37 research outputs found
The effect of thresholding on temporal avalanche statistics
We discuss intermittent time series consisting of discrete bursts or
avalanches separated by waiting or silent times. The short time correlations
can be understood to follow from the properties of individual avalanches, while
longer time correlations often present in such signals reflect correlations
between triggerings of different avalanches. As one possible source of the
latter kind of correlations in experimental time series, we consider the effect
of a finite detection threshold, due to e.g. experimental noise that needs to
be removed. To this end, we study a simple toy model of an avalanche, a random
walk returning to the origin or a Brownian bridge, in the presence and absence
of superimposed delta-correlated noise. We discuss the properties after
thresholding of artificial timeseries obtained by mixing toy avalanches and
waiting times from a Poisson process. Most of the resulting scalings for
individual avalanches and the composite timeseries can be understood via random
walk theory, except for the waiting time distributions when strong additional
noise is added. Then, to compare with a more complicated case we study the
Manna sandpile model of self-organized criticality, where some further
complications appear.Comment: 15 pages, 12 figures, submitted to J. Stat. Mech., special issue of
the UPoN2008 conferenc
On the 1/f Spectrum in the Solar Wind and Its Connection with Magnetic Compressibility
We discuss properties of Alfv\'enic fluctuations with large amplitude in
plasmas characterised by low magnetic field compression. We note that in such
systems power laws can not develop with arbitrarily steep slopes at large
scales, i.e. when becomes of the order of the background
field . In such systems there is a scale at which the spectrum
has to break due to the condition of weak compressibility. A very good example
of this dynamics is offered by solar wind fluctuations in Alfv\'enic fast
streams, characterised by the property of constant field magnitude. We show
here that the distribution of in the fast wind
displays a strong cut-off at , as expected for
fluctuations bounded on a sphere of radius . This is also
associated with a saturation of the rms of the fluctuations at large scales and
introduces a specific length above which the amplitude of the
fluctuations becomes independent on the scale . Consistent with that, the
power spectrum at is characterised by a -1 spectral slope, as expected
for fluctuations that are scale-independent. Moreover, we show that the
spectral break between the 1/f and inertial range in solar wind spectra indeed
corresponds to the scale at which \left\sim1. Such a
simple model provides a possible alternative explanation of magnetic spectra
observed in interplanetary space, also pointing out the inconsistency for a
plasma to simultaneously maintain const. at arbitrarily large
scales and satisfy a Kolmogorov scaling.Comment: 6 pages, 5 figures, Accepted by The Astrophysical Journal Letter
Does Turbulence along the Coronal Current Sheet Drive Ion Cyclotron Waves?
Evidence for the presence of ion cyclotron waves (ICWs), driven by turbulence, at the boundaries of the current sheet is reported in this paper. By exploiting the full potential of the joint observations performed by Parker Solar Probe and the Metis coronagraph on board Solar Orbiter, local measurements of the solar wind can be linked with the large-scale structures of the solar corona. The results suggest that the dynamics of the current sheet layers generates turbulence, which in turn creates a sufficiently strong temperature anisotropy to make the solar-wind plasma unstable to anisotropy-driven instabilities such as the Alfvén ion cyclotron, mirror-mode, and firehose instabilities. The study of the polarization state of high-frequency magnetic fluctuations reveals that ICWs are indeed present along the current sheet, thus linking the magnetic topology of the remotely imaged coronal source regions with the wave bursts observed in situ. The present results may allow improvement of state-of-the-art models based on the ion cyclotron mechanism, providing new insights into the processes involved in coronal heating
The Solar Orbiter Science Activity Plan: translating solar and heliospheric physics questions into action
Solar 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 missionâs 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 orbitâs 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, we introduce Solar Orbiterâs SAP through a series of examples and the strategy being followed
The CAESAR project for the ASI space weather infrastructure
This paper presents the project Comprehensive spAce wEather Studies for the ASPIS prototype Realization (CAESAR), which aims to tackle the relevant aspects of Space Weather (SWE) science and develop a prototype of the scientific data centre for Space Weather of the Italian Space Agency (ASI) called ASPIS (ASI SPace Weather InfraStructure). To this end, CAESAR involves the majority of the SWE Italian community, bringing together 10 Italian institutions as partners, and a total of 92 researchers. The CAESAR approach encompasses the whole chain of phenomena from the Sun to Earth up to planetary environments in a multidisciplinary, comprehensive, and unprecedented way. Detailed and integrated studies are being performed on a number of well-observed âtarget SWE eventsâ, which exhibit noticeable SWE characteristics from several SWE perspectives. CAESAR investigations synergistically exploit a great variety of different products (datasets, codes, models), both long-standing and novel, that will be made available in the ASPIS prototype: this will consist of a relational database (DB), an interface, and a wiki-like documentation structure. The DB will be accessed through both a Web graphical interface and the ASPIS.py module, i.e., a library of functions in Python, which will be available for download and installation. The ASPIS prototype will unify multiple SWE resources through a flexible and adaptable architecture, and will integrate currently available international SWE assets to foster scientific studies and advance forecasting capabilities
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The Solar Orbiter magnetometer
The 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
Innovative technique for separating proton core, proton beam, and alpha particles in solar wind 3D velocity distribution functions
Context. The identification of proton core, proton beam, and alpha particles in solar wind ion measurements is usually performed by applying specific fitting procedures to the particle energy spectra. In many cases, this turns out to be a challenging task due to the overlapping of the curves.
Aims. We propose an alternative approach based on the statistical technique of clustering, a standard tool in many data-driven and machine learning applications.
Methods. We developed a procedure that adapts clustering to the analysis of solar wind distribution functions. We first tested the method on a synthetic data set and then applied it to a time series of solar wind data.
Results. The moments obtained for the different particle populations are in good agreement with the official data set and with the statistical studies available in the literature.
Conclusions. Our method is shown to be a very promising technique that can be combined with the traditional fitting algorithms in working out difficult cases that involve the identification of particle species in solar wind measurements