Publications

2016

• Transport of Solar Wind H<SUP>+</SUP> and He<SUP>++</SUP> Ions across Earth's Bow Shock
  
  • Parks G. K.
  • Lee E.
  • Fu S. Y.
  • Kim H. E.
  • Ma Y. Q.
  • Yang Z. W.
  • Liu Y.
  • Lin N.
  • Hong J.
  • Canu Patrick
  • Dandouras I.
  • Rème H.
  • Goldstein M. L.
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2016, 825 (2), pp.L27. We have investigated the dependence of mass, energy, and charge of solar wind (SW) transport across Earth?s bow shock. An examination of 111 crossings during quiet SW in both quasi-perpendicular and quasi-parallel shock regions shows that 64 crossings had various degrees of heating and thermalization of SW. We found 22 crossings where the SW speed was <400 km s ?1 . The shock potential of a typical supercritical quasi-perpendicular shock estimated from deceleration of the SW and cutoff energy of electron flat top distribution is ?50 Volts. We find that the temperatures of H and He beams that penetrate the shock can sometimes be nearly the same in the upstream and downstream regions, indicating little or no heating had occurred crossing the bow shock. None of the models predict that the SW can cross the bow shock without heating. Our observations are important constraints for new models of collisionless shocks. (10.3847/2041-8205/825/2/L27)
  DOI : 10.3847/2041-8205/825/2/L27
• Electron Acceleration by Langmuir Waves Produced by a Decay Cascade
  
  • Krafft C.
  • Volokitin A. S.
  The Astrophysical Journal, American Astronomical Society, 2016, 821 (2), pp.99. It was recently reported that a significant part of the Langmuir waveforms observed by the STEREO satellite during type III solar radio bursts are likely consistent with the occurrence of electrostatic decay instabilities, when a Langmuir wave L resonantly interacts with another Langmuir wave L <SUP>\prime</SUP> and an ion sound wave S <SUP>\prime</SUP> through the decay channel L \to L <SUP>\prime</SUP> S <SUP>\prime</SUP> . Usually such wave-wave interactions occur in regions of the solar wind where the presence of electron beams can drive Langmuir turbulence to levels allowing waves L to decay. Moreover, such solar wind plasmas can present long-wavelength, randomly fluctuating density inhomogeneities or monotonic density gradients which can significantly modify the development of such resonant instabilities. If some conditions are met, the waves can encounter a second decay cascade (SDC) according to L <SUP>\prime</SUP> \to L <SUP>\prime\prime</SUP> S <SUP>\prime\prime</SUP> . Analytical estimates and observations based on numerical simulations show that the Langmuir waves L <SUP>\prime\prime</SUP> produced by this SDC can accelerate beam particles up to velocities and kinetic energies exceeding two times the beam drift velocity v<SUB>b</SUB> and half the initial beam energy, respectively. Moreover, this process can be particularly efficient if the scattering effects of waves on the background plasma inhomogeneities have already accelerated a sufficient amount of beam electrons up to the velocity range where the phase velocities of the L <SUP>\prime\prime</SUP> waves are lying. The paper shows that the conditions necessary for such process to occur can be easily met in solar wind plasmas if the beam velocities do not exceed around 35 times the plasma thermal velocity. (10.3847/0004-637X/821/2/99)
  DOI : 10.3847/0004-637X/821/2/99
• Diffusion of Energetic Electrons in Turbulent Plasmas of the Solar Wind
  
  • Volokitin A. S.
  • Krafft C.
  The Astrophysical Journal, American Astronomical Society, 2016, 833 (2), pp.166. A method of calculation of the diffusion coefficients D (v) of particles in velocity space, based on the statistical analysis of the motion of a great number of test electrons, is proposed. In the case of Langmuir turbulence developing in plasmas with fluctuating density inhomogeneities such as the solar wind, simulations provide coefficients D (v) which mainly depend on the Langmuir wave spectra and agree well with the analytical predictions D <SUB>th</SUB>(v) of the quasilinear theory of weak turbulence. Nevertheless, some noticeable differences exist with this theory: in the range of phase velocity of the short waves where the main part of the wave energy is concentrated, D (v) is noticeably smaller than D <SUB>th</SUB>(v), due to the scattering, the reflection, and the focusing effects encountered by the Langmuir waves when they interact with the plasma density inhomogeneities. Moreover, the probability of large velocity jumps in the particles' trajectories essentially exceeds the probability of a Gaussian distribution. These large jumps, which are connected with the waves' transformation processes, modify the nature of the particle diffusion, which is no more classical. These higher order effects cause the discrepancies observed with the quasilinear theory, which does not take them into account in its perturbative approach. The solar wind plasmas, which present fluctuating density inhomogeneities of noticeable average levels, are a very good laboratory to study such diffusion processes, which can eventually influence significantly on the development of essential physical phenomena, as electromagnetic radio emissions by type III solar radio bursts, for example. (10.3847/1538-4357/833/2/166)
  DOI : 10.3847/1538-4357/833/2/166
• Introduction to Modern Magnetohydrodynamics
  
  • Galtier Sébastien
  , 2016. Preface; Table of physical quantities; Part I. Foundations: 1. Introduction; 2. Magnetohydrodynamics; 3. Conservation laws; Part II. Fundamental Processes: 4. Magnetohydrodynamic waves; 5. Dynamo; 6. Discontinuities and shocks; 7. Magnetic reconnection; Part III. Instabilities and Magnetic Confinement: 8. Static equilibrium; 9. Linear perturbation theory; 10. Study of MHD instabilities; Part IV. Turbulence: 11. Hydrodynamic turbulence; 12. MHD turbulence; 13. Advanced MHD turbulence; Appendix 1. Solutions to the exercises; Appendix 2. Formulary; References; Index.
• Solar quiet current response in the African sector due to a 2009 sudden stratospheric warming event
  
  • Bolaji O. S.
  • Oyeyemi E. O.
  • Owolabi O. P.
  • Yamazaki Y.
  • Rabiu A. B.
  • Okoh D.
  • Fujimoto A.
  • Amory-Mazaudier Christine
  • Seemala G. K.
  • Yoshikawa A.
  • Onanuga O. K.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016. We present solar quiet (S q) variation of the horizontal (H) magnetic field intensity deduced from Magnetic Data Acquisition System (MAGDAS) records over Africa during an unusual strong and prolonged 2009 sudden stratospheric warming (SSW) event. A reduction in the S q H magnitude that enveloped the geomagnetic latitudes between 21.13°N (Fayum FYM) in Egypt and 39.51°S (Durban DRB) in South Africa was observed, while the stratospheric polar temperature was increasing and got strengthened when the stratospheric temperature reached its maximum. Another novel feature associated with the hemispheric reduction is the reversal in the north-south asymmetry of the S q H, which is indicative of higher S q H magnitude in the Northern Hemisphere compared to the Southern Hemisphere during SSW peak phase. The reversal of the equatorial electrojet (EEJ) or the counter electrojet (CEJ) was observed after the polar stratospheric temperature reached its maximum. The effect of additional currents associated with CEJ was observed in the Southern Hemisphere at middle latitude. Similar changes were observed in the EEJ at the South America, Pacific Ocean, and Central Asia sectors. The effect of the SSW is largest in the South American sector and smallest in the Central Asian sector. (10.1002/2016JA022857)
  DOI : 10.1002/2016JA022857