Simultaneous observation of sodium atoms, NLC and PMSE in the summer mesopause region above ALOMAR, Norway (69°N, 12°E)

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Abstract

Based on simultaneous observations of Na density, polar mesosphere summer echoes (PMSE) and noctilucent cloud (NLC) at ALOMAR, Norway (69°N) during the summers of 2001 and 2002, we report the strong anti-correlation of sodium atoms with NLC, suggesting that in the presence of large ice particles Na is always absent, and a weaker anti-correlation of sodium atoms with PMSE, suggesting that a population of sub-visible particles also deplete Na to a varied degree. This conclusion is in general agreement with recent reports on iron and potassium atom–ice particle anti-correlation. For this study, we use data taken with the Weber Na lidar, the ALWIN VHF radar, and the RMR lidar on 8 specific days in 2001 and 4 days in 2002 plus half-monthly average variation over the summer 2001 based on all 17 days of coincident measurements.

Introduction

The first lidar detection of a noctilucent cloud (NLC) was made in August 1989 at the Andøya rocket range, Norway (69°N, 16°E) by a ground-based, excimer-pumped narrowband dye laser operated at 589 nm (Hansen et al., 1989). Despite the fact that a Na-NLC vertical separation was noted, implicating that lidar cannot measure temperature at the NLC altitudes, there existed a desire in the fluorescence lidar community for concurrent temperature measurement, in the hope to investigate the atmospheric background in which the NLC ice particles form, grow and decay. This turned out to be a difficult proposition, since the early Na lidar (Kurzawa and von Zahn, 1990) was unable to make temperature measurements under sunlit conditions in the summer Arctic where NLCs reside. The size distribution of NLC particles was determined later by 3-color observations by the Rayleigh–Mie–Raman (RMR) lidar (von Cossart et al., 1999) at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR), Norway (69°N, 16°E); unfortunately, due to the poor signal-to-noise ratio (SNR) of Rayleigh scattering at high altitudes, the RMR lidar cannot make temperature measurements at the NLC altitudes (82–85 km) in summer. In this special issue, Thayer and Pan (2005) reported several years simultaneous observation of Na atoms and NLC, respectively, by a broadband Na resonance lidar and a Rayleigh lidar. They found on the average a 20% reduction of Na atoms below the Na peak altitude, when NLCs were detected. Though very strong radar echoes, termed “polar mesosphere summer echoes (PMSE)”, were first observed at frequencies of ∼50 MHz (Czechowsky et al., 1979; Ecklund and Balsley, 1981) about 10 years before lidar observation of NLC, the nature of PMSE has become clear only in recent years (Cho and Rottger, 1997; Rapp and Lübken, 2003). PMSE are believed to be related to small charged ice particles and therefore indicated the presence of ice particles; they also affect plasma diffusion and extend the scale of turbulent structures to that observable by a VHF radar.

The ALOMAR Weber sodium lidar is capable of measuring temperatures and winds under sunlit conditions near the mesopause. It was deployed in August 2000 and made one observation period lasting barely 24 h that first summer (She et al., 2002). The power per beam (0.5 W) was too weak to detect typical NLCs. The Weber lidar conducted temperature measurements of greater than 2 h duration on 17 days during the summer of 2001. These data when combined with the collocated VHF ALWIN radar, which is capable of investigating PMSE (Bremer et al., 2003), and the RMR lidar, which is capable of detecting the backscatter from NLC, will be used in this paper to investigate the anti-correlation between ice particles and sodium atoms in the polar summer mesopause.

There have been previous studies of ice particle removal of metals such as iron and potassium. Iron Boltzmann lidar observations at the South Pole (Plane et al., 2004) revealed a severe reduction of iron atoms at the altitudes of NLC. There were rather limited PMSE observations in the Antarctic, thus it was difficult to experimentally evaluate metal atoms and small ice particle anti-correlation. The question of ice particle–metal atoms interaction is much more complicated. This is because low temperatures favor both ice particle formation and lower metal density due to chemistry. Also the extent of metal depletion depends on the surface area and on the time that the ice particles have existed. A numerical model, such as Plane et al. (2004), that includes these elements, is required for the understanding of ice particle–metal atoms interaction. The IAP group recently presented experimental evidence for ice particle interaction with potassium atoms, conducting lidar observations in three summers between 2001 and 2003 in Spitzbergen (78°N). Lübken and Höffner (2004) concluded that the NLC (larger ice particles with radius greater than 10 nm) removed all available K atoms, while the PMSE (smaller particles with radius between 3 and 10 nm) reduced the number of K atoms. Since co-located observations of Na density, NLC and PMSE are still rare, a report on the observation of sub-visual ice particle–Na atom anti-correlation is timely, even if the existing data at this point do not permit a study of Na–ice interaction as extensive as that of Lübken and Höffner (2004) on K–ice particle interaction.

Sodium and potassium are both alkali metals and one might expect a very similar (if not identical) effect, with anti-correlation between ice particles and Na atoms. In a closer examination, the anticipated lack of difference between the interaction of Na and K atoms with ice particles was merely a conjecture, since the seasonal dependences in Na and K abundance in the mesopause region are well known, and they are very different. The fact that the K abundance is 2–3 times higher in summer than in winter, while the Na seasonal abundance variation is reversed, was experimentally established even in the first simultaneous lidar observation (Megie et al., 1978). This difference in both the seasonal variation of abundance and total abundance between Na and K is still not well understood (see Plane (2003) for a review of meteoric metal chemistry).

In the past, the ALOMAR RMR lidar and the VHF radar at Andenes have been used for the first simultaneous observations of PMSE and NLC (Nussbaumer et al., 1996), showing that the NLC is usually located at the lower edge of PMSE (von Zahn and Bremer, 1999). In this paper we extend these investigations and present data-sets with simultaneous, collocated Na density, PMSE and NLC observations taken in 2001 and 2002 at ALOMAR, in combination pointing to the effect of Na atom–ice particle anti-correlation.

Section snippets

Instrument and observation

The main instruments used were the Weber Na lidar, the RMR lidar and ALWIN VHF radar, all located at ALOMAR, close to the Andøya rocket range, Norway. We briefly describe the capability of these measurements under typical summer polar sunlit conditions. The Weber Na lidar is capable of measuring Na density, mesopause-region temperature and horizontal wind. The minimum detectable Na signal of this lidar is ∼3 photons in 75-m bin and 1 min. The temperature and wind measurement errors depend on the

Results and discussion

We present simultaneous observation of Na density, PMSE and NLC during the summer MaCWAVE (Mountain and Convective Waves Ascending Vertically) campaign (Goldberg et al., 2004) in July 2002, and regular summer observation between June and August 2001. The MaCWAVE campaign is a multiple instrument rocket campaign, focused on wave dynamics and interactions contributing to polar summer mesopause structure and variability. On July 02, 2002, a moderately high mesopause at 89 km altitude was observed

Conclusion

Based on the simultaneous observation of Na density and ice particles, i.e., PMSE (smaller sizes, with radius even below 10 nm) and NLC (larger sizes, with radius larger than ∼20 nm) at ALOMAR, Norway during summers of 2001 and 2002, we found that in the presence of NLC, Na is always absent at the same altitudes, and when PMSE is present, the lower edge of the Na layer most often than not overlap the peak of PMSE layer. This is in general agreement with recent reports on iron and potassium

Acknowledgments

This work was supported at CSU and CoRA in part by AFOSR under contracts F49620-00-C-0008 and F49620-03-C-0045, by NSF under Grants ATM-0137354 and ATM-0137555. Further support was given by the BMBF through the AFO2000 project OPOSSUM (FKZ: 07ATF41) and the ALOMAR ARI under the EU's 5th framework program. We acknowledge the important support of the Andoya Rocket Range and the ALOMAR observatory.

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