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I recently went to Antarctica where we made many shore excursions with Zodiacs. On one of them we visited the Ukrainian Vernadsky Research base on Galindez Island where they conduct geophysics research.

I was discussing this with one of the scientists there who was trying to describe his work. His English was not great and he was probably talking down to me as a non-specialist, but he very clearly indicated that he was studying the way charged particles from the sun cause perturbations along geomagnetic lines of force over great distances. And on a piece of paper he drew one such line that he was studying which he showed as descending to earth in New England (where I live) and said they were doing this research with Boston College.

My Question: I always thought that the idea of a "line" of magnetic force was conceptual convenience, an artifice we used to be able to talk about magnetism but that there are no actual, physical discreet "lines". And that the visual lines that form with, say, iron filings, are just due to the magnetized filings attracting others around them and so are just clumping artifacts of the filings themselves.

But this scientist seemed to be specifying a very specific line including a specific location near Boston where it descends to earth and drew it that way on a piece of paper and said they had been studying this particular line for years during different cycles of solar activity. What could he have been referring to?

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Geophysicists are not referring to specific lines of force, but instead study major long-term features or "lobes" of the earth's geomagnetic field. Apart from the main dipole, the field has more local features associated with more rapid change. These are hypothesized to originate somewhere in the core-mantle boundary and share some interesting relationships with the main dipole's excursions and flips.

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  • $\begingroup$ By lobes do you mean variations in magnetic field strength (e.g., the South Atlantic Anomaly)? OK, but how would that relate to what he described? He drew a line basically from the south end of the geomagnetic dipole in an arc entering the earth near Boston and showed how charged particles entering the earth's atmosphere at the polar end, near where we were, cause perturbations (he drew little squiggles along the line) propagate to the other end near Boston. $\endgroup$
    – user316117
    Mar 15 '17 at 19:25
  • $\begingroup$ It occurs to me the lines may have meant "connected-to conceptually", not "connected-to physically". I.e., "something changes here" (charged particles from the sun) "something changes there" (nanotesla strength changes). Sort of the way you might diagram a line between two quantum-entangled particles to show a relationship between them but not to imply any force was traveling along that line. It just goes to show that translation problems are not just with words, but also with drawings. $\endgroup$
    – user316117
    Mar 15 '17 at 19:35
  • $\begingroup$ Yes, local variations in the field strength and direction. And remember that these features are not just at the surface, they extend all the way from the core-mantle boundary to outer space. Solar wind absolutely does interact physically with the field in a literal sense. As a charged particle enters the field, the Lorentz force causes it to spiral along the axis of the field lines In places where the field strength is greater, this acts as a sort of conduit where charged particles aggregate and dump into our atmosphere. $\endgroup$ Mar 15 '17 at 19:51
  • $\begingroup$ physics.stackexchange.com/questions/239638/… $\endgroup$ Mar 15 '17 at 19:52
  • $\begingroup$ What do you mean by "the axis of the field lines"? Since we've already agreed that there's no "line", what can we mean by the "axis" of that line? $\endgroup$
    – user316117
    Mar 16 '17 at 2:51
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As pointed out already, the magnetic field is continuous and not made of discrete lines of force in a literal sense, but field lines are an effective way to represent the magnetic field vector, akin to stream lines for fluid flow.

What your scientist was likely talking about is a connection between New England and Antarctica as geomagnetic conjugate points at opposite ends of a magnetic field line where it meets Earth's surface. Indeed, for recent years, a field line from Galindez Island would land in New England.

A magnetic field line exiting at Galindez Island, as given by the IGRF-12 at 20170315T173300, landing somewhere in New England.

Various phenomena lead to charged particles and radio waves (like lightning whistlers) being guided along magnetic field lines. Studying conjugate points lets you observe aspects of the same phenomenon in two places

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There are no actual discrete physical lines, (except connected with superconductivity), there is a continous magnetic field. My guess he was simply placing a strong emphasis on a particular direction of the field in which the particles travelled. My knowledge of geophysics is very limited, perhaps there is some aspect of the terrain in the "landing zone" of the particles that is particularly important, but there are no field lines in reality.

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  • $\begingroup$ This map shows magnetic fields on a very local scale and in a very noisy city environment. I wonder what the purpose of this data set was . $\endgroup$ Mar 16 '17 at 6:30
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I always thought that the idea of a "line" of magnetic force was conceptual convenience, an artifice we used to be able to talk about magnetism but that there are no actual, physical discreet "lines".

You are correct, there are no magnetic field lines as actual physical quantities. Similar to vector field maps (i.e., plots with arrows indicating direction and magnitude by pointing and size, respectively), field lines are a useful visualization tool. Field lines are basically the "connect the dots" version of a vector field map (perhaps, overly simplified but basically true).

And that the visual lines that form with, say, iron filings, are just due to the magnetized filings attracting others around them and so are just clumping artifacts of the filings themselves.

Iron filings "line up" with the magnetic field to minimize the inner product $\boldsymbol{\mu} \cdot \mathbf{B}$, which is just the magnetic potential energy of a magnetic moment, $\boldsymbol{\mu}$, in an external magnetic field, $\mathbf{B}$. This happens to correspond to an approximate field line representation because the magnetic domains comprising the magnetic moment are aligned parallel to the length of the little cylindrical pieces composing the filings.

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