63 research outputs found
The north-south asymmetry of soft X-ray flare index during solar cycles 21, 22 and 23
In this paper the N-S asymmetry of the soft X-ray flare index during the
solar cycles 21, 22 and 23 has been analyzed. The results show the existence of
a real N-S asymmetry which is strengthened during solar minimum. The slope of
the regression lines fitted to the daily values of asymmetry time series has
been found to be negative in all the three cycles. The yearly asymmetry curve
can be fitted by a sinusoidal function with a period of eleven years. The power
spectral analysis of daily asymmetry time series reveals the significant
periods of around 28.26 days, 550.73 days and 3.72 years.Comment: 15 pages; 8 figures; Published in Solar Physic
Intermediate-term periodicities in soft X-ray flare index during solar cycles 21, 22 and 23
We have analyzed the intermediate term periodicities in soft X-ray flare
index () during solar cycles 21, 22 and 23. Power spectral analysis
of daily reveals a significant period of 161 days in cycle 21 which
is absent during cycle 22 and 23. We have found that in cycle 22 periodicities
of 74 and 83 days are in operation. A 123 day periodicity has been found to be
statistically significant during the part of the current solar cycle 23. The
existence of these periodicities has been discussed in the light of earlier
results.Comment: 9 pages, Appeared in the Solar Physic
North-South Asymmetry of Solar Activity during Cycle 23
In this paper, we have made a statistical analysis of solar H flares
that occurred during the period 1996 to 2005 to investigate their spatial
distribution with respect to northern and southern hemispheres of the Sun. The
analysis includes a total of 21608 single events. The study shows a significant
NS asymmetry which is persistent with the evolution of the solar cycle. The
flare activity favors the northern hemisphere in general during the rising and
maximum phase of the solar cycle (i.e., in 1997, 1999, and 2000), while the
declining phase (i.e., from 2001 to 2005) shows a southern dominance. Further,
the monthly NS asymmetry index for flares, sunspot numbers and sunspot areas
suggests similar variations for these phenomena with the progress of solar
cycle. We also find that in terms of asymmetric behavior of solar flares, cycle
23 seems to act quite differently from cycle 22 but comparably to cycle 21.Comment: Published in "Physics of Chromospheric Plasmas", ASP Conference
Series, 368, 539 (2007
Development of a Confined Circular-cum-parallel Ribbon Flare and Associated Pre-flare Activity
We study a complex GOES M1.1 circular ribbon flare and related pre-flare
activity on 26 January 2015 [SOL26-01-2015] in solar active region NOAA 12268.
This flare activity was observed by the AIA on board SDO and the RHESSI. The
examination of photospheric magnetograms during the extended period, prior to
the event, suggests the successive development of a so-called 'anemone' type
magnetic configuration. NLFFF extrapolation reveals a fan-spine magnetic
configuration with the presence of a coronal null-point. We found that the
pre-flare activity in the active region starts ~15 min prior to the main flare
in the form of localized bright patches at two locations. A comparison of
locations and spatial structures of the pre-flare activity with magnetic
configuration of the corresponding region suggests onset of magnetic
reconnection at the null-point along with the low-atmosphere magnetic
reconnection caused by the emergence and the cancellation of the magnetic flux.
The main flare of M1.1 class is characterized by the formation of a
well-developed circular ribbon along with a region of remote brightening.
Remarkably, a set of relatively compact parallel ribbons formed inside the
periphery of the circular ribbon which developed lateral to the brightest part
of the circular ribbon. During the peak phase of the flare, a coronal jet is
observed at the north-east edge of the circular ribbon which suggests
interchange reconnection between large-scale field lines and low-lying closed
field lines. Our investigation suggests a combination of two distinct processes
in which ongoing pre-flare null-point reconnection gets further intensified as
the confined eruption along with jet activity proceeded from within the
circular ribbon region which results to the formation of inner parallel ribbons
and corresponding post-reconnection arcade.Comment: 24 pages, 11 figures, Accepted for publication in Solar Physic
Signatures of magnetic reconnection in solar eruptive flares: A multi-wavelength perspective
In this article, we review some key aspects of a multi-wavelength flare which
have essentially contributed to form a standard flare model based on the
magnetic reconnection. The emphasis is given on the recent observations taken
by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on the
X-ray emission originating from different regions of the coronal loops. We also
briefly summarize those observations which do not seem to accommodate within
the canonical flare picture and discuss the challenges for future
investigations.Comment: 13 pages; Book chapter published in "Multi-scale Dynamical Processes
in Space and Astrophysical Plasmas", Springer-Verlag Berlin Heidelber
RHESSI and TRACE observations of multiple flare activity in AR 10656 and associated filament eruption
We present RHESSI and TRACE observations of multiple flare activity that
occurred in the active region NOAA 10656 over the period of two hours on 2004
August 18. Out of four successive flares, there were three events of class C
while the final event was a major X1.8 solar eruptive flare. The events during
the pre-eruption phase, i.e., before the X1.8 flare, are characterized by
localized episodes of energy release occurring in the vicinity of an active
region filament which produced intense heating along with non-thermal emission.
A few minutes before the eruption, the filament undergoes an activation phase
during which it slowly rises with a speed of ~12 km/s. The filament eruption is
accompanied with an X1.8 flare during which multiple HXR bursts are observed up
to 100-300 keV energies. We observe a bright and elongated coronal structure
simultaneously in E(UV) and 50-100 keV HXR images underneath the expanding
filament during the period of HXR bursts which provides strong evidence for
ongoing magnetic reconnection. This phase is accompanied with very high plasma
temperatures of ~31 MK and followed by the detachment of the prominence from
the solar source region. From the location, timing, strength, and spectrum of
HXR emission, we conclude that the prominence eruption is driven by the
distinct events of magnetic reconnection occurring in a current sheet formed
below the erupting filament. These multi-wavelength observations also suggest
that the localized magnetic reconnections associated with different
evolutionary stages of the filament in the pre-eruption phase play a crucial
role in destabilizing the filament by a tether-cutting process leading to
large-scale eruption and X-class flare.Comment: 16 pages, Accepted for publication in Ap
Successive flux rope eruptions from -Sunspots region of NOAA 12673 and associated X-class eruptive flares on 2017 September 6
In this paper, we present a multi-wavelength analysis of two X-class solar
eruptive flares of classes X2.2 and X9.3 that occurred in the sigmoidal active
region NOAA 12673 on 2017 September 6, by combining observations of Atmospheric
Imaging Assembly and Helioseismic Magnetic Imager instruments on board the
Solar Dynamics Observatory. On the day of the reported activity, the
photospheric structure of the active region displayed a very complex network of
-sunspots that gave rise to the formation of a coronal sigmoid observed
in the hot EUV channels. Both X-class flares initiated from the core of the
sigmoid sequentially within an interval of 3 hours and progressed as a
single "sigmoid--to--arcade" event. Differential emission measure analysis
reveals strong heating of plasma at the core of the active region right from
the pre-flare phase which further intensified and spatially expanded during
each event. The identification of a pre-existing magnetic null by
non-force-free-field modeling of the coronal magnetic fields at the location of
early flare brightenings and remote faint ribbon-like structures during the
pre-flare phase, which were magnetically connected with the core region,
provide support for the breakout model of solar eruption. The magnetic
extrapolations also reveal flux rope structures prior to both flares which are
subsequently supported by the observations of the eruption of hot EUV channels.
The second X-class flare diverged from the standard flare scenario in the
evolution of two sets of flare ribbons, that are spatially well separated,
providing firm evidence of magnetic reconnections at two coronal heights.Comment: 24 pages, 13 figures. Accepted for publication by The Astrophysical
Journal (ApJ
Identification of Pre-flare Processes and Their Possible Role in Driving a Large-scale Flux Rope Eruption with Complex M-class Flare in the Active Region NOAA 12371
In this article, we study the origin of precursor flare activity and
investigate its role towards triggering the eruption of a flux rope which
resulted into a dual-peak M-class flare (SOL2015-06-21T02:36) in the active
region NOAA 12371. The flare evolved in two distinct phases with peak flux
levels of M2.1 and M2.6 at an interval of 54 min. The active region
exhibited striking moving magnetic features (MMFs) along with sunspot rotation.
Non-linear force free field (NLFFF) modelling of the active region corona
reveals a magnetic flux rope along the polarity inversion line in the trailing
sunspot group which is observationally manifested by the co-spatial structures
of an active region filament and a hot channel identified in the 304 and 94
\AA\ images, respectively, from the Atmospheric Imaging Assembly (AIA). The
active region underwent a prolonged phase of flux enhancement followed by a
relatively shorter period of flux cancellation prior to the onset of the flare
which led to the build up and activation of the flux rope. Extreme ultra-violet
(EUV) images reveal localised and structured pre-flare emission, from the
region of MMFs, adjacent to the location of the main flare. Our analysis
reveals strong, localised regions of photospheric currents of opposite
polarities at the precursor location, thereby making the region susceptible to
small-scale magnetic reconnection. Precursor reconnection activity from this
location most likely induced a slipping reconnetion towards the northern leg of
the hot channel which led to the destabilization of the flux rope. The
application of magnetic virial theorem suggests that there was an overall
growth of magnetic free energy in the active region during the prolonged
pre-flare phase which decayed rapidly after the hot channel eruption and its
successful transformation into a halo coronal mass ejection (CME).Comment: 31 pages, 14 figures, 1 table. This is a pre-print of an article
published in Solar Physics. The final authenticated version is available
online at: https://doi.org/10.1007/s11207-020-1596-
Sequential Lid Removal in a Triple-Decker Chain of CME-Producing Solar Eruptions
We investigate the onsets of three consecutive coronal mass ejection (CME)
eruptions in 12 hours from a large bipolar active region (AR) observed by SDO,
STEREO, RHESSI, and GOES. Evidently, the AR initially had a triple-decker
configuration: three flux ropes in a vertical stack above the polarity
inversion line (PIL). Upon being bumped by a confined eruption of the middle
flux rope, the top flux rope erupts to make the first CME and its accompanying
AR-spanning flare arcade rooted in a far-apart pair of flare ribbons. The
second CME is made by eruption of the previously-arrested middle flux rope,
which blows open the flare arcade of the first CME and produces a flare arcade
rooted in a pair of flare ribbons closer to the PIL than those of the first
CME. The third CME is made by blowout eruption of the bottom flux rope, which
blows open the second flare arcade and makes its own flare arcade and pair of
flare ribbons. Flux cancellation observed at the PIL likely triggers the
initial confined eruption of the middle flux rope. That confined eruption
evidently triggers the first CME eruption. The lid-removal mechanism instigated
by the first CME eruption plausibly triggers the second CME eruption. Further
lid removal by the second CME eruption plausibly triggers the final CME
eruption.Comment: 27 pages, 12 figures, Accepted for publication in ApJ Journa
Observational and model analysis of a two-ribbon flare possibly induced by a neighbouring blowout jet
In this paper, we present unique observations of a blowout coronal jet that
possibly triggered a two-ribbon confined C1.2 flare in a bipolar solar active
region NOAA 12615 on 2016 December 5. The jet activity initiates at
chromospheric/transition-region heights with a small brightening that
eventually grows in a larger volume with well developed standard morphological
jet features, viz., base and spire. The spire widens up with a collimated
eruption of cool and hot plasma components, observed in the 304 and 94 A
channels of AIA, respectively. The speed of the plasma ejection, which forms
the jet's spire, was higher for the hot component (~200 km/s) than the cooler
one (~130 km/s). The NLFF model of coronal fields at pre- and post-jet phases
successfully reveal opening of previously closed magnetic field lines with a
rather inclined/low-lying jet structure. The peak phase of the jet emission is
followed by the development of a two-ribbon flare that shows coronal loop
emission in HXRs up to ~25 keV energy. The coronal magnetic fields rooted at
the location of EUV flare ribbons, derived from the NLFF model, demonstrate the
pre-flare phase to exhibit an "X-type" configuration while the magnetic fields
at the post-flare phase are more or less parallel oriented. The comparisons of
multi-wavelength measurements with the magnetic field extrapolations suggest
that the jet activity likely triggered the two-ribbon flare by perturbing the
field in the interior of the active region.Comment: 18 pages, 12 figures, accepted for publication in The Astrophysical
Journal (ApJ
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