Characteristics of Jupiter's X-ray auroral hot spot emissions using Chandra

Abstract

We present an extensive statistical study of all 29 Chandra High Resolution Camera (HRC-I) observations, covering ∼ 20 years worth of data from 18th December 2000 to 8th September 2019. Eight of these observations were pre-Juno and 21 while Juno was exploring the Jovian system providing in situ context. The first spatially resolved X-ray auroral "hot spot" was discovered by Gladstone et al. (2002) from Chandra observations of Jupiter’s North Pole. The emissions observed were found to pulsate with a 45-min quasi-periodic oscillation (QPO) and believed to originate in the outer magnetosphere (> 30 RJ) [1]. Since then, from using subsequent remote sensing X-ray observations in tandem with available in situ data, we know that the emissions from the hot spot consist of soft X-rays (SXRs, photons with energy < 2 keV) from charge exchange processes between precipitating ions and neutrals within the Jovian atmosphere [2]. However, the hot spot is found to be very variable both spatially and temporally [3] in all observations to date, thus proving determining the driver to be very difficult.

In this study, we characterise the typical and extreme behaviour of the hot spot emissions across the entire catalogue for the first time. Examining both types of behaviour allows us to determine the full extent of hot spot variability. We mainly focus on the northern hot spot (NHS) as: (1) the viewing geometry over the catalogue favours the North Pole best and (2) the NHS has been shown to be mostly non-conjugate with the south [4, 5], producing much stronger emission. We present heat maps and 2-D histograms to show the overall average hot spot morphology. The hot spot was defined using a numerical criterion of location and photon concentration (S3 longitude: 100° - 240°; latitude: 40° - 90°; concentration: > 7 photons per 5° S3 lon × 5° lat) [6]. From the catalogue, 26 out of 29 observations had emission within this threshold. Using a 2-D histogram with 3° S3 lon × 3° lat binning, we find a significant region of concentrated NHS emission at ∼ 162° - 171° S3 longitude and 60° - 66° latitude, herein referred to as the averaged hot spot nucleus (AHSNuc). The AHSNuc is found to mainly map to the noon magnetopause boundary while most events from the NHS are found to originate on the dusk-midnight boundary. This suggests that multiple drivers may produce the NHS. The mapping is carried out using a flux equivalence model [7, 8] with the Grodent Anomaly Model (GAM) [9], to model the internal field. We discuss the uncertainty in our mapping associated with possible fluctuations in ionospheric position and mapping limitations to improve the accuracy of our interpretations.

Finally, we apply the Rayleigh test technique discussed in Jackman et al. (2018) to find any significant QPOs within the hot spot and the AHSNuc.
We create a catalogue of all our timing analysis results noting all the significant QPOs found and test their robustness using a Jackknife test. We interpret the NHS and AHSNuc locations and compare the timescales of the QPOs to known possible drivers (such as magnetopause reconnection [10] and ultralow frequency (ULF) waves [11]) in an attempt to determine the overall hot spot driver/drivers.

 

References
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[9] Grodent, D., Bonfond, B., Gérard, J. C. et al.: Auroral evidence of a localized magnetic anomaly in Jupiter’s northern hemisphere, Journal of Geophysical Research: Space Physics, Vol. 113, pp. 1-10, 2008.

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