6,855 research outputs found
Kelvin-Helmholtz instability of a coronal streamer
The shear-flow-driven instability can play an important role in energy
transfer processes in coronal plasma. We present for the first time the
observation of a kink-like oscillation of a streamer probably caused by the
streaming kinkmode Kelvin-Helmholtz instability. The wave-like behavior of the
streamer was observed by Large Angle and Spectrometric Coronagraph Experiment
(LASCO) C2 and C3 aboard SOlar and Heliospheric Observatory (SOHO). The
observed wave had a period of about 70 to 80 minutes, and its wavelength
increased from 2 Rsun to 3 Rsun in about 1.5 hours. The phase speeds of its
crests and troughs decreased from 406 \pm 20 to 356 \pm 31kms^{-1} during the
event. Within the same heliocentric range, the wave amplitude also appeared to
increase with time. We attribute the phenomena to the MHD Kelvin-Helmholtz
instability which occur at a neutral sheet in a fluid wake. The free energy
driving the instability is supplied by the sheared flow and sheared magnetic
field across the streamer plane. The plasma properties of the local environment
of the streamer were estimated from the phase speed and instability threshold
criteria.Comment: ApJ, accepte
Vehicle Choices, Miles Driven, and Pollution Policies
Mobile sources contribute large percentages of each pollutant, but technology is not yet available to measure and tax emissions from each vehicle. We build a behavioral model of household choices about vehicles and miles traveled. The ideal-but-unavailable emissions tax would encourage drivers to abate emissions through many behaviors, some of which involve market transactions that can be observed for feasible market incentives (such as a gas tax, subsidy to new cars, or tax by vehicle type). Our model can calculate behavioral effects of each such price and thus calculate car choices, miles, and emissions. A nested logit structure is used to model discrete choices among different vehicle bundles. We also consider continuous choices of miles driven and the age of each vehicle. We propose a consistent estimation method for both discrete and continuous demands in one step, to capture the interactive effects of simultaneous decisions. Results are compared with those of the traditional sequential estimation procedure.
The Accretion Wind Model of the Fermi Bubbles (II): Radiation
In a previous work, we have shown that the formation of the Fermi bubbles can
be due to the interaction between winds launched from the hot accretion flow in
Sgr A* and the interstellar medium (ISM). In that work, we focus only on the
morphology. In this paper we continue our study by calculating the gamma-ray
radiation. Some cosmic ray protons (CRp) and electrons must be contained in the
winds, which are likely formed by physical processes such as magnetic
reconnection. We have performed MHD simulations to study the spatial
distribution of CRp, considering the advection and diffusion of CRp in the
presence of magnetic field. We find that a permeated zone is formed just
outside of the contact discontinuity between winds and ISM, where the
collisions between CRp and thermal nuclei mainly occur. The decay of neutral
pions generated in the collisions, combined with the inverse Compton scattering
of background soft photons by the secondary leptons generated in the collisions
and primary CR electrons can well explain the observed gamma-ray spectral
energy distribution. Other features such as the uniform surface brightness
along the latitude and the boundary width of the bubbles are also explained.
The advantage of this accretion wind model is that the adopted wind properties
come from the detailed small scale MHD numerical simulation of accretion flows
and the value of mass accretion rate has independent observational evidences.
The success of the model suggests that we may seriously consider the
possibility that cavities and bubbles observed in other contexts such as galaxy
clusters may be formed by winds rather than jets.Comment: 13 pages,6 figures, accepted for publication in Ap
Three-dimensional magnetohydrodynamical simulations of the morphology of head-tail radio galaxies based on magnetic tower jet model
The distinctive morphology of head-tail radio galaxies reveals strong
interactions between the radio jets and their intra-cluster environment, the
general consensus on the morphology origin of head-tail sources is that radio
jets are bent by violent intra-cluster weather. We demonstrate in this paper
that such strong interactions provide a great opportunity to study the jet
properties and also the dynamics of intra-cluster medium (ICM). By
three-dimensional magnetohydrodynamical simulations, we analyse the detailed
bending process of a magnetically dominated jet, based on the magnetic tower
jet model. We use stratified atmospheres modulated by wind/shock to mimic the
violent intra-cluster weather. Core sloshing is found to be inevitable during
the wind-cluster core interaction, which induces significant shear motion and
could finally drive ICM turbulence around the jet, making it difficult for jet
to survive. We perform detailed comparison between the behaviour of pure
hydrodynamical jets and magnetic tower jet, and find that the jet-lobe
morphology could not survive against the violent disruption in all of our pure
hydrodynamical jet models. On the other hand, the head-tail morphology is well
reproduced by using a magnetic tower jet model bent by wind, in which
hydrodynamical instabilities are naturally suppressed and the jet could always
keep its integrity under the protection of its internal magnetic fields.
Finally, we also check the possibility for jet bending by shock only. We find
that shock could not bend jet significantly, so could not be expected to
explain the observed long tails in head-tail radio galaxies.Comment: submitted to ApJ on December 9, 2016, and accepted on March 1st, 201
Comment on "Orientational Distribution of Free O-H Groups of Interfacial Water is Exponential"
In a recent letter (PRL,121,246101,2018), Sun et al. reported that combined
MD simulation and sum frequency generation vibrational spectroscopy (SFG-VS)
measurements led to conclusions of a broad and exponentially decaying
orientational distribution, and the presence of the free O-H group pointing
down to the bulk at the air/water interface. In this comment, we show that
their main conclusions are based on questionable interpretation of the SFG-VS
data presented in the letter [1], and are also contrary to the established data
analysis and interpretations in the literature [2-5].Comment: 2 pages, 0 figure
A General Theory for Direct Quantitative Analysis of Antigen
A theory for direct quantitative analysis of an antigen is proposed. It is
based on a potential homogenous immunoreaction system. It establishes an
equation to describe the concentration change of the antigen and antibody
complex. A maximum point is found in the concentration profile of the complex
which can be used to calculate the concentration of the antigen. An
experimental scheme was designed for a commercial time-resolved
fluoroimmunoassay kit for HBsAg, which is based heterogeneous immunoreaction.
The results showed that the theory is practically applicable.Comment: 7pages, 2 figure
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