When the Uruguayan tycoon Francisco Piria (1847-1933) undertook the creation of a seaside resort at the image of those in the Italian Riviera (where he had spent part of his youth) he insisted that the area he had chosen for it in Maldonado had “special energies”, something that befitted well both his alchemist ideologies and his real-estate marketing genius. However, though unknown to him, perhaps there was some justification for those ideas.
The name that Francisco chose for his resort was “Heliopolis”, but it has been known since as “Piriápolis”. Within a few days CX7RT will be back in Piriápolis (where he spent part of his youth). From his QTH there (GF25id) CX7RT likes to probe the HF bands with QRP/P signals and small wire antennas. In the past three years and in as many months he has logged from there short of 300 QRP/P QSOs (http://mylog.hamlog.eu/CX7RT). However, he noted that HF propagation predictions (using VOACAP) have not been too helpful in determining the best conditions for DX skeds and QSOs. Here is a possible reason: Piriápolis as well as all Uruguay and large portions of Brazil and Argentina, are within the “South Atlantic Anomaly”, a large and growing patch on the surface of the Earth with low strength in the geomagnetic field due to the fact that in that zone the axis of the magnetic dipole of the Earth is furthest away from its surface (https://en.wikipedia.org/wiki/South_Atlantic_Anomaly).
As a result, in this part of the South Atlantic, the Van Allen Belts (toroidal-like pockets of charged “high-energy” particles held around the Earth by the geomagnetic field) are closer to the surface of the Earth – (would modern alchemists wish to argue that this proximity of the Van Allen Belts might have been the source of the “energies” invoked by Francisco…?). In this zone the inner Van Allen Belt reaches as close as 200 Km from the Earth surface instead of the 500 Km of its general altitude elsewhere.
Also, normally, the inner Van Allen Belt touches the outside of the F2 layer (the main layer responsible for the retutn to Earth of HF electromagnetic waves). However, within the “South Atlantic Anomaly” area it reaches underneath it and its relation with the F2 layer is inverted (http://www.astrosurf.com/luxorion/qsl-perturbation4.htm). The general opinion seems to be that this condition does not affect HF propagation (http://ham.stackexchange.com/questions/693/how-does-the-south-atlantic-anomaly-affect-propagation-in-the-hf-bands). However, this seems difficult to visualize given the above considerations.
Stefanus Jansen van der Merwe, in his superb Master dissertation thesis (University of Pretoria, 2011) entitled “Characterisation Of The Ionosphere Over The South Atlantic Anomaly By Using A Ship-Based Dual-Frequency Gps Receiver” (http://repository.up.ac.za/xmlui/bitstream/handle/2263/30118/dissertation.pdf?sequence=1&isAllowed=y) states that “The abnormal behaviour of the ionosphere makes inter-continental high frequency (HF) propagation difficult to predict over this region.” He provides data showing that HF paths predicted on the basis of electron density maps generated from mobile GPS observables are better than those based on the IRI model (http://ccmc.gsfc.nasa.gov/modelweb/ionos/iri.html), which does not appear to be “well defined for the southern hemisphere” because of the lack of enough ionosonde data for the area.
Therefore, the “South Atlantic Anomaly” might affect HF paths more than is usually accepted and current HF path predictions may be less reliable in this area than in other parts of the World. Given the geographic span of the “South Atlantic Anomaly”, it is more likely to affect HF path predictions for stations within its boundaries, namely South America and South/West Africa. Hence, if you fail to hear CX7RT’s signal from Piriápolis, it could be because of its QRP nature and/or his use of small wire antennas. However, it could also be due to the “South Atlantic Anomaly”, something that may not be completely accounted for in HF propagation prediction models, and – who can say…? – which could also be contributing to the local “energies” that make Piriápolis such a compelling place…