Transcript UAH
UAH Ionospheric Sources of Storm-time RC and Plasmasheet Populations R. B. Sheldon The University of Alabama in Huntsville The Ring Current Origin UAH • 30’s/50’s Chapman-Ferraro / Alfven view from SW • 60’s Radiation Belt “diffusion” • 70’s Explorer 45, Hoffman “Nose events” – Increased Convection E-field, brings plasmasheet ions deep into the magnetosphere. The “notch filter” permits only certain energy/PA’s deepest access. Partial RC forms. Then cross-L transport circularizes the RC • 80’s Spjeldvik, Sheldon, Kozyra, Fok, Jordanova, Chen... • 90’s POLAR/IMAGE ENA satisfying confirmation So where’s the beef? UAH • Dessler-Parker-Sckopke doesn’t work like it should. – despite ENA’s and Dst tracking each other well • Great storms have 2 time-constant Dst recovery • Models often have an unexplained 2X factor around main phase (Fok, Jordanova, Chen) • RC densities can surpass plasmasheet densities in violation of Liouville’s theorem • Average Energy drops during a big storm injection • Composition Experiments don’t show plasmasheet UAH Ionosphere Contribution to RC • Before composition experiments we didn’t know – – – – – O+,He+ are Iono, He++, C5+,O6+ are SW/Magneto (1972) Shelley, Sharp: O+ exists in RC (E<30keV) AMPTE (1984) Gloeckler: 50% Iono in RC (1<keV<300) AMPTE (1986) Hamilton: O+ dominates CRRES/MICS (1989) Grande: O+/H+ a Dst • Somehow the ionosphere is getting involved in a really major way. But how? (Joe vs. Robert) – cleft ion fountain/auroral upflow not making RC energies – plasmasphere just doesn’t have enough O+ to do it either Some theories UAH • Plasmasphere drains into LLBL, flows over pole and makes a superdense plasmasheet, perfect for RC – it takes a few days. Storms take < 1 hour from SSC. • Storm heats atmosphere in auroral zone, sending upflows of O+ into plasmasheet that convect to RC – Daglis shows that RC O+ is very prompt, < 20 minutes – Sheldon showed that O+ & He+ are nearly simultaneous • Substorms preferentially heat O+ – Some storms have no substorms – The O+ dominance is at lower energy than usual UAH POLAR/CAMMICE Timing Study • Cross-correlations UAH Rosetta Stone, POLAR/CEPPAD April 15, 1996 (Sheldon GRL 98) 3 Populations UAH • The Ring Current is well understood. Radial transport adiabatically heats the particles • The 100 keV ions are probably “nose” ions – pancake distribution, predominately H+, “monoenergetic”, NOT adiabatically heated • What is the lowest trace!!!??? – field-aligned, 40 keV, enriched in O+ – Energy stays nearly constant at 1/2 of “nose” ions! • (Also seen March 21, 96; preferentially at equinox?) UAH It’s the Ionosphere, Stupid • • • • • What would have accelerated it to 40 keV? Why is it strongly field-aligned? Why does the energy track at 1/2 of nose ions? How can it last from L=7 down to L=3? Minimal Theory: – Nose ions generate field-aligned potential at 1/2 of their total kinetic energy which extract the ions from deep within the ionosphere. – Space Charge Review the Physics UAH • Injection of hot plasma into the dipolar magnetosphere produces separation of charge • When cold M’sph plasma is outnumbered, space charge results. Consider the ions. • Space charge expands the ions, mirror force compresses the ions • m DB = q DF ; D2F = 4pr • so if Ba 1/r3, then r a 1/r5 UAH Some simple consequences • • • • The potential looks like the magnetic field The electric field looks like derivative B The density of ions looks like 2nd deriv B Suppose field line is radial near the Earth B = Br = 1 / r cos 3 E = F 1 / r n = F 1/ r 2 6 5 Double Spikes UAH mirror pt mirror pt n,F S Iono Equator Iono UAH Double Layers & High Potential • The separation of charge leads to voltages proportional to the energy of the ions. • Ionospheric electrons would neutralize the ions, but for gradient B drifts. • Energy is conserved, but this geometry acts as a transformer, producing parallel fields • It should work wherever gradient B drifts are large enough (and gyroradii small) UAH Can a Magnet act as a Capacitor? • Standard Plasma books rarely do the dipole – Krall-Trivelpiece (1986) deal only with homogeneous magnetic fields. • MHD is not valid wherever grad-B drifts are important, e.g., inhomogeneous fields, hot plasma • “Malmberg trap” for non-neutral plasmas – Used to trap positrons, making anti-hydrogen – A dipole B-field w/ ionosphere “looks like” Malmberg • So is there any evidence for space charge in a dipole magnetic field? The UAH Spinning Terrella UAH Bell jar High Voltage Low cost Laboratory Space Plasma Simple Physics Cryo pump Sylvania Detectors 1 T Fe-B-Nd magnet Mechanical feedthrough UAH UAH UAH 2K 3k 4k Ion injection at 800 V UAH B/W, lower resolution UAH Higher Pressure He UAH 100 mTorr Helium UAH 400 mTorr Helium UAH UAH 200 mTorr Helium, stationary UAH 200 mTorr Helium, spinning Conclusions UAH • Space charge can accumulate in a magnetic dipole field, and discharges when it overcomes the “insulator”. This produces a periodic “relaxation” oscillator or discharge. • These discharges are at some fixed fraction of the injection energy. Simple theory (Whipple 77) argue for E|| component of Etotal or pitchangle of injection • At Earth, the “RC-time constant” is dynamic, depending on (injection rate - neutralization rate) – ri a ECON * nplasma More Conclusions UAH • Optimal Place for this discharge – Low magnetospheric electron density - Plasmasphere trough – Low ionospheric conductivity- trough at night • Equinoxes! (Check out the Russel-McPherron effect) – High plasmasheet ion density - pre-midnight • Optimal Time for this discharge – High convection Electric field - storm injection – High plasmasheet density - “primed” superdense PS – After plasmasphere is removed (double-dip?) Predictions UAH • We should see keV precipitating electrons in a highly localized region just before main phase of a geomagnetic storm: – PIXIE? (Schulz 98) • We should see 2-beam instability generated waves – Pc1 with gyrofrequency dropping to O+ at dusk (Mursula 98) • We should see correlation between L-shell & O+ • Correlation between recovery rate and size Astrophysical Jets UAH