In situ detection of the electron diffusion region of collisionless magnetic reconnection at the high-latitude magnetopause

Magnetic reconnection is the most fundamental energy-transfer mechanism in the universe that converts magnetic energy into heat and kinetic energy of charged particles.For reconnection to occur,the frozen-in condition must break down in a localized region,commonly called the ‘diffusion region'.In Earth's magnetosphere,ion diffusion regions have already been observed,while electron diffusion regions have not been detected due to their small scales(of the order of a few km)(Paschmann,2008).In this paper we report,for the first time,in situ observations of an active electron diffusion region by the four Cluster spacecraft at the Earth's highlatitude magnetopause.The electron diffusion region is characterized by nongyrotropic electron distribution,strong field-aligned currents carried by electrons and bi-directional super-Alfvénic electron jets.Also observed were multiple micro-scale flux ropes,with a scale size of about 5 c/ω_(pe)(12 km,with c/ωpe the electron inertial length),that are crucial for electron acceleration in the guide-field reconnection process(Drake et al.,2006 a).The data demonstrate the existence of the electron diffusion region in collisionless guide-field reconnection at the magnetopause.

Finding the magnetopause location using soft X-ray observations and a statistical inverse method

Variability in the location and shape of the dayside magnetopause is attributed to magnetic reconnection,a fundamental process that enables the transfer of mass,energy,and momentum from the solar wind into the magnetosphere.The spatial and temporal properties of the magnetopause,under varying solar and magnetospheric conditions,remain largely unknown because empirical studies using in-situ observations are challenging to interpret.Global wide field-of-view(FOV)imaging is the only means to simultaneously observe the spatial distribution of the plasma properties over the vast dayside magnetospheric region and,subsequently,quantify the energy transport from the interplanetary medium into the terrestrial magnetosphere.Two upcoming missions,ESA/CAS SMILE and NASA’s LEXI will provide wide-field imagery of the dayside magnetosheath in soft X-rays,an emission generated by charge exchange interactions between high charge-state heavy ions of solar wind origin and exospheric neutral atoms.High-cadence two-dimensional observations of the magnetosheath will allow the estimation of dynamic properties of its inner boundary,the magnetopause,and enable studies of its response to changes in the solar wind dynamic pressure and interplanetary magnetic field orientation.This work introduces a statistically-based estimation approach based on inverse theory to estimate the spatial distribution of magnetosheath soft X-ray emissivities and,with this,identify the location of the magnetopause over the Sun−Earth line.To do so,we simulate the magnetosheath structure using the MHD-based OpenGGCM model and generate synthetic soft X-ray images using LEXI’s orbit and attitude information.Our results show that 3-D estimations using the described statistically-based technique are robust against Poisson-distributed shot noise inherent to soft X-ray images.Also,our proposed methodology shows that the accuracy of both three-dimensional(3-D)estimation and the magnetopause standoff distance calculation highly depends on the observational point.

Estimating the subsolar magnetopause position from soft X-ray images using a low-pass image filter

The Lunar Environment heliospheric X-ray Imager(LEXI)and Solar wind Magnetosphere Ionosphere Link Explorer(SMILE)missions will image the Earth’s dayside magneto pause and cusps in soft X-rays after their respective launches in the near future,to specify glo bal magnetic reconnection modes for varying solar wind conditions.To suppo rt the success of these scientific missions,it is critical to develop techniques that extract the magnetopause locations from the observed soft X-ray images.In this research,we introduce a new geometric equation that calculates the subsolar magnetopause position(RS)from a satellite position,the look direction of the instrument,and the angle at which the X-ray emission is maximized.Two assumptions are used in this method:(1)The look direction where soft X-ray emissions are maximized lies tangent to the magnetopause,and(2)the magnetopause surface near the subsolar point is almost spherical and thus RSis nea rly equal to the radius of the magneto pause curvature.We create synthetic soft X-ray images by using the Open Geospace General Circulation Model(OpenGGCM)global magnetohydrodynamic model,the galactic background,the instrument point spread function,and Poisson noise.We then apply the fast Fourier transform and Gaussian low-pass filte rs to the synthetic images to re move noise and obtain accurate look angles for the soft X-ray pea ks.From the filte red images,we calculate RS and its accuracy for different LEXI locations,look directions,and solar wind densities by using the OpenGGCM subsolar magnetopause location as ground truth.Our method estimates RS with an accuracy of<0.3 RE when the solar wind density exceeds>10 cm-3.The accuracy improves for greater solar wind densities and during southward interplanetary magnetic fields.The method ca ptures the magnetopause motion during southwa rd interplaneta ry magnetic field turnings.Consequently,the technique will enable quantitative analysis of the magnetopause motion and help reveal the dayside reconnection modes for dynamic solar wind conditions.This technique will suppo rt the LEXI and SMILE missions in achieving their scientific o bjectives.

On the apparent line-of-sight alignment of the peak X-ray intensity of the magnetosheath and the tangent to the magnetopause,as viewed by SMILE-SXI

The Soft X-ray Imager(SXI)on board the Solar wind Magnetosphere Ionosphere Link Explorer(SMILE)spacecraft will be able to view the Earth’s magnetosheath in soft X-rays.Simulated images of the X-ray emission visible from the position of SMILE are created for a range of solar wind densities by using 3 years of the SMILE mission orbit,together with models of the expected X-ray emissivity from the Earth’s magnetosheath.Results from global magnetohydrodynamic simulations and a simple model for exospheric neutral densities are used to compare the locations of the lines of sight along which integrated soft X-ray intensities peak with the lines of sight lying tangent to surfaces(defined here to be the magnetopause)along which local soft X-ray intensities peak or exhibit their strongest gradients,or both,for strongly southward interplanetary magnetic field conditions when no depletion or low-latitude boundary layers are expected.Where,in the parameter space of the various times and seasons,orbital phases,solar wind conditions,and magnetopause models,the alignment of the X-ray emission peak with the magnetopause tangent is good,or is not,is presented.The main results are as follows.The spacecraft needs to be positioned well outside the magnetopause;low-altitude times near perigee are not good.In addition,there are seasonal aspects:dayside-apogee orbits are generally very good because the spacecraft travels out sunward at high altitude,but nightside-apogee orbits,behind the Earth,are bad because the spacecraft only rarely leaves the magnetopause.Dusk-apogee and dawnapogee orbits are intermediate.Dayside-apogee orbits worsen slightly over the first three mission years,whereas nightside-apogee orbits improve slightly.Additionally,many more times of good agreement with the peak-to-tangent hypothesis occur when the solar wind is in a high-density state,as opposed to a low-density state.In a high-density state,the magnetopause is compressed,and the spacecraft is more often a good distance outside the magnetopause.

Magnetopause properties at the dusk magnetospheric flank from global magnetohydrodynamic simulations,the kinetic Vlasov equilibrium,and in situ observations--Potential implications for SMILE

We derived the properties of the terrestrial magnetopause(MP)from two modeling approaches,one global–fluid,the other local–kinetic,and compared the results with data collected in situ by the Magnetospheric Multiscale 2(MMS2)spacecraft.We used global magnetohydrodynamic(MHD)simulations of the Earth’s magnetosphere(publicly available from the NASA-CCMC[National Aeronautics and Space Administration–Community Coordinated Modeling Center])and local Vlasov equilibrium models(based on kinetic models for tangential discontinuities)to extract spatial profiles of the plasma and field variables at the Earth’s MP.The global MHD simulations used initial solar wind conditions extracted from the OMNI database at the time epoch when the MMS2 observes the MP.The kinetic Vlasov model used asymptotic boundary conditions derived from the same in situ MMS measurements upstream or downstream of the MP.The global MHD simulations provide a three-dimensional image of the magnetosphere at the time when the MMS2 crosses the MP.The Vlasov model provides a one-dimensional local view of the MP derived from first principles of kinetic theory.The MMS2 experimental data also serve as a reference for comparing and validating the numerical simulations and modeling.We found that the MP transition layer formed in global MHD simulations was generally localized closer to the Earth(roughly by one Earth radius)from the position of the real MP observed by the MMS.We also found that the global MHD simulations overestimated the thickness of the MP transition by one order of magnitude for three analyzed variables:magnetic field,density,and tangential speed.The MP thickness derived from the local Vlasov equilibrium was consistent with observations for all three of these variables.The overestimation of density in the Vlasov equilibrium was reduced compared with the global MHD solutions.We discuss our results in the context of future SMILE(Solar wind Magnetosphere Ionosphere Link Explorer)campaigns for observing the Earth’s MP.

Using restored two-dimensional X-ray images to reconstruct the three-dimensional magnetopause

Astronomical imaging technologies are basic tools for the exploration of the universe,providing basic data for the research of astronomy and space physics.The Soft X-ray Imager(SXI)carried by the Solar wind Magnetosphere Ionosphere Link Explorer(SMILE)aims to capture two-dimensional(2-D)images of the Earth’s magnetosheath by using soft X-ray imaging.However,the observed 2-D images are affected by many noise factors,destroying the contained information,which is not conducive to the subsequent reconstruction of the three-dimensional(3-D)structure of the magnetopause.The analysis of SXI-simulated observation images shows that such damage cannot be evaluated with traditional restoration models.This makes it difficult to establish the mapping relationship between SXIsimulated observation images and target images by using mathematical models.We propose an image restoration algorithm for SXIsimulated observation images that can recover large-scale structure information on the magnetosphere.The idea is to train a patch estimator by selecting noise–clean patch pairs with the same distribution through the Classification–Expectation Maximization algorithm to achieve the restoration estimation of the SXI-simulated observation image,whose mapping relationship with the target image is established by the patch estimator.The Classification–Expectation Maximization algorithm is used to select multiple patch clusters with the same distribution and then train different patch estimators so as to improve the accuracy of the estimator.Experimental results showed that our image restoration algorithm is superior to other classical image restoration algorithms in the SXI-simulated observation image restoration task,according to the peak signal-to-noise ratio and structural similarity.The restoration results of SXI-simulated observation images are used in the tangent fitting approach and the computed tomography approach toward magnetospheric reconstruction techniques,significantly improving the reconstruction results.Hence,the proposed technology may be feasible for processing SXI-simulated observation images.

The high-latitude ionospheric phenomena observed by DPS-4 at Zhongshan Station, Antarctica

Ionograms, which were obtained from February 1995 to January 1999 by a Digisonde Portable Sounder-4 (DPS-4) at Zhongshan Station in Antarctica, have been scaled. The ionograms interpretation has shown various ionosphcric phcnomena occurred in the cusp region. They are particle precipitation effects in the E region. F region magnetic noon phenomena, slant Es phenomena and lacuna. F layer irregularity zone (FLIZ) phenomena, and auroral oval identification. Typical examples of ionograms and ionogram sequences observed at Zhongshan Station are displayed in this paper.

Model Simulation of Artificial Heating of the Daytime High-Latitude F-Region Ionosphere by Powerful High-Frequency Radio Waves

The large-scale disturbance of the spatial structure of the daytime high-latitude F-region ionosphere, caused by powerful high-frequency radio waves, pumped into the ionosphere by a groundbased ionospheric heater, is studied with the help of the numerical simulation. The mathematical model of the high-latitude ionosphere, developed earlier in the Polar Geophysical Institute, is utilized. The mathematical model takes into account the drift of the ionospheric plasma, strong magnetization of the plasma at F-layer altitudes, geomagnetic field declination, and effect of powerful high-frequency radio waves. The distributions of the ionospheric parameters were calculated on condition that an ionospheric heater, situated at the point with geographic coordinates of the HF heating facility near Tromso, Scandinavia, has been operated, with the ionospheric heater being located on the day side of the Earth. The results of the numerical simulation indicate that artificial heating of the ionosphere by powerful high-frequency waves ought to influence noticeably on the spatial structure of the daytime high-latitude F-region ionosphere in the vicinity of the ionospheric heater.

Determination of the Kelvin-Helmholtz Wave Parameters on the Magnetopause in Single-spacecraft Observations

Kelvin-Helmholtz(K-H) waves are formed from the triggering of the K-H instability on the magnetopause,which is a candidate mechanism for solar wind entry into the magnetosphere,especially under northward interplanetary magnetic field conditions.In this study,a K-H wave event was identified from the observation of probe B of the Time History of Events and Macroscale Interactions during Substorms(THEMIS) mission on 15 May 2008.A new method to determine the wave parameters of the K-H waves in single-spacecraft observations is proposed.The dominant wave period is determined by three kinds of spectrograms for three key parameters,namely the ion density,the ion temperature,and the z component of magnetic field.The phase velocity is estimated by calculating the center-of-mass velocity of the detected K-H vortex region.This approximation is validated by comparison with other alternative methods.The method to determine the wave parameters is a first step to further study K-H wave properties and their relationship with interplanetary conditions.

Endocrine response of early-hatching Asian Short-toed Lark nestlings exposed to cold temperature in a high-latitude grassland habitat

Background:In high latitude grassland habitats,altricial nestlings hatching in open-cup nests early in the breeding season must cope with cold temperature challenges.Thyroid hormones(triiodothyronine,T_(3)and thyroxine,T_(4))and corticosterone play a crucial role in avian thermoregulation response to cold.Investigating the endocrine response of altricial nestlings to temperature variation is important for understanding the adaptive mechanisms of individual variation in the timing of breeding in birds.Methods:We compared nest temperature,ambient temperature,body temperature,plasma T_(3),T_(4)and corticosterone levels in Asian Short-toed Lark(Alaudala cheleensis)nestlings hatching in the early-,middle-,and late-stages of the breeding season in Hulunbuir grassland,northeast China.Results:Mean nest temperature in the early-,middle-and late-stage groups was-1.85,3.81 and 10.23℃,respectively,for the 3-day-old nestlings,and 6.83,10.41 and 11.81℃,respectively,for the 6-day-old nestlings.The nest temperature significantly correlated with body temperature,plasma T_(3),T_(4)and corticosterone concentrations of nestlings.Body temperature of 3-day-old nestlings in the early and middle groups was significantly lower than that of the late group,but there was no significant difference between the nestlings in the early and middle groups.The T_(4)and T_(3)concentrations and the ratio of T_(3)/T_(4)of both 3-and 6-day-old nestlings in the early-stage group were significantly higher compared to the middle and late groups.The corticosterone levels of 3-day-old nestlings were significantly higher in the early-stage group compared to the middle-and late-stage groups.Conclusion:Nestlings hatching early responded to cold temperature by increasing thyroid hormones and corticosterone levels even in the early days of post hatching development when the endothermy has not been established.These hormones may play a physiological role in neonatal nestlings coping with cold temperature challenges.

Effect of the Interplanetary Electric Field on the Magnetopause From Global MHD Simulations

The north-south component B_z of the Interplanetary Magnetic Field(IMF) and solar wind dynamic pressure P_d are generally treated as the two main factors in the solar wind that determine the geometry of the magnetosphere.By using the 3D global MHD simulations,we investigate the effect of the Interplanetary Electric Field(IEF) on the size and shape of magnetopause quantitatively. Our numerical experiments confirm that the geometry of the magnetopause are mainly determined by P_d and B_z,as expected.However,the dawn-dusk IEFs have great impact on the magnetopause erosion because of the magnetic reconnection,thus affecting the size and shape of the magnetopause.Higher solar wind speed with the same B_z will lead to bigger dawn-dusk IEFs,which means the higher reconnection rate,and then results in more magnetic flux removal from the dayside. Consequently,the dayside magnetopause moves inward and flank magnetopause moves outward.

The global response of temperature to high-latitude vegetation greening in a two-dimensional energy balance model

The relationship between vegetation greening and climate change remains unclear due to its complexity, especially in drylands. Against the background of global warming, arid and semi-arid areas, including mid-latitude deserts, are most sensitive to climate change. In recent decades, the mechanisms underlying the relationship between vegetation greening and climate change have been widely discussed in the literature. However, the influence of vegetation greening in high latitudes on regional climate has not been fully studied. In this paper, a two-dimensional energy balance model was used to study the influence of greening in high latitudes on mid-latitude deserts. The authors found that when greening occurs in high latitudes, the mid-latitude desert recedes at the south boundary, while the polar ice belt and low-latitude vegetation belt both expand. Simultaneously, greening in high latitudes can induce a negative temperature anomaly in northern latitudes and a positive temperature anomaly in southern latitudes. The mid-latitude desert expands at its north and south boundaries until the CO2 concentration reaches 600 ppm(saturated state). The greening in high latitudes could result in a lower global-mean temperature in the ‘saturated’ state, due to the stronger cooling in high latitudes.

Global hybrid simulations of soft X-ray emissions in the Earth’s magnetosheath

Earth’s magnetopause is a thin boundary separating the shocked solar wind plasma from the magnetospheric plasmas,and it is also the boundary of the solar wind energy transport to the magnetosphere.Soft X-ray imaging allows investigation of the large-scale magnetopause by providing a two-dimensional(2-D)global view from a satellite.By performing 3-D global hybrid-particle-in-cell(hybrid-PIC)simulations,we obtain soft X-ray images of Earth’s magnetopause under different solar wind conditions,such as different plasma densities and directions of the southward interplanetary magnetic field.In all cases,magnetic reconnection occurs at low latitude magnetopause.The soft X-ray images observed by a hypothetical satellite are shown,with all of the following identified:the boundary of the magnetopause,the cusps,and the magnetosheath.Local X-ray emissivity in the magnetosheath is characterized by large amplitude fluctuations(up to 160%);however,the maximum line-of-sight-integrated X-ray intensity matches the tangent directions of the magnetopause well,indicating that these fluctuations have limited impact on identifying the magnetopause boundary in the X-ray images.Moreover,the magnetopause boundary can be identified using multiple viewing geometries.We also find that solar wind conditions have little effect on the magnetopause identification.The Solar wind Magnetosphere Ionosphere Link Explorer(SMILE)mission will provide X-ray images of the magnetopause for the first time,and our global hybrid-PIC simulation results can help better understand the 2-D X-ray images of the magnetopause from a 3-D perspective,with particle kinetic effects considered.

Simulation of the SMILE Soft X-ray Imager response to a southward interplanetary magnetic field turning

The Solar wind Magnetosphere Ionosphere Link Explorer(SMILE)Soft X-ray Imager(SXI)will shine a spotlight on magnetopause dynamics during magnetic reconnection.We simulate an event with a southward interplanetary magnetic field turning and produce SXI count maps with a 5-minute integration time.By making assumptions about the magnetopause shape,we find the magnetopause standoff distance from the count maps and compare it with the one obtained directly from the magnetohydrodynamic(MHD)simulation.The root mean square deviations between the reconstructed and MHD standoff distances do not exceed 0.2 RE(Earth radius)and the maximal difference equals 0.24 RE during the 25-minute interval around the southward turning.

2001年1月26日高纬磁层顶通量管事件的观测研究——空间电流密度计算及分析

讨论了三种根据ClusterⅡ四颗卫星的磁场测量数据计算空间电流的方法及其误差 ,论证了这几种方法的内在一致性 ,并得到了完全相同的计算结果 .进而依据ClusterⅡ磁场探测资料 ,计算了 2 0 0 1年 1月 2 6日多重磁通量管和FTE事件中高纬磁层顶边界层和磁鞘区的电流密度 .结果表明 ,磁通量管内电流密度较大 ,可达到约10 - 8A m2 ;计算精度较高 ,结果可靠 .本文还应用最小方差分析法 (MVA) ,发现电流方向与通量管的轴向基本一致 ;论证了电流MVA分析在研究通量管性质时的作用 。

2004年4月1日磁通量传输事件特性的研究——通量管内电流密度、粒子运动与管轴方向的对比分析

本文利用Cluster四颗卫星上磁通门磁力计(FGM)的同时观测,采用旋度器方法(Curlometer),计算和分析了2004年4月1日12:24到12:54UT期间Cluster卫星观测的多个磁通量传输事件(FTEs)的特性。结果表明:磁通量管内电流密度较大,可达到约10-7A/m2。应用最小方向微分法(MDD),发现这些FTE事件具有准二维结构,即为圆柱形结构,其通量管轴线方向与管内电流方向及粒子运动方向基本平行。

太阳风向磁层输入电磁能量研究

利用全球磁流体力学(MHD)模拟结果,通过确立包含磁层顶的太阳风流线内边界来识别三维磁层顶位形,并以极尖区位置作为磁层顶日侧与夜侧的分界线,在此基础上定量研究了不同条件下穿过磁层顶向磁层内输入的电磁能量.研究发现,磁层顶的能量传输与太阳风条件密切相关,磁重联是控制电磁能量传输的重要机制.结果表明,当IMF(行星际磁场)南向时,极尖区后方的磁尾附近存在电磁能输入最大值,当IMF北向时,电磁能输入最大值发生在极尖区附近;南向IMF条件下,在IMF强度增大或太阳风密度增大时,磁层顶电磁能传输的电磁能量比北向IMF条件时增加更显著.太阳风通过调节磁层顶面积间接影响到磁层顶能量传输大小.研究还发现,北向IMF与南向IMF条件下穿过磁层顶的电磁能输入的比值范围约为10%~30%,此比值一定程度上反映了北、南方向IMF与地磁场磁重联效率的比值.

涡旋诱发重联模型(Ⅱ)——通量传输事件理论和模拟

本文应用涡旋诱发重联理论研究了地球磁层顶区发生的瞬时局部重联现象.对向阳面磁顶区通量传输事件(FTEs)的形成、结构和运动进行了理论和模拟研究,并与卫星观测结果作了比较.结果表明,涡旋诱发重联可能是产生FTEs的重要机制.利用这一理论模型能解释FTEs的一些主要观测现象.此外,对背阳面磁顶区的局部重联从理论上作了分析,指出在背阳面磁顶区可能存在类似于向阳面磁顶区的通量传输事件.

2001年1月26日高纬磁层顶通量管事件的观测研究

20 0 1年 1月 2 6日 11:10～ 11:4 0UT ,ClusterⅡ卫星簇位于午后高纬磁鞘边界层和磁鞘区 ,此时行星际磁场Bz 为南向 .本文对在此期间观测到的多次磁通量管事件作了详细的研究 ,获得一系列的新发现 :(1)高纬磁鞘边界层磁通量管的出现具有准周期性 ,周期约为 78s,比目前已知的磁层顶向阳面FTE的平均周期 (8～ 11min)小得多 .(2 )这些通量管都具有强的核心磁场 ;其主轴多数在磁场最小变化方向 ,少数在中间变化方向 ,有些无法用PAA判定其方向 (需要用电流管PAA确定 ) ,这与卫星穿越通量管的相对路径有关 .(3)每个事件都存在很好的HT参考系 ,在HT参考系中这些通量管是准定常态结构 ;所有通量管都沿磁层顶表面运动 ,速度方向大体相同 ,都来自晨侧下方 .通量管的径向尺度为 1～ 2RE,与通常的FTE通量管相当 .(4 )起源于磁层的强能离子大体上沿着管轴方向由磁层向磁鞘运动 ;起源于太阳风的热等离子体沿管轴向磁层传输 .通量管为太阳风等离子体向磁层输运和磁层粒子向行星际空间逃逸提供了通道 .(5 )每个通量管事件都伴随有晨昏电场的反转 ,该电场为对流电场 .

磁层顶高纬边界层的瞬时重联过程

用二维可压缩的ＭＨＤ模型模拟研究了北半球背阳面磁层顶区的瞬时重联过程．结果表明，当行星际磁场（ＩＭＦ）具有南向Ｂｚ分量和背太阳向Ｂｘ分量时，ＩＭＦ与地磁场联接，磁层顶向外扩张，在等离子体幔区可形成流体旋涡，磁力线被扭曲，但不易形成磁涡旋；当ＩＭＦ具有北向Ｂｚ分量时，不论Ｂｘ分量是背向太阳或指向太阳，都可发生瞬时重联，而且当ＩＭＦＢｘ分量与地磁场Ｂｘ分量反向时，在等离子体幔区更易形成磁涡旋．这两种情况，在磁层顶边界层区都能产生多层的电流片．