Does the Peak Response of the Ionospheric F₂ Region Plasma Lag the Peak of 27‐Day Solar Flux Variation by Multiple Days?

Abstract

In this study, the in situ electron density measurements from the Challenging Minisatellite Payload (CHAMP) and solar extreme ultraviolet (EUV) radiation from the Solar Extreme Ultraviolet Experiment instrument on board the Thermosphere Ionosphere Mesosphere Energetic and Dynamics satellite, both with a time resolution of 1.5 hr, are used to explore the peak response of the ionospheric F2 region plasma to the peak of 27‐day solar EUV flux variation. The time delays of in situ electron density changes obtained from the CHAMP satellite in response to 27‐day solar EUV flux changes vary from 0 to about 3 days. Meanwhile, the Thermosphere Ionosphere Electrodynamics General Circulation Model simulations driven by the measured EUV flux and the actual geomagnetic activity show similar time delays as those observed in the CHAMP measurements. Further simulations reveal that the geomagnetic activity greatly affects the determination of the ionospheric time delay to the 27‐day solar EUV flux variations. Besides, the solar zenith angle change within the solar rotation interval can cause large latitudinal differences in the time delay. The ionospheric time delay to the pure 27‐day solar EUV flux variation is less than 1 day and slightly increases with latitude, when geomagnetic activity and seasonal variations are eliminated in the simulation. The simulation results further suggest that the ionospheric response time is associated with the photochemical, dynamic, and electrodynamic processes in the ionosphere‐thermosphere system.

Long-term trends in the ionospheric response to solar extreme-ultraviolet variations

“…Transport is one of the most critical parameters that control the behavior of the ionosphere. These results suggest that interannual variability depends not only on the solar activity but also on several other physical processes such as geomagnetic activity (Rich et al, 2003) and local ionospheric parameters such as neutral wind and lower atmospheric forcing through the vertical coupling. Lee et al (2012) analyzed electron density measurements from CHAMP and GRACE along with Global Ionosphere Maps (GIM) TEC maps in relation to the F10.7 index and showed the spatial distribution of delay and correlation coefficient during the years 2003 to 2007.…”

“…There is a tendency that during high solar activity, the delay is increased slightly at low latitudes, but strongly (up to 3 d) in the high-latitude region. A negative delay is observed during low solar activity, presumably associated with the meteorological effects as suggested by Ren et al (2018). Another possible reason is ionospheric saturation, which might reduce the transport process during high solar activity due to lower recombination rates.…”

“…To understand the underlying mechanisms of the delay observed in the ionospheric plasma, Jakowski et al (1991) used a one-dimensional numerical model to explain the ionospheric delay of about 1-2 d. They concluded that the ionospheric delay could be attributed to the delayed atomic oxygen density variation at 180 km height produced via O 2 photodissociation. Ren et al (2018) performed multiple numerical experiments using the Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM) to investigate the potential physical mechanisms responsible for the ionospheric delay. Their simulation results revealed that photochemical, dynamic, and electrodynamic processes, as well as the geomagnetic activity, can be associated with the ionosphere response time.…”

“…The interaction of solar radiation with the ionosphere is complicated due to several mechanisms with the potential to modulate the thermosphere-ionosphere (T-I) system at different timescales ranging from the 11-year solar cycle down to minutes (e.g., Liu et al, 2003;Afraimovich et al, 2008;Liu and Chen, 2009;Chen et al, 2012). The ionosphere plasma response to solar EUV and UV variations has been widely studied using ground-and space-based observations (e.g., Jakowski et al, 1991;Jacobi et al, 2016;Schmölter et al, 2018;Jakowski et al, 1999), as well as by numerical and empirical modeling (e.g., Ren et al, 2018;Vaishnav et al, 2018a, b). These studies have shown that the response of the ionosphere to solar EUV radiation variations is delayed by 1-2 d at the 27 d solar rotation period (e.g., Jakowski et al, 1991;Afraimovich et al, 2008;Jakowski et al, 2002;Min et al, 2009;Jacobi et al, 2016;Lee et al, 2012).…”

A Simulation Study on the Time Delay of Daytime Thermospheric Temperature Response to the 27‐Day Solar EUV Flux Variation

“…where t stands for the time in unit of days, and F10.7 is in the unit of solar flux unit (sfu). The model setting in this study makes it sure that the contribution from the 27-day solar EUV flux change to thermospheric temperature can be isolated, as is described in Ren et al (2018) in detail.…”