J. J. Mitchell

HELIOSPHERIC ASYMMETRIES AND 2-3 kHz RADIO EMISSION UNDER STRONG INTERSTELLAR MAGNETIC FIELD CONDITIONS

We discuss the asymmetry of the major heliospheric discontinuities obtained from the analysis of three-dimensional modeling of the solar wind interaction with the local interstellar medium (LISM). The flow of charged particles is governed by the ideal MHD equations and the transport of neutral particles is described by the Boltzmann equation. The emphasis is on the asymmetries of the termination shock (TS) and the heliopause under the combined action of the interstellar and interplanetary magnetic fields (ISMF and IMF) in the presence of neutral hydrogen atoms. It is shown that an ISMF of a strength greater than 4 μG can account for the 10 AU difference in the TS heliocentric distances observed by the Voyager 1 and Voyager 2 spacecraft. We analyze the deflection of neutral hydrogen flow from its original direction in the unperturbed LISM and conclude that it remains within the range measured in the SOHO SWAN experiment. The impact is analyzed of a strong ISMF on the 2-3 kHz radio emission observed by the Voyager spacecraft, using new estimates of the radio emission conditions.

New constraints and energy conversion efficiencies for plasma emission

Radio emission near the electron plasma frequency fp and 2 fp due to electron beams is important in many laboratory and space applications. Langmuir waves produced as a result of these beams undergo various interactions leading to radio emission. Two categories of such interactions are decay processes involving ion acoustic waves and processes involving scattering off thermal ions (STI). In this paper energy conversion efficiencies for STI emission processes are derived. These are then compared with existing expressions for emission via decay. It is found that decay dominates STI processes when decay is able to proceed. Conditions are derived for emission near fp, and 2 fp by decay processes. These depend on whether significant nonthermal levels of L′ or S waves are produced by the initial nonlinear processes. These conditions are determined by comparing nonlinear growth rates with Landau damping rates. It is found that where the ratio of the beam speed to the electron thermal speed exceeds roughly 3–6, L...

The local interstellar magnetic field direction from direction‐finding measurements of heliospheric 2–3 kHz radio emissions

Direction‐finding measurements of heliospheric 2–3 kHz radio emissions with the Voyager 1 and 2 spacecraft have shown that the source of the emissions is distributed along a line in the sky that passes near the nose of the heliosphere. It is now accepted that the radio emissions are generated near the heliopause by the interaction of an outward propagating interplanetary shock that originates from the sun during periods of intense solar activity. In this paper we suggest that the source is localized to regions where the interstellar magnetic field is nearly perpendicular to the shock normal, i.e., B⋅n = 0. For a spherical shock and the expected draping of the interstellar magnetic field over the heliopause this condition allows us to determine the projected direction of the local interstellar magnetic field as viewed from the sun looking toward the nose of the heliosphere. In this view the magnetic field (or its negative) is aligned at an angle of 44° ± 5° from the north ecliptic pole as measured clockwis...

Conditions for plasma emission in the solar wind and very local interstellar medium (VLISM)

[1] Plasma emission at radio frequencies is important in many regions of the solar system and very local interstellar medium (VLISM). This emission, induced by an unstable beam of electrons, can occur owing to either nonlinear three-wave decay interactions involving Langmuir waves, ion sound waves, and transverse waves or STI. Here recently obtained conditions for decay dominating STI and expressions for the combined efficiencies of decay and STI processes are applied to several phenomena: (1) type II and III solar radio bursts, both in the corona and in the solar wind, and (2) emission associated with global merged interaction region (GMIR) shocks which propagate through the solar system, and inner and outer heliosheath plasmas. The shock-associated electron distributions are computed using a shock drift acceleration model. It is found that decay should dominate STI for type II and III bursts and that the conditions for decay dominating STI reduce the emission predicted by decay processes by ? a factor of 2. Similar results are found for GMIR related emission in the solar wind. In the portion of the VLISM where the electron distribution is primed with a superthermal tail the conditions are less severe; however, outside the primed region the conditions are important.

Magnetic draping, 2–3 kHz radio emissions, and constraints on the interstellar magnetic field

The local interstellar magnetic field Bism is constrained by combining convected‐field calculations of magnetic draping over the heliopause with a recent theory for the 2–3 kHz radiation observed in the outer heliosphere. The two independent constraints considered involve priming of the outer heliosheath with a fast electron tail, produced by lower hybrid waves excited by pickup ions. Both yield lower limits of order 0.03 – 0.04 nT for Bism, not inconsistent with previous estimates. This implies that the priming process should proceed in the outer heliosheath and the theory is viable. While draping can produce a band‐like radiation source, Bism must be highly oblique to the flow and the priming process marginal. Plausibly another idea for the source shape is needed.