The Nature and Excitation Mechanisms of Acoustic Oscillations in Solar and Stellar Coronal Loops

Source: arXiv


In the recent work of Nakariakov et al. (2004), it has been shown that the time dependences of density and velocity in a flaring loop contain pronounced quasi-harmonic oscillations associated with the 2nd harmonic of a standing slow magnetoacoustic wave. That model used a symmetric heating function (heat deposition was strictly at the apex). This left outstanding questions: A) is the generation of the 2nd harmonic a consequence of the fact that the heating function was symmetric? B) Would the generation of these oscillations occur if we break symmetry? C) What is the spectrum of these oscillations? Is it consistent with a 2nd spatial harmonic? The present work (and partly Tsiklauri et al. (2004b)) attempts to answer these important outstanding questions. Namely, we investigate the physical nature of these oscillations in greater detail: we study their spectrum (using periodogram technique) and how heat positioning affects the mode excitation. We found that excitation of such oscillations is practically independent of location of the heat deposition in the loop. Because of the change of the background temperature and density, the phase shift between the density and velocity perturbations is not exactly a quarter of the period, it varies along the loop and is time dependent, especially in the case of one footpoint (asymmetric) heating. We also were able to model successfully SUMER oscillations observed in hot coronal loops.

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Available from: Markus J. Aschwanden, Aug 13, 2013
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    ABSTRACT: 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 $\ion{Fe}{xix}$ and $\ion{Fe}{xxi}$, 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.

    Astronomy and Astrophysics 08/2003; DOI:10.1051/0004-6361:20030858 · 4.38 Impact Factor
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    ABSTRACT: We present an analysis of hard X-ray imaging observations from one of the first solar flares observed with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, launched on 5 February 2002. The data were obtained from the 22 February 2002, 11:06UT flare, which occurred close to the northwest limb. Thanks to the high energy resolution of the germanium-cooled hard X-ray detectors on RHESSI we can measure the flare source positions with a high accuracy as a function of energy. Using a forward-fitting algorithm for image reconstruction, we find a systematic decrease in the altitudes of the source centroids z(ε) as a function of increasing hard X-ray energy ε, as expected in the thick-target bremsstrahlung model of Brown. The altitude of hard X-ray emission as a function of photon energy ε can be characterized by a power-law function in the ε=15–50keV energy range, viz., z(ε)≈2.3(ε/20keV)−1.3Mm. Based on a purely collisional 1-D thick-target model, this height dependence can be inverted into a chromospheric density model n(z), as derived in PaperI, which follows the power-law function n e(z)=1.25×1013(z/1Mm)−2.5cm−3. This density is comparable with models based on optical/UV spectrometry in the chromospheric height range of h≲1000km, suggesting that the collisional thick-target model is a reasonable first approximation to hard X-ray footpoint sources. At h≈1000–2500km, the hard X-ray based density model, however, is more consistent with the `spicular extended-chromosphere model' inferred from radio sub-mm observations, than with standard models based on hydrostatic equilibrium. At coronal heights, h≈2.5–12.4Mm, the average flare loop density inferred from RHESSI is comparable with values from hydrodynamic simulations of flare chromospheric evaporation, soft X-ray, and radio-based measurements, but below the upper limits set by filling-factor insensitive iron line pairs.
    Solar Physics 10/2002; 210(1):383-405. DOI:10.1023/A:1022472319619 · 4.04 Impact Factor
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    ABSTRACT: Observations of flaring loops in radio, visible and x-ray bands show quasi-periodic pulsations with periods from a few seconds to several minutes. Recent numerical studies have shown that some of these oscillations can be interpreted as standing slow magnetoacoustic waves. Energy deposition from the flare excites the second standing harmonic, with a period determined by the temperature and loop length. The excited longitudinal oscillations can be practically dissipationless and can, possibly, be considered MHD autowaves. Numerical simulations with a wide range of flare durations and choices of heat deposition location show that the second harmonic is a common feature of flaring loops.
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