Identification of a New Type of Mode Conversion Interference in VLF Propagation

Previously a mode conversion model was presented which allowed for both the vertical inhomogeneity and anisotropy of the ionosphere. Horizontal inhomogeneity along the direction of propagation was modeled by a slab approximation. The required height gain functions were determined by full‐wave solutions and their associated integrals evaluated numerically. In this paper results of a modified mode conversion model are compared with those of the original. In the modified model, height gain functions are discarded above some height h in the guide and are approximated below height h by Airy functions. Merits of the modified version are that a full‐wave program for height gains is not required, and that the associated integrals may be performed analytically. An obvious disadvantage is the free parameter h. The modified mode conversion model can be implemented with about the same ease as a WKB method and has one distinct advantage over the latter, namely, that mode numbering in any given slab is immaterial. Results of both mode conversion models are compared at several frequencies in the VLF band with experimental sunrise results obtained with a multifrequency oblique sounder system located on the island of Hawaii. In particular, comparisons are made with vertical field measurements in southern California. Approximate agreement is found. Discrepancies between modelling results and data are generally larger than the differences between the two mode conversion models.

Section: Comparisons Will Be Made In This Section Between Results Formentioning

confidence: 71%

Theoretical and experimental sunrise mode conversion results at VLF

Pappert¹,

Morfitt²

1975

“…The direct second mode is highly attenuated during transitions, since it rarely produces observable interference effects [Crombie, 1964;Bates and Albee, 1965]. Extensive interference effects are produced, however, by signals arising from mode conversion at the shadow line [Crombie, 1964;Kaiser, 1968]. These mode conversion signals are absent during daytime and nighttime, and so they affect the nightto-day phase change only when the interfering signals exceed the first-mode signal during transitions to produce cycle gains and losses.…”

Section: Introductionmentioning

confidence: 99%

An Explanation of VLF Diurnal Phase Change Observations

Kaiser¹

1969

Self Cite

The observed diurnal phase changes of VLF radio waves show large deviations from the theoretical first-mode phase change. The main features of the deviations are explained by the presence of a second mode that is larger than the first mode at night but that is small during transitions and during daytime. The data indicate that for frequencies above 16 kHz, nighttime second-mode dominance extends out to several megameters from the transmitter, in agreement with theoretical predictions.

“…In two of these stations, NPM and NWC, a daytime second-order mode was found to be present and to interfere with the lowest-order mode. This pattern of interference is consistent with the model proposed by Kaiser [1968] using the numerical modelling methods such as described by Mahmoud and Beal [1971]. A ground based phase-locked receiver at the launch site would also be useful to record a reference which could be used to correct for balloon drift and to observe signal changes from ionospheric disturbances such as sudden ionospheric disturbances and Trimpi events.…”

Section: Resultsmentioning

confidence: 99%

“…In the second model, proposed by Kaiser, a single nighttime first order mode is partially converted into a daytime second order mode. This second order daytime mode then interferes with the direct propagating dayside first order mode[Kaiser, 1968]. The minima occur relative to the dayside path length, and the relative phase between the two modes is given by [transition, and vd is the daytime phase velocity of the modes.…”

mentioning

confidence: 99%

VLF wave modes determined by the height gains measured during the Extended Life Balloon‐Borne Observatory test flight

Barnum¹,

Holzworth²,

Dowden³

1996

The changes with altitude of the vertical electric field from seven VLF ground‐based transmitters were measured during the ascent of the Extended Life Balloon‐Borne Observatory (ELBBO) balloon test flight from Ft. Sumner, New Mexico. The altitude profiles of the VLF vertical electric field allowed us to determine the propagation modes based on the relative amplitude changes of the VLF signals with height. Three of the VLF transmitters had signals which crossed through the dawn terminator on their paths to the balloon's receiver. The observed height gains from these three stations show that at least two models of sunrise modal interference, those proposed by Crombie [1964] and by Kaiser [1968 ], are occurring. For these three cases a time and altitude‐varying height gain signal (the ratio E(,z)/E(0) for altitude z above the surface z = 0 ) was measured during the balloon's 70‐min ascent to an altitude of 28.5 km. This method of balloon‐measured VLF height gains allows a high level of discrimination between modes, which is shown to be a useful method for studying the phenomena of sunrise and sunset terminator effects on VLF modal interference.