Jökull - 01.12.1962, Page 57

Geomagnetic Micropulsations with Periods /2 to 100 sec
Observed in Southern California and Iceland
BY
GUNNAR BODVARSSON
STATE ELECTRICITY AUTHORITY, REYKJAVIK, ICELAND
ABSTRACT
The present report covers an introductory
study of geomagnetic osciliations with periods
0.5 to 100 sec recorded in Southern California
in the period 1954 to 1957. Four main types of
oscillations are described: (1) frequent day
time oscillations with amplitudes to 20 micro-
gauss and periods 10 to 40 sec strongly corre-
lating with geomagnetic activity, (2) pulses and
irregular transients occurring preferably during
the night, (3) infrequent sinusoidal nighttime
oscillations with amplitudes up to 0.3 micro-
gauss and periods 0.5 to 2.5 sec occurring at
days of low geomagnetic activity and (4) oscilla-
tions witlr periods 0.5 to 3.0 sec associated pre-
l'erably with the pulses and the irregular trains.
A brief description is given of similar oscilla-
tions observed in Reykjavik, Iceland.
ACKN OWLEDGEMENT
Tlie present report is partially based on work
carried out by the author during liis stay in
1957 at the Seismological Laboratory of the
California fnstitute of Technology, Pasadena.
The research project was sponsorecl by the Of-
fice of Ordnance Research of the U. S. Army.
Tlie author is deeply inclebted to Dr. H. Benioff,
professor of seismology, for suggesting the pre-
sent study antl for the many helpful discus-
sions, criticism and suggestions.
INTRODUCTION
A programme of recording oscillations of the
earth’s magnetic field in the period range l/2
to 100 sec was initiated by Benioff in 1954 (see
Benioff, 1960). A number of recorders for this
purpose liave been installecl in Southern Cali-
fornia and this network was later greally ex-
tended. The first recorder was installed in 1954
at Flintridge, California. fn 1955 two additional
recorders were installed at Palomar and Mount
Wilson, California. In 1956 the recorder at
Mount Wilson was moved to Tinemaha ancl
later to Haiwee in Owens Valley, California.
An additiona! recorder was installed in 1957 at
Isabella, California.
During 1958 new recorders were installed in
Reykjavik, Iceland, in Uppsala, Sweclen, and
at Resolute Bay, Canada.
A comprehensive review of the observational
material recorded in this network has been
given by Benioff (1960). The present paper is
a delayed publication of a special report on an
introductory study carried out by the present
author and based partially on the same material.
Parts of the work were carried out under the
supervision of Dr. H. Benioff and the study is
confined mainly to the material obtained from
the California network in the period November
1954 to March 1957. A brief discussion is also
given of more recent recordings from Reykja-
vik, Iceland.
INSTRUMENTATION
The recorders are flux rate variographs con-
sisting of a circular coil, 192 cm in diameter,
with 750 turns and connected directly to galv-
anometers recording photographically with
standard seismograph recording drums. The
coil is installed such that the north-south com-
ponent of dH/dt is recorded. A further de-
scription of the equipment and the frequency
response is given by Benioff (1960).
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