Destruction and Observational Signatures of Sun-Impacting Comets

Abstract

Motivated by recent data on comets in the low corona, we discuss destruction of 'sun-impacting' comets in the dense lower atmosphere.Perihelion distances q ≲ R ! and masses M o >>" 10 12 " g are required to reach such depths.Extending earlier work on planetary atmosphere impacts to solar conditions, we evaluate the mechanisms and distribution of nucleus mass and energy loss as functions of M o and q, and of parameter X = 2Q/C H v o 2 . Q is the total specific energy for ablative mass-loss, C H the bow-shock heat-transfer efficiency, and v o the solar escape speed (619 km/s). We discuss factors affecting Q and C H and conclude that, for solar v o , X is most likely < 1 and solar-impactors mostly ablated before decelerating. Sun-impacting comets have energies M o v o 2 /2 ~ 2x10 30 ×(M o /10 15 g) erg, (comparable to magnetic flares ~ 10 29-33 ). This is released as a localised explosive airburst within a few scale heights H ≃ 200 km of the photosphere, depending weakly on M o , q and X. For X = 10 -2 and M o = 10 15 g, ! 2! a shallow incidence (e.g. polar θ ~ cos -1 (0.01)) Kreutz comet airburst occurs at atmospheric density n ~ 3x10 15 cm -3 -a height of 700 km (3.5 H) above the photosphere (where n = n o = 10 17 cm -3 ). The airburst n scales as ~ (M o Xcos 3 θ ) 1/2 (while height z(km) = 200ln(n o /n)) so n increases 1000× (700 km deeper) for vertical entry. Such airbursts drive flare-like phenomena including prompt radiation, hot rising plumes and photospheric ripples, the observability and diagnostic value of which we discuss.

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

“…Also plotted is a model of solar wind speed from Lamy et al (2003a) for a solar exobase temperature of 10 6 K. Inwards of ∼ 4.4 R (∼ 0.0205 AU), the heliocentric speed of a parabolic comet nucleus exceeds the expected local speed of the solar wind explosion deeper in the atmosphere than magnetic flares can readily generate helioseismic ripples ("Sun-quakes"), shedding light on that puzzling phenomenon (Lindsey and Donea 2008). Brown et al (2011Brown et al ( , 2015 have emphasized that the kinetic energy of very large sundivers far exceed that of the largest magnetic flares and coronal mass ejecta, CMEs, so could produce major terrestrial effects. Eichler and Mordecai (2012) suggest such an impact as the explanation of the major 7th Century isotopic abundance anomaly.…”

“…They are of great interest to both cometary and solar physics as discussed by Brown et al (2011Brown et al ( , 2015. For example, the spectrum of their abrupt total explosive destruction can shed light on their interior composition, while their Fig.…”

“…-q must be small enough to reach the dense atmospheric regime where fluid effects (bowshock and ram pressure driven ablation and deceleration) dominate over radiative sublimation and rapidly destroy the nucleus (Weissman 1983;Brown et al 2011Brown et al , 2015. This regime's outer boundary depends on various uncertain parameters discussed in Brown et al (2015) but is at a density n ∼ 10 14 cm −3 .…”

“…This regime's outer boundary depends on various uncertain parameters discussed in Brown et al (2015) but is at a density n ∼ 10 14 cm −3 . This is ∼ 10 −3 of photospheric values so ∼ 7 density scale heights above the photosphere, i.e., ∼ 1000 km, or a heliocentric distance of ∼ 1.0015 R (0.004657 AU).…”

“…-The incident nucleus mass must be high enough, >∼ 10 12 g, the exact value depending on nucleus density, latent heat, etc. (Brown et al 2011(Brown et al , 2015, to avoid total sublimation when inbound. -The overall nucleus strength must be sufficient to resist fragmentation by tidal gradient or by ablation flow pressure.…”