Ionospheric Effects of April 2002 Magnetic storms: Observations from the Network of Incoherent Scatter Radars

We present the first study of a major geomagnetic storm interval incorporating data from 8 incoherent scatter radars. The radars operated on Apr 15-17, 2002, with some radars continuing operations to Apr 18-20, 2002. The data was taken as a part of the CEDAR/TIMED collaborative storm study with the primary goal of investigating the extent of storm-related disturbances in latitude and altitude. We describe the variety of phenomena observed by different radars depending on their location relative to the auroral oval and on the local time zone. During the initial positive phase of the storm, an increase in electron density by 10-20% is observed at Irkutsk (52.8N) and Kharkov (50.0N). Large depletions of electron density due to changes in the neutral composition are seen at high and mid-latitudes, reaching a factor of 5-6 at Sondrestrom (67.0N) and a factor of 2-3 at Irkutsk (52.8N) and Millstone Hill (42.6N). We also identify several periods of penetration of magnetospheric electric fields from high to low latitudes and discuss their effects at different locations. Several intervals of rapidly changing electric fields, with the largest at ~12 UT and ~14 UT, are observed at EISCAT Svalbard (78.1N, L=20) and Sondrestrom (67.0N, L=12). Short spikes in electron density and temperature are observed at these locations during the time when the electric field direction and magnitude are changing, and represent regions of hot dense plasma moving in and out of the radar's field of view. At sub-auroral location (Millstone Hill, 42.6N, L=2.8), only a small increase in electron density is observed in response to the rapidly varying electric field, while electron temperature variations are hardly discernible. Similar electron density variations are observed at mid-latitude locations (Irkutsk, 52.8N, L=2.1, Kharkiv, 50.0N, L=2.1). In contrast to this, lower latitude observations (Arecibo, 18.3N, L=1.4), reveal variations in electron density of the order of ~50% and anti-correlated variations in electron temperature up to 500 K. The same penetrating eastward electric field is clearly seen at equatorial latitude (Jicamarca, 11.95S, L=1.1).