The most prominent feature of geospace, Earth's space environment, is the constant pushing of the solar wind against Earth's extended magnetic presence, the magnetosphere . A basic two-cell convection pattern forms in the ionosphere at high latitudes, inside which plasma drifts, or convects, from the dayside to the nightside . This large-scale drift of plasma essentially envelops geospace.

Inherent to this large-scale pattern is a complex energy cascade that originates in magnetospheric wave energy, starting with solar-wind-driven waves at the bow shock or magnetopause generating the ubiquitous ultra-low frequency (ULF) waves that traverse the magnetosphere . The unstable wave energy fulfills many roles in the system. Central to the present paper, they ultimately power the regions of structured, periodic high-energy charged particle precipitation in the ionosphere's auroral regions ; this flux of charged particles carries (on account of being electrons) electrical currents as well as a Poynting flux, the transfer of electromagnetic energy. At the same time, ULF waves produce a considerable Alfvénic Poynting flux directly, wherein the interchange instability and the Alfvén wave resonator cavity transports electromagnetic energy downwards into Earth's atmosphere, where it eventually dissipates by heating up the gas. Alfvén waves experience anomalous resistivity and accelerate electrons, producing aurorae . The total energy expenditure of the above constitute an important driver of electrodynamics in the ionosphere .

Common for the ways in which unstable wave energy is spent in the ionosphere-thermosphere system, all work crucial modulations into ionospheric conductivity and the electric fields that penetrate geospace, which in turn drives plasma dynamics directly and through particle precipitation, and this includes the essentially regulating role of the system on occurrences of small-scale structuring of the auroral ionosphere. As mentioned, this regulation occurs through modulations to electric fields and conductivities , providing driving and damping effects for the triggering of plasma instabilities and subsequent growth of small-scale turbulent structuring within the plasma , instabilities that are otherwise contributing to Landau damping at higher altitudes.

In addition, the embedding of a wave-cascade, or “structural imprint” into the ionospheric plasma , has been evoked to explain observations of density irregularities that appear directly driven by magnetospheric processes , representing kinetic-scale Alfvénic turbulence that is imprinted into the fields see also.

Figure  shows the first of the particular trends that we demonstrate in the present paper. We bin some 5 million precipitating particle spectra observed by all four DMSP satellites in the dawn- and pre-noon section of the northern hemisphere poleward of 60° magnetic latitude, collected during the local winter seasons of 2014–2016 (with winter defined as a 131 d period centered on the December solstice). In this region, spanning magnetic local times between 4 and 11 h, the four satellites achieve excellent coverage, avoiding both the narrow gap centered around noon at lower latitudes and the wide gap centered on midnight. The figure reproduces a distinct and characteristic magnetic local time-trend that occurs during disturbed conditions . The well-known result confirms that the various and differing degree to which electrons precipitation defines the shape of the ionosphere's bottomside; with increased levels of particle precipitation, the ionosphere sags towards midnight, where significant ionization (and free energy) is injected into the E-region. The result is a drastic decrease in the ratio of ionization experienced by the bottomside compared to the topside, as magnetic local time approaches noon.

The last two panels of Fig.  estimate statistically how the above basic mechanism evolves in time, through changes in geomagnetic activity and interplanetary magnetic field configuration. Figure c shows the altitude-dependent absolute changes in ionization rate implied by the DMSP-observed particle spectra, using a red-blue colorscale, where red indicates the excess ionization rate experienced during disturbed geomagnetic conditions, and where blue color indicates a reduction compared to geomagnetic quiet times (this time using the median value of 150 nT as deliminator). Figure d presents the same comparison, this time between southward and northward configurations of the interplanetary magnetic field (with red color indicating excess ionization during a southward field configuration). The results are likewise expected from comprehensive surveys and studies that are narrowly focused on the cusp , though we note the apparent and significant decrease in ionization rate at E-region altitudes around noon in Fig. c–d, implying an absent electrical load for the field-lines in this region.

Together, the four panels of Fig.  demonstrate that elevated levels of high-energy particle precipitation at dawn are accompanied by a decrease in such particle precipitation towards noon, causing a tilt (in magnetic local time) of the bottomside ionosphere during geomagnetic active times. Figure  explores this trend further, now showing altitude profiles for selected magnetic local times bins (a–e) as well as five geomagnetic activity bins in the noon-sector. The first row explores the tilting trend, accentuating how a general trend of a raised noon and lowered midnight are amplified during active times. In fact, for the most extreme geomagnetic activity bin (top quintile in the SME-index), precipitation-induced ionization below 150 km (that are caused by electrons with energies up to 30 keV) is almost completely absent in the noon-sector.

Next, we shall consider the tendency of the DMSP F18 satellite to consistently orbit through the noon-sector from the poleward-afternoon-side to the equatorward-morning-side of the cusp. This tendency is naturally exploited in a stretching and superposing orbital segment-scheme, comparing the median observations of ensembles of orbits. Figure a–c detail some 352 conjunctions between the DMSP F18 satellite and ground-based GNSS receivers on Svalbard, Norway, during which the cusp was directly inferred in the data. Figure d–g show two superposed orbit analyses, for both quiet and active conjunctions, where we compare the occurrence of GNSS phase fluctuations with observed ion drift speeds and the inferred ratio of E- to F-region conductance (Eq. ).

The stretched and superposed orbit analysis in Fig.  reveals a clear, characteristic pattern in the inferred ratio of E- to F-region conductance, η, where the poleward edge of the cusp marks the edge of a region in which E-region ionization is almost absent, leading to very low values of η (∼0.25). On the other hand, the equatorward edge of the cusp marks the transition of a distinct high-η region (>0.7): this is the characteristic diffuse aurora that we typically find equatorward of the cusp .

Whereas we are unable to capture the entire ionosphere-magnetosphere-thermosphere energy transfer in the relatively isolated dataset of precipitating particles, this analysis does demonstrate a systematic variation in penetration depth of precipitating particles, a quantity that is important by merit of its interconnectedness in geospace. A comparative analysis of the simultaneously observed occurrence rates of radio phase fluctuations ties the dramatic spatial gradient in η to the occurrence rate of ionospheric structuring in general. The next section will present a discussion of the consequences of these gradients in the ionospheric boundary condition, for the purpose of ionosphere-magnetosphere coupling and current closure, and of influencing when and where plasma instabilities can grow or are suppressed.

Comparing Fig. e and g we observe that the low-η region poleward of the cusp is accompanied by elevated occurrence rates of phase fluctuations. This action is caused by the proliferation of polar cap patches , drifting poleward from the cusp, for example created by a tongue-of-ionization action or seeded by poleward-moving auroral forms . The growth of turbulence on the edges of these patches are no doubt aided by low values of η, which should suppress the ability of the E-region to short-circuit the electric fields that sustain F-region plasma instabilities .

Adding to this picture are the ion drift observations shown in Fig. f–g. These drifts, driven by strong convection electric fields (and the process of dayside reconnection), represent the primary source of free energy that can generate F-region irregularities in the region. Notably, the drift speeds are highest not poleward or equatorward of the cusp, but directly inside it, peaking where the conductance ratio η is transitioning from low to high values. This allows us to view the cusp as an “instability factory”: it is the region that combines the strongest driver electric fields; see, e.g., with a relatively weak inhibitor (decreasing η values). The irregularities generated here then convect poleward into a region where, although the drift speed is lower, the inhibitor is almost nonexistent (η is at its lowest). This allows the turbulence that occurs during local winter to persist for long periods as it populates the polar cap . Consistent with the η-paradigm, occurrences of the “tongue of ionization”, a large, continuous blob of ionization that stretches from the cusp and into the polar cap are likewise associated with radio phase fluctuations . Conversely, equatorward of the cusp, both the driver (drift speed) and the turbulence itself are suppressed, both short-circuited by drastic increases in conductance (Figs. –). This dynamic interplay between the convection driver and the E-region conductance inhibitor is entirely consistent with established observations and inferences of the irregularity landscape poleward of the cusp .

A key aspect of the prevailing picture is the decrease, or even collapse, in energetic particle precipitation inside the noon-sector. It would then seem that the energetic particle precipitation from the radiation belts that normally sustain the dark, dayside E-region are effectively drained out from the cusp-region, consistent with recent observations . This draining is taken to account for the energy budget there being largely dominated by dayside magnetic reconnection, and parts of this energy expenditure will occur through broadband particle precipitation that is missed by the DMSP instrumentation, which is more efficient at probing intense high-energy diffuse aurorae. The mentioned collapse in high-energy electron precipitation is therefore marking the change in energy input from unstable wave energy on closed magnetic field-lines to a wave energy cascade controlled by magnetic reconnection.

We emphasize that this study focuses on the dark winter ionosphere where solar EUV conductance is minimal, an so, while the dayside cusp is pushed by Alfvénic Poynting flux see, e.g., and soft particle precipitation, the specific lack of high-energy precipitation results in a “hollow” conductivity profile (low η) distinct from the auroral oval.

Indeed, Fig.  shows that while the E-region at noon collapses, the E-region at dawn and dusk is significantly enhanced during active times, and especially so for the dawn sector. This reflects the substorm cycle, in which energetic particles injected near midnight drift around Earth, eastward towards dawn , providing a robust source of hot electrons that precipitate owing to pitch-angle scattering , evident as the eastward “sagging” of the ionosphere in Fig. .

At our onset, Fig.  illustrated a well-known fact, namely that lower conductivities in the ionosphere is generally associated with an increase in the detrimental effects caused by high-latitude plasma turbulence, capturing a rather small part of the entire energy cascade from large, unstable wave modes in the magnetosphere. Figures  and then recreated several known trends in the penetration heights of particle precipitation. Building on these established results, we described the spatial gradient in η and how it affects electrodynamics. Figure  applied a stretching and superposing orbit analysis to examine a prominent transition zone in η, the ionospheric cusp, showing a statistically clear relation between η and the occurrence rates of phase fluctuations, in accordance with expectations based on η as a crucial regulator of the effective growth rates of F-region instabilities. The results of this analysis demonstrated the explanatory value of examining boundary conditions in η as regulators of plasma instabilities in the ionosphere.

The regulation of turbulence by conductivity is not merely a passive damping process. Strong conductivity gradients, such as those found at the edges of auroral forms, necessitate the formation of polarization electric fields to maintain current continuity the Cowling effect, . These electrostatic causes can locally drive plasma drifts, effectively sustaining the turbulence initially triggered by magnetospheric inputs.

On that note, though, we point out that several important chemical and electrodynamic factors are broadly simplified in the present paper, and we isolate a narrow component of a large and complicated energy cascade. Our statistical application of the -equations is likewise simplified in terms of chemistry (see Appendix A). Furthermore, we note that DMSP spectra do not fully capture particles below 30 eV. However, such soft precipitation, typical of the cusp , would primarily ionize the F-region, further lowering the η ratio and reinforcing the contrast with the E-region-dominated auroral oval. Our results are therefore conservative with respect to the real magnitude of the η contrast.

At any rate, our albeit interesting results must be interpreted with caution, and more thorough investigations of their validity and ramification must follow. Here, we recommend the use of extensive magnetosphere-ionosphere coupling models that aim to capture the energy cascade, taking kinetic processes into account .

We have applied fast parameterisations of ionization rate altitude profiles due to and to a large database of precipitating particle observations from the high-latitude, northern hemisphere ionosphere, observed during local winter. The efforts have yielded several characteristic trends in the altitude-dependent ionization rate, distinguishing between magnetic local times and various levels of geomagnetic activity. While the calculations at times are based on a wide array of simplifications (the characteristic development in η on display in Fig. , for example), the trends uncovered are clear and convincing.

To see how these trends intersect with the current understanding of the greater energy- and momentum transfer that dominates geospace, we offer the following thoughts. The interconnected plasma environment as a whole is in a steady state MHD flow at all times. Excursions away from that equilibrium, in this case observations of particle precipitation, reflect an ongoing cascade of unstable wave energy from larger (ultra-low frequency) to smaller (kinetic) scales, a cascade whose purpose is to restore equilibrium through heat expenditure. Certain unstable modes interact in what are essentially complex and manifold ways to produce the observed proliferation of small-scale structuring in the auroral ionosphere's plasma, whether through kinetic Alfvén waves or wave-particle interactions.

The object of the present study, a database of particle precipitation observations, despite the deficiency in essentially being a dataset in isolate, contain a direct path for energetic particle precipitation to affect the triggering of F-region instabilities, and thereby regulate the occurrences of irregularities in the ionosphere plasma. We have arrived at our results through simplified calculations of ionization altitude and the physics of ambipolar diffusion in the kinetic plasma that forms on the boundary of Earth's dense atmosphere.

Prohibitively fast ambipolar diffusion caused by high conductance is efficient at smoothing out gradients at all scales . Recent demonstrations of the direct and largely instantaneous signal retention by the turbulent E-region are consistent with the origin of the signal being a structural imprint of magnetospheric wave processes, a pattern that dissipates through non-linear heating of the plasma . The very ephemeral quality of the turbulent processes (they die out very fast) points to the presence of fast, efficient dissipation enabled by enhancements in conductance.

The present study provides an exposition of the negating effect that high conductivities can have for instability growth in the F-region ionosphere, where an electrodynamic description of the ionosphere at impact isolate the influence that conductance demands over ambipolar diffusion in a plasma rich in ion-neutral collisions (roughly 90–150 km altitude). We show that when ionization sources cease and conductance is nominally low and decreasing, the structuring of F-region plasma responds in predictable ways, increasing in occurrence and magnitude.

We have reached these results through a simple statistical analysis of a large dataset, and further experiments are necessary to understand the plasma physics involved with damping , and the effects of non-linear conductivity on geospace as a whole .