分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Utilizing multipoint observations by the Cluster satellites, we investigated the ion distributions of the fast bulk flows (FBFs) in the plasma sheet. Simultaneous observation by C1 and C3 revealed that parallel-dominant and perpendicular-dominant components of the flows coexist and correspond to B-x-dominant and B-z-dominant magnetic field regions within the FBFs, respectively. In both cases, the ions distributions are characterized by a single-beam/crescent shape. In particular, no reflected ions are found within the FBFs. Statistical analysis showed that within the FBFs, the strength of the B-x component is typically less than 5 nT for B-z-dominant regions and above 10 nT for B-x-dominant regions. To distinguish between the parallel-dominant component of the FBFs and the field-aligned beams in the plasma sheet boundary layer (PSBL), we further statistically analyzed the tailward parallel flows (TPF) with positive B-z in the plasma sheet. The results indicated that the FBFs tend to have higher velocity, weaker B, and higher magnetic tilt angle (theta(MTA)) than the TPFs/PSBL beams. Statistically, in the region of B > 30 nT (theta(MTA) > 10 degrees), only PSBL beams can be observed, while in the region of B 30 degrees), the FBFs are dominant. In the intermediate region (10 degrees < theta(MTA) < 30 degrees) of the plasma sheet, the FBFs and the PSBL beams cooccur. These Cluster observations suggest that the X line can produce both perpendicular flow in central plasma sheet and parallel flow in the PSBL. In addition, the parallel-dominant component of the FBFs could be an important origin for the PSBL beams.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Soil Moisture and Ocean Salinity (SMOS) Level 3 (L3) sea surface salinity (SSS) products are provided by the Barcelona Expert Centre (BEC). Strong biases were observed on the SMOS SSS products, thus the data from the Centre Aval de Traitement des Donn,es SMOS (CATDS) were adjusted for biases using a large-scale correction derived from observed differences between the SMOS SSS and World Ocean Atlas (WOA) climatology data. However, this large-scale correction method is not suitable for correcting the large gradient of salinity biases. Here, we present a method for the correction of SSS regional bias of the monthly L3 products. Based on the stable characteristics of the large SSS biases from month to month in some regions, corrected SMOS SSS maps can be obtained from the monthly mean values after removing the regional biases. The accuracy of the SMOS SSS measurements is greatly improved, especially near the coastline, at high latitudes, and in some open ocean regions. The SMOS and ISAS SSS data are also compared with Aquarius SSS to verify the corrected SMOS SSS data. The correction method presented here only corrects annual mean biases. The measurement accuracy of the SSS may be improved by considering the influence of atmospheric and ocean circulation in different seasons and years.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Turbulence is a chaotic flow regime filled by irregular flows. The dissipation of turbulence is a fundamental problem in the realm of physics. Theoretically, dissipation ultimately cannot be achieved without collisions, and so how turbulent kinetic energy is dissipated in the nearly collisionless solar wind is a challenging problem. Wave particle interactions and magnetic reconnection (MR) are two possible dissipation mechanisms, but which mechanism dominates is still a controversial topic. Here we analyze the dissipation region scaling around a solar wind MR region. We find that the MR region shows unique multifractal scaling in the dissipation range, while the ambient solar wind turbulence reveals a monofractal dissipation process for most of the time. These results provide the first observational evidences for intermittent multifractal dissipation region scaling around a MR site, and they also have significant implications for the fundamental energy dissipation process.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
Liu, Ying D.
Hu, Huidong
Wang, Rui
Yang, Zhongwei
Zhu, Bei
Liu, Yi A.
Luhmann, Janet G.
Richardson, John D.
摘要： The largest geomagnetic storms of solar cycle 24 so far occurred on 2015 March 17 and June 22 with D-st minima of -223 and -195 nT, respectively. Both of the geomagnetic storms show a multi-step development. We examine the plasma and magnetic field characteristics of the driving coronal mass ejections (CMEs) in connection with the development of the geomagnetic storms. A particular effort is to reconstruct the in situ structure using a Grad-Shafranov technique and compare the reconstruction results with solar observations, which gives a larger spatial perspective of the source conditions than one-dimensional in situ measurements. Key results are obtained concerning how the plasma and magnetic field characteristics of CMEs control the geomagnetic storm intensity and variability: (1) a sheath-ejecta-ejecta mechanism and a sheath-sheath-ejecta scenario are proposed for the multi-step development of the 2015 March 17 and June 22 geomagnetic storms, respectively;(2) two contrasting cases of how the CME flux-rope characteristics generate intense geomagnetic storms are found, which indicates that a southward flux-rope orientation is not a necessity for a strong geomagnetic storm;and (3) the unexpected 2015 March 17 intense geomagnetic storm resulted from the interaction between two successive CMEs plus the compression by a high-speed stream from behind, which is essentially the "perfect storm" scenario proposed by Liu et al. (i.e., a combination of circumstances results in an event of unusual magnitude), so the "perfect storm" scenario may not be as rare as the phrase implies.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： MHD discontinuities are ubiquitous in the solar wind and are often found at the origin of turbulence intermittency. They may also play a key role in the turbulence dissipation and heating of the solar wind. The tangential discontinuities (TDs) and rotational discontinuities (RDs) are the two most important types of discontinuities. Recently, the connection between turbulence intermittency and proton thermodynamics has been observationally investigated. Here, we present numerical results from a three-dimensional MHD simulation with pressure anisotropy and we define new methods for identifying and distinguishing TDs and RDs. Three statistical results obtained for the relative occurrence rates and heating effects are highlighted: (1) RDs tend to take up the majority of the discontinuities along with time;(2) the thermal states embedding TDs tend to be associated with extreme plasma parameters or instabilities while RDs do not;(3) TDs have a higher average T as well as perpendicular temperature T.. The simulation shows that TDs and RDs evolve and contribute to solar wind heating differently. These results will improve our understanding of the mechanisms that generate discontinuities and cause plasma heating.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： We present high-resolution observations of a quiescent solar prominence that consists of a vertical and a horizontal foot encircled by an overlying spine and has ubiquitous counter-streaming mass flows. While the horizontal foot and the spine were connected to the solar surface, the vertical foot was suspended above the solar surface and was supported by a semicircular bubble structure. The bubble first collapsed, then reformed at a similar height, and finally started to oscillate for a long time. We find that the collapse and oscillation of the bubble boundary were tightly associated with a flare-like feature located at the bottom of the bubble. Based on the observational results, we propose that the prominence should be composed of an overlying horizontal spine encircling a low-lying horizontal and vertical foot, in which the horizontal foot consists of shorter field lines running partially along the spine and has ends connected to the solar surface, while the vertical foot consists of piling-up dips due to the sagging of the spine fields and is supported by a bipolar magnetic system formed by parasitic polarities (i.e., the bubble). The upflows in the vertical foot were possibly caused by the magnetic reconnection at the separator between the bubble and the overlying dips, which intruded into the persistent downflow field and formed the picture of counter-streaming mass flows. In addition, the counter-streaming flows in the horizontal foot were possibly caused by the imbalanced pressure at the both ends.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Mass is one of the most fundamental parameters characterizing the dynamics of a coronal mass ejection (CME). It has been found that CME apparent mass measured from the brightness enhancement in coronagraphs increases during its evolution in the corona. However, the physics behind it is not clear. Does the apparent mass gain come from the outflow from the dimming regions in the low corona, or from the pileup of the solar wind plasma around the CME? Here we analyze the mass evolution of six CME events. Based on the coronagraph observations from the Solar Terrestrial Relations Observatory, we find that their masses increased by a factor of 1.3-1.7 from 7 to 15 R-S, where the occulting effect is negligible. We then adopt the "snow-plow" model to calculate the mass contribution of the piled-up solar wind. The result gives evidence that the solar wind pileup probably makes a non-negligible contribution to the mass increase. In the height range from about 7 to 15 R-S, the ratio of the modeled to the measured mass increase is roughly larger than 0.55 though the ratios are believed to be overestimated. It is not clear yet whether the solar wind pileup is a major contributor to the final mass derived from coronagraph observations, but it does play an increasingly important role in the mass increase as a CME moves further away from the Sun.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Using a 2.5-dimensional MHD simulation, we investigate the role played by the inner coronal null point in the formation and evolution of solar quiescent prominences. The flux rope is characterized by its magnetic fluxes, the toroidal magnetic flux Phi(p) and the poloidal flux Phi(phi.) It is found that for a given Phi(p), the catastrophe does not occur in the flux rope system until Phi(phi) increases to a critical point. Moreover, the magnetic flux of the null point is the maximum value of the magnetic flux in the quadrupole background magnetic field, and represented by psi(N). The results show that the bigger psi(N) usually corresponds to the smaller catastrophic point, the lower magnetic energy of the flux rope system, and the lesser magnetic energy inside the flux rope. Our results confirm that catastrophic disruption of the prominence occurs more easily when there is a bigger psi(N). However, psi(N) has little influence on the maximum speed of the coronal mass ejections (CMEs) with an erupted prominence. Thus we argue that a topological configuration with the inner coronal null point is a necessary structure for the formation and evolution of solar quiescent prominences. In conclusion, it is easier for the prominences to form and to erupt as a core part of the CMEs in the magnetic structure with a greater psi(N.)
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： The superthermal ions at a quasi-parallel collisionless shock are considered to be generated during the reformation of the shock. Recently, hybrid simulations of a quasi-parallel shock have shown that during the reformation of a quasi-parallel shock the large-amplitude upstream low-frequency waves can trap the reflected ions at the shock front when they try to move upstream, and then these reflected ions can be accelerated several times to become superthermal ions. In this paper, with the Cluster observations of a quasi-parallel shock event, the relevance between the large-amplitude upstream low-frequency waves and the superthermal ions (about several keV) have been studied. The observations clearly show that the differential energy flux of superthermal ions in the upstream region is modulated by the upstream low-frequency waves, and the maxima of the differential energy flux are usually located between the peaks of these waves (including the shock front and the peak of the upstream wave just in front of the shock front). These superthermal ions are considered to originate from the reflected ions at the shock front, and the modulation is caused due to the trapping of the reflected ions between the upstream waves or the upstream waves and the shock front when these reflected ions try to travel upstream. It verifies the results from hybrid simulations, where the upstream waves play an important role in the generation of superthermal ions in a quasi-parallel shock.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： We study the role of the coronal magnetic field configuration of an active region (AR) in determining the propagation direction of a coronal mass ejection (CME). The CME occurred in the AR 11944 (S09W01) near the disk center on 2014 January 7 and was associated with an X1.2 flare. A new CME reconstruction procedure based on a polarimetric technique is adopted, which shows that the CME changed its propagation direction by around 28 degrees in latitude within 2.5 R-circle dot and 43 degrees in longitude within 6.5 R-circle dot with respect to the CME source region. This significant non-radial motion is consistent with the finding of Mostl et al. We use nonlinear force-free field and potential field source surface extrapolation methods to determine the configurations of the coronal magnetic field. We also calculate the magnetic energy density distributions at different heights based on the extrapolations. Our results show that the AR coronal magnetic field has a strong influence on the CME propagation direction. This is consistent with the "channeling" by the AR coronal magnetic field itself, rather than deflection by nearby structures. These results indicate that the AR coronal magnetic field configuration has to be taken into account in order to determine CME propagation direction correctly.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
Yang, Liping
Zhang, Lei
He, Jiansen
Tu, Chuanyi
Wang, Linghua
Marsch, Eckart
Wang, Xin
Zhang, Shaohua
Feng, Xueshang
摘要： Measurements of solar wind turbulence reveal the ubiquity of discontinuities. In this study we investigate how the discontinuities, especially rotational discontinuities (RDs), are formed in MHD turbulence. In a simulation of the decaying compressive three-dimensional (3D) MHD turbulence with an imposed uniform background magnetic field, we detect RDs with sharp field rotations and little variations of magnetic field intensity, as well as mass density. At the same time, in the de Hoffman-Teller frame, the plasma velocity is nearly in agreement with the Alfven speed, and is field-aligned on both sides of the discontinuity. We take one of the identified RDs to analyze its 3D structure and temporal evolution in detail. By checking the magnetic field and plasma parameters, we find that the identified RD evolves from the steepening of the Alfven wave with moderate amplitude, and that steepening is caused by the nonuniformity of the Alfven speed in the ambient turbulence.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： In this investigation, we first present a statistical result of the interplanetary sources of very strong solar wind dynamic pressure pulses (DPPs) detected by WIND during solar cycle 23. It is found that the vast majority of strong DPPs reside within solar wind disturbances. Although the variabilities of geosynchronous magnetic fields (GMFs) due to the impact of positive DPPs have been well established, there appears to be no systematic investigations on the response of GMFs to negative DPPs. Here, we study both the decompression effects of very strong negative DPPs and the compression from strong positive DPPs on GMFs at different magnetic local time sectors. In response to the decompression of strong negative DPPs, GMFs on the dayside near dawn and near dusk on the nightside, are generally depressed. But near the midnight region, the responses of GMF are very diverse, being either positive or negative. For part of the events when GOES is located at the midnight sector, the GMF is found to abnormally increase as the result of magnetospheric decompression caused by negative DPPs. It is known that under certain conditions magnetic depression of nightside GMFs can be caused by the impact of positive DPPs. Here, we find that a stronger pressure enhancement may have a higher probability of producing the exceptional depression of GMF at the midnight region. Statistically, both the decompression effect of strong negative DPPs and the compression effect of strong positive DPPs depend on the magnetic local time, which are stronger at the noon sector.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
Kilpua, E. K. J.
Olspert, N.
Grigorievskiy, A.
Kapyla, M. J.
Tanskanen, E. I.
Miyahara, H.
Kataoka, R.
Pelt, J.
Liu, Y. D.
摘要： We study the relation between strong and extreme geomagnetic storms and solar cycle characteristics. The analysis uses an extensive geomagnetic index AA data set spanning over 150 yr. complemented by the Kakioka magnetometer recordings. We apply Pearson correlation statistics and estimate the significance of the correlation with a bootstrapping technique. We show that the correlation between the storm occurrence and the strength of the solar cycle decreases from a clear positive correlation with increasing storm magnitude toward a negligible relationship. Hence, the quieter Sun can also launch superstorms that may lead to significant societal and economic impact. Our results show that while weaker storms occur most frequently in the declining phase, the stronger storms have the tendency to occur near solar maximum. Our analysis suggests that the most extreme solar eruptions do not have a direct connection between the solar large-scale dynamo-generated magnetic field, but are rather associated with smaller-scale dynamo and resulting turbulent magnetic fields. The phase distributions of sunspots and storms becoming increasingly in phase with increasing storm strength, on the other hand, may indicate that the extreme storms are related to the toroidal component of the solar large-scale field.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Standing sausage modes in flare loops are important for interpreting quasi-periodic pulsations(QPPs) in solar flare light curves. We propose an inversion scheme that consistently uses their periods P and damping times tau to diagnose flare loop parameters. We derive a generic dispersion relation governing linear sausage waves in pressure-less straight tubes, for which the transverse density inhomogeneity takes place in a layer of arbitrary width l and is of arbitrary form. We find that P and tau depend on the combination of [R/nu(Ai), L/R, l/R, rho(i)/rho(e)], where R is the loop radius, L is the looplength, nu(Ai) is the internal Alfven speed, and rho(i)/rho(e) is the density contrast. For all the density profiles examined, P and tau experience saturation when L/R >> 1, yielding an inversion curve in the [R/nu(Ai), l/R, rho(i)/rho(e)] space with a specific density profile when L/R is sufficiently large. When applied to a spatially unresolved QPP event, the scheme yields that R/nu(Ai) is the best constrained, whereas l/R corresponds to the other extreme. For spatially resolved QPPs, while L/R >> 1 cannot be assumed beforehand, an inversion curve remains possible due to additional geometrical constraints. When a spatially resolved QPP event involves another mode, as is the case for a recent event, the full set of [nu(Ai), l, rho(i)/rho(e)] can be inferred. We conclude that the proposed scheme provides a useful tool for magneto-seismologically exploiting QPPs.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： The spatial and temporal invariance in the spectra of energetic particles in gradual solar events is reproduced in simulations. Based on a numerical solution of the focused transport equation, we obtain the intensity time profiles of solar energetic particles (SEPs) accelerated by an interplanetary shock in three-dimensional interplanetary space. The shock is treated as a moving source of energetic particles with a distribution function. The time profiles of particle fluxes. with different energies are calculated in the ecliptic at 1 AU. According to our model, we find that shock acceleration strength, parallel diffusion, and adiabatic cooling are the main factors in forming the spatial invariance in SEP spectra, and perpendicular diffusion is a secondary factor. In addition, the temporal invariance in SEP spectra is mainly due to the effects of adiabatic cooling. Furthermore, a spectra invariant region, which agrees with observations but is different from. the one suggested by Reames et al. is proposed based on our simulations.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： In this work, a gradual solar energetic particle (SEP) event observed by multi-spacecraft has been simulated. The time profiles of SEP fluxes accelerated by an interplanetary shock in the three-dimensional interplanetary space are obtained by solving numerically the Fokker-Planck focused transport equation. The interplanetary shock is modeled as a moving source of energetic particles. By fitting the 1979 March 01 SEP fluxes observed by Helios 1, Helios 2, and IMP 8 with our simulations, we obtain the best parameters for the shock acceleration efficiency model. And we also find that the particle perpendicular diffusion coefficient with the level of similar to 1%-3% of parallel diffusion coefficient at 1 AU should be included. The reservoir phenomenon is reproduced in the simulations, and the longitudinal gradient of SEP fluxes in the decay phase, which is observed by three spacecraft at different locations, is more sensitive to the shock acceleration efficiency parameters than that is to the perpendicular diffusion coefficient.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
Yang, Liping
Zhang, Lei
He, Jiansen
Peter, Hardi
Tu, Chuanyi
Wang, Linghua
Zhang, Shaohua
Feng, Xueshang
摘要： The Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory has directly imaged the fast-propagating magnetosonic waves (FMWs) successively propagating outward along coronal magnetic funnels. In this study we perform a numerical investigation of the excitation of FMWs in the interchange reconnection scenario, with footpoint shearing flow being used to energize the system and drive the reconnection. The modeling results show that as a result of magnetic reconnection, the plasma in the current sheet is heated up by Joule dissipation to similar to 10 MK and is ejected rapidly, developing the hot outflows. Meanwhile, the current sheet is torn into plasmoids, which are shot quickly both upward and downward. When the plasmoids reach the outflow regions, they impact and collide with the ambient magnetic field there, which consecutively launches FMWs. The FMWs propagate outward divergently away from the impact regions, with a phase speed of the Alfven speed of similar to 1000 km s(-1). In the k-omega. diagram of the Fourier wave power, the FMWs display a broad frequency distribution with a straight ridge that represents the dispersion relation. With the WKB approximation, at the distance of 15Mm from the wave source region, we estimate the energy flux of FMWs to be E similar to 7.0 x 10(6) erg cm(-2) s(-1), which is similar to 50 times smaller than the energy flux related to the tube-channeled reconnection outflow. These simulation results indicate that energetically and dynamically the outflow is far more important than the waves.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Predicting the arrival times of coronal mass ejections (CMEs) and their related waves at Earth is an important aspect of space weather forecasting. The Shock Propagation Model (SPM) and its updated version (SPM2), which use the initial parameters of solar flare-Type II burst events as input, have been developed to predict the shock arrival time. This paper continues to investigate the influence of solar disturbances and their associated CMEs on the corresponding interplanetary (IP) shock's arrival at Earth. It has been found that IP shocks associated with wider CMEs have a greater probability of reaching the Earth, and the CME speed obtained from coronagraph observations can be supplementary to the initial shock speed computed from Type II radio bursts when predicting the shock's arrival time. Therefore, the third version of the model, i.e., SPM3, has been developed based on these findings. The new version combines the characteristics of solar flare-Type II events with the initial parameters of the accompanying CMEs to provide the prediction of the associated IP shock's arrival at Earth. The prediction test for 498 events of Solar Cycle 23 reveals that the prediction success rate of SPM3 is 70%-71%, which is apparently higher than that of the previous SPM2 model (61%-63%). The transit time prediction error of SPM3 for the Earth-encountered shocks is within 9 hr (mean-absolute). Comparisons between SPM3 and other similar models also demonstrate that SPM3 has the highest success rate and best prediction performance.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Voyager 2 (V2) observed multiple crossings of the heliospheric termination shock (TS) on 2007 August 31-September 1 at a distance of 84 AU from the Sun. Here, for the first time, we present two-dimensional particle-incell (PIC) simulations of the TS self-consistently including pickup ions (PUIs), and compare the simulation results with V2 observations. We find that (1) PUIs play a key role in the energy dissipation of the TS, and most of the incident ion kinetic energy is transferred to the thermal energy of PUIs. The PIC simulation indicates that, for the upstream parameters chosen for V2 conditions, the density of PUIs is about 25% and the PUIs gain the largest fraction (approximately 86.6%) of downstream thermal pressure. (2) The simulated heliosheath ion distribution function is a superposition of a cold core formed by transmitted solar wind ions (SWIs), with the shoulders contributed by the hot reflected SWIs and directly transmitted PUIs, and the wings of the distribution dominated by the very hot reflected PUIs. The V2 Faraday cups observed the cool core of the distribution, and so they only saw the tip of the iceberg. (3) The nonstationarity of the shock front is mainly caused by ripples along the shock front which form even if the percentage of PUIs is high. These simulation results agree reasonably well with the V2 experimental data. The relevance of the shock front ripples to the multiple TS crossings observed by V2 is also discussed in this paper.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： This paper investigates the onset time of solar energetic particle (SEP) events with numerical simulations and analyzes the accuracy of the velocity dispersion analysis (VDA) method. Using a three-dimensional focused transport model, we calculate the fluxes of protons observed in the ecliptic at 1 AU in the energy range between 10 MeV and 80 MeV. In particular, three models are used to describe different SEP sources produced by flare or coronal shock, and the effects of particle perpendicular diffusion in the interplanetary space are also studied. We have the following findings. When the observer is disconnected from the source, the effects of perpendicular diffusion in the interplanetary space and particles propagating in the solar atmosphere have a significant influence on the VDA results. As a result, although the VDA method is valid with impulsive source duration, low background, and weak scattering in the interplanetary space or fast diffusion in the solar atmosphere, the method is not valid with gradual source duration, high background, or strong scattering.
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