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Comment on “Nonlinear electron heating by resonant shear Alfvén waves in the ionosphere” by J. Y. Lu et al.
Author(s) -
St.Maurice J.P.
Publication year - 2005
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2005gl022513
Subject(s) - ionosphere , physics , nonlinear system , geophysics , electron , shear (geology) , computational physics , atomic physics , atmospheric sciences , geology , nuclear physics , quantum mechanics , petrology
[1] Lu et al. [2005] want to limit the amplitude of parallel currents in standing Alfven waves by having a new ionization term triggered by strong electric fields. The underlying logic seems to be that if the parallel currents are unable to close because the Pedersen conductivities are small, a perpendicular electric field, E?, will build up. The strong E? will pump up the electron temperatures, Te, through frictional heating from collisions with the neutrals. The hot electrons will then introduce appreciable ionization by overcoming the ionization potential of the ambient atoms and molecules. For large enough E? the ionization would become so effective that the conductivity would increase at the same pace as E?, causing the Pedersen currents to increase rapidly, thereby strongly slowing down any further increase in the E?. [2] I see many difficulties with this thesis. For one thing, the authors argue that 40 to 50 mV/m perpendicular fields heat the electrons to 10000 K (1eV). This result flies in the face of ionospheric observations and is obtained because Te is not properly calculated. Secondly, as far as I am concerned, the ionization mechanism is overestimated substantially: even if Te was as high as the authors claim, there would still be only an infinitesimal amount of extra ionization created. Thirdly, the nature of the parallel currents is not clear: I would think that with Alfven waves, we deal with soft electrons. However, soft electrons are stopped at 200 km and above. It would therefore seem that current closure would have to take place at 200 km or above. If closure is done lower down, then, how are the currents reaching 120 km? Are parallel ionospheric E fields required? Finally, if parallel fields are not used and closure is to take place in the F region, we have another problem: when E? becomes large, the F region ionization goes down, not up, owing to the fact that ion frictional heating with large E? fields produces hot O ions which rapidly turn into NO and recombine.