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

[James Hollander]

 Six-Regime 630m.  Fourth, last post.  73,  Jim H   W5EST   
 
VI.  NIGHT-TIME REGIME: PLATEAU
     D-region signal absorption reaches a minimum due to electron recombination with positive ions and attachment of electrons with some molecules to form negative ions.  630m WSPR SNR plateaus at a high average level subject to SNR variations.  Long distance DX receptions occur during the opportunity time window for each pair of locations-- transmit station location and receive station location--and time of year. See Notes 2-4 below.  Multipath in the F-region contributes to self-interference of signal at the receiving antenna and consequent 630m WSPR SNR variations.  Ionospheric convection and patches of varying electron concentration contribute somewhat less to the SNR variations than in the post-SS regime.  Geomagnetic variations indicated by K and A indices may contribute to SNR variations.  As pre-SR approaches, night-time 630m conditions blend into the pre-SR regime of section "I" of the first post.
 
NOTES:
Note1)  Regarding solar ionizing radiation, the ionospheric post-SR eastward F-region, E-region, and D-region are absorptive to different energy bands or wavelength bands of such solar radiation.  In that way, one explains how pre-SR F, E, and D regions westward do not ordinarily receive ionizing radiation except during major solar flares.  Why: Because the solar ionizing radiation in whichever wavelength band that could respectively ionize each westward F, E, and D region gets absorbed by that respective F, E, and D region farther east.  But visible sunlight, with its much lower energy, does reach the pre-SR westward D-region.  There, D-region negative ions have their electrons so loosely attached that even visible sunlight can detach the electrons and de-ionize the negative ions.  So 630m SNR starts to fall in the pre-SR regime "I."  The electron detachment in the D-region increases the electron concentration. So the pre-SR D-region increasingly diminishes WSPR signals that would earlier in the night have reached and been reflected from the F-region.  The D-region itself now lossily reflects enough 630m signal to nevertheless bring some signal to the receiving station.  (The low frequency of 630m signals make them more readily reflected by the lossy D-region than HF signals would be.  HF signals on the other hand experience less loss as well as less reflection in the D-layer than MF signals do.  So HF can continue to propagate via E and F regions as time goes on into sunrise and morning.)  

Note 2) The ionospheric D-region is less well understood than the E and F regions.  The D-region is too low for satellites, and getting data on the D-region using rockets is limited.

Note 3)  Ionospheric convection and advection ("winds" or ionospheric mass transfer) brought on by SS or SR at D-region altitudes also may cause SNR variations.  These processes are not fully understood.  Likewise, post-SS and pre-SR patches of varying electron concentration may contribute SNR variations that are difficult to predict.   
 
Note 4)  Tides in the ionosphere occur due to the moon and sun gravitational field gradients.  The sun/moon gravitations combine for maximum ocean and ionospheric tides around full moon and new moon days. The sun/moon gravitations somewhat conflict around first and third lunar quarters to deliver minimum tides.  The effects of such tides in the ionosphere are still debatable.  Possibly at night on 630m, the weekly dates of the lunar quarters may be more likely times for instances of trans-equatorial WSPR propagation over distances of 10,000 kilometers or more within a two-month interval around each equinox.

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