1,749 research outputs found
The emergence of magnetic flux through a partially ionised solar atmosphere
We present results from 2.5D numerical simulations of the emergence of magnetic flux from the upper convection zone through the photosphere and chromosphere into the corona. Certain regions of the solar atmosphere are at sufficiently low temperatures to be only partially ionised, in particular the lower chromosphere. This leads to Cowling resistivities orders of magnitude larger than the Coulomb values, and thus to anisotropic dissipation in Ohm’s law. This also leads to localised low magnetic Reynolds numbers (R m < 1). We find that the rates of emergence of magnetic field are greatly increased by the partially ionised regions of the model atmosphere, and the resultant magnetic field is more diffuse. More importantly, the only currents associated with the magnetic field to emerge into the corona are aligned with the field, and thus the newly formed coronal field is force-free
Testing predictors of eruptivity using parametric flux emergence simulations
Solar flares and coronal mass ejections (CMEs) are among the most energetic
events in the solar system, impacting the near-Earth environment. Flare
productivity is empirically known to be correlated with the size and complexity
of active regions. Several indicators, based on magnetic-field data from active
regions, have been tested for flare forecasting in recent years. None of these
indicators, or combinations thereof, have yet demonstrated an unambiguous
eruption or flare criterion. Furthermore, numerical simulations have been only
barely used to test the predictability of these parameters. In this context, we
used the 3D parametric MHD numerical simulations of the self-consistent
formation of the flux emergence of a twisted flux tube, inducing the formation
of stable and unstable magnetic flux ropes of Leake (2013, 2014). We use these
numerical simulations to investigate the eruptive signatures observable in
various magnetic scalar parameters and provide highlights on data analysis
processing. Time series of 2D photospheric-like magnetograms are used from
parametric simulations of stable and unstable flux emergence, to compute a list
of about 100 different indicators. This list includes parameters previously
used for operational forecasting, physical parameters used for the first time,
as well as new quantities specifically developed for this purpose. Our results
indicate that only parameters measuring the total non-potentiality of active
regions associated with magnetic inversion line properties, such as the
Falconer parameters , , and , as well as the
new current integral and length parameters, present a
significant ability to distinguish the eruptive cases of the model from the
non-eruptive cases, possibly indicating that they are promising flare and
eruption predictors.Comment: 46 pages, 16 figures, accepted for publication in Space Weather and
Space Climate on June, 8t
Group behaviour in financial markets
This thesis aims to revise the current understanding of the behaviour of different groups
of traders in financial markets. Research involves statistical analysis of historic
'Commitment of Traders' reports, a U.S government dataset providing the long and
short positions of core groups of traders, reported at weekly intervals over 17 years.
Empirical work identifies a surprising level of consistency amongst different groups
across 31 markets. A specific pattern is identified: speculators are found to increase their
buying interest when prices are rising whilst commercial traders (or 'hedgers') increase
their selling; the opposite pattern of behaviour occurs when prices are falling. The thesis
explores the implications of this behaviour for existing models of financial markets by
referencing a number of peer-reviewed studies. The agent-based computational model
of Alfarano, Lux, and Wagner (2005) is implemented and analysed. A lack of validity is
demonstrated in the interactions between the different types of traders in this model.
These theoretical components are further shown to be typical of much of the literature
in this area. An objective for the thesis is to correct this oversight by incorporating
genuine patterns of trading behaviour into an existing computational model. The
approach of Mike and Farmer (2008) is used for this purpose, being currently unique in
that core components are calibrated from real-world data and no group-level
representations are assumed. This model is extended to observe groups of traders with
different levels of order-aggression: speculators are found to rely on market orders
whereas commercial traders rely on limit orders. These preferences, in the absence of
deeper theoretical considerations, are sufficient to account for the identified behaviour.
A discussion is offered on the relevance of this finding for financial market regulators,
who have typically focused on regulating types of traders, specifically speculators,
rather than on types of trades
Multi-fluid simulations of chromospheric magnetic reconnection in a weakly ionized reacting plasma
We present results from the first self-consistent multi-fluid simulations of
chromospheric magnetic reconnection in a weakly ionized reacting plasma. We
simulate two dimensional magnetic reconnection in a Harris current sheet with a
numerical model which includes ion-neutral scattering collisions, ionization,
recombination, optically thin radiative loss, collisional heating, and thermal
conduction. In the resulting tearing mode reconnection the neutral and ion
fluids become decoupled upstream from the reconnection site, creating an excess
of ions in the reconnection region and therefore an ionization imbalance. Ion
recombination in the reconnection region, combined with Alfv\'{e}nic outflows,
quickly removes ions from the reconnection site, leading to a fast reconnection
rate independent of Lundquist number. In addition to allowing fast
reconnection, we find that these non-equilibria partial ionization effects lead
to the onset of the nonlinear secondary tearing instability at lower values of
the Lundquist number than has been found in fully ionized plasmas.These
simulations provide evidence that magnetic reconnection in the chromosphere
could be responsible for jet-like transient phenomena such as spicules and
chromospheric jets.Comment: 8 Figures, 32 pages tota
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