A Transient but Protracted Geomagnetic Anomaly in the Sudbury Basin Following Two Near-Contiguous Intense Geomagnetic Storms

doi:10.4236/ijg.2011.23038

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International Journal of Geosciences, 2011, 2, 363-365

doi:10.4236/ijg.2011.23038 Published Online August 2011 (http://www.SciRP.org/journal/ijg)

A Transient but Protracted Geomagnetic Anomaly in the

Sudbury Basin Following Two Near-Contiguous Intense

Geomagnetic Storms

Michael A. Persinger, Blake T. Dotta

Laurentian University, Sudbury, Onta rio, Canada

E-mail: [email protected]

Received March 28, 2011; revised June 7, 2011; accepted July 9, 2011

Abstract

During the maintained quiescence between solar cycle 23 and 24, two unusually intense (K-indices = 7)

global geomagnetic disturbances separated by 6 days occurred. They were followed by a protracted increase

of between 150 and 200 nT in the vertical component of our local magnetic field (Sudbury, Ontario). The

duration of the variation anomaly was unusually long, about 3 weeks, before returning to baseline following

a one week period of below average intensity characterized by approximately 50 min periodicities. We sug-

gest this anomaly supports previous research that specific temporal patterns of increased global geomagnetic

activity when matched with local impedance/reluctance of ore bodies created the condition for remarkable

transient changes in the surface static intensity of magnetic fields.

Keywords: Anomalous Transients, Sudbury Basin, Magnetic Storm, Periodicity

1. Introduction

Arrays of magnetometers have been employed to discern

geomagnetic variation anomalies [1]. They occur along

continental edges, within specific crustal compositions,

and over subsurface ore bodies. Although static magnetic

configurations are associated with mineral deposits, such

as nickel and magnetite [2] there are also additional ver-

tical and horizontal transients usually lasting for hours to

days during geomagnetic variations. The seminal work

by Porath and Dziewosnki [3] reported that the anoma-

lies corresponded to peaks in the energy of geomagnetic

variation and showed a strong dependence of its perio-

dicity, in the order of about 50 min.

We have been collecting N-S, E-W and vertical (z)

geomagnetic data once per min 24 hr/day for more than

10 years with a MEDA FVM 400 vector magnetometer.

The sensor for our instrument is positioned in the base-

ment of a building of the university which is built beside

the edge of the Sudbury Basin containing its large nickel

deposit. During the first two weeks of April 2010 we

measured an unprecedented transient geomagnetic ano-

maly. It was maintained for about three weeks and fol-

lowed the strongest and temporally contiguous series of

geomagnetic storms during the last two years. The period

was also significant because there had been no mining or

blasting, as verified by our RV-301 Helicorder seismo-

graphic unit, for more than 6 months because of a labour

dispute.

2. Methods and Materials

The magnetometer was located in our basement labora-

tory. X (N-S), Y (E-W) and Z (vertical) measurements

have been recorded every min to a Laptop computer for

the last three years in that specific locality as a compo-

nent of our research with nT range, mHz variations in

background photon densities as measured by a photon

multiplier tube. Geomagnetic information was obtained

by e-mail from SWPC Production Subscription service

from NOAA Space Weather. For the present analyses the

median values for the 1,440 measurements for the X, Y,

and Z directions per day were obtained for serial daily

comparisons. We employ median values to minimize the

effects of occasional cultural effects. Because the great-

est changes occurred in the vertical axis (Z), these data

were selected to be presented.

3. Measurements

As can be seen in Figure 1, a series of typical perturba-

M. A. PERSINGER ET AL.

364

tions began within the vertical component of the field

after the sudden impulse (S) on 2 April. However on 13

April the vertical intensity increased by between 150 and

200 nT and remained elevated until 3 May. Following

this maintained peak the value dropped below baseline

levels (indicated by “a” in Figure 1) for about a week

before returning to the typical local values. Figure 2

shows the approximately 50 min periodicity with peak-

to-peak amplitude of about 30 nT that occurred during

this interval.

Considering the importance of periodicity and large-

scale impedance/inductance factors for the creation of

transient variation anomalies [1,2], the actual sequence

of geomagnetic events were considered important. The

approximate times for the K events >4 are shown as ver-

tical dotted lines in Figure 1. The specific date and

Figure 1. Potential geomagnetic transient enhancement of

the vertical (Z) component of the geomagnetic field follow-

ing a specific sequence of geomagnetic storms (indicated by

vertical dotted lines) whose intensities are indicated by

k-values. The net protracted increase was about 150 nT.

Figure 2. Sample periodicity (about 50 min) that occurred

during the “undershoot” component (indicated by “a” in

Figure 1) of the potential geomagnetic transient shown in

Figure 1. This periodicity was no longer evident when the

intensity returned to baseline levels.

time of the events, according to alerts (K > 3) from

Boulder, were: 2 April, GSI 19 nT (0721 hr) 5 April, GSI

38 nT (0826 hr); 5 April, K-5 (0916 hr); 5 April, K-7

(0955 hr), 6 April, K-6 (0422 hr), 7 April, K-5 (0847 hr),

11 April, K-5 (0224 hr), K-6 (0225 hr), K-7 (0240 hr);

22 April, K-4 (0058 hr).

4. Calculations and Assumptions

The Sudbury Basin, due to an impact of a meteor of

about 10 km diameter about 1.8 billion years ago, is 62

km × 30 km × 15 km (deep). This constitutes an area of

1.8 × 109 m

2. The electric field induced in a space is

V=∂B/∂t·m2. Assuming the typical variation of about 100

s in periodicity and a field change in the order of 500 nT

(from the storm), the result would be: (5 × 10–7 T/102

s)·1.8 × 109 m2 or ~10 V. The first order current, inferred

by V/Ohm-m and with an average of ~102 Ohm-m for

deposits, would be ~10–1A/m. When multiplied by the

length of the basin there would be a potential ~5.6 × 103

A. The magnetic field strength from a current is B =

µi/2π r. Hence (1.26 × 10–6 N/A2 · 5.6 × 103 A) divided

by 6.28 ·1.0 × 104 m (the estimated distance of the labo-

ratory from the conductive range of the ore body), is 1.1

and 10–7 T or 110 nT. This is within the range observed

even with minimally rigorous assumptions.

5. Discussion and Implications

Our measurements suggest that following a particular

temporal pattern of enhanced global geomagnetic activ-

ity, an anomalous increase in of between 150 and 200 nT

occurred within the vertical component was maintained

for about 3 weeks. However, most variation anomalies'

durations have been in the order of days. We suggest that

Figure 3. Superimposed intensities of the north-south (X),

east-west (Y) and vertical (Z) components of the local geo-

magnetic field during the transient anomaly. During the

periodicities associated with the undershoot (Figure 2), the

magnetic field increase was displayed within the E-W di-

rection.

M. A. PERSINGER ET AL.

365

this specific temporal sequence of geomagnetic storms

may have maintained the current to produce this anomaly.

During this period there was also no mining (or blasting).

Another low probability possibility is that there was un-

detected homogeneous strain within the basin that al-

lowed a minimum dissipation of current. Strain accumu-

lation during sudden and prolonged termination of blast-

ing in some areas might be associated with in areas with

increased strain.

Local cultural artifacts from, for example machinery,

were not responsible for several reasons. First the ele-

vated magnitude was more or less consistent 24 hr per

day for the three week period. Second, the one week un-

dershoot was associated with a periodicity that is re-

markably similar values in the vicinity of 50 min that are

responsible for such anomalous fields in other ore-bear-

ing regions [3]. Within the Great Plains Anomaly during

various geomagnetic “storms” the various periods ranged

between 24 min and 85 min. In many ideal circuits the

temporal characteristics associated with the “charging” is

reflected during the period of “discharging”. In response

to queries by a referee for the manuscript the three-co-

ordinates of the magnetometer data were superimposed.

As can be see in Figure 3 only the E-W component was

markedly elevated during the periodicity of the “dis-

charging”. This component would have been effectively

parallel to the W/SW to E/NE orientation of the central

moment of the Basin. The fact that the marked increase

in this component of the static magnetic component oc-

curred only during the ~50 min periodicities and not

during the main transient might be considered support

for our explanation.

6. References

[1] D. J. Gough, “The Geophysical Significance of Geomag-

netic Variation Anomalies,” Physics of the Earth and

Planetary Interiors, Vol. 7, No. 3, 1983, pp. 379-388.

doi:10.1016/0031-9201(73)90062-9

[2] P. J. Gunn, M. C. Smith, “Magnetic Responses Associ-

ated with Mineral Deposits,” AGSO Journal of Australian

Geology & Geophysics, Vol. 17, 1997, pp. 145-158.

[3] H. Porath, A. Dziewonski, “Crustral Resistivity Anoma-

lies from Geomagnetic Deep-Sounding Studies,” Reviews

of Geophysics and Space Physics, Vol. 9, No. 4, 1971, pp.

891-915. doi:10.1029/RG009i004p00891