Distribution of mean Doppler shift, spectral width, and skewness of coherent 50‐MHz auroral radar backscatter

Watermann, J.; McNamara, A. G.; Sofko, G. J.; Koehler, J. A.

doi:10.1029/ja094ia06p06979

Cited by 16 publications

(19 citation statements)

References 5 publications

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“…4. Below these are occurrence distributions of echo Doppler shift versus spectral width, sometimes known as Watermann plots (Watermann et al, 1989). In these plots red indicates the greatest occurrence.…”

Section: Echo Characteristicsmentioning

confidence: 99%

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Multi-frequency observations of E-region HF radar aurora

2003

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Abstract. Multi-frequency observations of E-region coherent backscatter from decametre waves reveal that auroral echoes tend to comprise two spectral components superimposed, one at low Doppler shifts, below 250 m s −1 , and the other Doppler shifted to near the ion-acoustic speed or above, up to 800 m s −1 . The low Doppler shift component occurs at all look directions; Doppler shifts near the ion acoustic speed occur when looking at low flow angles along the direction of the electron drift in the electrojet, and Doppler shifts in excess of the ion acoustic speed occur at intermediate flow angles. The latter population appears most commonly at radar frequencies near 10-12 MHz, with its occurrence decreasing dramatically at higher frequencies. The velocity of the high Doppler shift echoes increases with increasing radar frequency, or irregularity wave number k. The velocity of the low Doppler shift population appears to be suppressed significantly below the line-of-sight component of the electron drift. Initial estimates of the altitude from which scatter occurs suggest that the high Doppler shift echoes originate from higher in the E-region than the low Doppler shift echoes, certainly in the eastward electrojet. We discuss these observations with reference to the theories of de/stabilization of twostream waves by electron density gradients and electrostatic ion cyclotron waves excited by field-parallel electron drifts.

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Section: Echo Characteristicsmentioning

confidence: 99%

“…Overall, the "type" nomenclature is perhaps now seen as too restrictive and poorly-defined to allow the physics behind the generation of irregularities to be properly elucidated, and sta-tistical approaches to classifying spectral populations can be more useful (e.g. Watermann et al, 1989).…”

Section: Introductionmentioning

confidence: 99%

Multi-frequency observations of E-region HF radar aurora

2003

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“…In the past, the grouping of echo types according to their spectral characteristics has been a useful tool in identifying the generation mechanisms of the irregularities observed by various radars (e.g. Watermann et al, 1989;Hanuise et al, 1991;Milan and Lester, 2001;Makarevitch et al, 2001).…”

Section: Echo Type Analysismentioning

confidence: 99%

E-region decameter-scale plasma waves observed by the dual TIGER HF radars

Carter

Makarevich

2009

Ann. Geophys.

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Abstract. The dual Tasman International Geospace Environment Radar (TIGER) HF radars regularly observe E-region echoes at sub-auroral magnetic latitudes 58 • -60 • S including during geomagnetic storms. We present a statistical analysis of E-region backscatter observed in a period of ∼2 years (late 2004-2006) by the TIGER Bruny Island and Unwin HF radars, with particular emphasis on storm-time backscatter. It is found that the HF echoes normally form a 300-km-wide band at ranges 225-540 km. In the evening sector during geomagnetic storms, however, the HF echoes form a curved band joining to the F-region band at ∼700 km. The curved band lies close to the locations where the geometric aspect angle is zero, implying little to no refraction during geomagnetic storms, which is an opposite result to what has been reported in the past. The echo occurrence, Doppler velocity, and spectral width of the HF echoes are examined in order to determine whether new HF echo types are observed at sub-auroral latitudes, particularly during geomagnetic storms. The datasets of both TIGER radars are found to be dominated by low-velocity echoes. A separate population of storm-time echoes is also identified within the datasets of both radars with most of these echoes showing similar characteristics to the low-velocity echo population. The stormtime backscatter observed by the Bruny Island radar, on the other hand, includes near-range echoes (r<405 km) that exhibit some characteristics of what has been previously termed the High Aspect angle Irregularity Region (HAIR) echoes. We show that these echoes appear to be a storm-time phenomenon and further investigate this population by comparing their Doppler velocity with the simultaneously measured F-and E-region irregularity velocities. It is suggested that the HAIR-like echoes are observed only by HF radars with relatively poor geometric aspect angles when electron density is low and when the electric field is particularly high.Correspondence to: R. A. Makarevich

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“…Apart from the ®rst two, observed during low backscatter, a relatively stable power distribution at negative velocities develops later. Compared to usual auroral radar spectra (Watermann et al, 1989) they exhibit more structure which is not surprising, regarding the small scattering volume, and their width is considerably smaller. In the ®rst three spectra aliasing is visible, since the mean Doppler velocity is close to the limits of the velocity range.…”

Section: Shape and Moments Of Spectramentioning

confidence: 99%

Non-magnetic aspect sensitive auroral echoes from the lower E region observed at 50 MHz

Rüster

Schlegel

1999

Ann. Geophys.

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Abstract.Backscatter from E-region irregularities was observed at aspect angles close to 90°(almost parallel to the direction of the magnetic ®eld) using the ALOMAR SOUSY radar at Andoya/Norway. Strong electric ®elds and increased E-region electron temperatures simultaneously measured with the incoherent scatter facility EISCAT proved that the Farley-Buneman plasma instability was excited. In addition, strong particle precipitation was present as inferred from EISCAT electron densities indicating that the gradient drift instability may have been active, too. Backscatter at such large aspect angles was not expected and has not been observed before. The characteristics of the observed echoes, however, are in many aspects completely dierent from usual auroral radar results: the Doppler velocities are only of the order of 10 m/s, the half-width of the spectra is around 5 m/s, the echoes originate at altitudes well below 100 km, and they seem to be not aspect-sensitive with respect to the magnetic ®eld direction. We, therefore, conclude that the corresponding irregularities are not caused by the mentioned instabilities and that other mechanism have to be invoked.

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Distribution of mean Doppler shift, spectral width, and skewness of coherent 50‐MHz auroral radar backscatter

Cited by 16 publications

References 5 publications

Multi-frequency observations of E-region HF radar aurora

Multi-frequency observations of E-region HF radar aurora

E-region decameter-scale plasma waves observed by the dual TIGER HF radars

Non-magnetic aspect sensitive auroral echoes from the lower E region observed at 50 MHz

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