Dynamics of the Radiation Belts & the Ring Current Ioannis A. Daglis Institute for Space Applications Athens

Dynamics of the near-space particle radiation environment Main issue: mechanism(s) that can efficiently accelerate and/or transport charged particles, leading to - build-up of storm-time ring current - enhanced fluxes of MeV radiation belt electrons.

Dynamics of the near-space particle radiation environment In both cases, the most obvious driver the magnetospheric substorm appears to be insufficient

Dynamics of Radiation Belts Substorms produce electrons with energies of 10s to 100s of keV, but only few of MeV energies.

Dynamics of Ring Current (Individual) Substorms inject plenty of hot ions to the inner magnetosphere, but not enough to create/sustain the ring current.

Dynamics of the near-space particle radiation environment Presumably, the ring current build-up and the radiation belt enhancement, being processes of a higher level of complexity, display properties not evident at the lower levels

Dynamics of Radiation Belts Close association of storm-time enhancements of relativistic electron fluxes with spacecraft failure. Spacecraft operational anomalies, SAMPEX data [Baker & Daglis, 2006]

Dynamics of Radiation Belts Each new mission in the inner MS brings new insights (SAMPEX, CRRES) Close correlation with storms / Large dynamic range: 10 to 10^4 (Li et al. 2001).

Location of the peak electron flux as a function of minimum Dst moves to lower L O’Brien et al., JGR2003 Dynamics of Radiation Belts

Association of MeV electrons with ULF waves / radial diffusion [Baker and Daglis, 2006] Dynamics of Radiation Belts

Green and Kivelson, 2004 – Polar/HIST data Dynamics of Radiation Belts – Internal/external

keV MeV Dst Kp GEO GPS 1.22 MeV equatorial flux (L=4.2) MeV Electron Flux evolution after a Storm  Equatorial fluxes reach max in: days at GEO orbit - 16 hours at GPS orbit  Equatorial fluxes reach max in: - 2 days at GEO orbit - 6 days at GPS orbit T=0 GPS max GEO max

Dynamics of Radiation Belts [Vassiliadis et al., JGR 2002] Region P 1 : Slow (2-3-day) response to hi-speed streams Characteristic of GEO orbit Prob. involves ULF waves Representative study: Paulikas and Blake, Region P 0 : Rapid (<1-day) response to magnetic clouds/ICMEs. Characterizes L<4. Representative events: January 1997, May 1998: - Baker et al., GRL 1998; - Reeves et al., GRL 1998 September 5, 1995 (Solar Min) May 4, 1998 (Solar Max)

O’Brien et al., JGR2003 SAMPEX, HEO-3 data / SAMNET, IMAGE Dynamics of Radiation Belts

Both ULF waves and microbursts are strongest during the main phase of storms, both progress to lower L during stronger magnetic activity, both continue to be active during the recovery phase of events, and both appear to be more active during intervals of high solar wind velocity. Dynamics of Radiation Belts

Close to GEO, ULF-wave enhanced radial diffusion is more important, pushing electrons inward and accelerating them. Around L~5, VLF chorus waves accelerate electrons (microbursts) without displacing them in L. Dynamics of Radiation Belts

1. Radial diffusion only: plasmapause 2100 MeV/G, equator, Kp =1.8 Iteration number L (MeV -3 s -3 ) 2. Plus chorus waves: Iteration number plasmapause L 2100 MeV/G, equator, Kp =1.8 (MeV -3 s -3 ) 1 MeV 3 MeV L L=5.5: Values 100 times higher! Distribution Functions (MeV -3 s -3 ) D LL and Chorus D LL L pp Varotsou et al. Dynamics of Radiation Belts - Salammbô model

Dynamics of Radiation Belts Not simply a superposition, but a synergy of various lower-level processes (combined effect > sum of individual effects) - feature of the emergent order of higher levels of complexity

Fully understand and specify radiation belt variability (CRRES, Bernie Blake) Dynamics of Radiation Belts - Future

Explain rapid acceleration of electrons to relativistic energies Identify loss mechanisms Develop accurate energetic electron model Dynamics of Radiation Belts - Future

The classical ring concept Image courtesy Hannu Koskinen, FMI

Ring Current Dynamics - RC sources (composition) / RC [a]symmetry - RC formation: IMF driver - RC formation: role of substorms

Ring Current Sources / Composition Daglis, Magnetic Storms Monograph [1997]

Fig. 6 of Daglis et al. JGR2003 Ring current asymmetry

A very asymmetric ring current distribution during the main and early recovery phases of an intense storm Near Dst minimum O + becomes the dominant ion in agreement with previous observations of intense storms Jordanova et al. [2003] Ring Current Asymmetry & Ion Composition

Ring Current Dynamics - RC sources (composition) / RC [a]symmetry - RC formation: IMF driver - RC formation: role of substorms

Ring Current Formation – IMF Driver Empirical certainty: Prolonged southward IMF drives strong convection (westward Ey) and therefore storms. Large IMF Bs => intense storms.

Modeling (RAM Code: Kozyra, Liemohn, et al.) Comparative study of a solar-max and a solar-min intense storms:  IMF comparable.  “Resulting” Dst different. Ring Current Formation – IMF Driver

Storm intensity defined by IMF Bs size and duration? Not exclusively! Ring Current Formation – IMF Driver

Ring Current Dynamics - RC sources (composition) / RC [a]symmetry - RC formation: IMF driver - RC formation: role of substorms

Ring Current Formation – Substorms Role of substorms  1960s Chapman and Akasofu: storms = cumulative result of substorms  1990s McPherron, Iyemori, et al.: purely solar driven, no substorm influence  2000s Daglis, Metallinou, Fok, Ganushkina, et al.: substorms act as catalysts Daglis [1997, 1999]

Fig. 5 of Ganushkina et al., AnnGeo2005 Ganushkina et al. showed that the observed H+ acceleration at high energies can be reproduced in modeling studies only through substorm-style induced E pulses.

Fig. 10 of Ganushkina et al., AnnGeo2005

Substorm-induced transient electric fields clearly contribute to particle acceleration Ring Current Formation – Substorms

Effect of recurrent (periodic) substorms on particle acceleration Ring Current Formation – Substorms

Dynamics of Ring Current Not simply a superposition, but a synergy of convection, substorm-induced electric fiels and wave-particle interactions (combined effect > sum of individual effects) - - a feature of the emergent order of higher levels of complexity

 The ring current is a very dynamic population, strongly coupling the inner magnetosphere with the ionosphere, which is an “increasingly important” source and modulator  IMF not the sole ruler: Plasma sheet density, ionospheric outflow, substorm occurrence, all have their role in storm development. Summary RC

 Substorms act catalytically: they accelerate ions to high(er) energies/ they preferentially accelerate O+ ions, which dominate during intense storms.  Storms, being phenomena of a higher level of complexity display properties not evident at the lower levels (substorms / convection) Summary RC

Dynamics of the Radiation Belts & the Ring Current End

RB models need better satellite measurements: Particle measurements with full pitch-angle information Comprehensive magnetic field measurements Wave measurements Particle measurements in inner zone Dynamics of Radiation Belts - Future

Radiation belt modeling - Problems Known problems:  AE8 models overestimate electron doses: shown by measurements in HEO, MEO orbits and by CRRES (Gussenhoven et al., 1992). Also by POLE model for GEO.

Radiation belt modeling - Problems Known problems:  AE8 models do not specify lower energy environment. Low energy electron (< 100 keV) flux intensity much higher than extrapolation of AE spectra.

Radiation belt modeling - Problems Known problems:  Magnetic field models are not accurate for disturbed times  Radial diffusion models: What is the source population? / Can r.d. transport particles efficiently enough to low L?

Ring Current Formation – IMF Driver

May 4, 1998, medium energies (20-80 keV) (a) (b) (c)(d)

May 4, 1998, high energies ( keV) (a) (b) (c) (d)