Relationship between Solar Wind-Magnetosphere Energy and Eurasian Winter Cold Events

The profound impact of solar irradiance variations on the decadal variability of Earth' s climate has been investigated by previous studies.However,it remains a challenge to quantify the energetic particle precipitation(EPP) influence on the surface climate,which is an emerging research topic.The solar wind is a source of magnetospheric EPP,and the total energy input from the solar wind into Earth' s magnetosphere(E_(in)) shows remarkable interdecadal and interannual variability.B ased on the new E_(in) index,this study reveals a significant interannual relationship between the annual mean E_(in)and Eurasian cold extremes in the subsequent winter.Less frequent cold events are observed over Eurasia(primarily north of 50°N) following the higher-than-normal E_(in) activity in the previous year,accompanied by more frequent cold events over northern Africa,and vice versa.This response pattern shows great resemblance to the first empirical orthogonal function of the variability of cold extremes over Eurasia,with a spatial correlation coefficient of 0.79.The pronounced intensification of the positive North Atlantic Oscillation events and poleward shift of the North Atlantic storm track associated with the anomalously higher E_(in) favor the anomalous extreme atmospheric circulation events,and thus less frequent extreme cold temperatures over northern Eurasia on the interannual time scale.It is further hypothesized that the wave-mean flow interaction in the stratosphere and troposphere is favorable for the connection of E_(in) signals to tropospheric circulation and climate in the following winter.     

The fate of ejecta from Hyperion

Ejecta from Saturn's moon Hyperion are subject to powerful perturbations from nearby Titan, which control their ultimate fate. We have performed numerical integrations to simulate a simplified system consisting of Saturn (including optical flattening as well as dynamical oblateness), its main ring system (treated as a massless flat annulus), the moons Tethys, Dione, Titan, Hyperion, and Iapetus, and the Sun (treated simply as a massive satellite). At several different points in Hyperion's orbit, 1050 massless particles, more or less evenly distributed over latitude and longitude, were ejected radially outward from 1 km above Hyperion's mean radius at speeds 10% faster than escape speed from Hyperion. Most of these particles were removed within the first few thousand years, but ∼3% of them survived the entire 100,000-year duration of the simulations. Ejecta from Hyperion are much more widely scattered than previously thought, and can cross the orbits of all of Saturn's satellites. About 9% of all the particles escaped from the saturnian system, but Titan accreted ∼78% of the total, while Hyperion reaccreted only ∼5%. This low efficiency of reaccretion may help to account for Hyperion's small size and rugged shape. Only ∼1% of all the particles hit other satellites, and another ∼1% impacted Saturn itself, while ∼3% of them struck its main rings. The high proportion of impacts into Saturn's rings is surprising; these collisions show a broad decline in impact speed with time, suggesting that Hyperion ejecta gradually spread inwards. Additional simulations were used to investigate the dependence of ejecta evolution on launch speed, the mass of Hyperion, and the presence of the Sun. In general, the wide distribution of ejecta from Hyperion suggests that it does contribute to “Population II” craters on the inner satellites of Saturn. Ejecta which escape from a satellite into temporary orbit about its planet, but later reimpact into the same moon or another one produce “poltorary” impacts, intermediate in character between primary and secondary impacts. It may be possible to distinguish poltorary craters from primary and secondary craters on the basis of morphology.     

The action of the satellite 1981 S13, and the optical depth profile of the Encke-gap ringlet in the Saturn A ring

The radial optical depth profile of the Encke ringlet obtained by the occultation experiment of the Voyager photopolarimeter (PPS data) is explained to be caused by the gravitational action of the recently discovered stallite 1981 S13 and a second smaller moonlet orbiting near one of its either libration points — L₄ or L₅. To this aim the results of previous and new numerical particle simulations as well as an extension of the scattering theory concerning a single moonlet to a pair of satellites have been used leading to a triple-peaked ringlet near the orbits of the moonlets. The width and the shape of that ringlet and its separate peaks depend on the mass ratio of both moonlets and on their orbital eccentricites. The best resemblance between the PPS data and the theoretical profile is obtained if the mass ratio of the either moonlets takes M₂/M_1981S13 ≈ (0.8 … 3.0) × 10⁻² and the eccentricities hold: e_1981S13 < h_1981S13; e₂ ≈ h₂ (the values h are the Hill scales of either moonlets defined by h_1981S13/2 ≈ (M_1981S13/2/3M_h)^1/3, M_h = Saturn mass). Furthermore, our results yield a size of 1981 S13 of < 15 km in diameter.     

Moonlet Collisions and the Effects of Tidally Modified Accretion in Saturn's F Ring

We both test and offer an alternative to a meteoroid bombardment model (M. R. Showalter 1998, Science282, 1099-1102) and suggest that anomalous localized brightenings in the F ring observed by Voyager result from disruptive collisions involving poorly consolidated moonlets, or “rubble piles.” This model can also explain the transient events observed during ring plane crossing. We have developed an evolutionary model that considers both the competing effects of accretion and disruption at the location of the F ring. Our numerical model is a Markov process where probabilities of mass transfer between the states of the system form a “transition matrix.” Successive multiplications of this matrix by the state vector generate expectation values of the distribution after each time step as the system approaches quasi-equilibrium. Competing effects of accretion and disruption in the F ring are found to lead to a bimodal distribution of ring particle sizes. In fact, our simulation predicts the presence of a belt of kilometer-sized moonlets in the F ring. These moonlets may continually disrupt one another and re-accrete on short time scales. We also agree with J. N. Cuzzi and J. A. Burns (1988, Icarus74, 284-324), who suggest that the classical F ring itself may be the consequence of a relatively recent collision between two of the largest of these yet unseen objects. Cassini observations can confirm the existence of the moonlet belt by directly observing these objects or the waves they create in the rings.     

A multilayer model for thermal infrared emission of Saturn’s rings. III: Thermal inertia inferred from Cassini CIRS

The thermal inertia values of Saturn’s main rings (the A, B, and C rings and the Cassini division) are derived by applying our thermal model to azimuthally scanned spectra taken by the Cassini Composite Infrared Spectrometer (CIRS). Model fits show the thermal inertia of ring particles to be 16, 13, 20, and 11 J m⁻² K⁻¹ s^−1/2 for the A, B, and C rings, and the Cassini division, respectively. However, there are systematic deviations between modeled and observed temperatures in Saturn’s shadow depending on solar phase angle, and these deviations indicate that the apparent thermal inertia increases with solar phase angle. This dependence is likely to be explained if large slowly spinning particles have lower thermal inertia values than those for small fast spinning particles because the thermal emission of slow rotators is relatively stronger than that of fast rotators at low phase and vise versa. Additional parameter fits, which assume that slow and fast rotators have different thermal inertia values, show the derived thermal inertia values of slow (fast) rotators to be 8 (77), 8 (27), 9 (34), 5 (55) J m⁻² K⁻¹ s^−1/2 for the A, B, and C rings, and the Cassini division, respectively. The values for fast rotators are still much smaller than those for solid ice with no porosity. Thus, fast rotators are likely to have surface regolith layers, but these may not be as fluffy as those for slow rotators, probably because the capability of holding regolith particles is limited for fast rotators due to the strong centrifugal force on surfaces of fast rotators. Other additional parameter fits, in which radii of fast rotators are varied, indicate that particles less than ∼1 cm should not occupy more than roughly a half of the cross section for the A, B, and C rings.     

基于Van Allen Probes EMFISIS波动仪器观测的内磁层下频带哨声合声波全球分布的统计分析

基于Van Allen Probes近三年的EMFISIS仪器波动观测数据,对内磁层下频带哨声模合声波幅度的全球分布特性对地磁活动水平的依赖性进行了详细的统计分析,着重研究下频带合声波平均场强幅度随磁壳值L、磁地方时、地磁纬度的分布特征及不同强度区间的合声波的发生概率.结果表明,下频带合声波的波动强度与地磁活动密切正相关,处于强磁扰期间的合声波具有更大的振幅,其发生率与地磁活动强度具有同样的正相关特性.下频带合声波主要发生于午夜至下午的磁地方时区间,其余的磁地方时时段下频带合声波较弱.赤道面附近的下频带合声波主要分布在夜侧至黎明这一时段内,随着磁纬度的增加逐步向日侧扩展.下频带合声波在午夜侧（21-03 MLT）主要出现在15°的磁纬范围内,在晨侧（03-09 MLT）可以到达15°磁纬甚至更高纬度.下频带合声波主要发生于L=~4.5的附近区域.随着地磁活动的增加,下频带合声波所覆盖的L-shell空间区域增大,趋势为向高、低L值区域同时扩展.建立的下频带哨声合声波的全球分布模型将有助于进一步深入理解该重要磁层波动对辐射带电子的波粒作用散射效应和对辐射带动力学过程的定量贡献.

     

Characteristics of Saturn’s FUV airglow from limb-viewing spectra obtained with Cassini-UVIS

This study reports the analysis of far ultraviolet (FUV) limb spectra of the airglow of Saturn in the 1150-1850 Å spectral window, obtained with the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini, spanning altitudes from −1200 to 4000 km. The FUV limb emission consists of three main contributions: (1) H Ly-α peaking at 1100 km with a brightness of 0.8 kilo-Rayleighs (kR), (2) reflected sunlight longward of 1550 Å which maximizes at −950 km with 16.5 kR and (3) H₂ bands in the 1150-1650 Å bandwidth, peaking at 1050 km reaching a maximum of 3.9 kR.A vertical profile of the local H₂ volume emission rate has been derived using the hydrocarbon density profiles from a model of the Saturn equatorial atmosphere. It is well matched by a Chapman function, characterized by a maximum value of 3.5 photons cm⁻³ s⁻¹ in the 800-1650 Å UV bandwidth, peaking at 1020 km.Comparisons between the observed spectra and a first-order synthetic airglow H₂ model in the 1150-1650 Å bandwidth show that the spectral shape of the H₂ bands is accounted for by solar fluorescence and photoelectron excitation. The best fits are obtained with a combination of H₂ fluorescence lines and 20 eV electron impact spectra, the latter contributing ∼68% of the total H₂ airglow emission.     

Heavy ion formation in Titan's ionosphere: Magnetospheric introduction of free oxygen and a source of Titan's aerosols?

Discovery by Cassini's plasma instrument of heavy positive and negative ions within Titan's upper atmosphere and ionosphere has advanced our understanding of ion neutral chemistry within Titan's upper atmosphere, primarily composed of molecular nitrogen, with ~2.5% methane. The external energy flux transforms Titan's upper atmosphere and ionosphere into a medium rich in complex hydrocarbons, nitriles and haze particles extending from the surface to 1200 km altitudes. The energy sources are solar UV, solar X-rays, Saturn's magnetospheric ions and electrons, solar wind and shocked magnetosheath ions and electrons, galactic cosmic rays (GCR) and the ablation of incident meteoritic dust from Enceladus’ E-ring and interplanetary medium. Here it is proposed that the heavy atmospheric ions detected in situ by Cassini for heights >950 km, are the likely seed particles for aerosols detected by the Huygens probe for altitudes <100 km. These seed particles may be in the form of polycyclic aromatic hydrocarbons (PAH) containing both carbon and hydrogen atoms C_nH_x. There could also be hollow shells of carbon atoms, such as C₆₀, called fullerenes which contain no hydrogen. The fullerenes may compose a significant fraction of the seed particles with PAHs contributing the rest. As shown by Cassini, the upper atmosphere is bombarded by magnetospheric plasma composed of protons, H₂⁺ and water group ions. The latter provide keV oxygen, hydroxyl and water ions to Titan's upper atmosphere and can become trapped within the fullerene molecules and ions. Pickup keV N₂⁺, N⁺ and CH₄⁺ can also be implanted inside of fullerenes. Attachment of oxygen ions to PAH molecules is uncertain, but following thermalization O+ can interact with abundant CH₄ contributing to the CO and CO₂ observed in Titan's atmosphere. If an exogenic keV O+ ion is implanted into the haze particles, it could become free oxygen within those aerosols that eventually fall onto Titan's surface. The process of freeing oxygen within aerosols could be driven by cosmic ray interactions with aerosols at all heights. This process could drive pre-biotic chemistry within the descending aerosols. Cosmic ray interactions with grains at the surface, including water frost depositing on grains from cryovolcanism, would further add to abundance of trapped free oxygen. Pre-biotic chemistry could arise within surface microcosms of the composite organic-ice grains, in part driven by free oxygen in the presence of organics and any heat sources, thereby raising the astrobiological potential for microscopic equivalents of Darwin's “warm ponds” on Titan.     

Negative ions in the Enceladus plume

During Cassini’s Enceladus encounter on 12th March 2008, the Cassini Electron Spectrometer, part of the CAPS instrument, detected fluxes of negative ions in the plumes from Enceladus. It is thought that these ions include negatively charged water group cluster ions associated with the plume and forming part of the ‘plume ionosphere’. In this paper we present our observations, argue that these are negative ions, and present preliminary mass identifications. We also suggest mechanisms for production and loss of the ions as constrained by the observations. Due to their short lifetime, we suggest that the ions are produced in or near the water vapour plume, or from the extended source of ice grains in the plume. We suggest that Enceladus now joins the Earth, Comet Halley and Titan as locations in the Solar System where negative ions have been directly observed although the ions observed in each case have distinctly different characteristics.