Multiscale, intense electric fields and density structures are frequently observed in the nightside subauroral ionospehere during intense geomagnetic substorms. We report results from comprehensive numerical studies of the origin and dynamics of these structures. The mechanism explaining a coexistence of the electric fields and plasma structures with different spatial scales is a strongly nonlinear interaction between the magnetospheric field-aligned currents and the ionospheric plasma driven by the mesoscale electric filed in the ionosphere.

Abstract Active high‐frequency (HF) heating experiments provide a controlled framework to investigate nonlinear wave coupling and plasma structuring in the ionosphere. In this study, we present a coordinated experiment utilizing the High Frequency Active Auroral Research Program (HAARP), combining HF heating of the F region with simultaneous extremely low/very‐low frequency (ELF/VLF) probing. Broadband ELF/VLF measurements obtained near the facility are analyzed together with SuperDARN radar observations sampling the heated volume. During periods of HF heating, the ELF/VLF spectra exhibit systematic enhancements and structured features that are absent or significantly weaker during heater‐off intervals. These spectral changes occur in close temporal correspondence with increased radar backscatter, indicating the development of heater‐induced plasma structuring. Guided by established HF heating theory and nonlinear lower‐hybrid/lower‐hybrid‐oblique‐resonance (LH/LOR)–whistler coupling mechanisms, we interpret these observations as evidence that HF heating created a plasma state favorable for (a) enhanced coupling to the LH/LOR branch near the lower‐hybrid resonance (LHR) scale and (b) subsequent scattering, mode conversion, and parametric interactions that populate the received VLF spectrum beyond the direct stepped transmission.

The mesoscale electric field is generated in the nightside equatorial magnetosphere near the plasmapause by the penetration of injected mesoscale plasma flows (MPFs) into the plasmasphere. Once created in the equatorial magnetosphere, the field penetrates into the ionosphere and produces there subauroral polarization streams (SAPS), subauroral ion drifts (SAID), strong thermal emission velocity enhancement (STEVE), stable auroral red (SAR) arcs, and dawnside auroral polarization streams (DAPS). It also drives the Ionospheric Feedback Instability (IFI) generating and amplifying small-scale ULF waves and fieldaligned currents standing along the subauroral magnetic field lines. These fields and currents modify parameters of the ionospheric plasma density and cause field-aligned irregularities and various plasma structures in the ionospheric E and F regions.

We present a unified approach to subauroral arcs within intense subauroral ion drifts (SAID), which explains the observed transition of a precursor Stable Auroral Red (SAR) arc into Strong Thermal Emission Velocity Enhancement (STEVE). This approach is based on the short-circuiting concept of fasttime SAID as an integral part of a magnetospheric voltage generator between the innermost boundaries of the freshly injected plasma sheet electrons and ring current ions. Here, enhanced plasma turbulence rapidly heats the bulk plasma and accelerates suprathermal non-Maxwellian "tails." Heat and suprathermal electron transport rapidly elevate the ionospheric electron temperature-the source of a bright SAR arc. Through a substorm, the density altitude profile within the evolving ionospheric SAID channel transforms into a "fresh" F-region trough with the E-region valley. The ionospheric feedback instability within the depleted-density SAID channel generates small-scale, field-aligned currents with parallel electric fields sufficient to produce the suprathermal electron population, exciting the STEVE and Picket Fence emissions. This approach also explains the inner electromagnetic structure of intense SAID, which is consistent with fine optical structures in STEVE and Picket Fence. Nightside Stable Auroral Red (SAR) arcs emerge and follow the evolution of the fasttime subauroral ion drifts (SAID) until the ensuing emergence of Strong Thermal Emission Velocity Enhancement (STEVE)Density trough formation with the E-region valley initiates the emergence of STEVE and Picket Fence underneath of the precursor SAR arcSmall-scale parallel electric fields created by the ionospheric feedback instability in the low-density SAID channel make STEVE and Picket Fence

Abstract We found the inner electromagnetic structure of subauroral ion drifts (SAID) in the SAID‐STEVE events documented by the Swarm spacecraft and numerically simulated the ionospheric feedback instability (IFI) development for one of the four similar events. Good quantitative agreement of the modeling results with the observed features shows that the ionospheric feedback mechanism captures their basic underlying physics. Simulations require nonlinear saturation of the IFI‐generated dispersive Alfvén waves. That is, a strong driving field of STEVE‐linked SAID with a deep density trough leads to a nonlinear system of dispersive Alfvén waves coupled with the density perturbation and parallel electric fields. As shown earlier, these fields produce the suprathermal electron population and energy balance necessary for the STEVE and Picket Fence radiation. Therefore, our results predict their inner structure.

An assessment of the status quo of fast subauroral flows—subauroral ion drifts (SAID) and subauroral polarization streams (SAPS), is presented. For a few decades, their development has been interpreted in terms of the voltage and current magnetospheric generators based largely on the drift motion of test particles. Recent multispacecraft observations revealed serious flaws in the generator paradigm and called for a new generation mechanism of fast-time subauroral flows and ring current (RC) injections. A novel model includes them in the overarching problem of the penetration of magnetotail plasma flow bursts (MPFs) into the plasmasphere and the substorm current wedge (SCW) development. SAID are created near the plasmapause, where inbound MPFs are short-circuited by the cold plasma. This stops the MPF’s electrons and forms the “dispersionless” plasma sheet (PS) boundary. The SAID electric field—the inherent part of the short-circuiting loop—stops the inward-moving MPF’s ions. In turn, SAPS are an integral part of the two-loop SCW system, or SCW2L, where the downward (R2) current emerges in response to the upward (R1) current in the SCW’s “head.” The meridional Pedersen current, which connects the R1 and R2 currents, leads to SAPS that ultimately drive the fast-time RC injections on the duskside.

The Rice Convection Model‐Equilibrium (RCM‐E) and SuperThermal Electron Transport (STET) are combined to investigate electron heat flux formation in the region of the diffuse aurora for the geomagnetic storms of 17 March 2013 and 17 March 2015. The primary electron precipitation into the atmosphere resulting from wave particle scattering in the magnetosphere are simulated by the magnetically and electrically RCM‐E during these two geomagnetic storms. The primary precipitating electron fluxes are modified by the STET model by taking into account atmospheric backscatter processes. The modified electron energy fluxes and their mean energies are coupled to the STET code to calculate electron thermal fluxes associated with diffuse aurora on a global scale. We use the simulated heat flux to estimate electron temperatures at the upper ionospheric altitudes and compare them with corresponding observations from the Defense Meteorological Satellite Program satellite.

We report on the development of plasma density irregularities in the subauroral ionosphere over the North American sector during the 17 March 2015 Saint Patrick's Day geomagnetic storm. Data from network of ground-based observation instruments including the National Oceanic and Atmospheric Administration Continuously Operating Reference Station global navigation satellite system receivers and Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imagers, as well as in situ measurements from the Defense Meteorological Satellite Program (DMSP) and the Van Allen Radiation Belt Storm Probes (RBSP) spacecrafts, were examined to characterize the spatial and temporal development of subauroral irregularities. The auroral electrojet (AE) index was used as primary indicator of substorm occurrences, aided by some cross-referencing with the SYM-H index and SYM-H time derivative. Special attention was given to substorms that happened at the beginning of this geomagnetic storm. Analysis of Global Positioning System (GPS) rate-of-total electron content index (ROTI) data along an east-west cut line near the US-Canada border indicates that subauroral ionospheric irregularities may start to form as early as a few minutes after a substantial increase in the AE index. Contemporaneous DMSP and RBSP observations confirmed the presence of subauroral polarization streams wave structures when the aforementioned enhancement in AE index and GPS ROTI occurred. An equatorward expansion of bright auroral arc and subsequent auroral breakups were also seen in the THEMIS all-sky imager observation data. The relatively short time scales for irregularities to develop suggest possible roles played by the penetration of magnetotail plasma flow bursts into the plasmasphere and the substorm current wedge development in their formation in the subauroral ionosphere.

We report on a novel scenario of subauroral arcs within strong subauroral ion drifts (SAID)‐STEVE and Picket Fence. Their explanation requires a local source of low‐energy, ε < 18.75 eV, suprathermal electrons, and N 2 vibrational and electronic excitation below ∼270 km. We show that the ionospheric feedback instability in strong SAID flows with depleted density troughs generates intense, small‐scale field‐aligned currents and parallel electric fields below the F 2 peak. With these fields, we employed a rigorous numerical solution of the Boltzmann kinetic equation for the distribution of ionospheric electrons and determined the power going to excitation and ionization of neutral gas (the energy balance). The obtained suprathermal electron population and energy balance at altitudes of ∼130–140 km are just what is necessary for Picket Fence. Concerning STEVE, the kinetic theory predictions are in a good qualitative agreement with its basic features, such as the enhanced continuum emissions. Besides, the theory predicts that subauroral arcs might have the transient phase with typical aurora‐like emissions that fade out afterward.

This paper presents results from the numerical investigation of nonlinear feedback interactions between ULF field‐aligned currents (FACs) and the ionospheric plasma in the global magnetospheric resonator with a non‐symmetrical distribution of the plasma density in the conjugate hemispheres. The density asymmetry is enhanced by the introduction of the ionospheric valley in the hemisphere where the plasma density is already lower. The main result from this study is that in the non‐symmetrical resonator, the ionospheric feedback mechanism, driven by the electric field with the maximum amplitude of 50 mV/m, develops nonlinear, intense, small‐scale upward currents with a characteristic quarter‐wavelength structure along the ambient magnetic field. The frequency of these waves is two times less than the fundamental frequency of the symmetrical resonator. The ionospheric valleys, which are depletions of the plasma density between the ionospheric E and F regions, enhance this effect, by reducing the effective ionospheric conductivity. This effect is important for the interpretation of ground, satellite, and sounding rocket observations of ULF waves and FACs in the auroral and subauroral geospace.

Generation of Very Low Frequency (VLF) electromagnetic whistler waves due to the parametric interaction of quasi-electrostatic VLF waves known as Lower Oblique Resonance (LOR) waves and Extremely Low Frequency (ELF) waves are analyzed in the frame of two possible scenarios. In the first scenario, quasi-electrostatic LOR waves and ELF waves are excited by conventional loop and dipole antennas. In t...

A review is given of the current state-of-the-art of experimental studies and the theoretical understanding of meso-scale and small-scale structure of the subauroral geospace, connecting ionospheric structures to plasma wave processes in the turbulent plasmasphere boundary layer (TPBL). Free energy for plasma waves comes from diamagnetic electron and ion currents in the entry layer near the plasma sheet boundary and near the TPBL inner boundary, respectively, and anisotropic distributions of energetic ions inside the TPBL and interior to the inner boundary. Collisionless heating of the plasmaspheric particles gives downward heat and suprathermal electron fluxes sufficient to provide the F-region electron temperature greater than 6000 K. This leads to the formation of specific density troughs in the ionospheric regions in the absence of strong electric fields and upward plasma flows. Small-scale MHD wave structures (SAPSWS) and irregular density troughs emerge on the duskside, coincident with the substorm current wedge development. Numerical simulations show that the ionospheric feedback instability significantly contributes to the SAPSWS formation. Antiparallel temperature and density gradients inside the subauroral troughs lead to the temperature gradient instability. The latter and the gradient-drift instability lead to enhanced decameter-scale irregularities responsible for subauroral HF radar backscatter.

This paper describes a system for multi‐position Doppler ionosphere sounding implemented at the beginning of 2018 in the Kharkiv region of Ukraine, the data processing technique and initial results of data analysis. The system comprises a high frequency (HF) transmitter and three receiving sites making almost a right triangle with catheti of 32.8 and 58 km. The transmitter continuously emitting a monochromatic signal of a ∼20 W power at a frequency in most cases slightly lower than foF2 is located in the city of Kharkiv. The foF2 value was permanently controlled by the ionosonde installed in about 45 km from the transmitter. We report and discuss the initial results of traveling ionospheric disturbances (TIDs) observations for three years since January 2018 until December 2020. Statistical distributions of the frequency of appearance, periods, horizontal speeds, and propagation directions of TIDs are presented. The range of their most probable wavelengths is from 120 to 260 km, while their propagation velocities are from 80 to 220 m/s. The preferred direction of the TID propagation changes during the daytime and shows a tendency to rotate clockwise in azimuth. Notably, such regular long‐term measurements in the Eastern Europe have been carried out for the first time.

Ultralow frequency (ULF) electromagnetic waves are regularly detected by satellites near the plasmapause during substorms. Usually, the small‐scale waves are observed embedded in the large‐scale, quasi‐stationary electric field. We suggest that the small‐scale waves are generated in the ionosphere by the interactions between the large‐scale field and irregularities in the ionospheric density/conductivity. Under certain conditions, these waves can be trapped in the global magnetospheric resonator and amplified by the positive feedback interactions with the ionosphere. To verify this hypothesis, we model with a two‐fluid magnetohydrodynamics code structure and amplitude of the ULF waves simultaneously observed near the plasmapause by the Defense Meteorological Satellite Program satellite at low altitudes and the Combined Release and Radiation Effects satellite at high altitudes. Simulations reproduce in good, quantitative detail the structure and amplitude of the observed waves. In particular, simulations reproduce a “spiky” character of the electric field observed by the Defense Meteorological Satellite Program satellite at low altitude, which is a characteristic feature of ULF waves produced by the ionospheric feedback instability.