Abstract
Intermediate layers (ILs) are regions of enhanced electron density located in the ionospheric valley that extends from the peak altitude of the daytime E-region to the bottom side of the F-region. This work presents the daytime behavior of the 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: −34.4°) for the deepest solar minimum of the last 500 years. In such a unique condition, this research reveals for the first time 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 decrease rapidly with the increase of geomagnetic activity mainly in the early morning hours. In the following hours, a smoothed rise of the IL was found in all seasons analyzed. Regarding to frequency, it was observed that after 12:00 LT, there is a tendency of it decreased with the increase of the magnetic disturbances, being this characteristic more intense after 16:00 LT, except in the equinox, when little or no response was found during all the interval analyzed. In addition, it stands out that the annual periodicity of the ftIL was observed while the h’IL presents semiannual component.
Highlights
The deep solar minimum of the solar cycle 23/24 provides an unprecedented opportunity to understand the variability of Earth's ambient ionosphere
It is well known that geomagnetic activity can drastically modify the low-latitude ionospheric dynamics
During the last solar minimum, a unique opportunity was available to investigate such 290 dynamics, since the effects of the solar activity, that dominates the temporal variability of ionospheric properties, could be practically disregarded due to very low solar radiation variation
Summary
The deep solar minimum of the solar cycle 23/24 provides an unprecedented opportunity to understand the variability of Earth's ambient ionosphere. Dos Santos et al (2019), for example, showed a case in which a daytime IL over the equatorial region of São Luis (2oS; 44oW) on October 9, 2009 presented a strong upward movement that carried the IL to the base of the F2 layer in ~ 1.5 hours This anomalous rise was probably caused by the joint action of the atmospheric gravity wave propagation and the PPEF. The focus of this paper is to investigate the geomagnetic activity effects on the intermediate layers over the Brazilian low latitude sector during the deep solar minimum of 2009 As mentioned previously, this epoch is especially suited to develop studies like the one proposed here due to the very low values of 95 the solar decimetric flux (10.7cm) that were observed.
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