42 research outputs found
Two Energy Release Processes for CMEs: MHD Catastrophe and Magnetic Reconnection
It remains an open question how magnetic energy is rapidly released in the
solar corona so as to create solar explosions such as solar flares and coronal
mass ejections (CMEs). Recent studies have confirmed that a system consisting
of a flux rope embedded in a background field exhibits a catastrophic behavior,
and the energy threshold at the catastrophic point may exceed the associated
open field energy. The accumulated free energy in the corona is abruptly
released when the catastrophe takes place, and it probably serves as the main
means of energy release for CMEs at least in the initial phase. Such a release
proceeds via an ideal MHD process in contrast with nonideal ones such as
magnetic reconnection. The catastrophe results in a sudden formation of
electric current sheets, which naturally provide proper sites for fast magnetic
reconnection. The reconnection may be identified with a solar flare associated
with the CME on one hand, and produces a further acceleration of the CME on the
other. On this basis, several preliminary suggestions are made for future
observational investigations, especially with the proposed KuaFu satellites, on
the roles of the MHD catastrophe and magnetic reconnection in the magnetic
energy release associated with CMEs and flares.Comment: 7 pages, 4 figures, Adv. Spa. Res., in press
Investigating the driving mechanisms of coronal mass ejections
The objective of this investigation was to first examine the kinematics of
coronal mass ejections (CMEs) using EUV and coronagraph images, and then to
make a comparison with theoretical models in the hope to identify the driving
mechanisms of the CMEs. We have studied two CMEs which occurred on 2006 Dec. 17
(CME06) and 2007 Dec. 31 (CME07). The models studied in this work were
catastrophe, breakout, and toroidal instability models. We found that after the
eruption, the accelerations of both events exhibited a drop before increasing
again. Our comparisons with the theories suggested that CME06 can be best
described by a hybrid of the catastrophe and breakout models while CME07 is
most consistent with the breakout model.Comment: 9 pages 7 figure
Photospheric flux cancellation and associated flux rope formation and eruption
We study an evolving bipolar active region that exhibits flux cancellation at
the internal polarity inversion line, the formation of a soft X-ray sigmoid
along the inversion line and a coronal mass ejection. The evolution of the
photospheric magnetic field is described and used to estimate how much flux is
reconnected into the flux rope. About one third of the active region flux
cancels at the internal polarity inversion line in the 2.5~days leading up to
the eruption. In this period, the coronal structure evolves from a weakly to a
highly sheared arcade and then to a sigmoid that crosses the inversion line in
the inverse direction. These properties suggest that a flux rope has formed
prior to the eruption. The amount of cancellation implies that up to 60% of the
active region flux could be in the body of the flux rope. We point out that
only part of the cancellation contributes to the flux in the rope if the arcade
is only weakly sheared, as in the first part of the evolution. This reduces the
estimated flux in the rope to or less of the active region flux. We
suggest that the remaining discrepancy between our estimate and the limiting
value of of the active region flux, obtained previously by the flux
rope insertion method, results from the incomplete coherence of the flux rope,
due to nonuniform cancellation along the polarity inversion line. A hot linear
feature is observed in the active region which rises as part of the eruption
and then likely traces out field lines close to the axis of the flux rope. The
flux cancellation and changing magnetic connections at one end of this feature
suggest that the flux rope reaches coherence by reconnection shortly before and
early in the impulsive phase of the associated flare. The sigmoid is destroyed
in the eruption but reforms within a few hours after a moderate amount of
further cancellation has occurred.Comment: Astron. Astrophys., in pres
Analysis and interpretation of a fast limb CME with eruptive prominence, C-flare and EUV dimming
Coronal Mass ejections or CMEs are large dynamical solar-corona events. The
mass balance and kinematics of a fast limb CME, including its prominence
progenitor and the associated flare, will be compared with computed magnetic
structures to look for their origin and effect.
Multi-wavelength ground-based and space-borne observations are used to study
a fast W-limb CME event of December 2, 2003, taking into account both on and
off disk observations. Its erupting prominence is measured at high cadence with
the Pic du Midi full H-alpha line-flux imaging coronagraph. EUV images from
space instruments are processed including difference imaging. SOHO/LASCO images
are used to study the mass excess and motions. A fast bright expanding coronal
loop is identified in the region recorded slightly later by GOES as a C7.2
flare, followed by a brightening and an acceleration phase of the erupting
material with both cool and hot components. The total coronal radiative flux
dropped by 5 percent in the EUV channels, revealing a large dimming effect at
and above the limb. The typical 3-part structure observed 1 hour later shows a
core shaped similarly to the eruptive filament/prominence. The total measured
mass of the escaping CME (1.5x10to16 g from C2 LASCO observations) definitely
exceeds the estimated mass of the escaping cool prominence material although
assumptions made to analyse the Ha erupting prominence, as well as the
corresponding EUV darkening of the filament observed several days before, made
this evaluation uncertain by a factor of 2. From the current free extrapolation
we discuss the shape of the magnetic neutral surface and a possible scenario
leading to an instability, including the small scale dynamics inside and around
the filament.Comment: 11 pages, 9 figure
Multiwavelength Study of M8.9/3B Solar Flare from AR NOAA 10960
We present a multi-wavelength analysis of a long duration white-light solar
flare (M8.9/3B) event that occurred on 4 June 2007 from NOAA AR 10960. The
flare was observed by several spaceborne instruments, namely SOHO/MDI,
Hinode/SOT, TRACE and STEREO/SECCHI. The flare was initiated near a small,
positive-polarity, satellite sunspot at the centre of the AR, surrounded by
opposite-polarity field regions. MDI images of the AR show considerable amount
of changes in a small positive-polarity sunspot of delta configuration during
the flare event. SOT/G-band (4305 A) images of the sunspot also suggest the
rapid evolution of the positive-polarity sunspot with highly twisted penumbral
filaments before the flare event, which were oriented in the counterclockwise
direction. It shows the change in orientation and also remarkable disappearance
of twisted penumbral filaments (~35-40%) and enhancement in umbral area
(~45-50%) during the decay phase of the flare. TRACE and SECCHI observations
reveal the successive activations of two helical twisted structures associated
with this sunspot, and the corresponding brightening in the chromosphere as
observed by the time-sequence images of SOT/Ca II H line (3968 A). The
secondary-helical twisted structure is found to be associated with the M8.9
flare event. The brightening starts 6-7 min prior to the flare maximum with the
appearance of secondary helical-twisted structure. The flare intensity
maximizes as this structure moves away from the AR. This twisted flux-tube
associated with the flare triggering, is found to be failed in eruption. The
location of the flare is found to coincide with the activation site of the
helical twisted structures. We conclude that the activations of successive
helical twists in the magnetic flux tubes/ropes plays a crucial role in the
energy build-up process and triggering of M-class solar flare without a CME.Comment: 22 pages, 12 figures, Accepted for Publication in Solar Physic
Triggering an eruptive flare by emerging flux in a solar active-region complex
A flare and fast coronal mass ejection originated between solar active
regions NOAA 11514 and 11515 on July 1, 2012 in response to flux emergence in
front of the leading sunspot of the trailing region 11515. Analyzing the
evolution of the photospheric magnetic flux and the coronal structure, we find
that the flux emergence triggered the eruption by interaction with overlying
flux in a non-standard way. The new flux neither had the opposite orientation
nor a location near the polarity inversion line, which are favorable for strong
reconnection with the arcade flux under which it emerged. Moreover, its flux
content remained significantly smaller than that of the arcade (approximately
40 %). However, a loop system rooted in the trailing active region ran in part
under the arcade between the active regions, passing over the site of flux
emergence. The reconnection with the emerging flux, leading to a series of jet
emissions into the loop system, caused a strong but confined rise of the loop
system. This lifted the arcade between the two active regions, weakening its
downward tension force and thus destabilizing the considerably sheared flux
under the arcade. The complex event was also associated with supporting
precursor activity in an enhanced network near the active regions, acting on
the large-scale overlying flux, and with two simultaneous confined flares
within the active regions.Comment: Accepted for publication in Topical Issue of Solar Physics: Solar and
Stellar Flares. 25 pages, 12 figure
Probing the Role of Magnetic-Field Variations in NOAA AR 8038 in Producing Solar Flare and CME on 12 May 1997
We carried out a multi-wavelength study of a CME and a medium-size 1B/C1.3
flare occurring on 12 May 1997. We present the investigation of magnetic-field
variations in the NOAA Active Region 8038 which was observed on the Sun during
7--16 May 1997. Analyses of H{\alpha} filtergrams and MDI/SOHO magnetograms
revealed continual but discrete surge activity, and emergence and cancellation
of flux in this active region. The movie of these magnetograms revealed two
important results that the major opposite polarities of pre-existing region as
well as in the emerging flux region (EFR) were approaching towards each other
and moving magnetic features (MMF) were ejecting out from the major north
polarity at a quasi-periodicity of about ten hrs during 10--13 May 1997. These
activities were probably caused by the magnetic reconnection in the lower
atmosphere driven by photospheric convergence motions, which were evident in
magnetograms. The magnetic field variations such as flux, gradient, and sunspot
rotation revealed that free energy was slowly being stored in the corona. The
slow low-layer magnetic reconnection may be responsible for this storage and
the formation of a sigmoidal core field or a flux rope leading to the eventual
eruption. The occurrence of EUV brightenings in the sigmoidal core field prior
to the rise of a flux rope suggests that the eruption was triggered by the
inner tether-cutting reconnection, but not the external breakout reconnection.
An impulsive acceleration revealed from fast separation of the H{\alpha}
ribbons of the first 150 seconds suggests the CME accelerated in the inner
corona, which is consistent with the temporal profile of the reconnection
electric field. In conclusion, we propose a qualitative model in view of
framework of a solar eruption involving, mass ejections, filament eruption,
CME, and subsequent flare.Comment: 8 figures, accepted for publication in Solar Physic
Electric current circuits in astrophysics
Cosmic magnetic structures have in common that they are anchored
in a dynamo, that an external driver converts kinetic energy into internal
magnetic energy, that this magnetic energy is transported as Poynting fl ux across the magnetically dominated structure, and that the magnetic energy
is released in the form of particle acceleration, heating, bulk motion,
MHD waves, and radiation. The investigation of the electric current system is
particularly illuminating as to the course of events and the physics involved.
We demonstrate this for the radio pulsar wind, the solar flare, and terrestrial
magnetic storms
The Origin, Early Evolution and Predictability of Solar Eruptions
Coronal mass ejections (CMEs) were discovered in the early 1970s when space-borne coronagraphs revealed that eruptions of plasma are ejected from the Sun. Today, it is known that the Sun produces eruptive flares, filament eruptions, coronal mass ejections and failed eruptions; all thought to be due to a release of energy stored in the coronal magnetic field during its drastic reconfiguration. This review discusses the observations and physical mechanisms behind this eruptive activity, with a view to making an assessment of the current capability of forecasting these events for space weather risk and impact mitigation. Whilst a wealth of observations exist, and detailed models have been developed, there still exists a need to draw these approaches together. In particular more realistic models are encouraged in order to asses the full range of complexity of the solar atmosphere and the criteria for which an eruption is formed. From the observational side, a more detailed understanding of the role of photospheric flows and reconnection is needed in order to identify the evolutionary path that ultimately means a magnetic structure will erupt