Excitation of Langmuir waves in interplanetary space

In this paper, the relation between the apparently stable or marginally sta ble electron reduced distribution and the excitation of Langmuir waves is d iscussed. It is found that the positive slope in the reduced distribution f unction is not always necessary for the excitation of Langmuir waves and th at under certain circumstances, Langmuir waves propagating in oblique direc tions with respect to the ambient magnetic field can be excited by the elec trons which may not have a bump in the reduced distribution.