Sunspot Seismology Research Articles

Inter-correlation between Sunspot Oscillations and Their Internal Structures

Three- and five-minute oscillations are commonly observed in any sunspot. Because they are modulated by the internal thermal and magnetic structures of a sunspot, they could be used as an effective tool for researching sunspot seismology. In this paper, we investigate the properties of oscillations in sunspot groups with varying sizes and magnetic fields, and aim to establish the relationships between sunspot oscillations and its internal structure comparatively. We selected three groups of the unipolar sunspot with approximately axial-symmetric magnetic field and calculated their Fourier spectra based on the ultraviolet/extreme ultraviolet emission intensity variations recorded by the Solar Dynamics Observatory/Atmospheric Imaging Assembly. We found that the distribution of three-minute oscillation is defined by the joint effect of diverging magnetic field and the stratification of the sunspot atmosphere. Its distribution could be modified by any invading magnetic structures in the umbra. In contrast, the five-minute oscillations are more prominent in small spots, implying that five-minute oscillation is very closely connected with umbral dynamics.

Penumbral thermal structure below the visible surface

$Context$. The thermal structure of the penumbra below its visible surface (i.e., $\tau_5 \ge 1$) has important implications for our present understanding of sunspots and their penumbrae: their brightness and energy transport, mode conversion of magneto-acoustic waves, sunspot seismology, and so forth. $Aims$. We aim at determining the thermal stratification in the layers immediately beneath the visible surface of the penumbra: $\tau_5 \in [1,3]$ ($\approx 70-80$ km below the visible continuum-forming layer). $Methods$. We analyzed spectropolarimetric data (i.e., Stokes profiles) in three Fe \textsc{i} lines located at 1565 nm observed with the GRIS instrument attached to the 1.5-meter solar telescope GREGOR. The data are corrected for the smearing effects of wide-angle scattered light and then subjected to an inversion code for the radiative transfer equation in order to retrieve, among others, the temperature as a function of optical depth $T(\tau_5)$. $Results$. We find that the temperature gradient below the visible surface of the penumbra is smaller than in the quiet Sun. This implies that in the region $\tau_5 \ge 1$ the penumbral temperature diverges from that of the quiet Sun. The same result is obtained when focusing only on the thermal structure below the surface of bright penumbral filaments. We interpret these results as evidence of a thick penumbra, whereby the magnetopause is not located near its visible surface. In addition, we find that the temperature gradient in bright penumbral filaments is lower than in granules. This can be explained in terms of the limited expansion of a hot upflow inside a penumbral filament relative to a granular upflow, as magnetic pressure and tension forces from the surrounding penumbral magnetic field hinder an expansion like this.

HELIOSEISMIC RESPONSE TO THE X2.2 SOLAR FLARE OF 2011 FEBRUARY 15

The X2.2-class solar flare of February 15, 2011, produced a powerful sunquake event, representing a helioseismic response to the flare impact in the solar photosphere, which was observed with the HMI instrument on the Solar Dynamics Observatory (SDO). The impulsively excited acoustic waves formed a compact wavepacket traveling through the solar interior and appearing on the surface as expanding wave ripples. The initial flare impacts were observed in the form of compact and rapid variations of the Doppler velocity, line-of-sight magnetic field and continuum intensity. These variations formed a typical two-ribbon flare structure, and are believed to be associated with thermal and hydrodynamic effects of high-energy particles heating the lower atmosphere. The analysis of the SDO/HMI and X-ray data from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) shows that the helioseismic waves were initiated by the photospheric impact in the early impulsive phase, observed prior to the hard X-ray (50-100 keV) impulse, and were probably associated with atmospheric heating by relatively low-energy electrons (~6-50 keV) and heat flux transport. The impact caused a short motion in the sunspot penumbra prior to the appearance of the helioseismic wave. It is found that the helioseismic wave front traveling through a sunspot had a lower amplitude and was significantly delayed relative to the front traveling outside the spot. These observations open new perspectives for studying the flare photospheric impacts and for using the flare-excited waves for sunspot seismology.

Effect of Suppressed Excitation on the Amplitude Distribution of 5 Minute Oscillations in Sunspots

Five minute oscillations on the Sun (acoustic and surface gravity waves) are excited by subsurface turbulent convection. However, in sunspots the excitation is suppressed because a strong magnetic field inhibits convection. We use three-dimensional simulations to investigate how the suppression of excitation sources affects the distribution of the oscillation power in sunspot regions. The amplitude of random acoustic sources was reduced in circular-shaped regions to simulate the suppression in sunspots. The simulation results show that the amplitude of the oscillations can be approximately 2-4 times lower in the sunspot regions in comparison to the quiet Sun, just because of the suppressed sources. Using SOHO MDI data we measured the amplitude ratio for the same frequency bands outside and inside sunspots and found that this ratio is approximately 3-4. Hence, the absence of excitation sources inside sunspots makes a significant contribution (about 50% or higher) to the observed amplitude ratio and must be taken into account in sunspot seismology.

Radiative Transfer Effects on Doppler Measurements as Sources of Surface Effects in Sunspot Seismology

We show that the use of Doppler shifts of Zeeman-sensitive spectral lines to observe waves in sunspots is subject to measurement-specific phase shifts arising from (1) the altered height range of spectral line formation and the propagating character of p-mode waves in penumbrae and (2) Zeeman broadening and splitting. We also show that these phase shifts depend on wave frequencies, strengths and line-of-sight inclinations of magnetic fields, and the polarization state used for Doppler measurements. We discuss how these phase shifts could contribute to local helioseismic measurements of surface effects in sunspot seismology.

Observed Frequency Variations of Solar p -Mode Travel Times as Evidence for Surface Effects in Sunspot Seismology

Using helioseismic holography, we measure acoustic ( -mode) travel-time perturbations, observed within solar p active regions, as functions of frequency and phase speed. We find evidence for a frequency variation, at fixed phase speed, of the travel times that has not previously been reported. This variation is not expected from typical sound-speed models of sunspots, which result from the inversion of travel times and may indicate a significant contribution to the travel times from structures with vertical scales smaller than about 1 Mm near the solar surface.

Simulation of [CLC][ITAL]f[/ITAL][/CLC]- and [CLC][ITAL]p[/ITAL][/CLC]-Mode Interactions with a Stratified Magnetic Field Concentration

The interaction of f- and p-modes with a slab of vertical magnetic field of sunspot strength is simulated numerically in two spatial dimensions. Both f-modes and p-modes are partially converted to slow magnetoatmospheric gravity (MAG) waves within the magnetic slab because of the strong gravitational stratification of the plasma along the magnetic lines of force. The slow MAG waves propagate away from the conversion layer guided by the magnetic field lines, and the energy they extract from the incident f- and p-modes results in a reduced amplitude for these modes as they exit from the back side of the slab. In addition, the incident p-modes are partially mixed into f-modes of comparable frequency, and therefore larger spherical harmonic degree, when they exit the magnetic flux concentration. These findings have important implications for the interpretation of observations of p-mode absorption by sunspots, both in terms of the successes and failures of this simple numerical simulation viewed in the sunspot seismology context.

Sunspot Seismology: The Interaction of Solar p-Modes with a Sunspot

AbstractProgress in sunspot seismology is briefly reviewed. New observations of the absorption of p-modes by a sunspot (Bogdan, Brown, Lites, and Thomas 1993) show that the absorption coefficient (averaged over frequency and azimuthal order) increases with horizontal wavenumber to a maximum near k = 0.9 Mm−1 and then decreases monotonically to zero at about k = 1.5 Mm−1. The absorption along individual p-mode ridges shows a sinusoidal-like modulation, which may be an important feature for comparison with theory. Results for the quiet Sun show no net absorption, but do show apparent emission along p1 over the same range of k where the sunspot shows its greatest absorption. This behavior, and the overall decrease in absorption at higher wavenumbers, can be explained as an effect of the decreasing lifetime of p-modes with increasing wavenumber k (increasing mode degree l).

Multiple scattering of acoustic waves by a pair of uniformly magnetized flux tubes

view Abstract Citations (24) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Multiple Scattering of Acoustic Waves by a Pair of Uniformly Magnetized Flux Tubes Bogdan, Thomas J. ; Fox, David C. Abstract The interaction of an acoustic plane wave with a pair of uniformly magnetized flux tubes of circular cross section is considered to determine whether sunspot seismology can distinguish between the competing monolithic flux-tube and spaghetti sunspot models. It is shown that this interaction falls within one of three distinct categories depending upon the separation of the magnetic flux tubes and their individual scattering strengths. Estimates suggest that the interaction of the solar acoustic oscillations with a spaghetti sunspot will almost surely show coherent phasing of the scattered waves and will probably exhibit multiple scattering effects. Publication: The Astrophysical Journal Pub Date: October 1991 DOI: 10.1086/170552 Bibcode: 1991ApJ...379..758B Keywords: Acoustic Scattering; Magnetosonic Resonance; Sound Waves; Sunspots; Coherent Scattering; Helioseismology; Incoherent Scattering; Magnetic Flux; Solar Magnetic Field; Solar Oscillations; Solar Physics; HYDROMAGNETICS; SUN: MAGNETIC FIELDS; SUN: OSCILLATIONS; SUN: SUNSPOTS; WAVE MOTIONS full text sources ADS |