极光光谱光发展史和现状

本文阐述了极光光谱光的发展历史和现状，并进一步说明极光绿线的解释对极光光谱学发展的贡献，以及激光过程的深入研究会出现的新问题。     

俏丽壮观而又迷人的极光

极光是地球上最蔚为壮观的景色之一，她色彩斑斓、摇曳多姿、美丽迷人。古今中外，多少人从极光的历史，形成的原因，形态和分类，以及极光的色彩和某些现象等多个角度对极光进行过观测和探索，因而使我们对这一奇特的自然景观有了比较深入的了解。     

南极中山站宁静日午后极光事件分析

对１９９７年６月２日１１２７－１７００ＵＴ，在南极中山站利用全天空极光摄像机和三波段极光扫描光度计观测到的午后极光进行了分析，发现宁静午后极光的形态随磁地方时变化。在午后早些时候，辉度较弱的冕状极光出现在中山站的极向侧和日向侧；随后出现的是来自日侧的带状极光，该结构东向传播，赤道向运动；     

去哪儿看极光？

你想看极光吗？想亲身体验大自然造出的美丽无比的绚丽景色吗？试想,站在荒郊野外,左等右等、就是不见其身影……突然,流光溢彩、铺天盖地的极光出现了,绵亘几百公里甚至上千公里,那是何等激动人心之事。亲眼看见极光,是人生中不可多得的一大幸事。 那么,何处能够看到极光,何处才是最佳地点？何时去看为好？低纬度和热带地区也能看到吗？     

行星都有极光吗？（上）——金星、火星和木星的极光

地球有极光,这是很多人都知道的事。那么太阳系的其他七大行星是否也有极光呢？地球的极光婀娜多姿、流光溢彩、变幻莫测、俏丽迷人,其他行星的极光又如何呢.     

纯毛服装极光的研究(第Ⅰ报)

用改制的织物极光发生仪对纯毛织物进行模拟实际穿着时的摩擦状况实验,对各试样的表面光泽度进行测试。确立了以光泽变化率(ε)这个指标作为人们视觉判断纯毛服装出现极光有效界限值,并结合各试样在受磨前后的织物、纱线及纤维结构的电镜片分析了纯毛服装生极光的机理。     

绚烂极光闪耀夜空

在北欧的传说中，极光是由“黎明女神”欧若拉掌管的，可我们是讲科学的旅行团，所以在观赏极光之前，有必要给大家普及一下关于它的知识。     

行星都有极光吗?(下)——土星、天王星和海王星的极光

笔者在上一期描述和讨论了金星、火星和木星的极光后,读者自然就会想到其他三颗地外行星——土星、天王星和海王星——是不是也有极光呢？如果有的话,它们的极光又是什么样子呢？都各自有哪些特点？现在的观测和研究工作进行到什么程度了？还有哪些问题有待进一步探索？下面让我们来一一细说。     

南北极的极光可以不对称

极光往往同时在北极和南极区域出现，其出现的地点由地磁场线连接，这种联系预计也许会将两个极光的图案、位置和时间关联起来。2001年5月12日，两个地球观测飞船“IMAGE”和“Polar”处在对两极同时进行观测的较好位置，当时对南极来说是黄昏，对北极来说是黎明。此次科学家对观测获得的图片所提供的清楚证据分析得出，两个极光可以是不对称的，     

极光带电导率的日夜变化对高纬电离层电场的影响

在同时考虑极盖区电导率日大夜小和极光区电导率升高日小夜大两种效应的情况下,本文求出了高纬电场的分析解,并用其讨论了几个具体模式和对结果进行了分析。模式计算表明,极光带电导率升高的日夜变化对对流圈形态,电场分布和场向电流分布均有一定的影响,模式计算与观测比较有较好的一致性,这表明本文模式具有较好的应用意义。     

An interplanetary shock traced by planetary auroral storms from the Sun to Saturn

A relationship between solar activity and aurorae on Earth was postulated long before space probes directly detected plasma propagating outwards from the Sun. Violent solar eruption events trigger interplanetary shocks that compress Earth's magnetosphere, leading to increased energetic particle precipitation into the ionosphere and subsequent auroral storms. Monitoring shocks is now part of the 'Space Weather' forecast programme aimed at predicting solar-activity-related environmental hazards. The outer planets also experience aurorae, and here we report the discovery of a strong transient polar emission on Saturn, tentatively attributed to the passage of an interplanetary shock--and ultimately to a series of solar coronal mass ejection (CME) events. We could trace the shock-triggered events from Earth, where auroral storms were recorded, to Jupiter, where the auroral activity was strongly enhanced, and to Saturn, where it activated the unusual polar source. This establishes that shocks retain their properties and their ability to trigger planetary auroral activity throughout the Solar System. Our results also reveal differences in the planetary auroral responses on the passing shock, especially in their latitudinal and local time dependences.     

Locating the polar cap boundary of postnoon sector from observations of 630.0 nm auroral emission at Zhongshan Station

We studied the ground observations of 630.0 nm auroral emission at Zhongshan Station to determine the polar cap boundary with the latitudinal profile of emission intensity. The open-closed field time boundary is assumed to lie at the boundary between polar rain and plasma sheet precipitation. We assume that nonprecipitation-dependent sources of 630.0 nm emission cause a spatially uniform luminosity in the polar cap and that auroral zone luminosity is also spatially uniform. Therefore we determine the location of the polar cap boundary of postnoon sector from the auroral emission data each time by finding the best fit of the observations to a step function in latitude and we produce a time series of the location of the polar cap boundary. The average error of the practice in the paper is less than 0.8 degree. Foundation item: Supported by the National Natural Science Foundation of China (No. 40044013) Biography: Liu Li-gang (1976-), male, Master candidate, research direction: auroral physics.     

Nonlinear dispersive scale Alfvén waves inmagnetosphere-ionosphere coupling: Physical processes and simulation results

Dispersive Alfvén waves(DAWs)have been demonstrated to play a significant role in auroral generation of the magnetosphereionosphere coupling system.Starting from a two fluid reduced MHD model,we summarize the frequency,temporal and spatial characteristics of magnetospheric DAWs.Then,the nonlinear kinetic and inertial scale Alfveén waves are studied,and we review some theoretical aspects and simulation results of dispersive Alfve′n waves in Earth’s magnetosphere.It is shown that dispersive standing Alfve′n waves can generate the field-aligned currents which transport energy into the auroral ionosphere,where it is dissipated by Joule heating and energy lost due to electron precipitation.The Joule dissipation can heat the ionospheric electron and produce changes in the ionospheric Pedersen conductivity.As a feedback,the conducting ionosphere can also strongly affect the magnetospheric currents. The ponderomotive force can cause the plasma to move along the field line,and generate ionospheric density cavity.The nonlinear structuring can lead to a dispersive scale to accelerate auroral particle,and the Alfvn waves can be trapped within the density cavity. Finally,we show the nonlinear decay of dispersive Alfvén waves related to two anti-propagating electron fluxes observed in the auroral zone.     

Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter

It has often been stated that Saturn's magnetosphere and aurorae are intermediate between those of Earth, where the dominant processes are solar wind driven, and those of Jupiter, where processes are driven by a large source of internal plasma. But this view is based on information about Saturn that is far inferior to what is now available. Here we report ultraviolet images of Saturn, which, when combined with simultaneous Cassini measurements of the solar wind and Saturn kilometric radio emission, demonstrate that its aurorae differ morphologically from those of both Earth and Jupiter. Saturn's auroral emissions vary slowly; some features appear in partial corotation whereas others are fixed to the solar wind direction; the auroral oval shifts quickly in latitude; and the aurora is often not centred on the magnetic pole nor closed on itself. In response to a large increase in solar wind dynamic pressure Saturn's aurora brightened dramatically, the brightest auroral emissions moved to higher latitudes, and the dawn side polar regions were filled with intense emissions. The brightening is reminiscent of terrestrial aurorae, but the other two variations are not. Rather than being intermediate between the Earth and Jupiter, Saturn's auroral emissions behave fundamentally differently from those at the other planets.     

Observations of thermospheric vector wind over Yellow River Station during auroral substorm events

Strong disturbances associated with auroral substorms originate from the ionosphere–magnetosphere owing to the effects of the solar wind, and the wind field in the ionosphere is related to such substorm activity. Here, we describe the analysis of four auroral substorm events, for which we employed an all-sky Fabry–Perot interferometer to observe the two-dimensional horizontal wind field and combined the results with data from an all-sky charge-coupled device imager, a fluxgate magnetometer installed at Yellow River Station, and the Super Dual Auroral Radar Network. The results demonstrate that, during auroral substorms, the vector wind field is related closely to variations in the ion drift and geomagnetic field. Moreover, we observed a changing wind field of approximately 300 m/s in response to variations in the electric and magnetic fields (likely caused by ion drag) and a disturbance of about 200 m/s that we attribute to the interaction of Joule heating and ion drag.     

Auroral substorm response to solar wind pressure shock

Two cases of auroral substorms have been studied with the Polar UVI data, which were associated with solar wind pressure shock arriving at the Earth. The global aurora activities started about 1–2 min after pressure shocks arrived at dayside magnetopause, then nightside auroras intensified rapidly 3–4 min later, with auroral substorm onset. The observations in synchronous orbit indicated that the compressing effects on magnetosphere were observed in their corresponding sites about 2 min after the pressure shocks impulse magnetopause. We propose that the auroral intensification and substorm onset possibly result from hydromagnetic wave produced by the pressure shock. The fast-mode wave propagates across the magnetotail lobes with higher local Alfven velocity, magnetotail was compressed rapidly and strong lobe field and cross-tail current were built in about 1–2 min, and furthermore the substorm was triggered due to an instability in current sheet.     

An Earth-like correspondence between Saturn's auroral features and radio emission

Saturn is a source of intense kilometre-wavelength radio emissions that are believed to be associated with its polar aurorae, and which provide an important remote diagnostic of its magnetospheric activity. Previous observations implied that the radio emission originated in the polar regions, and indicated a strong correlation with solar wind dynamic pressure. The radio source also appeared to be fixed near local noon and at the latitude of the ultraviolet aurora. There have, however, been no observations relating the radio emissions to detailed auroral structures. Here we report measurements of the radio emissions, which, along with high-resolution images of Saturn's ultraviolet auroral emissions, suggest that although there are differences in the global morphology of the aurorae, Saturn's radio emissions exhibit an Earth-like correspondence between bright auroral features and the radio emissions. This demonstrates the universality of the mechanism that results in emissions near the electron cyclotron frequency narrowly beamed at large angles to the magnetic field.     

Jovian-like aurorae on Saturn

Planetary aurorae are formed by energetic charged particles streaming along the planet's magnetic field lines into the upper atmosphere from the surrounding space environment. Earth's main auroral oval is formed through interactions with the solar wind, whereas that at Jupiter is formed through interactions with plasma from the moon Io inside its magnetic field (although other processes form aurorae at both planets). At Saturn, only the main auroral oval has previously been observed and there remains much debate over its origin. Here we report the discovery of a secondary oval at Saturn that is approximately 25 per cent as bright as the main oval, and we show this to be caused by interaction with the middle magnetosphere around the planet. This is a weak equivalent of Jupiter's main oval, its relative dimness being due to the lack of as large a source of ions as Jupiter's volcanic moon Io. This result suggests that differences seen in the auroral emissions from Saturn and Jupiter are due to scaling differences in the conditions at each of these two planets, whereas the underlying formation processes are the same.     

Scattering by chorus waves as the dominant cause of diffuse auroral precipitation

Earth's diffuse aurora occurs over a broad latitude range and is primarily caused by the precipitation of low-energy (0.1-30-keV) electrons originating in the central plasma sheet, which is the source region for hot electrons in the nightside outer magnetosphere. Although generally not visible, the diffuse auroral precipitation provides the main source of energy for the high-latitude nightside upper atmosphere, leading to enhanced ionization and chemical changes. Previous theoretical studies have indicated that two distinct classes of magnetospheric plasma wave, electrostatic electron cyclotron harmonic waves and whistler-mode chorus waves, could be responsible for the electron scattering that leads to diffuse auroral precipitation, but it has hitherto not been possible to determine which is the more important. Here we report an analysis of satellite wave data and Fokker-Planck diffusion calculations which reveals that scattering by chorus is the dominant cause of the most intense diffuse auroral precipitation. This resolves a long-standing controversy. Furthermore, scattering by chorus can remove most electrons as they drift around Earth's magnetosphere, leading to the development of observed pancake distributions, and can account for the global morphology of the diffuse aurora.