85 research outputs found
Photospheric Injection of Magnetic Helicity: Connectivity--based Flux Density Method
Magnetic helicity quantifies how globally sheared and/or twisted is the
magnetic field in a volume. This quantity is believed to play a key role in
solar activity due to its conservation property. Helicity is continuously
injected into the corona during the evolution of active regions (ARs). To
better understand and quantify the role of magnetic helicity in solar activity,
the distribution of magnetic helicity flux in ARs needs to be studied. The
helicity distribution can be computed from the temporal evolution of
photospheric magnetograms of ARs such as the ones provided by SDO/HMI and
Hinode/SOT. Most recent analyses of photospheric helicity flux derive an
helicity flux density proxy based on the relative rotation rate of photospheric
magnetic footpoints. Although this proxy allows a good estimate of the
photospheric helicity flux, it is still not a true helicity flux density
because it does not take into account the connectivity of the magnetic field
lines. For the first time, we implement a helicity density which takes into
account such connectivity. In order to use it for future observational studies,
we test the method and its precision on several types of models involving
different patterns of helicity injection. We also test it on more complex
configurations - from magnetohydrodynamics (MHD) simulations - containing
quasi-separatrix layers. We demonstrate that this connectivity-based helicity
flux density proxy is the best to map the true distribution of photospheric
helicity injection.Comment: Solar Physics, June 2013 (this is the version of the author, a
definitive version is now available in the online journal
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
The New 2018 Version of the Meudon Spectroheliograph
Daily full-disk observations of the solar photosphere and chromosphere
started at Meudon Observatory in 1908. After a review of the scientific context
and the historical background, we describe the instrumental characteristics and
capabilities of the new version operating since 2018. The major change is the
systematic recording of full line profiles over the entire solar disk providing
3D data cubes. Spectral and spatial sampling are both improved. Classical 2D
images of the Sun at fixed wavelength are still delivered. We summarize the
different processing levels of on-line data and briefly review the new
scientific perspectives.Comment: 14 pages, 5 figures; Published in Solar Physic
The origin of net electric currents in solar active regions
There is a recurring question in solar physics about whether or not electric
currents are neutralized in active regions (ARs). This question was recently
revisited using three-dimensional (3D) magnetohydrodynamic (MHD) numerical
simulations of magnetic flux emergence into the solar atmosphere. Such
simulations showed that flux emergence can generate a substantial net current
in ARs. Another source of AR currents are photospheric horizontal flows. Our
aim is to determine the conditions for the occurrence of net vs. neutralized
currents with this second mechanism. Using 3D MHD simulations, we
systematically impose line-tied, quasi-static, photospheric twisting and
shearing motions to a bipolar potential magnetic field. We find that such
flows: (1) produce both {\it direct} and {\it return} currents, (2) induce very
weak compression currents - not observed in 2.5D - in the ambient field present
in the close vicinity of the current-carrying field, and (3) can generate
force-free magnetic fields with a net current. We demonstrate that neutralized
currents are in general produced only in the absence of magnetic shear at the
photospheric polarity inversion line - a special condition rarely observed. We
conclude that, as magnetic flux emergence, photospheric flows can build up net
currents in the solar atmosphere, in agreement with recent observations. These
results thus provide support for eruption models based on pre-eruption magnetic
fields possessing a net coronal current.Comment: 14 pages and 11 figures (Accepted in The Astrophysical Journal
First observational application of a connectivity--based helicity flux density
Measuring the magnetic helicity distribution in the solar corona can help in
understanding the trigger of solar eruptive events because magnetic helicity is
believed to play a key role in solar activity due to its conservation property.
A new method for computing the photospheric distribution of the helicity flux
was recently developed. This method takes into account the magnetic field
connectivity whereas previous methods were based on photospheric signatures
only. This novel method maps the true injection of magnetic helicity in active
regions. We applied this method for the first time to an observed active
region, NOAA 11158, which was the source of intense flaring activity. We used
high-resolution vector magnetograms from the SDO/HMI instrument to compute the
photospheric flux transport velocities and to perform a nonlinear force-free
magnetic field extrapolation. We determined and compared the magnetic helicity
flux distribution using a purely photospheric as well as a connectivity-based
method. While the new connectivity-based method confirms the mixed pattern of
the helicity flux in NOAA 11158, it also reveals a different, and more correct,
distribution of the helicity injection. This distribution can be important for
explaining the likelihood of an eruption from the active region. The
connectivity-based approach is a robust method for computing the magnetic
helicity flux, which can be used to study the link between magnetic helicity
and eruptivity of observed active regions.Comment: 4 pages, 3 figures; published online in A&A 555, L6 (2013
ROAM: a Radial-basis-function Optimization Approximation Method for diagnosing the three-dimensional coronal magnetic field
The Coronal Multichannel Polarimeter (CoMP) routinely performs coronal
polarimetric measurements using the Fe XIII 10747 and 10798 lines,
which are sensitive to the coronal magnetic field. However, inverting such
polarimetric measurements into magnetic field data is a difficult task because
the corona is optically thin at these wavelengths and the observed signal is
therefore the integrated emission of all the plasma along the line of sight. To
overcome this difficulty, we take on a new approach that combines a
parameterized 3D magnetic field model with forward modeling of the polarization
signal. For that purpose, we develop a new, fast and efficient, optimization
method for model-data fitting: the Radial-basis-functions Optimization
Approximation Method (ROAM). Model-data fitting is achieved by optimizing a
user-specified log-likelihood function that quantifies the differences between
the observed polarization signal and its synthetic/predicted analogue. Speed
and efficiency are obtained by combining sparse evaluation of the magnetic
model with radial-basis-function (RBF) decomposition of the log-likelihood
function. The RBF decomposition provides an analytical expression for the
log-likelihood function that is used to inexpensively estimate the set of
parameter values optimizing it. We test and validate ROAM on a synthetic test
bed of a coronal magnetic flux rope and show that it performs well with a
significantly sparse sample of the parameter space. We conclude that our
optimization method is well-suited for fast and efficient model-data fitting
and can be exploited for converting coronal polarimetric measurements, such as
the ones provided by CoMP, into coronal magnetic field data.Comment: 23 pages, 12 figures, accepted in Frontiers in Astronomy and Space
Science
Open questions on prominences from coordinated observations by IRIS, Hinode, SDO/AIA, THEMIS, and the Meudon/MSDP
Context. A large prominence was observed on September 24, 2013, for three
hours (12:12 UT -15:12 UT) with the newly launched (June 2013) Interface Region
Imaging Spectrograph (IRIS), THEMIS (Tenerife), the Hinode Solar Optical
Telescope (SOT), the Solar Dynamic Observatory Atmospheric Imaging Assembly
(SDO/AIA), and the Multichannel Subtractive Double Pass spectrograph (MSDP) in
the Meudon Solar Tower. Aims. The aim of this work is to study the dynamics of
the prominence fine structures in multiple wavelengths to understand their
formation. Methods. The spectrographs IRIS and MSDP provided line profiles with
a high cadence in Mg II and in Halpha lines. Results. The magnetic field is
found to be globally horizontal with a relatively weak field strength (8-15
Gauss). The Ca II movie reveals turbulent-like motion that is not organized in
specific parts of the prominence. On the other hand, the Mg II line profiles
show multiple peaks well separated in wavelength. Each peak corresponds to a
Gaussian profile, and not to a reversed profile as was expected by the present
non-LTE radiative transfer modeling. Conclusions. Turbulent fields on top of
the macroscopic horizontal component of the magnetic field supporting the
prominence give rise to the complex dynamics of the plasma. The plasma with the
high velocities (70 km/s to 100 km/s if we take into account the transverse
velocities) may correspond to condensation of plasma along more or less
horizontal threads of the arch-shape structure visible in 304 A. The steady
flows (5 km/s) would correspond to a more quiescent plasma (cool and
prominence-corona transition region) of the prominence packed into dips in
horizontal magnetic field lines. The very weak secondary peaks in the Mg II
profiles may reflect the turbulent nature of parts of the prominence.Comment: 15 pages, 14 figure
Magnetic field in atypical prominence structures: Bubble, tornado and eruption
Spectropolarimetric observations of prominences have been obtained with the
THEMIS telescope during four years of coordinated campaigns. Our aim is now to
understand the conditions of the cool plasma and magnetism in `atypical'
prominences, namely when the measured inclination of the magnetic field
departs, to some extent, from the predominantly horizontal field found in
`typical' prominences. What is the role of the magnetic field in these
prominence types? Are plasma dynamics more important in these cases than the
magnetic support? We focus our study on three types of `atypical' prominences
(tornadoes, bubbles and jet-like prominence eruptions) that have all been
observed by THEMIS in the He I D_3 line, from which the Stokes parameters can
be derived. The magnetic field strength, inclination and azimuth in each pixel
are obtained by using the Principal Component Analysis inversion method on a
model of single scattering in the presence of the Hanle effect. The magnetic
field in tornadoes is found to be more or less horizontal, whereas for the
eruptive prominence it is mostly vertical. We estimate a tendency towards
higher values of magnetic field strength inside the bubbles than outside in the
surrounding prominence. In all of the models in our database, only one magnetic
field orientation is considered for each pixel. While sufficient for most of
the main prominence body, this assumption appears to be oversimplified in
atypical prominence structures. We should consider these observations as the
result of superposition of multiple magnetic fields, possibly even with a
turbulent field component.Comment: 13 pages, 9 figure
Can we explain non-typical solar flares?
We used multi-wavelength high-resolution data from ARIES, THEMIS, and SDO
instruments, to analyze a non-standard, C3.3 class flare produced within the
active region NOAA 11589 on 2012 October 16. Magnetic flux emergence and
cancellation were continuously detected within the active region, the latter
leading to the formation of two filaments.
Our aim is to identify the origins of the flare taking into account the
complex dynamics of its close surroundings.
We analyzed the magnetic topology of the active region using a linear
force-free field extrapolation to derive its 3D magnetic configuration and the
location of quasi-separatrix layers (QSLs) which are preferential sites for
flaring activity. Because the active region's magnetic field was nonlinear
force-free, we completed a parametric study using different linear force-free
field extrapolations to demonstrate the robustness of the derived QSLs.
The topological analysis shows that the active region presented a complex
magnetic configuration comprising several QSLs. The considered data set
suggests that an emerging flux episode played a key role for triggering the
flare. The emerging flux likely activated the complex system of QSLs leading to
multiple coronal magnetic reconnections within the QSLs. This scenario accounts
for the observed signatures: the two extended flare-ribbons developed at
locations matched by the photospheric footprints of the QSLs, and were
accompanied with flare loops that formed above the two filaments which played
no important role in the flare dynamics.
This is a typical example of a complex flare that can a-priori show standard
flare signatures that are nevertheless impossible to interpret with any
standard model of eruptive or confined flare. We find that a topological
analysis however permitted to unveil the development of such complex sets of
flare signatures.Comment: 13 pages, Accepted in A&
- …