410 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
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
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
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
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
CME Onset and Take-Off
For understanding and eventually predicting coronal mass ejections/eruptive flares, two critical questions must be answered: What is the mechanism for eruption onset, and what is the mechanism for the rapid acceleration? We address these questions in the context of the breakout model using 2.5D MHD simulations with adaptive mesh refinement (AMR). The AMR capability allowed us to achieve ultra-high numerical resolution and, thereby, determine the influence of the effective Lundquist number on the eruption. Our calculations show that, at least, for the breakout model, the onset of reconnection external to the highly sheared filament channel is the onset mechanism. Once this reconnection turns on, eruption is inevitable. However, as long as this is the only reconnection in the system, the eruption remains slow. We find that the eruption undergoes an abrupt "take-off" when the flare reconnection below the erupting plasmoid develops significant reconnection jets. We conclude that in fast CMEs, flare reconnection is the primary mechanism responsible for both flare heating and CME acceleration. We discuss the implications of these results for SDO observations and describe possible tests of the model
Consequences of the Breakout Model for Particle Acceleration in CMEs and Flares
The largest and most efficient particle accelerators in the solar system are the giant events consisting of a fast coronal mass ejection (CME) and an intense X-class solar flare. Both flares and CMEs can produce l0(exp 32) ergs or more in nonthermal particles. Two general processes are believed to be responsible: particle acceleration at the strong shock ahead of the CME, and reconnection-driven acceleration in the flare current sheet. Although shock acceleration is relatively well understood, the mechanism by which flare reconnection produces nonthermal particles is still an issue of great debate. We address the question of CME/flare particle acceleration in the context of the breakout model using 2.5D MHD simulations with adaptive mesh refinement (AMR). The AMR capability allows us to achieve ultra-high numerical resolution and, thereby, determine the detailed structure and dynamics of the flare reconnection region. Furthermore, we employ newly developed numerical analysis tools for identifying and characterizing magnetic nulls, so that we can quantify accurately the number and location of magnetic islands during reconnection. Our calculations show that flare reconnection is dominated by the formation of magnetic islands. In agreement with many other studies, we find that the number of islands scales with the effective Lundquist number. This result supports the recent work by Drake and co-workers that postulates particle acceleration by magnetic islands. On the other hand, our calculations also show that the flare reconnection region is populated by numerous shocks and other indicators of strong turbulence, which can also accelerate particles. We discuss the implications of our calculations for the flare particle acceleration mechanism and for observational tests of the models
Preflare magnetic and velocity fields
A characterization is given of the preflare magnetic field, using theoretical models of force free fields together with observed field structure to determine the general morphology. Direct observational evidence for sheared magnetic fields is presented. The role of this magnetic shear in the flare process is considered within the context of a MHD model that describes the buildup of magnetic energy, and the concept of a critical value of shear is explored. The related subject of electric currents in the preflare state is discussed next, with emphasis on new insights provided by direct calculations of the vertical electric current density from vector magnetograph data and on the role of these currents in producing preflare brightenings. Results from investigations concerning velocity fields in flaring active regions, describing observations and analyses of preflare ejecta, sheared velocities, and vortical motions near flaring sites are given. This is followed by a critical review of prevalent concepts concerning the association of flux emergence with flare
Ion-Neutral Coupling in Solar Prominence
Coupling between ions and neutrals in magnetized plasmas is fundamentally important to many aspects of heliophysics, including our ionosphere, the solar chromosphere, the solar wind interaction with planetary atmospheres, and the interface between the heliosphere and the interstellar medium. Ion-neutral coupling also plays a major role in the physics of solar prominences. By combining theory, modeling, and observations we are working toward a better understanding of the structure and dynamics of partially ionized prominence plasma. Two key questions are addressed in the present work: 1) what physical mechanism(s) sets the cross-field scale of prominence threads? 2) Are ion-neutral interactions responsible for the vertical flows and structure in prominences? We present initial results from a study investigating what role ion-neutral interactions play in prominence dynamics and structure. This research was supported by NASA
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