20 research outputs found
Flare Energy Release at the Magnetic Field Polarity Inversion Line During M1.2 Solar Flare of 2015 March 15. II. Investigation of Photospheric Electric Current and Magnetic Field Variations Using HMI 135-second Vector Magnetograms
This work is a continuation of Paper I [Sharykin et al., 2018] devoted to
analysis of nonthermal electron dynamics and plasma heating in the confined
M1.2 class solar flare SOL2015-03-15T22:43 revealing energy release in the
highly sheared interacting magnetic loops in the low corona, above the polarity
inversion line (PIL). The scope of the present work is to make the first
extensive quantitative analysis of the photospheric magnetic field and
photospheric vertical electric current (PVEC) dynamics in the confined flare
region near the PIL using new vector magnetograms obtained with the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory
(SDO) with high temporal resolution of 135 s. Data analysis revealed sharp
changes of the magnetic structure and PVEC associated with the flare onset near
the PIL. It was found that the strongest plasma heating and electron
acceleration were associated with the largest increase of the magnetic
reconnection rate, total PVEC and effective PVEC density in the flare ribbons.
Observations and non-linear force-free field (NLFFF) extrapolations showed that
the magnetic field structure around the PIL is consistent with the
tether-cutting magnetic reconnection (TCMR) geometry. We gave qualitative
interpretation of the observed dynamics of the flare ribbons, magnetic field
and PVEC, and electron acceleration, within the TCMR scenario
Azimuth ambiguity removal and non-linear force-free extrapolation of near-limb magnetic regions
Possibilities in principle for satisfactory removal of the 180-azimuthal
ambiguity in the transverse field of vector magnetograms and the extrapolation
of magnetic fields independently of their position on the solar disk are shown.
Revealed here is an exact correspondence between the estimated field and the
nonpotential loop structure on the limb. The Metropolis's algorithm modified to
work in spherical geometry is used to resolve the azimuthal ambiguity. Based on
a version of the optimization method from Rudenko and Myshyakov (2009), we use
corrected magnetograms as boundary conditions for magnetic field extrapolation
in the nonlinear force-free approximation
Evidence of the Relationship between the Emerging Magnetic Fields, Electric Currents, and Solar Flares Observed on May 10, 2012
We have analyzed multi-wavelength observations and magnetic-field data for
the solar flare of May 10, 2012 (04:18 UT) and have detected a sign inversion
of the signal in the line-of-sight magnetic measurements in the umbra of a
small spot. This effect is associated, at least partly, with the emergence of a
new magnetic field. Almost at the same time, a burst of hard X-rays was
recorded, and a wave in the vacuum ultraviolet (EUV) range (a "sunquake") was
generated due to the impact of the disturbance in the energy release range on
the photosphere. At the beginning of the event, a sigmoid flare was recorded,
but it did not spread, as it usually does, along the polarity inversion
(neutral) line. SDO/HMI full-vector measurements were used to extrapolate the
AR 11476 magnetic field to the corona, and the distribution of vertical
currents in the photosphere was obtained. The distribution of currents in
the active region shows that the relationship between them and the occurrence
of flares is very intricate. We have corroborated that the expected "ideal"
behavior of the current system before and after the flare (e.g., see (Sharykin
and Kosovichev, 2015)) is observed only in the sigmoid region. The results
obtained were compared with the observations of two other flares recorded in
this AR on the same day, one of which was similar to the flare under discussion
and the other was of different type. Our results confirm that the formation and
eruption of large-scale magnetic flux ropes in sigmoid flares are associated
with the shear motions in the photosphere and the emergence of twisted magnetic
tubes, as well as with the subsequent development of the torus instability.Comment: 16 pages, 6 figures; corrected versio
Flare SOL2012-07-06: on the origin of the circular polarization reversal between 17 GHz and 34 GHz
The new generations of multiwavelength radioheliographs with high spatial
resolution will employ microwave imaging spectropolarimetry to recover flare
topology and plasma parameters in the flare sources and along the wave
propagation paths. The recorded polarization depends on the emission mechanism
and emission regime (optically thick or thin), the emitting particle
properties, and propagation effects. Here, we report an unusual flare,
SOL2012-07-06T01:37, whose optically thin gyrosynchrotron emission of the main
source displays an apparently ordinary mode sense of polarization in contrast
to the classical theory that favors the extraordinary mode. This flare produced
copious nonthermal emission in hard X-rays and in high-frequency microwaves up
to 80 GHz. It is found that the main flare source corresponds to an interaction
site of two loops with greatly different sizes. We have investigated the three
possible known reasons of the circular polarization sense reversal - mode
coupling, positron contribution, and the effect of beamed angular distribution.
We excluded polarization reversal due to contribution of positrons because
there was no relevant response in the X-ray emission. We find that a beam-like
electron distribution can produce the observed polarization behavior, but the
source thermal density must be much higher than the estimate from to the X-ray
data. We conclude that the apparent ordinary wave emission in the optically
thin mode is due to radio wave propagation across the quasi-transverse (QT)
layer. The abnormally high transition frequency (above 35 GHz) can be achieved
reasonably low in the corona where the magnetic field value is high and
transverse to the line of sight. This places the microwave source below this QT
layer, i.e. very low in the corona.Comment: 15 pages, 7 figure
Probing Twisted Magnetic Field Using Microwave Observations in an M Class Solar Flare on 11 February, 2014
This work demonstrates the possibility of magnetic field topology
investigations using microwave polarimetric observations. We study a solar
flare of GOES M1.7 class that occurred on 11 February, 2014. This flare
revealed a clear signature of spatial inversion of the radio emission
polarization sign. We show that the observed polarization pattern can be
explained by nonthermal gyrosynchrotron emission from the twisted magnetic
structure. Using observations of the Reuven Ramaty High Energy Solar
Spectroscopic Imager, Nobeyama Radio Observatory, Radio Solar Telescope
Network, and Solar Dynamics Observatory, we have determined the parameters of
nonthermal electrons and thermal plasma and identified the magnetic structure
where the flare energy release occurred. To reconstruct the coronal magnetic
field, we use nonlinear force-free field (NLFFF) and potential magnetic field
approaches. Radio emission of nonthermal electrons is simulated by the GX
Simulator code using the extrapolated magnetic field and the parameters of
nonthermal electrons and thermal plasma inferred from the observations; the
model radio maps and spectra are compared with observations. We have found that
the potential magnetic field approach fails to explain the observed circular
polarization pattern; on the other hand, the Stokes map is successfully
explained by assuming nonthermal electrons to be distributed along the twisted
magnetic structure determined by the NLFFF extrapolation approach. Thus, we
show that the radio polarization maps can be used for diagnosing the topology
of the flare magnetic structures where nonthermal electrons are injected
A Challenging Solar Eruptive Event of 18 November 2003 and the Causes of the 20 November Geomagnetic Superstorm. III. Catastrophe of the Eruptive Filament at a Magnetic Null Point and Formation of an Opposite-Handedness CME
Our analysis in Papers I and II (Grechnev et al., 2014, Solar Phys. 289, 289
and 1279) of the 18 November 2003 solar event responsible for the 20 November
geomagnetic superstorm has revealed a complex chain of eruptions. In
particular, the eruptive filament encountered a topological discontinuity
located near the solar disk center at a height of about 100 Mm, bifurcated, and
transformed into a large cloud, which did not leave the Sun. Concurrently, an
additional CME presumably erupted close to the bifurcation region. The
conjectures about the responsibility of this compact CME for the superstorm and
its disconnection from the Sun are confirmed in Paper IV (Grechnev et al.,
Solar Phys., submitted), which concludes about its probable spheromak-like
structure. The present paper confirms the presence of a magnetic null point
near the bifurcation region and addresses the origin of the magnetic helicity
of the interplanetary magnetic clouds and their connection to the Sun. We find
that the orientation of a magnetic dipole constituted by dimmed regions with
the opposite magnetic polarities away from the parent active region
corresponded to the direction of the axial field in the magnetic cloud, while
the pre-eruptive filament mismatched it. To combine all of the listed findings,
we come to an intrinsically three-dimensional scheme, in which a spheromak-like
eruption originates via the interaction of the initially unconnected magnetic
fluxes of the eruptive filament and pre-existing ones in the corona. Through a
chain of magnetic reconnections their positive mutual helicity was transformed
into the self-helicity of the spheromak-like magnetic cloud.Comment: 30 pages, 14 figures. Accepted for publication in Solar Physics. The
final publication is available at Springer via
http://dx.doi.org/10.1007/s11207-014-0536-
Investigation of Relationship Between High-Energy X-ray Sources and Photospheric and Helioseismic Impacts of X1.8 Solar Flare of October 23, 2012
The X-class solar flare of October 23, 2012, generated continuum photospheric
emission and a strong helioseismic wave ("sunquake") that points to an
intensive energy release in the dense part of the solar atmosphere. We study
properties of the energy release with high temporal and spatial resolutions,
using photospheric data from the Helioseismic Magnetic Imager (HMI) onboard
Solar Dynamics Observatory (SDO), and hard X-ray observations made by the
Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). For this analysis we
use level-1 HMI data (filtergrams), obtained by scanning the Fe I line
(6731~\AA) with the time cadence of s and spatial resolution of
per pixel. It is found that the photospheric
disturbances caused by the flare spatially coincide with the region of hard
X-ray emission, but are delayed by seconds. This delay is
consistent with predictions of the flare hydrodynamics RADYN models. However,
the models fail to explain the magnitude of variations observed by the HMI. The
data indicate that the photospheric impact and helioseismic wave might be
caused by the electron energy flux substantially higher than that in the
current flare radiative hydrodynamic models.Comment: 23 pages, 7 figure
Flare Energy Release in the Magnetic Field Polarity Inversion Line During M1.2 Solar Flare of March 15, 2015. Paper I. Onset of Plasma Heating and Electrons Acceleration
We present the study of SOL2015-03-15 M1.2 flare, revealing acceleration of
electrons and plasma heating in the sheared twisted magnetic structure in the
polarity inversion line (PIL). The scope is to make the analysis of nonthermal
electrons dynamics and plasma heating in the highly stressed magnetic loops
interacting in the PIL by using X-ray, microwave, ultraviolet, and optical
observations. It is found that the most probable scenario for the energy
release in the PIL is the tether-cutting magnetic reconnection between the
low-lying (3 Mm above the photosphere) magnetic loops within a twisted magnetic
flux rope. Energetic electrons with the hardest spectrum were appeared at the
onset of plasma heating up to the super-hot temperature of 40 MK. These
electrons are localized in a thin magnetic channel with width of around 0.5 Mm
with high average magnetic field of about 1200 G. The plasma beta in the
super-hot region is less than 0.01. The estimated density of accelerated
electrons is about 10^9 cm^-3 that is much less than the super-hot plasma
density. The energy density flux of non-thermal electrons is estimated up to
3x10^12 ergs cm^-2s^-1 that is much higher than in the currently available
radiative hydrodynamic models. These results revealed that one need to develop
new self-consisting flare models reproducing 3D magnetic reconnection in the
PIL with strong magnetic field, spatial filamentation of energy release,
formation of high energy density populations of nonthermal electrons and
appearance of the super-hot plasma
Behaviour of oscillations in loop structures above active regions
In this study we combine the multiwavelength ultraviolet -- optical (Solar
Dynamics Observatory, SDO) and radio (Nobeyama Radioheliograph, NoRH)
observations to get further insight into space-frequency distribution of
oscillations at different atmospheric levels of the Sun. We processed the
observational data on NOAA 11711 active region and found oscillations
propagating from the photospheric level through the transition region upward
into the corona. The power maps of low-frequency (1--2 mHz) oscillations
reproduce well the fan-like coronal structures visible in the Fe ix 171A line.
High frequency oscillations (5--7 mHz) propagate along the vertical magnetic
field lines and concentrate inside small-scale elements in the umbra and at the
umbra-penumbra boundary. We investigated the dependence of the dominant
oscillation frequency upon the distance from the sunspot barycentre to estimate
inclination of magnetic tubes in higher levels of sunspots where it cannot be
measured directly, and found that this angle is close to 40 degrees above the
umbra boundaries in the transition region
Twin Null-Point-Associated Major Eruptive Three-Ribbon Flares with Unusual Microwave Spectra
On 23 July 2016 after 05:00\,UTC, the first 48-antenna stage of the Siberian
Radioheliograph detected two flares of M7.6 and M5.5 GOES importance that
occurred within half an hour in the same active region. Their multi-instrument
analysis reveals the following. The microwave spectra were flattened at low
frequencies and the spectrum of the stronger burst had a lower turnover
frequency. Each flare was eruptive, emitted hard X-rays and gamma-rays
exceeding 800\,keV, and had a rare three-ribbon configuration. An extended hard
X-ray source associated with a longest middle ribbon was observed in the second
flare. The unusual properties of the microwave spectra are accounted for by a
distributed multi-loop system in an asymmetric magnetic configuration that our
modeling confirms. Microwave images did not to resolve compact configurations
in these flares that may also be revealed incompletely in hard X-ray images
because of their limited dynamic range. Being apparently simple and compact,
non-thermal sources corresponded to the structures observed in the extreme
ultraviolet. In the scenario proposed for two successive three-ribbon eruptive
flares in a configuration with a coronal-null region, the first eruption causes
a flare and facilitates the second eruption that also results in a flare.Comment: 23 pages, 9 figures, "This is a pre-print of an article which will be
published in Solar Physics