Evening twilightglow of sodium 5893Å line emission at Calcutta and its typical relation with astronomical parameters

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1 Indian Journal of Radio & Space Physics Vol. 35, April 2006, pp Evening twilightglow of sodium 5893Å line emission at Calcutta and its typical relation with astronomical parameters S K Midya 1,2 & R Chattopadhyay 2,3 1 Department of Physics, Serampore College, Serampore Hooghly, West Bengal, India 2 Centre for Space Physics, 43 Chalantika, Garia Station Road, Kolkata , India 3 Haripal G D Institution, Khamarchandi, , Hooghly,West Bengal, India Received 19 April 2004; received 22 March 2005; accepted 22 February 2006 Sodium line (5893 Å) represents one of the most important airglow emission lines. The photometric observations of evening twilightglow for this line were taken at Narendrapur (Lat ' Long 'E), a town in extended Calcutta, during the period between 1983 and Two distinctly recognizable enhancements or peaks were found to exist in timeversus-emission-intensity curve in evening twilight. One enhancement around sunset and the other a few minutes later were found to be a regular feature in everyday observation. On plotting the time of occurrence of the first peak relative to the sunset time for a complete month we obtained a sinusoidal curve having its troughs and crests coinciding with the different phases of the moon. This observation shows that the time of occurrence of the peak depends on the moon's age. Keywords: Twilightglow; Sodium 5893Å line emission; Airglow emission lines PACS No: 94.10Q, Introduction In a dark moonless night at a place away from city light one observes that certain amount of light comes from above. Excluding galactic stellar radiations, Zodiacal radiations and atmospheric scattering one observes that the remaining part of the light coming from space is only 40% of total light flux and that is produced, as is well known now-a-days, by selfluminescence of the upper atmosphere. This is night air-glow. Sodium 5893 Å line is one of the most important among a few emission lines that are usually regularly observed by airglow observer during day, night and twilight. Enhancement of emission intensity is again a very noteworthy feature for airglow observation, which usually is observed around twilight period. Aburjania et al. 1 mentions that OI6300 Å line intensity had been observed to enhance 2-3 h before the start of local twilight and increases within an hour up to twice its original value. It has also been experimentally shown by them that about 65% of 6300 Å line intensity is explained by the direct collision of oxygen atoms with photoelectrons that are produced in the magneto-conjugate ionosphere. Rest of the enhanced emission for that particular line remains unaccounted for. Shefov 2 mentions that OH-band emission, which too is another important airglow emission, shows similar enhancement at around twilight period. Until now several authors have dealt with such enhancements mainly for the above-mentioned airglow emissions. But there are very few observations, still reported, regarding the enhancement of sodium 5893 Å line emission intensity. This enhancement of Na 5893 Å lineintensity has been observed very sharply and a very typical relationship between the time of occurrence of the first peak emission intensity and the local twilight time (i.e. time for actual sunset) has been obtained and reported in this paper. As a consequence of such observations of ours some very interesting relation of this variation with the moon's age and with some significant astronomical parameters has been found. 2 Experimental arrangement A Dunn-Manring type of photometer (Fig. 1) was set up to observe the Na 5893 Å twilight airglowintensity variation 3. The flux of incident light was allowed to pass through a band-pass filter and then to fall on the cathode of a photomultiplier tube. Integrated over a fraction of a second the incident photo flux produced a measurable photocurrent,

2 78 INDIAN J RADIO & SPACE PHYS, APRIL 2006 Fig. 1 Block diagram showing the schematic sketch of the instrument used for the observation which was recorded by a nano-ammeter. This photocurrent, due to its quantum nature, is obviously directly proportional to the intensity of incident radiation; and that is why variation in the photocurrent with time gives one the variation of intensity of incident light flux. 3 Observations Photoelectric measurements corresponding to the intensity of oxygen green line (5577 Å) and sodium 5893 Å line, were made. Observations of intensity enhancement of these lines were selectively few for oxygen green line and sufficient enough for Na 5893 Å line of evening twilight airglow 4. In case of 5577 Å, oxygen green line enhancement was found to begin after a few minutes from the local sunset time. 4 Some similar observation was taken by Srivastava et al. 5 from Allahabad. They reported that the time of occurrence of the enhancement varies with dates of a particular month. In case of Na 5893 Å line-intensity variation, two prominent peaks were found to occur during each evening-twilight. A few observations are presented for reference (Fig. 2). It is observed that the first enhancement reaches a maximum which is much higher than that reached by the second enhancement during each evening-twilight. Again the time of occurrence of second enhancement is found to occur always after sunset, while the first enhancement occurs alternately before and after the local sunset time, depending on the moon's age at sharp. Making the local sunset time-line as the reference axis, the time of occurrence of the first maximum enhancement has been plotted along y-axis, while the days are presented along x-axis, i.e. the sunset time-line axis. It is then observed that the mean curve passing through the plotted points is a sinusoidal curve having a period of about a month (Fig. 3). Moreover it is observed that, on an average the amplitude of variation is approximately 5 min and the maximum enhancement occurs 5 min after sunset on a New moon day, while the same thing occurs 5 min before sunset on a Full moon day. The time of occurrence of the second peak has also been found to be oscillatory in nature but with almost insignificant amplitude of less than a minute with respect to a mean time-axis, which is approximately min late from the local sunset time-line.

3 MIDYA & CHATTOPADHYAY: TWILIGHTGLOW OF Na 5893 Å AT CALCUTTA 79 Fig. 2 Graphical plot of photocurrent intensity corresponding to Na 5893Å twilightglow intensity against time showing two distinct peaks Fig. 3 Times of occurrences of 1 st and 2 nd peaks respectively, with respect to local sunset time plotted against the day of observation 4 Discussion There are a number of possible features that may, either singly or collectively, be responsible for the interrelation between the airglow enhancement and the moon's age. The features are as follows: (i) Effect of background moon light on airglow observation. Background moon light again are produced in the following two ways: (a) Lunar radiation (b) Reflection from moon's surface (ii) L-variation and tidal effect (iii) Gravity wave, SNE, Plasma bubble and Vertical wind distribution. (iv) Effect of magnetotail of Earth that extends beyond the moon and reversal of electric field and related E B drift. Measurements of weak emission lines at night are appreciably affected by moonlight in presence of moon in the sky above horizon. Measurement of oxygen red line emission on nights with moon of age less than 4.7 days or more than 24.8 days is little affected by moon light for D > 30 and similarly affected with moon of age 6 less than 5-7 days or more than 23.8 days for D > 50. Rays of light that come from the moon are produced mainly in two ways 7 as follows: (i) Approximately 7% of the incident light is reflected from the surface of the moon. (ii) As the temperature of lunar surface varies approximately from 170 to +130 C, radiations are emitted from the surface of the moon. According to Warwick 8, Mayer has found that lunar thermal emission does occur to an appreciable amount. Lunar surface radiations may include the oxygen red line emission. Atomic oxygen is produced on the lunar surface mainly from the sputtering and desorption of solar wind induced by absorption of solar photon. The calculated source rate is at least three orders of magnitude larger than the expected value of the same. In order to explain that discrepancy, Morgan et al. 9, proposed that low energy chemistry may affect accommodation rates

4 80 INDIAN J RADIO & SPACE PHYS, APRIL 2006 through the desorption of hot O atoms coupled with an additional loss process of charge transfer on the lunar dark side to form O. They also report that the moon has sodium exosphere, inferred from observations above the sunlit limb of the moon. Their calculations show that lunar atmosphere may contain approximately 50 cm -3 sodium atom, which by resonance scattering of sunlight emits Na-I radiation. Mostly derived from meteorite vaporization of the regolith the lunar atmospheric sodium has a high loss rate. Lunar radiations generally fade out 10 in the following two different ways: (i) Short period fading of lunar radiation caused by scintillation, which is proportional to libration rate of the moon. (ii) Long period fading of lunar radiation is presumed to be caused by ionospheric irregularities associated with absorption, Doppler effect and Faraday effect within the atmosphere. It is observed from the analysis of the daily records of h', h max and f o F 2 that their values oscillate with solar and lunar tides. According to the electrodynamical theory developed by Martyr, many observed anomalies in the F 2 -region are traceable to the effects of the electrodynamical action 11. Lunar tidal effects on the ionosphere, though of small amplitude, are free from complications. The variations of the magnetic elements contain a part which follows the lunar day and is called the lunar daily magnetic variation L. The curve representing this monthly mean L is a double sine wave 11. L-variation of magnetic field has been found to be very high at New moon phase at equinox and summer solistice. The L-variation is governed by Lunar time. The electric current responsible for L have intensity 12 on the sunlit side of the Earth, usually one-tenth of those for S q (solar daily magnetic variation). Semi-diurnal barometric variation due to the sun was found to be very much greater than that of the moon. But tidal force due to the moon is approximately twice that of the sun. According to Kelvin the period of free atmospheric oscillation being approximately 12 h magnifies solar semidiurnal tide by resonance 13. That's why although the amplitude of solar tidal variation of ionosphere is larger than those caused by the moon, lunar effects can more readily be determined. Lunar tidal variations have been observed in D, E, F 1 and F 2 regions of the ionosphere. Fesen 14 mentions that tidal effects on the ionosphere appear quite likely, because the neutral meridional winds are influenced by the tides. His calculations, based on purely solar model simulated function, show that η max increases rapidly at local sunrise and then building gradually to the largest densities at around 1600 hrs LT, decreases during the night to the lowest value just before sunrise. In tidal case η max is typically depressed relative to the solar values. From theoretical simulations, h max and η max are predicted to be more strongly perturbed by the tides than by geomagnetic activity during solar cycle minimum and therefore similar tidal force-induced perturbation may contribute to the observed daily variability in the peak electron-density and its height. Values of ( I/I) has been observed 14 to vary with moon's age having a maximum of 30%. Lunar tides leave signature in the upper atmosphere, which is revealed by the characteristic of the variation of atmospheric emission in the mesopause region. According to Sobral et al. 15 the gravity waves modulate the zenith OI 6300 Å nightglow emission rate. But observation utilizing this feature is scarce. Again South-to-North propagating waves like perturbation events (SNE), as Sobral et al. 15 mention, takes place during the early post-sunset hours. As Batista mentions 16, the vertical wind has been found to be the most important factor that determines the amplitude of the oscillation in sodium density at a fixed height. Therefore TEC enhancement too may have a dependence on the vertical wind speed distribution, which indirectly can affect the OI 6300 Å nightglow intensity. The Earth on its night side, has a magnetotail 17 extended beyond the lunar orbit. The dynamical processes within the magnetosphere are primarily governed by the flow of the solar wind, which ultimately gives rise to magnetospheric convection. Kolomiitsev et al. 18 mentions that a cavity of low electron density arises in the equatorial ionosphere at altitude between the peak of the night F-layer. Anomalous post-sunset enhancements in TEC have been observed during the period of low solar activity at the equatorial station Otackamund 19. It has been established on the basis of analysis of simultaneous observation of TEC at different stations during winter and equinoctial months that these enhancements are produced mainly due to the reversal of the E B drift at the equator along with a reversal of the equatorial fountain. According to Saha 20, in the Appleton

5 MIDYA & CHATTOPADHYAY: TWILIGHTGLOW OF Na 5893 Å AT CALCUTTA 81 anomally region the electric field is reversed after the sunset and consequently ionization process moves toward equator. As a result of this the recombination rate increases there, which gives rise to an enhancement of OI 6300 Å line emission intensity mainly on the quiet night. But Fig. 4 shows that there is no such enhancement observed in the twilight OI 6300 Å airglow intensity. This fact ensures that the E- field reversal produces little effect on the variation of airglow intensity directly. It has been established from different points of consideration including the most important one, i.e. the polarization of the line that the twilight enhancement in the NaI airglow is primarily 12, 21 caused by resonance scattering of sun light Na( 2 S) + hν(λ5893å) Na( 2 P) Na( 2 P) Na( 2 S) + hν(λ5893å) As this process does not involve any electrons or ions, photoelectrons can produce hardly any effect on it, photo dissociations and photo chemical reactions play any role only in cases of Day- and Night-NaI (5893 Å) airglow line emissions. Fig. 4 Intensity variation of 6300Å line during evening twilight at Calcutta The above process of resonance scattering depends only on the number density of free sodium atom at around its peak height of emission, i.e. at around 90 km. The maximum density of free Na has been found to lie below cm 3 at that height 22. Atomic sodium line (5893 Å) shows major twilight enhancement having a strong seasonal and diurnal variation characteristic. Atmospheric density of atomic sodium has been semi-empirically established to be altered due to input from extraterrestrial origin and not due to terrestrial origin 21. Twilight enhancement of NaI (5893 Å) airglow line intensity has generally been found to be about 15 times the night-time intensity. Again an East-West asymmetry in the distribution of atomic sodium has also been reported in general from some observing stations throughout the world, showing an average ratio of it in the West to that of East equal to 1.4. At around the time of sunset when the sun goes down by 6 o from horizon, the exciting sunlight suffers negligible extinction by the lower atmosphere, while large angles of solar depression the shadow of the Earth begins to affect the twilight airglow intensity. In the twilight time the NaI (5893 Å) airglow intensity first rise proportionately with the rise of number density of atomic sodium, but after reaching a peak the intensity falls down, because then the layer becomes optically thick enough to appreciably extinct the emerging fluxes of resonance lines. The enhancement peak has also been hypothesized and verified to be associated with multiple scattering 12. It is, therefore, being observed that the NaI (5893 Å) airglow line enhancement peak in the twilight occurs when the sodium layer within the atmosphere becomes optically thin. Optical thickness of such a layer within the atmosphere may certainly have bearing on the conjugate action of solar and lunar tidal forces. While this conjugate tidal force is greater, the atmosphere remains thinner around the middle of East and West over the surface of Earth for a while even after local sunset at the observing station. Similarly this conjugate tidal force being decreased (at time of Full moon) the atmosphere becomes relatively thick and optical thickness of each layer gets increased even before the sunset time on the side of observation. 5 Conclusion Faded out lunar radiations are effectively insignificant to produce any realizable change in the ionization rate within terrestrial ionosphere, which is

6 82 INDIAN J RADIO & SPACE PHYS, APRIL 2006 Fig. 5 Variation of 5577Å line intensity during evening twilight at Calcutta also indicated by the fact that in our experimental curve we see the enhancement peak to occur much before sunset in Full-moon phase than in other phases. Again the amplitude of variation of occurrencetime of enhancement peak before and after the sunset is relatively small enough for being considered to be caused by the perturbations due to the gravity-wave, SNE event, and plasma bubble. Moreover existence of an exact regularity in the pattern of variation of time for enhancement peak before and after sunset (Fig. 3) rules out any relation of that variation with the gravity-wave, SNE and plasma bubble, because those sorts of perturbation are highly irregular in a small scale of time interval as that of our experimental result. Spring tide occurs both at New-moon and Fullmoon phase, while neap tide occurs at intermediate phases. Hence tidal effect on terrestrial atmosphere in general, is expected to be maximum at New-moon and Full-moon phases of the moon. Our experimental curve shows that though maximum interval between the occurrences of sunset and corresponding Na 5893Å twilightglow enhancement peak (1st peak) occurs at New-moon and Full-moon phases, it occurs before sunset in Fullmoon phase and after sunset in New-moon phase. The reversal of electric field along with the equatorial fountain occurs sharply a little while after the sunset. Conjugate Solar-Lunar-tidal effect along with lunar daily magnetic variation strongly influence the reversal of electric field, which may probably influence through polarization of NaI line emissions produced from resonance scattering of Na, because the lunar magnetic variation shows a sinusoidal pattern in lunar time. Rao and Kulkarni 23 obtained a pretty good correlation between the seasonal variations of 5577 Å and 5893 Å airglow line emissions. Ghosh and Midya 4 obtained twilight enhancement of 5577 Å line intensity (Fig. 5), which may be useful in understanding the variation of intensity of NaI 5893 Å emission. Hence more detailed work on experimental search for an exact comparison between these two may help single out the exact cause of the type of lunar phase dependent observation as reported in this paper. There is a need to formulate a precise mathematical basis for explaining this result of our experiment reported in this paper. Acknowledgement The authors gratefully acknowledge the kind help offered by the Principal, Ramakrishna Mission Residential College, Narendrapur , WB, India in allowing them to take observations from their College premises. References 1 Aburjania G D, Machabeli G Z & Nanobashivili I S, Super thermal electron flux generation and optical airglow enhancement in mid latitude, Proceedings of the 2000 International Symposium on antennas and propagation (ISA- 2000), Fukuoka, Japan Aug [TOKYO, JAPAN, IEICE of Japan]. 2 Shefov N N, On the equilibrium nature of the rotational temperature of hydroxyl airglow, Planet Space Sci (Ireland), 25 (1977) Ghosh S N, Midya S K & Purkait S, Airglow, Mahavisva (Journal of Indian Astronomical Society, Calcutta) 1 & 2 (Combined) (1982) Ghosh S N & Midya S K, Evening twilight enhancements of airglow emission at Calcutta and covariation of emissions, Indian J Radio & Space Phys, 16 (1987) Srivastava N, Srivastava S, Dixit S D & Srivastava A N, Evening and morning twilight enhancement of λ 5577Å

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