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Journal articleManners HA, Masters A, 2020,
The global distribution of ultra-low-frequency waves in Jupiter's magnetosphere
, Journal of Geophysical Research, Vol: 125, ISSN: 0148-0227Jupiter's giant magnetosphere is a complex system seldom in a configuration approximating steady state, and a clear picture of its governing dynamics remains elusive. Crucial to understanding how the magnetosphere behaves on a large scale are disturbances to the system on length‐scales comparable to the cavity, which are communicated by magnetohydrodynamic waves in the ultra‐low‐frequency band (<1 mHz). In this study we used magnetometer data from multiple spacecraft to perform the first global heritage survey of these waves in the magnetosphere. To map the equatorial region, we relied on the large local‐time coverage provided by the Galileo spacecraft. Flyby encounters performed by Voyager 1 & 2, Pioneer 10 & 11 and Ulysses provided local‐time coverage of the dawn sector. We found several hundred events where significant wave power was present, with periods spanning ~5‐60 min. The majority of events consisted of multiple superposed discrete periods. Periods at ~15, ~30 and ~40 min dominated the event‐averaged spectrum, consistent with the spectra of quasi‐periodic pulsations often reported in the literature. Most events were clustered in the outer magnetosphere close to the magnetopause at noon and dusk, suggesting that an external driving mechanism may dominate. The most energetic events occurred close to the planet, though more sporadically, indicating an accumulation of wave energy in the inner magnetosphere or infrequent impulsive drivers in the region. Our findings suggest that dynamics of the system at large scales is modulated by this diverse population of waves, which permeate the magnetosphere through several cavities and waveguides.
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Journal articleMitchell JG, de Nolfo GA, Hill ME, et al., 2020,
Small Electron Events Observed by Parker Solar Probe/ISIS during Encounter 2
, ASTROPHYSICAL JOURNAL, Vol: 902, ISSN: 0004-637X- Author Web Link
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- Citations: 8
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Journal articleHorbury TS, OBrien H, Carrasco Blazquez I, et al., 2020,
The Solar Orbiter magnetometer
, Astronomy & Astrophysics, Vol: 642, Pages: A9-A9, ISSN: 0004-6361The magnetometer instrument on the Solar Orbiter mission is designed to measure the magnetic field local to the spacecraft continuously for the entire mission duration. The need to characterise not only the background magnetic field but also its variations on scales from far above to well below the proton gyroscale result in challenging requirements on stability, precision, and noise, as well as magnetic and operational limitations on both the spacecraft and other instruments. The challenging vibration and thermal environment has led to significant development of the mechanical sensor design. The overall instrument design, performance, data products, and operational strategy are described.
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Journal articleZouganelis I, 2020,
The Solar Orbiter Science Activity Plan: translating solar and heliospheric physics questions into action
, Astronomy & Astrophysics, Vol: 642, Pages: 1-19, ISSN: 0004-6361Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate?; (2) How do solar transients drive heliospheric variability?; (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere?; (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission’s science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit’s science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans, resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime. This allows for all four mission goals to be addressed. In this paper, w
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Journal articleMüller D, Cyr OCS, Zouganelis I, et al., 2020,
The solar orbiter mission. science overview
, Astronomy & Astrophysics, Vol: 642, Pages: 1-31, ISSN: 0004-6361Aims. Solar Orbiter, the first mission of ESA’s Cosmic Vision 2015–2025 programme and a mission of international collaboration between ESA and NASA, will explore the Sun and heliosphere from close up and out of the ecliptic plane. It was launched on 10 February 2020 04:03 UTC from Cape Canaveral and aims to address key questions of solar and heliospheric physics pertaining to how the Sun creates and controls the Heliosphere, and why solar activity changes with time. To answer these, the mission carries six remote-sensing instruments to observe the Sun and the solar corona, and four in-situ instruments to measure the solar wind, energetic particles, and electromagnetic fields. In this paper, we describe the science objectives of the mission, and how these will be addressed by the joint observations of the instruments onboard.Methods. The paper first summarises the mission-level science objectives, followed by an overview of the spacecraft and payload. We report the observables and performance figures of each instrument, as well as the trajectory design. This is followed by a summary of the science operations concept. The paper concludes with a more detailed description of the science objectives.Results. Solar Orbiter will combine in-situ measurements in the heliosphere with high-resolution remote-sensing observations of the Sun to address fundamental questions of solar and heliospheric physics. The performance of the Solar Orbiter payload meets the requirements derived from the mission’s science objectives. Its science return will be augmented further by coordinated observations with other space missions and ground-based observatories.
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Journal articleBeth A, Altwegg K, Balsiger H, et al., 2020,
ROSINA ion zoo at Comet 67P
, Astronomy and Astrophysics: a European journal, Vol: 642, Pages: 1-23, ISSN: 0004-6361Context. The Rosetta spacecraft escorted Comet 67P/Churyumov-Gerasimenko for 2 yr along its journey through the Solar System between 3.8 and 1.24 au. Thanks to the high resolution mass spectrometer on board Rosetta, the detailed ion composition within a coma has been accurately assessed in situ for the very first time.Aims. Previous cometary missions, such as Giotto, did not have the instrumental capabilities to identify the exact nature of the plasma in a coma because the mass resolution of the spectrometers onboard was too low to separate ion species with similar masses. In contrast, the Double Focusing Mass Spectrometer (DFMS), part of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis on board Rosetta (ROSINA), with its high mass resolution mode, outperformed all of them, revealing the diversity of cometary ions.Methods. We calibrated and analysed the set of spectra acquired by DFMS in ion mode from October 2014 to April 2016. In particular, we focused on the range from 13–39 u q−1. The high mass resolution of DFMS allows for accurate identifications of ions with quasi-similar masses, separating 13C+ from CH+, for instance.Results. We confirm the presence in situ of predicted cations at comets, such as CHm+ (m = 1−4), HnO+ (n = 1−3), O+, Na+, and several ionised and protonated molecules. Prior to Rosetta, only a fraction of them had been confirmed from Earth-based observations. In addition, we report for the first time the unambiguous presence of a molecular dication in the gas envelope of a Solar System body, namely CO2++.
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Journal articlePapini E, Cicone A, Piersanti M, et al., 2020,
Multidimensional Iterative Filtering: a new approach for investigating plasma turbulence in numerical simulations
, JOURNAL OF PLASMA PHYSICS, Vol: 86, ISSN: 0022-3778 -
Journal articleAlberti T, Laurenza M, Consolini G, et al., 2020,
On the Scaling Properties of Magnetic-field Fluctuations through the Inner Heliosphere
, ASTROPHYSICAL JOURNAL, Vol: 902, ISSN: 0004-637X- Author Web Link
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- Citations: 25
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Journal articleOwen CJ, Bruno R, Livi S, et al., 2020,
The Solar Orbiter Solar Wind Analyser (SWA) suite
, ASTRONOMY & ASTROPHYSICS, Vol: 642, ISSN: 0004-6361- Author Web Link
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- Citations: 113
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Journal articleMaksimovic M, Bale SD, Chust T, et al., 2020,
The Solar Orbiter Radio and Plasma Waves (RPW) instrument
, ASTRONOMY & ASTROPHYSICS, Vol: 642, ISSN: 0004-6361- Author Web Link
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- Citations: 64
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Journal articleRouillard AP, Pinto RF, Vourlidas A, et al., 2020,
Models and data analysis tools for the Solar Orbiter mission
, ASTRONOMY & ASTROPHYSICS, Vol: 642, ISSN: 0004-6361- Author Web Link
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- Citations: 42
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Journal articleWalsh AP, Horbury TS, Maksimovic M, et al., 2020,
Coordination of the in situ payload of Solar Orbiter
, Astronomy and Astrophysics: a European journal, Vol: 642, Pages: 1-7, ISSN: 0004-6361Solar Orbiter’s in situ coordination working group met frequently during the development of the mission with the goal of ensuring that its in situ payload has the necessary level of coordination to maximise science return. Here we present the results of that work, namely how the design of each of the in situ instruments (EPD, MAG, RPW, SWA) was guided by the need for coordination, the importance of time synchronisation, and how science operations will be conducted in a coordinated way. We discuss the mechanisms by which instrument sampling schemes are aligned such that complementary measurements will be made simultaneously by different instruments, and how burst modes are scheduled to allow a maximum overlap of burst intervals between the four instruments (telemetry constraints mean different instruments can spend different amounts of time in burst mode). We also explain how onboard autonomy, inter-instrument communication, and selective data downlink will be used to maximise the number of transient events that will be studied using high-resolution modes of all the instruments. Finally, we briefly address coordination between Solar Orbiter’s in situ payload and other missions.
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Journal articleRodriguez-Pacheco J, Wimmer-Schweingruber RF, Mason GM, et al., 2020,
The energetic particle detector: energetic particle instrument suite for the Solar Orbiter mission
, Astronomy and Astrophysics: a European journal, Vol: 642, Pages: 1-35, ISSN: 0004-6361After decades of observations of solar energetic particles from space-based observatories, relevant questions on particle injection, transport, and acceleration remain open. To address these scientific topics, accurate measurements of the particle properties in the inner heliosphere are needed. In this paper we describe the Energetic Particle Detector (EPD), an instrument suite that is part of the scientific payload aboard the Solar Orbiter mission. Solar Orbiter will approach the Sun as close as 0.28 au and will provide extra-ecliptic measurements beyond ∼30° heliographic latitude during the later stages of the mission. The EPD will measure electrons, protons, and heavy ions with high temporal resolution over a wide energy range, from suprathermal energies up to several hundreds of megaelectronvolts/nucleons. For this purpose, EPD is composed of four units: the SupraThermal Electrons and Protons (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) plus the Instrument Control Unit that serves as power and data interface with the spacecraft. The low-energy population of electrons and ions will be covered by STEP and EPT, while the high-energy range will be measured by HET. Elemental and isotopic ion composition measurements will be performed by SIS and HET, allowing full particle identification from a few kiloelectronvolts up to several hundreds of megaelectronvolts/nucleons. Angular information will be provided by the separate look directions from different sensor heads, on the ecliptic plane along the Parker spiral magnetic field both forward and backwards, and out of the ecliptic plane observing both northern and southern hemispheres. The unparalleled observations of EPD will provide key insights into long-open and crucial questions about the processes that govern energetic particles in the inner heliosphere.
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Journal articleVelli M, Harra LK, Vourlidas A, et al., 2020,
Understanding the origins of the heliosphere: integrating observations and measurements from Parker Solar Probe, Solar Orbiter, and other space- and ground-based observatories
, Astronomy and Astrophysics: a European journal, Vol: 642, Pages: 1-13, ISSN: 0004-6361Context. The launch of Parker Solar Probe (PSP) in 2018, followed by Solar Orbiter (SO) in February 2020, has opened a new window in the exploration of solar magnetic activity and the origin of the heliosphere. These missions, together with other space observatories dedicated to solar observations, such as the Solar Dynamics Observatory, Hinode, IRIS, STEREO, and SOHO, with complementary in situ observations from WIND and ACE, and ground based multi-wavelength observations including the DKIST observatory that has just seen first light, promise to revolutionize our understanding of the solar atmosphere and of solar activity, from the generation and emergence of the Sun’s magnetic field to the creation of the solar wind and the acceleration of solar energetic particles.Aims. Here we describe the scientific objectives of the PSP and SO missions, and highlight the potential for discovery arising from synergistic observations. Here we put particular emphasis on how the combined remote sensing and in situ observations of SO, that bracket the outer coronal and inner heliospheric observations by PSP, may provide a reconstruction of the solar wind and magnetic field expansion from the Sun out to beyond the orbit of Mercury in the first phases of the mission. In the later, out-of-ecliptic portions of the SO mission, the solar surface magnetic field measurements from SO and the multi-point white-light observations from both PSP and SO will shed light on the dynamic, intermittent solar wind escaping from helmet streamers, pseudo-streamers, and the confined coronal plasma, and on solar energetic particle transport.Methods. Joint measurements during PSP–SO alignments, and magnetic connections along the same flux tube complemented by alignments with Earth, dual PSP–Earth, and SO-Earth, as well as with STEREO-A, SOHO, and BepiColumbo will allow a better understanding of the in situ evolution of solar-wind plasma flows and the full three-dimensional distribution of
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Journal articleDesai R, Zhang H, Davies E, et al., 2020,
Three dimensional simulations of solar wind preconditioning and the 23 July 2012 Interplanetary Coronal Mass Ejection
, Solar Physics: a journal for solar and solar-stellar research and the study of solar terrestrial physics, Vol: 295, Pages: 1-14, ISSN: 0038-0938Predicting the large-scale eruptions from the solar corona and theirpropagation through interplanetary space remains an outstanding challenge in solar- and helio-physics research. In this article, we describe three dimensional magnetohydrodynamic simulations of the inner heliosphere leading up to and including the extreme interplanetary coronal mass ejection (ICME) of 23 July 2012, developed using the code PLUTO. The simulations are driven using the output of coronal models for Carrington rotations 2125 and 2126 and, given the uncertainties in the initial conditions, are able to reproduce an event of comparable magnitude to the 23 July ICME, with similar velocity and densityprofi les at 1 au. The launch-time of this event is then varied with regards to an initial 19 July ICME and the effects of solar wind preconditioning are found to be signi ficant for an event of this magnitude and to decrease over a time-window consistent with the ballistic re filling of the depleted heliospheric sector. These results indicate that the 23 July ICME was mostly unaffected by events prior, but would have travelled even faster had it erupted closer in time to the 19 July event where it would have experienced even lower drag forces. We discuss this systematic study of solar wind preconditioning in the context of space weatherforecasting.
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Journal articleSaito M, Yang P, Huang X, et al., 2020,
Spaceborne mid‐ and far‐infrared observations improving nighttime ice cloud property retrievals
, Geophysical Research Letters, Vol: 47, Pages: 1-10, ISSN: 0094-8276Two upcoming missions are scheduled to provide novel spaceborne observations of upwelling far‐infrared spectra. In this study, the accuracy of ice cloud property retrievals using spaceborne middle‐to‐far‐infrared (MIR‐FIR) measurements is examined toward a better understanding of retrieval biases and uncertainties. Theoretical sensitivity studies demonstrate that the MIR‐FIR spectra are sensitive to ice cloud properties, thereby providing a robust means for retrieving cloud properties under nighttime conditions. However, the temperature dependence of the ice refractive index and relevant ice particle shape models need to be incorporated into the retrieval procedure to avoid systematic biases in inferring cloud optical thickness and effective particle radius. Furthermore, prior information of subpixel cloud fractions is essential to mitigation of substantial systematic retrieval biases due to inconsistent subpixel cloud fractions.
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Journal articleMihailescu AT, Desai R, Shebanits O, et al., 2020,
Spatial variations of low mass negative ions in Titan's upper atmosphere
, The Planetary Science Journal, Vol: 1, Pages: 1-8, ISSN: 2632-3338Observations with Cassini’s Electron Spectrometer discovered negative ions in Titan’s ionosphere,at altitudes between 1400 and 950 km. Within the broad mass distribution extending up to severalt housand amu, two distinct peaks were identified at 25.8-26.0 and 49.0-50.1 amu/q, corresponding to the carbon chain anions CN−and/orC2H−for the first peak and C3N−and/orC4H−for the second peak. In this study we present the spatial distribution of these low mass negative ions from 28 Titanflybys with favourable observations between 26 October 2004 and 22 May 2012. We report a trend of lower densities on the night side and increased densities up to twice as high on the day side at small solar zenith angles. To further understand this trend, we compare the negative ion densities to the total electron density measured by Cassini’s Langmuir Probe. We find the low mass negative ion density and the electron density to be proportional to each other on the dayside, but independent of each other on the night side. This indicates photochemical processes and is in agreement with the primary production route for the low mass negative ions being initiated by dissociative reactions with suprathermal electron populations produced by photoionisation. We also find the ratio ofCN−/C2H−toC3N−/C4H−highly constrained on the day-side, in agreement with this production channel, but notably displays large variations on the nightside.
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Journal articleLavergne A, Sandoval D, Hare VJ, et al., 2020,
Impacts of soil water stress on the acclimated stomatal limitation of photosynthesis: insights from stable carbon isotope data.
, Global Change Biology, Vol: 26, Pages: 7158-7172, ISSN: 1354-1013Atmospheric aridity and drought both influence physiological function in plant leaves, but their relative contributions to changes in the ratio of leaf-internal to ambient partial pressure of CO2 (χ) - an index of adjustments in both stomatal conductance and photosynthetic rate to environmental conditions - are difficult to disentangle. Many stomatal models predicting χ include the influence of only one of these drivers. In particular, the least-cost optimality hypothesis considers the effect of atmospheric demand for water on χ but does not predict how soils with reduced water further influence χ, potentially leading to an overestimation of χ under dry conditions. Here we use a large network of stable carbon isotope measurements in C3 woody plants to examine the acclimated response of χ to soil water stress. We estimate the ratio of cost factors for carboxylation and transpiration (β) expected from the theory to explain the variance in the data, and investigate the responses of β (and thus χ) to soil water content and suction across seed plant groups, leaf phenological types and regions. Overall, β decreases linearly with soil drying, implying that the cost of water transport along the soil-plant-atmosphere continuum increases as water available in the soil decreases. However, despite contrasting hydraulic strategies, the stomatal responses of angiosperms and gymnosperms to soil water tend to converge, consistent with the optimality theory. The prediction of β as a simple, empirical function of soil water significantly improves χ predictions by up to 6.3 ± 2.3% (mean ± sd of adjusted-R2 ) over 1980-2018 and results in a reduction of around 2% of mean χ values across the globe. Our results highlight the importance of soil water status on stomatal functions and plant water-use efficiency, and suggest the implementation of trait-based hydraulic functions into the model to account for soil water stre
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Conference paperSouthwood D, Cao H, Hunt G, et al., 2020,
Discovery of Alfven waves planetward of the Rings of Saturn
, Europlanet Science Congress 2020, Publisher: American Geophysical UnionBetween April and September 2017 in the final stages of the Cassini Saturn Orbiter mission the spacecraft executed 22 orbits passing planetward of the innermost ring, the D-ring. During periapsis passes on all these orbits oscillations were detected in the azimuthal magnetic field components on typical time scales from a few minutes to 10 minutes. We argue that the time-varying signals detected on the spacecraft are also primarily time-varying in the plasma frame. Nonetheless, we show that nearly all signals exhibit a distinct spatial effect, namely a magnetic node near the effective field line equator. The oscillations thus have a standing structure along the background magnetic field and it follows that they are field line resonances associated with Alfvén waves. The form of the signals suggests that the local field line resonances are most likely pumped from global sources. This is the first detection in a giant planet magnetosphere of a phenomenon known to be important at Earth.
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Journal articleGaland M, Feldman PD, Bockelee-Morvan D, et al., 2020,
Far-ultraviolet aurora identified at comet 67P/ Churyumov-Gerasimenko
, Nature Astronomy, Vol: 4, Pages: 1084-1091, ISSN: 2397-3366Having a nucleus darker than charcoal, comets are usually detected from Earth through the emissions from their coma. The coma is an envelope of gas that forms through the sublimation of ices from the nucleus as the comet gets closer to the Sun. In the far-ultraviolet portion of the spectrum, observations of comae have revealed the presence of atomic hydrogen and oxygen emissions. When observed over large spatial scales as seen from Earth, such emissions are dominated by resonance fluorescence pumped by solar radiation. Here, we analyse atomic emissions acquired close to the cometary nucleus by the Rosetta spacecraft and reveal their auroral nature. To identify their origin, we undertake a quantitative multi-instrument analysis of these emissions by combining coincident neutral gas, electron and far-ultraviolet observations. We establish that the atomic emissions detected from Rosetta around comet 67P/Churyumov-Gerasimenko at large heliocentric distances result from the dissociative excitation of cometary molecules by accelerated solar-wind electrons (and not by electrons produced from photo-ionization of cometary molecules). Like the discrete aurorae at Earth and Mars, this cometary aurora is driven by the interaction of the solar wind with the local environment. We also highlight how the oxygen line O I at wavelength 1,356 Å could be used as a tracer of solar-wind electron variability.
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Journal articleKilpua EKJ, Fontaine D, Good SW, et al., 2020,
Magnetic field fluctuation properties of coronal mass ejection-driven sheath regions in the near-Earth solar wind
, ANNALES GEOPHYSICAE, Vol: 38, Pages: 999-1017, ISSN: 0992-7689- Author Web Link
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- Citations: 16
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Journal articleGreaves J, Richards A, Bains W, et al., 2020,
Phosphine gas in the cloud decks of Venus
, Nature Astronomy, Vol: 5, Pages: 655-664, ISSN: 2397-3366Measurements of trace gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbour, Venus, has cloud decks that are temperate but hyperacidic. Here we report the apparent presence of phosphine (PH3) gas in Venus’s atmosphere, where any phosphorus should be in oxidized forms. Single-line millimetre-waveband spectral detections (quality up to ~15σ) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH3 at ~20 ppb abundance is inferred. The presence of PH3 is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life. Other PH3 spectral features should be sought, while in situ cloud and surface sampling could examine sources of this gas.
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Journal articleWilliams RG, Ceppi P, Katavouta A, 2020,
Controls of the transient climate response to emissions by physical feedbacks, heat uptake and carbon cycling
, Environmental Research Letters, Vol: 15, ISSN: 1748-9326The surface warming response to carbon emissions is diagnosed using a suite of Earth system models, 9 CMIP6 and 7 CMIP5, following an annual 1\% rise in atmospheric CO$_2$ over 140 years. This surface warming response defines a climate metric, the Transient Climate Response to cumulative carbon Emissions (TCRE), which is important in estimating how much carbon may be emitted to avoid dangerous climate. The processes controlling these intermodel differences in the TCRE are revealed by defining the TCRE in terms of a product of three dependences: the surface warming dependence on radiative forcing (including the effects of physical climate feedbacks and planetary heat uptake), the radiative forcing dependence on changes in atmospheric carbon and the airborne fraction. Intermodel differences in the TCRE are mainly controlled by the thermal response involving the surface warming dependence on radiative forcing, which arise through large differences in physical climate feedbacks that are only partly compensated by smaller differences in ocean heat uptake. The other contributions to the TCRE from the radiative forcing and carbon responses are of comparable importance to the contribution from the thermal response on timescales of 50 years and longer for our subset of CMIP5 models and 100 years and longer for our subset of CMIP6 models. Hence, providing tighter constraints on how much carbon may be emitted based on the TCRE requires providing tighter bounds for estimates of the physical climate feedbacks, particularly from clouds, as well as to a lesser extent for the other contributions from the rate of ocean heat uptake, and the terrestrial and ocean cycling of carbon.
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Journal articleZhang YC, Dai L, Rong ZJ, et al., 2020,
Observation of the large‐amplitude and fast‐damped plasma sheet flapping triggered by reconnection‐induced ballooning instability
, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-11, ISSN: 2169-9380In this study, we reported the large‐amplitude and fast‐damped flapping of the plasma sheet, which co‐occurred with magnetic reconnection. Data from the Double Star TC‐1 and Cluster satellites were used to analyze the features of the plasma sheet flapping 1.4 RE earthward of an ongoing magnetic reconnection event. The flapping was rapidly damped, and its amplitude decreased from the magnetohydrodynamics scale to the subion scale in 5 min. The variation in the flapping period (from 224 to 20 s) indicated that the source of the flapping had highly dynamic temporal characteristics. The plasma sheet flapping propagated duskward through a kink‐like wave with a velocity of 100 km/s, which was in agreement with the group velocity of the ballooning perturbation. A correlation analysis between the magnetic reconnection and plasma sheet flapping indicated that the magnetic reconnection likely facilitated the occurrence of ballooning instability by altering the state of plasma in the downstream plasma sheet. In this regard, the reconnection‐induced ballooning instability could be a potential mechanism to generate the flapping motion of the plasma sheet.
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Journal articleBrown Z, Koskinen T, Muller-Wodarg I, et al., 2020,
A pole-to-pole pressure-temperature map of Saturn's thermosphere from Cassini Grand Finale data
, Nature Astronomy, Vol: 4, Pages: 872-879, ISSN: 2397-3366Temperatures of the outer planet thermospheres exceed those predicted by solar heating alone by several hundred degrees. Enough energy is deposited at auroral regions to heat the entire thermosphere, but models predict that equatorward distribution is inhibited by strong Coriolis forces and ion drag1,2. A better understanding of auroral energy deposition and circulation are critical to solving this so-called energy crisis. Stellar occultations observed by the Ultraviolet Imaging Spectrograph instrument during the Cassini Grand Finale were designed to map the thermosphere from pole to pole. We analyse these observations, together with earlier observations from 2016 and 2017, to create a two-dimensional map of densities and temperatures in Saturn’s thermosphere as a function of latitude and depth. The observed temperatures at auroral latitudes are cooler and peak at higher altitudes and lower latitudes than predicted by models, leading to a shallower meridional pressure gradient. Under modified geostrophy3, we infer slower westward zonal winds that extend to lower latitudes than predicted, supporting equatorward flow from approximately 70° to 30° latitude in both hemispheres. We also show evidence of atmospheric waves in the data that can contribute to equatorward redistribution of energy through zonal drag.
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Journal articleChakravorty S, Perez RC, Anderson BT, et al., 2020,
Testing the Trade Wind Charging Mechanism and Its Influence on ENSO Variability
, Journal of Climate, Vol: 33, Pages: 7391-7411, ISSN: 0894-8755<jats:title>Abstract</jats:title><jats:p>During the positive phase of the North Pacific Oscillation, westerly wind anomalies over the subtropical North Pacific substantially increase subsurface heat content along the equator by “trade wind charging” (TWC). TWC provides a direct pathway between extratropical atmospheric circulation and El Niño–Southern Oscillation (ENSO) initiation. Previous model studies of this mechanism lacked the ocean–atmospheric coupling needed for ENSO growth, so it is crucial to examine whether TWC-induced heat content anomalies develop into ENSO events in a coupled model. Here, coupled model experiments, forced with TWC favorable (+TWC) or unfavorable (−TWC) wind stress, are used to examine the ENSO response to TWC. The forcing is imposed on the ocean component of the model through the first winter and then the model evolves in a fully coupled configuration through the following winter. The +TWC (−TWC) forcing consistently charges (discharges) the equatorial Pacific in spring and generates positive (negative) subsurface temperature anomalies. These subsurface temperature anomalies advect eastward and upward along the equatorial thermocline and emerge as like-signed sea surface temperature (SST) anomalies in the eastern Pacific, creating favorable conditions upon which coupled air–sea feedback can act. During the fully coupled stage, warm SST anomalies in +TWC forced simulations are amplified by coupled feedbacks and lead to El Niño events. However, while −TWC forcing results in cool SST anomalies, pre-existing warm SST anomalies in the far eastern equatorial Pacific persist and induce local westerly wind anomalies that prevent consistent development of La Niña conditions. While the TWC mechanism provides adequate equatorial heat content to fuel ENSO development, other factors also play a role in determining whether an ENSO event develops.</jats:p>
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Journal articleBradley TJ, Cowley SWH, Bunce EJ, et al., 2020,
Saturn's Nightside Dynamics During Cassini's F Ring and Proximal Orbits: Response to Solar Wind and Planetary Period Oscillation Modulations
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380- Author Web Link
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- Citations: 12
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Journal articleMozer FS, Agapitov OV, Bale SD, et al., 2020,
DC and Low-Frequency Electric Field Measurements on the Parker Solar Probe
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380- Author Web Link
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- Citations: 21
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Journal articleLotekar A, Vasko IY, Mozer FS, et al., 2020,
Multisatellite MMS Analysis of Electron Holes in the Earth's Magnetotail: Origin, Properties, Velocity Gap, and Transverse Instability
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380- Author Web Link
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- Citations: 28
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Journal articleAla-Lahti M, Ruohotie J, Good S, et al., 2020,
Spatial Coherence of Interplanetary Coronal Mass Ejection Sheaths at 1 AU
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380- Author Web Link
- Cite
- Citations: 9
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