Low-Elevation Ionosphere Spatial Anomalies Discovered from the 20 November 2003 Storm

This paper presents data from low-elevation satellites on 20 November 2003 in the Ohio region, the area of the largest validated gradients seen to date. As viewed by multiple CORS stations in central and northeastern Ohio, SVN 26 came into view around 2030 UT on this day, rose to an elevation angle of about 15 degrees just after 2100 UT, and set between 2135 and 2200 UT. This observation window is very close to the time that peak ionosphere gradients were observed at those stations on higher-elevation satellites. An ionosphere gradient of about 300 mm/km was discovered on SVN 26 between CORS stations KNTN and SIDN at 2050 UT, when SVN 26 was at about 12.6° elevation relative to these two sites. Thirty minutes later, at 2120 UT, a gradient of about 360 mm/km was discovered between CORS stations GARF and WOOS, when SVN 26 was at about 12.0° elevation. Gradients between 200-300 mm/km were also discovered at other station pairs between 2100 and 2130 UT. Spatial gradients to another low elevation satellite with a similar azimuth angle, SVN 29, were observed to be about 100 −150 mm/km. While comparison of the L1-L2 ionosphere estimates with L1 code-minus-carrier measurements is hindered by higher noise and loss-oflock on L2, sufficient similarity between the two sets of ionosphere gradient estimates exists to allow us to state that at least some of these observations, including the two largest-gradient cases, can be declared "validated" based on the preponderance of the evidence. This paper details our recent study of low-elevation satellite observations from the 20 November 2003 ionosphere storm and shows how we arrived at the ionosphere gradient estimates cited above. These newfound observations imply that the means by which the Local Area Augmentation System (LAAS) mitigates anomalous ionosphere spatial gradients may need to be revised.