Radiation from electrons in magnetoplasma

Abstract

The radiation from an electron in a homogeneous magnetoplasma has some unu sual properti es as a consequence of the dispersive anisotropic nature of the medium. Attention is confine d to emi ssion in the ordinary (whistler) mode frequen cy band below the cyclotron resonance and the extraordinary mode frequency band around the plas ma freque ncy where the indices of refraction are ap preciably greater than one and vary signifi cantly. Due to the large indi ces, electrons can emit Cere nkov radiation over a limited band of nonrelativisti c e ne rgies . The cyclotron radiation which is generated by the gyration s of elec tron s is complicated also by thi s property of th e medium which permits both normal emi ssion due to "slower than li ght " moti on a nd a nom alous e mi ssion due to " fa ster than light" motion . In the ordinary mode, for example, th e anomalous cyclotron radi ation is emitted into the forward he misphere with res pect to the guiding ce nter motion of the electron wh ereas th e normal radiation is emitted into the backw ard he mi sph ere. In thi s paper th e freque ncy spectra and angular patterns of the average radia ted powe r are calcul ated by th e Ha milto nian method whi c h avoid s a direc t calc ulation of th e co mplicated electromagneti c fi eld vectors . The theory of e mi ssion in di s persive ani sotro pi c medi a with a hermiti an di electric te nso r by Kolomenskii and E idm an is th oroughl y re vie wed a nd exte nded to include relativi sti c e nergies; the co mpl icated anal yti c formulas for the power are evaluated for several spec ial cases; and th e res ults are appli ed to rece nt inte rpretations of ve ry low-freq ue ncy (V LF) and low-frequ e ncy (LF) emi ssions from elec trons in th e magnetos ph ere. The main co nclu sions of the work are as foll ows: (1 ) In th e ordin ary (whi stle r) mode mos t of the ene rgy is radi ated alon g wave normals at la rge angles to th e mag neti c fi eld at freque ncies othe r th an the rectilin early Dopp ler-s hifted fund a mental cyclotron harmonic whi ch is co ntrary to ass umptions of ce rtain VLF e mi ss ion theori es.(2) The reso nan ce s ingul ariti es in th e indi ces fo r a cold , colli sionless plas ma mu st be e liminated to ac hi e ve finit e power leve ls, but unfortunately the di electri c tenso r for th ermal motion is extre mely co mplex and for colli sions is non-hermiti a n; co nsequ e ntly, a n arbitrary upp er limit is imposed on th e indi ces in order to make a qu a ntitative es tim ate of th e powe r. (3) Based on th is approximation th e total power in the ordin ary mode is a slowl y varying fun ction of fr equ ency a nd electron energy with an average level of 10-30 W /(c/s) per electron. (4) This le vel is inadequate to exp lain observed VLF signals on th e bas is of in cohere nt emi ssion, but co herent e mi ssion from bunches of electrons can give th e obse rved power level of 10-14 W /c m 2 (c/s) above the ionos phere; he nce, the onu s of explaining the co mplex di spersion pattern s of VLF e mi ssions is l...

Radiation from pulsed electron beams in space plasmas

“…Consequently, the theory for radiation from single electrons plays an important part in our work. In the development of the radiation model we draw heavily on the previous work on single-particle radiation by a number of persons, in paiticular, that of Liemohn [1965] and McKenzie [19671. Basically, there are three types of radiation that can emanate from an electron beam: they are known as incoherent spontaneous emission, coherent spontaneous emission, and stimulated emission.…”

Influence of Magnetoplasma on Gyro-Synchrotron Radiation

“…Asymptotic behaviour of electromagnetic fields at great distance from the source is given by using a Green tensor and a Fourier transformation. The method developed here is different from that of Eidman(1958), Liemohn(1965) andMansfield (1967), and is similar to that of Bunkin (1957) and Mckenzie (1964). We find the frequency spectra of the radiation power of both ordinary and extraordinary waves for various values of electron energy and ambient plasma parameters in order to discuss the influence of the magnetoplasma on the gyro-synchrotron radiation.…”

“…Eidman (1958) solved the problem in a cold and collisionless magnetoplasma, using a Hamiltonian method. Liemohn (1965), modifying Eidman's results, applied them to VLF and LF emissions from an electron in the magnetosphere. Bunkin (1957) and Mckenzie (1964), who introduced a Green tensor and a Fourier transformation respectively, found an asymptotic expression of electromagnetic fields at great distance from a source in a cold and collisionless magnetoplasma.…”

Substorm aspects of magnetic pulsations

“…Besides, the computations are already so difficult for monocinetic beams, that no attempt has been made for introducing an arbitrary velocity distribution function. ** Cerenkov effects in a magnetoplasma has been studied in detail by Liemohn (1965) andMcKenzie (1967) with special reference to the magnetospheric plasma. They have given formulas for the spectral density and the radiation pattern of the electromagnetic energy which is emitted.…”