The great space weather washing machine:Examining the dynamics of high-latitude ionosphere-thermosphere coupling

The Earth’s upper atmosphere at high latitudes is a complicated region that is under the influence of many competing forces. For one, it is where the atmosphere becomes partially ionised, and thus subjected to the electromagnetic influences of the coupled solar wind-magnetosphere system. In contrast, neutral particles strongly feel the effects of non-magnetic forces, such as those due to temperature gradients and the Coriolis effect. Once ion-neutral collisions are then taken into account, the result is a global scale “washing machine” of charged and neutral particles, moving through various different spin cycles of coupling strength. Firstly, it is the inherent differences between neutral and plasma flows that result in one of the largest atmospheric sinks of magnetospheric energy: Joule heating. It has sometimes been assumed in previous studies that the neutral wind is slow enough to be treated as if it was stationary. However, we show in Chapter 3 that this is not the case statistically using plasma velocity data from the Super Dual Auroral Radar Network (SuperDARN) and an empirical neutral wind model (HWM14). Overall, the inclusion of neutral winds can lead to global Joule heating estimates that differ by as much as 18% from calculations assuming they are stationary. We also present common scenarios by which the neutral wind can provide both a net increase or decrease to heating rates, depending on season and geomagnetic activity level. In Chapter 4, neutral wind velocity measurements from an all-sky Fabry-Perot Interferometer known as SCANDI are presented for a period where the plasma velocity varied greatly in magnitude. We show that over the relatively small region observed (about 1000 km in diameter) for the event in question, the time it took for the neutrals to be fully accelerated or decelerated by a change in the plasma varied by as much as 30 minutes. Compared to the average neutral wind response time of around 75 minutes, this is quite a large amount of variability for regions at mesoscale separations. This has implications for how closely coupled the ionosphere and thermosphere are on a global scale, and shows the importance of carefully taking into account the current states of both. Finally, an aurorally active period that occurred towards the end of the event pre- sented in Chapter 4 is examined more closely in Chapter 5. We saw that coinciding with the onset of poleward moving auroral forms, the neutrals rapidly accelerated in the direction of the plasma - much faster than prior to the aurora. We propose that due to the increased ionisation from particle precipitation, combined with the rapid transient plasma bursts in the poleward direction, the strength of ion-neutral coupling was en- hanced significantly. This also happened during a transition of the IMF By component, showing for the first time a pseudo coupling of the thermosphere directly to the solar wind

Modeling Non-Force-Free and Deformed Flux Ropes in Titan’s Ionosphere

Previous work at Titan presented a set of 85 flux ropes detected during Cassini flybys of Titan from 2005 to 2017. In that study a force‐free model was used to determine the radii and axial magnetic field of the flux ropes. In this work we apply non‐force‐free models. The non‐force‐free model shows an improvement in the number of flux ropes that can be fitted with a model, along with improved uncertainties and χ2 values. A number of asymmetries and features in the magnetometer data cannot be reproduced by either model; therefore, we deform the force‐free model to show that small deformations can replicate these features. One such deformation is to use an elliptical cross section, which replicates a plateau in magnetic field strength along with asymmetries on either side of the center of the flux ropes. Additionally, we explore the properties of bending a flux rope, where we find that minimum variance analysis becomes increasingly degenerate with bending, along with a slight bend causing the switching of the axial field direction from intermediate to maximum variance direction. We conclude that the flux ropes at Titan show aspects of developing flux ropes, compared to other planetary bodies, which exhibit more agreement to the force‐free assumptions of mature flux ropes

GNSS Scintillations in the Cusp, and the Role of Precipitating Particle Energy Fluxes

Using a large data set of ground-based GNSS scintillation observations coupled with in situ particle detector data, we perform a statistical analysis of both the input energy flux from precipitating particles, and the observed occurrence of density irregularities in the northern hemisphere cusp. By examining trends in the two data sets relating to geomagnetic activity, we conclude that observations of irregularities in the cusp grows increasingly likely during storm-time, whereas the precipitating particle energy flux does not. We thus find a weak or nonexistent statistical link between geomagnetic activity and precipitating particle energy flux in the cusp. This is a result of a previously documented tendency for the cusp energy flux to maximize during northward IMF, when density irregularities tend not to be widespread, as we demonstrate. At any rate, even though ionization and subsequent density gradients directly caused by soft electron precipitation in the cusp are not to be ignored for the trigger of irregularities, our results point to the need to scrutinize additional physical processes for the creation of irregularities causing scintillations in and around the cusp. While numerous phenomena known to cause density irregularities have been identified and described, there is a need for a systematic evaluation of the conditions under which the various destabilizing mechanisms become important and how they sculpt the observed ionospheric “irregularity landscape.” As such, we call for a quantitative assessment of the role of particle precipitation in the cusp, given that other factors contribute to the production of irregularities in a major way

Spatially Resolved Neutral Wind Response Times During High Geomagnetic Activity Above Svalbard

It has previously been shown that in the high-latitude thermosphere, sudden changes in plasma velocity (such as those due to changes in interplanetary magnetic field) are not immediately propagated into the neutral gas via the ion-drag force. This is due to the neutral particles (O, O 2, and N 2) constituting the bulk mass of the thermospheric altitude range and thus holding on to residual inertia from a previous level of geomagnetic forcing. This means that consistent forcing (or dragging) from the ionospheric plasma is required, over a period of time, long enough for the neutrals to reach an equilibrium with regard to ion drag. Furthermore, mesoscale variations in the plasma convection morphology, solar pressure gradients, and other forces indicate that the thermosphere-ionosphere coupling mechanism will also vary in strength across small spatial scales. Using data from the Super Dual Auroral Radar Network and a Scanning Doppler Imager, a geomagnetically active event was identified, which showed plasma flows clearly imparting momentum to the neutrals. A cross-correlation analysis determined that the average time for the neutral winds to accelerate fully into the direction of ion drag was 75 min, but crucially, this time varied by up to 30 min (between 67 and 97 min) within a 1,000-km field of view at an altitude of around 250 km. It is clear from this that the mesoscale structure of both the plasma and neutrals has a significant effect on ion-neutral coupling strength and thus energy transfer in the thermosphere

AuroraWatch UK:an automated aurora alert system

The AuroraWatch UK aurora alert service uses a network of magnetometers from across the United Kingdom to measure the disturbance in the Earth's magnetic field caused by the aurora borealis (northern lights). The service has been measuring disturbances in the Earth's magnetic field from the UK and issuing auroral visibility alerts to its subscribers, since September 2000. These alerts have four levels, corresponding to the magnitude of disturbance measured, which indicate from where in the UK an auroral display might be seen. In the following, we describe the AuroraWatch UK system in detail and reprocess the historical magnetometer data using the current alert algorithm to compile an activity database. This data set is composed of over 150,000h (99.94% data availability) of magnetic disturbance measurements, including nearly 9,000h of enhanced geomagnetic activity. Plain Language Summary Witnessing the aurora borealis, more commonly known as the northern lights, is a much desired event, often featuring in people's "bucket lists." Although rarer than in more arctic regions, such as Scandinavia, Iceland, and Canada, the northern lights are seen from the UK too. To help with this aurora-hunting endeavor, the AuroraWatch UK service sends alerts to its followers when UK aurora sightings may be possible. The service has been running for 17 years and has over 100,000 subscribers. We have recorded over 150,000 h of magnetic field measurements including nearly 9,000 h where geomagnetic activity was large enough for an aurora to potentially be seen from at least some parts of the UK

The future of auroral E-region plasma turbulence research

The heating caused by ionospheric E-region plasma turbulence has documented global implications for the energy transfer from space into the terrestrial atmosphere. Traveling atmospheric disturbances, neutral wind motion, energy deposition rates, and ionospheric conductance have all been shown to be potentially affected by turbulent plasma heating. Therefore it is proposed to enhance and expand existing ionospheric radar capabilities and fund research into E-region plasma turbulence so that it is possible to more accurately quantify the solar-terrestrial energy budget and study phenomena related to E-region plasma turbulence. The proposed research funding includes the development of models to accurately predict and model the E-region plasma turbulence using particle-in-cell analysis, fluid-based analysis, and hybrid combinations of the two. This review provides an expanded and more detailed description of the past, present, and future of auroral E-region plasma turbulence research compared to the summary report submitted to the National Academy of Sciences Decadal Survey for Solar and Space Physics (Heliophysics) 2024–2033 (Huyghebaert et al., 2022a)

The ACS Nearby Galaxy Survey Treasury. X. Quantifying the Star Cluster Formation Efficiency of Nearby Dwarf Galaxies

We study the relationship between the field star formation and cluster formation properties in a large sample of nearby dwarf galaxies. We use optical data from the Hubble Space Telescope and from ground-based telescopes to derive the ages and masses of the young (t_age < 100Myr) cluster sample. Our data provides the first constraints on two proposed relationships between the star formation rate of galaxies and the properties of their cluster systems in the low star formation rate regime. The data show broad agreement with these relationships, but significant galaxy-to-galaxy scatter exists. In part, this scatter can be accounted for by simulating the small number of clusters detected from stochastically sampling the cluster mass function. However, this stochasticity does not fully account for the observed scatter in our data suggesting there may be true variations in the fraction of stars formed in clusters in dwarf galaxies. Comparison of the cluster formation and the brightest cluster in our sample galaxies also provide constraints on cluster destruction models.Comment: 16 pages, 9 figures, Accepted to Ap

The future of auroral E-region plasma turbulence research

The heating caused by ionospheric E-region plasma turbulence has documented global implications for the energy transfer from space into the terrestrial atmosphere. Traveling atmospheric disturbances, neutral wind motion, energy deposition rates, and ionospheric conductance have all been shown to be potentially affected by turbulent plasma heating. Therefore it is proposed to enhance and expand existing ionospheric radar capabilities and fund research into E-region plasma turbulence so that it is possible to more accurately quantify the solar-terrestrial energy budget and study phenomena related to E-region plasma turbulence. The proposed research funding includes the development of models to accurately predict and model the E-region plasma turbulence using particle-in-cell analysis, fluidbased analysis, and hybrid combinations of the two. This review provides an expanded and more detailed description of the past, present, and future of auroral E-region plasma turbulence research compared to the summary report submitted to the National Academy of Sciences Decadal Survey for Solar and Space Physics (Heliophysics) 2024–2033 (Huyghebaert et al., 2022a)