Chromospheric Oscillations Research Articles

Photospheric Origin of Three-minute Oscillations in a Sunspot

Abstract The origin of the three-minute oscillations of intensity and velocity observed in the chromosphere of sunspot umbrae is still unclear. We investigated the spatio-spectral properties of the 3 minute oscillations of velocity in the photosphere of a sunspot umbra as well as those in the low chromosphere using the spectral data of the Ni i λ5436, Fe i λ5435, and Na i D2 λ5890 lines taken by the Fast Imaging Solar Spectrograph of the 1.6 m New Solar Telescope at the Big Bear Solar Observatory. As a result, we found a local enhancement of the 3 minute oscillation power in the vicinities of a light bridge (LB) and numerous umbral dots (UDs) in the photosphere. These 3 minute oscillations occurred independently of the 5 minute oscillations. Through wavelet analysis, we determined the amplitudes and phases of the 3 minute oscillations at the formation heights of the spectral lines, and they were found to be consistent with the upwardly propagating slow magnetoacoustic waves in the photosphere with energy flux large enough to explain the chromospheric oscillations. Our results suggest that the 3 minute chromospheric oscillations in this sunspot may have been generated by magnetoconvection occurring in the LB and UDs.

Analysis of sudden variations in photospheric magnetic fields during a large flare and their influences in the solar atmosphere

The solar active region NOAA 11719 produced a large two-ribbon flare on 2013 April 11. We have investigated sudden variations in the photospheric magnetic fields in this active region during the flare by employing magnetograms obtained in the spectral line Fe I 6173 Å acquired by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) spacecraft. The analysis of the line-of-sight magnetograms from HMI show sudden and persistent magnetic field changes at different locations of the active region before the onset of the flare and during the flare. The vector magnetic field observations available from HMI also show coincident variations in the total magnetic field strength and its inclination angle at these locations. Using the simultaneous Dopplergrams obtained from HMI, we observe perturbations in the photospheric Doppler signals following the sudden changes in the magnetic fields in the aforementioned locations. The power spectrum analysis of these velocity signals shows enhanced acoustic power in these affected locations during the flare as compared to the pre-flare condition. Accompanying these observations, we have also used nearly simultaneous chromospheric observations obtained in the spectral line Hα 6562.8 Å by the Global Oscillation Network Group (GONG) to study the evolution of flare-ribbons and intensity oscillations in this active region. The Hα intensity oscillations also show enhanced oscillatory power during the flare in the aforementioned locations. These results indicate that the transient Lorentz force associated with sudden changes in the magnetic fields could drive localized photospheric and chromospheric oscillations, like the flare-induced oscillations in the solar atmosphere.

INTERFERENCE OF THE RUNNING WAVES AT LIGHT BRIDGES OF A SUNSPOT

ABSTRACT The observations of chromospheric oscillations of two umbral light bridges (LBs) within a sunspot from NOAA Active Region 12127 are presented. It was found that the running umbral waves with periods of 2.2–2.6 minutes underwent very fast damping before approaching umbral boundaries, while those with higher periods (>2.6 minutes) could propagate outside umbrae. On two sides of each LB adjacent to umbrae, the cross-wavelet spectra displayed that the oscillations on them had a common significant power region with dominant frequencies of 2–6 minutes and phase differences of ∼90°. A counterstream of two running umbral waves in the 2–6 minute frequency range propagated toward the LBs, where they encountered each other and gave rise to constructive or even destructive interference on the LBs. In addition, the velocity and density perturbations on the LBs were found in opposite phases suggesting that the perturbations were caused by the downward propagating waves.

Theoretical prediction of chromospheric oscillations in late-type stars

Self-consistent, nonlinear and time-dependent numerical computations of the excitation of chromospheric oscillations in a thin and non-isothermal magnetic flux tube embedded in magnetic-free atmospheres of late-type stars are performed. Longitudinal tube waves are considered and the wave energy spectra and fluxes generated in convective zones of these stars are calculated. The process of filtering the energy carried by longitudinal tube waves is investigated and both the local heating by shock waves as well as the excitation of chromospheric oscillations are studied. Cutoff frequencies of the resulting oscillations are computed numerically at different atmospheric heights in stars of different effective temperatures and gravities, and compared to three analytically derived cutoff frequencies. The obtained results show that the oscillation frequency ranges from 4 mHz for F5V stars to 20 MHz for M0V stars. It is pointed out that this frequency range may be relevant to the recent stellar p-mode observations made by the NASA space mission Kepler.

ACOUSTIC WAVES GENERATED BY IMPULSIVE DISTURBANCES IN A GRAVITATIONALLY STRATIFIED MEDIUM

Even though it is well-known from observations of the Sun that three-minute period chromospheric oscillations persist in the internetwork quiet regions and sunspot umbrae, until now their origin and persistence has defied clear explanation. Here we provide a clear and simple explanation for it with a demonstration of how such oscillations at the chromosphere's cutoff frequency naturally arise in a gravitationally stratified medium when it is disturbed. The largest-wavenumber vertical components of a chromospheric disturbance produce the highest-frequency wave packets, which propagate out of the disturbed region at group speeds that are close to the sound speed. Meanwhile, the smallest-wavenumber components develop into wave packets of frequencies close to the acoustic cutoff frequency that propagate at group speeds that are much lower than the sound speed. Because of their low propagation speed, these low-frequency wave packets linger in the disturbed region and nearby, and thus, are the ones that an observer would identify as the persistent, chromospheric three-minute oscillations. We emphasize that we can account for the power of the persistent chromospheric oscillations as coming from the repeated occurrence of disturbances with length scales greater than twice the pressure scale height in the upper photosphere.

DETECTION OF SHOCK MERGING IN THE CHROMOSPHERE OF A SOLAR PORE

It was theoretically demonstrated that a shock propagating in the solar atmosphere can overtake another and merge with it. We provide clear observational evidence that shock merging does occur quite often in the chromosphere of sunspots. Using Hα imaging spectral data taken by the Fast Imaging Solar Spectrograph of the 1.6 m New Solar Telescope at the Big Bear Soar Observatory, we construct time–distance maps of line-of-sight velocities along two appropriately chosen cuts in a pore. The maps show a number of alternating redshift and blueshift ridges, and we identify each interface between a preceding redshift ridge and the following blueshift ridge as a shock ridge. The important finding of ours is that two successive shock ridges often merge with each other. This finding can be theoretically explained by the merging of magneto-acoustic shock waves propagating with lower speeds of about 10 km s−1 and those propagating at higher speeds of about 16–22 km s−1. The shock merging is an important nonlinear dynamical process of the solar chromosphere that can bridge the gap between higher-frequency chromospheric oscillations and lower-frequency dynamic phenomena such as fibrils.

ON FIBRILS AND FIELD LINES: THE NATURE OF HαFIBRILS IN THE SOLAR CHROMOSPHERE

Observations of the solar chromosphere in the line-core of the \Halpha\ line show dark elongated structures called fibrils that show swaying motion. We performed a 3D radiation-MHD simulation of a network region, and computed synthetic \Halpha\ images from this simulation to investigate the relation between fibrils and the magnetic field lines in the chromosphere. The periods, amplitudes and phase-speeds of the synthetic fibrils are consistent with those observed. We analyse the relation between the synthetic fibrils and the field lines threading through them, and find that some fibrils trace out the same field line along the fibril's length, but there are also fibrils that sample different field lines at different locations along their length. Fibrils sample the same field lines on a time scale of $\sim200$~s. This is shorter than their own lifetime. We analysed the evolution of the atmosphere along a number of field lines that thread through fibrils and find that they carry slow-mode waves that load mass into the field line, as well as transverse waves that propagate with the Alfv\'en speed. Transverse waves propagating in opposite directions cause an interference pattern with complex apparent phase speeds. The relationship between fibrils and field lines is complex. It is governed by constant migration and swaying of the field lines, their mass loading by slow modes and subsequent draining, and their actual visibility in \Halpha. Field lines are visible where they lie close to the optical depth unity surface. The location of the latter is governed by the height at which the column mass in the chromosphere reaches a certain value. We conclude that using the swaying motion of fibrils as a tracer of chromospheric transverse oscillations must be done with caution.

DYNAMICS IN SUNSPOT UMBRA AS SEEN IN NEW SOLAR TELESCOPE AND INTERFACE REGION IMAGING SPECTROGRAPH DATA

We analyse sunspot oscillations using Interface Region Imaging Spectrograph (IRIS) slit-jaw and spectral data and narrow-band chromospheric images from the New Solar Telescope (NST) for the main sunspot in NOAA AR 11836. We report that the difference between the shock arrival times as measured the Mg II k 2796.35\AA\ and Si IV 1393.76\AA\ line formation levels changes during the observed period and peak-to-peak delays may range from 40~s to zero. The intensity of chromospheric shocks also displays a long term (about 20~min) variations. NST's high spatial resolution \ha\ data allowed us to conclude that in this sunspot umbral flashes (UFs) appeared in the form of narrow bright lanes stretched along the light bridges and around clusters of umbral bright points. Time series also suggested that UFs preferred to appear on the sunspot-center side of light bridges, which may indicate the existence of a compact sub-photospheric driver of sunspot oscillations. The sunspot's umbra as seen in the IRIS chromospheric and transition region data appears bright above the locations of light bridges and the areas where the dark umbra is dotted with clusters of umbral dots. Co-spatial and co-temporal data from the Atmospheric Imaging Assembly on board Solar Dynamics Observatory showed that the same locations were associated with bright footpoints of coronal loops suggesting that the light bridges may play an important role in heating the coronal sunspot loops. Finally, the power spectra analysis showed that the intensity of chromospheric and transition region oscillations significantly vary across the umbra and with height, suggesting that umbral non-uniformities and the structure of sunspot magnetic fields may play a role in wave propagation and heating of umbral loops.

Center-to-limb variation of low-frequency Ca II line brightness oscillations in the solar chromosphere

The goal of this paper is a detailed statistical analysis of the low-frequency Ca II line intensity oscillations containing information about the dynamics of the lower and middle chromosphere. A pixel-by-pixel analysis of the observed parameters has been performed. The following results have been obtained. (1) The low-frequency chromospheric oscillations (periods >400 s) are seen much more frequently in networks than in chromospheric network cells. (2) The relative fraction of the low-frequency chromospheric intensity oscillations increases with height. (3) The occurrence distribution of intensity oscillations as a function of the frequency is subdivided at least into two types. (4) In contrast to the low-frequency photospheric oscillations, the phase differences between the Ca II K and 849.8 nm line intensity oscillations do not give grounds to identify the low-frequency chromospheric oscillations with internal gravity waves. (5) The spectral composition of the oscillations in the network chromosphere resembles that expected in magnetic flux tubes in the nonlinear regime of conversion of transverse MHD waves at lower levels of the atmosphere into longitudinal MHD waves in its upper layer.

On vortex motion in chromospheric network boundaries

A large percent of spicules shows a surge-like behavior on the solar limb, supporting a multi-component model with twisted threads. The counterpart of limb spicules foot-points is investigated on the disk, and this re-examination indicates that the interpretation of transverse motion of off-limb spicules could directly be related to rotational motion at the feet of disk spicules. Related bright elements move and vary in brightness on the timescales of chromospheric oscillations. The motions are similar to random displacements of the bright elements along the network boundaries with amplitudes of about 200 to 400 km, with evidence of “spinning” or vortex motion. We find a clear evidence and in several cases for splitting process and suggesting a formation mechanism for doublet (or multi-component) spicules. A general inter chromospheric network velocity pattern with twists existing before the emergence and eruption of spicules seems to be required. In this paper a helical- kink mode propagation consistent with the new evidence of spicule multiple structure is presented and provides an explanation for the origin of the Alfvenic wave propagation along the spicules. The evidence of spinning spicules remains unclear from disk Dopplergram observations, we use a 3D time slice “column” diagrams (2D in x and y and time in z being the 3d dimension) by consecutive partly transparent slices put in perspective to show the rotational behavior at the chromospheric rosettes, and provide a wealth of information on spinning motion, helical wave propagation and splitting.

Properties of oscillatory motions in a facular region

We study the properties of waves in a facular region of moderate strength in the photosphere and chromosphere. Our aim is to analyse statistically the wave periods, power and phase relations as a function of the magnetic field strength and inclination. Our work is based on observations obtained at the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife) using two different instruments: the Triple Etalon SOlar Spectrometer (TESOS), in the BaII 4554 A line to measure velocity and intensity variations through the photosphere; and, simultaneously, the Tenerife Infrared Polarimeter (TIP-II), in the FeI 1.56 mm lines to the measure the Stokes parameters and magnetic field strength in the lower photosphere. Additionally, we use the simultaneous broad-band filtergrams in the CaIIH line to obtain information about intensity oscillations in the chromosphere. We find several clear trends in the oscillation behaviour: (i) the period of oscillation increases by 15-20 % with the magnetic field increasing from 500 to 1500 G; (ii) the temperature-velocity phase shifts show a strikingly different distribution in the facular region compared to the quiet region, a significant number of cases in the range from -180 to 180 degrees is detected in the facula. (iii) the most powerful chromospheric CaIIH intensity oscillations are observed at locations with strong magnetic fields (1.3-1.5 kG) inclined by 10-12 degrees, as a result of upward propagating waves with rather small phase speeds, and temperature-velocity phase shifts between 0 and 90 degrees; (iv) the power of the photospheric velocity oscillations from the \BaII\ line increases linearly with decreasing magnetic field inclination, reaching its maximum at strong field locations.

Atmosphere above a large solar pore

A large solar pore with a granular light bridge was observed on October 15, 2008 with the IBIS spectrometer at the Dunn Solar Telescope and a 69-min long time series of spectral scans in the lines Ca II 854.2 nm and Fe I 617.3 nm was obtained. The intensity and Doppler signals in the Ca II line were separated. This line samples the middle chromosphere in the core and the middle photosphere in the wings. Although no indication of a penumbra is seen in the photosphere, an extended filamentary structure, both in intensity and Doppler signals, is observed in the Ca II line core. An analysis of morphological and dynamical properties of the structure shows a close similarity to a superpenumbra of a sunspot with developed penumbra. A special attention is paid to the light bridge, which is the brightest feature in the pore seen in the Ca II line centre and shows an enhanced power of chromospheric oscillations at 3–5 mHz. Although the acoustic power flux in the light bridge is five times higher than in the "quiet" chromosphere, it cannot explain the observed brightness.

Chromospheric Sunspot Oscillations in Hα and Ca ii 8542 Å

We study chromospheric oscillations including umbral flashes and running penumbral waves in a sunspot of active region NOAA 11242 using scanning spectroscopy in Hα and Ca ii 8542 A with the Fast Imaging Solar Spectrograph (FISS) at the 1.6 meter New Solar Telescope at the Big Bear Solar Observatory. A bisector method is applied to spectral observations to construct chromospheric Doppler-velocity maps. Temporal-sequence analysis of these shows enhanced high-frequency oscillations inside the sunspot umbra in both lines. Their peak frequency gradually decreases outward from the umbra. The oscillation power is found to be associated with magnetic-field strength and inclination, with different relationships in different frequency bands.

Ca II 854.2 nm BISECTORS AND CIRCUMFACULAR REGIONS

Active regions appear bright in Ca II 854.2 nm line core intensity while the surrounding areas, referred to as circumfacular regions, are darker than the active region or the quiet Sun. We use Synoptic Optical Long-term Investigations of the Sun Vector Spectromagnetograph Ca II 854.2 nm data (photospheric and chromospheric full disk magnetograms as well as high spectral resolution Stokes I and V profiles) to study the connection between magnetic canopies, circumfacular regions, and Ca II 854.2 nm bisector amplitudes (spans). The line bisector amplitude is reduced in circumfacular regions, where the 3 minute period power in chromospheric Hα intensity oscillations is also reduced relative to the surrounding quiet Sun. The latter is consistent with magnetic canopies in circumfacular regions suppressing upward propagating steepening acoustic waves. Our results provide further strong evidence for shock waves as the cause of the inverse C-shaped bisector and explain the observed solar cycle variation of the shape and amplitude of Sun-as-a-star Ca II 854.2 nm bisectors.

SIMULTANEOUS OBSERVATION OF SOLAR OSCILLATIONS ASSOCIATED WITH CORONAL LOOPS FROM THE PHOTOSPHERE TO THE CORONA

The solar oscillations along one coronal loop in AR 11504 are observed simultaneously in white light emission and Doppler velocity by SDO/HMI, and in UV and EUV emissions by SDO/AIA. The technique of the time-distance diagram is used to detect the propagating oscillations of the emission intensities along the loop. We find that although all the oscillation signals were intercorrelated, the low chromospheric oscillation correlated more closely to the oscillations of the transition region and corona than to those of the photosphere. Situated above the sunspot, the oscillation periods were similar to 3 minutes in the UV/EUV emissions; however, moving away from the sunspot and into the quiet Sun, the periods became longer, e. g., up to similar to 5 minutes or more. In addition, along another loop we observe both the high-speed outflows and oscillations, which roughly had a one-to-one corresponding relationship. This indicates that the solar periodic oscillations may modulate the magnetic reconnections between the loops of the high and low altitudes that drive the high-speed outflows along the loop.

Solar faculae and waves propagating in the solar atmosphere

Solar faculae are among the most common manifestations of solar activity and can play an important role in the energy transfer from the lower solar atmosphere into the corona. However, the mechanisms by which energy is transferred remain insufficiently studied. Our work is based on observational data obtained with the AST telescope of the Sayan solar observatory. Simultaneous observations were performed in the Hα 6563 A and FeI 6569 A, BaII 4554 A and FeI 4551.6 A, and CaII 8542 A and FeI 8538 A pairs of spectral lines. The studies indicated that the spectral composition of the line-of-sight (LOS) velocity oscillations in faculae is inhomogeneous and is affected by the chromospheric network’s structural elements. The possible presence of short and low magnetic loops in the facula region makes it difficult to determine the unambiguous phase relations between chromospheric and photospheric oscillations. The LOS velocity oscillation signals registered at the chromospheric level both lead and lag behind the signals registered at the photospheric level. At the same time, signs of propagating waves are evidently registered in the chromosphere of individual faculae.

RAPID FLUCTUATIONS IN THE LOWER SOLAR ATMOSPHERE

The Rapid Oscillations in the Solar Atmosphere (ROSA) instrument reveals solar atmospheric fluctuations at high frequencies. Spectra of variations of the G-band intensity (IG) and CaII K-line intensity (IK) show correlated fluctuations above white noise to frequencies beyond 300 mHz and 50 mHz, respectively. The noise-corrected G-band spectrum for f = 28 - 326 mHz shows a power law with exponent -1.21 \pm, 0.02, consistent with the presence of turbulent motions. G-band spectral power in the 25 - 100 mHz ("UHF") range is concentrated at the locations of magnetic bright points in the intergranular lanes and is highly intermittent in time. The intermittence of the UHF G-band fluctuations, shown by a positive kurtosis {\kappa}, also suggests turbulence. Combining values of IG, IK, UHF power, and {\kappa}, reveals two distinct states of the solar atmosphere. State 1, including almost all the data, is characterized by low IG, IK, and UHF power and {\kappa} \approx 6. State 2, including only a very small fraction of the data, is characterized by high IG, IK, and UHF power and {\kappa} \approx 3. Superposed epoch analysis shows that the UHF power peaks simultaneously with spatio-temporal IG maxima in either state. For State 1, IK shows 3.5 min chromospheric oscillations with maxima occurring 21 s after IG maxima implying a 150 - 210 km effective height difference. However, for State 2 the IK and IG maxima are simultaneous; in this highly magnetized environment sites of G-band and K-line emission may be spatially close together.

Leakage of long-period oscillations from the chromosphere to the corona

Long-period oscillations in a coronal diffuse structure are detected with the Transition Region And Coronal Explorer (TRACE). The EUV images of the NOAA active region 8253 are available in 171 A and 195 A bandpasses from 30 June to 4 July 1998. The average intensity variation is found to be connected with the CCD temperature, which varies with the orbital motion of the spacecraft. Hence, oscillations with the orbital period and its higher harmonics appear as artifacts in the light curves. After the exclusion of the orbital effects, we identified several long-period oscillations in the diffuse fan-like structure of the active region. Similar periodicities were detected in the radio emission from the chromospheric part of that active region, observed with the ground-based Nobeyama Radioheliograph (NoRH) in the 17 GHz channel. It was found that 0.221, 0.312 and 0.573 mHz oscillations were present in both EUV emission lines in the corona and the radio signal from the sunspot in the chromosphere, just beneath the active region. From the frequency values, the 1st and 3rd detected oscillations could be associated with the l = 2, n = − 3o rl = 3, n = − 5a ndl = 1 gravity-driven solar interior modes, respectively. The appearance of these oscillations in the coronal part of the active region can be connected with the wave leakage or the evanescence of chromospheric oscillations.

The role of faculae in wave-energy transfer to upper layers of the solar atmosphere: Observations

The properties of Doppler-velocity oscillations in solar faculae are analyzed at the photospheric level (based on Fe I 6569 A and Fe I 8536 A lines) and chromospheric level (based on Hα and Ca II 8542 A lines) to search for upward propagating waves. The similarity of the averaged power spectra at 2.5–4 mHz is not found to be convincing proof of the presence of unidirectional wave-energy transfer from the photosphere to the chromosphere. Phase relations between the photospheric and chromospheric oscillations that are indicative of either upward or downward propagating waves are obtained for various areas in many faculae. This suggests that the wave energy of the five-minute oscillations returns to the photosphere, at least partially. The derived properties suggest that the role of faculae in the transfer of the five-minute oscillations to the chromosphere and overlying layers is not as obvious as could be expected. The relatively typical presence of low-frequency (0.5–2 mHz) oscillations in faculae and their possible important role in this energy transfer are noted.

Chromospheric oscillations in CaII lines

The dynamic regime of eight regions in the quiet solar chromosphere was analyzed. Observations in CaII lines were performed with a moderate spatial resolution of around 2″. The purpose of this study was to compare the diagnostic capabilities of these observations with high-precision measurements conducted in recent years by different authors with new-generation instruments. Being probably of importance in the chromospheric heating, the recently revealed structural formations (magnetoacoustic portals, magnetic aureoles, and magnetic shadows) were found to manifest themselves at moderate resolutions too. These findings are important in the context of investigations of coronal hole bases conducted by the authors with the same observational data.