scholarly journals The spectral characteristics of E-region radar echoes co-located with and adjacent to visual auroral arcs

2002 ◽  
Vol 20 (6) ◽  
pp. 795-805 ◽  
Author(s):  
S. E. Milan ◽  
N. Sato ◽  
M. Lester ◽  
T. K. Yeoman ◽  
Y. Murata ◽  
...  
Keyword(s):  
Electric Fields ◽  
Spectral Characteristics ◽  
Hf Radar ◽  
Radar Backscatter ◽  
Acoustic Instability ◽  
Radar Echoes ◽  
Close Relationship ◽  
E Region ◽  
Auroral Arcs ◽  
Gradient Drift

Abstract. Simultaneous all-sky camera and HF radar observations of the visual and E-region radar aurora in the west-ward electrojet suggest a close relationship between a pair of parallel east-west-aligned auroral arcs, separated by ~ 30 km, and a region of strong radar backscatter. Poleward of this a broader region of radar backscatter is observed, though the spectral characteristics of the echoes in these two regions differ considerably. We suggest that the visual aurorae and their radar counterparts are produced in a region of upward field-aligned current (FAC), whereas the backscatter poleward of this is associated with downward FAC. Relatively low electric fields ( ~ 10 mV m-1) are observed in the vicinity of the arc system, suggesting that in this case, two-stream waves are not directly generated through the electrodynamics of the arc. Rather, the generation of irregularities is most probably associated with the gradient drift instability operating within horizontal electron density gradients produced by the filamentary nature of the arc FAC system. The observation of high Doppler shift echoes superimposed on slow background flow within the region of backscatter poleward of the visual aurora is argued to be consistent with previous suggestions that the ion-acoustic instability threshold is reduced in the presence of upwelling thermal electrons carrying downward FAC.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities; particle precipitation)

scholarly journals E-region echo characteristics governed by auroral arc electrodynamics

2003 ◽  
Vol 21 (7) ◽  
pp. 1567-1575 ◽  
Author(s):  
S. E. Milan ◽  
N. Sato ◽  
M. Lester ◽  
Y. Murata ◽  
Y. Shinkai ◽  
...  
Keyword(s):  
Electric Field ◽  
Electric Fields ◽  
Doppler Shift ◽  
Hf Radar ◽  
Radar Echoes ◽  
Ion Acoustic ◽  
E Region ◽  
Electric Fields And Currents ◽  
Auroral Arcs ◽  
Acoustic Speed

Abstract. Observations of a pair of auroral arc features by two imagers, one ground- and one space-based, allows the associated field-aligned current (FAC) and electric field structure to be inferred. Simultaneous observations of HF radar echoes provide an insight into the irregularity-generating mechanisms. This is especially interesting for the E-region echoes observed, which form the focus of our analysis, and from which several conclusions can be drawn, summarized as follows. Latitudinal variations in echo characteristics are governed by the FAC and electric field background. Particularly sharp boundaries are found at the edges of auroral arcs. Within regions of auroral luminosity, echoes have Doppler shifts below the ion-acoustic speed and are proportional to the electric field, suggesting scatter from gradient drift waves. Regions of downward FAC are associated with mixed high and low Doppler shift echoes. The high Doppler shift component is greatly in excess of the ion-acoustic speed, but seems to be commensurate with the driving electric field. The low Doppler shift component appears to be much depressed below expectations.Key words. Ionosphere (ionospheric irregularities; electric fields and currents)

scholarly journals A classification of spectral populations observed in HF radar backscatter from the E region auroral electrojets

2001 ◽  
Vol 19 (2) ◽  
pp. 189-204 ◽  
Author(s):  
S. E. Milan ◽  
M. Lester
Keyword(s):  
Spectral Width ◽  
Hf Radar ◽  
Type I ◽  
Radar Backscatter ◽  
Vhf Radar ◽  
Flow Angle ◽  
E Region ◽  
Gradient Drift ◽  
Limiting Velocity

Abstract. Observations of HF radar backscatter from the auroral electrojet E region indicate the presence of five major spectral populations, as opposed to the two predominant spectral populations, types I and II, observed in the VHF regime. The Doppler shift, spectral width, backscatter power, and flow angle dependencies of these five populations are investigated and described. Two of these populations are identified with type I and type II spectral classes, and hence, are thought to be generated by the two-stream and gradient drift instabilities, respectively. The remaining three populations occur over a range of velocities which can greatly exceed the ion acoustic speed, the usual limiting velocity in VHF radar observations of the E region. The generation of these spectral populations is discussed in terms of electron density gradients in the electrojet region and recent non-linear theories of E region irregularity generation.Key words. Ionosphere (ionospheric irregularities)

scholarly journals On the altitude dependence of the spectral characteristics of decametre-wavelength E region backscatter and the relationship with optical auroral forms

2001 ◽  
Vol 19 (2) ◽  
pp. 205-217 ◽  
Author(s):  
S. E. Milan ◽  
M. Lester ◽  
N. Sato ◽  
H. Takizawa
Keyword(s):  
Doppler Shift ◽  
Spectral Characteristics ◽  
Coherent Scattering ◽  
Hf Radar ◽  
Type I ◽  
The North ◽  
Minute Period ◽  
E Region ◽  
Auroral Arcs ◽  
Auroral Luminosity

Abstract. Observations of E region backscatter by the Ice-land East SuperDARN HF radar from the 30 minute period 2330 to 2400 UT on 13 September 1999 are presented, along with simultaneous observations of auroral luminosity from two all-sky cameras. Interferometric techniques are employed to estimate the altitude of origin of each echo observed by the radar. Under investigation is a region of backscatter which is L-shell aligned and exists in a region of low auroral luminosity bounded to the north and the south by two auroral arcs. The spectral characteristics of the backscatter fall into three main populations: broad, low Doppler shift spectra; narrow, high Doppler shift spectra; and exceptionally narrow, low Doppler shift spectra. The first two populations are similar to type II and type I spectra observed with VHF radars, respectively. These populations scatter from near the peak of the E region. The high Doppler shift population appears to exist in a region of sub-critical electric field. The third population originates below the E region peak at altitudes between 80 and 100 km. We argue that a non-coherent scattering process is responsible for this backscatter.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities)

scholarly journals Effects of solar eclipse on the electrodynamical processes of the equatorial ionosphere: a case study during 11 August 1999 dusk time total solar eclipse over India

2002 ◽  
Vol 20 (12) ◽  
pp. 1977-1985 ◽  
Author(s):  
R. Sridharan ◽  
C. V. Devasia ◽  
N. Jyoti ◽  
Diwakar Tiwari ◽  
K. S. Viswanathan ◽  
...  
Keyword(s):  
Electric Fields ◽  
Solar Eclipse ◽  
Equatorial Ionosphere ◽  
Hf Radar ◽  
F Region ◽  
Large Enhancement ◽  
Temporal Structures ◽  
E Region ◽  
Field Lines ◽  
Electric Fields And Currents

Abstract. The effects on the electrodynamics of the equatorial E- and F-regions of the ionosphere, due to the occurrence of the solar eclipse during sunset hours on 11 August 1999, were investigated in a unique observational campaign involving ground based ionosondes, VHF and HF radars from the equatorial location of Trivandrum (8.5° N; 77° E; dip lat. 0.5° N), India. The study revealed the nature of changes brought about by the eclipse in the evening time E- and F-regions in terms of (i) the sudden intensification of a weak blanketing ES-layer and the associated large enhancement of the VHF backscattered returns, (ii) significant increase in h' F immediately following the eclipse and (iii) distinctly different spatial and temporal structures in the spread-F irregularity drift velocities as observed by the HF radar. The significantly large enhancement of the backscattered returns from the E-region coincident with the onset of the eclipse is attributed to the generation of steep electron density gradients associated with the blanketing ES , possibly triggered by the eclipse phenomena. The increase in F-region base height immediately after the eclipse is explained as due to the reduction in the conductivity of the conjugate E-region in the path of totality connected to the F-region over the equator along the magnetic field lines, and this, with the peculiar local and regional conditions, seems to have reduced the E-region loading of the F-region dynamo, resulting in a larger post sunset F-region height (h' F) rise. These aspects of E-and F-region behaviour on the eclipse day are discussed in relation to those observed on the control day.Key words. Ionosphere (electric fields and currents; equatorial ionosphere; ionospheric irregularities)

THE SPECTRAL CHARACTERISTICS AND TEMPORAL BEHAVIOR OF HF RADAR ECHOES FROM OVER-THE-HORIZON AIRCRAFT TARGETS

1965 ◽  
Author(s):  
J. R. Davis ◽  
J. F. Thomason ◽  
J. M. Hudnall ◽  
F. E. Boyd ◽  
F. H. Utley
Keyword(s):  
Spectral Characteristics ◽  
Hf Radar ◽  
Temporal Behavior ◽  
Radar Echoes

scholarly journals Simultaneous VHF radar backscatter and ionosonde observations of low-latitude E region

2005 ◽  
Vol 23 (3) ◽  
pp. 773-779 ◽  
Author(s):  
A. K. Patra ◽  
S. Sripathi ◽  
P. B. Rao ◽  
K. S. V. Subbarao
Keyword(s):  
Low Latitude ◽  
Vhf Radar ◽  
Radar Echoes ◽  
First Results ◽  
E Region ◽  
Gradient Drift ◽  
Drift Instability ◽  
Relationship Of ◽  
The Relationship ◽  
Field Aligned Irregularities

Abstract. The first results of simultaneous observations made on the low-latitude field-aligned irregularities (FAI) using the MST radar located at Gadanki (13.5° N, 79.2° E, dip 12.5°) and the Es parameters using an ionosonde at a nearby station Sriharikota (13.7° N, 80.1° E, dip 12.6°) are presented. The observations show that while the height of the most intense radar echoes is below the virtual height of Es (h'Es) during daytime, it is found to be either below or above during nighttime. The strength of the FAI is better correlated with the top penetration frequency (ftEs) and the blanketing frequency (fbEs) during the night (r=0.4 in both cases) as compared to the day (r=0.35 and -0.04, respectively). Furthermore, the signal strength of FAI is reasonably correlated with (ftEs-fbEs) during daytime (r=0.59) while very poorly correlated during nighttime (r=0.18). While the radar observations in general appear to have characteristics close to that of mid-latitudes, the relationship of these with the Es parameters are poorer than that of mid-latitudes. The observations reported here, nevertheless, are quite consistent with the expectations based on the gradient drift instability mechanism.

scholarly journals Investigation of the relationship between optical auroral forms and HF radar E region backscatter

2000 ◽  
Vol 18 (6) ◽  
pp. 608-617 ◽  
Author(s):  
S. E. Milan ◽  
M. Lester ◽  
N. Sato ◽  
H. Takizawa ◽  
J.-P. Villain
Keyword(s):  
Spectral Characteristics ◽  
Hf Radar ◽  
Vhf Radar ◽  
The Past ◽  
Optical Signature ◽  
E Region ◽  
Coherent Backscatter ◽  
First Time ◽  
The Relationship ◽  
East West

Abstract. The SuperDARN HF radars have been employed in the past to investigate the spectral characteristics of coherent backscatter from L-shell aligned features in the auroral E region. The present study employs all-sky camera observations of the aurora from Husafell, Iceland, and the two SuperDARN radars located on Iceland, Þykkvibær and Stokkseyri, to determine the optical signature of such backscatter features. It is shown that, especially during quiet geomagnetic conditions, the backscatter region is closely associated with east-west aligned diffuse auroral features, and that the two move in tandem with each other. This association between optical and radar aurora has repercussions for the instability mechanisms responsible for generating the E region irregularities from which radars scatter. This is discussed and compared with previous studies investigating the relationship between optical and VHF radar aurora. In addition, although it is known that E region backscatter is commonly observed by SuperDARN radars, the present study demonstrates for the first time that multiple radars can observe the same feature to extend over at least 3 h of magnetic local time, allowing precipitation features to be mapped over large portions of the auroral zone.Key words: Ionosphere (particle precipitation; plasma waves and instabilities)

scholarly journals Meridional motions of the afternoon radar aurora, auroral electrojets, and absorption patches under variable IMF conditions

2004 ◽  
Vol 22 (5) ◽  
pp. 1649-1664
Author(s):  
R. A. Makarevitch ◽  
F. Honary ◽  
A. V. Koustov ◽  
M. V. Uspensky
Keyword(s):  
Electric Fields ◽  
Temporal Correlation ◽  
Doppler Velocity ◽  
Plasma Convection ◽  
Motion Patterns ◽  
Aspect Angle ◽  
Radar Echoes ◽  
Meridional Motion ◽  
E Region ◽  
The Imf

Abstract. The meridional motions of the CUTLASS HF and STARE VHF coherent echoes, IMAGE equivalent electrojet currents, and IRIS absorption patches during the postnoon/early-evening event of 14 February 2000 are presented. The motions were found to be synchronous, to a first approximation, for all instruments. The temporal correlation between motions in the radar and magnetometer data was exceptionally good, although spatially the areas with the E-region backscatter and most intense equivalent currents were not coincident, with the HF (VHF) echoes being shifted 100–200km (20–50km) equatorward (poleward). The meridional motions of the radar echoes and electrojet currents appeared to be controlled by the IMF Bz changes; the meridional propagation direction was equatorward (poleward) during the intervals when the IMF was southward (northward), with one exception when the poleward progression continued after the IMF southward turning. We relate the observed meridional motion patterns to the polar cap expansion/contraction during variable IMF conditions and discuss the relative importance of two types of processes: the dayside reconnection and IMF-triggered substorms. We also investigate the irregularity Doppler velocity for the STARE (144MHz) and CUTLASS (12MHz) observations at large flow angles in the context of the eastward and westward electrojet systems. We show that the 144-MHz Doppler velocity is determined by a combination of two factors: the sense of electrojet currents and the aspect angle conditions within the STARE field of view. Finally, the behavior of small dayside enhancements of the IRIS absorption (up to 0.5dB at 38.2MHz) accompanying the radar echoes and electrojet currents is examined. Since the velocity of the meridional displacements was close to that of the poleward/equatorward progressing intense currents, it is suggested that the absorption patches observed during the event were related to the heating of the E-region plasma by the unstable plasma waves in the regions of enhanced electric fields. Key words. Ionosphere (auroral ionosphere; electric fields and currents; plasma convection)

scholarly journals CUTLASS HF radar observations of high-velocity E-region echoes

2001 ◽  
Vol 19 (4) ◽  
pp. 411-424 ◽  
Author(s):  
M. V. Uspensky ◽  
A. V. Koustov ◽  
P. Eglitis ◽  
A. Huuskonen ◽  
S. E. Milan ◽  
...  
Keyword(s):  
Electric Fields ◽  
High Velocity ◽  
Power Doppler ◽  
Spectral Width ◽  
Plasma Instability ◽  
Hf Radar ◽  
Doppler Velocity ◽  
Classical Type ◽  
E Region

Abstract. A short event of high-velocity E-region echo observations by the Pykkvibaer HF radar is analysed to study echo parameters and the echo relation to the Farley-Buneman plasma instability. The echoes were detected in several beams aligned closely to the magnetic L-shell direction. Two echo groups were identified: one group corresponded to the classical type 1 echoes with velocities close to the nominal ion-acoustic speed of 400 ms–1 , while the other group had significantly larger velocities, of the order of 700 ms–1 . The mutual relationship between the echo power, Doppler velocity, spectral width and elevation angles for these two groups was studied. Plotting of echo parameters versus slant range showed that all ~700 ms–1 echoes originated from larger heights and distances of 500–700 km, while all ~400 ms–1 echoes came from lower heights and from farther distances; 700–1000 km. We argue that both observed groups of echoes occurred due to the Farley-Buneman plasma instability excited by strong ( ~70 mVm–1 ) and uniformly distributed electric fields. We show that the echo velocities for the two groups were different because the echoes were received from different heights. Such a separation of echo heights occurred due to the differing amounts of ionospheric refraction at short and large ranges. Thus, the ionospheric refraction and related altitude modulation of ionospheric parameters are the most important factors to consider, when various characteristics of E-region decametre irregularities are derived from HF radar measurements.Key words. Ionosphere (ionospheric irregularities; plasma waves and instabilities; polar ionosphere)

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

1999 ◽  
Vol 17 (10) ◽  
pp. 1284-1292 ◽  
Author(s):  
R. Rüster ◽  
K. Schlegel
Keyword(s):  
Magnetic Field ◽  
Electric Fields ◽  
Middle Atmosphere ◽  
Magnetic Field Direction ◽  
Ionosphere Plasma ◽  
E Region ◽  
Strong Electric Fields ◽  
Gradient Drift ◽  
Drift Instability ◽  
The Magnetic Field

Abstract. Backscatter from E-region irregularities was observed at aspect angles close to 90° (almost parallel to the direction of the magnetic field) using the ALOMAR SOUSY radar at Andoya/Norway. Strong electric fields 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 different 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 field direction. We, therefore, conclude that the corresponding irregularities are not caused by the mentioned instabilities and that other mechanism have to be invoked.Key words. Ionosphere (plasma waves and instabilities; ionosphere irregularities; particle precipitaion) · Meteorology and atmospheric dynamics (middle atmosphere dynamics)

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