Topband: SSW propagation on 160m

[Eric Scace]

[My apologies to the Topband reflector if the following has been already
covered in this thread by other folks.  I'm a new
subscriber and wasn't sure if these items had already been considered.]

Hi Bob --

   While reading your article "On the SSW Path & 160-Meter Propagation" in
the 2000 Nov/Dec issue of "QEX", a few additional points
came to mind.

   According to my understanding, the article hypothesizes a 1999 December
solstice (solar cycle maximum) 160m propagation path
between XZ and USA via the short great circle route, with scattering from
turbulent regions over the central USA at the sunrise
ionospheric terminator.  The following considerations may be useful in
testing the hypothesis:

   a) Robin WA6CDR reports that operators at XZ0A were copying signals from
the USA not on the short great circle route (northerly
bearing) but rather on a skewed SSE bearing.  I've only seen Robin's recent
email to the Topband reflector [00 Nov 14] and do not
know how the operators were determining bearings.  However, a SSE bearing
from XZ does not appear to correspond with the hypothesis,
assuming a reciprocal propagation path... and the hypothesis does not
describe a non-reciprocal path mechanism.

   b) Fig 6 of the article shows scattering of a signal from XZ, arriving
from the NNW, resulting in an apparent SSW arrival at the
USA receiving station above the polar limit in Wisconsin.  The scattering
angle is acute in this case.  For an acute scattering to
occur, the critical frequency within the scattering mechanism must be close
to or above the 160m band.  I am not familiar enough
with the current state of knowledge about ion densities within such
turbulent regions to know if the critical frequency is that
high.  (Vertical incidence ionograms from the region might provide insight
into structure and persistence of any turbulent regions.
Has anyone looked for this yet?)  But one can simply note that oblique angle
scattering mechanisms rely on much lower critical
frequencies and are easier to form.

   c) If the hypothesized scattering was occurring, there should be reports
of XZ signal reception from a very wide range of
azimuths in the mid-USA.  If, at one instant, a turbulent scatterer existed
at the point shown in Fig 6, then stations in Oklahoma,
Texas and New Mexico should report signals from the NE while Colorado
stations simultaneously report signals from the east, etc.  Or
assume a band of mid-latitude turbulence along the terminator, with chaotic
turbulent scatterers forming and dissipating.  From the
perspective of a particular US station south of the polar limit and west of
the terminator of Fig 6, azimuths for reception at a
particular site would be varying widely and randomly throughout this phase
of the opening... and signals may arrive simultaneously
from several directions.  Have such reports been received?

   d) Signals from VK6 have been widely reported as arriving in the USA from
the SSW.  While it's certainly possible that entirely
different path mechanics are involved than for transmissions from XZ, it
seems worth noting that a southern hemispheric path from
VK6 to USA may closely parallels the path from XZ for signals leaving XZ in
the SSE direction.

   According to my understanding, the article also concludes that a southern
hemisphere terminator guided path is not supported by
3-D ray-tracing models based on reference ionospheres because of (1) a lack
of adequate horizontal ion density gradient focusing
mechanisms, and (2) E-layer absorption beyond 6000 - 7000 km from the XZ
path end for southward signals poised to enter the daylight
sector.  However, other mechanisms not represented in reference ionospheres
may contribute to a successful skew path through the
southern hemisphere, including:

   a) Plumes and blobs of ionization convected in high geomagnetic latitude
regions from the sunlit sector into the night sector.
Two-cell plasma convection zones over the polar caps continuously pump zones
of higher ionization into the night sector, into and
through the auroral zone, with maximum around 21h30m local time.  Such zones
are of adequate ionization levels to deviate signals at
HF frequencies (e.g., 8 MHz) as much as 50° from the great circle path over
distances as short as 700km.  Elevation angles can be
modulated plus/minus 10° as well.  Measured changes in foF2 critical
frequencies of these plumes shows variations of a few MHz
during low Kp values over periods of tens of minutes, and even larger
variations (as much as 8 MHz) when Kp is 3 or 4, as each blob
passes by.  Plumes extend the full height of the ionosphere.  [1] [2] [3]
Even modest, decaying remnants of such plumes and blobs,
advecting from Antarctica over the Southern Ocean below Australia and the
western South Pacific, would seem adequate for one or more
oblique refractions of a 160m signal on the dark side of the southern
hemisphere terminator... a curtain of irregular and changing
shape (blob and plume passages through a specific control point take 20
minutes in the early evening region to as much as two hours
close to the sunrise terminator) generally focusing signals back into the
night sector inside the terminator.

   b) Early evening range slant-F and other F-layer irregularities at
equatorial and low geomagnetic latitudes.  Equatorial
irregularities range in size from tens of centimeters to hundreds of meters,
appear concentrated in a thin layer from 200-400km, and
may persist for 5-6 hours on the evening side of the terminator.  Thin
irregularities may enhance the likelihood of Pederson rays
and other extended hop ranges, making it easier for signals to reach high
latitudes without fatal absorption.  [5]  Such ionospheric
irregularities also occur at increased frequency and intensity during the
high portion of the sunspot cycle... but, unfortunately
for the XZ0-USA path, the probability of irregularities in the Indian
Ocean/SE Asia region is relatively lower during Nov-Dec.

   Refraction from irregularities such as described in (a) and (b) is one
mechanism for coupling into whatever ionospheric ducts
might exist. [4]

   c) Magnetospheric waveguide:  Radio wave propagation between geomagnetic
conjugate points in the night sector via the
magnetosphere has been measured by satellite and terrestrial stations in the
1.6 - 2.0 MHz range with percentages of occurrence up
to about a third, especially at low geomagnetic latitudes.  (There is a
sharp drop in plasma density at mid-latitudes in the
magnetosphere, unless Kp is low, in which case the plasma density boundary
expands polewards.  Low plasma density is insufficient to
trap waves in ducts effectively.)  [4]  Quasi-stationary magnetospheric
irregularities elongate rapidly in the direction of the
geomagnetic field and form parallel ducts which focus signals in tubes of
200-500m diameter (at the lower extremities) between
conjugate points.  The frequency of these irregularities is maximum at
periods of sunrise and sunset.  If the critical frequency of
the F-layer is low enough, signals of terrestrial origin escape through the
ionosphere and may couple into the duct.

   Magnetospheric duct losses are extremely low and multiple reflections
between conjugate points within the duct are often
measured.  Attenuation in the magnetosphere is at a minimum between 2 and 3
MHz, just a fraction of a dB per passage, and is still
way below 1 dB at 160m.  Even a weak signal coupled into the duct will
emerge at the conjugate point almost intact.

   To exploit magnetospheric waveguides, a signal from XZ0A needs to
propagate (e.g., by ionospheric means) to a duct entry point...
enter any duct by leaking out of the top of the ionosphere at a 55-70°
takeoff angle (to intercept the magnetic field orientation at
1500km above the earth)... follow that duct around to the conjugate point
and reenter the top of the ionosphere at a similar
angle... refract upon its downward exit through the ionosphere (splaying
outwards from the vertical) and then couple into normal
ionospheric propagation for the last hop(s) to the USA receiver.  I believe
(but don't have a geomagnetic map with me to confirm)
that suitable low geomagnetic latitude conjugate points exist for the
XZ-North America path in late December at 12z near the
terminator.  XZ0, according to my recollection, is quite close to the
geomagnetic equator.  Signals could travel SSE near and
parallel to the terminator and leak above the ionosphere on any early hop
(e.g., NW of Australia) ... couple into a magnetospheric
duct ... and return to Earth in the terminator in a conjugate location
(e.g., southern Mexico or eastern equatorial Pacific north of
Easter Island).  Any splayed refractive exit from the F-layer towards the
earth's surface may allow coupling into ionospheric hops
for the last route to USA topbanders.  In this case, those signals would
appear to all stations to be arriving from the SSW... and
possibly from the south for stations located more west of the terminator at
the USA end.  The path would be reciprocal.

   (By the way, a magnetospheric duct wouldn't work on 160m if the duct
entry was too far from the terminator into the night sector,
as the exit would be onto the top of the ionosphere's F- and E-layers in the
daylight sector and signals could be fully absorbed or
reflected back up into the duct.)

   It's fun to hypothesize about methods for long distance signals on
160m... but there may be some observation methods we can use
to favor more likely solutions.  Some possible observational methods to
detect magnetospheric ducting between XZ0 and USA would
include careful timing of signals (it takes about 200 ms to travel once thru
a magnetospheric duct)... probably not very practical
on a 160m DXpedition!  Low geomagnetic latitude ducts that was open through
the ionosphere may exhibit weak echoes from
ground-scatter returns at the far end, spaced at intervals corresponding to
the duct transit time; that might be visible on a scope
trace of a received signal at few hundred ms intervals.  Stations using
ducts might see an azimuth shift as the low geomagnetic
latitude portion of the terminator approached, with the access at their end
to the ducts closing after sunrise absorption levels
rose enough to block their ability to reach the duct entry points.  Stations
at either side of the globe whose signals are unable to
reach the duct entry/exit points would not hear the distant signals at all,
if this were the only viable mode at the time... quite
different from high geomagnetic latitude refractive blobs, which would be
accessible to all stations along the route towards the
late evening area of the polar cap and auroral zone.  A concerted data
gathering effort, not just of logged QSOs but also of
received signal strength and azimuth vs time from many stations, could help
eliminate some hypotheses.

   And of course different modes may predominate on different days!

-- Eric Scace K3NA
eric@k3na.org


References:
[1]  Jenkins, R. W., "Preliminary Analysis of Kestrel Data", informal CRC
Tech Memo DRL/TM083/92, 1992.
[2]  Weber, E. J., "Auroral Zone Plasma Enhancements", Journal of
Geophysical Research, 90 6497-6573, 1985.
[3]  Jones, T. B. and Warrington, E. M.,  "Large Bearing Errors Observed at
a High Latitude DF Site", Radiolocation Techniques -
Papers presented at the Electromagnetic Wave Propagation Panel Symposium
held in London England 92 Jun 1-5, AGARD.
[4]  Gurevich, A. V. and Tsedilina, E. E., "Long Distance Propagation of HF
Radio Waves", translated from Russian and published by
Springer-Verlag, ISBN 3-540-15139-7, 1985.
[5]  Aarons, J., "Longitudinal Occurrence of Equatorial F-Layer
Irregularities",  Radiolocation Techniques - Papers presented at the
Electromagnetic Wave Propagation Panel Symposium held in London England 92
Jun 1-5, AGARD.


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