122 research outputs found
Recurrent solar jets in active regions
We study the emergence of a toroidal flux tube into the solar atmosphere and
its interaction with a pre-existing field of an active region. We investigate
the emission of jets as a result of repeated reconnection events between
colliding magnetic fields. We perform 3D simulations by solving the
time-dependent, resistive MHD equations in a highly stratified atmosphere. A
small active region field is constructed by the emergence of a toroidal
magnetic flux tube. A current structure is build up and reconnection sets in
when new emerging flux comes into contact with the ambient field of the active
region. The topology of the magnetic field around the current structure is
drastically modified during reconnection. The modification results in a
formation of new magnetic systems that eventually collide and reconnect. We
find that reconnection jets are taking place in successive recurrent phases in
directions perpendicular to each other, while in each phase they release
magnetic energy and hot plasma into the solar atmosphere. After a series of
recurrent appearance of jets, the system approaches an equilibrium where the
efficiency of the reconnection is substantially reduced. We deduce that the
emergence of new magnetic flux introduces a perturbation to the active region
field, which in turn causes reconnection between neighboring magnetic fields
and the release of the trapped energy in the form of jet-like emissions. This
is the first time that self-consistent recurrency of jets in active regions is
shown in a three-dimensional experiment of magnetic flux emergence.Comment: 4 pages, 3 figures, accepted for publication (A&A
Emergence of non-twisted magnetic fields in the Sun: Jets and atmospheric response
Aims. We study the emergence of a non-twisted flux tube from the solar
interior into the solar atmosphere. We investigate whether the length of the
buoyant part of the flux tube (i.e. {\lambda}) affects the emergence of the
field and the dynamics of the evolving magnetic flux system. Methods. We
perform three-dimensional (3D), time-dependent, resistive, compressible MHD
simulations using the Lare3D code. Results. We find that there are considerable
differences in the dynamics of the emergence of a magnetic flux tube when
{\lambda} is varied. In the solar interior, for larger values of {\lambda}, the
rising magnetic field emerges faster and expands more due to its lower magnetic
tension. As a result, its field strength decreases and its emergence above the
photosphere occurs later than in the smaller {\lambda} case. However, in both
cases, the emerging field at the photosphere becomes unstable in two places,
forming two magnetic bipoles that interact dynamically during the evolution of
the system. Most of the dynamic phenomena occur at the current layer, which is
formed at the interface between the interacting bipoles. We find the formation
and ejection of plasmoids, the onset of successive jets from the interface, and
the impulsive heating of the plasma in the solar atmosphere. We discuss the
triggering mechanism of the jets and the atmospheric response to the emergence
of magnetic flux in the two cases.Comment: 16 pages, 15 figure
Combining particle acceleration and coronal heating via data-constrained calculations of nanoflares in coronal loops
We model nanoflare heating of extrapolated active-region coronal loops via
the acceleration of electrons and protons in Harris-type current sheets. The
kinetic energy of the accelerated particles is estimated using semi-analytical
and test-particle-tracing approaches. Vector magnetograms and photospheric
Doppler velocity maps of NOAA active region 09114, recorded by the Imaging
Vector Magnetograph (IVM), were used for this analysis. A current-free field
extrapolation of the active-region corona was first constructed. The
corresponding Poynting fluxes at the footpoints of 5000 extrapolated coronal
loops were then calculated. Assuming that reconnecting current sheets develop
along these loops, we utilized previous results to estimate the kinetic-energy
gain of the accelerated particles and we related this energy to nanoflare
heating and macroscopic loop characteristics. Kinetic energies of 0.1 to 8 keV
(for electrons) and 0.3 to 470 keV (for protons) were found to cause heating
rates ranging from to 1 . Hydrodynamic
simulations show that such heating rates can sustain plasma in coronal
conditions inside the loops and generate plasma thermal distributions which are
consistent with active region observations. We concluded the analysis by
computing the form of X-ray spectra generated by the accelerated electrons
using the thick target approach that were found to be in agreement with
observed X-ray spectra, thus supporting the plausibility of our
nanoflare-heating scenario.Comment: 11 figure
The spectroscopic imprint of the pre-eruptive configuration resulting into two major coronal mass ejections
P.S acknowledges financial support from the programme Aristotelis/SIEMENS at the NOA.Aims: We present a spectroscopic analysis of the pre-eruptive configuration of active region NOAA 11429, prior to two very fast coronal mass ejections (CMEs) on March 7, 2012 that are associated with this active region. We study the thermal components and the dynamics associated with the ejected flux ropes. Methods: Using differential emission measure (DEM) analysis of Hinode/EIS and SDO/AIA observations, we identify the emission components of both the flux rope and the host active region. We then follow the time evolution of the flux rope emission components by using AIA observations. The plasma density and the Doppler and non-thermal velocities associated with the flux ropes are also calculated from the EIS data. Results: The eastern and western parts of the active region, in which the two different fast CMEs originated during two X-class flares, were studied separately. In both regions we identified an emission component in the temperature range of log T = 6.8-7.1 associated with the presence of flux ropes. The time evolution of the eastern region showed an increase in the mean DEM in this temperature range by an order of magnitude, 5 h prior to the first CME. This was associated with a gradual rise and heating of the flux rope as manifested by blue-shifts and increased non-thermal velocities in Ca xv 200.97 Å, respectively. An overall upward motion of the flux ropes was measured (relative blue-shifts of ~12 km s-1). The measured electron density was found to be 4× 109-2 × 1010 cm-3 (using the ratio of Ca xv 181.90 Å over Ca xv 200.97 Å). We compare our findings with other works on the same AR to provide a unified picture of its evolution.PostprintPeer reviewe
Emergence of small-scale magnetic flux in the quiet Sun
We study the evolution of a small-scale emerging flux region (EFR) in the
quiet Sun, from its emergence to its decay. We track processes and phenomena
across all atmospheric layers, explore their interrelations and compare our
findings with recent numerical modelling studies. We used imaging, spectral and
spectropolarimetric observations from space-borne and ground-based instruments.
The EFR appears next to the chromospheric network and shows all characteristics
predicted by numerical simulations. The total magnetic flux of the EFR exhibits
distinct evolutionary phases, namely an initial subtle increase, a fast
increase and expansion of the region area, a more gradual increase, and a slow
decay. During the initial stages, bright points coalesce, forming clusters of
positive- and negative-polarity in a largely bipolar configuration. During the
fast expansion, flux tubes make their way to the chromosphere, producing
pressure-driven absorption fronts, visible as blueshifted chromospheric
features. The connectivity of the quiet-Sun network gradually changes and part
of the existing network forms new connections with the EFR. A few minutes after
the bipole has reached its maximum magnetic flux, it brightens in soft X-rays
forming a coronal bright point, exhibiting episodic brightenings on top of a
long smooth increase. These coronal brightenings are also associated with
surge-like chromospheric features, which can be attributed to reconnection with
adjacent small-scale magnetic fields and the ambient magnetic field. The
emergence of magnetic flux even at the smallest scales can be the driver of a
series of energetic phenomena visible at various atmospheric heights and
temperature regimes. Multi-wavelength observations reveal a wealth of
mechanisms which produce diverse observable effects during the different
evolutionary stages of these small-scale structures.Comment: Accepted for publication in Astronomy & Astrophysics 14 pages, 14
figure
Sensitivity of solar off-limb line profiles to electron density stratification and the velocity distribution anisotropy
The effect of the electron density stratification on the intensity profiles
of the H I Ly- line and the O VI and Mg X doublets formed in solar
coronal holes is investigated. We employ an analytical 2-D model of the large
scale coronal magnetic field that provides a good representation of the corona
at the minimum of solar activity. We use the mass-flux conservation equation to
determine the outflow speed of the solar wind at any location in the solar
corona and take into account the integration along the line of sight (LOS). The
main assumption we make is that no anisotropy in the kinetic temperature of the
coronal species is considered. We find that at distances greater than 1 Rsun
from the solar surface the widths of the emitted lines of O VI and Mg X are
sensitive to the details of the adopted electron density stratification.
However, Ly-, which is a pure radiative line, is hardly affected. The
calculated total intensities of Ly- and the O VI doublet depend to a
lesser degree on the density stratification and are comparable to the observed
ones for most of the considered density models. The widths of the observed
profiles of Ly- and Mg X are well reproduced by most of the considered
electron density stratifications, while for the O VI doublet only few
stratifications give satisfying results. The densities deduced from SOHO data
result in O VI profiles whose widths and intensity ratio are relatively close
to the values observed by UVCS although only isotropic velocity distributions
are employed. These density profiles also reproduce the other considered
observables with good accuracy. Thus the need for a strong anisotropy of the
velocity distribution (i.e. a temperature anisotropy) is not so clear cut as
previous investigations of UVCS data suggested. ...Comment: 11 pages; 11 figure
Particle interactions with single or multiple 3D solar reconnecting current sheets
The acceleration of charged particles (electrons and protons) in flaring
solar active regions is analyzed by numerical experiments. The acceleration is
modelled as a stochastic process taking place by the interaction of the
particles with local magnetic reconnection sites via multiple steps. Two types
of local reconnecting topologies are studied: the Harris-type and the X-point.
A formula for the maximum kinetic energy gain in a Harris-type current sheet,
found in a previous work of ours, fits well the numerical data for a single
step of the process. A generalization is then given approximating the kinetic
energy gain through an X-point. In the case of the multiple step process, in
both topologies the particles' kinetic energy distribution is found to acquire
a practically invariant form after a small number of steps. This tendency is
interpreted theoretically. Other characteristics of the acceleration process
are given, such as the mean acceleration time and the pitch angle distributions
of the particles.Comment: 18 pages, 9 figures, Solar Physics, in pres
Study of the three-dimensional shape and dynamics of coronal loops observed by Hinode/EIS
We study plasma flows along selected coronal loops in NOAA Active Region
10926, observed on 3 December 2006 with Hinode's EUV Imaging Spectrograph
(EIS). From the shape of the loops traced on intensity images and the Doppler
shifts measured along their length we compute their three-dimensional (3D)
shape and plasma flow velocity using a simple geometrical model. This
calculation was performed for loops visible in the Fe VIII 185 Ang., Fe X 184
Ang., Fe XII 195 Ang., Fe XIII 202 Ang., and Fe XV 284 Ang. spectral lines. In
most cases the flow is unidirectional from one footpoint to the other but there
are also cases of draining motions from the top of the loops to their
footpoints. Our results indicate that the same loop may show different flow
patterns when observed in different spectral lines, suggesting a dynamically
complex rather than a monolithic structure. We have also carried out magnetic
extrapolations in the linear force-free field approximation using SOHO/MDI
magnetograms, aiming toward a first-order identification of extrapolated
magnetic field lines corresponding to the reconstructed loops. In all cases,
the best-fit extrapolated lines exhibit left-handed twist (alpha < 0), in
agreement with the dominant twist of the region.Comment: 17 pages, 6 figure
Z Cam stars: a particular response to a general phenomenon
We show that the disc instability model can reproduce all the observed
properties of Z Cam stars if the energy equation includes heating of the outer
disc by the mass-transfer stream impact and by tidal torques and if the
mass-transfer rate from the secondary varies by about 30% around the value
critical for stability. In particular the magnitude difference between outburst
maxima and standstills corresponds to observations, all outbursts are of the
inside-out type and can be divided into two classes: long (wide) and short
(narrow) outbursts, as observed. Mass transfer rate fluctuations should occur
in other dwarf novae but one can exclude variations similar to those observed
in magnetic systems (AM Her's and some DQ Her's) and some nova-like systems (VY
Scl's), in which \dot{M} become very small during low states; these would
produce mini-outburst which, although detectable, have never been observed.Comment: submitted to Astronomy & Astrophysics, 8 pages, 4 figure
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