Sunspot Oscillations Research Articles

Propagation Properties of Sunspots Umbral Oscillations in Horizontal and Vertical Directions

We present a study on investigating the propagation characteristics of umbral oscillations in sunspots. In sunspot 1 (located in NOAA AR 12127) with four umbrae, the analysis shows that the oscillations in different umbrae are correlated. The weak correlation (<20%) is attributed to the propagation of umbral oscillations across the umbral boundary to its adjacent umbra in the horizontal direction. We speculate that oscillations in two of the umbrae have a common origin in the sub-photosphere, resulting in a stronger correlation (>30%). Additionally, utilizing the TiO (photosphere), Hα (chromosphere) images provided by BBSO/GST, and the 304 Å (upper chromosphere and lower transition region), 171 Å (upper transition region), 193 Å (corona), and 211 Å (active region corona) images acquired by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO), we analyze the vertical propagation of oscillations in the sunspot umbra. Multi-channel observation shows that the umbral oscillations observed in the lower atmosphere of sunspot 1 cannot be detected in the upper atmosphere. However, in sunspot 2 (located in NOAA AR 12132), oscillations in the lower atmosphere can propagate to the upper atmosphere. Using photospheric magnetic field data provided by the Helioseismic and Magnetic Imager on board SDO, potential field extrapolation of the magnetic field for the two sunspots shows that open magnetic field structures allow sunspot oscillations to propagate to higher heights, while closed magnetic field structures do not.

Siberian Radioheliograph: sunspot oscillations in 3–6 GHz band

ABSTRACT We present the first observations of spatially resolved oscillation sources obtained with the Siberian Radioheliograph at 3–6 GHz. We have found significant flux oscillations with periods of about 3, 5, and 13 min emitted from AR12833. The 3-min periodicity dominates at higher frequencies. It was found that the apparent level of oscillations depends on the active region location on the disc, and scales down towards the limbs. The oscillations were studied in detail during 1 h interval on 2021 June 19. We found that sources of 3-min oscillations were located above the umbra and their emission is extraordinary polarized. The 5- and 13-min periods were manifested in emission at lower frequencies, down to 2.8 GHz. Sources with 5-min periodicity were located near the umbra/penumbra boundary and in the pore region. Positions of sources with 13-min oscillations were different at 3.1 and 4.7 GHz. We found consistency between spatial location of the oscillation sources in radio and ultraviolet at 171 and 304 Å. There is significant correlation of signals in two ranges. Time delays between microwave oscillations increase as the frequency decreases, which can be explained by upward propagation of periodic disturbances. The localization of oscillation sources is probably related to magnetic structures with different wave cut-off frequencies at different heights.

First looks at solar active regions with ALMA

During the first few years of observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA), the scientific community has acquired a number of observational datasets targeting various structures in active regions, including sunspot umbra and penumbra, active region pores, and plages. In this paper we review the results obtained from the extensive analysis of these interferometric millimeter data, together with the coordinated observations from IRIS, SDO, IBIS, and Hinode, that reveal information on the chromospheric thermal structure above active regions and properties of small-scale heating events near magnetic field concentrations. We discuss the properties of waves (especially the three-minute oscillations) in sunspots, plage, and network. We speculate how high-resolution millimeter data can supplement spectral line observations in the visible and UV and can improve chromospheric spectroscopic inversions. We identify challenges in the interpretation of the millimeter continuum emission due to the complex, non-local and time-dependent processes that determine the electron density through the chromosphere. Finally we overview the prospects for future active regions observations with ALMA during the ascending phase of the solar cycle.

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.

A Study of Sunspot 3 Minute Oscillations Using ALMA and GST

Abstract Waves and oscillations are important solar phenomena, not only because they can propagate and dissipate energy in the chromosphere, but also because they carry information about the structure of the atmosphere in which they propagate. The nature of the 3 minute oscillations observed in the umbral region of sunspots is considered to be an effect of propagation of magnetohydrodynamic waves upward from below the photosphere. We present a study of sunspot oscillations and wave propagation in NOAA Active Region 12470 using an approximately 1 hr long data set acquired on 2015 December 17 by the Atacama Large Millimeter/submillimeter Array (ALMA), the Goode Solar Telescope (GST) operating at the Big Bear Solar Observatory, the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, and the Interface Region Imaging Spectrograph. The ALMA data are unique in providing a time series of direct temperature measurements in the sunspot chromosphere. The 2 s cadence of ALMA images allows us to well resolve the 3 minute periods typical of sunspot oscillations in the chromosphere. Fourier analysis is applied to ALMA Band 3 (∼100 GHz, ∼3 mm) and GST Hα data sets to obtain power spectra as well as oscillation phase information. We analyzed properties of the wave propagation by combining multiple wavelengths that probe physical parameters of solar atmosphere at different heights. We find that the ALMA temperature fluctuations are consistent with that expected for a propagating acoustic wave, with a slight asymmetry indicating nonlinear steepening.

Effects of erupting magnetic flux rope on running penumbral waves

Context. It is widely known that solar flares have a substantial impact on the low atmosphere but the matter of how they affect sunspot waves and oscillations is generally unknown. In addition, there are ongoing debates on whether the flare-induced photospheric changes are due to the momentum conservation with coronal mass ejections or to magnetic reconnection. Aims. To shed light on the so-called “back reaction” of solar eruptions, we investigated how running penumbral waves (RPWs) at one foot of an erupting magnetic flux rope (MFR) respond to the rope buildup and subsequent erosion. Methods. We used UV/EUV images from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO) to explore the changing behaviors of RPWs in response to the MFR evolution, as well as 135-s vector magnetograms from the SDO Helioseismic and Magnetic Imager to analyze the changes in photospheric magnetic field during the eruption. Results. During the rope buildup stage, the western foot of the rope, which is completely enclosed by a hooked ribbon, expands rapidly and consequently ends up overlapping a sunspot penumbra. This converts the original penumbral field into the rope field, which is associated with a transient increase in electric currents flowing through the ribbon-swept penumbral region. During the rope erosion stage, the rope foot shrinks as the eastern section of the hooked ribbon slowly sweeps the same penumbral region, where the rope field is converted into flare loops. This conversion induces mixed effects on the photospheric field inclination but heats up the low atmosphere at the footpoints of these flare loops to transition-region temperatures, therefore resulting in the post-eruption RPWs with an enhanced contrast in the 1600 Å passband and an extended bandwidth to low frequencies at 3–5 mHz, compared with the pre-eruption RPWs that peak at 6 mHz. Conclusions. This observation clearly demonstrates that it is the magnetic reconnection in the corona that impacts the low atmosphere and leads to the changing behaviors of RPWs, which, in turn, offer a new window onto diagnosing flare reconnections.

Anticorrelation of Variations of the Magnetic Field of a Sunspot and the Brightness of Its Umbra in Long-Period Sunspot Oscillations

Changes in the umbra brightness and in the magnetic field over time should occur on long-period sunspot oscillations in accordance with Birman’s idea of inhibition of circulation (overturning) convection by a strong vertical magnetic field in antiphase mode.When the sunspot as a whole shifts upward, its power flux tube expands with a fixed magnetic flux, the magnetic field decreases, and the umbra brightness should increase. Vice versa, when the magnetic flux tube of the sunspot is lowered, it is compressed, the field grows, and the umbra brightness should decrease. Until now, this oscillatory process, which is described by the theoretical model of a shallow sunspot, has remained unexplored. This paper attempts to prove the Birman effect (Biermann, 1941) on the example of long-period oscillations of the sunspot 2010/12/07-13 (NOAA 11 133), for which there are available data from both the Solar and Heliospheric Observatory (SOHO) and Solar Dynamics Observatory (SDO).

Doppler shift oscillations of a sunspot detected by CYRA and IRIS

Context. The carbon monoxide (CO) molecular line at around 46655 Å in solar infrared spectra is often used to investigate the dynamic behavior of the cold heart of the solar atmosphere, i.e., sunspot oscillation, especially at the sunspot umbra. Aims. We investigated sunspot oscillation at Doppler velocities of the CO 7-6 R67 and 3-2 R14 lines that were measured by the Cryogenic Infrared Spectrograph (CYRA), as well as the line profile of Mg II k line that was detected by the Interface Region Imaging Spectrograph (IRIS). Methods. A single Gaussian function is applied to each CO line profile to extract the line shift, while the moment analysis method is used for the Mg II k line. Then the sunspot oscillation can be found in the time–distance image of Doppler velocities, and the quasi-periodicity at the sunspot umbra are determined from the wavelet power spectrum. Finally, the cross-correlation method is used to analyze the phase relation between different atmospheric levels. Results. At the sunspot umbra, a periodicity of roughly 5 min is detected at the Doppler velocity range of the CO 7-6 R67 line that formed in the photosphere, while a periodicity of around 3 min is discovered at the Doppler velocities of CO 3-2 R14 and Mg II k lines that formed in the upper photosphere or the temperature minimum region and the chromosphere. A time delay of about 2 min is measured between the strong CO 3-2 R14 line and the Mg II k line. Conclusions. Based on the spectroscopic observations from the CYRA and IRIS, the 3 min sunspot oscillation can be spatially resolved in the Doppler shifts. It may come from the upper photosphere or the temperature minimum region and then propagate to the chromosphere, which might be regarded as a propagating slow magnetoacoustic wave.

The origin of quasi-periodicities during circular ribbon flares

Context. Solar flares with a fan-spine magnetic topology are able to form circular ribbons. A previous study based on Hα line observations of the solar flares on 5 March 2014 revealed a uniform and continuous rotation of the magnetic fan-spine. A preliminary analysis of the flare time profiles revealed quasi-periodic pulsations (QPPs) with similar properties in hard X-rays, Hα, and microwaves. Aims. In this work, we address the question of whether the observed periodicities are related to periodic acceleration of electrons or plasma heating. Methods. We analysed QPPs in the Hα emission from the centre of the fan (inner ribbon R1), a circular ribbon (R2), a remote source (R3), and an elongated ribbon (R4) located between R2 and R3. We used methods of correlation, Fourier, wavelet, and empirical mode decomposition. We compared the QPPs in Hα emission with those in microwave and X-ray emission. Results. We found multi-wavelength QPPs with periods around 150 s, 125 s, and 190 s. The 150 s period is seen to co-exist in Hα, hard X-rays, and microwave emissions, which allowed us to connect it with flare kernels R1 and R2. These kernels spatially coincide with the site of the primary flare energy release. The 125 s period is found in the Hα emission of the elongated ribbon R4 and the microwave emission at 5.7 GHz during the decay phase. The 190 s period is present in the emission during all flare phases in the Hα emission of both the remote source, R3, and the elongated ribbon, R4, in soft X-rays and in microwaves at 4–8 GHz. Conclusions. We connected the dominant 150 s QPPs with the slipping reconnection mechanism occurring in the fan. We suggested that the period of 125 s in the elongated ribbon can be caused by a kink oscillation of the outer spine, connecting the primary reconnection site with the remote footpoint. The period of 190 s is associated with the three-minute sunspot oscillations.

Signatures of sunspot oscillations and the case for chromospheric resonances

Sunspots host a large variety of oscillatory phenomena, whose properties depend on the nature of the wave modes and the magnetic and thermodynamic structure of the spot. Umbral chromospheric oscillations exhibit significant differences compared to their photospheric counterparts. They show an enhanced power and a shorter dominant period, from waves with an amplitude of a few hundred meters per second in the five-minute band at the photosphere, to amplitudes of several kilometers per second in the three-minute band at the chromosphere. Various models have been proposed to explain this behaviour, including the presence of a chromospheric resonance cavity between the photosphere and the transition region. Jess et al. (2020, Nature Astronomy, 4, 220) claimed the detection of observational evidence supporting this model, obtained from the comparison of spectropolarimetric observations and numerical simulations. Here, it is shown that the observational insight reported by Jess et al. is not a common property of sunspots. More importantly, numerical modelling also shows that it is not an unequivocal signature of an acoustic resonator.

Reply to: Signatures of sunspot oscillations and the case for chromospheric resonances

Reply to: Signatures of sunspot oscillations and the case for chromospheric resonances

Spatial distributions of sunspot oscillation modes at different temperatures

Three- and five-minute sunspot oscillations have different spatial distributions in the solar atmospheric layers. The spatial distributions are crucial for revealing the physical origin of sunspot oscillations and to investigate their propagation. In this study, six sunspots observed by Solar Dynamics Observatory/Atmospheric Imaging Assembly were used to obtain the spatial distributions of three- and five-minute oscillations. The fast Fourier transform method is applied to represent the power spectra of oscillation modes. We find that, from the temperature minimum to the lower corona, the powers of the five-minute oscillation exhibit a circle-shape distribution around its umbra, and the shapes gradually expand with temperature increase. However, the circle-shape disappears and the powers of the oscillations appear to be very disordered in the higher corona. This indicates that the five-minute oscillation can be suppressed in the high-temperature region. For the three-minute oscillations, from the temperature minimum to the high corona, their powers mostly distribute within an umbra, and part of them are located at the coronal fan loop structures. Moreover, those relative higher powers are mostly concentrated in the position of coronal loop footpoints.

SDO Data Artifacts and Anticorrelation of Oscillations in the Sunspot Magnetic Field and Umbral Area

The paper considers a special class of artifacts that appears in the useful signal when the source moves along a discrete matrix of the receiver. A parasitic component forms in the signal due to the displacement of the extreme value of the distributed field of the physical quantity from pixel to pixel (p2p effect). It is shown that a stepwise discretization of Δt < T (p2p) is a necessary but not sufficient condition for the formation of the p2p-artifact. For small field gradients (in particular, with good spatial resolution), the parasitic signal in the time series of the studied variable is either weakened or does not form at all. This assumption has been qualitatively verified by comparative analysis of sunspot data obtained with SOHO/MDI and SDO/HMI. In addition, the Y-p2p effect, which is associated with sunspot displacement along the meridian and influences the process of long-period sunspot oscillations, is considered in detail based on SDO/HMI data. It was found that there is an inverse relationship between changes in the umbral area of a spot and variations of its magnetic field strength. This property of long-period sunspot oscillations as a single magnetic structure indicates the magnetic flux in the sunspot umbra is preserved, and it plays an extremely important role for our understanding of the physical nature of these oscillations.

An Analysis of Heavy Tail and Long-Range Correlation of Sunspot and El Nino-Southern Oscillation (ENSO) Cycles

Heavy tail analysis of Sunspot and El Nino-southern oscillation (ENSO) cycles is performed. Also, their significant behavior is investigated and the long-range correlation (persistency) is also determined. In a time series data, the heavy tail analysis helps to determine the persistency and long term dependency. All Sunspot Cycles and El Nino-southern oscillation (ENSO) Cycles are stationary in nature and each data value is strongly correlated to previous values. The purpose of the modeling performed is to evaluate the strength of long- range correlation as analyzed and to quantify the uncertainty which is hidden in Sunspots and El Nino-southern oscillation (ENSO) Cycles. All Sunspot Cycles and ENSO Cycles have the differencing parameter in the range 0 < d < 0.5 in both self-similar (dS) and self-affine (dA) cases. This means that the background dynamics are more regular. The heavy tail parameter βS (self-similar) as well as βA (self-affine) asymptotically follow the Pareto law which shows that the dynamics for all the Sunspot Cycles and El Nino-southern oscillation (ENSO) Cycles is regular and periodic. Heavy tail parameter (β) and differencing parameter (d = H – 0.5) is obtained from the Hurst Exponent ranging between 0.5 and 1 (persistent data). This study concludes that El Nino-southern oscillation (ENSO) Cycle data behave heavy tail and it is more elongated as compared to Sunspots time series data. All Sunspot Cycles and El Nino-southern oscillation (ENSO) Cycles demonstrate a strong long-range correlation ( $$\gamma $$ ). The strength of self-similar long-range correlation (1 < $$~{{\gamma }_{{\text{S}}}} < 3$$ ) and the self-affine strength of long-range correlation (–1 < $${{\gamma }_{{\text{A}}}} < 1$$ ) demonstrate persistency in the perspective that 0.5 < HS < 1 and 0.5 < HA < 1. This study shows that every value of El Nino-southern oscillation (ENSO) Cycles and Sunspot Cycles are strongly correlated to preceding values in both the self-similar and self-affine cases. Unit root test is applied to the tail parameter and the strength of long range-correlation of El Nino-southern oscillation (ENSO) and Sunspot Cycles confirms stationary behavior of the parameters. The variation of earth climatic has a strong influence in Sunspots Cycles and El Nino-southern oscillation (ENSO) Cycles. Sunspots and El Nino-southern oscillation (ENSO) have strong correlation with each other (Asma et al. 2018). The El Nino-southern oscillation (ENSO) cycles influence on the variation of the parameter of local climate which depends on the changes in solar activity.

The Observational Evidence for the Internal Excitation of Sunspot Oscillations Inferred from the Fe i 5435 Å Line

Abstract The umbral oscillations of velocity are commonly observed in the chromosphere of a sunspot. Their sources are considered to be either the external p-mode driving or the internal excitation by magnetoconvection. Even though the possibility of the p-mode driving has been often considered, the internal excitation has been rarely investigated. We report the identification of the oscillation patterns that may be closely related to the events of internal excitation from the observations of velocity oscillations in the temperature minimum region of two sunspots. The velocities were determined from the spectral data of the Fe i 5435 Å line, a magnetically insensitive line, taken with the Fast Imaging Solar Spectrograph of the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory. As a result, we identified four oscillation patterns of 2.0 × 103 km coherent size that were clearly identified for about 7.9 minutes with an oscillation amplitude of 9.3 × 10−2 km s−1. The power of the oscillations in these centers was concentrated in the 3 minute band. All the oscillation centers were located above the umbral dots undergoing noticeable morphological and dynamical changes that may be regarded as an observable signature of small-scale magnetoconvection inside the umbrae. Our results support the notion that magnetoconvection associated with umbral dots inside sunspots can drive the 3 minute umbral oscillations.

Quasi-periodic Pulsations before and during a Solar Flare in AR 12242

Abstract Quasi-periodic pulsations (QPPs) are frequently observed in solar flares, which may reveal some essential characteristics of both thermal and nonthermal energy releases. This work presents multi-wavelength imaging observations of an M8.7 flare in active region AR 12242 on 2014 December 17. We found that there were three different QPPs: UV QPPs with a period of about 4 minutes at 1600 Å images near the center of the active region lasting from the preflare phase to the impulsive phase; EUV QPPs with a period of about 3 minutes along the circular ribbon during the preflare phase; and radio QPPs with a period of about 2 minutes at frequencies of 1.2–2.0 GHz around the flaring source region during the impulsive phase. The observations include the radio images observed by the Mingantu Spectral Radioheliograph in China at frequencies of 1.2–2.0 GHz for the first time, microwave images by the Nobeyama Radioheliograph, UV and EUV images by AIA/SDO, and a magnetogram by HMI/SDO. We suggest that the 4 minute UV QPPs should be modulated by the sunspot oscillations, and the 3 minute EUV QPPs are closely related to the 2 minute radio QPPs for their source regions connected by a group of coronal loops. We propose that the intermittent magnetic reconnecting downward and upward plasmoids may be the possible trigger of both the preflare 3 minute EUV QPPs and the impulsive 2 minute radio QPPs. The other possible mechanism is LRC oscillation, which is associated with the current-carrying coronal loops. The latter mechanism implies that the existence of preflare QPPs may be a possible precursor to solar flares.

Local sunspot oscillations and umbral dots

Data analysis of sunspot oscillations based on a 6-hr SDO run of an observation showed that low frequency () oscillations are locally similar to three and five minute oscillations. The oscillations in the sunspot are concentrated in cells of a few arcsec, each of which has its own oscillation spectrum. The analysis of two scenarios for sunspot oscillations leads to a conclusion that local sunspot oscillations occur due to a subphotospheric resonator for slow MHD waves. Empirical models of a sunspot atmosphere and the theory of slow waves in thin magnetic flux tubes are applied to modeling the subphotospheric resonator. The spectrum of local oscillations consists of a great number of lines. This kind of spectrum can occur only if the subphotospheric resonator is a magnetic tube with a rather weak magnetic field. Magnetic tubes of this sort are umbral dots that appear due to the convective tongues in monolithic sunspots. The interrelation of local oscillations with umbral dots and wavefronts of traveling waves in sunspots is discussed.

Subphotospheric Resonator and Local Oscillations in Sunspots

The conditions under which the subphotospheric slow-wave resonator can be responsible for the local oscillations in a sunspot have been determined. A rich spectrum of local 3-min oscillations can be produced by the subphotospheric resonator only if the magnetic field in the resonator magnetic flux tube is much weaker than the surrounding sunspot magnetic field. Convective upflows of hot plasma in the sunspot magnetic field satisfy this condition. Consequently, there must be a correlation between the local oscillations and umbral dots, because the latter are produced by convective flows. Various modes of operation of the subphotospheric resonator give rise to wave packets of 3-min oscillations and umbral flashes. It is shown that giant local umbral flashes can emerge under certain conditions for the excitation of oscillations in the subphotospheric resonator.

Observations of Upward Propagating Waves in the Transition Region and Corona above Sunspots

Abstract We present observations of persistent oscillations of some bright features in the upper-chromosphere/transition region above sunspots taken by IRIS SJ 1400 Å and upward propagating quasi-periodic disturbances along coronal loops rooted in the same region taken by the AIA 171 Å passband. The oscillations of the features are cyclic oscillatory motions without any obvious damping. The amplitudes of the spatial displacements of the oscillations are about 1″. The apparent velocities of the oscillations are comparable to the sound speed in the chromosphere, but the upward motions are slightly larger than that of the downward. The intensity variations can take 24%–53% of the background, suggesting nonlinearity of the oscillations. The FFT power spectra of the oscillations show a dominant peak at a period of about 3 minutes, which is consistent with the omnipresent 3 minute oscillations in sunspots. The amplitudes of the intensity variations of the upward propagating coronal disturbances are 10%–15% of the background. The coronal disturbances have a period of about 3 minutes, and propagate upward along the coronal loops with apparent velocities in a range of 30 ∼ 80 km s−1. We propose a scenario in which the observed transition region oscillations are powered continuously by upward propagating shocks, and the upward propagating coronal disturbances can be the recurrent plasma flows driven by shocks or responses of degenerated shocks that become slow magnetic-acoustic waves after heating the plasma in the coronal loops at their transition-region bases.

Long-term oscillations of sunspots and a special class of artifacts in SOHO/MDI and SDO/HMI data

A specific type of artifacts, that originate due to displacement of the image of a moving object along the digital (pixel) matrix of receiver are analyzed in detail. The criteria of appearance and the influence of these artifacts on the study of long-term oscillations of sunspots are deduced. The obtained criteria suggest us methods for reduction or even elimination of these artifacts. It is shown that the use of integral parameters can be very effective against the artifact distortions. The simultaneous observations of sunspot magnetic field and ultraviolet intensity of the umbra have given the same periods for the long-term oscillations. In this way the real physical nature of the oscillatory process, which is independent of the artifacts have been confirmed again. A number of examples considered here confirm the dependence between the periods of main mode of the sunspot magnetic field long-term oscillations and its strength. The dependence was derived earlier from both the observations and the theoretical model of the shallow sunspot. The anti-phase behavior of time variations of sunspot umbra area and magnetic field of the sunspot demonstrates that the umbra of sunspot moves in long-term oscillations as a whole: all its points oscillate with the same phase.