Chinese Yellow River Station Research Articles

A Comparative Study on the Hot Dense Plasma and Cold Patch by Using Multi‐Instrument Observations

AbstractThe polar cap hot patch is an enhanced density structure which is associated with particle precipitation, ion upflow and flow shears. Based on combined observations from DMSP satellites, EISCAT radar, and all‐sky imager at the Chinese Yellow River Station, the plasma characteristics and evolution of hot dense plasma and classical polar cap patch are investigated. Both of them show enhanced F region density, but the hot dense plasma is associated with higher electron temperature and enhanced E region electron density caused by particle precipitation. Compared to the cold patch, the hot dense plasma is closer to the cusp region and is associated with strong auroral emissions. Based on the joint observations, the evolution of the hot/cold dense plasma is discussed. In the initial phase the dense plasma comes from the sunlit lower latitude region, or from particle precipitation in the duskside auroral oval, and is heated locally by poleward moving auroral structures in the dayside auroral oval. Next the patches move poleward into the polar cap where a decaying electron temperature (i.e., less precipitation) gradually transform them into a cold patch.

Representation and Classification of Auroral Images Based on Convolutional Neural Networks

Auroral forms are correlated with certain physical processes in the magnetosphere and ionosphere. It is, therefore, desirable to automatically classify the vast amount of observed auroral images and make large statistical studies. The key problem in classification tasks is image representation. In this article, using the adaptive feature learning ability of convolutional neural networks, an end-to-end auroral image classification network is proposed, which automatically classifies the auroral images observed at the Chinese Yellow River Station into four classes: arc, drapery corona, radial corona, and hotspot corona. Based on the AlexNet, our method exploits the advanced spatial transformer network (STN) and large margin Softmax (L-Softmax) loss function to extract auroral features. STN is able to learn invariance to translation, scaling, and rotation, whereas L-Softmax increases the difficulty of auroral feature learning so that it encourages the intraclass compactness and interclass separability between learned features. The proposed method was validated on the auroral image datasets by supervised classification, image retrieval, and statistical analysis of the temporal occurrence distributions of the four auroral categories. Experimental results showed that after trained on the winter auroral observations in 2003, the proposed model achieves an average classification accuracy of 93.7% on the auroral data of the following five winters (2004-2009) while maintaining high efficiency, which is superior to the previously reported articles.

Thermospheric Wind over Chinese Yellow River Station and Tromso during Auroral Substorm

中高层大气风场探测对研究大气物理过程具有极为重要的意义，尤其是在极地地区，风场对大气结构的影响更为剧烈.针对亚暴期间中国北极黄河站和日本Tromso站上空OI557.7nm气辉层（低热层）中性风场，利用全天空法布里-珀罗干涉仪（all-sky Fabry-Perot Interferometer，all-sky FPI）探测气辉谱线的多普勒频移，反演气辉层的大气风场信息.结果表明，低热层风场平均水平在100m·s^-1左右，热层风场在极地地区更为剧烈，纬度相对较低的Tromso站探测到的风速整体小于同期黄河站上空的风速.结合离子风数据，分析离子拖拽和焦耳加热对中性风的影响过程，发现极光亚暴不仅对低热层风场有增强作用，也有明显的抑制效果，但整体风向都垂直于极光弧变化.

The Behaviors of Ionospheric Scintillations Around Different Types of Nightside Auroral Boundaries Seen at the Chinese Yellow River Station, Svalbard

Dynamical nightside auroral structures are often observed by the all sky imagers (ASI) at the Chinese Yellow River Station (CYRS) at Ny-Alesund, Svalbard, located in the polar cap near poleward edge of the nightside auroral oval. The boundaries of the nightside auroral oval are stable during quiet geomagnetic conditions, while they often expand poleward and pass through the overhead area of CYRS during the substorm expansion phase. The motions of these boundaries often give rise to strong disturbances of satellite navigations and communications. Two cases of these auroral boundary motions have been introduced to investigate their associated ionospheric scintillations: one is Fixed Boundary Auroral Emissions (FBAE) with stable and fixed auroral boundaries, and another is Bouncing Boundary Auroral Emissions (BBAE) with dynamical and largely expanding auroral boundaries. Our observations show that the auroral boundaries, identified from the sharp gradient of the auroral emission intensity from the ASI images, were clearly associated with ionospheric scintillations observed by Global Navigation Satellite System (GNSS) scintillation receiver at the CYRS. However, amplitude scintillation (S4) and phase scintillation (σφ) respond in an entirely different way in these two cases due to the different generation mechanism as well as different IMF (Interplanetary Magnetic Field) condition. S4 and σφ have similar levels around the FBAE, while σφ was much stronger than S4 around BBAE. The BBAE were associated with stronger particle precipitation during the substorm expansion phase. IU/IL, appeared to be a good indicator of the poleward moving auroral structures during the BBAE as well as FBAE.

Dynamic aurora sequence recognition using Volume Local Directional Pattern with local and global features

Aurora event consists of the spatial structure and temporal evolution of aurora luminosity, which attributes to the effects of the solar wind-magnetosphere interaction and the physics of the magnetosphere-ionosphere interaction. Dynamic aurora event provides a meaningful projection of effects from plasma processes of outer space and also reveals some certain physical phenomenon and principle. Aurora sequence recognition is one of the key procedure in the analysis of dynamic aurora event. Lots of effective features for static aurora image classification are proposed. If these features for static image classification are utilized to recognize the dynamic aurora sequence, it will result in higher computational complexity. The dynamic features of aurora sequence are seldom proposed due to its complexity. To this end, this paper proposes an efficient aurora sequence descriptor which combines local and global spatial information with temporal location information, which is called as Volume Local Directional Patterns. The ring-section spatial pyramid partition structure is used in the VLDP code image which is coded by Volume Local Directional Patterns to obtain the local spatial feature. After combining the global feature of VLDP code image, the final RSPLDP feature is obtained. Finally, the STSC (self-tuning spectral clustering) method is used to classify the aurora sequence. The experimental results on the dataset which is captured from All-sky Imager (ASI) at the Chinese Yellow River Station demonstrate the effectiveness of the proposed classification scheme.

Widths of dayside auroral arcs observed at the Chinese Yellow River Station

Due to the altitudinal extent of an auroral arc, its observed width is different at different zenith angles even when its real width is the same. For that reason, former measurements of arc widths were obtained only for arcs located close to the geomagnetic zenith direction. A method to correct arc width is proposed in the paper, which considers the altitudinal extent of auroral arc. Then, we apply this method to the auroral arcs observed at the Chinese Yellow River Station, and analyze the widths of 17,571 dayside auroral arcs. The distributions of the widths are almost the same at different zenith angles with an average width of 18.5km. Arc widths are narrower as MLT is close to midday.

First observation of thermospheric neutral wind at Chinese Yellow River Station in Ny-Ålesund, Svalbard

A compact and cost-effective all-sky Fabry-Perot interferometer, which is produced by Wuhan University and used for the observation of the thermospheric neutral wind, was installed at Chinese Yellow River Station in Ny-Alesund, Svalbard in November 2010, and continuously operated in last two winter seasons. The 92-day all-sky interference data acquired from November 1, 2011 to February 26, 2012 were collected to get the velocity of thermospheric neutral wind, which was calculated from the Doppler shift caused by the movement of oxygen atom at the different layers. The database was divided into two periods: (1) The OI 557.7 nm emission was observed from November 1, 2011 to January 12, 2012. Observations showed that the velocity of horizontal wind is normally less than ∼40 m/s on the quiet condition, and exceeded 100 m/s on the disturbed condition; and (2) the OI 630.0 nm emission was observed from January 13, 2012 to February 26, 2012. Observations showed that the velocity of horizontal wind is normally less than ∼200 m/s, and enhanced to over 300 m/s on strong magnetic activities. It shows that the velocities of meridional and zonal wind are more consistent with the velocities calculated from the model HWM07 at the higher layer, especially for the zonal direction at nightside auroral regions. Ion drag and Joule heating were the two important processes considered in the analysis of the relationship between the wind pattern and aurora, suggesting that wind speed would be increased and accelerated in the direction perpendicular to the aurora arc when the aurora activity becomes strong.

Simultaneous optical and radar observations of poleward moving auroral forms under different IMF conditions

Using high temporal resolution optical data obtained from three-wavelength all-sky imagers at Chinese Yellow RiverStation in the Arctic, together with the EISCAT Svalbard radar (ESR) and SuperDARN radars, we investigated the dayside polewardmoving auroral forms (PMAFs) and the associated plasma features in the polar ionosphere under different interplanetarymagnetic field (IMF) conditions, between 0900 and 1010 UT on 22 December 2003. Simultaneous optical and ESR observationsrevealed that all PMAFs were clearly associated with pulsed particle precipitations. During northward IMF, particles can precipitateinto lower altitudes and reach the ionospheric E-region, and there is a reverse convection cell associated with these PMAFs.This cell is one of the typical signatures of the dayside high-latitude (lobe) reconnection in the polar ionosphere. These resultsindicate that the PMAFs were associated with the high-latitude reconnection. During southward IMF, the PMAFs show larger latitudinalmotion, indicating a longer mean lifetime, and the associated ionospheric features indicate that the PMAFs were generatedby the dayside low-latitude reconnection.

Poleward moving auroral forms (PMAFs) observed at the Yellow River Station: A statistical study of its dependence on the solar wind conditions

Poleward moving auroral forms (PMAFs) are statistically investigated by using continuous, high-quality, optical observations for six winters obtained from three-wavelength all-sky imagers at the Chinese Yellow River Station (78.92°N, 11.93°E), Svalbard, in the Arctic. In particular, we examined the dependence of the PMAF occurrence on solar wind conditions in the magnetic local time (MLT) range of 0900–1500 MLT. Comparing to the previous studies, our larger PMAFs data base enables us to examine how the PMAFs’ occurrence depends on each interplanetary magnetic field (IMF) component in different MLT sectors. It was found that 59%, 60%, and 57% of the PMAF events occurred under IMF conditions of negative Bz, positive By, and negative Bx, respectively. We also found that the PMAFs show a tendency to occur preferentially under large IMF |Bx| conditions. The MLT dependence of PMAFs presents a remarkable tendency of decreasing in the midday sector, showing a clear IMF By-related prenoon–postnoon asymmetry, which is consistent with the motion of reconnected flux tubes due to the release of magnetic field tension. For the first time, we found, however, that the asymmetry is more evident during conditions of Bz>0 than Bz<0, suggesting the importance of the effect of the lobe reconnection. Our statistical results suggest that the PMAFs are not caused by the solar wind pressure pulses, but by bursts of magnetic reconnection.

Automatic recognition of poleward moving auroras from all-sky image sequences based on HMM and SVM

We present an automatic method to recognize the poleward moving auroras (PMAs) from all-sky image sequences. A simplified block matching algorithm combined with an orientation coding scheme and histogram statistics strategy was utilized to estimate the auroral motion between interlaced images. An all-sky image sequence was first modeled by hidden Markov models (HMMs) and then represented by HMM similarities. The imbalanced classification problem, i.e., non-PMA events far outnumbering PMA events, was addressed by the metric-driven biased support vector machine (SVM). The proposed method was evaluated using auroral observations in 2003 at the Chinese Yellow River Station. Five days observations were manually labeled as PMA or non-PMA events considering both the keogram and all-sky image information. The supervised classification experiments were carried out and achieved satisfactory results. We further detected PMAs from auroral observations in the remaining days and the resultant double-peak occurrence distribution was compared with that of the well-known poleward moving auroral forms (PMAFs).

Simultaneous observations of reconnection pulses at Cluster and their effects on the cusp aurora observed at the Chinese Yellow River Station

Simultaneous observations of reconnection pulses at Cluster and their effects on the cusp aurora observed at the Chinese Yellow River Station

Simultaneous observations of reconnection pulses at Cluster and their effects on the cusp aurora observed at the Chinese Yellow River Station

While the Cluster spacecraft were located near the high‐latitude magnetopause, between 1010 and 1040 UT on 16 January 2004, three typical flux transfer event (FTE) signatures were observed. During this interval, simultaneous and conjugated all‐sky camera measurements, recorded at Yellow River Station, Svalbard, are available at 630.0 and 557.7 nm that show poleward‐moving auroral forms (PMAFs), consistent with magnetic reconnection at the dayside magnetopause. Simultaneous FTEs seen at the magnetopause mainly move northward, but having duskward (eastward) and tailward velocity components, roughly consistent with the observed direction of motion of the PMAFs in all‐sky images. Between the PMAFs meridional keograms, extracted from the all‐sky images, show intervals of lower intensity aurora which migrate equatorward just before the PMAFs intensify. This is strong evidence for an equatorward eroding and poleward moving open‐closed boundary associated with a variable magnetopause reconnection rate under variable IMF conditions. From the durations of the PMAFs, we infer that the evolution time of FTEs is 5–11 minutes from its origin on the magnetopause to its addition to the polar cap.