Geophysical Research Letters

First polar mesosphere summer echoes observed at Davis, Antarctica (68.6°S)

Authors


Abstract

[1] We report the first observations of polar mesosphere summer echoes (PMSE) above the high-latitude Southern Hemisphere (SH) station Davis, Antarctica (68.6°S, 78.0°E geographic; 74.6°S magnetic). Observations were obtained using a 55 MHz atmospheric radar, the first stage of which was commissioned late in the austral summer of 2002–2003. The radar commenced mesosphere observations with approximately 20 kW of transmitted power in October 2003. PMSE were recorded from 19 November to 3 December 2003 and, after a break in radar operation, from 27 January to 21 February 2004. We present the initial seasonal and diurnal occurrence morphology from 180 hours of Davis PMSE observations. Our initial findings reveal that SH PMSE show similar backscatter echo characteristics and occurrence properties to those reported for the Northern Hemisphere (NH).

1. Introduction

[2] The observation and investigation of polar mesosphere summer echoes (PMSE) has until recently been limited to the Northern Hemisphere (NH). PMSE are strong radar echoes scattered from Bragg scale structures within the plasma and neutral gases in the summer polar mesosphere [see Cho and Kelley, 1993; Cho and Röttger, 1997]. They were first observed using the 50 MHz radar at Poker Flat, Alaska by Ecklund and Balsley [1981].

[3] The climatology of Southern Hemisphere (SH) PMSE is largely unknown due to the lack of radars deployed at southern polar latitudes. The first observations of SH PMSE were carried out by Balsley et al. [1993] at Machu Picchu base on King George Island (62.1°S, 58.5°W) during the late summer of 1992–1993. Analysis of these observations suggested that there was no PMSE at this latitude [Balsley et al., 1993, 1995]. However, operation of this system the following summer in an upgraded form did identify PMSE [Woodman et al., 1999] albeit at a lower power and for a shorter period of time than expected from NH experience.

[4] It has been suggested that hemispheric differences in temperature, water vapour and dynamics are behind the relative absence of SH PMSE [Balsley et al., 1995; Woodman et al., 1999; Huaman and Balsley, 1999] although Lübken et al. [1999] question whether the temperature structure is dissimilar enough to be the cause. Further observations of PMSE at southern latitudes are needed to determine whether a hemispheric difference in PMSE does indeed exist.

[5] The recent installation of the VHF radar at Davis has enhanced our knowledge of the characteristics of SH PMSE. We report on observations of PMSE made at Davis (68.6°S) during the 2003–2004 austral summer and present an initial morphology of their occurrence properties. We also compare these findings with some NH observations obtained at Andenes (69.3°N) [Hoffmann et al., 1999; Bremer et al., 2003].

2. Experiment

[6] A 55 MHz atmospheric radar was commissioned at Davis, Antarctica (68.6°S, 78.0°E geographic; 74.6°S magnetic) late in the austral summer of 2002–2003. The radar was manufactured by Atmospheric Radar Systems and its specifications for the observations described in this paper are given in Table 1. The radar commenced mesosphere observations with ∼20 kW of transmitted power from October 2003. Unfortunately, as a consequence of technical difficulties the radar transmitter ceased operation on 4 December 2003 but resumed on 27 January 2004. The most southerly observations of PMSE were recorded at 68.6°S during the intervals 19 November to 3 December 2003 and 27 January to 21 February 2004. However, most of our discussion focuses on the later interval of 20 days of PMSE events when the radar transmission characteristics were more stable.

Table 1. Technical Parameters for the Davis VHF Radar During Mesospheric Observations, 2003–2004
Specifications
Frequency55.0 MHz
Peak power20 kW (nominal)
Transmitting antenna144 Yagis in sub-groups of 4
Antenna spacing0.7 λ
Effective antenna area2180 m2 (transmission)
Effective pulse width600 m
Pulse typeGaussian (no coding)
Range resolution300 m
Number of coherent integrations116
Number of incoherent integrations15 (per hour)
Pulse repetition frequency1160 Hz
Effective sampling time0.1 s
Antenna beam directionvertical
Antenna beam width

[7] The start-up power-aperture (PA) product of the Davis VHF radar was 2.5 × 106 Wm2 compared with a PA ∼6.25 × 105 Wm2 for the VHF radar used by Woodman et al. [1999] at Machu Picchu (62.1°S). This fourfold increase in PA, and the fact that Davis is 6.5° further south than Machu Picchu, leads to an expectation that the rate of detection of PMSE in the observations described in this paper will be greater than that found by Woodman et al. [1999].

3. Observations and Discussion

[8] The altitude-time plot of the backscattered signal-to-noise ratio (SNR) observed at Davis on 2 February 2004 that is presented in Figure 1 shows the typical structure of PMSE (where the scaling on the image bar is defined as SNR = 10 log10(S/N)). It can be seen that the PMSE appear as patches with multiple intensity peaks within individual patches [Kirkwood et al., 1995]. Intervals of PMSE with continuous layers as reported for NH PMSE [Cho and Röttger, 1997] are not evident in Figure 1, due in part to the smoothing applied to the hourly averaged spectra (15 per hour) during data processing for this initial study. However, an ongoing study of the fine structure of these PMSE patches has shown some of these patches to be comprised of layered structures.

Figure 1.

Altitude-time plot of the backscattered signal-to-noise ratio (SNR) on 2 February 2004 of PMSE observed at Davis.

[9] The Davis observations include 51 PMSE patches similar to those shown in Figure 1, with durations ranging between 0.5 and 11.5 hours. Most PMSE patches had durations of 1.0 to 3.5 hours. Furthermore, the PMSE patch thickness was in the range of 1.0 to 8.5 km, although two distinct maxima in thickness distribution were evident near 1.5 to 2.5 km and 3.5 to 4.5 km. The peak hourly SNR of the seven PMSE patches observed on 2 February 2004 (as shown in Figure 1) ranged between 5 and 24 dB and the altitude of maximum SNR ranged between 85 and 88.5 km. These characteristics were typical for Davis PMSE observations. Here and in what follows the SNR dB value is the backscatter magnitude above the background noise floor extracted from hourly average altitude-SNR profiles.

[10] PMSE were observed on 27 days for a total of 180 hours. The morphology of SH PMSE occurring during the 2003–2004 summer season was compiled from the occurrences of PMSE ‘events’. In this paper an event is recorded when PMSE unambiguously exceeded the background noise level by 5 dB on altitude-time plots of SNR variation for an interval of 30 minutes in a height bin 0.5 km wide. Figure 2 shows the seasonal occurrence histogram of the Davis PMSE events. The PMSE season commenced on 19 November 2003 (although lower altitude short duration PME were evident from late October but are not discussed here) and ended on 16 February 2004 (although a discrete low intensity event was observed on 21 February). Note that the data gap in the centre of Figure 2 is due to a technical problem with the radar. The start and end times of the PMSE season at Davis compare with 19 May and 28 August (or 19 November and 28 February – when translated by six months to compare with our SH observations) for Andenes (69.3°N) as reported by Hoffmann et al. [1999] and Bremer et al. [2003]. It was noted above that the transmitter characteristics were unstable during the early part of the season. For this reason, the statistical analysis that follows is limited to the period 27 January to 21 February 2004.

Figure 2.

Seasonal occurrence histogram of PMSE observed at Davis from 19 November until 3 December 2003 and from 27 January until 21 February 2004.

[11] The NH observations show that PMSE occur in a well defined layer between 80 and 93 km with a peak around 86 km [Cho and Kelley, 1993]. This is slightly below the summer mesopause temperature minimum at 88 km, and above the noctilucent cloud (NLC) or polar mesospheric cloud (PMC) layer at 83–84 km [Thomas, 1991]. Figure 3 shows the altitude distribution for (i) all PMSE events, and (ii) the most intense PMSE events with maximum SNR in each 30 minute time bin. This plot shows that SH PMSE peak at an altitude of 86 km with a distribution ranging between 81.5 and 92 km for all PMSE events. A large number of events occurred in the 85 to 88 km altitude range where most of the PMSE events with maximum SNR intensity were also found. However, two peaks are evident at 85 and 87 km altitude respectively for the most intense PMSE events. Although this distribution was compiled using a short data set, the altitude distribution of SH PMSE is also similar to those cited for the NH [Latteck et al., 1999; Hoffmann et al., 1999]. For instance Bremer et al. [2003] found the largest PMSE occurrence rates with a maximum near 85–86 km at Andenes (NH) between 1999 and 2001. Calibration of the Davis radar showed the range to be within ±300 m.

Figure 3.

Altitude distribution of PMSE with event resolution of 500 m by 30 minutes during the interval 27 January–21 February 2004: for all PMSE observations (grey); and for PMSE of maximum SNR (black).

[12] In Figure 4a the total number of PMSE events (30 minute bins) are plotted against time (UT) to establish the daily occurrence variation of PMSE at Davis between 27 January and 21 February 2004. We found a maximum occurrence peak near 0930 UT (which was part of a broader peak between 0830 and 1200 UT); a secondary maximum near 0300 UT; and a distinct minimum at 2000 UT (within a broader minimum between 1900 and 2100 UT). Note that at Davis, local solar time = UT + 0512 hours (i.e., local solar noon ∼0648 UT). The observed diurnal and semidiurnal variation in Davis PMSE occurrence is similar to NH occurrence distributions, although there are differences in the times of the respective maxima and minima: Hoffmann et al. [1999] found PMSE observations at Andenes from 1994–1997 had a clear maximum at 1300–1400 LT (1400–1730 LT at Davis) and a pronounced minimum at about 1900–2100 LT (0030–0230 LT at Davis) with only a small year-to-year variability, and a secondary maximum near midnight/early morning although unstable in time.

Figure 4.

The daily morphology of Davis PMSE observed during the interval 27 January–21 February 2004; (a) occurrence for 30 minute event bins, (b) maximum SNR, and (c) altitude at maximum SNR.

[13] The daily peak SNR for the latter part of the Davis PMSE season (not shown) revealed that for the majority of days the peak intensity was >15 dB above the noise level, with a maximum value of 32 dB on 3 February 2004. Note that the SNR variation as a function of UT hour contains an approximate 2 dB diurnal variation due to galactic cosmic noise. The daily SNR variation observed at Davis, averaged over the interval 27 January to 21 February 2004, is given in Figure 4b. These SNR intensity levels are comparable with PMSE observed at the equivalent NH latitude [see Hoffmann et al., 1999; Bremer et al., 2003]. We note that the Davis VHF radar must be fully calibrated and the SNR levels normalized against results from other radars prior to a definitive comparison being made. The measured echo strengths at Machu Picchu rarely exceeded ∼7 dB for hourly averages over the noise level during the short PMSE observation interval [Woodman et al., 1999]. This may be explained by a latitudinal distribution with PMSE intensities increasing with latitude [Huaman et al., 2001], since Davis is 6.5° poleward of Machu Picchu. Also of interest is the report by Sarango et al. [2003] of an 11 dB stronger PMSE intensity at Artigas station, Antarctica during 2001 compared with PMSE at Machu Picchu during 1998 (station separation 30 km). These authors suggest that annual variability of PMSE intensity is possible. Concurrent VHF radar observations at Davis and Machu Picchu will provide the opportunity to investigate both annual and latitude effects on PMSE occurrence and intensity.

[14] The altitude of maximum PMSE SNR for the Davis summer was found to decrease toward the end of the PMSE season, i.e., from 89.5 km on 29 January 2004 to 85 km on 16 February 2004, consistent with NH observations [Bremer et al., 2003]. In Figure 4c we plot the daily variation of the altitude of maximum SNR averaged over the period 27 January to 21 February 2004. This plot shows that PMSE at maximum SNR range in altitude from 85.8 to 87.7 km often with average temporal variations as short as two hours. Comparable daily altitude and temporal variations are clearly seen for the seven PMSE patches observed on 2 February 2004 (see Figure 1). Also evident is the characteristic altitude-time negative gradient of individual patches, of the order of 1 km/h, as reported for NH PMSE [see Cho and Kelley, 1993; Cho and Röttger, 1997]. We will report in future studies on dynamical processes occurring in the mesosphere region such as those driven by the zonal and meridional winds, atmospheric gravity waves, planetary waves and tides that impact on PMSE altitudes.

[15] Finally, in Figure 5 PMSE occurrence as a function of altitude and time of day are shown for the late-summer observations (27 January to 21 February 2004). This plot shows the occurrence trends discussed above, i.e., primary and secondary maxima and a distinct minimum.

Figure 5.

Frequency of PMSE occurrence for observations obtained at Davis for 20 days between 27 January and 21 February 2004. PMSE was defined to occur in an altitude-time bin of 500 m by 30 minutes where the SNR unambiguously exceeded the noise floor by 5 dB.

[16] It is noted that a complete observation season of PMSE at Davis is needed to substantiate our findings. Observations around the summer solstice are needed to extend the climatology of SH PMSE beyond the start and end of the PMSE season as reported in this initial Davis study. The proposed introduction of a beam steering capability will enable us to investigate the aspect sensitivity of PMSE.

4. Conclusions

[17] We have presented the first daily and seasonal occurrence morphology of SH PMSE. Although limited by technical difficulties, we have found that SH PMSE show similar backscattered echo characteristics and occurrence properties to those reported for the NH. A summary of our main observations of SH PMSE follows.

[18] (i) PMSE tend to occur as a series of individual patches, although altitude-SNR profiles show multiple peaks occur within patches;

[19] (ii) PMSE patches have durations from 1.0 to 3.5 hours, but can exist for up to 11.5 hours;

[20] (iii) PMSE patches have depths typically from 1.5 to 4.5 km, however, they can range from 1.0 to 8.5 km;

[21] (iv) The PMSE season starts on 19 November and ends on 16 February with the occasional outlier event;

[22] (v) We observed PMSE for 180 hours despite an extended data gap from 4 December 2003 until 26 January 2004;

[23] (vi) PMSE SNR intensities up to 32 dB above the background noise level are comparable to those reported for the NH;

[24] (vii) The diurnal occurrence distribution exhibits primary (0930 UT) and secondary maxima (0300 UT) with a distinct minimum (2000 UT); and

[25] (viii) PMSE occur most commonly at an altitude of 86 km but are observed in the altitude range 81.5 to 92 km similar to NH events.

[26] We conclude that PMSE observed at the SH station Davis, Antarctica during the 2003–2004 summer season exhibit little difference to observations at similar northern latitudes. In particular the SH PMSE altitude, intensity, diurnal and seasonal occurrence distributions are remarkably similar to their NH counterparts. This observation is consistent with the result of Lübken et al. [1999] that SH mesopause temperatures of 129 K at 87 km at Rothera (68.0°S) during January are favourable for the existence of NLC and PMSE. However we stress that additional observations from Davis and other southern polar latitudes are needed to verify this similarity.

Acknowledgments

[27] The authors would like to acknowledge the technical support with installation and operation of the Davis VHF radar by L. Symons, R. Groncki and D. Ward. The antenna array and infrastructure were installed with the excellent assistance of P. Saxby, A. Taylor, C. Heath, D. Ratcliffe, J. Lea and the expeditioners at Davis during the 2002–2003 austral summer. J. French and J. Whelan provided assistance with the preparation of the Figures. We also acknowledge the support given by Atmospheric Radar Systems (ATRAD) the supplier of the radar.

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