In situ measurements of plasma irregularity growth in the cusp ionosphere

Oksavik, K.; Moen, J.; Lester, M.; Bekkeng, T. A.; Bekkeng, Jan Kenneth

doi:10.1029/2012ja017835

Cited by 66 publications

(95 citation statements)

References 77 publications

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“…In this study, only the linear growth rate of GDI was considered. Even though radar backscatter power and occurrence trends are often interpreted using the GDI growth rates in the linear regime (e.g., Oksavik et al, 2012;Moen et al, 2012), there are other factors that may affect echo power and occurrence. In the polar cap, HF radars rely on beam refraction through a dense ionosphere to observe field-aligned irregularities (Bristow et al, 2011;Koustov et al, 2014).…”

Section: Discussionmentioning

confidence: 99%

A modeling study of asymmetries in plasma irregularity characteristics near gradient reversals

Lamarche

Makarevich

2016

Ann. Geophys.

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Abstract. Asymmetries in plasma density irregularity generation between the leading and trailing edges of the largescale plasma density structures in the high-latitude ionosphere are investigated. A model is developed that evaluates the gradient-drift instability (GDI) growth rate differences across the gradient reversal that is applicable at all propagation directions and for the broad range of altitudes spanning the entire lower ionosphere. In particular, the model describes asymmetries that would be observed by an oblique scanning radar near density structures in the polar cap such as elongated polar patches. The dependencies on the relative orientations between the directions of the gradient reversal, plasma convection, and wave propagation are examined at different altitudinal regions. At all altitudes, the largest asymmetries are expected for observations along the gradient reversals, e.g., when an elongated structure is oriented along the radar boresight. The convection direction that results in the strongest asymmetries exhibits a strong dependence on the altitude, with the optimal convection being parallel to the gradient reversal in the E region, perpendicular to it in the F region, and at some angle between these extremes in the transitional region. Implications for observations of polar patches by oblique scanning radars within the Super Dual Auroral Radar Network are discussed. It is demonstrated that the wave propagation direction relative to the prevalent convection and gradient directions plays a critical role in controlling both the irregularity growth rate and its asymmetries near gradient reversals.

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Section: Discussionmentioning

confidence: 99%

A modeling study of asymmetries in plasma irregularity characteristics near gradient reversals

Lamarche

Makarevich

2016

Ann. Geophys.

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“…By using electron density measurements from this rocket experiment, Moen et al [] estimated the linear growth rate of the GDI process and claimed that meter‐scale irregularities in the cusp can be produced very quickly through the GDI process. Oksavik et al [] carried out more rigorous test of the GDI process as a generation mechanism of cusp irregularities and obtained similar results. In addition, Moen et al [] clearly demonstrated that kilometer‐scale density perturbations are first created by the structured soft particle precipitations in the cusp and subsequently the GDI process breaks these irregularities down to meter scales.…”

Section: Introductionmentioning

confidence: 82%

“…As mentioned above, we assume zero neutral wind in the calculation of the linear growth rate, which is, of course, not always the case. As pointed out in Oksavik et al [], the neutral wind speed in the F region polar cap may reach 60% of the ion drift speed during extended intervals of southward IMF [ Heelis et al , ]. If the neutral wind speed is 60% of the plasma flow speed, the growth rate will be reduced by a factor of 0.4 for the collisional case; thus, the corresponding growth time will increase by a factor of 2.5.…”

Section: Discussion and Summarymentioning

confidence: 99%

“…Finally, we compare the current observations in the central polar cap with those in the dayside cusp region. Moen et al [] and Oksavik et al [] have suggested that in the case of cusp irregularities, kilometer‐scale structures are first created by the soft particle precipitations and these irregularities are broken down to meter scales. In contrast, during the present interval, the scale size of the patches was about 150 km [ Hosokawa et al , ] and that of the secondary structure (i.e., undulations or fingers) ranged from 50 to 100 km.…”

Section: Discussion and Summarymentioning

confidence: 99%

“…Current‐convective instability (CCI) [ Chaturvedi and Ossakow , , ; Huba , ; Ossakow and Chaturvedi , ] is a version of GDI but it considers electric currents flowing along the magnetic field (i.e., field‐aligned currents) to produce polarization electric fields which amplify the primary density perturbation and generate irregularities. Another class of plasma instability, Kelvin‐Helmholtz instability (KHI), is sometimes employed to account for the generation of irregularities under the existence of shear in the background plasma flow especially near the dayside cusp [ Carlson et al , ; Oksavik et al , ]. Even without the plasma instabilities described above, structuring of polar cap patches may be possible if there are structured precipitations [ Walker et al , ] or turbulences [ Kelley and Kintner , ].…”

Section: Introductionmentioning

confidence: 99%

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Two‐dimensional direct imaging of structuring of polar cap patches

et al. 2013

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[1] A highly sensitive all-sky electron multiplier charge-coupled device airglow imager has been operative in Longyearbyen, Norway (78.1 ı N, 15.5 ı E), since October 2011. The imager obtains the 630.0 nm all-sky images with an exposure time of 4 s, which is about 10 times shorter than the conventional cooled CCD airglow imagers. This new equipment allows us to image the ongoing structuring of polar cap patches in 2-D fashion. Here we report a case in which faint undulations appeared along the trailing edge of patches propagating in the central polar cap. The separation between the fingers in the undulations was about 50-100 km and the e-folding time of their growth was 5 min. We suggest that the gradient-drift instability (GDI) is one of the possible generation mechanisms of the undulating structures. The reasons for this interpretation are (1) the asymmetry in the preference of structuring between the leading and trailing edges is qualitatively consistent with the GDI mechanism and (2) the linear growth rate of GDI calculated by using electron density estimates from simultaneous European Incoherent Scatter Svalbard radar observations is roughly consistent with the observed growth time of the fingers. Such "unstable polar cap patches" could be important sources of seed irregularities, which would eventually be broken down to smaller-scale density perturbations affecting the transionospheric satellite communications in the central polar cap.

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Direct evidence of double‐slope power spectra in the high‐latitude ionospheric plasma

Spicher

Miloch

Moen

2014

Geophysical Research Letters

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We report direct observations of the double‐slope power spectra for plasma irregularities in the F layer of the polar ionosphere. The investigation of cusp irregularities ICI‐2 sounding rocket, which was launched into the polar cusp ionosphere, intersected enhanced plasma density regions with decameter‐scale irregularities. Density measurements at unprecedented high resolution with multi‐Needle Langmuir Probes allowed for a detailed study of the plasma irregularities down to kinetic scales. Spectral analysis reveals double‐slope power spectra for regions of enhanced fluctuations associated mainly with density gradients, with the steepening of the spectra occurring close to the oxygen gyrofrequency. These findings are further supported with the first results from the ICI‐3 rocket, which flew through regions with strong precipitation and velocity shears. Previously, double‐slope spectra have been observed in the equatorial ionosphere. The present work gives a direct evidence that the double‐slope power spectra can be common in the high‐latitude ionosphere.

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In situ measurements of plasma irregularity growth in the cusp ionosphere

Cited by 66 publications

References 77 publications

A modeling study of asymmetries in plasma irregularity characteristics near gradient reversals

A modeling study of asymmetries in plasma irregularity characteristics near gradient reversals

Two‐dimensional direct imaging of structuring of polar cap patches

Direct evidence of double‐slope power spectra in the high‐latitude ionospheric plasma

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