Sunspot Oscillations from the Chromosphere to the Corona N. Brynildsen, P. Maltby, T. Fredvik, O. Kjeldseth-Moe Institute for Theoretical Astrophysics, Univ. Oslo A the title tells this presentation is about oscillations above sunspots, in the transition region and their possible penetratiojn to the solar corona. A dominant peak in oscillatory power in the solar atmosphere above sunspots have been registered at periods around 3 minutes. They are seen in the CO2 band at 4.66 micrometer, i.e. In the coldest chromospheric regions. In the transition region the 3 minute oscillations were discovered and described by Joe Gurman 20 years ago. Recently the study of these sunspot oscillations have been much expanded by us in several papers, and by Andrzej Fludra. The (possible) penetration of the oscillations into the corona is a main point in the present investigation, but we shall also report on their properties in the transition region as well.

Observations Joint observing between CDS, on SOHO and TRACE, previously also incl. SUMER Normal and Grazing Incidence Spectro-meter(s) on CDS, (NIS/GIS) are both used Using the 171 Å channel on TRACE CDS/GIS for estimate of contributions in the TRACE 171 Å channel from coronal and transition region lines, respectively. 1. This is SOHO JOP 118. CDS – Coronal Diagnostic Spectrometer. TRACE – Transition Region and Coronal Explorer 3. The 171 Å channel on TRACE is supposed to cover the low corona, around and slightly below 1 MK. 4. Since we investigate whether the oscillations penetrate into the corona, we must be certain that the oscillations observed with TRACE really comes from the coronal emission. We shall return to this point later.

Oscillations occur above the umbra This slide show the oscillations in amplitude above the sunspot as registered with CDS/NIS. At the same time it illustrates how the observations are obtained. There is no attempt at following the sunspot with the spectropmeter slit, as this only introduces unwanted frequencies. Instead the solar rotation moves the spot accross the slit and intensities are measured as a function of time. The white contours outlines the umbra and penumbra of the spot. The yellow contour encircles a region where the peak line intensity exceed the average peak intensity in the FOV by a factor of 5. This defines the sunspot plume, if situated above the umbra or penumbra with a significant fraction of its area. 1. We note that oscillations are seen in the lines emitted from O IV, O V, and Ne VI (T ~ 180 000 K, 240 000 K, 430 000K). They are also prensent in He I. But they are not registered in M IX or Fe XVI (1 MK, 2.7 MK). 2. The oscillations in the transition region cover the sunspot umbra. They occur in the plume, but only so far as the plume exist above the umbra. They are thus a primarily property of the sunspot umbra. Intensity amplitudes are around 10%.

Amplitude with temperature 1. Amplitudes are largest at T ~200 000 K, or 1-200 000 K 2. Amplitudes decrease towards higher and lower temperatures 3. Note that these are amplitudes in average intensity oscillations

Deviations from linear oscillations Deviations from linear oscillations are present. They may be seen in the CDS data, but here we have included earlier observations with SUMER on SOHO from 1998, which demonstrate a strong case of non-linearity in both the intensity and the velocity oscillations. Velocity oscillations are much easier to detect with SUMER owing to its superior spectral resolution compared to CDS. These oscillations agree well with a non-liear sound disturbance propagating vertically in the sunspot.

Oscillations observed with NIS Oscillations observed with NIS in NOAA 9575 on 18 August 2001. The upper panel show the oscillations in the O IV line at 554 Å as the spot drifts past the spectrometer slit. The rectangle cuts out a time slot of 40 minutes. During this time the spot rotates approximatlely 6 arc seconds. This is comparable to the combined slit size (2 arc sec) and telescope resolution (~4 arc sec). The oscillations are then represntative for an area in the umbra, marked off with a rectangle in shown in the white light picture of the spot, upper right panel. Individual spectra are taken with an exposure time of 20 seconds. The remaining panels show the a) oscillation in relative integrated intensity, as a function of time (left), b) the powerspectra P[(DI/I)^2] (middle), and c) the phase and coherence (dashed) of the ocillations (right). Phases and coherences are relative to the O V line at 629 Å. He I and transition region lines are included. The coronal lines do not show significant oscillations. The oscillation amplitude is small in the Ne VI 562 Å line and its power spectrum is plotted on a different scale. In the power spectra we note a peak at 6 mHz, corrsponding to a period of 166 seconds. This peak is significant for all lines from, i.e. He I and the transition region lines, including the Ne VI 562 Å line. The significance is tested by randomizing the observed measured intensity values. A peak in the powerspectrum is deemed significant if its value exceeds three times the peaks in the powerspectra that emerge after randomization. The Ne VI line power peak exceeds this critical value by a factor of 2. The phase differences indicate that O IV leads O V qith 9 seconds, and O V leads Ne VI with 14 seconds.

Oscillations observed with GIS Oscillations observed with GIS in NOAA 9441 on 2 May 2001. During the observations the sun again drifts accross the GIS aperture, which is the 4x4 arc second slit. Note the slit size. The panels are organized the same way as in the previous viewgraph/image except that phases and coherences are relative to the O V line at 760 Å. Note that scales may differ among the various lines. Looking at the power spectra a clear peak is seen at 6 mHz (166 s) in the lines from N III, Ne IV, O V and Ne V (emitted at 80 000 K, 170 000 K, 235 000 K and 295 000K. The oscillations seem to penetrate to the level of the Ne VII line at 465 Å, emitting at 515 000 K. The Ne VIII 780 Å and the Fe IX 171 Å lines, emitted at 630 000 K, do not show any significant power. This suggests that the 6 mHz oscillations are in a sense ”lost” before reahing this level. However, the real explanation may be more complex and interesting, see below. The phase differences suggest that the 6 mHz oscillations reaches Ne IV 5 seconds before the O V line, and that the signal in the Ne VII line is delayed 14 seconds with respect to the signal in the O V line.

Sunspot chromosphere resonator - The observed power-spectra are remarkably similar in the chromosphere and through the TR. - This disagrees with the sunspot filter theory, which predicts peaks that are approximately equally spaced at distances of ~1 mHZ The figure above is from Settele, Staude and Zugzhda (Sol.Phys. 202,281,2002), and applies for the sunspor umbra model of Maltby et al. Other sunspot models give similar results, with a number of resonances.

Oscillations in the corona Introduction: Penetration of oscillations to the corona from observations with TRACE, i.e. Fe IX and Fe X, 630 000 K to 1 MK From GIS observations: Oscillations ”lost” before they reach the temperature level of Ne VIII or Fe IX, i.e. 630 000 K Is this a contradiction??

Sensitivity response of TRACE 171 Å (or 173 Å) channel Let us address the point of whether the TRACE intensities in the 171 Å channel contain contributions from lines other than those emitted at coronal temperatures. This graph shows the sensitivity response as a function of wavelength within the band. Note that the TRACE channel is 7 Å wide.

Spectrum in TRACE 171 Channel This is a spectrum above the umbra in one of the observed sunspots. We note that the lines from Fe IX and Fe X, formed at 630 000 K and 1 MK, respectively, are dominant in the band with respect to intensity, but that the band also contain lines from especially O VI as well, formed at 300 000 K. For reasons of insufficient angular resolution of the GIS, that we shall return to, we cannot directly see the oscillations in the Fe IX and Fe X lines. From the intensity contribution of the O V and O VI lines it is, however, clear that if these lines are the cuse of the oscillations that we see with TRACE, then the oscillations amplutides will have to be a factor 3-4 times higher than what we observe in the O V 629 Å line. (see p. 27 in paper) (see also p25ff for correction og ghosts etc in GIS)

TRACE Oscillations This graph shows the oscillations observed with TRACE in the 171 Å channel on 19 August 2001. The spot is NOAA 9575. The three top panels show the white light spot, the loop structures as observed with TRACE overlaid the spot contour, and the directions. IT IS IMPORTANT TO NOTE THAT THE TRACE ANGULAR RESOLUTION IS 1 ARC SEC. This is much better than the 4x4 arc sec aperture of the CDS/GIS The middle panes show the oscillations in intensity, relative intensity and the power spectrum. There is a clear power peak at 6 mHz. The bottom panels show the location of the power function at three different frequencies. Note the concentration of power at 5.9 mHz in limited areas inside the umbra but smaller than the umbra, and situated at the footpoins of coronal loops. This indicates that the oscillations indeed penetrates to temperatures of 0.6-1 MK. We shall demonstrate this in a final movie sequence as well. But let me mention that we recently observe oscillations with TRACE in the 195 Å band as well. In this band the radiation mainly comes from Fe XII lines at 1.5 MK, albeit with som possible contribution from transition region lines, but this latter contribution is unlikely to be significant.

Summary Oscillations occur in the transition region above the umbra and covers the umbra Amplitudes largest at 200 000 K, samller at lower and higher temperature values Deviations from pure linear oscillations are present

Summary (cont) Power spectra are similar in the chromo-sphere and through the transition region This disagrees with the sunspot filter theory Oscillations penetrates to the corona In the corona they are channeled into smaller areas, coinciding with the endpoints of coronal loops