8 Discussions

Using UVSP data obtained in emission lines formed at temperature of $7 \times 10^{4}$ K to $1.3 \times 10^{5}$ K, Gurman et al. (1982) observed transition region oscillations in sunspots with frequencies in the range of 5.8-7.8 mHz. Their in-phase intensity and velocity oscillations lead them to interpret the oscillations in terms of upward propagating acoustic waves. For the first time Thomas et al. (1987) made simultaneous detection of umbral oscillation at different heights, starting from the chromosphere to the transition region. Their power spectra of intensity and velocity both show multiple peaks at the 3 min band.

Figure 13: Frequencies measured as a function of spatial position along the slit (X-F slice) for the O  V 629 Å line (left panels) and the s19336r00 dataset.

   

Table 4: Summary of the oscillation frequencies observed corresponding to the different dataset.

Dataset	Lines	Pixel	Intensity results			Velocity results
			Freq. maxima	Prob. level	Duration	Freq. maxima	Prob. level	Duration
s19332r00	He  I	67	6.2 mHz	99-100%	20-30, 40-65	6.2 mHz	99-100%	20-25, 40-65
	O  III	67-68	5.6 mHz	99.2%	20-25, 45-50
	O  V	67	6.7 mHz	99-100%	20-30, 40-65	6.7 mHz	99-100%	20-30, 40-50, 60-65
s19334r00	He  I	66	5.7 mHz	99-100%	10-30, 40-50	5.2 mHz	98.8%	15-30, 40-50
	O  III	66-67	5.2 mHz	95.6%	15-20, 45-55
	O  V	66	5.7 mHz	99-100%	15-30, 40-55	5.7 mHz	99-100%	15-30, 40-50
s19336r00	He  I	67	6.2 mHz	99-100%	5-10, 20-30, 40-60
	O  III	66-67	5.7 mHz	98.4%	5-10, 40-50
	O  V	67	6.2 mHz	99-100%	5-15, 25-60	6.2 mHz	99-100%	20-55
s19378r00	He  I	20	4.8 mHz	99-100%	5-20, 25-45
	O  III	21-22	4.8 mHz	99-100%	10-20, 25-50	4.8 mHz	98.8%	10-20, 40-50
	O  V	21	4.8 mHz	99-100%	10-20, 25-45	4.8 mHz	99-100%	0-20
s19380r00	He  I	21	4.8 mHz	99-100%	5-20, 25-30, 45-55
	O  III	21-22	5.2 mHz	99-100%	5-20, 45-55
	O  V	21	4.8 mHz	99-100%	5-20, 45-55	4.0 mHz	99.8%	5-20
s19382r00	He  I	21	5.2 mHz	99-100%	45-70
	O  III	21-22	5.6 mHz	99-100%	45-60
	O  V	21	5.2 mHz	99-100%	10-20, 25-35, 45-65	4.0 mHz	99-100%	25-35
s19388r00	He  I	33	3.5-5.0 mHz	99-100%	0-35	3.5-5.0 mHz	99-100%	5-35
	O  III	32-33	4.0-5.0 mHz	99-100%	0-45	5.7 mHz	95.0%	5-10, 35-45
	O  V	33	4.0-5.0 mHz	99-100%	5-20, 35-55	5.2 mHz	99-100%	40-55
s19390r00	He  I	33	3.5-5.0 mHz	99-100%	5-35	4.5-5.0 mHz	95.0%	0-20
	O  III	32-33	3.5-5.0 mHz	95.0%	5-10, 25-30
	O  V	33	3.0-5.0 mHz	99-100%	10-45
s19392r00	He  I	32	3.0-6.0 mHz	99-100%	30-75	4.4 mHz	99.2%	10-20, 40-45
	O  III	31-32	4.8 mHz	98.4%	30-40
	O  V	32	3.0-5.0 mHz	99-100%	0-10, 35-50, 65-70	3.5-5.0 mHz	99-100%	0-20, 40-45

With the launch of SoHO there has been renewed interest in the study of umbral oscillations. Fludra (1999, 2001) investigated 3 min intensity oscillations with CDS by observing the chromospheric line He  I and several transition region lines. He concluded that the 3 min umbral oscillations can occur both in the so called sunspot plumes (bright features seen in the transition region above sunspot) or in the lower intensity plasma closely adjacent to the plumes. He found the spectral power to be contained in the 5.55-6.25 mHz range. No oscillations were detected by him in the Mg  IX 368 Å line, suggesting that the 3 min oscillation does not propagate into the corona. Tziotziou et al. (2002) have presented two-dimensional intensity and Doppler shift images computed at different wavelengths within the Ca  II 8542 Å line. Their power spectrum analysis shows a 6 mHz frequency, for the standing umbral oscillations only for the upper half part of the umbra. For the penumbra they report a 3 mHz frequency. They also conclude that the umbral oscillations are a localized phenomena. SUMER observations (in both intensity and velocity) have confirmed that the sunspot oscillations are prominent in transition region lines above the umbra (Maltby et al. 2001). They also state that the umbral oscillations are a localized phenomenon and that the 3 min oscillations fill the sunspot umbra in the transition region and tends to stop at the umbral rim. Support for the acoustic wave hypothesis was presented by Brynildsen et al. (1999a,b). They observed oscillations in intensity and velocity to test the hypothesis and found the oscillations to be compatible with upwardly propagating waves. More recently O'Shea et al. (2002) and Brynildsen et al. (2002) have both presented joint observations of the 3 min umbral oscillations with TRACE and CDS. O'Shea et al. (2002) find oscillations at all temperatures from the temperature minimum, as observed by TRACE 1700 Å up to the upper corona, as measured by the Fe  XVI 335 Å line with CDS. Both these authors report that the oscillation amplitude above the umbra increases with increasing temperature, reaches a maximum in the transition region and decreases for higher temperature lines, though O'Shea et al. (2002) finds evidence for another increase in amplitude for lines formed above 1 MK. O'Shea et al. interpreted their observations in terms of slow magneto-acoustic waves propagating upwards (as confirmed from their time delays) along magnetic field lines. In a recent theoretical paper, Zhukov (2002) calculated the spectrum of eigenmodes of umbral oscillations. It was shown that the 3 min umbral oscillations are the p-modes modified by the magnetic field.

The salient feature of our observation is that we have detected both intensity and velocity oscillations in chromospheric and transition region lines as observed by CDS. We should point out that the velocity resolution of CDS is, at best, 5 km s^-1, and generally it is quite difficult to detect velocity oscillations with any confidence from noisy data. But with inclusion of a reliable probability test and wavelet technique we were able to extract velocity information in most of the cases with a 95% confidence level or higher. Most of the earlier work on sunspot oscillations with CDS (Fludra et al. 1999, 2001; Brynildsen et al. 2002; O'Shea et al. 2002) presented only intensity results. Our results clearly show that the 3 min intensity and velocity oscillations are a property of the umbra, and not just the sunspot plume (Figs. 9 and 13, shows that power peak around 6 mHz is present over several pixels in the umbra). We also detect 3 mHz oscillations corresponding to the penumbra, which supports the recent observation by THEMIS, Tziotziou et al. (2002). We should point out that the He  I line is thought to have a complex formation history and its emission may not correspond to that which one might expect from a chromospheric line. It is believed that there are two main mechanisms by which He  I can form (Andretta & Jones 1997); either by collisional excitation in the lower transition region from electrons with kinetic temperature higher than the local temperature of the helium atom or by a process in which coronal photons penetrate into the chromosphere and photoionize helium atoms which then recombine to form He  I. Thus the emission from He  I can reflect conditions at temperatures above that of its putative formation temperature. Furthermore, the definition of where the chromosphere ends and transition region begins is a bit arbitrary. Thus there exists an uncertainty and controversy over the He  I formation height. Moreover, the O  III and O  V lines are formed close to each other in temperature. This does not allow us to make a time-delay analysis for the calculation of the wave propagation speed using this data.