Ze-Jun Hu

Hemispheric asymmetry of the structure of dayside auroral oval

A comprehensive analysis of long-term and multispectral auroral observations made in the Arctic and Antarctica demonstrates that the dayside auroral ovals in two hemispheres are both presented in a two-peak structure, namely, the prenoon 09:00 magnetic local time (MLT) and postnoon 15:00 MLT peaks. The two-peak structures of dayside ovals, however, are asymmetric in the two hemispheres; i.e., the postnoon average auroral intensity is more than the prenoon one in the Northern Hemisphere but less in the Southern Hemisphere. The hemispheric asymmetry cannot be accounted for by the effect of the interplanetary magnetic field By component and the seasonal difference of ionospheric conductivities in the two hemispheres, which were used to interpret satellite-observed real-time auroral intensity asymmetries in the two hemispheres in previous studies. We suggest that the hemispheric asymmetry is the combined effect of the prenoon-postnoon variations of the magnetosheath density and local ionospheric conductivity.

An extensive survey of dayside diffuse aurora based on optical observations at Yellow River Station

By using 7 years optical auroral observations obtained at Yellow River Station (magnetic latitude 76.24°N) at Ny-Alesund, Svalbard, we performed the first extensive survey for the dayside diffuse auroras (DDAs) and acquired observational results as follows. (1) The DDAs can be classified into two broad categories, i.e., unstructured and structured DDAs. The unstructured DDAs are mainly distributed in morning and afternoon, but the structured DDAs predominantly occurred around the magnetic local noon (MLN). (2) The unstructured DDAs observed in morning and afternoon present obviously different properties. The afternoon ones are much stable and seldom show pulsating property. (3) The DDAs are more easily observed under geomagnetically quiet times. (4) The structured DDAs mainly show patchy, stripy, and irregular forms and are often pulsating and drifting. The drifting directions are mostly westward (with speed ~5 km/s), but there are cases showing eastward or poleward drifting. (5) The stripy DDAs are exclusively observed near the MLN and, most importantly, their alignments are confirmed to be consistent with the direction of ionospheric convection near the MLN. (6) A new auroral form, called throat aurora, is found to be developed from the stripy DDAs. Based on the observational results and previous studies, we proposed our explanations to the DDAs. We suggest that the unstructured DDAs observed in the morning are extensions of the nightside diffuse aurora to the dayside, but that observed in the afternoon are predominantly caused by proton precipitations. The structured DDAs occurred near the MLN are caused by interactions of cold plasma structures, which are supposed to be originated from the ionospheric outflows or plasmaspheric drainage plumes, with hot electrons from the plasma sheet. We suppose that the cold plasma structures for producing the patchy DDAs are in lumpy and are more likely from the plasmaspheric drainage plumes. The cold plasma structure for producing the stripy DDAs should be in wedge like and is generated by conveying the cold plasmas from lower L-shell toward higher L-shell with magnetospheric convection, and that for producing the irregular DDAs is resulted from deforming the wedge-like structure by disturbance. The throat aurora is supposed to be projection of a newly opened flux of reconnection. In addition, we also found that structured DDAs correspond to structured electron precipitations in the ionosphere, which implies that the cold plasma structures in the magnetosphere are magnetically mapped to the ionosphere and act as a duct for producing the structured DDAs. We argue that we have presented some new observational results about DDA in this paper, which will be useful for fully understanding the DDAs.

Auroral Sequence Representation and Classification Using Hidden Markov Models

The naturally occurring aurora phenomenon is a dynamically evolving process. Taking temporal information into consideration, the auroral image sequence analysis is more reasonable and desirable than using static images only. However, the enormous richness of space structures and temporal variations make automatic auroral sequence analysis a particularly challenging task. In this paper, a hidden Markov model (HMM) based representation method including features of spatial texture and dynamic evolution is presented to characterize auroral image sequences captured by all-sky imagers (ASIs). The uniform local binary patterns are employed to describe the 2-D space structures of ASI images. HMM is feasible to characterize the doubly stochastic process involved in the auroral evolution-measurable polar light activities and hidden dynamic plasma processes. We present an affine log-likelihood normalization technique to manage the sequences with different lengths. The proposed method is used in the automatic recognition of four primary categories of ASI auroral observations between the years 2003 and 2009 at the Yellow River Station, Ny-Ålesund, Svalbard. The supervised classification results on manually labeled data in 2003 demonstrate the effectiveness of the proposed technique. Compared with frame-based classification, the higher accuracies and the lower rejection rates show the advantages of the sequence-based method. The occurrence distributions of the four aurora categories were obtained through automatic classification of data gathered from 2004 to 2009. Their agreement with the multiple-wavelength intensity distribution of the dayside aurora and the conclusions made from the frame-based method further illustrate the validity of our method on auroral representation and classification.

Modulation of the dayside diffuse auroral intensity by the solar wind dynamic pressure

Compared to the recently improved understanding of the nightside diffuse aurora, the mechanism(s) responsible for the dayside diffuse auroral precipitation remains limitedly understood. We investigate the dayside diffuse aurora observed by the all-sky imagers of Chinese Arctic Yellow River Station in the time interval of 02:00–10:00 UT (05:00–13:00 magnetic local time) on 2 January 2006. In this interval, the intensity of dayside diffuse aurora is highly correlated with the solar wind dynamic pressure with a maximum coefficient of 0.89. Moreover, there are similar spectra characteristics in the Pc5 range between the intensity of dayside diffuse aurora and solar wind dynamic pressure (proton density) during a portion of the time interval, in which the interplanetary magnetic field Bz is northward. The observation indicates that changes in solar wind dynamic pressure can efficiently modulate the magnitude of the dayside diffuse aurora, except when the interplanetary magnetic field is southward. The enhancement of the solar wind dynamic pressure can provide favorable circumstances for dayside chorus wave generation, so we consider that the dayside chorus could be a candidate for the production of the dayside diffuse aurora. Furthermore, since the compressional Pc4-Pc5 pulsations can also modulate the intensity of whistler mode chorus waves, the solar wind dynamic pressure modulates the dayside diffuse aurora through affecting dayside chorus wave activity and the associated scattering process.

Conjugate Observations of the Evolution of Polar Cap Arcs in Both Hemispheres

We report results from the analysis of a case of conjugate polar cap arcs (PCAs) observed on February 5, 2006 in the northern hemisphere by the ground based Yellow River Station all-sky imager (Svalbard) and in both hemispheres by the space based DMSP/SSUSI and TIMED/GUVI instruments. The PCAs motion in dawn-dusk direction shows a clear dependence on the interplanetary magnetic field (IMF) By component and presents a clear asymmetry between southern and northern hemispheres, i.e., formed on the duskside and moving from dusk to dawn in the northern hemisphere and vice versa in the other hemisphere. The already existing PCAs’ motion is influenced by the changes in the IMF By with a time delay of ~70 minutes. We also observed strong flow shears/reversals around the PCAs in both hemispheres. The precipitating particles observed in the ionosphere associated with PCAs showed properties of boundary layers plasma. Based on these observations, we might reasonably expect that the topological changes in the magnetotail can produce a strip of closed field lines and local processes would be set up conditions for the formation and evolution of PCAs.

Coordinated observations of two types of diffuse auroras near magnetic local noon by Magnetospheric Multiscale mission and ground all-sky camera

Structured diffuse auroras are often observed near magnetic local noon (MLN), but their generation mechanisms are poorly understood. We have found that two types of structured diffuse auroras with ...

Diurnal variation of winter F-region ionosphere for solar minimum at both Zhongshan Station, Antarctica and Svalbard Station, Arctic†

Diurnal variation features of winter-time F2-peak electron density (NmF2) representative for solar minimum at both Zhongshan station, Antarctica, and Svalbard station, are compared and analyzed. Both stations are located around cusp latitude, and are almost on the same geomagnetic meridian plane in both hemispheres. For quiet time period, typical NmF2 diurnal variation features at Svalbard station show double peaks with a decrease of NmF2 around magnetic local noon (~UT+3 hour), NmF2 diurnal variation at Zhongshan station shows one major peak around magnetic local noon (~UT+1.75 hour), followed by a sharp decrease of NmF2 and a subpeak around 1500 UT. Simulation results of the high-latitude ionospheres in both hemispheres agree well with observations at both stations. It is found that the major difference of NmF2 variation between both stations can be explained by the unique location of each station relative to the sunlit demarcation line during the day. For quiet time period, photoionization from lower latitude contributes to the major peak of NmF2 in the diurnal variation at Zhongshan station, while the interaction between horizontal convection and auroral precipitation is the main cause for NmF2 variation at Svalbard station. For active time period, both stations show the increase of NmF2 due to transportation of higher plasma density from lower latitudes on the dayside with the expansion of the polar cap, and the additional ionization from soft precipitating electrons.

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.