scholarly journals Large-scale imaging of high-latitude convection with Super Dual Auroral Radar Network HF radar observations

1998 ◽  
Vol 103 (A9) ◽  
pp. 20797-20811 ◽  
Author(s):  
J. M. Ruohoniemi ◽  
K. B. Baker
Keyword(s):  
High Latitude ◽  
Large Scale ◽  
Hf Radar ◽  
Radar Observations ◽  
Radar Network

scholarly journals A Statistical study of the Doppler spectral width of high-latitude ionospheric F-region echoes recorded with SuperDARN coherent HF radars

2002 ◽  
Vol 20 (11) ◽  
pp. 1769-1781 ◽  
Author(s):  
J.-P. Villain ◽  
R. André ◽  
M. Pinnock ◽  
R. A. Greenwald ◽  
C. Hanuise
Keyword(s):  
High Latitude ◽  
Statistical Study ◽  
Spectral Width ◽  
Auroral Oval ◽  
F Region ◽  
Radar Network ◽  
Period 2 ◽  
Time Period ◽  
High Latitude Ionosphere ◽  
Data Points

Abstract. The HF radars of the Super Dual Auroral Radar Network (SuperDARN) provide measurements of the E × B drift of ionospheric plasma over extended regions of the high-latitude ionosphere. We have conducted a statistical study of the associated Doppler spectral width of ionospheric F-region echoes. The study has been conducted with all available radars from the Northern Hemisphere for 2 specific periods of time. Period 1 corresponds to the winter months of 1994, while period 2 covers October 1996 to March 1997. The distributions of data points and average spectral width are presented as a function of Magnetic Latitude and Magnetic Local Time. The databases are very consistent and exhibit the same features. The most stringent features are: a region of very high spectral width, collocated with the ionospheric LLBL/cusp/mantle region; an oval shaped region of high spectral width, whose equator-ward boundary matches the poleward limit of the Holzworth and Meng auroral oval. A simulation has been conducted to evaluate the geometrical and instrumental effects on the spectral width. It shows that these effects cannot account for the observed spectral features. It is then concluded that these specific spectral width characteristics are the signature of ionospheric/magnetospheric coupling phenomena.Key words. Ionosphere (auroral ionosphere; ionosphere-magnetosphere interactions; ionospheric irregularities)

scholarly journals Mapping ionospheric backscatter measured by the SuperDARN HF radars – Part 2: Assessing SuperDARN virtual height models

2008 ◽  
Vol 26 (4) ◽  
pp. 843-852 ◽  
Author(s):  
T. K. Yeoman ◽  
G. Chisham ◽  
L. J. Baddeley ◽  
R. S. Dhillon ◽  
T. J. T. Karhunen ◽  
...  
Keyword(s):  
Large Scale ◽  
Elevation Angle ◽  
Field Measurements ◽  
Companion Paper ◽  
Hf Radar ◽  
Propagation Path ◽  
Angle Of Arrival ◽  
Virtual Height ◽  
F Region ◽  
Propagation Paths

Abstract. The Super Dual Auroral Radar Network (SuperDARN) network of HF coherent backscatter radars form a unique global diagnostic of large-scale ionospheric and magnetospheric dynamics in the Northern and Southern Hemispheres. Currently the ground projections of the HF radar returns are routinely determined by a simple rangefinding algorithm, which takes no account of the prevailing, or indeed the average, HF propagation conditions. This is in spite of the fact that both direct E- and F-region backscatter and 1½-hop E- and F-region backscatter are commonly used in geophysical interpretation of the data. In a companion paper, Chisham et al. (2008) have suggested a new virtual height model for SuperDARN, based on average measured propagation paths. Over shorter propagation paths the existing rangefinding algorithm is adequate, but mapping errors become significant for longer paths where the roundness of the Earth becomes important, and a correct assumption of virtual height becomes more difficult. The SuperDARN radar at Hankasalmi has a propagation path to high power HF ionospheric modification facilities at both Tromsø on a ½-hop path and SPEAR on a 1½-hop path. The SuperDARN radar at Þykkvibǽr has propagation paths to both facilities over 1½-hop paths. These paths provide an opportunity to quantitatively test the available SuperDARN virtual height models. It is also possible to use HF radar backscatter which has been artificially induced by the ionospheric heaters as an accurate calibration point for the Hankasalmi elevation angle of arrival data, providing a range correction algorithm for the SuperDARN radars which directly uses elevation angle. These developments enable the accurate mappings of the SuperDARN electric field measurements which are required for the growing number of multi-instrument studies of the Earth's ionosphere and magnetosphere.

scholarly journals Small-scale characteristics of extremely high latitude aurora

2009 ◽  
Vol 27 (9) ◽  
pp. 3335-3347 ◽  
Author(s):  
J. A. Cumnock ◽  
L. G. Blomberg ◽  
A. Kullen ◽  
T. Karlsson ◽  
Keyword(s):  
High Latitude ◽  
Large Scale ◽  
Small Scale ◽  
Strong Correlations ◽  
Polar Cap ◽  
Plasma Flows ◽  
Ionospheric Convection ◽  
Local Closure ◽  
Scale Characteristics

Abstract. We examine 14 cases of an interesting type of extremely high latitude aurora as identified in the precipitating particles measured by the DMSP F13 satellite. In particular we investigate structures within large-scale arcs for which the particle signatures are made up of a group of multiple distinct thin arcs. These cases are chosen without regard to IMF orientation and are part of a group of 87 events where DMSP F13 SSJ/4 measures emissions which occur near the noon-midnight meridian and are spatially separated from both the dawnside and duskside auroral ovals by wide regions with precipitating particles typical of the polar cap. For 73 of these events the high-latitude aurora consists of a continuous region of precipitating particles. We focus on the remaining 14 of these events where the particle signatures show multiple distinct thin arcs. These events occur during northward or weakly southward IMF conditions and follow a change in IMF By. Correlations are seen between the field-aligned currents and plasma flows associated with the arcs, implying local closure of the FACs. Strong correlations are seen only in the sunlit hemisphere. The convection associated with the multiple thin arcs is localized and has little influence on the large-scale convection. This also implies that the sunward flow along the arcs is unrelated to the overall ionospheric convection.

scholarly journals An Interpretation of the Distribution of Metre Wavelength Radio Emission

1968 ◽  
Vol 21 (2) ◽  
pp. 167 ◽  
Author(s):  
KW Yates
Keyword(s):  
High Latitude ◽  
Large Scale ◽  
Galactic Halo ◽  
Galactic Plane ◽  
Absolute Calibration ◽  
Fundamental Difficulty ◽  
The Galaxy ◽  
Calibration Accuracy ◽  
Limit Estimate ◽  
Spiral Arm

A recent 85 MHz survey of the southern sky had an absolute calibration accuracy and resolution comparable with a number of surveys made for the northern skies. By combining the results of these surveys in both hemispheres a complete sky map has been produced, and in this paper an analysis is made of the distribution of the medium and high latitude emission. A fundamental difficulty encountered is the identification and isolation of the spurs of emission projecting from the galactic plane. Two hypotheses are proposed. The first attributes the spurs to a large-scale feature associated with the galactic core and the remaining emission to a galactic halo. The second postulates the origin of the spurs within the local spiral arm, which is itself considered to contribute significantly to the high latitude background. An upper-limit estimate of the emissivity of the local arm is made from currently available independent data. Using this result a model local arm is proposed, which, together with an isotropic component from beyond the Galaxy and a small additional galactic component, explains the observed distribution.

scholarly journals Expansion and diversification of high-latitude radiolarian assemblages in the late Eocene linked to a cooling event in the Southwest Pacific

2015 ◽  
Vol 11 (4) ◽  
pp. 2977-3018 ◽  
Author(s):  
K. M. Pascher ◽  
C. J. Hollis ◽  
S. M. Bohaty ◽  
G. Cortese ◽  
R. M. McKay
Keyword(s):  
Oxygen Isotope ◽  
High Latitude ◽  
Large Scale ◽  
Middle Eocene ◽  
Late Eocene ◽  
Climate History ◽  
Southwest Pacific ◽  
Early Oligocene ◽  
Radiolarian Assemblages

Abstract. The Eocene was characterised by "greenhouse" climate conditions that were gradually terminated by a long-term cooling trend through the middle and late Eocene. This long-term trend was determined by several large-scale climate perturbations that culminated in a shift to "ice-house" climates at the Eocene–Oligocene Transition. Geochemical and micropaleontological proxies suggest that tropical-to-subtropical sea-surface temperatures persisted into the late Eocene in the high-latitude Southwest Pacific Ocean. Here, we present radiolarian microfossil assemblage and foraminiferal oxygen and carbon stable isotope data from Deep Sea Drilling Project (DSDP) Sites 277, 280, 281 and 283 from the middle Eocene to early Oligocene (~ 40–33 Ma) to identify oceanographic changes in the Southwest Pacific across this major transition in Earth's climate history. The Middle Eocene Climatic Optimum at ~ 40 Ma is characterised by a negative shift in foraminiferal oxygen isotope values and a radiolarian assemblage consisting of about 5 % of low latitude taxa Amphicraspedum prolixum group and Amphymenium murrayanum. In the early late Eocene at ~ 37 Ma, a positive oxygen isotope shift can be correlated to the Priabonian Oxygen Isotope Maximum (PrOM) event – a short-lived cooling event recognized throughout the Southern Ocean. Radiolarian abundance, diversity, and preservation increase during the middle of this event at Site 277 at the same time as diatoms. The PrOM and latest Eocene radiolarian assemblages are characterised by abundant high-latitude taxa. These high-latitude taxa also increase in abundance during the late Eocene and early Oligocene at DSDP Sites 280, 281 and 283 and are associated with very high diatom abundance. We therefore infer a~northward expansion of high-latitude radiolarian taxa onto the Campbell Plateau towards the end of the late Eocene. In the early Oligocene (~ 33 Ma) there is an overall decrease in radiolarian abundance and diversity at Site 277, and diatoms are absent. These data indicate that, once the Tasman Gateway was fully open in the early Oligocene, a frontal system similar to the present day was established, with nutrient-depleted subantarctic waters bathing the area around DSDP Site 277, resulting in a more oligotrophic siliceous plankton assemblage.

scholarly journals Intercomparison of snowfall estimates derived from the CloudSat Cloud Profiling Radar and the ground based weather radar network over Sweden

2015 ◽  
Vol 8 (8) ◽  
pp. 8157-8189
Author(s):  
L. Norin ◽  
A. Devasthale ◽  
T. S. L'Ecuyer ◽  
N. B. Wood ◽  
M. Smalley
Keyword(s):  
High Latitude ◽  
Precipitation Amount ◽  
Weather Radar ◽  
Probability Of Detection ◽  
Polar Regions ◽  
Rain Gauges ◽  
Radar Network ◽  
Skill Scores ◽  
Satellite Sensors ◽  
Observing Systems

Abstract. To be able to estimate snowfall accurately is important for both weather and climate applications. Ground-based weather radars and space-based satellite sensors are often used as viable alternatives to rain-gauges to estimate precipitation in this context. The Cloud Profiling Radar (CPR) onboard CloudSat is especially proving to be a useful tool to map snowfall globally, in part due to its high sensitivity to light precipitation and ability to provide near-global vertical structure. The importance of having snowfall estimates from CloudSat/CPR further increases in the high latitude regions as other ground-based observations become sparse and passive satellite sensors suffer from inherent limitations. Here we intercompared snowfall estimates from two observing systems, CloudSat and Swerad, the Swedish national weather radar network. Swerad offers one of the best calibrated data sets of precipitation amount at very high latitudes that are anchored to rain-gauges and that can be exploited to evaluate usefulness of CloudSat/CPR snowfall estimates in the polar regions. In total 7.2×105 matchups of CloudSat and Swerad over Sweden were inter-compared covering all but summer months (October to May) from 2008 to 2010. The intercomparison shows encouraging agreement between these two observing systems despite their different sensitivities and user applications. The best agreement is observed when CloudSat passes close to a Swerad station (46–82 km), when the observational conditions for both systems are comparable. Larger disagreements outside this range suggest that both platforms have difficulty with shallow snow but for different reasons. The correlation between Swerad and CloudSat degrades with increasing distance from the nearest Swerad station as Swerad's sensitivity decreases as a function of distance and Swerad also tends to overshoots low level precipitating systems further away from the station, leading to underestimation of snowfall rate and occasionally missing the precipitation altogether. Further investigations of various statistical metrics, such as the probability of detection, false alarm rate, hit rate, and the Hanssen–Kuipers skill scores, and the sensitivity of these metrics to snowfall rate and the distance from the radar station, were carried out. The results of these investigations highlight the strengths and the limitations of both observing systems at the lower and upper ends of snowfall distributions and the range of uncertainties that could be expected from these systems in the high latitude regions.

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