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High time resolution (~13 s cadence) Solar and Heliospheric Observatory (SOHO) Coronal Diagnostic Spectrometer (CDS) Grazing Incidence Spectrometer (GIS) time series data is analysed for evidence of quasi-periodic variability in the solar corona. The observations are obtained from a GIS study of a coronal hole boundary region taken in October 1997. A wavelet analysis is applied to EUV emission lines covering a temperature range of log Te=5.3-6.1 K. Wavelet analysis allows the frequency and temporal information of any periods present within the data to be analysed. Statistically significant periods are found within the range 100-900 s and 1500 s. Observations of short wavepackets with periods of the order 50-100 s with durations of 2-5 cycles are reported. Observations of a decay in the driving period over time are also described.


Coronal Polarization Measurements with CDS - by Roger J. Thomas

As mentioned in an earlier newsletter ( issue 3), an attempt was made to validate our understanding of basic emission mechanisms in the solar corona during a special SOHO roll maneuver last year, by making use of the polarization sensitivity of CDS. Preliminary analyses of these observations show that the expected EUV polarization signatures are indeed seen in coronal emission of low temperature lines above the limb, at the expected locations and with the proper sign. But the observed strength of these signals is much higher than expected, and on the verge of what seems physically possible. A similar result was obtained from the 1997 SOHO roll, although at the time there was concern that the very high intensity modulations observed may have been due to time-variability instead of due to polarization effects. The 2001 data do not allow that interpretation, so something very peculiar may actually be going on in the solar corona.

Figure 1 shows intensity gradients up to 220 arcsec above the limb measured before, during, and after the roll maneuver. Since the entire sequence took nearly two and a half days to complete, observations were made at the north solar pole to minimize effects of temporal variations. In all cases, the pre- and post-roll curves are nearly identical, confirming this lack of variability. The first three lines shown, He I 584A, O IV 555A, and O V 630A, are all formed at temperatures below logT = 5.4. All three have a break in their exponential intensity falloff at about 20-30 arcsec above the limb, indicative of a change between emission dominated by collisional excitation (with an Ne^2 dependence) to radiation from resonance scattering (which varies as Ne to the first power). This latter radiation should be linearly polarized in the plane parallel to the limb because of the asymmetric illumination field incident on the scattering ions. And because the CDS normal-incidence spectrometer is fed by sectors of a telescope with two grazing-incidence reflections, its response should then decrease at the 90-degree roll position for such polarized light, which is exactly what is observed.

This intensity modulation can not be due to some internal instrumental effect, because the other three lines plotted in Figure 1, Mg IX 368A, and Mg X 610 & 625A, show no such strong variation with roll angle, though their wavelengths are interspersed with the others. However, these latter three lines are all formed at temperature above logT = 6.0 with much larger density scale-heights, and so are expected to be collisionally dominated and thus un-polarized over the full range of coronal heights measured, again exactly matching what we see.