[en] Penning traps have been used in recent years for a variety of experiments: measurement of electronic or nuclear g factors, high precision mass measurements, or plasma studies. Three dimensional confinement of charged particles is obtained by a static electric field applied between electrodes and superposition of a magnetic field. Storage times of many hours or even days can be obtained under well defined conditions and the stored particles are then subject to investigations. We have measured the storage instabilities of electrons in a Penning trap at low magnetic fields. These measurements are carried out. as a function of the trapping voltage, for different magnetic fields. It is seen that these instabilities occur at the same positions when the trapping voltage is expressed as a percentage of the maximum voltage, given by the stability limit. The characteristic frequencies at which these instabilities occur, obey a relation that is given by η_zω_z + η₊ω₊ + η_-ω_- = 0, where ω_z,ω₊ and ω_- are the axial, perturbed cyclotron and the magnetron frequencies of the trapped electrons respectively, and the n's are integers. The reason for these instabilities are attributed to higher order static perturbations in the trapping potential. (authors)