Wave Driver Research Articles

Kelvin-Helmholtz instability and heating in oscillating loops perturbed by power-law transverse wave drivers

Context. Instabilities in oscillating loops are believed to be essential for dissipating the wave energy and heating the solar coronal plasma. Aims. Our aim is to study the development of the Kelvin-Helmholtz (KH) instability in an oscillating loop that is driven by random footpoint motions. Methods. Using the PLUTO code, we performed 3D simulations of a straight gravitationally stratified flux tube. The loop footpoints are embedded in chromospheric plasma, in the presence of thermal conduction and an artificially broadened transition region. Using drivers with a power-law spectrum, one with a red noise spectrum and one with the low-frequency part subtracted, we excited standing oscillations and the KH instability in our loops, after one-and-a-half periods of the oscillation. Results. We see that our broadband drivers lead to fully deformed, turbulent loop cross-sections over the entire coronal part of the loop due to the spatially extended KH instability. The low RMS velocity of our driver without the low-frequency components supports the working hypothesis that the KH instability can easily manifest in oscillating coronal loops. We report for the first time in driven transverse oscillations of loops the apparent propagation of density perturbations due to the onset of the KH instability, from the apex towards the footpoints. Both drivers input sufficient energy to drive enthalpy and mass flux fluctuations along the loop, while also causing heating near the driven footpoint of the oscillating loop, which becomes more prominent when a low-frequency component is included in the velocity driver. Finally, our power-law driver with the low-frequency component provides a RMS input Poynting flux of the same order as the radiative losses of the quiet-Sun corona, giving us promising prospects for the contribution of decayless oscillations in coronal heating.

The magnetohydrodynamic equations in terms of waveframe variables

Generalising the Elsässer variables, we introduce the $Q$ -variables. These are more flexible than the Elsässer variables, because they also allow us to track waves with phase speeds different than the Alfvén speed. We rewrite the magnetohydrodynamics (MHD) equations with these $Q$ -variables. We consider also the linearised version of the resulting MHD equations in a uniform plasma, and recover the classical Alfvén waves, but also separate the fast and slow magnetosonic waves into upward- and downward-propagating waves. Moreover, we show that the $Q$ -variables may also track the upward- and downward-propagating surface Alfvén waves in a non-uniform plasma, displaying the power of our generalisation. In the end, we lay the mathematical framework for driving solar wind models with a multitude of wave drivers.

Wave transformations near a coronal magnetic null point

Context. Null points are often invoked in studies of quasi-periodic coronal jets and in connection with periodic signals preceding actual reconnection events. Although the periodicity of these events spans a wide range of periods, most show a 2- to 5-min periodicity compatible with the global p-modes. Aims We investigate whether magnetohydrodynamics (MHD) waves, in particular, acoustic p-modes, can cause strong current accumulation at the null points. This can in turn drive localized periodic heating in the solar corona. Methods. To do this, we began with a three-dimensional numerical setup incorporating a gravitationally stratified solar atmosphere and an axially symmetric magnetic field including a coronal magnetic null point. To excite waves, we employed wave drivers mimicking global p-modes. Using our recently developed wave-mode decomposition technique, we investigated the process of mode conversion, mode transmission, and wave reflection at various important layers of the solar atmosphere, such as the Alfvén acoustic equipartition layer and transition region. We examined the energy flux distribution in various MHD modes or in acoustic and magnetic components, as the waves propagate and interact with a magnetic field of null topology. We also examined current accumulation in the surroundings of the null point. Results. We found that most of the vertical velocity is transmitted through the Alfvén acoustic equipartition layer and maintains an acoustic nature, while a small fraction generates fast waves via the mode conversion process. The fast waves undergo almost total reflection in the transition region due to sharp gradients in density and Alfvén speed. There are only weak signatures of Alfvén wave generation near the transition region through the fast-to-Alfvén mode conversion. Because the slow waves propagate with the local sound speed, they are not much affected by the density gradients in the transition region and undergo secondary mode conversion and transmission at the Alfvén-acoustic equipartition layer surrounding the null point. This leads to fast-wave focusing at the null point. These fast waves have associated perturbations in current density and show oscillatory signatures that are compatible with the second harmonic of the driving frequency. This might result in resistive heating and in an enhanced intensity in the presence of finite resistivity. Conclusions. We conclude that MHD waves are a potential source for oscillatory current dissipation around the magnetic null point. We conjecture that in addition to oscillatory magnetic reconnection, global p-modes could lead to the formation of various quasi-periodic energetic events.

A Beach Profile Evolution Model Driven by the Hybrid Shock-Capturing Boussinesq Wave Solver

An existing Boussinesq wave model, solved in a hybrid format of the finite-difference method (FDM) and finite-volume method (FVM), with good merits of stability and shock-capturing, was used as the wave driver to simulate the beach evolution under nearshore wave action. By coupling the boundary layer model, the sand transport model, and the terrain updating model, the beach evolution model is established. Based on the coupled model, the interaction process between sandbars and waves was simulated, reproducing the process of the original sand bars diminishing, new sandbars creating, and finally disappearing. At the same time, the formation and movement process of sand bars under solitary and regular waves are numerically simulated, in the breaking zone, the water bottom has a larger shear stress, which promotes the sediment activation, transport and erosion formation, and near the breaking point, the decrease of sand-carrying capacity is the main reason for the formation of sandbars, the numerical model can accurately describe the changes in the shoreline profile under wave action.

Understanding the properties, wave drivers, and impacts of electron microburst precipitation: current understanding and critical knowledge gaps

Understanding the properties, wave drivers, and impacts of electron microburst precipitation: current understanding and critical knowledge gaps

Development and Application of an Infragravity Wave (InWave) Driver to Simulate Nearshore Processes

AbstractInfragravity waves are key components of the hydro‐sedimentary processes in coastal areas, especially during extreme storms. Accurate modeling of coastal erosion and breaching requires consideration of the effects of infragravity waves. Here, we present InWave, a new infragravity wave driver of the Coupled Ocean‐Atmopshere‐Waves‐Sediment Transport (COAWST) modeling system. InWave computes the spatial and temporal variation of wave energy at the wave group scale and the associated incoming bound infragravity wave. Wave group‐varying forces drive free infragravity wave growth and propagation within the hydrodynamic model of the coupled modeling system, which is the Regional Ocean Modeling System (ROMS) in this work. Since ROMS is a three‐dimensional model, this coupling allows for the combined formation of undertow currents and infragravity waves. We verified the coupled InWave‐ROMS with one idealized test case, one laboratory experiment, and one field experiment. The coupled modeling system correctly reproduced the propagation of gravity wave energy with acceptable numerical dissipation. It also captured the transfer of energy from the gravity band to the infragravity band, and within the different infragravity bands in the surf zone, the measured three‐dimensional flow structure, and dune morphological evolution satisfactorily. The idealized case demonstrated that the infragravity wave variance depends on the directional resolution and horizontal grid resolution, which are known challenges with the approach taken here. The addition of InWave to COAWST enables novel investigation of nearshore hydro‐sedimentary dynamics driven by infragravity waves using the strengths of the other modeling components, namely the three‐dimensional nature of ROMS and the sediment transport routines.

Alfvénic Motions Arising from Asymmetric Acoustic Wave Drivers in Solar Magnetic Structures

Alfvénic motions are ubiquitous in the solar atmosphere and their observed properties are closely linked to those of photospheric p-modes. However, it is still unclear how a predominantly acoustic wave driver can produce these transverse oscillations in the magnetically dominated solar corona. In this study we conduct a 3D ideal MHD numerical simulation to model a straight, expanding coronal loop in a gravitationally stratified solar atmosphere which includes a transition region and chromosphere. We implement a driver locally at one foot-point corresponding to an acoustic–gravity wave which is inclined by θ = 15° with respect to the vertical axis of the magnetic structure and is similar to a vertical driver incident on an inclined loop. We show that transverse motions are produced in the magnetic loop, which displace the axis of the waveguide due to the breaking of azimuthal symmetry, and study the resulting modes in the theoretical framework of a magnetic cylinder model. By conducting an azimuthal Fourier analysis of the perturbed velocity signals, the contribution from different cylindrical modes is obtained. Furthermore, the perturbed vorticity is computed to demonstrate how the transverse motions manifest themselves throughout the whole non-uniform space. Finally we present some physical properties of the Alfvénic perturbations and present transverse motions with velocity amplitudes in the range 0.2–0.75 km s−1 which exhibit two distinct oscillation regimes corresponding to 42 and 364 s, where the latter value is close to the period of the p-mode driver in the simulation.

Heating and Cooling in Transversely Oscillating Coronal Loops Powered by Broadband, Multi-Directional Wave Drivers

Recent studies have identified the potential for coronal wave heating to balance radiative losses in a transversely oscillating low-density loop undergoing resonant absorption, phase mixing and the Kelvin–Helmholtz instability. This result relied on a continuous, resonant oscillatory driver acting on one of the loop footpoints and similar setups with non-resonant driving produce insufficient heating. Here, we consider broadband and multi-directional drivers with power in both resonant and non-resonant frequencies. Using three-dimensional magnetohydrodynamic simulations, we impose transverse, continuous velocity drivers at the footpoints of a coronal loop, which is dense in comparison to the background plasma. We include the effects of optically thin radiation and a uniform background heating term that maintains the temperature of the external plasma but is insufficient to balance energy losses within the loop. For both broadband and multi-directional drivers, we find that the energy dissipation rates are sufficient to balance the average energy losses throughout the simulation volume. Resonant components of the wave driver efficiently inject energy into the system and these frequencies dominate the energetics. Although the mean radiative losses are balanced, the loop core cools in all cases as the wave heating rates are locally insufficient, despite the relatively low density considered here.

Understanding the properties, wave drivers, and impacts of electron microburst precipitation: Current understanding and critical knowledge gaps

Microbursts are impulsive (<1s) injections of very energetic to relativistic electrons (energies from a few keV to MeV) into Earth’s atmosphere. They are important because they may represent a major loss process for the outer radiation belt (Ripoll and Claudepierre and Ukhorskiy and Colpitts and Li and Fennell and Crabtree, J. Geophys. Res. Space Physics, 2020, 125–e2019JA026735). Understanding and quantifying the underlying causes and consequences plus relative importance of microburst precipitation represent outstanding questions in radiation belt physics and may have significant implications ranging from space weather to atmospheric chemistry. Chorus waves are the likely dominant cause of microburst precipitation, but important questions remain regarding the exact nature of the resonance generating the microbursts and the overall importance of the precipitation. These important questions are limited by lack of systematic coordination of simultaneous observations of causative waves in the magnetosphere and resulting precipitating particles at low altitudes. Multi-spacecraft missions dedicated to answering these questions, themselves required to make progress in radiation belt physics, are critical.

What drives decayless kink oscillations in active-region coronal loops on the Sun?

Here, we present a study of the phenomena of decayless kink oscillations in a system of active-region (AR) coronal loops. Using high-resolution observations from two different instruments, namely the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, we follow these AR loops for an hour each on three consecutive days. Our results show significantly more resolved decayless waves in the higher resolution EUI data compared with the AIA data. Furthermore, the same system of loops exhibits many of these decayless oscillations on Day 2, while we detect very few oscillations on Day 3 , and find none at all on Day 1. Analysis of photospheric magnetic field data reveals that, most of the time, these loops were rooted in sunspots, where supergranular flows are generally absent. This suggests that supergranular flows, which are often invoked as drivers of decayless waves, are not necessarily driving such oscillations in our observations. Similarly, our findings also cast doubt on other possible drivers of these waves, such as a transient driver or mode conversion of longitudinal waves near the loop footpoints. In conclusion, our analysis suggests that none of the commonly suspected sources proposed to drive decayless oscillations in active-region loops seem to be operating in this event, and therefore the search for that elusive wave driver needs to continue.

3D MHD wave propagation near a coronal null point: New wave mode decomposition approach

Context. Ubiquitous vortex flows at the solar surface excite magnetohydrodynamic (MHD) waves that propagate to higher layers of the solar atmosphere. In the solar corona, these waves frequently encounter magnetic null points. The interaction of MHD waves with a coronal magnetic null in realistic 3D setups requires an appropriate wave identification method. Aims. We present a new MHD wave decomposition method that overcomes the limitations of existing wave identification methods. Our method allows for an investigation of the energy fluxes in different MHD modes at different locations of the solar atmosphere as waves generated by vortex flows travel through the solar atmosphere and pass near the magnetic null. Methods. We used the open-source MPI-AMRVAC code to simulate wave dynamics through a coronal null configuration. We applied a rotational wave driver at our bottom photospheric boundary to mimic vortex flows at the solar surface. To identify the wave energy fluxes associated with different MHD wave modes, we employed a wave decomposition method that is able to uniquely distinguish different MHD modes. Our proposed method utilizes the geometry of an individual magnetic field-line in the 3D space to separate the velocity perturbations associated with the three fundamental MHD waves. We compared our method with an existing wave decomposition method that uses magnetic flux surfaces instead. Over the selected flux surfaces, we calculated and analyzed the temporally averaged wave energy fluxes, as well as the acoustic and magnetic energy fluxes. Our wave decomposition method allowed us to estimate the relative strengths of individual MHD wave energy fluxes. Results. Our method for wave identification is consistent with previous flux-surface-based methods and provides the expected results in terms of the wave energy fluxes at various locations of the null configuration. We show that ubiquitous vortex flows excite MHD waves that contribute significantly to the Poynting flux in the solar corona. Alfvén wave energy flux accumulates on the fan surface and fast wave energy flux accumulates near the null point. There is a strong current density buildup at the spine and fan surface. Conclusions. The proposed method has advantages over previously utilized wave decomposition methods, since it may be employed in realistic simulations or magnetic extrapolations, as well as in real solar observations whenever the 3D fieldline shape is known. The essential characteristics of MHD wave propagation near a null – such as wave energy flux accumulation and current buildup at specific locations – translate to the more realistic setup presented here. The enhancement in energy flux associated with magneto-acoustic waves near nulls may have important implications in the formation of jets and impulsive plasma flows.

Modeling of Barrier Breaching During Hurricanes Sandy and Matthew

AbstractPhysical processes driving barrier island change during storms are important to understand to mitigate coastal hazards and to evaluate conceptual models for barrier evolution. Spatial variations in barrier island topography, landcover characteristics, and nearshore and back‐barrier hydrodynamics can yield complex morphological change that requires models of increasing resolution and physical complexity to predict. Using the Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport (COAWST) modeling system, we investigated two barrier island breaches that occurred on Fire Island, NY during Hurricane Sandy (2012) and at Matanzas, FL during Hurricane Matthew (2016). The model employed a recently implemented infragravity (IG) wave driver to represent the important effects of IG waves on nearshore water levels and sediment transport. The model simulated breaching and other changes with good skill at both locations, resolving differences in the processes and evolution. The breach simulated at Fire Island was 250 m west of the observed breach, whereas the breach simulated at Matanzas was within 100 m of the observed breach. Implementation of the vegetation module of COAWST to allow three‐dimensional drag over dune vegetation at Fire Island improved model skill by decreasing flows across the back‐barrier, as opposed to varying bottom roughness that did not positively alter model response. Analysis of breach processes at Matanzas indicated that both far‐field and local hydrodynamics influenced breach creation and evolution, including remotely generated waves and surge, but also surge propagation through back‐barrier waterways. This work underscores the importance of resolving the complexity of nearshore and back‐barrier systems when predicting barrier island change during extreme events.

Bispectral analysis of nonlinear mixing in a periodically driven Korteweg–de Vries system

The nonlinear response of a periodically driven Korteweg–de Vries model system is studied using a variety of nonlinear drivers and compared to previous results obtained for a purely time-dependent sinusoidal driver by Mir et al. [Phys. Plasmas 27, 113701 (2020)]. It is found that a nonlinear driver in the form of a cnoidal-square wave or a traveling wave driver produces a spectral response that is closer to experimental observations of Nosenko et al. [Phys. Rev. Lett. 92, 085001 (2004)] than that predicted by the simple sinusoidal driver. Using a bispectral analysis, we also firmly establish that the nature of the nonlinear oscillations, due to the interaction between the periodic source and the inherent collective mode of the system, is predominantly governed by a three-wave mixing process. Furthermore, by studying the variation in mixing patterns, from a broad to a sparse frequency spectrum, as a function of the driver frequency and its functional form, we propose a means of tailoring the nature of such patterns. Our results could find useful applications in the experimental interpretation and manipulation of nonlinear wave mixing patterns in weakly nonlinear and dispersive plasma systems or similar phenomena in neutral fluids.

Experimental investigation of supertransmission for an intrinsic localized mode in a cyclic nonlinear transmission line.

In this experimental study of the nonlinear loss mechanism between traveling localized excitation and the underlying extended normal mode spectrum for a 1D lattice, three types of cyclic, electric, nonlinear transmission lines (NLTLs) are used. They are nonlinear capacitive, inductive, and capacitive+inductive NLTLs. To maintain a robust, steady-state traveling intrinsic localized mode (ILM), a traveling wave driver is used. The ILM loses energy because of a resonance between it and the extended NLTL modes. A wake field excitation is detected directly from ILM velocity experiments by the decrease in ILM speed and by the observation of the wake. Its properties are quantified via a two-dimensional Fourier map in the frequency-wavenumber domain, determined from the measured spatial-time voltage pattern. Simulations support and extend these experimental findings. We find for the capacitive+inductive NLTL configuration, when the two nonlinear terms are theoretically balanced, the wake excitation is calculated to become very small, giving rise to supertransmission over an extended driving frequency range.

Numerical Analysis of Stem Wave Control Effect of a Curved Slit Caisson Breakwater

Curved slit caisson has been the preferred structural type of breakwater in South Korea, and effective control of stem waves is a crucial design factor significantly affecting the performance of a curved slit caisson breakwater. Most of the past studies on stem waves heavily relied on wave drivers like the cubic Schrödinger Eq. due to the intrinsic difficulties in analyzing stem waves. However, considering the perturbation method evoked in the derivation of cubic Schrödinger Eq., the wave driver mentioned above could give erroneous results in the rough sea due to the higher-order waves that appeared in the wave field by resonance wave-wave interaction. In this rationale, in this study, the numerical simulation was implemented to verify the stem wave control effect of curved slit caisson using the ihFoam, toolbox having its roots on OpenFoam. It was shown that curved slit caisson breakwater effectively alleviates the scope and height of stem waves.

Empirically revealed properties of Rieger-type cycles of stellar activity

Context. The Rieger cycles were discovered in the Sun as a specific 154-day periodicity of flare occurrence; they strongly influence terrestrial space weather. This phenomenon is far from being understood. Various proposed mechanisms for this periodicity need further verification in stars with stellar parameters different from those of the Sun. Aims. In this work, we study the Rieger-type cycle (RTC) periods PRTC of stellar activity surveyed in the photometric data of the Kepler space telescope. Methods. The processing of 1726 stellar light curves reveals statistics of PRTC values for different main-sequence stars with different effective temperatures Teff and periods of rotation P. This study uses as an index of stellar activity the squared amplitude of the first rotational harmonic A12 of the stellar light curve variability. Results. The obtained information on PRTC of the considered stars confirms the phenomenological analogy between stellar RTCs and the solar Rieger cycles. Two types of RTCs were found: (1) activity cycles with PRTC independent on the stellar rotation, which are typical for the stars with Teff ≲ 5500 K, and (2) activity cycles with PRTC proportional to the stellar rotation period P, which take place on stars with Teff ≳ 6300 K. These two types of RTCs can be driven by the Kelvin and Rossby waves, respectively. The Rossby wave-driven RTCs show a relation with the location of tachocline at shallow depths in the hot stars. This confirms the theoretical predictions of the connection of the RTC with the tachocline. At the same time, the Kelvin wave-driven RTCs do not show this connection. Apparently, both types of wave drivers of RTCs can coexist, resulting in the joint modulation of the magnetic flux tubes emergence by Kelvin and Rossby waves, and the corresponding behavior of PRTC. Conclusions. The signatures of two types of wave drivers discovered for RTCs and their different relations with the tachocline call for a revision and further elaboration of the theory of RTCs.

Very rare heat extremes: quantifying and understanding using ensemble re-initialization

AbstractHeat waves such as the one in Europe 2003 have severe consequences for the economy, society, and ecosystems. It is unclear whether temperatures could have exceeded these anomalies even without further climate change. Developing storylines and quantifying highest possible temperature levels is challenging given the lack of long homogeneous time series and methodological framework to assess them. Here, we address this challenge by analysing summer temperatures in a nearly 5000-year pre-industrial climate model simulation, performed with the Community Earth System Model CESM1. To assess how anomalous temperatures could get, we compare storylines, generated by three different methods: (1) a return-level estimate, deduced from a generalized extreme value distribution, (2) a regression model, based on dynamic and thermodynamic heat wave drivers, and (3) a novel ensemble boosting method, generating large samples of re-initialized extreme heat waves in the long climate simulation.All methods provide consistent temperature estimates, suggesting that historical exceptional heat waves as in Chicago 1995, Europe 2003 and Russia 2010 could have been substantially exceeded even in the absence of further global warming. These estimated unseen heat waves are caused by the same drivers as moderate observed events, but with more anomalous patterns. Moreover, altered contributions of circulation and soil moisture to temperature anomalies include amplified feedbacks in the surface energy budget. The methodological framework of combining different storyline approaches of heat waves with magnitudes beyond the observational record may ultimately contribute to adaptation and to the stress testing of ecosystems or socio-economic systems to increase resilience to extreme climate stressors.

Laser synthesis of Si nanopillars-based SERS for efficient chlorpyrifos detection

A uniform and high-density Si nanopillars-based SERS layer was synthesized; as an efficient means, for detecting ultra-low concentration of chlorpyrifos. A low (20 %) laser pulse duty cycle of 405 nm wavelength and 600mW/cm2 intensity from a laser diode was used to achieve laser–induced etching. A simple, low-cost, and stable current wave driver circuit was utilized to drive the laser diode. Structural and spectroscopic features of the based SERS layer and AuNPs/Si nano-pillars sensor were studied by scanning probe microscopy, (FESEM) images, x-ray diffraction patterns and Raman spectroscopy. The results revealed that the Si nano-pillars layer provided superior features for creating high density hot spot gaps. A uniform, high population and unique size distributions of (AuNPs) layers on the AuNPs/Si nano-pillars layer were achieved by ion reduction process. The sensor performance displays an excellent detection of chlorpyrifos with an exponential relationship with Raman signal. The highest chlorpyrifos enhancement factor (EF= 1.1*106) with minimum limit of detection (LOD= 22*10−8 M); equivalent to 0.07 mg/Kg, was obtained with Si nano pillars of high value altitude populated with partially three dimensions AuNPs layer. This limit of detection is much lower than the recognized (0.1 mg/kg) value by the European Union.

Multicomponent gas detection technology of FDM and TDM based on photoacoustic spectroscopy.

In this paper, a multicomponent gas detection system based on photoacoustic spectroscopy (PAS) is proposed with a combination of frequency division multiplexing (FDM) and time division multiplexing (TDM), combining a resonance photoacoustic cell and broadband microphone. A PAS gas cell with a wide frequency response bandwidth was used to achieve the FDM by selecting a specific modulation frequency of each component gas. The sawtooth wave driver current of each laser was output at a constant time interval for achieving the TDM. Compared with the laser channel control using a photoswitch, the driver current control was a simpler and more convenient means to implement TDM. The four gas components of methane (CH4), water (H2O) vapor, carbon dioxide (CO2), and acetylene (C2H2) were selected as sample gases for testing the feasibility of the method. The experimental results showed that the gas detection limits of CH4, H2O vapor, CO2, and C2H2 were 75.435, 2.502, 341.960, and 4.284 ppm, respectively. In addition, the linear fittings of gas concentration were 0.99386, 0.99772, 0.98995, and 0.98955, respectively.

Statistical Evidence for EMIC Wave Excitation Driven by Substorm Injection and Enhanced Solar Wind Pressure in the Earth's Magnetosphere: Two Different EMIC Wave Sources

AbstractSubstorm injection and solar wind dynamic pressure have long been considered as two main drivers of electromagnetic ion cyclotron (EMIC) wave excitation, but clear observational evidence is still lacking. With Van Allen Probes data from 2012–2017, we have investigated the roles of the two EMIC wave drivers separately, by using time‐modified AE+ and . Both the occurrence rate and magnetic amplitude of waves significantly increase with the enhancement of each index. During large AE+, EMIC waves are mainly generated in the dusk sector (16 ≤ MLT ≤ 20) and near the magnetic equator (|MLAT| < 10°). This is presumably due to substorm‐injected protons drifting from midnight sector to the plasmaspheric bulge. While during large , EMIC waves mainly occur in the noon sector (9 ≤ MLT ≤ 15). But there exist higher‐latitude (10° < |MLAT| < 20°) source regions besides equatorial source, possibly due to the minimum B regions. Our results provide strong observational support to existing generation mechanisms of EMIC waves in the Earth's magnetosphere.