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Register a new userCandidates for detecting exoplanetary radio emissions generated by magnetosphere-ionosphere coupling
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J. D. Nichols
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In this paper we consider the magnetosphere-ionosphere (M-I) coupling at Jupiter-like exoplanets with internal plasma sources such as volcanic moons, and we have determined the best candidates for detection of these radio emissions by estimating the maximum spectral flux density expected from planets orbiting stars within 25 pc using data listed in the NASA/IPAC/NExScI Star and Exoplanet Database (NStED). In total we identify 91 potential targets, of which 40 already host planets and 51 have stellar X-ray luminosity 100 times the solar value. In general, we find that stronger planetary field strength, combined with faster rotation rate, higher stellar XUV luminosity, and lower stellar wind dynamic pressure results in higher radio power. The top two targets for each category are $epsilon$ Eri and HIP 85523, and CPD-28 332 and FF And.
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A number of ultra-cool dwarfs emit circularly polarised radio waves generated by the electron cyclotron maser instability. In the solar system such radio is emitted from regions of strong auroral magnetic field-aligned currents. We thus apply ideas developed for Jupiters magnetosphere, being a well-studied rotationally-dominated analogue in our solar system, to the case of fast-rotating UCDs. We explain the properties of the radio emission from UCDs by showing that it would arise from the electric currents resulting from an angular velocity shear in the fast-rotating magnetic field and plasma, i.e. by an extremely powerful analogue of the process which causes Jupiters auroras. Such a velocity gradient indicates that these bodies interact significantly with their space environment, resulting in intense auroral emissions. These results strongly suggest that auroras occur on bodies outside our solar system.
All magnetized planets are known to produce intense non thermal radio emissions through a mechanism known as Cyclotron Maser Instability (CMI), requiring the presence of accelerated electrons generally arising from magnetospheric current systems. In return, radio emissions are a good probe of these current systems and acceleration processes. The CMI generates highly anisotropic emissions and leads to important visibility effects, which have to be taken into account when interpreting the data. Several studies showed that modeling the radio source anisotropic beaming pattern can reveal a wealth of physical information about the planetary or exoplanetary magnetospheres that produce these emissions. We present a numerical tool, called ExPRES (Exoplanetary and Planetary Radio Emission Simulator), which is able to reproduce the occurrence in time-frequency plane of CMI-generated radio emissions from planetary magnetospheres, exoplanets or star-planet interacting systems. Special attention is given to the computation of the radio emission beaming at and near its source. We explain what physical information about the system can be drawn from such radio observations, and how it is obtained. These information may include the location and dynamics of the radio sources, the type of current system leading to electron acceleration and their energy and, for exoplanetary systems, the magnetic field strength, the orbital period of the emitting body and the rotation period, tilt and offset of the planetary magnetic field. Most of these parameters can be remotely measured only via radio observations. The ExPRES code provides the proper framework of analysis and interpretation for past (Cassini, Voyager, Galileo), current (Juno, groundbased radiotelescopes) and future (BepiColombo, Juice) observations of planetary radio emissions, as well as for future detection of radio emissions from exoplanetary systems.
Decametric (DAM) radio emissions are one of the main windows through which one can reveal and understand the Jovian magnetospheric dynamics and its interaction with the moons. DAMs are generated by energetic electrons through cyclotron-maser instability. For Io (the most active moon) related DAMs, the energetic electrons are sourced from Io volcanic activities, and quickly trapped by neighboring Jovian magnetic field. To properly interpret the physical processes behind DAMs, it is important to precisely locate the source field lines from which DAMs are emitted. Following the work by Hess et al. [2008, 2010], we develop a method to locate the source region as well as the associated field lines for any given DAM emission recorded in a radio dynamic spectrum by, e.g., Wind/WAVES or STEREO/WAVES. The field lines are calculated by the state-of-art analytical model, called JRM09 [Connerney et al., 2018]. By using this method, we may also derive the emission cone angle and the energy of associated electrons. If multiple radio instruments at different perspectives saw the same DAM event, the evolution of its source region and associated field lines is able to be revealed. We apply the method to an Io-DAM event, and find that the method is valid and reliable. Some physical processes behind the DAM event are also discussed.
Moon-magnetosphere interaction stands for the interaction of magnetospheric plasma with an orbiting moon. Observations and modeling of moon-magnetosphere interaction is a highly interesting area of space physics because it helps to better understand the basic physics of plasma flows in the universe and it provides geophysical information about the interior of the moons. Moon-magnetosphere interaction is caused by the flow of magnetospheric plasma relative to the orbital motions of the moons. The relative velocity is usually slower than the Alfven velocity of the plasma around the moons. Thus the interaction generally forms Alfven wings instead of bow shocks in front of the moons. The local interaction, i.e., the interaction within several moon radii, is controlled by properties of the atmospheres, ionospheres, surfaces, nearby dust-populations, the interiors of the moons as well as the properties of the magnetospheric plasma around the moons. The far-field interaction, i.e., the interaction further away than a few moon radii, is dominated by the magnetospheric plasma and the fields, but it still carries information about the properties of the moons. In this chapter we review the basic physics of moon-magnetosphere interaction. We also give a short tour through the solar system highlighting the important findings at the major moons.
Titans ionosphere contains a plethora of hydrocarbons and nitrile cations and anions as measured by the Ion Neutral Mass Spectrometer and Cassini Plasma Spectrometer (CAPS) onboard the Cassini spacecraft. Data from the CAPS Ion Beam Spectrometer (IBS) sensor have been examined for five close encounters of Titan during 2009. The high relative velocity of Cassini with respect to the cold ions in Titans ionosphere allows CAPS IBS to function as a mass spectrometer. Positive ion masses between 170 and 310 u/q are examined with ion mass groups identified between 170 and 275 u/q containing between 14 and 21 heavy (carbon/nitrogen/oxygen) atoms. These groups are the heaviest positive ion groups reported so far from the available in situ ion data at Titan. The ion group peaks are found to be consistent with masses associated with Polycyclic Aromatic Compounds (PAC), including Polycyclic Aromatic Hydrocarbon (PAH) and nitrogen-bearing polycyclic aromatic molecular ions. The ion group peak identifications are compared with previously proposed neutral PAHs and are found to be at similar masses, supporting a PAH interpretation. The spacing between the ion group peaks is also investigated, finding a spacing of 12 or 13 u/q indicating the addition of C or CH. Lastly, the occurrence of several ion groups is seen to vary across the five flybys studied, possibly relating to the varying solar radiation conditions observed across the flybys. These findings further the understanding between the low mass ions and the high mass negative ions, as well as with aerosol formation in Titans atmosphere.
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