Radiation Belt Event Studies Reeves et al. (2003) Baker et al. (2004) Radiation Belt Working Group 28 January 2009 Brian Larsen Chia-Lin Huang

Large Context

Sometimes you have increases, decreases, and no change in flux because of a storm. We found that about half (53%, 145 events) of the geomagnetic storms increased the fluxes; about one in five storms (19%, 53 events) decreased the fluxes; and the remaining storms (28%, 78 events) produced changes that were less than a factor of two either up or down.

Slot region is totally filled in.

Very small plasmasphere plays a role, but the PS can be small other times too, without this dramatic effect.

This is the cartoon of the changes in the overlap between the plasmasphere and radiation belt.

On the basis of the exponential decay of electron fluxes seen in November 2003 at L < 2.5 (as shown here in Fig. 1d), we can estimate the ‘e-folding’ lifetime of 2–6MeV electrons from the following equation: J(E 1/4 2–6 MeV) 1/4 Kexp(2t/t 0) where J is electron flux, K is a proportionality constant, and t 0 is the ‘lifetime’ of the electrons against precipitation loss (or radial diffusion away from L 1/4 2.5). From a simple fitting procedure, we find that from 3 to 20 November, t 0 < 4.6 days. Following the other major storm enhancement that occurred on 20 November 2003, there was another decay over the interval 25 November to 20 December 2003. This led to t 0 < 2.9 days. Thus, the ‘active’ acceleration and decay episodes produced within the inner magnetosphere by the Sun’s output gives us a direct measure of the loss lifetimes of electrons in the inner magnetosphere.

Borrowed from here – CIR and CME storms

Inner edge of outer belt & plasmapause mostly lineup

Zoom in to see how well they line up

Event Studies (Brian)  Geomagnetic storms  Reeves (2003): storm category  Baker (2004): Halloween storm

 High speed stream intervals  Paulikas and Blake (1979)

 Others  Li (2003): plasmapause location and inner boundary of outer belt

Radiation belt physics bibles  T. G. Northrop, The adiabatic motion of charged particles, 1963.  J. G. Roederer, Dynamics of geomagnetically trapped radiation, 1970.  M. Schulz and L. J. Lanzerotti, Particle diffusion in the radiation belts, 1974.  M. Walt, Introduction to geomagnetically trapped radiation, 1994.

Dst effect (Kim and Chen, 1997)  During storm main phase, electron flux drops by orders of magnitude due to adiabatic or reversible changes  Ring current goes up, net magnetic flux goes down, then electrons move outward to conserve 3rd adiabatic invariant  Need to identify adiabatic and non-adiabatic effects

Adiabatic invariants  Gyro, bounce and drift motions  Gyro ~ms, bounce ~ 0.1-1 s, drift ~1-10 min

 Adiabatic invariants p⊥2 µ= 2m0 B J =∫

bounce

Φ=∫

drift

p|| ds

B dS

 To change particle energy, must violate one or more invariants → non-adiabatic

Geomagnetic coordinate system based on adiabatic invariants  Irregular and time-dependent geomagnetic field complicates the tabulation of particle flux as a function of position  Geographic coordinate system does not lead to insights into the relationships of fluxed at different locations  Need coordinate system based on trapped particle motion which will have identical values for the coordinates of magnetically equivalent position  McIlwain (1966): L RE = f (I, B, ME)  B is magnetic field, I is 2nd invariant, ME is magnetic moment  L is distance from the center of the Earth’s tilted, off-center, equivalent dipole to the equatorial crossing of the field line

Phase space density, f (µ, K, L*, t)  Convert measurement j (E, α, x, t) to PSD: Green (2004)  Step 1: change j (E, α, x, t) to f (E, α, x, t)  Step 2: calculate K using a magnetic field model to obtain f (E, K, x, t)  Step 3: express E as a function of µ and K to get f (µ, K, x, t)  Step 4: calculate L* using a magnetic field model to get f (µ, K, L*, t)  Errors: poor data fits (E, α) and imperfect magnetic field model  Very time consuming, but necessary; Yue Chen (LANL) p⊥2 µ= 2m0 B Sm ' J J =∫ p|| ds , K = =∫ bounce Sm 2 2m0 µ

Φ=∫

drift

B dS , L* =

2π M Φ RE

∫

Bm − B ( s ) ds

Example: electron acceleration event (Top) Polar data plotted versus L* and are color coded in time with each color corresponding to a different orbit

(Bottom) IMF By and Bz, dynamic pressure, and Dst

April 22, 1998 event: µ = 1000 MeV/G, K = 2500 G1/2km

Reference    

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Baker, D. N. et al., 2004, An extreme distortion of the Van Allen belt arising from the Halloween solar storm in 2003, Nature, 432, 878-880. Green, J.C., Kivelson, M.G., 2004, Relativistic electrons in the outer radiation belt: differentiating between acceleration mechanisms, JGR, 109(A3), A03213. Kim, H.-J., Chan, A.A., 1997, Fully adiabatic changes in storm time relativistic electron fluxes, JGR, 102(A10), 22107-22116. Li, X., D. N. Baker, T. P. O'Brien, L. Xie, and Q. G. Zong (2006), Correlation between the inner edge of outer radiation belt electrons and the innermost plasmapause location, GRL, 33(14), L14107,10.1029/2006GL026294. McIlwain, C.E., 1966, Magnetic coordinates, Space Science Review, 5, 585-598. Reeves, G.D. et al., 2003, Acceleration and loss of relativistic electrons during geomagnetic storms, GRL, 30(10), 1529. Paulikas, G.A., J.B. Blake, 1979, Effects of the solar wind on magnetospheric dynamics: energetic electrons at the synchronous orbit, Geophysical Monograph (21), Quantitative modeling of magnetospheric processes, 180.