Responses of intermediate layers to geomagnetic activity during the 2009 deep solar minimum over the Brazilian low-latitude sector

Abstract. This work presents the daytime behavior of the intermediate layer (ILs) parameters (the virtual height – h'IL, and the top frequency – ftIL) over the low-latitude region of Cachoeria Paulista (CP, 22.42∘ S; 45∘ W, I: −34.59∘) during the 2009 deep solar minimum. Under such a unique condition, this research reveals the ILs' quiet state seasonal behavior as well as its responses to moderate changes in the geomagnetic activity. The main results show that even small variations of geomagnetic activity (quantified by the planetary Kp index) are able to modify the dynamics of the ILs parameters. For the first time, it was observed that during the summer, the h'IL decreases rapidly with the increase of geomagnetic activity, mainly in the early morning hours, while in the following hours, a smoothed rise of the IL was found in all seasons analyzed. Regarding the IL frequency, it was observed that after 12:00 LT, there is a tendency to decrease with the increase of magnetic disturbances, this characteristic being more intense after 16:00 LT for summer and winter. For the equinox, such variation was detected, however with half of the amplitude of the other seasons. In addition, the domain of the annual periodicity of the ftIL stands out, while the h'IL presents a semiannual component under the condition of geomagnetic quiet.