Flux transfer events (FTEs) are magnetic flux ropes that are produced via magnetic reconnection at the planetary magnetopause where the solar wind directly interacts with the magnetosphere. Previous ...observations show that FTEs with a duration of several seconds, corresponding to a spatial scale of ∼0.5-1 RM, can occur at Mercury. However, the formation of these macroscale FTEs at a small dimensional magnetopause with a radius of ∼1.5 RM remains unclear. Here, we report the observations of active magnetic reconnection events at Mercury's magnetopause by the MESSENGER spacecraft. The reconnection process is dominated by the formation of a series of multi-scale FTEs. Ion-scale flux ropes, typically with durations of ∼1 s or less, may be produced by the tearing instability in the thin current sheet near the subsolar position. Moreover, the commonly observed macroscale FTEs consist of three to tens of successive small-scale FTEs. We propose that macroscale FTEs at Mercury are generated by the interaction and merging of multiple ion-scale flux ropes, probably through two or more steps. This is distinct from the formation of typical FTEs, mainly between a pair of X-lines, at Earth's magnetopause. Thus, the formation and evolution of FTEs may differ among planetary magnetospheres with a vast range of scale sizes. We further conclude that Mercury's magnetopause is a natural plasma laboratory to study flux rope dynamics and evolution for the upcoming Bepi-Colombo mission.
Sudden impulses (SIs) are an important source of ultra low frequency (ULF) wave activity throughout the Earth's magnetosphere. Most SI‐induced ULF wave events have been reported in the dayside ...magnetosphere; it is not clear when and how SIs drive ULF wave activity in the nightside plasma sheet. We examined the ULF response of the nightside plasma sheet to SIs using an ensemble of 13 SI events observed by THEMIS (Timed History of Events and Macroscale Interactions during Substorms) satellites (probes). Only three of these events resulted in ULF wave activity. The periods of the waves are found to be 3.3, 6.0, and 7.6 min. East‐west magnetic and radial electric field perturbations, which typically indicate the toroidal mode, are found to be stronger and can have phase relationships consistent with standing waves. Our results suggest that the two largest‐amplitude ULF responses to SIs in the nightside plasma sheet are tailward‐moving vortices, which have previously been reported, and the dynamic response of cross‐tail currents in the magnetotail to maintain force balance with the solar wind, which has not previously been reported as a ULF wave driver. Both mechanisms could potentially drive standing Alfvén waves (toroidal modes) observed via the field‐line resonance mechanism. Furthermore, both involve frequency selection and a preference for certain driving conditions that can explain the small number of ULF wave events associated with SIs in the nightside plasma sheet.
Key Points
In 3 of 13 SI events, steady ULF wave is induced in the nightside plasma sheet
Field perturbations are usually consistent with standing toroidal Alfven waves
Tailward moving vortices or dynamic response of tail currents may explain
How magnetic reconnection drives Mercury's magnetospheric dynamics under extreme solar wind conditions is not well understood. Here we report MESSENGER observations of an active reconnection event in ...Mercury's magnetotail driven by an interplanetary coronal mass ejection on 2011 November 23. The primary Hall magnetic field, sequential passage of X-lines with Hall field perturbations, and flux ropes (FRs) provide unambiguous evidence of multiple X-line reconnection in an unstable ion diffusion region. In addition, large FRs consisting of multiple successive small-scale FRs are ejected tailward at quasi-periodic intervals of ∼1 minute, which is comparable to the Dungey cycle time. We propose that these large FRs are generated by the interaction and coalescence of multiple ion-scale FRs. This is distinct from the commonly accepted Earth-like substorm process where plasmoids are created by widely separated X-lines in the magnetotail. These observations suggest that during extreme solar wind conditions multiple X-line reconnection may dominate the tail reconnection process and control the global dynamics of Mercury's magnetosphere.
Magnetic flux transfer events (FTEs) are signatures of unsteady magnetic reconnection, often observed at planetary magnetopauses. Their generation mechanism, a key ingredient determining how they ...regulate the transfer of solar wind energy into magnetospheres, is still largely unknown. We report THEMIS spacecraft observations on 2007‐06‐14 of an FTE generated by multiple X‐line reconnection at the dayside magnetopause. The evidence consists of (1) two oppositely‐directed ion jets converging toward the FTE that was slowly moving southward, (2) the cross‐section of the FTE core being elongated along the magnetopause normal, probably squeezed by the oppositely‐directed jets, and (3) bidirectional field‐aligned fluxes of energetic electrons in the magnetosheath, indicating reconnection on both sides of the FTE. The observations agree well with a global magnetohydrodynamic model of the FTE generation under large geomagnetic dipole tilt, which implies the efficiency of magnetic flux transport into the magnetotail being lower for larger dipole tilt.
A new method is described which calculates the velocity of observed, quasi‐stationary structures at every moment in time from multi‐point magnetic field measurements. Once the magnetic gradient ...tensor G = ∇ and the time variation of the magnetic field have been estimated at every moment, the velocity can then be determined, in principle, as a function of time. One striking property of this method is that we can calculate the velocity of structures for any dimensionality: for three‐dimensional structures, all three components of the velocity vector can be calculated directly; for two‐dimensional (or one‐dimensional) structures, we can calculate the velocity along two (or one) directions. The advantage of this method is that the velocity is determined instantaneously, point by point through any structure, and so we can see the time variation of the velocity as the spacecraft traverse the structure. In this paper, the feasibility of the method is tested by calculating the motion velocity of a three‐dimensional, near cusp structure and a two‐dimensional magnetotail current sheet. The results for one‐dimensional structures in the magnetopause and cusp boundaries are compared to calculations for the standard techniques for analyzing discontinuities.
Summary
Occult hepatitis B infection (OBI) is characterized by serum hepatitis B surface antigen (HBsAg) negative and hepatitis B virus (HBV) DNA positive (HBsAg‐/HBV DNA+). Occult hepatitis B ...infection in community‐based populations has been scarcely investigated, and OBI outcomes remain unclear, especially in Wuwei, a region located in Northwest China. This region is one of the areas in China that has the highest prevalence of chronic HBV infection. A prospective study was performed in the general population of 2 towns of Wuwei from June 2011 to May 2014. A questionnaire was used to collect demographic and medical data, and serum samples were collected from the participants and stored until analysis. DNA was detected using quantitative PCR (qPCR) or nested PCR, the HBV DNA from HBV DNA‐positive or possible positive (below the detection limit) subjects was extracted and amplified by nested PCR, and the PCR products were sequenced. Sequence analysis was performed using the Mega 6.0 program and CLC sequence viewer software. Hepatitis B virus DNA was detected in 90 of 3,080 HBsAg‐negative subjects, and the prevalence of OBI in the study population was 2.92% (90/3,080, 95% CI: 2.33%‐3.51%). Hepatitis B virus genomes in 51 of 80 objects (63.75%) contained mutations in the “a” determinant of HBsAg. After 2 years follow‐up, 42 of 90 HBV DNA of OBI subjects remained positive, and the natural clearance rate of OBI subjects was 53.3%. Occult hepatitis B infection prevalence in this cohort was much lower than chronic HBV infection in the same region. HBV DNA was cleared in most OBI subjects during the 2 year period. Our data suggest that some OBI may represent a late stage of resolving the HBV infection process.
This paper reports for the first time the identification of a magnetic structure around a magnetic null in a magnetic reconnection region in the magnetotail. Magnetic reconnection is one of the ...fundamental processes in astrophysical and solar‐terrestrial plasmas. Though the concept of reconnection has been studied for many years, the process that really occurs has not been fully revealed by direct measurements. In particular, the lack of a description of three‐dimensional (3‐D) reconnecting magnetic field from observations makes the task more difficult. The Cluster spacecraft array provide an opportunity to reconstruct the 3‐D magnetic reconnection structure based on magnetic field vectors simultaneously measured at four positions. The identification of this structure comes from a new method of analysis of in situ measurements proposed here. Applying a fitting model of 10 spherical harmonic functions and a Harris current sheet function, plus a constant field, we reconstruct a 3‐D magnetic field configuration around the magnetic null in an reconnection event observed by Cluster in the geo‐magnetotail.
Magnetic reconnection is one of the most important processes in astrophysical, space and laboratory plasmas. Identifying the structure around the point at which the magnetic field lines break and ...subsequently reform, known as the magnetic null point, is crucial to improving our understanding of reconnection. But owing to the inherently three-dimensional nature of this process, magnetic nulls are only detectable through measurements obtained simultaneously from at least four points in space. Using data collected by the four spacecraft of the Cluster constellation as they traversed a diffusion region in the Earth's magnetotail on 15 September 2001, we report here the first in situ evidence for the structure of an isolated magnetic null. The results indicate that it has a positive-spiral structure whose spatial extent is of the same order as the local ion inertial length scale, suggesting that the Hall effect could play an important role in 3D reconnection dynamics. PUBLICATION ABSTRACT
This paper presents THEMIS measurements of two substorm events to show how the substorm current wedge (SCW) is generated. In the late growth phase when an earthward flow burst in the near‐Earth ...magnetotail brakes and is diverted azimuthally, pressure gradients in the X‐ and Y‐directions are observed to increase in the pileup and diverting regions of the flow. The enhanced pressure gradient in the Y‐direction is dawnward (duskward) on the dawnside (duskside) where a clockwise (counter‐clockwise) vortex forms. This dawn‐dusk pressure gradient drives downward (upward) field‐aligned current (FAC) on the dawnside (duskside) of the flow, which, when combined with the FACs generated by the clockwise (counter‐clockwise) vortex, forms the SCW. Substorm auroral onset occurs when the vortices appear, Near‐Earth dipolarization onset is observed by the THEMIS spacecraft (probes) when a rapid jump in the Y‐component of pressure gradient is detected. The total FACs from the vortex and the azimuthal pressure gradient are found to be comparable to the DP‐1 current in a typical substorm.
Key Points
Two dimensional pressure gradient could be estimated using three satellites
Dawn‐dusk pressure gradient was generated after flow diversion
FAC generated by azimuthal pressure gradient is enough for SCW formation
Using the Magnetospheric Multiscale spacecraft, we report observations of the pressure‐balanced structures in the form of slow magnetosonic waves rather than the mirror mode description in the ...Earth's magnetosheath. The slow magnetosonic waves are identified with three criteria: (1) The significant inverse correlation coefficients between the plasma thermal pressure (Pplasma) and the magnetic pressure (Pmag) with the constant total pressure (Ptot = Pplasma+Pmag); (2) the wave frequency Doppler shifted to the plasma flow frame is not zero to within the measurement error, which distinguishes the slow magnetosonic mode from the mirror mode; and (3) the observational dispersion relation is in a reasonable agreement with the linear theoretical result of slow magnetosonic waves. We find that slow magnetosonic waves can propagate over many wavelengths with negligible decay due to their transverse propagation which dramatically decreases their kinetic damping rate. Furthermore, we propose a possible generation mechanism that a high‐speed ion beam resonates with the local ion acoustic mode contributing to the inverse Landau damping process that converts particle energy into waves.
Key Points
We report observational evidence of a large‐amplitude slow magnetosonic wave propagating transverse to the magnetic field in the magnetosheath
To within the measurement error, the wave frequency Doppler shifted to the plasma flow frame (0.1±0.017)ωci is not zero, which distinguishes the slow magnetosonic mode from the mirror mode
Both the dispersion relation and the anti‐correlation between magnetic and plasma thermal pressures are used to uncover the existence of slow magnetosonic waves
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