRACT

Context. Several authors predicted an outburst of the Draconid meteor shower in 2018, but with an uncertain level of activity.

Aims. Optical meteor observations were used to derive the population and mass indices, flux, and radiant positions of Draconid meteors.

Methods. 90 minutes of multi-station observations after the predicted peak of activity were performed using highly sensitive Electron Multiplying Charge Coupled Device (EMCCD) cameras. The data calibration is discussed in detail. A novel maximum likelihood estimation method of computing the population and mass index with robust error estimation was developed. We apply the method to observed Draconids and use the values to derive the flux. Meteor trajectories are computed and compared to predicted radiant positions from meteoroid ejection models.

Results. We found that the mass index was 1.74 ± 0.18 in the 30 minute bin after the predicted peak, and 2.32 ± 0.27 in the next 60 minutes. The location and the dispersion of the radiant matches well to modeled values, but there is an offset of 0.4° in solar longitude.

1. Introduction

The Draconids are an annual meteor shower whose parent body is the Jupiter-family comet 21P/Giacobini-Zinner. The shower usually has a very low activity with a Zenithal Hourly Rate (ZHR) of ∼ 1 (Jenniskens 2006). It produced large meteor storms in 1933 and 1946 and strong outbursts in a number of other years (Egal et al. 2019). Many of these outbursts were not predicted beforehand. The shower’s occasionally high intensity and unpredictability have made it a focus of research, particularly as it can pose a significant impact risk to spacecraft in the near-Earth environment (Beech et al. 1995; Cooke & Moser 2010; Egal et al. 2018).

In recent years, the shower has produced several notable outbursts. In 2011 an outburst was predicted in advance (Watanabe & Sato 2008; Maslov 2011; Vaubaillon et al. 2011) and well observed by both radar (Kero et al. 2012; Ye et al. 2013a) and optical methods (Trigo-Rodr´ıguez et al. 2013; Boroviˇcka et al. 2014; Koten et al. 2014; Segon ˇ et al. 2014; Vaubaillon et al. 2015). That year the outburst reached a ZHR of 350, and had an average mass index of 2.0±0.1, but which varied between 1.84 to 2.30 in one hour during the peak (Koten et al. 2014).

In contrast, the 2012 outburst was not predicted and was only well observed by the Canadian Meteor Orbit Radar (CMOR) (Brown & Ye 2012). The shower produced a meteor storm at radar sizes (ZHR ≈ 9000 ± 1000), but visual observers reported a ZHR almost 2 orders of magnitude lower, ZHR ∼ 200, suggesting a high mass distribution index, i.e. that the stream was rich in small meteoroids. Unfortunately, due to the bad timing of the peak (maximum over central Asia) and unfavorable weather conditions elsewhere, no optical orbits associated with the outburst were secured.The 2012 outburst was also peculiar in that modeling suggested a very high (> 100 m/s) meteoroid ejection velocity from the parent comet was needed (Ye et al. 2013b).

The 2018 outburst was predicted by various authors (Kres´ak 1993; Maslov 2011; Ye et al. 2013b; Kastinen & Kero 2017; Egal et al. 2018), but the predicted activity varied from weak (ZHR 10 - 20) (Maslov 2011) to possible meteor storm levels (Kastinen & Kero 2017). The most recent work by Egal et al. (2018), which reproduced well most historic Draconids activity, predicted a peak ZHR of ∼ 80 at 00:00 UTC on October 9, 2018.

In this work we analyze 1.5 hours of optical observations from Southwestern Ontario just after the peak1 of the 2018 outburst, from 00:00 UTC to 01:30 UTC. We also develop a novel method of population and mass index estimation, and compute these indices using our observations. Finally, we compare model predicted radiants with our multi-station observations and compute the shower flux.