March 31, 2001 geomagnetic storm
Distribution of QSO distances
by Volker Grassmann, DF5AI, May 31, 2005
Distribution function of the QSO distances
In Aurora backscatter, the radiowaves travel from the transmitter into the E region of the ionosphere and finally return to the receiver on ground. However, VHF radio amateurs are mainly interested in the terrestrial QSO distance which is measured along the great circle path connecting the two radio stations.
Fig. 1 displays the distribution of the great circle QSO distances in the European sector on 50, 144 and 432 MHz, respectively. On 144 MHz (see the blue curve), the number of QSOs increases almost proportional with the QSO distance and reaches a peak maximum around 800 to 900 kilometers; beyond that distance the number of QSOs drops sharply. Nevertheless, a certain amount of QSOs represent long distance backscatter communication exceeding 1.000 kilometers considerably, some QSOs have even manged distances of 2.000 kilometers.
The 50 MHz data shows a very similar distribution, the peak maximum is however less distinct and appears shifted to shorter distances. This shift is definitely available with the 432 MHz data where the peak maximum is found between 500 and 600 kilometers, at longer distances the number of QSOs drops almost abruptly. No 432 MHz Aurora QSO has been reported at distances longer than 800 kilometers - remarkable exceptions (see the arrows in fig. 1) will be discussed further below.
A very different picture is obtained when analysing the great circle distances in the North American sector, see fig. 2. Although dealing with a much smaller database, there is strong indication of shorter dx distances in average, which is however difficult to understand. Mack, NA3T, has communiated a similar result when analysing dx reports from radio amateurs in the U.S. and in Canada associated with the July 15/16, 2000 Aurora band opening , i.e. great circle distances peaking around 600 kilometers (nevertheless, he also reports QSO distances in 50 and 144 MHz exceeding 2.000 kilometers contrary to the above findings).
The author has analysed the geographical variability of the maximum dx range in field-aligned backscatter which shows a gradual change from long to short distances when moving from southern to northern Europe, see  and . The maximum dx range is controlled by the actual inclination of the geomagnetic field (dip angle) which varies from 50 degree in the south to more than 80 degree in the north of Europe. At very high dip angles, Aurora dx communication is not available at all, i.e. radio amateurs in the far north of Europe cannot benefit even from strong Aurora band openings. Because the geomagnetic pole is located in Canada, the dip angles in North America are higher at lower geographical latitudes compared to Europe (see the bulge over North America in fig. 3) which might reduce the maximum dx range in Aurora dx communication, perhaps. However, this assumption does not appear justified because U.S. radio amateurs are generally located at lower geographical latitudes compared to their European colleagues, i.e. the geomagnetic latitude of U.S. radio amateurs and European ham operators does not differ significantly, in fact (see fig. 3). Thus, the shorter Aurora QSO distances in North America must be considered a feature which cannot be explained, so far.
Occurence of long dx
Long dx QSOs occured in all major phases of the Auroral activity and do not show any systematical features, see those blue dots in fig. 4 which indicate distances longer than, say 1.500 kilometers.
Azimuthal direction of the great circle paths
Great circle distances shorter than, say 1.000 kilometers represent Aurora QSOs which do not show any preferred direction in azimuth, i.e. we may find north-south as well as east-west propagation paths and any other direction in between. This feature is shown in the left panel of fig. 5 which displays all great circle paths between 700 and 800 kilometers, i.e. shorter and longer QSO distances are all excluded in the graphics. Note that the blue criss-cross pattern is composed of the terrestrial great circle paths, i.e. the true radiowave paths are not shown here.
However, considering QSO distances of more than 1.000 kilometers, the azimuthal direction of the great circle paths changes considerably. North-south great circle paths become more and more unlikely if the QSO distance increases and become even impossible at very long path lengths. In fact, Aurora dx communication does not support very long dx QSOs in the north-south direction, see the discussion given by, e.g., Grayer, G3NAQ,  and Newton, G2FKZ, , .
The right panel of fig. 5 shows this feature in practice by selecting all Aurora QSOs corresponding to great circle path lengths between 1300 and 1400 kilometers, i.e. QSOs corresponding to shorter and longer distances, respectively, are all excluded. Evidently, the dx scenario is dominated by QSOs directed from east to west and vice versa, i.e. north-south directions are absent altogether. Thus, extinguished north-south directions may explain the sharp decrease in the number of QSOs beyond the 900km-peak in fig. 1.
Unusual maxima in the QSO distance distribution
The two arrows in fig. 1 indicate a striking feature, i.e. two maxima at 1300-1400 kilometers and at 1600-1700 kilometers, respectively. This maxima are visible in 50 and in 144 MHz and, most surprising, even in 432 MHz - we are apparently facing a systematical feature rather than an accidental result here. A very similar feature was found, for example, by Newton, G2FKZ, (see fig. 7.22 in ) and by Mack, NA3T, see , when analysing amateur radio dx reports associated with the geomagnetic storms in March 1989 and in July 2000, respectively.
Considering the dx data from the U.S. and Canada, see fig. 2, the existence of similar maxima appears difficult to judge. The 144 MHz curve, however, appears to show a maximum around 1.600 kilometers and another unusual maximum at 1.100 kilometers - both maxima could also result from statistical effects though because of too few QSO data.
The feature of the two maxima is still under investigation, any comments, ideas and suggestions capable to solve the puzzle are very much appreciated.