[600MRG] 630m WSPR Propagation Over a Typical Day: Six Regimes

[James Hollander]

All,  I've been trying to learn how 630m propagation "works" compared to 160m and HF bands. It's different, that's for sure. 
      Would you let me know any errors in the description. It's posted in part-day pieces to fit this reflector. Use mrsocion at aol.com. Please send me any exceptional experiences that don't fit the description, too. 
      Seasonality on 630m is not covered. I discussed seasonality of 630m reception opportunity time windows in two posts on Sept. 19. 
      I've appreciated e-mail dialogs about this subject with John Langridge WG2XIQ and Carl Luetzelschwab K9LA.  I wrote the description and any errors are mine.
73, Jim H  W5EST
 
 
SIX REGIMES OF 630M WSPR RECEPTION OVER 24 HOURS BETWEEN TWO STATIONS
 
GROUND WAVE:  630m ground wave reception continues unchanged within variations only a few dB day and night at the short distances to which it extends.  Storms and significant noise level variations may affect wspr SNR.  Ground wave varies with location, because of spatially-varying attenuation and location-dependent destructive interference by reflection from large structures.  Time-wise, ground wave reception may also suffer some destructive interference over time at a given location due to arriving sky waves of the same signal if they are strong.

SKYWAVE: ["SS" is local sunset time UTC at the later station to achieve common darkness between them (usually the westward of two stations). "SR" is local sunrise time UTC at the first station to lose the common darkness (usually the eastward station).  SNR is signal-to-noise ratio.  

     I.  PRE-SR REGIME
    The pre-dawn sun's visible light passes high overhead the eastward station and reaches into the ionosphere's D-region where RF signal has been passing through from higher regions.  The sun's visible light progressively sweeps from East to West and releases electrons from easily de-ionized negative ions at a negative ion de-ionization rate that begins to spoil the high-SNR night-time regime VI sketched later.  The D-region absorption rises due to interaction of the released electrons with the 630m RF signal, and SNR consequently falls.  By contrast with the sun's visible light, the sun's ionizing radiation (EUV, X-ray) is blocked due to positive ion production in the ionizable ionospheric regions eastward of SR.  An exception occurs during some solar flares (M, X-class) that may penetrate westward. (See Note 1 posted later.)  
     Negative ions eastward of SR are already de-ionized and do not impede the sun's visible light from extending into the westward pre-SR region.  In duration, this pre-SR regime is quite variable and may begin about a half-hour or even an hour or more before sunrise. Cirrus and other ordinary water vapor clouds in the troposphere far below the D-region may be involved. Such clouds may even deliver underside reflection of visible sunlight ("anti-crepuscular light") from east of SR very deeply into the pre-SR region westward. So some de-ionization of negative ions in the D-region can occur more than hour before SR.  Overall, the pre-SR and post-SR regimes deliver a reverse S-shaped declining SNR pattern subject to SNR variations and cutoff by the WSPR decoder's decodability threshold itself.  As time approaches SR, more WSPR signals vanish from decodability.  Multipath self-interference of signal in the F-region contributes to sky wave SNR variations as much as 15 dB or more.
(Next post: II. POST-SR REGIME.)