Magnetospheric Radio Emission Research Articles

ROME. IV. An Arecibo Search for Substellar Magnetospheric Radio Emissions in Purported Exoplanet-hosting Systems at 5 GHz

Plasma flow–obstacle interactions, such as those between an exoplanet’s magnetosphere and the host star’s stellar wind, may lead to detectable radio emissions. Despite many attempts to detect magnetospheric (auroral) radio emissions from exoplanets, a reproducible, unambiguous detection remains elusive. This fourth paper of the Radio Observations of Magnetized Exoplanets (ROME) series presents the results of a targeted radio survey of nine nearby systems that host exoplanet, brown dwarf, or low-mass-stellar companions conducted with the Arecibo radio telescope at ∼5 GHz. This search for magnetospheric radio emissions has the greatest sensitivity (∼1 mJy during <1 s integration times) and collected full Stokes parameters over the largest simultaneous bandpass of any survey to date. It is also the first survey to search for radio emission from brown dwarfs of spectral class Y, which may illuminate open questions regarding their magnetism, interior and atmospheric structure, and formation histories. No magnetospheric radio emissions from substellar companions were detected. These results are examined within the context of recent theoretical work on plasma flow–obstacle interactions, and radio emissions observed from the solar system planets and ultracool dwarfs.

Rotational and radio emission properties of PSR J0738−4042 over half a century

ABSTRACT We present a comprehensive study of the rotational and emission properties of PSR J0738−4042 using a combination of observations taken by the Deep Space Network, Hartebeesthoek, Parkes (Murriyang) and Molonglo observatories between 1972 and 2023. Our timing of the pulsar is motivated by previously reported profile/spin-down events that occurred in 2005 September and 2015 December, which result in an anomalously large braking index of n = 23 300 ± 1800. Using a Gaussian process regression framework, we develop continuous models for the evolution of the pulsar spin-down rate ($\dot{\nu }$) and profile shape. We find that the pulse profile variations are similar regardless of radio observing frequency and polarization. Small-scale differences can be ascribed to changes in the interstellar medium along the line of sight and frequency-dependent changes in magnetospheric radio emission height. No new correlated spin-down or profile events were identified in our extended data set. However, we found that the disappearance of a bright emission component in the leading edge of archival profiles between 1981 and 1988 was not associated with a substantial change in $\dot{\nu }$. This marks a notable departure from the previous profile/spin-down events in this pulsar. We discuss the challenges these observations pose for physical models and conclude that interactions between the pulsar and in-falling asteroids or a form of magnetospheric state-switching with a long periodicity are plausible explanations.

ROME. III. The Arecibo Search for Star–Planet Interactions at 5 GHz

After nearly three decades of discovery, many exoplanetary systems have been studied and characterized in detail with one important exception: exoplanet magnetism. Although many surveys sought to detect magnetospheric radio emissions from exoplanets to directly measure their magnetic field strengths, they have yet to reveal an unambiguous detection. However, the indirect detection of exoplanet magnetic fields by measuring their influence on their host stars via magnetic star–planet interactions has recently gained prominence as an alternative method of discovery. This third paper of the Radio Observations of Magnetized Exoplanets series presents the results of a targeted radio survey of eight nearby exoplanet-hosting systems that may engage in star–planet interactions. This survey, conducted with the Arecibo radio telescope at ∼5 GHz, has the greatest frequency coverage of any to date while providing millijansky-level sensitivity over <1 s integration times. No exoplanet-induced stellar radio bursts were detected. The orbital phase coverage of candidate systems for magnetic star–planet interactions is described, and the survey results are examined within the context of the plasma flow–obstacle paradigm and searches for star–planet interactions at other wavelengths.

Space environment and magnetospheric Poynting fluxes of the exoplanet τ Boötis b

Context. The first tentative detection of a magnetic field on the hot-Jupiter-type exoplanet τ Boötis b was recently reported by Turner et al. (A&amp;A, 645, A59). The magnetic field was inferred from observations of circularly polarized radio emission obtained with the LOFAR telescopes. The observed radio emission is possibly a consequence of the interaction of the surrounding stellar wind with the planet's magnetic field. Aims. We aim to better understand the near space environment of τ Boötis b and to shed light on the structure and energetics of its near-field interaction with the stellar wind. We are particularly interested in understanding the magnetospheric energy fluxes powered by the star-planet interaction and in localizing the source region of possible auroral radio emission. Methods. We performed magnetohydrodynamic simulations of the space environment around τ Boötis b and its interaction with the stellar wind using the PLUTO code. We investigated the magnetospheric energy fluxes and effects of different magnetic field orientations in order to understand the physical processes that cause the energy fluxes that may lead to the observed radio emission given the magnetic field strength proposed in Turner et al. (A&amp;A, 645, A59). Furthermore, we study the effect of various stellar wind properties, such as density and pressure, on magnetospheric energy fluxes given the uncertainty of extrasolar stellar wind predictions. Results. We find in our simulations that the interaction is most likely super-Alfvénic and that energy fluxes generated by the stellar wind-planet interaction are consistent with the observed radio powers. Magnetospheric Poynting fluxes are on the order of 1–8 × 1018 W for hypothetical open, semi-open, and closed magnetospheres. These Poynting fluxes are energetically consistent with the radio powers in Turner et al. (A&amp;A, 645, A59) for a magnetospheric Poynting flux-to-radio efficiency &gt;10−3 when the magnetic fields of the planet and star are aligned. In the case of lower efficiency factors, the magnetospheric radio emission scenario is, according to the parameter space modeled in this study, not powerful enough. A sub-Alfvénic interaction with decreased stellar wind density could channel Poynting fluxes on the order of 1018W toward the star. In the case of a magnetic polarity reversal of the host star from an aligned to anti-aligned field configuration, the expected radio powers in the magnetospheric emission scenario fall below the observable threshold. Furthermore, we constrain the possible structure of the auroral oval to a narrow band near the open-closed field line boundary. The strongest emission is likely to originate from the night side of the planet. More generally, we find that stellar wind variability in terms of density and pressure does significantly influence magnetospheric energy fluxes for close-in magnetized exoplanets.

Detecting Magnetospheric Radio Emission from Giant Exoplanets

As radio astronomy enters a golden age, ground-based observatories are reaching sensitivities capable of unlocking a new and exciting field of exoplanet observation. Radio observation of planetary auroral emission provides unique and complementary insight into planetary science not available via orthodox exoplanet observation techniques. Supplying the first measurements of planetary magnetic fields, rotation rates, and orbital obliquities, we gain necessary and crucial insight into our understanding of the star–planet relationships, geophysics, composition, and habitability of exoplanets. Using a stellar-wind-driven Jovian approximation, we present analytical methods for estimating magnetospheric radio emission from confirmed exoplanets. Predicted radio fluxes from cataloged exoplanets are compared against the wavelengths and sensitivities of current and future observatories. Candidate exoplanets are downselected based on the sky coverage of each ground-based observatory. Orbits of target exoplanets are modeled to account for influential orbit-dependent effects in anticipating time-varying exoplanet radio luminosity and flux. To evaluate the angular alignment of exoplanetary beamed emission relative to Earth’s position, the equatorial latitude of exoplanetary auroral emission is compared against Earth’s apparent latitude on the exoplanet. Predicted time-dependent measurements and recommended beamformed observations for ground-based radio arrays are provided, along with a detailed analysis of the anticipated emission behavior for τ Boo b.

Radio Emission from Binary Ultracool Dwarf Systems

Well-characterized binary systems will provide valuable opportunities to study the conditions that are necessary for the onset of both auroral and nonauroral magnetospheric radio emission in the ultracool dwarf regime. We present new detections of nonauroral “quiescent” radio emission at 4–8 GHz of the three ultracool dwarf binary systems GJ 564 BC, LP 415-20, and 2MASS J21402931+1625183. We also tentatively detect a highly circularly polarized pulse at 4–6 GHz that may indicate aurorae from GJ 564 BC. Finally, we show that the brightest binary ultracool dwarf systems may be more luminous than predictions from single-object systems.

Modeling Star–Planet Interactions in Far-out Planetary and Exoplanetary Systems

Abstract The magnetized wind from a host star plays a vital role in shaping the magnetospheric configuration of the planets it harbors. We carry out three-dimensional (3D) compressible magnetohydrodynamic simulations of the interactions between magnetized stellar winds and planetary magnetospheres corresponding to a far-out star–planet system, with and without planetary dipole obliquity. We identify the pathways that lead to the formation of a dynamical steady-state magnetosphere and find that magnetic reconnection plays a fundamental role in the process. The magnetic energy density is found to be greater on the nightside than on the dayside, and the magnetotail is comparatively more dynamic. It is found that stellar wind plasma injection into the inner magnetosphere is possible through the magnetotail. We further study magnetospheres with extreme tilt angles, keeping in perspective the examples of Uranus and Neptune. High dipole obliquities may also manifest due to polarity excursions during planetary field reversals. We find that global magnetospheric reconnection sites change for large planetary dipole obliquity, and more complex current sheet structures are generated. We discuss the implications of these findings for atmospheric erosion, the introduction of stellar and interplanetary species that modify the composition of the atmosphere, auroral activity, and magnetospheric radio emission. This study is relevant for exploring star–planet interactions and its consequence on atmospheric dynamics and habitability in solar system planets and exoplanets.

Constraints on magnetospheric radio emission from Y dwarfs

Abstract As a pilot study of magnetism in Y dwarfs, we have observed the three known infrared variable Y dwarfs WISE J085510.83−071442.5, WISE J140518.40+553421.4, and WISEP J173835.53+273258.9 with the NSF’s Karl G. Jansky Very Large Array in the 4–8 GHz frequency range. The aim was to investigate the presence of non-bursting quiescent radio emission as a proxy for highly circularly polarized radio emission associated with large-scale auroral currents. Measurements of magnetic fields on Y dwarfs may be possible by observing auroral radio emission, and such measurements are essential for constraining fully convective magnetic dynamo models. We do not detect any pulsed or quiescent radio emission, down to rms noise levels of 7.2 µJy for WISE J085510.83−071442.5, 2.2 µJy for WISE J140518.40+553421.4, and 3.2 µJy for WISEP J173835.53+273258.9. The fractional detection rate of radio emission from T dwarfs is ∼10 per cent suggesting that a much larger sample of deep observations of Y dwarfs is needed to rule out radio emission in the Y dwarf population. We discuss a framework that uses an empirical relationship between the auroral tracer Hα emission and quiescent radio emission to identify brown-dwarf auroral candidates. Finally, we discuss the implications that Y dwarf radio detections and non-detections can have for developing a picture of brown dwarf magnetism and auroral activity.

Effect of the exoplanet magnetic field topology on its magnetospheric radio emission

Context. The magnetized wind from stars that impact exoplanets should lead to radio emissions. According to the scaling laws derived in the solar system, the radio emission should depend on the stellar wind, interplanetary magnetic field, and topology of the exoplanet magnetosphere. Aims. The aim of this study is to calculate the dissipated power and subsequent radio emission from exoplanet magnetospheres with different topologies perturbed by the interplanetary magnetic field and stellar wind, to refine the predictions from scaling laws, and to prepare the interpretation of future radio detections. Methods. We use the magnetohydrodynamic (MHD) code PLUTO in spherical coordinates to analyze the total radio emission level resulting from the dissipation of the kinetic and magnetic (Poynting flux) energies inside the exoplanet’s magnetospheres. We apply a formalism to infer the detailed contribution in the exoplanet radio emission on the exoplanet’s day side and magnetotail. The model is based on Mercury-like conditions, although the study results are extrapolated to exoplanets with stronger magnetic fields, providing the lower bound of the radio emission. Results. The predicted dissipated powers and resulting radio emissions depend critically on the exoplanet magnetosphere topology and interplanetary magnetic field (IMF) orientation. The radio emission on the exoplanet’s night and day sides should thus contain information on the exoplanet magnetic field topology. In addition, if the topology of an exoplanet magnetosphere is known, the radio emission measurements can be used as a proxy of the instantaneous dynamic pressure of the stellar wind, IMF orientation, and intensity.

Numerical simulations of unbounded cyclotron-maser emissions

AbstractNumerical simulations have been conducted to study the spatial growth rate and emission topology of the cyclotron-maser instability responsible for stellar/planetary auroral magnetospheric radio emission and intense non-thermal radio emission in other astrophysical contexts. These simulations were carried out in an unconstrained geometry, so that the conditions existing within the source region of some natural electron cyclotron masers could be more closely modelled. The results have significant bearing on the radiation propagation and coupling characteristics within the source region of such non-thermal radio emissions.

Observation of planetary radio emissions using large arrays

Planetary radio astronomy mostly concerns plasma phenomena at low frequencies (i.e., below a few hundred MHz). The low frequency limit for ground‐based observations of these phenomena is given by the Earth's ionosphere, which limits ground based radio observations to frequencies ≥10 MHz. We give an overview and update on the status of a few representative ground‐based radio arrays that have been used for planetary studies within the frequency range 10–200 MHz, and we discuss their potential for the four types of planetary radio emissions that can be observed within this frequency range: (1) synchrotron emission from Jupiter's radiation belts, (2) radio bursts caused by solar system planetary lightning, (3) Jupiter's magnetospheric emission, and (4) magnetospheric radio emission from extrasolar planets, for which we also give an update to previous predictive studies. Comparing the four emission modes with the characteristics of existing ground‐based radio arrays, we show that the Low Frequency Array (LOFAR) has the potential to bring considerable advances to those four fields of planetary radio science.

Predicting low-frequency radio fluxes of known extrasolar planets

Context: Close-in giant extrasolar planets (``Hot Jupiters'') are believed to be strong emitters in the decametric radio range. Aims: We present the expected characteristics of the low-frequency magnetospheric radio emission of all currently known extrasolar planets, including the maximum emission frequency and the expected radio flux. We also discuss the escape of exoplanetary radio emission from the vicinity of its source, which imposes additional constraints on detectability. Methods: We compare the different predictions obtained with all four existing analytical models for all currently known exoplanets. We also take care to use realistic values for all input parameters. Results: The four different models for planetary radio emission lead to very different results. The largest fluxes are found for the magnetic energy model, followed by the CME model and the kinetic energy model (for which our results are found to be much less optimistic than those of previous studies). The unipolar interaction model does not predict any observable emission for the present exoplanet census. We also give estimates for the planetary magnetic dipole moment of all currently known extrasolar planets, which will be useful for other studies. Conclusions: Our results show that observations of exoplanetary radio emission are feasible, but that the number of promising targets is not very high. The catalog of targets will be particularly useful for current and future radio observation campaigns (e.g. with the VLA, GMRT, UTR-2 and with LOFAR). Appendices A and B are only available in electronic form at http://www.aanda.org Table 1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/475/359

Magnetospheric radio emission from extrasolar giant planets: the role of the host stars

We present a new analysis of the expected magnetospheric radio emission from extrasolar giant planets (EGPs) for a distance limited sample of the nearest known extrasolar planets. Using recent results on the correlation between stellar X-ray flux and mass-loss rates from nearby stars, we estimate the expected mass-loss rates of the host stars of extrasolar planets that lie within 20 pc of the Earth. We find that some of the host stars have mass-loss rates that are more than 100 times that of the Sun and, given the expected dependence of the planetary magnetospheric radio flux on stellar wind properties, this has a very substantial effect. Using these results and extrapolations of the likely magnetic properties of the extrasolar planets, we infer their likely radio properties. We compile a list of the most promising radio targets and conclude that the planets orbiting Tau Bootes, Gliese 86, Upsilon Andromeda and HD 1237 (as well as HD 179949) are the most promising candidates, with expected flux levels that should be detectable in the near future with upcoming telescope arrays. The expected emission peak from these candidate radio emitting planets is typically ∼40–50 MHz. We also discuss a range of observational considerations for detecting EGPs.

Chaos in magnetospheric radio emissions

Abstract. A three-wave model of auroral radio emissions near the electron plasma frequency was proposed by Chian et al. (1994) involving resonant interactions of Langmuir, whistler and Alfvén waves. Chaos can occur in the nonlinear evolution of this three-wave process in the magnetosphere. In particular, two types of intermittency, due to either local or global bifurcations, can be observed. We analyze the type-I Pomeau-Manneville intermittency, arising from a saddle-node bifurcation, and the crisis-induced intermittency, arising from an interior crisis associated with a global bifurcation. Examples of time series, power spectrum, phase-space trajectory for both types of intermittency are presented through computer simulations. The degree of chaoticity of this three-wave process is characterized by calculating the maximum Lyapunov exponent. We suggest that the intermit-tent phenomena discussed in this paper may be observed in the temporal signal of magnetospheric radio emissions.

An interpretation of banded magnetospheric radio emissions

Recently published Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) banded magnetospheric emissions, commonly referred to as “(n + 1/2)fce” emissions, where fce is the electron gyrofrequency, are analyzed by treating them as analogous to sounder‐stimulated ionospheric emissions. We show that both individual spectra of magnetospheric banded emissions and a statistically derived spectra observed over the 2‐year lifetime of the mission can be interpreted in a self‐consistent manner. The analysis, which predicts all spectral peaks within 4% of the observed peaks, interprets the higher‐frequency emissions as due to low group velocity Bernstein‐mode waves and interprets the lower‐frequency emissions as eigenmodes of cylindrical‐electromagnetic plasma oscillations. The demarcation between these two classes of emissions is the electron plasma frequency fpe where an emission is often observed. This fpe emission is not necessarily the strongest. None of the observed banded emissions were attributed to the upper hybrid frequency. We present Alouette 2 and ISIS 1 plasma resonance data and model electron temperature (Te) values to support the argument that the frequency spectrum of ionospheric sounder‐stimulated emissions is not strongly temperature dependent and thus that the interpretation of these emissions in the ionosphere is relevant to other plasmas (such as the magnetosphere) where Ne and Te can be quite different but where the ratio fpe/fce is identical. The Ne values deduced from the spectral interpretation do not agree with the values determined from the AMPTE/IRM three‐dimensional plasma instrument. The latter, which represent a lower bound, are found to be higher than the former by a factor of 3.2 – 3.5. All values were less than 1 cm−3, a domain known for measurement difficulties. One possible explanation is that the wave and plasma techniques respond to different components of a non‐Maxwellian magnetospheric electron distribution.

Synthesis of magnetospheric radio emissions during and after the Jupiter/SL-9 collision

A worldwide network of ground-based and spaced-based radiotelescopes monitored the continuum emission of Jupiter from millimetric to kilometric wavelengths throughout the SL-9 impacts in July 1994. Within 2 days of the first impact, an increase of flux density was detected and confirmed by many observatories at wavelengths from 3 to 90 cm. The increase involved only the synchrotron emission from the energetic electrons trapped in the Jovian radiation belts. At decameter and kilometer wavelengths, normally from radio sources located in the polar regions, no abnormal emission can be associated with certainty with the impacts. At short centimeter and millimeter wavelengths a search was made for impact-associated changes in Jupiter's deep atmosphere, but no detections have been reported and confirmed. The present knowledge of the synchrotron radio emission from the Jovian radiation belts is reviewed and the changes which occurred during and after the collision are commented on. During the week of cometary impacts, the synchrotron flux density increased, depending on the wavelength, by 10–40% and the radio spectrum hardened. Following the week of impacts, the flux density decreased at all wavelengths, more rapidly at 36–90 cm than at 6–22 cm. The observed decline was not a simple exponential decay, and the timescale was more than 100 days at 6–20 cm and less than 30 days at 36–90 cm. The beaming curve was flattened and distorted during and after the impacts, with long-lasting enhancements localized in longitude. Two-dimensional images show that the belts became brighter than before the impacts, and they remained bright at least one week after the first impact. The increase of brightness at the magnetic equator was concentrated in a limited range of longitude, from ≈80 to ≈300°. The radial distance of the peak intensity decreased over that week. Images in 3-D, constructed from observations with the Australia Telescope Compact Array, reveal distinct brightenings confined in longitude that can be attributed to the longitudes of individual impacts or group of impacts.

Time‐variable magnetospheric radio emissions from Jupiter

Jupiter is the source of a large number of independent nonthermal radio sources, all of which vary with time. The known causes of the variations include planetary rotation modulation, modulation by Io and/or its torus, and influence by the solar wind which can reach surprisingly deep into the Jovian magnetosphere. However, a significant number of radio variations, both short‐term and long‐term, are not currently explained by any known mechanism.

A relativistic theory of the R wave cut-off

R wave propagation at frequencies near the cut-off frequency is studied in detail based on a weakly relativistic approximation. It is pointed out that the deviation of the relativistic cut-off frequency from the corresponding value of the cut-off frequency predicted by the theory of wave propagation in a cold plasma is particularly noticeable in a rarefied plasma. Our theory predicts that the R wave in the frequency range under consideration can propagate without collisionless damping or amplification. The obtained results are applicable to the analysis of low frequency cut-off in the dynamic spectra of natural magnetospheric radio emissions at frequencies above the electron plasma frequency (non-thermal continuum).

Prediction of radio frequency power generation of Neptune's magnetosphere from generalized radiometric Bode's law

Magnetospheric radio frequency emission power has been shown to vary as a function of both solar wind and planetary values such as magnetic field by Kaiser and Desch. Planetary magnetic fields have been shown to scale with planetary variables such as density and angular momentum by numerous researchers. This paper combines two magnetic scaling laws (Busse's and Curtis Ness') with the radiometric law to yield ‘Bode's’‐type laws governing planetary radio emissions. Further analysis allows the reduction of variables to planetary mass and orbital distance. These generalized laws are then used to predict the power output of Neptune to be about 1.6 × 107W; with the intensity peaking at about 3MHz.

Coupling between auroral kilometric radiation and auroral radio noise based on wave-particle resonance interaction

The existence of a coupling, due to wave-particle resonance interaction, between magnetospheric radio emissions, i.e. auroral kilometric radiation (AKR), and auroral radio noise(AN), separated in an inhomogeneous magnetospheric plasma by an opacity barrier, is studied. It is proved that the ballistic process can only take place on the electrons of the superthermal component of the plasma, i.e. on particles of auroral electron fluxes of the inverted V type, due to finite width of the AKR spectral lines, which amounts to 1 kHz. Analyses indicate that its effectiveness is sufficient to explain radio emission coupling and that it increases with the degree of auroral activity.