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

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)

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Simultaneous HF measurements of E- and F-region Doppler velocities at large flow angles

Annales Geophysicae ◽

10.5194/angeo-22-1177-2004 ◽

2004 ◽

Vol 22 (4) ◽

pp. 1177-1185 ◽

Cited By ~ 18

Author(s):

R. A. Makarevitch ◽

F. Honary ◽

A. V. Koustov

Keyword(s):

Hf Radar ◽

Doppler Velocity ◽

Plasma Convection ◽

Aspect Angle ◽

F Region ◽

Significant Difference ◽

E Region ◽

The Difference ◽

New Feature ◽

Large Flow

Abstract. Data collected by the CUTLASS Finland HF radar are used to illustrate the significant difference between the cosine component of the plasma convection in the F-region and the Doppler velocity of the E-region coherent echoes observed at large flow angles. We show that the E-region velocity is ~5 times smaller in magnitude and rotated by ~30° clockwise with respect to convection in the F-region. Also, measurements at flow angles larger than 90° exhibit a completely new feature: Doppler velocity increase with the expected aspect angle and spatial anticorrelation with the backscatter power. By considering DMSP drift-meter measurements we argue that the difference between F- and E-region velocities cannot be interpreted in terms of the convection change with latitude. The observed features in the velocity of the E-region echoes can be explained by taking into account the ion drift contribution to the irregularity phase velocity as predicted by the linear fluid theory. Key words. Ionosphere (auroral ionosphere; ionospheric irregularities; plasma convection)

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Particle-in-cell plasma simulations of the modified two-stream instability

Annales Geophysicae ◽

10.1007/s00585-994-1091-3 ◽

1994 ◽

Vol 12 (10/11) ◽

pp. 1091-1100 ◽

Cited By ~ 11

Author(s):

K. Schlegel ◽

H. Thiemann

Keyword(s):

Doppler Velocity ◽

Plasma Parameters ◽

Particle In Cell ◽

Aspect Angle ◽

Similar Work ◽

Cell Plasma ◽

E Region ◽

Unstable Plasma ◽

Stream Instability ◽

Good Agreement

Abstract. We model the modified two-stream plasma instability occurring in the ionospheric E-region using a 2.5-dimensional particle-in-cell code. Compared to previous similar work we concentrate on simulated quantities that can easily be measured in the real ionosphere by coherent radars or rockets, such as the Doppler velocity, the backscattered power, backscattered spectra, aspect angle behaviour and electron temperature enhancement. Despite using a relatively small simulation model, we obtain remarkably good agreement between actual observed and simulated plasma parameters. The advantage of such a small system is that we were able to perform (other than in previous related work) many simulation runs with different sets of input parameters, thus studying the unstable plasma under various conditions.

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

Annales Geophysicae ◽

10.5194/angeo-19-411-2001 ◽

2001 ◽

Vol 19 (4) ◽

pp. 411-424 ◽

Cited By ~ 10

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)

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Auroral E-region electron density height profile modificationby electric field driven vertical plasma transport:some evidence in EISCAT CP-1 data statistics

Annales Geophysicae ◽

10.5194/angeo-22-901-2004 ◽

2004 ◽

Vol 22 (3) ◽

pp. 901-910 ◽

Cited By ~ 2

Author(s):

T. Bösinger ◽

G. C. Hussey ◽

C. Haldoupis ◽

K. Schlegel

Keyword(s):

Electric Field ◽

Electron Density ◽

Electric Fields ◽

Plasma Transport ◽

Plasma Convection ◽

Ionosphere Plasma ◽

E Region ◽

Eiscat Radar ◽

Electric Fields And Currents ◽

Electron Density Profiles

Abstract. A model developed several years ago by Huuskonen et al. (1984) predicted that vertical transport of ions in the nocturnal auroral E-region ionosphere can shift the electron density profiles in altitude during times of sufficiently large electric fields. If the vertical plasma transport effect was to operate over a sufficiently long enough time, then the real height of the E-region electron maximum should be shifted some km upwards (downwards) in the eastward (westward) auroral electrojet, respectively, when the electric field is strong, exceeding, say, 50 mV/m. Motivated by these predictions and the lack of any experimental verification so far, we made use of the large database of the European Incoherent Scatter (EISCAT) radar to investigate if the anticipated vertical plasma transport is at work in the auroral E-region ionosphere and thus to test the Huuskonen et al. (1984) model. For this purpose a new type of EISCAT data display was developed which enabled us to order a large number of electron density height profiles, collected over 16 years of EISCAT operation, according to the electric field magnitude and direction as measured at the same time at the radar's magnetic field line in the F-region. Our analysis shows some signatures in tune with a vertical plasma transport in the auroral E-region of the type predicted by the Huuskonen et al. model. The evidence brought forward is, however, not unambiguous and requires more rigorous analysis. Key words. Ionosphere (auroral ionosphere; plasma convection; electric fields and currents)

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

Annales Geophysicae ◽

10.5194/angeo-20-795-2002 ◽

2002 ◽

Vol 20 (6) ◽

pp. 795-805 ◽

Cited By ~ 6

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)

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Assessing the contamination of SuperDARN global convection maps by non-F-region backscatter

Annales Geophysicae ◽

10.5194/angeo-20-13-2002 ◽

2002 ◽

Vol 20 (1) ◽

pp. 13-28 ◽

Cited By ~ 20

Author(s):

G. Chisham ◽

M. Pinnock

Keyword(s):

Doppler Velocity ◽

Plasma Convection ◽

Data Set ◽

Radio Science ◽

Ionosphere Plasma ◽

F Region ◽

Mesoscale Structure ◽

Mapping Process ◽

E Region ◽

Instruments And Techniques

Abstract. Global convection mapping using line-of-sight Doppler velocity data from the Super Dual Auroral Radar Network (SuperDARN) is now an accepted method of imaging high-latitude ionospheric convection. This mapping process requires that the flow measured by the radars is defined solely by the convection electric field. This is generally only true of radar backscatter from the ionospheric F-region. We investigate the extent to which the E-region and ground backscatter in the SuperDARN data set may be misidentified as F-region backscatter, and assess the contamination of global convection maps which results from the addition of this non-F-region backscatter. We present examples which highlight the importance of identifying this contamination, especially with regard to the mesoscale structure in the convection maps.Key words. Ionosphere (plasma convection) – Radio science (radio wave propagation; instruments and techniques)

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Numerical simulation of mid-latitude ionospheric <i>E</i>-region based on SEEK and SEEK-2 observations

Annales Geophysicae ◽

10.5194/angeo-23-2377-2005 ◽

2005 ◽

Vol 23 (7) ◽

pp. 2377-2384 ◽

Cited By ~ 7

Author(s):

T. Yokoyama ◽

M. Yamamoto ◽

S. Fukao ◽

T. Takahashi ◽

M. Tanaka

Keyword(s):

Numerical Simulation ◽

Electric Field ◽

Electric Fields ◽

Theoretical Models ◽

Vhf Radar ◽

Sounding Rockets ◽

Radar Echoes ◽

Polarization Electric Field ◽

E Region ◽

Sporadic E

Abstract. Observational campaigns of the mid-latitude ionospheric E-region with sounding rockets and ground-based instruments were conducted in 1996 (SEEK) and 2002 (SEEK-2). Both of them were successfully conducted to bring important findings about the mid-latitude E-region and quasi-periodic (QP) VHF radar echoes. The observational results in the SEEK and the SEEK-2 are compared with numerical simulations and discussed in this paper. While sporadic-E (Es)-layers are actually formed by the observed neutral wind, it is difficult for the constant wind shear to produce the sharp Es-layer gradient. However, once they are formed in the lower E-region, they cannot easily be dissipated by the simple diffusive motion. The polarization electric field, calculated under the condition at the rocket launch time, shows similar amplitude and structure to the measurement around the Es-layer altitude. The structure of the plasma density and the electric field above the Es-layer observed in the SEEK-2 showed a wave-like pattern up to an altitude of 150 km. Considering a mapping of the polarization electric field generated within the Es-layers, gravity waves are the possible source of the wave-like structure of the measured electric fields and sub-peaks of the electron density above the main Es-layers. Fluctuation of the measured magnetic field is reproduced by Hall or field-aligned current driven by the polarization electric field. The current theoretical models for QP echoes and the polarization electric field are basically verified by the discussion in this paper. Keywords. Ionospheric irregularities – Mid-latitude ionosphere – Numerical simulation studies

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

Annales Geophysicae ◽

10.5194/angeo-27-261-2009 ◽

2009 ◽

Vol 27 (1) ◽

pp. 261-278 ◽

Cited By ~ 9

Author(s):

B. A. Carter ◽

R. A. Makarevich

Keyword(s):

Geomagnetic Storms ◽

Wide Band ◽

Spectral Width ◽

Doppler Velocity ◽

Geometric Aspect ◽

Low Velocity ◽

Aspect Angle ◽

F Region ◽

Evening Sector ◽

E Region

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 storm-time 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.

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Plasma drift estimates from the Dynasonde: comparison with EISCAT measurements

Annales Geophysicae ◽

10.1007/s00585-998-1138-y ◽

1998 ◽

Vol 16 (10) ◽

pp. 1138-1143 ◽

Cited By ~ 10

Author(s):

K. J. F. Sedgemore ◽

J. W. Wright ◽

P. J. S. Williams ◽

G. O. L. Jones ◽

M. T. Rietveld

Keyword(s):

Drift Velocity ◽

Rate Of Change ◽

Plasma Convection ◽

Operating Modes ◽

Plasma Drift ◽

Ionosphere Plasma ◽

Radar Echoes ◽

E Region ◽

Convection Patterns ◽

Scattering Volume

Abstract. Modern ionosondes make almost simultaneous measurements of the time rate of change of phase path in different directions and at different heights. By combining these 'Doppler' measurements and angles of arrival of many such radar echoes it is possible to derive reliable estimates of plasma drift velocity for a defined scattering volume. Results from both multifrequency and kinesonde-mode soundings at 3-min resolution show that the Dynasonde-derived F-region drift velocity is in good agreement with EISCAT, despite data loss during intervals of 'blanketing' by intense E-region ionisation. It is clear that the Tromsø Dynasonde, employing standard operating modes, gives a reliable indication of overall convection patterns during quiet to moderately active conditions.Key words. Auroral ionosphere · Plasma convection · Instruments and techniques

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E-region echo characteristics governed by auroral arc electrodynamics

Annales Geophysicae ◽

10.5194/angeo-21-1567-2003 ◽

2003 ◽

Vol 21 (7) ◽

pp. 1567-1575 ◽

Cited By ~ 4

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)

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