Main Aurora Research Articles

Drifting discrete Jovian radio bursts reveal acceleration processes related to Ganymede and the main aurora

Radio detection at high time-frequency resolutions is a powerful means of remotely studying electron acceleration processes. Radio bursts have characteristics (polarization, drift, periodicity) making them easier to detect than slowly variable emissions. They are not uncommon in solar system planetary magnetospheres, the powerful Jovian “short bursts (S-bursts)" induced by the Io-Jupiter interaction being especially well-documented. Here we present a detection method of drifting radio bursts in terabytes of high resolution time-frequency data, applied to one month of ground-based Jupiter observations. Beyond the expected Io-Jupiter S-bursts, we find decameter S-bursts related to the Ganymede-Jupiter interaction and the main Jovian aurora, revealing ubiquitous Alfvénic electron acceleration in Jupiter’s high-latitude regions. Our observations show accelerated electron energies are distributed in two populations, kilo-electron-Volts and hundreds of electron-Volts. This detection technique may help characterizing inaccessible astrophysical sources such as exoplanets.

How Bi‐Modal Are Jupiter's Main Aurora Zones?

AbstractUsing Juno‐measured >30 keV electrons, three regions with substantial ultraviolet emissions were identified previously for Jupiter's main aurora (excluding the polar cap): low‐latitude diffuse aurora, mid‐latitude Zone I of downward acceleration, and higher latitude Zone II of bi‐directional acceleration. Zone I, associated with upward magnetic field‐aligned currents, was represented as bimodal: sometimes supporting coherent downward electron electrostatic acceleration and sometimes downward electron broadband acceleration, with broadband acceleration usually delivering the most intense electron energy flux at Juno. Recent observations of up‐going ion beams within Zone I represent a challenge as to whether coherent electrostatic acceleration invariably accompanies broadband acceleration. Is this region strictly bi‐modal, or is there a continuum between these two modes? We address these questions by combining multiple ion and electron data sources to diagnose electrostatic potentials both above and below the spacecraft. We find: (a) During Zone I downward electron broadband events, there are examples where evidence of downward electron electrostatic acceleration completely disappears and examples where it endures at some level. (b) Most often, evidence of downward electron electrostatic acceleration is strongly suppressed with strong downward electron broadband acceleration. Residual potentials most often (not always) have values small (<10 kV) compared to the electron characteristic energies of 100–400 keV. (c) Care must be exercised in these studies because plasmasheet electron precipitation spectra can mimic broadband acceleration spectra. At least for weaker auroral broadband accelerations, there is likely to be a continuum of electrostatic and broadband participation. Why either process is favored at any one time is unknown.

A Tumor Suppressive Role of CYLD as a Novel Potential DUB of Aurora B in Cervical Cancer.

Cervical cancer is a common leading cause of cancer related to women death worldwide. Cylindromatosis (CYLD) is known as an important tumor suppressor in various human cancers, and a deubiquitination enzyme (DUB) as well. Previously, we identified Skp2 as an E3 ligase of Aurora B ubiquitination, but the DUB of Aurora B still remains unknown. Aurora B ubiquitination site is identified through in vivo ubiquitination assay. Activity of Aurora B and CENPA was detected by immunoblotting (IB) and immunofluorescence (IF) assay. Protein-to-protein interaction was investigated by immunoprecipitation (IP). Cell chromosome dynamics was monitored by live-cell time-lapse Imaging. Cancer cell proliferation, colony formation, apoptosis, and cell invasion and migration assays were also performed. Protein level was checked by immunohistochemical (IHC) staining in clinical cervical cancer samples. We identified Lysine 115 (K115) as the main Aurora B ubiquitination site for Skp2. We could also detect an interaction of Aurora B with the DUB CYLD. We found that CYLD promoted deubiquitination of Aurora B, and regulated Aurora B activity and function as well. Compared with control, we found it took more time for the cells to finish cell mitosis with CYLD over-expression. Furthermore, we found that CYLD deficiency promoted cervical cancer cell proliferation, colony formation, cell migration and invasion, and inhibited apoptosis instead, whereas it is just opposite with CYLD over-expression. In clinical cervical cancer samples, we showed a negative correlation of CYLD expression with Aurora B activation and histological cancer cell invasion. Furthermore, there was less CYLD abundance and higher Aurora B activity in advanced cancer samples compared with early stage. Our findings uncover CYLD as a novel potential DUB of Aurora B, which inhibits Aurora B activation and its subsequent function in cell mitosis, and also provide more evidence for its tumor suppressor function in cervical cancer.

Jupiter's Low-Altitude Auroral Zones: Fields, Particles, Plasma Waves, and Density Depletions.

The Juno spacecraft's polar orbits have enabled direct sampling of Jupiter's low‐altitude auroral field lines. While various data sets have identified unique features over Jupiter's main aurora, they are yet to be analyzed altogether to determine how they can be reconciled and fit into the bigger picture of Jupiter's auroral generation mechanisms. Jupiter's main aurora has been classified into distinct “zones”, based on repeatable signatures found in energetic electron and proton spectra. We combine fields, particles, and plasma wave data sets to analyze Zone‐I and Zone‐II, which are suggested to carry upward and downward field‐aligned currents, respectively. We find Zone‐I to have well‐defined boundaries across all data sets. H+ and/or H3 + cyclotron waves are commonly observed in Zone‐I in the presence of energetic upward H+ beams and downward energetic electron beams. Zone‐II, on the other hand, does not have a clear poleward boundary with the polar cap, and its signatures are more sporadic. Large‐amplitude solitary waves, which are reminiscent of those ubiquitous in Earth's downward current region, are a key feature of Zone‐II. Alfvénic fluctuations are most prominent in the diffuse aurora and are repeatedly found to diminish in Zone‐I and Zone‐II, likely due to dissipation, at higher altitudes, to energize auroral electrons. Finally, we identify significant electron density depletions, by up to 2 orders of magnitude, in Zone‐I, and discuss their important implications for the development of parallel potentials, Alfvénic dissipation, and radio wave generation.

Jupiter's Double‐Arc Aurora as a Signature of Magnetic Reconnection: Simultaneous Observations From HST and Juno

AbstractJupiter's powerful auroral emission is usually divided into the polar, main, and equatorward components. The driver of Jupiter's main aurora is a central question for the community. Previous investigations reveal many distinct substructures on the main auroral oval, which are indicators of fundamentally different magnetospheric processes. Understanding these substructures could provide key constraints for uncovering the driver of Jupiter's main aurora emission. In this study, we show the evolution of a double‐auroral arc on the dawnside from observations by the Hubble Space Telescope (HST). Simultaneous in situ observations from the Juno spacecraft provide direct evidence of magnetic reconnection and magnetic dipolarization. By analyzing the datasets from Juno and HST, we suggest that the evolution of the double‐arc structure is likely a consequence of the non‐steady progress of magnetic reconnection.

Six Pieces of Evidence Against the Corotation Enforcement Theory to Explain the Main Aurora at Jupiter

AbstractThe most remarkable feature of the ultraviolet auroras at Jupiter is the ever‐present and almost continuous curtain of bright emissions centered on each magnetic pole and called the main emissions. According to the widely accepted theory, it results from an electric current loop transferring momentum from the Jovian ionosphere to the magnetospheric plasma. However, predictions based on this theory have been recently challenged by observations from Juno and the Hubble Space Telescope. Here we review the main contradictory observations, expose their implications for the theory, and discuss promising paths forward.

Energetic Particles and Acceleration Regions Over Jupiter's Polar Cap and Main Aurora: A Broad Overview

AbstractPrevious Juno mission event studies revealed powerful electron and ion acceleration, to 100s of kiloelectron volts and higher, at low altitudes over Jupiter's main aurora and polar cap (PC; poleward of the main aurora). Here we examine 30–1200 keV JEDI‐instrument particle data from the first 16 Juno orbits to determine how common, persistent, repeatable, and ordered these processes are. For the PC regions, we find (1) upward electron angle beams, sometimes extending to megaelectron volt energies, are persistently present in essentially all portions of the polar cap but are generated by two distinct and spatially separable processes. (2) Particle evidence for megavolt downward electrostatic potentials are observable for 80% of the polar cap crossings and over substantial fractions of the PC area. For the main aurora, with the orbit favoring the duskside, we find that (1) three distinct zones are observed that are generally arranged from lower to higher latitudes but sometimes mixed. They are designated here as the diffuse aurora (DifA), Zone‐I (ZI(D)) showing primarily downward electron acceleration, and Zone‐II (ZII(B)) showing bidirectional acceleration with the upward intensities often greater than downward intensities. (2) ZI(D) and ZII(B) sometimes (but not always) contain, respectively, downward electron inverted Vs and downward proton inverted Vs, (potentials up to 400 kV) but, otherwise, have broadband distributions. (3) Surprisingly, both ZI(D) and ZII(B) can generate equally powerful auroral emissions. It is suggested but demonstrated for intense portions of only one auroral crossing, that ZI(D) and ZII(B) are associated, respectively, with upward and downward electric currents.

Comparing Electron Energetics and UV Brightness in Jupiter's Northern Polar Region During Juno Perijove 5.

We compare electron and UV observations mapping to the same location in Jupiter's northern polar region, poleward of the main aurora, during Juno perijove 5. Simultaneous peaks in UV brightness and electron energy flux are identified when observations map to the same location at the same time. The downward energy flux during these simultaneous observations was not sufficient to generate the observed UV brightness; the upward energy flux was. We propose that the primary acceleration region is below Juno's altitude, from which the more intense upward electrons originate. For the complete interval, the UV brightness peaked at ~240 kilorayleigh (kR); the downward and upward energy fluxes peaked at 60 and 700 mW/m2, respectively. Increased downward energy fluxes are associated with increased contributions from tens of keV electrons. These observations provide evidence that bidirectional electron beams with broad energy distributions can produce tens to hundreds of kilorayleigh polar UV emissions.

Diverse Electron and Ion Acceleration Characteristics Observed Over Jupiter's Main Aurora

AbstractTwo new Juno‐observed particle features of Jupiter's main aurora demonstrate substantial diversity of processes generating Jupiter's mysterious auroral emissions. It was previously speculated that sometimes‐observed potential‐driven aurora (up to 400 kV) can turn into broadband stochastic acceleration (dominating at Jupiter) by means of instability. Here direct evidence for such a process is revealed with a “mono‐energetic” electron inverted‐V rising in energy to 200 keV, transforming into a region of broadband acceleration with downward energy fluxes tripling to 3,000 mW/m2, and then transforming back into a mono‐energetic structure ramping down from 200 keV. But a second feature of interest observed nearby is unlikely to have operated in the same way. Here a downward accelerated proton inverted‐V, with inferred potentials to 300–400 kV, occurred simultaneously with downward accelerated broadband electrons with downward energy fluxes as high as any observed (~3,000 mW/m2). This latter feature has no known precedent with Earth auroral observations.

Discrete and broadband electron acceleration in Jupiter's powerful aurora.

The most intense auroral emissions from Earth's polar regions, called discrete for their sharply defined spatial configurations, are generated by a process involving coherent acceleration of electrons by slowly evolving, powerful electric fields directed along the magnetic field lines that connect Earth's space environment to its polar regions. In contrast, Earth's less intense auroras are generally caused by wave scattering of magnetically trapped populations of hot electrons (in the case of diffuse aurora) or by the turbulent or stochastic downward acceleration of electrons along magnetic field lines by waves during transitory periods (in the case of broadband or Alfvénic aurora). Jupiter's relatively steady main aurora has a power density that is so much larger than Earth's that it has been taken for granted that it must be generated primarily by the discrete auroral process. However, preliminary in situ measurements of Jupiter's auroral regions yielded no evidence of such a process. Here we report observations of distinct, high-energy, downward, discrete electron acceleration in Jupiter's auroral polar regions. We also infer upward magnetic-field-aligned electric potentials of up to 400 kiloelectronvolts, an order of magnitude larger than the largest potentials observed at Earth. Despite the magnitude of these upward electric potentials and the expectations from observations at Earth, the downward energy flux from discrete acceleration is less at Jupiter than that caused by broadband or stochastic processes, with broadband and stochastic characteristics that are substantially different from those at Earth.

Variation of Jupiter's aurora observed by Hisaki/EXCEED: 2. Estimations of auroral parameters and magnetospheric dynamics

AbstractJupiter's auroral parameters are estimated from observations by a spectrometer EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) on board Japanese Aerospace Exploration Agency's Earth‐orbiting planetary space telescope Hisaki. EXCEED provides continuous auroral spectra covering the wavelength range over 80–148 nm from the whole northern polar region. The auroral electron energy is estimated using a hydrocarbon color ratio adopted for the wavelength range of EXCEED, and the emission power in the long wavelength range 138.5–144.8 nm is used as an indicator of total emitted power before hydrocarbon absorption and auroral electron energy flux. The quasi‐continuous observations by Hisaki provide the auroral electron parameters and their relation under different auroral activity levels. Short‐ (within < one planetary rotation) and long‐term (> one planetary rotation) enhancements of auroral power accompany increases of the electron number flux rather than the electron energy variations. The relationships between the auroral electron energy (~70–400 keV) and flux (1026–1027/s, 0.08–0.9 μA/m2) estimated from the observations over a 40 day interval are in agreement with field‐aligned acceleration theory when incorporating probable magnetospheric parameters. Applying the electron acceleration theory to each observation point, we explore the magnetospheric source plasma variation during these power‐enhanced events. Possible scenarios to explain the derived variations are (i) an adiabatic variation of the magnetospheric plasma under a magnetospheric compression and/or plasma injection, and (ii) a change of the dominant auroral component from the main emission (main aurora) to the emission at the open‐closed boundary.

Open flux in Saturn’s magnetosphere

We characterise the interaction between the solar wind and Saturn’s magnetosphere by evaluating the amount of ‘open’ magnetic flux connected to the solar wind. This is deduced from a large set of Hubble Space Telescope images of the ultraviolet aurora, using the poleward boundary of the main aurora as a proxy for the open-closed field line boundary in the ionosphere. The amount of open flux is found to be 10–50GWb, with a mean of 35GWb. The typical change in open flux between consecutive observations separated by 10–60h is -5 or +7GWb. These changes are a result of imbalance between open flux creation at the dayside magnetopause and its closure in the magnetotail. The 5GWb typical decrease in open flux is consistent with in situ measurements of the flux transported following a reconnection event. Estimates of average, net reconnection rates are found to be typically a few tens of kV, with some extreme examples of unbalanced magnetopause or tail reconnection occurring at ∼300kV. The range of values determined suggest that Saturn’s magnetosphere does not generally achieve a steady state between flux opening at the magnetopause and flux closure in the magnetotail. The percentage of magnetic flux which is open in Saturn’s magnetosphere is similar to that measured at the Earth (2–11%), but the typical percentage that is closed between observations is significantly lower (13% compared to 40–70%). Therefore, open flux is usually closed in smaller (few GWb) events in Saturn’s magnetosphere. The exception to this behaviour is large, rapid flux closure events which are associated with solar wind compressions. While the rates of flux opening and closure should be equal over long timescales, they are evidently different on shorter (up to tens of hours) timescales. The relative independence of the magnetopause and tail reconnection rates can be attributed to the long loading timescales required to transport open field lines into the tail.

Observations of a high-latitude stable electron auroral emission at ∼16 MLT during a large substorm

[1] During an interval when the interplanetary magnetic field was large and primarily duskward and southward, a stable region of auroral emission was observed on 17 August 2001 by IMAGE at ∼16 magnetic local time, poleward of the main aurora, for 1 h, from before the onset of a large substorm through the recovery phase. In a region where ions showed the energy dispersion expected for the cusp, strong field-aligned currents and Poynting flux were observed by Polar (at 1.8 RE in the Southern Hemisphere) as it transited field lines mapping to the auroral spot in the Northern Hemisphere. The data are consistent with the hypothesis that the long-lasting electron auroral spot maps to the magnetopause region where reconnection was occurring. Under the assumption of conjugacy between the Northern and Southern hemispheres on these field lines, the Polar data suggest that the electrons on these field lines were accelerated by Alfvén waves and/or a quasi-static electric field, primarily at altitudes below a few RE since the in situ Poynting flux (mapped to 100 km) is comparable to the energy flux of the emission while the mapped in situ electron energy flux is much smaller. This event provides the first example of an emission due to electrons accelerated at low altitudes at the foot point of a region of quasi-steady dayside reconnection. Cluster data in the magnetotail indicate that the Poynting flux from the reconnection region during this substorm is large enough to account for the observed nightside aurora.

AZD1152 negatively affects the growth of anaplastic thyroid carcinoma cells and enhances the effects of oncolytic virus dl922-947

Novel therapeutic approaches are required for the treatment of anaplastic thyroid carcinoma (ATC), an incurable disease resistant to current available therapies. Aurora B is an important mitotic kinase involved in chromosome segregation and cytokinesis. It is overexpressed in many cancers including ATC and represents a potential target for chemotherapy. The effects of AZD1152, a specific Aurora B kinase inhibitor, have been evaluated against ATC, showing G(2)/M accumulation, polyploidy and subsequent cell death by mitotic catastrophe upon drug treatment. Only three administrations of AZD1152 significantly reduced the growth of ATC tumour xenogratfs. Oncolytic viruses in association with other forms of treatment have proven highly promising in preclinical and clinical reports. The oncolytic adenovirus dl922-947 is active against ATC cells, and we have evaluated the effects of the association between AZD1152 and dl922-947. In cells treated with virus and drug, we report additive/synergistic killing effects. Interestingly, the phosphorylation of histone H3 (Ser10), the main Aurora B substrate, is inhibited by dl922-947 in a dose-dependent manner, and completely abolished in association with AZD1152. The combined treatment significantly inhibited the growth of ATC tumour xenografts with respect to single treatments. Our data demonstrate that the Aurora B inhibitor AZD1152, alone or in combination with oncolytic virus dl922-947, could represent a novel therapeutic option for the treatment of ATC.

Plasma wave characteristics of the Jovian magnetopause boundary layer: Relationship to the Jovian aurora?

The Jovian magnetopause boundary layer (BL) plasma wave spectra from 10−3 to 102 Hz have been measured for the first time. For one intense event the magnetic (B′) and electric (E′) spectra were 2 × 10−4 ƒ2.4 nT2/Hz and 4 × 10−9 ƒ2.4 V2/m2 Hz, respectively. Although no measurable wave amplitudes were detected above the electron gyrofrequency, ∼140 Hz, this finding may be due to the low signal strength characteristic of this region. The B′/E′ ratio is relatively frequency independent. It is possible that waves are obliquely propagating whistler mode waves. The B′ and E′ spectra are broadband with no obvious spectral peaks. The waves are sufficiently intense to cause cross‐field diffusion of magnetosheath plasma to create the BL itself. A Jovian BL thickness of 10,700 km is predicted, which is consistent with past in situ measurements. The Jovian boundary layer wave properties are quite similar to the BL waves at Earth (however, the Jovian waves are orders of magnitude less intense). It appears that the solar wind/magnetosphere dynamos at the two planets are similar enough to be consistent with a common wave generation mechanism. The predicted ionospheric latitudinal width of the BL of ∼100–200 km is quite similar to the Jovian auroral high‐latitude ring measured by Hubble. The location of the BL at and inside the foot point of the last closed field line may place the boundary layer and the aurora on approximately the same magnetic field lines. The Jovian BL waves are sufficiently intense to cause strong pitch angle diffusion for <5‐keV electrons and 1‐keV to 1‐MeV protons. The estimated energy precipitation rate from this interaction <1 erg cm−2 s−1, sufficient for a weak high‐latitude auroral ring. This intensity is 2 to 3 orders of magnitude too low to cause the main aurora ring, however. If it is found that this main aurora maps into the boundary layer, then other mechanisms such as (ionospheric) double layers must be responsible for the particle energization and precipitation.