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Z. Nemecek, J. Safrankova, L. Prech, O. Goncharov, O. Gutynska, P. Cagas, A. Komarek, F. Nemec, K. Jelinek

OVERVIEW OF INTERESTING RESULTS FROM BMSW. Z. Nemecek, J. Safrankova, L. Prech, O. Goncharov, O. Gutynska, P. Cagas, A. Komarek, F. Nemec, K. Jelinek Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic G. Zastenker, I. Koloskova, M. Riazantseva

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Z. Nemecek, J. Safrankova, L. Prech, O. Goncharov, O. Gutynska, P. Cagas, A. Komarek, F. Nemec, K. Jelinek

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  1. OVERVIEW OF INTERESTING RESULTS FROM BMSW Z. Nemecek, J. Safrankova, L. Prech, O. Goncharov, O. Gutynska, P. Cagas, A. Komarek, F. Nemec, K. Jelinek Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic G. Zastenker, I. Koloskova, M. Riazantseva IKI RAN, Moscow, Russia The authors would like to acknowledge the effort of numerous collaborators that participate in the BMSW development and calibration, namely: J. Vaverka – FC modeling, I. Cermak – hardware development, L. Chesalin – telemetry interface, N. Shevyrev – FC modeling, A. Leibov – device calibration

  2. Bright Monitor of the Solar Wind (BMSW) 4 • Launch July 18, 2011 • Apogee ~50 RE • Inclination 65o • Orbital period ~8.5 days • Time resolution 31 ms • Three axis stabilized Developed by Charles University in Prague and Space Research Institute in Moscow for Spektr-R 5 3 2 1 0 • Three Faraday cups for determination of the speed and temperature - 0, 1, 2 • Three declined Faraday cups for determination of the density and velocity direction – 3, 4, 5 Safrankova et al. (SSR, 2013)

  3. Overview of new results The contribution deals with: Helium abundance and its variations Shock front, its structure, and thickness Solar wind turbulence 1 3 2

  4. An example of measurements and their comparison with other spacecraft BMSW-Spektr-R 3DP-Wind ESA-THEMIS B 2.5 hours 25 minutes 60 seconds • All spacecraft measure similar features on a scale of hours • Notable differences on scale of minutes • BMSW shows a high level of fluctuations • at the kinetic scale 15 seconds

  5. BMSW in the sweeping mode Protons Alphas • BMSW in the sweeping mode; speed of measurements – 0.031 s • a full set of solar wind parameters – 1-3 s • details of distributions – protons and alphas

  6. Helium abundance in the solar wind • An average content of the alpha particles is ~3-4% • He relative content varies with the solar cycle • He content is statistically larger within fast solar wind streams that originate from coronal holes. • Na/Np ratio ranges from 0.5 to 10%; even larger values were observed within ICMEs • Helium content rises with the solar wind speed but there is no direct correlation (correlation 0.94 for the slow solar wind only - Kasper et al., 2007) • Smaller attention was devoted to fast abundance variations connected probably with different waves originated on the Sun (Rakowski and Laming; Ebert et al.; Bourouaine et al., 2012)

  7. Alpha particle computation An example of He content variations Derivation of retarding characteristics – ion distribution function • There are large variations of Na/Np ratios – from 2 to 12% • These variations cannot be connected with solar cycle and with a change of the solar wind stream Protons Alpha particles

  8. Abrupt changes of helium content The speed of large variations • The change of the Na/Np ratio from 0.5 to 4.5% lasts 10 s, i.e., ~0.5 RE • Gyroradius of alphas is ~0.2 RE • How can be such short front formed? • Is it a signature of two different streams? An example (April 11, 2012) of large variations in the He content without any change of the solar wind speed; but with changes of the density and thermal velocity

  9. Statistical analysis An example of observation probability of different Na/Np ratios as a function of the solar wind speed All ratios • Limited set of observations (~120 hours) within one year • Limited solar wind speed • (< 600 km/s) 0 < Na/Np < 3 3 < Na/Np < 6 6 < Na/Np No clear dependence on the solar wind type, maybe three different groups?

  10. Analysis of He abundance Statistics of Na/Np ratio changes as a function of solar wind parameters – the density, bulk and thermal speeds 3 1 2 4 • A low proton density implies low relative He content regardless sw speed or temperature • A rise of mean He content with speed only for large densities (> 15 cm-3) • Enhanced He content for limited ranges of the proton speed and density • A decrease of the He abundance with proton temperature • A very low He abundance at large sw speeds • (> 525 km/s) 4 5

  11. Temporary conclusion – He abundance In spite of still small amount of the data, we have found: • Abrupt changes of the relative He abundance without changes of the speed are frequent • There are some indices in favor of a “third state” of the solar wind • Already reported rise of the He abundance with the speed is observed only in a sufficiently dense solar wind • The He abundance depends on the proton temperature or, more probable, the He abundance controls the proton temperature

  12. Solar wind turbulence Solar wind is heated by dissipation of turbulent structures but the nature of dissipation processes would depend on the scale Until present, only MHD scale was accessible for experimental investigation of plasma turbulence due to insufficient time resolution of sounding devices The experimental evidence was based on analysis of magnetic field measurements but the coupling between plasma and magnetic field is principally different at MHD and kinetic scales BMSW device provides the plasma moments with 31 ms time resolution – well inside the kinetic scale

  13. How to reach the ion kinetic scale ? FC0 FC1 FC2 • Three identical FCs • Voltages on deceleration grids of FC1 and FC2 set by a feedback to obtain ~ 50% and 30% of the FC0 current • 3 points of the distribution are enough to calculate basic plasma moments • Maximum time resolution is given by telemetry bit rate FC current

  14. Expectation on solar wind turbulence ωc • Power law frequency spectrum with several segments • -1 slope on large scale determined by the solar activity • -5/3 (Kolgomorov) slope at the MHD scale (MHD waves) • -7/3 slope at the ion kinetic scale (KAW, ion cyclotron waves) • A steeper slope at the electron scale -1 -5/3 -7/3 Power spectral density -? MHD kinetic Years to days ion electron • Our knowledge is based on theoretical considerations and on analysis of magnetic field fluctuations or • On the analysis of the spacecraft potential that is used as a proxy of the electron density (e.g., Chen et al., 2012, 2013)

  15. First example of frequency spectra First spectra revealed several peculiarities: • Clearly different slopes at low- and high-frequency ranges • Slightly different slopes for analyzed parameters • A plateau (sometimes a peak) near the break frequency • Another hump at the end of our frequency range

  16. Instrumental noise • When the solar wind is extremely quiet the velocity spectra are spoiled by the preamplifier noise • This effect decreases the available frequency range Frequency spectrum of speed Noise spectrum determined onboard Frequency spectrum of preamplifier noise

  17. Spectral analysis of plasma moments 1.01 1.01 1.28 N V Vth 3.18 2.69 3.57 • Spectral slopes of density and velocity fluctuations are different • Spectral slopes in the MHD range are close to 5/3 but often lower • Spectral slopes in the kinetic range are always larger than 7/3 • The break frequency of density fluctuations is larger, whereas that of velocity (temperature) fluctuations is lower than ion cyclotron frequency 0.29 Hz 0.1 Hz 0.14 Hz

  18. First statistics Kinetic scale Density Spectral break [Hz] Spectral slope (density) Velocity MHD scale Ion inertial length Ion inertial length • The break between scales is above the cyclotron frequency for density fluctuations but below it for the velocity • The break frequency decreases with the ion inertial length • The slope is about 5/3 in the MHD scale and about 3 in the kinetic scale • Both slopes decrease with the ion inertial length Spectral break [Hz] Ion cyclotron frequency Safrankova et al. (PRL, 2013)

  19. A new emission? N V Vth ωc ωc ωc • The spectra of moments often exhibit a plateau or a peak at a fraction of ion cyclotron frequency • This peak/plateau is more often seen at density spectra • A similar plateau can be found in the Wind magnetic field magnitude B

  20. Spectral analysis of FC currents FC0 FC1 FC2 Power spectra of FC currents allow us to determine which part of the distribution is responsible for a particular feature • It can be clearly seen that the hump on the density spectrum is caused by the low energy part of the distribution in this particular case • The presence of the hump increases the break frequency FC2 FC1 FC0

  21. A novel description of the spectra • An example of precise processing of density fluctuations for statistical results • This example is taken from June 2, 2012, 01-06 UT (5 hours) • FFT is computed on ~20-min time interval with shift of ~2 minutes Ion inertial length Three different spectral slopes are clearly revealed -5/3

  22. Temporary conclusion - turbulence BMSW provides reliable moments of the ion distribution with 31 ms time resolution The instrumental noise does not spoil the frequency spectra that can be determined down to ion kinetic scale for all moments A mean slope of ~1.5 (i.e., close to 5/3) was found for the MHD scale, whereas a mean value of 3.2 at the kinetic scale is much larger than 7/3 The break frequency is larger than ion cyclotron frequency for density fluctuations, whereas it is lower for velocity and temperature – does it mean that the compressible fluctuations survive longer? The break frequency decreases with the ion inertial length – why? A significant enhancement of the fluctuation level below the ion cyclotron frequency was often observed in spectra of all plasma moments Similar enhancement can be identified in magnetic field spectra

  23. Interplanetary and bow shock studies MOTIVATION ~2 min • INTERSHOCK • BIFRAM • Bow shock on May 12, 1985 • 0.6 s resolution • Plasma deceleration in front of the shock • Large amplitude oscillations downstream Do you remember this figure? Zastenker et al. (1986)

  24. An example of IP shock registration • A shock ramp duration ~0.48 s • It corresponds to 355 km • Ion gyroradius – ~800 km • What is the mechanism of the shock creation? • Is the shock built by electrons? • An evolution of the ion distribution on the scale shorter than ion gyroradius seems to be impossible – faster measurements of the full ion distributions are required

  25. Analysis of the distribution function Ion distributions measured 2.4 s apart(4 proton gyro-periods) Prior to shock ramp After shock ramp • Plasma parameters are computed from 3 points of distribution – how reliable is this method at shocks? • The normalization of the FC current to the full ion flux enables determination of plasma parameters even in highly disturbed environment of the shock • The three-point method (adaptive mode of BMSW) is based on this normalization • The first ion distribution measured after the shock ramp reveals complete thermalization within ~2 ion gyro-periods

  26. Two extreme cases of IP shocks • The ramp width varies by an order of amplitude • The width correlates with the proton thermal gyro-radius (1-4 Rt) • Reflected ions cannot participate in a ramp formation • A role of electrons should be investigated • Shock ramp duration - 0.48 s • Shock speed – 743 km/s • Shock ramp width – 355 km/s • Thermal proton gyroradius – 19 km • Ion inertial length – 87 km • Shock ramp duration - 0.1 s • Shock speed – 476 km/s • Shock ramp width – 46 km/s • Thermal proton gyroradius – 16 km • Ion inertial length – 32 km

  27. Upstream and downstream waves • Non-phase standing precursor whistler waves and steepened magnetosonic waves with leading whistler – shocklets were observed upstream of quasi-perpendicular IP shocks (Wilson et al., 2009, 2010, 2012) • Downstream – irregular ULF fluctuations and regular high-frequency waves were reported • (Kajdic et al., 2012) A preliminary analysis shows that wavelength of downstream waves increases with the IPS ramp thickness. Upstream waves Downstream waves

  28. Temporary conclusion - shocks • Gradients of ion parameters observed at the IP shock are as step as those of the magnetic field or electron temperature (Schwartz et al., 2012) • The shock ramp thickness is only 2 - 4 ion inertial lengths (or thermal proton gyroradii) • What is the mechanism of the proton heating on such a small scale? • No clear correlation of the shock ramp thickness with the magnetic field, solar wind parameters and/or their combinations was found • A preliminary analysis shows that wavelength of downstream waves increases with the IPS ramp thickness

  29. Directions of further investigations • Collection of a significantly larger data set • Statistical analysis of spectral slopes and investigation of dependences of slopes and break frequencies on other parameters • Search for the sources of enhanced fluctuations • Search for the cause of fast large variations of the He abundance • A further analysis of the IP shock (and bow shock) ramp and waves connected with it Thank you for your attention

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