| 61. |
- Carozzi, TD, et al.
(författare)
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Full polarimetry measurements of stimulated electromagnetic emissions: First results
- 2001
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 106:A10, s. 21395-21407
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Tidskriftsartikel (refereegranskat)abstract
- A theory and technique for the measurement of the full state of polarization (SOP) for wideband electromagnetic HF emissions from the ionosphere are introduced. The technique was employed in a recent experiment at the Sura ionospheric HF pumping facility
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| 62. |
- Chan, Phoebe, et al.
(författare)
-
Freshening of the Alaska Coastal Current recorded by coralline algal Ba/Ca ratios
- 2011
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Ingår i: Journal of Geophysical Research-Biogeosciences. - 0148-0227. ; 116
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Tidskriftsartikel (refereegranskat)abstract
- Arctic Ocean freshening can exert a controlling influence on global climate, triggering strong feedbacks on ocean-atmospheric processes and affecting the global cycling of the world's oceans. Glacier-fed ocean currents such as the Alaska Coastal Current are important sources of freshwater for the Bering Sea shelf, and may also influence the Arctic Ocean freshwater budget. Instrumental data indicate a multiyear freshening episode of the Alaska Coastal Current in the early 21st century. It is uncertain whether this freshening is part of natural multidecadal climate variability or a unique feature of anthropogenically induced warming. In order to answer this, a better understanding of past variations in the Alaska Coastal Current is needed. However, continuous long-term high-resolution observations of the Alaska Coastal Current have only been available for the last 2 decades. In this study, specimens of the long-lived crustose coralline alga Clathromorphum nereostratum were collected within the pathway of the Alaska Coastal Current and utilized as archives of past temperature and salinity. Results indicate that coralline algal Mg/Ca ratios provide a 60 year record of sea surface temperatures and track changes of the Pacific Decadal Oscillation, a pattern of decadal-to-multidecadal ocean-atmosphere climate variability centered over the North Pacific. Algal Ba/Ca ratios (used as indicators of coastal freshwater runoff) are inversely correlated to instrumentally measured Alaska Coastal Current salinity and record the period of freshening from 2001 to 2006. Similar multiyear freshening events are not evident in the earlier portion of the 60 year Ba/Ca record. This suggests that the 21st century freshening of the Alaska Coastal Current is a unique feature related to increasing glacial melt and precipitation on mainland Alaska.
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| 63. |
- Chaufray, J. Y., et al.
(författare)
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Mars solar wind interaction : Formation of the Martian corona and atmospheric loss to space
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:E9, s. E09009-
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Forskningsöversikt (refereegranskat)abstract
- A three- dimensional ( 3- D) atomic oxygen corona of Mars is computed for periods of low and high solar activities. The thermal atomic oxygen corona is derived from a collisionless Chamberlain approach, whereas the nonthermal atomic oxygen corona is derived from Monte Carlo simulations. The two main sources of hot exospheric oxygen atoms at Mars are the dissociative recombination of O-2(+) between 120 and 300 km and the sputtering of the Martian atmosphere by incident O+ pickup ions. The reimpacting and escaping fluxes of pickup ions are derived from a 3- D hybrid model describing the interaction of the solar wind with our computed Martian oxygen exosphere. In this work it is shown that the role of the sputtering crucially depends on an accurate description of the Martian corona as well as of its interaction with the solar wind. The sputtering contribution to the total oxygen escape is smaller by one order of magnitude than the contribution due to the dissociative recombination. The neutral escape is dominant at both solar activities ( 1 x 10(25) s(-1) for low solar activity and 4 x 10(25) s(-1) for high solar activity), and the ion escape flux is estimated to be equal to 2 x 10(23) s(-1) at low solar activity and to 3.4 x 10(24) s(-1) at high solar activity. This work illustrates one more time the strong dependency of these loss rates on solar conditions. It underlines the difficulty of extrapolating the present measured loss rates to the past solar conditions without a better theoretical and observational knowledge of this dependency.
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| 64. |
- Chen, Deliang, 1961, et al.
(författare)
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Recent Recovery of the Siberian High Intensity
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116
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Tidskriftsartikel (refereegranskat)abstract
- This study highlights the fast recovery of the wintertime Siberian High intensity (SHI) over the last two decades. The SHI showed a marked weakening trend from the 1970s to 1980s, leading to unprecedented low SHI in the early 1990s according to most observational data sets. This salient declining SHI trend, however, was sharply replaced by a fast recovery over the last two decades. Since the declining SHI trend has been considered as one of the plausible consequences of climate warming, the recent SHI recovery seemingly contradicts the continuous progression of climate warming in the Northern Hemisphere. We suggest that alleviated surface warming and decreased atmospheric stability in the central Siberia region, associated with an increase in Eurasian snow cover, in the recent two decades contributed to this rather unexpected SHI recovery. The prominent SHI change, however, is not reproduced by general circulation model (GCM) simulations used in the IPCC AR4. The GCMs indicate the steady weakening of the SHI for the entire 21st century, which is found to be associated with a decreasing Eurasian snow cover in the simulations. An improvement in predicting the future climate change in regional scale is desirable.
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| 65. |
- Chen, Li-Jen, et al.
(författare)
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Evidence of an extended electron current sheet and its neighboring magnetic island during magnetotail reconnection
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A12, s. A12213-
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Tidskriftsartikel (refereegranskat)abstract
- We have identified a spatially extended electron current sheet (ECS) and its adjacent magnetic island during a magnetotail reconnection event with no appreciable guide field. This finding is based on data from the four Cluster spacecraft and is enabled by detailed maps of electron distribution functions and DC electric fields within the diffusion region. The maps are developed using two-dimensional particle-in-cell simulations with a mass ratio m(i)/m(e) = 800. One spacecraft crossed the ECS earthward of the reconnection null and, together with the other three spacecraft, registered the following properties: (1) The ECS is colocated with a layer of bipolar electric fields normal to the ECS, pointing toward the ECS, and with a half width less than 8 electron skin depths. (2) In the inflow region up to the ECS and separatrices, electrons have a temperature anisotropy (Te-parallel to/Te-perpendicular to > 1), and the anisotropy increases toward the ECS. (3) Within about 1 ion skin depth (d(i)) above and below the ECS, the electron density decreases toward the ECS by a factor of 3-4, reaching a minimum at edges of the ECS, and has a local distinct maximum at the ECS center. (4) A di-scale magnetic island is attached to the ECS, separating it from another reconnection layer. Our simulations established that the electric field normal to the ECS is due to charge imbalance and is of the ECS scale, and ions exhibit electron-scale structures in response to this electric field.
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| 66. |
- Chiacchio, Marc, et al.
(författare)
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On the links between meteorological variables, aerosols, and tropical cyclone frequency in individual ocean basins
- 2017
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 122:2, s. 802-822
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Tidskriftsartikel (refereegranskat)abstract
- A generalized linear model based on Poisson regression has been used to assess the impact of environmental variables modulating tropical cyclone frequency in six main cyclone development areas: the East Pacific, West Pacific, North Atlantic, North Indian, South Indian, and South Pacific. The analysis covers the period 1980-2009 and focuses on widely used meteorological parameters including wind shear, sea surface temperature, and relative humidity from different reanalyses as well as aerosol optical depth for different compounds simulated by the Goddard Chemistry Aerosol Radiation and Transport model. Circulation indices are also included. Cyclone frequency is obtained from the International Best Track Archive for Climate Stewardship. A strong link is found between cyclone frequency and the relative sea surface temperature, Atlantic Meridional Mode, and wind shear with significant explained log likelihoods in the North Atlantic of 37%, 27%, and 28%, respectively. A significant impact of black carbon and organic aerosols on cyclone frequency is found over the North Indian Ocean, with explained log likelihoods of 27%. A weaker but still significant impact is found for observed dust aerosols in the North Atlantic with an explained log likelihood of 11%. Changes in lower stratospheric temperatures explain 28% of the log likelihood in the North Atlantic. Lower stratospheric temperatures from a subset of Coupled Model Intercomparison Project Phase 5 models properly simulate the warming and subsequent cooling of the lower stratosphere that follows a volcanic eruption but underestimates the cooling by about 0.5 degrees C.
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| 67. |
- Collier, A. B., et al.
(författare)
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Correlation between global lightning and whistlers observed at Tihany, Hungary
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114, s. A07210-
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Tidskriftsartikel (refereegranskat)abstract
- Although the generation and propagation mechanisms for whistlers are fairly well understood, the location and extent of the lightning source region for the whistlers observed at a given station are currently unknown. The correlation of whistler observations against global lightning data allows an estimate of the size and position of the source region. For whistlers detected at Tihany, Hungary, an area of positive correlation with radius of similar to 1000 km was found to be centered on the conjugate point. Although the maximal sample correlation coefficient was relatively low, r = 0.065, it has a high statistical significance, indicating that it is extremely improbable that the whistlers and lightning in this region are actually uncorrelated. Other smaller areas of positive correlation were found further afield in South America and the Maritime Continent. Lightning in the northern hemisphere displayed a negative correlation with whistlers at Tihany.
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| 68. |
- Cossu, R., et al.
(författare)
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Influence of the Coriolis force on the velocity structure of gravity currents in straight submarine channel systems
- 2010
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Ingår i: JOURNAL OF GEOPHYSICAL RESEARCH. - 0148-0227. ; 115
-
Tidskriftsartikel (refereegranskat)abstract
- Large-scale turbidity currents in submarine channels often show a significant asymmetry in the heights of their levee banks. In the Northern Hemisphere, there are many observations of the right-hand channel levee being noticeably higher than the left-hand levee, a phenomenon that is usually attributed to the effect of Coriolis forces upon turbidity currents. This article presents results from an analog model that documents the influence of Coriolis forces on the dynamics of gravity currents flowing in straight submarine channels. The observations of the transverse velocity structure, downstream velocity, and interface slope show good agreement with a theory that incorporates Ekman boundary layer dynamics. Coriolis forces will be important for most large-scale turbidity currents and need to be explicitly modeled when the Rossby number of these flows (defined as Ro = ∣U/Wf∣, where U is the mean downstream velocity, W is the channel width, and f is the Coriolis parameter defined as f = 2Ω sin(), with Ω being the Earth's rotation rate and being the latitude) is less than order 1. When Ro 1, the flow is substantially slower than a nonrotating flow with the same density contrast. The secondary flow field consists of frictionally induced Ekman transports across the channel in the benthic and interfacial boundary layers and a return flow in the interior. The cross-channel velocities are of the order of 10% of the along-channel velocities. The sediment transport associated with such transverse flow patterns should influence the evolution of submarine channel levee systems.
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| 69. |
- Cravens, T. E., et al.
(författare)
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Dynamical and magnetic field time constants for Titan's ionosphere : Empirical estimates and comparisons with Venus
- 2010
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 115:8, s. A08319-
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Tidskriftsartikel (refereegranskat)abstract
- Plasma in Titan's ionosphere flows in response to forcing from thermal pressure gradients, magnetic forces, gravity, and ion-neutral collisions. This paper takes an empirical approach to the ionospheric dynamics by using data from Cassini instruments to estimate pressures, flow speeds, and time constants on the dayside and nightside. The plasma flow speed relative to the neutral gas speed is approximately 1 m s(-1) near an altitude of 1000 km and 200 m s(-1) at 1500 km. For comparison, the thermospheric neutral wind speed is about 100 m s(-1). The ionospheric plasma is strongly coupled to the neutrals below an altitude of about 1300 km. Transport, vertical or horizontal, becomes more important than chemistry in controlling ionospheric densities above about 1200-1500 km, depending on the ion species. Empirical estimates are used to demonstrate that the structure of the ionospheric magnetic field is determined by plasma transport (including neutral wind effects) for altitudes above about 1000 km and by magnetic diffusion at lower altitudes. The paper suggests that a velocity shear layer near 1300 km could exist at some locations and could affect the structure of the magnetic field. Both Hall and polarization electric field terms in the magnetic induction equation are shown to be locally important in controlling the structure of Titan's ionospheric magnetic field. Comparisons are made between the ionospheric dynamics at Titan and at Venus.
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| 70. |
- Cui, J., et al.
(författare)
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Diurnal variations of Titan's ionosphere
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114:6, s. A06310-
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Tidskriftsartikel (refereegranskat)abstract
- We present our analysis of the diurnal variations of Titan's ionosphere (between 1000 and 1300 km) based on a sample of Ion Neutral Mass Spectrometer (INMS) measurements in the Open Source Ion (OSI) mode obtained from eight close encounters of the Cassini spacecraft with Titan. Although there is an overall ion depletion well beyond the terminator, the ion content on Titan's nightside is still appreciable, with a density plateau of similar to 700 cm(-3) below similar to 1300 km. Such a plateau is a combined result of significant depletion of light ions and modest depletion of heavy ones on Titan's nightside. We propose that the distinctions between the diurnal variations of light and heavy ions are associated with their different chemical loss pathways, with the former primarily through "fast'' ion-neutral chemistry and the latter through "slow'' electron dissociative recombination. The strong correlation between the observed night-to-day ion density ratios and the associated ion lifetimes suggests a scenario in which the ions created on Titan's dayside may survive well to the nightside. The observed asymmetry between the dawn and dusk ion density profiles also supports such an interpretation. We construct a time-dependent ion chemistry model to investigate the effect of ion survival associated with solid body rotation alone as well as superrotating horizontal winds. For long-lived ions, the predicted diurnal variations have similar general characteristics to those observed. However, for short-lived ions, the model densities on the nightside are significantly lower than the observed values. This implies that electron precipitation from Saturn's magnetosphere may be an additional and important contributor to the densities of the short-lived ions observed on Titan's nightside.
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