501 research outputs found
Toward detailed prominence seismology - I. Computing accurate 2.5D magnetohydrodynamic equilibria
Context. Prominence seismology exploits our knowledge of the linear
eigenoscillations for representative magnetohydro- dynamic models of filaments.
To date, highly idealized models for prominences have been used, especially
with respect to the overall magnetic configurations.
Aims. We initiate a more systematic survey of filament wave modes, where we
consider full multi-dimensional models with twisted magnetic fields
representative of the surrounding magnetic flux rope. This requires the ability
to compute accurate 2.5 dimensional magnetohydrodynamic equilibria that balance
Lorentz forces, gravity, and pressure gradients, while containing density
enhancements (static or in motion).
Methods. The governing extended Grad-Shafranov equation is discussed, along
with an analytic prediction for circular flux ropes for the Shafranov shift of
the central magnetic axis due to gravity. Numerical equilibria are computed
with a finite element-based code, demonstrating fourth order accuracy on an
explicitly known, non-trivial test case.
Results. The code is then used to construct more realistic prominence
equilibria, for all three possible choices of a free flux-function. We quantify
the influence of gravity, and generate cool condensations in hot cavities, as
well as multi- layered prominences.
Conclusions. The internal flux rope equilibria computed here have the
prerequisite numerical accuracy to allow a yet more advanced analysis of the
complete spectrum of linear magnetohydrodynamic perturbations, as will be
demonstrated in the companion paper.Comment: Accepted by Astronomy & Astrophysics, 15 pages, 15 figure
Convective magneto-rotational instabilities in accretion disks
We present a study of instabilities occuring in thick magnetized accretion
disks. We calculate the growth rates of these instabilities and characterise
precisely the contribution of the magneto-rotational and the convective
mechanism. All our calculations are performed in radially stratified disks in
the cylindrical limit. The numerical calculations are performed using the
appropriate local dispersion equation solver discussed in Blokland et al.
(2005). A comparison with recent results by Narayan et al. (2002) shows
excellent agreement with their approximate growth rates only if the disks are
weakly magnetized. However, for disks close to equipartition, the dispersion
equation from Narayan et al. (2002) loses its validity. Our calculations allow
for a quantitative determination of the increase of the growth rate due to the
magneto-rotational mechanism. We find that the increase of the growth rate for
long wavelength convective modes caused by this mechanism is almost neglible.
On the other hand, the growth rate of short wavelength instabilities can be
significantly increased by this mechanism, reaching values up to 60%.Comment: 10 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
Accelerators and their ghosts
The issue of particle accelerator reliability is a problem that currently is not fully defined, understood nor addressed. Conventional approaches to reliability (e.g., RBDs) struggle due to a lack of data about specific component/system reliability and failure.
There is a large body of beam current data retrievable from operating accelerators that contains detailed information about the accelerator behaviour, both before and after a
machine trip has occurred.
Analysing this data could provide insight and help develop a new approach to address accelerator reliability. In this paper, we propose a data-driven approach to detecting emergent behaviour in particle accelerators. Instead of attempting to identify every possible failure of a machine
we propose an alternative approach based around a change in perspective, to knowing the normal default operational behaviour of a machine. Taking action when a “ghost in the machine” emerges that causes accelerator wide aberrant changes to normal machine behaviour
Toward detailed prominence seismology - II. Charting the continuous magnetohydrodynamic spectrum
Starting from accurate MHD flux rope equilibria containing prominence
condensations, we initiate a systematic survey of their linear
eigenoscillations. To quantify the full spectrum of linear MHD eigenmodes, we
require knowledge of all flux-surface localized modes, charting out the
continuous parts of the MHD spectrum. We combine analytical and numerical
findings for the continuous spectrum for realistic prominence configurations.
The equations governing all eigenmodes for translationally symmetric,
gravitating equilibria containing an axial shear flow, are analyzed, along with
their flux-surface localized limit. The analysis is valid for general 2.5D
equilibria, where either density, entropy, or temperature vary from one flux
surface to another. We analyze the mode couplings caused by the poloidal
variation in the flux rope equilibria, by performing a small gravity parameter
expansion. We contrast the analytical results with continuous spectra obtained
numerically. For equilibria where the density is a flux function, we show that
continuum modes can be overstable, and we present the stability criterion for
these convective continuum instabilities. Furthermore, for all equilibria, a
four-mode coupling scheme between an Alfvenic mode of poloidal mode number m
and three neighboring (m-1, m, m+1) slow modes is identified, occurring in the
vicinity of rational flux surfaces. For realistically prominence equilibria,
this coupling is shown to play an important role, from weak to stronger gravity
parameter g values. The analytic predictions for small g are compared with
numerical spectra, and progressive deviations for larger g are identified. The
unstable continuum modes could be relevant for short-lived prominence
configurations. The gaps created by poloidal mode coupling in the continuous
spectrum need further analysis, as they form preferred frequency ranges for
global eigenoscillations.Comment: Accepted by Astronmy & Astrophysics, 21 pages, 15 figure
A large-scale longitudinal structured dataset of the dark web cryptomarket Evolution (2014-2015)
Dark Web Marketplaces (DWM) facilitate the online trade of illicit goods. Due
to the illicit nature of these marketplaces, quality datasets are scarce and
difficult to produce. The Dark Net Market archives (2015) presented raw scraped
source files crawled from a selection of DWMs, including Evolution. Here, we
present, specifically for the Evolution DWM, a structured dataset extracted
from Dark Net Market archive data. Uniquely, many of the data quality issues
inherent to crawled data are resolved. The dataset covers over 500 thousand
forum posts and over 80 thousand listings, providing data on forums, topics,
posts, forum users, market vendors, listings, and more. Additionally, we
present temporal weighted communication networks extracted from this data. The
presented dataset provides easy access to a high quality DWM dataset to
facilitate the study of criminal behaviour and communication on such DWMs,
which may provide a relevant source of knowledge for researchers across
disciplines, from social science to law to network science.Comment: 19 pages, 5 figure
Unstable magnetohydrodynamical continuous spectrum of accretion disks. A new route to magnetohydrodynamical turbulence in accretion disks
We present a detailed study of localised magnetohydrodynamical (MHD)
instabilities occuring in two--dimensional magnetized accretion disks. We model
axisymmetric MHD disk tori, and solve the equations governing a
two--dimensional magnetized accretion disk equilibrium and linear wave modes
about this equilibrium. We show the existence of novel MHD instabilities in
these two--dimensional equilibria which do not occur in an accretion disk in
the cylindrical limit. The disk equilibria are numerically computed by the
FINESSE code. The stability of accretion disks is investigated analytically as
well as numerically. We use the PHOENIX code to compute all the waves and
instabilities accessible to the computed disk equilibrium. We concentrate on
strongly magnetized disks and sub--Keplerian rotation in a large part of the
disk. These disk equilibria show that the thermal pressure of the disk can only
decrease outwards if there is a strong gravitational potential. Our theoretical
stability analysis shows that convective continuum instabilities can only
appear if the density contours coincide with the poloidal magnetic flux
contours. Our numerical results confirm and complement this theoretical
analysis. Furthermore, these results show that the influence of gravity can
either be stabilizing or destabilizing on this new kind of MHD instability. In
the likely case of a non--constant density, the height of the disk should
exceed a threshold before this type of instability can play a role. This
localised MHD instability provides an ideal, linear route to MHD turbulence in
strongly magnetized accretion disk tori.Comment: 20 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
Early warning signals for predicting cryptomarket vendor success using dark net forum networks
In this work we focus on identifying key players in dark net cryptomarkets.
Law enforcement aims to disrupt criminal activity conducted through these
markets by targeting key players vital to the market's existence and success.
We particularly focus on detecting successful vendors responsible for the
majority of illegal trade. Our methodology aims to uncover whether the task of
key player identification should center around plainly measuring user and forum
activity, or that it requires leveraging specific patterns of user
communication. We focus on a large-scale dataset from the Evolution
cryptomarket, which we model as an evolving communication network. While user
and forum activity measures are useful for identifying the most successful
vendors, we find that betweenness centrality additionally identifies those with
lesser activity. But more importantly, analyzing the forum data over time, we
find evidence that attaining a high betweenness score comes before vendor
success. This suggests that the proposed network-driven approach of modelling
user communication might prove useful as an early warning signal for key player
identification
Magnetohydrostatic solar prominences in near-potential coronal magnetic fields
We present numerical magnetohydrostatic solutions describing the
gravitationally stratified, bulk equilibrium of cool, dense prominence plasma
embedded in a near-potential coronal field. These solutions are calculated
using the FINESSE magnetohydrodynamics equilibrium solver and describe the
morphologies of magnetic field distributions in and around prominences and the
cool prominence plasma that these fields support. The equilibrium condition for
this class of problem is usually different in distinct subdomains, separated by
free boundaries, across which solutions are matched by suitable continuity or
jump conditions describing force balance. We employ our precise finite element
elliptic solver to calculate solutions not accessible by previous analytical
techniques with temperature or entropy prescribed as free functions of the
magnetic flux function, including a range of values of the polytropic index,
temperature variations mainly across magnetic field lines and photospheric
field profiles sheared close to the polarity inversion line. Out of the many
examples computed here, perhaps the most noteworthy is one which reproduces
precisely the three-part structure often encountered in observations: a cool
dense prominence within a cavity/flux rope embedded in a hot corona. The
stability properties of these new equilibria, which may be relevant to solar
eruptions, can be determined in the form of a full resistive MHD spectrum using
a companion hyperbolic stability solver.Comment: To appear in ApJ August 200
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