Slow-Mode Magnetoacoustic Waves in Coronal Loops (Review) |
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Tongjiang Wang Submitted: 2021-02-24 11:24
Rapidly decaying long-period oscillations often occur in hot coronal loops of active regions associated with small (or micro-) flares. This kind of wave activity was first discovered with the SOHO/SUMER spectrometer from Doppler velocity measurements of hot emission lines, thus also often called "SUMER" oscillations. They were mainly interpreted as global (or fundamental mode) standing slow magnetoacoustic waves. In addition, increasing evidence has suggested that the decaying harmonic type of pulsations detected in light curves of solar and stellar flares are likely caused by standing slow-mode waves. The study of slow magnetoacoustic waves in coronal loops has become a topic of particular interest in connection with coronal seismology. We review recent results from SDO/AIA and Hinode/XRT observations that have detected both standing and reflected intensity oscillations in hot flaring loops showing the physical properties (e.g., oscillation periods, decay times, and triggers) in accord with the SUMER oscillations. We also review recent advances in theory and numerical modeling of slow-mode waves focusing on the wave excitation and damping mechanisms. MHD simulations in 1D, 2D and 3D have been dedicated to understanding the physical conditions for the generation of a reflected propagating or a standing wave by impulsive heating. Various damping mechanisms and their analysis methods are summarized. Calculations based on linear theory suggest that the non-ideal MHD effects such as thermal conduction, compressive viscosity, and optically thin radiation may dominate in damping of slow-mode waves in coronal loops of different physical conditions. Finally, an overview is given of several important seismological applications such as determination of transport coefficients and heating function.
Authors: Tongjiang Wang, Leon Ofman, Ding Yuan, Fabio Reale, Dmitrii Y. Kolotkov, Abhishek K. Srivastava
Projects: None
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Publication Status: Accepted for publication in Space Science Reviews in 2021
Last Modified: 2021-02-24 13:10
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Effect of transport coefficients on excitation of flare-induced standing slow-mode waves in coronal loops |
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Tongjiang Wang Submitted: 2018-05-10 14:02
Standing slow-mode waves have been recently observed in flaring loops by the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). By means of the coronal seismology technique transport coefficients in hot (∼10 MK) plasma were determined by Wang et al.(2015, Paper I), revealing that thermal conductivity is nearly suppressed and compressive viscosity is enhanced by more than an order of magnitude. In this study we use 1D nonlinear MHD simulations to validate the predicted results from the linear theory and investigate the standing slow-mode wave excitation mechanism. We first explore the wave trigger based on the magnetic field extrapolation and flare emission features. Using a flow pulse driven at one footpoint we simulate the wave excitation in two types of loop models: model 1 with the classical transport coefficients and model 2 with the seismology-determined transport coefficients. We find that model 2 can form the standing wave pattern (within about one period) from initial propagating disturbances much faster than model 1, in better agreement with the observations. Simulations of the harmonic waves and the Fourier decomposition analysis show that the scaling law between damping time (τ) and wave period (P) follows τ∝P2 in model 2, while τ∝P in model 1. This indicates that the largely enhanced viscosity efficiently increases the dissipation of higher harmonic components, favoring the quick formation of the fundamental standing mode. Our study suggests that observational constraints on the transport coefficients are important in understanding both, the wave excitation and damping mechanisms.
Authors: Tongjiang Wang, Leon Ofman, Xudong Sun, Sami K Solanki, Joseph M Davila
Projects: SDO-AIA,SDO-HMI
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Publication Status: Accepted by ApJ
Last Modified: 2018-05-11 16:11
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Variation of Coronal Activity from the Minimum to Maximum of Solar Cycle 24 using Three Dimensional Coronal Electron Density Reconstructions from STEREO/COR1 |
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Tongjiang Wang Submitted: 2017-06-19 14:15
Three dimensional electron density distributions in the solar corona are reconstructed for 100 Carrington Rotations (CR 2054-2153) during 2007/03-2014/08 using the spherically symmetric method from polarized white-light observations with the STEREO/COR1. These three-dimensional electron density distributions are validated by comparison with similar density models derived using other methods such as tomography and a MHD model as well as using data from SOHO/LASCO-C2. Uncertainties in the estimated total mass of the global corona are analyzed based on differences between the density distributions for COR1-A and -B. Long-term variations of coronal activity in terms of the global and hemispheric average electron densities (equivalent to the total coronal mass) reveal a hemispheric asymmetry during the rising phase of Solar Cycle 24, with the northern hemisphere leading the southern hemisphere by a phase shift of 7-9 months. Using 14-CR (~13-month) running averages, the amplitudes of the variation in average electron density between Cycle 24 maximum and Cycle 23/24 minimum (called the modulation factors) are found to be in the range of 1.6-4.3. These modulation factors are latitudinally dependent, being largest in polar regions and smallest in the equatorial region. These modulation factors also show a hemispheric asymmetry, being somewhat larger in the southern hemisphere. The wavelet analysis shows that the short-term quasi-periodic oscillations during the rising and maximum phases of Cycle 24 have a dominant period of 7-8 months. In addition, it is found that the radial distribution of mean electron density for streamers at Cycle 24 maximum is only slightly larger (by ~30%) than at cycle minimum.
Authors: Tongjiang Wang, Nelson L. Reginald, Joseph M. Davila, O. Chris St. Cyr, William T. Thompson
Projects: STEREO
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Publication Status: Accepted for publication in Solar Physics, in June 2017
Last Modified: 2017-06-20 16:06
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Evidence of thermal conduction suppression in hot coronal loops: Supplementary results |
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Tongjiang Wang Submitted: 2015-10-13 11:38
Slow magnetoacoustic waves were first detected in hot (>6 MK) flare loops by the SOHO/SUMER spectrometer as Doppler shift oscillations in Fe XIX and Fe XXI lines. Recently, such longitudinal waves have been found by SDO/AIA in the 94 and 131 Å channels. Wang et al. (2015) reported the first AIA event revealing signaturesin agreement with a fundamental standing slow-mode wave, and found quantitative evidence for thermal conduction suppression from the temperature and density perturbations in the hot loop plasma of > 9 MK. The present study extends the work of Wang et al. (2015) by using an alternative approach. We determine the polytropic index directly based on the polytropic assumption instead of invoking the linear approximation. The same results are obtained as in the linear approximation, indicating that the nonlinearity effect is negligible. We find that the flare loop cools slower (by a factor of 2-4) than expected from the classical Spitzer conductive cooling, approximately consistent with the result of conduction suppression obtained from the wave analysis. The modified Spitzer cooling timescales based on thenonlocal conduction approximation are consistent with the observed, suggesting that nonlocal conduction may account for the observed conduction suppression in this event. In addition, the conduction suppression mechanism predicts that larger flares may tend tobe hotter than expected by the EM-T relation derived by Shibata & Yokoyama (2002)
Authors: Tongjiang Wang, Leon Ofman, Xudong Sun, Elena Provornikova, Joseph M. Davila
Projects: SDO-AIA
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Publication Status: submitted to Proceedings of IAU29 GA Symposium 320 (2015)
Last Modified: 2015-10-15 07:27
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Three-dimensional MHD modeling of propagating disturbances in fan-like coronal loops |
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Tongjiang Wang Submitted: 2013-08-13 10:30
Quasi-periodic propagating intensity disturbances (PDs) have been observed in large coronal loops in EUV images over a decade, and are widely accepted to be slow magnetosonic waves. However, spectroscopic observations from Hinode/EIS revealed their association with persistent coronal upflows, making this interpretation debatable. Motivated by the scenario that the coronal upflows could be cumulative result of numerous individual flow pulses generated by sporadic heating events (nanoflares) at the loop base, we construct a velocity driver with repetitive tiny pulses, whose energy frequency distribution follows the flare power-law scaling. We then perform 3D MHD modeling of an idealized bipolar active region by applying this broadband velocity driver at the footpoints of large coronal loops which appear open in the computational domain. Our model successfully reproduces the PDs with similar features as the observed, and shows that any upflow pulses inevitably excite slow magnetosonic wave disturbances propagating along the loop. We find that the generated PDs are dominated by the wave signature as their propagation speeds are consistent with the wave speed in the presence of flows, and the injected flows rapidly decelerate with height. Our simulation results suggest that the observed PDs and associated persistent upflows may be produced by small-scale impulsive heating events (nanoflares) at the loop base, and that the flows and waves may both contribute to the PDs at lower heights.
Authors: Tongjiang Wang, Leon Ofman, and Joseph M. Davila
Projects: Hinode/EIS,SDO-AIA
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Publication Status: Accepted by ApJ Letters in August 2013
Last Modified: 2013-08-13 11:02
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Growing transverse oscillations of a multistranded loop observed by SDO/AIA |
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Tongjiang Wang Submitted: 2012-04-09 18:38
The first evidence of transverse oscillations of a multistranded loop with growing amplitudes and internal coupling observed by the Atomspheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is presented. The loop oscillation event occurred on 2011 March 8, triggered by a CME. The multiwavelength analysis reveals the presence of multithermal strands in the oscillating loop, whose dynamic behaviors are temperature-dependent, showing differences in their oscillation amplitudes, phases and emission evolution. The physical parameters of growing oscillations of two strands in 171 Å are measured and the 3-D loop geometry is determined using STEREO-A/EUVI data. These strands have very similar frequencies, and between two 193 Å strands a quarter-period phase delay sets up. These features suggest the coupling between kink oscillations of neighboring strands and the interpretation by the collective kink mode as predicted by some models. However, the temperature dependence of the multistarnded loop oscillations was not studied previously and needs further investigation. The transverse loop oscillations are associated with intensity and loop width variations. We suggest that the amplitude-growing kink oscillations may be a result of continuous non-periodic driving by magnetic deformation of the CME, which deposits energy into the loop system at a rate faster than its loss
Authors: Tongjiang Wang, Leon Ofman, Joseph M. Davila, Yang Su
Projects: SDO-AIA
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Publication Status: Accepted by ApJ Letter (2012)
Last Modified: 2012-04-10 11:29
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Underflight calibration of SOHO/CDS and Hinode/EIS with EUNIS-07 |
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Tongjiang Wang Submitted: 2011-09-30 09:49
Flights of Goddard Space Flight Center's Extreme-Ultraviolet Normal-IncidenceSpectrograph (EUNIS) sounding rocket in 2006 and 2007 provided updatedradiometric calibrations for SOHO/CDS and Hinode/EIS. EUNIS carried twoindependent imaging spectrographs covering wavebands of 300-370 A in firstorder and 170-205 A in second order. After each flight, end-to-end radiometriccalibrations of the rocket payload were carried out in the same facility usedfor pre-launch calibrations of CDS and EIS. During the 2007 flight, EUNIS, SOHOCDS and Hinode EIS observed the same solar locations, allowing the EUNIScalibrations to be directly applied to both CDS and EIS. The measured CDS NIS 1line intensities calibrated with the standard (version 4) responsivities withthe standard long-term corrections are found to be too low by a factor of 1.5due to the decrease in responsivity. The EIS calibration update is performed intwo ways. One is using the direct calibration transfer of the calibratedEUNIS-07 short wavelength (SW) channel. The other is using the insensitive linepairs, in which one member was observed by EUNIS-07 long wavelength (LW)channel and the other by EIS in either LW or SW waveband. Measurements fromboth methods are in good agreement, and confirm (within the measurementuncertainties) the EIS responsivity measured directly before the instrument'slaunch. The measurements also suggest that the EIS responsivity decreased by afactor of about 1.2 after the first year of operation. The shape of the EIS SWresponse curve obtained by EUNIS-07 is consistent with the one measured inlaboratory prior to launch. The absolute value of the quiet-Sun He II 304 Aintensity measured by EUNIS-07 is consistent with the radiance measured by CDSNIS in quiet regions near the disk center and the solar minimum irradianceobtained by CDS NIS and SDO/EVE recently.
Authors: Tongjiang Wang, Roger J. Thomas, Jeffrey W. Brosius, Peter R. Young, Douglas M. Rabin, Joseph M. Davila, Giulio Del Zanna
Projects: EUNIS
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Publication Status: accepted by ApJ Supplement, publication in December 2011, V197-2 issue
Last Modified: 2011-09-30 10:49
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Absolute radiometric calibration of the EUNIS-06 170-205 A channel and calibration update for CDS/NIS |
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Tongjiang Wang Submitted: 2009-12-11 15:04
The Extreme-Ultraviolet Normal-Incidence Spectrograph sounding-rocket payload was flown on 2006 April 12 (EUNIS-06), carrying two independent imaging spectrographs covering wave bands of 300-370 A in first order and 170-205 A in second order, respectively. The absolute radiometric response of the EUNIS-06 long-wavelength (LW) channel was directly measured in the same facility used to calibrate CDS prior to the SOHO launch. Because the absolute calibration of the short-wavelength (SW) channel could not be obtained from the same lab configuration, we here present a technique to derive it using a combination of solar LW spectra and density- and temperature-insensitive line intensity ratios. The first step in this procedure is to use the coordinated, cospatial EUNIS and SOHO/CDS spectra to carry out an intensity calibration update for the CDS NIS-1 waveband, which shows that its efficiency has decreased by a factor about 1.7 compared to that of the previously implemented calibration. Then, theoretical insensitive line ratios obtained from CHIANTI allow us to determine absolute intensities of emission lines within the EUNIS SW bandpass from those of cospatial CDS/NIS-1 spectra after the EUNIS LW calibration correction. A total of 12 ratios derived from intensities of 5 CDS and 12 SW emission lines from Fe Fe X - Fe XIII yield an instrumental response curve for the EUNIS-06 SW channel that matches well to a relative calibration which relied on combining measurements of individual optical components. Taking into account all potential sources of error, we estimate that the EUNIS-06 SW absolute calibration is accurate to about 20%.
Authors: Tongjiang Wang, Jeffrey W. Brosius, Roger J. Thomas, Douglas M. Rabin, Joseph M. Davila
Projects: EUNIS
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Publication Status: ApJ Suppl. 2009, accepted
Last Modified: 2009-12-14 09:29
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Hinode/EIS observations of propagating low-frequency slow magnetoacoustic waves in fan-like coronal loops |
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Tongjiang Wang Submitted: 2009-08-03 12:43
We report the first observation of multiple-periodic propagating disturbances along a fan-like coronal structure simultaneously detected in both intensity and Doppler shift in the Fe XII 195 A line with the EUV Imaging Spectrometer (EIS) onboard Hinode. A new application of coronal seismology is provided based on this observation. We analyzed the EIS sit-and-stare mode observation of oscillations using the running difference and wavelet techniques. Two harmonics with periods of 12 and 25 min are detected. We measured the Doppler shift amplitude of 1-2 km s-1, the relative intensity amplitude of 3%-5% and the apparent propagation speed of 100-120 km s-1. The amplitude relationship between intensity and Doppler shift oscillations provides convincing evidence that these propagating features are a manifestation of slow magnetoacoustic waves. Detection lengths (over which the waves are visible) of the 25 min wave are about 70-90 Mm, much longer than those of the 5 min wave previously detected by TRACE. This difference may be explained by the dependence of damping length on the wave period for thermal conduction. Based on a linear wave theory, we derive an inclination of the magnetic field to the line-of-sight about 59pm8 deg, a true propagation speed of 128pm25 km s-1 and a temperature of 0.7pm0.3 MK near the loop's footpoint from our measurements.
Authors: T. J. Wang, L. Ofman, J. M. Davila, J. T. Mariska
Projects: Hinode/EIS,TRACE
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Publication Status: A&A Letter, 2009, in press
Last Modified: 2009-08-04 08:07
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Propagating slow magnetoacoustic waves in coronal loops observed by Hinode/EIS |
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Tongjiang Wang Submitted: 2009-02-25 16:09
We present the first Hinode/EIS observations of 5 min quasi-periodic oscillations detected in a transition-region line (He II) and five coronal lines (Fe X, Fe XII, Fe XIII, Fe XIV, and Fe XV) at the footpoint of a coronal loop. The oscillations exist throughout the whole observation, characterized by a series of wave packets with nearly constant period, typically persisting for 4-6 cycles with a lifetime of 20-30 min. There is an approximate in-phase relation between Doppler shift and intensity oscillations. This provides evidence for slow magnetoacoustic waves propagating upwards from the transition region into the corona. We find that the oscillations detected in the five coronal lines are highly correlated, and the amplitude decreases with increasing temperature. The amplitude of Doppler shift oscillations decrease by a factor of about 3, while that of relative intensity decreases by a factor of about 4 from Fe X to Fe XV. These oscillations may be caused by the leakage of the photospheric p-modes through the chromosphere and transition region into the corona, which has been suggested as the source for intensity oscillations previously observed by TRACE. The temperature dependence of the oscillation amplitudes can be explained by damping of the waves traveling along the loop with multithread structure near the footpoint. Thus, this property may have potential value for coronal seismology in diagnostic of temperature structure in a coronal loop.
Authors: T. J. Wang, L. Ofman, and J. M. Davila
Projects: Hinode/EIS
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Publication Status: ApJ, May 2009 - v696 issue, (in press)
Last Modified: 2009-03-11 00:19
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Direct observation of high-speed plasma outflows produced by magnetic reconnection
in solar impulsive events |
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Tongjiang Wang Submitted: 2007-04-25 21:55
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 about 900-3500 km s-1, consistent
with theoretical predictions of the Alfvénic 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.
Authors: TONGJIANG WANG, LINHUI SUI and JIONG QIU
Projects: SoHO-SUMER,TRACE,RHESSI
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Publication Status: ApJ Lett. Accepted
Last Modified: 2007-04-26 08:05
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Determination of the Coronal Magnetic Field by Hot Loop Oscillations Observed by SUMER and SXT |
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Tongjiang Wang Submitted: 2006-10-30 21:29
We apply a new method to determine the magnetic field in coronal loops using
observations of coronal loop oscillations. We analyze seven Doppler shift
oscillation events detected by SUMER in the hot flare line Fe XIX
to obtain oscillation periods of these events. The geometry, temperature, and electron
density of the oscillating loops are measured from coordinated multi-channel soft X-ray
imaging observations from SXT. All the oscillations are consistent with standing slow
waves in their fundamental mode. The parameters are used to calculate the magnetic field
of coronal loops based on MHD wave theory. For the seven events, the plasma
eta is in the range 0.15-0.91 with a mean of 0.33pm0.26, and the estimated
magnetic field varies between 21-61 G with a mean of 34pm14 G. With background
emission subtracted, the estimated magnetic field is reduced by 9%-35%. The
maximum backgroud subtraction gives a mean of 22pm13 G in the range 12-51 G. We discuss
measurement uncertainties and the prospect of determining coronal loop magnetic fields
from future observations of coronal loops and Doppler shift oscillations.
Authors: Tongjiang Wang, Davina E. Innes, and Jiong Qiu
Projects: SoHO-SUMER
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Publication Status: ApJ, V656, No. 1, Feb 10 2007 issue
Last Modified: 2006-10-31 12:06
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Fe XIX observations of active region brightenings in the corona |
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Tongjiang Wang Submitted: 2006-05-22 11:05
Small flarelike brightenings seen in the hot flare line, FeXIX, by the spectrometer SUMER on SOHO are analysed. We observe active region coronae about 30 Mm off the limb of the Sun for a period of several days. Brightenings are observed with a frequency 3-14 per hour and their lifetimes range from 5-150 min, with an average of about 25 min. The measured size of the events along the spectrometer slit range from 2-67 Mm, but most are around 7 Mm. Like soft X-ray active region transient brightenings, they range in estimated thermal energy from 1026 to 1029 ergs with a power law index of 1.7 to 1.8, beyond 1027 ergs. We conclude that they are the coronal parts of loops heated to > 6 MK by soft X-ray microflares.
Authors: T. J. Wang, D.E. Innes, and S.K. Solanki
Projects: SoHO-SUMER
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Publication Status: A&A, 2006, in press
Last Modified: 2006-05-24 09:48
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