Neutral Air Turbulence Research Articles

The role of charged ice particles for the creation of PMSE: A review of recent developments

We review the current understanding of the role of charged ice particles for the creation of PMSE. We briefly describe the historical background leading to the Cho et al. [1992]-theory of PMSE which tried to explain the radar echoes as a turbulent scatter where the scattering electrons have a high Schmidt number due to the fact that somewhat more than 50% of the overall negative charge is bound to heavy aerosol particles of low mobility. We then discuss recent experimental results showing that 1. neutral air turbulence is regularly found only in the upper part of a PMSE layer and 2. that PMSE exist even if only a minor fraction of the overall negative charge is bound to aerosol particles. We then present the solution of this contradiction between experiment and theory by discussing the recent updated electron diffusion theory by Rapp and Lübken [2003]. Their main result is that electron diffusivity is reduced due to the presence of charged aerosol particles almost independently of the ratio between the aerosol charge number density and the electron number density. Furthermore, the diffusive lifetime of electron irregularities can be hours depending on the aerosol radius such that it appears likely that at low altitudes, where we expect the presence of large particles, irregularities exist even though neutral air turbulence might have ceased a considerable time ago. Thus the question for the spatial and temporal variability of mesospheric turbulence is identified as the most pressing task for future research in order to finally understand PMSE.

Morphology of turbulence in the polar summer mesopause region during the MIDAS/SOLSTICE campaign 2001

Two rocket borne in situ measurements of neutral air turbulence profiles in the polar summer mesosphere were performed during the MIDAS/SOLSTICE campaign in June 2001 from the Andøya Rocket Range (69°N). Both profiles reveal typical energy dissipation rates for summer conditions with maximum values of ∼1000mW/kg near the mesopause corresponding to heating rates of ∼100 K/d. The morphology of the turbulent layers in both flights is very different. In the first flight almost the complete altitude range from 81 to 90 km is filled with turbulence. With the help of accompanying high resolution wind measurements with foil clouds we identify dynamical instabilities as sources for these turbulent events. The second flight shows only two single turbulent layers of 1 km thickness. A dynamical instability near the upper turbulent layer also suggests that an unstable flow created this turbulent event.

On the nature of PMSE: Electron diffusion in the vicinity of charged particles revisited

Triggered by recent experimental evidence showing that some parts of the Cho et al. [1992] theory describing electron diffusion in the vicinity of charged aerosol particles cannot be correct, we reconsider the process of electron diffusion under the conditions of the polar summer mesopause region. The key idea is that perturbations in the distribution of charged aerosol particles created for example by neutral air turbulence almost immediately lead to (anticorrelated) perturbations in the electron number density due to simple charge neutrality and zero net current arguments. We obtain analytical solutions of the coupled diffusion equations for electrons, charged aerosol particles, and positive ions subject to the initial condition of anticorrelated perturbations in the charged aerosol and electron distribution. The main signatures of these solutions are in line with available in situ evidence of small‐scale plasma structures in the vicinity of polar mesosphere summer echoes (PMSE), i.e., electron perturbations are anticorrelated to both perturbations in the distributions of negatively charged aerosol particles and positive ions. The lifetime of these perturbations is proportional to the square of the aerosol particle radius such that the presence of particles with radii larger than ∼10 nm allows for the existence of electron number density perturbations up to several hours after the initial creation mechanism has stopped. These results are almost independent of the ratio between the aerosol charge number density and the number density of free electrons. These electron perturbations potentially give rise to a radar reflectivity comparable to values observed with 50 MHz VHF radars. Our model results can readily explain why in situ measurements of neutral air turbulence have repeatedly shown active turbulence only in the upper part of the PMSE layer whereas turbulence was basically absent in the lower part. Furthermore, our model concept qualitatively yields the correct altitude profile of the mean PMSE occurrence frequency based on the measured altitude profile of the turbulence occurrence frequency.

Neutral air turbulence and temperatures in the vicinity of polar mesosphere summer echoes

A total of 8 sounding rocket flights with measurements of neutral air turbulence in the upper mesosphere have been performed in the past 10 years with simultaneous and nearly co‐located radar measurements of polar mesosphere summer echoes (PMSE). These measurements took place close to the rocket ranges in northern Norway (Andøya Rocket Range, 69°N) and in northern Sweden (Esrange, 68°N). A detailed comparison demonstrates that there is no apparent correlation between PMSE and neutral air turbulence and that in fact turbulence is absent in the majority of all PMSE events (no turbulence in 7 out of 10 PMSE layers). This suggests that neutral turbulence and other mechanisms affecting the neutral atmosphere at very small spatial scales play a minor role in creating PMSE, contrary to the speculations published in the literature. The main mechanism for creating PMSE remains unidentified. A comparison of PMSE with simultaneous temperature profiles derived from falling sphere and ionization gauge measurements shows that PMSE are practically always present at altitudes where the temperature is low enough for water ice particles to exist. This supports the general understanding that PMSE are closely related to charged water ice particles. On the other hand, the measurements also demonstrate that low enough temperatures are not sufficient for PMSE to exist. Temperature lapse rates were deduced from the high‐altitude‐resolution ionization gauge measurements. Within the PMSE layers the temperature lapse rate is typically +1–2 K/km with a rather large variability of ±5–10 K/km. Adiabatic lapse rates have never been found within a PMSE layer, which suggests that turbulence cannot have been active for a substantial period. This again supports the idea that neutral air turbulence plays a minor role in creating PMSE. Probably the only common physical reason for PMSE and turbulence is the background temperature profile, which supports the creation of ice particles (since temperatures are very low) and which provokes the breaking of gravity waves and creation of turbulence since the temperature gradient changes at the mesopause.

Identification of possible ion-drag induced neutral instability in the lower thermosphere over Svalbard

The EISCAT Svalbard Radar (ESR) has been used to obtain ion velocities in the lower thermosphere. By using beam swinging and assuming homogeneity and stationarity of the plasma, first approximations to the electric field have been deduced, and thus the thermospheric neutral wind has been estimated. From these derived parameters, we have estimated the gradient Richardson Number. Although many assumptions must be made, there is an indication that electrodynamics is able to contribute to enhancement or even production of neutral-air turbulence in the lower thermosphere. Finally, we outline a proposal for an analogy to the Reynolds Number, but reflecting the relative importance’s of the contribution of ion-drag to the neutral dynamics and the kinematic viscosity.

Small scale plasma and charged aerosol variations and their importance for polar mesosphere summer echoes

Since 1990 a number of campaigns have been conducted in order to study the small scale structure of charged as well as neutral species within Polar Mesosphere Summer Echo (PMSE) layers. Several VHF radars have been used to study this phenomenon from the ground (EISCAT, CUPRI, ALOMAR SOUSY) and sounding rockets have been launched through different types of PMSE layers. The radar observations appear to occur in two classes: (I) One characterized by broad spectral widths and almost no aspect sensitivity, associated with neutral air turbulence; and (II) another by narrow spectral widths and strong aspect sensitivity, which is observed in the absence of neutral air turbulence. Both radar and in situ measurements show a clear dominance of the non-turbulent type II echoes. We will discuss how localised layers of charged aerosols, ions and electrons observed in situ might lead to single and multiple Fresnel type reflections at VHF wavelengths. We will also discuss the possibility that a two-stream type plasma instability may explain the type II echoes. The important parameter in this respect is the electrical field and a horizontal electric field of about 40 mV/m is needed to excite small scale plasma waves. Such a field may have its origin in a differential drift between the heavy aerosols and the much lighter plasma, caused by a varying background wind, windshears and/or waves of different types, that give rise to a polarisation electric field.

The importance of charged aerosols in the polar mesosphere in connection with Noctilucent Clouds and Polar Mesosphere Summer Echoes

A number of campaigns have been conducted in order to study Polar Mesosphere Summer Echos (PMSE) and Noctilucent Clouds (NLC) in the period 1991–1994. Several sounding rockets have been launched through these layers with measurements being performed on upleg as well as downleg. These include measurements of positive ions and electrons in both ram and wake positions, as well as measurements of charged aerosols in ram on upleg. In this paper we will review these measurements and make a preliminary classification of the data based upon the presence of PMSE and/or NLC. One of the mechanisms responsible for PMSE is the presence of neutral air turbulence in combination with a high Schmidt number. We will briefly discuss this type of echo using in situ rocket data. Differences and similarities of PMSE and NLC as observed both in the Arctic and the Antarctic will be discussed. Observations show that especially PMSE are much more frequent in the Arctic. This may be due to a difference in the water vapour content or the temperature at mesopause heights. Lack of data in the Antarctic makes it difficult to decide which of these two factors are the most important. More measurements, especially co-ordinated in situ and ground-based lidar and radar measurements, are needed to discuss the Arctic and Antarctic similarities and differences in further detail.

Microphysical and turbulent measurements of the Schmidt number in the vicinity of polar mesosphere summer echoes

During the ECHO campaign in 1994 neutral and electron density fluctuations were measured together with charged aerosols on the same sounding rocket launched close to a VHF radar detecting polar mesosphere summer echoes (PMSE). For the first time this combination of measurements allows for an independent test of the microphysical and the turbulence interpretations of the Schmidt number (Sc). The Schmidt number characterizes the reduction of the electron diffusivity by charged aerosols, which leads to an enhancement of the electron density fluctuations at small spatial scales. In one of the flights charged aerosols were observed at ∼83–89km together with correlated depletions in electron density (‘biteouts’). We have applied a model of aerosol charging to the measured plasma profiles and determined a mean aerosol radius of ∼8nm and a mean aerosol charge of 1e‐. In the microphysical description of electron diffusion these parameters correspond to Sc∼420. Spectral analysis of the electron density fluctuations showed enhancements of spectral densities at small scales suggesting likewise a Schmidt number much larger than unity. Using an energy dissipation rate of 67mW/kg as derived from neutral air turbulence measurements on the same rocket we get from the electron spectra Sc=385 which is in excellent agreement with the microphysical result. Apart from this turbulent layer we observe no significant disturbances in neutral air number densities below ∼87km which confirms earlier indications that processes must exist to create PMSEs which are not directly coupled to neutral air turbulence.

TOTAL: a rocket-borne instrument for high resolution measurements of neutral air turbulence during DYANA

An improved version of a rocket-borne instrument (‘TOTAL’), optimized for high resolution measurements of relative density variations, was successfully employed during the DYANA campaign in winter 1990. Both the inertial-convective subrange and the viscous-diffusive subrange of turbulence were observed in the power spectra derived from density fluctuations. An extended spectral model which comprises both subranges has been used to analyse the data. In this paper we present altitude profiles of turbulent parameters, such as turbulent energy dissipation rates ε and turbulent diffusion coefficients K, which were derived from a total of eight successfully launched instruments at high (Andoya, 69°N) and middle (Biscarosse, 44°N) latitudes. The limitations of the measurement technique as well as instrumental errors are discussed. The results mainly show small values of ε and K throughout the whole campaign period. The turbopause was found at an altitude of 95 ± 3 km.

In‐situ measurement of the Schmidt number within a PMSE layer

During the SCALE campaign in July and August 1993, rocket borne in situ measurements of neutral and electron small scale density fluctuations were performed in the mesosphere over Andøya (69° N16°E) at the same time as the EISCAT 224 MHz radar in Tromsø observed strong polar mesosphere summer echoes (PMSE). Strong neutral air turbulence was observed in the height of the PMSE with a turbulent energy dissipation rate of ϵ = 630mW/kg. In the same altitude range, electron density fluctuations were observed extending to scales smaller than found in the neutrals. From a comparison of the neutral and electron density fluctuation spectrum we obtained a Schmidt number Sc of 6.8 (Sc = ν/D ; ν = kinematic viscosity ; D = molecular diffusion coefficient for the electrons). From the above numbers we deduced ℓoD&K = 5.2m (ℓoD&K is the ‘break‐off’ scale between the viscous‐convective and the viscous‐diffusive subrange of the turbulence spectrum). The half wavelength of the EISCAT 224 MHz radar (λ/2 = 0.67m) is therefore well located in the viscous‐diffusive subrange of the turbulence spectrum, despite the fact that Sc > 1, which shifts ℓoD&K towards smaller scales. Our results indicate that the theoretical model of Driscoll and Kennedy [1985], frequently used in this context, is not appropriate under PMSE conditions at scales significantly smaller than ℓoD&K.

Neutral air turbulence during DYANA: First results

During the DYANA campaign in early 1990 neutral air turbulence was measured by an improved version of a rocket-borne instrument (‘TOTAL’) at high (Andøya, 69°N) and middle (Biscarosse, 44°N) latitudes. A first preliminary analysis of the flights shows that at both latitudes typical small scale (1 km – 1 m) density fluctuations are less than 0.1–0.2% (RMS) in the entire mesosphere, apart from single altitude bins where values up to 1% were observed. The overall turbulent activity is thus significant less than what we have observed at high latitudes in winter. This result is in agreement with the model from Garcia and Solomon /1/. The temperature profiles and scale heights needed to convert small scale density fluctuations to turbulent parameters (e.g. eddy diffusion coefficients) were obtained from passive falling sphere measurements. An intercomparison of temperature profiles simultaneously measured by falling sphere and sodium LIDAR shows excellent agreement. In this report we concentrate on data from 6 Mar 1990 where measurements were performed both in Andøya and Biscarosse.

Middle atmosphere measurements of small-scale electron density irregularities and ion properties during the MAC/Epsilon Campaign

Rocket payloads designed to measure small-scale electron density irregularities and ion properties in the middle atmosphere were flown with each of the three main salvos of the MAC/Epsilon Campaign conducted at the Andøya Rocket Range, Norway, during October to November 1987. Fixed-bias, hemispheric nose tip probes measured small-scale electron density irregularities, indicative of neutral air turbulence, during the rocket's ascent; and subsequently, parachute-borne Gerdien condensers measured the region's polar electrical conductivity, ion mobility and density. One rocket was launched during daylight (15 October, 1052:20 UT), and the other two launches occurred at night (21 October, 2134 UT; 12 November, 0021:40 UT) under moderately disturbed conditions which enhanced the detection and measurement of turbulence structures. A preliminary analysis of the data indicates the presence of small-scale electron density irregularities in the general altitude range of 60 to 90 km. Ongoing data reduction will determine turbulence parameters and also the region's electrical properties below 90 km.

Turbulent small-scale neutral and ion density fluctuations: First results of the MAC/EPSILON campaign

In-situ measurements of small scale density fluctuations were performed during the recent rocket campaign MAC/EPSILON. Spectral analysis of both neutral and positive ion number density fluctuations show that positive ions can be used as passive tracers for neutral air turbulence. Temperature profiles obtained simultaneously indicate that in one example regions of turbulent activity coincide with regions of convective instability. Mean turbulent vertical velocities exhibit a trend towards a local minimum around 85 km.

Electrodynamics of the high‐latitude mesosphere

The discovery of apparent large (V/m) electric fields within the mesosphere suggests that this region is more active electrically than originally suspected. High‐latitude observations have been particularly productive in developing new concepts regarding mesospheric electrodynamics. Several high‐latitude observations of large mesospheric fields have been made under both quiet and aurorally active conditions but always below heights where enhanced ionizing radiations could significantly penetrate. Two measurements from Andøya, Norway, have also produced an anticorrelation of horizontal electric field directions with neutral wind velocities, leading to the theoretical description of a newly defined mechanism for V/m electric field generation involving wind‐induced separation of charged aerosols. Evidence for mesospheric aerosols and winds exists at all latitudes but is most evident at high latitudes during the appearance of noctilucent and/or polar mesospheric clouds. New measurements show an influence of such clouds on electric fields mapping downward from the ionosphere, modifying the electric field in both magnitude and direction. Recent results also demonstrate that middle atmospheric electrical parameters can be used to track neutral air turbulence and wave structure at high latitudes, thereby providing a powerful tool for study of mesospheric neutral dynamics and its relationship to the electrodynamic system. This overview concentrates on the various subjects outlined above.

First observations of summer polar mesospheric backscatter with a 224 MHz radar

Exceptionally strong echoes from the mesopause region have been observed with the EISCAT 224 MHz radar during the summer of 1987. The echoes are so strong that they cannot be explained by incoherent scatter theory but may instead be associated with neutral air turbulence. We discuss the variability of these echoes in terms of time and height distribution, and outline some characteristics of vertical velocity oscillations observed at the echo height.

Simultaneous observations of E-region irregularities by ground-based and rocket-borne techniques

During the Energy Budget Campaign (November–December 1980) E-region irregularities were observed under various geomagnetic conditions. Under disturbed conditions on the night of 30 Nov./1 Dec. the SAFARI auroral radar detected 10m wavelength irregularities north of Lycksele and measured their south to north phase velocity. Simultaneous observations of 1 m irregularities using one of the STARE auroral radars provided measurements of their SW-NE phase velocity. Associated rocket-borne measurements of electron densities over Kiruna using Langmuir probes showed vertical/temporal structure indicating irregularities in ionisation between 90 and 110 km, having a wide range of scale sizes. The power spectra of electron current variability were ‘white’ above about 90 km. 100 min later irregularities in the electric field were detected below 110 km by an E-field probe and an associated TMA release indicated the presence of turbulent regions in the neutral atmosphere between 90 and 105 km. A separate set of rocket-borne measurements under moderately disturbed conditions on the night of 15–16 November 1980 showed the presence of VLF electric fields in the frequency bands 168–5519 Hz at altitudes between 103 and 145 km at Kiruna. On the same occasion measurements of ion density fluctuations were obtained using a rocket-borne positive ion probe at Andøya. Power spectra derived from both these experiments have spectral indices near − 5 3 at those heights where the irregularities were strong. Although this would be consistent with the generation of plasma inhomogeneities by neutral air turbulence below about 95 km, these observations were at too great an altitude for this mechanism to be effective. The results are therefore tentatively interpreted as due to the decay of ion acoustic waves generated by plasma instabilities in the auroral electrojet.

Neutral air turbulence in the upper atmosphere observed during the Energy Budget Campaign

A number of different experimental techniques employed in the campaign provided measurements on the fine scale structure of the upper atmosphere, from which information about turbulent intensity, eddy transport and eddy dissipation rates may be extracted. The turbulent state of the mesosphere was shown to be highly variable and significant differences were found between observations obtained during the four salvoes launched during different degrees of geomagnetic disturbance.

Arecibo Middle Atmosphere Experiment

An experiment was performed using the powerful radar at Arecibo (18°N 67°E, 51° dip) to look for stratospheric and mesospheric echoes in the UHF frequency band. Strong echoes were returned from the stratospheric region, whereas no distinct returns were obtained from mesospheric heights. These results, when compared to those reported for the VHF radar at Jicamarca, support wavenumber dependences of the scattering mechanisms, such that scattering due to neutral air turbulence should be more effective at UHF frequencies than at VHF in the stratosphere, while the reverse should be true in the mesosphere. The results suggest the feasibility of utilizing the present UHF incoherent scatter radar for remote sensing of winds in the stratosphere.