Ultraviolet Imager Images Research Articles

Long‐Term Variability of Mean Winds and Planetary‐Scale Waves Around Venusian Cloud Top Observed With Akatsuki/UVI

AbstractSince December 2015, Ultraviolet Imager (UVI) onboard Akatsuki has been observing Venus clouds at the wavelengths of 283 and 365 nm. Horizontal winds near the cloud top derived from the UVI images over ∼7 earth years are analyzed to elucidate spatial and temporal variability of the superrotation and planetary‐scale waves. Zonal winds averaged over the analysis period are asymmetric with respect to the equator, being faster in the southern hemisphere. This asymmetry varied temporarily and was occasionally reverted. Comparison of the winds from the two wavelengths suggests that it is uncertain whether the asymmetry is in the wind distribution or in the sensing altitude for winds. Mean zonal winds representing the superrotation exhibited broad low‐frequency variability with spectra resembling the red noise spectra. This is indicative of the presence of internal variability rather than responses to periodical external forcing. Planetary‐scale waves with zonal‐wavenumber 1 at periods around 4 and 5 days, which have been interpreted as equatorial Kelvin and Rossby waves, respectively, are quantified. While the ∼5‐day waves have nearly constant frequencies, the ∼4‐day waves have variable phase speeds that follow the superrotation speed. This result indicates that the ∼5‐day waves are likely to extend over a large depth below the cloud top and that the ∼4‐day waves are likely to be confined near the cloud top. Their possible generation mechanism through the coupling of Kelvin and Rossby waves is discussed. This study further reports wind variability of 10 to 15‐day periodicity, thermal‐tide structure, and comparison with minor species observations.

Sunglint reflection facilitates performance of spaceborne UV sensor in oil spill detection.

Ultraviolet Imager (UVI) onboard Haiyang-1C/D (HY-1C/D) satellites has been providing ultraviolet (UV) data to detect marine oil spills since 2018. Although the scale effect of UV remote sensing has been preliminarily interpreted, the application characteristics of spaceborne UV sensors with medium spatial resolution in oil spill detection deserve further investigation, especially the role of sunglint in the process of detection. In this study, the performance of the UVI is thoroughly assessed by the following aspects: image features of oils under sunglint, sunglint requirement for spaceborne UV detection of oils, and the stability of the UVI signal. The results indicate that in UVI images, it is sunglint reflection that determines the image features of spilled oils, and the appearance of sunglint can strengthen the contrast between oils and seawater. Besides, the required sunglint strength in spaceborne UV detection has been deduced to be 10-3 - 10-4 sr-1, which is higher than that in the VNIR wavelengths. Moreover, uncertainties in the UVI signal can meet the demand to discriminate between oils and seawater. The above results can confirm the capability of the UVI and the critical role of sunglint in spaceborne UV detection of marine oil spills, and provide new reference for spaceborne UV remote sensing.

Modeling of Ultraviolet Aurora Intensity Associated With Interplanetary and Geomagnetic Parameters Based on Neural Networks

AbstractThe spatial distribution of aurora intensity is an important manifestation of solar wind‐magnetosphere‐ionosphere energy coupling process, and it oscillates with the change of space environment parameters and geomagnetic index. It is of great significance to establish an appropriate aurora intensity model for the prediction of space weather and the study of magnetosphere dynamics. Based on Ultraviolet Imager (UVI) data of Polar satellite, we constructed two auroral models by using two different neural networks, that is, the generalized regression neural network (GRNN), and the conditional generation adversarial network (CGAN). Input parameters of the models include interplanetary magnetic field, solar wind velocity and density, and the geomagnetic AE index. Output result is the spatial distribution of auroral intensity in altitude adjusted corrected geomagnetic (AACGM) coordinates. The structural similarity index (SSIM) of image quality is used as an evaluation standard of detail similarity between the prediction results of auroral intensity model and corresponding UVI images (complete similarity is 1, dissimilarity is 0, SSIM is generally considered to have good similarity if it is greater than 0.5). Based on the respective training datasets of GRNN and CGAN models, the evaluating results showed that the mean values (standard deviation) of SSIM were 0.5409 (0.0912) and 0.5876 (0.0712), respectively, so the prediction results from both models can restore the auroral intensity distribution of the original images of UVI. In addition, the value of SSIM can increase with the increase of the number of training data. Therefore, more training data will help improve the effectiveness of these models.

Geometric stitching of a HaiYang-1C ultra violet imager with a distorted virtual camera.

The HaiYang-1C (HY-1C) ultra violet imager (UVI) consists of five independent cameras with a designed total image swath of approximately 3000 km. In order to obtain a complete seamless image formed by the five sub-images, a feasible geometric stitching method for the HY-1C UVI with a distorted virtual camera is proposed. First, we perform the absolute geometric calibration of camera 3 and the relative geometric calibration of cameras 1, 2, 4, and 5. Then, a distorted virtual camera is assigned. Finally, the five sub-images are stitched together with the distorted virtual camera. Three HY-1C UVI images were tested. The experimental results showed that the georeferencing accuracy of the stitched images was better than 1 pixel. Compared with the conventional stitching method with an undistorted virtual camera, the ground sampling distance differences of the five cameras obtained by the proposed method were reduced from 23%, 37%, 53%, 37%, and 25% to 6%, 6%, 1%, 7%, and 8%, respectively.

Sensor orientation of HaiYang-1C ultraviolet imager based on a piecewise rational function model

Due to the large field angle and large geometric distortions of the HaiYang-1C ultraviolet imager (UVI), a single rational function model (SRFM) is unable to fully absorb the geometric distortions. The SRFM-based sensor orientation accuracy of the UVI images is thereby worse than expected. In order to improve the sensor orientation accuracy, a feasible sensor orientation method for the UVI images based on a piecewise rational function model (PRFM) is proposed. A complete UVI image is first logistically divided into five subimages, and the adjacent subimages have an overlap. Then, an SRFM is used to mathematically fit the physical sensor model of each subimage. Finally, the five SRFMs form a continuous PRFM, and the PRFM-based sensor orientation of the UVI images is performed. Three HaiYang-1C UVI images were tested. The experimental results showed that the unabsorbed geometric distortions fully propagated into the SRFM-based sensor orientation results. The orientation accuracy of the three images reached ∼7 pixels. In the PRFM-based sensor orientation, both the PRFM fitting errors and the inconsistent errors between the adjacent subimages could be negligible. The PRFM-based orientation accuracy was thereby noticeably improved and reached >1 pixel.

Ultraviolet imager on Venus orbiter Akatsuki and its initial results

The ultraviolet imager (UVI) has been developed for the Akatsuki spacecraft (Venus Climate Orbiter mission). The UVI takes ultraviolet (UV) images of the solar radiation reflected by the Venusian clouds with narrow bandpass filters centered at the 283 and 365 nm wavelengths. There are absorption bands of SO2 and unknown absorbers in these wavelength regions. The UV images provide the spatial distribution of SO2 and the unknown absorber around cloud top altitudes. The images also allow us to understand the cloud top morphologies and haze properties. Nominal sequential images with 2-h intervals are used to understand the dynamics of the Venusian atmosphere by estimating the wind vectors at the cloud top altitude, as well as the mass transportation of UV absorbers. The UVI is equipped with off-axial catadioptric optics, two bandpass filters, a diffuser installed in a filter wheel moving with a step motor, and a high sensitivity charge-coupled device with UV coating. The UVI images have spatial resolutions ranging from 200 m to 86 km at sub-spacecraft points. The UVI has been kept in good condition during the extended interplanetary cruise by carefully designed operations that have maintained its temperature maintenance and avoided solar radiation damage. The images have signal-to-noise ratios of over 100 after onboard desmear processing.

Auroral precipitating energy during long magnetic storms

AbstractThe power energy input carried by precipitating electrons into the auroral zone is an important parameter for understanding the solar wind‐magnetosphere energy transfer processes and magnetic storms triggering. Some magnetic storms present a peculiar long recovery phase, lasting for many days or even weeks, which can be associated with the intense and long‐duration auroral activity named HILDCAA (High Intensity Long Duration Continuous AE Activity). The auroral energy input during HILDCAAs has been pointed out as an essential key issue, although there have been very few quantitative studies on this topic. In the present work, we have estimated the auroral electron precipitating energy during the events of long (LRP) and short (SRP) storm recovery phase. The energy has been calculated from the images produced by the Ultraviolet Imager (UVI) on board the Polar satellite. In order to obtain accurate energy values, we developed a dayglow estimate method to remove solar contamination from the UVI images, before calculating the energy. We compared the UVI estimate to the Hemispheric Power (HP), to the empirical power obtained from the AE index, and to the solar wind input power. Our results showed that the UVI electron precipitating power for the LRP events presented a quasiperiodic fluctuation, which has been confirmed by the other estimates. We found that the LRP events are a consequence of a directly driven system, where there is no long‐term energy storage in the magnetosphere, and the auroral electrojets during these events are directly affected by the electron precipitating power.

A shape-initialized and intensity-adaptive level set method for auroral oval segmentation

Auroral oval segmentation is of great significance in the field of spatial physics. However, random noise, low contrast and dayglow contamination in ultraviolet imager (UVI) images lead to various difficulties and challenges. To address these issues, in this paper we develop a shape-initialized and intensity-adaptive level set method. Firstly, morphological component analysis (MCA) based preprocessing is applied to reduce noise effect on UVI images. Subsequently, a saliency morphological map is constructed to capture the shape knowledge for auroral oval and used as the initial curve for level set. Finally, an intensity-adaptive level set evolution is implemented to construct the auroral oval boundary. Experimental results demonstrate that the proposed method is superior to the existing auroral oval segmentation methods for both full oval and gap oval images. In addition, the subjective and objective evaluations for segmentation results further validate the favorable performance of the proposed method.

Correlation of LBH intensities with precipitating particle energies

Precipitating particle data from the DMSP F12 and F13 satellites has been merged with image data from the Ultraviolet Imager (UVI) on the Polar satellite. UVI images were combined within single transpolar passes of the DMSP F12 and F13 satellites. Pixel irradiances were then averaged into 1° × 1° bins along the satellite tracks, thus generating along‐track irradiance profiles. Similarly, energy fluxes of precipitating electrons were averaged into 1° bins along the trajectory. The peaks in each set of profiles were isolated in order to compare photon irradiance with electron energy flux. The irradiance and energy flux data of hundreds of peaks were linearly correlated for January–February 1997 when the northern polar regions were in darkness and dayglow effects were minimal. In the pre‐midnight and dawn sectors, good correlations (r > 0.7) arise between the photon irradiance and electron energy fluxes, while correlations for other sectors are not as good. The good correlation suggests that UVI fluxes can be directly calibrated to measure energy input without the additional complication of modeling, at least for some MLT sectors. Using this calibration, global maps of energy input into the aurora may be derived empirically.

The dependence of winter aurora on interplanetary parameters

The dependence of the northern winter aurora on interplanetary parameters is examined for images obtained from the Polar spacecraft Ultraviolet Imager (UVI) during the month of January 1997. Linear correlation analysis is used to relate auroral brightness to interplanetary parameters as a function of magnetic latitude (MLAT) and magnetic local time (MLT). Spatial maps of maximum magnitude correlation coefficient and optimal lag times are presented. In addition, the Method of Natural Orthogonal Components (MNOC) is applied to UVI images and an orthogonal set of eigenmodes of auroral behavior is produced. The dominant eigenmodes are determined to be (1) the main auroral oval brightness, (2) polar cap contraction/expansion, (3) dawn/dusk aurora, and (4) midnight sector brightening/quenching. The temporal coefficients for the various eigenmodes are correlated separately with solar wind parameters and interplanetary magnetic field (IMF) components. The dominant influence of the negative Bz component is confirmed, but the timescale of its influence is found to vary widely. The MNOC procedure allows some of the more subtle relationships between the aurora and the interplanetary parameters to be isolated, particularly those associated with the solar wind density and IMF Bx and By.

Scale‐free statistics of spatiotemporal auroral emissions as depicted by POLAR UVI images: Dynamic magnetosphere is an avalanching system

We report first results from a spatiotemporal statistical analysis of ionospheric emissions as observed by the Ultraviolet Imager (UVI) onboard the POLAR spacecraft during 4 months of 1997 and 1998. Approximately 12,300 individual emission events near local midnight with durations exceeding the sampling time of the image sequences are investigated. The probability distributions of these events over the lifetime T, maximum area A, integrated area S, maximum power W, and integrated energy output are shown to obey distinct power law relations p ∼ T−2.2, p ∼ A−1.8, p ∼ S−1.6, p ∼ W−1.7, p ∼ E−1.5 over a wide range of scales. The observed behavior is consistent with the behavior of statistical–physical avalanche models near a stationary critical state. These results support the hypothesis of self‐organized critical dynamics of the magnetosphere and suggest an important role for cross‐scale coupling effects in the development of geomagnetic disturbances.

The quantitative relationship between auroral brightness and solar EUV Pedersen conductance

A quantitative relationship between auroral brightness and solar EUV produced Pedersen conductance is established by using Lyman‐Birge‐Hopfleld long and short bands of auroral emissions from Polar Ultraviolet Imager (UVI). We used a conductance model characterized by solar zenith angle, solar F10.7 radio flux, and the local magnetic field to estimate solar EUV Pedersen conductance for each pixel of the images. The hourly average IMP 8 solar wind data set is used to derive corresponding solar wind conditions. Pixels of the auroral brightness in Polar UVI images have been binned in terms of magnetic latitude and local time. We relate the auroral brightness to the conductance and find that a positive correlation occurs in the early morning sector, indicating increasing brightness with increasing conductance. An anticorrelation appears in the premidnight region, supporting the previously reported suppression of the discrete aurora occurrence in sunlight. The correlation becomes stronger for the southward interplanetary magnetic field (IMF) condition than for the all Bz condition in the early morning and premidnight sectors, indicating that the conductance effect on the auroral brightness becomes significant when the IMF is southward. The afternoon auroral bright spot is not eminent when the ionospheric conductance is low.

Energy characteristics of auroral electron precipitation: A comparison of substorms and pressure pulse related auroral activity

The Polar Ultraviolet Imager (UVI) observes auroral responses to incident solar wind pressure pulses and interplanetary shocks such as those associated with coronal mass ejections (CMEs). The arrival of a CME pressure pulse at the front of the magnetosphere results in highly disturbed geomagnetic conditions and a substantial increase in both dayside and nightside auroral precipitation. Our observations show a simultaneous brightening over broad areas of the dayside and nightside aurora in response to a pressure pulse, indicating that more magnetospheric regions participate as sources for auroral precipitation than during isolated substorms. We estimate the average energies of incident auroral electrons using Polar UVI images and compare the precipitation energies during pressure pulse associated events with those during isolated auroral substorms. Electron precipitation during substorms has average energies greater than 10 keV and is structured both in local time and magnetic latitude. For auroral intensifications following the arrival of a pressure pulse or interplanetary shock, electron precipitation is less spatially structured and has greater flux of lower‐energy electrons (Eave ≤ 7 keV) than during isolated substorm onsets. The average energies of the precipitating electrons inferred from UVI are consistent with those measured in situ by the Fast Auroral Snapshot (FAST) spacecraft. These observations quantify the differences between global and local auroral precipitation processes and will provide a valuable experimental check for models of sudden storm commencements and magnetospheric response to perturbations in the solar wind.

Auroral polar cap boundary ion conic outflow observed on FAST

We observe large ion outflow fluxes (> 108 cm−2s−1 mapped to 100 km) flowing from the auroral polar cap boundary on many FAST auroral passes near midnight. The outflow is in the form of ion conies composed mostly of light ions with energies of hundreds of eV. A statistical study of 606 FAST orbits during January and February 1997 was done to determine the MLT distribution and ion outflow properties relative to substorms. We find that the ion conic events occur most frequently near midnight, and therefore their direct magnetic connection to the nightside central plasma sheet makes this ion outflow a candidate for the ionospheric source of the nightside plasma sheet. To study the relationship between the ion conic outflow events and auroral substorm conditions, we use global auroral images acquired from the ultraviolet imager (UVI) on Polar. We find that the outflow events are well correlated with substorm expansion phase, but ion conic outflow events also occur during generally active aurora, when onset and expansion phases were not clearly identifiable. By combining FAST data to determine ion outflow fluxes and ILAT extent and Polar UVI images to determine MLT extent, we estimate the total outflow due to these ion conic events to be 1022 to 1024 ions/s. By comparing with earlier ion outflow studies, we conclude that these polar cap boundary ion conic events are not the dominant or only ionospheric source of ion outflow over the entire auroral oval but are the dominant nightside auroral ion outflow.

The nightside poleward boundary of the auroral oval as seen by DMSP and the Ultraviolet Imager

A lack of reliable error estimates for poleward auroral emission (PAE) boundaries derived from satellite‐borne auroral imagers has hampered the application of these instruments in quantitative magnetospheric energy balance and substorm analysis. In this study, PAE boundaries from Polar Ultraviolet Imager (UVI) images are compared with precipitation boundaries from Defense Meteorological Satellite Program (DMSP) satellite spectrograms. In particular, the study quantifies the accuracy with which UVI images can be used to reproduce the DMSP poleward auroral oval (b5e) boundary. Most of the DMSP b5e boundaries were obtained in the evening sector. It has been determined that a UVI PAE boundary defined by a fixed ratio to the maximum in the auroral oval at each magnetic local time correlates better with the DMSP b5e boundary than one defined by a constant brightness threshold (0.90 versus 0.80 maximum correlation). The optimal threshold and ratio values are found to be 4.3 photons cm−2S−1 and 0.30 normalized flux, respectively. The study also reveals a systematic latitudinal offset between UVI and DMSP in the evening sector with a magnitude of approximately 1°. This offset might represent a real height‐dependent geomagnetic influence (e.g., active magnetic topology) or result from systematic errors in the analysis (e.g., removal of the UVI platform wobble). It is demonstrated that the offset can be partially removed with a linear calibration model, allowing the reproduction of DMSP b5e boundaries from UVI images with a standard deviation error of approximately 1°.

High time resolution study of the hemispheric power carried by energetic electrons into the ionosphere during the May 19/20,1996 auroral activity

The ultraviolet imager (UVI) on board the POLAR satellite offers the opportunity to obtain high time resolution global auroral images. The spectral discrimination of the imager is sufficient to separate the auroral far ultraviolet emissions from the scattered sunlight, even when the entire auroral zone is sunlit. The energy flux of the precipitating electrons is derived from the surface brightness through the LBH‐long filter. Global images which have the dayglow removed are spatially integrated to yield the total rate of energy input into the northern hemisphere. This parameter, the hemispheric power, has found much application in ionospheric modeling. It can also be derived from electron spectra measured along the track of the NOAA/TIROS satellites that are combined with average empirical auroral precipitation patterns. We show that the hemispheric power derived from the two‐dimensional images represents a substantial improvement in the temporal variability of this parameter. We present an example for the period of 19/20 May 1996 by comparing the hemispheric power derived from NOAA/TIROS measurements with those derived from the UVI images.

Initial comparison of POLAR UVI and Sondrestrom IS radar estimates for auroral electron energy flux

Calibrated images from the POLAR satellite ultraviolet imager (UVI) in the 165.5 to 174.5 nm portion of the N2 Lyman‐Birge‐Hopfield band (LBH‐long) can be used to estimate the energy flux (FE) of auroral electrons precipitating into the high‐latitude ionosphere. Similarly, electron density profiles, as measured by ground‐based incoherent‐scatter radar, can be used to estimate FE and mean energy (Eo) by solving a system of linear equations relating the E‐region ionization rate profile to a family of monoenergetic ion production profiles. A coordinated POLAR/Sondrestrom radar experiment, designed as an initial comparison of POLAR UVI and ground‐based estimates of FE for a stable auroral arc, was executed during a POLAR apogee on May 20, 1996 at the Sondrestrom radar facility (lat. 66.99°N, long. 50.95°W). Reconstructed energy distributions, based on radar‐measured Ne profiles, indicate an approximately 2 keV Maxwellian source with an energy flux of from 6.4 to 14 mW m−2. LBH‐long images, binned over 0.5° of latitude and 1.0° of longitude, were used to derive energy flux as well. The UVI‐derived FE time history agrees favorably with radar estimates both in absolute magnitude and in the trend for this period. This experiment suggests that reliable estimates for the precipitating electron source energy and its ionospheric response can be derived from either ground‐based radar or POLAR UVI images during summertime conditions.

Auroral observations by the polar Ultraviolet Imager (UVI)

The Ultraviolet Imager (UVI) experiment on the Polar spacecraft has been operating since 21 March 1996, and is returning high quality global images of auroral activity. This article presents first images of global aurora from the UVI experiment. We discuss the capabilities of the UVI instrument and how the data can be used to study substorm physics. A preliminary survey of 80 days of UVI images indicates that the dayside auroral activity is intense and dynamic, and occurs as frequently as the nightside activity. These features are demonstrated using the images obtained on 9 April 1996 when the UVI observed simultaneous dayside and nightside activities.