Polar Mesosphere Summer Echoes Research Articles

Climatological comparison of polar mesosphere summer echoes over the Arctic and Antarctica at 69°

Abstract. Polar mesosphere summer echoes (PMSE) have been observed for more than 30 years with 50 MHz VHF radars at various locations in the Northern Hemisphere. Continuous observation of PMSE is conducted on the northern Norwegian island of Andøya (69.3° N) using the ALWIN radar (1999–2008) and MAARSY (since 2010). The same kind of PMSE measurements began in 2004 in the Southern Hemisphere with the Australian Antarctic Division's VHF radar at Davis Station in Antarctica (68.6° S), which is at an opposite latitude to Andøya. Since the radars at both sites are calibrated, the received echo strength of PMSE from more than 1 decade of mesospheric observations on both hemispheres could be converted to absolute signal power, allowing for direct comparison of the measurements. Comparison of PMSE observations obtained at both radar sites during a period of 23 boreal summers (Andøya) and 15 austral summers (Davis) shows that their PMSE signal strengths are of the same order of magnitude, but significantly fewer PMSE are observed in the Southern Hemisphere than in the Northern Hemisphere. Compared to Andøya, the PMSE season over Davis starts about 7 d later on average and ends 9 d earlier, making it 16 d shorter. PMSE over Davis occur less frequently but with greater variability in seasonal, diurnal, and altitudinal occurrence. For example, PMSE over Davis reach maximum altitudes about 1.5 km higher than those over Andøya.

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.

Using Deep Learning Methods for Segmenting Polar Mesospheric Summer Echoes

Polar mesospheric summer echoes (PMSE) are radar echoes that are observed in the mesosphere during the arctic summer months in the polar regions. By studying PMSE, researchers can gain insights into physical and chemical processes that occur in the upper atmosphere—specifically, in the 80 to 90 km altitude range. In this paper, we employ fully convolutional networks such as UNET and UNET++ for the purpose of segmenting PMSE from the EISCAT VHF dataset. First, experiments are performed to find suitable weights and hyperparameters for UNET and UNET++. Second, different loss functions are tested to find one suitable for our task. Third, as the number of PMSE samples used is relatively small, this can lead to poor generalization. To address this, image-level and object-level augmentation methods are employed. Fourth, we briefly explain our findings by employing layerwise relevance propagation.

A Simulation of Artificial Neutral Gas‐Dust Plasma in the Ionosphere

AbstractDusty plasma is generally defined as a plasma system containing a large number of nano‐ or micron‐sized dust particles. Its plasma properties and propagation properties of wave travel through it is of great scientific significance to the understanding of many fundamental problems in space physics and astrophysics. Based on previous studies on rocket exhaust and Polar Mesosphere Summer Echoes, fluid mechanics and dust microphysics are combined to simulate the ionospheric plasma structure generated by neutral gases and charged aerosols using fluid theory and kinetic theory. The Total Electron Content of the in‐situ detection beacon is simulated and compared with the experimental phenomenon given in Charged Aerosol Release Experiment by Bernhardt et al. The ray tracing algorithm is used to simulate the path deflection of the radio wave when it passes through the artificial disturbance area generated by the gas‐solid mixture, and the simulation results of ionosonde and analysis are given. Since the neutral gas expands substantially isotropically after being released, the resulting depleted structure is an approximately isotropic sphere, or an acorn‐like structure. The solid particles are constrained by the nozzle, and particles will distribute within a spray angle, and the depletion structure will appear as a “broom‐like” or “cone‐like” structure. If the size of dust particles is small enough, the particle movement will be restricted by the geomagnetic field, resulting in a “squid‐like” ionospheric structure. Because of the fishtail‐like sharp boundary, radio waves passing through it are deflected in a specific path, which is markedly different from the effect of pure neutral gas injected into the ionosphere.

Modulation of polar mesospheric summer echoes (PMSEs) with high-frequency heating during low solar illumination

Abstract. Polar mesospheric summer echo (PMSE) formation is linked to charged dust/ice particles in the mesosphere. We investigate the modulation of PMSEs with radio waves based on measurements with EISCAT VHF radar and EISCAT heating facility during low solar illumination. The measurements were made in August 2018 and 2020 around 20:02 UT. Heating was operated in cycles with intervals of 48 s on and 168 s off. More than half of the observed heating cycles show a PMSE modulation with a decrease in PMSE when the heater is on and an increase when it is switched off again. The PMSE often increases beyond its initial strength. Less than half of the observed modulations have such an overshoot. The overshoots are small or nonexistent at strong PMSE, and they are not observed when the ionosphere is influenced by particle precipitation. We observe instances of very large overshoots at weak PMSE. PMSE modulation varies strongly from one cycle to the next, being highly variable on spatial scales smaller than a kilometer and timescales shorter than the timescales assumed for the variation in dust parameters. Average curves over several heating cycles are similar to the overshoot curves predicted by theory and observed previously. Some of the individual curves show stronger overshoots than reported in previous studies, and they exceed the values predicted by theory. A possible explanation is that the dust-charging conditions are different either because of the reduced solar illumination around midnight or because of long-term changes in ice particles in the mesosphere. We conclude that it is not possible to reliably derive the dust-charging parameters from the observed PMSE modulations.

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.

Studies of noctilucent clouds from the stratosphere during the SONC balloon-borne experiment in 2021

On the night 16–17 August 2021, a balloon-borne experiment called Stratospheric Observations of Noctilucent Clouds (SONC) was successfully performed. A big scientific balloon, having onboard three automated cameras for studies of noctilucent clouds (NLC), was launched to 32.7 km altitude from Esrange (northern Sweden). All three NLC cameras and electronics were completely operational in the stratosphere for more than 10 h at low temperatures of about −30 °C. Two wide angle cameras registered an extended NLC field of about 1700 km long in the twilight sky sector from the north-west to the north-east of Esrange. NLC were of a moderate brightness and were located at high latitudes between 68° and 71°N. The NLC field was located in a cold area (138–142 K) below the frost point temperature (145–148 K) in the mesopause region that was confirmed by Aura/MLS satellite and Esrange lidar measurements. The balloon-borne NLC measurements were accompanied by ground-based lidar and radar measurements. The latter have registered Polar Mesosphere Summer Echoes (PMSE) in the same volume of the summer mesopause along with NLC observed from the stratosphere that has been performed for the first time above northern Scandinavia. We describe the technique and method of the NLC observation from the stratosphere as well as present the first scientific results of the SONC experiment.

Segmentation of PMSE Data Using Random Forests

EISCAT VHF radar data are used for observing, monitoring, and understanding Earth’s upper atmosphere. This paper presents an approach to segment Polar Mesospheric Summer Echoes (PMSE) from datasets obtained from EISCAT VHF radar data. The data consist of 30 observations days, corresponding to 56,250 data samples. We manually labeled the data into three different categories: PMSE, Ionospheric background, and Background noise. For segmentation, we employed random forests on a set of simple features. These features include: altitude derivative, time derivative, mean, median, standard deviation, minimum, and maximum values corresponding to neighborhood sizes ranging from 3 by 3 to 11 by 11 pixels. Next, in order to reduce the model bias and variance, we employed a method that decreases the weight applied to pixel labels with large uncertainty. Our results indicate that, first, it is possible to segment PMSE from the data using random forests. Second, the weighted-down labels technique improves the performance of the random forests method.

Mid-Latitude Detection of High Schmidt-Number Turbulent Echoes, and Comparison to PMSE and Geomagnetic Variations

Unexpected observations of strong radiowave scatter at a ~85–90 km altitude with very high frequency radars were explained in the early 1990s, when it was demonstrated that these were due to special turbulent and small-scale scatterers with high Schmidt number. Studies of these phenomena have primarily been concentrated in polar regions, and the events seem most prominent in regions of very cold air (below 140 K). Such radar echoes are referred to as polar mesosphere summer echoes (PMSE), and are rare at lower latitudes. In this paper we report observations of similar scatterers at sites below 50° latitude. The nature of these scatterers is discussed and results are compared to observations at the polar site of Eureka, Canada. Mid-latitude observations at frequencies of 48.92 and 45.47 MHz were made, respectively, at Abitibi Canyon (49.9° N latitude) and Markstay (46.5° N latitude) in Ontario, Canada. In particular, we look at the relationship of these scatterers to geophysical parameters, especially the Ap index. Our results suggest that mesospheric air with temperatures less than 140 K now exists below 50° latitude. This may be an indication of an equator-ward creep of global mesospheric cooling (which is associated with the well-known tropospheric global warming), but the scatterers at lower latitudes also demonstrate correlation with the Ap index. On the other hand, the polar scatterers at Eureka demonstrated no correlation of any significance with Ap. The importance of these results in regard to the global distribution of mesospheric temperatures is discussed, and comparisons to other measurements are made.

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.

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)

Polar Mesosphere Summer Echoes and Possible Signatures of Pulsating Aurora Detected by the Meteor Radar

AbstractUsing data of the all‐sky interferometric meteor radar (SKiYMET, 36.9 MHz) operating in the Sodankylä Geophysical Observatory (67°22′N, 26°38′E, Finland) we found a specific type of polar mesosphere summer echo (PMSE), the power of which exhibits irregular variations at a frequency of the order of a few Hz. We classified such radar echoes as pulsating PMSE. These echoes were observed in late June‐July in the morning sector (4–12 MLT) during geomagnetic storms. They were received from a narrow range of altitudes near 82 km, which corresponds to the altitude of noctilucent clouds where ice particles of about 50‐nm radii exist. During pulsating PMSE, the SGO ionosonde showed an electron density of the order of 3 × 1011 m−3 around 82 km, and enhanced D‐region ionization was manifested in the cosmic noise absorption. We suggest that the power of PMSE is modulated by bursts of electron precipitation corresponding to the few‐Hz internal modulation of pulsating aurora. During a short precipitation burst of 50–100 keV electrons, additional electrons can attach to the ice particles due to the presence of hyperthermal electrons according to the hypothesis proposed by Rosenberg et al. (2012, https://doi.org/10.1016/j.jastp.2011.10.011). This leads to an increase of the power of PMSE. After the burst is ended, the ice particles are deionized with a characteristic time of about 0.2 s due to attractive interaction with ions.

Radar Observation of Extreme Vertical Drafts in the Polar Summer Mesosphere

AbstractThe polar summer mesosphere is the Earth's coldest region, allowing the formation of mesospheric ice clouds. These ice clouds produce strong polar mesospheric summer echoes (PMSE) that are used as tracers of mesospheric dynamics. Here, we report the first observations of extreme vertical drafts (50 ) in the mesosphere obtained from PMSE, characterized by velocities more than five standard deviations larger than the observed vertical wind variability. Using aperture synthesis radar imaging, the observed PMSE morphology resembles a solitary wave in a varicose mode, narrow along propagation (3–4 km) and elongated ( km) transverse to propagation direction, with a relatively large vertical extent (13 km). These spatial features are similar to previously observed mesospheric bores, but we observe only one crest with much larger vertical extent and higher vertical velocities.

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.

Study on the Strong Radar Echoes at Polar Mesosphere Using a New Dataset Analysis Software

Polar mesosphere echoes and polar mesosphere clouds data extract and analyze software (PMEC_DEAS) was used to deal with the echoes and clouds dataset, then the characteristics of radar echoes were studied in the paper. By analyzing the polar mesosphere summer echoes (PMSE) and polar mesospheric clouds (PMC) data in the short-and long-terms as an example based on the PMEC_DEAS, the correlation between the occurrence rate (OR) of PMSE and PMC is studied. It is found that the correlation between the short-term PMSE OR and PMC is not significant, the long-term double-layer PMSE OR is positively correlated with the PMC. It shows that double-layer PMSE is closely related to PMC. This is consistent with existing conclusions. PMEC_DEAS can effectively adapt to the complex characteristic changes of the events and has good robustness. The data analyzed by PMEC_DEAS was associated with better stability and compatibility. The convenience and utilitarian nature show superiority compared with the current mainstream software.

Comparison of modulated UHF PMSE characteristics at different altitude ranges

Heating of electrons in PMSE region with powerful high frequency (HF) radio waves is called artificial electron heating. This is used as a diagnostic tool to study the interesting radar phenomenon of Polar Mesosphere Summer Echoes (PMSE). Using the PMSE observations of EISCAT UHF (930 MHz) radar from 2003 to 2015, the effect of artificial electron heating on PMSE is studied for the first time in terms of modulated PMSE characteristics. The modulated PMSE characteristics including the PMSE intensity reduction, PMSE intensity recovery, and PMSE overshoot, are analyzed and compared between different height ranges. Our results indicate variability in these characteristics at different height ranges, however, statistics reveal that unlike VHF PMSE these characteristics show a clear difference between the comparatively lower (below 84.0 km) and upper (above 84.0 km) height ranges. It is found that in below 84.0 km height ranges a greater number of heating cycles show PMSE intensity reduction, PMSE intensity recovery, and PMSE overshoot as compared to above 84.0 km height ranges. All characteristics found are in good agreement with literature especially at just after the heater-on time where no opposite behavior of PMSE intensity reduction (onset overshoot) is found, and at the heater-off time where the unheated PMSE intensity never found smaller than the heated PMSE intensity. The observed response of the PMSE to heating by a powerful radio wave is used to estimate the associated electron temperature enhancement and PMSE modulation index. PMSE modulation index is the ratio of heated PMSE to unheated PMSE, which describes that how much the heated PMSE varies around its unheated PMSE. In below 84 km height ranges, the PMSE modulation index and enhanced electron temperatures are greater than that in above 84 km height ranges. The difference in modulated properties of PMSE below and above 84 km is due to the difference in electron temperature enhancement, which depends on the electron density below the PMSE altitudes.

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.

Radar imaging with EISCAT 3D

Abstract. A new incoherent scatter radar called EISCAT 3D is being constructed in northern Scandinavia. It will have the capability to produce volumetric images of ionospheric plasma parameters using aperture synthesis radar imaging. This study uses the current design of EISCAT 3D to explore the theoretical radar imaging performance when imaging electron density in the E region and compares numerical techniques that could be used in practice. Of all imaging algorithms surveyed, the singular value decomposition with regularization gave the best results and was also found to be the most computationally efficient. The estimated imaging performance indicates that the radar will be capable of detecting features down to approximately 90×90 m at a height of 100 km, which corresponds to a ≈0.05∘ angular resolution. The temporal resolution is dependent on the signal-to-noise ratio and range resolution. The signal-to-noise ratio calculations indicate that high-resolution imaging of auroral precipitation is feasible. For example, with a range resolution of 1500 m, a time resolution of 10 s, and an electron density of 2×1011m-3, the correlation function estimates for radar scatter from the E region can be measured with an uncertainty of 5 %. At a time resolution of 10 s and an image resolution of 90×90 m, the relative estimation error standard deviation of the image intensity is 10 %. Dividing the transmitting array into multiple independent transmitters to obtain a multiple-input–multiple-output (MIMO) interferometer system is also studied, and this technique is found to increase imaging performance through improved visibility coverage. Although this reduces the signal-to-noise ratio, MIMO has successfully been applied to image strong radar echoes as meteors and polar mesospheric summer echoes. Use of the MIMO technique for incoherent scatter radars (ISRs) should be investigated further.

Investigation of Polar Mesospheric Summer Echoes Using Linear Discriminant Analysis

Polar Mesospheric Summer Echoes (PMSE) are distinct radar echoes from the Earth’s upper atmosphere between 80 to 90 km altitude that form in layers typically extending only a few km in altitude and often with a wavy structure. The structure is linked to the formation process, which at present is not yet fully understood. Image analysis of PMSE data can help carry out systematic studies to characterize PMSE during different ionospheric and atmospheric conditions. In this paper, we analyze PMSE observations recorded using the European Incoherent SCATter (EISCAT) Very High Frequency (VHF) radar. The collected data comprises of 18 observations from different days. In our analysis, the image data is divided into regions of a fixed size and grouped into three categories: PMSE, ionosphere, and noise. We use statistical features from the image regions and employ Linear Discriminant Analysis (LDA) for classification. Our results suggest that PMSE regions can be distinguished from ionosphere and noise with around 98 percent accuracy.