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In this thesis ground-based observations of the terrestrial magnetic field have been used to studythe characteristics of a special type of magnetospheric pulsation called Pi2. The magnetic fielddata used was observed by the SANAE pulsation magnetometer, located at Antarctica. Severalcharacteristics of the pulsations have been examined for 137 events. For all the events aninjection of energetic particles could be seen simultaneously in the magnetosphere, which is anindication of a substorm onset. The particle injection was confirmed by data from theLANL/SOPA electron flux satellites, located in geosynchronous orbits at 6.6 RE height. Themain frequency of the events was concentrated in the lower part of the defined Pi2 range (7-25mHz) with a mean of 11.3 mHz. The polarisation parameters and the rate of damping wereexamined for 20 clear events. The damping increment had typical values of 0.05-2.Thepolarisation azimuth was directed in the North-East/South-West direction for most of the eventsand the sense of polarisation showed a transition from counter clockwise before 0200 local timeto clockwise afterward.

Mercury is the tiniest planet in the solar system and that closest tothe Sun. It has a similar magnetosphere like the Earth, which to firstorder can be described as a magnetic dipole. The center of the dipoleis shifted 0.2 Mercury radii toward north, hence, the magnetic strengthare not equal at same latitude on the north and the south hemisphere.This asymmetric magnetosphere in reference to Mercury's equatorialplan, leads to the consequences that only some of the photoelectronsemitted from the south surface will reach the conjugate surface, andsome of them will mirror, whereas all the photoelectrons from the northwill reach the south surface. This phenomenon induces a potential difference,an electric field, and a current. It is the aim of this project thatto study the structure of Mercury's magnetosphere, the distribution ofthe photoelectrons, the electric field and the photoelectron currents.

A model for the static current voltage characteristic of a thin collisionless plasma inwich an electrical double layer is present is used for simulating the behaviour of anelectrical circuit in wich the plasma is a component. The numerical results thusobtained are compared to experimental results and found to be quite accurate forlow frequencies and amplitudes. The discrepancy between simulations andexperiments are found to be caused mainly by effects of hysteresis in the plasma.The hysteresis is then shown to be caused by 'slow ions'. The hysteresis is dividedinto three typical cases and a method to predict wich case to expect for a certaincombination of applied potential and frequeny is presented. The formation of thedouble layer in time and space is also studied by means of Langmuir probe measurements. Comments are also made concerning the internal plasma potential for different frequencies of the applied potential.