Venus Lyα: A Morphological and Radiative Transfer Analysis

Abstract

The Venus Lya corona is caused by resonance scattering of the solar 1215.67 A Lya line by hydrogen atoms in the Venus upper atmosphere. The atmospheric atomic hydrogen content is probed remotely via Lya observations. On 1990 February 10 the Galileo spacecraft flew by Venus, obtaining a series of Venus scans with the Ultraviolet Spectrometer Experiment. The Pioneer Venus (PV) Orbiter Ultraviolet Spectrometer obtained Venus Lya images approximately weekly throughout its 14 year mission (1978–1992), spanning the 11 year solar cycle. I develop a morphology of the behavior of the Venus Lya signal with respect to location on Venus and solar activity level. The predawn hydrogen density enhancement extends to high latitudes (160 ). I find an equatorial minimum of hydrogen and evidence for a polar hood of enhanced hydrogen abundance. This hood may be produced by a hydrogen influx from the predawn region that is not removed as efficiently as hydrogen in the subsolar region. All features examined persist throughout the solar cycle and increase in hydrogen abundance with solar activity. A search for small-scale (1000 km) features produced a null result. However, the timescale for ballistic transport of atomic hydrogen in the exosphere is ∼8 minutes, too short for the detection of nonequilibrium density features. I analyze the data using a two-dimensional nonisothermal complete-frequency-redistribution multiple-scattering code modified from the LYAB code provided by James Bishop for the geocorona. I employ the VTS3 neutral thermosphere model (A. Hedin, H. Niemann, W. Kasprzak, & A. Seiff, J. Geophys. Res., 88, 73 [1983]) and calculate diffusive profiles for the vertical distribution of atomic hydrogen, characterized by the hydrogen number density and vertical flux at the exobase n f 0 0 (L. Paxton, D. Anderson, & A. Stewart, J. Geophys. Res., 93, 1766 [1988]). The flux parameter regulates the amount of hydrogen in the lower thermosphere, and the exobase density controls the amount in the upper thermosphere and exosphere. The almost linear relationship between the PV Langmuir probe photoelectron current and the observed solar Lya output provides a daily proxy for solar illumination of Venus hydrogen. I determine the parameter values that best fit the data for selected segments of the sunlit disk and examine their behavior with respect to location and solar activity. Numerical results are limited to the region of spatially uniform temperature in the exosphere and thermosphere. For high solar activity ( ), subsolar values are F ∼ 200 n (5.6 10.7 0 cm 3 and cm 2 s , cor4 7 0.8) # 10 f (5.8 0.9) # 10 0 responding to hydrogen column densities of (2.0 0.3) # cm 2 above 200 km and between 135 12 13 10 (1.0 0.2) # 10 km and the CO2 Lya absorbing layer at 112 km. These values agree with the subsolar work of Paxton et al. and with densities derived from in situ measurements by H. Brinton, H. Taylor, H. Niemann, H. Mayr, A. Nagy, T. Cravens, & D. Strobel (Geophys. Res. Lett., 7, 865 [1980]). Exobase density, , den0 creases from a local solar time (LST) of 10 hr to 15 hr by to cm . At 12 hr LST, increases from the 4 4 7 # 10 4 # 10 n0 equator to 60 latitude by to cm . This exo4 4 4 # 10 7 # 10 base density minimum at low latitudes on the dayside is consistent with solar EUV heating driving the transport removal of hydrogen atoms from the dayside upper atmosphere. The hydrogen abundance in the lower thermosphere, , does not f0 vary significantly. However, because of the opacity of the overlying upper thermosphere and exosphere, the variation of hydrogen column abundance for the values observed would n0 not be detectable in . Both and increase with solar f n f 0 0 0 activity, and there is evidence suggesting solar cycle phase dependence. A detailed interpretation of these quantitative results requires a photochemical model of the Venus upper atmosphere. The details of suprathermal hydrogen atom production are essential to understanding the transport behavior of hydrogen in the upper thermosphere and exosphere.