Physics Photo of the Week - November 8, 2024

Physics Photo of the Week - November 8, 2024

Aurora from Swannanoa

On May 10, 2024, much of North America experienced a "solar storm" that produced aurora as far south of the Carolinas.  Aurora (northern lights) are usually confined to the Earth's polar regions (northern Canada, Alaska, Norway, Sweeden, Russia, ...).  The aurora are produced by charged particles (electrons and protons) that are emitted by the Sun (solar wind) which are guided by magnetic fields in the solar system.  The Sun had emitted a massive amount of high energy electrons and protons a couple of days earlier - called a "Coronal mass ejection".  When these particles reach the Earth, they strike the atoms and molecules in the Earth's upper atmosphere and cause the atmosphere to glow.  This is analogous to the glow on an old-fashioned TV picture tube when electrons strike the coating on the inside of the tube.  

The charged particles of the solar wind are guided by magnetic fields.  The streaks of light slanting down toward the northern horizon in this week's photo show the direction of the Earth's magnetic field - slanted down toward the Earth's magnetic pole which is deep within the Earth.  Thus the auroras show a 3-d rendering of the Earth's magnetic field lines.

Physics labs almost always have a special compass called a "dip needle".  A dip needle is small magnet like a compass needle that is balanced.  The dip needle however is constrained to rotate in a vertical plane, allowing the balanced needle to be rotated by the dipped magnetic field down toward the center of the Earth.

The photo at right was made by my Bill Mosher, my neighbor.  His camera was pointed up into the sky roughly in the direction of the Earth's magnetic field.  The streamers are actually almost parallel - like the streamers in the upper photo -  but appear to diverge away from an origin - analogous to looking along the parallel rails of a railroad.

The cameras in modern cell phones have very sensitive sensors.  Bill's and my cameras also had a time-exposure feature.  Bill showed the feature to me!  We did not use any tripod for these photos, which required 3 sec exposures, we braced our phones on a tree, or fence post.

The Sun approaching the maximum activity in the 11 year sunspot cycle.  When there are more sunspots on the sun, there are more coronal mass ejections and more aurora.  Be on the lookout for auroras on clear, moonless nights (away from city lights) during the following year or so.

This last photo was made on October 10.  The power failure from the hurricane Helene made for darker skies allowing the stars and aurora to shine with more contrast.  Through the auroral glow we can see the Pleiades star cluster near the bottom of the photo.  The Cassiopeia constellation can be seen in the upper left of the photo (a sideways "M" or "W").

 

 

The drawing at right shows a schematic diagram of the Earth (green circle), with a bar magnet embedded that creates a simulated magnetic field.  The magnetic field is simulated by the blue lines that resemble a "butterfly" pattern.  Notice that the magnetic field lines are perpendicular to the surface of the Earth near the poles.  At the equator, the field lines are parallel to the surface of the Earth.  Finally, at mid-latitudes (Swannanoa), the field lines intersect the surface of the Earth at an angle to the horizontal.  This angle is the "dip angle", the angle of the aurora streamers in the top photos

The butterfly pattern of the magnetic fields can be visualized by placing a cardboard over a bar magnet on a wooden table, and sprinkling iron filings over the paper that covers the bar magnet.  Tapping the paper would allow the iron filings to align with the butterfly-shaped magnetic field.  Watch the  video link below that demonstrates the iron filings and a magnet.

https://www.youtube.com/watch?v=IJgzVaTPEow

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Physics Photo of the Week is published periodically during the academic year on Fridays by Donald F. Collins, professor emeritus of Warren Wilson College. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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