Download Supplement to Activity 9: A Soda Bottle Magnetometer

Solar Storms and You!
Studying the Magnetosphere in the Classroom
By Dr. Sten Odenwald NASA IMAGE Satellite Project
Supplement to Activity 9: A Soda Bottle Magnetometer
Introduction
Abstract: The magnetosphere is, at once, one of the most familiar and the least understood elements to the
earth's environment for grades 7-12 in the typical Earth and physical science curriculum. Students learn that
it resembles a bar magnet, but what they seldom encounter is the concept of the geomagnetic storm and its
connections to solar activity and human technological impacts. We will demonstrate a series of classroom
activities that have been designed by teachers for the NASA IMAGE satellite program. These activities
bring geomagnetic storms and 'geospace' into the classroom for direct investigation. They provide a valuable
'third dimension' to typical classroom discussions, and follow the new educational practices recommended
by local and national education guidelines. This article is specifically designed to provide information to
middle school and high school teachers and students about the history and application of magnetometers.
Introduction In the 1740's, George Graham (1674-1751) in London, and Anders Celsius (1701-1744) in
Uppsala, Sweden began taking detailed hourly measurements of changes in the Earth's magnetic declination.
The fact that this quantity varied at all was known as early as 1634 by Gellibrand's observation of the
'variation of the (magnetic) variation' (Fleming, 1939). It didn't take very long before Celsius and his
assistant Olof Hiorter uncovered in the 6638 hourly readings, a correlation between these disturbances and
local auroral activity. Moreover, comparing the records between Uppsala and London, it became quite
apparent that the magnetic disturbances occurred at the same times at both locations. By 1805, the
independently wealthy, scientific traveler, Baron von Humbolt (1769-1859) had also noted these magnetic
disturbances and called them magnetic storms' since they caused the same gyrations of his compass needles
as local lightning storms would do. Just as Celsius and Hiorter nearly 100 years earlier, during a 13 month
period, Humbolt and his assistant also made thousands of half-hourly readings of a compass needle.
Using his considerable influence and popularity, following a two-decade hiatus caused by European wars,
von Humbolt acquired the resources needed to set up a number of magnetic 'observatories' in Paris,
Freiburg, and later across Russia in the 1830's. The first magnetometers were quite crude affairs. A human
'reader' would peer into a microscope at a needle on a graduated scale, little more than an ordinary compass.
At half-hourly intervals, day and night, the position of the needle would be noted. By the 1850's networks of
observatories amassed millions of these observations.
The magnetic field of the Earth can be described as a three-dimensional vector
B = Bx X + ByY+ BzZ
at each point in space. Near the surface of the Earth, the X, Y and Z coordinate unit vectors are defined in
such a way that X follows the lines of longitude, Y follows the latitude great circles and Z is in the vertical
direction towards the local zenith. The Bx and By components lie in the local horizontal plane and the angle
between them is the so-called magnetic declination angle D measured positively eastward. This angle is
familiar to anyone that has had to use a magnetic compass to navigate with a map. One can also define the
magnitude of the horizontal component of the magnetic field as
2
2
1/2
H = (Bx + By
)
The remaining component along the Z-axis measured to be positive downwards, gives the Dip Angle, I,
according to
Bz
tan(I) = ( --- )
H
The total magnitude of the magnetic field vector is about 0.5 Gauss units or equivalently 50,000 nanoTeslas
(nT). To find the components of the magnetic field where you live you can visit the Standard magnetic Field
Model and enter the date, and your geographic latitude, longitude and elevation. Table 1 shows the
representative components for June 1, 1999 at sea level. Bx, By and Bz are the components in units of nT, B
is the total field strength also in units of nT, D is the declination angle between geographic and magnetic
north, and I is the inclination or Dip Angle, in degrees below the local horizontal plane.
Table 1: Average Magnetic Components <td.56.5< td=""></td.56.5<>
City
Bx
By
Bz
B
D
I
New York
19308 -4643 50289 54068 -13.5 68.5
Boston
18006 -1566 53490 56461 -4.9 71.3
Chicago
18686 -803 52908 56117 -2.5 70.5
Miami
25478 -2182 38586 46290 -4.9
Huston
24892 2050 42441 49245 4.7
Denver
20895 3878 49938 54272 10.5 66.9
59.5
San Francisco 23004 6411 43851 49932 15.5 61.4
Los Angeles
24276 5996 41636 48568 13.9 59.0
For example, in Miami the three components of the field are 25,478 nT, -2182 nT and 38,586 nT. The total
magnitude of the field at the surface is then 46,290 nT or since there are 10,000 Gauss units per tessla, this
equals 0.4629 Gauss. The angle between geographic north and magnetic north at this location is -4.9
degrees, so that you compass will point 4.9 degrees west of true north. The needle of the compass will dip
56.5 degrees from the horizontal plane.You can actually see this if you have a compass with a needle
suspended at its middle point.
Since Kristian Birkeland (1867-1917) first proposed the distinction, magnetic disturbances have been
categorized as either magnetic storms, or sub-storms. The former are typically very large events during
which time the local magnetic field conditions change abruptly during the so-called Storm Sudden
Commencement (SSC) phase. Within a matter of minutes, measurements of the field may change from
quiescent conditions to very disturbed conditions, and the new level of activity can persist for hours or days.
Auroral displays may be seen in many localities across the globe, especially the Great Aurora which can be
seen as far south as the Mediterranian or Japan.
Magnetic storms are apparently spawned by violent events in the solar corona which send clouds of plasma
called Coronal Mass Ejections (CMEs) into interplanetary space. If the Earth happens to be in the wrong
place in its orbit, within a few days, these million kilometer/hour plasma clouds reach the Earth and impact
its magnetic field. The momentary compression of the field caused an increase in the field strength at the
Earth's surface causing the SSC. Many physical processes are then precipitated as the CME particles and
magnetic fields invade geospace, especially the amplification of the equatorial Ring Current. This current
induces its own magnetic field which interacts with the Earth's field to cause fluctuations in the geomagnetic
field near ground level and a net decrease in the field strength. Magnetometers then notice complex field
changes which last until the CME plasma passes the Earth and geospace conditions return to normal. Major
magnetic storm events also lead to spectacular auroral displays even at low geographic latitudes.
Sub-storms were first documented in 1964 by Syun-Ichi Akasofu of the University of Alaska using a
network of all-sky cameras. They are generally less dramatic than magnetic storms, and may come and go
within a few hours or so, often with accompanying auroral displays seen in the upper latitudes in Canada,
Scandinavia and Alaska. Although there is considerable variation on a central theme, the evolution of substorm aurora (also called auroral sub-storms) follows a non-random basic script. Beginning with quiet
auroral curtains near the horizon in the late evening, they brighten and pick up streaks or rays. Then a series
of sweeping folds or spirals appear near the eastern horizon and surge westward as the 'expansion phase'
begins. Near local midnight, the sky brigntens again and dissolves into a myriad of rapidly moving forms,
followed by a 'recovery phase' where conditions return to a vague diffuse cloudiness.
Sub-storms are thought to be produced by minor changes in the orientation of the solar wind magnetic field
as it collides with the geomagnetic field. If magnetic 'kinks' in the solar wind field meet up with the
geomagnetic field, rapid polarity changes can lead to reconnection events in the magnetopause and geotail
regions. These events can cause particles to be accelerated to high energy and flow into the atmosphere to
produce aurora. Sub-storms cannot be anticipated in advance because the interplanetary magnetic field is a
complex phenomenon that is largely invisible. Major magnetic storms, however, are known to follow the
sun spot cycle; a fact uncovered by Edward Sabine (18.. - 18..) in 1839, but not formally recognized by the
scientific community until the turn of the 20th century. The best time to observe magnetic storms is when
the solar surface is active, or has large sunspot groups transiting its surface.
As we approach the maximum of sunspot cycle #23 between 2000 - 2001, there will be many opportunities
for observing magnetic storms, sub-storms and aurora, provided you are equipped to do so. In what follows,
we will describe in detail how to construct a simple 'soda bottle' magnetometer, and use it under classroom
conditions, to track the invisible consequences of solar activity on the Earth.
Construction
Although the IMAGE satellite hardware costs millions of dollars to construct and calibrate, we will now
provide detailed directions for assembling a high-precission optical magnetic compass at a cost of less than
$5.00. This is probably the first time that a book of this kind has ever provided detailed hardware
descriptions and assembly instructions!
The basic operating principle is that a suspended magnet, free to move in the local horizontal plane but not
vertically, will orient itself so that it is aligned with the horizontal 'H' component of the Earth's local
magnetic field. If it is mounted, instead, on a pivot so that it can move vertically but not horizontally, it will
dip in the local 'Z' direction. At the north magnetic pole, for example, this dip causes the needle to nearly
stand vertically on its end following the field lines which are disappearing beneath your feet. As this field is
disturbed and changes its orientation, the suspended magnet will track the direction changes. By recording
the orientation of the magnet over time, you can then follow the changes in the local field.
The original design is based on the 'jam-jar' magnetometer devised by Livesey (1982, 1989) of the BAA
Auroral Section in the 1980's. The current design replaces the original design with more readily available
components. Detailed instructions can also be found at the IMAGE/POETRY web site Solar Activity and
YOU!.
Calibration and Sensitivity
When properly set up, these simple magnetometers are known to be exceptionally sensitive. The movement
of automobiles on nearby road surfaces can perturb the local magnetic field in much the same way as a
magnetic storm, although the time scale for the disturbance is very much shorter than for an actual storm.
Metal detectors measure the same kinds of deviations caused by ferro-magnetic substances. With a 1-meter
separation between the magnetic sensor and the light spot on the screen, a 1 centimeter movement
corresponds to a 0.28 degree deflection in the direction of the field from its ambient orientation. A good
student exercise, by the way, is to show that the deflection angle in degrees will be twice the angle
computed from 57.4 x displacement/distance. Magnetic storms often produce deviations of 10 degrees or
more at large magnetic and geographic latitudes on the Earth. For the latitudes of North America, positional
shifts in the few-degree range should be seen. Because all of the magnetometer measurements are
differential in nature, magnetic storm events can only be seen clearly in relation to at least several previous
'null' measurements of the field direction. The null position needs to be clearly determined so that the onset,
climax and termination of the magnetic storm can be discerned in the data.
To follow the overall change in the geomagnetic H component during a storm, the optimal sample rate is
approximately 1 hour so that 10 - 20 points can trace out the envelope of the disturbance. If you want to
resolve potential sub-storm activity, even shorter update intervals approaching 10 minutes may be needed.
For manual recording, hourly measurements of the light spot location may be conducted until the SSC is
spotted; usually an abrupt change in the spot location relative to the null position. Thereafter, measurements
every 10 minutes may be carried out to capture the high-frequency variations, and sub-storm events.
Figure 2 is an example of an actual magnetic storm event recorded by a high precision magnetometer. We
show the 3 components of the local magnetic field, X = Bx, Y= By and Z= Bz recorded at the Baker Lake
magnetic observatory on September 26, 1998 at a latitude of 64.3^o North. The plots show the strength of
the magnetic field in each direction, and it can be verified from the plot that the magnitude of the total field
remains essentially constant ( B = ( X^2 + Y^2 + Z^2)^1/2). To compute the change in the local direction of
the field between the north geographic pole and the north magnetic pole, you simply use corresponding
points in the X and Y traces and calculate the magnetic declination angle,
D = arcTan(X/Y).
Figure 3 shows the resulting magnetic deviation along with the expected deviation of the reflective spot in
the standard 2-meter magnetometer configuration. Although the magnetic measurements in Figure 2 were
obtained each minute with automatic recording instruments at the observatory, it is clear that sampling this
kind of data at hourly intervals will be enough to detect such events. Greater information can be obtained
with 10 minute sampling after the SSC is detected from the hourly studies.
Actual Observations
A magnetometer of this type was constructed at the NASA Goddard Space Flight Center in Greenbelt,
Maryland, and operated from December 1998 to January 1999. To avoid disturbing the light when turning it
on and off for a reading, substantial quantities of duct tape was used to anchor the lamp base and soda
bottle. The light was then turned on and off by unplugging it at the wall socket.
We monitored the state of the magnetosphere by visiting the NOAA Space Environments Center web site at
the beginning of each working day to see if the field was either disturbed or quiet. On quiet days, no
measurements were attempted. On disturbed days, we visited the magnetometer at hourly intervals until an
SSC was observed, at which time we began measuring the spot deviation from the null position.
Since we could not keep the instrument under constant supervision, every SSE event was scrutinized to
insure that someone had not accidentally disturbed the instrument's geometry. If a potential SSC was
observed, we watched it continuously for 30 minutes to detect the common short-term variations that
invariably accompany the recovery phase of the magnetic storm or sub-storm events. If these were not
spotted, we recorded the new position as the new 'null' and returned to the hourly watch schedule. We
resumed the process the next day until the SSC announced the return to quiet magnetic field conditions.
During the period from TBD to TBD we were able to detect TBD magnetic storm events at GSFC.
Comparing these against the records from the magnetic observatory at TBD it is quite obvious that even a
crude soda bottle magnetometer can perform in an acceptable way, provided that the observer prudently
adopts the safeguards we have mentioned in our observing procedure.
Application
There are a number of exciting research questions that could now be formulated with this instrument,
especially when data from professional magnetic observatories are used in conjunction. For example, do the
measurements made with the SBM at your latitude look similar to those made at other geographic locations
at the same time? Does the magnetic storm onset, and principle large deviations, occur at the same times at
different geographic locations? How strong is the magnetic or geographic latitude effect in determining the
amplitude of the biggest deflection? Are some geographic locations better than others in seeing magnetic
storms? How does the storm correlate with auroral displays at your location? Do you see more magnetic
storms at certain times of the year than others? Is the magnetic field more disturbed when the Sun is up than
at night time?
With a little forethought, students can use the data to search for trends, compute averages and standard
deviations, and consider what factors can influence the quantity of the measurements, especially local
environmental effects, cars, earthquakes etc.
References:
Fleming, J. A. 1939, 'Physics of the Earth: Terrestrial Magnetism
and Electricity', (TBD: TBD), pg. 5
Livesey, R., 1982, 'A Jam-Jar Magnetometer', Journal of the
British Astronomical Society, v. 93, p. 17.
Livesey, R. 1989,
'A Jam-Jar magnetometer as an Aurora Detector',
Sky and Telescope,
October, 1989, p. 426
Pettitt, D. O., 1984, 'A Fluxgate Magnetometer', Journal of the British
Astronomical Society, v. 94, p. 55.
Savage, C. 1994, 'Aurora: The mysterious northern lights',
(Sierra Club Books:SF), p. 59-67.
Solar Storms and You!
Activity 9: A Soda Bottle Magnetometer
Introduction
Solar storms can affect the Earth's magnetic field causing small changes in its direction at the surface which
are called 'magnetic storms'. A magnetometer operates like a sensitive compass and senses these slight
changes. The soda bottle magnetometer is a simple device that can be built for under $5.00 which will let
students monitor these changes in the magnetic field that occur inside the classroom. When magnetic storms
occur, you will see the direction that the magnet points change by several degrees within a few hours, and
then return to its normal orientation pointing towards the magnetic north pole. Please refer to the attached
primer Studying the magnetosphere in the Classroom for additional background information.
Objective
The students will create a magnetometer to monitor changes in the Earth's magnetic field for signs of
magnetic storms.
The following instructions provide directions for making a simple magnetometer. For various
enhancements, please have a look at the Mark 2 Design page which uses the same setup but with a laser
pointer. Also, in the January 1999 issue of Scientific American, there is a design for magnetometer that uses
a torsion wire and laser pointer developed by amateur scientist Roger Baker. You can visit the Scientific
american pages online to get more information on these other designs.
Design 1 | Design 2
Materials
One clean 2 liter soda bottle
2 pounds of sand
2 feet of sewing thread
A small bar magnet
Get this from the Magnet Source. They offer a Red Ceramic Bar Magnet with 'N' and
'S' marked. It is 1.5" long. Price $0.48 each. Catalog Number DMCPB. Call 1-800-5253536 or 1-888-293-9190 for ordering and details.
A 3x5 index card
A 1 inch piece of soda straw
A mirrored dress sequin, or small craft mirror.
Darice, Inc. 1/2-inch round mirror, item No. 1613-41, $0.99 for 10.
PO Box 360284, Strongsville, Ohio, 44136-6699. Available at Crafts
Stores under trademark 'Darice Craft Designer'
Super glue (be careful!)
2 inch clear packing tape
A meter stick
An adjustable high intensity lamp with a clear...not frosted..bulb.
Procedure
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Clean the soda bottle thoroughly and remove labeling.
Slice the bottle 1/3 of the way from the top.
Pierce a small hole in the center of the cap.
Fill the bottom section with sand.
Cut the index card so that it fits inside the bottle (See Figure 1).
Glue the magnet to the center of the top edge of the card.
Glue a 1 inch piece of soda straw to the top of the magnet.
Glue the mirror spot to the front of the magnet.
Thread the thread through the soda straw and tie it into a small triangle with 2 inch sides.
Tie a 6 inch thread to the top of the triangle in #9 and thread it through the hole in the cap.
Put the bottle top and bottom together so that the 'sensor card' is free to swing with the mirror spot above
the seam (See Figure 2).
12. Tape the bottle together and glue the thread through the cap in place.
13. Place the bottle on a level surface and point the lamp so that a reflected spot shows on a nearby wall about
2 meters away. Measure the changes in this spot position to detect magnetic storm events (See Figure 3).
For additional pictures of the assembled magnetometer, have a look at these images:
Photo 1 Photo 2 Photo 3
For detailed instructions about how to set-up the magnetometer and record data, have a look at our Set-up
and Data-Taking Procedures file.
As a supplementary activity in applied geometry, you may want to show that the angular deflection you will
see on the wall will equal TWICE the actual angular deflection of the magnet and its deviation from
magnetic north. Here's how to think about this problem.
First, imagine holding the mirror so that it is parallel to the wall, with the light beam also 'skimming the
surface' of the mirror. The point where the glancing beam hits the wall will define 'zero degrees'. Now
imagine slowly rotating the mirror so that it is at right angles to the wall. The beam will be reflected directly
back to the light source located at '180 degrees'. So, by rotating the mirror (magnet) by 90 degrees, the light
beam spot on the wall will scan through 180 degrees. At a mirror tilt angle of 45 degrees, the beam will be
reflected at a 90 degree angle and the spot on the wall will be at 90 degrees to the light source. For small
deviations about this point, you can use the 'skinny triangle' approximation to convert the spot displacement
in centimeters to a spot displacement in degrees. From the geometry, the relevant formula is:
Angle in degrees
=
57. 307 x
deflection in centimeters
------------------------------distance in centimeters
BUT the true deflection angle will be 1/2 of this amount because of the discussion above. For example, if
the distance between the mirror and the wall is 1 meter ( 100 centimeters) and you notice a deflection of 1
centimeter from the spots previous position, then the deflection angle of the magnetic field is just
1.0
Deflection in degrees = 1/2 x 57.307 x ------100
or 0.28 degrees. If you prefer using minutes of arc ( there are 60 in a degree) then this equals 60 x 0.28 or
17.2 minutes of arc.
Sample Data
Here are some records of data taken with this magnetometer. Note that the 'Relative Position' column is an
arbitrary numerical scale, in this case numbered in inches. Only the differences between pairs of numbers
are significant and correspond to the magnetic deflection!
Adams, Massachusetts December 24,25, 1998
Greenbelt, Maryland January 25-29, 1999
Greenbelt, Maryland February 1-5, 1999
Greenbelt, Maryland February 8-12, 1999
Greenbelt, Maryland February 15-19, 1999
Greenbelt, Maryland February 22-26, 1999
Greenbelt, Maryland March 1-5, 1999
Greenbelt, Maryland March 8-13, 1999
Greenbelt, Maryland March 15-17, 1999
Greenbelt, Maryland March 22-26, 1999
Tips
It is important that when you adjust the location of the sensor card inside the bottle that its edges do not
touch the inside of the bottle. Be sure that the mirror spot is above the seam and the taping region of this
seam, so that it is unobstructed and free to spin around the suspension thread.
The magnetometer must be placed in an undisturbed location of the classroom where you can also set up the
high intensity lamp so that a reflected spot can be cast on a wall within 1 meter of the center of the bottle.
This allows a 1 centimeter change in the light spot position to equal 1/4 degree in angular shift of the
magnetic north pole. At half this distance, 1 centimeter will equal 1/2 a degree. Because magnetic storms
produce shifts up to 5 or more degrees for some geographic locations, you will not need to measure angular
shifts smaller than 1/4 degrees. Typically, these magnetic storms last a few hours or less.
To begin a measuring session which could last for several months, note the location of the spot on the wall
by a small pencil mark. Measure the magnetic activity from day to day by measuring the distance between
this reference spot and the current spot whose position you will mark, and note the date and the time of day.
Measure the distance to the reference mark and the new spot in centimeters. Convert this into degrees of
deflection for a 1 meter distance by multiplying by 1/4 degrees for each centimeter of displacement.
You can check that this magnetometer is working by comparing the card's pointing direction with an
ordinary compass needle, which should point parallel to the magnet in the soda bottle. You can also note
this direction by marking the position of the light spot on the wall.
If you must move the soda bottle, you will have to note a new reference mark for the light spot and the
resume measuring the new deflections from the new reference mark as before.
Most of the time there will be few detectable changes in the spot's location, so you will have to exercise
some patience. However, as we approach sun spot maximum in the year 2000, there should be several good
storms each month, and perhaps as often as once a week. Large magnetic storms are accompanied by major
aurora displays, so you may want to use your magnetometer in the day time to predict if you will see a good
aurora display after sunset. Note: Professional photographers use a similar device to get ready for
photographing aurora in Alaska and Canada.
This magnetometer is sensitive enough to detect cars moving on a street outside your room. With a 1-meter
distance between the mirror and the screen, a car moving 30-50 feet away produces a sudden deviation by
up to 1 inch from its refeence position. The oscillation frequency of the magnet on the card is about 4
seconds and after a car passes, the amplitude of the spot motion will decrease for 5-10 cycles before
returning to its rest position. You can even determine the direction of the car's motion by seeing if the spot
initially moves east or west! Also, by moving a large mass of metal...say 30 lbs of iron nails...at distances of
1 meter to 5 meters from the magnet, you can measure the amount of deflection you get on the spot, and by
plotting this, you may attempt to recover the 'inverse-cube' law for magnetism. This would be an advanced
project for middle-school students, but they would see that magnetism falls-off with distance, which is the
main point of the plotting exercise.
Conclusions
Just as students may be asked to monitor their classroom barometer for signs of bad weather approaching,
this magnetometer will allow students to monitor the Earth's environment in space for signs of bad space
weather caused by solar activity.
Related Web Resources
Visit our page of links to professional Magnetic Observatories to see the latest measurements from stations
around the world.
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