GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L02106, doi:10.1029/2003GL018776, 2004
Statistics of Antarctic mesospheric echoes observed with the SuperDARN Syowa Radar K. Hosokawa,1 T. Ogawa,2 A. S. Yukimatu,3 N. Sato,3 and T. Iyemori4 Received 5 October 2003; revised 21 November 2003; accepted 2 December 2003; published 20 January 2004.
[1] An oblique-sounding coherent HF radar of the SuperDARN (Super Dual Auroral Radar Network) at about 11 MHz sometimes receives peculiar backscatter returns, which are suspected as Polar Mesosphere Summer Echoes (PMSE), from ranges very close to the radar site. To disclose their statistical features, we looked for such echoes that were observed, under quiet geomagnetic conditions, during 46 months from 1997 until 2000 with the SuperDARN radar at Syowa Station, Antarctica (69.0S, 38.6E). With some strict criteria for echo selection, we identified 22 events in summer months and 2 events in August and September. Their distinct seasonal variation evidences that these near range echoes in summer months are indeed the Antarctic PMSE. Origin of the two spring echoes is not clear at the moment. The occurrence probability of the summer echoes seems to increase with year. We, however, need more data accumulation to confirm this trend. These results indicate that the SuperDARN radars can be used to routinely monitor the activity of PMSE in a wide area of INDEX the polar mesosphere in both hemispheres. T ERMS : 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0340 Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342). Citation: Hosokawa, K., T. Ogawa, A. S. Yukimatu, N. Sato, and T. Iyemori (2004), Statistics of Antarctic mesospheric echoes observed with the SuperDARN Syowa Radar, Geophys. Res. Lett., 31, L02106, doi:10.1029/ 2003GL018776.
1. Introduction [2] Polar Mesosphere Summer Echoes (PMSE), which are regularly observed in summer months at polar latitudes, are strong radar backscatter from the upper mesosphere (Cho and Ro¨ttger [1997] and references therein). PMSE in the Northern Hemisphere have been detected with ground-based radars whose frequency ranges from HF to UHF. In contrast to the Arctic PMSE,
1 Department of Information and Communication Engineering, The University of Electro-Communications, Tokyo, Japan. 2 Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi, Japan. 3 National Institute of Polar Research, Tokyo, Japan. 4 Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto University, Kyoto, Japan.
Copyright 2004 by the American Geophysical Union. 0094-8276/04/2003GL018776
observational history of the Antarctic PMSE is very short and our knowledge of them is still poor. This could be due to the lack of VHF radar facilities suitable for the detection of PMSE and/or lower occurrence of PMSE itself in the Southern Hemisphere [Huaman and Balsley, 1999]. Further detailed investigations are required to clarify the characteristics of the Antarctic PMSE and to estimate possible interhemispheric asymmetry of PMSE occurrence probability. However, a problem is that only a small number of VHF radar have been operative in Antarctica [Woodman et al., 1999]. Alternative way of monitoring the Antarctic mesosphere is indispensable for more detailed analysis of PMSE. [3] Recently, Ogawa et al. [2002] identified peculiar radar echoes at the near ranges of the SuperDARN Syowa radars in Antarctica (hereinafter referred as peculiar near range echo: PNRE). Their origin can not be explained by backscatters from E region field-aligned irregularities (FAIs) and their morphological features are very similar to those of the Arctic PMSE. However, amount of the dataset surveyed was just 4 months, which is not sufficient to ascertain the PMSE hypothesis. In the present paper, we look for PNRE events in 46-months observations of the SuperDARN Syowa East radar and compare echo characteristics such as seasonal variation and local time distribution with those of the Arctic PMSE. Long-term variability of the echo occurrence is also investigated.
2. SuperDARN Syowa Radar [4] Data from the Super Dual Auroral Radar Network [Greenwald et al., 1995] Syowa East radar in Antarctica (69.0S, 38.6E) are employed. Operating frequency could be set from 8 to 20 MHz, but most often is set to 11 MHz. The radar beam in the normal operation is sequentially scanned from beam 0 to beam 15 with a step in azimuth of 3.33. It takes approximately 7 s to integrate backscatter returns in one direction and about 120 s is needed for a full scan of all directions. The first range gate is set to 180 km with a range resolution of 45 km. Most of PMSE have been detected by using vertical incidence radars with short altitude resolution, which is due to the strong aspect sensitivity and thin layer of PMSE. Range resolution of the SuperDARN radars is obviously thicker than the PMSE layer. However, the SuperDARN employs the oblique sounding system, in which maximum sensitivity occurs at elevation angles of 15– 35 depending on frequency [Greenwald et al., 1985]. The oblique incidence enable the radars to detect the backscatter echoes at mesospheric altitudes. Ogawa et al. [2003] investigated angle of arrival of PNRE backscatters and demonstrated that they come
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Figure 1. A collection of the range versus time backscatter power plots along beam 7 of the events selected. Duration of the echoes in minutes and operating frequency are also shown in the bottom of each panel. The panels with black background are the events that occurred during summer months from November to February.
from upper mesosphere. Details of the radar operation and geometry of the radar field-of-view are given by Ogawa et al. [2002].
3. Statistical Analysis 3.1. Event Selection [5] There exist three competitive backscatter targets at the near ranges (say 180– 500 km) of the SuperDARN Syowa radar. Those are meteor trails, E region field-aligned irregularities (FAIs) and PNRE reported by Ogawa et al. [2002]. Simple calculation of echo occurrence rate is not effective in deriving reliable seasonal distribution of PNRE. In order to distinguish E region FAIs and meteor echoes from PNRE correctly, we performed the following procedures. In general, radars observe E region FAIs when their fields-of-view intersect the region of auroral electrojet. Then, they can be excluded by comparing echo appearance with local geomagnetic field variations. Here, K-index and H-component magnetogram at Syowa station are employed. In addition, the slant range where E region FAIs appear systematically changes with increasing beam number for the Syowa East radar (see Ogawa et al. [2002] in detail). These two diagnostic checks are firstly applied to the dataset. Then, we cautiously omitted near range echoes which are accompanied by apparent E region FAIs at farther ranges up to 500 km. Meteor trails are also one of the major backscatter targets in the near ranges of the SuperDARN radars. Average characteristics of the parameters are very similar to
those of the PNRE. However, meteor echoes appear randomly both in time and space because they are short-lived and range-isolated. In contrast, the PNRE last longer than 20 minutes in a fixed range. Hence, meteor echoes can be accurately excluded. [6] In summary, we have selected echoes satisfying the following conditions simultaneously; (1) quiet geomagnetic conditions (local K index is 0 or 1, and no corresponding variation in H-component magnetogram of Syowa station), (2) echoes existing between 180 and 315 km in slant ranges on almost all beams, (3) no random appearance in time and space, (4) no E region backscatters and meteor echoes together with. Although strict criteria were applied to distinguish PNRE from the other backscatter targets correctly, total 24 events were identified during 46 months from March 1997 to December 2000. Figure 1 is a collection of the backscatter plots (along beam 7) of these events in range and time coordinates. Duration of the echoes in minutes and operating frequency are also shown in the bottom of each panel. Most of the 22 events occurred in summer months from November to February, which are displayed as a panel of black background. However, two echoes that appeared in spring (August and September) passed the selection criteria (Events 11 and 16). 3.2. Statistical Characteristics [7] Main objective of this paper is to clarify the seasonal distribution of the PNRE occurrence in a statistical
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Figure 2. Seasonal occurrence probability of the echoes summarized in Figure 1. Histogram indicates the total duration in minutes binned into monthly intervals. The solid line with open circle indicates the percentage of the occurrence probability. fashion. Occurrence probability is estimated by using the total duration time of the events in minutes. It might be reasonable to use the number of the day when the echoes appeared as an indicator of the occurrence distribution. However, the number of the events identified is just 24, which is clearly insufficient to discuss the occurrence probability on the basis of the number of the days. The bottom panel of Figure 2 shows seasonal occurrence probability of the 24 echoes summarized in Figure 1. Histogram indicates the total duration in minutes binned into monthly intervals. The solid line with open circles indicates the occurrence probability in percent, which was derived by dividing the total duration by the period of quiet geomagnetic conditions (local K-index is 0 or 1). Due to the strict selection criteria, significant number of potential PNRE could be rejected. Absolute values do not correspond to the real occurrence rate of PNRE. Only the trend of the derived values should be argued in the following discussions. [8 ] Occurrence probability is enhanced in summer months, while there is no clear peak in other seasons except for September 1998 and August 1999. Noteworthy is that the occurrence probability seems to increase with increasing year. Peak values of the monthly occurrence probability for three summer seasons from 1997 to 2000 are 1.07%, 2.98% and 3.19%, respectively. The occurrence in summer months from November to February are also likely to increase with year (0.48%, 1.00% and 1.61%). Figure 3 shows how the occurrence of the echoes varies with UT and local time (LT = UT + 3 hours; magnetic local time UT). Filled bars indicate duration of the events that occurred during summer and open bars do that of the events in the other seasons (Events 11 and 16). Most of the echoes in summer months are found to appear between 12 and 18 LT, although some of them appear before local midnight (between 19 and 01 LT).
4. Discussion 4.1. Seasonal Dependence [9] The seasonal variation of PNRE shown in Figure 2 is in good agreement with the seasonal dependence of
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PMSE presented in the previous studies [e.g., Palmer et al., 1996]. The first and last dates when summer PNRE were observed in each season are presented in Figure 2. Bremer et al. [2003] investigated 7-years data from the Andenes VHF radar and reported that averaged date of the first seasonal appearance of PMSE is May 19 (Julian day 139), and that of the last is August 28 (Julian day 240). These dates correspond to November 19 and February 27 in the Southern Hemisphere, respectively. All of the PNRE in summer months are well inside of this period. Our statistical results evidence the PMSE hypothesis by Ogawa et al. [2002], and suggest that near range observations of the SuperDARN radars actually contain PMSE. [10] We have found two echoes that occurred in early spring. Balsley et al. [1983] have identified mesospheric echoes in winter months which appeared to be correlated with the high-energy particle precipitation into the mesospheric altitudes. Czechowsky et al. [1989] reported that these echoes are detectable only during periods when electron densities are enhanced by energetic particle precipitation. During the intervals of the two spring echoes, however, enhancement of energetic proton and electron flux was not observed by the geosynchronous satellites GOES and no signature of the radio wave absorption was identified by a riometer at Syowa station, which indicate that the two near range echoes in spring are not associated with the high-energy particles precipitation. In addition, these spring echoes did not correspond to meteor shower bursts. Origin of these echoes is not clear, and more detailed study is needed. 4.2. Local Time Distribution [11] Balsley et al. [1983] reported that the occurrence of PMSE has a maximum value 1 – 2 hours after the local noon and that secondary peak usually appears around the local midnight. This semi-diurnal pattern is confirmed by many authors [e.g., Palmer et al., 1996]. Our statistical result is basically consistent with those of previous studies. However, there exist some differences
Figure 3. Occurrence distribution of the echoes listed in Figure 1 with UT and local time. Filled bars indicate duration of events that occurred in summer months (November to February), and open bars do that of the events in the other seasons.
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among them. One is that the secondary peak around midnight is not so prominent in our dataset, particularly after 1997. This could be because we have eliminated near range echoes that appear together with the E region FAIs. E region FAIs are predominantly the nighttime phenomena and their occurrence is enhanced with increasing year. Then, some PNRE on the nightside might be filtered out together with the nighttime E region auroral FAIs, which implies that the occurrence on the nightside might be underestimated in this analysis. The other difference is that few events are identified before local noon. One possible reason for this inconsistency is that the SuperDARN observations of PNRE are contaminated by backscatters from meteor at mesospheric heights. Meteor echoes are dominative backscatter target of the SuperDARN near range observations, particularly on the dawn hemisphere (for details of the occurrence distribution of meteor echoes at Syowa Station, see Ogawa et al. [1985]). In the pre-noon sectors the radar often observes PNRE and meteor echoes simultaneously. In such cases, PNRE cannot be identified due to a contamination of meteor echoes. Then occurrence of PNRE could be underestimated. Although some differences exist, local time distribution shown in Figure 3 belongs to the semi-diurnal cycle deduced from the previous Arctic observations. Recently, MF radar has been operative at Syowa Station. Comparison between PNRE appearance and semi-diurnal tide will be done in the future. 4.3. Long-Term Variabilities [12] Cho et al. [1992] suggested that PMSE are smallscale structures which are maintained by reduced electron diffusivity due to the effect of charged ice particles and aerosols. The existence of such particles is supported by low temperature at polar summer mesosphere [Lu¨bken et al., 1999] and the observations of noctilucent clouds consisting of water ice particles in the lower part of the PMSE structure [von Zahn and Bremer, 1999]. Thus, long-term trends in the occurrence of PMSE might be an indicator of negative temperature trends and/or positive trends of the mesospheric water vapor concentration. [13] Recently, Bremer et al. [2003] investigated longterm variation of PMSE using a VHF radar in Norway. They presented that the occurrence increased gradually from 1996 to 2001. However, this trend was found to be positively correlated with the mesospheric ionization level mainly indexed by solar cycle variations of the Lyman a radiation and also by the flux of precipitating high energetic particles. They pointed out that amount of the database is not enough (6 years) to conclude that long-term trend is directly connected to the mesospheric climate change. The period of the dataset used in this study also well corresponds to the phase of increasing solar activities. Mesospheric ionization level as indexed by the Lyman a or F10.7 is considered to be increasing monotonically. Then, even if the occurrence of PMSE increases by the effect of the mesospheric climate change, their signature might not be identified because mesospheric ionization level biases the long-term trend of PMSE. In order to acquire more reliable and con-
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clusive result, occurrences of the echoes must be investigated over the period longer than at least one solar cycle.
5. Summary and Conclusion [14] We have investigated statistical characteristics of the peculiar near range echoes (PNRE), first reported by Ogawa et al. [2002], using 46-months measurements of the SuperDARN Syowa East radar in Antarctica. Although strict criteria were applied to distinguish PNRE from the E region FAIs and meteor echoes correctly, 24 events were identified. Most of them (22 events) appear in summer months from November to February with a maximum in December or January. This distinct seasonal variation implies that PNRE events in summer months indeed correspond to the Antarctic PMSE. Two events occurred in spring (August and September), whose origin is not clear at the moment. Occurrence of PNRE seems to increase with year, which might suggest that conditions for generating PMSE (mesopause temperature and water vapor concentration) are becoming more favorable in the recent years. However, amount of the data employed is clearly not sufficient to extract essential long-term trend due to the biases such as the increasing solar activities. Then, long-term trend of PNRE (and PMSE) occurrence is still an open question. However, 15 SuperDARN radars are currently operative in both hemispheres. More detailed analysis of the long-term variabilities and interhemispheric (or regional) asymmetry of PMSE activity will be done in near future. [15] Acknowledgments. The Ministry of Education, Culture, Sports, Science and Technology supports the SENSU Syowa HF radar systems. The 39th, 40th, and 41th JAREs (Japanese Antarctic Research Expedition) have carried out the HF radar operation.
References Balsley, B. B., W. L. Ecklund, and D. C. Fritts (1983), Mesospheric radar echoes at Poker Flat, Alaska: Evidence for seasonally dependent generation mechanisms, Radio Sci., 18, 1053 – 1058. Bremer, J., P. Hoffmann, R. Lattechk, and W. Singer (2003), Seasonal and long-term variations of PMSE from VHF radar observations at Andenes, Norway, J. Geophys. Res., 108(8), 8438, doi:10.1029/ 2002JD002369. Czechowsky, P., I. M. Reid, R. Ru¨ster, and G. Schmidt (1989), VHF radar echoes observed in the summer and polar winter mesosphere over Andoya, J. Geophys. Res., 94(D4), 5199 – 5217. Cho, J. Y. N., T. M. Hall, and M. C. Kelley (1992), On the role of charged aerosols in polar mesosphere summer echoes, J. Geophys. Res., 97(D1), 875 – 886. Cho, J. Y. N., and J. Ro¨ttger (1997), An updated review of polar mesosphere summer echoes: Observation, theory and their relationship to noctilucent clouds and subvisible aerosols, J. Geophys. Res., 102(D2), 2001 – 2020. Greenwald, R. A., K. B. Baker, R. A. Hutchins, and C. Hanuise (1985), An HF phased-array radar for studying small-scale structure in the highlatitude ionosphere, Radio Sci., 20, 63 – 79. Greenwald, R. A., et al. (1995), DARN/SuperDARN: A global view of the dynamics of high-latitude convection, Space Sci. Rev., 71, 761 – 796. Huaman, M. M., and B. B. Balsley (1999), Difference in near-mesopause summer winds, temperatures, and water vapor at northern and southern latitudes as possible causal factors for inter-hemispheric PMSE differences, Geophys. Res. Lett., 26(11), 1529 – 1532. Lu¨bken, F.-J., M. J. Jarvis, and G. O. L. Jones (1999), First in situ temperature measurements at the Antarctic summer mesopause, Geophys. Res. Lett., 26(24), 3581 – 3884.
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Ogawa, T., K. Igarashi, Y. Kuratani, R. Fujii, and T. Hirasawa (1985), Some initial results of 50 MHz meteor radar observation at Syowa Station, Mem. Natl Inst. Polar Res., Spec. Issue, 36, 254 – 263. Ogawa, T., N. Nishitani, N. Sato, H. Yamagishi, and A. S. Yukimatu (2002), Upper mesosphere summer echoes detected with the Antarctic Syowa HF radar, Geophys. Res. Lett., 29(7), doi:10.1029/ 2001GL014094. Ogawa, T., N. F. Arnold, S. Kirkwood, N. Nishitani, and M. Lester (2003), Finland HF and Esrange MST radar observation of polar mesosphere summer echoes, Ann. Geophys., 21, 1047 – 1055. Palmer, J. R., H. Rishbeth, G. O. L. Jones, and P. J. S. Williams (1996), A statistical study of polar mesosphere summer echoes observed by EISCAT, J. Atmos. Terr. Phys., 58, 307 – 315. Woodman, R. F., B. B. Balsley, F. Aquino, L. Flores, E. Vazquez, M. Sarango, M. M. Huaman, and H. Soldi (1999), First observations of polar mesosphere summer echoes in Antarctica, J. Geophys. Res., 104(A10), 22,577 – 22,590.
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von Zahn, U., and J. Bremer (1999), Simultaneous and common-volume observations of noctilucent clouds and polar mesosphere summer echoes, Geophys. Res. Lett., 26(11), 1521 – 1524.
K. Hosokawa, Department of Information and Communication Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofushi, Tokyo 182-8585, Japan. (
[email protected]) T. Ogawa, Solar – Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi, Japan. (
[email protected] – u.ac.jp) N. Sato and A. S. Yukimatu, National Institute of Polar Research, Tokyo, Japan. (
[email protected];
[email protected]) T. Iyemori, Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto University, Kyoto, Japan. (
[email protected] – u.ac.jp)
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