| The 11 August 1999 total solar eclipse had been studied using a large array
of stations in Central Europe (Bencze et al., 2005). According to the result
of this study, the amplitudes of the field line resonance (FLR)-type
pulsations decreased in and around the dark spot by about a factor of 2, and
this decrease moved with the velocity of the dark spot in the same
direction. This decrease was interpreted as a switch-off of the FLR-type
pulsations, due to a change in the eigenperiod of the field line as a
consequence of a change in the charged particle distribution along the field
line. An effect was also found in the phase of the (magnetic or electric)
perpendicular components.
At the Nagycenk (NCK) observatory lying in the zone of totality, both
magnetic and electric records were available. The magnetotelluric (MT)
sounding curve computed by the usual method for the eclipse interval
(08:00-14:00 UT) fits the previously known standard curve extremely well. During
the eclipse, however, impedance values in the FLR period range were highly
scattered. The scatter remained as long as the eclipse lasted. Coherence
values between magnetic and electric components decreased significantly. In
contrast, an earlier similar switch-off of the FLR-type activity on the same
day did not cause a similar scatter, in spite of a comparably low coherence.
Thus, the lack of FLR-type activity disturbed the usual MT connection
between the magnetic and electric components during the eclipse.
The induction vector (tipper), especially its real part, shows a clear effect
of the eclipse in the FLR period range (24-29 s), too. Both at NCK and at
Bad Bergzabern (BBZ, westernmost station and longest FLR period), a definite
decrease in the real tipper was ascertained during the totality. The average
direction of the tipper did not change.
Concerning both parameters, a random effect cannot fully explain the
observed phenomena. The scatter of the EM induction parameters is most
likely due to the switch-off of the FLR activity. The possibility of such an
effect should be considered in induction studies. Pilipenko and Fedotov
(1993) supposed an opposite effect and emphasised lower quality data, if
resulting from FLR-type pulsations, while we claim high quality data just
from such an activity. |