A document by Samson Tilahun Moges. Click on the document to view its contents.

During periods of northward interplanetary magnetic field (IMF), the magnetospheric structure and dynamics are dramatically different compared to the southward IMF case. Previous studies using both observations and simulations have shown that the magnetotail becomes dominated by closed magnetic flux and associated trapped particle populations. In this study, we analyse three intervals of plasma observed in the high latitude magnetotail during a period of prolonged northward IMF, coinciding with observations of cusp-aligned arcs in the polar region. We observe that the plasma is typically continuous with some substructure observed on length scales of 0.5 - 1.5 R$_E$ which may be linked to the substructure observed in cusp-aligned arcs. The plasma characteristics in each of the three intervals studied are similar. The ion and electron densities are on the order of 0.2 - 1 cm$^{-3}$. The electron energies typically vary between 10$^2$ - 10$^3$ eV. The ion energies are higher compared to the electron energies and range between 10$^2$ - 10$^4$ eV. The ion temperatures are on the order of 5 - 18 MK. The speed of the plasma crossing the Cluster spacecraft is between 4 - 21 km/s which is on the order of magnitude of convection speeds in the magnetotail plasma sheet. We tentatively suggest that this may provide supporting evidence for dual lobe reconnection acting to trap plasma within the magneotsphere and resulting cusp-aligned arc formation.

Understanding global space weather effects is of great importance to the international scientific community, but more localised space weather predictions are important on a national level. In this study, data from a ground magnetometer at Valentia Observatory is used to characterise space weather effects on the island of Ireland. The horizontal component of magnetometer observations and its time derivative are considered, and extreme values of these are identified. These extremes are fit to a generalised extreme value distribution, and from this model return values (the expected magnitude of an observation within a given time window) are predicted. The causes of extreme values are investigated both in a case study, and also statistically by looking at contributions from geomagnetic storms, substorms, and sudden commencements. This work characterises the extreme part of the distribution of space weather effects on Ireland (and at similar latitudes), and hence examines those space weather observations which are likely to have the greatest impact on susceptible technologies.

High-latitude ionospheric convection is a useful diagnostic of solar wind-magnetosphere interactions and nightside activity in the magnetotail. For decades, the high-latitude convection pattern has been mapped using the Super Dual Auroral Radar Network (SuperDARN), a distribution of ground-based radars which are capable of measuring line-of-sight (l-o-s) ionospheric flows. From the l-o-s measurements an estimate of the global convection can be obtained. As the SuperDARN coverage is not truly global, it is necessary to constrain the maps when the map fitting is performed. The lower latitude boundary of the convection, known as the Heppner-Maynard boundary (HMB), provides one such constraint. In the standard SuperDARN fitting, the HMB location is determined directly from the data, but data gaps can make this challenging. In this study we evaluate if the HMB placement can be improved using data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), in particular for active time periods when the HMB moves to latitudes below 55°. We find that the boundary as defined by SuperDARN and AMPERE are not always co-located. SuperDARN performs better when the AMPERE currents are very weak (e.g. during non-active times) and AMPERE can provide a boundary when there is no SuperDARN scatter. Using three geomagnetic storm events, we show that there is agreement between the SuperDARN and AMPERE boundaries but the SuperDARN-derived convection boundary mostly lies ~3° equatorward of the AMPERE-derived boundary. We find that disagreements primarily arise due to geometrical factors and a time lag in expansions and contractions of the patterns.

We examine the average evolution of precipitation-induced height-integrated conductances, along with field-aligned currents, in the nightside sector of the polar cap over the course of a substorm. Conductances are estimated from the average energy flux and mean energies derived from auroral emission data. Data are binned using a superposed epoch analysis on a normalised time grid based on the time between onset and recovery phase ($\delta$t) of each contributing substorm. We also examine conductances using a fixed time binning of width 0.25 hr. We split the data set by magnetic latitude of onset. We find that the highest conductances are observed for substorms with onsets that occur between 63 and 65 degrees magnetic latitude, peaking at around 11 mho (Hall) and 4.8 mho (Pedersen). Substorms with onsets at higher magnetic latitudes show lower conductances and less variability. Changes in conductance over the course of a substorm appear primarily driven by changes (about 40% at onset) in the average energy flux, rather than the average energy of the precipitation. Average energies increase after onset slower than energy flux, later these energies decrease slowly for the lowest latitude onsets. No clear expansion of the main region 1 and region 2 field-aligned currents is observed. However, we do see an ordering of the current magnitudes with magnetic latitude of onset, particularly for region 1 downwards FAC in the morning sector. Peak current magnitudes occur slightly after or before the start of the recovery phase for the normalised and fixed-time grids.