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Plasma Physics (Physics/ECE/NE 922) Seminar
Formation and Particle Acceleration in Large-Scale Diamagnetic Cavities at the Boundary of Earth’s Magnetic Shield
Date: Monday, September 12th
Time: 12:00 pm
Place: 1610 Engineering Hall
Speaker: Katariina Nykyri, Embry-Riddle University
Abstract: Earth’s magnetic field resembles that of the bar-magnet where magnetic field converges (diverges) toward northern (southern) hemisphere at the high-latitudes creating two funnel-like structures called geomagnetic cusps. Because the magnetic field around the cusps rotates by 360 degrees, there always exist regions where geomagnetic field and interplanetary magnetic field (IMF) are anti-parallel in the vicinity of the cusps, leading to magnetic reconnection if plasma flow is sub-Alfvenic along the k-vector of the tearing instability. On the other hand, the magnetosheath flow is perpendicular to cusp field both at the dawn and dusk sectors of the high-altitude cusps, leading to Kelvin-Helmholtz Instability and associated secondary processes.
Cluster and MMS spacecraft observations have revealed that magnetic reconnection (both at the high- and the low-latitudes) can create large-scale diamagnetic cavities in the high-latitude magnetosphere where particles can be accelerated to 100s of keV in the reconnection quasi-potential. The acceleration mechanism creates strongly perpendicular pitch angle distributions for ions and electrons, likely contributing to the source for the observed ion temperature anisotropy in the high-latitude magnetosphere. Recently, 4 MMS spacecraft flew through a dusk- sector southern hemispheric diamagnetic cavity observing strong ULF wave activity, driven by the drift-mirror instability, at the edge of the cavity. The high-energy electrons within these wave field depressions showed counter-streaming pitch angle distributions while ions remained trapped. MMS spacecraft also have detected high-energy trapped electrons and occasionally parallel streaming high-energy ions within Kelvin-Helmholtz waves at high-latitudes, in the vicinity of the Southern hemispheric dawn-sector cusp.
In this talk I demonstrate the formation mechanism of these diamagnetic cavities, show the observed and simulated signatures of particle acceleration, as well as discuss the possible physical mechanisms leading to their escape from the cavities and their subsequent faith and consequences for magnetospheric energy and mass budget, and on the origin of seed population of the radiation belts.
As cusp-like structures are universal, these cavity-associated acceleration mechanisms may be important also for plasma energization in other systems, e.g., in the atmospheres of magnetized stars. The ~four-orders of magnitude energy increase, associated with this process, may also be important for plasma energization in laboratory plasmas with cusp-like configurations.
Host: Benedikt Geiger
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