Jim Koehler
02-27-2000, 08:14 PM
Hi all,
I don't have my testbooks down here with me but I'll try to
summarize what I can remember.
The diurnal variation in the earth's magnetic field is very
small - typically just a few nT and, because it takes 24 hours to go
through its cycle, it is not much of a problem anywhere - even for
single mags. Near the magnetic equator, there is a phenomenon
called the equatorial electrojet which occurs daily, has a magnitude
of (I'm not sure of this number) about 50 nT or so. But, you have
to be close to the equator to see this and it only lasts for a brief
period - less than an hour. I'm somehwat hazy about these numbers
because I've never been much interested in the equatorial
electrojet..
Globally, you can have magnetic storms - these are large scale
things and affect the whole world at higher northern and southern
latitudes. These occur fairly rarely - only a few times per year
but are more frequent as the sunspot numbers get larger. We happen
to be just entering the peak of the sunspot cycle. Magnetic storms
have magnitudes of perhaps approx. 100 nT and last for days. That
is, for days, the magnetic field will vary over ~100 nT range. The
variations are typcially fairly slow, taking place over hours of
time.
Magnetic substorms are seen at auroral latitudes (i.e., Alaska
and most of western Canada)and are probably the most troublesome.
They can appear over a period of just a few minutes, have magnitudes
of approx. 500 nT and last for up to a few hours. They also are
more common as sunspot numbers increase. During peak sunspot years,
they can occur very frequently. In Saskatoon, Sask., where I lived
most of my life, we might get as many as 100 of these per year.
They occur mostly around magnetic midnight (contrary to what Dale
Seppa's reference says) but can be as early as 6PM or as late as
6AM. Because the onset is so rapid, these would pose a real problem
for users of single mags. However, they rarely occur during the day
(instances are known).
With a gradiometer, you are insulated from ALL of these - none
of them have any effect whatsoever! That is the advantage of using a
gradiometer.
Jim
J.A. Koehler
3290 N. Koehler Rd.
Florence, AZ, 85232
Ph: (520) 868-1156
I don't have my testbooks down here with me but I'll try to
summarize what I can remember.
The diurnal variation in the earth's magnetic field is very
small - typically just a few nT and, because it takes 24 hours to go
through its cycle, it is not much of a problem anywhere - even for
single mags. Near the magnetic equator, there is a phenomenon
called the equatorial electrojet which occurs daily, has a magnitude
of (I'm not sure of this number) about 50 nT or so. But, you have
to be close to the equator to see this and it only lasts for a brief
period - less than an hour. I'm somehwat hazy about these numbers
because I've never been much interested in the equatorial
electrojet..
Globally, you can have magnetic storms - these are large scale
things and affect the whole world at higher northern and southern
latitudes. These occur fairly rarely - only a few times per year
but are more frequent as the sunspot numbers get larger. We happen
to be just entering the peak of the sunspot cycle. Magnetic storms
have magnitudes of perhaps approx. 100 nT and last for days. That
is, for days, the magnetic field will vary over ~100 nT range. The
variations are typcially fairly slow, taking place over hours of
time.
Magnetic substorms are seen at auroral latitudes (i.e., Alaska
and most of western Canada)and are probably the most troublesome.
They can appear over a period of just a few minutes, have magnitudes
of approx. 500 nT and last for up to a few hours. They also are
more common as sunspot numbers increase. During peak sunspot years,
they can occur very frequently. In Saskatoon, Sask., where I lived
most of my life, we might get as many as 100 of these per year.
They occur mostly around magnetic midnight (contrary to what Dale
Seppa's reference says) but can be as early as 6PM or as late as
6AM. Because the onset is so rapid, these would pose a real problem
for users of single mags. However, they rarely occur during the day
(instances are known).
With a gradiometer, you are insulated from ALL of these - none
of them have any effect whatsoever! That is the advantage of using a
gradiometer.
Jim
J.A. Koehler
3290 N. Koehler Rd.
Florence, AZ, 85232
Ph: (520) 868-1156