Determination of O₂density from Lyman α ion chambers

Abstract

Molecular oxygen data during spring and fall are deduced from Lyman a ion chamber measurements. The data show the existence of wave type structure. Random errors in the technique are estimated from simultaneous independent measurements and are found to be low. A major uncertainty in the densities results from lack of information on the appropriate absorption cross sections of molecular oxygen for the temperatures encountered in the atmosphere. Potentially large overestimates in the densities could result from absorption by nitric oxide above 105 km and to a lesser extent by nitric oxide at lower altitudes and water.vapor.

Observations of the O2column density between 120 and 70 km and absorption cross section in the vicinity of H Lyman alpha

“…All of the observations which have resolved the solar Lyman alpha line either have been made from altitudes above most of the O2 or had exposure times too large to achieve any altitude resolution for O2 absorption measurements. Conversely, other observations which have been made at the appropriate altitudes with good altitude resolution have not resolved the Lyman alpha line [Carver et al, 1964;Weeks and Smith, 1968;Quesette, 1970;Subbaraya et al, 1972;Hall, 1972;Weeks, 1975]. These observations were made by using ion chambers fitted with MgF2 or LiF windows.…”

“…Many observers have assumed that Lyman alpha was the principal contributor to the signal, that O2 was the principal absorber, and that the O2 cross section was constant over the Lyman alpha line. Refinements in these assumptions have been made by Quesette [1970], Hall [1972], Smith and Miller [1974], and Weeks [1975], who allowed for variation in the O2 cross section by using the values of Ogawa [1968]. For the Copyright ¸ 1977 by the American Geophysical Union.…”

Molecular oxygen concentrations and absorption cross sections in the thermosphere derived from extreme ultraviolet occultation profiles

“…The effects of minor absorbers are considered to be insignificant at 1335 and 1032,4, because of the magnitude of the Os absorption cross section. However, at 1216 •, absorption by NO can be significant at altitudes above 110 km, as is discussed by Weeks [1975]. Nevertheless, even using the high concentration of NO reported by Meira [1971], we calculate that the maximum error on the Os concentration at the altitude of unit optical depth is about 2.5%.…”