(Published in the Solar Bulletin)
Below are magnetograms of the 22 - 23 September, 1999, magnetic storm by Jim Mandaville, A-90 and Danie Overbeek, A52. Jim has downloaded a magnetogram of the same storm from the USGS magnetic observatory in nearby Tucson for comparison. There is little doubt that Jim's simple homemade McWilliams magnetometer makes very accurate recordings of magnetic storms. Below Jim has very kindly provided detailed instructions how other amateurs can compare their magnetograms with those made by a nearby professional magnetic observatory.
(Oversized version of image)
(Oversized version of image)
I've recently discovered that numerous magnetograms from government observatories are made available on the Internet by Intermagnet, an international organization that pools magnetic observatory data from member governments and makes it available through several data centers. Some of these observatory records, including those from the United States, are available in near-real time, with a lag of only 10-15 minutes. This makes it possible to compare one's amateur magnetograms quickly and directly with those of the nearest professional observatory and help answer the vexing question of whether one's trace is "real" or resulting from instrument error or local interference. The main site can be reached at:
This page brings up a long table with observatories shown in the left column. In the right column are the data centers that hold the present data for each. Clicking on the observatory brings up some written information about the station. For data, however, one clicks on one of the data centers listed to the right, opposite the station of interest. Each data center has its own format, but the procedure for obtaining data for individual stations is usually self-evident. The procedure for the Golden, Colorado center (Gol) is one of the simplest. Clicking on "Gol" brings up a map showing U.S. observatories. In the boxes below one simply selects the station and day, and a magnetograrn will come up on the screen. With some data centers, one has to be careful to select plotted data; otherwise only numbers will be provided. Some centers require either a user ID (which is whatever name you want to show on the record of users).
The Golden center does not appear to encourage downloading of its magnetograms (which are based on raw un-calibrated data), and the only way I found to copy them was by performing a screen dump. Their resolution is quite low, and they cannot be enlarged greatly without showing blocky pixels. They are nevertheless: extremely useful for comparison with home traces.
When picking an observatory to compare with home data it is usually preferable to choose the nearest one that is similar in latitude to the home station. Observatories considerably to the North or South will often show rather different trace shapes because of latitude effects.
(Oversized version of image)
The graph above shows 10.7 cm flux plotted against Zurich sunspot numbers computed from observations of seven AAVSO sunspot observers who count according to the Zurich system. The Zurich reduction formula was used to reduce the counts to true Zurich Relative Sunspot Numbers, Rz. The graph was prepared by AAVSO sunspot observer, Tom Lizak.
Horizontal component (H) at Oro Valley, Arizona (geographic 32o 23.4'N, 110o 56.8'W; magnetic latitude 40o N), 24-26 September 1998. Torsion variometer; observer J. Mandaville. Time scale in hours UTC.
Disturbances between 16 and 18 hours, 25 September resulted from movement of motor vehicles near the observing site.
Note: This is the most intense magnetic storm so far monitored here by this observer. Main phase was entered about 0200 hours, 25 September. The spike between 06 and 07 hours had an amplitude of over 220 nT. The H field recovered gradually and had nearly reached normal level for the station by about 0300 hours, 26 September.
Two recordings above are of a strong magnetic storm on 25 Sepiember, 1998. Jim Mandaville, A-90, used the classic design by Al MacWilliams for his magnetometer. A big magnetic compass needle is suspended as a torsion pendudum that is balanced against the Earth's magnetic field. A shadow vane on one end of the compass needle shades two Cadmium Sulfide photoconductive cells equally. The cells are variable resistors and their resistance depends on how much light falls on them. They make two elements of a four element Wheatstone bridge in which the bridge is a strip chart recorder. So long as the cells are shaded equally from light bulb above them no current flows in the bridge and the chart recorder draws a straight line. Solar activity, such as a big solar flare, can send charged particles to the Earth's magnetosphere changing the strength of the Earth's magnetic field. This causes the torsion balanced compass needle to move and its shadow vane imbalances the light falling on the cells thereby unbalancing the bridge and causing current to flow in the chart recorder to record the magnetic storm. MacWilliams's simple design has been in use for over 20 years and although Jerry Winkler, A-50, replaced the photo sensitive resistor cells with modern Hall Effect sensors, he was not able to improve on the sensitivity of the traditional design. The following page shows details of the MacWIlliams design as built by A-90.
The above drawing shows Jim Mandaville's magnetometer in sufficient detail that anyone wishing to build one could use it as a guide to build their own. Everything you need to know is shown in the drawing and the four details. The main drawing shows the simplicity of the device, a bar magnet suspended on a steel guitar string about 0.008 inches in diameter. Normally the magnet will line up with the Earth's magnetic field as any compass would and point North and South. The top end of the guitar string is fastened to the inside of a PVC pipe cap that can be rotated to wind up the guitar string to produce enough torque to force the magnet to point East and West instead of the North-South position it would prefer. It thus becomes a torsion balance in which its East-West position is balanced against the Earth's magnetic field. So long as the Earth1s magnetic field remains constant the magnet remains stationary. Charged particles from solar flares set up currents in the magnetosphere that change the strength of the magnetic field and therefore the position of the balanced magnetic compass needle.
Details B and C show how a shadow vane attached to the magnet shades two Cadmium Sulfide photo cells equally from light from the 12-volt bulb above them. Their resistance depends on how much light falls on them and so long as they are lit equally the Wheatstone bridge shown in detail D remains balanced and the strip chart recorder connected across the bridge draws a straight line. A magnetic storm upsets the torsion balance moving the shadow vane. That unbalances the resistance of the CdS cells causing current to flow in the bridge so the strip chart recorder records the magnetic storm as shown on the recording on the previous page.
The Wheatstone bridge of which the two CdS photocells are a part is a simple device that drives the recorder without amplification and yet provides more sensitivity than is usually needed. It has the further advantage that the 1000 ohm resistor can be adjusted to provide a bias current to place the strip chart recorder's pen in the middle of the chart so both negative and positive changes produced by the magnetic storm are truly recorded.
Materials to build the magnetometer cost very little. Magnets are available from Edmund Scientific Company, 101 East Gloucester Pike, Barrington, NJ 08007 USA, The Cadmium Sulfide photocells and parts for the power supply can be found at Radio Shack. If anyone has difficulty finding the parts I will be glad to ship them the parts at cost. Please contact me at my address above. If you are interested in recording magnetic storms on a computer rather than a strip chart recorder a kit to build an A/D converter and free software are available from Joseph Lawrence at his address above. C.H.H.