observations were also available. We have used them to determine physical parameters, including the temperature, density and line-of-sight (LOS) Doppler shift. By projecting a 3D plume onto the SUMER density map, its density scale height could be calculated. The temperature corresponding to this scale height and the temperature derived spectroscopically from SUMER has been compared.

The data
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*STEREO* was launched somewhat after the solar activity minimum which provides us with good opportunities to observe polar plumes. We selected two data sets for this study, one in the south polar cap on 2007-04-07 22:01:17 UTC and the other in the north polar cap on 2007-06-01 00:09:00 UTC. Both were recorded by the two almost identical SECCHI/EUVI telescopes at $\lambda = 17.1~nm$ corresponding to a formation temperature of roughly 1 MK. The position information of both spacecraft, the exposure time and the compression mode are given in Table \[tab:orbit\_plume\].

On April 7, the separation of the two spacecraft was $3.6^ \circ$. The HEEQ (Heliocentric Earth EQuatorial coordinate system; @Thompson:2006) latitudes of the two spacecraft were well below the Sun’s equatorial plane. At that time the solar south pole was tilted towards the spacecraft which was very appropriate for observing as much 