The formation of ducts and spread F and the initiation of bubbles by field‐aligned currents

Abstract

Magnetospheric ducts for medium frequency (MF) radio waves are formed, for L •> 1.2, between about 20 and 22 hours local time. Their occurrence frequency remains constant for the remainder of the night and begins to decrease after sunrise. This decrease continues until near sunset. The seasonal and longitudinal variation of the ducts and of low-latitude spread F implies that both are related to hemispheric asymmetry of the winds in the dynamo region and hence are probably generated by field-aligned currents as suggested by K. D. Cole. These currents generate significant irregularities in the E region of the ionosphere at night when E region densities are low; these irregularities in turn produce perpendicular electric fields due to winds. The small-scale electric fields (•< few kilometers) responsible for ducts can only be transferred to the F region and magnetosphere along magnetic field lines when the parallel conductivity is fairly large between the E and F layers. These electric fields interchange tubes of flux and hence can change tube densities. The transfer of electric fields occurs before the formation of the deep E- F density valley. The valley forms about 22 hours, which explains why ducts with L •> 1.2 can only form after the E region has low density (after 19 hours) and before the E-F valley forms. For L <• 1.2 the percentage occurrence of MF ducts increases from about 20 to roughly 02 hours local time and decreasesfrom roughly 02 to about 10 hours. It is thought that these ducts are associated with ionospheric 'bubbles.' The frequency of occurrence of MF ducts with L •< 1.2 continues to increase for roughly 3 hours after the frequency of occurrence of ducts with L •> 1.2 stops increasing. Since the estimated rise time for bubbles from the bottom of the F layer to the topside is about 3 hours, these ducts may also have been initiated before about 22 hours. INTRODUCTION Medium frequency (MF) radio waves transmitted by topside sounder satellites can become trapped in magnetic-fieldaligned ionization ducts. Observations of the traces on topside ionograms, resulting from the trapped waves, enable characteristics of the ducts to be determined [Muldrew, 1963, 1967a; Loftus et al., 1966; Dyson, 1967; Ramasastry, 1971; Nishizaki and Matuura, 1972; $harma and Muldrew, 1973, 1975]. The ionization ducts observed with the topside sounders have a diameter of about one to a few kilometers and an electron density deviation from ambient of the order of 1%. The occurrence of ducts is either independent of Kp or has a slight negative correlation. Whistler ducts are much larger and have been studied by Ondoh [1976]. MF ducts are closely related to ionization irregularities, i.e., to spread F[Calvert and $chmid, 1964; Dyson, 1968, 1977] and ionospheric bubbles [Dyson and Benson, 1978]. Bubbles have recently received much study [Woodman and La Hoz, 1976; Kelley et al., 1976; McClure et al., 1977; Ossakow and Chaturvedi, 1978; Ott, 1978].It has been suggested that ionospheric and magnetospheric field-aligned irregularities...

Plasma density enhancements associated with equatorial spread F: ROCSAT‐1 and DMSP observations

“…However, the ROCSAT spacecraft at ∼600 km can see density enhancements associated with the depletion structures that rise to ∼1000 km. Previous observations showed that most depletion structures rise to maximum height up to ∼1000 km, although they have been observed up to 3000 km [ Burke et al , 1979; Muldrew , 1980a]. Thus the lower orbit ROCSAT spacecraft would observe the density enhancements more frequently than the DMSP spacecraft.…”

Observations in equatorial anomaly region of total electron content enhancements and depletions

“…The characteristics and morphology of these depletions, that has been obtained mainly with the Jicamarca radar (e.g. Woodman and La Hoz, 1976), and substantiated by the satellite-based instruments and Altair radar observations (McClure et al, 1977;Dyson and Benson, 1978;Muldrew, 1980;Tsunoda, 1980;Tsunoda et al, 1982;Kil and Heelis, 1998;Yeh et al, 2001), are very well documented. Theoretical and observational investigations indiCorrespondence to: R. Pandey (pandey r@yahoo.com) cate that these depletions are generated on the bottomside of the nighttime equatorial F region and rise to higher altitudes due to the non-linear evolution of the generalized RayleighTaylor and E×B plasma instabilities (Kelley, 1989;Fejer et al, 1999).…”

Guided echoes in the magnetosphere: Observations by Radio Plasma Imager on IMAGE

“… Calvert [1981] suggested that R ‐ X waves ducted in FAI would be readily detectable. Evidence of ducting has been observed in the ionosphere for L ‐ O and R ‐ X modes [see, e.g., Muldrew , 1963, 1967, 1980] and in the magnetosphere for whistlers [e.g., Angerami , 1970; Carpenter , 1981; Carpenter and Sulic , 1988]. Only a few percent N e enhancement is required for effective whistler ducting [e.g., Smith , 1961; Helliwell , 1965], and a similar N e depletion for L ‐ O and R ‐ X mode ducting [see, e.g., Muldrew , 1963, 1967; Platt and Dyson , 1989; Calvert , 1995].…”