Occurrence of F region echoes for the polar cap SuperDARN radars

Abstract Observations by six Super Dual Auroral Radar Network (SuperDARN) polar cap radars, three in the northern hemisphere and three in the southern hemispheres, are considered to assess F region echo occurrence rates over solar, season, and day cycles and to establish relationship between the echo occurrence rate and the background electron density and plasma flow velocity magnitude. The echo occurrence rate is shown to increase toward the solar cycle maximum, more distinctly on the nightside, consistent with a general trend of the background electron density. Over the last 5 years, the echo occurrence rates decline at a rate of 5–10% per year. The pattern of seasonal and diurnal variations in echo occurrence is found to be consistent with previous SuperDARN publications. Minor dips in echo occurrence rate are observed in winter solstices, and these are related to an overall decrease in the electron density. In most of the time sectors, the echo occurrence rate increases with the electron density but only up to a certain threshold value after which the dependence saturates. The level of the saturation depends on season, local time, and average plasma flow velocity magnitude. For the summer daytime observations, the echo occurrence rate correlates with variations of both electron density and plasma flow velocity magnitude.

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Polar patches observed by ESR and their possible origin in the cusp region

Annales Geophysicae ◽

10.1007/s00585-000-1043-5 ◽

2000 ◽

Vol 18 (9) ◽

pp. 1043-1053 ◽

Cited By ~ 12

Author(s):

A. M. Smith ◽

S. E. Pryse ◽

L. Kersley

Keyword(s):

Energy Distribution ◽

Electron Density ◽

Plasma Flow ◽

Plasma Temperature ◽

Soft Particle ◽

Polar Ionosphere ◽

Polar Cap ◽

Particle Precipitation ◽

Cusp Region ◽

Reconnection Site

Abstract. Observations by the EISCAT Svalbard radar in summer have revealed electron density enhancements in the magnetic noon sector under conditions of IMF Bz southward. The features were identified as possible candidates for polar-cap patches drifting anti-Sunward with the plasma flow. Supporting measurements by the EISCAT mainland radar, the CUTLASS radar and DMSP satellites, in a multi-instrument study, suggested that the origin of the structures lay upstream at lower latitudes, with the modulation in density being attributed to variability in soft-particle precipitation in the cusp region. It is proposed that the variations in precipitation may be linked to changes in the location of the reconnection site at the magnetopause, which in turn results in changes in the energy distribution of the precipitating particles.Key words: Ionosphere (ionosphere-magnetosphere interactions; plasma temperature and density; polar ionosphere)

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On the factors controlling occurrence of F-region coherent echoes

Annales Geophysicae ◽

10.5194/angeo-20-1385-2002 ◽

2002 ◽

Vol 20 (9) ◽

pp. 1385-1397 ◽

Cited By ~ 23

Author(s):

D. W. Danskin ◽

A. V. Koustov ◽

T. Ogawa ◽

N. Nishitani ◽

S. Nozawa ◽

...

Keyword(s):

Density Distribution ◽

Radio Wave ◽

Electron Density ◽

Electron Density Distribution ◽

Propagation Path ◽

Occurrence Rate ◽

Wave Refraction ◽

F Region ◽

D Region ◽

Field Lines

Abstract. Several factors are known to control the HF echo occurrence rate, including electron density distribution in the ionosphere (affecting the propagation path of the radar wave), D-region radio wave absorption, and ionospheric irregularity intensity. In this study, we consider 4 days of CUTLASS Finland radar observations over an area where the EISCAT incoherent scatter radar has continuously monitored ionospheric parameters. We illustrate that for the event under consideration, the D-region absorption was not the major factor affecting the echo appearance. We show that the electron density distribution and the radar frequency selection were much more significant factors. The electron density magnitude affects the echo occurrence in two different ways. For small F-region densities, a minimum value of 1 × 1011 m-3 is required to have sufficient radio wave refraction so that the orthogonality (with the magnetic field lines) condition is met. For too large densities, radio wave strong "over-refraction" leads to the ionospheric echo disappearance. We estimate that the over-refraction is important for densities greater than 4 × 1011 m-3. We also investigated the backscatter power and the electric field magnitude relationship and found no obvious relationship contrary to the expectation that the gradient-drift plasma instability would lead to stronger irregularity intensity/echo power for larger electric fields.Key words. Ionosphere (ionospheric irregularities; plasma waves and instabilities; auroral ionosphere)

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Seasonal and diurnal variations of PolarDARN F region echo occurrence in the polar cap and their causes

Journal of Geophysical Research Space Physics ◽

10.1002/2014ja020726 ◽

2014 ◽

Vol 119 (12) ◽

Cited By ~ 10

Author(s):

M. Ghezelbash ◽

A. V. Koustov ◽

D. R. Themens ◽

P. T. Jayachandran

Keyword(s):

Diurnal Variations ◽

Polar Cap ◽

F Region ◽

Seasonal And Diurnal Variations

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Statistical Analysis of the Electron Density Gradients in the Polar Cap F Region Using the Resolute Bay Incoherent Scatter Radar North

Journal of Geophysical Research Space Physics ◽

10.1029/2017ja025156 ◽

2018 ◽

Vol 123 (5) ◽

pp. 4066-4079 ◽

Cited By ~ 3

Author(s):

Victoriya V. Forsythe ◽

Roman A. Makarevich

Keyword(s):

Statistical Analysis ◽

Electron Density ◽

Density Gradients ◽

Polar Cap ◽

Incoherent Scatter Radar ◽

Incoherent Scatter ◽

F Region ◽

Scatter Radar ◽

Resolute Bay

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On the possible role of cusp/cleft precipitation in the formation of polar-cap patches

Annales Geophysicae ◽

10.1007/s00585-999-1298-4 ◽

1999 ◽

Vol 17 (10) ◽

pp. 1298-1305 ◽

Cited By ~ 40

Author(s):

I. K. Walker ◽

J. Moen ◽

L. Kersley ◽

D. A. Lorentzen

Keyword(s):

Electron Density ◽

Source Region ◽

Convection Flow ◽

Substantial Part ◽

Soft Particle ◽

Polar Cap ◽

Particle Precipitation ◽

Flow Measurements ◽

F Region

Abstract. The work describes experimental observations of enhancements in the electron density of the ionospheric F-region created by cusp/cleft particle precipitation at the dayside entry to the polar-cap convection flow. Measurements by meridian scanning photometer and all-sky camera of optical red-line emissions from aurora are used to identify latitudinally narrow bands of soft-particle precipitation responsible for structured enhancements in electron density determined from images obtained by radio tomography. Two examples are discussed in which the electron density features with size scales and magnitudes commensurate with those of patches are shown to be formed by precipitation at the entry region to the anti-sunward flow. In one case the spectrum of the incoming particles results in ionisation being created, for the most part below 250 km, so that the patch will persist only for minutes after convecting away from the auroral source region. However in a second example, at a time when the plasma density of the solar wind was particularly high, a substantial part of the particle-induced enhancement formed above 250 km. It is suggested that, with the reduced recombination loss in the upper F-region, this structure will retain form as a patch during passage in the anti-sunward flow across the polar cap.Key words. Ionosphere (ionospheric irregularities; particle precipitation; polar ionosphere)

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VHF scintillations, orientation of the anisotropy of F-region irregularities and direction of plasma convection in the polar cap

Annales Geophysicae ◽

10.5194/angeo-26-1725-2008 ◽

2008 ◽

Vol 26 (7) ◽

pp. 1725-1730 ◽

Cited By ~ 3

Author(s):

E. D. Tereshchenko ◽

N. Yu. Romanova ◽

A. V. Koustov

Keyword(s):

Magnetic Field ◽

Plasma Flow ◽

Flow Direction ◽

Ionospheric Irregularities ◽

Plasma Convection ◽

Polar Cap ◽

Data Set ◽

Multiple Peaks ◽

F Region ◽

The Magnetic Field

Abstract. Scintillation data recorded at the polar cap station Barentsburg are shown to occasionally exhibit two or more peaks in the latitudinal profiles of the amplitude dispersion. Comparison with concurrent SuperDARN radar convection maps indicates that multiple peaks occur when Barentsburg is located within the area of strong changes in the plasma flow direction. When parameters of the ionospheric irregularities are inferred from the scintillation data, the orientation of the irregularity anisotropy in a plane perpendicular to the magnetic field is found to coincide well with the E×B flow direction, individually for each peak of the scintillation data. The differences were found to be mostly less than 20° for a data set comprised of 104 events. The conclusion is made that analysis of scintillation data allows one to infer the direction of plasma flow with a certain degree of detail.

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Imaging riometer observations of drifting absorption patches in the morning sector

Annales Geophysicae ◽

10.5194/angeo-22-3461-2004 ◽

2004 ◽

Vol 22 (10) ◽

pp. 3461-3478 ◽

Cited By ~ 9

Author(s):

R. A. Makarevitch ◽

F. Honary ◽

I. W. McCrea ◽

V. S. C. Howells

Keyword(s):

Plasma Flow ◽

Magnetic Flux Tube ◽

Ion Drift ◽

Velocity Magnitude ◽

F Region ◽

Noise Absorption ◽

Morning Sector ◽

Magnetic Perturbations ◽

Intensity Images ◽

Cosmic Noise

Abstract. Observations by a 7x7-beam imaging riometer in Kilpisjärvi, Finland (~66° MLAT) of the drifting cosmic noise absorption (CNA) structures in the morning sector near the zonal drift reversals are presented. The examination of the absorption intensity images revealed several regions with enhanced CNA (absorption patches) slowly drifting through the riometer field of view (FoV). The absorption patches were found to vary in shape, orientation (for elongated arc-like patches), and drift direction. The latter was calculated from the regression lines to positions of the absorption maxima in the FoV images and compared with the direction of electrojet plasma flow from horizontal magnetic perturbations and (for one event) tristatic ion drift velocities in the F-region. A reasonable agreement was found between these directions both in point-by-point comparisons and in terms of direction reversal timings. The absorption patches of lower intensity appear to have smaller drift velocities and to be associated with weaker magnetic perturbations. These results are consistent with the notion that relatively slow motions of the auroral absorption near the zonal drift reversals are associated with the drift of the entire magnetic flux tube as opposed to the gradient-curvature drift of energetic electrons injected into the ionosphere at the substorm onset. The absorption drift velocity magnitude, on the other hand, was found to be substantially lower than that of the plasma flow based on the results of limited comparison with tristatic ion drift measurements. A comparison of the drift directions with those of the patch elongation showed that a considerable number of patches had these directions close to each other. Using this observation, we demonstrate a satisfactory agreement between the patch drift velocities (both in direction and magnitude) as determined from the absorption images and keograms under the assumption that some patches were propagating in a direction that was significantly different from the perpendicularity to elongation.

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A comparison of optical and coherent HF radar backscatter observations of a post-midnight aurora

Annales Geophysicae ◽

10.1007/s00585-997-1388-0 ◽

1997 ◽

Vol 15 (11) ◽

pp. 1388-1398 ◽

Cited By ~ 2

Author(s):

S. E. Milan ◽

M. Lester ◽

J. Moen

Keyword(s):

Flow Velocity ◽

Plasma Flow ◽

Hf Radar ◽

Polar Cap ◽

Common Features ◽

Optical Feature ◽

Radar Measurements ◽

Coherent Backscatter ◽

East West ◽

Poleward Expansion

Abstract. A poleward-progressing 630 nm optical feature is observed between approximately 0100 UT and 0230 UT (0400 MLT to 0530 MLT) by a meridian-scanning photometer (MSP) located at Ny Ålesund, Svalbard. Simultaneous coherent HF radar measurements indicate a region of poleward-expanding backscatter with rapid sunward plasma flow velocity along the MSP meridian. Spatial maps of the backscatter indicate a stationary backscatter feature aligned obliquely with respect to the MSP meridian, which produces an impression of poleward-expansion as the MSP progresses to later MLT. Two interpretations of the observations are possible, depending on whether the arc system is considered to move (time-dependent) or to be stationary in time and apparent motion is produced as the MSP meridian rotates underneath it (time-independent). The first interpretation is as a poleward motion of an east-west aligned auroral arc. In this case the appearance of the region of backscatter is not associated with the optical feature, though the velocities within it are enhanced when the two are co-located. The second interpretation is as a polar arc or theta aurora, common features of the polar cap under the prevailing IMF northwards conditions. In this case the backscatter appears as an approximately 150 km wide region adjacent to the optical arc. In both interpretations the luminosity of the optical feature appears related to the magnitude of the plasma flow velocity. The optical features presented here do not generate appreciable HF coherent backscatter, and are only identifiable in the backscatter data as a modification of the flow by the arc electrodynamics.

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Observations of AGW/TID propagation across the polar cap: a case study

Annales Geophysicae ◽

10.5194/angeo-29-1355-2011 ◽

2011 ◽

Vol 29 (8) ◽

pp. 1355-1363 ◽

Cited By ~ 11

Author(s):

H. T. Cai ◽

F. Yin ◽

S. Y. Ma ◽

I. W. McCrea

Keyword(s):

Large Scale ◽

Source Region ◽

Polar Cap ◽

Night Time ◽

F Region ◽

Atmospheric Disturbances ◽

Geomagnetic Pole ◽

Ionization Waves ◽

North Polar ◽

Propagation Properties

Abstract. In this paper, we present observational evidence for the trans-polar propagation of large-scale Traveling Ionospheric Disturbances (TIDs) from their nightside source region to the dayside. On 13 February 2001, the 32 m dish of EISCAT Svalbard Radar (ESR) was directing toward the geomagnetic pole at low elevation (30°) during the interval 06:00–12:00 UT (MLT ≈ UT + 3 h), providing an excellent opportunity to monitor the ionosphere F-region over the polar cap. The TIDs were first detected by the ESR over the dayside north polar cap, propagating equatorward, and were subsequently seen by the mainland UHF radar at auroral latitudes around geomagnetic local noon. The propagation properties of the observed ionization waves suggest the presence of a moderately large-scale TIDs, propagating across the northern polar cap from the night-time auroral source during substorm conditions. Our results agree with the theoretical simulations by Balthazor and Moffett (1999) in which poleward-propagating large-scale traveling atmospheric disturbances were found to be self-consistently driven by enhancements in auroral heating.

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