Bursty Emission Research Articles

The search for radio emission from the exoplanetary systems 55 Cancri,υAndromedae, andτBoötis using LOFAR beam-formed observations

Context.The detection of radio emissions from exoplanets will open up a vibrant new research field. Observing planetary auroral radio emission is the most promising method to detect exoplanetary magnetic fields, the knowledge of which will provide valuable insights into the planet’s interior structure, atmospheric escape, and habitability.Aims.We present LOFAR (LOw-Frequency ARray) Low Band Antenna (LBA: 10–90 MHz) circularly polarized beamformed observations of the exoplanetary systems 55 Cancri,υAndromedae, andτBoötis. All three systems are predicted to be good candidates to search for exoplanetary radio emission.Methods.We applied theBOREALISpipeline that we have developed to mitigate radio frequency interference and searched for both slowly varying and bursty radio emission. Our pipeline has previously been quantitatively benchmarked on attenuated Jupiter radio emission.Results.We tentatively detect circularly polarized bursty emission from theτBoötis system in the range 14–21 MHz with a flux density of ~890 mJy and with a statistical significance of ~3σ. For this detection, we do not see any signal in the OFF-beams, and we do not find any potential causes which might cause false positives. We also tentatively detect slowly variable circularly polarized emission fromτBoötis in the range 21–30 MHz with a flux density of ~400 mJy and with a statistical significance of >8σ. The slow emission is structured in the time-frequency plane and shows an excess in the ON-beam with respect to the two simultaneous OFF-beams. While the bursty emission seems rather robust, close examination casts some doubts on the reality of the slowly varying signal. We discuss in detail all the arguments for and against an actual detection, and derive methodological tests that will also apply to future searches. Furthermore, a ~2σmarginal signal is found from theυAndromedae system in one observation of bursty emission in the range 14–38 MHz and no signal is detected from the 55 Cancri system, on which we placed a 3σupper limit of 73 mJy for the flux density at the time of the observation.Conclusions.Assuming the detected signals are real, we discuss their potential origin. Their source probably is theτBoötis planetary system, and a possible explanation is radio emission from the exoplanetτBoötis b via the cyclotron maser mechanism. Assuming a planetary origin, we derived limits for the planetary polar surface magnetic field strength, finding values compatible with theoretical predictions. Further observations with LOFAR-LBA and other low-frequency telescopes, such as NenuFAR or UTR-2, are required to confirm this possible first detection of an exoplanetary radio signal.

Bursty emission of whistler waves in association with plasmoid collision

Abstract. A new mechanism to generate whistler waves in the course of collisionless magnetic reconnection is proposed. It is found that intense whistler emissions occur in association with plasmoid collisions. The key processes are strong perpendicular heating of the electrons through a secondary magnetic reconnection during plasmoid collision and the subsequent compression of the ambient magnetic field, leading to whistler instability due to the electron temperature anisotropy. The emissions have a bursty nature, completing in a short time within the ion timescales, as has often been observed in the Earth's magnetosphere. The whistler waves can accelerate the electrons in the parallel direction, contributing to the generation of high-energy electrons. The present study suggests that the bursty emission of whistler waves could be an indicator of plasmoid collisions and the associated particle energization during collisionless magnetic reconnection.

Pulsating fireballs with high-frequency sheath–plasma instabilities

High-frequency instabilities are observed in connection with unstable fireballs. Fireballs are discharge phenomena near positively biased electrodes in discharge plasmas. They are bounded by a double layer whose potential is of order of the ionization potential. Fireballs become unstable when plasma losses and plasma production are not in balance, resulting in periodic fireball pulses. High-frequency instabilities in the range of the electron plasma frequency have been observed. These occur between fireball pulses, hence are not due to electron beam–plasma instabilities since there are no beams without double layers. The instability has been identified as a sheath–plasma instability. Electron inertia creates a phase shift between high-frequency current and electric fields which destabilizes the sheath–plasma resonance. High-frequency signals are observed in the current to the electrode and on probes near the sheath of the electrode. Waveforms and spectra are presented, showing bursty emissions, phase shifts, frequency jumps, beat phenomena between two sheaths, and nonlinear effects such as amplitude clipping. These reveal many interesting properties of sheaths with periodic ionization phenomena.

Pitch angle transport of electrons due to cyclotron interactions with the coherent chorus subelements

Chorus is a right‐hand, circularly‐polarized electromagnetic plane wave. Dayside chorus is a bursty emission composed of rising frequency “elements” with duration of ∼0.1 to 1.0 s. Each element is composed of coherent subelements with durations of ∼1 to 100 ms or more. Due to the coherent nature of the chorus subelements/wave packets, energetic electrons with pitch angles near the loss cone may stay in resonance with the waves for more than one wave cycle. The electrons could therefore be “transported” in pitch across a relatively large angle from a single wave‐particle interaction. Here we study the cyclotron resonance of the energetic electron with the coherent chorus subelements. We consider a Gaussian distribution for the time duration of the chorus subelements and derive an expression for the pitch angle transport due to this interaction. For typical chorus subelement parameters, the average pitch angle diffusion coefficients ∼(0.5–8.5) s−1 are found. Such rapid pitch angle scattering may provide an explanation for the ionospheric microbursts of ∼0.1 to 0.5 s in bremsstrahlung x‐rays formed by ∼10–100 keV precipitating electrons. The model is applicable to the cases when R = ttr/Δt = [(ω + Ω/2) ttr/ωτ] > 1 and inhomogeneity factor S = ttr2/tinh2 < 1, where Ω is the electron cyclotron frequency in the ambient magnetic field, B0, ω is the frequency of chorus, ttr is the trapping time (or phase oscillation period), tinh is the time for the passage through the resonance in the inhomogeneous magnetic field, and τ is the duration of the chorus subelement. For the typical parameters at L = 5, the energetic electrons having pitch angles of α ≤ π/3 can satisfy both the condition R > 1 and S < 1 for a range of chorus wave amplitudes.

ON THE BRIGHTNESS AND WAITING-TIME DISTRIBUTIONS OF A TYPE III RADIO STORM OBSERVED BY STEREO/WAVES

Type III solar radio storms, observed at frequencies below approximately 16 MHz by space borne radio experiments, correspond to the quasi-continuous, bursty emission of electron beams onto open field lines above active regions. The mechanisms by which a storm can persist in some cases for more than a solar rotation whilst exhibiting considerable radio activity are poorly understood. To address this issue, the statistical properties of a type III storm observed by the STEREO/WAVES radio experiment are presented, examining both the brightness distribution and (for the first time) the waiting-time distribution. Single power law behavior is observed in the number distribution as a function of brightness; the power law index is approximately 2.1 and is largely independent of frequency. The waiting-time distribution is found to be consistent with a piecewise-constant Poisson process. This indicates that during the storm individual type III bursts occur independently and suggests that the storm dynamics are consistent with avalanche type behavior in the underlying active region.

The Bursty Nature of Solar Flare X‐Ray Emission

The complex and highly varying temporal nature of emission from an X4.8 flare is studied across seven X-ray energy bands. A wavelet transform modulus maxima method is used to obtain the multifractal spectra of the temporal variation of the X-ray emission. As expected from the Neupert effect, the time series of the emission at low energies (3-6, 6-12 keV; thermal) is smooth. The peak Holder exponent, around 1.2, for this low-energy emission is indicative of a signal with a high degree of memory and suggestive of a smooth chromospheric evaporation process. The more bursty emission at higher energies (100-300, 300-800 keV; nonthermal) is described by a multifractal spectrum that peaks at a smaller Holder exponent (less than 0.5 for the largest singularities), indicative of a signal with a low degree of memory. This describes an antipersistent walk and indicates an impulsive, incoherent driving source. We suggest that this may arise from bursty reconnection, with each reconnection event producing a different and uncorrelated nonthermal particle source. The existence of a power-law scaling of wavelet coefficients across timescales is in agreement with the creation of a fractal current sheet diffusion region.

New regimes of stochastic wave growth: Theory, simulation, and comparison with data

Stochastic growth theory (SGT) of bursty waves is generalized and it is shown that the theory of “elementary bursts,” previously used to describe bursty emission in certain solar plasmas, is a limiting case of the generalized theory. New regimes of strong and weak stochastic growth are found, the boundaries separating the regimes are elucidated, and a reduced-parameter quasilinear model is used to constrain growth dynamics. The analytic results are then compared with simulations using the reduced-parameter model. Upon re-analysis of data from situations previously studied using SGT or other theories, including spacecraft data and results of particle-in-cell and quasilinear simulations, good agreement is found with the predictions of the generalized theory. In particular, data collapse of stochastic wave statistics is accomplished onto a universal curve with no free parameters.

The plasma wave environment of Europa

The Galileo spacecraft has executed nine close flybys of Jupiter's moon Europa for which plasma wave observations were obtained. This paper presents an analysis of the observations from these flybys taking into consideration the variable geometry of the trajectories in an attempt to characterize the general plasma-wave environment associated with the interaction of the Jovian magnetosphere with the moon. A wide variety of plasma-wave phenomena are found to be associated with this interaction. While there are apparently temporal variations which complicate the analysis, a crude model of the distribution of these phenomena around Europa is derived. Primarily on the upstream side of Europa, and working inward to the moon, electron–cyclotron harmonics are first observed, followed by a region within about two Europa radii of the moon with whistler-mode hiss or chorus, and culminating in a region closest to the moon where a band at the upper hybrid resonance frequency is sometimes enhanced over its ambient intensity. The wake region is approximately two Europa radii across and comprises a broadband, highly variable, and bursty electrostatic phenomenon. Upon closer inspection, these bursty emissions appear as solitary structures similar to those in Earth s auroral zone and plasma sheet boundary layer. In addition to the survey of wave phenomena in the vicinity of Europa, we provide density profiles derived primarily from the upper hybrid resonance frequency which is readily apparent throughout most of each of the flybys. Finally, we suggest that the whistler mode, electron cyclotron harmonic, and upper hybrid resonance emissions are driven by some combination of factors including variations in the magnetic field near Europa and the loss and production of plasma at Europa as a result of the interaction of the Jovian magnetosphere with the moon. By analogy with studies of the ion and electron holes and broadband electrostatic noise at Earth and Jupiter, we argue that the electrostatic solitary structures in the wake are associated with currents and beams coupling Europa to Jupiter's ionosphere.

Bursty wave instabilities in open driven plasmas.

Wave instabilities in open, driven systems cause bursty emission. Local burst properties are combined with global power balance to predict the source dynamics, energy conversion efficiency, and distribution of power levels. Sporadic growth near marginal stability is characterized by noninteger scaling exponents and an exponential distribution of power levels, unlike self-organized criticality or stochastic growth. Predictions are confirmed by particle-in-cell simulations of driven cyclotron instabilities. Solar microwave bursts are found to have the predicted distribution of power levels. {copyright} {ital 1996 The American Physical Society.}

Observations of electron plasma waves upstream of the Jovian bow shock

The Ulysses spacecraft encountered the planet Jupiter in February 1992, on its journey towards high heliospheric latitude. During the approach to the planet, as well as on the outbound pass, while receding from the Jovian bow shock, the Plasma Frequency Receiver that is part of the Unified Radio and Plasma Wave experiment (URAP) recorded bursts of plasma waves in the frequency range of a few kHz. These emissions, first observed by the PWS experiment onboard the Voyager spacecraft, have been identified as upstream electron plasma waves. In this paper, we present the first analysis of the characteristics of these emissions, which are very similar to those found in the Earth's electron foreshock, upstream of the Earth's bow shock. These bursty emissions, with a peak frequency very close to the local electron plasma frequency F pe, have a typical electric field amplitude in the range 0.01–0.1 mV m −1, with some bursts above 1 mV m −1. The frequency bandwidth over which significant power can be found above the instrument background noise ranges from below 0.2 F pc to about 2 F pc. On the basis of our present knowledge of similar emissions observed at Earth, we suggest that the broadband emissions are triggered by suprathermal (a few tens of eV) electrons, streaming back from Jupiter's bow shock.

Numerical simulations of bursty radio emissions from planetary magnetospheres

The Voyager spacecraft observed both smooth and bursty radio emissions from Uranus and Neptune. These emissions are known to be freely propagating primarily in the right‐hand circularly polarized (RCP) mode with the bursty emissions having burst periods as short as a few tenths of a second and the smooth emissions being observed over periods of a few hours. While the smooth emission is probably due to the electron cyclotron maser instability, some other processes must be at work to produce the bursty emissions. It is proposed that one important difference in mechanisms is that the smooth emissions are associated with continuous injection of electrons while the bursty emissions are associated with impulsive injection. In the latter case, the electron distribution can develop a beam feature with a temperature anisotropy. It is shown via one‐dimensional (three velocity) relativistic particle simulations that while the beam may be initially unstable to an electrostatic instability, this instability quickly saturates and eventually the beam is unstable to a strong electromagnetic beam instability which utilizes the temperature anisotropy as free energy; a beam feature is not explicitly needed for the growth of this electromagnetic instability. The radiation generated by the instability is able to convert particle energy to wave energy at similar levels as the maser instability. For ωpe/Ωe ≳ 1, most of the wave energy is LCP and is trapped. However, for ωpe/Ωe ≲ 1 the dominant mode is RCP. The induced waves are in the form of modified whistlers when the beam speed is low and ωpe/Ωe ≃ 1 but are in the freely propagating x mode branch when ωpe/Ωe ≃ 0.4. The amount of radiation with frequencies above the local x mode cutoff increases with beam speed. It is also postulated that some of the radiation generated below the local x mode cutoff may also be able to escape the plasma and be detected remotely via mode conversion between regions where field‐aligned currents produce local perturbations in the magnetic field.

Neptune's radio emissions

Neptune's radio emission was discovered by the Planetary Radio Astronomy experiment (PRA) a few days before Voyager 2 closest approach. However, its detection was possible as early as 30 days before encounter and 22 days after encounter. At least two types of emissions, a smooth and a bursty emission were recorded in the frequency range of 20 kHz to 1300 kHz. They differ by their spectral, temporal and polarization characteristics. The smooth emission exhibits both polarizations depending on the frequency of occurrence and on the longitude of Neptune (NLS); the pattern which is repeated before and after encounter is located in the same NLS range. The bursty emission consists of very short duration bursts, narrow-banded, and also strongly polarized. The bursts, as well as the smooth emission, can be described in terms of rotation in a period of 16.10 hours. The fore shock of Neptune's magnetosphere, the impacts of dust at ring-crossing and many different low frequency waves were also detected by the PRA experiment during the period of the encounter. We describe here the polarization response of the instrument in order to determine the true polarization of the incoming waves. At the closest approach the smooth radio source was occulted by the planet. By modeling the intensity profile at the encounter we have been able to conclude that there is at least one radio source in the auroral zone of the northern magnetic hemisphere where a good fit to the observations can be obtained when assuming an emission cone model. The implications on the magnetic field strength will be discussed.

An anomalous component of neptune radio emission: Implications for the auroral zone

The Voyager planetary radio astronomy experiment detected a bursty, narrow‐band radio emission originating in Neptune's magnetosphere. The time of occurrence of nearly all of the episodes of this bursty radio emission can be explained on the basis of a radio source located just above and to the east of the south magnetic offset tilted dipole (OTD) tip (Farrell et al., 1990). However, several episodes of bursty emission do not occur at the usual frequency and planetary rotation phase for emissions of this type. We show that the occurrences of these rarely seen anomalous episodes are shifted systematically in planetary longitude so as to be consistent with a source of emission to the southwest of the southern magnetic OTD pole. Owing to the proximity of these sources to the magnetic polar region we associate them with an active auroral region. Therefore, at least from the standpoint of the radio emission, the picture that emerges is of an auroral zone with two emission hot spots approximately diametrically east and west of the south magnetic pole. The possibility of a complete radio‐active auroral oval is discussed.

Ray tracing of broadband bursty radio emissions from Uranus

To determine the source position of the broad‐band bursty emission, we traced rays of X‐mode emission from source positions along magnetic field lines with footprints that form a large grid centered approximately on the south magnetic pole of Uranus. We assumed the emission is just above the RX cutoff frequency, and we included sources only if the ratio of plasma to gyrofrequency was less than about 0.25. Contrary to the results of others who have assumed straight‐line propagation, we found, for large wave normal angles, source regions different from those producing b‐smooth emission, and along field lines with footprints in a relatively large area associated with the south magnetic pole. The emission which was observed prior to closest approach has a source along field lines that are distinct from those which generate emissions observed after closest approach. In order to understand these findings and the bursty nature of the emission we speculate the plasma distribution in the source region and/or the source mechanism is distinct from that which generates the b‐smooth emission.

Overview of the Uranian radio emissions: Polarization and constraints on source locations

Radio emission from Uranus was discovered a few days before the closest approach of Voyager 2 with Uranus on January 24, 1986. The planetary radio astronomy experiment recorded several types of emissions in the frequency range 1–900 kHz; they differ by their spectral, temporal, and polarization characteristics. The analysis of the observations led us to distinguish two smooth radio components, one occurring in the lowest frequency range (SLF) from 20 kHz to 347 kHz, and the other one reaching the highest observed frequencies, i.e., 865 kHz (SHF). The SLF component is left‐hand polarized when observed in the northern magnetic hemisphere and right‐handed in the southern magnetic hemisphere, without polarization reversal from dayside to nightside. The SHF component is only observed in the nightside of Uranus, and it is left‐hand polarized. Several bursty emissions were observed: some of them are broadband bursts and occur in the 78‐ to 750‐kHz range (b‐bursty), others are narrow‐band bursts and occur in the 40‐ to 270‐kHz range (n‐bursty). The broadband bursts were observed at low magnetic latitudes of the spacecraft and mainly when it was in the southern magnetic hemisphere; their polarization is left‐handed. The narrow‐band bursts were observed in the northern magnetic hemisphere and are not correlated with the magnetic latitude of the spacecraft; their polarization is right‐handed. Sporadic bursts of short duration were observed mainly in the northern magnetic hemisphere with right‐hand polarization. Two periodic events were observed, ahead of the inbound bow shock and inside the magnetosphere of Uranus. Very low frequency emissions at 1.2 and 20.4 kHz are also discussed; some of them are correlated with in situ phenomena. From the polarization study we deduce the possible source locations of the main components in the frame of our hypothesis. The source regions are characterized by the latitude and longitude of the footprints of the magnetic field lines through the source expressed in the Uranographic longitude system (ULS). The SLF source would be fixed in magnetic longitude at the northern magnetic pole in the range 13°< latitudeULS<38°, 26°<west longitudeULS<70°, and 1.23<altitude (78 kHz)<1.39 RU. The emission mode would be ordinary. The SHF and b‐bursty components would be fixed in magnetic longitude at the southern magnetic pole. The SHF (at 300 kHz) would be in the range −52°< latitudeULS<−44°, 228°<west longitudeULS<256°, and 1.77<altitude<1.85 RU. The b‐bursty component would be located in a smaller region included in the SHF region in the range −48°<latitudeULS<−45° and 233°<longitudeULS<243°. For both high‐frequency components the emission mode would be extraordinary.

Beaming of Uranian nightside kilometric radio emission and inferred source location

Taking advantage of the large range of magnetic latitudes explored by Voyager 2, due to the 60° tilt of the Uranian dipole field with respect to the rotation axis of the planet, we study the beaming pattern of the broadband kilometric radio emission detected over Uranus' nightside. Using the simple physical assumptions of an emission close to the local gyrofrequency and in a time‐stationary average lobe (i.e., assuming that the 10 min‐averaged intensity received does not vary with time for a given observing geometry), we constrain the radiation source to a high south magnetic latitude region (∼80°, or L∼40) magnetically connected to the magnetospheric tail. We discuss the different source locations derived by other authors from different analyses. We determine then the beaming pattern of the main components (smooth and bursty) of the Uranian nightside radio emission and their variations with frequency. It appears to be a hollow cone whose axis is tangent to the magnetic field in the source region, with a large aperture (semi‐apical angle ∼50°) and with an angular thickness (at −3 dB) increasing with decreasing frequency from about 30° at 750 kHz to more than 55° below ∼350 kHz; the bursty emission occurs about 90° away from the magnetic field. The motion of the spacecraft relative to the radiation lobe explains most of the observed emission characteristics. The origin of the bursty emission and its possible link to the smooth component are discussed. The results are compared with the present experimental and theoretical knowledge of planetary radio emission beaming.

Sporadic narrowband radio emissions from Uranus

Among several different types of radio emissions discovered at Uranus during the Voyager 2 encounter in January 1986 is a very sporadic, bursty signal which consists of very narrow bands lying in the frequency range from about 3 to 10 kHz. The bursty emission was virtually undetectable from the dayside portion of the Voyager 2 trajectory but was observed out to beyond 300 RU during the outbound trajectory through the predawn sector. While the narrowband tones making up this emission are reminiscent of escaping continuum radiation observed near earth, Jupiter, and Saturn, the Uranian signals show large amplitude variations on time scales of 1 s, suggesting a very different type of generation mechanism.