721 research outputs found
Can Thermal Nonequilibrium Explain Coronal Loops?
Any successful model of coronal loops must explain a number of observed
properties. For warm (~ 1 MK) loops, these include: 1. excess density, 2. flat
temperature profile, 3. super-hydrostatic scale height, 4. unstructured
intensity profile, and 5. 1000--5000 s lifetime. We examine whether thermal
nonequilibrium can reproduce the observations by performing hydrodynamic
simulations based on steady coronal heating that decreases exponentially with
height. We consider both monolithic and multi-stranded loops. The simulations
successfully reproduce certain aspects of the observations, including the
excess density, but each of them fails in at least one critical way. Monolithic
models have far too much intensity structure, while multi-strand models are
either too structured or too long-lived. Our results appear to rule out the
widespread existence of heating that is both highly concentrated low in the
corona and steady or quasi-steady (slowly varying or impulsive with a rapid
cadence). Active regions would have a very different appearance if the dominant
heating mechanism had these properties. Thermal nonequilibrium may nonetheless
play an important role in prominences and catastrophic cooling events (e.g.,
coronal rain) that occupy a small fraction of the coronal volume. However,
apparent inconsistencies between the models and observations of cooling events
have yet to be understood.Comment: 40 pages, 10 figures, accepted by the Astrophysical Journal (vol.
714
Formation of Solar Filaments by Steady and Nonsteady Chromospheric Heating
It has been established that cold plasma condensations can form in a magnetic
loop subject to localized heating of the footpoints. In this paper, we use
grid-adaptive numerical simulations of the radiative hydrodynamic equations to
parametrically investigate the filament formation process in a pre-shaped loop
with both steady and finite-time chromospheric heating. Compared to previous
works, we consider low-lying loops with shallow dips, and use a more realistic
description for the radiative losses. We demonstrate for the first time that
the onset of thermal instability satisfies the linear instability criterion.
The onset time of the condensation is roughly \sim 2 hr or more after the
localized heating at the footpoint is effective, and the growth rate of the
thread length varies from 800 km hr-1 to 4000 km hr-1, depending on the
amplitude and the decay length scale characterizing this localized
chromospheric heating. We show how single or multiple condensation segments may
form in the coronal portion. In the asymmetric heating case, when two segments
form, they approach and coalesce, and the coalesced condensation later drains
down into the chromosphere. With a steady heating, this process repeats with a
periodicity of several hours. While our parametric survey confirms and augments
earlier findings, we also point out that steady heating is not necessary to
sustain the condensation. Once the condensation is formed, it can keep growing
also when the localized heating ceases. Finally, we show that the condensation
can survive continuous buffeting by perturbations resulting from the
photospheric p-mode waves.Comment: 43 pages, 18 figure
Spectral evolution of multiply-impulsive solar bursts
Hard X-ray and microwave observations of multiply-impulsive solar bursts, identified in the OSO-5 data were analyzed. Spectra in both frequency ranges were used to determine whether or not the source properties change from peak to peak within individual bursts. Two categories of microwave spectral behavior were identified: those events during which the microwave turnover frequency and spectral shape remain the same from peak to peak, and those during which the turnover frequency and spectral shape change significantly. These categories correspond to two classes of multiply-impulsive bursts: those for which the emission can be characterized by a constant magnetic field and therefore a single source region, in which case the multiplicity may be due to modulation of the emission process; and those in which groups of component spikes appear to originate in regions of different magnetic-field strengths, corresponding to separate source regions which flare sequentially. Examples of the latter type of events are presented. The discrete flaring regions are analyzed and their spatial separations estimated
The effects of magnetic-field geometry on longitudinal oscillations of solar prominences: Cross-sectional area variation for thin tubes
Solar prominences are subject to both field-aligned (longitudinal) and
transverse oscillatory motions, as evidenced by an increasing number of
observations. Large-amplitude longitudinal motions provide valuable information
on the geometry of the filament-channel magnetic structure that supports the
cool prominence plasma against gravity. Our pendulum model, in which the
restoring force is the gravity projected along the dipped field lines of the
magnetic structure, best explains these oscillations. However, several factors
can influence the longitudinal oscillations, potentially invalidating the
pendulum model. The aim of this work is to study the influence of large-scale
variations in the magnetic field strength along the field lines, i.e.,
variations of the cross-sectional area along the flux tubes supporting
prominence threads. We studied the normal modes of several flux tube
configurations, using linear perturbation analysis, to assess the influence of
different geometrical parameters on the oscillation properties. We found that
the influence of the symmetric and asymmetric expansion factors on longitudinal
oscillations is small.}{We conclude that the longitudinal oscillations are not
significantly influenced by variations of the cross-section of the flux tubes,
validating the pendulum model in this context.Comment: Accepted for publication in Astronomy & Astrophysic
Magnetic-Island Contraction and Particle Acceleration in Simulated Eruptive Solar Flares
The mechanism that accelerates particles to the energies required to produce
the observed high-energy impulsive emission in solar flares is not well
understood. Drake et al. (2006) proposed a mechanism for accelerating electrons
in contracting magnetic islands formed by kinetic reconnection in multi-layered
current sheets. We apply these ideas to sunward-moving flux ropes (2.5D
magnetic islands) formed during fast reconnection in a simulated eruptive
flare. A simple analytic model is used to calculate the energy gain of
particles orbiting the field lines of the contracting magnetic islands in our
ultrahigh-resolution 2.5D numerical simulation. We find that the estimated
energy gains in a single island range up to a factor of five. This is higher
than that found by Drake et al. for islands in the terrestrial magnetosphere
and at the heliopause, due to strong plasma compression that occurs at the
flare current sheet. In order to increase their energy by two orders of
magnitude and plausibly account for the observed high-energy flare emission,
the electrons must visit multiple contracting islands. This mechanism should
produce sporadic emission because island formation is intermittent. Moreover, a
large number of particles could be accelerated in each
magnetohydrodynamic-scale island, which may explain the inferred rates of
energetic-electron production in flares. We conclude that island contraction in
the flare current sheet is a promising candidate for electron acceleration in
solar eruptions.Comment: Accepted for publication in The Astrophysical Journal (2016
The effects of magnetic-field geometry on longitudinal oscillations of solar prominences
We investigate the influence of the geometry of the solar filament magnetic
structure on the large-amplitude longitudinal oscillations. A representative
filament flux tube is modeled as composed of a cool thread centered in a dipped
part with hot coronal regions on either side. We have found the normal modes of
the system, and establish that the observed longitudinal oscillations are well
described with the fundamental mode. For small and intermediate curvature radii
and moderate to large density contrast between the prominence and the corona,
the main restoring force is the solar gravity. In this full wave description of
the oscillation a simple expression for the oscillation frequencies is derived
in which the pressure-driven term introduces a small correction. We have also
found that the normal modes are almost independent of the geometry of the hot
regions of the tube. We conclude that observed large-amplitude longitudinal
oscillations are driven by the projected gravity along the flux tubes, and are
strongly influenced by the curvature of the dips of the magnetic field in which
the threads reside
On the origin of multiply-impulsive emission from solar flares
A set of solar hard X-ray bursts observed with the hard X-ray burst spectrometer on board the OSO-5 satellite was analyzed. The multiply-impulsive two stage events were selected on the basis of both morphological characteristics and association with appropriate phenomena at other wavelengths. Coincident radio, soft X-ray, H-alpha interplanetary particle, and magnetographic data were obtained from several observatories, to aid in developing a comprehensive picture of the physical processes underlying these complex bursts. Two classes of multiply impulsive bursts were identified: events whose components spikes apparently originate in one location, and events in which groups of spikes appear to come from separate regions which flare sequentially. The origin of multiplicity in the case of a single source region remains unidentified. Purely impulsive emissions show no sign of betatron acceleration, thus eliminating this mechanisn as a candidate for inducing multiply spiked structure. The majority of the two stage bursts, however, exhibited spectral behavior consistent with the betatron model, for the first few minutes of the second stage. Betatron acceleration thus has been identified as a common second stage phenomenon
A model for straight and helical solar jets: II. Parametric study of the plasma beta
Jets are dynamic, impulsive, well-collimated plasma events that develop at
many different scales and in different layers of the solar atmosphere.
Jets are believed to be induced by magnetic reconnection, a process central
to many astrophysical phenomena. Within the solar atmosphere, jet-like events
develop in many different environments, e.g., in the vicinity of active regions
as well as in coronal holes, and at various scales, from small photospheric
spicules to large coronal jets. In all these events, signatures of helical
structure and/or twisting/rotating motions are regularly observed. The present
study aims to establish that a single model can generally reproduce the
observed properties of these jet-like events.
In this study, using our state-of-the-art numerical solver ARMS, we present a
parametric study of a numerical tridimensional magnetohydrodynamic (MHD) model
of solar jet-like events. Within the MHD paradigm, we study the impact of
varying the atmospheric plasma on the generation and properties of
solar-like jets.
The parametric study validates our model of jets for plasma ranging
from to , typical of the different layers and magnetic
environments of the solar atmosphere. Our model of jets can robustly explain
the generation of helical solar jet-like events at various . This
study introduces the new result that the plasma modifies the morphology
of the helical jet, explaining the different observed shapes of jets at
different scales and in different layers of the solar atmosphere.
Our results allow us to understand the energisation, triggering, and driving
processes of jet-like events. Our model allows us to make predictions of the
impulsiveness and energetics of jets as determined by the surrounding
environment, as well as the morphological properties of the resulting jets.Comment: Accepted in Astronomy and Astrophysic
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