Observations of sunspot transition region oscillations

Oscillations with a period of 3 minutes are observed in the transition region of six sunspots with the Solar and Heliospheric Observatory - SOHO joint observing programme for velocity fields in sunspot regions. Observations of the transition region lines Ov 629 and Nv 1238, 1242 with the SUMER instrument show significant differences in the amplitude of the 3-minute oscillations from one sunspot to another, both in intensity and line-of-sight velocity. In four sunspots the central part of the umbra is observed. Two of these sunspots show coincidence between the maxima in peak line intensity and velocity directed towards the observer, as is expected for an upward-propagating acoustic wave. The two other sunspots show large oscillation amplitudes and a difference of 25° between maxima in intensity and blue shift. The possible effect of partial wave reflection on the observed phase relation is discussed. For one sunspot only a part of the umbra, close to the penumbra, was observed and the observations show a difference of 50° between maxima in intensity and blueshift. For the smallest sunspot the observations are found to be contaminated by contributions from an area without oscillations. Observed oscillations in line width are small, but probably significant in two sunspots. The observations of NOAA 8378 allow us to compare simultaneous recordings of the oscillations in the chromospheric Siii 1260 line with the oscillations in the transition region lines. We question the suggestion by Fludra (1999) that the sunspot transition region oscillations are a typical feature of the sunspot plumes.     

On the sunspot transition region

The EUV line emission and relative line-of-sight velocity in the transition region between the chromosphere and corona of 36 sunspot regions are investigated, based on observations with the Coronal Diagnostic Spectrometer – CDS and the Solar Ultraviolet Measurements of Emitted Radiation – SUMER on the Solar and Heliospheric Observatory – SOHO. The most prominent features in the transition-region intensity maps are the sunspot plumes. In the temperature range between log T=5.2 and log T=5.6 we find that 29 of the 36 sunspots contain one or two sunspot plumes. The relative line-of-sight velocity in sunspot plumes is high and directed into the Sun in the transition region, for 19 of the sunspots the maximum velocity exceeds 25 km s^?1. The velocity increases with increasing temperature, reaches a maximum close to log T=5.5 and then decreases abruptly.

Attention is given to the properties of oscillations with a period of 3 min in the sunspot transition region, based on observations of six sunspots. Comparing loci with the same phase we find that the 3-min oscillations affect the entire umbral transition region and part of the penumbral transition region. Above the umbra the observed relation between the oscillations in peak line intensity and line-of-sight velocity is compatible with the hypothesis that the oscillations are caused by upward-propagating acoustic waves. Information about intensity oscillations in the low corona is obtained from observations of one sunspot in the 171 Å channel with the Transition Region And Coronal Explorer – TRACE. We conclude that we observe the 3-min sunspot oscillations in the chromosphere, the transition region and the low corona. The oscillations are observable over a wider temperature range than the sunspot plumes, and show a different spatial distribution than that of the plumes.

     

Discussion of a possible magnetic transient in UVSP observations

As part of a search for possible magnetic transients in the transition region, polarization observations were made in the C iv line at 1548 Å during a flare on 1980, July 13. In contradiction to earlier informal reports, it is not possible to state that magnetic transients of several thousand gauss have really been detected. This conclusion is based primarily on the lack of correlation in the circular polarization signals observed in the two halves of the spectral line and also on the possible effects of rapid fluctuations of intensity, velocity, and line width and shape.     

High resolution EUV structure of the chromosphere-corona transition region above a sunspot

Ion emission line intensities between 1170 and 1700 Å allow one to determine the differential emission measure (DEM) and electron pressure of the plasma in the solar transition region (TR). These line intensities together with their Doppler shifts and line widths are measured simultaneously for the first time above a sunsport from data obtained with the NRL High Resolution Telescope and Spectrograph with 0.06 Å spectral and 1 spatial resolution.The Doppler shifts show both subsonic and supersonic flow in the same line of sight over the umbra. The temperature structure for 40 resolution elements in the sunspot umbra and penumbra is derived from the DEM and the observed electron pressures.Extrapolation of the emission measure curves supports the previous EUV and X-ray observations that coronal plasma above sunspots with T _e>10⁶ K is reduced while emission from TR plasma between 2×10⁵ and 10⁶ K is greatly enhanced relative to quiet or active regions. This enhancement shifts the minimum of the DEM to lower temperatures and increases the slope at 2×10⁵ K by a factor of two.New pressure diagnostics using the emission line intensity ratios of C iv to N iv are presented, and applied to the data.The energy balance in the TR for the sunspot umbra is dominated by radiative losses from the large amount of TR plasma.An estimate of the energy budget shows that an energy input is required to balance the radiative energy losses above the umbra. The observed divergence of the enthalpy flux for the umbral downflows can balance these radiative losses for T _e between 30000 and 200 000 K.A typical umbral model of T _e versus reduced mass column density is compared with one for chromospheric temperatures determined from the Ca H and K lines.Institute of Theoretical Astrophysics, University of Oslo, Norway.     

Observations of the longitudinal magnetic field in the transition region and photosphere of a sunspot

The Ultraviolet Spectrometer and Polarimeter on the Solar Maximum Mission spacecraft has observed for the first time the longitudinal component of the magnetic field by means of the Zeeman effect in the transition region above a sunspot. The data presented here were obtained on three days in one sunspot, have spatial resolutions of 10 arc sec and 3 arc sec, and yield maximum field strengths greater than 1000 G above the umbrae in the spot. The method of analysis, including a line-width calibration feature used during some of the observations, is described in some detail in an appendix; the line width is required for the determination of the longitudinal magnetic field from the observed circular polarization.The transition region data for one day are compared with photospheric magnetograms from the Marshall Space Flight Center. Vertical gradients of the magnetic field are computed from the two sets of data; the maximum gradients of 0.41 to 0.62 G km^–1 occur above the umbra and agree with or are smaller than values observed previously in the photosphere and low chromosphere.     

The properties of velocity oscillations in vicinities of sunspot penumbra

It is suggested that the problem of the extent of the outer penumbra could be considered by investigating the range over which the influence of the penumbra on the oscillation regime of the surrounding medium extends. In summer 1998, velocity oscillations were observed in the vicinities of the penumbra of the NOAA 8263 sunspot. The observations were carried out in the H line (chromosphere) and in the far wing of Nii 4857.2 Å (lower photosphere) simultaneously. The oscillation regime typical for sunspot penumbrae is observed within distances of about 15 from the visible outer boundary of the penumbra.     

A study of the relation between intensity oscillations and magnetic field parameters in a sunspot: Hinode observations

We present properties of intensity oscillations of a sunspot in the photosphere and chromosphere using G band and Ca II H filtergrams from Hinode. Intensity power maps as function of magnetic field strength and frequency reveal reduction of power in the G band with an increase in photospheric magnetic field strength at all frequencies. In Ca II H, however, stronger fields exhibit more power at high frequencies, particularly in the 4.5–8.0 mHz band. Power distributions in different locations of the active region show that the oscillations in Ca II H exhibit more power compared to that of the G band. We also relate the power in intensity oscillations with different components of the photospheric vector magnetic field using near simultaneous spectro-polarimetric observations of the sunspot from the Hinode spectropolarimeter. The photospheric umbral power is strongly anti-correlated with the magnetic field strength and its line-of-sight component but there is a good correlation with the transverse component. A reversal of this trend is observed in the chromosphere except at low frequencies(ν≤ 1.5 mHz). The power in sunspot penumbrae is anti-correlated with the magnetic field parameters at all frequencies(1.0 ≤ν≤ 8.0 mHz) in both the photosphere and chromosphere, except that the chromospheric power shows a strong correlation in the frequency range 3–3.5 mHz.     

Physical conditions in a quiescent prominence derived from UV spectra obtained with the UVSP instrument on the SMM

A quiescent prominence observed above the north-west limb on November 20, 1980, is analyzed using data obtained with the Ultraviolet Spectrometer and Polarimeter (UVSP) on the Solar Maximum Mission (SMM). The spectral data include the lines 1215 Å of Hi, 1401 Å of Oiv, 1402 Å of Siiv, 1548 Å of Civ, 1640 Å of Hei, and 1655 Å of Ci. From an analysis of these lines and their emission patterns we deduce physical characteristics of the prominence plasma, and suggest in particular that the prominence consisted of flux tubes at various temperatures. In the hotter parts of the plasma the number density reached values of about 3 × 10¹¹ cm^#X2212;3.     

Analysis of UVSP Dopplergrams and magnetograms and their calibration using the orbital velocity of the SMM spacecraft

To infer velocities and longitudinal magnetic fields from Dopplergram and magnetogram signals obtained by the Ultraviolet Spectrometer and Polarimeter on the Solar Maximum Mission, one must know the width of the observed emission line. Although the instrument control system provided for a line-width calibration feature which utilized periodically commanded shifts of the line, it was not always used. However, it is possible to use the time-varying line-of-sight component of the orbital velocity of the spacecraft as a means of calibrating the line width for each pixel in a raster. Such a method based on a least-squares fit of the observed Doppler signal to the line-of-sight component of the spacecraft velocity is described here; it then applied to magnetogram observations. As background, the theoretical expressions for the interpretation of the observed Dopplergram and magnetogram signals for both wide and narrow exit slits are also summarized.     

Can oscillations grow in a sunspot umbra?

Umbral flashes and running penumbral waves have been attributed by Moore (1972) to overstable oscillations in the umbra. His numerical results were derived by inserting physical conditions at two particular depths beneath the umbral surface. Seven variables must be specified at each point. We have extended Moore's analysis to examine the depth-dependence of overstable oscillations in a recently computed umbral model. Electrical conductivity is evaluated taking full account of partial ionization and magnetic fields. In the surface layers, within 250 km of the top of the umbral convection zone, the conductivity is so low that Joule dissipation is more rapid than the growth rate of oscillations. In these layers, Moore's results are therefore not applicable. At greater depths, oscillations can grow and we agree with Moore that both umbral flashes and penumbral waves may be due to overstable oscillations. However, we suggest that both phenomena can arise at the same depth in the spot, and not in two layers, as Moore suggests.The umbral model we used is based on Öpik's cellular convection model. The interaction between the vertical magnetic field and convection is included by varying the diameter of the cell, and not its height. The diameter is assumed to be proportional to the distance that gas diffuses relative to the field during its upward convection.Work supported by NASA Contract NGR-39-005-066.     

An emission measure analysis of two sunspots observed by the UVSP instrument on the SMM spacecraft

The EUV observations from the SMM satellite of two sunspots are presented here. These observations show the sunspots (a) to be regions of lower intensity than the surrounding plage, contrary to that found by previous authors, and (b) to have line intensities which vary little over a period of several hours. An upper limit to mass flows of 2km s^-1 is derived, indicating a relatively simple energy balance for the chromosphere-corona transition zone with thermal conduction being balanced by radiative losses. Electron densities derived from Niv to Civ line ratios imply electron pressures (log N _eT_e) of 15.0 to 15.3.     

On the inhomogeneous structure of the chromosphere-corona transition region above sunspot umbrae

Multiple wavelength observations of sunspot umbrae can only be expalined by an inhomogeneous, two-component model for the structure of the umbral transition region and lower corona. The ‘Wroclaw-Ondrejov sunspot model’ was a first step in this direction. This working model has now been improved using analytic expressions for the atmospheric structure in each component and fitting the free parameters to recent sunspot observations, particularly in EUV lines. The main component has a shallow transition region and a deep-set corona. The second, ‘active’ component has a vast transition region in relatively cool fine structure elements embedded in the coronal main component. The spatial filling factor of this active component amounts to 5–10% in sunspots with bright EUV plumes, but is is more than ten times smaller in sunspot without such plumes. Observations with high spatial and temporal resolutions are necessary to understand in more detail the basic physical processes.     

Emission measures and structure of the transition region of a sunspot from emission lines in the far ultraviolet

Absolute intensities of emission lines in the wavelength range from 1200 Å to 1817 Å from the large sunspot in McMath region 12510 near Sun center are presented. The intensities are averaged across the umbra and penumbra of the sunspot. The observations were made with the NRL slit spectrograph on Skylab. Emission measures are derived from the measured intensities. Assuming a balance between the divergence of the conductive energy flux and the radiative energy losses, a self-consistent model of the lower transition region in the sunspot is constructed. The model gives a constant pressure of about 0.19 dyne cm^-2, and a conductive flux which decreases approximately one order of magnitude between 2 × 10⁵ K and 4 × 10⁴ K. The temperature gradient is relatively constant, increasing slowly with decreasing temperature.Ball Brothers Research Corporation.     

Search for magnetic field oscillations in a sunspot umbra

In order to search for oscillations in velocity and magnetic field strength within a sunspot umbra, a time series of spectra has been obtained through a circular analyzer and the Gregory-Coudé telescope at the Observatorio del Teide, Tenerife. The velocity oscillations clearly show peaks of power at periods between 2 and 7 minutes, with a maximum at 5 minutes. The apparent variations of the magnetic field strength, however, don't exhibit significant oscillations; these fluctuations are rather produced by the influence of parasitic stray light from the surrouding quiet sun which are also visible in the measured time variations of the umbral contrast of continuum intensity.     

The interpretation of oscillations in sunspot umbrae

We review possibilities for an interpretation of oscillations observed in several period bands (3 min., 5 min., 20 min.) and at different heights in sunspot umbrae. At subphotospheric depths two independent resonators are acting: A resonator for slow, quasi-transverse waves can explain the lifetimes of bright umbral dots (≥20 sec.), while a resonator for fast (acoustic), quasi-longitudinal waves could result in the 5-min. oscillations. The acoustic resonator strongly couples with the slow-mode longitudinal resonator at photospheric and chromospheric heights, the latter produces the resonance peaks in the 3-min. period band. The whole scheme of resonance levels generalizes and corroborates a chromospheric resonator model earlier proposed by the present authors. Comparisons with alternative models and recent measurements show that the present model most naturally explains the majority of observed data.     

Periodic and non-periodic phenomena in a sunspot region

We have studied running penumbral waves, the homogeneous Evershed effect, and the spatial relation between intensity and Doppler velocity penumbral features of a chromospheric sunspot. The observations were obtained with the multichannel subtractive double-pass spectrograph (MSDP) operating in H at the Vacuum Tower Telescope (VTT) installed at Tenerife (Canary Islands). We derived intensity and Doppler velocity maps at H ± 0.3 Å over a two-dimensional field of view. We have computed the components of the velocity vector (radial, azimuthal, vertical) as a function of distance from the center of the spot under the assumption of axial symmetry. The results show the well-known, from previous observations, general large-scale characteristics of the chromospheric Evershed flow. Our measurements show that the axes along the discrete structures, where the Evershed flow is confined, are not spatially related to the axes along H ± 0.3 Å intensity features, and we suggest that either the flow is confined in flow channels or that it takes place along sheared magnetic field lines. We also detected, for the first time in velocity images, running penumbral waves, which started in the outer 0.3 of the umbral radius and propagated through the penumbra with propagation velocities 13–24 km s^–1. The propagation velocity, as well as the velocity amplitude, is greater for the waves closer to the center of the spot and diminishes as one moves outward.     

Temperature distribution in the transition region and inner corona

The energy balance equation for a general solar atmosphere without assuming the plane-parallel approximation is solved analytically. This leads to models for both the transition region and inner corona as well as for the outer corona. The form of the latter is very similar to that of the hydrostatic conduction model of Chapman (1957). However, in this paper we confine ourselves only to the former.Model I is electron-pressure dependent but model II depends particularly on the maximum coronal temperature T _m and its corresponding altitude h _m. Both the models are compared with recently constructed temperature models of Chiuderi and Riani (1974), McWhirter et al. (1975), and Gabriel (1976a). It is concluded that our model II reproduces these models within a factor of not more than 2.     

Heat flux as a source of ion-acoustic oscillations in the transition region of the solar atmosphere

Based on model calculations, we show that ion-acoustic oscillations can be excited by heat fluxes in a plasma. We discuss the probable effect of ion-acoustic oscillations on the formation of temperature gradients at critical heat fluxes. The local critical heat flux in the transition region of the solar atmosphere is close to the well-known experimental heat flux from the corona into the chromosphere.     

On spatial characteristics of five-minute oscillations in the sunspot umbra

Using a differential method we have carried out observations of oscillations in six sunspots. Spectral lines Fe i 5434 Å and Fe i 5576 Å were used. Horizontal waves are not observed in the sunspot umbra photosphere. Results obtained indicate that, at least, the sunspot umbra oscillates as a single whole.