Direct Measurements of the Poynting Flux Associated with Convection Electric Fields
Toshi Nishimura1, Takashi Kikuchi1, Atsuki Shinbori1, John Wygant2, Yuji Tsuji1, Tomoaki Hori1,
Takayuki Ono1, Sigeru Fujita1, and Takashi Tanaka1
1
2
Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Aichi, Japan
School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA
Observations of Poynting fluxes associated with onset of convection electric fields are
essential for understanding of electromagnetic energy transport from the solar wind toward the
magnetosphere leading to changes in the convection electric field, and for evaluation of effects
of the ionosphere. We present Cluster and THEMIS observations in the Earth’s magnetosphere
and discuss implications in the Mercury’s magnetosphere. The Cluster spacecraft detected
convection electric field enhancements and associated Poynting fluxes dominated by the fieldaligned upward component during the preliminary impulse of sudden commencements and
southward turning of the IMF. The upward Poynting flux indicates existence of Alfvén waves
transporting electromagnetic energy from the ionosphere toward the magnetosphere leading to
magnetospheric convection changes. The waveguide model and global magnetohydrodynamic
(MHD) simulation calculating evolution of the Poynting flux following solar wind pressure
enhancements also show upward Poynting fluxes propagating from the ionosphere toward the
magnetosphere faster than the propagation of compressional waves. We conclude that the
ionosphere acts as a channel to transmit electromagnetic energy supplied as field-aligned
currents toward a wide region in the magnetosphere-ionosphere system instantaneously, leading
to changes in magnetospheric convection electric fields. Such prompt response of convection
electric fields and Poynting flux from the ionosphere are expected to be measured in the
Mercury’s magnetosphere if the ionosphere or conducting surface exists. Perpendicular Poynting
fluxes will be dominated in the opposite case.