Transient brightenings in the quiet Sun detected by ALMA at 3 mm |
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Alexander Nindos Submitted: 2020-04-17 00:56
Using ALMA observations, we performed the first systematic survey for transient brightenings (i.e. weak, small-scale episodes of energy release) in the quiet solar chromosphere at 3 mm. Our dataset included images of six 87'' x 87'' regions of the quiet Sun obtained with angular resolution of a few arcsec at a cadence of 2 s. The transient brightenings were detected as weak enhancements above the average intensity after we removed the effect of the p-mode oscillations. A similar analysis, over the same regions, was performed for simultaneous 304 and 1600 Å data obtained with the Atmospheric Imaging Assembly. We detected 184 3 mm transient brightening events with brightness temperatures from 70 K to more than 500 K above backgrounds of ∼7200-7450 K. Their mean duration and maximum area were 51.1 s and 12.3 Mm2, respectively, with a weak preference of appearing at network boundaries rather than in cell interiors. Both parameters exhibited power-law behavior with indices of 2.35 and 2.71, respectively. Only a small fraction of ALMA events had either 304 or 1600 Å counterparts but the properties of these events were not significantly different from those of the general population except that they lacked their low-end energy values. The total thermal energies of the ALMA transient brightenings were between 1.5×1024 and 9.9×1025 erg and their frequency distribution versus energy was a power law with an index of 1.67. We found that the power per unit area provided by the ALMA events could account for only 1% of the chromospheric radiative losses (10% of the coronal ones). Therefore, their energy budget falls short of meeting the requirements for the heating of the upper layers of the solar atmosphere and this conclusion does not change even if we use the least restrictive criteria possible for the detection of transient brightenings.
Authors: A. Nindos, C.E. Alissandrakis, S. Patsourakos, T.S. Bastian
Projects: ALMA
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Publication Status: A&A, in press
Last Modified: 2020-04-17 12:01
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First high-resolution look at the quiet Sun with ALMA at 3 mm |
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Alexander Nindos Submitted: 2018-10-19 05:47
We present an overview of high resolution quiet Sun observations, from disk center to the limb, obtained with the Atacama Large mm and sub-mm Array (ALMA) at 3 mm. Seven quiet Sun regions were observed with resolution of up to 2.5" by 4.5". We produced both average and snapshot images by self-calibrating the ALMA visibilities and combining the interferometric images with full disk solar images. The images show well the chromospheric network, which, based on the unique segregation method we used, is brighter than the average over the fields of view of the observed regions by ∼305 K while the intranetwork is less bright by ∼280 K, with a slight decrease of the network/intranetwork contrast toward the limb. At 3 mm the network is very similar to the 1600 Å images, with somewhat larger size. We detected for the first time spicular structures, rising up to 15" above the limb with a width down to the image resolution and brightness temperature of ∼ 1800 K above the local background. No trace of spicules, either in emission or absorption, was found on the disk. Our results highlight ALMA's potential for the study of the quiet chromosphere.
Authors: A. Nindos, C.E. Alissandrakis, T.S. Bastian, S. Patsourakos, B. De Pontieu, H. Warren, T. Ayres, H.S. Hudson, T. Shimizu, J.-C. Vial, S. Wedemeyer, V. Yurchyshyn
Projects: ALMA
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Publication Status: A&A (Letters), in press
Last Modified: 2018-10-24 10:43
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Pulsating Solar Radio Emission |
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Alexander Nindos Submitted: 2006-02-01 04:29
A status report of current research on pulsating radio
emission is given, based on working group discussions at the CESRA
2004 workshop. Quasi-periodic pulsations have been observed at all
wavelength ranges of the radio band. Usually, they are associated with
flare events; however since the late 90s, pulsations of the
slowly-varying component of the Sun's radio emission have also been
observed. Radio pulsations show a large variety in their periods,
bandwidths, amplitudes, temporal and spatial signatures. Most of them
have been attributed to MHD oscillations in coronal loops, while
alternative interpretations consider intrinsic oscillations of a
nonlinear regime of kinetic plasma instabilities or modulation of the
electron acceleration. Combined radio spectroscopic observations with
radio imaging and X-ray/EUV data have revived interest in the
subject. We summarize recent progress in using radio pulsations as a
powerful tool for coronal plasma and magnetic field diagnostics. Also
the latest developments on the study of the physical processes leading
to radio emission modulation are summarized.
Authors: A. Nindos, H. Aurass
Projects: None
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Publication Status: Proceedings of CESRA Workshop 2004 The high-energy solar corona: waves, eruptions, particles, to be published in Lecture Notes in Physics, 2006 (paper accepted for publication on July 12, 2005)
Last Modified: 2006-02-01 04:29
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Spatially resolved microwave oscillations above a sunspot |
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Alexander Nindos Submitted: 2002-06-28 00:29
Using high quality VLA observations, we detected for the first time
spatially resolved oscillations in the microwave total intensity (I)
and circular polarization (V) emission of a sunspot-associated
gyroresonance (g-r) source. Oscillations were detected at 8.5 and 5
GHz during several time intervals of our 10-hour-long dataset. The
oscillations are intermittent: they start suddenly and are damped
somehow more gradually. Despite their transient nature when they are
observed they show significant positional, amplitude and phase
stability. The spatial distribution of intensity variations is
patchy and the location of the patches of strong oscillatory power is
not the same at both frequencies. The strongest oscillations are
associated with a small region where the 8.5 GHz emission comes from
the second harmonic of the gyrofrequency while distinct peaks of
weaker oscillatory power appear close to the outer boundaries of the
8.5 and 5 GHz g-r sources, where the emissions come from the third
harmonic of the gyrofrequency. Overall, the 5 GHz oscillations are
weaker than the 8.5 GHz oscillations (the rms amplitudes of the I
oscillations are 1.3-2.5 imes 104 K and 0.2-1.5 imes 105 K,
respectively). At both frequencies the oscillations have periods in
the three-minute range: the power spectra show two prominent peaks at
6.25-6.45 mHz and 4.49-5.47 mHz. Our models show that the microwave
oscillations are caused by variations of the location of the third
and/or second harmonic surfaces with respect to the base of the
chromosphere-corona transition region (TR), i.e. either the magnetic
field strength or/and the height of the base of the TR oscillates.
The best-fit model to the observed microwave oscillations can be
derived from photospheric magnetic field strength oscillations with an
rms amplitude of 40 G or oscillations of the height of the base of the
TR with an rms amplitude of 25 km. Furthermore small variations of the
orientation of the magnetic field vector yield radio oscillations
consistent with the observed oscillations.
Authors: A. Nindos, C.E. Alissandrakis, G.B. Gelfreikh, V.M. Bogod, C. Gontikakis
Projects:
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Publication Status: A&A, 386, 658
Last Modified: 2002-06-28 00:29
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