2,995 research outputs found
Scalable Persistent Storage for Erlang
The many core revolution makes scalability a key property. The RELEASE project aims to improve the scalability of Erlang on emergent commodity architectures with 100,000 cores. Such architectures require scalable and available persistent storage on up to 100 hosts. We enumerate the requirements for scalable and available persistent storage, and evaluate four popular Erlang DBMSs against these requirements. This analysis shows that Mnesia and CouchDB are not suitable persistent storage at our target scale, but Dynamo-like NoSQL DataBase Management Systems (DBMSs) such as Cassandra and Riak potentially are. We investigate the current scalability limits of the Riak 1.1.1 NoSQL DBMS in practice on a 100-node cluster. We establish for the first time scientifically the scalability limit of Riak as 60 nodes on the Kalkyl cluster, thereby confirming developer folklore. We show that resources like memory, disk, and network do not limit the scalability of Riak. By instrumenting Erlang/OTP and Riak libraries we identify a specific Riak functionality that limits scalability. We outline how later releases of Riak are refactored to eliminate the scalability bottlenecks. We conclude that Dynamo-style NoSQL DBMSs provide scalable and available persistent storage for Erlang in general, and for our RELEASE target architecture in particular
Cross and magnetic helicity in the outer heliosphere from Voyager 2 observations
Plasma velocity and magnetic field measurements from the Voyager 2 mission
are used to study solar wind turbulence in the slow solar wind at two different
heliocentric distances, 5 and 29 astronomical units, sufficiently far apart to
provide information on the radial evolution of this turbulence. The magnetic
helicity and the cross-helicity, which express the correlation between the
plasma velocity and the magnetic field, are used to characterize the
turbulence. Wave number spectra are computed by means of the Taylor hypothesis
applied to time resolved single point Voyager 2 measurements. The overall
picture we get is complex and difficult to interpret. A substantial decrease of
the cross-helicity at smaller scales (over 1-3 hours of observation) with
increasing heliocentric distance is observed. At 5 AU the only peak in the
probability density of the normalized residual energy is negative, near -0.5.
At 29 AU the probability density becomes doubly peaked, with a negative peak at
-0.5 and a smaller peak at a positive values of about 0.7. A decrease of the
cross-helicity for increasing heliocentric distance is observed, together with
a reduction of the unbalance toward the magnetic energy of the energy of the
fluctuations. For the smaller scales, we found that at 29 AU the normalized
polarization is small and positive on average (about 0.1), it is instead zero
at 5 AU. For the larger scales, the polarization is low and positive at 5 AU
(average around 0.1) while it is negative (around - 0.15) at 29 AU.Comment: 14 pages 5 figures. Accepted for publication on European Journal of
Mechanics B/Fluids (5/8/2015
The Whole Heliosphere Interval in the Context of a Long and Structured Solar Minimum: An Overview from Sun to Earth
Throughout months of extremely low solar activity during the recent extended solar-cycle minimum, structural evolution continued to be observed from the Sun through the solar wind and to the Earth. In 2008, the presence of long-lived and large low-latitude coronal holes meant that geospace was periodically impacted by high-speed streams, even though solar irradiance, activity, and interplanetary magnetic fields had reached levels as low as, or lower than, observed in past minima. This time period, which includes the first Whole Heliosphere Interval (WHI 1: Carrington Rotation (CR) 2068), illustrates the effects of fast solar-wind streams on the Earth in an otherwise quiet heliosphere. By the end of 2008, sunspots and solar irradiance had reached their lowest levels for this minimum (e.g., WHI 2: CR 2078), and continued solar magnetic-flux evolution had led to a flattening of the heliospheric current sheet and the decay of the low-latitude coronal holes and associated Earth-intersecting high-speed solar-wind streams. As the new solar cycle slowly began, solar-wind and geospace observables stayed low or continued to decline, reaching very low levels by June – July 2009. At this point (e.g., WHI 3: CR 2085) the Sun–Earth system, taken as a whole, was at its quietest. In this article we present an overview of observations that span the period 2008 – 2009, with highlighted discussion of CRs 2068, 2078, and 2085. We show side-by-side observables from the Sun’s interior through its surface and atmosphere, through the solar wind and heliosphere and to the Earth’s space environment and upper atmosphere, and reference detailed studies of these various regimes within this topical issue and elsewhere
Multi-scale harmonic model for solar and climate cyclical variation throughout the Holocene based on Jupiter-Saturn tidal frequencies plus the 11-year solar dynamo cycle
The sunspot record since 1749 is made of three major cycles (9.98, 10.9 and
11.86 yr). The side frequencies are related to the spring tidal period of
Jupiter and Saturn (9.93 yr) and to the tidal sidereal period of Jupiter (11.86
yr). A simplified harmonic constituent model based on the above two planetary
tidal frequencies and on the exact dates of Jupiter and Saturn planetary tidal
phases, plus a theoretically deduced 10.87-year central cycle reveals complex
quasi-periodic interference/beat patterns at about 115, 61 and 130 years, plus
a quasi-millennial large beat cycle around 983 years. We show that equivalent
synchronized cycles are found in cosmogenic records used to reconstruct solar
activity and in proxy climate records throughout the Holocene. The
quasi-secular beat oscillations hindcast reasonably well the known prolonged
periods of low solar activity during the last millennium known as Oort, Wolf,
Sporer, Maunder and Dalton minima, as well as 17 115-year long oscillations
found in temperature reconstructions during the last 2000 years. The millennial
three-frequency beat cycle hindcasts equivalent solar and climate cycles for
12,000 years. Prolonged solar minima in 1900-1920 and 1960-1980, the secular
solar maxima around 1870-1890, 1940-1950 and 1995-2005, and a secular upward
trending during the 20th century is recovered: this modulated trending agrees
well with some solar proxy model, with the ACRIM TSI satellite composite and
with the global surface temperature modulation since 1850. The model forecasts
a new prolonged solar grand minimum during 2020-2045, which would be produced
by the minima of both the 61 and 115-year reconstructed cycles. Solar and
climate oscillations are linked to planetary motion and, furthermore, their
timing can be reasonably hindcast and forecast for decades, centuries and
millennia. The critique by Smythe and Eddy (1977) is rebutted.Comment: Journal of Atmospheric and Solar-Terrestrial Physics (2012
The predictability of advection-dominated flux-transport solar dynamo models
Space weather is a matter of practical importance in our modern society.
Predictions of forecoming solar cycles mean amplitude and duration are
currently being made based on flux-transport numerical models of the solar
dynamo. Interested in the forecast horizon of such studies, we quantify the
predictability window of a representative, advection-dominated, flux-transport
dynamo model by investigating its sensitivity to initial conditions and control
parameters through a perturbation analysis. We measure the rate associated with
the exponential growth of an initial perturbation of the model trajectory,
which yields a characteristic time scale known as the e-folding time .
The e-folding time is shown to decrease with the strength of the
-effect, and to increase with the magnitude of the imposed meridional
circulation. Comparing the e-folding time with the solar cycle periodicity, we
obtain an average estimate for equal to 2.76 solar cycle durations.
From a practical point of view, the perturbations analysed in this work can be
interpreted as uncertainties affecting either the observations or the physical
model itself. After reviewing these, we discuss their implications for solar
cycle prediction.Comment: 33 pages, 12 figure
A Gyrochronology and Microvariability Survey of the Milky Way's Older Stars Using Kepler's Two-Wheels Program
Even with the diminished precision possible with only two reaction wheels,
the Kepler spacecraft can obtain mmag level, time-resolved photometry of tens
of thousands of sources. The presence of such a rich, large data set could be
transformative for stellar astronomy. In this white paper, we discuss how
rotation periods for a large ensemble of single and binary main- sequence
dwarfs can yield a quantitative understanding of the evolution of stellar
spin-down over time. This will allow us to calibrate rotation-based ages beyond
~1 Gyr, which is the oldest benchmark that exists today apart from the Sun.
Measurement of rotation periods of M dwarfs past the fully-convective boundary
will enable extension of gyrochronology to the end of the stellar
main-sequence, yielding precise ages ({\sigma} ~10%) for the vast majority of
nearby stars. It will also help set constraints on the angular momentum
evolution and magnetic field generation in these stars. Our Kepler-based study
would be supported by a suite of ongoing and future ground-based observations.
Finally, we briefly discuss two ancillary science cases, detection of
long-period low-mass eclipsing binaries and microvariability in white dwarfs
and hot subdwarf B stars that the Kepler Two-Wheels Program would facilitate.Comment: Kepler white pape
Extreme Longitudinal Variability of Plasma Structuring in the Equatorial Ionosphere on a Magnetically Quiet Equinoctial Day
We investigate the extreme longitudinal variability of equatorial scintillation under quiet magnetic conditions during 22–23 March 2002. Scintillation Network Decision Aid (SCINDA) observations show intense activity in the South American–Atlantic sector during local evening hours, whereas an absence of scintillation is seen in the far east Asian sector. Ground- and space-based measurements from SCINDA, the Global Ultraviolet Imager (GUVI), TOPEX, and a chain of GPS receivers are used in combination with the Utah State University Global Assimilation of Ionospheric Measurements (USU-GAIM) model to explore the relationship between the large-scale ionization distribution and small-scale irregularities at low latitudes in both the scintillating and nonscintillating longitude sectors. Our analysis shows that there are significant differences in the evolution of the ionization distributions during the evening hours, which are likely the result of differences in the daytime and postsunset vertical plasma drift in the two sectors. This study demonstrates the importance of USU-GAIM as a new tool for investigating longitudinal as well as day-to-day variability that is observed in the large-scale distribution of the ionosphere and how this relates to the occurrence of scintillation
Theoretical limits on magnetic field strengths in low-mass stars
Observations have suggested that some low-mass stars have larger radii than
predicted by 1-D structure models. Some theoretical models have invoked very
strong interior magnetic fields (of order 1 MG or more) as a possible cause of
such large radii. Whether fields of that strength could in principle by
generated by dynamo action in these objects is unclear, and we do not address
the matter directly. Instead, we examine whether such fields could remain in
the interior of a low mass object for a significant time, and whether they
would have any other obvious signatures. First, we estimate timescales for the
loss of strong fields by magnetic buoyancy instabilities. We consider a range
of field strengths and simple morphologies, including both idealized flux tubes
and smooth layers of field. We confirm some of our analytical estimates using
thin flux tube magnetohydrodynamic (MHD) simulations of the rise of buoyant
fields in a fully-convective M-dwarf. Separately, we consider the Ohmic
dissipation of such fields. We find that dissipation provides a complementary
constraint to buoyancy: while small-scale, fibril fields might be regenerated
faster than they rise, the dissipative heating associated with such fields
would in some cases greatly exceed the luminosity of the star. We show how
these constraints combine to yield limits on the internal field strength and
morphology in low-mass stars. In particular, we find that for stars of 0.3
solar masses, no fields in flux tubes stronger than about 800 kG are
simultaneously consistent with both constraints.Comment: 19 pages, 10 figures, accepted to Ap
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