The Experts below are selected from a list of 294 Experts worldwide ranked by ideXlab platform
Gareth Chisham - One of the best experts on this subject based on the ideXlab platform.
-
A statistical comparison of SuperDARN Spectral Width boundaries and DMSP particle precipitation boundaries in the morning sector ionosphere
Annales Geophysicae, 2005Co-Authors: Gareth Chisham, M. Lester, Mervyn P. Freeman, Thomas Sotirelis, R. A. Greenwald, J.-p. VillainAbstract:Determining reliable proxies for the ionospheric signature of the open-closed field line boundary (OCB) is crucial for making accurate ionospheric measurements of many magnetospheric processes (e.g. magnetic reconnection). This study compares the latitudes of Spectral Width Boundaries (SWBs), identified in the morning sector ionosphere using the Super Dual Auroral Radar Network (SuperDARN), with Particle Precipitation Boundaries (PPBs) determined using the low-altitude Defense Meteorological Satellite Program (DMSP) spacecraft, in order to determine whether the SWB represents a good proxy for the ionospheric projection of the OCB. The latitudes of SWBs and PPBs were identified using automated algorithms applied to 5 years (1997-2001) of data measured in the 00:00-12:00 Magnetic Local Time (MLT) range. A latitudinal difference was measured between each PPB and the nearest SWB within a ±10min Universal Time (UT) window and within a ±1h MLT window. The results show that the SWB represents a good proxy for the OCB close to midnight (~00:00-02:00 MLT) and noon (~08:00-12:00 MLT), but is located some distance (~2°-4°) equatorward of the OCB across much of the morning sector ionosphere (~02:00-08:00 MLT). On the basis of this and other studies we deduce that the SWB is correlated with the poleward boundary of auroral emissions in the Lyman-Birge-Hopfield ``Long" (LBHL) UV emission range and hence, that Spectral Width is inversely correlated with the energy flux of precipitating electrons. We further conclude that the combination of two factors may explain the spatial distribution of Spectral Width values in the polar ionospheres. The small-scale structure of the convection electric field leads to an enhancement in Spectral Width in regions close to the OCB, whereas increases in ionospheric conductivity (relating to the level of incident electron energy flux) lead to a reduction in Spectral Width in regions just equatorward of the OCB.
-
On the probability distributions of SuperDARN Doppler Spectral Width measurements inside and outside the cusp
Geophysical Research Letters, 2004Co-Authors: Mervyn P. Freeman, Gareth ChishamAbstract:The Doppler Spectral Width of backscatter from the SuperDARN HF radars has been routinely used to identify the ionospheric cusp. The probability distribution of Spectral Width values has been described as Gaussian ( normal) in the cusp and exponential equatorward of it. Here we re-examine the empirical distributions and show that they are better described by a log-Levy distribution in both the cusp and non-cusp regions, with the same Levy index a approximate to 1.85 but varying Levy location parameter. The Spectral Width distribution may also be log-Levy at other magnetic local times, but with generally different parameters.
-
An investigation of latitudinal transitions in the SuperDARN Doppler Spectral Width parameter at different magnetic local times
Annales Geophysicae, 2004Co-Authors: Gareth Chisham, Mervyn P. FreemanAbstract:Abstract. Latitudinal transitions from low to high Doppler Spectral Width in backscatter measured by the Super Dual Auroral Radar Network (SuperDARN) are now routinely used as proxies for the polar cap boundary (PCB) in the cusp-region ionosphere. In this paper we perform a statistical study of the nature of similar Spectral Width transitions at other magnetic local times (MLTs). This analysis illustrates that these latitudinal Spectral Width transitions exist at all magnetic local times, and that the latitude, gradient, and amplitude of the transitions vary systematically with MLT. In particular, the probability of a transition occurring at any latitude, identified independently in each MLT sector, is continuous with MLT from the cusp, through the morning sector, to the nightside. This suggests that the transition represents the PCB, as this is known to be what it represents in the cusp region. However, the picture in the afternoon sector (12:00-18:00 MLT) is more complex with no clearly preferred transition latitudes. Key words. Ionosphere (ionosphere-magnetosphere interactions; instruments and techniques). Magnetospheric physics (magnetopause, cusp, and boundary layers.)
-
A statistical comparison of SuperDARN Spectral Width boundaries and DMSP particle precipitation boundaries in the nightside ionosphere: NIGHTSIDE Spectral Width BOUNDARIES
Geophysical Research Letters, 2004Co-Authors: Gareth Chisham, M. P. Freeman, Thomas SotirelisAbstract:The boundary between quasi-dipolar (closed) geomagnetic field lines and those connected to the interplanetary magnetic field (open) is a key diagnostic for the magnetospheric system. This study presents an HF radar technique for determining the location of this boundary in the nightside ionosphere and calibrates it with the boundary determined from simultaneous particle precipitation data. The latitudes of Spectral Width boundaries (SWBs) identified from 5 years (1997-2001) of data from the Halley HF radar of the Super Dual Auroral Radar Network (SuperDARN), in the range 1800-0200 magnetic local time (MLT), have been compared with nightside particle precipitation boundaries (PPBs) identified from measurements from the Defense Meteorological Satellite Program (DMSP) spacecraft. Latitudinal differences were measured between the PPBs and the nearest SWB within a +/-10 min UT window and within a +/-1 hr MLT window. These differences were then organised into 2-hour MLT sectors. The statistical distributions of the latitudinal differences show that the nightside SWB correlates best with the b6 PPB, which is a good marker for the open-closed field line boundary (OCB) in the nightside ionosphere. When considering SWBs determined using different Spectral Width threshold values, those determined using the lower threshold values (
B. Carissimo - One of the best experts on this subject based on the ideXlab platform.
-
TURBULENT DISSIPATION RATE IN THE BOUNDARY LAYER VIA UHF WIND PROFILER DOPPLER Spectral Width MEASUREMENTS
Boundary-Layer Meteorology, 2002Co-Authors: Sandra Jacoby-koaly, Bernard Campistron, S. Bernard, B. Bénech, F. Ardhuin-girard, J. Dessens, E. Dupont, B. CarissimoAbstract:The study is focused on the retrieval and validation of the turbulent kinetic energy dissipation rate efrom Spectral Width measurements made by a UHF wind profiler in the convective atmospheric boundary layer. The possibility to deduce e, which is one of the most important parameters for boundary-layer study and monitoring, from Doppler radar Spectral Width measurements has a firm theoretical basis established by numerous earlier works. However, the major drawback of this approach lies in various meteorological and instrumental sources of non-turbulent Spectral Width broadening which have to be recognised and accounted for. In the first part of the study, the theoretical background of the e retrieval is detailed and all possible causes of Spectral broadening are listed and evaluated. In the second part, the method is applied to four days of UHF diurnal boundary-layer observations, collected during the TRAC-98 experiment, for which in situ aircraft e measurements were available. Comparison between radar-retrieved eand in situ aircraft measurements yields a fairly good agreement with a linear correlation coefficient of about 0.9 and a residual bias less than 2 × 10-4 m2 s-3. The analysis of ederived from vertical and off-zenith observation leads to the recommendation, that in the boundary layer, where the wind is usually moderate, data collected by off-zenith beams should be used. Indeed, the measurement of the vertical Spectral Width, less affected by the large-scale broadening factor, can still be altered by the ground clutter removal.
Thomas Sotirelis - One of the best experts on this subject based on the ideXlab platform.
-
A statistical comparison of SuperDARN Spectral Width boundaries and DMSP particle precipitation boundaries in the morning sector ionosphere
Annales Geophysicae, 2005Co-Authors: Gareth Chisham, M. Lester, Mervyn P. Freeman, Thomas Sotirelis, R. A. Greenwald, J.-p. VillainAbstract:Determining reliable proxies for the ionospheric signature of the open-closed field line boundary (OCB) is crucial for making accurate ionospheric measurements of many magnetospheric processes (e.g. magnetic reconnection). This study compares the latitudes of Spectral Width Boundaries (SWBs), identified in the morning sector ionosphere using the Super Dual Auroral Radar Network (SuperDARN), with Particle Precipitation Boundaries (PPBs) determined using the low-altitude Defense Meteorological Satellite Program (DMSP) spacecraft, in order to determine whether the SWB represents a good proxy for the ionospheric projection of the OCB. The latitudes of SWBs and PPBs were identified using automated algorithms applied to 5 years (1997-2001) of data measured in the 00:00-12:00 Magnetic Local Time (MLT) range. A latitudinal difference was measured between each PPB and the nearest SWB within a ±10min Universal Time (UT) window and within a ±1h MLT window. The results show that the SWB represents a good proxy for the OCB close to midnight (~00:00-02:00 MLT) and noon (~08:00-12:00 MLT), but is located some distance (~2°-4°) equatorward of the OCB across much of the morning sector ionosphere (~02:00-08:00 MLT). On the basis of this and other studies we deduce that the SWB is correlated with the poleward boundary of auroral emissions in the Lyman-Birge-Hopfield ``Long" (LBHL) UV emission range and hence, that Spectral Width is inversely correlated with the energy flux of precipitating electrons. We further conclude that the combination of two factors may explain the spatial distribution of Spectral Width values in the polar ionospheres. The small-scale structure of the convection electric field leads to an enhancement in Spectral Width in regions close to the OCB, whereas increases in ionospheric conductivity (relating to the level of incident electron energy flux) lead to a reduction in Spectral Width in regions just equatorward of the OCB.
-
A statistical comparison of SuperDARN Spectral Width boundaries and DMSP particle precipitation boundaries in the nightside ionosphere: NIGHTSIDE Spectral Width BOUNDARIES
Geophysical Research Letters, 2004Co-Authors: Gareth Chisham, M. P. Freeman, Thomas SotirelisAbstract:The boundary between quasi-dipolar (closed) geomagnetic field lines and those connected to the interplanetary magnetic field (open) is a key diagnostic for the magnetospheric system. This study presents an HF radar technique for determining the location of this boundary in the nightside ionosphere and calibrates it with the boundary determined from simultaneous particle precipitation data. The latitudes of Spectral Width boundaries (SWBs) identified from 5 years (1997-2001) of data from the Halley HF radar of the Super Dual Auroral Radar Network (SuperDARN), in the range 1800-0200 magnetic local time (MLT), have been compared with nightside particle precipitation boundaries (PPBs) identified from measurements from the Defense Meteorological Satellite Program (DMSP) spacecraft. Latitudinal differences were measured between the PPBs and the nearest SWB within a +/-10 min UT window and within a +/-1 hr MLT window. These differences were then organised into 2-hour MLT sectors. The statistical distributions of the latitudinal differences show that the nightside SWB correlates best with the b6 PPB, which is a good marker for the open-closed field line boundary (OCB) in the nightside ionosphere. When considering SWBs determined using different Spectral Width threshold values, those determined using the lower threshold values (
M. Lester - One of the best experts on this subject based on the ideXlab platform.
-
A statistical comparison of SuperDARN Spectral Width boundaries and DMSP particle precipitation boundaries in the morning sector ionosphere
Annales Geophysicae, 2005Co-Authors: Gareth Chisham, M. Lester, Mervyn P. Freeman, Thomas Sotirelis, R. A. Greenwald, J.-p. VillainAbstract:Determining reliable proxies for the ionospheric signature of the open-closed field line boundary (OCB) is crucial for making accurate ionospheric measurements of many magnetospheric processes (e.g. magnetic reconnection). This study compares the latitudes of Spectral Width Boundaries (SWBs), identified in the morning sector ionosphere using the Super Dual Auroral Radar Network (SuperDARN), with Particle Precipitation Boundaries (PPBs) determined using the low-altitude Defense Meteorological Satellite Program (DMSP) spacecraft, in order to determine whether the SWB represents a good proxy for the ionospheric projection of the OCB. The latitudes of SWBs and PPBs were identified using automated algorithms applied to 5 years (1997-2001) of data measured in the 00:00-12:00 Magnetic Local Time (MLT) range. A latitudinal difference was measured between each PPB and the nearest SWB within a ±10min Universal Time (UT) window and within a ±1h MLT window. The results show that the SWB represents a good proxy for the OCB close to midnight (~00:00-02:00 MLT) and noon (~08:00-12:00 MLT), but is located some distance (~2°-4°) equatorward of the OCB across much of the morning sector ionosphere (~02:00-08:00 MLT). On the basis of this and other studies we deduce that the SWB is correlated with the poleward boundary of auroral emissions in the Lyman-Birge-Hopfield ``Long" (LBHL) UV emission range and hence, that Spectral Width is inversely correlated with the energy flux of precipitating electrons. We further conclude that the combination of two factors may explain the spatial distribution of Spectral Width values in the polar ionospheres. The small-scale structure of the convection electric field leads to an enhancement in Spectral Width in regions close to the OCB, whereas increases in ionospheric conductivity (relating to the level of incident electron energy flux) lead to a reduction in Spectral Width in regions just equatorward of the OCB.
-
Interhemispheric comparison of Spectral Width boundary as observed by SuperDARN radars
Annales Geophysicae, 2003Co-Authors: K. Hosokawa, Emma Woodfield, M. Lester, S. E. Milan, A. S. Yukimatu, Noriaki K. Sato, T. IyemoriAbstract:Abstract. Previous studies have shown that dayside equatorward edge of coherent HF radar backscatter having broad Doppler Spectral Width is coincident with the equatorward edge of the cusp particle precipitation. This enables the boundary between broad and narrow Spectral Width backscatters (Spectral Width boundary) in the dayside magnetic local time sector to be used as a proxy for the open/closed field line boundary. The present case study employs magnetically conjugate SuperDARN coherent HF radars to make an inter-hemispheric comparison of the location and variation of the Spectral Width boundaries. Agreement between the magnetic latitudes of the boundaries in both hemispheres is remarkable. Correlation coefficients between the latitudes of the boundaries are larger than 0.70. Temporal variation of the Spectral Width boundary follows the same equatorward trend in both hemispheres. This is consistent with the accumulation of open flux in the polar cap by dayside low-latitude magnetopause reconnection, expected when IMF Bz is negative. Boundaries in both hemispheres also exhibit short-lived poleward motions superposed on the general equator-ward trend, which follows the onset of substorm expansion phase and a temporary northward excursion of IMF Bz during substorm recovery phase. There is an interhemispheric difference in response time to the substorm occurrence between two hemispheres. The Spectral Width boundary in the Southern Hemisphere starts to move poleward 10 min earlier than that in the Northern Hemisphere. We discuss this difference in terms of interhemispheric asymmetry of the substorm breakup region in the longitudinal direction associated with the effect of IMF By. Key words. Ionosphere (ionosphere-magnetosphere interactions; plasma convection) – Magnetospheric physics (magnetopause, cusp, boundary layers)
-
Interhemispheric comparison of Spectral Width boundary as observed by SuperDARN radars
Annales Geophysicae, 2003Co-Authors: K. Hosokawa, E. E. Woodfield, M. Lester, S. E. Milan, N. Sato, A. S. Yukimatu, T. IyemoriAbstract:Previous studies have shown that dayside equatorward edge of coherent HF radar backscatter having broad Doppler Spectral Width is coincident with the equatorward edge of the cusp particle precipitation. This enables the boundary between broad and narrow Spectral Width backscatters (Spectral Width boundary) in the dayside magnetic local time sector to be used as a proxy for the open/closed field line boundary. The present case study employs magnetically conjugate SuperDARN coherent HF radars to make an inter-hemispheric comparison of the location and variation of the Spectral Width boundaries. Agreement between the magnetic latitudes of the boundaries in both hemispheres is remarkable. Correlation coefficients between the latitudes of the boundaries are larger than 0.70. Temporal variation of the Spectral Width boundary follows the same equatorward trend in both hemispheres. This is consistent with the accumulation of open flux in the polar cap by dayside low-latitude magnetopause reconnection, expected when IMF Bz is negative. Boundaries in both hemispheres also exhibit short-lived poleward motions superposed on the general equator-ward trend, which follows the onset of substorm expansion phase and a temporary northward excursion of IMF Bz during substorm recovery phase. There is an interhemispheric difference in response time to the substorm occurrence between two hemispheres. The Spectral Width boundary in the Southern Hemisphere starts to move poleward 10 min earlier than that in the Northern Hemisphere. We discuss this difference in terms of interhemispheric asymmetry of the substorm breakup region in the longitudinal direction associated with the effect of IMF By.
-
Statistical characteristics of Doppler Spectral Width as observed by the conjugate SuperDARN radars
Annales Geophysicae, 2002Co-Authors: K. Hosokawa, E. E. Woodfield, M. Lester, S. E. Milan, N. Sato, A. S. Yukimatu, T. IyemoriAbstract:We performed a statistical analysis of the occurrence distribution of Doppler Spectral Width around the day-side high-latitude ionosphere using data from the conjugate radar pair composed of the CUTLASS Iceland-East radar in the Northern Hemisphere and the SENSU Syowa-East radar in the Southern Hemisphere. Three types of Spectral Width distribution were identified: (1) an exponential-like distribution in the lower magnetic latitudes (below 72°), (2) a Gaussian-like distribution around a few degrees magnetic latitude, centered on 78°, and (3) another type of distribution in the higher magnetic latitudes (above 80°). The first two are considered to represent the geophysical regimes such as the LLBL and the cusp, respectively, because they are similar to the Spectral Width distributions within the LLBL and the cusp, as classified by Baker et al. (1995). The distribution found above 80° magnetic latitude has been clarified for the first time in this study. This distribution has similarities to the exponential-like distribution in the lower latitude part, although clear differences also exist in their characteristics. These three Spectral Width distributions are commonly identified in conjugate hemispheres. The latitudinal transition from one distribution to another exhibits basically the same trend between two hemispheres. There is, however, an interhemispheric difference in the form of the distribution around the cusp latitudes, such that Spectral Width values obtained from Syowa-East are larger than those from Iceland-East. On the basis of the Spectral Width characteristics, the average locations of the cusp and the open/closed field line boundary are estimated statistically.
-
An inter-hemispheric, statistical study of nightside Spectral Width distributions from coherent HF scatter radars
Annales Geophysicae, 2002Co-Authors: E. E. Woodfield, N. Sato, K. Hosokawa, S. E. Milan, M. LesterAbstract:A statistical investigation of the Doppler Spectral Width parameter routinely observed by HF coherent radars has been conducted between the Northern and Southern Hemispheres for the nightside ionosphere. Data from the SuperDARN radars at Thykkvibær, Iceland and Syowa East, Antarctica have been employed for this purpose. Both radars frequently observe regions of high (>200 ms-1) Spectral Width polewards of low (
Mervyn P. Freeman - One of the best experts on this subject based on the ideXlab platform.
-
A statistical comparison of SuperDARN Spectral Width boundaries and DMSP particle precipitation boundaries in the morning sector ionosphere
Annales Geophysicae, 2005Co-Authors: Gareth Chisham, M. Lester, Mervyn P. Freeman, Thomas Sotirelis, R. A. Greenwald, J.-p. VillainAbstract:Determining reliable proxies for the ionospheric signature of the open-closed field line boundary (OCB) is crucial for making accurate ionospheric measurements of many magnetospheric processes (e.g. magnetic reconnection). This study compares the latitudes of Spectral Width Boundaries (SWBs), identified in the morning sector ionosphere using the Super Dual Auroral Radar Network (SuperDARN), with Particle Precipitation Boundaries (PPBs) determined using the low-altitude Defense Meteorological Satellite Program (DMSP) spacecraft, in order to determine whether the SWB represents a good proxy for the ionospheric projection of the OCB. The latitudes of SWBs and PPBs were identified using automated algorithms applied to 5 years (1997-2001) of data measured in the 00:00-12:00 Magnetic Local Time (MLT) range. A latitudinal difference was measured between each PPB and the nearest SWB within a ±10min Universal Time (UT) window and within a ±1h MLT window. The results show that the SWB represents a good proxy for the OCB close to midnight (~00:00-02:00 MLT) and noon (~08:00-12:00 MLT), but is located some distance (~2°-4°) equatorward of the OCB across much of the morning sector ionosphere (~02:00-08:00 MLT). On the basis of this and other studies we deduce that the SWB is correlated with the poleward boundary of auroral emissions in the Lyman-Birge-Hopfield ``Long" (LBHL) UV emission range and hence, that Spectral Width is inversely correlated with the energy flux of precipitating electrons. We further conclude that the combination of two factors may explain the spatial distribution of Spectral Width values in the polar ionospheres. The small-scale structure of the convection electric field leads to an enhancement in Spectral Width in regions close to the OCB, whereas increases in ionospheric conductivity (relating to the level of incident electron energy flux) lead to a reduction in Spectral Width in regions just equatorward of the OCB.
-
On the probability distributions of SuperDARN Doppler Spectral Width measurements inside and outside the cusp
Geophysical Research Letters, 2004Co-Authors: Mervyn P. Freeman, Gareth ChishamAbstract:The Doppler Spectral Width of backscatter from the SuperDARN HF radars has been routinely used to identify the ionospheric cusp. The probability distribution of Spectral Width values has been described as Gaussian ( normal) in the cusp and exponential equatorward of it. Here we re-examine the empirical distributions and show that they are better described by a log-Levy distribution in both the cusp and non-cusp regions, with the same Levy index a approximate to 1.85 but varying Levy location parameter. The Spectral Width distribution may also be log-Levy at other magnetic local times, but with generally different parameters.
-
An investigation of latitudinal transitions in the SuperDARN Doppler Spectral Width parameter at different magnetic local times
Annales Geophysicae, 2004Co-Authors: Gareth Chisham, Mervyn P. FreemanAbstract:Abstract. Latitudinal transitions from low to high Doppler Spectral Width in backscatter measured by the Super Dual Auroral Radar Network (SuperDARN) are now routinely used as proxies for the polar cap boundary (PCB) in the cusp-region ionosphere. In this paper we perform a statistical study of the nature of similar Spectral Width transitions at other magnetic local times (MLTs). This analysis illustrates that these latitudinal Spectral Width transitions exist at all magnetic local times, and that the latitude, gradient, and amplitude of the transitions vary systematically with MLT. In particular, the probability of a transition occurring at any latitude, identified independently in each MLT sector, is continuous with MLT from the cusp, through the morning sector, to the nightside. This suggests that the transition represents the PCB, as this is known to be what it represents in the cusp region. However, the picture in the afternoon sector (12:00-18:00 MLT) is more complex with no clearly preferred transition latitudes. Key words. Ionosphere (ionosphere-magnetosphere interactions; instruments and techniques). Magnetospheric physics (magnetopause, cusp, and boundary layers.)
