Citizen Scientist Magnetometer potential pitfalls

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HL Serra

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Jun 8, 2021, 2:06:09 PMJun 8
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Please pass to all involved with the HAMSCI Magnetometer project:


Kindly read this gentleman's results from his citizen magnetometer project in AK.

His GitHub site is:

https://engrinak.github.io/

With the first part of his project at:

https://engrinak.github.io/python/2021/01/21/Geomagnetic-Observatory-Part1.html


This gentleman's experience with his magnetometer shows likely pitfalls for a citizen-science magnetometer, to the point that I'm not sure it is worth trying on a mass scale unless the sensor can be designed and engineered in a way to physically isolate and protect it from local thermal, magnetic and electromagnetic effects. 


Also, his data comparison to the USGS magnetometer did not show as much "correlation" as I would have hoped, in fact some of his data goes the opposite direction from the calibrated USGS magnetometer data recorded 600 miles away. Can magnetometer data vary that much 600 miles from USGS magnetometer site? (I don't know that much about the sensitivity of magnetometers except in degaussing our ship in the navy, but learned a lot from this gentleman's experiments.)


73, Larry N6NC

 

Dr. Nathaniel A. Frissell Ph.D.

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Jun 8, 2021, 2:21:28 PMJun 8
to ham...@googlegroups.com, Julius Madey, David Witten, Hyomin Kim

Hi Larry,

 

Thank you for your concerns; we are doing our best to test and mitigate all of those things that you talk about. Jules K2KGJ has already done extensive thermal testing, and has developed and enclosure/mount that allows for temperature stabilization when buried in the ground. Bill AB4EJ has just recently been conducting EM/RFI testing, and results and discussion have been posted just over the past few days on the TangerineSDR Listserv.

 

Please take a look at the presentations given at the HamSCI workshop this March by Jules Madey K2KGJ and Dave Witten KD0EAG:

 

Jules’ presentation shows excellent agreement of his Hillsdale, NY PN3100 magnetometer with the professional magnetometer in Fredericksburg, VA.

 

We are testing at additional locations as well, and will be analyzing the data carefully from all of these stations.

 

73 Nathaniel W2NAF

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Jonathan

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Jun 8, 2021, 3:38:53 PMJun 8
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Hi Larry,

I remember learning the magnetometer developed by HamSCI outputs changes in the magnetic field. You’ll have to integrate the data to get the magnitude. Is the USGS magnetometer data the differential data too?

Jonathan
KC3EEY

HL Serra

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Jun 8, 2021, 3:59:23 PMJun 8
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Thank you, Jonathan and Nathan and all the magnetometer participants. 

Apparently these concerns are being investigated, and I hope the final configuration will be well field-tested at various types of sites before offering to the HAMSCI participants. 
(I live in a commercial and residential urban village, and particularly the EMI sources are increasing almost daily, trying to eliminate them is like playing "Whack-a-Mole." I can imagine all the local magnetic influences here from urban sources and Average Daily Traffic of 25,000 commercial and residential vehicles.)
73, Larry N6NC

HL Serra

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Jun 8, 2021, 6:41:19 PMJun 8
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Thanks, Nathan. I will review that material, but I think the magnetometer committee has already ruled out calibrated absolute HAMSCI magnetometer readings as a project objective, having realized they will never get the units isolated from local magnetic (and perhaps EMI) effects. They said in their project PPT title page: "Time-varying field measurement is sufficient: absolute measurement is not necessary."

I still wonder from personal experiences with antenna arrays over 62 years of ham radio, how the project will EMI isolate the magnetometers in a buried plastic epoxy box without a faraday shield, unless a faraday shield is fashioned from a non-magnetic material like aluminum screening wire or such. And burying a plastic box still leaves it susceptible to all kinds of direct or condensation-induced water intrusion despite the box having O-ring seals.

Cheers, and 73, Larry N6NC


James Secan

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Jun 8, 2021, 7:00:55 PMJun 8
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To answer your question about differences to be expected due to the separation of the sensors (Anchorage to Sitka) you might look at the USGS magnetometer page <https://geomag.usgs.gov/plots/?scale=Trace> and compare the trace for Sitka (SIT) to that for College (CMO). These stations are roughly 1,000km apart, as are Sitka and Anchorage. During auroral substorms the current systems which cause the changes in the magnetic field can be very localized and rapidly-changing.

Jim
3222 NE 89th St
Seattle, WA 98115
(206) 430-0109
> To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAM4%3DgM-Gjvho-YHTibL8twpW_rmXwLvZE%2Br-qn7K6zyJX1JE0Q%40mail.gmail.com.

Phil Erickson

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Jun 8, 2021, 7:14:21 PMJun 8
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Hi Larry,

  In geospace research, we often are looking only for relative B measurements as we're trying to figure out whether currents are flowing in for example the E region.  For those reasons, the science case is satisfied by measuring the change in B to figure out curl (B) and by Ampere's law sense the flowing current, and the absolute measurement does not matter as much.  So it's not so much "realized they will never get the units isolated from local magnetic (and perhaps EMI) effects" but rather the science target that dictates a good relative measurement.

  Having said that, of course, there are transients in B that are not due to currents flowing in the ionosphere as I think you pointed out, but due to various local effects.  Sorting those out is the domain of the back end analysis as long as the instrument maintains a good relative calibration.

73
Phil W1PJE



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HL Serra

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Jun 8, 2021, 7:49:24 PMJun 8
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Thank you, Phil. I've got it. The magnetometer group said in its project PPT title page:
 "Time-varying field measurement is sufficient: absolute measurement is not necessary."
So all they are looking for is the delta change percentage in the magnetic field at any one time from each of the discrete HAMSCI magnetometer sites, correct?

From 62 years of ham radio experience building antenna arrays, I'm also even more concerned about water/condensation intrusion into a buried underground box, even with O-ring seals. I see that undergrounding is a great idea for thermal stability, but getting all that gear wet will eventually create a problem. (I have no confidence in supposedly waterproof NEMA outdoor boxes-- they always leak either directly at their seams and cable feed-throughs, or indirectly due to condensation from fluctuations of pressure, humidity and temperature. 

For EMI, I think the box should have a (reasonably non-magnetic) faraday shield of something like fine mesh aluminum screen wire. There is just too much EMI in urban and suburban commercial and residential neighborhoods these days not to shield the sensitive magnetometer gear from EMI.

Cheers, and 73, Larry N6NC
PS I don't like to throw my opinions or anything else around on the internet, and I have no energy for group email pissing contests. But when I stumbled on the citizen magnetometer attempt by Stephen Bell in AK, I was alarmed by the difficulty of keeping out external influences on magnetometers aimed to produce serious citizen data. I kind of know a little bit from whence I speak, since besides my six+ decades of ham experience, I gained a masters from Scripps Institution of Oceanography/UCSD in 2016, and also do occasional propagation experiments with our ham radio propagation engineer/guru Carl K9LA.

HL Serra

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Jun 8, 2021, 7:55:06 PMJun 8
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Thanks, Jim. But can these currents show opposite trends at stations 600 miles apart? I wish I could visualize the flux lines of those auroral currents to see if they fluctuate at locations that distance apart. I always imagined the lines rising up from the poles like the cut view of layers of an onion. 
73, Larry N6NC

James Secan

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Jun 8, 2021, 8:12:00 PMJun 8
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Larry,

Yes, but the variations that are being measured are due to current systems flowing in the E-region of the ionosphere, about 80-100 km up. If you visualize these as kms-thick “wires” in the ionosphere, there are time-varying magnetic fields wrapped around these currents with field strength that drops off with a 1/r dependence. For example, a current that’s flowing directly over Anchorage (100 km from the overhead source) might be small-scale enough that it doesn’t make more than a blip at Sitka (~1,000 km from the source).

You can see opposite trends if, for example, the current system is between the two stations. The B field wraps around the current flow direction, and is pointing in different directions on opposite sides of the current.

Jim
3222 NE 89th St
Seattle, WA 98115
(206) 430-0109

> To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAM4%3DgM9RvXT1%2B%3DjrZHSpt0s9WHDonOcMnxRhnkMmB7PzR91VSg%40mail.gmail.com.

HL Serra

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Jun 8, 2021, 8:14:56 PMJun 8
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>You can see opposite trends if, for example, the current system is between the two stations.  The B field >wraps around the current flow direction, and is pointing in different directions on opposite sides of the current.

Thanks, Jim, now I can visualize it and the potential variances.
Cheers, Larry N6NC

James Secan

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Jun 8, 2021, 8:24:09 PMJun 8
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In the parlance of the field, these are referred to as positive or negative “bays” depending on the direction the change is WRT the quiet-day curve. That gives a rough indication of where the primary current system is WRT the station. This is complicated (a little) by the fact that these current flow in one direction (roughly along geomagnetic latitude lines) on the evening side of the polar ionosphere and in the other direction on the morning side.

Jim
3222 NE 89th St
Seattle, WA 98115
(206) 430-0109

> To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAM4%3DgM-seH3Tv4XqRBi-Cn0FBnxP%3DAgfXNdPXmNrC7NB2ANxAA%40mail.gmail.com.

Jonathan

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Jun 8, 2021, 8:31:47 PMJun 8
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Hi Jules,

Thank you for clearing that up for me!

Larry,

The magnetometer is being installed in a PVC pipe that is sticking out of
the ground. With the magnetometer buried, the temperature is much more
stable and constant from what was discussed in the Zoom meetings.

As for a Faraday cage, I asked this question to the Zoom meeting back in
the fall. I was told that the magnetometer is shielded from electric
fields from the ground's conductivity. This makes sense and is even
employed in loops used for sensing Schumann resonances that are buried
underground. With my VLF loops, I use copper or aluminum tape as a shield.

The electrical noise was determined to be coming from the inside. The Pi
itself creates a lot of emissions and it's easy for those emissions to
find there way to the magnetometer outside, tens or hundreds of feet away
being conducted through the cable. In my VLF receivers I install outside.
I use magnetic and optical isolation to ensure mains hum doesn't find its
way into the VLF receiver frontend.

Jonathan
KC3EEY


On Tue, 8 Jun 2021, James Secan wrote:

> Date: Tue, 8 Jun 2021 17:08:15 -0700
> From: James Secan <james...@gmail.com>
> Reply-To: ham...@googlegroups.com
> To: ham...@googlegroups.com
> Subject: Re: [HamSCI] Citizen Scientist Magnetometer potential pitfalls
>
> Larry,
>
> Yes, but the variations that are being measured are due to current systems flowing in the E-region of the ionosphere, about 80-100 km up. If you visualize these as kms-thick ?wires? in the ionosphere, there are time-varying magnetic fields wrapped around these currents with field strength that drops off with a 1/r dependence. For example, a current that?s flowing directly over Anchorage (100 km from the overhead source) might be small-scale enough that it doesn?t make more than a blip at Sitka (~1,000 km from the source).
>
> You can see opposite trends if, for example, the current system is between the two stations. The B field wraps around the current flow direction, and is pointing in different directions on opposite sides of the current.
>
> Jim
> 3222 NE 89th St
> Seattle, WA 98115
> (206) 430-0109
>
>> On Jun 8, 2021, at 4:54 PM, HL Serra <hls...@gmail.com> wrote:
>>
>> Thanks, Jim. But can these currents show opposite trends at stations 600 miles apart? I wish I could visualize the flux lines of those auroral currents to see if they fluctuate at locations that distance apart. I always imagined the lines rising up from the poles like the cut view of layers of an onion.
>> 73, Larry N6NC
>>
>> On Tue, Jun 8, 2021 at 4:01 PM James Secan <james...@gmail.com> wrote:
>> To answer your question about differences to be expected due to the separation of the sensors (Anchorage to Sitka) you might look at the USGS magnetometer page <https://geomag.usgs.gov/plots/?scale=Trace> and compare the trace for Sitka (SIT) to that for College (CMO). These stations are roughly 1,000km apart, as are Sitka and Anchorage. During auroral substorms the current systems which cause the changes in the magnetic field can be very localized and rapidly-changing.
>>
>> Jim
>> 3222 NE 89th St
>> Seattle, WA 98115
>> (206) 430-0109
>>
>>> On Jun 8, 2021, at 3:41 PM, HL Serra <hls...@gmail.com> wrote:
>>>
>>> Thanks, Nathan. I will review that material, but I think the magnetometer committee has already ruled out calibrated absolute HAMSCI magnetometer readings as a project objective, having realized they will never get the units isolated from local magnetic (and perhaps EMI) effects. They said in their project PPT title page: "Time-varying field measurement is sufficient: absolute measurement is not necessary."
>>>
>>> I still wonder from personal experiences with antenna arrays over 62 years of ham radio, how the project will EMI isolate the magnetometers in a buried plastic epoxy box without a faraday shield, unless a faraday shield is fashioned from a non-magnetic material like aluminum screening wire or such. And burying a plastic box still leaves it susceptible to all kinds of direct or condensation-induced water intrusion despite the box having O-ring seals.
>>>
>>> Cheers, and 73, Larry N6NC
>>>
>>>
>>> On Tue, Jun 8, 2021 at 11:21 AM Dr. Nathaniel A. Frissell Ph.D. <nathaniel...@scranton.edu> wrote:
>>> Hi Larry,
>>>
>>>
>>>
>>> Thank you for your concerns; we are doing our best to test and mitigate all of those things that you talk about. Jules K2KGJ has already done extensive thermal testing, and has developed and enclosure/mount that allows for temperature stabilization when buried in the ground. Bill AB4EJ has just recently been conducting EM/RFI testing, and results and discussion have been posted just over the past few days on the TangerineSDR Listserv.
>>>
>>>
>>>
>>> Please take a look at the presentations given at the HamSCI workshop this March by Jules Madey K2KGJ and Dave Witten KD0EAG:
>>>
>>> ? https://drive.google.com/drive/folders/1-dvJsu6rNgiILPHP57RQNoKoREI9zDn7?usp=sharing
>>>
>>>
>>> Jules? presentation shows excellent agreement of his Hillsdale, NY PN3100 magnetometer with the professional magnetometer in Fredericksburg, VA.
> To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/7EAAE2A1-9432-40C8-ACA6-CD652CC3336C%40gmail.com.
>

Gerald Creager

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Jun 8, 2021, 8:47:15 PMJun 8
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I'll try, if there's interest, to dig out my old notes from the Shuttle Amateur Radio Experiment, and from my real-world day-job flight experiment on the EMI/EMC testing and the issues we had with conducted emissions on both. I was the customer and had to fix the problems on SAREX, while the Imed corporation fixed the problems base on the reports for the IV pump we flew in Spacelab. However, I can tell you that ferrite beads, appropriately applied are your friend. And... the location, type and number isn't always what ham radio tells you it is. 

Let me know if I've got to go into the attic and look. I can narrow it down to one of about 15 boxes...

gerry n5jxs

Capt Gerry Creager, CAP

SWR Health Services Officer
Weather and Environmental Support Officer -- Incident Management Team
OKWG Asst Dir Communications Planning


Kim, Hyomin

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Jun 8, 2021, 9:16:23 PMJun 8
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I echo Jim's comments. Magnetic fields and their variations affected by the ionospheric currents at locations separated by 600 miles at low and mid latitudes may not be so remarkable. However, at high latitudes, especially near/in the auroral region (e.g., AK), 600 mile separation can be quite large in terms of their magnetic field signatures. Whether or not his experiment at AK was done properly is a different story, of course. 

Hyomin

NJIT logoHyomin Kim
Assistant Professor
Physics
Center for Solar-Terrestrial Research
Institute for Space Weather Sciences

hm...@njit.edu • (973) 596-5704
https://web.njit.edu/~hmkim/
104 Tiernan Hall, 161 Warren Street, Newark, NJ 07102


Kim, Hyomin

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Jun 8, 2021, 9:37:35 PMJun 8
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Hi Larry,
I also echo Phil's comments about "delta B" measurements. 
In addition, how much we are concerned about EMI nearby also depends on our science target. There are many levels of "variations" in the geomagnetic field that we are interested in. There are "waves" in the ULF range (a few mHz to a few Hz). Some of these waves are quite small in terms of their spectral power (even down to sub nano Tesla (nT)). While these are quite important phenomena that many space physicists are interested in, these are not what we are aiming for not only because of our different science focus but also because of the performance level of our magnetometer (we'd need a much more sophisticated and much more expensive magnetometer). Rather, we are focusing more on "space weather" effects, often associated with large-scale variations of geomagnetic fields (due to currents in the ionosphere). These are in the range of tens to hundreds of nT range which can be discerned quite reasonably by our RM3100 magnetometers without worrying too much about any intermittent EMI sources. We are looking at variations in the time scale of approx. half an hour to a few hours. Therefore, having some intermittent noise may be okay unless the noise source is persistent for a long time. 


Hyomin

NJIT logoHyomin Kim
Assistant Professor
Physics
Center for Solar-Terrestrial Research
Institute for Space Weather Sciences

hm...@njit.edu • (973) 596-5704
https://web.njit.edu/~hmkim/
104 Tiernan Hall, 161 Warren Street, Newark, NJ 07102

HL Serra

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Jun 9, 2021, 1:57:11 PMJun 9
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Thanks, Jim. Then for measurements taken at the same point in time, two stations sited roughly along the same longitude line should not have opposite magnetometer readings, correct?
73, Larry

HL Serra

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Jun 9, 2021, 2:11:14 PMJun 9
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Thanks, Jonathan. I suppose accessing the HAMSCI magnetometer through a fiberoptic system would help, but perhaps make the unit too expensive for citizen use? 

Wouldn't appropriate toroids/filters eliminate or at least reduce the influences most outside EMI? Seems to me (from practical experience) filtering and/or at least a few square inches of copper or aluminum faraday shielding would be a good investment for the unit. I note that in Mr. Stephen Bell's AK citizen magnetometer experiment he at one point put the magnetometer within a copper cylinder, but it still was subject to outside interference.

My only point here is that I hope there are enough safeguards for the HAMSCI citizen magnetometer so they will be useful in EMI dense urban and suburban areas, not just low EMI and low magnetic influence sites. We want everybody to participate as best possible for more granular data collection.

73, Larry N6NC

HL Serra

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Jun 9, 2021, 2:34:48 PMJun 9
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Thanks, Hyomin. But my question was, is it likely to expect that Mr. Bell's recorded magnetometer data would show the opposite trend of data recorded 600 miles away in Sitka? The sites are basically on the same longitude line, and I thought (in my simple "sliced onion" view of the Earth's magnetic lines) that the data should have at least shown the same trend direction, and not an opposite trend direction. Is that kind of opposite variation likely, or possible?

Regards,
Larry N6NC


HL Serra

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Jun 9, 2021, 2:51:16 PMJun 9
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Thank you, Hyomin, for the very good explanation. I'm just concerned to make sure that the HAMSCI citizen-magnetometers will be useful for participants (like me) in EMI- and magnetic influence-dense areas of the country, and not only for "quiet" areas. 

I have done HF propagation experiments here in San Diego with some classmates and colleagues from Scripps Oceanography/UCSD and have been astonished at the density of HF RF signals and noise here, at one point requiring FCC Field Engineers to come out to help track down mystery RF signals and interferences. I'm hoping the final HAMSCI magnetometer will produce useful data in environments such as this, or simply to realize these interference-dense areas are simply not suitable environments for the HAMSCI magnetometer.

Cheers, Larry N6NC

James Secan

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Jun 9, 2021, 3:31:44 PMJun 9
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Larry,

You’ve forced me to go back and dig into books I’ve not looked at in decades (literally!). A good, but somewhat dense and a bit wonky, book on geomagnetic variations in general and specifically about the many indexes that are derived from these data (Kp, AE, AU, AL, Dst, etc) is:

Derivation, Meaning, and Use of Geomagnetic Indices, P. N. Mayaud, ISBN 0-87590-022-4, 1980 (AGU Geophysical Monograph 22).

I’m reminded in reading the pertinent parts of this book that care needs to be taken in specifying exactly what is being discussed, the total field measurement (typically denoted F) or a component of that field. At high latitudes, a coordinate system often used is called H-D-Z, where Z is the down component of F, H is the total horizontal component, and D is the angle between north (geographic or geomagnetic) and the direction of the horizontal component (F vector = H vector + Z vector). In this coordinate system, changes in Z can be in opposite directions for two stations at the same longitude on different sides of a simple E-region current “wire”, but changes in H will be positive at all stations on a fixed longitude for an eastward flowing current and negative for a westward flowing current regardless of where the current is. You can verify this by using the old right-hand rule of pointing your thumb in the direction of the current flow and the resulting B field is in the direction around the current in which your fingers curl.

So, as in many things in this complex life of ours, the answer varies. For stations on the same longitude, changes are:

H component : same direction regardless of current latitude WRT stations
Z component : opposite direction if current is between the stations, same direction otherwise
F (total field): depends on the relative magnitudes and directions of delta-H and delta-Z

I’m pretty sure this is correct - could one of the others on this list who’s currently active in this field double-check this?

Jim
3222 NE 89th St
Seattle, WA 98115
(206) 430-0109

> To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAM4%3DgM-ZeSKZj9Gy%3Dee1rBpgU066cc0vqLaust%2BG%2BcoTWxLxmQ%40mail.gmail.com.

HL Serra

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Jun 9, 2021, 3:47:58 PMJun 9
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Thanks, Jim. Good stuff. This begins to narrow down the issue. Here is Stephen Bell's graph of his magnetometer data v. the Sitka site's data for the same period:
image.png
His magnetometer is the RM3100, “Military Grade” 3 axis accelerometer from Wit Motion, a 3-axis magnetometer chip from Amazon for about $37. 
Q: Does this information tell you whether his data should move in the opposite direction than the Sitka data?

73, Larry N6NC


James Secan

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Jun 9, 2021, 4:06:20 PMJun 9
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The quick answer (which is all I have time for at the moment) is that it’s hard to tell. Are both traces measuring the same thing, such as the total field (F) or the total horizontal field (H), or what? Have to make sure he’s comparing apples with apples, and in the same coordinate system.

If he could find a place in Fairbanks to let his system run for a couple of days that would put it very close to the USGS magnetometer in College. Make for an easier comparison with fewer variables.

Jim
3222 NE 89th St
Seattle, WA 98115
(206) 430-0109

> On Jun 9, 2021, at 12:47 PM, HL Serra <hls...@gmail.com> wrote:
>
> Thanks, Jim. Good stuff. This begins to narrow down the issue. Here is Stephen Bell's graph of his magnetometer data v. the Sitka site's data for the same period:
> To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/CAM4%3DgM_hzanPMZ5Mh%3DiRDO8ZMswiSPz714pdUvCG8q29EUDZzA%40mail.gmail.com.

Dana Whitlow

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Jun 9, 2021, 5:44:17 PMJun 9
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I'm curious now:

In what bandwidth are these magnetometer measurements made?

And what level of sensitivity is required to produce "interesting" (and
useful) data?

Thanks,

Dana


Dr. Nathaniel A. Frissell Ph.D.

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Jun 10, 2021, 10:37:00 AMJun 10
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Hi Larry,

 

I just wanted to clarify one other point… there is not an expectation that the ground magnetometer will be able to make effective geophysical measurements in dense urban and suburban areas. I know that this has been tried many times with professional instruments, and has not turned out well. There may be other applications for the magnetometer in these environments, but the best data is likely going to come from rural locations.

 

73 Nathaniel

HL Serra

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Jun 10, 2021, 11:40:24 AMJun 10
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Thank you, Nathaniel, for your candor. I had sort of reached the same conclusion after reading Stephen Bell's experiment, and some of the comments from the enclosure team-- but especially based on decades of dealing with EMI/RFI and weather problems at amateur installations.

Despite EMI problems here we've been successful in recording relative HF signal strengths over time and, with proper equipment, Doppler frequency shifts during eclipse events, and at sunrise/sunset reception of WWV and WWVH. So I hope the magnetometer team will think of adding not very costly protective features-- onboard toroids for RFI suppression, possible Faraday shielding with simple aluminum mesh, and practical waterproofing treatment-- such as spray-on coatings, or liquid electrical tape-- for PCBs and parts which will be placed underground (where in my experience water always eventually intrudes in the form of condensation or enclosure leakage).

I am grateful for your candid assessment and hope that a similar warning or caution will be provided in the marketing of the magnetometer product to the HAMSCI group. 

Cheers, and 73, Larry N6NC

HL Serra

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Jun 10, 2021, 12:20:53 PMJun 10
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Hi Dana- I don't know. You can read Mr. Bell's project notes here:
73 Larry N6NC


Jonathan

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Jun 10, 2021, 2:35:51 PMJun 10
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Hi Larry,

Plastic fiber won’t add much cost. The thick plastic fiber made for low speed data would be adequate. I purchased a 100ft spool for $70 for use with my optical audio link for my VLF receivers. The only thing is that a battery will be required to be used outside. This gives you a great excuse to check the magnetometer when the battery is low. 

It’s not as simple as using toroids/ferrite beads and shielding. A comprehensive noise study is required to understand the source of the noise and how it is coupled into the magnetometer. I’ve seen this in my VLF adventures. Noise can be electrically coupled it radiates, and have uncommon sources that you’d never think about. A feedline from the inside creates a complex situation that must be studied and understood. 

And as I mentioned before, the ground’s conductivity provides electric field shielding. Remember, even non-magnetic shielding placed close to the coils could cause magnetic interference. And even with e-field shielding, that doesn’t filter out b-field noise. This b-field noise can be found in urban environments and may overload the magnetometer and mask interesting signals. There may be little that can be done to mitigate this, so rural environments may be the best.

I’ve seen many of these interference issues tinkering with VLF reception and there is rarely one, simple, blanket solution. It is often site specific to obtain optimum results.

Jonathan
KC3EEY

HL Serra

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Jun 10, 2021, 3:01:42 PMJun 10
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Thanks, Jonathan-
>There may be little that can be done to mitigate this, so rural environments may be the best.
Yep, and Nathaniel thinks so, too.
73, Larry N6NC
> To view this discussion on the web visit https://groups.google.com/d/msgid/hamsci/66781A94-64FA-49E3-BC02-6676F463B0FE%40gmail.com.

Dana Whitlow

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Jun 10, 2021, 7:40:13 PMJun 10
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Thanks, Larry.  That answered my questions nicely.

Dana


Gwyn Griffiths

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Aug 27, 2021, 12:47:46 PMAug 27
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With no expectation of producing data worthy of even amateur science I've installed the combination of a PNI RM3100 magnetometer and a Bosch BME280 environmental sensor in an aluminium box buried 50 cm deep in a small English suburban garden.
The box has a gasket seal, as does the cable gland connector with a rubber compression tube around the incoming cable. The cable is connected to a surface gasket-sealed ABS box with a Pi Zero W for data acquisition and processing.
The humidity sensor in the BME280 allows me to monitor the relative humidity, and with the temperature and pressure sensor also the absolute humidity (g/m^3). The attached graph shows an incredibly linear rise in relative humidity from about 0 to about 12 percent over three weeks. The box was closed when in the cold dry air in a fridge at 3 deg C and there is a silica gel pouch. This reinforces Larry, N6NC that moisture will get in. 
A graph of three weeks of magnetometer vertical component data comprising 1 minute medians of up to 500 samples each over 800 cycles is also shown. I'd be interested to hear if other have observed the form of slow drift shown here. The large spikes are my taking out and then replacing my small lawnmower out of the nearby (2 m) shed. A Hampel median absolute distance filter has been used prior to calculating each 1 minute median; the count of samples replaced each minute is also logged. 

Gwyn G3ZIL

G3ZIL_Vertical_Magnetometer.png
G3ZIL_Humidity_Magnetometer.png

Julius Madey

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Aug 27, 2021, 1:20:25 PMAug 27
to 'Gwyn Griffiths' via HamSCI
All useful information, but with regard to quality of data and what is needed from and for the project, I think we need to hear from Hyomin Kim, who is the geomagnetometry expert in the group. I'd like to see RH data over at least a 60 day period from Gwyin's experiment.  And, specifically, what is the dewpoint in the box?  High humidity without condensing moisture is not, by itself, necessarily a problem.
JM

Julius Madey

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Aug 27, 2021, 1:37:17 PMAug 27
to 'Gwyn Griffiths' via HamSCI
Bottom line:
If you think you have an improvement or a better idea; more power to you.
Build it
Test it
Share the results
JM


On 8/27/2021 12:47 PM, 'Gwyn Griffiths' via HamSCI wrote:

Kim, Hyomin

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Sep 1, 2021, 4:00:26 PMSep 1
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Gwyn - Thank you for sharing the interesting data with us. I am curious where the slow deviation came from. ~150 nT over the course of 20 days seems a lot. It appears that the nominal diurnal variations (mainly due to magnetospheric activity, I guess) are well caught by the sensor, though. BTW, where is this located?

Hyomin


NJIT logoHyomin Kim
Assistant Professor
Physics
Center for Solar-Terrestrial Research
Institute for Space Weather Sciences

hm...@njit.edu • (973) 596-5704
https://web.njit.edu/~hmkim/
104 Tiernan Hall, 161 Warren Street, Newark, NJ 07102

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