Coupling of the interstellar and interplanetary magnetic fields at the heliospheric interface: The effect of neutral hydrogen atoms

We present numerical results showing the effect of neutral hydrogen atoms on the solar wind (SW) interaction with the local interstellar medium (LISM), where the interstellar magnetic field (ISMF) is coupled to the interplanetary magnetic field (IMF) at the surface of the heliopause. The IMF on the inner boundary surrounding the Sun is specified in the form of a Parker spiral and self-consistently develops in accordance with the SW motion inside the heliopause. The model of the SW–LISM interaction involves both plasma and neutral components which are treated as fluids. The configuration is chosen where the ISMF is orthogonal to the LISM velocity and tilted 60° to the ecliptic plane. This orientation of the magnetic field is a possible explanation of the 2–3 kHz emission data which is believed to originate ahead of the heliopause. It is shown that the topology of the heliospheric current sheet is substantially affected by the ISMF. The interaction pattern dependence on the neutral hydrogen density is analyzed.     

Radio emissions from the outer heliosphere

For nearly fifteen years the Voyager 1 and 2 spacecraft have been detecting an unusual radio emission in the outer heliosphere in the frequency range from about 2 to 3 kHz, Two major events have been observed, the first in 1983–84 and the second in 1992–93. In both cases the onset of the radio emission occurred about 400 days after a period of intense solar activity, the first in mid-July 1982, and the second in May–June 1991. These two periods of solar activity produced the two deepest cosmic ray Forbush decreases ever observed. Forbush decreases are indicative of a system of strong shocks and associated disturbances propagating outward through the heliosphere. The radio emission is believed to have been produced when this system of shocks and disturbances interacted with one of the outer boundaries of the heliosphere, most likely in the vicinity of the the heliopause. The emission is believed to be generated by the shock-driven Langmuir-wave mode conversion mechanism, which produces radiation at the plasma frequency (f _p ) and at twice the plasma frequency (2f _p ). From the 400-day travel time and the known speed of the shocks, the distance to the interaction region can be computed, and is estimated to be in the range from about 110 to 160 AU.Abbreviations PWS Plasma Wave Subsystem - AU Astronomical Unit - DSN Deep Space Network - NASA National Aeronautics and Space Administration - GMIR Global Merged Interaction Region - MHD Magnetohydrodynamic - CME coronal mass ejection - f _p plasma frequency - R radial distance - AGC automatic gain control