Phase Space Density Analysis of Energy Transport in the Earth’s Magnetotail

Two energetic events in the Earth’s magnetotail detected by Geotail are examined with detailed analysis of three-dimensional velocity phase space density. It is found that the occurrence of multiple ion components is high during these dynamic episodes. Different populations evolve independently of each other, suggesting particles from multiple activity sites contributing to the observed phase space density. The transport properties with consideration of multiple components are evaluated, with the result showing significant differences from those based on a single fluid approach. This comparison indicates that precise evaluation of the energy and magnetic flux transport of energetic events in the magnetotail requires resolving individual populations in the phase space density.     

Cluster observations showing the indication of the formation of a modified-two-stream instability in the geomagnetic tail

This study presents several observations of the Cluster spacecraft on September 24, 2003 around 15:10 UT, which show necessary prerequisites and consequences for the formation of the so-called modified-two-stream instability (MTSI). Theoretical studies suggest that the plasma is MTSI unstable if (1) a relative drift of electrons and ions is present, which exceeds the Alfvèn speed, and (2) this relative drift or current is in the cross-field direction. As consequences of the formation of a MTSI one expects to observe (1) a field-aligned electron beam, (2) heating of the plasma, and (3) an enhancement in the B-wave spectrum at frequencies in the range of the lower-hybrid-frequency (LHF). In this study we use prime parameter data of the CIS and PEACE instruments onboard the Cluster spacecraft to verify the drift velocities of ions and electrons, FGM data to calculate the expected LHF and Alfvèn velocity, and the direction of the current. The B-wave spectrum is recorded by the STAFF instrument of Cluster. Finally, a field aligned beam of electrons is observed by 3D measurements of the IES instrument of the RAPID unit. Observations are verified using a theoretical model showing the build-up of a MTSI under the given circumstances.     

Do we understand the inner magnetotail dynamics during substorms?

Substorm evolution of the near-Earth (|X|<15 R_E) plasma sheet has been emphasized recently because the inner tail is thought to link closely to the substorm auroral activity in the ionosphere during the early stage of substorms. In this paper, we discuss how the inner tail substorm phenomena during the late substorm growth phase and early expansion phase are accounted for by the two prevailing substorm models, namely, the near-Earth neutral line model and the current disruption model. We find that the late growth phase features are more satisfactorily accounted for by the current disruption model than by the near-Earth neutral line model. In addition, detailed observations on current disruption show evidence inconsistent with the proposed idea of dipolarization being due to plasma flow braking from reconnection in the mid-tail region, which poses a difficulty to the near-Earth neutral line model as well.     

Controlling factors of Region 2 field-aligned current and its relationship to the ring current: Model results

One essential component of magnetosphere and ionosphere coupling is the closure of the ring current through Region 2 field-aligned current (FAC). Using the Comprehensive Ring Current Model (CRCM), which includes magnetosphere and ionosphere coupling by solving the kinetic equation of ring current particles and the closure of the electric currents between the two regions, we have investigated the effects of high latitude potential, ionospheric conductivity, plasma sheet density and different magnetic field models on the development of Region 2 field-aligned currents, and the relationship between R2 FACs and the ring current. It is shown that an increase in high latitude potential, ionospheric conductivity or plasma sheet density generally results in an increase in Region 2 FACs’ intensity, but R2 FACs display different local time and latitudinal distributions for changes in each parameter due to the different mechanisms involved. Our simulation results show that the magnetic field configuration of the inner magnetosphere is also an important factor in the development of Region 2 field-aligned current. More numerical experiments and observational results are needed in further our understanding of the complex relationship of the two current systems.     

Grad-Shafranov Reconstruction of Magnetic Flux Ropes in the Near-Earth Space

Electric currents permeate space plasmas and often have a significant component along the magnetic field to form magnetic flux ropes. A larger spatial perspective of these structures than from the direct observation along the satellite path is crucial in visualizing their role in plasma dynamics. For magnetic flux ropes that are approximately two-dimensional equilibrium structures on a certain plane, Grad-Shafranov reconstruction technique, developed by Bengt Sonnerup and his colleagues (see Sonnerup et al. in J. Geophys. Res. 111:A09204, 2006), can be used to reveal two-dimensional maps of associated plasma and field parameters. This review gives a brief account of the technique and its application to magnetic flux ropes near the Earth’s magnetopause, in the solar wind, and in the magnetotail. From this brief survey, the ranges of the total field-aligned current and the total magnetic flux content for these magnetic flux ropes are assessed. The total field-aligned current is found to range from ∼0.14 to ∼9.7×10⁴ MA, a range of nearly six orders of magnitude. The total magnetic flux content is found to range from ∼0.25 to ∼2.3×10⁶ MWb, a range of nearly seven orders of magnitude. To the best of our knowledge, this review reports the largest range of both the total field-aligned current and the total magnetic flux content for magnetic flux ropes in space plasmas.     

高硬韧材料切削温度的解析预测

以传热学为基础，用有限差分数值方法，二元切削加工过程中切削区域温度场进行了计算机模拟，并以金刚石和硬质合金刀具切削钛合金炙例，进行了切削温度计算，经分析，计算结果与实测切削温度值吻合良好。这不但表明切削温度的计算机模拟可行的，同时人加工材料的切削加工特性提供了一种新的解析方法，可节省大量实验，为进一步预测最佳切削过程，指导新型刀具材料的开发奠定了基础。     

Statistical properties of the IMF clock angle in the solar wind with northward and southward interplanetary magnetic field based on ACE observation from 1998 to 2009: Dependence on the temporal scale of the solar wind

Utilizing ACE satellite observations from 1998 to 2009, we performed the elaborate study on the properties of the clock angle θ_CA (arctan(By/Bz) (?90° to 90°) of the interplanetary magnetic field (IMF) in the solar wind at 1?AU. The solar wind with northward IMF (NW-IMF) and southward IMF (SW-IMF) are analyzed, independently. Statistical analysis shows that the solar wind with SW-IMF and NW-IMF has similar properties in general, including their durations, the IMF Bz and By components, and the IMF θ_CA. Then, the solar wind with NW-IMF (SW-IMF) is classified into five different temporal scales according to the duration of the NW-IMF (SW-IMF), i.e., very-short wind of 10–30?min, short-scale wind of 0.5–1?h, moderate-scale wind of 1–3?h, long-scale wind of 3–5?h, and super-long wind >5?h. Our analysis reveals that the IMF θ_CA has a distinct decrease with increase of the temporal scale of the solar wind. Next, the solar wind is classified into two groups, i.e., the high-speed solar wind (>450?km/s) and the low-speed solar wind (<450?km/s). Our analysis indicates that the IMF θ_CA depends highly on the solar wind speed. Statistically, high-speed solar wind tends to have larger IMF θ_CA than low-speed solar wind. The evolutions of the solar wind and IMF with the solar activity are further studied, revealing no clear solar variation of the IMF θ_CA. Finally, we analyze the monthly variation of the IMF θ_CA. Superposed epoch result strongly suggests the seasonal variation of the IMF θ_CA.     

Evaluation of substorm models with Cluster observations of plasma flow reversal in the magnetotail

We evaluate two prevailing substorm models with an event of plasma flow reversal from tailward to Earthward detected by Cluster at the downstream distance of ∼19 R_E in the magnetotail during a substorm on August 22, 2001. We use the unique capability of Cluster measurements in determining gradients to examine the associated current density, Lorentz force, and current dissipation/dynamo term. In association with plasma flow reversal, it is found that (1) there was no clear quadrupole magnetic perturbation signature, (2) the x-component of the Lorentz force did not change sign, (3) the y-component of the product of the current density and the electric field was occasionally negative indicative of a dynamo effect, and (4) the timing sequence of flow reversal from the Cluster configuration did not match tailward motion of a single plasma flow source. These observations are consistent with the near-Earth initiation model for substorms with multiple current disruption sites moving progressively tailward near the late stage of substorm expansion.     

A Multiscale Model for Substorms

Most substorm researchers assume substorms to be caused by a unique large-scale process. However, a critical evaluation of substorm observations indicates that a new paradigm is needed to understand the substorm phenomenon and the magnetospheric dynamics in general. It is proposed here that substorms involve a number of physical processes covering over a wide range of spatial and temporal scales. Potential candidates include the kinetic or shear ballooning instability, the Kelvin-Helmholtz instability, the cross-field current instability, the tearing instability, and magnetic reconnection. An observational constraint on the qualified process for substorm onset is that it must be associated with magnetic field lines of auroral arcs since substorm onsets start with brightening of a pre-existing auroral arc. Which particular process dominates in a given substorm depends on the present and past states of the magnetosphere as well as the external solar wind. The magnetosphere is almost perpetually driven by the solar wind to be near a critical point and in a metastable state. Magnetospheric disturbances occur sporadically in multiple localized sites. A substorm is realized when the combined effect of these localized disturbances become global in extent, much like the system-wide activity in a sandpile or avalanche model.