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Second harmonic generation is the lowest-order wave-wave nonlinear interaction occurring in, e.g., optical, radio, and magnetohydrodynamic systems. As a prototype behavior of waves, second harmonic generation is used broadly, e.g., for doubling Laser frequency. Second harmonic generation of Rossby waves has long been believed to be a mechanism of high-frequency Rossby wave generation via cascade from low-frequency waves. Here, we report the observation of a Rossby wave second harmonic generation event in the atmosphere. We diagnose signatures of two transient waves at periods of 16 and 8 days in the terrestrial middle atmosphere, using meteor-radar wind observations over the European and Asian sectors during winter 2018–2019. Their temporal evolution, frequency and wavenumber relations, and phase couplings revealed by bicoherence and biphase analyses demonstrate that the 16-day signature is an atmospheric manifestation of a Rossby wave normal mode, and its second harmonic generation gives rise to the 8-day signature. Our finding confirms the theoretically-anticipated Rossby wave nonlinearity.

During sudden stratospheric warming events, the ionosphere exhibits phase‐shifted semidiurnal perturbations, which are typically attributed to vertical coupling associated with the semidiurnal lunar tide (M2). Our understanding of ionospheric responses to M2 is limited. This study focuses on fundamental vertical coupling processes associated with the latitudinal extent and hemispheric asymmetry of ionospheric M2 signatures using total electron content data from the American sector. Our results illustrate that the asymmetry maximizes at 15°N and 20°S magnetic latitudes. In the Southern Hemisphere, the M2‐like signatures extend deep into midlatitude and encounter the Weddell Sea Anomaly. The time evolution of the Anomaly exhibits a distortion, which is attributed to an M2 modulation. The hemispheric asymmetry of M2 signatures in the low latitude can be primarily explained by the transequatorial wind modulation of the equatorial plasma fountain. Other physical processes could also be relevant, including hemispheric asymmetry of the M2 below the F‐region, the ambient thermospheric composition and ionospheric plasma distribution, and the geomagnetic field configuration.

A thermospheric O and N₂column density ratio (∑O/N₂) depletion with long‐duration (>16 hr) was observed by the Global‐scale Observations of the Limb and Disk at the Atlantic longitudes (75W–20W) and middle latitudes (20N–50N) during the recovery phase of the 8 June 2019 geomagnetic storm. The National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations reproduced the ∑O/N₂depletion patterns with a similar magnitude, and indicated that the composition recovery at middle latitudes began several hours after the beginning of the recovery phase of the geomagnetic storm. The TIEGCM simulations enable quantitative analysis of the physical mechanisms driving the middle‐latitude composition changes during the storm recovery phase. This analysis indicates that vertical advection and molecular diffusion dominated the initial recovery of composition perturbations at middle latitudes. Horizontal advection was also a main driver in the initial recovery of composition, but its contribution decreased rapidly. In the late recovery phase, the composition recovery was mainly determined by horizontal advection. In comparison, vertical advection and molecular diffusion played a much less important role.

A suite of general circulation models is used to investigate the surface magnetic perturbations due to the ionospheric currents driven by an eastward‐propagating ultrafast Kelvin wave (UFKW) packet with periods between 2 and 4 days and zonal wave number. The simulated daytime UFKW‐driven meridional magnetic perturbations dBn (∼±5 nT) (or zonal currents) between about 5° and 20° magnetic latitude in each hemisphere are opposite in sign to those equatorward of±5° and produced by the equatorial electrojet (EEJ), with the directions on any given day determined by the phase of the UFKW as it propagates eastward with respect to the sunlit ionosphere. Since the nominal daytimeSqzonal current between∼±30° is uniformly eastward flowing, the present results are consistent with the hypothesis that the EEJ is part of a local current vortex with oppositely directed currents near the equator versus those between 5° and 20° at low latitudes. UFKWs are a special wave type wherein meridional winds are relatively small, which leads to our finding that the EEJ dBn constitutes a simple quantitative proxy forE‐region UFKW neutral winds near the 107‐km peak height of the Hall conductivity, including the variable wave period of the UFKW packet. Numerical experiments are also performed tomore »understand the longitude distribution of actual ground magnetometer measurements that are needed to reliably extract the UFKW dBn signal and hence the neutral winds, both of which are closely linked to plasma drifts and electron densities in the equatorialFregion. Using actual magnetometer data it is moreover shown that the UFKW dBn signal is easily measurable. Therefore measurements of EEJ dBn can potentially be used to infer UFKW activity for scientific investigations focusing on coupling between the tropical troposphere and the ionosphere‐thermosphere.

Mesospheric winds from three longitudinal sectors at 65°N and 54°N latitude are combined to diagnose the zonal wave numbers (m) of spectral wave signatures during the Southern Hemisphere sudden stratospheric warming (SSW) 2019. Diagnosed are quasi‐10‐ and 6‐day planetary waves (Q10DW and Q6DW,m = 1), solar semidiurnal tides withm = 1, 2, 3 (SW1, SW2, and SW3), lunar semidiurnal tide, and the upper and lower sidebands (USB and LSB,m = 1 and 3) of Q10DW‐SW2 nonlinear interactions. We further present 7‐year composite analyses to distinguish SSW effects from climatological features. Before (after) the SSW onset, LSB (USB) enhances, accompanied by the enhancing (fading) Q10DW, and a weakening of climatological SW2 maximum. These behaviors are explained in terms of Manley‐Rowe relation, that is, the energy goes first from SW2 to Q10DW and LSB, and then from SW2 and Q10DW to USB. Our results illustrate that the interactions can explain most wind variabilities associated with the SSW.

TIMED/GUVI limb measurements and first‐principles simulations from the Thermosphere Ionosphere Electrodynamics Global Circulation Model (TIEGCM) are used to investigate thermospheric atomic oxygen (O) and molecular nitrogen (N₂) responses in the middle thermosphere on a constant pressure surface (∼160 km) to the November 20 and 21, 2003 superstorm. The consistency between GUVI observations and TIEGCM simulated composition changes allows us to utilize TIEGCM outputs to investigate the storm‐time behaviors of O and N₂systematically. Diagnostic analysis shows that horizontal and vertical advection are the two main processes that determine the storm‐induced perturbations in the middle thermosphere. Molecular diffusion has a relatively smaller magnitude than the two advection processes, acting to compensate for the changes caused by the transport partly. Contributions from chemistry and eddy diffusion are negligible. During the storm initial and main phases, composition variations at high latitudes are determined by both horizontal and vertical advection. At middle‐low latitudes, horizontal advection is the main driver for the composition changes where O mass mixing ratiodecreases (N₂mass mixing ratioincreases); whereas horizontal and vertical advection combined to dominate the changes in the regions whereincreases (decreases). Over the entire storm period, horizontal advection plays a significant rolemore »in transporting high‐latitude composition perturbations globally. Our results also demonstrate that storm‐time temperature changes are not the direct cause of the composition perturbations on constant pressure surfaces.