Modulated electron-acoustic waves in auroral density cavities: FAST observations

We report on FAST observations of large amplitude (up to 500 mV m(-1)) enve lope solitary waves at the edges of the AKR source region. These edges are characterized by the presence of two electron populations: a dominant hot ( similar to keV) component and a minority cold (< 60 eV) component. The nonl inear waves are recorded when the spacecraft passes the base of the paralle l auroral acceleration region. They form intense packets of electron acoust ic waves. The modulation is due to ion acoustic waves. These structures are electrostatic and propagate along the magnetic field at speeds of a few 10 0 km s(-1). They may play a crucial. role in the acceleration processes tak ing place in these regions.