Solar wind and interplanetary magnetic field effects in the magnetosphere-ionosphere system
The research presented in this thesis investigates several effects in the magnetosphere-ionosphere system related to specific solar wind and interplanetary magnetic field drivers. For the southward Interplanetary Magnetic Field (IMF) when the coupling between the solar wind and the magnetosphere is efficient, the energy accumulation in the magnetotail is often interrupted by a spontaneous energy release into various parts of the magnetosphere-ionosphere system through a substorm process. When the IMF turns northward, the dayside reconnection is inefficient and the lobe reconnection becomes important. Onset of sunward flows in the dayside portion of the polar cap ionosphere is one effect related to this process. Another phenomenon under northward IMF is the occurrence of auroral arcs at very high latitudes. This thesis is focused of these three phenomena: substorms, dayside sunward plasma flows and the onset of polar cap arcs. First, the relationship between the substorm occurrence and the solar wind driving is statistically investigated. Four independent lists of substorm events are considered. The events in these lists are inferred from jumps in the SuperMAG AL index for 1979–2015, from electron injections into geosynchronous orbit for 1989–2007, from positive bay events in 1982–2012, and from ground-based magnetometer data with the SOPHIE algorithm for 1981–2015. Additionally, a well-known list of substorms identified from IMAGE and Polar satellite imagery is considered. The different lists are investigated and the two lists with the most robust substorm onsets are chosen for further study. Substorm occurrence rates and substorm recurrence-time distributions are examined as functions of the phase of the solar cycle, the season of the year, the Russell-McPherron favorability, the type of solar wind plasma at Earth, the geomagnetic-activity level, and as functions of various solar and solar wind properties. Three populations of substorm occurrences are seen: (1) quasiperiodically occurring substorms with recurrence times of 24 hr, (2) randomly occurring substorms with recurrence times of about 6–15 hr, and (3) long intervals where no substorms occur. A working model is suggested where (1) the period of periodic substorms is set by the magnetosphere with variations in the actual recurrence times caused by the need for a solar wind driving interval to occur, (2) the mesoscale structure of the solar wind magnetic field triggers the occurrence of the random substorms, and (3) the large-scale structure of the solar wind plasma is responsible for the long intervals where no substorms occur. Statistically, the recurrence time of periodically occurring substorms is slightly shorter when the ram pressure of the solar wind is high, when the magnetic field strength of the solar wind is strong, when the Mach number of the solar wind is low, and when the polar-cap potential saturation parameter is high. Second, SuperDARN radar data are used to investigate polar cap ionospheric flows under strongly dominant northward IMF. By considering line-of-sight velocities from SuperDARN radars looking in the meridional direction, it is shown that the near-noon flow is predominantly sunward in summer. The sunward velocity increases with intensification of the flow driver (the reverse convection electric field); the effect is stronger in summer and in the Southern hemisphere. Statistical patterns of the sunward flows along the noon-midnight meridian clearly indicate seasonal differences in the intensity and flow direction. Simultaneous SuperDARN convection maps in both hemispheres, averaged over periods of approximately two hours, show that sunward flows are faster in the summer hemisphere. In addition to this, while the sunward flows are aligned with the midnight-noon line in a winter hemisphere, they are oriented toward earlier magnetic local hours in a summer hemisphere. Finally, data from the SuperDARN radars and DMSP and Swarm satellite are used to investigate plasma flows around polar cap arcs. Two cases of arcs, that are just detached from the auroral oval, are considered — one in the morning sector and one in the evening sector. It is shown that polar cap arcs introduce mesoscale structuring of the global convection pattern in the polar cap, and clear flow shears occur. The shears are interpreted as a superposition of the background plasma flow and relatively narrow channels of plasma flow intrinsically related to the polar cap arcs. PolarDARN radar observations at close ranges consistently indicate sunward flows collocating with the arcs. This result is additionally supported by DMSP satellite observations. According to the SuperDARN data, the detached arcs are preceded in time by the flow of the plasma from the auroral oval to the polar cap. It is hypothesised that such inflows are indicators that the formation of polar cap arcs is related to instability processes in the plasma sheet.
substorms, polar cap arcs, sunward polar cap flows
Doctor of Philosophy (Ph.D.)
Physics and Engineering Physics