Characteristics of TEC variations observed at Chofu for geomagnetic storms

Since the middle of 1996, we have made a routine observation of TEC using C PS in Chofu (35.65 degreesN, 139.54 degreesE), Tokyo. We examine in this pa per the relationship between the perturbation components of TEC and geomagn etic field variations during 11 storm events for 1997-1998 to clarify the m id-latitude characteristics of TEC variations at the time of magnetic storm s. The perturbation components of TEC were derived every 20 seconds by subt racting quiet-time TEC values which are estimated as the average of both 3 days just before and after the storm period. The magnetic field data from M emambetsu Magnetic Observatory (45.92 degreesN, 144.20 degreesE) are used f or the identification of the storm-time variations. Our results show that ( 1) the amplitude level of the TEC variation tends to increase during the fi rst 24 hr of storm and then decrease below its usual-day level with recover y in one or two days later for the typical magnetic storm, and (2) during a negative-value phase of the TEC variation, which follows the initial posit ive hump structure, the perturbation amplitude of TEC shows a remarkable re duction in summer compared to in winter.