Highly varying daytime sodium airglow emissions over an equatorial station: a case study based on the measurements using a grating monochromator
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A case study is performed to investigate the probable reasons behind substantial daytime sodium (Na) D1 airglow intensity (589.6 nm) variations measured using a ground-based monochromator during the three near consecutive days of February 2007 from Trivandrum (8.5°N, 77°E), India. The roles of both the resonance fluorescence and the chemistry have been considered in this study. It appears that fluorescence plays only a minor role towards the observed five to nine times of large intensity variations among these days. From investigations on the role of chemistry, it seems that through the Chapman chemical scheme, Na abundance contribute favorably, while the O3 concentrations and the ambient temperature do not play any role as such for the observed intensity variations. From further investigations, it transpires that because of pressure differences (approximately 0.0002 to 0.0003 hPa/day) in the emitting altitude region among these days, the variations in collisional quenching of excited NaO*(A2Σ+) (first excited electronic state of NaO that produces D line) contribute considerably towards the observed intensity variations. From consideration of all the possible factors, it appears that whereas resonance fluorescence plays only a minor role, chemistry has contributed to greater extent towards the observed significant intensity differences among these days.
KeywordsDaytime sodium airglow Resonance fluorescence Collisional quenching Mesospheric chemistry
The atmospheric sodium (Na) layer was discovered in 1929 by Vesto Melvin Slipher, when radiation at 589 nm was observed in the night sky spectrum (Slipher 1929), and later identified as airglow emitted by neutral sodium atoms within the atmosphere (Bernard 1939). It is now well established that the major source of Na in the mesosphere lower thermosphere (MLT) region is the sporadic ablation of the meteorites entering the Earth atmosphere. The neutral/ionic forms of sodium constitute the mesospheric sodium layer that typically extends from 80 to 110 km in altitude and peaks in concentration at around 90 km. Since the discovery of Na layer in the late 1930s, several investigators have studied the origin and variation of this layer and the Na airglow originated therein (Blamont and Donahue 1961; Gibson and Sandford 1972; Kirchhoff and Clemesha 1983; Zahn et al. 1988; Clemesha et al. 1995; She et al. 2000; Fan et al. 2007; Kumar 2007; Sarkhel et al. 2009). However, all these investigations were confined to either the night-time or satellite-based daytime studies. Since daytime ground-based measurements of sodium airglow are particularly difficult to perform, there were no contemporary systematic studies so far. Only recently, a ground-based high-resolution monochromator (with maximum resolution of 0.05 Å using 2,400 g/mm grating) has been used for systematic measurement of the daytime sodium D1 airglow (589.6 nm) (Hossain et al. 2010).
where, ‘RD’ (=ID2/ID1) is the ratio of D2 to D1 line intensity. It is evident from the expression (7) that VER is dependent on the parameters k1, α, and RD in addition to the prevailing [Na], [O3].
The role of the Chapman chemical scheme in producing the Na airglow at the MLT region has been recognized over the years. Plane (2003) had presented a review on the validity of this scheme. However, in the context of explaining the observed Na airglow intensity and its variation, there has been widespread debate over the values of α and RD. Clemesha et al. had estimated the value of α to be within 0.05 to 0.2 with a best estimate of 0.1 based on the rocket‒borne photometry and ground-based lidar measurements (Clemesha et al. 1995). Also, Hecht et al. had found the value of α in between 0.02 and 0.04, even though temperature of the mesopause region on that night was very low (165 K) and the Na layer and the airglow emission layers were peaked around 95 km (Hecht et al. 2000). In contrast to these evaluations, considerably smaller value (less than 0.01) of α was found in an early laboratory measurement (Plane and Husain 1986). The reasons for these disagreement in the values of α have been looked at through a series of laboratory experiments, and it has been found that the earlier laboratory estimate of Plane and Husain was smaller, probably because reaction (1) produces most of the NaO in the (A2Σ+) excited electronic state rather than in the (X2Π) ground state (Shi et al. 1993; Wright et al. 1993), and excited NaO*(A2Σ+) (‘*’ denoting excited state) has a longer radiative lifetime and is not quenched effectively by N2. Hence, reaction (2) should involve mainly the NaO*(A) (Joo et al. 1999). Also, in 2001, through an elegant experiment, Griffin et al. had shown that α for NaO*(A) + O is 0.14 ± 0.04 (Griffin et al. 2001). This indicates that the earlier laboratory estimate of α by Plane and Husain was smaller because it involved the reaction between ground-state NaO(X) and O. Hence, the inconsistency in the values of α is not explainable with the original Chapman mechanism of reactions (1 to 2).
where, M represents primarily the ambient N2 and O2 molecules. The modified Chapman scheme of reactions (8 to 11) assumes that NaO is mostly produced in the excited state NaO*(A) rather than in ground-state NaO(X) following Shi et al. (1993) and Wright et al. (1993). In the modified scheme of reactions (8 to 11), the first possibility is that the excited NaO*(A) produced can be reduced by atomic oxygen to yield the excited Na atoms, which in turn, de-excites to give the airglow emissions. Secondly, NaO*(A) may be converted to NaO(X) through collisional quenching with M before being reacted with O. In a recent paper, Plane et al. (2012) have used a zenith-sky viewing telescope and a Czerny-Turner spectrograph for a series of ground-based long-term measurements of RD at several locations and found that RD varies between 1.5 and 2.0, with an average value of 1.67. These results were interpreted using a statistical model of the Na nightglow which involves initial production of electronically excited NaO*(A) from the reaction between Na and O3, followed either by reaction with O to generate Na(2P J ) with a branching ratio α of 1/6 and a J = 3/2 to 1/2 spin-orbit propensity of 2.0 or quenching of NaO*(A) to NaO(X) by O2. The resulting NaO(X) then reacts with O to generate Na(2P J ) with a branching ratio of 1/6 and a J = 3/2 to 1/2 spin-orbit propensity of 1.5 (Plane et al. 2012). In the modified Chapman scheme, variations in the values of α and RD are suggested to depend on whether NaO*(A) or NaO(X) are primarily available in the concerned altitude region. The value of α is on the higher side when NaO*(A) directly reacts with O (Herschbach et al. 1992; Griffin et al. 2001). But, if NaO*(A) is quenched by M [following reaction (10)] to produce NaO(X) before its reactions with O, the value of α becomes less (Herschbach et al. 1992; Joo et al. 1999). Furthermore, it has been shown by Slanger et al. (2005, 2006) that quenching of NaO*(A) to NaO(X) also affects the value of RD. The reactions (9) and (11) are likely to produce Na(2P1/2) and Na(2P3/2) in different proportions depending on the extent of quenching of NaO*(A) with the ambient gas, hence leading to the variations in RD. Hence, it is evident that the ambient collisional quenching plays important role in deciding the variations in α and RD.
As the supporting evidence for the mechanism proposed by Slanger et al. (2005) owing to the possible effects of collisional quenching on RD, a recent paper by Sarkhel et al. (2009) have suggested that the quenching process due to the ambient gas affects the variations in branching ratio α and RD leading to the observed differences in the Na airglow. Also, in an another related work to study the influence of altitude-dependent collisional quenching on the observed Na airglow intensity variation, Sarkhel et al. (2009) has shown that the Na atom concentration and the Na airglow intensity are well correlated at a particular altitude that is about one scale height higher than the altitude of maximum Na concentration. They also have found through the Na airglow VER estimation using the contribution from the mesospheric ozone and temperature data that the peak emission altitude does not match the altitude of the maximum correlation between the Na atoms and the Na airglow, and hence suggested that the altitude variation of the pressure-dependent collisional quenching needs to be considered, on this occasion, to account for the observed Na airglow intensity variation. In a recent paper, Hossain et al. (2010) has reported that daytime Na airglow exhibits significant variability within a day and from one day to another. Also, it has been shown that the dynamics and the chemistry affect its temporal variation. However, in that study, the roles of the possible factors in controlling the Na airglow emission rate and its highly varying behavior have not been looked into. In this work, we present the highly varying nature of the daytime Na airglow intensities measured using a ground-based high-resolution monochromator during the three near consecutive days, i.e., 9, 13, and 15 February 2007 on a case study basis. Here, we have considered the roles of the resonance fluorescence through Na atom concentration and solar radiation, and the chemistry, in terms of the concerned ambient species, and the relevant parameters including α and RD for the qualitative estimation of VER of Na airglow to find out the possible reasons behind considerable intensity variations among these days. Variations of α and RD has been considered in line with the suggestion given by Sarkhel et al. (2009). From the detailed analysis, it appears that whereas resonance fluorescence plays only a minor role, it is the chemistry that plays larger role in controlling the intensity variations among the mentioned days. This work discusses these aspects in detail.
Instruments and data used
Along with the monochromator, data from a collocated all sky interferometric meteor (SKiYMET) radar (Mardoc Inc., London, Ontario, Canada; Genesis Software Pty. Ltd, North Adelaide, South Australia, Australia) has also been used to get the information about the abundance of Na. This radar operates at a frequency of 35.25 MHz and provides hourly mean meteor counts, neutral winds, and temperature in the altitude range of 80 to 110 km with a height resolution of 3 km (Kumar et al. 2007a). Also, daytime OH airglow emission intensity at 731.6 nm wavelength (that originates from about the same altitude region as the Na airglow) measured using the collocated multi-wavelength dayglow photometer (MWDPM) was used to derive the strength of the gravity waves activity. Details on the MWDPM instrumentation and the dayglow data acquisition/analysis can be found in Sridharan et al. 1998. Further, the concentration of [O3] and temperature data obtained from the Microwave Limb Sounder (MLS) instrument onboard the Aura satellite of NASA, USA has also been used (Waters et al. 2006). The pressure data used in this study has been taken from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard TIMED satellite (Remsberg et al. 2008). The MLS and SABER data available for the locations nearest to and the Na abundance information over the region of study have been used for the qualitative evaluation of the VER of the Na airglow. Further, in order to check the variation of solar flux, the F10.7-cm radio flux data, taken from the Space Physics Interactive data resource (http://spidr.ngdc.noaa.gov/spidr/), has also been looked into.
Methodology of investigation
To study the rationale behind the highly varying nature of the daytime Na airglow intensity on the three near-consecutive days of February 2007, the roles of resonance fluorescence and chemistry in the MLT region have been considered. Since the relevant mesospheric fluorescence depends on the available concentration of neutral Na atom and the prevailing solar radiation, the information on the daytime Na abundance and solar flux variations on these days have been used to investigate qualitatively the role of fluorescence on the observed intensity variations. Then to investigate the role of chemistry, information on Na abundance, O3 altitude profiles, and altitude profiles of temperature and pressure have been used to qualitatively appraise the variations of the VER of Na airglow among these days.
Results and discussions
However, since the direct quantitative measurements of mesospheric Na concentration were not available over or nearest to the region of study and also the quantitative detailed data on the dependence of α and RD on pressure were not available in the literature, contributions of Na abundance and the pressure-dependent collisional quenching towards the resonance fluorescence/chemistry could not be quantitatively established. Nonetheless, taking into considerations of all the possible factors, it emerges that whereas resonance fluorescence, through only the Na abundance variations, plays only a minor role, the chemistry through the Na abundance variations and the variations in the collisional quenching of NaO (A) due to significant pressure differences, has contributed to greater extent towards the observed significant intensity differences among these days. This work supports the observations made by Sarkhel et al. (2009) and reaffirms the role of collisional quenching towards controlling the Na airglow intensity variations.
In the present work, in the absence of direct Na atom density measurement, meteor data has been used as the indicator of the Na abundances, and it is seen that the meteor counts are consistent with the airglow intensities during the mentioned 3 days. It is known that although the sources of neutral Na atoms are meteors and cosmic dust, the day-to-day variation of Na atoms in the mesosphere is mostly controlled by the dynamics (tides and gravity waves). Hence, as per the results of the present work, it seems that dynamics did not cause the day-to-day variation of Na density significantly on those days. However, using the wavelet analysis and the tidal analysis, as shown early in the results, it seems that the dynamics might have played a role in controlling the day-to-day Na density variation on those days, but its role could not be clearly established.
A ground-based 1-m scanning monochromator has been used to measure the significant variations in daytime sodium D1 airglow intensity (589.6 nm) from Trivandrum (8.5°N, 77°E), a near-equatorial station in India during the three near-consecutive days of February 2007. The possible reasons for the significant intensity differences have been investigated through considerations of the roles of both the resonance fluorescence and the mesospheric chemistry. From deliberations of all the possible factors, it appears that whereas resonance fluorescence, through only the Na abundance variations, plays only a minor role, its the chemistry, through the Na abundance variations and the variations in the collisional quenching of excited NaO (A) due to significant pressure differences, that has played greater role towards the observed significant intensity differences among these days. However, since the direct quantitative measurements of mesospheric Na concentration were not available and also the quantitative detailed data on the dependence of α and RD on pressure were not available in the literature, contributions of resonance fluorescence and chemistry could not be quantitatively established. This work supports the inference made by Sarkhel et al. (2009) that the collisional quenching controls the Na airglow intensity variability through the variations of α and RD.
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