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.

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.

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.

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.

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.

Assessing the performance of magnetopause models based on THEMIS data

The magnetopause is the boundary between the Earth’s magnetic field and the interplanetary magnetic field(IMF),located where the supersonic solar wind and magnetospheric pressure are in balance.Although empirical models and global magnetohydrodynamic simulations have been used to define the magnetopause,each of these has limitations.In this work,we use 15 years of magnetopause crossing data from the THEMIS(Time History of Events and Macroscale Interactions during Substorms)spacecraft and their corresponding solar wind parameters to investigate under which solar wind conditions these models predict more accurately.We analyze the pattern of large errors in the extensively used magnetopause model and show the specific solar wind parameters,such as components of the IMF,density,velocity,temperature,and others that produce these errors.It is shown that(1)the model error increases notably with increasing solar wind velocity,decreasing proton density,and increasing temperature;(2)when the cone angle becomes smaller or|Bx|is larger,the Shue98 model errors increase,which might be caused by the magnetic reconnection on the dayside magnetopause;(3)when|By|is large,the error of the model is large,which may be caused by the east-west asymmetry of the magnetopause due to magnetic reconnection;(4)when Bz is southward,the error of the model is larger;and(5)the error is larger for positive dipole tilt than for negative dipole tilt and increases with an increasing dipole tilt angle.However,the global simulation model by Liu ZQ et al.(2015)shows a substantial improvement in prediction accuracy when IMF Bx,By,or the dipole tilt cannot be ignored.This result can help us choose a more accurate model for forecasting the magnetopause under different solar wind conditions.

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.

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.

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.

Ionospheric polarization electric field guiding magnetopause reconnection: A conceptual model of throat aurora

Frequently observed throat auroras have been suggested to be correspondent to indentations on the subsolar magnetopause,but how these indentations can be generated is unknown yet.Based on analyzing the detailed observational features of throat aurora,a conceptual model for generation of throat aurora is proposed.This model suggests that precipitation of a north-south aligned stripy diffuse aurora can lead to an ionospheric conductivity enhancement and thus produce a polarization electric field in dusk-to-dawn direction in the ionosphere.After mapping to the magnetosphere along closed field lines,this electric field can guide a magnetopause reconnection to develop inward the magnetosphere and result in a throat aurora.Because this model can comprehensively explain all the observational results that have been presented up to now,we argue that the assumption of ionospheric polarization electric field affecting magnetopause reconnection should be true and be worthy of further investigations.

Observations of the step-like accelerating processes of cold ions in the reconnection layer at the dayside magnetopause

Cold ions of plasmaspheric origin have been observed to abundantly appear in the magnetospheric side of the Earth's magnetopause. These cold ions could affect the magnetic reconnection processes at the magnetopause by changing the Alfvén velocity and the reconnection rate, while they could also be heated in the reconnection layer during the ongoing reconnections. We report in situ observations from a partially crossing of a reconnection layer near the subsolar magnetopause. During this crossing, step-like accelerating processes of the cold ions were clearly observed, suggesting that the inflow cold ions may be separately accelerated by the rotation discontinuity and slow shock inside the reconnection layer.

Equilibrium structure of low latitude magnetopause

A new magnetopause model is proposed based on observation results, in which the magnetic-field includes one constant component and one monotomcally increasing component, so that magnetic field shear exists during the intervals when the interplanetary magnetic field has a northward component. Under this model, a completely analytical self-consistent equilibrium distribution function is derived based on the Vlasov-Maxwell equations. Through the analysis of this distribution function and its moments, the following can be concluded: (i) The equilibrium distribution function consists of a homogeneous Maxwellian distribution and two inhomogeneous shifted-Maxwellian distributions. There are 'hole' and 'bump' structures which have different cross-field drift velocities in the velocity distribution, (ii) The density decreases monotonously from the outside of the magneto-pause to the inside, (iii) The electrical current distribution is a typical current sheet structure, (iv) Under a certain condition of temperature parameters, the thickness of the magnetopause may be larger than the ion gyroradius. These results are applicable to the region of the low-latitude magnetopause during the intervals when the interplanetary magnetic field has a northward component.

Numerical study of asymmetric driven reconnection at dayside magnetopause

A two-dimensional compressible MHD code has been used to numerically study the asymmetric driven reconnection processes in the vicinity of the magnetopause. The initial magnetic field configuration is assumed to be in a mechanical equilibrium state. The cases with identical temperatures (T m0/T s0=1.0) and four different ratios of magnetic field strength (Q=B m0/B s0=1.0, 1.5, 2.0, 2.5), and the case withT m0/T s0=2.0 andQ=1.5 are investigated (B m0,T m0 andB s0,T s0 are the initial magnetic strength and temperature outside the current sheet on the magnetosphere and the magnetosheath, respectively). When the magnetic field on the magnetosheath side is set as southward, a recurrent formation of multiple magnetic bubbles with various scales occurs under the action of the inward plasma flow imposed at the left and right boundaries. In the simulation, some bubbles coalesce into a bigger one and then it is convected out of the simulation domain; the others are convected through the top boundary all alone. Thus, the plasmoid events with different scales and different time intervals take place intermittently and the impulsive features of magnetic reconnection are clearly shown. The multiple magnetic islands are all high-temperature and large-density regions in comparison with the ambient environment. The bipolar signatures or fluctuant variations of normal magnetic field component are generated by the formation of multiple magnetic islands. This result is similar to the FTEs signature.

Magnetic Field Configuration at the Magnetopause for Different Planetary Conditions

We have simulated the processes of transient reconnection at the nightside magnetopause by using a two-dimensional compressible MHD model.According to the simulation results,three types of global reconnection configuration at the magnetopause are proposed.The main results are summarized as follows.

On the loss mechanisms of radiation belt electron dropouts during the 12 September 2014 geomagnetic storm

Radiation belt electron dropouts indicate electron flux decay to the background level during geomagnetic storms,which is commonly attributed to the effects of wave-induced pitch angle scattering and magnetopause shadowing.To investigate the loss mechanisms of radiation belt electron dropouts triggered by a solar wind dynamic pressure pulse event on 12 September 2014,we comprehensively analyzed the particle and wave measurements from Van Allen Probes.The dropout event was divided into three periods:before the storm,the initial phase of the storm,and the main phase of the storm.The electron pitch angle distributions(PADs)and electron flux dropouts during the initial and main phases of this storm were investigated,and the evolution of the radial profile of electron phase space density(PSD)and the(μ,K)dependence of electron PSD dropouts(whereμ,K,and L^*are the three adiabatic invariants)were analyzed.The energy-independent decay of electrons at L>4.5 was accompanied by butterfly PADs,suggesting that the magnetopause shadowing process may be the major loss mechanism during the initial phase of the storm at L>4.5.The features of electron dropouts and 90°-peaked PADs were observed only for>1 MeV electrons at L<4,indicating that the wave-induced scattering effect may dominate the electron loss processes at the lower L-shell during the main phase of the storm.Evaluations of the(μ,K)dependence of electron PSD drops and calculations of the minimum electron resonant energies of H+-band electromagnetic ion cyclotron(EMIC)waves support the scenario that the observed PSD drop peaks around L^*=3.9 may be caused mainly by the scattering of EMIC waves,whereas the drop peaks around L^*=4.6 may result from a combination of EMIC wave scattering and outward radial diffusion.

Earth＇s Magnetosphere Impinged by Interplanetary Shocks of Different Orientations

Using a recently developed PPMLR-MHD code, we carry out a global numerical simulation of the interaction between interplanetary shocks and Earth＇s magnetosphere. The initial magnetosphere is in a quasi-steady state, embedded in a uniform solar wind and a spiral interplanetary magnetic field （IMF）. An interplanetary （IP） shock interacts in turn with the bow shock, the magnetosheath, the magnetopause, and the magnetosphere, and changes the magnetosphere in shape and structure, and the distribution of the electric current and potential in the ionosphere as well. A preliminary comparison is made between two IP shocks of the same solar wind dynamic pressure and a vanishing IMF Bz on the downstream side, but with different propagation directions, one parallel and the other oblique to the Sun-Earth line. The numerical results show that both shocks cause a compression of the magnetosphere, an enhancement of magnetic field strength and field-aligned current in the magnetosphere, and an increase of the dawn-dusk electric potential drops across the polar ionosphere. Moreover, the magnetosphereionosphere system approaches a similar quasi-steady state after the interaction, for the downstream states are very close for the two shocks. However, the evolution processes of the system are remarkably different during the interaction with the two shocks of different orientations. The shock with the normal oblique to the Sun-Earth line results in a much longer evolution time for the system. This demonstrates that the shock orientation plays an important role in determining the associated geophysical effects and interpreting multisatellite observations of IP shock-magnetosphere interaction events.

MAGNETOSPHERIC PHYSICS

Magnetospheric physics has been one of the most active areas in Chinese space research in past two years. The major project "Energy Transport Processes in the Solar-Terrestrial System" (1993-1997) sponsored by the National Natural Science Foundation in China (NSFC) has been successfully completed. Prestudies relevant to the key scientific engineering program "Meridian Chain at One Hundred Twenty Degree East Multi-Station and Multi-Instrument Observatory System" have started. A new key project "Study of Auroral Magnetospheric and Ionospheric Physics" (1997-1999) sponsored by the NSFC has begun. The Space Active Experiment Program has been carrying on further.Collaborations between Chinese and international magnetospheric physicists have proceeded forward. More than 40 papers covering a variety of subjects in the magnetospheric physics were published in Chinese and international academic journals. Most of these works were supported by the NSFC. This report provides a brief summary of aforementioned advances made in China in the past two years.

New method for determining central axial orientation of flux rope embedded within current sheet using multipoint measurements

A new method for determining the central axial orientation of a two-dimensional coherent magnetic flux rope(MFR)via multipoint analysis of the magnetic-field structure is developed. The method is devised under the following geometrical assumptions:(1) on its cross section, the structure is left-right symmetric;(2) the projected structure velocity is vertical to the line of symmetry. The two conditions can be naturally satisfied for cylindrical MFRs and are expected to be satisfied for MFRs that are flattened within current sheets. The model test demonstrates that, for determining the axial orientation of such structures, the new method is more efficient and reliable than traditional techniques such as minimum-variance analysis of the magnetic field,Grad-Shafranov(GS) reconstruction, and the more recent method based on the cylindrically symmetric assumption. A total of five flux transfer events observed by Cluster are studied using the proposed approach, and the application results indicate that the observed structures, regardless of their actual physical properties, fit the assumed geometrical model well. For these events, the inferred axial orientations are all in excellent agreement with those obtained using the multi-GS reconstruction technique.

Multiple magnetic topologies in flux transfer events: THEMIS measurements

Flux transfer events （FTEs） are local transient magnetic reconnections at the magnetopause （MP） that provide channels for transport of solar wind energy and plasma into the magnetosphere （MSP）. All current theoretical models suggest that FTEs are open-flux ropes; however, global simulations show that they contain both open and closed magnetic fields. To clarify this to- pology, we analyzed 441 events observed by THEMIS and investigated their magnetic topologies. Only one type of open field line was detected in most magnetosheath （MSH） FTEs, independent of the polarity of the Bn bipolar signatures. Newly formed MSH field lines were also observed. In the all MP boundary layers FTEs and most MSP FTEs, multiple types of topologies were observed, irrelevant to the Bn bipolar polarity. Closed field lines were found in all MP boundary layers and MSP FTEs. Meanwhile very few boundary FTEs contained the newly formed MSH flux. In some situations, only closed field lines were seen in MSP FTEs, which are referred to as the fossil FTEs. These results, which largely differ from the traditional view, demonstrate the existence of complex magnetic topologies in FTEs. Based on these results, we propose a new 3D FTE picture to modify the current FTE models.