The conductivity of the ionospheric E region is known to cause effective dissipation of plasma structures in the F region. We use 3.5 years of 16-Hz sampling rate electron density measurements from the Swarm advanced data set to investigate seasonal dependencies of plasma structure dissipation. Using a novel algorithm to infer plasma structure dissipation through detection of spectral breaks in density fluctuation power spectra, we analyze 100,000 spectra based on data from Swarm A in both the northern and southern polar caps. For the first time, we can present long-term development of small-scale (∼1-10 km) plasma structure diffusion in the high-latitude ionospheric F region. We discuss possible reasons for these variations. This study presents evidence for the E region as an important factor in the seasonal variation of F region plasma irregularity amplitudes.

We present a new index to process break-points and subsequent steepening in plasma density spectra at high latitudes on a systematic basis.• When the E-region conductance is high, there is a strong tendency for spectra to steepen at the scale interval between 40 km and 1 km. • There is a tendency for spectra not to steepen near the cusp, where F-region plasma density and turbulence is enhanced.

Electromagnetic energy carried by magnetohydrodynamic modes is an important mechanism in the energy transfer between the magnetosphere and the ionosphere. Alfvén waves are known to carry field-aligned currents and thus play an important role in the dynamics of the ionosphere-magnetosphere coupling. The role of Hall conductance in this interplay has been explored in magnetohydrodynamic models of the ionosphere but has hitherto not been observed in situ. We use 5 years of observations from the Swarm mission to shed light on this interplay. We present a high-latitude climatology of both the measured Poynting flux and the measured Alfvén wave reflection coefficient. Our results indicate that high-energy precipitation, which penetrates deep into the ionosphere and directly leads to strongly enhanced Hall conductance, is an important cause of positively interfering Alfvén wave reflection. We present such observational evidence and, with that, suggest that Hall conductance is substantially more important in the ionospheric wave reflection climatology than hitherto believed.

We present an investigation of polar cap plasma structure lifetimes. We analyze both simulated data from ionospheric models (International Reference Ionosphere model and Mass Spectrometer Incoherent Scatter model) and in situ data from the Swarm satellite mission (the 16 Hz Advanced Plasma Density dataset). We find that the theoretical prediction that E‐region conductance is a predictor of F‐region polar cap plasma structure lifetimes is indeed supported by both in situ‐based observations and by ionospheric models. In situ plasma structure lifetimes correlate well with the ratio of F‐ to E‐region conductance. We present explicit predictions of small scale (∼1 km) structure lifetimes, which range from less than 1 h during local summer to around 3 h during local winter. We highlight a large discrepancy between the observational and theoretical scale‐dependency of decay due to diffusion.

Context. Alternative theories of gravity typically invoke an environment-dependent screening mechanism to allow phenomenologically interesting deviations from general relativity (GR) to manifest on larger scales, while reducing to GR on small scales. The observation of the transition from screened to unscreened behavior would be compelling evidence for beyond-GR physics. Aims. We show that pairwise peculiar velocity statistics, in particular the relative radial velocity dispersion, σ , can be used to observe this transition when they are binned by some measure of halo environment. Methods. We established this by measuring the radial velocity dispersion between pairs of halos in N-body simulations for three f (R) gravity and four symmetron models. We developed an estimator involving only line-of-sight velocities to show that this quantity is observable, and binned the results in halo mass, ambient density, and the isolatedness of halos. Results. Ambient density is found to be the most relevant measure of environment; it is distinct from isolatedness, and correlates well with theoretical expectations for the symmetron model. By binning σ in ambient density, we find a strong environment-dependent signature for the symmetron models, with the velocities showing a clear transition from GR to non-GR behavior. No such transition is observed for f (R), as the relevant scales are deep in the unscreened regime. Conclusions. Observations of the relative radial velocity dispersion in forthcoming peculiar velocity surveys, if binned appropriately by environment, therefore offer a valuable way of detecting the screening signature of modified gravity.