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Nitric Oxides in the Mesosphere

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Background

A major source of nitric oxide NOx (= NO + NO2) in the high latitude mesosphere and thermosphere is through Energetic Particle Precipitation (EPP). NOx in the MLT is produced via a cascade of dissociation, ionization, and recombination processes, which are described in, e.g., Thorne [1980] and [Rusch et al., 1981]. This is known to occur through two separate mechanisms: The first mechanism is by direct particle impact that has the ultimate effect of dissociating nitrogen molecules into excited nitrogen atoms: NOx is produced by dissociation of molecular nitrogen by energetic particles and solar photons, in particular auroral and precipitating electrons from the outer trapping region of the magnetosphere, and a subsequent reaction of N(4S,2D) with oxygen to form NO. NO can be produced particularly by current induced ion-neutral collisions known as Joule heating. This may be important in the ~140–200 km range, where ground state atomic nitrogen reacting with molecular oxygen becomes an important source of NO [Solomon et al., 1999]. The second mechanism is through heating that accelerates the chemical reactions that create NO from those atoms: The chemical reaction between nitrogen atoms (ground state) and molecular oxygen that produces nitric oxide at MLT altitudes, and in particular at the peak of the NOx layer, at ~110 km, is very sensitive to temperature. In particular, during Joule heating events, the temperature of the thermosphere may increase by 50 to 100 K leading to an increase in the nitric oxide density at this altitude by a factor of two. Thus the density of nitric oxide may be used as an indicator of Joule heating in the thermosphere [Barth et al., 2005]. The theoretical relationship between the aurora and NO has been investigated [e.g., Siskind et al., 1989] and measurements by AE [Cravens and Stewart, 1978], SME [Barth, 1992; 1996], HALOE [Siskind et al., 1998], and MAHRSI Stevens et al., 1997] have demonstrated the high-latitude enhancement of NO. Figure 1.44 shows simulation results of the expected response of Nitric Oxide number density following the EUV flare on the Bastille Day event, using a coupled thermosphere ionosphere model:


Figure1.44: Simulation of the height/time profiles of the expected response of NOx number density at low latitude on the dayside of the Earth to the EUV flare on the Bastille Day event using a coupled thermosphere ionosphere model [After Fuller-Rowell et al., 2004].
Figure1.44: Simulation of the height/time profiles of the expected response of NOx number density at low latitude on the dayside of the Earth to the EUV flare on the Bastille Day event using a coupled thermosphere ionosphere model [After Fuller-Rowell et al., 2004].


Transport of Nitric Oxide


As described above, Energetic Particle Precipitation (EPP) in the MLT region can produce nitric oxide (NOx = NO + NO2) in the mesosphere and thermosphere: NOx is produced by dissociation of molecular nitrogen by solar photons and energetic particles, in particular auroral and precipitating electrons from the outer trapping region of the magnetosphere, and subsequent reaction of N(4S,2D) with oxygen to form NO. NOx enhancements have been measured by the MIPAS, LIMS and HALOE instruments and have been modelled by Chemical Transport Models, such as the TOPOZ III, 2005 model. From these measurements it was found that, depending on solar and geomagnetic activity, maximum thermospheric NO VMRs can vary from 100 up to 2000 ppmv, which, in terms of number densities, becomes comparable to stratospheric NOx abundances [Funke et al., 2005]. It is known that in the middle mesosphere and above NOx has a lifetime of days or less, as it is destroyed by photodissociation and recombination in processes that where described in more detail above; however, in particular during the polar winter when sunlight duration is short, throughout the mesosphere and into the thermosphere lifetimes are long enough that it is speculated that NOx can descent to the lower mesosphere without being photochemicaly destroyed [Solomon et al., 1982; and Frederick and Orsini 1982], even though the magnitude and variability of this process is not known. Once NO reaches the lower mesosphere, its lifetime becomes longer because of less photodissociation, due to absorption in the O2 Schumann-Runge band, and self-absorption in the δ band [Minschwaner and Siskind, 1993]; therefore in the lower mesosphere there is sufficient time for the NOx to descend to the stratosphere, where it has a lifetime as long as a year [Solomon et al., 1982]. Once in the stratosphere, NOx can participate in the catalytic processes controlling ozone. This mechanism for coupling the upper and lower atmosphere has been referred to as the EPP Indirect Effect [Randall et al., 2007]: it was first proposed through modeling using a 2D model [Solomon et al., 1982; Brasseur et al., 1984], and has since been observed many times in connection to Solar Proton Events (SPEs) [e.g., Callis et al., 1996, 1998; Siskind et al., 2000a; Jackman et al., 2001; Randall et al., 2001; Randall et al., 2005]. However the degree to which NOx descent depends on varying EPP and/or varying solar UV flux is still a controversial issue [e.g., Rozanov et al., 2005], which needs satellite measurements of NOy, O3, and related parameters covering the high latitude stratosphere to the thermosphere throughout the polar winter in order to be conclusively resolved.

Destruction of Nitric Oxide


A pronounced minimum is observed in NOx in the mesosphere due to NO destruction caused by the following reactions, which reduce the chemical lifetime of NO:

NO + hν -> N + O

NO + N -> N2 + O

The time constant for photodissociation of NO in the lower thermosphere has been calculated to be about 2 days under nominal sunlight [Minschwaner and Siskind, 1993], which is shorter than the time for transport to the stratosphere due to vertical eddy diffusion, which is thought to last for several days. However during polar winter, when sunlit duration is short and photodissociation is slow, NO produced in the lower thermosphere by auroral particle precipitation is expected to last long enough to pass through the mesosphere and to reach the stratosphere. The fact that NO is more abundant in the lower thermosphere at high latitudes than at lower latitudes has been confirmed since early on: Several satellite measurements such as the Atmosphere Explorers C and D [e.g. Rusch and Barth, 1975] found 3 to 10 times larger NO abundance with high variability in the lower thermosphere at high latitudes than at lower latitudes. However, there is only limited experimental evidence for downward transport of NO during polar winter. This is because those satellite measurements did not detect NO densities below 100 km, and just a few rocket measurements in the mesosphere have been performed at high latitudes. If NO succeeds in reaching the lower mesosphere, its lifetime becomes longer because of less photodissociation, due to absorption in the O2 Schumann-Runge band, and self-absorption in the δ band [Minschwaner and Siskind, 1993]; further down in the stratosphere, NOx has a lifetime as long as a year [Solomon et al., 1982].


Measurements of Nitric Oxide


NOx can be measured by various methods and various instruments have provided a wealth of information:

(i) Rocket Measurements of NOx: Nitric Oxides have been measured in-situ by rockets: for example, Iwagami and Ogawa [1980] used a rocket-born γ band gas-correlation radiometer in austral winter at Syowa (69°S), and found NO densities of about 1 × 108 cm–3 in the 70–110 km region. Horvath and Frederick [1985] using a rocket-born chemiluminescence sensor at Poker Flat (65°N) found NO mixing ratios more than 100 ppbv at 52 km (1.7 × 109 cm–3) and their steep positive gradient above 50 km in winter. [Iwagami et al. [1998] using two-rocket measurements, found NO densities in the 70–90 km region produced by auroral precipitation to be one to two orders of magnitude larger than those at middle latitudes. They also found the influence on ozone densities in the 70–90 km region due to such enhanced nitric oxide abundance to be still insignificant as compared to that due to transport in the middle of February, one month after the end of polar night and one month before the spring equinox. They concluded that larger ozone densities found in February (in spite of longer sunlit duration) than in November in the 40–60 km region again support predominance of transport over photochemical destruction.

(ii) HALOE Measurements of NOx: Measurements from HALOE on UARS have validated the hypothesis that large amounts of NO existing above 90 km can be transported to the polar stratosphere in winter. HALOE data have improved our knowledge of NO in the MLT and the stratosphere and aspects of their coupling; however, they suffer from lack of global coverage: Because of the 57 deg inclination of UARS and the requirement for sunlight, polar latitudes are never sampled in winter. The highest latitude sampled in winter is around 50-55 deg N; this is an important constraint of the dataset, as the highest NO densities are predicted to occur in winter northward of 60 deg N.

Figure 1.45: Average of HALOE zonal mean nitric oxide mixing ratios (ppbv) for 1992-1996 for June- August. [After Siskind, 2000a]
Figure 1.45: Average of HALOE zonal mean nitric oxide mixing ratios (ppbv) for 1992-1996 for June- August. [After Siskind, 2000a]

(iii) ENVISAT/MIPAS and TIMED/SABER Measurements of NOx: The synergistic findings of two different satellite instruments of the NO 5.3 μm emissions in the thermosphere were studied and compared in the study performed by Gardner et al., 2007. Agreement between the NO emissions found was very good, within 25% over the entire latitude range from -58° to + 4°N from the MIPAS instrument on board ENVISAT satellite and the SABER instrument on board TIMED. The agreement was found to be quite satisfactory using a non-Local Thermodynamic Equilibrium radiative transfer model for the NO energy distributions in the thermosphere except that the model kinetic temperatures, based on the NRLMSISE-00 empirical model, were found to be low near 110 km by approximately 50–100 K for the locations investigated. From the MIPAS high spectral resolution measurements a set of correction factors were derived and applied to the SABER data in order to calculate the total volume emission rates as well as the densities of NO(v = 1). The NO 5.3 mm night-time emissions were found to be greatest at high latitudes, to decrease at mid-latitudes, and to increase again near the equator. The peak NO volume emission rates were observed between 120 and 130 km altitude. A feasibility study was also conducted to investigate the use of the NO 5.3 μm emission data to derive NO(v = 0) densities in the thermosphere under night-time conditions using a simplified steady state expression and using non-LTE modeling of the NO vibrational energy distributions. The results, shown here in Figure 1.46 indicate that NO(v = 0) densities can be accurately retrieved from NO 5.3 μm mission data if there are simultaneous measurements of the kinetic temperatures and the O atom concentrations.


Figure 1.46: Comparison of [NO(v = 0)] densities calculated from the SABER NO 5.3 μm data using a steady state expression (solid lines) with [NO(v= 0)] densities calculated from the SABER NO 5.3 μm data using non-LTE model vibrational distributions (dashed lines) as a function of latitude. After Gardner et al., 2007.
Figure 1.46: Comparison of [NO(v = 0)] densities calculated from the SABER NO 5.3 μm data using a steady state expression (solid lines) with [NO(v= 0)] densities calculated from the SABER NO 5.3 μm data using non-LTE model vibrational distributions (dashed lines) as a function of latitude. After Gardner et al., 2007.


(iv) SME Measurements of NOx: The hypothesis that the variation in the density of low latitude nitric oxide at 110 km is caused by the variation in the solar output of soft X-rays in the wavelength range 2–10 nm and that the solar soft X-rays vary with a greater amplitude than does the solar extreme ultraviolet radiation was first proposed by Siskind et al. [1990] and the evidence for this hypothesis came from three years of observations of thermospheric nitric oxide from the Solar Mesosphere Explorer [Barth et al., 1998]. The SME observations showed that the nitric oxide density at low latitudes varies with the 27-day solar rotation period and with the 11-year solar cycle. The variation of nitric oxide correlates with the solar 10.7 cm radio flux which is a solar index measured from the ground. The correlation is due to the partial ability of the 10.7 cm flux to track solar EUV and soft X-rays. As shown in Figure 1.47, Nitric oxide (NO) has a maximum density of about 3x107 cm-3 near 110 km whereas in the polar region the mean density is several times greater and highly variable, sometimes as much as 10 times larger. However, Figure 1.48 shows that the solar 10.7 cm flux is an imperfect index of the solar radiation that is causing the changes in nitric oxide density.

Figure 1.47:Latitudinal distribution of nitric oxide. Contour plot of the average nitric oxide density shows that the maximum density occurs in the auroral region.
Figure 1.47:Latitudinal distribution of nitric oxide. Contour plot of the average nitric oxide density shows that the maximum density occurs in the auroral region.


Figure 1.48:Variation of nitric oxide with solar activity. The nitric oxide density at 110 km is plotted as a function of the solar 10.7 cm flux which is an index of solar activity.
Figure 1.48:Variation of nitric oxide with solar activity. The nitric oxide density at 110 km is plotted as a function of the solar 10.7 cm flux which is an index of solar activity.


(v) SNOE Measurements of NOx: Nitric Oxides were the primary focus of the SNOE satellite [Solomon et al., 1996], which performed simultaneous observation of nitric oxide in the lower thermosphere, the solar irradiance in the soft X-ray region of the spectrum, and ultraviolet emissions from the auroral zone, in order to show the functional relationship between nitric oxide density and solar variation, used to test and revise photochemical models, and in addition to determine how auroral activity produces increased nitric oxide near the poles. The second objective was accomplished by measuring the intensity of the ultraviolet aurora in the 130–180 nm region, which includes atomic oxygen emission lines and the Lyman-Birge-Hopfield bands of N2. The relationship between the auroral region nitric oxide density and the time history of auroral intensity was used to determine if bombardment by auroral particles is the dominant process producing polar nitric oxide.

(vi) GOMOS Measurements of NOx: NOx has been measured by solar occultation, by GOMOS on ENVISAT. With this method, NO2 and NO3 vertical profiles up to around 85 km altitude have been obtained [Hauchecorne et al., 2005]. As an example, resent results from GOMOS, shown in Figure 1.49, show the NO2 mixing ratios up to the Mesosphere. Regarding the region from 60-70 km, NO2 is involved in a number of processes, but is currently under-sampled.




References

Figure 1.49: Zonal and yearly median NO2 mix ratio (left) and mixing ratio as a function of time MLT (right).


References
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