Sunspot Oscillations Research Articles

Linear resistive magnetohydrodynamic computations of resonant absorption of acoustic oscillations in sunspots

A numerical study of the resonant absorption of p-modes by sunspots is carried out in linear resistive MHD. The sunspot is idealized as a cylindrical axisymmetric flux tube stratified only in the radial direction and surrounded by a uniform unmagnetized plasma. First, the results of Lou in viscous MHD are reproduced in resistive MHD, and, hence, it is shown that the absorption efficiency is independent of the actual dissipation mechanism. Next, the parameter domain investigated by Lou is substantially enlarged to higher m-values, larger sunspots, and sunspot models with a twisted magnetic field

Photoelectric observations of chromospheric sunspot oscillations. III - Spatial distribution of power and frequency in umbrae

view Abstract Citations (47) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photoelectric Observations of Chromospheric Sunspot Oscillations. III. Spatial Distribution of Power and Frequency in Umbrae Lites, B. W. Abstract On the basis of analyses of the spatial distribution of oscillation power and frequency of data from Lites (1984), the multiple peaks frequently observed in average power spectra are noted to be due to the presence of cospatial modes with different frequencies. Changes in these modes' amplitudes and frequencies with time and place within an umbra appear to arise from an interference of modes in the driving force in the photosphere. High oscillatory power regions in the 3-min band are uncorrelated with high oscillatory power regions in the 5-min band within the umbrae, suggesting that the 5-min oscillations in the photosphere do not drive the 3-min chromospheric oscillations. These results are used to develop a consistent view of the umbral oscillation mechanism: attention is given to a photospheric resonator for fast magnetoatmospheric waves, driving the chromospheric resonator through nonlinear wave interaction. Publication: The Astrophysical Journal Pub Date: February 1986 DOI: 10.1086/163963 Bibcode: 1986ApJ...301..992L Keywords: Chromosphere; Electrophotometry; Resonant Frequencies; Solar Oscillations; Spatial Distribution; Sunspots; Power Spectra; Spectral Energy Distribution; Stellar Spectrophotometry; Umbras; Solar Physics; SUN: CHROMOSPHERE; SUN: OSCILLATIONS; SUN: SUNSPOTS full text sources ADS | Related Materials (4) Part 1: 1982ApJ...253..386L Part 2: 1984ApJ...277..874L Part 4: 1986ApJ...301.1005L Part 5: 1988ApJ...334.1054L

Photoelectric observations of chromospheric sunspot oscillations. IV - The CA II H line and He I 10830 A

view Abstract Citations (65) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photoelectric Observations of Chromospheric Sunspot Oscillations. IV. The CA II H Lines and He i lambda 10830 Lites, B. W. Abstract Simultaneous observations of profiles of He I λ10830 and the Ca II H line in sunspot umbrae have been made with the Tower telescope, echelle spectrograph, and multidiode array at the National Solar Observatory/Sunspot. These observations reveal the nature of fluid motions in the upper chromosphere during the cycle of umbral oscillations. Time sequences of profiles show that the chromospheric He I line is in weak absorption during all phases of the umbral oscillations, and that it undergoes periodic Doppler shifts of high amplitude (up to 11 km s-1). The amplitudes and waveforms of the Doppler shifts in the He I line indicate that the oscillations are frequently nonlinear (i.e., forming shocks) and therefore contribute to the heating of the upper umbral chromosphere. Publication: The Astrophysical Journal Pub Date: February 1986 DOI: 10.1086/163964 Bibcode: 1986ApJ...301.1005L Keywords: Abundance; Chromosphere; Electrophotometry; Solar Oscillations; Stellar Spectrophotometry; Sunspots; Calcium; Doppler Effect; Helium Ions; Hydrogen; Line Shape; Line Spectra; Temporal Distribution; Umbras; Solar Physics; SUN: CHROMOSPHERE; SUN: OSCILLATIONS; SUN: SUNSPOTS full text sources ADS | Related Materials (4) Part 1: 1982ApJ...253..386L Part 2: 1984ApJ...277..874L Part 3: 1986ApJ...301..992L Part 5: 1988ApJ...334.1054L

Oscillations in Sunspots

Recent observational and theoretical work on oscillations in sunspots is reviewed. The characteristic 3-minute umbral oscillations and flashes are reSonant modes of the sunspot itself, whereas the 5-minute oscillations in the umbra are a passive response to forcing by p modes in the surrounding convection zone. The observational evidence suggests that the fundamental cause of the 3-minute oscillations is the photospheric fast-mode resonance, with chromospheric slow-mode resonances perhaps producing additional oscillation frequencies in the chromosphere. Observations and theoretical models of the interaction of 5-minute p-mode oscillations with a sunspot offer a means of probing the structure of a sunspot magnetic flux tube beneath the solar surface. The observed differences between running penumbral waves in the chromosphere and in the photosphere may be explained by the effect of the Evershed flow on trapped magneto-atmospheric waves in the penumbra.

Photoelectric observations of chromospheric sunspot oscillations. II - Propagation characteristics

Measurements of oscillations in umbrae and penumbrae using the line pairs (Ca II lambda8498--lambda8542) and (Fe I lambda5434--Ca II lambda8498)= are presented for seven sunspots and 11 observing runs. In the umbral chromosphere, the observations reported here, taken with other measurements of umbral oscillations, suggest that the oscillations are upward propagating acoustic (or slow mode) disturbances, and that they become nonlinear and develop into shock waves in the upper layers. They cause a significant increase in the radiative output of the umbral chromosphere, indicating the possibility of nonthermal heating at these levels. On the basis of this picture, umbral chromosphere, indicating the possibility of nonthermal heating at these levels. On the basis of this picture, umbral chromospheres have a greater physical extent in height than predicted by umbral models with coronal pressures greater than those found in the quiet Sun. Oscillations are present in the outer regions of penumbrae with frequencies and phase relationships that suggest the possibility of magneto-gravity waves.

Transition region oscillations in sunspots

Time series observations of the profile of the C IV resonance line 1548.19 A obtained in eight sunspots with the Ultraviolet Spectrometer and Polarimeter (UVSP) on the Solar Maximum Mission are discussed. All of the sunspots exhibit significant oscillations in line-of-sight velocity with frequencies in the range from 5.8 mHz to 7.8 mHz (periods of 129-173 s). Significant intensity oscillations are observed at the same periods in four of the time series; the maximum intensity is in phase with maximum blueshift. Difference spectroheliograms ('Dopplergrams') of the two halves of the C IV line, as well as observations in the Si IV resonance line 1402.77 A and the O IV intersystem line 1401.16 A, also reveal velocity oscillations at similar frequencies but only over sunspots.

Photoelectric observations of propagating sunspot oscillations

Repeated intensity and velocity images of a large, isolated sunspot in both the chromospheric Ca II 8542 A and photospheric Fe I 5576 line were performed. It is shown by means of a movie of the digital data for the chromospheric line that a relationship exists between the propagating umbral disturbances and the running penumbral waves. Power spectra of the oscillations show a sharp peak at a period of about 170 sec in both the velocity and intensity signals, and the oscillations at any point in the sunspot are found to be very regular. The phase relationship between the velocity and the intensity of the chromospheric oscillations contrasts with that for the quiet sun. The mechanical energy flux carried by the observed umbral disturbances does not appear to be a significant contributor to the overall energy budget of the sunspot or the surrounding active region.

Umbral oscillations as resonant modes of magneto-atmospheric waves

Umbral oscillations in sunspots are identified as a resonant response of the umbral atmosphere to forcing by oscillatory convection in the subphotosphere. The full, linearized equations for magneto-atmospheric waves are solved numerically for a detailed model of the umbral atmosphere, for both forced and free oscillations. Resonant ‘fast’ modes are found, the lowest mode having a period of 153 s, typical of umbral oscillations. A comparison is made with a similar analysis by Uchida and Sakurai (1975), who calculated resonant modes using an approximate (‘quasi-Alfven’) form of the wave equations. Whereas both analyses give an appropriate value for the period of oscillation, several new features of the motion follow from the full equations. The resonant modes are due to upward reflection in the subphotosphere (due to increasing sound speed) and downward reflection in the photosphere and low chromosphere (due to increasing Alfven speed); downward reflection at the chromosphere-corona transition is unimportant for these modes.

The major pressure oscillation, 1875 to 1960

Pressure anomaly maps for the world for different periods have been constructed from barometric information received as a result of international appeals. Maps of changes by 5-year periods have been shown to reflect a pressure see-saw between West Greenland and the Indian Ocean. The sign of this pattern is defined by the «pressure parameter», approximately the reversed pressure in the latter area. The parameter seems to represent much more reliably than sunspot numbers the effective changes in solar radiation. Changes by 30-year periods, at least between 1876/1905 and 1906/35, follow a geographical pattern very similar to that of the ordinary southern oscillation, with at least one marked «discontinuity» in the nodal zone off south-east Australia. In contrast to the map of changes by five-year periods, West Greenland now acts as a negative area, that is, its pressure fluctuates in sympathy with the Indian Ocean. However, there is a phase difference, associated with «south-steering», and a difference of wave-length between the Arctic and the Tropics, and historical evidence suggests that Greenland can also be a «positive» area. In low latitudes, the major pressure oscillation has for over a century been closely in phase with the major sunspot oscillation, that is, an oscillation of the order of 60 to 90 years. A sudden change to low parameter phase in the next decade or so is therefore expected. This will be associated presumably with severe droughts in many low latitude regions and the characteristic changes in circulation patterns.