Topband: Enhanced electron recombination

[Robert Brown]

Friends in Radio Land -
 
Before the DXpedition to Mongolia was cancelled, I received
inquiries concerning the two ion chemistry reactions that I cited,
giving enhanced electron recombination due to ionospheric electric
fields in the polar cap.  While the question is still fresh, let
me tell you what I know about them.
 
Both reactions involve major neutral constituents in the region,
N2 and O2, and an important ion, O+:
 
                    O+ + N2 -----> NO+ + N
                    O+ + O2 -----> O2+ + O
 
and the reactions are well-known, cited in all modern ionospheric
references.  Probably the most complete discussion, short of the
research literature, is found in "Physics and chemistry of the
upper atmosphere" by M.H. Rees, Cambridge University Press, 1989.
And it should be noted those are the only two ion reactions cited
as contributing factors to ion density depressions at satellite
altitudes, discussed in the book "The Earth's Ionosphere: Plasma
Physics and Electrodynmics" by M.C. Kelley, Academic Press, 1989.
 
     Of the two reactions, the one with molecular nitrogen is the
most inportant and its reactions rate, measured in the laboratory,
increases with the square of the temperature:
 
                    (5.0*E-13), Tr<1000 K
                    (4.5*E-14)*(Tr/300)^2, Tr>1000 K
 
where Tr+=(Ti+Tn)/2, an average of ion and neutral temperatures,
Ti and Tn..  Rees comments that reaction rate is about two orders
of magnitude less than simple theory would predict at thermal
energies, i.e., Tr = 300 K.
 
     He goes on to say that if the reaction were actually as
rapid as simple theory did predict, then ionization in the
F-region would decay quickly by recombination after sunset and
drop to levels that would be insufficient to maintain radio
propagation.  In short, DXing would cease at night.
 
     For the case of interest in connection with 160 meter DX
propagation across the polar cap to Mongolia, the value of Ti was
taken from EISCAT radar data as about 1,500 K, a factor of 5
greater than 300 K for Kp=1+ or 2-.  With Tn=726 K, the reaction
rate would be higher than at thermal energies by a factor of 14,
but not the two orders of magnitude that would devastate the
F-region if simple theory prevailed.
 
     That being the case, a 15%-20% reduction in electron density
at 125-175 km is not unreasonable as a result from applying the
theory to the problem of MF propagation across the polar cap at
low levels of geophysical activity.  Details of the calculations
will be found in a future issue of QEX.