Results 1 - 10 of 1523
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[en] Complete text of publication follows. One of the most important problems in solar-terrestrial physics concerns understanding the response of the ionosphere to a variety of physical drivers from the magnetotail. In particular, determining the ionospheric response to magnetotail disturbances such as current disruptions and Earthward-directed bursty bulk flows is key to understanding the causal sequence of events during substorms. We present a series of case studies using data from combined networks of magnetometers in the Canadian sector, including those from CARISMA (www.carisma.ca) and the THEMIS ground-based observatory (GBO) network and supporting arrays, to examine the current response in the ionosphere to substorm expansion phase onset. These analyses highlight the importance of extensive magnetometer coverage in order to correctly identify and characterise the initiation and temporal dynamics of substorm-time ionospheric electrodynamics and current systems. We present details of a magnetic disturbance diagnostic which may distinguish between tail drivers. We suggest that the structure of the resulting current systems may enable these processes to be distinguished using a new set of local magnetometer derived disturbance indices. We suggest forms for these new ionospheric disturbance indices, as an extension to the traditional AE, AL and AU indices. We show how these diagnostics can provide important input into substorm studies, especially in partnership with in-situ measurements from the THEMIS probes, and contribute towards resolving the causal sequence of energy release in the substorm cycle. Finally, we reiterate the importance of understanding the limitations of the inferences made from the standard AE indices, and how they may be used (or misused) in the course of substorm diagnosis.
[en] In taillike configurations magnetic reconnection necessarily leads to the formation of plasmoids. This paper analyses the dynamical evolution of the developing plasmoids and the influence of magnetic reconnection on the propertie of plasmoids. By two dimensional resistive MHD calculations it is shown that plasmoid properties depend very much on the reconnection process especially when the reconnection rate is large. Thus the early plasmoid formation is dominated by magnetic reconnection. At later times the main plasmoid acceleration is due to pressure forces while the tension of open interplanetary fieldlines is negligible. The comparison of different resistivity and equilibrium modells reveals a definite influence of the microscopic dissipation and the initial state on magnetic reconnection and the resulting evolution of plasmoids. (author). 8 refs.; 6 figs
[en] The role of entropy conservation and loss in magnetospheric dynamics, particularly in relation to substorm phases, is discussed on the basis of MHD theory and simulations, using comparisons with PIC simulations for validation. Entropy conservation appears to be a crucial element leading to the formation of thin embedded current sheets in the late substorm growth phase and the potential loss of equilibrium. Entropy loss (in the form of plasmoids) is essential in the earthward transport of flux tubes (bubbles, bursty bulk flows). Entropy loss also changes the tail stability properties and may render ballooning modes unstable and thus contribute to cross-tail variability. We illustrate these effects through results from theory and simulations. Entropy conservation also governs the accessibility of final states of evolution and the amount of energy that may be released.
[en] The structure of the electric field in the magnetotail appearing as a result of interaction of transition region plasma with the magnetic field of the magnetotail is considered. On the base of the plane problem solution the distribution is obtained of the electric field along the boundary of the magnetotail and its dependence on the magnetic field direction and plasma parameters in the transition region. It is pointed out that the data obtained allow describing the electric field asymmetry in the polar cap, Hall current direction from the magnetic field sector and plasma mantle formation in the magnetotail inside its boundary
[en] Complete text of publication follows. An accurate estimate of the time required for a disturbance in the magnetotail to propagate to the ionosphere is needed in order to establish the time history of phenomena related to substorms. Recent THEMIS observations of the 26 February 2008 substorm have found that the time delay from a substorm onset at X = -20 RE in the magnetotail to the corresponding auroral intensification in the ionosphere can be as short as 96 sec, calling into question whether or not Alfven waves can propagate fast enough to serve as an agent connecting the two phenomena. Using empirical models of magnetic field and plasma parameters, we have calculated the minimum time for an MHD wave to travel from a substorm onset in the magnetotail to the ionosphere. The shortest time delay can be shown to correspond to propagation along the so-called Tamao travel path, which consists of an earthward-propagating segment in the equatorial plane in which the disturbance is carried by the fast mode and a field-aligned route to the ionosphere in which the disturbance propagates in the Alfven mode. Our results show that the travel time of an MHD wave along the Tamao path is a strong function of the source location and the latitude of the ground observer. For a source at X = -20 RE, the Tamao travel time can within 90 sec to ground stations connecting to magnetic field lines within 10 RE but outside the plasmasphere. It is therefore feasible for an impulse generated by a substorm onset at X = -20 RE to be carried by MHD waves and reach the ionosphere within 100 sec. However, travel delays on paths to the high-latitude ionosphere and the low-latitude ionosphere are longer. The fact that auroral intensification usually starts at low auroral latitudes and propagates poleward is a natural consequence of MHD wave propagation, and the onset location in the magnetotail should not be estimated by mapping along the magnetic field between the first appearance of auroral intensification in the ionosphere and the magnetotail.