Satgen256 HF Propagation - Causes and Effects ? by GM4IHJ 19 Feb 94 BID of this msg is SGEN256 Please use this BID if you retransmit this msg For some time now it has been clear that total confusion is present in our descriptions of how events on the Sun affect HF Radio propagation and Aurora at the Earth.Assumptions which have recently been discredited are:- 1. That Solar Flux measured at 10.7cms , directly relates to maximum usable HF propagation frequency, in the ionosphere ? Frankly, 10.7 cm radiation can never ionise anything, and it seems to be much more likely that solar X ray output is the phenomena affecting the ionosphere. 2. That Solar Flares cause large magnetic storms around the Earth ? A recent paper by J T Gosling titled Coronal Mass Ejection - The link between solar and geomagnetic activity ( Paper 41G1 Bull Am Phys Soc 37 1466 1992 ), makes it very clear that Solar Flares are not a cause of anything very much at all.Many of them are simply an effect consequent on a Solar Coronal Mass Ejection. Indeed as a mass ejection rises off the Sun , it draws out oppositely directed magnetic fields in the legs of its loop like base. These mag fields can subsequently reconnect and produce a flare. But the flare is a secondary effect, which, compared to the Mass Ejection and the tremendous shock wave it propagates into the solar wind, is hardly of much consequence. Equally important. The output of the flare covers only a narrow cone angle, whereas the mass ejection can cover up to half of interplanetary space, thereby having a much bigger chance of hitting the earth. 3. That Planetary Magnetic Index is directly related to the size and scope of Visual Aurora, or propagation trans aurorally or, VHF back scatter off the Auroral plasma ? The events of 4th to 15th of February 1994 have completely shattered that myth. A tremendous "Coronal Hole" ( note this is very different from a Coronal Mass Ejection ), transitted across the face of the Sun for 11 days, seriously affecting radio propagation conditions around the Earth. During this event planetary magnetic indices were sometimes reported to be as high as Kp5 and Kp6, but viewed from Scotland and Northern UK the activity was rarely much better than which had previously been assumed to be the norm for Kp3 conditions. Indeed the Visual Auroras never got anywhere near overhead GM, and the radio propagation effects were also typical of Kp3 condition with trans auroral propagation reaching stations inside the typical Kp3 auroral oval but getting nowhere near those inside the much bigger Kp6 auroral oval. So the premise that a Kp5 report means that one can anticipate radio auroral propagation begining in mid afternoon in GM , then followed from about 1900 local by visual aurora up to 60 degrees elevation, is clearly not valid for all events, and the link between Kp and propagation conditions must henceforth be treated as a very weak link at best. There are lessons for all of us here. An excellent review article (Nature 17 Feb 94 p595 by N Crooker) shows why we must stop presuming that if event B follows event A, we can presume event A caused event B. Auroral reporting has been plagued for years by this kind of false logic. Eg Visual Aurora is a result of particulate bombardment of the ionosphere but it is not related to the misnamed Radio Aurora. This latter term should have been reserved for the natural relatively low frequency radiation generated by auroral plasma. What we call radio aurora is properly refered to as Propagation of radio signals Trans auroral or as back scatter off the Aurora. Can we hope that soon we will drop all this loose talk which is so misleading. After all it is now 37 years since Sputnik 1 first reported on the ionosphere 73 de GM4IHJ@GB7SAN (sat spotter5 next week)