Observation of a mesospheric front in a dual duct over King George Island , Antarctica

Introduction Conclusions References


Introduction
Mesospheric bore events have been extensively reported at low and mid latitudes (Taylor et al., 1995;Smith et al., 2003;Medeiros et al., 2005;Fechine et al., 2005).Taylor et al. (1995) described a spectacular bore event observed in four mesospheric nightglow emissions at Hawaii (20.8 • N) during the ALOHA-93 campaign.Intensity measurements showed that the front was characterized by a sudden increase in the OH brightness and a concurrent decrease in the OI emission.Smith et al. (2003) reported a large-scale, long-duration mesospheric bore over the Southern US which was visible to the naked eye.Medeiros et al. (2005) and Fechine et al. (2005) have reported a large number of mesospheric bore or "front" events (>60) over northeastern Brazilian (7 • S) sector over a period of two years.
In contrast, bores are relatively uncommon events at Antarctic latitudes.Nielsen et al. (2006) observed five potential bores in four winter seasons, with only one clear event.That event was seen in airglow images from Halley Station (75.5 • S), Antarctica, with responses in three different airglow layers (OH, Na, and O 2 ), persisted for ∼3 h, and exhibited a number of trailing wave crests behind the main front, with an increase in the number of wave crests estimated at ∼6.6 crests/hr.The bore characteristics in this case were determined using a standard 2-D spectral analysis and observed wave parameters in the three airglow layers.Because they did not have information about the temperature structure above Halley Station, they could neither infer the duct structure in which the bore was propagating nor identify the presence of inversion layers.Stockwell et al. (2006) applied a S-Transform analysis to the same event reported by Nielsen et al. (2006) in order to investigate the dynamical properties of the bore.They were able to infer the bore parameters (horizontal wavelength, observed horizontal phase speed, and period) and also described its evolution as it propagated.They observed that the horizontal wavelength and horizontal phase speed decreased, and the bore amplitude increased, as the bore evolved.In our observations at Ferraz Station during 2007, more than 230 gravity wave events were observed (Bageston et al., 2009).The report of only a single bore event, among a large number of gravity wave events, confirms the low occurrence rate of bores at high southern latitudes compared to lower latitudes (Fechine et al., 2005(Fechine et al., , 2009)).This event was classified as a mesospheric bore according to the main characteristics described by Dewan and Picard (1998).It comprised a mesospheric bore having a sharp "front" that extended across the entire all-sky image, with a series of crests following the front and an observed phase speed of 92 m s −1 .These features, especially the high phase speed, are in agreement with the characteristics expected for such events (Dewan and Picard, 1998).Zonal and meridional winds observed with a MF radar at Rothera Station (68 • S, 68 • W), and temperature profiles obtained by the SABER instrument on the TIMED satellite were used to characterize the propagation environment.The results show that the bore was supported by a ducting structure, which is a primary requirement for such events (Isler et al., 1997;Dewan and Picard, 1998;Laughman et al., 2009Laughman et al., , 2011)).Our analysis and discussion of the bore characteristics and ducting conditions are discussed in detail below.

Airglow measurements
A system (Bageston et al., 2009(Bageston et al., , 2010)).The quality of the airglow data and its usage were limited by weather conditions to clear sky and new moon periods.Our data analysis used here was a slightly modified version of the method described in the literature (Garcia and Taylor, 1997;Medeiros et al., 2003).Initially, a sequence of images was selected in order to analyze the wave event that was previously identified in an animation that contains all the images collected in one night.The star field was subtracted from each image that was selected for analysis, and flat field corrections were applied to the image series.Each image in the data set was spatially filtered with a second-order high-pass Butterworth filter, with cutoff frequencies at 8 and 14 km for projections onto 512×512 km (resolution of 1 km pixel −1 ) and 768×768 km (resolution of 1.5 km pixel −1 ) images, respectively.This filtering process was the modification introduced in the methodology.Finally, the observed images were processed using a 2-D Fast Fourier Transform (FFT) analysis, and a cross spectrum was applied to the sequence of images in order to obtain the horizontal wavelength, observed phase speed, and observed period.

Mesospheric bore event
The mesospheric bore was observed from ∼23:20 to 00:00 LT on the night of 9-10 July 2007 at Ferraz Station at the tip of the Antarctic Peninsula as it progressed from southwest to northeast.Figure 1 presents a sequence of three all-sky images for the event observed in the OH emission.The first image at 23:29 LT (LT = UT-3) shows the first clear view of the bore as it crossed the image field.The arrows just ahead of the bore front indicate its propagation direction, i.e., towards the northeast.We emphasize here the difficulty in identifying the bore event in individual images, due to the presence of a thin fog, identified because most of the stars appear defocused (not due to the focus calibration), and the position of the Milky Way approximately centered in the images.These factors made it much more difficult to identify the bore event that otherwise would have been the case.However, the bore structure and propagation were seen clearly in an animation of the event.Due to the limitation of the imager, which Figures observed the bore with only one filter (OH emission), we were not able to observe the complementarity effect described by Taylor et al. (1995), i.e., an anti-phase relation in the brightness associated with the bore response seen in different airglow layers.From FFT analysis applied to a set of six images from 23:32 to 23:38 (LT), the following bore parameters were obtained: (a) a horizontal wavelength of 33 km, (b) an observed period of 6 min, and (c) an observed phase speed of 92 m s −1 .These parameters are very similar to those reported for the bore observed at Halley Station (Nielsen et al., 2006), especially the horizontal wavelength and period, with 31 km and 6.8 min, respectively, seen in the OH layer.The observed phase speed for the bore reported by Nielsen et al. (2006) had a lower limit of 76 m s −1 , at the OH airglow layer and lasted for three hours in the images.The parameters calculated by Nielsen et al. (2006) were obtained during the first hour of the bore observation.

Mesospheric winds
Winds in the mesosphere and lower thermosphere (MLT, ∼80-100 km) are known to have large influences on gravity wave and bore propagation in MLT, depending on their propagation direction.Wind measurements were not available at Ferraz during the bore observation.However, mesospheric wind measurements were available with an MF radar at Rothera Station ∼765 km to the south, as employed in the study by Bageston et al. (2010).The Rothera MF radar measures horizontal winds in the MLT through analysis of D-region partial reflection echoes (Jarvis et al., 1999;Hibbins et al., 2007).
The radar operates at a frequency of 1.98 MHz with a transmitter power of 25 kW and full-width half-maximum pulse width of 25 µs, which corresponds to a height resolution of ∼4 km, with returns oversampled at 2 km intervals (Hibbins et al., 2007).Data used here are restricted to altitudes below 94 km as several authors have observed that MF radars tend to underestimate wind speeds compared to those observed by meteor radars at higher altitudes (e.g.Manson et al., 2004 andPortnyagin et al., 2004).Rothera winds surely do not describe the small-scale winds over Ferraz, but very similar tidal structures are seen at sites with similar spacing extending across the Drake 16190 Figures

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Full Passage (Fritts, 2011), and large-scale inertia-gravity waves (IGWs) having vertical wavelengths of ∼10 km or more imply similar structures somewhat shifted in phase at distances up to ∼1000 km or more away.Assuming (as shown below) that tidal and IGW structures dominate the bore ducting environment, Rothera winds will allow us to estimate roughly the intrinsic parameters for the bore.
Winds along the bore propagation direction obtained with the Rothera MF radar during the bore event are shown in Fig. 2a.This profile was obtained by the hourly wind observed during the time duration of bore event, interpolating to 1-km vertical resolution, and then averaging over 3 km in the vertical in order to remove vertical structure expected to not be coherent over large distances.The dominant structure seen has a vertical wavelength of ∼10 km and a near-inertial period (see data analysis and discussion below), suggesting a dominant large-scale IGW with perhaps tidal contributions.Further analysis and discussion of these data and their implications for the bore environment will be provided below.

Mesospheric temperatures
During Southern Hemisphere winter, SABER views towards high southern latitudes, so measurements are occasionally available very near Ferraz (Russell et al., 1999;Mertens et al., 2004).For this study, the nearest SABER profile was obtained ∼1300 km north of Ferraz, and about 140 min before the bore was seen.This distance is larger than desired, but it is the only profile sufficiently close to Ferraz to be useful, it is likely much more representative of the thermal structure over Ferraz than a climatology without tides would be, and the extent of the bore noted above is greater than the imager field-of-view, suggesting that the ducting environment has a large horizontal extent.This temperature profile is shown in Fig. 2b.The implications of this temperature profile for ducting and bore propagation will be explored further below.
An imaging spectrometer was collocated with the all-sky imager at Ferraz during the bore observation.However, the field-of-view over which the spectrometer temperatures were averaged was ∼70 km in diameter about the zenith, more than twice the 16191 Figures

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Full wavelength of the bore identified in the OH images shown in Fig. 1.Hence, the spectrometer showed no evidence of the 6-min bore period inferred from the airglow imager data, and therefore the spectrometer data were not employed in this analysis.

Bore analysis and discussion
To investigate the large-scale MLT propagation environment during the bore observation, we use the Rothera MF radar winds and the SABER temperature profile discussed briefly above to compute the vertical profile of m 2 , where m = 2π/λ z is the vertical wavenumber and λ z is the local vertical wavelength for positive m 2 .The resulting m 2 profile suggests a duct for gravity waves and bores in a region where m 2 > 0 that is between two regions having m 2 < 0 (in which the relevant motions are evanescent in the vertical) having sufficient depth to cause effective trapping of the disturbance at the m 2 maximum.This was not done in several previous bore studies, including that by Nielsen et al. (2006) at Antarctic latitudes, because of the absence of temperature and/or wind data.The approximate dispersion relation, including wind curvature effects, but neglecting shear effects, is given by where N is the buoyancy frequency, u 0 is the horizontal wind in the direction of bore propagation, u 0 is the second derivative of the wind with altitude, c is the observed phase speed in the direction of propagation, k h = 2π/λ h is the horizontal wavenumber, and λ h is the horizontal wavelength in the direction of propagation.This dispersion relation is valid for gravity waves propagating in an environment where the effects of horizontal winds and temperature gradients cannot be neglected (Chimonas and Hines, 1986;Isler et al., 1997;Fritts and Yuan , 1989).Figures

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Full The buoyancy frequency, N, can be estimated using the relation 2: where g is the acceleration due to gravity, T is temperature, C p = 1005 J kg −1 K −1 is the specific heat at constant pressure, ∇T z is the vertical temperature gradient, and The vertical profile of m 2 obtained from Eq. 1 for the horizontal wind and temperature profiles shown in Fig. 2a and b is shown in Fig. 2c.Sensitivity to the details of the SABER temperature profile was evaluated by shifting it vertically by ±3 km, that showed a consistence of the duct structure which was also shifted vertically, above and below of the nominal OH peak, but not changing significantly its shape.This suggested that the ducting environment is much more dependent on the structure of the wind profile than on the temperature profile, with the curvature term being particularly important (see below).It is clear from these profiles that the bore event was supported by a large-scale duct between ∼83 and 90 km, within which (m 2 > 0), with evanescent regions (m 2 < 0) above and below.These profiles also suggest that the duct coincides closely with the altitude of the OH airglow layer (Marsh et al., 2006), though we also note that the true ducting environment over Ferraz may have exhibited a different phase structure of the low-frequency motions than observed over Rothera, potentially resulting in a ducting structure centered at somewhat higher or lower altitudes than shown in Fig. 2c.
The observed phase speed of the bore estimated from the 2-D FFT cross spectrum of the airglow images discussed above was 92 m s −1 .This is ∼50 m s −1 faster than the wind along the bore propagation direction at any altitude, and ∼70 m s −1 faster than the mean wind at the center of the primary ducting region (see Fig. 2a and c), where the maximum values of m 2 are implied by the maximum positive temperature gradients (see Fig. 2b).Indeed, temperature inversions, or thermal ducts, have often been associated with mesospheric bores (Dewan andPicard, 1998, 2001;Smith et , 2003;She et al., 2004;Smith et al., 2005;Narayanan et al., 2009;Laughman et al., 2009Laughman et al., , 2011)).The intrinsic phase speed along the direction of propagation implied by the estimated m 2 profile is from ∼50 to 70 m s −1 , depending on whether we take the relevant background wind to be the maximum in the direction of propagation or that centered in the ducting region (with m 2 > 0).Assuming the latter is most relevant, we infer a corresponding intrinsic period of ∼8 min.The intrinsic parameters of the bore reported by Nielsen et al. (2006) had an opposite variation, since the wind in that case was opposite to the propagation direction, yielding a higher intrinsic phase speed (104 m s −1 ) and a lower period (5.9 min), compared to our observed parameters.
In a related study, Fechine et al. (2009) performed a comparative analysis of the terms in the dispersion relation for a similar bore observation.They inferred that it was likely supported by a Doppler duct, with apparent influences by the semidiurnal tide.
More recently, Bageston et al. (2010) analyzed a mesospheric wall event over Ferraz and concluded that it was supported by a thermal duct, with the winds having little effect on the duct configuration.
We now examine the influences of the wind field we have assumed to apply in the present case in greater detail.Fig. 3 shows a contour plot for the hourly-averaged wind component along the bore propagation direction observed over 24 h from 12:00 LT on 9 July to 12:00 LT on 10 July 2007.The vertical dashed red line at 23:30 LT on 9 July denotes the time when the bore front was near zenith in the all-sky images.
This figure indicates both shorter-period motions (periods of ∼4 to 6 h) and longerperiod motions (likely IGW or semidiurnal tide winds), with the former achieving the largest amplitudes prior to observation of the bore event.Of the observed motions, we expect that only the lower-frequency components would have had the potential to also occur over Ferraz without large phase and amplitude variations.This is because large horizontal winds imply relatively large vertical wavelengths and shorter observed periods imply smaller horizontal wavelengths.Assuming that the observed periods are indicative of comparable intrinsic periods, this would result in much less coherence at large horizontal distances.Thus, we focus on the longer-period motions hereafter.A Morlet wavelet analysis (Torrence and Compo, 1998) was used to identify the dominant low-frequency oscillations centered on the day and time of the bore observation.We used a wind time series at 86 km near the center of the duct extending over ∼3 days, from 00:00 LT on 9 July to 18:00 LT on 11 July, for this purpose.
Figure 4a shows the result of the Morlet wavelet spectral analysis of the wind time series presented in Figure 4b (black line).The power spectrum describes periods from 4 and 33 h as functions of time.Solid black lines indicate regions where the confidence level is higher than 90%, while the dashed black lines define the region where the results are confident excluding the border effects.The white vertical dashed line indicates the time when the mesospheric bore was seen in the zenith by the all-sky imager.Clearly seen in Fig. 4a is a maximum in spectral density centered at a period of ∼10 h, consistent with dominance of the wind field by some combination of IGWs and semidiurnal and terdiurnal tides at this time.Motivated by this result, a band-pass filter was applied to the original wind time series at 86 km high for periods between 6 and 14 h.The filtered time series is shown as a red line in Fig. 4b, and is seen to be consistent with the periodicities observed in the unfiltered time series in Fig. 4b.These results suggest that if the wind was important in defining the ducting structure, then the low-frequency IGW and tidal contributions would have played key roles.The relative importance of the tidal winds and the thermal structure in defining the ducting environment is discussed further below.
In order to assess the tidal contributions to the m 2 profile and potential influences on bore ducting, we employ the least mean square method, which makes use of the well established classical harmonic fitting.Details of this method and the equation employed for fitting the winds (and consequentely to obtain the tidal amplitudes and phases) are found in the work of Andrioli et al. (2009).Following this methodology we are able to reconstruct the wind profiles by means of the amplitude and phase associated to the tidal modes.In our case, it was identified that only the semi-diurnal and terdiurnal modes were present around the observation time of the mesospheric bore and these periodicities were used to reconstruct the vertical wind profiles near the time Figures in which the wave was seen in the all-sky images.We did not evaluate the contribution from the low-frequency IGW apart of the tides, since the winds reconstruction using just the tidal oscillations produced a good fit in comparison to the observed wind (see Fig. 5b).The winds reconstruction was performed in two ways, at first the tidal components were included, and later the wind contribution due to the tides was removed in order to observe the relative contribution of the tides in the observed duct.Here, the mean wind field was defined as the wind field smoothed as described above after removal of the tides.The mean wind defined here did not includes the low-frequency motions, since its periods were not included in the harmonic fitting.
The resulting m 2 profiles, wind profiles averaged from 23:00 to 24:00 LT on 9 July, and curvature terms are shown in Fig. 5a, b, and c, respectively.In the plots, black lines denote the profiles employing the full wind field as observed with the MF radar (which include tides, low-frequency IGWs and other low-frequency motions), blue lines denote results using only the mean wind, and red lines denote results using the tidal winds added to the mean wind.The buoyancy frequency squared and the corresponding term in the dispersion relation assuming the full wind field with filtered low-frequency motions are shown with red and black lines, respectively, in Fig. 5d.
Comparing the various profiles in Fig. 5, we see that all exhibit a region of maximum (positive) m 2 at altitudes ∼84 to 90 km, despite the varying contributions of wind magnitudes and curvature.This implies a significant role of the large and sustained temperature gradient spanning these altitudes in defining the ducting structure.We also see much smaller differences between the black and red profiles in panels (a), (b), and (c) than between the black and blue profiles.The blue profile of m 2 suggests that there is no evanescent region adjacent to the duct at lower altitudes, and that this structure may not support ducted or bore responses at all.This indicates that the tides and low-frequency motions make a significant contribution to the ducting structure enabling bore propagation, particularly in combination with the enhanced stability (and temperature gradient) at the ducting level.Even if the velocity field over Ferraz is somewhat different in phase (and altitude) than measured over Rothera, we expect Introduction

Conclusions References
Tables Figures

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Full the low-frequency IGW and tidal motions to impose ducting structure similar to that shown in Fig. 5, given the apparent importance of the wind and curvature terms in the dispersion relation, provided that some portion of the implied increase in m 2 due to the wind field coincides with the stability enhancement between ∼84 and 90 km.Together, these results imply a duct that depends on both the wind and temperature fields, consistent with expectations of theoretical studies of these dynamics (Fritts and Yuan , 1989;Laughman et al., 2009).

Conclusions
We described a bore event observed by an all-sky airglow imager over Ferraz Station on the night of 9-10 July 2007.The ducting environment was assumed to be of large spatial extent, given the extent of the bore itself across the full field of view of the imager.We thus employed winds measured at Rothera Station at the south of Ferraz and a temperature profile measured by SABER on TIMED at north, and somewhat earlier than the bore observation, to evaluate the environmental factors likely to have enabled the bore propagation.Our analysis revealed that tidal motions and low-frequency IGW were likely responsible for the wind structure contributing to the ducting environment, but that a maximum in N 2 due to a large positive temperature gradient (∼40 K extending over ∼6 km) also contributed to the dual thermal and wind duct supporting bore propagation.In particular, the winds were found to be essential in defining the ducting environment in this case, in contrast to other bore events where the temperature structure alone appeared to be sufficient to enable ducting.Future observations at Ferraz Station will benefit from local wind measurements by a new meteor radar recently installed at that site.Introduction

Conclusions References
Tables Figures

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Full Discussion Paper | Discussion Paper | Discussion Paper | mesospheric gravity waves campaign was carried out at Ferraz Station (62.1 • S, 58.4 • W), located on the King George Island, Antarctica, from May to October 2007, using an all-sky CCD imager.The imaging system uses a 2-inch wide band near infrared (NIR) optical filter (715-930 nm), with a notch at 865.5 nm to suppress the O 2 (0-1) emission to measure OH airglow emissions near the mesopause (∼87 km).An exposure time of 20 s was used for each image, yielding a sampling interval of ∼38 s since the imager does not have a filter wheel.The original images (with 1024×1024 pixels) were not binned, but cropped to 512×512 pixels, due to limitations of the optical Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | al.
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Fig. 1 .Fig. 3 .
Fig. 1.All-sky OH airglow images at three times spanning 24 min on the night of 9-10 July 2007 showing a mesospheric bore propagating from southwest to northeast.

Fig. 5 .
Fig. 5. (a) Profiles of m 2 for different wind profiles as shown in (b): observed wind (black); mean wind (without tides, blue) and tidal winds alone (red).(c) Curvature terms due to the winds in (b).(d) N 2 (red, upper axis) and the buoyancy term contribution to m 2 (black, lower axis) for comparison with panels (a) and (c).