Search for: All records

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.

 

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 N₂vibrational 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 F₂peak. 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.

 

Observations show that magnetic pulsations with frequencies around 1 mHz are frequently detected simultaneously at different latitudes on the ground, in the inner magnetosphere, and in the solar wind. The coupling between oscillations in the dynamic pressure or magnetic field carried by the solar wind and the ultra‐low frequency (ULF) waves detected on the ground at high latitudes has been suggested in several studies. We present results from a numerical study of ultra‐low‐frequency waves detected by the ground magnetometers at middle latitudes during substorm. We investigate the hypothesis that these waves are generated by the ionospheric feedback instability driven by the large‐scale electric field in the ionosphere. This field is associated with the surface waves propagating along the ambient magnetic field on a strong perpendicular gradient in the plasma density occurring in the equatorial magnetosphere. The gradient in the plasma density is associated with the plasmapause. The plasmapause moves to the middle latitude when the plasmasphere erodes during substorm. The energy from the external driver can be coupled to the large‐scale surface Alfvén waves traveling along the field lines into the ionosphere and generating small‐scale intense ULF waves and field‐aligned currents at middle latitudes. The simulations of the two‐fluid magnetohydrodynamics model confirm this scenario, and the numerical results show a good quantitative agreement with the observations.

 

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.

 

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