Possible manifestation of large-scale transverse oscillations of coronal loops in solar microwave emission (original) (raw)
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Long-Periodic Transverse Oscillations of Coronal Loops and Modulations of Solar Microwave Radiation
The paper deals with interpretation of long-periodic (minutes, e.g. < 0.01 Hz) modulations, detected in the microwave records during flaring events on the Sun, as the signatures of large scale transverse oscillations of coronal loops, observed at the same time by TRACE. In the case of transverse large scale oscillatory motions of a loop a properly located observer, besides of the modulation caused by motion of the emission diagram pattern at the main oscillation frequency, may see also a modulation at double frequency of the loop oscillation, resulted by a varying magnetic field during each inclination of the loop, as well as weaker higher harmonics. Identification of such "modulation pairs" in the dynamic spectra of solar microwave emission and their association with the observed oscillating coronal loops form the major result of the undertaken investigation. Three different events with the detected by TRACE post-flare oscillating loops were considered in that study (M...
MHD-Oscillations of Coronal Loops and Diagnostics of Flare Plasma
Effects of ballooning and radial oscillations of coronal magnetic loops on the modulations of microwave and X-ray emission from flare loops are considered. The damping mechanisms of loop MHD modes are analyzed. The method for diagnostics of flare plasma parameters using peculiarities of the microwave and X-ray pulsations is proposed. The diagnostic method was applied for two solar flares: on May 8, 1998 and August 28, 1999 observed with the Nobeyama Radioheliograph.
Hot coronal loop oscillations observed with SUMER: Examples and statistics
Astronomy and Astrophysics, 2003
We give an extensive overview of Doppler shift oscillations in hot active region loops obtained with SUMER. The oscillations have been detected in loops sampled 50−100 arcsec off the limb of the Sun in ultraviolet lines, mainly Fe and Fe , with formation temperature greater than 6 MK. The spectra were recorded along a 300 arcsec slit placed at a fixed position in the corona above the active regions. Oscillations are usually seen along an extended section of the slit and often appear to be from several different portions of the loops (or from different loops). Different portions are sometimes in phase, sometimes out of phase and sometimes show phase shifts along the slit. We measure physical parameters of 54 Doppler shift oscillations in 27 flare-like events and give geometric parameters of the associated hot loops when soft X-ray (SXR) images are available. The oscillations have periods in the range 7−31 min, with decay times 5.7−36.8 min, and show an initial large Doppler shift pulse with peak velocities up to 200 km s −1 . The oscillation periods are on average a factor of three longer than the TRACE transverse loop oscillations. The damping times and velocity amplitude are roughly the same, but the derived displacement amplitude is four or five times larger than the transverse oscillation amplitude measured in TRACE images. Unlike TRACE oscillations, only a small fraction of them are triggered by large flares, and they often recur 2−3 times within a couple of hours. All recurring events show initial shifts of the same sign. These data provide the following evidence to support the conclusion that these oscillations are slow magnetoacoustic standing waves in hot loops: (1) the phase speeds derived from observed periods and loop lengths roughly agree with the sound speed; (2) the intensity fluctuation lags the Doppler shifts by 1/4 period; (3) The scaling of the dissipation time of slow waves with period agrees with the observed scaling for 49 cases. They seem to be triggered by micro-or subflares near a footpoint, as revealed in one example with SXR image observations. However other mechanisms cannot as yet be ruled out. Some oscillations showed phase propagation along the slit in one or both directions with apparent speeds in the range of 8−102 km s −1 , together with distinctly different intensity and line width distributions along the slit. These features can be explained by the excitation of the oscillation at a footpoint of an inhomogeneous coronal loop, e.g. a loop with fine structure.
Coronal Loop Oscillations Observed with theTransition Region and Coronal Explorer
The Astrophysical Journal, 1999
We report here, for the Ðrst time, on spatial oscillations of coronal loops, which were detected in extreme-ultraviolet wavelengths (171 with the T ransition Region and Coronal Explorer, in the tem-A) perature range of MK. The observed loop oscillations occurred during a Ñare that began at T e B 1.0È1.5 1998 July 14, 12 : 55 UT and are most prominent during the Ðrst 20 minutes. The oscillating loops connect the penumbra of the leading sunspot to the Ñare site in the trailing portion. We identiÐed Ðve oscillating loops with an average length of L \ 130,000^30,000 km. The transverse amplitude of the oscillations is A \ 4100^1300 km, and the mean period is T \ 280^30 s. The oscillation mode appears to be a standing wave mode (with Ðxed nodes at the footpoints). We investigate di †erent MHD wave modes and Ðnd that the fast kink mode with a period q \ 205(L /1010 cm~3)1@2 cm)(n e /109 (B/10 G)~1 s provides the best agreement with the observed period. We propose that the onset of loop oscillations in distant locations is triggered by a signal or disturbance that propagates from the central Ñare site with a radial speed of B700 km s~1. Because the observed loop oscillation periods are comparable to photospheric 5 minute oscillations, a resonant coupling between the two systems is possible. We further Ðnd evidence for global extreme-UV dimming in the entire active region possibly associated with a coronal mass ejection.
Transverse oscillations in coronal loops observed with TRACE
2001
TRACE discovered transverse oscillations in coronal loops associated with a flare three years ago, and until recently only two such events were known. We have now identified a total of 17 events that trigger some form of loop oscillations. Oscillation periods are estimated to range over a factor of ~ 15, with most values between 2 and 7 min. The
Astronomy and Astrophysics, 2007
Context. Analysing the structure of solar coronal loops is crucial to our understanding of the processes which heat and maintain the coronal plasma at multimillion degree temperatures. The determination of the physical parameters of the loops remains both an observational and theoretical challenge. Aims. The present work is aimed at tackling some of these challenges such as the determination of the average loop temperature and its distribution along a given loop. Methods. A novel diagnostic technique for quiescent coronal loops based on the analysis of power spectra of Doppler shift time series is proposed. It is assumed that the loop is heated randomly both in space and time by small-scale discrete impulsive events of unspecified nature. The loop evolution is characterized by longitudinal motions caused by the random heating events. These random motions can be represented as a superposition of the normal modes of the loop, i.e., its standing acoustic wave harmonics. The idea is borrowed from helioseismology where a similar approach resulted in a deep understanding of the solar interior. Results. It is shown that depending on the heliographic position of the loop and its geometrical orientation, various harmonics can be identified in the power spectra of the line shift time series. The highest power peak corresponds to the fundamental mode. The peaks become smaller as the frequency increases. The frequencies of the harmonics are determined by the loop length and temperature and thus are suggested to be used as a temperature diagnostic tool. It is demonstrated that the analysis of the power spectra allows the distinction between uniformly heated loops from loops heated near their foot-points and to estimate the average energy of a single heating event. The proposed new method could in principle be used to study the multi-thermal structure of coronal loops. Conclusions. The power spectrum analysis is a potentially powerful technique for coronal loop diagnostics.
The Astrophysical Journal, 2018
Observational detection of quasi-periodic drifting fine structures in a type III radio burst associated with a solar flare SOL2015-04-16T11:22, with Low Frequency Array, is presented. Although similar modulations of the type III emission have been observed before and were associated with the plasma density fluctuations, the origin of those fluctuations was unknown. Analysis of the striae of the intensity variation in the dynamic spectrum allowed us to reveal two quasi-oscillatory components. The shorter component has the apparent wavelength of ∼ 2 Mm, phase speed of ∼ 657 km s −1 , which gives the oscillation period of ∼ 3 s, and the relative amplitude of ∼ 0.35%. The longer component has the wavelength of ∼ 12 Mm, and relative amplitude of ∼ 5.1%. The short frequency range of the detection does not allow us to estimate its phase speed. However, the properties of the shorter oscillatory component allowed us to interpret it as a fast magnetoacoustic wave guided by a plasma non-uniformity along the magnetic field outwards from the Sun. The assumption that the intensity of the radio emission is proportional to the amount of plasma in the emitting volume allowed us to show that the superposition of the plasma density modulation by a fast wave and a longer-wavelength oscillation of an unspecified nature could readily reproduce the fine structure of the observed dynamic spectrum. The observed parameters of the fast wave give the absolute value of the magnetic field in the emitting plasma of ∼ 1.1 G which is consistent with the radial magnetic field model.
TRACE Observation of Damped Coronal Loop Oscillations: Implications for Coronal Heating
Science, 1999
The imaging telescope on board the Transition Region and Coronal Explorer (TRACE) spacecraft observed the decaying transversal oscillations of a long [(130 ± 6) × 10 6 meters], thin [diameter (2.0 ± 0.36) × 10 6 meters], bright coronal loop in the 171 angstrom Fe IX emission line. The oscillations were excited by a solar flare in the adjacent active region. The decay time of the oscillations is 14.5 ± 2.7 minutes for an oscillation with a frequency 3.90 ± 0.13 millihertz. The coronal dissipation coefficient is estimated to be eight to nine orders of magnitude larger than the theoretically predicted classical value. The larger dissipation coefficient may solve existing difficulties with wave heating and reconnection theories.
Coronal Loop Oscillations and Flare Shock Waves
The Astrophysical Journal, 2004
A statistical analysis of coronal loop oscillations observed by the Transition Region and Coronal Explorer (TRACE) shows that 12 of 28 cases were associated with metric type II bursts. The timing is consistent with the idea that in many cases the loop oscillations result from the passage of a large-scale wave disturbance originating in a flare in the nearby active region. The GOES classifications for these flares range from C4.2 to X20. Typically, the oscillating structures are not disrupted, implying that the disturbance has passed through the medium, which has returned to an equilibrium near that seen prior to the event. This is consistent with the Uchida interpretation of the disturbance as a weak fast-mode blast wave (i.e., a simple wave at a low Alfvénic Mach number) propagating in the ambient corona. We note that all 12 of the associated events were also associated with coronal mass ejections (CMEs) and conclude that the CME eruptions in these cases corresponded to only partial openings of the active-region magnetic fields.
Decaying and decayless transverse oscillations of a coronal loop
Astronomy & Astrophysics, 2013
Aims. We investigate kink oscillations of loops observed in an active region with the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory (SDO) spacecraft before and after a flare. Methods. The oscillations were depicted and analysed with time-distance maps, extracted from the cuts taken parallel or perpendicular to the loop axis. Moving loops were followed in time with steadily moving slits. The period of oscillations and its time variation were determined by best-fitting harmonic functions. Results. We show that before and well after the occurrence of the flare, the loops experience low-amplitude decayless oscillations. The flare and the coronal mass ejection associated to it trigger large-amplitude oscillations that decay exponentially in time. The periods of the kink oscillations in both regimes (about 240 s) are similar. An empirical model of the phenomenon in terms of a damped linear oscillator excited by a continuous low-amplitude harmonic driver and by an impulsive high-amplitude driver is found to be consistent with the observations.