Oscillations above sunspots: Evidence for propagating waves?

Abstract

Abstract. We present results of an analysis of time series data observed in sunspot umbral regions. The data were obtained in the context of the SOHO Joint Observing Program (JOP) 97 in September 2000. This JOP included the Coronal Diagnostic Spectrometer (CDS) and the Michelson Doppler Imaging (MDI) instrument, both part of SOHO, the TRACE satellite and various ground based observatories. The data was analysed by using both Fourier and wavelet time series analysis techniques. We find that oscillations are present in the umbra at all temperatures investigated, from the temperature minimum as measured by TRACE 1700Å up to the upper corona as measured by CDS Fe xvi 335Å (log T = 6.4 K). Oscillations are found to be present with frequencies in the range of 5.4 mHz (185 s) to 8.9 mHz (112 s). Using the techniques of cross-spectral analysis time delays were found between low and high temperature emission suggesting the possibility of both upward and downward wave propagation. It is found that there is typically a good correlation between the oscillations measured at the different emission temperatures, once the time delays are taken into account. We find umbral oscillations both inside and outside of sunspot plume locations which indicates that umbral oscillations can be present irrespective of the presence of these sunspot plumes. We find that a number of oscillation frequencies can exist co-spatially and simultaneously i.e. for one pixel location three different frequencies at 5.40, 7.65 and 8.85 mHz were measured. We investigate the variation of the relative amplitudes of oscillation with temperature and find that there is a tendency for the amplitudes to reach a maximum at the temperature of O iii (and less typically O v and Mg ix) and then to decrease to reach a minimum at the temperature of Mg x (log T = 6.0 K), before increasing again at the temperature of Fe xvi. We discuss a number of possible theoretical scenarios that might explain these results. From a measurement of propagation speeds we suggest that the oscillations we observe are due to slow magnetoacoustic waves propagating up along the magnetic field lines.

Chromospheric Resonances Above Sunspot Umbrae

“…Results similar to the observations by Centeno et al (2006) were produced, as well as normalized amplitudes of a few percent in the transition region (McEwan & De Moortel 2006) that are larger than those in the corona (O'Shea et al 2002). The results demonstrate that three-minute oscillations in the chromospheric cavity are obtained without the need for a clear three-minute source in the photosphere.…”

“…As one moves higher up in the chromospheric cavity, the oscillation amplitudes increase so that amplitudes are largest at the top of the chromosphere (2.2 Mm). This numerical result agrees with observations that relative oscillation amplitudes reach a maximum in the transition region, decrease with height up till a coronal temperature of 10 6 K, before increasing again as one moves to higher coronal temperatures (O'Shea et al 2002).…”

Observations of the Sun at Vacuum-Ultraviolet Wavelengths from Space. Part II: Results and Interpretations

“…These oscillations are much more prominent in the blue wing of the emission lines than in the red wing (Brynildsen et al 2003). Umbral oscillations with periods between 112 s and 185 s could be seen with CDS and TRACE inside and outside of sunspot plumes; in some cases different frequencies have been observed cospatially and simultaneously (O'Shea et al 2002).…”

The damping of slow MHD waves in solar coronal magnetic fields

“…Using a combination of different instruments, a number of authors have recently found evidence for the propagation of slow magneto-acoustic waves through the transition region and into the corona. Using mainly CDS/SOHO, MDI/SOHO and TRACE, O'Shea et al (2002) found oscillations present in a sunspot umbra at all investigated temperatures, from the temperature minimum (TRACE 1700 passband) to the coronal temperature of log T = 6.4 (CDS FE XVI 335 Å). The measured propagation speeds suggest that the observed oscillations are slow magneto-acoustic waves propagating from the footpoints along the magnetic field lines.…”