PLANETARY CORONAE AND ATMOSPHERIC EVAPORATION

Journal Article · Thu Aug 01 04:00:00 EDT 1963 · Planetary Space Sci.

DOI:https://doi.org/10.1016/0032-0633(63)90122-3· OSTI ID:4697469

Chamberlain, J W

9 ; : = 7 > : ; = : 7 8 8 4 planetary atmosphere where collisions are rare and where the controlling factors are gravitational attraction and thermal energy conducted from below. The classic assumption of a sharply defined critical level above which the atmosphere is completely free of collisions is used initially. Throughout, the different types of particle orbits are treated separately; coronal particles are either ballistic (meaning captive particles whose orbits intersect the critical level), satellite (captive particles orbiting above the critical level), or escaping. Liouville's equation leads to exact expressions for the density distributions and escape flux. The latter is a simple analytic expression, but the density integrals are more complex and numerical evaluations are provided in tables. At large distances from the planet all the integrals may be evaluated in their asymptotic limits. Expressions are given for the different density components, the kinetic temperature, and heat flux. The latter two cannot generally be evaluated with the thermal conductivity as normally computed. The integrated density in a column above a specified height and orientated in a specified direction is given particular attention, because it fixes the critical level and because it is essentially an observable quantity. Numerical calculations are given for relating such observations in either the radial or transverse directions to the basic parameters of the corona (the critical-level density, temperature, and height above the planet). The radial integrated density at the critical level is shown to deviate sensibly from the simple expression applicable to a plane- parallel atmosphere unless the gravitational energy is very much greater than the mean thermal energy. Results of the preceding theory are utilized to justify the adoption of the concept of a critical level. The principal question focuses on the outward flux of escaping particles. A detailed treatment of escape as governed by collisions agrees with earlier similar analyses in showing that (with certain approximations) the critical-level concept predicts the accurate flux. However, it is noted that this result, except for confirming the height of the critical level, is trivial: it is a consequence of an assumed Maxwellian distribution and the special properties accruing to it. A variety of related problems dealing with production and loss mechanisms are discussed after some orbital properties and flight times are investigated. The critical-level theory cannot cope with the important question of the abundance of satellite orbits. Here it is shown that the satellite contribution may be included throughout the theory by omitting the exprsssions for the satellite component as such and evaluating the ballistic component, not with the critical level, but with a higher satellite critical level instead. For the Earth's hydrogen corona this level is near 2.5 Earth radii. The effect of photoionization on the density distribution of escaping orbits is also treated and shown to be negligible for the Earth. The concentration of hot interplanetary gas around a bare planet oifers an interesting effect in that screening of orbits by the planet may more than counteract condensation due to the gravitational attraction. A hot, stationary interplanetary gas intermingled with a planetary corona will not have any appreciable effect on the coronal density or temperature distributions. Specifically, it is inappropriate to regard heat as being conducted (in the usual meaning of the word) from the interplanetary medium through the corona. Doppler profiles of coronal particles in a column along a specified direction were examined. Asymptotic expressions are given along with the general formulas, which will have to be evaluated numerically if and when the pertinent spectral observations are made. Even at large distances from the planet where the population consists largely of escaping particles, the shape of the spectral profile does not deviate very seriously

• Research Organization: Kitt Peak National Observatory, Tucson, Ariz.
• NSA Number: NSA-17-041696
• OSTI ID: 4697469
• Journal Information: Planetary Space Sci., Journal Name: Planetary Space Sci. Vol. Vol: 11
• Country of Publication: Country unknown/Code not available
• Language: English

Related Subjects

ASTROPHYSICS
ATMOSPHERE
BOLTZMANN EQUATION
COLLISIONS
DENSITY
DIFFERENTIAL EQUATIONS
DISTRIBUTION
EARTH
ENERGY
EQUATIONS
GASES
GRAVITATION
HELIUM
HYDROGEN
IONIZATION
LEVELS
LIGHT
LOSSES
MAXWELL DISTRIBUTION
MOTION
NUMERICALS
ORBITS
PARTICLES
PHYSICS
PLANETS
PRODUCTION
SPACE
SUN
TABLES
TEMPERATURE
THERMAL CONDUCTIVITY
THERMODYNAMICS
VELOCITY