SA11A-1105 0800h
The Maunder Minimum Ionosphere
The Maunder Minimum epoch, from 1645 to 1715 A.D. was characterized by a near absence of sunspots. Studies of cosmogenic isotopes and Sun-like stars suggest that solar chromospheric and coronal emissions during this period were significantly lower than contemporary solar minima. To study the effect of such a reduction on the Earth's upper atmosphere, we employ a 1-D global average ionosphere and thermosphere model that accounts for the photon flux between 3 and 360 nm. Within the ionosphere, an unexpected transition occurs as the irradiance falls below normal solar minimum levels. The concentration of O$^{+}$ ions decreases rapidly relative to the other ions, such that NO$^{+}$ becomes the dominant F-region ion. The state of the underlying thermosphere, particularly the neutral gas temperature, is largely responsible for this behavior.
SA11A-1106 0800h
Collisional Removal of O$_2$($b^1\Sigma^+_g$, $v$ = 1) by Atomic Oxygen
In the thermosphere, energy transfer between excited O atoms and ground-state molecular oxygen produces O$_2$ in the first two vibrational levels of the $b^1\Sigma^+_g$ state: O($^1D$) + O$_2$ $\rightarrow$ O($^3P$) + O$_2$($b^1\Sigma^+_g$, $v$ = 0, 1). Subsequent radiative decay of O$_2$($b^1\Sigma^+_g$, $v$ = 0, 1) to the ground state O$_2$($X^3\Sigma^-_g$) results in the Atmospheric Band emissions. Atmospheric observations suggest that above $\sim$120 km O($^3P$) plays an important role in removing O$_2$($b^1\Sigma^+_g$, $v$ = 1). Therefore, knowledge of the rate coefficient for collisional removal of O$_2$($b^1\Sigma^+_g$, $v$ = 1) by O($^3P$) is important for detailed understanding of the Atmospheric Band emissions. Measurements are reported of the room-temperature rate coefficient for removal of O$_2$($b^1\Sigma^+_g$, $v$ = 1) by O($^3P$). A commercial F$_2$ laser with pulsed energy output of up to 50 mJ at 157 nm is used to photodissociate a large fraction of molecular oxygen in a O$_2$/N$_2$ mixture. Photodissociation of an O$_2$ molecule produces a ground-state oxygen atom O($^3P$) and an excited oxygen atom O($^1D$), and O($^1D$) rapidly transfers energy to the remaining O$_2$ to produce O$_2$($b^1\Sigma^+_g$, $v$ = 0, 1). The O$_2$($b^1\Sigma^+_g$, $v$ = 1) population is monitored by observing emission in the O$_2$ ($b-X$) 1--1 band at 771 nm. To extract the O$_2$($b^1\Sigma^+_g$, $v$ = 1) + O($^3P$) rate coefficient, knowledge of the O($^3P$) partial pressure or, equivalently, the fraction of dissociated O$_2$, is necessary. Based on the F$_2$ laser fluence, the signal dependence on the fraction of dissociation, and computer modeling, the signals measured in our experiments correspond to about 50% dissociation. Our measurements yield a preliminary value of the rate coefficient for O$_2$($b^1\Sigma^+_g$, $v$ = 1) removal by O($^3P$) of 6 $\times$ 10$^{-12}$ cm$^3$s$^{-1}$. The results will be compared to the rate coefficients for corresponding processes in the ground and $a^1\Delta_g$ states of O$_2$, and implications of the results for modeling of the upper atmosphere will be discussed. This work is supported by the NSF Aeronomy Program under grant ATM-0209229. The F$_2$ laser was purchased under grant ATM-0216583 from the NSF Major Research Instrumentation Program.
SA11A-1107 0800h
New Measurement of the Rate Coefficient for Three-Body Recombination of Oxygen Atoms in Presence of N$_2$
In the atmospheres of Earth, Venus, and Mars photodissociation of O$_2$ and CO$_2$ produces oxygen atoms that eventually undergo three-body recombination: O + O + M $\rightarrow$ O$_2^*$ + M. The competition between photodissociation, recombination, and diffusive vertical transport controls the atomic and molecular composition of the mesosphere and lower thermosphere. Knowledge of the rate coefficient for recombination of atomic oxygen is essential for modeling atmospheric composition. The most recent measurement of O-atom recombination rate coefficient is over thirty years old [1]. The published values of this rate coefficient have large divergence for both M = O$_2$ and M = N$_2$. For N$_2$ as the third body, the room temperature coefficient varies between about 3 $\times$ 10$^{-33}$ cm$^6$s$^{-1}$, which is the value recommended in the combustion science community, and 5 $\times$ 10$^{-33}$ cm$^6$s$^{-1}$, a value used in the atmospheric modeling community. Previous laboratory investigations [2] of the process O + O + N$_2$ $\rightarrow$ O$_2^*$ + N$_2$ shared the same basic approach, which was to use N$_2$ discharge flow system with NO added downstream to generate O-atoms in the absence of O$_2$ through the reaction N + NO $\rightarrow$ O + N$_2$. This approach is vulnerable to heterogeneous recombination and other processes that may obscure the reaction of interest, mostly due to the low O-atom densities and, consequently, long reaction times. We employ an F$_2$ laser with up to 50 mJ of 157 nm pulsed output to achieve nearly complete photodissociation of molecular oxygen. In a high-pressure (760 Torr) background of N$_2$ the oxygen atoms recombine in a time scale of several milliseconds. Oxygen atom population is monitored by detecting 845-nm fluorescence, which is induced by the 226 nm output of the second laser via a two-photon process O($2p^4$ $^3P$) + $2h\nu$ $\rightarrow$ O($2p^33p$ ^3P$). Our measurements give a preliminary value for the O + O + N$_2$ recombination rate coefficient of approximately 3 $\times$ 10$^{-33}$ cm$^6$s$^{-1}$, which favors the value recommended in the combustion community. Implications of this result for atmospheric modeling will be discussed. This work is supported by the NASA Geospace Sciences Program under grant NAG5-12992. The F$_2$ laser was purchased under grant ATM-0216583 from the NSF Major Research Instrumentation Program. [1] I. M. Campbell and C. N. Gray, Chem. Phys. Lett. 18, 607 (1973). [2] D. L. Baulch, D. D. Drysdale, J. Duxbury, and S. J. Grant, Evaluated Kinetic Data for High Temperature Reactions Vol. 3 (Butterworths, London, 1976).
SA11A-1108 0800h
On the Role of Vibrationally Excited H$_2$ as a Source of OH in the Mesosphere
It has recently been suggested that$^{1}$ the reaction between vibrationally excited H$_2(v=1)$ and O($^3P$) atoms may be an important source of OH in the upper mesosphere. Here, we report accurate quantum calculations of rate coefficients for the O($^3P$)+H$_2(v=1)$ reaction and show that the proposed mechanism is not an important source of OH in the mesosphere$^{2}$. The reaction of vibrationally excited H$_2(v=1-3)$ with oxygen atoms has also been suggested as a source of OH in photon dominated regions of the interstellar medium. We present results for $v=2$ and 3 of the H$_2$ molecule using chemically accurate potential energy surfaces for the O+H$_2$ system and quantum mechanical method for the calculation of the relevant cross sections and rate coefficients. This work was supported by NSF grant No. ATM-0205199. $^1$ L. M. Reynard and D. J. Donaldson, Geophys. Res. Lett. {\bf 28}, 2157 (2001). $^2$ N. Balakrishnan, Geophys. Res. Lett. {\bf 31}, L04106 (2004).
SA11A-1109 0800h
Vibrational Relaxation of OH($v$) in Collisions With O$_2$ Molecules
The reaction between hydrogen atoms and ozone is the single most important source of vibrationally excited hydroxyl radicals near the mesopause. The reaction yields OH molecules in vibrational levels up to $v=9$. The subsequent chemistry of OH depends on its vibrational excitation. Collisions with O$_2$ molecules are considered to be the dominant mechanism of vibrational relaxation of OH in the mesosphere. We have carried out mixed-quantum classical calculations of vibrational relaxation of OH($v=1-6$) in collisions with O$_2$ molecules. We present cross sections and rate coefficient for the vibrational quenching process at temperatures of interest in the mesosphere. This work was supported by NSF grant No. ATM-0205199.
SA11A-1110 0800h
Laboratory Measurements of HOx Radical Rate Constants
Measurements of chemically active species in the mesosphere and upper stratosphere - O3, OH, HO2 - are poorly predicted by model calculations and some rate constant changes for OH + O (1), OH + HO2 (2), and O + HO2 (3) have been proposed as remedies. We have performed a series of laser photolysis experiments on mixtures of O3, N2, O2, and H2O or H2, using laser induced fluorescence measurements of OH or O atom decay rates to provide new determinations of these rate constants. Our method relies upon complete photodissociation of ozone at 248 nm to produce known amounts of O atoms at temperatures of 230 K to 385 K. For (1), we measure k = 1.36 x 10(-11) e (261/T) cm3/molec/s in agreement with recommended values. A second experiment which requires computer modeling of results was designed to be sensitive to k(2)*k(3)/k(1). Initial analysis of decays suggests a 15% increase in k(3) and 15% decrease in k(2) from NASA panel values. These differences are within uncertainties and are insufficient to modify model predictions significantly. Research supported by the NASA Geosciences ITM Program and NSF Aeronomy Program.
SA11A-1111 0800h
CROSS-SECTIONS FOR EXCITED STATE NONADIABATIC THERMOSPHERIC REACTIONS
New methods for treating nonadiabatic excited state reactions are tested using accurate ab initio potentials and nonadiabatic couplings in studies of excited state atom quenching reactions including N($^{2}$D) + O($^{3}$P) \rightarrow N($^{4}$S) + O($^{3}$P) and O($^{1}$D) + O($^{3}$P) \rightarrow O($^{3}$P) + O($^{3}$P). These collision induced electronic transitions (CIET) processes play a crucial role in quenching excited atoms produced by photodissociation, photoionization, and dissociative recombination processes in the thermosphere. Potential energy surfaces and non-adiabatic couplings for these processes computed using the ab initio MOLPRO package will be presented. Energy dependent cross-sections for these CIET processes, obtained from a nonadiabatic mixed quantum-classical surface hopping approach, a nonadiabatic extension to quasiclassical trajectory (QCT) methods, will be compared with the results of exact full quantum close coupled (CC) calculations, to develop an understanding of the conditions under which these nonadiabatic trajectory-based dynamics methods can be reliable. Cross sections calculated for O($^{1}$D) quenching will be compared to currently available values and preliminary computed cross sections for the N($^{2}$D) quenching will be presented.