Neutral Air Turbulence Research Articles

Neutral Air Turbulence in the Mesosphere and Associated Polar Mesospheric Summer Echoes (PMSEs)

The first true common volume observations of the PMSE source region with 4 radars are presented in this paper. Radar frequencies of 8, 56, 224, and 930 MHz are used in this study. Three days of experimental observations at EISCAT are presented. Numerical simulations of mesospheric dusty/ice plasma associated with the observed radar frequencies are presented. The effect of neutral air turbulence on the generation and strength of plasma density perturbations associated with PMSE using four radar frequencies and in the presence of various dust parameters is investigated. Using the model it is shown that the well-known neutral air turbulence in the presence of heavy dust particles and neutral air turbulence combined with dust density (dusty turbulence) can largely explain the observed radar cross-section at four radar frequencies. The effect of neutral air turbulence amplitude along with dust charging and diffusion in the presence of various dust parameters is investigated using the computational model. Specifically, the response of diffusion to charging time scales, plasma density fluctuation amplitude and background dusty plasma parameters are discussed. Several key parameters in the dusty plasma responsible for the PMSE observations are determined. Qualitative comparison of radar echo strength at 4 frequencies with numerical results is provided. Unlike the previous studies that required large dust particles of 20 nm for PMSE formation, the present work demonstrates the possibility of small dust particles to explain the experimental observations.

Neutral air turbulence in the mesosphere and associated polar mesospheric summer echoes (PMSEs)

Neutral air turbulence in the mesosphere and associated polar mesospheric summer echoes (PMSEs)

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.

On the temporal evolution of turbopause altitude, 1996–2021, 70°N, 19°E

Abstract Medium frequency radars with multiple receivers are able to track the movement of the interference pattern on the ground from echoes from irregularities in refractive index. In particular, refractive index in the mesosphere is determined by electron density – commonly known as the ionospheric D-region. Thus using this technique it is possible to determine winds in the height regime 70-90 km, depending on the degree of ionization throughout the year. In addition, by examining the fading times of the passage of these structures, it is possible to deduce metrics pertaining to neutral air turbulence. Here, we employ a well-established method to this effect. Thereafter, comparing the turbulent intensity to the kinematic viscosity of the neutral atmosphere, we determine the turbopause altitude. Above this height, atmospheric constituents behave independently, whereas below, all components are mixed. Contrary to earlier analyses, we present evidence the turbopause altitude has been constant since approximately 2004.

Observation of Polar Mesosphere Summer Echoes using the northernmost MST radar at Eureka (80°N)

We investigate long-term Polar Mesosphere Summer Echoes (PMSEs) observations conducted by the northernmost geographically located MST radar at Eureka (80°N, 86°W). While PMSEs are a well recognized summer phenomenon in the polar regions, previous calibrated studies at Resolute Bay and Eureka using 51.5MHz and 33MHz radars respectively, showed that PMSE backscatter signal strengths are relatively weak in the polar cap sites, compared to the auroral zone sites (Swarnalingam et al., 2009b; Singer et al., 2010). Complications arise with PMSEs in which the echo strength is controlled by the electrons, which are, in turn, influenced by heavily charged ice particles as well as the variability in the D-region plasma. In recent years, PMSE experiments were conducted inside the polar cap utilizing a 51MHz radar located at Eureka. In this paper, we investigate calibrated observations, conducted during 2009–2015. Seasonal and diurnal variations of the backscatter signal strengths are discussed and compared to previously published results from the ALOMAR radar, which is a radar of similar design located in the auroral zone at Andenes, Norway (69°N, 16°E). At Eureka, while PMSEs are present with a daily occurrence rate which is comparable to the rate observed at the auroral zone site for at least two seasons, they show a great level of inter-annual variability. The occurrence rate for the strong echoes tends to be low. Furthermore, comparison of the absolute backscatter signal strengths at these two sites clearly indicates that the PMSE backscatter signal strength at Eureka is weak. Although this difference could be caused by several factors, we investigate the intensity of the neutral air turbulence at Eureka from the measurements of the Doppler spectrum of the PMSE backscatter signals. We found that the level of the turbulence intensity at Eureka is weak relative to previously reported results from three high latitude sites.

Simultaneous observations of a Mesospheric Inversion Layer and turbulence during the ECOMA-2010 rocket campaign

Abstract. From 19 November to 19 December 2010 the fourth and final ECOMA rocket campaign was conducted at Andøya Rocket Range (69° N, 16° E) in northern Norway. We present and discuss measurement results obtained during the last rocket launch labelled ECOMA09 when simultaneous and true common volume in situ measurements of temperature and turbulence supported by ground-based lidar observations reveal two Mesospheric Inversion Layers (MIL) at heights between 71 and 73 km and between 86 and 89 km. Strong turbulence was measured in the region of the upper inversion layer, with the turbulent energy dissipation rates maximising at 2 W kg−1. This upper MIL was observed by the ALOMAR Weber Na lidar over the period of several hours. The spatial extension of this MIL as observed by the MLS instrument onboard AURA satellite was found to be more than two thousand kilometres. Our analysis suggests that both observed MILs could possibly have been produced by neutral air turbulence.

A case study of the evolution of a Kelvin–Helmholtz wave and turbulence in noctilucent clouds

Bright and extensive noctilucent clouds (NLC) were observed in Århus (Denmark) on 3/4 July of 2008 with an automatic digital camera taking images every minute. This event was unique in the sense that bright NLC were seen at high elevation angles (more than 30°) that allowed observing the evolution of a Kelvin–Helmholtz (KH) wave, resulted in well-developed turbulence. In particular, coherent vortex structures of a horseshoe-shaped form were observed for the first time in noctilucent clouds. The turbulent diffusion coefficient and turbulent energy dissipation rate around the mesopause are estimated in the range 162–667 m 2/s and 300–1235 mW/kg, respectively, representing a case of strong neutral air turbulence in noctilucent clouds. Turbulent structures were observed to be in the vicinity of breaking small-scale gravity waves that seems to be responsible for a high level of turbulence. At the same time, it has been demonstrated that it is of importance to take into account non-turbulent process such as the gravity wave motion that is always present in NLC layers. Unless non-turbulent process is taken into account, this certainly leads to overestimating of the value of the turbulent diffusion coefficient. More accurate characteristics of turbulence in NLC can be obtained by analyzing a sequence of high-resolution images with a high frame-rate high-resolution digital camera.

PMSE and E-region plasma instability: In situ observations

From 29 June to July 6, 2003, the ROMA-Svalrak (Rocketborne Observations of the Middle Atmosphere at the Svalrak facilities) sounding rocket campaign took place at Ny-Ålesund (Spitsbergen, geographical coord. 79 ∘ N , 12 ∘ E , geomagnetic coord. 76 ∘ N , 110 ∘ E ). Three sounding rockets were launched to study neutral air turbulence and small scale plasma dynamics around polar mesosphere summer echoes (PMSE). During all three flights both PMSE and plasma instability events were observed. It is known that small-scale field aligned structures in the E-region plasma density can be created by unstable electromagnetic waves. The mechanism responsible for creating the structures causing radar echoes (PMSE) is believed to be neutral air turbulence in the presence of heavy charged particles. E-region plasma irregularities recorded during the last rocket flight (labeled RO-MI-03) were observed only during the upleg of the trajectory but not during the downleg. Also, on the upleg there was no clear spatial separation between PMSE and the plasma instability regions. In the current paper we consider this transition region in detail.

Relationship of electric field and charged particle density fluctuations to air turbulence in the mesosphere

A theoretical model is developed that is applicable to the electric field fluctuations that arise in the polar summer mesosphere as a result of the coupling of the charged species to the neutral air turbulence. The motions of electrons, ions, and charged aerosol particles are described as harmonic oscillators both driven and damped by the drag force exerted by the neutral air. The relative fluctuations in the ion density are found to be nearly the same as those in the neutral air as a consequence of the ions' high‐momentum transfer relaxation frequency. The aerosol density fluctuations follow those of the neutral air at frequencies below their relaxation frequency, which is in the acoustic range. The electrons move primarily in response to the electric force to partially cancel the net charge density of ions and aerosol particles except at wavelengths shorter than the Debye length. Electric field and charge density fluctuations are calculated for several sets of conditions. In “bite‐out” regions in which the electron density is reduced as a consequence of attachment to the aerosol particles the electric field fluctuations are found to be enhanced, which is consistent with observations.

Imaging of Polar Mesosphere Summer Echoes with the 450 MHz Poker Flat Advanced Modular Incoherent Scatter Radar

Polar Mesosphere Summer Echoes (PMSE) occur near the mesopause during the polar summer months. PMSE are primarily studied at VHF, however there have been some detections at higher frequencies. Here, we report on some of the first detections of PMSE with the 450 MHz (67 cm) Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR). Echoes were observed with volume reflectivities (radar scattering cross section per unit volume) near 2–3 × 10−17 m−1. On 11 June 2007, PFISR was operating in a 26‐beam position mode, with look directions spread over an approximately 80 by 80 km2 region at 85 km altitude with elevation angles as low as ∼50°. The measurements showed patchy (tens of kilometer) irregularity regions drifting in from the north, in addition to smaller, more localized structures. There was no evidence for strong aspect sensitivity of these UHF echoes, as PMSE was observed in all look directions with relatively uniform intensity. The observations indicate the presence of fossilized irregularities drifting with the background wind field as well as areas of developing irregularities possibly associated with the presence of active neutral air turbulence.

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.

Spectrum of ionospheric irregularities below the turbopause

In this paper we consider density irregularities induced by neutral air turbulence in the low temperature ionospheric plasma under day-time and night-time conditions. Analysis based on the analytical expressions for the irregularity spectrum and the mean-square level of plasma fluctuations has shown that the shape of the spectrum and the fluctuation level have to change from day to night. The slope of the spectrum and the rms fluctuation level have to be larger at night.

The thermal and dynamical state of the atmosphere during polar mesosphere winter echoes

Abstract. In January 2005, a total of 18 rockets were launched from the Andøya Rocket Range in Northern Norway (69° N) into strong VHF radar echoes called "Polar Mesosphere Winter Echoes" (PMWE). The echoes were observed in the lower and middle mesosphere during large solar proton fluxes. In general, PMWE occur much more seldom compared to their summer counterparts PMSE (typical occurrence rates at 69° N are 1–3% vs. 80%, respectively). Our in-situ measurements by falling sphere, chaff, and instrumented payloads provide detailed information about the thermal and dynamical state of the atmosphere and therefore allow an unprecedented study of the background atmosphere during PMWE. There are a number of independent observations indicating that neutral air turbulence has caused PMWE. Ion density fluctuations show a turbulence spectrum within PMWE and no fluctuations outside. Temperature lapse rates close to the adiabatic gradient are observed in the vicinity of PMWE indicating persistent turbulent mixing. The spectral broadening of radar echoes is consistent with turbulent velocity fluctuations. Turbulence also explains the mean occurrence height of PMWE (~68–75 km): viscosity increases rapidly with altitude and destroys any small scale fluctuations in the upper mesosphere, whereas electron densities are usually too low in the lower mesosphere to cause significant backscatter. The seasonal variation of echoes in the lower mesosphere is in agreement with a turbulence climatology derived from earlier sounding rocket flights. We have performed model calculations to study the radar backscatter from plasma fluctuations caused by neutral air turbulence. We find that volume reflectivities observed during PMWE are in quantitative agreement with theory. Apart from turbulence the most crucial requirement for PMWE is a sufficiently large number of electrons, for example produced by solar proton events. We have studied the sensitivity of the radar echo strength on various parameters, most important electron number density and turbulence intensity. Our observational and theoretical considerations do not provide any evidence that charged aerosol particles are needed to explain PMWE, in contrast to the summer echoes which owe their existence to charged ice particles.

Polar mesosphere summer echoes (PMSE): Review of observations and current understanding

Abstract. Polar mesosphere summer echoes (PMSE) are very strong radar echoes primarily studied in the VHF wavelength range from altitudes close to the polar summer mesopause. Radar waves are scattered at irregularities in the radar refractive index which at mesopause altitudes is effectively determined by the electron number density. For efficient scatter, the electron number density must reveal structures at the radar half wavelength (Bragg condition for monostatic radars; ~3 m for typical VHF radars). The question how such small scale electron number density structures are created in the mesopause region has been a longstanding open scientific question for almost 30 years. This paper reviews experimental and theoretical milestones on the way to an advanced understanding of PMSE. Based on new experimental results from in situ observations with sounding rockets, ground based observations with radars and lidars, numerical simulations with microphysical models of the life cycle of mesospheric aerosol particles, and theoretical considerations regarding the diffusivity of electrons in the ice loaded complex plasma of the mesopause region, a consistent explanation for the generation of these radar echoes has been developed. The main idea is that mesospheric neutral air turbulence in combination with a significantly reduced electron diffusivity due to the presence of heavy charged ice aerosol particles (radii ~5–50 nm) leads to the creation of structures at spatial scales significantly smaller than the inner scale of the neutral gas turbulent velocity field itself. Importantly, owing to their very low diffusivity, the plasma structures acquire a very long lifetime, i.e., 10 min to hours in the presence of particles with radii between 10 and 50 nm. This leads to a temporal decoupling of active neutral air turbulence and the existence of small scale plasma structures and PMSE and thus readily explains observations proving the absence of neutral air turbulence at PMSE altitudes. With this explanation at hand, it becomes clear that PMSE are a suitable tool to permanently monitor the thermal and dynamical structure of the mesopause region allowing insights into important atmospheric key parameters like neutral temperatures, winds, gravity wave parameters, turbulence, solar cycle effects, and long term changes.

Temperatures, polar mesosphere summer echoes, and noctilucent clouds over Spitsbergen (78°N)

Simultaneous measurements of temperatures, polar mesosphere summer echoes (PMSE), and noctilucent clouds (NLC) took place in the polar cap (78°N) during the Rocketborne Observations in the Middle Atmosphere campaign (ROMA) in summer 2001. PMSE were observed practically permanently from the beginning and prior to the ROMA campaign (mid‐July) until mid‐August and disappeared by the end of August. PMSE occur between 81 and 92 km but have maximum occurrence rates between 83 and 89 km (up to 80% per day). PMSE are compared with temperatures from falling spheres, with frost point temperatures (Tf), and with degrees of saturation (S) using water vapor mixing ratios from models. PMSE occur nearly exclusively at altitudes with supersaturation, but the reverse is not true. Temperatures within PMSE layers can be up to 20–25 K below Tf. There is no correlation between the radar echo power and the magnitude of S. Around the mesopause we frequently find S values significantly larger than 1 (up to 13000) at altitudes with the confirmed absence of PMSE. This could indicate that the actual water vapor mixing ratio is substantially smaller than our assumed model values in line with the “freeze‐drying effect.” The mean of all NLC peak altitudes is 83.6 km (variability: ±1.1 km), and the occurrence rate is 77% in the main summer season. Most of the time the lower edges of PMSE and NLC are colocated within a few hundred meters or less, which can be explained by a rapid evaporation of ice particles. This close agreement also indicates a rather homogeneous horizontal distribution of ice particles at scales below a few kilometers. The seasonal and height variation of PMSE nicely agrees with the time/height range of S > 1 and confirms the overwhelming importance of low enough temperatures for the existence of PMSE. The variation of NLC also agrees with the seasonal variation of S but covers only the lower height range with supersaturation in line with the different sensitivity of NLC and PMSE on particle radius. From the combined observations of PMSE and NLC, mean occurrence rates of neutral air turbulence of larger than ∼50% are deduced. The experimental results at Spitsbergen confirm the standard scenario of PMSE and NLC, namely that particles start to nucleate around the mesopause, and grow and sediment until they reach “warm” atmospheric regions around 82 km where they quickly evaporate. Small ice particles can affect the plasma leading to PMSE, whereas they need to grow to radii larger than approximately 20 nm to be seen by lidar.

Electrodynamic structure of charged dust clouds in the earth's middle atmosphere

Noctilucent clouds (NLCs) and polar mesospheric summer echoes (PMSEs) are two phenomena at the forefront of near-earth space science. NLCs are high-altitude clouds in the earth's mesosphere that are formed from visible aerosol particles. The occurrence rate of NLCs over time is believed to have profound implications on global climate change. NLCs are often associated with PMSEs which are strong 50 MHz–1.3 GHz radar echoes from mesospheric electron irregularities. Therefore, PMSEs may be an important remote-sensing diagnostic for the evolution of NLCs and the earth's middle atmosphere in general. The electron irregularities that produce PMSEs are generally believed to result from charging of electrons onto subvisible aerosol irregularities, the source of which is currently a debated issue. Neutral air turbulence has long been considered a primary source of the irregularities. However, there are clearly fundamental characteristics of the irregularities in past and recent observations that cannot be explained by neutral air turbulence and most probably involve plasma processes. This work considers the latter mechanism and the possibility of production of aerosol irregularities in the boundary region between the charged aerosol layer and the background mesospheric plasma. First a model for investigating the electrodynamics of this boundary layer is described. This model indicates that plasma flows are expected to exist in the electrodynamic equilibrium. An initial assessment of the possible role of these plasma flows in producing irregularities that may ultimately result in PMSEs is provided.

Small scale density variations of electrons and charged particles in the vicinity of polar mesosphere summer echoes

Abstract. We present small scale variations of electron number densities and particle charge number densities measured in situ in the presence of polar mesosphere summer echoes. It turns out that the small scale fluctuations of electrons and negatively charged particles show a strong anticorrelation down to the smallest scales observed. Comparing these small scale structures with the simultaneously measured radar signal to noise profile, we find that the radar profile is well described by the power spectral density of both electrons and charged particles at the radar half wavelength (=the Bragg scale). Finally, we consider the shape of the power spectra of the observed plasma fluctuations and find that both charged particles and electrons show spectra that can be explained in terms of either neutral air turbulence acting on the distribution of a low diffusivity tracer or the fossil remnants of a formerly active turbulent region. All these results are consistent with the theoretical ideas by Rapp and Lübken (2003) suggesting that PMSE can be explained by a combination of active and fossil neutral air turbulence acting on the large and heavy charged aerosol particles which are subsequently mirrored in the electron number density distribution that becomes visible to a VHF radar when small scale fluctuations are present.

Comment on “Neutral air turbulence and temperatures in the vicinity of polar mesosphere summer echoes” by F.‐J. Lübken, M. Rapp, and P. Hoffmann

Comment on “Neutral air turbulence and temperatures in the vicinity of polar mesosphere summer echoes” by F.‐J. Lübken, M. Rapp, and P. Hoffmann

Reply to comment by J. Klostermeyer on “Neutral air turbulence and temperatures in the vicinity of polar mesosphere summer echoes” by F.‐J. Lübken, M. Rapp, and P. Hoffmann

Reply to comment by J. Klostermeyer on “Neutral air turbulence and temperatures in the vicinity of polar mesosphere summer echoes” by F.‐J. Lübken, M. Rapp, and P. Hoffmann