Decrease inD-region O₂(¹Δ_g) Photoionization rates resulting from CO₂absorption

Abstract

Photoionization of the metastable Os (xzXg) molecule has been proposed as an important source of ionization in the D region. The wavelength region of interest is from 1027 to 1118 A, where ground state O• is the primary absorber of the incident solar flux but is not ionized itself. This paper presents improved ion production rates based on new laboratory data, including new O• (•zXg) photoionization cross sections and more detailed curves in several ground state O•. windows. Recent solar flux measurements reduce the continum intensity by about a factor of 5. This reduction is partly compensated by including in the calculations the Si III multipier, which is at the deepest O•. window (1108.2 A, minimum cross section _--4.4 X 10 -• cm •) and by the new ionization cross sections, which are generally larger than the previously assumed values. An important factor not previously considered is absorption by carbon dioxide, which has a much larger absorption cross section than ground state O•. throughout this region. The ground level mixing ratio has been assumed. The ion production rates are somewhat less than Hunten and McElroy's [1968] curves if CO• is not included. With CO•, a production rate of I ion cm -8 sec -1 is reached at approximately 80, 84, and 86 km for zenith angles of 0', 45', and 60', respectively. Hunten and McElroy find this rate to be 10 to 15 km lower. Concentrations of O_•* are calculated for a zenith angle of 45* for a variety of conditions, including a dry atmosphere and an atmosphere with 5 ppm I-I•O. Hunten and McElroy [1968] proposed photoionization of molecular oxygen in the metastable •A• state as an important source of O• * ions in the D region. This process was suggested after •A relatively large concentrations of O• • were detected in the upper atmosphere [Evans etal., 1968]. This ionization source could be important between 90 and 70 km, a region where the photoionization of NO by hydrogen Lyman a radiation has generally been considered the primary ionization process. It was previously noted that an additional ionization source was needed in this region to explain both the observed O• * and the total ion densities [Donahue, 1966]. The O•A• ionization could also be important at altitudes lower than 70 km in providing an O• • source capable of initiating a sequence of reactions leading to water vapor cluster ion formation [Ferguson and Fehsen[eld, •A A, which are the ionization thresholds of the O•Ag roetastable and the 02Z,-ground states, respectively. Shorter wavelength radiation is mainly absorbed by ground state 0•, which is the primary source of 0•* at higher altitudes. Since the photoionization rate calculations of Hunten and McElroy [1968] and Swider [1969], several of the input parameters have been improved. The OIA, ionization cross sections have been measured [Clarke and Wayne, 1970a, b], and we now present more detailed results. The solar flux has also been remeasured, leading to a significant decrease in the continuum region observed between strong lines [Hall and Hinteregger, 1...

Photochemistry of Ions at D-region Altitudes of the Ionosphere: A Review

“…ions in the D-region. This process is not observed at any wavelength longer than 111.8 nm, the value of which corresponds to the ionization potential of O 2 ( 1 D g ) (Huffman et al 1971). Only the solar flux at wavelengths from about 103 to 111.8 nm where there are deep absorption minima in the O 2 cross section can be a potential additional source of O 2 ?…”

A study of the enhanced electron concentrations in the mid-latitude D region on winter days in terms of the positive-ion chemistry.

“…It is concerned with the production of water cluster ions from N0+ ions, on the assumption that the production of O2+ for a solar zenith angle of 80 is negligible by comparison. On the basis of the results of HUFFMAN et al (1971), this is justifiable unless the concentration of 02(14g) is greatly enhanced; the small ratio of 02+ to N0+ concentrations observed during a winter anomaly day at S. Uist confirms the small production of 02t Particular attention is paid to the successive hydration of N0+ to produce N0+. (H20)3, as initially proposed by FEHSENFELD and FERGUSON (1969).…”

Uncertainties in Stratospheric–Mesospheric Modeling

“…This appears to be an area which requires further experimental work. Absorption by CO, is also important in the upper mesosphere where CO, shields O,('A) from ionization (24). In addition to solar fluxes and cross sections, the solution of [ I ] requires a fairly detailed knowledge of the reactions involved in the chemistry of n,.…”