Dartmouth Space Physics Theory Program
W. Lotko, PI
Dartmouth SPTP researchers are evolving models of magnetospheric
resonance phenomena and Alfvénic aurora
that provide (at
least partial) answers to these questions. We know that selective,
efficient coupling between incoherent, ultra-low-frequency (ULF)
fluctuations at the magnetopause, and in the magnetotail, and surface
waves propagating along magnetospheric boundary layers concentrates
energy in quasi-coherent Alfvén waves standing between northern and
southern ionospheres---the magnetospheric analogues to resonances of a
tied string.
In effect, the magnetosphere rings at natural frequencies determined by the length of the geomagnetic field lines between ionospheres and the shear wave propagation speed (Alfvén speed) along them. The intensity of these``field line resonances'' (FLRs) is amplified within magnetospheric boundary layers that serve as collectors, lenses, and conduits for energy contained in background fluctuations whose frequencies match the resonant frequencies of field lines threading the boundary layer. Ubiquitous background fluctuations are stimulated by magnetospheric buffets arising from solar wind variability and magnetotail flapping and internal dynamics.
Field line resonances have been studied for over two decades, but only
recently has their connection with discrete auroral arcs become
observationally and theoretically compelling. The new insight that
emerges from our theoretical development
is the behavior
of FLRs at kilometer-scale, horizontal resolution (referenced to the
ionospheric height), wherein they acquire a substantial magnetic
field-aligned electric field and the capability of accelerating auroral
particles. Magnetospheric resonance determines where Alfvénic arcs
occur and that they will be long in longitude and narrow in latitude.
The inhomogeneity along an auroral flux tube magnetically focuses and
traps resonant electromagnetic energy at low altitude,
making this
region the primary site for particle acceleration in Alfvén
aurora. And the observed multiplicity of auroral arcs appears to be
related to the harmonic number of the dominant FLR, although in a way
that is not yet fully understood.
``Measurements'' on ``virtual satellites'' traversing simulated FLRs
reveal a new paradigm for large-amplitude electric field spikes
routinely observed on satellite auroral overpasses: The spikes are
highly Doppler-shifted, standing, dispersive Alfvén waves (figure,
next page), while their signatures on longer dwell-time, equatorial
satellites take the form of classical toroidal micropulsations. The
concept has also been tested through signal analysis of DE 1 satellite data, showing quadrature of the wave electric
and magnetic fields in such spikes, precisely as expected in an FLR.
It has also been shown that geomagnetic curvature enhances the power delivered to FLRs
by dayside boundary oscillations, especially on flux tubes near the
magnetospheric boundary and projecting to the dayside auroral oval.
And first numerical simulations of the poloidal field line
resonance, whose properties show considerable promise in explaining the
more isotropic Alfvénic auroral forms sometimes observed near
nightside ``inverted V'' precipitation events, have now been carried
out.
Because FLR is a global feature of an auroral flux tube, understanding
its modes of stimulation and its influence on auroral luminosity offers
a potentially powerful diagnostic for identifying the signatures of
auroral arcs on equatorial satellites, and, conversely, for using the
aurora, especially satellite images, to remote sense magnetospheric
dynamics, and ultimately specific features of magnetospheric storm and
substorm phases. A practical objective of this research is to enable,
through quantitative model development, reliable and routine use of
these proxy diagnostics.
