783 research outputs found
X-ray and EUV Observations of Simultaneous Short and Long Period Oscillations in Hot Coronal Arcade Loops
We report decaying quasi-periodic intensity oscillations in the X-ray (6-12
keV) and extreme ultraviolet (EUV) channels (131, 94, 1600, 304 \AA) observed
by the Fermi GBM (Gamma-ray Burst Monitor) and SDO/AIA, respectively, during a
C-class flare. The estimated period of oscillation and decay time in the X-ray
channel (6-12 keV) was about 202 s and 154 s, respectively. A similar
oscillation period was detected at the footpoint of the arcade loops in the AIA
1600 and 304 \AA channels. Simultaneously, AIA hot channels (94 and 131 \AA)
reveal propagating EUV disturbances bouncing back and forth between the
footpoints of the arcade loops. The period of the oscillation and decay time
were about 409 s and 1121 s, respectively. The characteristic phase speed of
the wave is about 560 km/s for about 115 Mm loop length, which is roughly
consistent with the sound speed at the temperature about 10-16 MK (480-608
km/s). These EUV oscillations are consistent with the SOHO/SUMER Doppler-shift
oscillations interpreted as the global standing slow magnetoacoustic wave
excited by a flare. The flare occurred at one of the footpoints of the arcade
loops, where the magnetic topology was a 3D fan-spine with a null-point.
Repetitive reconnection at this footpoint could cause the periodic acceleration
of non-thermal electrons that propagated to the opposite footpoint along the
arcade and precipitating there, causing the observed 202-s periodicity. Other
possible interpretations, e.g. the second harmonics of the slow mode are also
discussed.Comment: ApJ (in press), 13 pages, 6 figure
Multiwavelength Observations of a Flux Rope Formation by Series of Magnetic Reconnection in the Chromosphere
Using high-resolution observations from the 1.6 m New Solar Telescope (NST)
operating at the Big Bear Solar Observatory (BBSO), we report direct evidence
of merging/reconnection of cool H loops in the chromosphere during two
homologous flares (B- and C-class) caused by a shear motion at the footpoint of
two loops. The reconnection between these loops caused the formation of an
unstable flux rope which showed counterclockwise rotation. The flux rope could
not reach the height of torus instability and failed to form a coronal mass
ejection. The HMI magnetograms revealed rotation of the negative/positive
(N1/P2) polarity sunspots in the opposite directions, which increased the right
and left-handed twist in the magnetic structures rooted at N1/P2. Rapid
photospheric flux cancellation (duration20-30 min,
rate3.4410 Mx h) was observed during and even
after the first B6.0 flare and continued until the end of the second C2.3
flare. The RHESSI X-ray sources were located at the site of the loop's
coalescence. To the best of our knowledge, such a clear interaction of
chromospheric loops along with rapid flux cancellation has not been reported
before. These high-resolution observations suggest the formation of a small
flux rope by a series of magnetic reconnection within chromospheric loops
associated with very rapid flux cancellation.Comment: A&A, in press, 12 pages, 12 figure
Quasi-periodic radio bursts associated with fast-mode waves near a magnetic null point
This paper presents an observation of quasi-periodic rapidly propagating waves observed in the Atmospheric Image Assembly (AIA) 171/193 Å channels during the impulsive phase of an M1.9 flare that occurred on 2012 May 7. The instant period was found to decrease from 240 to 120 s, and the speed of the wavefronts was in the range of ~664–1416 km s−1. Almost simultaneously, quasi-periodic bursts with similar instant periods, ~70 and ~140 s, occur in the microwave emission and in decimetric type IV and type III radio bursts, and in the soft X-ray emission. The magnetic field configuration of the flare site was consistent with a breakout topology, i.e., a quadrupolar field along with a magnetic null point. The quasi-periodic rapidly propagating wavefronts of the EUV emission are interpreted as a fast magnetoacoustic wave train. The observations suggest that the fast-mode waves are generated during the quasi-periodic magnetic reconnection in the cusp region above the flare arcade loops. For the first time, we provide evidence of a tadpole wavelet signature at about 70–140 s in decimetric (245/610 MHz) radio bursts, along with the direct observation of a coronal fast-mode wave train in EUV. In addition, at AIA 131/193 Å we observed quasi-periodic EUV disturbances with periods of 95 and 240 s propagating downward at apparent speeds of 172–273 km s−1. The nature of these downward propagating disturbances is not revealed, but they could be connected to magnetoacoustic waves or periodically shrinking loops
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