The single‐layered D‐region model based only on Lyman α photoionization of nitric oxide is inadequate to explain certain aspects of VLF radio propagation. Empirical evidence indicates the existence of a second and lower D layer which is important for VLF propagation. It is shown that normal cosmic rays produce sufficient ionization for such a layer. The electron‐production and ‐depletion processes effective below 100 km and the latitudinal distribution of cosmic‐ray primaries due to the geomagnetic field are discussed. A series of electron‐density profiles obtained from photoionization and cosmic‐ray ionization processes is compared with the two‐layered D‐region model deduced from electromagnetic measurements. The calculated phase and amplitude characteristics of VLF signals reflected from the lower cosmic‐ray‐induced layer during the presunrise hour are in good agreement with those measured.
The hypothesis is proposed that the vertical gradient of the refractive index is strongly affected by the vertical motion in the atmosphere. It is shown that changes in this gradient are due principally to changes in the moisture stratification and temperature lapse rate resulting from the vertical velocity and its horizontal gradient within the air mass. Mean radio signal strengths are plotted against the time position of 500‐mb troughs and ridges relative to the radio link. A series of 500‐mb charts is given, with the vertical velocity field obtained by the Sawyer‐Bushby method. The vertical velocity over two radio links is compared with the signal strength. Cross‐sections taken through several of the troughs illustrate the stratification changes on the forward and rear sides of the troughs due to vertical motion.
A wave guide mode computer model is used to calculate ELF radio fields in the earth-ionosphere wave guide. Results are compared with those calculated by Galejs and by Johler and Lewis for both homogeneous and diffuse inhomogeneous ionospheres as well as with measurements of Sanguine test transmissions. Good agreement is found in all comparisons except for those with the Johler and Lewis calculations for an inhomogeneous ionosphere.
~r an s ho ri zoll vhf a nd uhf fi elds ex hi bit d e~p fa des o r la rge s ig nal enhance men ts of several hours .duratlOll, as the propagatlOll mec hams m a ltern ates between pa rtia l refl ectio n and scattenng caused by t urbulen t dielectri c fluctu ations in t he at mosph ere. Such a ltern ations occ ur when str ong refractIve layers de velop below 3,000 ft. Surface wind strea mlin e a na lyses s how ~hat mes? scale ce?ters of co nverge nce or dIv ergence cause local redis tribution of refractIve laye nng, tendmg to produce t he cha nge from on e mec ha ni sm to t he other.Current scatterin g t heory and t he empirical findin gs of others a re exam in ed to determin e ~h e gross meteorological factors t hat influence changes in scattered fi elds . Th e two vari ables 111 t he t urbulent scatterin g coeffi cients, t he scattering a ngle a nd t he in te ns ity o f di electri c fluctua t IOns at hI gh wave numbers, .are found to be d ependen t upon the refrac t ive laye rin g a nd the t h ermal s~a bllLty of t he a ll·mass . It h as bee n s hown elsewhe re that refr activ ity a nd s tablh ty are pnnClpally fun ctIO ns of the ver tical velocity in t he at mosphere. It is shown here that t he dIrectIOn a nd relatIve magn it ude of t he vertical velocity can be inferred f rom t he uppe r-troposp her:ic wind velocity diyerge.nce . R eceived scattere· d signa ls are found to be well co rrelated WIth co mputed velocIty divergence. I t IS s uggested t hat the va ri ations of scattered signal level or ra nge can be pred icted in a ro utIne manne r by r egul a r meteo rological p ersonn el usin g ordin a ril y available meteoro-logIcal d ata.
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