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The errors inherent to the use of the standard "ionization correction factor" ("i CF ") method of calculating nebular conditions and relative abundances of H, He, N, O, Ne, S, and Ar in emission line nebulae have been investigated under conditions typical for planetary nebulae. The photoionization code Cloudy was used to construct a series of model nebulae with properties spanning the range typical of PNe. Its radial "profiles" of bright, frequently observed optical emission lines were then summed over a variety of "apertures" to generate sets of emission line measurements. These resulting line ratios were processed using the i CF method to "derive" nebular conditions and abundances. We find that for lines which are summed over the entire nebula the i CF -derived abundances differ from the input abundances by ≤ 5% for He and O up to ≤ 25% or more for Ne, S, and Ar. For resolved observations, however, the discrepancies are often much larger and are systematically variable with radius. This effect is especially pronounced in low-ionization zones where nitrogen and oxygen are neutral or once-ionized such as in FLIERs, ansae and ionization fronts. We argue that the reports of stellar-enriched N in the FLIERs of several PNe are probably specious.

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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