First observational study during a solar eclipse event on variations in the horizontal winds simultaneously in the troposphere-stratosphere-mesosphere-lower-thermosphere region over the equatorial station Thumba (8.5°N, 77°E)
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The longest annular solar eclipse of the millennium occurred on 15 January, 2010, and was visible over the equatorial station Thumba (8.5°N, 77°E) around noon time. A host of experiments were carried out to study the variations due to the solar eclipse event on various geophysical parameters, from the Earth’s surface to ionospheric heights. The present study focuses on the variation in the horizontal winds in the height regions of 0–65 km and 80–100 km, using GPS-sondes, rocket-sondes and meteor wind radar. The observations were made during, and after, the maximum obscuration on the day of the eclipse, as well as at the same time on a control day. The observations showed a strengthening/weakening of winds along with directional changes both in zonal and meridional winds in the selected height domains. A drastic change from easterly to westerly is observed at 98 km during, and after, the maximum phase, but, for the meridional wind, the reversal is observed only after the maximum phase. Variations due to the eclipse were also observed around the tropopause and stratopause in both wind components. However, the observed changes in winds around the tropopause and stratopause could not be attributed unambiguously to the eclipse as day-to-day wind variability is not available in these height regions. The significance of the present study lies in reporting the variations in the horizontal wind components from the ground to the 100-km height region (with a gap around 65–80 km), and the characteristics of the atmospheric waves generated due to the mid-day annular solar eclipse.
Key wordsSolar eclipse gravity waves troposphere-stratosphere-mesosphere-lower-thermosphere region
Solar eclipses are rare events and each eclipse is unique with respect to the time of their occurrence, the percentage of the maximum obscuration, and the path of the eclipse. Despite continuous efforts from various researchers, the regular effects of solar eclipses on atmospheric processes are still not well understood and, hence, cannot be generalized. The movement of the eclipsed region at supersonic speed clearly differs from that of regular solar terminators at sunrise and sunset times. Solar eclipse events provide us with rare opportunities to study and understand wave propagation characteristics, their generation mechanisms and the dynamical processes associated with sudden changes in the thermal structure of the atmosphere. Chimonas (1970) was the first to report that a sudden cooling effect could act as a potential source of gravity waves that are likely to propagate upwards, even into the thermosphere, when the source mechanism changes rapidly. Later, Chimonas and Hines (1970), 1971) and Fritts and Luo (1993) described the gravity wave structure induced by the cooling of the ozone layer in response to a solar eclipse, and the associated forcing of the middle atmosphere. Randhawa et al. (1970) reported the effect of a partial solar eclipse, which took place on 24 December, 1973, on the temperature and winds at the equatorial station of Fort Sherman, Panama Canal Zone (9°N, 79°67′W). These observations were based on Locki rocket flights that provided wind and temperature measurements in the limited height region of 47–57 km. Cooling by 3–5°C in the 50–52-km layer, and a decrease in wind speed close to the maximum obscuration and an increase in wind speed towards the end of the eclipse, were observed during this particular event.
Appu et al. (1982), making use of rocket measurements from the equatorial station Thumba (8.5°N, 77°E) in connection with the partial solar eclipse of 16 February, 1980, reported a warming of 10°C around 30 km, and a cooling of 14°C around 58 km. From balloon measurements, these authors also reported a warming of 5°C around 13 km at Thumba, and a systematic cooling in the 3–20-km height region at Hyderabad. Dutta et al. (1999), conducting balloon measurements from Hyderabad during the 24 October, 1995, eclipse, reported a ~10°C cooling below the tropopause with no significant change in wind components. Zerefos et al. (2007) conducted concurrent measurements in the troposphere, stratosphere, and ionosphere, from three mid-latitude stations, viz., Athens, Kastelorizo and Thessaloniki, to study eclipse-induced thermal fluctuations during the 29 March, 2006, eclipse event, and reported dominant oscillations of a 30–40-minute period in the parameters related to the ozone layer and the ionosphere, and the main mechanism of generating gravity waves was attributed to thermal stratospheric ozone forcing.
The present study focuses on the variation in horizontal winds, the resulting wave motions, and the dynamical processes observed throughout the tropospheric-stratospheric-mesospheric-lower-thermospheric regions due to a solar eclipse over the equatorial station Thumba (8.5°N, 77°E). This particular study is the first of its kind in the sense that the earlier eclipse-related studies of neutral winds were limited to the lower-mesospheric region and the present study reports the variations in wind, due to a solar eclipse, from near ground to the 100-km region, using balloon-borne GPS sondes, rocket- and radar-based measurements. The perturbations in the temperature structure in the tropospheric-stratospheric region, which was part of the present study conducted over Thumba during the solar eclipse, have already been published (Subrahmanyam et al., 2011); however, a few results from that paper, relevant to the present study, will be discussed later.
2. Experimental Setup
The first contact of the annular eclipse at Thumba was at 11:04 IST (= GMT + 5:30 hr), the maximum obscuration of ~92% was at 13:14IST and the last contact was at 15:05 IST. This spectacular event provided a rare opportunity to study the dynamical response of the middle atmosphere at noon-time due to an annular eclipse. Measurements of temperature and winds in the middle atmospheric region were made over the equatorial station Thumba (8.5°N, 77°E), using balloons, rockets and meteor wind radar during, and after, the maximum obscuration of the Sun. High-altitude balloon ascents carrying in-house-developed Pisharoty GPS sondes (Subrahmanyam et al., 2011) were carried out at 08:00, 10:00, 12:30, and 14:30 IST on 14 January (control day) and 15 January (annular eclipse), 2010, to study the variations in temperature and horizontal winds during, and after, the eclipse in the tropospheric-stratospheric (0–30 km) height region. The accuracies of the Pisharoty GPS sonde wind measurements are 1–2 m s−1 in wind speed and 5° in wind direction for wind speeds above 25 m s−1 and 10° for lower wind speeds.
RH-200 rockets carrying a chaff payload were launched at 13:07 and 15:00 IST on the eclipse day, and the control day, to measure the horizontal winds in the stratospheric-mesospheric (30–65 km) region. RH-200 is a two-stage rocket, which uses chaff as a payload to measure the winds in the 20–65-km altitude region with a height resolution of 1 km (Devarajan et al., 1984). Chaff consists of a large number of very thin strips of metal (usually copper or aluminum) or metalized nylon, cut to half the wavelength size of the radio wave from the tracking radar. Chaff, with its very low ballistic coefficient, is well suited as a wind tracer.
The chaff released at the apogee point of the rocket trajectory is tracked by the ground-based radar to measure the wind velocity in the mesosphere and stratosphere as it descends with time. The chaff gradually spreads in space after its release and is a diffusive radar target. The radar automatically tracks the strongest portion of the diffused echo, and, hence, the position coordinates measured by the radar correspond to the densest part of the diffused chaff cloud. The wind velocity is calculated from the radar measured position coordinates of the chaff (R,θ,φ) as a function of time, where R,θ, and φ are the range, elevation and azimuth angles respectively. Standard errors involved in the wind measurements are 2.7 m s−1 in 20–30-km altitudes, 1.9 m s−1 in the 31–50-km and 3.8 m s−1 in the 50–65-km altitude regions (Devarajan et al., 1984).
In addition to the above two techniques, round the clock meteor radar observations at Thumba provided high resolution (15 min) zonal and meridional winds in the 82–98-km region and the daily mean mesopause temperature. The winds derived from the Thumba meteor radar are well comparable with MF radar (Kumar et al., 2007) and TIDI observations (John et al., 2011), and the meteor-radar-derived mesopause temperature is well comparable with SABER and dayglow photometer (Das et al., 2012). For the present study, we have used the wind measurements on the control and eclipse day. A general description of the meteor wind radar can be found in Hocking et al. (2001), and the Thumba meteor radar description and initial results can be found in Deepa et al. (2006) and Kumar et al. (2007). In the case of meteor radar, the radial velocity can be measured with an accuracy of 5% or better.
3. Observational Results
The zonal and meridional wind measurements on 14 and 15 January, 2010, using balloon (12:30–13:30 IST), rocket (13:07–13:30 IST), and meteor wind radar (12:30–13:30 IST), observations around the maximum obscuration of the Sun in the height regions of 0–65 and 82–98 km are analysed to look for the perturbations in wind components and the resulting dynamical processes. For balloon- and rocket-based measurements, the 14 January, 2010, observations were taken as a reference. However, as the meteor radar observations were available round the clock, the reference profile is based on the data collected during the entire month (January 2010). There is a data gap between 65–82 km, which is due to observational limitations (RH-200 provides wind information up to 65 km and meteor radar provides data from 82 km onwards).
In order to characterize and quantify the short-scale fluctuations, and to get a better picture of eclipse-induced wave characteristics, the height profiles of the wind fluctuations were subjected to Fourier analysis, after subtracting a second-order polynomial fit from all the profiles. The choice of the order of the polynomial is subjective and the possible effects of the order of the polynomial on the vertical wavelength spectrum can be found in Babu et al. (2008). The wavelength spectra for the troposphere and stratosphere are shown in Figs. 5(a) and (b), respectively. Upper panels are for the zonal wind and lower panels are for the meridional winds. The left panels are during the maximum obscuration, and the right panels are after the maximum obscuration. Only those spectral peaks, which are above the 95% significance level are considered. In general, a substantial increase in the amplitude of shorter (2–8 km) vertical wavelengths in the zonal and meridional wind components in the troposphere and stratosphere during the maximum obscuration, and even after 2 hours of the maximum obscuration, is found on the eclipse day compared to that on control day. In the troposphere, the zonal wind component shows a predominant amplitude of 6 m s−1 on the eclipse day and 4 m s−1 on the control day, which correspond to a ~6 km vertical wavelength. This feature is observed during, and after 2 hrs from, the time of maximum obscuration. Similarly, a 6-km vertical wavelength in the meridional component shows an enhanced amplitude of ~2 m s−1 during the eclipse phase compared with ~1 m s−1 on the control day, whereas, after the maximum obscuration, the amplitude corresponding to a 6-km vertical wavelength becomes 2.5 m s−1 compared with 1.3 m s−1 on the control day. The increase in amplitude at stratospheric heights is found to be comparatively less. The amplitude of larger (10–16 km) vertical wavelengths remains almost the same during, and after, the maximum obscuration in the troposphere and stratospheric region and this can be attributed to short-period planetary waves not associated with the eclipse. Thus, the enhanced amplitudes of short vertical wavelengths of (2–8 km) throughout the troposphere and stratosphere can be attributed to gravity waves triggered by the eclipse-induced wind shear and thermal perturbations. However, this is only a possibility as we don’t have the day-to-day variability of winds in these height regions.
Figure 8 shows the height profiles of the amplitude and phase of a prominently present 80-min-period gravity wave in the zonal and meridional components. The dotted lines in the height profiles of the phase indicate the propagation, which is fitted by a least-square method. It is clear from the figures that the wave propagates upward from the region below 80 km and downwards from the region above 98 km. This is indicative of the fact that there are two source regions: one above the 98-km, and the other below the 80-km, height region. The following section presents a discussion on the possible reasons for the observed features of the wind components on the eclipse day.
4. Discussion and Concluding Remarks
The Moon’s shadow moves through the Earth’s atmosphere at supersonic speed during an eclipse event and, thus, screens the solar radiation, eventually blocking the direct solar heating of ambient air on, and above, the ground. This, in turn, reduces upward long-wave heating of the ambient air above the ground, generating an effective cooling spot. During the solar eclipse, the absorption of ultraviolet radiation by ozone that heats the middle atmosphere is cut off in the shadow region. The cooling of the upper layers produces subsidence/contraction in the lower atmospheric regions, which induces a downward movement. The downward motion adiabatically heats the lower atmosphere resulting in a warming of that region. The sudden change in temperature and the associated wind shears can act as a source for the generation of gravity waves. Thus, one should expect changes in the thermal structure, as well as the wind system, due to the eclipse effect. Recently, Subrahmanyam et al. (2011) reported a decrease of temperature of the order of 2–4°C in the lower and upper troposphere, and 6–8°C in the lower stratosphere, during the annular eclipse of 15 January, 2010, over the same location of Thumba. Also, it is seen that towards the end of the eclipse, the temperature slowly returned to its normal value, except in the lower stratosphere, where the difference in the temperature fluctuation was beyond the diurnal temperature fluctuations. Using balloon flights carrying ozone sondes from the same location, Manchanda (personal communication) observed a cooling of the order of 5°C in the 36–40-km height region along with a decrease in the stratospheric ozone peak value by ~8%. From high resolution mesopause (~90 km) temperature measurements over Thumba using a Multi Wavelength Dayglow Photometer, Sumod et al. (2011) reported enhancements of the mesopause temperature of the order of 35 K, and the generation of gravity waves with periods from 30 minutes to 2 hours.
The change in temperature can also cause changes in pressure over the totality footprint. The sudden change in the stratospheric pressure during the solar eclipse may also be responsible for the generation of gravity waves in the stratosphere (Zerefos et al., 2007). Thus, these variations throughout the middle atmosphere, in turn, can result in variations in neutral winds and wind shears which, in turn, give rise to short-scale fluctuations. The horizontal wind measurements throughout the middle atmospheric region over the equatorial station Thumba during the annular eclipse event of 15 January, 2010, are used to study the dynamical processes during the eclipse which are associated with wave motions produced in the region of local temperature changes of the atmosphere as the Moon’s shadow advances at supersonic speeds. Eclipse-induced perturbations could be found in the vertical profiles of the zonal and meridional winds throughout the middle atmospheric region over Thumba. The MLT region zonal wind showed a reversal at 98 km during the maximum phase of the eclipse and the reversal is further strengthened after the maximum phase. The meridional wind in the MLT region showed noticeable changes only after the maximum phase of the eclipse. These observed changes in the wind system are attributed to eclipse-induced cooling and subsequent changes in the pressure in the middle atmosphere over Thumba. Winkler et al. (2001) have argued that factors such as the cooling rate, the inversion strength and height, and the pressure gradient could induce strong wind shear and turbulent mixing, thus substantiating the wind variations associated with the eclipse.
The Fourier analysis of height profiles of wind fluctuations revealed the presence of 2–8-km-vertical wavelengths in the wind components during, and after, the maximum phase of the eclipse in the troposphere and stratosphere. Gravity waves, generated away from this location earlier, along the path of the eclipse can also reach this location, thus manifesting in the form of multiple wavelengths. The spectral analysis of the time series of wind data in the MLT region could reveal the presence of 30-minute- and 80–100-minute-period gravity waves.
The authors acknowledge the support provided by scientists and engineers of the ATV Project, Thumba Rocket launching station and Meteorological Facility, Vikram Sarabhai Space Centre, Trivandrum. The support and encouragement provided by Prof. R. Sridharan, Former Director, Space Physics Laboratory (SPL) and Dr. K. Krishnamoorthy, Director, SPL are greatly appreciated. The authors, Uma, Veena, Sherin and Asha wish to thank the Indian Space Research Organisation (ISRO) for their research fellowship during the study presented here.
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