[P-EM17] Space Plasma Science

[PEM17-15] Observation of VLF Radio Emission upon the Reentry of OSIRIS-REx Sample-Return Cuspule on 24 September 2023

*Takashi Watanabe¹, Miki Kobayashi², Yasuo Katoh³, Kazuo Shiokawa³, Hiroyo Ohya⁴, Kazuhiro Suzuki² (1.Institute of Information and Communications Technology, 2.Japan Meteor Society, 3.Institute of Space-Earth Environmental Research, 4.Graduate School of Engeneering, Chiba University)

Keywords:Space Craft, Reentry, VLF Radio Wave, Direction Finding

After the detection of peculiar VLF radio noises upon the reentry of Hayabusa-2 Sample-Return Capsule (SRC) RC 2020 (WGN, the Journal of the IMO 51:3, 2023), direction-finding radio observations were performed at Pinto Summit, Eureka, Nevada upon the reentry of the OSIRIS-REx SRC on 24 Sep. 2023 (UTC) in the frequency region of 100 Hz - 96 kHz. Crossed loop aerials were employed to estimate the azimuthal angle (Az) of an observing radio source. The expected accuracy of the measurement is +/- 10 degrees. One vertical rod antenna was also used to measure the vertical component of the electric field of the radio wave. The initial polarity of Ez can eliminate the ambiguity in the Az estimation. We employed a simplified method to estimate the altitudinal angle (Alt) of the radio source by assuming the directivity of the vertical antenna, but an error of +/-20 degrees or more is expected near the zenith. We checked the consistency between the estimated (Az, Alt) and those of SRC predicted by a simple model of its trajectory (based on the nominal parameters given in NTRS- 20150000809). This model is only applicable in the interval before the onset of significant deceleration of the SRC. The SRC's Entry Interface (EI) epoch was finally adjusted to 14:42:00 UTC, after an announcement from NASA. More than 20 weak non-impulsive radio noises, having similar values of (Az, Alt) to those predicted by the trajectory model, were detected during the interval of about +/- 25 sec around the peak-heating phase of the reentry (+51 sec from EI). These signals showed relatively broader frequency spectra than those of daytime long-distance sferics. Although the actual trajectory of the reentry has not been available at this stage, the presence of VLF radio emission upon the reentry of the SRC is plausible within the level of uncertainty. Intermittent electric discharges in the environmental plasma of SRC in the fireball phase of the reentry are suggested to be the sources of observed radio noises.