Large variations in the thermosphere and ionosphere during minor geomagnetic disturbances in April 2002 and their association with IMF <i>B</i> <sub> <i>y</i> </sub>

<p>We investigate the variations in the thermosphere and ionosphere using multi-instrument observations during the April 2002 period, with a particular focus on periods during small geomagnetic disturbances. Large and long-lasting reductions in the daytime electron density were observed at midlatitudes by incoherent scatter radars, ionosondes, and GPS receivers. These reductions reached 30\textendash50\% and were observed over an extended longitudinal area. They propagated to middle latitudes (35\textendash40\textdegreeN) in the case of a weak geomagnetic disturbance (<em>K</em><sub><em>p</em></sub> = 3-) and to low latitudes (0\textendash10\textdegreeN) in the case of a stronger disturbance (<em>K</em><sub><em>p</em></sub> = 5-). Data from the GUVI instrument aboard the TIMED satellite reveal a reduction in the daytime O/N<sub>2</sub> ratio in the coincident area. Similar decreases are also predicted by the TIMEGCM/ASPEN model in both O/N<sub>2</sub> ratio and electron density, though the magnitude of the decrease from the model is smaller than observed. We suggest that these ionospheric and thermospheric disturbances result from high-latitude energy input and efficient transport of regions with reduced O/N<sub>2</sub> to lower latitudes. We discuss the possible role of a strong positive <em>B</em><sub><em>y</em></sub> component of the interplanetary magnetic field in the transport of regions with reduced O/N<sub>2</sub>.</p>
Year of Publication
Journal of Geophysical Research
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