Small lightning flashes from shallow electrical storms on Jupiter

Abstract

Jovian lightning flashes were characterized by a number of missions that visited Jupiter over the past several decades. Imagery from the Voyager 1 and Galileo spacecraft led to a flash rate estimate of ~4×10 -3 flashes/km 2 /yr on Jupiter. 1,2 The spatial extent of Voyager flashes was estimated to be ~30 km at half-width half-maximum intensity (HWHM), but the camera was unlikely to have detected the dim outer edges of the flashes given weak response to the brightest spectral line of Jovian lightning emission, the 656.3 nm H-alpha line of atomic hydrogen (known from lab experiments). 1,3-6 The spatial resolution of Galileo and New Horizons cameras allowed investigators to confirm twenty-two flashes with HWHM >42 km and estimate one between 37-45 km. 1,7,8,9 These flashes, with optical energies only comparable to terrestrial "superbolts" (2×10 8 -1.6×10 10 Joules), have historically been interpreted as tracers of moist convection originating near the 5 bar level of Jupiter's atmosphere (assuming photon scattering from points beneath the clouds). 1-3,7,8,10-12 All previous optical observations of Jovian lightning have been limited by camera sensitivity, proximity to Jupiter, and long exposures (~680 ms to 85 s) hence some measurements were likely superimposed flashes reported as one. 1,2,7,9,10,13 Here we report optical observations of lightning flashes by Juno's Stellar Reference Unit 14 with energies of ~10 5 -10 8 Joules, flash durations as short as 5.4 ms, and inter-flash separations of tens of milliseconds. The observations exposed Jovian flashes with typical terrestrial energies. The flash rate is ~6.1×10 -2 flashes/km 2 /yr, more than an order of magnitude greater than hitherto seen. Several flashes are of such small spatial extent they must originate above the 2 bar level, where there is no liquid water. 15,16 Juno's Stellar Reference Unit (SRU) captured images of Jovian lightning on the dark side of Jupiter from a unique perspective of as close as 53,000 km above the 1 bar level (30 km/pixel resolution). The SRU is a broadband (450 -1100 nm) imager designed to detect dim stars in support of spacecraft attitude determination. The camera's point spread function (PSF) spreads the optical signal of a point source over ~5 × 5 pixels, allowing unambiguous identification of small optical sources (see Extended Data Fig. 1). Fourteen lightning flashes (see Extended Data

Storms and the Depletion of Ammonia in Jupiter: II. Explaining the Juno Observations

“…In a companion paper (Guillot et al., 2020) (hereafter Paper I), we have shown that during strong storms able to loft water ice into a region located at pressures between 1.1 and 1.5 bar and temperatures between 173 and 188 K, ammonia vapor can dissolve into water ice to form a low‐temperature liquid phase containing about 1/3 ammonia and 2/3 water. The presence of this liquid mixture is consistent with the observation of lightning flashes originating from low pressure levels (Becker et al., 2020). The subsequent formation of ammonia‐rich hail that we call “mushballs” leads to an effective transport of the ammonia to deep levels (between 7 and 25 bar, depending on poorly known ventilation coefficients).…”

Storms and the Depletion of Ammonia in Jupiter: I. Microphysics of “Mushballs”

“…We also note that the minimum in the derived NH 3 abundances (Bolton et al., 2017; Li et al., 2017) is very close to the minimum NH 3 abundance below which the mushball mechanism cannot work (i.e., from Figure 1, a partial pressure bar, corresponding to a ∼100 ppmv NH 3 mole fraction in Jupiter). Finally, recent Juno observations in the optical show lightning flashes that are formed between 1 and 2 bar, consistent with the presence of liquid NH 3 ·H 2 O and large particles in the mushball formation region (Becker et al., 2020). In a subsequent paper, we develop a model of Jupiter's deep atmosphere to attempt to reproduce the dominant features of Juno's observations.…”

Possible Transient Luminous Events Observed in Jupiter's Upper Atmosphere

“…The third instrument that has so far reported lightning observations is the Stellar Reference Unit (SRU), a low light visible imager designed for attitude determination. As with previous optical observations, the SRU lightning detections consist of bright flashes on the planet's nightside (Becker et al., 2020).…”

“…Any bright flashes detected by UVS therefore must occur much higher in the atmosphere than both the water cloud and the “shallow” lightning observed by Becker et al. (2020), and must be driven by a different mechanism than intracloud discharge. In this paper, we conclude that these bright flashes are consistent with TLEs in Jupiter's upper atmosphere, a phenomenon that has been predicted for Jupiter (Yair et al., 2009) but has not been previously observed.…”

“…Becker et al. (2020) found that some of the lightning observed by the SRU occurs at pressures of 1.4–1.9 bar, where pure liquid water cannot exist. Instead, they suggest that a mixed ammonia‐water liquid plays an important role in generating this “shallow” lightning.…”