N. Brynildsen

Nonlinear sunspot transition region oscillations in NOAA 8378

Observations obtained with the Solar and Heliospheric Observatory joint observing program for velocity fields in sunspot regions are used to study the 3 minute transition region oscillations above four sunspots. Here we report mainly on the results obtained for NOAA 8378, based on simultaneous recordings of the transition region lines O V λ629 and N V λλ1238, 1242 and the chromospheric Si II λ1260 line with the Solar Ultraviolet Measurements of Emitted Radiation instrument. The 3 minute transition region oscillations in NOAA 8378 occur mainly above the umbra and show (1) larger peak line intensity amplitudes than reported before, (2) clear signs of nonlinearities, (3) significant oscillations in line width, (4) maxima in peak line intensity and maxima in velocity directed toward the observer that are nearly, but not exactly in phase, and (5) a clear connection to the oscillations in the sunspot chromosphere. The suggestion that the waves are upward-propagating acoustic waves is confronted with the observations by a simple test.

Sunspot transition region oscillations in NOAA 8156

Based on observations obtained with the Solar and Heliospheric Observatory joint observing program for velocity fields in sunspot regions, we have detected 3 minute transition region umbral oscillations in NOAA 8156. Simultaneous recordings of O V λ629 and N V λ1238, λ1242 with the SUMER instrument give the spatial distribution of power in the 3 minute oscillations, both in intensity and in line-of-sight velocity. Comparing loci with the same phase, we find that the entire umbral transition region oscillates. The observed maxima in peak line intensity are nearly in phase with the maxima in velocity directed toward the observer. We discuss the suggestion that the waves are upward-propagating acoustic waves.

Gas flows in the transition region above sunspots

Strong downflows and moderate upflows in the transition region over a sunspot have been observed with the Spacelab 2 HRTS in 1985. The flows are found to be persistent, and it is suggested that they are common. Data show that the downflows are supersonic and that there is more than one characteristic flow speed in the downflows. Evidence is provided for constant downflows in the 30,000-230,000-K temperature range.

Plumes and oscillations in the sunspot transition region

The suggestion that sunspot transition region oscillations are a typical feature of the sunspot plumes is examined. The present observations show 3 min oscillations in the umbra that end at the umbral rim. We nd that sunspot plumes located above the umbra show these oscillations, in contrast to plumes above the penumbra. These two ndings suggest that the oscillations may be a property of the umbral transition region.

MULTIPLE FLOWS AND THE FINE STRUCTURE OF THE TRANSITION REGION AROUND SUNSPOTS

The fine structure in the flow field in the transition region above and surrounding a sunspot is determined fromCIV 1548 Å line profiles, observed with the High Resolution Telescope and Spectrograph (HRTS) during the Spacelab 2 mission. The observed line profiles show one, two, or three distinct velocity components within the resolution element of 1″ × 1″. Supersonic flows occur in small regions where the line profile has two or three components. The line component that shows supersonic speed often is weaker than the subsonic line component, which may explain why some observers have been unable to detect the supersonic flow component. The broadening of individual line components shows non-thermal velocities close to 20 km s−1. This suggests that turbulence is less important than usually considered.The presence of multiple flows, which also occurs in quiet solar regions, suggests that the transition region above sunspots has a sub-arc-second fine structure, perhaps consisting of thin fibrils. The Evershed flow in the transition region appears to have a correspondingly complex character, possibly consisting of sub- and supersonic siphon flows along the individual fibrils. Time changes in the flow field over 5 min may correspond to characteristic times of individual fine structures. Possible explanations of the net downward directed mass flux are presented.

Soho Observations of the Connection Between Line Profile Parameters in Active and Quiet Regions and the Net Red Shift in Euv Emission Lines

We present high spatial and spectral resolution observations of one active and one quiet-Sun region, obtained with CDS and SUMER on SOHO. The connections between the line profile parameters are studied and a systematic wavelength shift towards the red with increasing peak line intensity (line broadening) is detected. The large scatter in the data calls for another approach. We apply conditional probability analysis to a series of EUV emission lines and find significant correlations between line profile parameters. For a given interval in wavelength shift we find that: (1) line profiles with large intensities (line widths) and red shifts above the average constitute an increasing fraction of the profiles as the relative wavelength shift increases, (2) line profiles with large intensities (line widths) and blue shifts compared to the average, on the other hand, constitute a decreasing fraction of the profiles as the relative wavelength shift increases. These results extend the findings of an earlier quiet-Sun study from one to several emission lines and expand the validity to include the active region. Interestingly, the active region observations show correlations between peak line intensity and wavelength shift in the coronal lines.The tendency for red-shifted profiles to be more intense than blue-shifted profiles will shift line profiles derived by integrating along the slit towards the red. From the present observations we are not able to determine the fraction of the net red shift that emerges from this correlation. We suggest that the same mechanism is responsible both for the correlation between the line profile parameters and for the differential red shift between the transition region and chromospheric emission lines.

Euv Spectroscopy of the Sunspot Region Noaa 7981 Using Soho – II. Velocities and Line Profiles

We have studied the dynamics in the sunspot transition region between the chromosphere and the corona and investigated the extension of the flow field into the corona. Based on EUV spectra of a medium size sunspot and its surroundings, NOAA 7981, observed with CDS and SUMER on SOHO, we derive line-of-sight velocities and study the line profiles for a series of emission lines.The flow field in the low corona is found to differ markedly from that in the transition region. In the transition region the relative line-of-sight velocity shows an upflow in the umbra and relatively large areas with downflow that cover part of the penumbra. The spatial extent of these areas with upflow and downflow increases with increasing temperature in the transition region, but the whole flow field changes character as the temperature increases from the upper transition region to the low corona. Based on a calibration of the SUMER wavelength scale we find that the entire sunspot transition zone appears to be moving downwards towards the chromosphere. The relation between this finding and the general tendency for transition-region lines to show a net red shift is discussed.Several of the transition-region spectral line profiles are observed to show two line components with Gaussian shape and line-of-sight velocities that differ markedly. Several of the line profiles that are composed of two spectral line components occur close to the dividing line between up- and downflow. A discussion of this observation is presented. In small regions with spatial extent of a few arc sec we detect enhanced continuum emission underlying explosive events. The similarities between explosive events with continuum emission and the moustaches observed in Hα close to sunspots are so striking that we are tempted to introduce the notation ‘transition-region moustaches’.

Search for a chromospheric resonator above sunspots

Abstract We have recently observed thirteen sunspots, both with CDS on SOHO and TRACE. For each sunspot we investigate the oscillations in the chromosphere, transition region and corona. Above the umbra the power spectra show one dominant peak corresponding to a period of 3 minutes. The intensity oscillation amplitude increases with increasing temperature, reaches a maximum for lines emitting close to 1–2 × 10 5 K, and decreases for higher temperatures. Part of the wave energy penetrates into the corona, channeled into small areas that appear to coincide with the endpoints of sunspot coronal loops. The observations support the hypothesis that the oscillations are caused by upwardly propagating acoustic waves and show no signs of the resonances, equally spaced ≈1 mHz in frequency, predicted by the chromospheric resonator theories.

Quiet-Sun Connection between Intensity, Doppler Shift, and Line Broadening in Solar Ultraviolet Emission Lines

The quiet-Sun relations between the C IV line parameters, intensity, wavelength shift, and line broadening, are studied with high spatial and spectral resolution. The material is ordered into equal intervals for one of the line parameters, and conditional probabilities are calculated. As the Doppler velocity increases, we find that redshifted profiles with a large intensity or line width constitute an increasing fraction of the total number of profiles in a given velocity interval. For blueshifted profiles, on the other hand, the conditional probabilities for finding a profile with considerable intensity or line width decrease as the line shift increases. Similar relations are found when the material is ordered according to intensity or line-width values.

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.