An unusual geometry of the ionospheric signature of the cusp: implications for magnetopause merging sites

Chisham, G.; Pinnock, M.; Coleman, I. J.; Hairston, M. R.; Walker, A. D. M.

doi:10.5194/angeo-20-29-2002

Cited by 17 publications

(16 citation statements)

References 45 publications

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“…Therefore, one must be careful when interpreting spatial and temporal variations of the proxies in the cusp region. These can be due to variations in the offset of the proxy from the PCB, as well as to variations in the PCB itself (Chisham et al, 2002). It is also unclear what effect the transient nature of reconnection will have on the variation of this proxy offset and on the location of the observable cusp (e.g.…”

Section: Introductionmentioning

confidence: 99%

A technique for accurately determining the cusp-region polar cap boundary using SuperDARN HF radar measurements

Chisham

Freeman

2003

Ann. Geophys.

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Abstract. Accurately measuring the location and motion of the polar cap boundary (PCB) in the high-latitude ionosphere can be crucial for studies concerned with the dynamics of the polar cap, e.g. the measurement of reconnection rates. The Doppler spectral width characteristics of backscatter received by the SuperDARN HF radars have been previously used for locating and tracking the PCB in the cusp region. The boundary is generally observed in meridional beams of the SuperDARN radars and appears as a distinct change between low spectral width values observed equatorward of the cusp region, and high, but variable spectral width values observed within the cusp region. To identify the spectral width boundary (SWB) between these two regions, a simple algorithm employing a spectral width threshold has often been applied to the data. However, there is not, as yet, a standard algorithm, or spectral width threshold, which is universally applied. Nor has there been any rigorous assessment of the accuracy of this method of boundary determination. This study applies a series of threshold algorithms to a simulated cusp-region spectral width data set, to assess the accuracy of different algorithms. This shows that simple threshold algorithms correctly identify the boundary location in, at the most, 50% of the cases and that the average boundary error is at least ∼1-2 range gates (∼1 • latitude). It transpires that spatial and temporal smoothing of the spectral width data (e.g. by median filtering), before application of a threshold algorithm can increase the boundary determination accuracy to over 95% and the average boundary error to much less than a range gate. However, this is sometimes at the cost of temporal resolution in the motion of the boundary location. The algorithms are also applied to a year's worth of spectral width data from the cusp ionosphere, measured by the Halley SuperDARN radar in Antarctica. This analysis highlights the increased accuracy of the enhanced boundary determination algorithm in the cusp region. Away from the cusp, the resulting SWB locations are often dependent on the choice of threshold. This suggests that there is not a sharp latituCorrespondence to: G. Chisham (G.Chisham@bas.ac.uk) dinal SWB in regions of the dayside ionosphere away from the cusp, but that there is a shallower latitudinal gradient in spectral width near the boundary location.

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Section: Introductionmentioning

confidence: 99%

A technique for accurately determining the cusp-region polar cap boundary using SuperDARN HF radar measurements

Chisham

Freeman

2003

Ann. Geophys.

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“…The poleward edge of this ion dispersion represents a good proxy for the ionospheric projection of the merging line, as it marks the arrival in the ionosphere of the first magnetosheath-like ions from the reconnection site following an interval of reconnection. However, there exists a small (typically <0.5 • ) latitudinal offset of this proxy from the "true" projection of the merging line, as the footprint will have convected sunward during the finite travel time of the fastest ions from the reconnection site to the ionosphere (Rodger, 2000;Chisham et al, 2002b). We assume that this offset is negligible when compared to other uncertainties in our determination of the location and motion of the merging line and discuss the effect of these uncertainties later in the paper.…”

Section: Location Of the Ionospheric Projection Of The Merging Linementioning

confidence: 99%

Measuring the dayside reconnection rate during an interval of due northward interplanetary magnetic field

et al. 2004

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Abstract. This study presents, for the first time, detailed spatiotemporal measurements of the reconnection electric field in the Northern Hemisphere ionosphere during an extended interval of northward interplanetary magnetic field. Global convection mapping using the SuperDARN HF radar network provides global estimates of the convection electric field in the northern polar ionosphere. These are combined with measurements of the ionospheric footprint of the reconnection X-line to determine the spatiotemporal variation of the reconnection electric field along the whole X-line. The shape of the spatial variation is stable throughout the interval, although its magnitude does change with time. Consequently, the total reconnection potential along the X-line is temporally variable but its typical magnitude is consistent with the cross-polar cap potential measured by low-altitude satellite overpasses. The reconnection measurements are mapped out from the ionosphere along Tsyganenko model magnetic field lines to determine the most likely reconnection location on the lobe magnetopause. The X-line length on the lobe magnetopause is estimated to be ∼6-11 R E in extent, depending on the assumptions made when determining the length of the ionospheric X-line. The reconnection electric field on the lobe magnetopause is estimated to be ∼0.2 mV/m in the peak reconnection region.

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“…These include optical measurements from all-sky cameras, imaging spacecraft and meridian-scanning photometers (Blanchard et al, 1995(Blanchard et al, , 1997Sandholt et al, 1998;Brittnacher et al, 1999), particle precipitation signatures from low-altitude spacecraft (Newell et al, 1991(Newell et al, , 1996Sotirelis and Newell, 2000) and measurements from incoherent and coherent scatter radars (Baker et al, 1995;Blanchard et al, 1996Blanchard et al, , 2001Milan et al, 1999;Chisham et al, 2001Chisham et al, , 2002Lester et al, 2001;Chisham and Freeman, 2003), or a combination of all of these . Unfortunately, most of these measurements suffer from only partial coverage of the auroral oval at any one time, making a global determination of the OCB difficult.…”

Section: Introductionmentioning

confidence: 99%

On the use of IMAGE FUV for estimating the latitude of the open/closed magnetic field line boundary in the ionosphere

et al. 2008

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Abstract.A statistical comparison of the latitude of the open/closed magnetic field line boundary (OCB) as estimated from the three far ultraviolet (FUV) detectors onboard the IMAGE spacecraft (the Wideband Imaging camera, WIC, and the Spectrographic Imagers, SI-12 and SI-13) has been carried out over all magnetic local times. A total of over 400 000 OCB estimations were compared from December 2000 and January and December of [2001][2002]. The modal latitude difference between the FUV OCB proxies from the three detectors is small, <1 • , except in the predawn and evening sectors, where the SI-12 OCB proxy is found to be displaced from both the SI-13 and WIC OCB proxies by up to 2 • poleward in the predawn sector and by up to 2 • equatorward in the evening sector. Comparing the IMAGE FUV OCB proxies with that determined from particle precipitation measurements by the Defense Meteorological Satellites Program (DMSP) also shows systematic differences. The SI-12 OCB proxy is found to be at higher latitude in the predawn sector, in better agreement with the DMSP OCB proxy. The WIC and SI-13 OCB proxies are found to be in better agreement with the DMSP OCB proxy at most other magnetic local times. These systematic offsets may be used to correct FUV OCB proxies to give a more accurate estimate of the OCB latitude.

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An unusual geometry of the ionospheric signature of the cusp: implications for magnetopause merging sites

Cited by 17 publications

References 45 publications

A technique for accurately determining the cusp-region polar cap boundary using SuperDARN HF radar measurements

A technique for accurately determining the cusp-region polar cap boundary using SuperDARN HF radar measurements

Measuring the dayside reconnection rate during an interval of due northward interplanetary magnetic field

On the use of IMAGE FUV for estimating the latitude of the open/closed magnetic field line boundary in the ionosphere

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