Whistler excitation by short current pulses in a magnetoplasma

In the frame of laboratory studies devoted to whistler excitation by a modu lated electron beam injected in a magnetoplasma under conditions relevant t o space experiments, fast processes have been investigated using the modula ted electron beam as a train of short-time current pulses injected in the p lasma. Single pulses of duration of the order of tau similar or equal to 15 ns as well as unmodulated beams with sharp fronts (step functions with ris e time of the order of tau similar or equal to 10 ns) have been injected in the afterglow plasma with an energy of the order of 300 eV. Plasma respons es to the fast perturbations are transported by whistler waves in the frequ ency range omega(lh) much less than 1/tau < omega(c) omega(p). Different ty pes of responses have been evidenced and characterized, depending on the na ture of the whistler excitation mechanism involved: Nonresonant transition radiation from the beam injection point and Cherenkov resonant emission. Bo th emissions could be distinguished one from each other owing to adequate c hoices of plasma parameters and observation conditions. The formation of wa ke fields associated with the propagating current pulses has also been obse rved for the first time in the whistler range. This study should shed light on the physical mechanisms connected with the injection of modulated and p ulsed electron beams in space plasmas, as well as on fast processes induced in the whistler frequency range by the presence of suprathermal fluxes of electrons traveling in the ionospheric and magnetospheric plasmas. (C) 1999 American Institute of Physics. [S1070-664X(99)02006-6].