1207 research outputs found
Sort by
STIX X-ray microflare observations during the Solar Orbiter commissioning phase
Get PDFContext. The Spectrometer/Telescope for Imaging X-rays (STIX) is the hard X-ray instrument onboard Solar Orbiter designed to observe solar flares over a broad range of flare sizes.
Aims. We report the first STIX observations of solar microflares recorded during the instrument commissioning phase in order to investigate the STIX performance at its detection limit.
Methods. STIX uses hard X-ray imaging spectroscopy in the range between 4–150 keV to diagnose the hottest flare plasma and related nonthermal electrons. This first result paper focuses on the temporal and spectral evolution of STIX microflares occuring in the Active Region (AR) AR12765 in June 2020, and compares the STIX measurements with Earth-orbiting observatories such as the X-ray Sensor of the Geostationary Operational Environmental Satellite (GOES/XRS), the Atmospheric Imaging Assembly of the Solar Dynamics Observatory, and the X-ray Telescope of the Hinode mission.
Results. For the observed microflares of the GOES A and B class, the STIX peak time at lowest energies is located in the impulsive phase of the flares, well before the GOES peak time. Such a behavior can either be explained by the higher sensitivity of STIX to higher temperatures compared to GOES, or due to the existence of a nonthermal component reaching down to low energies. The interpretation is inconclusive due to limited counting statistics for all but the largest flare in our sample. For this largest flare, the low-energy peak time is clearly due to thermal emission, and the nonthermal component seen at higher energies occurs even earlier. This suggests that the classic thermal explanation might also be favored for the majority of the smaller flares. In combination with EUV and soft X-ray observations, STIX corroborates earlier findings that an isothermal assumption is of limited validity. Future diagnostic efforts should focus on multi-wavelength studies to derive differential emission measure distributions over a wide range of temperatures to accurately describe the energetics of solar flares.
Conclusions. Commissioning observations confirm that STIX is working as designed. As a rule of thumb, STIX detects flares as small as the GOES A class. For flares above the GOES B class, detailed spectral and imaging analyses can be performed
Anelasticity and Lateral Heterogeneities in Earth's Upper Mantle: Impact on Surface Displacements, Self‐Attraction and Loading, and Ocean Tide Dynamics
Get PDFSurface displacement and self-attraction and loading (SAL) elevation induced by ocean tides are known to be affected by material properties of the solid Earth. Recent studies have shown that, in addition to elasticity, anelasticity considerably impacts surface displacements due to ocean tide loading (OTL). We employ consistent 3D seismic elastic and attenuation tomography models to construct 3D elastic and anelastic earth models, and derive corresponding averaged 1D elastic/anelastic models. We apply these models to systematically study the impact of anelasticity and lateral heterogeneity on M2 OTL displacements and SAL elevation. We find that neglecting lateral heterogeneities highly underestimates displacements and SAL elevation in mid-ocean-ridge regions and in some coastal areas of North and Central America. In comparison to PREM, 3D anelastic models can increase the predicted amplitudes of the vertical displacement and SAL elevation by up to 1.5 mm. The increased amplitudes reduce the discrepancy between GPS-observed OTL displacements and their predictions based on PREM in places like Cornwall (England), Brittany (France), and the Ryukyu Islands (Japan). Applying our results to ocean tides, we discover that the impact on ocean tide dynamics exceeds the predicted SAL elevation correction with an RMS of about 1 mm, reaching an RMS of more than 5 mm in areas like North Atlantic or East Pacific. Due to the fact that such a value reaches the accuracy of modern data-constrained tidal models, we regard the impact of anelastic shear relaxation as significant in tidal modeling
New seismic noise interferometry methods for imaging crustal and mantle reflectivity and monitoring seismic velocity changes
Get PDFThe nature of the southern West African craton lithosphere inferred from its electrical resistivity
No full textThe West-African craton is defined by a combination of Archean and Palaeoproterozoic rocks that stabilised at ~2 Ga towards the end of the Paleoproterozoic Eburnean Orogeny, and therefore may reflect the transition from Archean to modern tectonic processes. Exploring its present lithospheric architecture aids further understanding of not only the craton’s stability through its history but also its formation. We investigate the lithospheric structure of the craton through analysing and modelling magnetotelluric (MT) data from a 500-km-long east-west profile in northern Ghana and southern Burkina Faso crossing part of the Baoulé-Mossi Domain and reaching the Volta Basin in the south-eastern part of the craton. Although the MT stations are along a 2D profile, due to the complexity of the structures characterising the area, 3D resistivity modelling of the data is performed to obtain insights on the thermal signature and composition of the subcontinental lithosphere beneath the area. The thermal structure and water content estimates from different resistivity models highlight a strong dependence on the starting model in the 3D inversions, but still enable us to put constraints on the deep structure of the craton. The present‐day thermal lithosphere‐asthenosphere boundary (LAB) depth is estimated to be at least 250 km beneath the Baoulé-Mossi domain. The area likely transitions from a cold and thick lithosphere with relatively low water content into thinner, more fertile lithosphere below the Volta Basin. Although the inferred amount of water could be explained by Paleoproterozoic subduction processes involved in the formation of the Baoulé-Mossi domain, later enrichment of the lithosphere cannot be excluded
Optimal resolution tomography with error tracking and the structure of the crust and upper mantle beneath Ireland and Britain
Get PDFThe classical Backus–Gilbert method seeks localized Earth-structure averages at the shortest
length scales possible, given a data set, data errors, and a threshold for acceptable model
errors. The resolving length at a point is the width of the local averaging kernel, and the
optimal averaging kernel is the narrowest one such that the model error is below a specified
level. This approach is well suited for seismic tomography, which maps 3-D Earth structure
using large sets of seismic measurements. The continual measurement-error decreases and
data-redundancy increases have reduced the impact of random errors on tomographic models.
Systematic errors, however, are resistant to data redundancy and their effect on the model is
difficult to predict. Here, we develop a method for finding the optimal resolving length at every
point, implementing it for surface-wave tomography. As in the Backus–Gilbert method, every
solution at a point results from an entire-system inversion, and the model error is reduced by
increasing the model-parameter averaging. The key advantage of our method stems from its
direct, empirical evaluation of the posterior model error at a point. We first measure inter-
station phase velocities at simultaneously recording station pairs and compute phase-velocity
maps at densely, logarithmically spaced periods. Numerous versions of the maps with varying
smoothness are then computed, ranging from very rough to very smooth. Phase-velocity curves
extracted from the maps at every point can be inverted for shear-velocity (V S ) profiles. As
we show, errors in these phase-velocity curves increase nearly monotonically with the map
roughness. We evaluate the error by isolating the roughness of the phase-velocity curve that
cannot be explained by any Earth structure and determine the optimal resolving length at a point
such that the error of the local phase-velocity curve is below a threshold. A 3-D V S model is then
computed by the inversion of the composite phase-velocity maps with an optimal resolution
at every point. The estimated optimal resolution shows smooth lateral variations, confirming
the robustness of the procedure. Importantly, the optimal resolving length does not scale with
the density of the data coverage: some of the best-sampled locations display relatively low
lateral resolution, probably due to systematic errors in the data. We apply the method to image
the lithosphere and underlying mantle beneath Ireland and Britain. Our very large data set
was created using new data from Ireland Array, the Irish National Seismic Network, the UK
Seismograph Network and other deployments. A total of 11 238 inter-station dispersion curves,
spanning a very broad total period range (4–500 s), yield unprecedented data coverage of the
area and provide fine regional resolution from the crust to the deep asthenosphere. The lateral
resolution of the 3-D model is computed explicitly and varies from 39 km in central Ireland to
over 800 km at the edges of the area, where the data coverage declines. Our tomography reveals
pronounced, previously unknown variations in the lithospheric thickness beneath Ireland and Britain, with implications for their Caledonian assembly and for the mechanisms of the British
Tertiary Igneous Province magmatism
High-frequency seismic interferometry: broadband measurements of surface-wave phase velocities using “large-N” arrays
Get PDFHigh-frequency seismic surface waves sample the top few tens of meters to the top few kilometres
of the subsurface. They can be used to determine three-dimensional distributions of shear-wave
velocities and to map the depths of discontinuities (interfaces) within the crust. Passive seismic
imaging, using ambient noise as the source of signal, can thus be an effective tool of exploration
for mineral, geothermal and other resources, provided that sufficient high-frequency signal is
available in the ambient noise wavefield and that accurate, high-frequency measurements can be
performed on this signal. Ambient noise imaging using the ocean-generated noise at 5-30 s
periods is now a standard method, but less signal is available at frequencies high enough for
deposit-scale imaging (0.2-30 Hz), and few studies have reported successful measurements in
broad frequency bands. Here, we develop a workflow for the measurement of high-frequency,
surface-wave phase velocities in very broad frequency ranges. Our workflow comprises (1) a new
noise cross-correlation procedure that accounts for the non-stationary properties of the high
frequency noise sources, removes bandpass filtering, replaces temporal normalization with short
time window stacking, and drops the explicit spectral normalization by adopting cross-coherence;
(2) a new phase-velocity measurement method that extends the bandwidth of reliable
measurements by exploiting the (resolved) 2π ambiguity of phase-velocity measurements; (3)
interstation-distance-dependent quality control that uses the similarity of subgroups of dispersion
curves to reject outliers and identify the frequency ranges with accurate measurements. The
workflow is highly automated and applicable to large arrays. Applying our method to data from a
large-N array that operated for one month near Marathon, Ontario, Canada, we use rectangular
subarrays with 150-m station spacing and, typically, 1 hour of data and obtain Rayleigh-wave
phase-velocity measurements in a 0.55-23.8 Hz frequency range, spanning over 5.4 octaves, nearly
twice the typical frequency range of 1.5-3 octaves in previous studies. Phase-velocity maps and the
subregion-average 1D velocity models they constrain show a high-velocity anomaly consistent with
the known, west-dipping gabbro intrusions beneath the area. The new structural information can
improve our understanding of the geometry of the gabbro intrusions, hosting the Cu-PGE
Marathon deposit
LOFAR imaging of the solar corona during the 2015 March 20 solar eclipse
Get PDFThe solar corona is a highly-structured plasma which can reach temperatures of more than ∼2 MK. At low frequencies (decimetric and metric wavelengths), scattering and refraction of electromagnetic waves are thought to considerably increase the imaged radio source sizes (up to a few arcminutes). However, exactly how source size relates to scattering due to turbulence is still subject to investigation. The theoretical predictions relating source broadening to propagation effects have not been fully confirmed by observations due to the rarity of high spatial resolution observations of the solar corona at low frequencies. Here, the LOw Frequency ARray (LOFAR) was used to observe the solar corona at 120−180 MHz using baselines of up to ∼3.5 km (corresponding to a resolution of ∼1−2′) during the partial solar eclipse of 2015 March 20. A lunar de-occultation technique was used to achieve higher spatial resolution (∼0.6′) than that attainable via standard interferometric imaging (∼2.4′). This provides a means of studying the contribution of scattering to apparent source size broadening. It was found that the de-occultation technique reveals a more structured quiet corona that is not resolved from standard imaging, implying scattering may be overestimated in this region when using standard imaging techniques. However, an active region source was measured to be ∼4′ using both de-occultation and standard imaging. This may be explained by the increased scattering of radio waves by turbulent density fluctuations in active regions, which is more severe than in the quiet Sun
Observing Jupiter's radio emissions using multiple LOFAR stations: a first case study of the Io-decametric emission using the Irish IE613, French FR606 and German DE604 stations
Get PDFThe Low Frequency Array (LOFAR) is an international radio telescope array, consisting of 38 stations in the Netherlands and 14 international stations spread over Europe. Here we present an observation method to study the jovian decametric radio emissions from several LOFAR stations (here DE604, FR606 and IE613), at high temporal and spectral resolution. This method is based on prediction tools, such as radio emission simulations and probability maps, and data processing. We report an observation of Io-induced decametric emission from June 2021, and a first case study of the substructures that compose the macroscopic emissions (called millisecond bursts). The study of these bursts make it possible to determine the electron populations at the origin of these emissions. We then present several possible future avenues for study based on these observations. The methodology and study perspectives described in this paper can be applied to new observations of jovian radio emissions induced by Io, but also by Ganymede or Europa, or jovian auroral radio emissions
Method to observe Jupiter’s radio emissions at high resolution using multiple LOFAR stations: a first case study of the Io-decametric emission using the Irish IE613, French FR606 and German DE604 stations
Get PDFThe Low Frequency Array (LOFAR) is an international radio telescope array, consisting of 38 stations in the Netherlands and 14 international stations spread over Europe. Here we present an observation method to study the jovian decametric radio emissions from several LOFAR stations (here DE604, FR606 and IE613), at high temporal and spectral resolution. This method is based on prediction tools, such as radio emission simulations and probability maps, and data processing. We report an observation of Io-induced decametric emission from June 2021, and a first case study of the substructures that compose the macroscopic emissions (called millisecond bursts). The study of these bursts make it possible to determine the electron populations at the origin of these emissions. We then present several possible future avenues for study based on these observations. The methodology and study perspectives described in this paper can be applied to new observations of jovian radio emissions induced by Io, but also by Ganymede or Europa, or jovian auroral radio emissions