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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.

“Ultra‐fast” Kelvin waves (UFKWs) serve as a mechanism for coupling the tropical troposphere with the mesosphere, thermosphere and ionosphere. Herein, solutions to the linearized wave equations in a dissipative thermosphere in the form of “Hough Mode Extensions (HMEs)” are employed to better understand the vertical propagation of the subset of these waves that most effectively penetrate into the thermosphere above about 100 km altitude; namely, UFKWs with periods ≲4 days, vertical wavelengths (λ_z) ≳30 km, and zonal wavenumbers = −1. Molecular dissipation is found to broaden latitude structures of UFKWs with increasing height while their vertical wavelengths (λ_z) increase with latitude. Collisions with ions fixed to Earth's magnetic field (“ion drag”) are found to dampen UFKW amplitudes, increasingly so as the densities of those ions increase with increased solar flux. The direct effect of ion drag is to decelerate the zonal wind. This leads to suppression of vertical velocity and the velocity divergence, and related terms in the continuity and thermal energy equations, respectively, that lead to diminished perturbation temperature and density responses. Access is provided to the UFKW HMEs analyzed here in tabular and graphical form, and potential uses for future scientific studies are noted.

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.

Numerical experiments are performed using a suite of general circulation models that enable the interaction between a Kelvin wave packet and the ionosphere‐thermosphere (IT) to be elucidated. Focus is on an eastward‐propagating ultra‐fast Kelvin wave (UFKW) packet with periods between 2 and 4 days and zonal wavenumbers=−1 during day of year (DOY) 266–281, 2009. Dissipative processes modify the classic UFKW dynamics (equatorially trapped, small meridional wind component) in three ways: (1) molecular diffusion acts to spread the UFKW zonal (u) and meridional (v) wind fields meridionally, pole to pole, asuandv, respectively, decay and grow with increasing height; (2) due to molecular diffusion, the UFKW spectrum at longer periods and with shorter vertical wavelengths preferentially dissipates with height; and (3) interaction with the diurnally varying IT introduces a westward‐propagatings=+2 component to the wind field that significantly modifies its longitude‐UT structure to include a diurnal modulation. The F‐region ionosphere also responds withs=+2, which originates from the influence of diurnally varying E‐region conductivity onE×Bdrifts. Additional spectral peaks invand ionospheric parameters arise due to longitude variations in the magnetic field. Maximum excursions in NmF2 (as compared with those from a simulation without UFKW forcing) achieve values as large as ±50% but more commonly occur inmore »the range of ±20–30%. The combination of positive and negative responses, and their relative magnitudes, depends on the phasing of the UFKW as it moves zonally relative to the Sun‐synchronous diurnal variation of the ionosphere, in addition to its changing amplitude between DOY 266 and 282. Modifications of order 10 ms⁻¹and −7% to zonal‐mean zonal winds and NmF2, respectively, also result from dissipation of the UFKW packet.

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.