169 research outputs found
Gradual Solar Coronal Dimming and Evolution of Coronal Mass Ejection in the Early Phase
We report observations of a two-stage coronal dimming in an eruptive event of
a two-ribbon flare and a fast coronal mass ejection (CME). Weak gradual dimming
persists for more than half an hour before the onset of the two-ribbon flare
and the fast rise of the CME. It is followed by abrupt rapid dimming. The
two-stage dimming occurs in a pair of conjugate dimming regions adjacent to the
two flare ribbons, and the flare onset marks the transition between the two
stages of dimming. At the onset of the two-ribbon flare, transient brightenings
are also observed inside the dimming regions, before rapid dimming occurs at
the same places. These observations suggest that the CME structure, most
probably anchored at the twin dimming regions, undergoes a slow rise before the
flare onset, and its kinematic evolution has significantly changed at the onset
of flare reconnection. We explore diagnostics of the CME evolution in the early
phase with analysis of the gradual dimming signatures prior to the CME
eruption
Direct observation of high-speed plasma outflows produced by magnetic reconnection in solar impulsive events
Spectroscopic observations of a solar limb flare recorded by SUMER on SOHO
reveal, for the first time, hot fast magnetic reconnection outflows in the
corona. As the reconnection site rises across the SUMER spectrometer slit,
significant blue- and red-shift signatures are observed in sequence in the Fe
XIX line, reflecting upflows and downflows of hot plasma jets, respectively.
With the projection effect corrected, the measured outflow speed is between
900-3500 km/s, consistent with theoretical predictions of the Alfvenic outflows
in magnetic reconnection region in solar impulsive events. Based on theoretic
models, the magnetic field strength near the reconnection region is estimated
to be 19-37 Gauss.Comment: 5 pages, 6 color figures, 1 animation onlin
Heating of Flare Loops With Observationally Constrained Heating Functions
We analyze high cadence high resolution observations of a C3.2 flare obtained
by AIA/SDO on August 1, 2010. The flare is a long duration event with soft
X-ray and EUV radiation lasting for over four hours. Analysis suggests that
magnetic reconnection and formation of new loops continue for more than two
hours. Furthermore, the UV 1600\AA\ observations show that each of the
individual pixels at the feet of flare loops is brightened instantaneously with
a timescale of a few minutes, and decays over a much longer timescale of more
than 30 minutes. We use these spatially resolved UV light curves during the
rise phase to construct empirical heating functions for individual flare loops,
and model heating of coronal plasmas in these loops. The total coronal
radiation of these flare loops are compared with soft X-ray and EUV radiation
fluxes measured by GOES and AIA. This study presents a method to
observationally infer heating functions in numerous flare loops that are formed
and heated sequentially by reconnection throughout the flare, and provides a
very useful constraint to coronal heating models.Comment: This paper is revise
Properties and Energetics of Magnetic Reconnection: I. Evolution of Flare Ribbons
In this article, we measure the mean magnetic shear from the morphological
evolution of flare ribbons, and examine the evolution of flare thermal and
non-thermal X-ray emissions during the progress of flare reconnection. We
analyze three eruptive flares and three confined flares ranging from GOES class
C8.0 to M7.0. They exhibit well-defined two ribbons along the magnetic polarity
inversion line (PIL), and have been observed by the Atmospheric Imaging
Assembly and the Ramaty High Energy Solar Spectroscopic Imager from the onset
of the flare throughout the impulsive phase. The analysis confirms the
strong-to-weak shear evolution in the core region of the flare, and the flare
hard X-ray emission rises as the shear decreases. In eruptive flares in this
sample, significant non-thermal hard X-ray emission lags the ultraviolet
emission from flare ribbons, and rises rapidly when the shear is modest. In all
flares, we observe that the plasma temperature rises in the early phase when
the flare ribbons rapidly spread along the PIL and the shear is high. We
compare these results with prior studies, and discuss their implications, as
well as complications, related to physical mechanisms governing energy
partition during flare reconnection.Comment: 28 pages, 8 figures; Solar Physic
IRIS Observations of the Mg II h & k Lines During a Solar Flare
The bulk of the radiative output of a solar flare is emitted from the
chromosphere, which produces enhancements in the optical and UV continuum, and
in many lines, both optically thick and thin. We have, until very recently,
lacked observations of two of the strongest of these lines: the Mg II h & k
resonance lines. We present a detailed study of the response of these lines to
a solar flare. The spatial and temporal behaviour of the integrated
intensities, k/h line ratios, line of sight velocities, line widths and line
asymmetries were investigated during an M class flare (SOL2014-02-13T01:40).
Very intense, spatially localised energy input at the outer edge of the ribbon
is observed, resulting in redshifts equivalent to velocities of ~15-26km/s,
line broadenings, and a blue asymmetry in the most intense sources. The
characteristic central reversal feature that is ubiquitous in quiet Sun
observations is absent in flaring profiles, indicating that the source function
increases with height during the flare. Despite the absence of the central
reversal feature, the k/h line ratio indicates that the lines remain optically
thick during the flare. Subordinate lines in the Mg II passband are observed to
be in emission in flaring sources, brightening and cooling with similar
timescales to the resonance lines. This work represents a first analysis of
potential diagnostic information of the flaring atmosphere using these lines,
and provides observations to which synthetic spectra from advanced radiative
transfer codes can be compared.Comment: 12 pages, 14 figures, Accepted for publication in Astronomy and
Astrophysic
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