Junyi Wang

On the basis of observations from Sanya incoherent scatter radar (SYISR), we investigated the vertical structures of multiple medium‐scale traveling ionospheric disturbances (MSTIDs) over low latitudes on December 28, 2022. Nine MSTIDs were observed to occur continuously during the night (17:18–00:18 LT) at 150–250 km altitude. These MSTIDs are generated by gravity waves (GWs) propagating from the lower atmosphere. Among them, five GWs exhibit periods of 34–49 min and horizontal phase velocities of 148–324 m/s, while the remaining four displayed periods of 17–22 min and horizontal phase velocities of 134–276 m/s. 8 of 9 GWs propagated northwestward with azimuths of 289°–339°, whereas one 4th GW propagated northeastward at an azimuth of 59°. Three‐dimensional reconstructions of the multiple MSTIDs reveal phase fronts that initially appeared at higher altitudes and subsequently at lower altitudes, which is consistent with the phase progression of upward‐propagating GWs. All GWs exhibit downward vertical phase velocities of 91–265 m/s and amplitudes of ∼3%–9%. The vertical and horizontal wavelengths of these GWs increased with altitude, whereas the periods and amplitudes initially increased with altitude, peaked near the bottom of the F layer, and then decreased. This altitudinal variation reflects the dissipative filtering of upward‐propagating GWs through viscosity and thermal conduction. Ray‐tracing results suggest that the nine MSTIDs likely originated from secondary waves, which were excited by primary waves generated by deep convection.

This study investigated the response of the ionospheric electron temperature ( T _e ) over Sanya to the super geomagnetic storm of May 2024. The observations, conducted with the Sanya Incoherent Scatter Radar (SYISR; 18.3°N, 109.6°E, dip latitude: 12.8°N) using a zenith‐directed uncoded long pulse, provided the T _e from May 10 to 12, 2024. On May 11, the daytime (nighttime) T _e at 400 km increased ∼40% (∼30%) relative to the quiet‐time reference, and a wave‐like variation of T _e was also observed. On May 12, a unique T _e increase occurred above ∼270 km, with the T _e at 400 km increasing ∼90% relative to the quiet‐time reference and remaining increased for ∼7 hr. Further Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations reproduced the observed T _e relatively well under realistic geophysical conditions. The TIEGCM simulations indicated that the decrease in the ionospheric electron density, which was associated with the unique T _e increase, was dominated by chemistry at ∼270–380 km, while E × B drifts and neutral winds contributed most successively at higher altitudes. The simulations also revealed that the relative contribution of the electron‐neutral cooling was comparable with that of the electron‐ion cooling at 400 km and 09:00 LT on May 12, when the unique T _e increase mainly occurred.

Previous observations have demonstrated that rocket exhaust can trigger ionospheric electron density holes and cause electron temperature enhancements during launches. Using high‐precision measurements from the Sanya incoherent scatter radar, we discovered a previously overlooked decrease in electron temperature. We report a daytime event and a nighttime event. During daytime, the electron temperature first decreased sharply by ∼650 K within 5.5 min, then increased by ∼1000 K before gradually recovering over ∼1 hr. Multi‐beam observations indicated that this cooling was localized near the rocket trajectory, much narrower than the accompanying electron density depletion. Moreover, the ion temperature increased by 550 K. The nighttime electron and ion temperatures decreased by ∼450 and ∼150 K, respectively, without subsequent electron temperature increase. Analysis indicated that these temperature perturbations resulted from the initial expansion cooling following the exhaust injection and the subsequent hole formation, which disrupted the thermal equilibrium in the ionosphere.

Sporadic E (Es) layer plays essential role in ionosphere‐atmosphere coupling. Theoretical simulations show that the atmospheric gravity waves (GWs) in the lower thermosphere are the main modulatory source on Es dynamics at small spatial scales and short periods. Based on Sanya Incoherent Scatter Radar, direct observational evidence for the mesoscale GW modulations on low‐latitude Es layer is firstly obtained by four‐dimensional electron density with extreme fine resolutions of 37.5 m in range and 50 s in time. The wave‐like horizontal structures, with ∼10 min period and ∼55 km wavelength, indicate the typical features of mesoscale GWs. Our results confirm that, interacting with the background tidal winds, the upward propagating GWs modulate the horizontal structures and drift velocity of Es layers at different altitudes with increasing amplitude to cause the rapid oscillations in range.

Day‐to‐day variability is a persistent feature of the ionosphere, but distinguishing contributions from the atmosphere below or geospace above remains unclear. Here, simultaneous observations of thermospheric meridional winds and ionospheric electron density from the Sanya Incoherent Scatter Radar (SYISR) reveal that the terdiurnal tide (TDT), a crucial agent in atmosphere‐ionosphere coupling, is strongly correlated with ionospheric F‐layer oscillations on a day‐to‐day basis. During a major sudden stratospheric warming in 2024, day‐to‐day variability of both the TDT amplitudes and F‐layer oscillations exhibited pronounced modulation by enhanced planetary waves (PWs). Bispectral analysis revealed nonlinear interactions between PWs, the TDT, and secondary waves at lower and upper sidebands. The ionospheric bispectrum showed peaks shifted toward shorter periods than the PWs themselves. Numerical simulations indicate that variations in magnetospheric forcing modify the ionospheric response to PW‐modulated tides. Our results clarify how the ionosphere responds to coupled lower atmospheric and geospace disturbances on daily timescales.

New high‐resolution measurements by Sanya incoherent scatter radar (ISR) employing a wide‐bandwidth chirp signal now enable spatiotemporal resolutions of 37.5 m and 50 s. We have applied these new capabilities for observations of Kelvin‐Helmholtz instability (KHI) dynamics in the low‐latitude ionosphere E region. The KHI evolution exhibited ∼±150 m oscillations in altitude, billows “periods” of ∼3–8 min due to horizontal advection, and 1–3 km vertical extents in the sporadic E layers, consistent with the classic understanding of KHI at these altitudes. Our results confirm the quasi‐period character of KHI and first reveal the KHI modulates the plasma layers by horizontal advection in direct observation. They also demonstrate the benefits of improved spatiotemporal resolutions for more general studies of the low‐latitude E region ionosphere.

Collisions between oxygen atoms and oxygen ions play a crucial role in the momentum and energy exchange between the ionosphere and the thermosphere. However, direct measurement of O ⁺ ‐O collision frequency or parameterizing this factor has been challenging over years. By utilizing the newly built Sanya incoherent scatter radar wind profiles, we have proposed a new method to calculate the O ⁺ ‐O collision frequency factors, that is, Burnside factor, for the first time from the whole year SYISR data set. Our results show that the mean Burnside factor is 1.31, although it ranges from 1.07 to 1.41 as a function of local time and from 1.19 to 1.41 with respect to the seasons. The reliability of the method has been validated through an independent estimation by a co‐located Fabry‐Perot interferometer and SYISR winds. To our knowledge, this is the first comprehensive analysis of Burnside factor, which are of great importance in collision frequency parameterization.

Previous observations shows that during rocket launches, the water in the plume released by rockets usually caused a wide range of ionosphere electron density depletion. While a number of publications devoted to the horizontal variations of such ionosphere holes, observation of the vertical features of ionosphere holes has seldom been reported. In this paper, we used the Sanya Incoherent Scattering Radar (SYISR) to observe the vertical variations of the ionosphere hole during two rocket launch events in year 2022. Both the rockets passed through the topside ionosphere over the China South Sea, with an estimated minimum distance of ~100-300 km from SYISR. About ~15 min after the rocket launch, we observed the ionosphere hole with an altitude range of ~200-800 km. The maximum electron density depletion of -20% occurred at ~420 km, with a duration of 2.2 hours. Based on the observations, we discussed the diffusion of water molecules and its influences on altitude distribution of the ionosphere holes during the rocket launches.

This study investigated the day‐to‐day variability of ionospheric electron temperature (T_e) over Sanya using the newly built Sanya Incoherent Scatter Radar (SYISR; 18.3°N, 109.6°E, dip latitude: 12.8°N). The 24 days of observations with transmitted beams encoded by the alternating code and directed toward the zenith provided T_e profiles during April–May 2022. The spatial‐temporal resolution was as precise as 4.5 km and ∼120 s. The day‐to‐day variability of T_e over Sanya thus revealed, which was strong in the sunrise T_e peak region (above ∼260 km, duration increases from ∼0.5 to ∼3.5 hr with altitude) and the top of the 200–300 km T_e bulge (∼260 to ∼300 km, lasting from ∼09:00 LT to ∼18:30 LT). The day‐to‐day variability of T_e in the two regions could reach ∼26% and ∼16% of the average T_e, respectively. Although the day‐to‐day variability of T_e is negatively correlated with the variation of ionospheric electron densities (N_e) in general, a positive correlation still occurred during the daytime, especially at 400–480 km from ∼17:00 LT to ∼18:00 LT. Further Thermosphere Ionosphere Electrodynamics General Circulation Model simulations reproduced the positive correlation, which may result from the internal heat exchange between electrons, ions, and neutrals. The simulations suggested that the variation of solar radiation/geomagnetic activity contributes most to the daytime/sunrise day‐to‐day variability of T_e over Sanya.