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Medium-scale traveling ionospheric disturbances (MSTIDs) are one of the ionospheric plasma density structures and are observable through 630-nm airglow images. Previous studies using airglow images at Tromso (69.6°N, 19.2°E; magnetic latitude: 66.7°N), Norway, reported high-latitude MSTIDs (here we call them as polar-type MSTIDs) whose propagation direction changes associated with auroral brightening and magnetic field disturbances. However, there has been little statistical analysis on the connection of MSTIDs occurring at high and middle latitudes. In this study, we statistically analyzed the MSTIDs observed by an airglow imager at Nyrola (62.3°N, 25.5°E; magnetic latitude: 59.4°N), Finland, which is located ~7° south of Tromso, corresponding to subauroral latitudes. The period analyzed was from 23 January 2017, to 30 September 2021. We found 11 cases of MSTIDs during this period. Eight cases were found to be the polar-type MSTIDs whose motion changes associated with auroral brightening and magnetic field disturbances. We found that 9 cases of MSTID show the low-latitude boundary at 61° ± 2°N for geographic latitude and 58° ± 2°N for magnetic latitude, indicating disconnection between high- and mid-latitude MSTIDs. We also derived occurrence probability, velocity, wavelength, period, wave front direction, and propagation direction of these MSTIDs. The occurrence probability of MSTIDs at Nyrola is 1.9%, which is much lower than those at high (Tromsoe, more than 50%) and middle (Japan, ~30%) latitudes. We discuss these MSTID characteristics at subauroral latitudes based on possible difference of generation mechanisms of nighttime MSTIDs at high and middle latitudes. [ABSTRACT FROM AUTHOR]

The low latitude ionosphere and thermosphere are strongly disturbed during and shortly after geomagnetic storms. We use novel Jicamarca radar measurements, ACE satellite solar wind, and SuperMAG geomagnetic field observations to study the electrodynamic response of the equatorial ionosphere to the 23, 24 April 2023 geomagnetic storm. We also compare our data with results from previous experimental and modeling studies of equatorial storm-time electrodynamics. We show, for the first time, unusually large equatorial vertical and zonal plasma drift (zonal and meridional electric field) perturbations driven simultaneously by multi storm-time electric field mechanisms during both the storm main and recovery phases. These include daytime undershielding and overshielding prompt penetration electric fields driven by solar wind electric fields and dynamic pressure changes, substorms, as well as disturbance dynamo electric fields, which are not well reproduced by current empirical models. Our nighttime measurements, over an extended period of large and slowly decreasing southward IMF Bz, show very large, substorm-driven, vertical and zonal drift fluctuations superposed on large undershield driven upward and westward drifts up to about 01 LT, and the occurrence of equatorial spread F irregularities with very strong spatial and temporal structuring. These nighttime observations cannot be explained by present models of equatorial storm-time electrodynamics. [ABSTRACT FROM AUTHOR]

To establish the statistical behavior of ionospheric TEC variations from high latitudes to midlatitudes during the main and recovery phases of geomagnetic storms, we conducted a superposed epoch analysis of interplanetary magnetic field, solar wind, geomagnetic indices (AE and SYM-H), and global navigation satellite system (GNSS)-total electron content (TEC) data for 20 yr (2000-2019). In this study, we identify 663 geomagnetic storm events with the minimum SYM-H value of less than -40 nT and investigate the characteristics of the TEC variations for the weak (-60 = SYM-Hmin < -40 nT), moderate (-100 = SYMHmin < -60 nT), and strong (-150 = SYM-Hmin < -100 nT) geomagnetic storms. The main results obtained from the present study are as follows: (a) The TEC enhancements related to the tongue of ionization (TOI), auroral oval, and storm-enhanced density (SED) plume are more dominant in winter than in summer during the main phase of geomagnetic storms. (b) The structure of the mid-latitude trough in the nighttime sector becomes unclear in winter. (c) The TEC depletion at auroral and mid-latitudes (40°-70° GMLAT: geomagnetic latitude) starts to appear in the morning sector (8-10 hr GMLT: geomagnetic local time) during the main phase of geomagnetic storms and the decreased region extends in the lower latitude and GMLT directions with time. The negative storm activity tends to be enhanced significantly as the storm intensity becomes larger. The activity of the TEC depletion is dominant in summer than in winter, which is agreement with the classical ionospheric storm scenario. Plain Language Summary The ionosphere is the region where Earth's upper atmosphere (80-1,600 km) is partially ionized by solar extreme ultraviolet and ultraviolet radiation and is sandwiched between the lower atmosphere and the magnetosphere. For that reason, a spatial distribution of electron density in the ionosphere varies significantly due to atmospheric waves propagating from the lower atmosphere and input of electromagnetic energy from the magnetosphere. Furthermore, the structure of electron density in the ionosphere gives a strong influence on the nature of propagation of electromagnetic waves and its variation degrades the accuracy of all radio communication and navigation transmissions traveling the ionosphere. Therefore, we need to understand what causes the temporal and spatial variation of the electron density in the ionosphere during geomagnetic storms using long-term multi-point observation data. In this article, to establish the statistical behavior of global total electron content (TEC) during the main and recovery phases of geomagnetic storms, we conducted a super epoch analysis of interplanetary magnetic field (IMF), solar wind, geomagnetic indices (AE and SYM-H), and global navigation satellite system (GNSS)-TEC data for 20 yr (2000-2019). As a result, the structure of ionospheric electron density is different between the main and recovery phases. [ABSTRACT FROM AUTHOR]

We applied a novel three-dimensional spectral analysis method to GPS-TEC perturbation (GPSTECP) maps to study the propagation direction of daytime and nighttime medium-scale traveling ionospheric disturbances (MSTIDs) over North America. By this method, we can automatically calculate the phase velocity spectrum and directionality of MSTIDs. We focused on the periods of high MSTIDs occurrence, namely, June-July 2006 and November-December 2006, to study nighttime and daytime MSTIDs. We divided North America into the west (100°-130°W, 25°-55°N) and east (70°-100°W, 25°-55°N) parts. Our results show that both daytime and nighttime MSTID propagations exhibit strong longitudinal variations as a function of local time and day-to-day variations. The power peaks of daytime MSTIDs are from 10:00-16:00 LT in the west part and 10:00-14:00 LT in the east part. The predominant propagation directions of daytime MSTIDs are southward (southeastward) in the west (east). The daytime local time variations demonstrate that the MSTIDs display directional change in the west part; however, a similar directional change is not very pronounced in the east part. The local time variations of nighttime MSTIDs shows the power peaks from 22:00-02:00 LT in the west and 20:00-00:00 LT in the east. We found that the predominant propagation direction in the west part is westward with a wider azimuthal band (~210°-300°) than the east part (~210°-240°). By comparing nighttime propagation directions between the western and eastern parts, we reached the conclusion that the magnetic declination angle affects the propagation direction. [ABSTRACT FROM AUTHOR]

We have studied atmospheric gravity waves (AGWs) and nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) by applying three‐dimensional spectral analysis technique to 557.7‐ and 630.0‐nm airglow images at Shigaraki (SGK) (35°N, 136°E, 1999–2017) and Rikubetsu (RIK) (44°N, 144°E, 1999–2017), Japan, Athabasca (ATH), Canada, (55°N, 247°E, 2005–2017), and Magadan (MGD), Russia (60°N, 151°E, 2008–2017), focusing on their horizontal wavenumber spectra. For the AGWs in 557.7‐nm images, the power spectra in summer are stronger than in other seasons, probably due to stronger tropospheric convection. The highest energy content of the waves are mostly at wavelengths between 20 and 300 km at MGD, ATH, and RIK, while it is above 200 km at SGK. The largest power spectral density is obtained at RIK at wavelengths of 30–100 km and then ATH. The slopes of the horizontal wavenumber spectra varies from −2.77 to −3.22. From the MSTIDs in 630.0‐nm images, the power spectra in summer at RIK and SGK are stronger than those in other seasons regardless of solar activity. The power spectra in solar quiet time are stronger than those in solar active time at all four stations. These features can be explained by the Perkins instability with coupling between sporadic E and F layers. The spectral slope decreases with increasing latitudes. Weak positive correlations were obtained between the daily wave power of AGWs in 557.7‐nm images and MSTIDs in 630.0‐nm images, suggesting that the MSTIDs in the thermosphere may be partially generated by the AGWs from the mesopause region. Plain Language Summary: In this paper we study atmospheric gravity waves (AGWs) and nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) observed at four stations in Japan, Canada, and Russia, in 557.7‐ and 630.0‐nm airglow images over more than 10 years. The 557.7‐nm airglow has an emission layer at altitudes of 90–100 km (mesopause region). The waves seen in the 557.7‐nm airglow images mainly indicates AGWs. The 630.0‐nm airglow has an emission layer at altitudes of 200–300 km (bottomside ionosphere). The waves seen in the 630.0‐nm airglow images mainly indicates MSTIDs in the ionosphere. The AGWs in the mesopause region are the main driver of global atmospheric circulation in the middle atmosphere. The MSTIDs in the bottomside ionosphere are one of the causes of the satellite positioning error. We show typical energy content, propagation direction, and wavelengths of these waves at these two altitudes and discuss possible reason of the observed characteristics. These results contribute to our understanding of generation and propagation of AGWs and MSTIDs in the upper atmosphere. Key Points: We conducted the statistical analysis of the horizontal wavenumber spectra of waves in airglow images in Japan, Canada, and RussiaDifferences of horizontal wavenumber spectra at different locations, seasons, and solar activities are quantitatively obtainedWeak positive correlation of the power between atmospheric gravity waves and medium‐scale traveling ionospheric disturbances was observed [ABSTRACT FROM AUTHOR]

The solar radiation focusing effect is related to the specific phenomenon of propagation of the Sun-emitted HF and VHF waves through terrestrial ionosphere. This natural effect is observed with ground-based radio instruments running within the 10–200 MHz range as distinctive patterns—the spectral caustics (SCs)—on the solar dynamic spectra. It has been suggested that SCs are associated with medium-scale traveling ionospheric disturbances (MSTIDs). In this paper, we present the first direct observations of SCs induced by MSTIDs, using solar dynamic spectra with SCs obtained by different European radio telescopes on 2014 January 8 and simultaneous two-dimensional detrended total electron content (dTEC) maps over Europe. Spatial examination of dTEC maps as well as precise timing analysis of the maps and the dynamic spectra have been performed. First, we found several pairs of one-to-one (TID-SC) correspondences. The study provides strong observational evidence supporting the suggestion that MSTIDs are the cause of SCs. [ABSTRACT FROM AUTHOR]