GIS science highlights
Oscillations above sunspots
Observations of oscillations in coronal loops are sought in order to determine the prescence of waves in the corona and to investiagte a possible source of coronal heating. GIS is well suited to such studies as it can simultaneoulsy monitor the intensity of spectral lines through the transition region and the corona to look for signatures of oscillations.
Due to this spectral coverage GIS has been used to study oscillations in the corona and transition region above sunspots. Combined with NIS and high cadence TRACE observations results have shown a 3 minute oscillation above the sunspot umbra; caused by upwardly propogating acoustic waves.
Further work has shown that both the 3 minute and the 5 minute oscillations are observed in coronal loops away from the footpoints (previous work showed the oscillations at the footpoints only) indicating that magneto-acoutic or pure acoutic waves are propogating up the loops.
For further information see:
- Brynildsen, N., Maltby, P, Foley, C. R., Fredvik, T, & Kjeldseth-Moe, O., Oscillations in the umbral atmosphere, 2004, Solar Physics, 221, 237.
- Lin, C.-H., Banerjee, D., Doyle, J. G., O'Shea, E., Foley, C. R., Coronal
oscillations in the vicinity of a sunspot as observed by GIS/CDS, 2005, Astronomy and Astrophysics, 434, 751.
Coronal holes have long been known to be the source region of the fast solar wind. Accurate temperature measurements within the coronal holes are required to determine the acceleration mechanism which creates velocities of the order of 800 km/s. Observations of coronal holes were difficult are difficult due to the very low intensities and SOHO provided the first opportunity to make such measurements with the required accuracy.
Observations with GIS showed that the temperature within a coronal hole is lower than that of closed field regions (eg. at the equator) and that within the coronal hole the temperature rises to a maximum around 1.15 solar radii above the photosphere. The temperature never exceeds 1 MK and falls to around 0.4 MK at 1.3 solar radii. This result indicates that the fast solar wind is not thermally driven but that the acceleration mechanism is likely to be a transfer of momentum from MHD waves. This process does not require high temperatures of around 3-4 MK at the base of the coronal holes as does the thermal process.
For further information see:
- David, C., Gabriel, A. H., Bely-Dubau, F., Fludra, A., Lemaire, P., Wilhelm, K., Measurement of the electron temperature gradient in a solar coronal hole, 1998, Astronomy and Astrophysics, 336, 90.
Plumes are ray-like structures that are an instrinsic feature of polar coronal holes. The generation of the fast solar wind may take place in these structures or in the surrounding coronal hole regions The plumes extend to distances of many solar radii above the limb and appear to trace the paths of open field lines. Using GIS, investigations can be carried out to determine the plasma charateristics of plumes and how they differ to the surrounding coronal hole or inter-plume regions. Solar wind models are highly dependant on the temperature and density of the source region so these must be accurately determined.
GIS has recorded the first observations of the base of a polar plume and has produced more accurate elemental abundances and differential emission measures due to the large number of spectral lines that GIS observes. Analysis has shown that the plume plasma is close to iso-thermal with a temperature of around 700,000-800,000 K which is a little less then the surrounding coronal hole region. There is a small increase in temperature along the plume, but not across it. Work with GIS has also shown that the technique used to analyse plume plasma must be carefully chosen.
For futher information see:
- Del Zanna, G., Bromage, B.J.I., and Mason, H.E., Spectroscopic characteristics of polar plumes, 2003, Astronomy and Astrophysics, 398, 743.