Morning Twilight Research Articles

Twilight observation by the naked eye of the dawn sincere at Hail and other areas in Saudi Arabia

Naked eye observations of morning twilight phenomenon at Hail and other areas of Saudi Arabia are recorded. During the interval from 2014 to 2015, about 80 morning twilight observations were carried out in Hail at longitude λ = 41°42′E and latitude φ = 27°31′N for a desert background. The phenomena was followed over azimuth angles ranged from 0° to 20° of solar vertical direction and from 0° to 10° along the altitudinal range. By selecting 32 days with a very good visibility, it was found that Sun depression, Do, lies in a range between 13.48° and 14.69° with an average of 14.014° ±0.317. The difference between our obtained value and that one which is currently applicable in Saudi Arabia is about 4°. The results indicate that dawn (white thread browser) occurs at a sun vertical depression angle Do = 14.66° (mean + 2SD) according to the normal eye estimations. The results at different areas in the deep desert in KSA showed that the beginning of morning twilight and true dawn is at sun vertical depression Do = 14.88° (mean + 1SD). The current study shows significant results, which are comparable with both naked eye observations and photoelectric measurements of true dawn in both Egypt and Libya for desert background.

Nocturnal bird migration at Besh Barmag bottleneck in Azerbaijan as revealed by means of acoustic monitoring

SummaryBird migration studies are sparse in the Caucasus region, but have received more interest in recent years. To date, these studies have focused on diurnal migration and no information about nocturnal bird migration is available from this region. Therefore, nocturnal bird migration in the Besh Barmag bottleneck (Azerbaijan) was acoustically analysed on the basis of 1,464 h 44 min of sound recordings cost-efficiently obtained with an autonomously operating recorder and an omnidirectional microphone between sunset and sunrise on 63 nights in autumn 2011 and 67 nights in spring 2012. In total, 88,455 calls of 106 migrating species were detected. Of these, 2,172 calls could not be identified due to recording deficiencies or imperfect familiarity with some of the vocalisations and may involve as many as 20 species. The calls and songs of another 13 non-migratory species were not counted. Due to organisational or technical constraints some nights in the study periods could not be analysed and so the ensuing data gaps were repaired by interpolation, resulting in an estimated total of 108,986 calls in autumn 2011 and 33,348 calls in spring 2012. In both seasons the most vocally productive and species-rich phase was civil morning twilight, containing as it does the onset of diurnal migration. In autumn 2011, 54.7% of the recorded calls occurred in civil evening and morning twilight and 68.8% in spring 2012. But species and call numbers were also high in the darkest twilight and night phases. The interpretation of the data is, however, partly conjectural and any future access to truly reliable information on migration densities is conceivable only through radar studies.

Vertical profile of polarization over Vladivostok using horizon shadowing: Clues to understanding the altitude variation of reflectance of aerosol particles

We report polarimetric measurements of atmospheric aerosol in evening and morning twilight over Vladivostok (Russia) in late winter–early spring of 2017 using a horizon-shadowing technique. Motion of the Sun beneath the horizon changes the altitude of the boundary of the shadowed atmosphere h, making it possible to constrain the altitude of aerosol particles contributing to the polarimetric response. We investigate the degree of linear polarization P in aerosol particles at different altitudes, ranging from 0 km up to 17 km. In two out of four experiments we found significant variations of the polarimetric response (ΔP ∼ 10%) with altitude; whereas, in the other measurements, the polarization appeared nearly the same (ΔP ∼ 2–3%) throughout the entire range of the studied altitude. Polarization P was measured in the zenith direction with the scattering angle being θ ∼ 90° At such a scattering angle, the polarization is near its maximum value Pmax. We analyze our measurements using the Umov effect that describes an inverse correlation between Pmax and the geometric albedo A. On 2017-02-21, we estimate A ≈ 0.0120 ± 0.0004 at h = 0 km and A ≈ 0.0154 ± 0.0026 at h = 10 km. On 2017-02-27, we find the opposite trend, A ≈ 0.0108 ± 0.0003 (h = 0 km) and A ≈ 0.0084 ± 0.0006 (h = 10 km). On 2017-03-02 and 2017-03-03 we find A ≈ 0.0084 ± 0.0009 and A ≈ 0.0090 ± 0.0010, respectively. Conclusions drawn from polarimetric measurements appear in qualitative accordance with results of remote sensing with a three-wavelength lidar.

The Effect of atmospheric humidity level to the determination of Islamic Fajr/morning prayer time and twilight appearance

Islamic prayer times are based on the astronomical position of the Sun in the sky. One of them is the Fajr prayer. It is marked by some indicators in the morning twilight which is white light spread in the Eastern horizon. However, determining the true time of twilight can be difficult. One of the reasons is the effect of atmospheric humidity to the appearance of morning twilight. The higher the humidity, the redder twilight sky appearance. This paper discusses this effect. It is shown that despite of the same Sun's position, sky color can vary considerably. Observations of various solar dip angle have been made to study this effect. Visibility for different angle can change accordingly. We obtained that the average solar dip for Fajr prayer by morning twilight images was -18°39'29.4".

Daily activity time budgets of leaf-beetles (Coleoptera, Chrysomelidae) at different stages of their generation cycle

Native (natural) motor responses in the active life phases (adult and larvae) of 4 species of leaf-beetles (Coleoptera, Chrysomelidae) of a one-year generation were studied in natural and laboratory conditions in the Samara region during 19742010. Depending on the dominant biological program at each stage of ontogenesis, type of motor activity (specific movements) and time budgets for the implementation of these movements, the entire generation (reproductive) cycle was divided into 4 stages the functional unit of behavioral reactions. These powers include precopulatory period (puberty adult), stage of copulation (mating adults), the stage of ovulation (the maturation and oviposition) and the stage of embryogenesis (hatching and larvae development). The dynamics of the duration of the light period of the day (when the beetles are active) is considered throughout the observation period. The duration of useful time of the light period of the day (for this latitude) is calculated, i.e. the astronomical day length minus the length of morning and evening twilight (when it is relatively cold, the dew falls, and the insects are inactive), exactly the period when the beetles are most mobile. Real time budgets of motor activity in the period of daylight hours at different stages of the lasting cycle are estimated. The main types of motor responses of leaf beetles are recorded and examined. It is established that all in all there are 13 such reactions, of them 5 are common for all stages, 8 are specific.

Diurnal asymmetry in slant column density of NO2, O3, H2O and O4 during CAIPEEX–IGOC over Mahabubnagar, a rural site in Southern Peninsular India

In order to study the column densities of atmospheric trace gases over a rural environment, zenith-sky scattered light observations have been carried out by employing a high-precision, portable UV-V-IR spectrometer (Ocean Optics Model HR2000) at Mahabubnagar (16°42′N, 77°58′E) during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment–Integrated Ground Observational Campaign during October 1, 2011–November 11, 2011. The observed and calculated differential optical density spectra of NO2, O3, H2O and O4 are compared in the spectral range 462–498 nm and found good agreement within a percent deviation up to 1, 1, 0.5 and 0.8 %, respectively. Differential slant column densities (SCDdiff) of NO2, O3, H2O and O4 are retrieved to present the diurnal variation at morning and evening hours between 65° and 95° solar zenith angles (SZAs). The SCDdiff at morning and evening 90° SZA are observed to be 6.4 × 1016 and 9.4 × 1016 mol cm−2 for NO2; 1.03 × 1020 and 1.38 × 1020 mol cm−2 for O3; 1.7 × 1024 and 1.8 × 1024 mol cm−2 for H2O, 1.23 × 1044 and 1.57 × 1044 mol cm−2 for O4, respectively. The diurnal variations of NO2 in twilight period are observed to vary from 36 to 75 %, O3 from 23 to 40 %, H2O from 2 to 20 % and O4 from 23 to 53 % during the study period. The SCDs of NO2, O3 and O4 are observed to be higher in the evening twilight hours compared to the morning twilight hours, which may be due to higher temperature observed at evening as compared to morning between 65° and 95° SZAs.

High-speed solar wind streams and polar mesosphere winter echoes at Troll, Antarctica

Abstract. A small, 54 MHz wind-profiler radar, MARA, was operated at Troll, Antarctica (72° S, 2.5° E), continuously from November 2011 to January 2014, covering two complete Antarctic winters. Despite very low power, MARA observed echoes from heights of 55–80 km (polar mesosphere winter echoes, PMWE) on 60% of all winter days (from March to October). This contrasts with previous reports from radars at high northern latitudes, where PWME have been reported only by very high power radars or during rare periods of unusually high electron density at PMWE heights, such as during solar proton events. Analysis shows that PWME at Troll were not related to solar proton events but were often closely related to the arrival of high-speed solar wind streams (HSS) at the Earth, with PWME appearing at heights as low as 56 km and persisting for up to 15 days following HSS arrival. This demonstrates that HSS effects penetrate directly to below 60 km height in the polar atmosphere. Using local observations of cosmic-noise absorption (CNA), a theoretical ionization/ion-chemistry model and a statistical model of precipitating energetic electrons associated with HSS, the electron density conditions during the HSS events are estimated. We find that PMWE detectability cannot be explained by these variations in electron density and molecular-ion chemistry alone. PWME become detectable at different thresholds depending on solar illumination and height. In darkness, PWME are detected only when the modelled electron density is above a threshold of about 1000 cm−3, and only above 75 km height, where negative ions are few. In daylight, the electron density threshold falls by at least 2 orders of magnitude and PWME are found primarily below 75 km height, even in conditions when a large proportion of negative ions is expected. There is also a strong dawn–dusk asymmetry with PWME detected very rarely during morning twilight but often during evening twilight. This behaviour cannot be explained if PMWE are caused by small-scale structure in the neutral/molecular-ion gas alone but may be explained by the presence of charged meteoric dust.

A comparison of traffic estimates of nocturnal flying animals using radar, thermal imaging, and acoustic recording.

There are several remote-sensing tools readily available for the study of nocturnally flying animals (e.g., migrating birds), each possessing unique measurement biases. We used three tools (weather surveillance radar, thermal infrared camera, and acoustic recorder) to measure temporal and spatial patterns of nocturnal traffic estimates of flying animals during the spring and fall of 2011 and 2012 in Lewes, Delaware, USA. Our objective was to compare measures among different technologies to better understand their animal detection biases. For radar and thermal imaging, the greatest observed traffic rate tended to occur at, or shortly after, evening twilight, whereas for the acoustic recorder, peak bird flight-calling activity was observed just prior to morning twilight. Comparing traffic rates during the night for all seasons, we found that mean nightly correlations between acoustics and the other two tools were weakly correlated (thermal infrared camera and acoustics, r = 0.004 ± 0.04 SE, n = 100 nights; radar and acoustics, r = 0.14 ± 0.04 SE, n = 101 nights), but highly variable on an individual nightly basis (range = -0.84 to 0.92, range = -0.73 to 0.94). The mean nightly correlations between traffic rates estimated by radar and by thermal infrared camera during the night were more strongly positively correlated (r = 0.39 ± 0.04 SE, n = 125 nights), but also were highly variable for individual nights (range = -0.76 to 0.98). Through comparison with radar data among numerous height intervals, we determined that flying animal height above the ground influenced thermal imaging positively and flight call detections negatively. Moreover, thermal imaging detections decreased with the presence of cloud cover and increased with mean ground flight speed of animals, whereas acoustic detections showed no relationship with cloud cover presence but did decrease with increased flight speed. We found sampling methods to be positively correlated when comparing mean nightly traffic rates across nights. The strength of these correlations generally increased throughout the night, peaking 2-3 hours before morning twilight. Given the convergence of measures by different tools at this time, we suggest that researchers consider sampling flight activity in the hours before morning twilight when differences due to detection biases among sampling tools appear to be minimized.

Atmospheric transmission and thermal background emission in the mid-infrared at Mauna Kea

We present results of a preliminary study intended to quantitatively estimate the atmospheric transmission and thermal background emission in the mid-infrared (MIR), 7 μm - 26 μm, at the 13N TMT site in Mauna Kea. This is in the interest of supporting the planning of MIR instrumentation for the posible second-generation of astronomical instruments for the Thirty Meter Telescope (TMT) project. Mauna Kea, located at high altitude (4,050 m above sea level), enjoys natural conditions that make it an outstanding location for astronomical observations in the mid-infrared. The goal of this work is to produce a dataset and model that shows the atmospheric transmission and thermal emission for two cases of precipitable water vapor (PWV), a low value of 0.3 mm, and at 1.5 mm which represent near median conditions at the site. Besides, and driven by the interest of the MIR community to exploit the daily twilight times, we look at the specific atmospheric conditions around twilight as a function of season. The best conditions are found for cold and dry winter days, and in particular the morning twilight offers the best conditions. The analysis of PWV data, shows the median value for the site (all year conditions between 6:00 PM and 7:30AM) is 1.8 mm and that periods of water vapor lower than 1.0 mm are common, these supports the opportunity and discovery potential of the TMT project in the mid-infrared bands.

A transient event in AD 775 reported by al‐Tabarī: A bolide – not a nova, supernova, or kilonova

AbstractGiven that the cause for the strong increase in 14C in AD 774/5 in Japanese and German trees is still a matter of debate (e.g. short gamma‐ray burst or solar super‐flare), we have searched in Arabic chronicles for reports about unusual transient celestial events. In the History of al‐Tabarī we found two (almost identical) reports about such an event. The group around caliph al‐Mansūr observed a transient event while on the way from Baghdad to Mecca on AD 775 Augusst 29– September 1 (Julian calendar). A celestial object (kawkab) was seen to fall or set (inqadda), and its trace (atharuhu) was seen for at least tens of minutes (up to 70–90 min) during morning twilight. The reports use the Arabic words kawkab and athar(uhu), which were also used in the known Arabic reports about supernovae SN 1006 and 1054, so that one might consider an interpretation as a nova‐like event. The kawkab (celestial object) was observed only during the morning twilight at a brightness of probably between about –3 and 0 mag. Such a brightness and time‐scale would be expected for optical kilonovae (at ∼3 to 9 kpc) in the context of short gamma‐ray bursts. There are no similar reports from eastern Asia for this time. However, the short reports are fully consistent with a bolide: The word kawkab can be used for meteor, the verb inqad. d. a normally means falling down, the word atharuhu can mean its trace. We therefore prefer the interpretation as bolide. We discuss in detail how to convert the Muslim calendar date to a date in the Julian calendar using first the calculated Islamic calendar and then considering the time when the crescent new moon could be visible at the given location. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Brightness and color variation for evening and morning twilights at Bahria of Egypt IV

Photoelectric observations of twilight (evening and morning) Bahria (φ=28° 42.94′ N, λ=28° 59.99′ E) in Egypt were done in the period between 1983 and 1985. A semiautomatic photoelectric scanner equipped with a refractor of diameter D=10cm and focal length f=24cm was used. The phenomena were followed in altitude and azimuth each 10°. For evening twilight, we find a minimum value in the color index (CI) curves at Do=12.5°. The color index (CI) is found to be in the range between −1.5 and 3. A decrease on both sides of the CI toward large and small depressions is seen indicating a red background. This red color decreases on both sides of the maximum although being positive. No change of the maximum values is noticed with the azimuth. The (B–R) curves show positive maximum values at 11°⩽Do⩽13° with a decrease on both sides. Color indices are studied for the three bands (B–V), (B–R) and (V–R) for morning and evening twilights. For morning twilight, the (B–V) color index curves show minimum (CI) values with positive (B–V) at Do around 8°. This can denote a dominating yellow color of the sky till Do≈8–10°, where the sky is as bright in the yellow as in the blue. Stability is reached at Do=12–14°. We believe that, the dawn shows itself at Do⩽15°, while the nightfall shows itself at Do⩽18°.

Naked eye observations for morning twilight at different sites in Egypt

Twilight observations were carried out in the period between 1984 and 1987 in different seasons at different sites of Egypt (Baharia, Matrouh, Kottamia and Aswan) through a cooperation project between Dar El-Iftaa’ and the Egyptian Academy of Scientific Research and Technology. Naked eye observations of the first light of the dawn were done in parallel to the photoelectric measurements of the twilight phenomena. The depression of the sun below the horizon corresponding to the first light was calculated from the time of observations. Our estimates show that the normal eye can just discriminate the dawn (the first white light thread) at a depression of 14.7° with a maximum value of 15.08° and a minimum value of 12.01°. This result agrees with result obtained by our previous photoelectric measurements.

Near-Earth object 2012XJ112 as a source of bright bolides of achondritic nature

We analyze the likely link between the recently discovered Near Earth Object 2012XJ112 and a bright fireball observed over the south of Spain on December 27, 2012. The bolide, with an absolute magnitude of -9 +- 1, was simultaneously imaged during the morning twilight from two meteor stations operated by the SPanish Meteor Network (SPMN). It was also observed by several casual witnesses. The emission spectrum produced during the ablation of the meteoroid in the atmosphere was also recorded. From its analysis the chemical nature of this particle was inferred. Although our orbital association software identified several potential parent bodies for this meteoroid, the analysis of the evolution of the orbital elements performed with the Mercury 6 symplectic integrator supports the idea that NEO 2012XJ112 is the source of this meteoroid. The implications of this potential association are discussed here. In particular, the meteoroid bulk chemistry is consistent with a basaltic achondrite, and this emphasizes the importance to deduce from future Earth approaches the reflectance spectrum and taxonomic nature of 2012XJ112.

Navigational Efficiency of Nocturnal Myrmecia Ants Suffers at Low Light Levels

Insects face the challenge of navigating to specific goals in both bright sun-lit and dim-lit environments. Both diurnal and nocturnal insects use quite similar navigation strategies. This is despite the signal-to-noise ratio of the navigational cues being poor at low light conditions. To better understand the evolution of nocturnal life, we investigated the navigational efficiency of a nocturnal ant, Myrmecia pyriformis, at different light levels. Workers of M. pyriformis leave the nest individually in a narrow light-window in the evening twilight to forage on nest-specific Eucalyptus trees. The majority of foragers return to the nest in the morning twilight, while few attempt to return to the nest throughout the night. We found that as light levels dropped, ants paused for longer, walked more slowly, the success in finding the nest reduced and their paths became less straight. We found that in both bright and dark conditions ants relied predominantly on visual landmark information for navigation and that landmark guidance became less reliable at low light conditions. It is perhaps due to the poor navigational efficiency at low light levels that the majority of foragers restrict navigational tasks to the twilight periods, where sufficient navigational information is still available.

Long-term changes in the nightly behaviour of the oxygen red 630.0 nm line nightglow intensity and trends in the thermospheric meridional wind velocity

Long-term changes in the nightly behaviour of the monthly/seasonal mean of the oxygen red 630.0 nm line intensity were observed at Abastumani (41.75° N, 42.82° E) from 1957 to 1993. The long-term increase in the red-line intensity after twilight and decrease at midnight is also mostly accompanied by an increase in the intensity before morning twilight. Using changes in mean seasonal nightly behaviour of the red-line intensity combined with the observed night-time negative trend in the ionosphere F2 layer peak height (hmF2) at Tbilisi (41.65° N, 44.75° E) from 1963 to 1986, the trend in the meridional component of thermospheric wind velocity is estimated. The obtained positive trend in the northward wind velocity (or decrease in the southward wind velocity) has a higher value in winter, where the hmF2 height decrease and the midnight/after midnight decrease in the red-line intensity are larger, and has lower values in spring and summer, where the decrease in hmF2 and midnight red-line intensity are comparatively smaller.

Variability of stratospheric and tropospheric nitrogen dioxide observed by the visible spectrophotometer at Zvenigorod, Russia

We present an analysis of the results of measurements of nitrogen dioxide (NO2) content at the Zvenigorod Research Station of the A.M. Obukhov Institute of Atmospheric Physics, which is located 50 km west of Moscow. Systematic NO2 observations at Zvenigorod started in 1990 and are carried out with a zenith viewing grating spectrophotometer in the 435–450 nm wavelength range during morning and evening twilight. Measurements within a wide range of air-mass factors allow the retrieval of vertical profiles of NO2 and the determination of stratospheric and tropospheric NO2 content. Analysis shows that stratospheric and boundary-layer NO2 exhibit very different temporal behaviours. Stratospheric NO2 undergoes very prominent and regular diurnal and annual cycles associated with photochemical processes, as well as less regular intra-annual and inter-annual variations. Boundary-layer NO2 is very changeable and is significantly affected by pollution episodes. Boundary-layer NO2 exhibits irregular diurnal, weekly, intra-annual (within 15–100 day periods) and annual variations. Intra-annual variations of stratospheric and boundary-layer NO2 content with similar time scales can be coherent, but the time lag between these variations varies with time.

Determining Circadian Response of Adult MaleAcrobasis nuxvorella(Lepidoptera: Pyralidae) to Synthetic Sex Attractant Pheromone Through Time-Segregated Trapping With a New Clockwork Timing Trap

Mate finding is a key lifecycle event for the pecan nut casebearer, Acrobasis nuxvorella Neunzig, as it is for virtually all Lepidoptera, many of which rely on long-range, species-specific sex pheromones, regulated largely by circadian clocks. Adult male moths were trapped at discrete time intervals during the first two seasonal flights for 6 yr to determine times of peak activity associated with male response to pheromones. From 1997 to 2002, the Harris-Coble automated clockwork timing trap was used for hourly time-segregated sampling. Analysis of variance with linear contrasts determined that circadian response of A. nuxvorella males to pecan nut casebearer pheromone began at approximately 2100 hours, the first hour of total darkness, lasting for 6-7 h. It peaked from midnight to 0400 hours and ended at the onset of morning twilight, approximately 0500 hours. The hours of peak activity are hours of minimal bat predation. The study shows that pecan nut casebearer males become responsive to pheromone several hours before females start calling and remain responsive for at least 1 h after they stop. The extended response period conforms to studies of other polygamous Lepidoptera in which a selective advantage is conferred on early responding males in scramble competition for available females.

Selection of winter roosts in the Great Tit Parus major: influence of microclimate

The success in energy saving in roosting Great Tits (Parus major) is potentially influenced by roost microclimatic characteristics. We tried to learn about the pattern in cavity microclimate influence on night roosting of birds, under natural weather conditions. An experimental aviary offered two artificial tree-roosts with different microclimatic conditions which varied according to the outdoor conditions (mainly due to solar radiation) and microclimate retention capability of roosts. Birds actively explored roosts before the final choice of sleeping site. Two hours prior to sunset, there were significant differences between the roosts (n = 11 adult males tested, each six nights). Selected roosts had higher average temperature (Wilcoxon matched pairs test, P = 0.017) and temperature at the time of sunset (P = 0.028). Simultaneously, inside temperature decreased slowly (P = 0.047). During the night, when birds slept in one of the roosts, the average temperature was higher (P < 0.001) and the fluctuation range of temperatures was smaller (P = 0.003) there. Moreover, at the morning twilight, the temperature of the occupied roost was higher (P < 0.001). Humidity did not have a significant influence at all. The effect of metabolic heating was demonstrated. When birds slept in the insulated roost (n = 30 nights), the indoor average temperature significantly differed from the non-insulated and outdoor temperatures (Kruskal–Wallis ANOVA, P = 0.0001). Selection of an actually warmer and thermally more stable roost according to the course of weather indicates demand for a wide spectrum of potential roosts in the natural environment.

Time budgets and at-sea behaviour of lactating female Antarctic fur seals Arctocephalus gazella at Bouvetøya

We present the first data on attendance patterns, at-sea movements and diving behaviour of Antarctic fur seals breeding at Bouvetoya (Bouvet Island), Southern Ocean. While other colonies have been extensively studied, this remote and second largest global population remains relatively unknown. Time depth recorders and satellite relay data loggers were deployed on breeding females during the 2000-2001 and 2001-2002 summers. Attendance and foraging patterns were similar to those observed at colonies in the Scotia Sea region where Antarctic krill Euphausia superba is the pre- dominant prey. Early to mid-lactation trips ranged within ~100 km of the island, usually towards the west; the dominant direction shifted later in the season and the range also increased markedly to a peak between early February and early March. Solar elevation influenced arrivals and departures from the island, with most departures occurring around sunset. Diurnal variations in diving behaviour were consistent with the vertical migration of krill. Diving frequency was higher at night and diving effort peaked around morning twilight. Afternoon deep diving was common, suggesting that females might target dense daytime krill aggregations between the photic zone and the thermocline. Trip durations increased throughout early to mid-lactation, peaking in late January to early March, before again decreasing towards the end of lactation. Our results illustrate the substantial variability, both between individuals and within individuals over time, that is likely to reflect variations in prey distrib- ution and in the growth requirements of pups. Such variations need to be taken into account when estimating habitat use and resource utilisation in marine top predators.

Retrieval of stratospheric O3 and NO2 vertical profiles using zenith scattered light observations

Daily zenith scattered light intensity observations were carried out in the morning twilight hours using home-made UV-visible spectrometer over the tropical station Pune (18‡31′, 73‡51′) for the years 2000–2003. These observations are obtained in the spectral range 462–498 nm for the solar zenith angles (SZAs) varying from 87‡ to 91.5‡. An algorithm has been developed to retrieve vertical profiles of ozone (O3) and nitrogen dioxide (NO2) from ground-based measurements using the Chahine iteration method. This retrieval method has been checked using measured and recalculated slant column densities (SCDs) and they are found to be well matching. O3 and NO2 vertical profiles have been retrieved using a set of their air mass factors (AMFs) and SCDs measured over a range of 87–91.5‡ SZA during the morning. The vertical profiles obtained by this method are compared with Umkehr profiles and ozonesondes and they are found to be in good agreement. The bulk of the column density is found near layer 20–25 km. Daily total column densities (TCDs) of O3 and NO2 along with their stratospheric and tropospheric counterparts are derived using their vertical profiles for the period 2000–2003. The total column, stratospheric column and tropospheric column amounts of both trace gases are found to be maximum in summer and minimum in the winter season. Increasing trend is found in column density of NO2 in stratospheric, tropospheric and surface layers, but no trend is observed in O3 columns for above layers during the period 2000–2003