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+ BzZat 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.
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 sub-storm 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.
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!.
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.
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.
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.
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.