Magnetic Reversals and Variation.

Scientists have studied the way that Earth's magnetic field is generated deep within its core by conducting theoretical investigations, and advanced supercomputer modeling. (see the review of Earth's field provided by Dr. Andrew Fortes at the University College London.)

The conclusion is that all objects that create their magnetic fields by flowing currents of matter deep inside, a process called the Dynamo Mechanism, will fall victim to these reversals over time. The span of time between these reversal episodes depends on how fast the body is spinning, how large the body is, whether the body is solid or gaseous, and how electrically-conducting the material happens to be. The Earth and Sun differ in many ways from each other, but both have magnetic fields. The solar magnetic field reverses with every sunspot cycle (11-years) while the solid Earth takes much longer (300,000 years or more). For more details, see the American Geophysical Union article by Prof. Kenneth Hoffmann (Physics Department, California Polytechnic State University).

In this activity, students will plot the changes in Earth's magnetic field during the last 800,000 years, and investigate the answers to some important questions about past magnetic variations and future magnetic reversals. The data for this study are from the research by Yohan Guyodo and Jean-Pierre Valet at the Instuitute de Physique in Paris published in the journal Nature on May 20, 1999 (page 249-252)

 

This is a plot of the change in the main field strength of Earth for the last 800,000 years. The Brunhes-Matuyama Reversal ended 980,000 years ago when the polarity of the field actually did 'flip'. Since that time, the polarity of Earth's field has remained the same as what we measure today with the Northern Hemisphere Arctic Region containing a 'South-Type' magnetic polarity, and the Antarctic Region containing a 'North-type' polarity. You will note that the last reversal ended when the magnetic intensity reached near-zero levels. Since then, there was a near-reversal about 200,000 years ago labeled 'Jamaica/Pringle Falls' after the geologic stratum in which these intensity measurements were first identified. Scientists do not know just how low our field has to fall in intensity before a reversal is triggered, but the threshold seems to be below 2.0 units on the scale of the above 'VADM' plot.

Students will use tabulated data based on the figure above, and re-plot a version of the above graph. They will then answer specific questions about the changes in Earth's magnetic field during the last 800,000 years.

 The following table gives the strength of Earth's main field in units such that the Earth's current strength (Time = 0.0) has a value of 11.5 on this intensity scale. This data point is not included in the table below, but should be included in the student plots.

The data are based on the accumulated measurements by geologists from a variety of geological deposits, strata, and ocean sediment samples. The 'Time' columns indicate how many thousands of years before the present time the field was at the indicated strength. The first table entry '20' means 20,000 years ago, at which time the strength was '7.0' which corresponds to the physical unit of '7.0 x 1022 Ampere-meter2

 

Time

Strength

Time

Strength

Time

Strength

Time

Strength

20

 7.0

220

 5.7

420

 8.4

620

 5.5

40

 3.2

240

 6.5

440

 6.3

640

 8.5

60

 5.0

260

 4.5

460

 7.0

660

 7.0

80

 6.6

280

 5.0

480

 6.0

680

 3.5

100

 3.8

300

 6.0

500

 5.7

700

 5.0

120

 4.3

320

 5.8

520

 4.6

720

 5.5

140

 6.5

340

 6.4

540

 3.8

740

 8.2

160

 6.3

360

 8.5

560

 4.2

760

 6.5

180

 2.2

380

 5.0

580

 4.7

780

 0.5

200

 6.0

400

 7.5

600

 6.0

800

 3.4

 From the tabulated entries above, students will answer the following questions:

1...What is the minimum and maximum range of the intensity range?

2...What is the average value of the intensity range?

3...What is the median of the intensity values?

4...What is the mode of the intensity values?

 

 Now plot the data and answer these questions:

5...How many times has the field intensity dipped below 1/2 of its current value?

6...When was the last time that the field intensity reached 1/5th of its current level?

7...When was the last time the field intensity was close to zero?

8...When did the fastest change in the field strength occur in the last 800,000 years? (steepest slope)

9...The current field is decreasing at about 5% per century, in how many years will it reach zero using a straight-line extrapolation?

10...Do you think the field strength will actually reach zero? Explain your reasoning below.