Comparison of observed vlf attenuation rates and excitation factors with theory

The properties of VLF propagating mod es are bri efl y reviewed and simplified eq uation s are presented whi ch can be employed in calculating t he fi elds p roduced.Expe rimentall y determin ed exc itation factors are co mpared with t heoretical curves obtained by Wa it an d fo un d to agree rathcr closely.At tenu ation rates are shown as a function of frequenc y and ar e fou nd t o agree ra t her closel y wit h calculated valu es us ing a proposed simplified p er turbation solut ion for atten uation ra tes based on r eflec t ion coeffic ients at t he ground an d io nosphere.When e mploying values of ionospheric re flection coeffi cients recent ly presen ted by vVa it and vValters, the nonrec iprocal effects i n atte nuation ra te are fo und both t heore t ically and exper imen ta ll y to be much grea ter i n t he 10 kc/s region t han in t he 20 t o 30 kc/s r egion.Fin all y, expcr imcn La l field strengt h ve rsus d istance data are compa red wit h mode calculat ions and fo un ei to compare well all t he way from 1 m ega meter ou t t o and i nclud ing fi elds at t he a nt ipode (20 megame ter s) .I At VLF tbe n~O mode whieh is actuall y a TEM mode is bighly a ttenna ted and is not expected to contribute to t be observed fields.