Polar Mesosphere Summer Echoes Layers Research Articles

Observations of polar mesospheric summer echoes resembling kilometer-scale varicose-mode flows

Abstract. The mesosphere and lower thermosphere (MLT) region represents a captivating yet challenging field of research. Remote sensing techniques, such as radar, have proven invaluable for investigating this domain. The Middle Atmosphere Alomar Radar System (MAARSY), located in northern Norway (69∘ N, 16∘ E), uses polar mesospheric summer echoes (PMSEs) as tracers to study MLT dynamics across multiple scales. Chau et al. (2021) recently discovered a spatiotemporally highly localized event showing a varicose mode (simultaneous upward and downward movements), which is characterized by extreme vertical velocities (|w|≥3σ) of up to 60 m s−1 in the vertical drafts. Motivated by this finding, our objective is to identify and quantify similar extreme events or comparable varicose structures, i.e., defined by quasi-simultaneous updrafts and downdrafts, that may have been previously overlooked or filtered out. To achieve this, we conducted a thorough manual search through a MAARSY dataset, considering the PMSE months (i.e., May, June, July, August) spanning from 2015 to 2021. This search has revealed that these structures do indeed occur relatively frequently with an occurrence rate of up to 2.5 % per month. Over the 7-year period, we observed and recorded more than 700 varicose-mode events with a total duration of about 265 h and documented their vertical extent, vertical velocity characteristics, duration, and their occurrence behavior. Remarkably, these events manifest throughout the entire PMSE season with pronounced occurrence rates in June and July, while the probability of their occurrence decreases towards the beginning and end of the PMSE seasons. Furthermore, their diurnal variability aligns with that of PMSEs. On average, the observed events persisted for 20 min, while the varicose mode caused an average expansion of the PMSE layer by a factor of 1.5, with a maximum vertical expansion averaging around 8 km. Notably, a careful examination of the vertical velocities associated with these events confirmed that approximately 17 % surpassed the 3σ threshold, highlighting their non-Gaussian velocity distribution and extreme nature.

Effects of particle precipitation on the polar mesospheric summer echoes observed by EISCAT VHF 224 MHz radar

We present the results from investigation of spectra of polar mesospheric summer echoes (PMSE) observed with EISCAT VHF 224 MHz radar during energetic particle precipitation events in the summer of 2019. We used the sudden enhancements in electron densities derived from the VHF observations above 90 km as indicators of particle precipitation. We find that the altitude extent of the PMSE increased along with an enhancement of the strength of the pre-existing PMSE. On some occasions, PMSE suddenly appeared during particle precipitation events. After the particle precipitation subsided, the PMSE intensities continued to be stronger for a few minutes and then decreased. At the altitudes where the maximum enhancement in PMSE backscatter occurred, there is no corresponding broadening in the spectral widths. Interestingly, the most intense PMSE echoes almost always coincided with the lower values of spectral widths both during particle precipitations and other times. In some cases, the spectral widths show well separated regimes between the upper and lower parts of the PMSE layers and this also happened irrespective of particle precipitation. The frequency Doppler shifts showed alternating upward and downward motions without much difference before and after the particle precipitation. Based on these observations, we conclude that the sudden enhancement in the electron densities during particle precipitation intensifies PMSE structures that match the Bragg scale of the incident radio wave thereby enabling an enhancement in the backscatter strength. Similarly, PMSE backscattering may get initiated in the altitudes in which the structures were already present but where electron density was not sufficient before the onset of precipitation. Spectral widths and Doppler velocities does not show sudden variations with particle precipitation because they depend on the neutral dynamics.

Aspect sensitivity of double-layer polar mesosphere summer echoes at VHF based on an experimental case

Using the EISCAT VHF (224 MHz) radar data, an experimental case of aspect sensitivity and the characteristics of volume reflectivity of Polar Mesosphere Summer Echoes (PMSE) are presented. The data from the PMSE region during 24–28 June and 1 July 2013, when the beam pointed the off-vertical elevation angles (the largest is 28°), are used. It is found that the distribution curve of the volume reflectivity occurrence rate (OR) at different elevation angles obeys a Gaussian distribution as a whole, but there are some differences at different elevation angles. We investigated the aspect sensitivity parameters (θs) at different elevation angles, and found that θs shows a decrease at the altitude range of 82–84 km. In addition, the aspect sensitivity parameters (θs) of the upper and lower layers were calculated respectively when there is double-layer PMSE. The experimental results showed that in the case of double-layer PMSE, the aspect sensitivity of the lower PMSE layer is stronger than that of the upper PMSE layer. This indicates that the lower PMSE layers are associated with more anisotropic turbulence scattering. It provides new evidence on the dependence of PMSE aspect sensitivity on altitude. And possible explanations of the presented results are discussed.

Dynamically Unstable Strong Wind Shears Observed in the Polar Mesosphere Summer Echo Layer Associated With Geomagnetic Disturbances

AbstractWe report observations of dynamically unstable strong wind shear (Richardson number < 0.25) capable of inducing Kelvin‐Helmholtz instability in the polar mesospheric summer echoes (PMSE) layer (80–90 km) using very high frequency radar measurements in Kiruna (67.8°N, 20.4°E), Sweden, in 2006. The unstable strong wind shear can play an important role in producing PMSE by inducing turbulence and adiabatic cooling. We find that the strong shears take up 64% of the observed wind profiles and are frequently composed of systematically single‐shear/multishear (layer) structures, which gradually or abruptly vary in wind directions, so‐called wind shifts, through heights at intervals of 4–8 km. The strong shear rate normalized by PMSE counts has a good correlation (R = 0.7) in day‐to‐day variation with energetic electron (>30 keV) precipitations that were related to high‐speed solar wind streams. The observations of strong wind shear can be supported by satellite‐measured temperature modulations matching with the peaks of the first three high‐speed solar wind stream events. This study suggests that PMSE production is closely associated with the strong shear that is in turn linked to the effects of energetic electron precipitation.

Statistical study of PMSE response to HF heating in two altitude regions

Polar mesospheric summer echoes (PMSE) often show different layers. Artificial electron heating experiments are an important diagnostic tool to investigate the parameters in the PMSE regions. The response of PMSE to high frequency heating in the lower (80–85 km) and upper (85–90 km) PMSE layers is studied by analyzing PMSE observations carried out by the EISCAT VHF radar in July 2007. Different characteristics of modulated PMSE (e.g., PMSE intensity reduction, recovery, overshoot, ratios during the heating-on and heating-off times) are analyzed and compared at different situations such as only in one layer (lower or upper) and in both layers simultaneously without high-energy particle precipitation. Based on statistical results, it is found that in the lower layer all characteristics of modulated PMSE except PMSE recovery are greater than that in the upper layer. Electron temperature enhancement and PMSE modulation index due to ionospheric heating were calculated to show the enhanced electron temperature effect on PMSE. It is found that in both layers usually higher electron temperature enhancement corresponds to higher PMSE modulation index. However, the comparison of statistical results shows that in the lower layer electron temperature enhancement and PMSE modulation index are greater than that in the upper layer. Based on the relation of electron temperature enhancement to the PMSE modulation index, it is concluded that the variability of electron temperature enhancement may be responsible for the variations in different modulated PMSE characteristics between the lower and upper layers.

Dusty Space Plasma Diagnosis Using the Behavior of Polar Mesospheric Summer Echoes During Electron Precipitation Events

AbstractThe behavior of polar mesospheric summer echoes (PMSEs) during an electron precipitation event is investigated by including dusty plasma effects for the first time. The observational data recorded with the very high frequency (224 MHz) and ultrahigh frequency (930 MHz) radars at the European Incoherent SCATter Scientific Association on 10 and 11 July 2012 are presented. The observed radar echoes show that the PMSEs are both correlated and anticorrelated with the increased electron density associated with electron precipitation events on the two consecutive days. The experimental observations are compared with numerical simulations of the temporal evolution of PMSE with different background dusty plasma parameters during the electron precipitation event. Specifically, the effect of dust radius, dust density, recombination/photoionization rates, photo‐detachment current, and electron density enhancement ratio on the behavior of a PMSE layer and the associated dust charging process in the course of electron precipitation events is studied. It is observed that the ratio of electron density fluctuation amplitude δne to the plasma density (ne) plays a critical role in the appearance/disappearance of the layer. The simulation results revealed that the existence of PMSE is mainly determined by dust radius and dust density. The dusty plasma parameters associated with each event are estimated. The condensation nuclei of the ice particles such as proton hydrate clusters (H+(H2O)n) or meteoric smoke particles can be determined by employing the microphysical models along with the dusty plasma simulations. This can resolve any discrepancy in the description of the observed phenomena.

Influence of Solar and Lunar Tides on the Mesopause Region as Observed in Polar Mesosphere Summer Echoes Characteristics

AbstractLong‐term observations of polar mesosphere summer echoes (PMSE) from 2002 to 2012 are investigated with the aim to statistically study the effects of solar thermal migrating and lunar gravitational tides on aerosol layers and their environment at altitudes 80–90 km. The solar and lunar tidal periodicities are clearly present in PMSE data. For the first time, both amplitudes and phases of solar and lunar tides are estimated using PMSE data from the ESRAD radar located at Esrange (Sweden). The diurnal, semidiurnal, and terdiurnal solar migrating tides show pronounced periodicities in the PMSE strength and wind velocity components. Lunar tides demonstrate clear oscillations in the PMSE strength and wind velocities as well. “canonical” lunar gravitational tides, corresponding to the lunar gravitational potential, produce rather large amplitudes and are comparable to the solar thermal tides, whereas “noncanonical” lunar oscillations have minor effects on PMSE layers, but are still statistically significant. The influence of diurnal/semidiurnal tides and monthly/semimonthly tidal components is studied separately. Our estimations of solar thermal and lunar tidal amplitudes are in good agreement with those of previous model and experimental studies. A new mechanism of quadratic demodulation of the solar semidiurnal and lunar semidiurnal tides is shown to be valid at the summer mesopause and can explain periodical PMSE oscillations due to the lunar synodic semimonthly tide with period of 14.77 days. Two harmonics with periods of 27.0 and 13.5 days supposedly representing the solar rotation cycle are also clearly present in PMSE data.

A comparison of overshoot modelling with observations of polar mesospheric summer echoes at radar frequencies of 56 and 224 MHz

Abstract. We have compared radar observations of polar mesospheric summer echoes (PMSEs) modulated by artificial electron heating, at frequencies of 224 MHz (EISCAT VHF) and 56 MHz (MORRO). We have concentrated on 1 day of observation, lasting ~ 3.8 h. The MORRO radar, with its much wider beam, observes one or more PMSE layers all the time while the VHF radar observes PMSEs in 69% of the time. Statistically there is a clear difference between how the MORRO and the VHF radar backscatter reacts to the heater cycling (48 s heater on and 168 s heater off). While MORRO often reacts by having its backscatter level increased when the heater is switched on, as predicted by Scales and Chen (2008), the VHF radar nearly always sees the "normal" VHF overshoot behaviour with an initial rapid reduction of backscatter. However, in some heater cycles we do see a substantial recovery of the VHF backscatter after its initial reduction to levels several times above that just before the heater was switched on. For the MORRO radar a recovery during the heater-on phase is much more common. The reaction when the heater was switched off was a clear overshoot for nearly all VHF cases but less so for MORRO. A comparison of individual curves for the backscatter values as a function of time shows, at least for this particular day, that in high layers above ~ 85 km height, both radars see a reduction of the backscatter as the heater is switched on, with little recovery during the heater-on time. These variations are well described by present models. On the other hand, the backscatter in low layers at 81–82 km can be quite different, with modest or no reduction in backscatter as the heater is switched on, followed by a strong recovery for both radars to levels several times above that of the undisturbed PMSEs. This simultaneous, nearly identical behaviour at the two very different radar frequencies is not well described by present modelling.

Geometric considerations of polar mesospheric summer echoes in tilted beams using coherent radar imaging

Abstract. We present observations of polar mesospheric summer echoes (PMSE) using the Middle Atmosphere Alomar Radar System in Northern Norway (69.30° N, 16.04° E). The radar is able to resolve PMSE at high spatial and temporal resolution and to perform pulse-to-pulse beam steering. In this experiment, 81 oblique beam directions were used with off-zenith angles up to 25°. For each beam pointing direction and range gate, coherent radar imaging was applied to determine the mean backscatter location. The location of the mean scatterer in the beam volume was calculated by the deviation from the nominal off-zenith angle of the brightest pixel. It shows that in tilted beams with an off-zenith angle greater than 5°, structures appear at the altitudinal edges of the PMSE layer. Our results indicate that the mean influence of the location of the maximum depends on the tilt of the beam and on the observed area of the PMSE layer. At the upper/lower edge of the PMSE layer, the mean backscatter has a greater/smaller off-zenith angle than the nominal off-zenith angle. This effect intensifies with greater off-zenith beam pointing direction, so the beam filling factor plays an important role in the observation of PMSE layers for oblique beams.

Wave influence on polar mesosphere summer echoes above Wasa: experimental and model studies

Abstract. Comprehensive analysis of the wave activity in the Antarctic summer mesopause is performed using polar mesospheric summer echoes (PMSE) measurements for December 2010–January 2011. The 2-day planetary wave is a statistically significant periodic oscillation in the power spectrum density of PMSE power. The strongest periodic oscillation in the power spectrum belongs to the diurnal solar tide; the semi-diurnal solar tide is found to be a highly significant harmonic oscillation as well. The inertial-gravity waves are extensively studied by means of PMSE power and wind components. The strongest gravity waves are observed at periods of about 1, 1.4, 2.5 and 4 h, with characteristic horizontal wavelengths of 28, 36, 157 and 252 km, respectively. The gravity waves propagate approximately in the west-east direction over Wasa (Antarctica). A detailed comparison between theoretical and experimental volume reflectivity of PMSE, measured at Wasa, is made. It is demonstrated that a new expression for PMSE reflectivity derived by Varney et al. (2011) is able to adequately describe PMSE profiles both in the magnitude and in height variations. The best agreement, within 30%, is achieved when mean values of neutral atmospheric parameters are utilized. The largest contribution to the formation and variability of the PMSE layer is explained by the ice number density and its height gradient, followed by wave-induced perturbations in buoyancy period and the turbulent energy dissipation rate.

A statistical survey of electron temperature enhancements in heater modulated polar mesospheric summer echoes at EISCAT

A statistical analysis has been made of 26 Polar Mesospheric Summer Echoes (PMSE) active modulation experiments between 2002 and 2007. Observed with the EISCAT VHF radar the PMSE signature can be reduced by means of heating the ionosphere with powerful high frequency (HF) radio waves. However, PMSE modulation experiments sometimes fail. We use a computational model to estimate the enhanced electron temperatures due to ionospheric heating from HF radio waves in the D-region. We show that the statistical PMSE modulation for a fixed HF heater-induced electron temperature enhancement appears to be independent of altitude. In addition, for experiments where the PMSE modulation experiment failed, we show that the atmospheric attenuation of the HF heater wave was too great for the HF wave to have any effect on the PMSE layer.

Long-term aspect-sensitivity measurements of polar mesosphere summer echoes (PMSE) at Resolute Bay using a 51.5 MHz VHF radar

Polar Mesosphere Summer Echoes (PMSE) are now a well recognized summer phenomenon in the polar regions. For more than a decade, the echoes have been continuously monitored at Resolute Bay (75.0°N, 95.0°W) using a 51.5 MHz VHF radar, which has a narrow vertically pointing beam as well as four tilted beams at 10.9° from zenith. The radar site is located inside the northern polar cap, and remains a true polar cap radar at any level of geomagnetic activity. PMSE at this location show some unique features compared with other high latitude radars operating at similar frequencies. In this paper, we investigate the aspect-sensitivity of the PMSE at Resolute Bay. This feature of PMSE, which led to debate among researchers regarding the scattering mechanisms of these echoes, has not been extensively studied, especially on a long-term basis. In this study, we use a decade-long data-set collected during 1998–2009 to investigate the aspect-sensitivity parameter by comparing the absolute backscattered powers in four off-zenith beams with the vertical beam. We concentrate on conditions when electron precipitation is weak or not present, in order to simplify comparison (precipitation is present less than 8% of the time during PMSE). It was found that the aspect-sensitivity of PMSE can be broadly categorized into three groups based on the PMSE signal strengths of strong, moderate and weak. The height profiles of the calculated aspect angle show a good agreement among all four off-zenith beams, especially in the cases of strong and moderate PMSE strengths. Our long-term investigation reveals that when PMSE are strong, they show a median aspect angle of 8° at around 84 km and a 11° at around 88 km, meaning lower PMSE layers are associated with more anisotropic scattering. These values are about 2–3° higher, when PMSE are only “moderate” strength at Resolute Bay. The height dependence of aspect-sensitivity remains consistent in the cases of strong and moderate PMSE strengths. The results are compared with the observations made by a similar radar at another polar cap location at Eureka (80.0°N, 86.0°W). The comparison shows agreement in terms of the shape of the aspect-sensitivity height profiles, but the values are about 2–3° smaller. Possible explanations are discussed.

Multifrequency observations of Polar Mesosphere Summer Echoes using Alaskan radar facilities: Comparisons and scattering calculations

We present polar mesospheric radar observations at three frequency bands (MF/HF/VHF) and eight radar frequencies: 2.43, 3.3, 4.53, 4.9, 7.6, 28, 50, and 139 MHz, in order to better understand the well known but still not fully understood Polar Mesosphere Summer Echoes (PMSE). The echo morphology at the different frequencies is described by means of case studies where PMSE events were observed concurrently using at least two radar systems deployed over the Alaskan central region. The identity of MF and HF radar echoes as PMSE is resolved for the first time by means of simultaneous measurements made with VHF radars, the reference sensors employed traditionally for PMSE studies. On the basis of echo duration and radar reflectivity estimates, we suggest that low‐power HF radars would be more appropriate for PMSE monitoring. This is confirmed by a radar target analysis of turbulent scattering mechanisms in the polar summer mesosphere. MF radars show highly organized PMSE layers quite often but are more susceptible to ionospheric absorption and higher‐altitude returns associated with geomagnetic activity. Both phenomena produce a blanking effect in MF PMSE, which at times can persist for hours. HF and VHF radars are less affected by absorption events, but the PMSE echoes become weaker as the radar frequency increases.

Influence of tides and gravity waves on layering processes in the polar summer mesopause region

Abstract. Polar Mesosphere Summer Echoes (PMSE) have been studied at Andenes (69° N, 16° E), Norway, using VHF radar observations since 1994. One remarkable feature of these observations is the fact that {during 50% of the time,} the radar echoes occur in the form of two or more distinct layers. In the case of multiple PMSE layers, statistical analysis shows that the lower layer occurs at a mean height of ~83.4 km, which is almost identical to the mean height of noctilucent clouds (NLC) derived from observation with the ALOMAR Rayleigh/Mie/Raman lidar at the same site. To investigate the layering processes microphysical model simulations under the influence of tidal and gravity waves were performed. In the presence of long period gravity waves, these model investigations predict an enhanced formation of multiple PMSE layer structures, where the lower layer is a consequence of the occurrence of the largest particles at the bottom of the ice cloud. This explains the coincidence of the lowermost PMSE layers and NLC. During periods with enhanced amplitudes of the semidiurnal tide, the observed NLC and PMSE show pronounced tidal structures comparable to the results of corresponding microphysical simulations. At periods with short period gravity waves there is a tendency for a decreasing occurrence of NLC and for variable weak PMSE structures.

Time evolution of artificial electron heating in polar mesosphere summer echo layers

Artificial electron heating by the use of high‐energy transmitters can have a strong effect on the observed strength of the radar polar mesospheric summer echoes (PMSE). This heating can make PMSE go through a sequence where it nearly disappears as the heater is turned on and can also cause PMSE to overshoot its preheating strength by a large factor as the heater is switched off. Modeling the effect of heating on the PMSE scattering has shown that the form of the variation in PMSE intensity during the heater cycling is crucially dependent on the amount of electron heating and on the dust and plasma conditions. In the present paper we examine if and when the model assumption that the electron temperature Te stays constant during the heating after the first sudden rise when the heater is switched on is correct. We find that this is valid for low values of the ratio between the charge density of dust and electron charge density throughout the PMSE layer since dust has little effect on the electron density. However, when this ratio is not small compared to 1, so we have an electron bite‐out, the increased electron temperature and collision rate of electrons and dust particles can now lead to a significant reduction in the electron density. This will lead to a reduction in the absorption of heater wave energy in the bite‐out, and more wave energy will become available in its upper parts and in the PMSE above it where the electron temperature now will increase. This situation does, however, require that a substantial fraction of the heater wave energy reach the PMSE layer so that at least the lower parts of it are strongly heated.

The effect of electron bite-outs on artificial electron heating and the PMSE overshoot

Abstract. We have considered the effect that a local reduction in the electron density (an electron bite-out), caused by electron absorption on to dust particles, can have on the artificial electron heating in the height region between 80 to 90km, where noctilucent clouds (NLC) and the radar phenomenon PMSE (Polar Mesospheric Summer Echoes) are observed. With an electron density profile without bite-outs, the heated electron temperature Te,hot will generally decrease smoothly with height in the PMSE region or there may be no significant heating effect present. Within a bite-out Te,hot will decrease less rapidly and can even increase slightly with height if the bite-out is strong. We have looked at recent observations of PMSE which are affected by artificial electron heating, with a heater cycling producing the new overshoot effect. According to the theory for the PMSE overshoot the fractional increase in electron temperature Te,hot/Ti, where Ti is the unaffected ion temperature=neutral temperature, can be found from the reduction in PMSE intensity as the heater is switched on. We have looked at results from four days of observations with the EISCAT VHF radar (224 MHz), together with the EISCAT heating facility. We find support for the PMSE overshoot and heating model from a sequence of observations during one of the days where the heater transmitter power is varied from cycle to cycle and where the calculated Te,hot/Ti is found to vary in proportion to the transmitter power. We also looked for signatures of electron bite-outs by examining the variation of Te,hot/Ti with height for the three other days. We find that the height variation of Te,hot/Ti is very different on the three days. On one of the days we see typically that this ratio can increase with height, showing the presence of a bite-out, while on the next day the heating factor mainly decreases with height, indicating that the fractional amount of dust is low, so that the electron density is hardly affected by it. On the third day there is little heating effect on the PMSE layer. This is probably due to a sufficiently high electron density in the atmosphere below the PMSE layer, so that the transmitted heater power is absorbed in these lower layers. On this day the D-region, as given by the UHF (933MHz) observations, extends deeper down in the atmosphere than on the other two days, indicating that the degree of ionization in and below the PMSE layers is higher as well.

Observations of the overshoot effect during the 2004 EISCAT PMSE campaign

The radar phenomenon PMSE (Polar Mesospheric Summer Echoes), which is associated with charged dust particles, can be affected by artificial electron heating. Chilson et al., (2000) showed that if the heater is run in a cycle with equal, and comparatively, short (10–20 s) off and on periods, the PMSE strength is observed to weaken when the heater is on, and recover to approximately the same strength when the heater is switched off. With a new heater cycling, where the heater is on for a short time and then off for a long enough time for the dusty plasma conditions to return to its undisturbed conditions, a PMSE overshoot effect is produced. In the overshoot the PMSE strength, when the heater is switched off, can increase by a factor of several fold compared to what it was directly before the heater was switched on. By analyzing the shape of the overshoot characteristic curve as the PMSE varies through a weakening as the heater is switched on, an overshoot as it is switched off and a subsequent relaxation back to an unaffected strength, we can obtain a considerable amount of information on the state of the PMSE dusty plasma. In the present paper, we will show results from a large EISCAT overshoot campaign in July 2004 where the PMSE overshoot was observed frequently. The shape of the PMSE power variations in each overshoot cycle is shown to vary considerably with height in the PMSE layer and also from cycle to cycle. We will show that very different PMSE overshoot shapes can occur at various occasions, and heights, and how they can be dependent on the local dust and plasma conditions.

In situ observations of small scale neutral and plasma dynamics in the mesosphere/lower thermosphere at 79°N

From June 29 to July 6 2003, the German-Norwegian ROMA-SvalRak campaign (ROMA = Rocket borne Observations in the Middle Atmosphere) took place at the SvalRak rocket range, Ny-Ålesund (78.9°N, 11.9°E; Spitzbergen). The main scientific aim of this campaign was to study polar mesosphere summer echoes (PMSE) and mesospheric turbulence. During this campaign a total of three instrumented sounding rockets was launched. In addition, ground based observations with a VHF radar and a potassium lidar at Longyearbyen complemented the in situ measurements. All three sounding rockets were equipped with the CONE (COmbined sensor for Neutrals and Electrons) instrument to measure small scale structure of neutral air and electron density, and neutral temperature. The PIP (positive ion probe) instrument was used to measure small scale structure of the positive ion density. Furthermore, two cold plasma probes were flown to measure electron temperature, and a particle detector was employed to detect signatures of charged aerosols. During the first launch, an electric field experiment was also incorporated, while during the other two launches, Faraday rotation experiments yielded absolute electron number densities. During all three rocket flights a PMSE was observed by the VHF radar, whereas the potassium lidar detected a noctilucent cloud (NLC) only during the second launch. Signatures of charged particles forming the PMSE and NLC layers were recorded by the onboard particle detectors.

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.

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.