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The most fantastic optical phenomena in the Earth's upper atmosphere are the auroras. They are highly informative indicators of solar activity, geomagnetic activity, upper atmospheric structures and dynamics, and magnetospheric energetic particles. An area where the geomagnetic field differs significantly from the expected symmetric dipole, such as at the South Atlantic Anomaly, where the magnetic field intensity is low, gives rise to stronger precipitation of energetic particles into the upper atmosphere. Impact excitation and the subsequent airglow emissions exhibit aurora-like dynamic signatures. Nomenclatures of nonpolar aurora or equatorial auroras are similar to those used with the polar auroras owing to their similar excitation mechanisms. This paper provides an overview of the knowledge and the challenges concerning auroral activity at the South Atlantic Anomaly, or more generally, at the negative magnetic anomaly. We emphasize systematic investigation of the equatorial auroras to reveal the temporal and spatial evolution of the magnetic anomaly and the behaviour of energetic particles in near-Earth space. [ABSTRACT FROM AUTHOR]

Sudden stratospheric warming events (SSWs) are the most spectacular atmospheric vertical coupling processes, well‐known for being associated with diverse wave activities in the upper atmosphere and ionosphere. The first four solar tidal harmonics have been reported as being engaged. Here, combining mesospheric winds detected by three midlatitude radars, we demonstrate at least the first six harmonics that occurred during SSW 2018. Wave number diagnosis demonstrates that all six harmonics are dominated by migrating components. Wavelet analyses reveal that the fourth, fifth, and sixth harmonics quench after the SSW onset. The six harmonics and the quenching appear also in a statistical analysis based on near‐12‐year observations from one of the radars. We attribute the quenching to reversal of the background eastward wind. Plain Language Summary: Solar tides are the most predictably occurring waves in the upper atmosphere. Although the dynamical theory can be dated back to Laplace in the sixteenth century, upper atmospheric tides were rarely studied observationally until satellites and ground‐based radars became common. To date, observational studies have mainly dealt with low‐order solar‐day harmonics. Here, we combine mesospheric wind observations from three longitudinal sectors to investigate high‐order harmonics. Results illustrate that the first six harmonics appear in early 2018, all of which are dominated by Sun‐synchronous components. Among these harmonics, the fourth, fifth, and sixth quench at the sudden stratospheric warming onset, which we attribute to variations in the background zonal wind. Key Points: First six solar tidal harmonics occur in the mesospheric wind during SSW 2018 among which the fourth, fifth, and sixth harmonics quench at the SSW onsetWave number diagnosis using multistation techniques suggests that all six harmonics are dominated by migrating tidesIn a near‐12‐year statistics, the six harmonics and quenching also occur [ABSTRACT FROM AUTHOR]

Abstract: This work studies the sudden increases in total electron content of the ionosphere caused by the very intense solar flare on July 14, 2000. Total electron content (TEC) data observed from a Global Positioning System (GPS) network are used to calculate the flare-induced TEC increment, δTECf, and variation rate, dTECf/dt. It is found that both dTECf/dt and δTECf are closely related with the solar zenith angles. To explain the observation results, we derived a simple relationship between the partial derivative of the flare-induced TEC, ∂TECf/∂t, which is a good approximation for dTECf/dt, and the solar zenith angle χ, as well as the effective flare radiation flux I f, according to the well-known Chapman theory of ionization. The derived formula predicted that ∂TECf/∂t is proportional to I f and inverse proportional to Chapman function ch(χ). This theoretical prediction not only explains the correlation of dTECf/dt and δTECf with χ as shown in our TEC observation, but also gives a way to deduce I f from TEC observation of GPS network. Thus, the present work shows that GPS observation is a powerful tool in the observation and investigation of solar flare effects on the ionosphere, i.e., the sudden ionospheric disturbances, which is a significant phenomenon of space weather. [Copyright &y& Elsevier]

Abstract: The annual and semi-annual variations of the ionosphere are investigated in the present paper by using the daytime F2 layer peak electron concentration (NmF2) observed at a global ionosonde network with 104 stations. The main features are outlined as follows. (1) The annual variations are most pronounced at magnetic latitudes of 40–60° in both hemispheres, and usually manifest as winter anomalies; Below magnetic latitude of 40° as well as in the tropical region they are much weaker and winter anomalies that are not obvious. (2) The semi-annual variations, which are usually peak in March or April in most regions, are generally weak in the near-pole regions and strong in the far-pole regions of both hemispheres. (3) Compared with their annual components, the semi-annual variations in the tropical region are more significant. In order to explain the above results, we particularly analyze the global atomic/molecular ratio of [O/N2] at the F2 layer peak height by the MSIS90 model. The results show that the annual variation of [O/N2] is closely related with that of NmF2 prevailing in mid-latitudes and [O/N2] annual variation usually may lead to the winter anomalies of NmF2 occurring in the near-pole region. Moreover, NmF2 semi-annual variations appearing in the tropical region also have a close relationship with the variation of [O/N2]. On the other hand, the semi-annual variations of NmF2 in the far-pole region cannot be simply explained by that of [O/N2], but the variation of the solar zenith angle may also have a significant contribution. [Copyright &y& Elsevier]

The solar cycle variability of thermospheric circulation is one of the outstanding questions involving the upper atmosphere. However, it needs to be identified due to the latitude and longitude dependence of neutral winds. This article examines the solar activity trends in the vertical component of equivalent winds (VEWs) derived from ionosonde data during 1966–1985 over Wuhan (114.4° E, 30.6° N, 45.2° dip) of China. The monthly median data of routine ionosonde measurements in published tables are used to derive VEWs which are contributions from both neutral winds and zonal electric fields. The solar activity trends of VEWs over Wuhan have an obvious local time and seasonal dependence. Over Wuhan, the magnitudes of VEWs decrease with increasing solar activity around midnight for all seasons and around midday in winter and autumn. In contrast, little variation is found throughout the solar cycle around midday in summer and spring. The magnitudes of daily means and amplitudes of diurnal and semidiurnal components of VEWs over Wuhan decrease with solar activity, which also confirms that it is not a feature of a particular location. There is a more pronounced decrease in diurnal amplitudes of VEWs than other harmonic components over Wuhan. The decrease in diurnal amplitudes with increasing solar activity can be reasonably explained, if the greater ion drag at solar maximum compensates for pressure gradients due to the greater EUV input. [Copyright &y& Elsevier]

A theoretical ionospheric model is employed to investigate the ionospheric behavior as observed by the incoherent-scatter radar (ISR) at Millstone Hill during the September 21–27, 1998 storm. The observed NmF2 presented a significant negative phase on September 25, and a G condition (hmF2<200 km) was also observed. The model results based on the standard input parameters (climatological model values) are in good agreement with the observed electron densities under quiet conditions, but there are large discrepancies during disturbed periods. The exospheric temperature Tex, neutral winds, atomic oxygen density [O] and molecular nitrogen density [N2], and solar flux are inferred from the ISR ion temperature profiles and from the electron density profiles. Our calculated results show that the maximum Tex is higher than 1700 K, and an averaged decrease in [O] is a factor of 2.2 and an increase in [N2] at 300 km is about 1.8 times for the disturbed day, September 25, relative to the quiet day level. Therefore, the large change of [N2]/[O] ratio gives a good explanation for the negative phase at Millstone Hill during this storm. Furthermore, at the disturbed nighttime the observations show a strong NmF2 decrease, accompanied by a significant hmF2 increase after the sudden storm commencement (SSC). Simulations are carried out based on the inferred Tex. It is found that the uplift of F2 layer during the period from sunset to post-midnight is mainly associated with the large equatorward winds, and a second rise in hmF2 after midnight results from the depleted Ne in the bottom-side of F2 layer due to the increased recombination, while the “midnight collapse” of hmF2 is attributed to the large-scale traveling atmospheric disturbances. [Copyright &y& Elsevier]

Abstract: In this paper, we investigate the solar flare effects of the ionosphere at middle latitude with a one-dimensional ionosphere theoretical model. The measurements of solar irradiance from the SOHO/Solar EUV Monitor (SEM) and GOES satellites have been used to construct a simple time-dependent solar flare spectrum model, which serves as the irradiance spectrum during solar flares. The model calculations show that the ionospheric responses to solar flares are largely related to the solar zenith angle. During the daytime most of the relative increases in electron density occur at an altitude lower than 300km, with a peak at about 115km, whereas around sunrise and sunset the strongest ionospheric responses occur at much higher altitudes (e.g. 210km for a summer flare). The ionospheric responses to flares in equinox and winter show an obvious asymmetry to local midday with a relative increase in total electron content (TEC) in the morning larger than that in the afternoon. The flare-induced TEC enhancement increases slowly around sunrise and reaches a peak at about 60min after the flare onset. [Copyright &y& Elsevier]