Can we Detect the Auroras of Proxima B?

The Detectability of Radio Auroral Emission from Proxima B

Authors:

Burkhart et al

Abstract:

Magnetically active stars possess stellar winds whose interaction with planetary magnetic fields produces radio auroral emission. We examine the detectability of radio auroral emission from Proxima b, the closest known exosolar planet orbiting our nearest neighboring star, Proxima Centauri. Using the Radiometric Bode's Law, we estimate the radio flux produced by the interaction of Proxima Centauri's stellar wind and Proxima b's magnetosphere for different planetary magnetic field strengths. For plausible planetary masses, Proxima b produces 6-83 mJy of auroral radio flux at frequencies of 0.3-0.8 MHz for planetary magnetic field strengths of 1-3 B⊕. According to recent MHD models that vary the orbital parameters of the system, this emission is expected to be highly variable. This variability is due to large fluctuations in the size of Proxima b's magnetosphere as it crosses the equatorial streamer regions of the dense stellar wind and high dynamic pressure. Using the MHD model of Garraffo et al. 2016 for the variation of the magnetosphere radius during the orbit, we estimate that the observed radio flux can vary nearly by an order of magnitude over the 11.2 day period of Proxima b. The detailed amplitude variation depends on the stellar wind, orbital, and planetary magnetic field parameters. We discuss observing strategies for proposed future space-based observatories to reach frequencies below the ionospheric cut off (∼10 MHz) as would be required to detect the signal we investigate.

Auroral Radio Emission from Ultracool Dwarfs: a Jovian Model

Auroral Radio Emission from Ultracool Dwarfs: a Jovian Model

Authors:

Turnpenney et al

Abstract:
A number of fast-rotating ultra cool dwarfs (UCDs) emit pulsed coherent radiation, attributed to the electron cyclotron maser instability, a phenomenon that occurs in the solar system at planets with strong auroral emission. In this paper we examine magnetosphere-ionosphere coupling currents in UCDs, adopting processes used in models of Jovian emission. We consider the angular velocity gradient arising from a steady outward flux of angular momentum from an internal plasma source, as analogous to the jovian main oval current system, as well as the interaction of a rotating magnetosphere with the external medium. Both of these mechanisms are seen in the solar system to be responsible for the production of radio emission. We present the results of an investigation over a range of relevant plasma and magnetosphere-ionosphere coupling parameters to determine regimes consistent with observed UCD radio luminosities. Both processes are able to explain observed UCD luminosities with ionospheric Pedersen conductances of ∼1 − 2 mho, either for a closed magnetosphere with a plasma mass outflow rate of ∼105 kg s−1, i.e. a factor of ∼100 larger than that observed at Jupiter’s moon Io, or for a dwarf with an open magnetosphere moving through the interstellar medium at ∼50 km s−1 and a plasma mass outflow rate of ∼1000 kg s−1. The radio luminosity resulting from these mechanisms have opposing dependencies on the magnetic field strength, a point which may be used to discriminate between the two models as more data become available.

A Method to Characterize Proxima Centauri b Using its Aurora

The Pale Green Dot: A Method to Characterize Proxima Centauri b using Exo-Aurorae

Authors:

Luger et al

Abstract:

We examine the feasibility of detecting auroral emission from the potentially habitable exoplanet Proxima Centauri b. This planet's active, late-type M dwarf host makes detection of aurorae more favorable than around a solar-type star, primarily by increasing auroral power and improving the planet-star contrast in the visible wavelength range due to strong TiO absorption in the star. Detection of aurorae would yield an independent confirmation of the planet's existence, constrain the presence and composition of its atmosphere, and determine the planet's eccentricity and inclination, thereby breaking the mass-inclination degeneracy. If Proxima b is a terrestrial world with an atmosphere and magnetic field, we estimate that its auroral power at the 5577\AA\ OI auroral line is on the order of 0.1 TW under steady-state stellar wind, or ~100 times stronger than that on Earth. This corresponds to a planet-star contrast ratio of 10−6−10−7 in a narrow band about the 5577\AA\ line, although higher contrast (10−4−10−5) may be possible during periods of strong magnetospheric disturbance (auroral power 1-10 TW). We searched the Proxima b HARPS data for the 5577\AA\ line, but find no signal, indicating that the OI auroral line power must be lower 3,000 TW, consistent with our predictions. We find that observations of 0.1 TW auroral emission lines are likely infeasible with current and planned telescopes. However, future observations with a large-aperture, space-based coronagraphic telescope or a ground-based extremely large telescope (ELT) with a coronagraph could push sensitivity down to terawatt oxygen aurorae (contrast 7×10−6) with exposure times of ~1 day at high spectral resolution. If a coronagraph design contrast of 10−7 can be achieved with negligible instrumental noise, a future concept ELT could observe steady-state auroral emission in a few nights.

The Auroras of hot Jupiters

Stellar wind–magnetosphere interaction at exoplanets: computations of auroral radio powers

Authors:

Nichols et al

Abstract:

We present calculations of the auroral radio powers expected from exoplanets with magnetospheres driven by an Earth-like magnetospheric interaction with the solar wind. Specifically, we compute the twin cell-vortical ionospheric flows, currents, and resulting radio powers resulting from a Dungey cycle process driven by dayside and nightside magnetic reconnection, as a function of planetary orbital distance and magnetic field strength. We include saturation of the magnetospheric convection, as observed at the terrestrial magnetosphere, and we present power-law approximations for the convection potentials, radio powers and spectral flux densities. We specifically consider a solar-age system and a young (1 Gyr) system. We show that the radio power increases with magnetic field strength for magnetospheres with saturated convection potential, and broadly decreases with increasing orbital distance. We show that the magnetospheric convection at hot Jupiters will be saturated, and thus unable to dissipate the full available incident Poynting flux, such that the magnetic Radiometric Bode's Law (RBL) presents a substantial overestimation of the radio powers for hot Jupiters. Our radio powers for hot Jupiters are ∼5–1300 TW for hot Jupiters with field strengths of 0.1–10 BJ orbiting a Sun-like star, while we find that competing effects yield essentially identical powers for hot Jupiters orbiting a young Sun-like star. However, in particular, for planets with weaker magnetic fields, our powers are higher at larger orbital distances than given by the RBL, and there are many configurations of planet that are expected to be detectable using SKA.

Radio Emissions From T and L Dwarf Auroras

Auroral radio emission from late L and T dwarfs: A new constraint on dynamo theory in the substellar regime

Authors:

Kao et al

Abstract:

We have observed 6 late-L and T dwarfs with the Karl G. Jansky Very Large Array (VLA) to investigate the presence of highly circularly polarized radio emission, associated with large-scale auroral currents. Previous surveys encompassing ~60 L6 or later targets in this spectral range have yielded only one detection. Our sample includes the previously detected T6.5 dwarf 2MASS 10475385+2124234 as well as 5 new targets selected for the presence of H-alpha emission or optical/infrared photometric variability, which are possible manifestations of auroral activity. We detect 2MASS 10475385+2124234, as well as 4 of the 5 targets in our biased sample, including the strong IR variable SIMP J01365662+0933473 and bright H-alpha emitter 2MASS 12373919+6526148, reinforcing the possibility that activity at these disparate wavelengths is related. The radio emission frequency corresponds to a precise determination of the lower-bound magnetic field strength near the surface of each dwarf and this new sample provides robust constraints on dynamo theory in the low mass brown dwarf regime. Magnetic fields greater than 2.5 kG are confirmed for 5/6 targets. Our results provide tentative evidence that the dynamo operating in this mass regime may be inconsistent with predicted values from a recently proposed model. Further observations at higher radio frequencies are essential for verifying this assertion.

Brown Dwarf LSRJ1835+3259's Aurora Detected

Brown dwarf stars host powerful aurora displays just like planets, astronomers have discovered.

The so-called failed stars, which are difficult to detect and also remain hard to classify, are too massive to be planets but physicists from the Universities of Sheffield and Oxford have revealed that they host powerful auroras just like Earth.

The international team of researchers made the discovery by observing a brown dwarf 20 light years away using both radio and optical telescopes. Their findings provide further evidence that suggests these stars act more like supersized planets.

Dr Stuart Littlefair, from the University of Sheffield's Department of Physics and Astronomy, said: "Brown dwarfs span the gap between stars and planets and these results are yet more evidence that we need to think of brown dwarfs as beefed-up planets, rather than "failed stars".

"We already know that brown dwarfs have cloudy atmospheres - like planets - although the clouds in brown dwarfs are made of minerals that form rocks on Earth now we know brown dwarfs host powerful auroras too."

He added: "Sometimes the best thing about a scientific result is simply the thrill of discovering something exciting and cool. The northern lights on Earth are one of the most spectacular and beautiful things you can see.

"I've always wanted to see them, but have never got the chance. It's particularly ironic that I got to discover an auroral light show which is vastly more powerful and many light years away!"

Auroral displays result when charged particles manage to enter a planet's magnetic field. Once within the magnetosphere, those particles get accelerated along the planet's magnetic field lines to the planet's poles where they collide with gas atoms in the atmosphere, producing the bright emissions associated with auroras.

During the study the international research team, led by Professor Gregg Hallinan from the California Institute of Technology, conducted an extensive observation campaign of a brown dwarf called LSRJ1835+3259.

The team used the most powerful radio telescope in the world, the National Radio Astronomy Observatory's Karl G. Jansky Very Large Array (JVLA) in New Mexico, as well as optical telescopes including Palomar's Hale Telescope and the W.M Keck Observatory's telescopes to make their ground breaking observations.

Using the JVLA they detected a bright pulse of radio waves that appeared as the brown dwarf rotated around. The object rotates every 2.84 hours, so the team were able to watch nearly three full rotations over the course of a single night.

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Hot Jupiter HD 209458b's Aurora Releases One *MILLION* Times the X Rays of Jupiter's

Stellar wind induced soft X-ray emission from close-in exoplanets

Authors:

Kislyakova et al

Abstract:

In this paper, we estimate the X-ray emission from close-in exoplanets. We show that the Solar/Stellar Wind Charge Exchange Mechanism (SWCX) which produces soft X-ray emission is very effective for hot Jupiters. In this mechanism, X-ray photons are emitted as a result of the charge exchange between heavy ions in the solar wind and the atmospheric neutral particles. In the Solar System, comets produce X-rays mostly through the SWCX mechanism, but it has also been shown to operate in the heliosphere, in the terrestrial magnetosheath, and on Mars, Venus and Moon. Since the number of emitted photons is proportional to the solar wind mass flux, this mechanism is not very effective for the Solar system giants. Here we present a simple estimate of the X-ray emission intensity that can be produced by close-in extrasolar giant planets due to charge exchange with the heavy ions of the stellar wind. Using the example of HD~209458b, we show that this mechanism alone can be responsible for an X-ray emission of ≈1022~erg~s−1, which is 106 times stronger than the emission from the Jovian aurora. We discuss also the possibility to observe the predicted soft X-ray flux of hot Jupiters and show that despite high emission intensities they are unobservable with current facilities.