Multiband Electrostatic Waves below and above the Electron Cyclotron Frequency in the Near-Sun Solar Wind

Using the Parker Solar Probe measurements, this Letter reports two new types of multiband electrostatic waves in and near the heliospheric current sheet. They are classified into the f < f (ce) and f > f (ce) multiband electrostatic waves, in which most (or all) of the bands in the former type are lower than f (ce), and all of the bands in the latter type are higher than f (ce), where f and f (ce) denotes the wave frequency and the electron cyclotron frequency, respectively. This Letter also exhibits observational evidence of the existence of nonlinear wave-wave interactions of both types of electrostatic waves. In particular, the f > f (ce) multiband electrostatic waves are found to be modulated in the presence of low-frequency oblique ion-scale waves. According to the observed frequency distribution, this Letter proposes that the mode nature of the f < f (ce) multiband electrostatic waves could be the oblique ion acoustic wave or the lower-hybrid wave, and the f > f (ce) multiband electrostatic waves are the electron Bernstein mode wave. These findings provide a challenge to understand the complex electron and ion dynamical processes in and near the heliospheric current sheet.