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PRODID:UW-Madison-Physics-Events
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SEQUENCE:0
UID:UW-Physics-Event-4791
DTSTART:20180409T170500Z
DURATION:PT1H0M0S
DTSTAMP:20260419T002754Z
LAST-MODIFIED:20180321T190203Z
LOCATION:2241 Chamberlin Hall
SUMMARY:Mechanisms for onset of the whistler chorus in Earth’s magne
 tosphere \, Plasma Physics (Physics/ECE/NE 922) Seminar\, Dr. Ge Wang\
 , UW-Madison Engineering Physics
DESCRIPTION:We have extended the formalism that describes chirping phe
 nomena in a fusion plasma to model chirping of whistler waves in the m
 agnetosphere\, and developed a code that uses realistic physical scale
 s and high phase space resolution near the particle resonance regions 
 to study the onset of a whistler chorus. Pressure anisotropy in the ma
 gnetosphere excites convective whistler instability with the most unst
 able mode at a frequency and then the excited wave packet moves away f
 rom the equator at the group velocity. The wave amplitude is spatially
  linearly amplified\, until the nonlinearity of particles near the res
 onance region sets in. This nonlinearity arises from trapping of the r
 esonance particles\, so that the trapped distribution will transport w
 ith a resonant velocity toward the equator (opposite to the propagatio
 n direction of the wave packet) in accord with the local resonance con
 dition\, while the ambient non-trapped distribution is constrained to 
 oscillate about the equilibrium particle orbit with the energy and mag
 netic moment conserved. This causes a hole to form in the trapped regi
 on\, with a hole depth\, which steadily increases as the hole moves to
 ward the equator with the field frequency still oscillating at the ini
 tial frequency\, while transferring particle free energy to the waves.
  Thus a chain of holes move into an environment where the field amplit
 udes are spatially decreasing until the wave amplitude matches a nonli
 near BGK condition\, which allows the BGK mode to form at a shifted fr
 equency\, that initiates a chirping signal as the the holes move in ph
 ase space. The chirping holes then serve as antennas that radiate new 
 whistler frequencies amplified as the group velocities transmit the ne
 w frequencies toward a magnetic pole. The simulation is consistent wit
 h our conjecture that these mechanisms are what enable the emergence o
 f the observed rising tone whistler chorus in the magnetosphere.
URL:https://www.physics.wisc.edu/events/?id=4791
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