Abstract
Abstract. Ionospheric scintillations of radio waves at low-latitudes are associated with electron density irregularities. These irregularities are field-aligned and can provide excitation energy all along the field line to non-local field-aligned oscillations, such as the local field line oscillations. Eigen-periods of toroidal field line oscillations at low-latitudes, computed by using the dipole magnetic field and ion distributions obtained from the International Reference Ionosphere (IRI) for typical nighttime conditions, fall in the range of 20–25 s. When subjected to spectral analysis, signal strength of the radio waves recorded on the 250 MHz beacon at Pondicherry (4.5° N dip), Mumbai (13.4° N dip) and Ujjain (18.6° N dip) exhibit periodicities in the same range. For the single event for which simultaneous ground magnetic data were available, the geomagnetic field also oscillated at the same periodicity. The systematic presence of a significant peak in the 20–25 s range during periods of strong radio wave scintillations, and its absence otherwise suggests the possibility that field line oscillations are endogenously excited by the irregularities, and the oscillations associated with the excited field line generate the modulation characteristics of the radio waves received on the ground. The frequency of modulation is found to be much lower than the characteristic frequencies that define the main body of scintillations, and they probably correspond to scales that are much larger than the typical Fresnel scale. It is possible that the refractive mechanism associated with larger scale long-lived irregularities could be responsible for the observed phenomenon. Results of a preliminary numerical experiment that uses a sinusoidal phase irregularity in the ionosphere as a refracting media are presented. The results show that phase variations which are large enough to produce a focal plane close to the ground can reproduce features that are not inconsistent with our observations.Key words. Magnetospheric physics (magnetosphere – ionosphere interactions) Ionosphere (ionosphere – magnetoshere interactions; ionospheric irregularities)
Highlights
(Fedorov, 1999), there has been a renewal of interest in the suggestion of Saito (1983) that the near equatorial region may be passively amplifying the current system imposed from elsewhere, but could be an active source of the generation of ULF geomagnetic disturbances
Though Pc3 geomagnetic pulsations are supposed to be associated with hydromagnetic oscillations excited on geomagnetic field line, their characteristics at low- and equatorial-latitudes are not yet understood either observationally or theoretically (Yumoto, 1986)
The concept of an independent field line oscillation was introduced by Dungey (1954) who proposed for the first time that the long-period geomagnetic pulsations might be the result of standing Alfven waves being excited on geomagnetic field lines
Summary
(Fedorov, 1999), there has been a renewal of interest in the suggestion of Saito (1983) that the near equatorial region may be passively amplifying the current system imposed from elsewhere, but could be an active source of the generation of ULF geomagnetic disturbances. It has been suggested that larger scales corresponding to the typical scale size in the ionosphere can contribute as they may be associated with amplitudes of phase variation as high as hundreds to thousands of radians (Booker and Majidiahi, 1981) In this case, the irregularity acts as a refracting media and the interference of the refracted waves on the ground produce the spatial pattern whose movement causes scintillations in a fixed location radio wave record. For the right choice of phase shift amplitude and scale size, the refraction of radio waves can come to focus close to the ground plane (Titheridge, 1971) and even small movements of the irregularity pattern in the ionosphere can cause measurable changes in the received radio wave signal at a fixed station Such movements can be provided by transverse field line oscillations.
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