Zenith Research Articles

Improving Otsu Method Parameters for Accurate and Efficient in LAI Measurement Using Fisheye Lens

The leaf area index (LAI) is an essential indicator for assessing vegetation growth and understanding the dynamics of forest ecosystems and is defined as the ratio of the total leaf surface area in the plant canopy to the corresponding surface area below it. LAI has applications for obtaining information on plant health, carbon cycling, and forest ecosystems. Due to their price and portability, mobile devices are becoming an alternative to measuring LAI. In this research, a new method for estimating LAI using a smart device with a fisheye lens (SFL) is proposed. The traditional Otsu method was enhanced to improve the accuracy and efficiency of foreground segmentation. The experimental samples were located in Gansu Ziwuling National Forest Park in Qingyang. In the accuracy parameter improvement experiment, the variance of the average LAI value obtained by using both zenith angle segmentation and azimuth angle segmentation methods was reduced by 50%. The results show that the segmentation of the front and back scenes of the new Otsu method is more accurate, and the obtained LAI values are more reliable. In the efficiency parameter improvement experiment, the time spent is reduced by 17.85% when the enhanced Otsu method is used to ensure that the data anomaly rate does not exceed 10%, which improves the integration of the algorithm into mobile devices and the efficiency of obtaining LAI. This study provides a fast and effective method for the near-ground measurement of forest vegetation productivity and provides help for the calculation of forest carbon sequestration efficiency, oxygen release rate, and forest water and soil conservation ability.

Zenith angle dependence on cosmic-ray background in HPGe gamma spectrometers by using GEANT4 simulation

In this study, we investigate the impact of zenith angle variations on cosmic-ray induced background in High-Purity Germanium (HPGe) gamma spectrometers using a coincidence technique based on plastic scintillator-Germanium detectors. We utilize an HPGe detector (Model GC2018 Mirion Ge Detector) enclosed within a low-activity cylindrical lead shield (Model 747E Mirion Lead Shield). For cosmic ray detection, a coincidence detection system with plastic scintillator detectors was positioned on top of the lead shielding. The zenith angle at the Germanium detector is computed using the dimensions of the square plastic scintillator and its distance from the Germanium detector center. We carried out measurements of cosmic-ray induced background in an HPGe gamma spectrometer with a square plastic configuration (80cm x 80cm), equivalent to a 45° zenith angle. The experimental measurements were compared with GEANT4 simulation data. The results demonstrate a good agreement between the measured energy spectrum and the simulated data across the energy range of 0.05 to 47 MeV. Further investigations into the effects of varying zenith angles provide valuable insights for optimizing HPGe spectrometer setups with minimized background interference.

The dayside ionosphere of Mars observed by MAVEN NGIMS

The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has provided a unique opportunity to study the variability of the Martian upper atmosphere and ionosphere, as well as their interaction with the solar wind under various conditions. This study characterizes the large-scale spatial-temporal evolution of the dayside ionosphere as observed by the Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard MAVEN. NGIMS measures the composition and structure of both the ions and neutrals in the upper ionosphere and thermosphere at altitudes below ∼500 km. Thanks to the precessing orbit of MAVEN, NGIMS has accumulated millions of ion and neutral measurements over a wide range of spatial and temporal parameters. In this study, we present what we have learned from the NGIMS measurements during the first 3.5 Mars years (2015–2021), including the spatial and compositional variation of the median dayside ionosphere, the observed ionospheric peak layer at high solar zenith angles (SZAs), which complements the observations by previous missions at lower SZAs, and the variability of the topside ionosphere.

Considerations on measurement of bidirectional transmittance distribution function of thick samples over a wide range of viewing zenith angles

Abstract We delve into theoretical and experimental considerations for determining the spectral bidirectional transmittance distribution function (BTDF) of thick samples across a broad viewing zenith angle range. Nominally, BTDF is defined as the ratio of transmitted radiance to incident irradiance measured from the same plane. However, when employing thick samples for BTDF measurements, the viewing plane of the transmitted beam may shift from the front to the rear surface of the sample, altering the measurement geometry compared to using the sample front surface as the reference plane. Consequently, the viewing zenith angle from the sample rear surface increases relative to the sample front surface, and the sample-to-detector-aperture distance decreases by an amount corresponding to the sample thickness. We introduce a method for determining the BTDF of thick samples, considering the transformation of practical measurement results to a scenario where the measurements are conducted at a very large distance from the sample. To validate the method, we utilize a BTDF facility equipped with two instruments that significantly differ in their sample-to-detector-aperture distances. We evaluate the impact of a 2 mm sample thickness on the BTDF by assessing the ratio of transmitted and incident radiant fluxes as a function of viewing zenith angle relative to the sample rear surface. The evaluation is conducted in the wavelength range from 550 nm to 1450 nm in 300 nm steps, and in the viewing zenith angle range from −70° to 70° in 5° steps. Measurements are performed in-plane at an incident zenith angle of 0°. It is concluded that consistent determination of BTDF of a thick sample is possible by converting the experimental parameters of the real measurements at relatively short distances from the sample to correspond to those that would be obtained from measurements at very large distances from the sample.

Numerical investigation of optical characterization of polycarbonate panels

Polycarbonate panels (PC panels) are state-of-the-art transparent insulating materials widely used in the construction industry due to their cavity structure, which provides exceptional thermal insulation and optimal optical performance. However, the inherent anisotropy of the three-dimensional cavity structure complicates radiative transfer and requires consideration of both azimuth and zenith angles in optical performance evaluation. This aspect has received limited attention in existing research. This study aims to accurately characterize the optical performance of PC panels through numerical simulations. A three-dimensional radiative transfer model based on the discrete ordinate radiation model is developed to solve the radiation transfer equation. The model's independency regarding mesh division, angular discretization, and accuracy is validated. The effects of incidence angle, geometric parameters, and optical properties of PC panels on optical performance are analyzed. The findings reveal a strong correlation between transmittance and absorption with variations in incident zenith and azimuth angles. The transmittance exhibits a consistent monotonic variation expressible as a rational bifunction. Notably, absorption peaks occur within specific solid angle ranges, with increased structural complexity resulting in heightened absorption and greater uncertainty. For conventional PC materials, maximum transmittance ranges from 46.9 % to 73 %, while maximum absorption ranges from 2.3 % to 13.5 %. Increasing absorption coefficients, refractive index, and surface scattering coefficients nonlinearly decrease transmittance while increasing absorption. Additionally, deviations in transmittance and absorption with azimuth angle amplify with an increase in non-horizontal structures. Sensitivity analysis indicates a significant influence of zenith angle on transmittance, and absorption coefficient predominantly affects absorption.

A new method for estimating megacity NOx emissions and lifetimes from satellite observations

Abstract. We present a new method for estimating NOx emissions and effective lifetimes from large cities based on NO2 measurements from the TROPOspheric Monitoring Instrument (TROPOMI) (PAL dataset, May 2018–November 2021). As in previous studies, the estimate is based on the downwind plume evolution for different wind directions separately. The novelty of the presented approach lies in the simultaneous fit of downwind patterns for opposing wind directions, which makes the method far more robust (i.e., less prone to local minima with nonphysically high or low lifetimes) than a single exponential decay fit. In addition, the new method does not require the assumption of a city being a “point source” but also derives the spatial distribution of emissions. The method was successfully applied to 100 cities worldwide on a seasonal scale. Fitted emissions generally agree reasonably with the Emissions Database for Global Atmospheric Research (EDGAR) v6 (R=0.72) and are on average 14 % lower, while estimated uncertainties are still rather large (≈ 30 %–50 %). Lifetimes were found to be rather short (2.44 ± 0.68 h) and show no distinct dependency on season or latitude, which might be a consequence of discarding observations at high solar zenith angles (>65°). The main limitations of this and similar methods are the underlying assumptions of steady state (meaning constant emissions, wind fields and chemical conditions) within about 100 km downwind from a city, which is probably a simplification that is too strong in order to reach higher accuracies.

Evaluation of Two Satellite Surface Solar Radiation Products in the Urban Region in Beijing, China

Surface solar radiation, as a primary energy source, plays a pivotal role in governing land–atmosphere interactions, thereby influencing radiative, hydrological, and land surface dynamics. Ground-based instrumentation and satellite-based observations represent two fundamental methodologies for acquiring solar radiation information. While ground-based measurements are often limited in availability, high-temporal- and spatial-resolution, gridded satellite-retrieved solar radiation products have been extensively utilized in solar radiation-related studies, despite their inherent uncertainties in accuracy. In this study, we conducted an evaluation of the accuracy of two high-resolution satellite products, namely Himawari-8 (H8) and Moderate Resolution Imaging Spectroradiometer (MODIS), utilizing data from a newly established solar radiation observation system at the Beijing Normal University (BNU) station in Beijing since 2017. The newly acquired measurements facilitated the generation of a firsthand solar radiation dataset comprising three components: Global Horizontal Irradiance (GHI), Direct Normal Irradiance (DNI), and Diffuse Horizontal Irradiance (DHI). Rigorous quality control procedures were applied to the raw minute-level observation data, including tests for missing data, the determination of possible physical limits, the identification of solar tracker malfunctions, and comparison tests (GHI should be equivalent to the sum of DHI and the vertical component of the DNI). Subsequently, accurate minute-level solar radiation observations were obtained spanning from 1 January 2020 to 22 March 2022. The evaluation of H8 and MODIS satellite products against ground-based GHI observations revealed strong correlations with R-squared (R2) values of 0.89 and 0.81, respectively. However, both satellite products exhibited a tendency to overestimate solar radiation, with H8 overestimating by approximately 21.05% and MODIS products by 7.11%. Additionally, solar zenith angles emerged as a factor influencing the accuracy of satellite products. This dataset serves as crucial support for investigations of surface solar radiation variation mechanisms, future energy utilization prospects, environmental conservation efforts, and related studies in urban areas such as Beijing.

Loss of Coherence and Change in Emission Physics for Radio Emission from Very Inclined Cosmic-Ray Air Showers.

Next-generation radio experiments such as the radio detector of the upgraded Pierre Auger Observatory and the planned GRAND and BEACON arrays target the detection of ultra-high-energy particle air showers arriving at low elevation angles. These inclined cosmic-ray air showers develop higher in the atmosphere than vertical ones, enhancing magnetic deflections of electrons and positrons inside the cascade. We evidence two novel features in their radio emission: a new polarization pattern, consistent with a geosynchrotron emission model and a coherence loss of the radio emission, both for showers with zenith angle θ≳65° and strong enough magnetic field amplitude (typical strength of B∼50 μT). Our model is compared with both ZHAireS and CoREAS MonteCarlo simulations. Our results break the canonical description of a radio signal made of Askaryan and transverse current emission only, and provide guidelines for the detection and reconstruction strategies of next-generation experiments, including cosmic-ray or neutrino discrimination.

Accounting for the effect of aerosols in GHGSat methane retrieval

Abstract. GHGSat comprises a constellation of satellites with high spatial and spectral resolution that specialize in monitoring methane emissions at 1.65 µm. This study investigates the ability to accurately retrieve both the methane mixing-ratio enhancement (ΔXCH4) and the aerosol optical depth (AOD) simultaneously from simulated GHGSat observations that incorporate angle-dependent scattering information. Results indicate that the sign of the ΔXCH4 bias when neglecting aerosols changes from negative to positive as surface albedo increases, which is consistent with previous studies. The bias in ΔXCH4 is most pronounced when AOD is not simultaneously retrieved, ranging from −3.0 % to 6.3 % with an AOD of 0.1, a 60° solar zenith angle, and a surface albedo of 0.2 for the nadir-only retrieval. Using multiple satellite viewing angles during the GHGSat observation sequence with a scattering angle ranging from 100 to 140°, the study shows that the mean bias and standard deviation of ΔXCH4 are within 0.3 % and 2.8 % relative to the background. The correlation between simultaneously retrieved ΔXCH4 and AOD shifts from being positive to negative as surface albedo increases and the aerosol asymmetry factor decreases, signifying a transition of the dominant aerosol effect from aerosol-only scattering to aerosol–surface multiple scattering. The variety of scattering angle ranges has little impact on the performance of the multi-angle viewing method. This study improves the understanding of the impact of aerosols on the GHGSat ΔXCH4 retrieval and provides guidance for improving future GHGSat-like point-source imagers.

Disk Images of Neutral Temperature From the Global‐Scale Observations of the Limb and Disk (GOLD) Mission

AbstractObservations of far‐ultraviolet (FUV) dayglow by the Global‐scale Observations of Limb and Disk (GOLD) mission provide an opportunity for quantifying the global‐scale response of the thermosphere to solar extreme‐ultraviolet variability and geomagnetic activity. Relative temperature changes can be measured by monitoring changes in the rotational structure observed in molecular nitrogen Lyman‐Birge‐Hopfield (LBH) band emissions. We present a new technique for deriving effective neutral temperatures from GOLD FUV observations using optimal estimation fits to spectra containing LBH band emissions. We provide an overview of the theoretical basis for the GOLD Level 2 TDISK algorithm. Effects on derived effective neutral temperatures from instrument artifacts and particle background are reviewed. We also discuss GOLD Level 1C DAY and Level 2 TDISK data products and present representative examples of each. We show that effective neutral temperatures vary with local time, exhibit a strong dependence on season and solar zenith angle, and correlate strongly with geomagnetic and solar activity. Finally, we present results from a preliminary data product validation that show good agreement with coincident GOLD exospheric temperatures and predictions from a global reference atmospheric model.

Fluctuations in the “static” atmosphere and their effects on tropospheric ozone distribution

Atmospheric fluctuation can be seen everywhere. This study focuses on the record-breaking increase of O3 concentration during the summer in some sensitive areas in recent years. The findings indicate that in the vicinity of the East Asian continent near western Pacific ocean, when the atmospheric conditions are stable or neutral, it is conducive to the maintenance and propagation of atmospheric oscillations near the height of the pollutant mixed layer (H_PML). Accompanied by the "peak-trough" effect of external gravity wave oscillations, due to the abundant water vapor of the cloud system (there are low pressure or typhoon disturbances in summer) near the large-scale cloud belt at the edge of the subtropical high in the western Pacific, the bright temperature at cloud top shows "light and dark changes" on satellite images, forming a wave-like cloud system. The novelty of this study lies in the fact that atmospheric fluctuations near the H_PML is not only related to the known aggravation of heavy rainfall, but also leads to the additional value-added effect of aerosols. Under static atmospheric conditions, the impact of atmospheric fluctuations near the H_PML on additional rise of O3 concentration helps us to deepen our understanding of the so-called "entrained ozone (EZ) effect" in the atmosphere. Due to the external gravity waves, the concentration of O3 increased further. Diurnal variations of solar zenith angle and H_PML are key meteorological factors influencing the significant increase in near-surface O3 concentration entrainment. The formation mechanism of solar photochemical O3 is further deepened and supplemented by analyzing the record-breaking increase of O3 concentration in summer observed in recent years.

Comparison of Solar Wind Interaction with Mars and Earth: Energy Transfer

Using a global magnetohydrodynamics numerical simulation, this work compares the interaction of the solar wind with Mars and Earth from the perspective of energy transfer under north–south interplanetary magnetic fields (IMFs). Mars lacks a global dipole magnetic field like Earth’s and instead has a small-scale crustal magnetic field near highlands in the southern hemisphere. Unlike Earth’s magnetopause reconnection (at the subsolar point or tail under a southward IMF, or behind the cusp under a northward IMF), the reconnection of the Martian magnetic pileup boundary (MPB) occurs near solar zenith angle (SZA) ≈ 45° (SZA ≈ 30° and 60°) for a southward (northward) IMF, resulting in asymmetric energy transfer between the northern and southern hemispheres. The Martian outflow of mechanical energy appears near SZA ≈ 45° (SZA ≈ 30° and 60°) under a southward (northward) IMF, accompanied by an inflow due to the process of “solar wind pickup.” For energy transfer across the MPB, whether the IMF is northward or southward, the input of electromagnetic energy is twice as large as the input of mechanical energy, which is similar to Earth’s magnetopause for a southward IMF, but opposite to it for a northward IMF. The energy transfer rate of the MPB is slightly higher in a northward IMF than in a southward one, whereas the energy transfer rate of Earth’s magnetopause is far higher in a southward IMF than in a northward one.

An Explainable Deep-learning Model of Proton Auroras on Mars

Proton auroras are widely observed on the dayside of Mars, identified as a significant intensity enhancement in the hydrogen Lyα (121.6 nm) emission at altitudes of ∼110 and 150 km. Solar wind protons penetrating as energetic neutral atoms into Mars’ thermosphere are thought to be primarily responsible for these auroras. Recent observations of spatially localized “patchy” proton auroras suggest a possible direct deposition of protons into Mars’ atmosphere during unstable solar wind conditions. Improving our understanding of proton auroras is therefore important for characterizing the interaction of the solar wind with Mars’ atmosphere. Here, we develop a first purely data-driven model of proton auroras using Mars Atmosphere and Volatile Evolution (MAVEN) in situ observations and limb scans of Lyα emissions between 2014 and 2022. We train an artificial neural network that reproduces individual Lyα intensities and relative Lyα peak intensity enhancements with Pearson correlations of ∼94% and ∼60% respectively for the test data, along with a faithful reconstruction of the shape of the observed altitude profiles of Lyα emission. By performing a Shapley Additive Explanations (SHAP) analysis, we find that solar zenith angle, solar longitude, CO2 atmosphere variability, solar wind speed, and temperature are the most important features for the modeled Lyα peak intensity enhancements. Additionally, we find that the modeled peak intensity enhancements are high for early local-time hours, particularly near polar latitudes, and the induced magnetic fields are weaker. Through SHAP analysis, we also identify the influence of biases in the training data and interdependences between the measurements used for the modeling, and an improvement of those aspects can significantly improve the performance and applicability of the ANN model.

EMM EMUS Observations of FUV Aurora on Mars: Dependence on Magnetic Topology, Local Time, and Season

AbstractWe present a comprehensive study of the nightside aurora phenomenon on Mars, utilizing observations from EMUS onboard Emirates Mars Mission. The oxygen emission at 130.4 nm is by far the brightest FUV auroral emission line observed at Mars. Our statistical analysis reveals geographic, solar zenith angle, local time, and seasonal dependencies of auroral occurrence. Higher occurrence of aurora is observed in regions of open magnetic topology, where crustal magnetic fields are either very weak or both strong and vertical. Aurora occurs more frequently closer to the terminator and is more likely on the dusk side than on the dawn side of the night hemisphere. A pronounced auroral feature appears close to midnight local times in the southern hemisphere, consistent with the spot of energetic electron fluxes previously identified in the Mars Global Surveyor data. This auroral spot is more frequent after midnight than before. Additionally, some regions on Mars are “aurora voids” where essentially no aurora occurs. Aurora exhibits a seasonal dependence, with a major enhancement near perihelion. Non–crustal field aurora additionally shows a secondary enhancement near Ls 30°. This seasonal variability is a combination of the variability in ionospheric photoelectrons and thermospheric atomic oxygen abundance. Auroral occurrence also shows an increase with the rise of Solar Cycle 25. The brightest auroral pixels are observed during space weather events such as Coronal Mass Ejections and Stream Interaction Regions. These observations not only shed light on where and when Martian aurora occurs, but also add to our understanding of Mars' magnetic environment and its interaction with the heliosphere.

An effective parameterization of broadband ocean surface albedo applicable to all skies

The ocean surface albedo (OSA) is an important parameter in ocean and climate models for air-sea heat flux calculations. Current OSA schemes are either too simple, making them only suitable for clear sky conditions, or too complex, because they depend on parameters that are not often measured in conventional ocean observations. Using radiation observations at a fixed offshore platform, we propose a simple but effective parameterization scheme of OSA, in which the broadband OSA is an analytical function of both the solar zenith angle and atmospheric transparency. It depends only on the downward shortwave radiation measured at the ocean surface and applies to all sky conditions. During our 15-month radiation observations, the correlation coefficient between the calculated OSA and the observations reached 0.90 for all skies, and the root mean square deviation was 0.0130. Three other OSA observation datasets are also introduced to verify this scheme.

Circular polarization characterization analysis: A case study of the scaled model of the ship and water body

Addressing the inadequate analysis of circular polarization characteristics, this study constructs a laboratory detection system. Utilizing the ship model against the water body background, this study focuses on analyzing the circular polarization characteristics and proposes a quantitative algorithm for analyzing circular polarization contrast. Results indicate that the circular polarization of the target increases in the forward scattering and decreases in the backward scattering region as the zenith angle of observation increases. The band characteristics of circular polarization in the water body are evident, with the peak located in the specular reflection zone. In contrast, the values in the backward scattering region exhibit reduced variability. Quantitative analysis of circular polarization contrast reveals peak values exceeding 0.7, which can effectively separate the target from the water body information. This paper provides critical insights for utilizing circular polarization data and establishes a theoretical foundation for target detection by incorporating comprehensive polarization information.

A method of determining distance to moon and its zenith angle from a pair of moon-images

A method of determining distance to moon and its zenith angle from a pair of moon-images

Analyzing meteorological parameters using Pearson correlation coefficient and implementing machine learning models for solar energy prediction in Kuching, Sarawak

Solar energy is one of the clean renewable energy sources that can offset the rising consumption of fossil fuels. However, the meteorological parameters, such as solar irradiance, ambient and solar module temperatures, relative humidity, etc., constantly change, and so does the solar power generation. Such variations cause instability in the power grid operation due to injecting an unpredicted amount of power. Hence, solar energy prediction models capable of learning from past weather data and predicting future energy generation are highly desired for grid operation and planning. The objective of this study is to determine the suitable meteorological parameters for the solar energy prediction model based on the Pearson correlation coefficient and to implement them in different machine learning models. It is found in this study that five meteorological parameters, namely Air temperature, cloud opacity, global tilted irradiance, relative humidity, and zenith angle, correlate highly with solar energy generation. Later, based on the correlations, four machine-learning models were implemented to predict the solar power for Kuching, Sarawak. The accuracy of the models is measured through standard matrices such as root mean square error, mean square error, mean absolute error, and R-squared value.

Directional reflectance of light from landscapes on a long transect in Australia – forest to desert

The reflectance of land and vegetation observed in satellite imagery depends on sun and viewing geometry. This bidirectional reflectance requires correction for monitoring changes in vegetation cover and condition. We used a digital camera mounted in a light aircraft, and fitted with a fisheye lens, to measure directional reflectance of a diverse range of landscapes along a long transect in Australia — between Brisbane and the Simpson desert. All, except one, of the measured directional reflectances were able to be characterised accurately (adjusted r2 > 0.95) by the product of two analytical functions. The first, G(θ1,θ2), which represents volume scattering, is a function of illumination and viewing zenith angles, θ1andθ2, and has one parameter k:

Ratio of photosynthetically active radiation to global solar radiation above forest canopy in complex terrain: measurements and analyses based on Qingyuan Ker Towers

BackgroundUnderstanding of the ratio of photosynthetic photon flux density (Qp) to global solar radiation (Rs) (Qp/Rs) is crucial for applying Rs to ecology-related studies. Previous studies reported Qp/Rs and its variations based on measurements from a single observatory tower, instead of multi-site-based measurements over complex terrains. This may neglect spatial heterogeneity in the terrain, creating a gap in an understanding of how terrain affects Qp/Rs and how this effect interacts with meteorological factors.MethodsHere the Qingyuan Ker Towers (three towers in a valley with different terrains: T1, T2, and T3) were utilized to measure Qp and Rs over mountainous forests of Northeast China. An airborne LiDAR system was used to generate a digital elevation model, and sky view factor of sectors (SVFs) divided from the field of view of tower’s pyranometer was calculated as a topographic factor to explain the variations of Qp/Rs.ResultsThe results identified significant differences in Qp/Rs of the three towers at both daily and half-hour scales, with larger differences on clear days than on overcast days. Qp/Rs was positively correlated with SVFs of T1 and T3, while this correlation was negative with that of T2. The effect of SVFs on Qp/Rs interacted with clearness index, water vapor pressure and solar zenith angle. Random forest-based importance assessment demonstrated that explanation (R2) on Qp/Rs was improved when SVFs was included in the predictor variable set, indicating that incorporating terrain effects enhances the prediction accuracy of Qp/Rs. The improvement in the R2 values was more pronounced on clear days than on overcast days, suggesting that the effect of terrain on Qp/Rs depended on sky conditions.ConclusionsAll findings suggested that Qp/Rs is affected by terrain, and integrating terrain information into existing Qp/Rs models is a feasible solution to improve Qp/Rs estimates in mountainous areas.