Solar Activity Driven 27‐Day Signatures in Ionospheric Electron and Molecular Oxygen Densities

Abstract

Solar extreme ultraviolet (EUV) radiation is absorbed in different ionization and dissociation processes in the upper atmosphere driving the creation of the ionospheric plasma. The composition of this plasma is significantly affected by variations of the solar activity but geomagnetic (driven by the solar wind) and meteorological variability contribute to ionospheric variability as well (Rishbeth & Mendillo, 2001). Thus, changes of the plasma occur on different spatial and temporal scales, which may further depend on additional parameters (e.g., solar zenith angle). Nevertheless, significant signatures in the ionospheric plasma are related to corresponding changes in the solar EUV radiation. The 27-day solar rotation period (appearance and disappearance of active regions on the solar disc) causes continuous variations of the solar EUV spectrum which in turn cause observable 27-day signatures for different ionospheric parameters (Ma et al., 2012).Photoionization of O to O + at heights from 150 to 500 km defines the so-called F region with the maximum of the plasma density profile. The amount of O + dominates the ionospheric plasma in this region. Photoionization of O 2 to 𝐴𝐴 O + 2 and photodissociation of O 2 to O at heights from 90 to 150 km (as well as further absorption processes or charge exchange) cause a different plasma composition in the lower ionosphere. Additionally, recombination processes occur depending on the existing plasma composition at different heights (Kelley, 2009). The 27-day signatures, which are driven by the solar EUV radiation, thus also show height-dependent variations based on the contribution of the different processes (Schmölter et al., 2022).In the upper ionosphere the 27-day signatures of the plasma density are controlled by production and loss of O + (Ren et al., 2018;Schmölter et al., 2022). The production due to photoionization of O is almost immediate and increased due to the photodissociation of O 2 and upward transport processes during the time of increasing solar activity (first phase of 27-day solar rotation period). The recombination is not immediate and therefore an accumulation of O + may occur resulting in an observable time difference between the solar and ionospheric 27-day signatures (Schmölter et al., 2022). This delayed ionospheric response and its variability has been characterized

Ionospheric Response to Solar EUV Radiation Variations Using GOLD Observations and the CTIPe Model

“…Therefore, additional O is produced in the lower ionosphere, which in turn by transport (diffusion) contributes to an increase of O and O + at the F region peak, resulting in a delayed electron density response to the changes in the solar EUV flux. This has been successfully modeled (Schmölter et al., 2022) and confirmed by observations (Schmölter & von Savigny, 2022). In addition, several other factors can also affect the ionospheric density and influence the delayed ionospheric response, such as geomagnetic activity, photochemical, dynamical, and electrodynamic processes (e.g., Ren et al., 2018; Schmölter et al., 2020; Vaishnav et al., 2018, 2022).…”

Spatial Features of the Delayed Ionospheric Response During Low and High Solar Activity

“…Future studies could extend the SEA method to cover all dimensions of interest, since time‐, wavelength‐, latitude‐, longitude‐ and height‐dependent results would allow to identify the extent of specific processes (e.g., photoionization of O and O 2 via wavelength and height or plasma transport in equatorial ionization anomaly via latitude and height). All of these are covered separately with observational results in the present study and results by Schmölter and von Savigny (2022), but a combined analysis could provide an comprehensive reference for modeling efforts on the delayed ionospheric response.…”

“…Schmölter and von Savigny (2022) have also shown a significant increase of the delay at even longer wavelengths, which is due photodissociation occurring at wavelengths shorter than 242 nm. These varying response times of the ionospheric plasma are further reflected with height-dependent electron density changes (Schmölter & von Savigny, 2022) and the integrated response via TEC (see Figure 2). The results in Figures 9 and 11 are in good agreement with the established understanding of the delayed ionospheric response and, based on this, the temporal and spatial variations during low and high solar activity are investigated in more detail below.…”

“…The differences that occur are important for the understanding of the delayed ionospheric response, since different absorption processes in the upper atmosphere occur dependent on the wavelength (Fennelly & Torr, 1992). An increase of the delay at longer wavelengths affects the photodissociation of O 2 in the lower ionosphere (Schmölter & von Savigny, 2022) and may cause changes that do not correlate with variations of F10.7.…”

“…In the first phase of the 27‐day solar rotation period, production and loss of O + in the upper ionosphere increase due to the rise of solar activity. This process is enhanced when more O is produced in the lower ionosphere by photodissociation of O 2 and transported upward (Schmölter & von Savigny, 2022; Schmölter et al., 2022). In the second phase of the 27‐day solar rotation period, production and also loss of O + in the upper ionosphere decrease due to the drop of solar activity.…”