The changing field line direction of the IMF is believed to cause "magnetic reconnection" to occur when the IMF field line is opposite to that of the earths magnetic field.
In 1961 James Dungey in England proposed that when the field lines were in
opposite directions a neutral point occurred that allowed IMF field lines
to join with field lines of the earth.
The earths magnetosphere is never stable and periods of increased activity in the earths magnetotail have been labeled "magnetospheric substorms". The most visible signs of magnetic substorms is increased auroral activity in the polar regions at night. The activity may build up for half an hour then decay slowly away. During "sub storms" the magnetic field is drawn into the tail and the tail lobes expand storing additional energy. It is believed the lobes store energy that power the sub storms.
Geomagnetic storms result from large changes in IMF intensity and direction.
These storms usually follow intense solar flare activity. The auroral
activity becomes extremely active and is seen more toward the middle latitudes.
These intense magnetic storms can cause current surges in power lines causing
flickering lights and electrical equipment damage. There is also an increase in
radio, telephone and television interference.