Abstract
We have generated off-limb polar synoptic charts of polar plume evolution at various solar altitudes using EUV Imaging Telescope and Large Angle and Spectrometric Coronagraph data from 1996 December. The charts allow direct measurement of the altitude expansion of the solar minimum coronal holes. We find expansion values that are consistent with the conventional picture of superradial expansion and inconsistent with radial expansion. Using visible red line data as a bridge between EUV and white-light images of the corona, we are able to confirm that the coronal structure seen at the base of the corona is preserved throughout the considered altitude range of 1.1-3.0 R☉. We show that polar plumes are episodic in nature, lasting perhaps 24 hr but recurring for up to weeks at a time; this strengthens the picture that they are caused by magnetic heating under the influence of supergranulation.
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