Journal notes, Kp5 Event, More on System Grounding
GELLER Labs "Backyard Science"
Thoughts on a proton precession magnetometer design - a Proton Magnetometer Project. Build an Earth's field magnetometer.
The FDM MAGNETOMETER1 project is a low cost high performance proton magnetometer (a digital magnetometer) kit under development for universities and amateur scientists to be able to accurately measure and monitor changes in the Earth's total magnetic F field and to observe geomagnetic storms. Magnetic storms can cause large excursions in the field and are of concern to interests ranging from electrical power grids, radio communications, and satellite operations, to aurora watchers and amateur radio operators.
1 Filter Diagonalization Method "FDM" (harmonic inversion), see Jan 21 and Jan 23 entries, based on: Vladimir A. Mandelshtam, Howard S. Taylor, Harmonic inversion of time signals and its applications, Journal of Chemical Physics (1997), Volume 107, Issue 17, 1997, Pages 6756-6769
(be sure to hit refresh to pick up our latest changes and entries)
Project Documentation, Links and References (very early stages)
Journal Notes:
Sunday, January 2, 2011
Overnight: PDF, TXT The geomagnetic field continued to be quiet overnight. There was one relatively small negative going pulse after 10 pm. figure of merit (FOM) PDF, amplitude PDF. Beginning of the normal diurnal cycle PDF
After a discussion with a reader about powered coil self resonance, I raised the value of our secondary dump resistor to 1.1 kohms from 402 ohms. Since the FET actively clamps the powered coil discharge at 55 V for most of the powered coil fast linear current discharge (constant di/dt) in the avalanche mode, it is unlikely that the secondary dump resistor plays much of a role until the powered coil voltage falls below 55 V and the FET ends conduction in the avalanche mode and the secondary dump resistor takes over for the final L/R exponential discharge (only for the last milli Joule or so of about 22 mJ of powered coil stored energy). This aspect of the circuit could use some further study, however instrument performance is already far beyond acceptable. It is unclear if the increased success rate overnight (77% to 82%) is related to the changed dump resistor or to the dramatic increase in outdoor air temperature 40F to 50F after weeks in the low 20s. Outdoor temperature could also be a factor in observing fine aspects of instrument performance, since tau 1 and tau 2 of the working fluid are believed to vary with temperature. (Also, note that we are discussing the relatively slow low amplitude signal component of the discharge current (10 to 20 kHz), not the 10 cycles or so of very high frequency (>100 kHz) switching noise (another equivalent RLC circuit previously undamped oscillation somewhere in the system) suppressed by a snubber capacitor on December 21.)
Note that even at the earlier 60% success rate of months past, the FDM proton precession magnetometer instrument has been properly measuring the field at the desired measurement rate and desired resolution and probably with higher accuracy than is needed for a geomagnetic observatory. (It is very difficult to quantify absolute accuracy below about 1 nT.) The recent changes are all fine tuning only to improve instrument efficiency.
For new readers, I posted a brief discussion of success rate and the auto-retry process with examples.
Evening: The temperature has been falling since yesterday from the mid 40s to now 26F (-3 c). The plot of amplitude (peak value of the filtered envelope) for successive plotted measurement points shows a steady increase with decrease in temperature. PDF If temperature continues to be of interest, particularly during instrument development, we should probably install an aluminum cased RTD at the counter-wound coil sensor pair. It might be practical to use one of the unused USB 6008 analog inputs for a temperature monitor. Such temperature studies are only for academic interest and for measuring instrument efficiency, but have no relation to the accuracy of the precession frequency and thus the magnetic field value (other than perhaps indirectly as related to the average FOM values). Speaking of FOM values, here is the corresponding plot PDF, still averaging about 5x10-7, (do not confuse the present average FOM value with the equation for the automatic linear fit on the excel page) and as can be seen in this snapshot PDF, the percent success rate is now up to about 83%. This is the highest success rate seen to date during the many months of development. There is probably still more improvement to be had. Sample evening spectra PDF, Log Spectra PDF.
Monday, January 3, 2011
Overnight: PDF, TXT The geomagnetic field continued to be quiet overnight. Figure of merit (FOM) PDF, amplitude PDF. Negative going spikes are all vehicles, the largest, the usual Monday morning garbage trucks. There were very small sinusoidal and triangular shaped waveforms (e.g. after 5 am). Here is a magnified view in time and vertical scale (1 nT/div) at the waveforms at about 5 and 5:40 am PDF. Similar waveforms were observed throughout North America (1000 UTC) USGS PDF, NRCan OTT PDF.
At some point, we need to develop a method to quantify the noise floor of the FDM proton precession magnetometer. The system is very sensitive and minor magnetic disturbances, many of which survive our (single shot) auto-retry filter, are recorded. At brief times of quiet, some of our magnetograms show little perceptible noise, for example the recorded curve between about 11:25 am and 11:40 am this morning appears to be relatively smooth at 1 nT / div PDF.
The IAGA publication Guide for Magnetic Measurements and Observatory Practice by J Jankowski and C Sucksdorff (1996), an excellent free e-book (in PDF), has been added to our new Links and References page. This is a must read for those thinking about building an amateur geomagnetic observatory. Probably anyone interested in magnetometer instruments in general will find this book a good read as well.
Tuesday, January 4, 2011
Overnight: PDF, TXT. The average figure of merit (FOM) has been running about 4x10-7 (.02 nT) PDF, and the percent measurement success rate at 85%. The resolution of the original Wadsworth proton precession gradiometer instrument, which kindled my interest in proton precession magnetometers and inspired this project, was about 20 nT. Evening: All is very quiet, it would be a great evening to calibrate fluxgate magnetometers outdoors on a wood table using the free calibration standard of the Earth's quiet geomagnetic field! PDF.
Our current best estimate of the total load resistance as a function of outdoor temperature for the polarization switch control system is R=0.0076 x (Temp deg. F) + 5.604 ohms PDF. The cable is 100' of well shielded #22 pairs (Alpha Xtreme) run partly outdoors, through a semi-heated basement, and into the lab. The powered coil (600 turns of #18 wire) is about 2.9 ohms at room temperature. The graph is based on measurements of voltage used to achieve a 1.6 A constant current (read from the Agilent E3615A voltmeter after crossover to constant current mode during a polarizaton cycle) and outdoor temperature (via a wireless thermometer).
Wednesday, January 5, 2011 - More on System Grounding, PART VI Article! (very early rough draft)
Overnight: PDF, TXT. Classic view of the diurnal cycle from yesterday's magnetogram PDF.
Update on FDM Magnetometer electronics system grounding. Our PART IV article on the coils and sensor stand has been updated to reflect the current grounding configuration (the current file name begins with "PART IV _d"). We make changes often, always do a refresh on your browser to update links before downloading a document to be sure to get the latest version.
Also, I posted a very early rough draft of the PART VI, Operation, Building and Testing the FET-Relay Control Board, article on our articles page.
Thursday, January 6, 2011
Overnight: PDF, TXT. Early evening, just changed over to our fast cycling (first time, it does not show yet in this first PDF) PDF, perhaps some interesting activity to follow? NOAA Costello index PDF, SWN PDF. So, far here, just one modest ~10 nT negative going swing in the total field. The minor event seems some what stronger almost everywhere else in North America USGS PDF, we are hundreds of miles from the nearest FRD station, which is also showing less activity. The problem with fast cycling for a minor even is that the normal plot becomes very dense PDF. Fast cycling might prove more useful for very large excursions during a more severe geomagnetic storm. It has not been called yet, but this will probably end up a G1 storm PDF Costello Index PDF. The later data, taken at our normal 2 minute measurement interval, shows that the slower rate gives a better view of these slow events on our 12 hour plots. 9:35 pm, the storm might be winding now, PDF, USGS PDF, NOAA K index PDF, NRCan OTT PDF. That was an interesting geomagnetic event, mostly over by 11:29 pm (est) PDF, TXT (our raw data). The Costello index performed very well for this event! PDF. Yet another slow pulse around 1 am (est) 6 UTC PDF.
Friday, January 7, 2011 - estimated global Kp 5 Event! (G1?) - Calibration Check
Overnight: PDF, TXT. The NOAA Space Weather page had not called a G1 minor magnetic storm PDF, not sure why since the estimated global Kp index reached 5 (G1), NOAA Costello Index PDF. It might be because the maximum Boulder K value was K4. Note that the first half of the event is not related to the normal down turn which occurs with the diurnal variation (a normal morning event generally on the order of 10 to 20 nT). This expanded view shows our early use of the 20 second fast cycle during the first half of the event. While a faster measurement rate might be useful for large field excursions, the higher density of plotted points appears to be less useful on our normal plots PDF. Afternoon, the field remains unsettled PDF, TXT.
We are revisiting the topics of absolute calibration and calibration constants. Note that we presently use a +18 nT offset to compensate for a nearby parked vehicle (at about 33 feet). Our reported magnetic field value is the value calculated using the Larmor equation plus 18 nT, presumably a more correct value for our local total "F" magnetic field. This is a less than an ideal situation, however amateur geomagnetic observatories simply have to cope with available resources, including plot and property sizes.
I performed a regular calibration check of the USB 6008 digitization rate yesterday. The test was performed by applying a 50 ohm terminated sine wave 2 V peak across the psuedo differential analog input. We used an Agilent 33120A synthesizer locked to a Hewlett Packard Z3801A GPS stabilized 10 MHz reference signal. The frequency was verified to 8 digits using a Hewlett Packard 5335A Universal Counter having as its external reference the 10 MHz output of a second independent Z3801A. After many months of running, we found the frequency was reading high by about 0.001 Hz (~.02 nT). Our new measured USB 6008 10 kHz sample frequency is 10.000 347 7 kHz. (See also: The IAGA publication, Guide for Magnetic Measurements and Observatory Practice by J Jankowski and C Sucksdorff (1996), Testing and adjusting absolute instruments, section 8.5, pages 167, 168.) This level of USB 6008 digitizer crystal oscillator drift over many months is both impressive (for a relatively inexpensive instrument <$200) and is quite acceptable for our geomagnetic observatory application.
Saturday, January 8, 2011
Overnight: PDF, TXT. There were several slow undulations of 5 nT or less in the geomagnetic field overnight. Afternoon: There is a noticable, albiet still minor, increase in activity this afternoon compared with this morning PDF (disregard the ~ -4 nT vehicle offset from about 10:40 to 11:20) PDF, however by the evening, the field was very quiet again PDF.
Beta Tester Carl Olsen prepared a detailed NBLNA assembly note, which can now be found on our Docs page under NBLNA. Many thanks!
We posted a very brief article, PART IX, The Gyromagnetic Constant and the Larmor Equation, showing how to use the Larmor Equation for Earth's field NMR (EFNMR) (These constants are for low field NMR).
Sunday, January 9, 2011
Overnight: PDF, TXT, morning sample spectra PDF, Log spectra PDF. There was noticable increase in activity after about 4 pm followed by an interesting well defined positive going pulse about 8 pm this evening (all at sub-storm levels) PDF , which turned out a little later to be more of an assymetric sinewave like waveform PDF.
Project Documentation, Links and References (very early stages)
QUESTIONS/COMMENTS/notice of typos, etc. send email to joegeller at gellerlabs dot com
COPYRIGHT © 2009, 2010, 2011 JOSEPH M. GELLER, All rights reserved.