Ray Tracing over a Transequatorial Path

Ray tracing procedures including the magnetic field were employed in an attempt to explain the mechanism of transequatorial propagation. The analysis was based upon (a) 41 MHz backscatter soundings south from Mayaguez, Puerto Rico and (b) vertical-incidence observations from the ionosonde chain near 75 °W. The latter were converted into electron density versus true height profiles. Data from both sources obtained during the same month were utilized.The computed ray tracings show the expected effects for refraction from the F layer: skip and horizon focusing, predawn blackout (0200–0600 LST), escape of all rays launched above 18° irrespective of time of day, diurnal variation in one-hop propagation distances, etc. Some calculated rays attain TE distances (6000–11 000 km without intervening ground reflections) at 0800 LST, 1600–2000 LST and 2400 LST. Others are trapped to distances exceeding 11 000 km at 0800 LST and 1400–2400 LST. Fair agreement is found between TE observations and TE calculated ray paths. Specific hours and distances showed some correlation. Qualitatively the general features of TE seem clarified. The calculations imply that rays launched within 9° of the horizon southward across the (magnetic) equator are responsible for TE propagation. These rays are injected into an ionospheric trapped mode by a strong electron density gradient. For a ray launched at the ground to propagate to TE distances, two requirements must be satisfied: (a) vertical refractivity gradients propitious for radiowave trapping, and (b) horizontal refractivity gradients allowing injection and ejection of the ray into and out of the duct. TE concurrences near 0800 LST may arise because of the rapid strengthening of the postsunrise electron density gradient near 20° geomagnetic. This strong horizontal gradient then disappears, possibly because of an atmospheric expansion, and does not reappear until late afternoon. The trapping conditions, however, remain from about sunrise to midnight.The results imply that at the same or a higher frequency more TE would be observed if more energy was emitted at lower launch angles.