Searching for coronal plumes in Ulysses observations of the far solar wind

In the past, from the analysis of data acquired by the Helios spacecrafts within distances ≤ 1 AU, some evidence has been found of the presence of coronal plumes in the solar wind. Ulysses observations offer a unique opportunity to search for plume remnants in the polar wind at larger distances. Pressure balanced structures (PBS), which might possibly be a signature of those features, have in fact been recently identified in its data. On the basis of previous work, which detected significant peaks (possibly related to plumes) in power spectra of solar wind parameters, we present here the results from a similar research. However, our analysis does not confirm previous findings, because power spectra bear no evidence of significant periodicities. This result allows for different interpretations, but does not rule out the presence of a typical periodicity in the data. By developing a simple 2-D model for structures traversed by Ulysses, we show how easily, even if they were regularly distributed, the original periodicity may become hardly identifiable in power spectra analyses. We conclude that this is not a viable technique for tracing plumes in the solar wind and we suggest alternative means for an unambiguous identification of these features.