In observations of penumbra oscillations there is the challenging problem of separating the oscillations associated with Evershed flows from those of a different nature. The authors used the differential method which is best suited to this problem as it can be used to filter out the wave motions in the direction and wavelength already at the stage of observation. Our findings indicate that, in

addition to the 10–12 min periods, periods near 30–35 minutes are present in the velocity variations of Evershed flows. Furthermore, the possible candidates for this role are the 8 min oscillations, which we have also observed at two height levels of sunspot penumbra

Kobanov, N.I.

Makarchik, D.V.

English

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IL NUOVO CIMENTO Vol. 25 C, N. 5-6 Settembre-Dicembre 2002Flows and oscillations in sunspot penumbra(∗)N. I. Kobanov and D. V. MakarchikInstitute of Solar-Terrestrial Physics - P.O. Box 4026, Irkutsk, Russia(ricevuto il 10 Giugno 2002; approvato il 24 Settembre 2002)Summary. — In observations of penumbra oscillations there is the challengingproblem of separating the oscillations associated with Evershed flows from those ofa different nature. The authors used the differential method which is best suitedto this problem as it can be used to filter out the wave motions in the directionand wavelength already at the stage of observation. Our findings indicate that, inaddition to the 10–12 min periods, periods near 30–35 minutes are present in thevelocity variations of Evershed flows. Furthermore, the possible candidates for thisrole are the 8 min oscillations, which we have also observed at two height levels ofsunspot penumbra.PACS 96.60.Qc – Sunspots, faculae, plages.PACS 96.60.Ly – Oscillations and waves; helioseismology.PACS 01.30.Cc – Conference proceedings.1. – IntroductionTwo types of regular motion of solar plasma are most often observed in sunspotpenumbra. One of them is represented by the Evershed effect [1, 2] implying a radialoutflow of material to the outside at the photospheric level and to the inside in thechromosphere. The other type of regular motion includes waves and oscillations [3].Oscillatory processes in sunspot penumbra have been extensively investigated in recentyears. But in what interrelationship are the observed oscillations with Evershed flows?Do the flows themselves undergo periodic variations? Rimmele [4] and Shine [5] foundthat Evershed flows experience quasi-periodic variations with a period of 12–15 min. Thecomplexity of such observations implies that in line-of-sight velocity observations usingconventional methods, a total set of motions are recorded, in which the area of the solarsurface under investigation (of all possible spatial scales and directions) participates.If, however, oscillations propagating only in a radial direction are recorded, then there(∗) Paper presented at the International Meeting on THEMIS and the New Frontiers of SolarAtmosphere Dynamics, Rome, Italy, March 19-21, 2001.c© Societa` Italiana di Fisica 609610 N. I. KOBANOV and D. V. MAKARCHIKTable I. – Oscillation periods in the observed sunspots.NOAA Coord. Part Chromosphere Photosphere Duration Fig.T (min) T (min) (min)8585 N38:W19 N ≈ 10–14, 7 21, 6 54 1a, b8585 N38:W19 W ≈ 30*, 17, 8.5, 7, 6* ≈ 40*, 17, 6* 70 2a, b8602 N18:E22 W 28, 14, 6–5* ≈ 28, 6* 85 3a, b8602 N18:E22 E 14, 7 ≈ 14 43 3c, d8602 N18:E08 N 42*, 14, 8.5*, ≈ 6* 42*, 21, 8.5*, 6* 75 4a, b8602 N18:E08 N 21, 10.7, ≈ 7–5.3 ≈ 6–5.3 43 4c, d8636 N20:E61 N ≈ 30*, ≈ 7–5.3 ≈ 30*, 10.7, 7 43 5a, bwill appear more chances to establish a connection of such oscillations with Evershedflows. Differential methods [6, 7] make it possible to separate from a noisy mixture theoscillations of our interest by filtering the waves in the propagation length and direction.Taking into account these properties as well as the fact that in differential measurementsthe instability influence of the spectrograph is reduced to a minimum, these methodsare best suited for investigating the oscillations in sunspot penumbra. In simultaneousobservations at two altitude levels (photosphere + chromosphere) the Sent-John effectcan be used as an additional means to establish the possible connection of measuredoscillations with Evershed flows. Generally, the Sent-John effect leads to a different signof measured line-of-sight signals in the photosphere and chromosphere because of theopposite direction of the motion.2. – Observations and resultsLarge round sunspots with well-developed umbra and penumbra, as a rule, were se-lected for the observation. The observations reported here were carried out in June,1999 at the telescope AST of the Sayan Observatory with differential method [6]. Wehave investigated the line-of-sight velocity oscillations in penumbrae of several sunspots.When investigating the line-of-sight velocity oscillations in sunspot penumbra, the dis-Fig. 1. – The northern penumbra of NOAA 8585 (21.06.1999). a) Value of differential velocity(the thin lines show the smoothed signals); b) power spectrum.FLOWS AND OSCILLATIONS IN SUNSPOT PENUMBRA 611Fig. 2. – The western penumbra of NOAA 8585 (21.06.1999). a) Value of differential velocity;b) power spectrum.tance between the elements observed was taken to be 5.5′′. Simultaneously at two heightlevels, Ni I 4857 and Hβ, measurements were made of the line-of-sight velocity differencein the radial direction of penumbra. The polarization optics parameters of the electronicslits positions are specified such that the measurements are made in the wings of spectrallines at a distance of ±40 mA˚ away from the core. This corresponds to at least 1500–2000 km altitude for Hβ and to about 100–200 km for Ni I. The areas under observationlay radially along the direction of filaments of penumbra, in its middle part, and theheight of the entrance slit was taken to be 4′′ to 8′′ and wide 2′′. Such spatial resolu-tion was especially chosen by these authors, although atmospheric conditions providedhigher resolution. By this means the observations can be got rid of the fine-structureinfluence, and more general properties of oscillatory processes in penumbra can be con-sidered. Time intervals with minimum image motion and minimum scatter light wereused. Therefore, the length of the temporal series varied from 40 to 80 min. Such aduration is nearly ideally. On the one hand, it is clear that as long observational seriesas possible should be used to obtain power spectra with high resolution in frequency.On the other, observational practice indicates that physical conditions in sunspots aresufficiently dynamic and the spectrum of a long series will be smeared by this dynam-ics. Furthermore, in differential measurements of the line-of-sight velocity in sunspotsthe contribution to the signal is made by short-wavelength spatial harmonics, which inthis case are formally equivalent to acoustic p-modes. The original temporal series wereprocessed following the standard procedure of removing the linear trend, applying cosinespectral window, and of performing a fast Fourier transform. We have used the heightinversion of Evershed effect to identify the oscillations associated with Evershed flows,and to separate them from oscillations of a different origin [8, 9]. When analyzing thespectral composition, the range 0.2–3 mHz was considered.Table I contains a list of results of observations presented in this paper. The firstcolumn (NOAA) contains the active region number in accordance with a classificationgenerally accepted in modern solar physics; the second column includes the approximatelocation of this region, the third column shows the part of the sunspot penumbra inwhich the observations were made, the fourth and fifth columns contain a list of periodsobserved, respectively, in the chromosphere and photosphere of a given sunspot, the datain the sixth column indicate the length of the time series, and, finally, the seventh columnrefers to the figure reflecting this observation. The symbol (*) marks those periods whichare observed in the chromosphere and the photosphere simultaneously.612 N. I. KOBANOV and D. V. MAKARCHIKFig. 3. – The western (a, b) and the eastern (c, d) penumbra of NOAA 8602 (28.06.1999). a), c)Value of differential velocity; b), d) power spectrum.Fig. 4. – Two temporary sequences carried out in the north part of penumbra of NOAA 8602(29.06.1999, the second at two hours after the first). a), c) Value of differential velocity; b), d)power spectrum.FLOWS AND OSCILLATIONS IN SUNSPOT PENUMBRA 613Fig. 5. – The north part of penumbra of NOAA 8636 (19.07.1999). a) Value of differentialvelocity; b) power spectrum.Figures 1 and 2 show temporary series received at the NOAA 8585, 21.06.1999. In thefirst case 7 min fluctuations of chromosphere obviously dominate. In the photosphere theoscillations with the periods 4, 6 and 20 minutes are observed. In the second case we cansee an obvious prevalence of two periods in oscillations of the chromosphere: 28 and 17minutes, therewith a wide band from 9 to 4 minutes is present. In the photosphere level,the 43 min oscillations are clearly expressed along with the 5 min band oscillations. Thesignals of differential velocity at different levels of heights shown in fig. 2 demonstratean opposite behaviour in fluctuations (on the photosphere their frequency is more than40 minutes, on the chromosphere about 30 minutes). For establishing a connection withlow-frequency oscillations, of special interest are the data shown in figs. 2, 4a, and 5. Inthese spectra both levels of atmospheric altitudes show oscillations within 30–40 min,and these oscillations have a synchronized character, which can be seen even withoutanalyzing the spectra. Of the examples mentioned above, in two cases (2 and 4a) and inthe case portrayed in fig. 3a there is a similar behavior for the oscillations with periodson the order of 6–7 min. Sometimes (for instance, in figs. 3a-e) oscillations with periodsof 30–40 min are present in both spectra, but it is difficult to estimate their behavior.Specifically in this first case (fig. 1a) this is because the photospheric level does notshow any well-defined peak but shows a “plateau” with periods from 40 min or longerto 20 min. Besides, the amplitude of these oscillations is also small. In the second case(1c), however, oscillations with periods of 40–30 min are weakly pronounced in both thephotosphere and the chromosphere. We were unable to find instrumental effects thatwere responsible for the periods observed. Therefore, there is a reason to consider themreal and to attribute their synchronous character to the Evershed and Sent-John effects.On the other hand, the proper motion of sunspots (such as rotational) can also havesimilar periods. In this case, however, it will be difficult to explain the consistency of thespatial structures of the velocity field in the photosphere and the chromosphere.3. – ConclusionsIn our opinion, the above observations indicate that, in addition to the 10–12 minperiods, a period of 30–35 minutes is present in the velocity variations of Evershedflows. What kind of waves are these variations? To ascertain this, it will require fur-ther investigations using dopplerograms and magnetograms of the entire active region.Furthermore, we are going to investigate the possible connection with Evershed flows614 N. I. KOBANOV and D. V. MAKARCHIKof the 8 min oscillations, which we have also observed sometime at two height levels ofsunspot penumbra. At the present moment there is no sufficient information to make anunequivocal conclusion of this period.∗ ∗ ∗The work is supported in part by the Russian Foundation for Fundamental Research,grant No. 00-15-96659 and Federal program “Astronomy”. We thank Mr. V. G.Mikhalkovsky for his assistance in preparing the English version of the manuscript.REFERENCES[1] Degenhardt D. and Weihr E., Astron. Astrophys., 287 (1994) 620.[2] Westendorp Plaza et al., Nature, 387 (1997) 47.[3] Lites B. W., Sunspots: Theory and Observations, in NATO ASIC Proc., 375 (1992), p. 261.[4] Rimmele T. R., Astrophys. J., 445 (1995) 511.[5] Schine R. A. et al., Astrophys. J., 430 (1994) 413.[6] Kobanov N. I., Solar Phys., 82 (1983) 237.[7] Kobanov N. I., Astron. Astrophys., 143 (1985) 99.[8] Kobanov N. I., Astron. Rep., 44 (2000) 202.[9] Kobanov N. I. and Makarchik D. V., Do the Evershed flows oscillate?, in Proceedingsof the International Conference on Large-scale Structure of Solar Activity: Advances andProspects, San. Pet., 1999, Vol. 109, p. 114.

Flows and oscillations in sunspot penumbra

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Flows and oscillations in sunspot penumbra

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