Ionospheric Alfvén Resonator (IAR) - Ultra and Extremely Low Frequency Electromagnetic Fields - page 162

and producing the geomagnetic perturbations with the spectral resonance structure
(SRS) signature. As it was shown by Belyaev et al. ( 1987 , 1990 ), the shear Alfvén
mode connected with the TE mode in the atmosphere can exhibit SRS observed on
the ground far from the thunderstorm center.
Recently Surkov et al. ( 2005a , b , 2006 ) and Fedorov et al. ( 2006 ) have reported
that the calculated IAR spectra due to the tropic thunderstorm activity are on one or
two order of magnitude lower than that observed at middle latitudes. The model
calculations of the power spectra are in favor of the nearby thunderstorms as a
possible cause for the IAR excitation at middle latitudes. Perhaps, an impulsive
magnetic background from regional thunderstorms makes a significant contribution
to the low-frequency part of SRS (Fedorov et al. 2006 ; Schekotov et al. 2011 ;
Pilipenko 2011 ). The upper atmospheric discharges, associated with TLEs, can
be even more effective in the IAR excitation (Sukhorukov and Stubbe 1997 ). We
note that at middle latitudes there is a natural source of free energy stored in the
ionospheric neutral wind motions which can excite the IAR similar to the operation
of a police whistle (Surkov et al. 2004 ; Molchanov et al. 2004 ).
On the contrary, at high latitudes other sources of the free energy can come
into play. The basic mechanism of the IAR excitation at high latitudes usually
refers to the resonant energy transfer from the magnetospheric convective flow to
the IAR eigenmodes (Trakhtengertz and Feldstein 1981 , 1984 , 1987 , 1991 ) and
development of the fast feedback instability induced by the large-scale ionospheric
shear flows (Lysak 1991 ; Trakhtengertz and Feldstein 1991 ; Pokhotelov et al. 2000 ,
2001 ). Lysak ( 1991 , 1993 , 1999 ) has included a self-consistent analysis of the “fast
feedback instability” due to the modification of the ionospheric conductivity by
precipitating energetic electrons that may increase the rate of energy transfer from
the convective flow to the IAR. The term “fast” was used in order to distinguish
this type of instability from the slow feedback instability of the global ionosphere-
magnetosphere resonator studied by Atkinson ( 1970 ), Sato and Holzer ( 1973 ), and
Sato ( 1978 ). The energetics of the feedback instability was reviewed by Lysak
and Song ( 2002 ). It should be noted that the feedback instability can serve as a
basic mechanism of the IAR excitation only at high latitudes where the convection
electric fields can reach quite strong values. In some cases the IAR manifests itself
as the anomalous ULF transients and can be observed in the upper ionosphere
on board the low-orbiting satellites above strong atmospheric weather systems
(Fraser-Smith 1993 ). In due time Sukhorukov and Stubbe ( 1997 ) considered the
nonlinear conversion of the lightning discharges energy into the IAR eigenmodes.
The nonlinear theory of the IAR was recently developed by Pokhotelov et al. ( 2003 ,
2004 ) and Onishchenko et al. ( 2004 ).
To summarize, we note that the spectral structure and attenuation factors of
IAR have been successfully modeled by analytical (e.g., see Belyaev et al. 1989 ;
Lysak 1991 ; Surkov et al. 2004 ) and numerical (e.g., see Pokhotelov et al. 2001 ;
Polyakov et al. 2003 ) models, while the excitation mechanisms have not been firmly
established yet.