Fengsi Wei

Energetic Electrons Associated with Magnetic Reconnection in the Magnetic Cloud Boundary Layer

Here is reported in situ observation of energetic electrons (∼100-500 keV) associated with magnetic reconnection in the solar wind by the ACE and Wind spacecraft. The properties of this magnetic cloud driving reconnection and the associated energetic electron acceleration problem are discussed. Further analyses indicate that the electric field acceleration and Fermi-type mechanism are two fundamental elements in the electron acceleration processes and the trapping effect of the specific magnetic field configuration maintains the acceleration status that increases the totally gained energy.

Global distribution of coronal mass outputs and its relation to solar magnetic field structures

The basic characteristics of the coronal mass output near the Sun are analyzed with the statistic and numerical methods by using observational data from K corona brightness, interplanetary scintillation, and photospheric magnetic field during the descending phases (1983) and the minimum (1984) of solar activity. The methods used here are based on the global distribution of the solar magnetic field on the source surface (at 2.5 solar radii (R-S)). Our main results are the following: (1) There are certain regular persistent patterns in the global distributions of coronal mass outputs flux F-m (density rho x speed V), which shows that the highest F-m in 1983 and 1984 display more regularly double peaks and single-peak wave-like patterns on the source surface (2.5 R-S), respectively. The highest and the lowest F-m are associated with the coronal current sheet and the polar corona regions, respectively, and the other regions are associated with a moderate F-m. (2) The speed dependence of F-m is different for various magnetic structures. The dependence is nearly constant in the polar coronal region and monotonically rises in the current sheet regions both for the descending ( 1983) and the ascending (1976) phases. ( 3) The different frequency number distributions of F-m also correspond to different magnetic field structures, with average values (F) over bar (m,p) = 8.3 x 10(11) particles/cm(2)(s) for the polar coronal region and (F) over bar (m,c) = 17.7 x 10(11) particles/cm(2)(s) for the coronal current sheet. (4) As a theoretical test, a preliminary numerical study of the global distribution near 2.5 R-S for the Carrington rotation 1742 in 1983 has been made by solving a self-consistent MHD system based on the observations of K coronal brightness and the photospheric magnetic fields. The numerical results indicate that the global distributions of the coronal mass outputs on the source surface could be used to understand/predict the change of the interplanetary conditions.

Observations of an interplanetary slow shock associated with magnetic cloud boundary layer

The observations of the slow shocks associated with the interplanetary coronal mass ejections near 1 AU have seldom been reported in the past several decades. In this paper we report the identification of an interplanetary slow shock observed by Wind on September 18, 1997. This slow shock is found to be just the front boundary of a magnetic cloud boundary layer. A self-consistent method based on the entire R-H relations is introduced to determine the shock normal. It is found that the observations of the jump conditions across the shock are in good agreement with the R-H solutions. The intermediate Mach number M-I = U-n/(V(A)cos theta(Bn)) is less than 1 on both sides of the shock. In the upstream region, the slow Mach number M-s1 = U-n1/V-s1 is 1.44 ( above unity), and in the downstream region, the slow Mach number M-s2 = U-n2/ V-s2 is 0.8 ( below unity). Here V-s and V-A represent the slow magnetoacoustic speed and Alfven speed respectively. In addition, the typical interior magnetic structure inside the shock layer is also analyzed using the 3s time resolution magnetic field data since the time for the spacecraft traversing the shock layer is much longer ( about 17s). As a potential explanation to the formation of this kind of slow shock associated with magnetic clouds, this slow shock could be a signature of reconnection that probably occurs inside the magnetic cloud boundary layer.

Observations of reconnection exhausts associated with large-scale current sheets within a complex ICME at 1 AU

During 26-27 November 2000 a complex interplanetary coronal mass ejection, composed of four flux ropes, was detected by Wind and ACE at 1 AU. We identify two Petschek-like exhaust events within the interiors of the second and third flux ropes, respectively. In the first event, Wind and ACE detected an exhaust at the same side from the reconnection site, which was associated with a large-scale bifurcated current sheet with a spatial width of similar to 10,000 ion inertial lengths and the magnetic shear was 155 degrees. In the second event, the two spacecraft observed the oppositely directed exhausts from a single reconnection X line. The exhausts were also related to a large-scale current sheet with a spatial width of similar to 3000 ion inertial lengths and a shear angle of about 135 degrees. The two exhaust events resulted from fast and quasi-stationary reconnection. The related current sheets were both flat on the scale of a few hundred Earth radii and located close to the centers of subflux ropes. The decrease of radial expansion speed of each flux rope might account for the formation of the two current sheets. Reconnections at the centers of flux ropes may change the entire topology of the flux ropes and may fragment them into smaller ones.

Plasmoid-like structures in multiple X line Hall MHD reconnection

Driven by a waveform inflow, multiple X line reconnection is initiated in a long current layer, and the plasmoids are bound between two neighboring reconnection sites. We investigate the behaviors of the plasmoid-like structures in the absence and presence of an initial guide field B-y0 normalized by B-0 (B-0 is the initial intensity of B-x field at the top and bottom boundaries of the simulation domain) using a Hall magnetohydrodynamic (MHD) code. For the case with B-y0 = 0 the profiles of the out-of-plane B-y component are the bipolar signature or the bipolar wavelike signature which is caused by Hall effect and independent of the external mechanism. Such B-y features are in line with the observed signature of a closed-loop-like plasmoid in the magnetotail. The bipolar and fluctuation signatures of B-y have an asymmetric feature in the presence of a small B-y0(= 0.1), and the B-y profile becomes a positive signature as B-y0 reaches or exceeds 0.3. In the case of B-y0 = 0.5, a B-y bulge appears in the B-y signature when the enhanced B-y regions caused by Hall effect take place in the plasmoid. The B-y bulge evolves into a peaking signature, whose maximum (B-y vertical bar(max)) is quickly raised and approaches the lobe magnetic field strength. Such a significant enhancement of the B-y component in the central region of the plasmoid might be representative of the observed strong core field in the magnetic flux rope. The present results indicate the following implications: (1) Hall effect and a preexisting cross-tail component B-y are two important factors controlling the occurrence of various plasmoid-like structures in the magnetotail. (2) In the later phase the nonlinear interaction between Hall effect and the B-y flux added by the plasma inflow makes a most important contribution to the growth of the core B-y field.

EVIDENCE FOR NEWLY INITIATED RECONNECTION IN THE SOLAR WIND AT 1 AU

We report the first evidence for a large-scale reconnection exhaust newly initiated in the solar wind using observations from three spacecraft: ACE, Wind, and ARTEMIS P2. We identified a well-structured X-line exhaust using measurements from ARTEMIS P2 in the downstream solar wind. However, in the upstream solar wind, ACE detected the same current sheet that corresponds to the exhaust identified by ARTEMIS P2 data without showing any reconnection signals. We cannot find any reconnection signals from Wind located between ACE and ARTEMIS P2. Within the exhaust, a magnetic island is identified, which is not consistent with the quasi-steady feature as previously reported and provides further evidence that the reconnection is newly initiated. Our observations show that the entering of energetic particles, probably from Earths bow shock, makes the crucial difference between the non-reconnecting current sheet and the exhaust. Since no obvious driving factors are responsible for the reconnection initiation, we infer that these energetic particles probably play an important role in the reconnection initiation. Theoretical analysis also shows support for this potential mechanism.

Direct evidence for kinetic effects associated with solar wind reconnection

Kinetic effects resulting from the two-fluid physics play a crucial role in the fast collisionless reconnection, which is a process to explosively release massive energy stored in magnetic fields in space and astrophysical plasmas. In-situ observations in the Earths magnetosphere provide solid consistence with theoretical models on the point that kinetic effects are required in the collisionless reconnection. However, all the observations associated with solar wind reconnection have been analyzed in the context of magnetohydrodynamics (MHD) although a lot of solar wind reconnection exhausts have been reported. Because of the absence of kinetic effects and substantial heating, whether the reconnections are still ongoing when they are detected in the solar wind remains unknown. Here, by dual-spacecraft observations, we report a solar wind reconnection with clear Hall magnetic fields. Its corresponding Alfvenic electron outflow jet, derived from the decouple between ions and electrons, is identified, showing direct evidence for kinetic effects that dominate the collisionless reconnection. The turbulence associated with the exhaust is a kind of background solar wind turbulence, implying that the reconnection generated turbulence has not much developed.

VARIATIONS OF SOLAR ELECTRON AND PROTON FLUX IN MAGNETIC CLOUD BOUNDARY LAYERS AND COMPARISONS WITH THOSE ACROSS THE SHOCKS AND IN THE RECONNECTION EXHAUSTS

The magnetic cloud boundary layer (BL) is a dynamic region formed by the interaction of the magnetic cloud (MC) and the ambient solar wind. In the present study, we comparatively investigate the proton and electron mean flux variations in the BL, in the interplanetary reconnection exhaust (RE), and across the MC-driven shock by using the Wind data from 1995 to 2006. In general, the proton flux has higher increments at lower energy bands compared with the ambient solar wind. Inside the BL, the core electron flux increases quasi-isotropically and the increments decrease monotonously with energy from similar to 30% (at 18 eV) to similar to 10% (at 70 eV); the suprathermal electron flux usually increases in either parallel or antiparallel direction; the correlation coefficient of electron flux variations in parallel and antiparallel directions changes sharply from similar to 0.8 below 70 eV to similar to 0 above 70 eV. Similar results are also found for RE. However, different phenomena are found across the shock where the electron flux variations first increase and then decrease with a peak increment (>200%) near 100 eV. The correlation coefficient of electron flux variations in parallel and antiparallel directions is always around 0.8. The similar behavior of flux variations in BL and RE suggests that reconnection may commonly occur in BL. Our work also implies that the strong energy dependence and direction selectivity of electron flux variations, which were previously thought to have not enough relevance to magnetic reconnection, could be considered as an important signature of solar wind reconnection in the statistical point of view.

X-ray imaging of Kelvin-Helmholtz waves at the magnetopause

This paper simulates the Kelvin-Helmholtz wave (KHW)-induced X-ray emissions at the low-latitude magnetopause based on a global MHD code. A method is proposed to extract the KHW information from the X-ray intensity measured by a hypothetical X-ray telescope onboard a satellite assumed with a low Earth orbit. Specifically, the X-ray intensity at high latitude is subtracted from the intensity map as a background to highlight the role of KHW. Using this method, global features of KHW such as the vortex velocity, perturbation degree, spatial distribution, and temporal evolution could be evaluated from the X-ray intensity map. The validity of this method during intervals of solar wind disturbances is also verified. According to the simulation results, X-ray imaging of KHW is suggested as a promising observation technique to essentially see the large-scale configuration and evolution of KHW for the first time.

Magnetic reconnection phenomena in interplanetary space

Interplanetary magnetic reconnection(IMR) phenomena are explored based on the observational data with various time resolutions from Helios, IMP-8, ISEE3, Wind, etc. We discover that the observational evidence of the magnetic reconnection may be found in the various solar wind structures, such as at the boundary of magnetic cloud, near the current sheet, and small-scale turbulence structures, etc. We have developed a third order accuracy upwind compact difference scheme to numerically study the magnetic reconnection phenomena with high-magnetic Reynolds number (R M = 2000-10000) in interplanetary space. The simulated results show that the magnetic reconnection process could occur under the typical interplanetary conditions. These obtained magnetic reconnection processes own basic characteristics of the high R M reconnection in interplanetary space, including multiple X-line reconnection, vortex velocity structures, filament current systems, splitting, collapse of plasma bulk, merging and evolving of magnetic islands, and lifetime in the range from minutes to hours, etc. These results could be helpful for further understanding the interplanetary basic physical processes.