Asymmetries in auroral electron precipitation and magnetospheric dynamics

Aurorae are caused by charged particles precipitating into the atmosphere, originally from the solar wind, flown into the region of space dominated by the Earth’s magnetic field, called the magnetosphere. Solar wind is a stream of plasma particles originating from the Sun, carrying the Sun’s magnetic field with it. The magnetic field of the solar wind is in constant interaction with the Earth’s magnetic field, causing the Earth’s magnetic field to vary over time. This is called geomagnetic activity.

Geomagnetic activity mostly depends on the direction and strength of the magnetic field of the solar wind. The interaction between the solar wind’s and the Earth’s magnetic fields happens via so-called magnetic reconnection, where two oppositely directed magnetic field lines flow towards each other, break, and reconnect with each other. Since the Earth’s magnetic field points north on the dayside, geomagnetic activity is strong when the magnetic field of the solar wind points southward. Reconnection can also happen when the magnetic field of the solar wind points eastward or westward. In such situation, the magnetic field lines of the solar wind are askew relative to Earth’s, which causes eastward and westward twisting of the Earth’s magnetosphere. Typically, geomagnetic activity is assumed equally strong for both eastward and westward directions of the solar wind’s magnetic field. However, the measurements analyzed in this thesis show that this symmetry breaks during winter and summer seasons.

This thesis shows a strong asymmetry, where auroral particle precipitation was stronger for the eastward solar wind’s magnetic field in winter, and for westward magnetic field in summer. This thesis also shows for the first time that the asymmetry is considerably strong in auroral particle precipitation caused by high speed solar wind streams.

Even though the asymmetries caused by the eastward and westward solar wind’s magnetic fields are an elementary problem in magnetospheric dynamics, its physical mechanism is still unknown. This thesis studies the temporal development of the asymmetry, which suggested that the asymmetries in auroral particle precipitation and ionospheric horizontal currents first require a 2-3 hours of asymmetry build-up in the magnetosphere. The results give evidence that the asymmetry is caused by a systematic difference in the dayside reconnection.