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Radio signal detection from exoplanets

Problem description:

In analogy to our solar system in which Jupiter with a dominating planetary magnetic field acts as a powerful radio emitter, it is reasonable that giant exoplanets exhibit comparable radiation in the decametric wavelength regime. State-of-the-art instrumentation incorporated in huge ground-based radio telescopes may be able to discover these electromagnetic signatures, applying sophisticated observational methods. Some of these aspects are addressed in view of recent developments.

By analogy with our solar system where the radially expanding solar atmosphere, the so-called solar wind (Hundhausen, 1972), interacts with magnetic planets forming the magnetically dominating planetary magnetospheres, it is generally assumed that extrasolar planets also have planetary magnetic fields, thus forming exoplanetary magnetospheres within a streaming stellar wind plasma. Again by analogy with the magnetic planets of our solar system (Earth and the four giant planets Jupiter, Saturn, Uranus and Neptune) where nonthermal long-wavelength radiation is generated within and emitted out of the respective magnetospheres, it is likely possible that extrasolar planets may also be capable of emitting nonthermal electromagnetic radiation.

Nonthermal planetary radio emission, first discovered at Jupiter (Burke & Franklin, 1955), is the manifestation of a complex waveparticle interaction. The emission results from energetic electrons being accelerated along converging magnetic field lines, e.g. into active auroral regions. Among several plasma and field conditions necessary for the generation of nonthermal radio emission, the highest efficiency of conversion into EM radiation is obtained within magnetized regions where the local electron gyrofrequency fce significantly exceeds the local electron plasma frequency fpe. By means of nonthermal processes thes keV electron fluxes are in an unstable non-Maxwellian particle distribution and in the course of re-distribution free kinetic energy is transformed into electromagnetic radiation, i.e. coherent cyclotron emission. (Wu & Lee, 1979)

Estimates on radio emission flux densities from exoplanets:

Jupiter-like planets with properties comparable to the radio planets of our solar system might be detectable by decametric observations of cyclotron radiation if the level of the respective flux densities is within the reach of at present or near future radio telescope sensitivity. (Burke, 1992)

As shown in Fig. 1 below, the spectral characteristics of various radio sources are displayed. Sources like Crab Nebula, Virgo A, and Jupiter DAM (upper left part of Fig. 1) have spectra where the flux density S falls off with frequency (or increases with wavelength λ). The variation of S with wavelength λ follows with a positive spectral index α > 0 indicating a nonthermal type of emission. (Moon and Mars e.g., exhibit spectral slopes similar to the characteristic thermal radiation from a blackbody with α=-2 (Kraus, 1986))

The estimates on the flux densities of exoplanets orbiting Tau Bootis and Rho Coronae Borealis have been stretched about one order of magnitude around the most likely median flux density levels, due to the fact that variations in solar (and by analogy also stellar) wind speed by a factor of 2 may increase the level of radio power output by a factor of 100. (Farrell, 1999) These flux density estimates as indicated in Fig. 1 are placed within the decametric wavelength band were the estimated cyclotron frequencies range from 9 - 84 MHz (for τ Boo) and from 8 - 77 Mhz (for ρ CrB), respectively. (Kassim et al.) The maximum estimate Smax (τ Boo) is well below the 1 Jy level, and Smax (ρ CrB) is around 10 mJy.

Fig.1: Spectra of selected radio sources and estimates on exoplanets orbiting Tau Bootis and Rho Coronae Borealis (adapted and combined from Farrell, 1999, 2003; Kraus, 1986). Kharkov UTR-2 sensitivity levels and 2002 VLA measurements see text.

These flux density levels have to be evaluated in view of the at present possible radio telescope sensitivities. Investigations in 1999 and again in 2002 by means of the Very Large Array (VLA, 27 radio antennas in Y-shaped configuration, located close to Socorro, New Mexico, USA), have been performed surveying the region near Tau Bootis with the sofar unsurpassed sensitivity. (Farrell, 2003) As displayesd in Fig. 1, the search at 74 MHz with a sensitivity of < 0.12 Jy would have been within the margins of the estimates, but the negative results may place upper limits on stellar winds, exo-interplanetary magnetic fields. Even the world-largest telescope UTR-2 (located near Kharkov, Ukraine) and connected to state-of-the-art acousto-optic (AOS) and digital receivers (DSP) (Rucker, 2001; Lechacheux, 1998), is at present unable to cope with those necessary low sensitivities.

Taking into account the UTR-2 effective area (∼5*104 m2), the spectrometer bandwidth (1 kHz) and an integration time of 10s, the UTR-2 sensitivity is about 25 Jy, with an integration time of 100s down at ∼8 Jy. As will be shortly addressed below, the development of a new generation of receivers with increased bandwidth and integration time will yield sensitivities below the 1 Jy level. Considering the dependence of flux density S with wavelength λ the probability of detecting exoplanetary radio emission may increase at lower frequencies, i.e. at comparable sensitivties like the VLA 2002 measurements, but considerably below 74 MHz.

Conclusion and future prospects

Summarizing the many aspects of exoplanetary radio astronomy, nonthermal radio emission from stellar orbiting companions at solar/stellar distances of ≤20 parsec may be within the reach of near future technology, under favorable conditions like strong stellar wind and/or strong exo-interplanetary magnetic field interactions with exoplanetary magnetic fields.

Within the frame of the French-Austrian-Ukrainian cooperation in the field of decameter radio astronomy a new generation of radio receiver is under development to be installed at UTR-2. An interference robust digital radio wave analyzer with pure digital techniques, 14 MHz bandwidth, center frequency between 8 - 80 Mhz, 2 channels for polarization measurements and real time processing, the so-called Robin 2, will provide sensitivities at the 100 mJy level.  An extremely ambitious project entitled LOFAR (= Low Frequency Array, proposes a multi-element, interferometric imaging telescope operating in the frequency range of 10 - 240 MHz, with a collecting area of 106 m2 at 15 MHz. Sensitivities of 1 mJy (at 15 MHz) or even 0.3 mJy (at 150 MHz) may be possible.

Reaching these new frontiers of low frequency radio astronomy, new insights will be obtained in the physics of extremely remote worlds, e.g. on the magnetic properties of exoplanets and probably on their rotation periods. Beyond that, these investigations touch the question of our uniqueness, the possible existance of magnetic planets within the habitable zones. It is reasonable that strong planetary magnetic fields can also be maintained by planets with solid surfaces (Paschke, priv. comm.), thus providing the necessary shielding effect against cosmic rays. In this sense, planetary radio astronomy significantly contributes in the search for extrasolar planets.

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