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Abstract: High resolution ~1 arcminute astronomical imaging at low frequency below150 MHz has only recently become practical with the development of newcalibration algorithms for removing ionospheric distortions. In addition toopening a new window in observational astronomy, the process of calibrating theionospheric distortions also probes ionospheric structure in an unprecedentedway. Here we explore one aspect of this new type of ionospheric measurement,the differential refraction of celestial source pairs as a function of theirangular separation. This measurement probes variations in the spatial gradientof the line-of-sight total electron content TEC to 0.001 TECU-km accuracyover spatial scales of under 10 km to over 100 km. We use data from the VLALow-frequency Sky Survey VLSS; Cohen et al. 2007, AJ 134, 1245, a nearlycomplete 74 MHz survey of the entire sky visible to the Very Large Array VLAtelescope in Socorro, New Mexico. These data comprise over 500 hours ofobservations, all calibrated in a standard way. While ionospheric spatialstructure varies greatly from one observation to the next, when analyzed overhundreds of hours, statistical patterns become apparent. We present a detailedcharacterization of how the median differential refraction depends on sourcepair separation, elevation and time of day. We find that elevation effects arelarge, but geometrically predictable and can be -removed- analytically using a-thin-shell- model of the ionosphere. We find significantly greater ionosphericspatial variations during the day than at night. These diurnal variationsappear to affect the larger angular scales to a greater degree indicating thatthey come from disturbances on relatively larger spatial scales 100s of km,rather than 10s of km.



Autor: A.S. Cohen NRL, H.J.A. Röttgering Leiden Observatory

Fuente: https://arxiv.org/







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