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| Titel |
Southern high-latitude Digisonde observations of ionosphere E-region Bragg scatter during intense lacuna conditions |
| VerfasserIn |
D. P. Monselesan, R. J. Morris, P. L. Dyson, M. R. Hyde |
| 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 ; 22, no. 8 ; Nr. 22, no. 8 (2004-09-07), S.2819-2835 |
| Datensatznummer |
250014968
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| Publikation (Nr.) |
 copernicus.org/angeo-22-2819-2004.pdf |
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| Zusammenfassung |
| During summer months at solar cycle minimum, F-region lacuna
and slant-Es conditions (SEC) are common features of daytime ionograms
recorded around local magnetic noon at Casey, Antarctica. Digisonde
measurements of drift velocity height profiles show that the occurrence of
lacuna prevents the determination of F-region drift velocities and also
affects E-region drift velocity measurements. Unique E-region spectral
features revealed as intervals of Bragg scatter superimposed on typical
background E-region reflection were observed in Digisonde Doppler spectra
during intense lacuna conditions. Daytime E-region Doppler spectra recorded
at carrier frequencies from 1.5 to 2.7MHz, below the E-region critical
frequency foE, have two side-peaks corresponding to Bragg scatter at
approximately ±1-2Hz symmetrically located on each side of a central-peak
corresponding to near-zenith total reflections. Angle-of-arrival information
and ray-tracing simulations show that echo returns are coming from oblique
directions most likely resulting from direct backscatter from just below the
total reflection height for each sounding frequency. The Bragg backscatter
events are shown to manifest during polar lacuna conditions, and to affect
the determination of E-region background drift velocities, and as such must
be considered when using standard Doppler-sorted interferometry (DSI)
techniques to estimate ionospheric drift velocities. Given the Doppler and
spatial separation of the echoes determined from high-resolution Doppler
measurements, we are able to estimate the Bragg scatter phase velocity
independently from the bulk E-region motion. The phase velocity coincides
with the ExB direction derived from
in situ fluxgate magnetometer records. When ionospheric refraction is
considered, the phase velocity amplitudes deduced from DSI are comparable to
the ion-acoustic speed expected in the E-region. We briefly consider the
plausibility that these previously unreported polar cap E-region Bragg
scatter Doppler spectral signatures, observed at Casey in December 1996
during SEC/lacuna conditions may be linked to ionosphere irregularities.
These irregularities may possibly be generated by primary plasma waves
triggered by current-driven instabilities, that is to say, a hybrid of the
"modified two-stream" and "gradient drift" instability mechanisms. |
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