Multiple dipolarization fronts were observed by THEMIS spacecraft in the near‐Earth magnetotail during a substorm. The dipolarization fronts were located at the leading edge of earthward propagating ...plasma bubbles. Major energetic electron flux enhancements were observed at the dipolarization fronts, which were also associated with large wave fluctuations extending from below the lower hybrid frequency to above the electron cyclotron frequency. Intense electric field wave packets, primarily contributed by the Hall electric field and lower hybrid drift (LHD) wave, were observed right at the front, which was a thin current layer with size of the order of the ion inertial length. The LHD wave was believed to be generated by a diamagnetic current in the presence of density and temperature gradients. Electrostatic electron cyclotron harmonic (ECH) waves were detected slightly after the front. The ECH waves were probably generated by the positive slope of the electron perpendicular velocity distribution. Both of these waves are suggested to be able to heat electrons. The observation of these waves at the dipolarization front could be important for the understanding of electron energization during substorm injection, as well as the mechanism of current disruption.
Oxygen impacts on dipolarization fronts and reconnection rate Liang, Haoming; Ashour‐Abdalla, Maha; Lapenta, Giovanni ...
Journal of geophysical research. Space physics,
February 2016, 2016-02-00, 20160201, Letnik:
121, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Spacecraft observations near a magnetotail X line show that oxygen (O+) ions are minor species during nonstorm substorms, but they can become major species during some of the storm time substorms. ...Dipolarization fronts (DFs), which are characterized by a sharp increase northward magnetic field in the magnetotail, are commonly observed during magnetospheric substorms. In this study, we investigated the O+ effects on DFs and the reconnection rate during magnetotail reconnection. We used a 2.5‐D implicit particle‐in‐cell simulation in a 2‐D Harris current sheet in the presence of H+ and O+ ions. Simulation runs with equal number densities of O+ and H+ (O+ run) and with pure H+ ion species (H+ run) were performed. Comparing the two different runs, we found that both the reconnection rate and the DF speed in the O+ run were much less than those in the H+ run. By studying the force balance and plasma composition at the DF, we found that the outflow magnetic flux and DF propagation were encumbered by the current sheet O+ inertia, which reduced the DF speed and delayed the reconnection rate in the O+ run. We also found an ambipolar electric field in the O+ run due to the different inflow and outflow speeds of O+ and electrons in the O+ diffusion region. As a result, this ambipolar electric field induced O+ drag on the convective magnetic field in the O+ diffusion region. The small reconnection rate determined in the O+ run can be attributed to the current sheet inertia and the O+ drag on the convective magnetic flux.
Key Points
DF propagation has to overcome a current sheet O+ inertia
DF thickness is mainly related to the penetration of current sheet ions
O+ inertia can impede the magnetic flux propagation and thus reduce reconnection rate
Understanding particle acceleration in the magnetotail during substorms requires knowledge of changes in the global magnetospheric configuration and the local regions of intense fields and ...microinstabilities caused by processes associated with reconnection. We simulated a substorm on 15 February 2008 by coupling the University of California, Los Angeles global magnetohydrodynamic simulation code and a two‐dimensional version of the iPIC3D implicit particle‐in‐cell code. The MHD code provides realistic initial and boundary conditions for the particle‐in‐cell (PIC) code, while the PIC code models the reconnection and evolution of the dipolarization front (DF) self‐consistently with full kinetic physics. In the PIC simulation, after a few seconds, an active X point forms and DF‐like structures form about every 2 s and propagate earthward. In the near‐Earth tail, the earthward moving fronts combine to form thicker structures. The presence of the macroscopic‐scale magnetic field, featuring a significant dipolar component nearer the Earth, affects the reconnection process, chocking the flow that cannot freely propagate earthward, causing the production of multiple repeating DFs. In the region away from the equator and equatorward of the separatrices, the DFs become associated with a series of spatial stripes that are visible in the electron temperature and the magnetic and electric fields and are also caused by the unsteady nature of the process of reconnection. The electrons are preferentially accelerated in the dipolarization region in the extension of the dipolarization field lines to high latitudes and reach energies of 100 keV or more. A streaming instability may be responsible for the parallel acceleration. This acceleration of the plasma associated with the DFs is greater than that occurring near the X point for this substorm.
Key Points
Acceleration of plasma near DFs is greater than that occurring near the X point
Multiple dipolarization fronts are caused by unsteady reconnection
The electrons are preferentially accelerated in the separatrices
The idea of antiangiogenic therapy was the brainchild of Dr. Judah Folkman in the early 1970s. He proposed that by cutting off the blood supply, cancer cells would be deprived of nutrients and, ...hence, treated. His efforts paid off when bevacizumab, a monoclonal antibody targeting vascular endothelial growth factor, was approved as antiangiogenic therapy in 2004 for the treatment of colon cancer. Since then, an array of antiangiogenic inhibitors, either as monotherapy or in combination with other cytotoxic and chemotherapy drugs, have been developed, used in clinical trials, and approved for the treatment of cancer. Despite this important breakthrough, antiangiogenic therapy for cancer met with a number of hurdles on its way to becoming an option for cancer therapy. In this article, we summarize the most current information on the mechanisms of tumor angiogenesis, proangiogenic and antiangiogenic factors, potential targets and their mechanisms of action, and experimental evidences, as well as the most recent clinical trial data on antiangiogenic agents for cancer therapy.
•CAM use is popular among patients receiving cancer treatment.•Patients’ view of life from cancer affects their decision in using the CAM.•Majority of the patients were guided by family and friends’ ...suggestions towards CAM use.•The rate of non-disclosure was high indicating the need for enhancement of doctor – patient relationship for effective patients’ care.
Complementary and alternative medicine (CAM) use among cancer patients is increasing over the years. Some types of CAM may interact with conventional cancer therapy or may cause serious adverse effects. This study was designed to determine the prevalence of CAM use among cancer patients during cancer therapy and to identify CAM users’ characteristics, perception, reasoning, expectation and disclosure of CAM use.
This cross-sectional study was conducted between July 2017 to October 2017, on 121 cancer patients in oncology and hematology clinics and daycare of a tertiary hospital in Kuala Lumpur, Malaysia. Study data was collected using a self-administered questionnaire.
The prevalence of CAM use among cancer patients was found to be 61.2 %. Multivariate logistic regression analysis showed that CAM use was associated with change in outlook on life after development of cancer. The most frequently used types of CAM were vitamins and Islamic medical practices. Majority of patients were guided by family and friends’ suggestions towards using CAM. Most patients expected CAM to cure their disease. The percentage of patients who disclosed CAM use to the doctors was slightly higher than those who did not disclose CAM use.
The use of CAM was found to be remarkably high. Rate of non-disclosure of CAM use was high indicating the need for enhancement of doctor- patient relationship for effective patient’s care. Future research is needed with large sample of patients to provide better representation of the population and to help understand patients’ beliefs and perception more.
Ion heating during a substorm on 15 February 2008, starting at 0348 UT, is studied with a new approach recently described in Ashour‐Abdalla et al. (2015). The general conditions of the magnetotail ...are obtained from a global magnetohydrodynamic (MHD) model and are used to drive a full kinetic particle‐in‐cell (PIC) simulation of a 3‐D region of the tail. Within the kinetic box, the ions, the electrons, and the fields evolve self‐consistently. The large scales are captured by the MHD model and the small scales by the PIC model based on the MHD state. This approach is used to study ion heating. Different heating mechanisms were analyzed by examining the velocity distributions at different locations. In the x direction heating occurs as the reconnection‐generated ion jet interacts with the environment in which it propagates. The heating is found mostly in the separatrices and increases downstream of the reconnection region. In the y direction the heating is less intense and is found near the dipolarization fronts. It occurs as ions become magnetized and gyrotropize the distribution function. In addition, ions can be heated in the y direction by the reconnection electric field near the reconnection site. In the z direction the ions are heated by the formation of beams moving along z between the separatrices.
Key Points
Coupled kinetic‐fluid simulations predict ion distributions observed by THEMIS
The localization and causes of ion heating during one substorm are found
Ions are unmagnetized with anisotropic processes of energization
Study of magnetic reconnection has been focused on two‐dimensional geometry in the past decades, whereas three‐dimensional structures and dynamics of reconnection X line are poorly understood. In ...this paper, we report Cluster multispacecraft observations of a three‐dimensional magnetic reconnection X line with a weak guide field (~25% of the upstream magnetic field) in the Earth's magnetotail. We find that the X line not only retreated tailward but also expanded across the tail following the electron flow direction with a maximum average speed of (0.04–0.15) VA,up, where VA,up is the upstream Alfvén speed, or (0.14–0.57) Vde, where Vde is the electron flow speed in the out‐of‐plane direction. An ion diffusion region was observed by two spacecraft that were separated about 10 ion inertial lengths along the out‐of‐plane direction; however, these two spacecraft observed distinct magnetic structures associated with reconnection: one spacecraft observed dipolarization fronts, while the other one observed flux ropes. This indicates that reconnection proceeds in drastically different ways in different segments along the X line only a few ion inertial lengths apart.
Plain Language Summary
Two‐dimensional reconnection model has been employed for decades to interpret the observations in space. However, reconnection sometimes happens in a three‐dimensional manner that is dramatically different from the 2‐D model. The 3‐D dynamics of reconnection is poorly known. By using observations from multiple points in Earth's magnetosphere, we are able to reveal some important aspects of the three‐dimensional dynamics of magnetic reconnection X line, which is characterized by expansion and large gradient in the out‐of‐plane direction. For the first time, we estimate the X line expansion speed using the in situ data. We found that the X line expanded along the electron out‐of‐plane flow direction with a speed much less than the upstream Alfvén speed and a small fraction of the electron flow speed in the out‐of‐plane direction. An ion diffusion region was observed by two spacecraft that were separated about one Earth radius along the out‐of‐plane direction; however, these two spacecraft observed distinct magnetic structures associated with reconnection. This indicates that the characteristic length of X line is on the order of the ion scale. Reconnection produces 3‐D structures on the ion scale in Earth's magnetosphere.
Key Points
Cluster multispacecraft observations of a three‐dimensional magnetic reconnection X line with a weak guide field in the magnetotail
The X line expanded in the out‐of‐plane direction with a maximum speed less than both the upstream Alfven speed and electron flow speed
Reconnection proceeds in drastically different ways in different segments along the X line only a few ion inertial lengths apart
Various plasma waves have been observed in the vicinity of dipolarization fronts (DFs) and the rarefaction regions behind them. It was suggested that these waves not only play crucial roles in ...regulating particle kinetics at the DFs but also may potentially affect the large‐scale dynamics of the magnetotail. In this paper, we present the observations of large‐amplitude electromagnetic waves at DFs that occurred during magnetospheric substorms. The DFs were embedded in either the tailward or earthward flows in the near‐Earth magnetotail. The wave frequencies were near the local proton cyclotron frequency. The waves propagated at highly oblique angles with respect to the ambient magnetic field (~80°–100°). Their corresponding wavelengths were on the order of the local ion gyroradii. The major magnetic field fluctuations were along the background magnetic field, while the electric field fluctuations were predominantly perpendicular to the background magnetic field. The waves were compressional waves as there was an anticorrelation between the plasma density and the wave magnetic field strength. The electric potential associated with the waves reached to more than half of the electron temperature, indicating the waves are nonlinear. We suggest that the waves were magnetosonic or ion Bernstein mode waves driven by the ion ring distribution. The waves were able to provide significant anomalous resistivity at the front, with major contributions from the electric field fluctuations. The effects of these waves on the electron pitch angle scattering and energy diffusion are also discussed.
Key Points
The first report of magnetosonic wave or ion Bernstein wave at the DF
They are nonlinear waves
The waves are probably excited by the ion ring distribution formed at the front
A comprehensive examination of particle and wave data from multiple Thermal Emission Imaging System (THEMIS) satellites has been made of an electron injection structure in the magnetotail as it ...propagated earthward from −20 RE to −11 RE on 27 February 2009. The electron injection, which was closely associated with a dipolarization front and bursty bulk flows, occurred within a thin plasma boundary layer and had both perpendicular and parallel energization, with very little energy dispersion. The thin plasma boundary layer had a thickness comparable to the ion inertial length and displayed different plasma characteristics at different locations. Strong electromagnetic waves between the lower hybrid frequency and the electron gyrofrequency, as well as electrostatic waves up to the electron plasma frequency, were observed within the thin plasma boundary layers. The two outermost spacecraft at X = −20.1 RE and X = −16.7 RE detected intense whistler waves, most likely driven by an observed electron temperature anisotropy with T⊥/T∥ > 1. Closer to Earth at X = −11.1 RE, whistlers were not seen, consistent with the observed electron distribution having T⊥/T∥ < 1. Near the electron injection region, nonlinear electrostatic structures such as electrostatic solitary waves and double layers were also observed. These nonlinear electrostatic structures can interact with the electron distribution and accelerate electrons; high energy distributions could be generated if the electrons encountered a large number of these structures. The observations show that nonideal MHD, nonlinear, and kinetic behavior is intrinsic to the electron injections with multiscale coupling.
Fast plasma flows are believed to play important roles in transporting mass, momentum, and energy in the magnetotail during active periods, such as the magnetospheric substorms. In this paper, we ...present Cluster observations of a plasma‐depleted flux tube, i.e., a plasma bubble associated with fast plasma flow before the onset of a substorm in the near‐Earth tail around X = −18 RE. The bubble is bounded by both sharp leading (∂bz/∂x < 0) and trailing (∂bz/∂x > 0) edges. The two edges are thin current layers (approximately ion inertial length) that carry not only intense perpendicular current but also field‐aligned current. The leading edge is a dipolarization front (DF) within a slow plasma flow, while the trailing edge is embedded in a super‐Alfvénic convective ion jet. The electron jet speed exceeds the ion flow speed thus producing a large tangential current at the trailing edge. The electron drift is primarily given by the E × B drift. Interestingly, the trailing edge moves faster than the leading edge, which causes shrinking of the bubble and local flux pileup inside the bubble. This resulted in a further intensification of Bz, or a secondary dipolarization. Both the leading and trailing edges are tangential discontinuities that confine the electrons inside the bubble. Strong electron acceleration occurred corresponding to the secondary dipolarization, with perpendicular fluxes dominating the field‐aligned fluxes. We suggest that betatron acceleration is responsible for the electron energization. Whistler waves and lower hybrid drift waves were identified inside the bubble. Their generation mechanisms and potential roles in electron dynamics are discussed.
Key Points
The bubble is bounded by both leading and trailing edges
The interaction between leading and trailing edge leads to a secondary DF
Electron acceleration occurred at the secondary DF
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