Outflow Velocity Structure at the Base of the Solar Corona

A.H. Gabriel1, F. Bely-Dubau2, L. Abbo3

1IAS, Orsay, France
2OCA, Nice, France
3Observatory of Turin, Italy

Recent work on coronal polar plumes (Gabriel et al. 2003, 2005) has aimed at determining the outflow velocity in plume and interplume regions, using the Doppler dimming technique on oxygen VI observations from SUMER and UVCS data from SOHO. It shows that the outflow velocity in plumes is faster out to around 1.5 R, beyond which the situation reverses and the interplume regions become faster. More recent work is directed towards the relation between plumes, the supergranule network and potential heating and acceleration mechanisms. Using a comparison between observations from EIT on SOHO and plume modelling, we show that the major population component of plumes is the result of chance alignments along the line-of-sight of small enhancements in intensity. This confirms the so-called, 'curtain' model. These can be attributed to reconnection activity along the boundaries of supergranule cells. A second population of plumes has a lower abundance and arises from surface bright points having a particular magnetic configuration. New observations using the Hinode/EIS spectrometer are in progress, with the aim of providing further support for this model.