Schmucker, 1970 (Scripps) - MTNet

Schmucker: Geomagnetic Variations 49
4.6 Coastal Anomaly in California during Magnetic Storms
Various sections of magnetic storms have been assembled in figure 22. They
reveal the same general increase of the Z -amplitude from inland toward the
coast which we have found already during bays and other short-period events.
Since the overhead current distribution is less uniform during storms than
during bays, a concurrent analysis of their anomalous behavior could indicate
how the nonuniformity of the primary field influences the correlation between
anomalous and normal parts (source-field dependence of the transfer functions).
For that purpose a typical storm as recorded on the southern profile (PI.
II) has been analyzed according to the method outlined in section 3.7. Power
spectra have been calculated for the horizontal variations at the reference
station Cameron and for the Z -variations of all survey stations. Cross
spectra were derived between Z (all stations) and DCAM and between Z and
HCAM. The resulting spectral values for the frequencies 0, 1/2, 1 ..• 12 cph
were then inserted into the basic formulae of the correlation analysis (eqs.
3.22 and 3. 17), yielding the transfer functions zH(f) and zD(f) together with
the pertinent residual EZ (f). They are listed in table 6 z*. A comparison
with corresponding values for single events (tab. 6 z) shows that both sets of
transfer values agree within the statistical limits set by the residuals, a convincing
demonstration that the analysis of storms and single events leads to
comparable results.
A minor but consistent discrepancy deserves our attention. We infer from
figure23 that the in-phase transfers of LaJolla are at frequencies below 4 cph
a bit smaller for the storm than for the single events, while the out-of-phase
transfers agree fairly well. This implies that the subterranean eddy currents
which are responsible for the anomaly are somewhat weaker during storms
than during single events and a bit more out-of-phase with respect to the
source field. Both observations are in perfect agreement with the increased
nonuniformityof the large-scale storm field and the resulting reduction and
phase shift of the internal induction.
Of particular interest is a comparison of the reSiduals, since the analysis
of the storm involve about the same amount of independent information as
the analysis of the single events. It turns out that the residuals are of comparable
size. Only beyond 4 cph do the residuals of the storm correlation
analysis seem to outweight those of single events.
The power spectra of Z in figure 24 summarize impressively the decrease
of the coastal Z-amplitude toward inland and toward the open ocean as function
of frequency. Notice in particular that the slope of the Z -spectra between
0.5 and 2 cph is clearly steeper at Cameron than at La Jolla. This
verifies the statement above, namely, that the anomalous Z -amplitudes at
the coast disappear for high frequencies more rapidly toward inland than for
low frequencies. Above 6 cph the Z -spectra flatten out and merge into a
general noise level. Lowest residuals and therefore maximum coherence
between Z and the normal horizontal variations are found near 1 cph.