A&A 380, L39-L42 (2001)
D. Banerjee1 - E. O'Shea2 - J. G. Doyle 3 - M. Goossens 1
1 - Centre for Plasma Astrophysics, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
2 - ESA Space Science Dept, ESTEC Solar System Div., Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
3 - Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
Received 17 September 2001 / Accepted 8 November 2001
We examine long spectral time series of a coronal hole observed on the 7th March 2000 with the Coronal Diagnostic Spectrometer (CDS) on-board SoHO. The observations were obtained in the chromospheric He I, and a series of higher temperature oxygen lines. In this letter we report on the presence of long period oscillations in a polar coronal hole region on the disk. Our observations indicate the presence of compressional waves with periods of 20-30 min or longer.
Key words: Sun: polar coronal holes - ultraviolet: SoHO - Sun: oscillations
For these observations we have used the normal incidence spectrometer (NIS)
(Harrison et al. 1995), which is one of the components of the Coronal Diagnostic
Spectrometer (CDS) on-board the Solar Heliospheric Observatory (SoHO). In order
to get good time resolution the rotational compensation was switched off
(sit-and-stare mode) and so it is important to calculate the lowest possible
frequency we can detect from this long time sequence after taking the solar rotation
into account (see Doyle et al. 1998 for details). For our dataset, s18778r00
x=127, y=781), this rotation amounts to 3 arcsec per hour.
Thus for a 2 arcsec wide slit width, the lowest
frequency resolution is 0.42 mHz.
|Figure 1: Wavelet results corresponding to the He I 584 Å line in the s18778r00 dataset at pixel 22. Panels a) and b) represent intensity and velocity results respectively. The middle row left panels show the time frequency phase plot corresponding to the variations shown in the top panels. The middle row right hand panels show the average of the wavelet power spectrum over time, i.e. the global wavelet spectrum. The continuous dashed horizontal lines in the wavelet spectra indicate the lower cut off frequency (0.42 mHz). The lowest panels show the variation of the probability with time from the randomisation test, with the dot-dash line indicating the 95% significance level.|
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|Figure 2: Intensity wavelet results for the O III 599 Å line corresponding to pixel location 28.|
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|Figure 3: Intensity wavelet results for the O IV 554 Å line corresponding to pixel location 28.|
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|Figure 4: Intensity wavelet results for the O V 629 Å line corresponding to pixel location 28.|
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Turning to the higher temperature oxygen lines, the intensity wavelet results of O III 599 Å, O IV 554 Å and O V 629 Å are presented in Figs. 2, 3 and 4 respectively, for a single pixel location, px 28. All three oxygen lines show intensity power around 0.7 mHz, with a peak at 0.64 mHz, in the global wavelet spectrum plots, at a very high probability level (see value in figures). The velocity oscillation shows a similar trend but with a much smaller probability level. For the O V velocity data, the strongest global peak is at 0.7 mHz, with a 99.6% probability level. For the other two oxygen lines the velocity oscillations are much weaker and are not considered to be significant. Note that the nature and period of the intensity oscillations, corresponding to the three oxygen lines, formed over the temperature range 100000 to 250000 K, behave more or less in a similar way.
To emphasize the fact that these low frequency oscillations are not only coming from one or two particular pixel locations but rather from all over the coronal hole across our slit, we show, in Fig. 5, the spatial behaviour of the oscillation frequencies measured from the O V 629 Å line for a section of the slit. This figure shows the measured frequencies as a function of position along the slit (X-F slice). The frequencies in the left panels, crosses and plus symbols, correspond to the primary and secondary maxima (from the global wavelet spectrum) respectively. The total number of counts in a pixel (summed counts) during the observation is shown in the right column, and is useful in identifying the network brightening (the peaks correspond to the network pixels). The intensity and velocity results both show that the primary maxima in the global wavelet spectra lies in the range 0.5-1.0 mHz. The secondary maxima of intensity often appears in the 1.2-1.4 mHz range. The appearance of a few more crosses in the intensity X-F slice as compared to the velocity also indicates that the intensity oscillations are slightly stronger and more reliable (>95% probability level). Note that these low frequency oscillations come from both bright and dark pixels, implying that they are present both in the network and internetwork, if that structure is present in the coronal hole.
|Figure 5: Frequencies measured as a function of spatial position along the slit (X-F slice) for the O V 629Å line (left panels). The right panels show the total number of counts in a pixel (summed counts) over the observation time.|
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High-cadence EIT/SoHO observations indicate that quasi periodic fluctuations with periods of 10-15 min are present in polar plumes (DeForest & Gurman 1998). These authors conclude that the fluctuations are caused either by sound waves or slow magneto-acoustic waves propagating along the plumes at 75-150 kms-1. Ofman et al. (2000) detected quasi periodic variations in the polarization brightness (pB) at 1.9 , in both plume and inter-plume regions. Their Fourier power spectrum shows significant peaks around 1.6-2.5 mHz and additional smaller peaks at longer and shorter time-scales. Recently, Banerjee et al. (2000, 2001) reported on the existence of long period slow magneto-acoustic waves in the plumes and inter-plumes respectively, as observed by CDS/SoHO. Compressional modes reveal themselves in the form of intensity oscillations, through variations in the emission measure, and also as velocity oscillations through fluctuations in the plasma density. This fact allowed them to interpret the measured oscillations as being due to slow magneto-acoustic waves. It is likely that the waves detected at 1.9 by Ofman et al. (2000) using UVCS/SoHO and the waves detected by DeForest & Gurman (1998) around 1.2 using EIT/SoHO are the same as those reported by Banerjee et al. (2000, 2001) in the polar plumes and inter-plumes very close to the solar limb (off-limb). Thus it is important to find a source region for these long period waves.
It was conjectured that these waves can originate from the network boundaries in the polar coronal hole. In this short contribution we show that these long period waves do indeed originate from the disk part of the coronal hole but the important point to note is that they are also present at several locations in the coronal hole, namely in the network and internetwork regions. We find the presence of long period oscillations in bright pixels (corresponding to the network locations) and also in the darker pixels corresponding to the inter-network. The nature of the waves are very similar to the ones reported by Banerjee et al. (2000, 2001) and thus we also interpret these waves as slow magneto-acoustic. We should also point out here that we do not have information about the phase speed, k and the phase relations, which confirms if the waves are slow or fast. But since we do not observe any steepening of the wave amplitudes or shocks we suggest that these waves are slow waves. Furthermore, it is interesting to note that these long period slow waves are detected in plasma ranging in temperature from 20000 K to 250000 K, which implies that theses waves are present much lower in the atmosphere and are able to propagate upwards. However, we have not been able to pin-point the location where these waves originate and whether they are present in the corona. We hope to address these questions in a wider diagnostic study in a future observing campaign.
DB wishes to thank the ONDERZOEKSRAAD of K.U. Leuven for a fellowship (F/99/42). EOS is a member of the European Solar Magnetometry Network (www.astro.su.se/~dorch/esmn/). We would like to thank the CDS and EIT teams at Goddard Space Flight Center for their help in obtaining the present data. CDS and EIT are part of SoHO, the Solar and Heliospheric Observatory, which is a mission of international cooperation between ESA and NASA. Research at Armagh Observatory is grant-aided by the N. Ireland Dept. of Culture, Arts and Leisure. This work was supported by PPARC grant PPA/G/S/1999/00055. The original wavelet software was provided by C. Torrence and G. Compo, and is available at URL: http://paos.colorado.edu/research/wavelets/