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| Titel |
Low frequency modulation of transionospheric radio wave amplitude at low-latitudes: possible role of field line oscillations |
| VerfasserIn |
A. K. Sinha, B. M. Pathan, R. Rajaram, D. R. K. Rao |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 20, no. 1 ; Nr. 20, no. 1, S.69-80 |
| Datensatznummer |
250014343
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| Publikation (Nr.) |
 copernicus.org/angeo-20-69-2002.pdf |
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| Zusammenfassung |
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) |
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