1. Small Introduction Truth, we know, is so delicate that, if we make the slightest deviation from it, we fall into error; but this alleged error is so extremely fine-spun that, if we diverge from it in the slightest degree, we fall back upon the truth. Blaise Pascal, “The Provincial Letters” (Letter III)‏

2. Baranov V.B. Institute for Problem in Mechanics, RAS (Moscow); Moscow State University (Faculty of Mechanics and Mathematics, Aeromechanics and Gasdynamics Department) Interplanetary magnetic field penetration into a cometary ionosphere ISSI Workshop, Bern, Switzerland, 20 – 24 January 2014

3. Geometrical pattern of the solar wind interaction with a cometary atmosphere (Biermann, Brosowski, Schmidt, Solar Physics, 1967) BS – the bow shock of the solar wind deceleration, CD – contact discontinuity («ionopause»), IS – the inner shock formed due to the deceleration of the supersonic cometary ion flow, -uncoupling radius (cometocentric distance, where cometary molecule will be collisionless) IMF – interplanetary magnetic field IMF

4. PART I Interplanetary magnetic field in the vicinity of the comet Halley. Experimental data obtained on “Giotto” (March, 1986)

5. Discovery of the Interplanetary Magnetic Field Cavity on “Giotto” spacecraft in First Minutes of 14 March 1986 (Neubauer et al., Science, 1986)

6. One minute average magnetic field observations (during 16 h) in Halley centered solar ecliptic coordinates (Neubauer, JGR, 1988) BS – bow shock (≈ ) PB – pile-up boundary (the boundary of the magnetic field Increasing ( ) PU – region of the increased magnetic field С – region of the magnetic field cavity ( ) nT

7. Interplanetary magnetic field for 10 sec before the cavity (Neubauer, JGR, 1988) Region C is the ionopause magnetic structure

8. Magnetic ionopause structure measured for 1 sec (Neubauer, 1988) ∆~25km

9. Part II Interplanetary magnetic field penetration into the comet Halley ionosphere. Theoretical interpretation Experimental facts 1. Left boundary of the region C separates solar wind and cometary plasma flows (Schwenn et al., 1986; Balsiger at al., 1986; Cravens, 1986 and 1989; Reme et al., 1994) 2. Interplanetary magnetic field is parallel to this boundary C (Neubauer, 1988), i.e. C is tangential discontinuity rather than contact one (in MHD normal component of the magnetic field is not zero for contact discontinuity)

10. Ohm law for fully ionized plasma taking into account Hall currents Dimensionless equation of the magnetic field induction in this case Hall dispersion rather than magnetic field diffusion determines «cavity transition layer» in the vicinity of the comet Halley!! (Baranov, 2013, Astronomy Letters, No 11, pp. 792 - 796 Magnetic field diffusion Hall dispersion - in astrophysics !! However for the problem of the solar wind interaction with the cometary ionosphere

11. Small coefficients at second derivations determine thin physical layers A viscous boundary layer is a hydrodynamic analogy or O y xOx – along the ionopause Oy – normal to the ionopause Solar wind flow Cometary ion flow B=0 or Penetration of the magnetic field due to diffusion process

12. Penetration of the cometary ion flow into interplanetary magnetic field due the hall dispersion effect because Cometary ion parameters at the comet Halley ionopause are equal (Schwenn et al., 1986, Cravens, 1986, 1989) (Neubauer, 1988) As a result we obtain We have at and

13. Bulk velocity and temperature of comet Halley ions as a function of the distance from the cometary nucleus. Point С was interpreted as the contact discontinuity (Balsiger et al., 1986, Nature)

14. Ion bulk velocity and electron number density of the comet Halley measured on Giotto (Schwenn et al., 1986; Cravens, 1989)

15. Rosetta mission. Theoretical predictions (Alexashov, Baranov, Lebedev, 2014, not published) (d) – 1.3 AU: (c) – 2.0 AU: ionopause (IP)

16. Possible depth of IMF penetration into the coma of the comet 67P/CG Case comet at IP IMF (nT) at HP c – at 2.0 AU 6.25 1.2 31.8 d – perihelion (1.3 AU) 1.1 1.2 50.5 case (c)case (d) Table of the numerical modelling (Alexashov, Baranov, Lebedev, 2014) Thank you!