Radio echoes from field-aligned ionization above the magnetic equator and their resemblance to auroral echoes

Bowles, Kenneth L.; Cohen, Robert; Ochs, G. R.; Balsley, B. B.

doi:10.1029/jz065i006p01853

Cited by 92 publications

(27 citation statements)

References 5 publications

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“…It was not until 1960 that it seemed even remotely possible to explain field-aligned radar echoes seen at the magnetic equator by the mechanism used to explain auroral radar echoes. Bowles, Cohen, Ochs, and Balsley [1960] suggested that the irregularities are plane waves whose fronts are parallel to the earth's magnetic field lines. [1963a] have pointed out the possibility that the ionospheric current system as it exists in the ionosphere is similar to a 'two-stream' instability system as described in modern plasma theory.…”

Section: Proposed Reflection Meci-ianislv[mentioning

confidence: 99%

Simultaneous very high frequency and ultra high frequency observations of the aurora at Fraserburgh, Scotland

Leadabrand¹,

Schlobohm²,

Baron³

1965

J. Geophys. Res.

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Simultaneous three‐frequency (30, 401, and 800 Mc/s) radar studies of the aurora were made from a site in northern Scotland. A completely steerable 142‐foot‐diameter antenna was used at all frequencies, providing a 15° antenna beam at 30 Mc/s and identical scaled beams of 1.2° at 401 and 800 Mc/s. Detailed results on the position in space indicate that most echoes arise from a layerlike form of auroral scatterers whose thickness is about 10–20 km. The height distribution of the auroral return was peaked at about 100 to 120 km, but heights as great as 200 km were observed. The highest amplitude and greatest percentage of auroral echoes were found to arise from the intersection of the maximum of the visual auroral zone and the 100‐ to 120‐km orthogonality loci. Wavelength dependence and aspect sensitivity measurements at 401 and 800 Mc/s resulted in a power law dependence of λ7 and aspect sensitivity of 10 db per degree of off‐perpendicular angle. Interpretation of these results in terms of the Booker scattering formula indicates that the size of the irregularities is 45 to 90 meters along the magnetic field lines and 0.7 meter across the field lines. The wavelength dependence results from the 30‐ and 401‐Mc/s data indicate that questionable beam filling at 30 Mc/s introduces an unresolvable uncertainty of 45 db. The 30‐Mc/s aspect sensitivity results indicate an amplitude drop of 10 db per 5° of off‐perpendicular angle, or a length of the scatterers of 120 meters along the magnetic field line. Detailed comparison of magnetic activity auroral Doppler shifts and auroral echo amplitude indicates that the auroral Doppler spectrums and the amplitude of the echoes have a close relationship to the auroral current system. The auroral echoes seen at 401 Mc/s appear at times to be completely depolarized, possibly owing to Faraday rotation, but in general have a considerable degree of polarization retention. At 30 Mc/s, long‐range auroral echoes propagated by an intervening F‐layer reflection were seen as well as ground backscatter propagated by auroral sporadic E. The data indicate that the present theories for auroral scattering are insufficient to explain all the experimental characteristics observed. The plasma acoustic wave model developed for equatorial field‐aligned irregularities appears to fit most of the auroral data obtained but warrants further investigation.

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Section: Proposed Reflection Meci-ianislv[mentioning

confidence: 99%

Simultaneous very high frequency and ultra high frequency observations of the aurora at Fraserburgh, Scotland

Leadabrand¹,

Schlobohm²,

Baron³

1965

J. Geophys. Res.

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“…In the equatorial regions, radar experiments in the early 1960s linked coherent echoes to field-aligned irregularities associated with intensifications of the current systems found at these latitudes (Bowles et al, 1960;Cohen et al, 1962 and references therein). Since then, numerous E-region radar observations have been extensively reviewed (e.g.…”

Section: Introductionmentioning

confidence: 99%

Southern high-latitude Digisonde observations of ionosphere E-region Bragg scatter during intense lacuna conditions

et al. 2004

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Abstract. 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 Eregion reflection were observed in Digisonde Doppler spectra during intense lacuna conditions. Daytime E-region Doppler spectra recorded at carrier frequencies from 1.5 to 2.7 MHz, below the E-region critical frequency f o E, have two side-peaks corresponding to Bragg scatter at approximately ±1-2 Hz symmetrically located on each side of a centralpeak corresponding to near-zenith total reflections. Angleof-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 E×B 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/lacunaCorrespondence to: R. J. Morris (ray.morris@aad.gov.au) 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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“…The restriction on the valid prediction of the vertical profile of current density on high jet days arises mainly from the possible modification of the profile due to the stabilisation of two stream instability in the jet. It may be recalled that the studies of BOWLES et al (1960BOWLES et al ( , 1963, FARLEY (1963) and the work of many others reviewed by CoHEN (1967) show that the drift velocities of the irregularities as obtained from doppler shift of back scatter radar echoes from type I irregularities remain close to the threshold ion acoustic velocity (360 m/sec) even when the electrojet strength becomes exceedingly large, as seen from the large H variation amplitude at the ground. RoGISTER (1971) and SATO (1972SATO ( , 1973 have explained this observation by suggesting that the stabilisation of two stream instability will restrict the growth of Hall potential, and consequently the peak current density in the electrojet, to such a value that the electron drift velocity does not exceed the threshold.…”

Section: Discussionmentioning

confidence: 99%

Results from in situ measurements of ionospheric currents in the equatorial region. I.

Sampath

Sastry

1979

J. geomagn. geoelec

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Several rocket borne magnetometer experiments have been conducted from Thumba to measure the equatorial electrojet currents under different solar and geophysical conditions. The data from these experiments have been used to study the vertical structure of the electrojet, the relation between strength of the jet and the magnetic field variation at the ground and the causes of large day-to-day variability of the amplitude of H variation. The results show that the experimentally measured current density peaks at a significantly different altitude compared to the altitude of the peak of Cowling conductivity calculated using model values of Ionospheric parameters. The analysis indicates that given the magnetogram from a station under the electrojet, it is possible to predict the vertical distribution of current density in the electrojet on a quiet day. The mechanism of stabilisation of two stream instability suggested by the theoretical workers ro explain the back scatter radar echoes from type I irregularities on high jet days does not become operative atleast upto jet strengths equivalent to 4H<140nT.

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Radio echoes from field-aligned ionization above the magnetic equator and their resemblance to auroral echoes

Cited by 92 publications

References 5 publications

Simultaneous very high frequency and ultra high frequency observations of the aurora at Fraserburgh, Scotland

Simultaneous very high frequency and ultra high frequency observations of the aurora at Fraserburgh, Scotland

Southern high-latitude Digisonde observations of ionosphere E-region Bragg scatter during intense lacuna conditions

Results from in situ measurements of ionospheric currents in the equatorial region. I.

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