[SADXA] Gray-line Semantics - from Bill - W7EXG

[W7EXG (Bill)]

Gray-line Semantics
From: Bill-- W7EXG

During my SADXA Presentation entitled -- Gray-line Propagation: Lessons
Learned from the Tucson/Heard-Island Experience, a question was raised
regarding the title, since the propagation paths discussed did not appear to
be in the actual Gray-line region. 

In fact, all of the propagations reported, were either after Sunrise or
Sunset, so technically, none of the propagation paths were Gray-line, even
with an almost simultaneous sunrise and sunset between Tucson and Heard.
(See below) 

I would like to clear up any possible confusion regarding this question, by
first defining some basic terms regarding Gray-line. 

	1) Terminator - A line dividing the illuminated from the dark part
of a moon or planet.
	2) Twilight Zone - A region [with refractive and reflective
atmospheric sunlight] that is parallel to and moving with the Terminator
after Sunset, and before Sunrise.  
	The Twilight Zone has 3 zone classifications, with the total zone
width being latitude dependent, and based on the Sun being 18 Deg below the
Horizon.  At the Equator, the Vernal Equinox Twilight Zone is 1245 mi wide,
traveling at 1054 mph, and thus having only 1:11 Hours of Twilight time? 
	3) Gray-line - is synonymous with the Twilight Zone, and hence its
name. In reality, Gray-line enhancements require upper layer ionization in
this region, which is solar zenith and layer height dependent. 

There are several confusion factors associated with these definitions. 
	A) Gray-line IS NOT A LINE, IT'S A ZONE. 
	B) Some authors use Terminator to mean Gray-line. and Gray-line to
mean a line -- which further adds to the confusion. 

Strictly speaking -- ANY propagation between stations located IN the
Twilight Zone at the time, would be considered Gray-line Propagation.  Any
propagation that strays outside the Twilight Zone, would NOT be considered
Gray-line Propagation. 

The Semantics can get even more confusing, since most hams who work far side
DX during Sunrise or Sunset, [where the stations are somewhat aligned to the
terminator], will argue Gray-line Propagation.  

However strictly speaking, the DX propagation may NOT have been Gray-line at
all.  They would need to dissect the propagation path, and clearly
demonstrate that both stations were IN the Gray-line during the DX contact. 

Further, if the propagation was a N/S path, then any propagation exceeding
about 1:11 Hrs, would be partially propagating outside the equatorial
Gray-line. 

Let us consider two stations having simultaneous Sunrise and Sunset.   This
would seem an ideal case for Gray-line propagation. However with time, the
sunrise station would appear to move OUT of the twilight region, while the
other station would appear to move INTO the twilight region. These two
stations will NEVER be in the Twilight Region at the same time. While some
of the great circle path propagation will be in the twilight zone, the
remaining path will be remain outside the twilight zone. This was almost
exactly the case that the Tucson-Heard propagation was suggesting - thus
prompting the question. 

Jerry,[W6XI]reports that Low Band (160m) DX stations report signal
enhancement just before, during, and just AFTER sunrise.  In order to
declare this Gray-line propagation, the great circle path must remain in the
Gray-line region, even AFTER sunrise. 

In my opinion, most authors who discuss Gray-line propagation are using the
term loosely, or generically to suggest Propagation Enhancement due to the
Terminator. Actual Gray-line propagation is ONLY a small subset of the
Terminator Enhanced Propagation. 

It was in this context, I had titled my presentation. 

Part 2 in Sept SADXA meeting, will discuss Terminator Enhanced band
dependent Propagation Corridors, which will clearly include unique
propagation enhancements on both sides of the terminator, and unique for all
bands.  Almost ALL of us use Terminator Enhanced propagation along the
terminator during sunrise or sunset. Few of us actually use TRUE Gray-line
propagation, and may not really know. If one made a rare DX contact, at
sunrise or sunset -- most would not care to know?

Tucson / Heard Island propagation was classic for Terminator Enhanced
propagation, both during sunrise and sunset. As mentioned in the title, it
was ONE of the many lessons learned by this study.  

73s, 
Bill W7EXG