Ultra-low frequency waves in the magnetosphere

Ultra-low frequency waves, i.e. pulsations between 0.1 and 1,000 mHz, are an inherent feature of space plasmas. These plasmas are largely collisionless and hence waves play an important role as an agent to transfer energy, mass and momentum. This work explores some of the wave's features in the Earth's magnetosphere using several different measurement techniques located in different regions. The first of the three case studies presented describes results from an experiment involving an ionospheric heating facility located on Svalbard. The SPace Exploration by Active Radar (SPEAR) facility periodically changed the ionospheric conductivities, superimposing a AC component on a background DC current. Thus the heated patch acted as a giant virtual antenna, emitting Alfvén waves which were detected by ground-based magnetometers in the vicinity of the heater. The following two case studies investigate naturally occurring waves, using both space- and ground-based instruments. In both cases waves generated at the bow shock penetrated into the magnetospheric cavity where they interacted with the local plasma and magnetic field. Whereas in the first case the propagation path of one individual wave packet is analysed, the last data chapter discusses the generation of Alfvénic continuum by upstream generated waves

Ultra-low frequency waves in the magnetosphere

Ultra-low frequency waves, i.e. pulsations between 0.1 and 1,000 mHz, are an inherent feature of space plasmas. These plasmas are largely collisionless and hence waves play an important role as an agent to transfer energy, mass and momentum. This work explores some of the wave's features in the Earth's magnetosphere using several different measurement techniques located in different regions. The first of the three case studies presented describes results from an experiment involving an ionospheric heating facility located on Svalbard. The SPace Exploration by Active Radar (SPEAR) facility periodically changed the ionospheric conductivities, superimposing a AC component on a background DC current. Thus the heated patch acted as a giant virtual antenna, emitting Alfvén waves which were detected by ground-based magnetometers in the vicinity of the heater. The following two case studies investigate naturally occurring waves, using both space- and ground-based instruments. In both cases waves generated at the bow shock penetrated into the magnetospheric cavity where they interacted with the local plasma and magnetic field. Whereas in the first case the propagation path of one individual wave packet is analysed, the last data chapter discusses the generation of Alfvénic continuum by upstream generated waves.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Automatic Classification of Auroral Images From the Oslo Auroral THEMIS (OATH) Data Set Using Machine Learning

Based on their salient features we manually label 5,824 images from various Time History of Events and Macroscale Interactions during Substorms (THEMIS) all‐sky imagers; the labels we use are clear/no aurora, cloudy, moon, arc, diffuse, and discrete. We then use a pretrained deep neural network to automatically extract a 1,001‐dimensional feature vector from these images. Together, the labels and feature vectors are used to train a ridge classifier that is then able to correctly predict the category of unseen auroral images based on extracted features with 82% accuracy. If we only distinguish between a binary classification aurora and no aurora, the true positive rate increases to 96%. While this study paves the way for easy automatic classification of all auroral images from the THEMIS all‐sky imager chain, we believe that the methodology shown here is readily applied to all images from any other auroral imager as long as the data are available in digital form. Both the neural network and the ridge classifier are free, off‐the‐shelf computer codes; the simplicity of our approach is demonstrated by the fact that our entire analysis comprises about 50 lines of Python code. Automatically attaching labels to all available all‐sky imager data would enable statistical studies of unprecedented scope

On the symmetry of ionospheric polar cap patch exits around magnetic midnight

In this paper we examine how polar cap patches, which have been frozen into the antisolar flow over the polar cap, are transported into the nighttime auroral oval. First we present a detailed case study from 12 January 2002, with continuous observations of polar cap patches exiting into the nighttime auroral oval in the Scandinavian sector. Satellite images of the auroral oval and all-sky camera observations of 630.0 nm airglow patches are superimposed onto Super Dual Auroral Radar Network convection maps. These composite plots reveal that polar cap patches exit on both the dusk and on the dawn convection cells. Then we present statistics based on 8 years of data from the meridian scanning photometer at Ny-Aalesund, Svalbard, to investigate the possible interplanetary magnetic field (IMF) By influence on the distribution of patch exits around magnetic midnight. The magnetic local time distribution of patch exits is almost symmetric around magnetic midnight, independent of IMF By polarity. Synthesizing these observations with previous results, we propose a three-step mechanism for why patch material exits symmetrically around midnight. First, intake of patch material occurs on both convection cells for both IMF By polarities. Second, plasma intake by transient magnetopause reconnection stretches the newly cut polar cap patches into dawn-dusk elongated forms during their transport into the polar cap. And finally at exit, dawn-dusk elongated patches are split and diverted toward both the dawn and dusk flanks when grabbed by transient tail reconnection

On the symmetry of ionospheric polar cap patch exits around magnetic midnight

In this paper we examine how polar cap patches, which have been frozen into the antisolar flow over the polar cap, are transported into the nighttime auroral oval. First we present a detailed case study from 12 January 2002, with continuous observations of polar cap patches exiting into the nighttime auroral oval in the Scandinavian sector. Satellite images of the auroral oval and all-sky camera observations of 630.0 nm airglow patches are superimposed onto Super Dual Auroral Radar Network convection maps. These composite plots reveal that polar cap patches exit on both the dusk and on the dawn convection cells. Then we present statistics based on 8 years of data from the meridian scanning photometer at Ny-Aalesund, Svalbard, to investigate the possible interplanetary magnetic field (IMF) By influence on the distribution of patch exits around magnetic midnight. The magnetic local time distribution of patch exits is almost symmetric around magnetic midnight, independent of IMF By polarity. Synthesizing these observations with previous results, we propose a three-step mechanism for why patch material exits symmetrically around midnight. First, intake of patch material occurs on both convection cells for both IMF By polarities. Second, plasma intake by transient magnetopause reconnection stretches the newly cut polar cap patches into dawn-dusk elongated forms during their transport into the polar cap. And finally at exit, dawn-dusk elongated patches are split and diverted toward both the dawn and dusk flanks when grabbed by transient tail reconnection

Direct Evidence for the Dissipation of Small-Scale Ionospheric Plasma Structures by a Conductive E Region

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

Ionospheric Plasma Fluctuations Induced by the NWC Very Low Frequency Signal Transmitter

The Australian NWC (North West Cape) signal transmitter is known to strongly interfere with the topside ionosphere. We analyze 456 conjunctions between Swarm A, B and NWC, in addition to 58 conjunctions between NorSat-1 and NWC. The in-situ measurements provided by these satellites include the 16 Hz Swarm Advanced Plasma Density data set, and the novel 1,000 Hz plasma density measurements from the m-NLP system aboard NorSat-1. We subject the data to a detailed PSD analysis and subsequent superposed epoch analysis. This allows us to present comprehensive statistics of the NWC-induced plasma fluctuations, both their scale-dependency, and their climatology. The result should be seen in the context of VLF signal transmitter-induced plasma density fluctuations, where we find counter-evidence for the existence of turbulent structuring induced by the NWC transmitter

Solar cycle and seasonal variations of the GPS phase scintillation at high latitudes

We present the long-term statistics of the GPS phase scintillation in the polar region (70°–82° magnetic latitude) by using the GPS scintillation data from Ny-Ålesund for the period 2010–2017. Ny-Ålesund is ideally located to observe GPS scintillations modulated by the ionosphere cusp dynamics. The results show clear solar cycle and seasonal variations, with the GPS scintillation occurrence rate being much higher during solar maximum than during solar minimum. The seasonal variations show that scintillation occurrence rate is low during summer and high during winter. The highest scintillation occurrence rate is around magnetic noon except for December 2014 (solar maximum) when the nightside scintillation occurrence rate exceeds the dayside one. In summer, the dayside scintillation region is weak and there is a lack of scintillations in the nightside polar cap. The most intriguing features of the seasonal variations are local minima in the scintillation occurrence rate around winter solstices. They correspond to local minima in the F2 peak electron density. The dayside scintillation region migrates equatorward from summer to winter and retreats poleward from winter to summer repetitively in a magnetic latitude range of 74°–80°. This latitudinal movement is likely due to the motion of the cusp location due to the tilt of the Earth’s magnetic field and the impact of the sunlight

Polar cap patch prediction in the expanding contracting polar cap paradigm

Space weather can cause serious disturbances of global navigation satellite systems (GNSS) used for positioning and navigation purposes. This paper describes a new method to forecast space weather disturbances on GNSS at high latitudes, in which we describe the formation and propagation of polar cap patches and predict their arrival at the nightside auroral oval. The space weather prediction model builds on the expanding/contracting polar cap (ECPC) paradigm and total electron content (TEC) observations from the Global Positioning System (GPS) network. The input parameter is satellite observations of the interplanetary magnetic field at the first Lagrange point. To validate our prediction model, we perform a case study in which we compare the results from our prediction model to observations from the GPS TEC data from the MIT's Madrigal database, convection data from Super Dual Auroral radar network, and scintillation data from Svalbard. Our results show that the ECPC paradigm describes the polar cap patch motion well and can be used to predict scintillations of GPS signals at high latitudes

Interhemispheric study of polar cap patch occurrence based on Swarm in situ data

The Swarm satellites offer an unprecedented opportunity for improving our knowledge about polar cap patches, which are regarded as the main space weather issue in the polar caps. We present a new robust algorithm that automatically detects polar cap patches using in situ plasma density data from Swarm. For both hemispheres, we compute the spatial and seasonal distributions of the patches identified separately by Swarm A and Swarm B between December 2013 and August 2016. We show a clear seasonal dependency of patch occurrence. In the Northern Hemisphere (NH), patches are essentially a winter phenomenon, as their occurrence rate is enhanced during local winter and very low during local summer. Although not as pronounced as in the NH, the same pattern is seen for the Southern Hemisphere (SH). Furthermore, the rate of polar cap patch detection is generally higher in the SH than in the NH, especially on the dayside at about 77° magnetic latitude. Additionally, we show that in the NH the number of patches is higher in the postnoon and prenoon sectors for interplanetary magnetic field (IMF) By0, respectively, and that this trend is mirrored in the SH, consistent with the ionospheric flow convection. Overall, our results confirm previous studies in the NH, shed more light regarding the SH, and provide further insight into polar cap patch climatology. Along with this algorithm, we provide a large data set of patches automatically detected with in situ measurements, which opens new horizons in studies of polar cap phenomena