ULF amplitude observations at the dawn/dusk terminators

. Since 2003 continuous measurements of natural ultra low frequency (ULF) magnetic fields have been made in the southern region of Israel. An analysis of the diurnal cycle over 21 months was made in order to examine the daily behavior of the ULF magnetic field amplitude. A comparison of the ULF magnetic field measurements and the time of local sunrise shows a sharp daily rise in amplitude that starts about 2 hours before sunrise, reaching a peak just after the terminator crossing. On the other hand, across the sunset terminator there is no parallel sharp decrease, but only a steady decrease, which is seen normally during the day. As a result no significant sunset effect can be observed from the ULF amplitude measurements. A possible explanation of this asymmetric behavior can be the recombination time difference between sunrise and sunset in the ionospheric terminator. The amplitude changes at dawn were also better observed in the East-West (EW) component than in the North-South (NS) or Vertical components of the magnetic field, possibly due to North-South currents along the ionospheric terminator. The regularity of these diurnal changes may allow us not only to monitor ionospheric currents along the terminator using ground based measurements, but also to define a polarization of the initial MHD-wave incident on the ionosphere.