Fwd: Fw: [EXTERNAL] NASA's Van Allen Probes Spot Man-Made Barrier Shrouding Earth - NASA

[Jonathan]

Older, but extremely interesting article! Thanks Phil for contributing to this!

https://www.nasa.gov/missions/van-allen-probes/nasas-van-allen-probes-spot-man-made-barrier-shrouding-earth/

Jonathan

From: Bill Engelke
Date: Sun, Sep 7, 2025 at 9:56 AM
Subject: Fw: [EXTERNAL] NASA's Van Allen Probes Spot Man-Made Barrier Shrouding Earth - NASA
To: Jonathan <emum...@gmail.com>

Jonathan - in case you didn't already see this... a potential whole new area of VLF research - 73 - Bill    AB4EJ

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Subject: [EXTERNAL] NASA's Van Allen Probes Spot Man-Made Barrier Shrouding Earth - NASA

 

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[Phil Erickson]

Hi all,

  Yes, from a few years ago.  It has been known for decades that VLF transmitters interact with the inner edge of the outer radiation belt electrons by changing their pitch angle (angle with respect to the magnetic field).  Lowering the pitch angle means the electrons penetrate deeper into the atmosphere as they gyrate around the magnetic field (1st adiabatic invariant = gyromotion), and eventually reverse direction, bouncing between north and south hemispheres; this is the 2nd adiabatic invariant.  When the reflection altitude is low enough, the electrons get lost out of the radiation belt as they scatter due to atmospheric collisions, creating a loss mechanism.

  The study shows that electric field measurements on Van Allen Probes as they orbited in and out of the inner edge of the outer radiation belt were well aligned with the edge of VLF propagation.  This is highly suggestive that the VLF interactions do in fact play a role in shaping the inner radiation belt.

  I will say (including for my colleagues who are on this list; hi, Jacob Bortnik) that as in many things, the real story is quite a bit more complicated and variable because there are a number of factors that go into radiation belt loss mechanisms, and these operate on different time scales.  Quasi-linear diffusion steps in at longer time scales to reshape the belts.  Nonlinear phase trapping / phase bunching works at shorter time scales.  Still, when a dynamic storm event happens, the initial inward movement of the outer radiation belt inner edge definitely sees the VLF 'bubble' as a significant influencing factor.

  We learned a great deal from Van Allen Probes and maybe someday, there will be more radiation belt missions, especially to help unravel the mysteries of how the cold plasma lifecycle interacts with the higher energy populations.

Cheers/73

Phil

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Phil Erickson
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