Polar Mesosphere Winter Echoes Research Articles

Comparison of the reaction of polar mesosphere winter echoes and polar mesosphere summer echoes to high-frequency heating in terms of modulated characteristics

In this work, for the first time, we have analyzed and compared the responses of polar mesosphere winter echoes (PMWE) and their summer counterpart, polar mesosphere summer echoes (PMSE), to high-frequency (HF) heating in terms of modulated characteristics (i.e., backscatter intensity reduction, recovery, and overshoot). Both PMWE and PMSE observations were from the same site (Tromsø, Norway; 69.6°N, 19.2°E) and radar (EISCAT [European Incoherent Scatter Scientific Association] very high frequency, 224 MHz). The heating patterns of both PMWE and PMSE were found to be similar; however, PMSE was more greatly affected by HF heating. Polar mesosphere summer echoes showed recovery and overshoot more frequently than did PMWE. In addition, the mean recovery and overshoot of PMSE were greater than those of PMWE. The associated electron temperature enhancement was estimated for both PMWE and PMSE and showed that, compared with PMWE, the electron temperature enhancement was more significant in PMSE. The strong heating effects on PMSE may be due to the considerable increase in electron temperature.

Sounding rocket project “PMWE” for investigation of polar mesosphere winter echoes

A first sounding rocket campaign dedicated to investigate the creation mechanism of Polar Mesosphere Winter Echoes (PMWE) was conducted in April 2018 from the north Norwegian Andøya Space Center (69 °N, 16 °E). Two instrumented sounding rockets were launched on 13th and 18th of April under PMWE and non-PMWE conditions, respectively. In this paper we give an overview of the PMWE sounding rocket mission. We describe and discuss some results of combined in situ and ground-based measurements which allow to verify existing PMWE theories. Our measurements ultimately show that: a) polar winter mesosphere is abounded with meteor smoke particles (MSP) and intermittent turbulent layers, b) all PMWE observed during this campaign can be explained by neutral air turbulence, c) turbulence creates small-scale structures in all D-region constituents, including free electrons; d) MSP ultimately influence the radar volume reflectivity by distorting the turbulence spectrum of electrons, e) the influence of MSP and of background electron density is just to increase SNR.

Turbulence generated small-scale structures as PMWE formation mechanism: Results from a rocket campaign

Simultaneous measurements of rocket-borne and ground-based instruments in a common volume were performed from Andøya, Norway (69° N, 16° E) in frame of the recent PMWE-project, devoted to clarify the formation mechanism behind polar mesosphere winter echoes (PMWE). This article focuses on measurements of April, 13th 2018. Despite low solar activity, we observe several radar echoes, giving the launch criterion. Combining precise in-situ ionization gauge and wave propagation measurements with ground-based radar measurements on 53.5 and 3.17 MHz, as well as lidar, we were able to measure key parameters of PMWE formation. Carefully analyzing the atmospheric background (i.e., temperature, viscosity, Brunt-Väisälä frequency, and scale heights of electron and neutral density), deriving turbulence parameters by means of radar and rocket, as well as estimating particle sizes of meteor smoke particles (MSP), we got a deep insight into the physical processes behind the PMWE phenomenon. Measurements clearly show that the coherent structures in refractive index variations (forming PMWE) are accompanied by neutral air turbulence, which is reflected in small-scale structures (down to some meters) of neutral and electron density. We analyze and discuss the temporal development of the radar echos by means of spectral width and wind measurements. We show that the behavior of the structures under investigation together with the atmospheric background is consistent with the interpretation, that PMWE were created by turbulence. Furthermore, it becomes clear that charged Meteor Smoke Particles (MSP) and background electron density can only enhance SNR, while turbulence is a prerequisite for their formation.

Two decades of long-term observations of polar mesospheric echoes at 69°N

Since 1999 radar continuous observations of polar mesospheric echoes have been conducted on the Norwegian island of Andøya (69.30°N, 16.04°E), with the ALWIN radar (1999–2008) and MAARSY (since 2011). Traditionally these observations have been named after their seasonal occurrence, i.e., Polar Mesosphere Summer Echoes (PMSE) and Polar Mesosphere Winter Echoes (PMWE). PMSE are much stronger than PMWE and are known to be due to contributions of charged-dust (ice) particles and turbulence. On the other hand, most PMWE are mainly due to turbulence. Both echoes depend on electron density and its gradients. The use of MAARSY, more 17 dB sensitive than the ALWIN radar, makes it possible to observe turbulence-dominated echoes, usually below 80 km, all year round with a mean seasonal occurrence frequency of about 14%. Their minimum occurrence rate is 2% in July and August, while there are two maxima, one in March/April (22%) and the second one in October (26%). On average the dust-dominated summer echoes starts on 14 May and ends on 26 August (i.e. 105 days) with an average occurrence in June/July of 95%. These summer echoes occur mainly between 80 and 90 km, and present a maximum daily occurrence around 13:00 LT. On the other hand, the turbulence-dominated winter echoes occur mainly between 55 and 80 km with maximum daily occurrence around 11:30 LT. Besides the seasonal and daily occurrence of these echoes, we present the variability of occurrence frequency rates for the summer layers since 1994. After eliminating the effects of geomagnetic and/or solar activity the occurrence of the summer dust-dominated echoes show a positive trend of about 0.32%̇/yr over the last twenty seven years which might be related to the observed negative mesospheric temperature trends at polar latitudes.

A Statistical Analysis of the Energy Dissipation Rate Estimated From the PMWE Spectral Width in the Antarctic

AbstractThe radar volume reflectivity and turbulent kinetic energy dissipation rate in the Antarctic mesosphere were estimated from the polar mesosphere winter echoes (PMWE) recorded using a vertical beam of the PANSY radar, a Mesosphere‐Stratosphere‐Troposphere radar at Syowa Station (69°S, 40°E), over a period of 4 years. The observed radar volume reflectivity exhibits a lognormal distribution in the range of 2 × 10−18 to 5 × 10−15 m−1 for a height region of 55–82 km. The turbulent energy dissipation rate estimated from the spectral widths of the PMWE ranges from 10−4 to 100 m2 s−3. From monthly histograms of the turbulent energy dissipation rate for a fixed solar zenith angle (SZA) and height, it was found that the summer‐to‐winter transition of the turbulent energy dissipation rate occurs in March, while the winter‐to‐summer transition occurs in September. This seasonal variation agrees well with that of gravity wave activity, suggesting that the turbulence in the mesosphere is likely caused by gravity wave breaking.

Direct Comparison Between Magnetospheric Plasma Waves and Polar Mesosphere Winter Echoes in Both Hemispheres

AbstractWe present the first and direct comparison between magnetospheric plasma waves and polar mesosphere winter echoes (PMWE) simultaneously observed by the conjugate observation with Arase satellite and high‐power atmospheric radars in both hemispheres, namely, the Program of the Antarctic Syowa Mesosphere, Stratosphere, and Troposphere/Incoherent Scatter Radar at Syowa Station (SYO; −69.00°S, 39.58°E), Antarctica, and the Middle Atmosphere Alomar Radar System at Andøya (AND; 69.30°N, 16.04°E), Norway. The PMWE were observed during 03–07 UT on 21 March 2017, just after the arrival of corotating interaction region in front of high‐speed solar wind stream. An isolated substorm occurred at 04 UT during this interval. Electromagnetic ion cyclotron (EMIC) waves and whistler mode chorus waves were simultaneously observed near the magnetic equator and showed similar temporal variations to that of the PMWE. These results indicate that chorus waves as well as EMIC waves are drivers of precipitation of energetic electrons, including relativistic electrons, which make PMWE detectable at 55‐ to 80‐km altitude. Cosmic noise absorption measured with a 38.2‐MHz imaging riometer and low‐altitude echoes at 55–70 km measured with an medium‐frequency radar at SYO also support the relativistic electron precipitation. We suggest a possible scenario in which the various phenomena observed in near‐Earth space, such as magnetospheric plasma waves (EMIC waves and chorus waves), pulsating auroras, cosmic noise absorption, and PMWE, can be explained by the interaction between the high‐speed solar wind containing corotating interaction regions and the magnetosphere.

Photocurrent modelling and experimental confirmation for meteoric smoke particle detectors on board atmospheric sounding rockets

Abstract. Characterising the photoelectron current induced by the Sun's UV radiation is crucial to ensure accurate daylight measurements from particle detectors. This article lays out the methodology used to address this problem in the case of the meteoric smoke particle detectors (MSPDs), developed by the Leibniz Institute of Atmospheric Physics in Kühlungsborn (IAP) and flown on board the PMWEs (Polar Mesosphere Winter Echoes) sounding rockets in April 2018. The methodology focuses on two complementary aspects: modelling and experimental measurements. A detailed model of the MSPD photocurrent was created based on the expected solar UV flux, the atmospheric UV absorption as a function of height by molecular oxygen and ozone, the photoelectric yield of the material coating the MSPD as a function of wavelength, the index of refraction of these materials as a function of wavelength and the angle of incidence of the illumination onto the MSPD. Due to its complex structure, composed of a central electrode shielded by two concentric grids, extensive ray-tracing calculations were conducted to obtain the incidence angles of the illumination on the central electrode, and this was done for various orientations of the MSPD in respect to the Sun. Results of the modelled photocurrent at different heights and for different materials, as well as for different orientations of the detector, are presented. As a pre-flight confirmation, the model was used to reproduce the experimental measurements conducted by Robertson et al. (2014) and agrees within an order of magnitude. An experimental setup for the calibration of the MSPD photocurrent is also presented. The photocurrent induced by the Lyman-alpha line from a deuterium lamp was recorded inside a vacuum chamber using a narrowband filter, while a UV-sensitive photodiode was used to monitor the UV flux. These measurements were compared with the model prediction, and also matched within an order of magnitude. Although precisely modelling the photocurrent is a challenging task, this article quantitatively improved the understanding of the photocurrent on the MSPD and discusses possible strategies to untangle the meteoric smoke particles (MSPs) current from the photocurrent recorded in-flight.

Tristatic observation of polar mesosphere winter echoes with the EISCAT VHF radar on 8 January 2014: a case study

Polar mesosphere winter echoes (PMWE) were observed at 70 km over Tromsø, Norway, on 8 January 2014 using the tristatic configuration of the European incoherent scatter VHF radar. For the interval 11:00–13:00 UT where the strongest patch of PMWE of about 6-min duration was detected, the spectra of the received signal were analysed for the Tromsø site and altitude profiles of spectral parameters were derived. For the remote sites Kiruna and Sodankylä, the Doppler velocities and their vertical shear were determined by using the measured autocorrelation functions. Ducted gravity waves with periods of 5–10 min were found in the vertical wind velocity between 66 and 81 km altitudes. The duct might be formed around 70 and 77 km altitude where horizontal wind maxima were observed with the Kiruna receiver. However, we did not find any close relation between wind shear at 70 km altitude and PMWE at the same height: the wind shear was present for 2 h, but PMWE for only 6 min. Enhanced spectral width in the vertical Tromsø beam was observed for the PMWE patch. We discussed these experimental findings in relation to the winter echo generation mechanism. Our conclusion is that the presence of patchy negatively charged small-sized dust might explain the observations although a gravity wave breaking mechanism cannot be completely rejected.

Simultaneous Observations of Polar Mesosphere Winter Echoes and Cosmic Noise Absorptions in a Common Volume by the PANSY Radar (69.0°S, 39.6°E)

AbstractThis study focuses on the one‐to‐one relationship between the morphology of polar mesosphere winter echo (PMWE) and cosmic noise absorption (CNA) as determined by measurements made with a single atmospheric radar, the Program of the Antarctic Syowa mesosphere‐stratosphere‐troposphere/incoherent scatter (PANSY) radar. CNA was calculated using the noise level in radar signal data collected during May 2013, including data of a solar proton event on 23 May. Using PMWE and CNA data in a common volume, their temporal variations and relation were examined in detail. PMWE altitude was clearly anticorrelated with CNA magnitude in a statistical sense: When a large CNA exceeding 0.50 dB took place, PMWE seemed to concentrate around 65 km and disappear above 70 km. The electron density behind the PMWE was estimated by using the ionospheric model for the auroral zone for the solar proton event. PMWE occurrence roughly coincided with a high electron density in the model, except that no PMWE was observed above 70 km at 0730 UT despite the electron density being higher than 108 m−3. Additionally, the estimated radar volume reflectivity with the Schmidt number Sc less than or equal to 1 is qualitatively consistent with the observed PMWE. Although weak turbulent energy dissipation rate can also play a dominant role in the observed PMWE decay, a plausible mechanism was small Sc or reduction of Sc that is equal to an increase in electron diffusivity resulting from an unusually high electron density, which significantly reduced radar volume reflectivity above 70 km.

Extended observations of polar mesosphere winter echoes over Andøya (69°N) using MAARSY

AbstractContinuous observations of Polar Mesosphere Winter Echoes (PMWE) have been conducted at the Norwegian island Andøya (69.30°N, 16.04°E) since autumn 2004 using the ALWIN VHF radar (until 2008) and the Middle Atmosphere Alomar Radar System (MAARSY) (since 2011). Using the more sensitive MAARSY compared to the ALWIN radar results in more detections characterized by smaller volume reflectivity values down to 4·10−18m−1 and a greater altitudinal coverage, 50–88 km compared to previous observations between 65 and 75 km. The results obtained by MAARSY show that the PMWE season starts clearly at the beginning of September with a mean seasonal occurrence rate of about 16%, but a strong seasonal variability with maxima up to 70% in the seasonal variation of the individual years. The end of the winter season is hard to determine since mesospheric echoes have also been observed below altitudes of 80km during nonwinter months, particularly around the end of May, i.e., the beginning of the polar mesospheric summer echo season, indicating that the physical mechanism for creating the lower mesospheric echoes is present during the early summer months as well.

High-speed solar wind streams and polar mesosphere winter echoes at Troll, Antarctica

Abstract. A small, 54 MHz wind-profiler radar, MARA, was operated at Troll, Antarctica (72° S, 2.5° E), continuously from November 2011 to January 2014, covering two complete Antarctic winters. Despite very low power, MARA observed echoes from heights of 55–80 km (polar mesosphere winter echoes, PMWE) on 60% of all winter days (from March to October). This contrasts with previous reports from radars at high northern latitudes, where PWME have been reported only by very high power radars or during rare periods of unusually high electron density at PMWE heights, such as during solar proton events. Analysis shows that PWME at Troll were not related to solar proton events but were often closely related to the arrival of high-speed solar wind streams (HSS) at the Earth, with PWME appearing at heights as low as 56 km and persisting for up to 15 days following HSS arrival. This demonstrates that HSS effects penetrate directly to below 60 km height in the polar atmosphere. Using local observations of cosmic-noise absorption (CNA), a theoretical ionization/ion-chemistry model and a statistical model of precipitating energetic electrons associated with HSS, the electron density conditions during the HSS events are estimated. We find that PMWE detectability cannot be explained by these variations in electron density and molecular-ion chemistry alone. PWME become detectable at different thresholds depending on solar illumination and height. In darkness, PWME are detected only when the modelled electron density is above a threshold of about 1000 cm−3, and only above 75 km height, where negative ions are few. In daylight, the electron density threshold falls by at least 2 orders of magnitude and PWME are found primarily below 75 km height, even in conditions when a large proportion of negative ions is expected. There is also a strong dawn–dusk asymmetry with PWME detected very rarely during morning twilight but often during evening twilight. This behaviour cannot be explained if PMWE are caused by small-scale structure in the neutral/molecular-ion gas alone but may be explained by the presence of charged meteoric dust.

Height and time characteristics of seasonal and diurnal variations in PMWE based on 1 year observations by the PANSY radar (69.0°S, 39.6°E)

AbstractWe report height and time variations in polar mesosphere winter echoes (PMWE) based on the Program of the Antarctic Syowa mesosphere‐stratosphere‐troposphere/incoherent scatter (PANSY) radar observations. PMWE were identified for 110 days from March to September 2013. PMWE occurrence frequency increased abruptly in May when two solar proton events occurred. PMWE were also observed even during periods without any solar proton events, suggesting that a possible cause of the PMWE is ionization by energetic electron precipitations. The monthly mean PMWE characteristics showed that occurrence of PMWE were mainly restricted to sunlit time. This fact indicates that electrons detached from negatively charged particles play an important role. While PMWE below 72 km in altitude completely disappeared before sunset, it was detected above that altitude for a few hours even after sunset. This height dependence in the altitude range of 60–80 km can be explained qualitatively by empirical effective recombination rates.

EISCAT and ESRAD radars observations of polar mesosphere winter echoes during solar proton events on 11–12 November 2004

Abstract. Polar mesosphere winter echoes (PMWE) were detected by two radars, ESRAD at 52 MHz located near Kiruna, Sweden, and EISCAT at 224 MHz located near Tromsø, Norway, during the strong solar proton event on 11–12 November 2004. PMWE maximum volume reflectivity was estimated to be 3 × 10−15 m−1 for ESRAD and 2 × 10−18 m−1 for EISCAT. It was found that the shape of the echo power spectrum is close to Gaussian inside the PMWE layers, and outside of them it is close to Lorentzian, as for the standard ion line of incoherent scatter (IS). The EISCAT PMWE spectral width is about 5–7 m s−1 at 64–67 km and 7–10 m s−1 at 68–70 km. At the lower altitudes the PMWE spectral widths are close to those for the IS ion line derived from the EISCAT data outside the layers. At the higher altitudes the PMWE spectra are broader by 2–4 m s−1 than those for the ion line. The ESRAD PMWE spectral widths at 67–72 km altitude are 3–5 m s−1, that is, 2–4 m s−1 larger than ion line spectral widths modelled for the ESRAD radar. The PMWE spectral widths for both EISCAT and ESRAD showed no dependence on the echo strength. It was found that all these facts cannot be explained by turbulent origin of the echoes. We suggested that evanescent perturbations in the electron gas generated by the incident infrasound waves may explain the observed PMWE spectral widths. However, a complete theory of radar scatter from this kind of disturbance needs to be developed before a full conclusion can be made.

First observations of Southern Hemisphere polar mesosphere winter echoes including conjugate occurrences at ∼69°S latitude

[1] We present the first account of polar mesosphere winter echoes (PMWE) observed from the Southern Hemisphere (SH), using measurements from Davis, Antarctica (68.6°S; 78.0°E). PMWE were observed by mesosphere-stratosphere-troposphere (MST) radar during solar proton events (SPEs) and auroral substorms. We supplement the MST radar measurements with profiles of temperature and turbulent velocity from co-located Rayleigh lidar and medium frequency (MF) radar, respectively, as well as temperature data from the Microwave Limb Sounder (MLS) onboard the Aura satellite. We establish that SH PMWE exhibit similar characteristics to their well-studied northern hemisphere counterpart. Significantly our observations reveal that these radar echoes in the lower mesosphere (∼50–80 km) can occur year-round, and on occasions simultaneously in both hemispheres. We report the seasonal occurrence distribution of SH PMWE is linked to the seasonal distribution of atmospheric turbulence in the lower mesosphere. We hypothesise that given sufficient turbulent velocities in the neutral atmosphere and co-located gradients in electron density, PMWE can occur throughout the year. However they are more likely in winter when turbulent velocities in the lower mesosphere maximise.

Artificial modification of polar mesospheric winter echoes with an RF heater: Do charged dust particles play an active role?

An investigation of polar mesosphere winter echoes (PMWE) has been carried out with the mesosphere‐stratosphere‐troposphere Mobile Rocket and Radar Observatory (MST MORRO) radar operating at 56 MHz. MORRO has been deployed recently at the European Incoherent Scatter (EISCAT) installation near Tromsø in northern Norway, home of two other large incoherent scatter radars and an RF heating facility. The main object of the investigation is to examine whether, and if so, how, RF heating influences PMWE. In particular, an experimental confirmation of the overshoot effect would indicate the presence of charged dust particles. On 11 February 2008 we measured several weak and variable PMWE layers and we used the RF heater with an on period of 20 s and off period of 100 s to modulate the radar scatter from the layers. We chose one layer, which for 44 min was the strongest and most stable layer, for further analysis. The signal intensity variation during an averaged on/off heater period shows the expected weakening of the signal intensity when heating is turned on followed by a significant small recovery of the signal during the on phase and a corresponding small overshoot of the signal strength of about 13–15% over the background level when heating is switched off. The recovery and overshoot are attributable to charge accumulation on the dust particles due to electron heating. The overshoot characteristic curve shows that a considerable increase in the electron temperature did take place during heating and that charged dust particles should be present, probably with radii of some nanometers.

First observation of the overshoot effect for polar mesosphere winter echoes during radiowave electron temperature modulation

Polar mesosphere winter echoes (PMWE), observed with the EISCAT VHF radar located near Tromsø, Norway on October 24, 2006, were modulated using the EISCAT Heating facility. The heater was turned on for 20 s and then off for 160 s. The experiment confirmed previous results that when the heater was switched on, the PMWE power dropped significantly compared to the undisturbed level measured before heating. The experiment showed for the first time that, after the heater was switched off, the PMWE strength overshot the value that it had before heating. By using data from 6 heating cycles we find that the overshoot effect is about 50% of the pre‐heating value, which is rather weak compared to 3–5 times for PMSE. No, or little recovery of PMWE strength during heating periods was observed. This may indicate the presence of very small dust particles, such as meteoric smoke particles.

Radar measurements of turbulence, electron densities, and absolute reflectivities during polar mesosphere winter echoes (PMWE)

On January 21, 2005, strong radar echoes known as ‘polar mesosphere winter echoes’ (PMWE) were observed by the ALWIN VHF radar at 69°N. Peak reflectivites of ∼10 −14/m were observed at approximately 13 UT. At the same time the Saura MF radar which is only ∼15 km away from ALWIN measured electron densities ( N e) and turbulence energy dissipation rates ( ϵ). This combination is rather seldom because PMWE are rare (occurrence rate is 1–2%) and electron densities are critical: too low values do not give a PMWE, and too large values prohibit to derive N e due to signal absorption. At PMWE altitudes (∼62 km) typical energy dissipation rates before, during, and after the PMWE are ϵ∼100 mW/kg. The electron density during the PMWE is typically 10 9/m 3 and much smaller prior to PMWE. We have applied a theoretical model based on turbulence theory to derive absolute reflectivities for these ϵ and N e values and arrive at ∼2 × 10 −15/m during PMWE (in nice agreement with measurements) and ∼3 × 10 −17/m outside PMWE. The latter value is indeed below the detection limit of the ALWIN radar. The nice quantitative agreement between measured and calculated absolute volume reflectivities confirms earlier conclusions that neutral air turbulence is the main cause for PMWE. Furthermore, we have analyzed the autocorrelation function of the EISCAT 224 MHz PMWE observations from November 10, 2004, and find that the spectral form is different inside the PMWE compared to outside. The shape is Gaussian inside PMWE (compatible with turbulent scatter), whereas it is Lorentzian above the PMWE, indicating non-turbulent, incoherent scatter.

Polar mesosphere winter echoes – A review of recent results

Polar mesosphere winter echoes (PMWE) are strong radar returns from heights between 50 and 80 km recorded by high-latitude VHF radars during the winter months. They are seen when ionization is increased above background levels during solar protons events or by high-energy electron precipitation from within the magnetosphere. Interest in PMWE has increased substantially in recent years since it has been suggested that the traditional explanation for their existence – turbulence in the neutral atmosphere – cannot explain their characteristics. An alternative theory has been proposed in which PMWE are caused by infrasound from the oceans being reflected at wind-shears and temperature gradients in the mesosphere. The observed characteristics of PMWE are reviewed in relation to the two theories. Although some characteristics of PMWE are consistent with either theory, other characteristics are not, particularly the spectral characteristics of the radar echoes at 224 MHz and the high horizontal travel velocity of the scatterer diffraction pattern at 52 MHz. The theory based on infrasound can qualitatively explain these characteristics but much remains to be done to test whether there is also quantitative agreement.

Rocket measurements of positive ions during polar mesosphere winter echo conditions

Abstract. On 18 January 2005, two small, instrumented rockets were launched from Andøya Rocket Range (69.3° N, 16° E) during conditions with Polar Mesosphere Winter Echoes (PMWE). Each of the rockets was equipped with a Positive Ion Probe (PIP) and a Faraday rotation/differential absorption experiment, and was launched as part of a salvo of meteorological rockets measuring temperature and wind using falling spheres and chaff. Layers of PMWE were detected between 55 and 77 km by the 53.5 MHz ALWIN radar. The rockets were launched during a solar proton event, and measured extremely high ion densities, of order 1010 m−3, in the region where PMWE were observed. The density measurements were analyzed with the wavelet transform technique. At large length scales, ~103 m, the power spectral density can be fitted with a k−3 wave number dependence, consistent with saturated gravity waves. Outside the PMWE layers the k−3 spectrum extends down to approximately 102 m where the fluctuations are quickly damped and disappear into the instrumental noise. Inside the PMWE layers the spectrum at smaller length scales is well fitted with a k−5/3 dependence over two decades of scales. The PMWE are therefore clearly indicative of turbulence, and the data are consistent with the turbulent dissipation of breaking gravity waves. We estimate a lower limit for the turbulent energy dissipation rate of about 10−2 W/kg in the upper (72 km) layer.

Polar mesosphere winter echoes during MaCWAVE

Abstract. During the MaCWAVE winter campaign in January 2003, layers of enhanced echo power known as PMWE (Polar Mesosphere Winter Echoes) were detected by the ESRAD 52 MHz radar on several occasions. The cause of these echoes is unclear and here we use observations by meteorological and sounding rockets and by lidar to test whether neutral turbulence or aerosol layers might be responsible. PMWE were detected within 30 min of meteorological rocket soundings (falling spheres) on 5 separate days. The observations from the meteorological rockets show that, in most cases, conditions likely to be associated with neutral atmospheric turbulence are not observed at the heights of the PMWE. Observations by instrumented sounding rockets confirm low levels of turbulence and indicate considerable small-scale structure in charge density profiles. Comparison of falling sphere and lidar data, on the other hand, show that any contribution of aerosol scatter to the lidar signal at PMWE heights is less than the detection threshold of about 10%.