All-sky Imager Research Articles

Solar Power Ramp Event Grading Forecasting for Solar Power Plants Based on All-Sky Images

Solar Power Ramp Event Grading Forecasting for Solar Power Plants Based on All-Sky Images

Development of a new hybrid cloud cover detection method for all sky mapping

Local precise cloud detection techniques in the daytime are finding a wide range of applications in solar farms and for tracking the factory flue gases. Additionally, the cloud coverage is a measure for the site quality of astronomical observatories. The moon is itself a problem for astronomical observations. If there are astronomical objects near to moon, the light scattered by clouds from moon in the sky also has to be determined to extract from the observation schedule of robotic telescope. A novel hybrid method to deal with all-sky images in any conditions in daytime and nighttime is developed, and related algorithms are implemented in MATLAB. A GUI has been presented which interactively analyzes the images taken from all-sky cams. It plots a radar scan of the sky with clouds in polar coordinate systems. The systematic mean error is estimated at less than 3%.

Poleward-Motion Aware Network for Poleward Moving Auroral Forms Recognition

Poleward moving auroral forms (PMAFs) are a common dayside auroral phenomenon, and the study of PMAFs has important implications for the exploration of the near-earth space physical processes for geosciences. In the all-sky imager (ASI) image sequence, PMAFs show a tendency to move northward in the northern hemisphere. Therefore, this particular motion pattern can be used for PMAF recognition. Previous works for automatic recognition of PMAFs tend to rely on optical flow. However, both the traditional and the deep learning-based optical flow estimation methods are time- and memory-expensive. In view of the large number of auroral images generated every year, it is impractical to estimate the optical flow for all auroral data with limited computational resources. In this letter, a poleward-motion aware network (PA-Net) is proposed to extract the motion features directly from ASI images. PA-Net computes the correlation between each point in an image and the points at the poleward direction in the following image by means of a poleward-motion aware operation (PA-Operation), to verify whether the point under consideration has undergone poleward motion. In addition, a channel attention mechanism is applied to the features obtained by PA-Operation to suppress information less helpful for recognizing PMAFs. The PA-Net achieves the best performance on the PMAFs recognition dataset over other commonly used action recognition models, validating the superiority of our approach. More importantly, the complicated optical flow estimation is avoided, making it possible to apply the proposed method to large-scale auroral data.

Upwelling coherent backscatter plumes observed with ionosondes in low-latitude region

Investigations of upwelling backscatter plumes are mostly from observations of VHF radars. This study reports the first observation of upwelling backscatter plumes (backscatter echoes beyond the critical frequency of the F2 layer, foF2) recorded by ionosondes at low latitudes on 13 March 2015. With a pair of ionosondes (Puer, 22.7° N, 101.05° E, Dip Lat 12.9° N, and Chiang Mai, 18.76° N, 98.93° E, Dip Lat 9.04° N), Swarm satellites flying side-by-side (longitudinal separation of about 1.4°, about 150 km), and an all-sky imager (25° N, 104° E, Dip Lat 15.1° N), the evolution of plasma bubbles is presented in this study. Observations show that ionosonde backscatter plumes originating from a local-scale upwelling could be observed. In addition, this study also reported ionosonde backscatter plumes from other regions with approaching and receding characteristics. Results show that characteristics of backscatter plumes with ionosondes are consistent with observations from VHF radars. It suggests that ionosonde backscatter plumes might also be used to study the characteristics of upwelling backscatter plumes.

Unsupervised Learning of Auroral Optical Flow for Recognition of Poleward Moving Auroral Forms

Poleward moving auroral forms (PMAFs) are one of the most common dayside auroral phenomena, which are important to study the dynamics of the polar cusp. In this article, we propose an unsupervised auroral optical flow (UnAurFlow) model for recognizing the PMAFs from auroral image sequences captured by all-sky imagers. Since auroral shape and illumination change continuously and randomly during the evolution, and auroral motion violates the brightness constancy assumption, UnAurFlow designs an auroral deformation detection module based on bidirectional optical flow and exploits the census transform to compensate for the auroral luminosity changes. UnAurFlow is trained in an unsupervised manner, which circumvents the difficulty of manually labeling auroral optical flow. The validity of UnAurFlow is qualitatively and quantitatively evaluated using auroral observations in 2003–2005 at the Arctic Yellow River Station. Based on the auroral optical flow computed by UnAurFlow, and combining with ResNet and the attention mechanism, PMAFs are automatically recognized with a precision of 84.77% from continuous auroral observations, which outperform the state-of-the-art unsupervised optical flow algorithms. The temporal occurrence distribution of PMAFs is prenoon–postnoon asymmetric under negative interplanetary magnetic field (IMF) By conditions, which is in good agreement with the previous statistical study.

Joint observation of the concentric gravity wave event on the Tibetan Plateau

A concentric gravity wave event was captured by a photographer in Nagarzê County (90.28°N, 28.33°E) between 02:00 and 04:00 (local time) on May 11, 2019. This concentric gravity wave event was also observed by the Suomi National Polar-orbiting Partnership satellite and the all-sky airglow imager at Yangbajing station (90.5°E, 30.1°N). The temporal and spatial information on gravity waves from the photographs provided a rare opportunity to study the propagation of gravity waves over the Tibetan Plateau. According to wind and temperature data from the MERRA-2 reanalysis (Modern-Era Retrospective analysis for Research and Applications, Version 2) and empirical models (NRLMSISE-00 [Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar Exosphere] and HWM [horizontal wind model]), we inversely derived the propagation trajectory from the observed wave pattern to the source region by using the ray-tracing method. The source of the concentric gravity wave was identified as deep convection in Bangladesh (90.6°E, 25.0°N). The maximum background wind speed in the propagation direction (31.05 m/s) was less than the phase speed of 53 m/s, which is consistent with the wind-filtering theory.

Auroral Image Classification With Very Limited Labeled Data Using Few-Shot Learning

Auroral image classification is an important task for polar research. At present, the auroral image classification mainly focuses on common auroral types with a lot of labeled samples, where the classification performance is heavily dependent on large labeled datasets for supervised training. However, in many instances, such as for rare auroras or newly discovered auroras, it is not feasible to obtain ample category labels. This letter proposes a few-shot learning-based solution for auroral image classification with very limited labeled samples. Since auroral data are always collected by ground-based all-sky imagers with little domain differences, we incorporate prior knowledge into auroral image representation with limited labeled samples using the labels from previous auroral image classification studies. Also, the cosine classifier is employed to avoid the overfitting problem caused by the very few labels of the auroral images to be classified. When providing one and five labeled samples for each category, our solution achieves an average classification accuracy of 87.14% and 95.33% on the novel auroral category dataset, respectively. The effectiveness of this solution is further demonstrated by auroral image retrieval and temporal occurrence distributions. The experimental results show that the proposed solution provides a new idea for the automatic classification and retrieval of auroral images, especially for those cases where there are only very few labeled samples.

Transfer Learning Aurora Image Classification and Magnetic Disturbance Evaluation.

We develop an open source algorithm to apply Transfer learning to Aurora image classification and Magnetic disturbance Evaluation (TAME). For this purpose, we evaluate the performance of 80 pretrained neural networks using the Oslo Auroral THEMIS (OATH) data set of all‐sky images, both in terms of runtime and their features' predictive capability. From the features extracted by the best network, we retrain the last neural network layer using the Support Vector Machine (SVM) algorithm to distinguish between the labels “arc,” “diffuse,” “discrete,” “cloud,” “moon” and “clear sky/ no aurora”. This transfer learning approach yields 73% accuracy in the six classes; if we aggregate the 3 auroral and 3 non‐aurora classes, we achieve up to 91% accuracy. We apply our classifier to a new dataset of 550,000 images and evaluate the classifier based on these previously unseen images. To show the potential usefulness of our feature extractor and classifier, we investigate two test cases: First, we compare our predictions for the “cloudy” images to meteorological data and second we train a linear ridge model to predict perturbations in Earth's locally measured magnetic field. We demonstrate that the classifier can be used as a filter to remove cloudy images from datasets and that the extracted features allow to predict magnetometer measurements. All procedures and algorithms used in this study are publicly available, and the code and classifier are provided, which opens possibility for large scale studies of all‐sky images.

Visibility Domain Direction-of-Arrival Optimization for Arbitrary 2-D Arrays

A visibility-modelling based method is introduced to determine the elevation and azimuth angles for a signal received at a quasi-randomly arranged (2-D) array. The method is optimal for the number of antennas when the array is configured such that no antenna baselines are repeated. Simulation shows that our method has comparable accuracy to subspace-based acrlong doa algorithms like acrfull music and measured data shows that it is both more accurate and faster than visibility based methods like acrlong ra all-sky imaging.

Long-lasting stagnant equatorial plasma bubble event and the related scintillation over the Brazilian region

Diverse studies about equatorial plasma bubble structures and their relation with ionospheric scintillation have been performed in the last decades. Among many findings, the investigations usually reported dominant plasma bubble eastward velocity with a magnitude of few tens of m/s and larger amplitude scintillation for transionospheric signals aligned with these depleted structures. However, an uncommon long-lasting event with negligible average zonal drift prevailing for hundreds of minutes was registered over the Brazilian region, allowing a case study of the scintillation pattern under this particular condition. Data from ionosondes, all-sky imager (6300 Å filter), geostationary satellites and Global Navigation Satellite System were used here, covering the eastern portion of the Brazilian ionosphere. The results show that the scintillation was less intense than in other nights around the event, suggesting that larger plasma density gradients and sequential bubble structures seem to cause more severe scintillation scenario than the alignment condition alone, even though the last also contributes to worsen the scintillation effects. Regarding the stagnant bubble pattern, the results suggest prevailing E region Hall conductivity and equatorward wind as potential agents.

Ionospheric Plasma Vertical Drift and Zonal Wind Variations Cause Unusual Evolution of EPBs During a Geomagnetically Quiet Night

AbstractPrevious studies show equatorial plasma bubbles (EPBs) mainly occur after sunset and usually drift from west to east. In this work, two EPBs occurred after midnight and lasted to sunrise, and were observed by multi‐instruments (two all‐sky imagers (ASIs), global positioning system, Swarm satellite, and a digisonde) during a geomagnetically quiet night of December 18–19, 2018. Our observations also show that plasma depletion structure (PDS) also occurred at that night. EPBs occurred in the region of PDS and showed a special evolution process during the night. Those PDS gradually disappeared with time, but strip‐like EPBs remained clearly in airglow images. Near sunrise, new PDS appeared to merge with those EPBs to form large PDS. Meanwhile, EPBs showed different zonal drifts within a very narrow longitudinal zone (∼2°). One EPB remained stationary while the other showed a small eastward drift. These results are different from post‐sunset EPBs in previous studies. During the lifetime of those ionospheric plasma irregularities, upward and downward motion of the peak height and virtual height of the ionospheric F region was observed by a digisonde, which indicates that the evolution of EPBs should be related to the vertical drift of ionospheric plasma. Besides, based Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM) simulations, we suggest the different zonal drifts of those EPBs might related to plasma zonal drifts caused by zonal winds.

Westward Drift of Ionospheric Plasma Irregularities over a Low to Mid-latitude Transition Region in Indian Sector

We report the observation of plasma depletions/plumes in the F region ionosphere over a low to middle latitude transition region in the Indian sector. The observation of these plasma depletions is based on the data obtained in May 2019 through the all-sky airglow CCD imager installed in the campus of University of Kashmir, Srinagar (34.12 °N, 74.83 °E, magnetic latitude 25.91 °N). The depletions on the two consecutive nights of 05 and 06 May 2019 are aligned along the North-South (N-S) direction and drift westward. Several depletion bands along with some enhancement bands are seen in the 630-nm airglow images throughout the two nights. The observed structures show certain characteristics similar to Medium Scale Traveling Ionospheric Disturbances (MSTIDs) but these airglow features are not completely periodic. Further, in the observed depletion bands some East-West asymmetries are observed along with the structured tree-like branches of the airglow depletions. Some depletion bands even bifurcate leading to the inference that the structures are signatures of plasma irregularities rather than the usual MSTIDs observed in low-mid latitude transition region. The westward drift of the depletions especially during geomagnetic quiet times over this region makes this study significant since it offers a possible evidence that shows extension of spread F irregularities from the mid latitude region to the low-mid latitude transition region. In this paper, we point out some possible mechanisms related to the occurrence of plasma depletions at this region and their westward movement during geomagnetic quiet times.

Mesoscale Features in the Global Geospace Response to the March 12, 2012 Storm

The geospace response to coronal mass ejections includes phenomena across many regions, from reconnection at the dayside and magnetotail, through the inner magnetosphere, to the ionosphere, and even to the ground. Phenomena occurring in each region are often connected to each other through the magnetic field, but that field undergoes dynamic changes during storms and substorms. Improving our understanding of the geospace response to storms requires a global picture that enables us to observe all the regions simultaneously with both spatial and temporal resolution. Using the Energetic Neutral Atom (ENA) imager on the Two Wide-Angle Imaging Neutral-Atom Spectrometers (TWINS) mission, a temperature map can be calculated to provide a global view of the magnetotail. These maps are combined with in situ measurements at geosynchronous orbit from GOES 13 and 15, auroral images from all sky imagers (ASIs), and ground magnetometer measurements to examine the global geospace response of a coronal mass ejection (CME) driven event on March 12th, 2012. Mesoscale features in the magnetotail are observed throughout the interval, including prior to the storm commencement and during the main phase, which has implications for the dominant processes that lead to pressure buildup in the inner magnetosphere. Auroral enhancements that can be associated with these magnetotail features through magnetosphere-ionosphere coupling are observed to appear only after global reconfigurations of the magnetic field.

Estimating Precipitating Energy Flux, Average Energy, and Hall Auroral Conductance From THEMIS All-Sky-Imagers With Focus on Mesoscales

Recent attention has been given to mesoscale phenomena across geospace (∼10 s km to 500 km in the ionosphere or ∼0.5 RE to several RE in the magnetosphere), as their contributions to the system global response are important yet remain uncharacterized mostly due to limitations in data resolution and coverage as well as in computational power. As data and models improve, it becomes increasingly valuable to advance understanding of the role of mesoscale phenomena contributions—specifically, in magnetosphere-ionosphere coupling. This paper describes a new method that utilizes the 2D array of Time History of Events and Macroscale Interactions during Substorms (THEMIS) white-light all-sky-imagers (ASI), in conjunction with meridian scanning photometers, to estimate the auroral scale sizes of intense precipitating energy fluxes and the associated Hall conductances. As an example of the technique, we investigated the role of precipitated energy flux and average energy on mesoscales as contrasted to large-scales for two back-to-back substorms, finding that mesoscale aurora contributes up to ∼80% (∼60%) of the total energy flux immediately after onset during the early expansion phase of the first (second) substorm, and continues to contribute ∼30–55% throughout the remainder of the substorm. The average energy estimated from the ASI mosaic field of view also peaked during the initial expansion phase. Using the measured energy flux and tables produced from the Boltzmann Three Constituent (B3C) auroral transport code (Strickland et al., 1976; 1993), we also estimated the 2D Hall conductance and compared it to Poker Flat Incoherent Scatter Radar conductance values, finding good agreement for both discrete and diffuse aurora.

Identifying distinct metrics for assessing night sky brightness

ABSTRACT Studying light pollution is an interest to researchers in a wide range of fields including astronomy, biology, civil engineering, ecology, and social science. Consequently, numerous sky brightness metrics have been developed over the years. However, what metrics are truly representative of the night sky quality and unique to the measured feature? The US National Park Service Night Skies Program has collected more than 1500 sets of night skies data throughout the United States. For each data set, a maximum of 56 metrics were measured through the combination of the captured images, Sky Quality Meter readings, and visual observations. This paper analyses these measurements and identifies a distinctive set of night sky brightness metrics based on the principal component analysis. Three major findings emerge. First, the commonly used metrics, such as the zenith brightness, horizontal illuminance, maximum vertical illuminance, all-sky light pollution ratio, Bortle class, and limiting magnitude, are highly correlated. Secondly, the observed sky brightness often offers a good estimate of the artificial light level despite the natural varying night sky background. Thirdly, a set of six metrics that consists of the zenith brightness and sky brightness percentiles are more distinctive when used to concisely describe night sky characteristics. These findings suggest that long-term night sky monitoring can be efficiently carried out by measuring the sky brightness percentiles on the observed all-sky images.

A Hybrid Solar Irradiance Nowcasting Approach: Combining All Sky Imager Systems and Persistence Irradiance Models for Increased Accuracy

The share of distributed solar power generation is continuously growing. This increase, combined with the intermittent nature of the solar resource creates new challenges for all relevant stakeholders, from generation to transmission and demand. Insufficient consideration of intra‐minute and intra‐hour variabilities might lead to grid instabilities. Therefore, the relevance of nowcasts (shortest‐term forecasts) is steadily increasing. Nowcasts are suitable for fine‐grained control applications to operate solar power plants in a grid‐friendly way and to secure stable operations of electrical grids. In space and time, highly resolved nowcasts can be obtained by all sky imager (ASI) systems. ASI systems create hemispherical sky images. The associated software analyzes the sky conditions and derives solar irradiance nowcasts. Accuracy is the decisive factor for the effective use of nowcasts. Therefore, the goal of this work is to increase the nowcast accuracy by combining ASI nowcasts and persistence nowcasts, which persist with the prevailing irradiance conditions, while maintaining the spatial coverage and resolution obtained by the ASI system. This hybrid approach combines the strengths while reducing the respective weaknesses of both approaches. Results of a validation show reductions of the root mean square deviation of up to 12% due to the hybrid approach.

Cloud height measurement by a network of all-sky imagers

Abstract. Cloud base height (CBH) is an important parameter for many applications such as aviation, climatology or solar irradiance nowcasting (forecasting for the next seconds to hours ahead). The latter application is of increasing importance for the operation of distribution grids and photovoltaic power plants, energy storage systems and flexible consumers. To nowcast solar irradiance, systems based on all-sky imagers (ASIs), cameras monitoring the entire sky dome above their point of installation, have been demonstrated. Accurate knowledge of the CBH is required to nowcast the spatial distribution of solar irradiance around the ASI's location at a resolution down to 5 m. To measure the CBH, two ASIs located at a distance of usually less than 6 km can be combined into an ASI pair. However, the accuracy of such systems is limited. We present and validate a method to measure the CBH using a network of ASIs to enhance accuracy. To the best of our knowledge, this is the first method to measure the CBH with a network of ASIs which is demonstrated experimentally. In this study, the deviations of 42 ASI pairs are studied in comparison to a ceilometer and are characterized by camera distance. The ASI pairs are formed from seven ASIs and feature camera distances of 0.8…5.7 km. Each of the 21 tuples of two ASIs formed from seven ASIs yields two independent ASI pairs as the ASI used as the main and auxiliary camera, respectively, is swapped. Deviations found are compiled into conditional probabilities that tell how probable it is to receive a certain reading of the CBH from an ASI pair given that the true CBH takes on some specific value. Based on such statistical knowledge, in the inference, the likeliest actual CBH is estimated from the readings of all 42 ASI pairs. Based on the validation results, ASI pairs with a small camera distance (especially if <1.2 km) are accurate for low clouds (CBH<4 km). In contrast, ASI pairs with a camera distance of more than 3 km provide smaller deviations for greater CBH. No ASI pair provides the most accurate measurements under all conditions. The presented network of ASIs at different distances proves that, under all cloud conditions, the measurements of the CBH are more accurate than using a single ASI pair.

Angular Calibration of Visible and Infrared Binocular All-Sky-View Cameras Using Sun Positions

Visible and infrared binocular all-sky-view cameras can provide continuous and complementary ground-based cloud observations. Accurate angular calibration for every pixel is an essential premise to further cloud analysis and georeferencing. However, most current calibration methods mainly rely on calibration plates, which still remains difficult for simultaneously calibrating visible and infrared binocular cameras, especially with different imaging resolutions. Thus, in this study, we present a simple and convenient angular calibration method for wide field-of-view visible and infrared binocular cameras. Without any extra instruments, the proposed method only utilizes the relation between the angular information of direct sun lights and the projected sun pixel coordinates to compute the geometric imaging parameters of the two cameras. According to the obtained parameters, the pixel-view-angle for the visible and infrared all-sky images is efficiently computed via back projection. Meanwhile, the projected pixel coordinates for the incident lights at any angle can also be computed via reprojection. Experimental results show the effectiveness and accuracy of the proposed angular calibration through the error estimation of reprojection and back projection. As a novel application, we successfully achieve visible and infrared binocular image registration at the pixel level after finishing angular calibration, which not only verifies the accuracy of calibration results, but also contributes to further cloud parameter analysis under these two different imaging features. The registration results, to our knowledge, also provide a reference for the current blank in visible and infrared binocular cloud image registration.

An experimental investigation into the possible connections between the zonal neutral wind speeds and equatorial plasma bubble drift velocities over the African equatorial region

There is increasing interest to understand thermospheric neutral wind and ionized plasma coupling, especially by using empirical observations. This is the first study that reports simultaneous measurements of thermospheric neutral wind and equatorial plasma bubble (EPB) speeds over the African equatorial region. Neutral wind speeds were obtained using a Fabry-Perot interferometer (FPI), while EPB speeds were obtained using an all-sky optical imager. Both instruments are co-located in Abuja (Geographic: 8.99°N, 7.38°E; Geomagnetic: 1.60°S). Data for 11 nights of coincident neutral wind and EPB speed observations were used. The neutral wind speeds were in the range of about −30 to 300 m/s, while EPB speeds were in the range of about 15–400 m/s. The speeds of both neutral winds and EPBs were generally observed to be faster during the early night hours and to slow down towards midnight. In some instances, the neutral wind and EPB speeds were comparable, an observation that is in agreement with full development of the F region dynamo. There were also instances in which the EPBs were faster or slower than the neutral winds. In one of the nights, a southward turning of the IMF-Bz was observed to cause a sudden increase in the EPB speed. For some nights, the neutral winds were faster than the EPBs during earlier hours of the nights, and the disparity reduced during the later hours of the nights. This was attributed to the possibility that the F region dynamo was yet to be fully developed during the earlier hours.

Airglow-imager based observation of possible influences of subtropical mesospheric gravity waves on F-region ionosphere over Jammu & Kashmir, India

As a joint research collaboration between the National Atmospheric Research Laboratory (NARL), and the University of Kashmir (KU), NARL installed an all-sky airglow CCD imager (with centre wavelengths of 630 nm, 557.7 nm [2 nm band widths] and 840 nm [150 nm wide band with blocking notch at 866 nm to avoid the contamination of molecular oxygen emissions]) in the University campus in Srinagar (75°E, 34°N, geographic), Jammu and Kashmir, India (western Himalayan region). To understand the upper atmospheric dynamics and ionospheric electrodynamics and their associated physical coupling mechanisms, the imager observes airglow emissions of OH molecules (~ 85 km height; 840 nm) and atomic oxygen occurring at the heights of ~ 97 km (557.7 nm) and ~ 250 km (630 nm). Airglow observations in Kashmir commenced in the night of August 11, 2017 and the present work reports on the characteristics of first-time observation of Medium Scale Travelling Ionospheric Disturbances (MSTIDs with horizontal wavelengths of ~ 100–300 km) over Kashmir region during 20:30—22:30 IST (Indian standard time) on August 15, 2017 (India independence day). Initially, the phase front of MSTIDs was aligned along the north-west and south-east direction and moved at ~ 57 m/s towards the south-west direction and finally the westward direction by aligning along the meridian before they disappeared. Along with SAMI-3 ionospheric model simulations, simultaneous multiwavelength airglow observations indicate that secondary gravity waves generated due to dissipation of upward propagating mesospheric gravity waves in the heights of ~ 85–95 km would have contributed to the generation of MSTIDs in the F region ionospheric plasma through electrodynamical coupling between the E and F region (Perkins instability) ionosphere.