Keywords

The Carrington storm (1859 September 1/2) is one of the largest magnetic storms ever observed, and it caused global auroral displays in low-latitude areas, together with a series of multiple magnetic storms from 1859 August 28 to September 4. In this study, we revisit contemporary auroral observation records to extract information on their elevation angle, color, and direction to investigate this stormy interval in detail. We first examine the equatorward boundary of the "auroral emission with multiple colors" based on descriptions of elevation angle and color. We find that their locations were 365 ILAT on August 28/29 and 327 ILAT on September 1/2, suggesting that trapped electrons moved to, at least, L ∼ 1.55 and L ∼ 1.41, respectively. The equatorward boundary of "purely red emission" was likely located at 308 ILAT on September 1/2. If the "purely red emission" was a stable auroral red arc, it would suggest that trapped protons moved to, at least, L ∼ 1.36. This reconstruction with observed auroral emission regions provides conservative estimations of magnetic storm intensities. We compare the auroral records with magnetic observations. We confirm that multiple magnetic storms occurred during this stormy interval, and that the equatorward expansion of the auroral oval is consistent with the timing of magnetic disturbances. It is possible that the August 28/29 interplanetary coronal mass ejections (ICMEs) cleared out the interplanetary medium, making the ICME for the Carrington storm on September 1/2 more geoeffective.

The first nearby very-low-mass star–planet-host discovered, TRAPPIST-1, presents not only a unique opportunity for studying a system of multiple terrestrial planets, but a means to probe magnetospheric interactions between a star at the end of the main sequence and its close-in satellites. This encompasses both the possibility of persistent coronal solar-like activity, despite cool atmospheric temperatures, and the presence of large-scale magnetospheric currents, similar to what is seen in the Jovian system. Significantly, the current systems include a crucial role for close-in planetary satellites analogous to the role played by the Galilean satellites around Jupiter. We present the first radio observations of the seven-planet TRAPPIST-1 system using the Karl G. Jansky Very Large Array, looking for both highly circularly polarized radio emission and/or persistent quiescent emissions. We measure a broadband upper flux density limit of <8.1 μJy across 4–8 GHz, and place these observations both in the context of expectations for stellar radio emission, and the possible electrodynamic engines driving strong radio emissions in very-low-mass stars and brown dwarfs, with implications for future radio surveys of TRAPPIST-1 like planet-hosts. We conclude that the magnetic activity of TRAPPIST-1 is predominantly coronal and does not behave like the strong radio emitters at the stellar/substellar boundary. We further discuss the potential importance of magnetic field topology and rotation rates, demonstrating that a TRAPPIST-1 like planetary system around a rapidly rotating very-low-mass star can generate emission consistent with the observed radio luminosities of very-low-mass stars and brown dwarfs.

Auroral emissions provide opportunities to study the tenuous atmospheres of solar system satellites, revealing the presence and abundance of molecular and atomic species as well as their spatial and temporal variability. Far-UV aurorae have been used for decades to study the atmospheres of the Galilean satellites. Here we present the first detection of Europa's visible-wavelength atomic oxygen aurora at 6300/6364 Å arising from the metastable ${\rm{O}}{(}^{1}{\rm{D}}$) state, observed with the Keck I and Hubble Space Telescope while Europa was in eclipse by Jupiter on six occasions in 2018 February–April. The disk-integrated O(¹D) brightness varies from <500 R up to more than 2 kR between dates, a factor of 15 higher than the O i 1356 Å brightness on average. The ratio of emission at 6300/5577 Å is diagnostic of the parent molecule; the 5577 Å emission was not detected in our data set, which favors O₂ as the dominant atmospheric constituent and rules out an O/O₂ mixing ratio above 0.35. For an O₂ atmosphere and typical plasma conditions at Europa's orbit, the measured surface brightness range corresponds to column densities of (1–9) × 10¹⁴ cm⁻².

We have used NSF's Karl G. Jansky Very Large Array to observe a sample of five known radio-emitting late-L and T dwarfs ranging in age from ∼0.2 to 3.4 Gyr. We observed each target for seven hours, extending to higher frequencies than previously attempted and establishing proportionally higher limits on maximum surface magnetic field strengths. Detections of circularly polarized pulses at 8–12 GHz yield measurements of 3.2–4.1 kG localized magnetic fields on four of our targets, including the archetypal cloud variable and likely planetary-mass object T2.5 dwarf SIMP J01365663+0933473. We additionally detect a pulse at 15–16.5 GHz for the T6.5 dwarf 2MASS 10475385+2124234, corresponding to a localized 5.6 kG field strength. For the same object, we tentatively detect a 16.5–18 GHz pulse, corresponding to a localized 6.2 kG field strength. We measure rotation periods between ∼1.47–2.28 hr for 2MASS J10430758+2225236, 2MASS J12373919+6526148, and SDSS J04234858–0414035, supporting (i) an emerging consensus that rapid rotation may be important for producing strong dipole fields in convective dynamos, and/or (ii) rapid rotation is a key ingredient for driving the current systems powering auroral radio emission. We observe evidence of variable structure in the frequency-dependent time series of our targets on timescales shorter than a rotation period, suggesting a higher degree of variability in the current systems near the surfaces of brown dwarfs. Finally, we find that age, mass, and temperature together cannot account for the strong magnetic fields produced by our targets.

The study of historical great geomagnetic storms is crucial for assessing the possible risks to the technological infrastructure of a modern society, caused by extreme space–weather events. The normal benchmark has been the great geomagnetic storm of 1859 September, the so-called "Carrington Event." However, there are numerous records of another great geomagnetic storm in 1872 February. This storm, which occurred about 12 years after the Carrington Event, resulted in comparable magnetic disturbances and auroral displays over large areas of the Earth. We have revisited this great geomagnetic storm in terms of the auroral and sunspot records in historical documents from East Asia. In particular, we have surveyed the auroral records from East Asia and estimated the equatorward boundary of the auroral oval to be near 242 invariant latitude, on the basis that the aurora was seen near the zenith at Shanghai (20° magnetic latitude, MLAT). These results confirm that this geomagnetic storm of 1872 February was as extreme as the Carrington Event, at least in terms of the equatorward motion of the auroral oval. Indeed, our results support the interpretation of the simultaneous auroral observations made at Bombay (10° MLAT). The East Asian auroral records have indicated extreme brightness, suggesting unusual precipitation of high-intensity, low-energy electrons during this geomagnetic storm. We have compared the duration of the East Asian auroral displays with magnetic observations in Bombay and found that the auroral displays occurred in the initial phase, main phase, and early recovery phase of the magnetic storm.

Dim red aurora at low magnetic latitudes is a visual and recognized manifestation of magnetic storms. The great low-latitude auroral displays seen throughout East Asia on 1770 September 16–18 are considered to manifest one of the greatest storms. Recently found, 111 historical documents in East Asia attest that these low-latitude auroral displays appeared in succession for almost nine nights during 1770 September 10–19 in low magnetic latitude areas (<30°). This suggests that the duration of the great magnetic storm is much longer than usual. Sunspot drawings from 1770 reveal that the sunspot areas were twice as large as those observed in another great storm of 1859, which substantiates these unusual storm activities in 1770. These spots likely ejected several huge, sequential magnetic structures in short duration into interplanetary space, resulting in spectacular worldwide aurorae in mid-September of 1770. These findings provide new insight into the history, duration, and effects of extreme magnetic storms that may be valuable for those who need to mitigate against extreme events.

Stellar coronal activity has been shown to persist into the low-mass star regime, down to late M-dwarf spectral types. However, there is now an accumulation of evidence suggesting that at the end of the main sequence, there is a transition in the nature of the magnetic activity from chromospheric and coronal to planet-like and auroral, from local impulsive heating via flares and MHD wave dissipation to energy dissipation from strong large-scale magnetospheric current systems. We examine this transition and the prevalence of auroral activity in brown dwarfs through a compilation of multiwavelength surveys of magnetic activity, including radio, X-ray, and optical. We compile the results of those surveys and place their conclusions in the context of auroral emission as a consequence of large-scale magnetospheric current systems that accelerate energetic electron beams and drive the particles to impact the cool atmospheric gas. We explore the different manifestations of auroral phenomena, like Hα, in brown dwarf atmospheres and define their distinguishing characteristics. We conclude that large-amplitude photometric variability in the near-infrared is most likely a consequence of clouds in brown dwarf atmospheres, but that auroral activity may be responsible for long-lived stable surface features. We report a connection between auroral Hα emission and quiescent radio emission in electron cyclotron maser instability pulsing brown dwarfs, suggesting a potential underlying physical connection between quiescent and auroral emissions. We also discuss the electrodynamic engines powering brown dwarf aurorae and the possible role of satellites around these systems both to power the aurorae and seed the magnetosphere with plasma.

We examine the feasibility of detecting auroral emission from the potentially habitable exoplanet Proxima Centauri b. 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 Centauri b is a terrestrial world with an Earth-like atmosphere and magnetic field, we estimate that the power at the 5577 Å O i auroral line is on the order of 0.1 TW under steady-state stellar wind, or ∼100× stronger than that on Earth. This corresponds to a planet–star contrast ratio of ${10}^{-6}\mbox{--}{10}^{-7}$ in a narrow band about the 5577 Å line, though higher contrast (${10}^{-4}\mbox{--}{10}^{-5}$) may be possible during periods of strong magnetospheric disturbance (auroral power 1–10 TW). We searched the Proxima Centauri b HARPS data for the 5577 Å line and for other prominent oxygen and nitrogen lines, but find no signal, indicating that the O i auroral line contrast must be lower than $2\times {10}^{-2}$ (with power ≲3000 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 space-based coronagraphic telescope or a ground-based extremely large telescope (ELT) with a coronagraph could push sensitivity down to terawatt oxygen aurorae (contrast $7\times {10}^{-6}$) with exposure times of ∼1 day. 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 role of electrostatic electron cyclotron harmonic (ECH) waves in producing diffuse auroral emission O i 1356 Å on Ganymede is investigated. Electron precipitation flux entering the atmosphere of Ganymede due to pitch-angle diffusion by ECH waves into the atmospheric loss-cone is calculated. The analytical yield spectrum approach for electron energy degradation in gases is used for calculating diffuse auroral intensities. It is found that calculated O i 1356 Å intensity resulting from the precipitation of magnetospheric electrons observed near Ganymede is insufficient to account for the observed diffuse auroral intensity. This is in agreement with estimates made in earlier works. Heating and acceleration of ambient electrons by ECH wave turbulence near the magnetic equator on the field line connecting Ganymede and Jupiter are considered. Two electron distribution functions are used to simulate the heating effect by ECH waves. Use of a Maxwellian distribution with temperature 100 eV can produce about 50–70 Rayleigh O i 1356 Å intensities, and the kappa distribution with characteristic energy 50 eV also gives rise to intensities with similar magnitude. Numerical experiments are performed to study the effect of ECH wave spectral intensity profile, ECH wave amplitude, and temperature/characteristic energy of electron distribution functions on the calculated diffuse auroral intensities. The proposed missions, joint NASA/ESA Jupiter Icy Moon Explorer and the present JUNO mission to Jupiter, would provide new data to constrain the ECH wave and other physical parameters near Ganymede. These should help confirm the findings of the present study.