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The Outer Heliospheric Radio Emission: Observations and Theory

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The Outer Heliospheric Radio Emission: Observations and Theory — 12 — The Outer Heliospheric Radio Emission: Observations and Theory Rudolf A. Treumann1 Geophysics Section, Ludwig-Maximilians-University, Munich, Germany Raymond Pottelette CETP/CNRS, St. Maur des Foss´es,France Abstract. This chapter provides a brief review of the observation and interpre- tation of radio emission from the outer heliosphere in the view of the new knowledge obtained about the outer heliosphere after the Voyager passage of the termination shock. It is discussed whether the termination shock and inner heliosheath could serve as the radiation sources. It is argued that the conditions at both places are not favourable for emitting observable radio waves. Moreover, the observation of electron plasma waves without radiation in the foreshock of the termination shock suggests that the plasma there is unable to excite radiation. It is too cold to excite ion-acoustic waves for scat- tering Langmuir waves or modulating the plasma, while the mere existence of plasma oscillations identifies the termination shock as a non-perpendicular or at least a non-stationary shock that is supercritical enough to reflect elec- tron beams – and probably also ions beams. In this case it must be a shock that is mediated by the hot pick-up ion component. We therefore confirm the proposal that the radiation can only be produced by interaction between the heliopause plasma and a strong global interplanetary shock wave. We present arguments that put the electron beam primed mechanism of stochastically growing lower-hybrid wave for the production of the observed radiation in question. A possible replacement for this mechanism is the combination of shock acceleration of electrons and reconnection between the spiral inter- planetary field and the interstellar field. Electrons accelerated by the purely perpendicular global shock can be released from the shock by reconnection and are injected into the outer heliosheath where in the sufficiently dense plasma they excite sufficiently intense radiation. The low frequency cut-off of the radiation maps the density of the driver piston of the global shock. 1Visiting scientist at the International Space Science Institute, Bern, Switzerland The Physics of the Solar Wind Termination Region, V. Izmodenov and R. Kallenbach (eds.), ISSI Scientific Report No. 5, pp. 355 - 389, ESA-ESTEC, Paris 2006 356 12. Outer Heliospheric Radio Emission 12.1 Introduction The outer heliosphere (a numerical three-component fluid simulation model of which is shown in Figure 12.1) is a huge – on the planetary scale – plasma reservoir [for collections of timely reviews see the two COSPAR Proceedings volumes edited by Grzedzielski & Page, 1990; Scherer et al., 2001] filled with a highly conducting, highly diluted and weakly magnetized plasma that consists of several different par- ticle components [for more details see Axford, 1990; Suess, 1990; Zank, 1999; Wood et al., 2000; Zank and Mueller, 2003; Izmodenov et al., 2003a, b, 2005]. The prop- erties of these components have been described in detail elsewhere in this book. In brief, towards interstellar space the outer heliosphere is bound by the interaction of the solar wind with the interstellar gas which retards the solar wind at the position of the termination shock and confines it inside the heliopause. A presumably highly turbulent region, the inner heliosheath, is caused between the termination shock and the heliopause, while outside the heliosphere the retardation of the interstellar gas may generate an external bow shock in front of the heliosphere and an outer heliosheath between the heliopause and this heliospheric bow shock. The matter in this entire region consists of a mixture of solar wind electrons and protons, a neu- tral, non-ionized interstellar gas component that penetrates into the heliosphere, interstellar pick-up ions which result from charge exchange interaction between the solar wind and the interstellar gas and become accelerated in the magnetized solar wind stream, and fast neutrals created in the same process from the solar wind protons and which continue to fly in the anti-sunward direction until entering the interstellar space and either undergoing secondary charge exchange or collisional ionization there to contribute to plasma in the outer heliosheath. In Figure 12.2 we show the density and temperature profiles along the Sun-stagnation point line as taken from the simulations in Figure 12.1. We have also included there the plasma frequency fpe and an estimate for the Debye length λD. The latter is a function of electron temperature, a very uncertain quantity in the outer heliosphere that does not coincide with the proton temperature given in Figure 12.2 unless some mechanism is responsible for isothermalization. In the absence of collisions, such mechanisms are basically unknown. In the expanding solar wind the electron tem- perature would decreases about adiabatically until outside the planetary system interstellar pick-up ions are created. However, on the large scale 10 AU colli- sions come into play again and the electron and ion temperatures∼ may approach each other. In the interstellar pick-up ion region the ion temperature will become dominated by the pick-up component such that for isothermalization the electron temperature may be anywhere between fractions of eV and few keV (the pick-up ion energy), being approximately constant. For simplicity we have taken the elec- tron temperature to be roughly of the order of Te several eV just up to the termination shock. ∼ A mixture of matter like that in the outer heliosphere is subject to different forms of radiation. Here we are basically interested in radiation that is generated in the outer heliosphere at solar radial distances far outside the planetary system. Radiation emitted there is thus independent of the particular conditions prevail- ing in the vicinity of the different planets. As the emission measure of the outer heliospheric plasma is small (radiation is generated by non-collisional processes) 12.1. Introduction 357 Figure 12.1: Two-dimensional (2-D) steady-state, two-shock heliosphere showing (top) the colour-coded temperature distribution of the solar wind and interstellar plasma and (bottom) the density distribution of neutral hydrogen. The termination shock, heliopause, bow shock and neutral hydrogen wall are identified. The solid lines (top plot) show stream- lines. The plasma temperature is plotted logarithmically and the neutral density linearly. Distances along x and y are in AU [from Zank and Mueller, 2003]. the focus is on radio emission [e.g. Cairns et al., 1992]. We note, however, that radio emission is not the only form of radiation the outer heliospheric plasma is able to support. Even though free energy sources for generating radiation in the outer heliosphere are scarce or unknown, X-ray emission from charge transfer with interstellar neutrals has been considered [Cravens, 2000] as a viable radiation from the heliosphere with maximum source strength in the outer heliosphere. In addi- tion, the particle population in the outer heliosphere can scatter short wavelength radiation produced in other places. This is known for solar H Lyman-α radiation, which in recent years has been found to be scattered from the interstellar neu- tral gas component in the heliosphere. Measurement of the spatial and temporal variation of solar H Lyman-α has allowed detection of the radial gradient of the neutral gas component in the outer heliosphere and inference of global properties 358 12. Outer Heliospheric Radio Emission Figure 12.2: Top: A 1-D profile of the plasma density np(r), the corresponding plasma frequency (inner left scale) in kHz, and temperature Tp(r) as a function of radial heliocen- tric distance r along the direction of the Sun-stagnation point [from Zank and Mueller, 2003]. The density profile exhibits the typical radial decrease up to the termination shock, the downstream heliosheath compression, and the increase at the heliopause. The temper- ature profile shows the temperature control by the anomalous cosmic ray component for r > 8 AU, downstream plasma heating in the heliosheath, and the steep temperature drop at the heliopause. This picture is not changed substantially if the LISM is supersonic and a heliospheric bow shock develops at a few Hundred AU. Also shown is the presumable variation of the electron Debye length λD (in m) for the calculated density profile. The shaded area shows the frequency range of heliospheric radio emissions. Clearly, if the emis- sions are related to the plasma density then they could only come either from the region inside r < 10 AU or from the heliopause region near r ≥ 100 AU. Bottom: Actual Voyager 2 measurements of the solar wind ion temperature within the first 70 AU heliocentric distance compared to different model of the relation between the solar wind speed and temperature [from Richardson and Smith, 2003]. The green adiabatic line corresponds to the simulations in the top part. It underestimates the actual temperature. Average measured values do not drop far below 1 eV and though increasing with distance remain close to this value. of the heliosphere from these measurements [Qu´emeraiset al., 2003; Lallement et al., 2005]. Radio emission from astrophysical objects is usually attributed to the single- particle mechanisms of gyro- or synchrotron emission processes in magnetic fields. Radiation produced by collective processes in high-temperature plasmas like the so- lar corona [for a collection of reviews see, e.g., Labrum, 1985] assumes much higher
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