Sunspot Oscillations Research Articles

Torsional oscillations of sunspots: magnetic and velocity fields observations

Torsional oscillations of sunspots were investigated in a penumbra on the observations in the photosphere of the longitudinal magnetic and velocity fields.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Oscillations in Sunspots observed in the Near Infrared

Spectropolarimetric time series of two sunspots are investigated to search for magnetic field oscillations. While the existence of velocity oscillations in the five-minute band is clearly confirmed, periodic variations of the magnetic field strength or the magnetic angles inclination and azimuth are small and restricted to very narrow areas. They occur in single frequency bins, but different for magnetic field strength and angles. Small dark structures embedded in one penumbra or in the near surroundings of the other spot exhibit enhanced power for the magnetic variations at all frequencies. Phase differences are rather unsure. The obtained values are in agreement with intrinsic magnetic field variations produced by slow magnetoacoustic modes as well as with an opacity mechanism connected with fast modes.

Coronal Oscillations above Sunspots?

Observational data clearly indicate the presence of 3-min oscillations in sunspots in spectral lines covering a vast temperature range from the low chromosphere to those lines normally associated with coronal temperatures. We show that after folding in the sunspot plume emission measure distribution, the contribution functions for lines normally formed just below 1×106 K are shifted to lower temperatures. For example, the Fe ix 171 A line is shifted to 6×105 K for a Maxwellian distribution and to less than 5×105 K with a non-Maxwellian distribution. Other lines such as Mg ix 368 A will also be affected. This then questions some previous work regarding the suggested detection of 3-min oscillations in the corona above sunspots.

A comparison between model calculations and observations of sunspot oscillations

We investigate the signal which is expected to be produced by magnetic field oscillations in sunspots umbrae due to the combination of the oscillation model, radiative transfer and observing procedure. For this purpose we investigate the signal expected to be produced by theoretical models of sunspot oscillations and compare them with the signal seen in observed power spectra of sunspot magnetograms. We show that the amplitudes of the observed oscillations are compatible with the predictions of the theoretical model of magnetoacoustic oscillations for the 5-min as well as for the 3-min band. For the particular sunspot umbral oscillation models used, our analysis suggests that most of the expected observed power in the magnetogram signal oscillations is actually due to cross-talk from the temperature and density oscillations associated with the magnetoacoustic wave. A detailed modelling of the observing procedure turns out to be of central importance for the assignment of the observed oscillations to a specific wave type.

Temporal and spatial variations of the magnetic field vector in sunspots

In order to look for magnetic field vector oscillations in sunspots we used data measured with the Advanced Stokes Polarimeter at the Dunn Solar Telescope. We analyzed two time series of 65 and 110 min that were taken by scanning repeatedly a 62 field of view, while obtaining the full Stokes vector in the lines FeI 630.15 nm and 630.25 nm. An inversion was carried out and a Fast Fourier Transform (FFT) analysis was used to find oscillatory phenomena. We discuss possible unwanted effects that lead to apparent magnetic field oscillations and find an average amplitude of () 5.8 G/0.23° rms by excluding these effects, which also means that only 6% and 22% of the two sunspot umbrae respectively remained for analysis. If we smooth the power spectra over pixels, all significant power disappears.

Magnetic field oscillations of sunspots?

Magnetic field oscillations of sunspots?

Differential rotation of the sun determined tracing sunspots and oscillations of sunspot tilt angle

The time and spatial characteristics of 324 large sunspots (S≥50 millionths of the solar hemisphere) selected from the Abastumani Astrophysical Observatory photoheliogram collection (1950–1990) have been studied. The variations of sunspot angular rotation velocity residuals and oscillations of sunspot tilt angle were analyzed. It has been shown that the differential rotation rate of selected sunspots correlates on average with the solar cycle. The deceleration of differential rotation of large sunspots begins on the ascending arm of the activity curve and ends on the descending arm reaching minimum near the epochs of solar activity maxima. This behavior disappears during the 21st cycle. The amplitudes and periods of sunspot tilt-angle oscillations correlate well with the solar activity cycle. Near the epochs of activity maximum there appear sunspots with large amplitudes and periods showing a significant scatter while the scatter near the minimum is rather low. We also found evidence of phase difference between the sunspot angular rotation velocity and the amplitudes and periods of tilt-angle oscillations.

$\ion{Ca}{ ii}$ 8542 Å sunspot oscillations observed with THEMIS

Oscillations in the umbra and the penumbra of an isolated sunspot located near the solar disk centre were investigated. The observations were obtained with the Multichannel Subtractive Double Pass (MSDP) spectrograph operating in the $\ion{Ca}{ii}$ 8542 A line and installed at the focus of THEMIS (Tenerife). From the MSDP data, two-dimensional intensity and Doppler shift images were computed at different wavelengths within the line. Intensity and Doppler shift oscillations in the umbra and the penumbra of the sunspot showing up as umbral flashes and penumbral waves were analyzed using a 23 min time series with a cadence of 46 s. The $\ion{Ca}{ii}$ umbral flash intensity profile shows an emission core in its blue wing. We investigate the relation between umbral flashes and running penumbral waves by a power spectrum analysis which shows a 6 mHz frequency for the standing umbral oscillations (flashes) which are observed only on the upper half part of the umbra. The running penumbral waves propagate with an average phase velocity of 16 and their frequency is constant in the penumbra and equal to 3 mHz. Although the time slice images suggest that umbral flashes and running penumbral waves are probably due to the same resonator, the power analysis shows no direct relationship between the two phenomena.

Waves in Sunspots: Resonant Transmission and the Adiabatic Coefficient

We investigate linear acoustic-gravity waves in three different semi-empirical model atmospheres of large sunspot umbrae. The sunspot filter theory is applied, that is, the resonant transmission of vertically propagating waves is modelled. The results are compared with observed linear sunspot oscillations. For three umbral models we present the transmission coefficients and the energy density of the oscillations with the maxima of transmission. The height dependence of the adiabatic coefficient (the ratio of specific heats) γ strongly influences the calculated resonance frequencies. The variable γ can explain the observed closely spaced resonance period peaks. The first resonance in the 3 min range is interpreted as a resonance of the upper chromosphere only, while the higher order peaks are resonances of the whole chromosphere.

The EUV spectrum of sunspot plumes observed by SUMER on SOHO

We present results from sunspot observations obtained by SUMER on SOHO. In sunspot plumes the EUV spectrum differs from the quiet Sun; continua are observed with different slopes and intensities; emission lines from molecular hydrogen and many unidentified species indicate unique plasma conditions above sunspots. Sunspot plumes are sites of systematic downflow. We also discuss the properties of sunspot oscillations

Sunspot Oscillations: a Review (Invited Review)

The current state of our knowledge, and ignorance, of the nature of oscillations in sunspots is surveyed. An effort is made to summarize the robust aspects of both the observational and theoretical components of the subject in a coherent, and common, conceptual framework. Detailed discussions of the various controversial issues are avoided except in instances where new viewpoints are advanced. Instead, extensive references are made to the growing literature on the subject, and generous explanatory remarks are made to guide the reader who wishes to delve more deeply into the underpinnings of the subject matter.

The EUV Spectrum of Sunspot Plumes Observed by SUMER on SOHO

AbstractWe present results from sunspot observations obtained by SUMER on SOHO. In sunspot plumes the EUV spectrum differs from the quiet Sun; continua are observed with different slopes and intensities; emission lines from molecular hydrogen and many unidentified species indicate unique plasma conditions above sunspots. Sunspot plumes are sites of systematic downflow. We also discuss the properties of sunspot oscillations.

Observational Study of Sunspot Oscillations in Stokes I, Q, U, and V

Time series of 2-dimensional spectro-polarmetric data were obtained with the intent of studying the temporal behavior of velocity, magnetic flux, and characteristics of the Stokes V profile in a small region of a larger sunspot. Full Stokes profiles in I, Q, U, and V were obtained. Velocity oscillations were found at frequencies of 3.3 mHz in each of the profiles. Acoustic power maps indicate that locations of highest power correspond to areas in which the polarization signal was greatest, therefore no conclusion about the type of wave mode participating in the oscillations can be made. Velocity amplitudes were I: 71 m s−1, Q: 47 m s−1, U: 65 m s−1 and V: 86 m s−1. Oscillatory behavior was also detected in longitudinal field strength, with an r.m.s. amplitude of 22 G, at 2.6 and 3.3 mHz. The power was localized at the umbral/penumbral boundary. A phase analysis indicates a −130° phase difference with Stokes V velocity oscillations at 3.3 mHz and a 75° difference at 2.6 mHz. Results are consistent with magnetic field lines swaying in response to a p-mode driver. No oscillatory behavior was seen in Stokes V asymmetry or amplitude splitting.

A new method to calculate the resonant response of a sunspot model atmosphere to magneto-atmospheric waves

In order to understand the observed oscillations in sunspots we present a new method for calculating the resonant response of a realistic semi-empirical model of the sunspot umbral atmosphere and subphotosphere to magneto-atmospheric waves in a vertical magnetic field. The depth dependence of both the adiabatic coefficient and the turbulent pressure is taken into account. This requires an extension of the wave equations by Ferraro & Plumpton (1958). We compare the coefficients of wave transmission, re flection, and conversion between fast mode and slow mode waves for different assumptions, compare the results with those from earlier modelling efforts, and point out possible sources of mistakes. The depth dependence of the adiabatic coefficient strongly influences the resulting spectrum of resonance frequencies. The condition of a conservation of wave flux is violated if the depth dependence of the turbulent pressure is not properly considered.

Characteristics of Magnetohydrodynamic Oscillations Observed with the Michelson Doppler Imager

We report on the spatial distribution of the magnetogram oscillatory power and the phase angles between velocity and magnetogram signals as observed with the Michelson Doppler Imager. The data set is 15125 × 15125, containing the sunspot from 1997 December 2 with a temporal sampling interval of 60 s and spatial sampling of 0605. The simultaneously observed continuum intensity and surface velocity accompany the magnetic information. We focus on three frequency regimes: 0.5-1.0, 3.0-3.5, and 5.5-6.0 mHz, corresponding roughly to timescales of magnetic evolution, p-modes, and the 3 minute resonant sunspot oscillation. Significant low-frequency magnetogram power is found in lower flux pixels, 100-300 G, in a striking ring with filamentary structure surrounding the sunspot. The 5 minute magnetogram power peaks in extended regions of flux that measure 600-800 G. The 3 minute oscillation is observed in the sunspot umbra in pixels whose flux measures 1300-1500 G. Phase angles of approximately -90° between velocity and magnetic flux in the 3.0-3.5 and 5.5-6.0 mHz regimes are found in regions of significant cross amplitude.

The Seismology of Sunspots: A Comparison of Time‐Distance and Frequency‐Wavenumber Methods

A pair of formulae are developed that relate the absorption coefficient and partial-wave phase shift concepts of frequency-wavenumber local helioseismology to the center-annulus cross-correlation function of time-distance helioseismology, under the general circumstances that both induced and spontaneous sunspot oscillations may be present. These formulae show that spontaneous emission of p-modes by magnetic and Reynolds stresses within the spot and the mode mixing between incoming and outgoing p-modes affect only the outgoing center-annulus cross-correlation time τ+, and they caution that real or spurious phase lags of the umbral oscillation signal lead to differences in the incoming and outgoing correlation times, resulting in τ- ≠ τ+. The application of these methods to actual helioseismic data obtained by the Global Oscillation Network Group (GONG) project is carried out in order to provide a tangible illustration of how time-distance and frequency-wavenumber ideas can profitably be combined to yield deeper insight into the seismic probing of sunspots. By using the helioseismic GONG data in conjunction with concurrent observations of Doppler velocities and vector magnetic fields obtained by the High Altitude Observatory/National Solar Observatory (HAO/NSO) Advanced Stokes Polarimeter (ASP) for the 1995 October disk passage of active region NOAA 7912, we demonstrate that the inferred GONG umbral signal actually originates from the umbra-penumbra boundary about 6 Mm distant from the center of the spot. Further, the ASP observations show that the 5 minute oscillations at the umbra-penumbra boundary lag behind those in the center of the umbra by approximately 1 minute, which is precisely the difference between the incoming and outgoing correlation times for NOAA 7912 recently determined by Braun. This remarkable result underscores the perils of using umbral oscillations in time-distance helioseismology, and it calls into question previous claims that correlation time differences constitute direct evidence for the existence of a steady downflow in and around sunspots. Taken together, the observational and theoretical evidence suggest that the p-mode forcing of the spot leads to the generation of upwardly propagating slow magnetoatmospheric waves. These waves are in turn responsible for the decreased amplitudes of the outwardly propagating p-modes in the surrounding quiet Sun, and the dispersion in their travel times between the hidden subsurface layer where they are forced and the overlying level where the Doppler signals originate leads to the observed phase lag between the umbral and penumbral oscillations and the corresponding correlation time differences.

Umbral oscillations in sunspots: Absorption of p-modes and active region heating by mode conversion

Umbral oscillations in sunspots: Absorption of p-modes and active region heating by mode conversion

Total resonant absorption of acoustic oscillations in sunspots

The question of total resonant absorption of acoustic oscillations in sunspots is studied for cylindrical 1-D flux tubes that are stratified only in the radial direction and surrounded by a uniform, non-magnetic plasma. The numerical investigation of Goossens and Poedts (1992) in linear resistive MHD is taken further by increasing the strength of the azimuthal magnetic field in the equilibrium flux tubes. For relatively strong azimuthal magnetic fields, total absorption is found over a relatively wide range of spot radii.

Correlation of Human Longevity Oscillations with Sunspot Cycles

An examination of past human mortality trends revealed that the mean longevity of birth cohorts from 1740 to 1900 for United States of America (U.S.) Congressional Representatives exhibited oscillations that coincided with the 9- to 12-year sunspot cycle. Cohort mean longevities were 2-3 years greater during times of low sunspot activity than at peak activity. This phenomenon was confirmed in data from members of the House of Commons of the United Kingdom Parliament and from University of Cambridge alumni. An additional longevity oscillation with a longer period was visible in the data and may also be related to sunspot cycles. The amplitude and frequency modulations in the longevity and sunspot oscillations aligned when a 20-year phase shift was incorporated. This shift requires the existence of a lag between solar changes and the affected birth cohorts. Several possible causes of the effect are discussed, in particular: radiation on primordial germ cells in developing embryos; influenza epidemics and pandemics; and weather. The size of the longevity oscillation requires that the solar effect must be considered in studies that examine longevity trends and risk estimation.

The conversion of p-modes to slow modes and the absorption of acoustic waves by sunspots

The possibility that the conversion or p-modes to flux tube-guided slow modes (s-modes) is responsible for the reported absorption of the solar acoustic oscillations in sunspots and neighboring plage is discussed in the context of a simple model atmosphere plane-parallel stratified adiabatic polytrope threaded by a uniform vertical magnetic field. The normal modes of oscillation of this atmosphere have complex eigenfrequencies. It is argued that the acoustic absorption suffered by an incident p-mode scales like the imaginary part of these complex eigenfrequencies of the internal sunspot atmosphere