Magnetic shell enhancements during magnetic disturbances

Magnetic shell enhancements during magnetic field disturbances from Langmuir probe observations of electron density on Ariel I satellit

Transient and localized processes in the magnetotail: a review

Many phenomena in the Earth's magnetotail have characteristic temporal scales of several minutes and spatial scales of a few Earth radii (<I>R<sub>E</sub></I>). Examples of such transient and localized mesoscale phenomena are bursty bulk flows, beamlets, energy dispersed ion beams, flux ropes, traveling compression regions, night-side flux transfer events, and rapid flappings of the current sheet. Although most of these observations are linked to specific interpretations or theoretical models they are inter-related and can be the different aspects of a physical process or origin. Recognizing the inter-connected nature of the different transient and localized phenomena in the magnetotail, this paper reviews their observations by highlighting their important characteristics, with emphasis on the new results from Cluster multipoint observations. The multi-point Cluster measurements have provided, for the first time, the ability to distinguish between temporal and spatial variations, and to resolve spatial structures. Some examples of the new results are: flux ropes with widths of 0.3 <I>R<sub>E</sub></I>, transient field aligned currents associated with bursty bulk flows and connected to the Hall current at the magnetic reconnection, flappings of the magnetotail current sheet with time scales of 100 s–10 min and thickness of few thousand km, and particle energization including velocity and time dispersed ion structures with the latter having durations of 1–3 min. The current theories of these transient and localized processes are based largely on magnetic reconnection, although the important role of the interchange and other plasma modes are now well recognized. On the kinetic scale, the energization of particles takes place near the magnetic X-point by non-adiabatic processes and wave-particle interactions. The theory, modeling and simulations of the plasma and field signatures are reviewed and the links among the different observational concepts and the theoretical frameworks are discussed. The mesoscale processes in the magnetotail and the strong coupling among them are crucial in developing a comprehensive understanding of the multiscale phenomena of the magnetosphere

The drivers and timescales of solar wind-magnetosphere-Ionosphere coupling in global MHD simulations

The interaction between the solar wind and the terrestrial magnetosphere-ionosphere system is highly dynamic and non-linear, strongly influencing conditions in near-Earth space. Understanding the coupling between each component of the system is crucial to mitigating societal effects, known as space weather. Global magnetohydrodynamic (MHD) simulations are an invaluable tool in studying this interaction. This thesis entails the use of the Gorgon MHD code for simulating the Earth’s magnetosphere. An updated version of the code is presented, including a newly developed ionosphere module which is tested and benchmarked to validate its proper coupling to the magnetosphere. The model is applied to study the effect of the geomagnetic dipole tilt angle on magnetopause reconnection and ionospheric current systems. The location of the reconnection line is identified for tilt angles up to 90°, with reconnection found to be weaker and more unsteady at large tilt angles. The tilt introduces a North-South asymmetry driving more FAC in the sunward-facing hemisphere, highlighting the sensitivity to onset time in the potential impact of a severe space weather event. An idealised example of such an event is then simulated by impacting the magnetosphere with an interplanetary shock. The location and intensity of dayside reconnection is found to be highly time-dependent following impact, with reconnection enhanced in the vicinity of the shock. These results suggest that steady models of reconnection may not be reliable immediately after onset. Finally, an extended version of the code is implemented to simulate a real geomagnetic storm. The key response timescales of the magnetosphere-ionosphere system to the varying solar wind are investigated, and found to be consistent with those of global convection, being sensitive to the particular mode of driving. It is shown that Gorgon is a capable space weather modelling tool, forming a crucial step towards future operational forecasting purposes.Open Acces

Solar Terrestrial Physics: Present and Future

The following topics relating to solar-terrestrial interactions are considered: (1) reconnection of magnetic fields; (2) particle acceleration; (3) solar magnetic flux; (4) magnetohydrodynamic waves and turbulence in the Sun and interplanetary medium; (5) coupling of the solar wind to the magnetosphere; (6) coronal transients; (7) the connection between the magnetosphere and ionosphere; (8) substorms in the magnetosphere; (9) solar flares and the solar terrestrial environment; (10) shock waves in the solar terrestrial environment; (11) plasma transport and convection at high latitudes; and (12) high latitude ionospheric structure

Dynamics of Particle Precipitation in the Auroral Ionosphere.

Energetic electrons deposit significant amounts of energy into the ionosphere during precipitation events. Riometers provide a means of monitoring this precipitation by measuring the associated cosmic noise absorption. Individually however they cannot provide any details about the energies of the precipitating electrons. The first study in this thesis looks at estimating the characteristic energy of the precipitating electrons by the means of two imaging riometers with overlapping fields of view. Two methods of calculating the height of maximum cosmic noise absorption are developed, a method of height triangulation and tomographic reconstruction of the absorption events. These methods show a marked improvement when compared with a previously published method. A case study comparing the calculated height of maximum cosmic noise absorption with a deduced absorption profile from an EISCAT electron density profile shows good correlation. Using the height of maximum cosmic noise absorption estimates are made of the characteristic energy for three case studies; a morning absorption event, a substorm spike and an afternoon absorption event. The estimated energies for these events were 5keV, 17-20keV and 100+keV respectively. The second study concerns the statistics and mechanisms of daytime absorption events. Statistics of absorption during the course of a day show a deep minimum during the afternoon sector. However there are a number of discrete cases that do occur during this afternoon minimum. A statistical analysis of the time period, 12-16UT at Kilpisjarvi is undertook. They are found to be short lived, highly localised events. This is in agreement with previous studies. They tend to occur during periods of weak geomagnetic activity. A portion of these are found to be early onset substorms, and account for 7.4% of the events. To understand the mechanisms behind the rest of the events one year of data was analyised in greater detail. A portion of these events seem to agree with previous studies, that these events are reltavistic preciptiation events caused by ElectroMagnetic Ion Cyclotron (EMIC) wave scattering. However a greater number of the events seem to be due to the precipitation of lower energy electrons during dispersed electron injections of the radiation belts; a more localized and later occurring version of morning absorption caused by the eastward drift and scattering of lower energy (10-100keV) substorm injected electrons

RF wave observations in beam-plasma discharge

The Beam Plasma Discharge (BPD) was produced in the large vacuum chamber at Johnson Space Center (20 x 30 m) using an energetic electron beam of moderately high perveance. A more complete expression of the threshold current I sub c taking into account the pitch angle injection dependence is given. Ambient plasma density inferred from wave measurements under various beam conditions are reported. Maximum frequency of the excited RF band behaves differently than the frequency of the peak amplitude. The latter shows signs of parabolic saturation consistent with the light data. Beam plasma state (pre-BPD or BPD) does not affect the pitch angle dependence. Unexpected strong modulation of the RF spectrum at half odd integer of the electron cyclotron frequency (n + 1/2)f sub ce is reported (5 n 10). Another new feature, the presence of wave emission around 3/2 f sub ce for I sub b is approximate I sub c is reported

Energetic Ions at Earth's Quasi-Parallel Bow Shock

Zugl.: München, Univ., Diss., 200

Auroral radio absorption:modelling and prediction

Energetic particle precipitation with (>keV) energetic electrons from closed field lines and (>MeV) protons from the solar wind are responsible for enhanced high frequency radiowave absorption in the high latitude ionosphere. Measuring the propagation of radio waves through the ionosphere has been utilised for investigating particle precipitation into the upper atmosphere. Although various methods and models previously proposed for auroral and polar cap absorptions have significantly contributed to our knowledge, there are progressive efforts to improve these models as a result of improved understanding of already made assumptions, development of more efficient equipment and availability of real-time data. This study seeks to contribute to this field. The method utilised combines data from ground-based imaging riometer during solar cycle 23 (1996-2009) and solar wind parameters measured at the L1 point between the Earth and Sun to provide empirical relationship as the basis for a statistical model of auroral absorption. The variation of cosmic noise absorption (CNA) has been modelled using the Akasofu epsilon parameter which characterises the energy transfer between the solar wind and the magnetosphere. The result shows that the absorption model based on the epsilon parameter is reliable for periods with low to moderate solar activity but breaks down during periods of high solar activity such as solar flare and interplanetary coronal mass ejections (ICMEs). Hence, separate models for ICME and flare induced absorptions are produced. The modelled results have been compared with IRIS data. On ICME absorption, key observation shows that absorption associated with ICMEs exhibits different character depending on ICME start times. ICMEs were categorised into day time events (solar zenith of riometer station _ _ 80_) and as night time events (_ _ 100_). Differing absorption signatures were observed for day and night ICME events. This work ranked various solar wind parameters to obtain the best coupling parameter for day and night time absorption. For example, day time ICME model is based on solar wind dynamic pressure and V Bz, while night time ICME model is based on Bz and nV 3. In the case of modelling of solar flare induced absorption, the magnitude and duration of absorption is seen to be dependent on different classes of solar flares. Properties of solar flare such as the rise time, the maximum intensity and decay time were used as the building block of the flare model. Comparing ICME induced absorption with absorption induced by solar flares, it was observed that ICME induced absorption is seen to have longer duration (_ of hours) and stronger magnitude than those associated with solar flares (_ of minutes)