Research note

The F-layer dynamo

Space based radio wave communication and navigation has become need of the society. Atmospheric (thermospheric-ionospheric) processes, such as Equatorial Plasma Bubbles (EPBs), affect the radio waves propagating through this region, causing heavy perturbations on signals received at ground. This paper investigates the causative mechanism of EPB through the ground based remotely sensed imaging observations of O (¹S) 557.7 nm and O (¹D) 630.0 nm emissions emanating from the upper mesosphere (∼100 km altitudes) and thermosphere-ionosphere (∼250 km altitudes) over a low-latitude station, Kolhapur (16.8° N, 74.2° E, and dip lat. 10.6°N). Our investigation revealed that the gravity waves evident in OI557.7 nm images exhibit a close association with the observed EPB structures. These mesospheric gravity waves were found to travel from the South to North with horizontal wavelengths ∼35 and ∼56 km on 13–14 April and 26–27 April 2015, respectively. The thermosphere-ionosphere measurements exhibited occurrence of the North–South aligned EPB moving to the east with an inter depletion distance (IDD) equal to ∼44 km and ∼64 km. These results provide evidences on association of the gravity waves with the EPB.

An extensive year-long evaluation of Total Electron Content (TEC) obtained from NCAR Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM) simulations along with TEC obtained from International Reference Ionosphere (IRI-2020) is carried out with respect to observations made by a GPS TEC receiver over the dip equatorial location of Thiruvananthapuram (8.5° N, 77° E, 0.5° magnetic), in the Indian region during solar minimum period of 2005–2006. The TIE-GCM model and IRI model were run for the period of 2005 (November December) and till October 2006 for each day, and the hourly values of TEC were compared to GPS TEC observations. Overall the temporal and seasonal characteristics of TEC over Thiruvananthapuram were simulated reasonably well by the TIE-GCM model and IRI model. The skill scores of normalized Root Mean Square Error (nRMSE), Prediction Efficiency (P.E), and Ratio between maximum and minimum values were estimated for both models to evaluate their performance. The mean percentage difference between the measured and the simulated TEC was found to be around −25% for IRI, indicating underestimation of the observations, and between −40% and −50% for TIE-GCM again indicating underestimation. A comprehensive analysis revealed that there is good conformity between the modeled and measured values for the whole observation period, with a correlation coefficient R-value of 0.8 and 0.9 for the TIE-GCM and IRI models respectively. This study brings out the competence of the two models in simulating the temporal and seasonal pattern of variability of ionospheric TEC over the equatorial Indian region.

Equatorial Electrojet (EEJ) and Equatorial Ionization Anomaly (EIA) are two distinct ionospheric phenomena which are caused by the horizontal configuration of the geomagnetic field at the equator. On the day side of the earth, these activities are controlled by a common zonal electric field, and their interactions must be researched for a number of scientific reasons. In this study, we used multiple Global Navigation Satellite System (GNSS) stations at Al Wajh, Sola village, Nama, and Sheba in 2012 to investigate the correlations between the instantaneous and integrated equatorial electrojet (EEJ) and the variations of vertical total electron content (vTEC) at the northern crest of EIA in the East Africa and Middle East longitudinal sector. The equinoctial and solstitial months have the highest and lowest values of the monthly mean EEJ and vTEC, respectively, and as a result, both parameters show a semi-annual variability. The monthly mean vTEC also demonstrates equinoctial and solstitial asymmetries, resulting in vTEC values that are higher at the September equinox and the December solstice than at the March equinox and the June solstice, respectively. These asymmetries have been also observed in the composition of $[O]/[N_2]$ ratio. On days with strong counter electrojet (CEJ), EEJ peaks are smaller and occur later than on days with weak CEJ. Additionally, when CEJ is high, EIA is suppressed and vTEC exhibits low values at all crest points. In the early hours (10:00–12:00 Lt), Nama and Sheb showed higher correlation coefficient values with significant levels than Alwj and Sola. According to the correlation analysis, the anomaly crest develops at the lower latitude stations between 10:00 and 12:00 Lt and then moves to the higher latitude stations later. Generally, the correlations are higher and more significant in the equinoxes than the solstices. The correlations between integrated EEJ (IEEJ) and vTEC were also found to be strong and significant at the lower latitude stations throughout all seasons. These findings suggest that both the instantaneous and integrated EEJ can be used as a proxy for the eastward zonal equatorial electric field and must be considered in regional and global models of total electron content of equatorial and low latitude ionosphere.

We have investigated equinoctial asymmetry of scintillation occurrence and zonal irregularity drift velocity observed with closely-spaced GPS receivers at Kototabang (0.20°S, 100.32°E; geomagnetic latitude 10.6°S), Indonesia during a period from 2003 to 2016, and found that the scintillation occurrence rate is higher in Mar. equinox than in Sep. equinox, and that the eastward drift velocity at post-sunset is higher in Mar. equinox than in Sep. equinox. This result is consistent with previous work done by Otsuka et al. (2006), who have analyzed scintillation and zonal drift velocity data obtained at Kototabang in 2003–2004, and suggests that the eastward drift velocity corresponding to the downward electric field at post-sunset may be related to the pre-reversal enhancement of eastward electric field, which could play an important role in generating plasma bubble. In this study, we have investigated vertical drift velocity by analyzing the ionosonde data obtained near magnetic equator (Chumphon, Bac Lieu, and Cebu) during a period from 2003 to 2016, and found that the upward drift velocity at post-sunset is larger in Mar. equinox than Sep. equinox. We also find that solar activity dependence of the zonal and vertical drift velocities is more clearly seen in Mar. equinox than in Sep. equinox. In order to consider the mechanism causing the equinoctial asymmetry of the drift velocities, foF₂ obtained by the ionosondes is also analyzed. We find that f_oF₂ also depends on solar activity more clearly in Mar. equinox than in Sep. equinox, and that f_oF₂ during high solar activity conditions is higher in Mar. equinox than in Sep. equinox. These results suggest that in Mar. equinox, due to high electron density, intense electric field could be generated through the F-region dynamo so that plasma bubble likely occurs.

An extensive intercomparison of ionospheric foF2 observations and NCAR Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM) simulations has been carried out for the dip equatorial location of Thiruvananthapuram. Ionosonde measurements for geomagnetically quiet days of 2002, 2006 and 2008, representing solar maximum, solar minimum and deep solar minimum conditions have been used for the analysis.

In general TIE-GCM simulations reproduced the temporal and seasonal characteristics of foF2 over Thiruvananthapuram reasonably well for all the three solar activity conditions. Seasonally the difference between the measured and the simulated foF2 tended to be higher during winter (maximum of 25%). Additionally, it is found that TIE-GCM is not reproducing the reduction in the foF2 values in the noon hours i.e. the bite out, which is very prominent in the foF2 observations predominantly during 2002. A detailed analysis revealed that, there is good agreement between the modeled and measured values for the whole observation period, with an R value of 0.81. From the comparison it is clear that the model underestimates the observations in general but for the periods when bite out is prominent, the model gives an over estimation.

The comprehensive comparisons during different solar activity conditions have shown that the difference between modeled and measured ionospheric peak densities lies in the range of.

10 to −25%. This study brings out the efficacy of the model in simulating the temporal seasonal and solar cycle variability of ionospheric foF2 over the equatorial Indian region.

Section 7.1

The ion-neutral interaction makes the terrestrial ionosphere and thermosphere a tightly coupled system, which is of critical importance to both applications and scientific understanding. In this section, we present the current understanding of ionosphere-thermosphere interactions, from mass, momentum, and energy coupling point of views between neutrals and ions, for both quiet and storm conditions. This survey begins with how the thermosphere drives the ionosphere through photochemistry, wind transport, and neutral wind dynamo. The ionosphere imprints on the thermosphere are discussed from both dynamic and energetic processes. We also describe the ionosphere-thermosphere coupling during transient solar flux changes such as solar eclipses and solar flares. Besides the climatology of ionospheric and thermospheric variations, the ion-neutral coupling also plays an important role in the small- and medium-scale ionospheric perturbations.

Section 7.2

The Earth’s upper atmosphere is a mixture of neutral and ionized particles, with the neutrals composing the thermosphere and the ionized the ionosphere. The thermosphere and ionosphere are closely coupled together via dynamical (the wind-driven field-aligned movement through ion drag), electrodynamical (the wind dynamo), and chemical (composition and photochemistry) processes. This section reviews some of these fundamental processes, in particular, the large-scale ionospheric and thermospheric structures that they produce. We also provide an outlook on research concerning the thermosphere-ionosphere system, its day-to-day variability, and its difference between Earth and other planets that deserves more future efforts.

Section 7.3

Large-scale traveling ionospheric disturbances (LSTIDs) are quasiperiodical plasma density fluctuations that propagate globally, typically following geospace disturbances. They are manifestation of traveling atmospheric disturbances (TADs) excited at high latitudes, predominantly in the auroral zone where various heating processes are dramatically intense. Storm-time gravity waves (GWs) are a specific type of TADs originating from geomagnetic storm and substorm disturbances; however, GWs are ubiquitous in the atmosphere and their presence in the thermosphere driving ionospheric disturbances as TIDs of various scale sizes play a fundamental role in the transportation and dispersion of the magnetosphere-ionosphere-thermosphere coupling energy from “above” and terrestrial perturbation energy from “below.” Here we provide a quick overview of the thermospheric GW theory and discuss the ionospheric response to and manifestation of GWs and TADs. We describe GW excitation processes in the auroral latitude during geospace disturbances and characterize LSTID global distribution with recent observations. We also provide some new perspectives on TIDs at subauroral and high latitudes.

Section 7.4

Medium-scale traveling ionospheric disturbances (MSTIDs) are wave-like ionospheric disturbances with wavelengths of a few hundred kilometer scale that propagate preferentially toward certain directions. MSTIDs are phenomena observed in all latitudes, but they are encountered more frequently in mid-latitudes. Depending on the occurrence local times, MSTIDs are divided into nighttime and daytime MSTIDs. As well as the difference in their occurrence local times, the characteristics and generation mechanisms of nighttime and daytime MSTIDs are also different. The climatological behavior of MSTIDs varies with geographic location, but there exist common features applicable worldwide. Knowledge of the characteristics of MSTIDs in the global context provides a useful tool for the evaluation of the physical processes underlying MSTIDs and of the impact of MSTIDs on space weather. This section reviews the characteristics of MSTIDs and their variability in the global context by comparing and synthesizing observations worldwide.