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J.‐p. Ampuero - One of the best experts on this subject based on the ideXlab platform.

  • On the statistical significance of Foreshock sequences in Southern California
    Geophysical Research Letters, 2020
    Co-Authors: Martijn P.a. Van Den Ende, J.‐p. Ampuero
    Abstract:

    Earthquake Foreshocks may provide information that is critical to short‐term earthquake forecasting. However, Foreshocks are far from ubiquitously observed, which makes the interpretation of ongoing seismic sequences problematic. Based on a statistical analysis, Trugman & Ross (2019) suggested that as much as 72 % of all mainshocks in Southern California is preceded by Foreshock sequences. In this study, we re‐assess the analysis of Trugman & Ross (2019), and we evaluate the impact of the assumptions made by these authors. Using an alternative statistical approach, we find that only 15 out of 46 mainshocks (33 %) are preceded by significantly elevated seismicity rates. When accounting for temporal fluctuations in the background seismicity, only 18 % of the analysed Foreshock sequences remain unexplained by the background seismicity. These results imply that even in a highly complete earthquake catalogue, the majority of earthquakes do not exhibit detectable Foreshock sequences.

  • on the statistical significance of Foreshock sequences in southern california
    Geophysical Research Letters, 2020
    Co-Authors: Martijn P.a. Van Den Ende, J.‐p. Ampuero
    Abstract:

    Earthquake Foreshocks may provide information that is critical to short‐term earthquake forecasting. However, Foreshocks are far from ubiquitously observed, which makes the interpretation of ongoing seismic sequences problematic. Based on a statistical analysis, Trugman and Ross (2019, https://doi.org/10.1029/2019GL083725) suggested that as much as 72% of all mainshocks in Southern California is preceded by Foreshock sequences. In this study, we reassess the analysis of Trugman and Ross (2019, https://doi.org/10.1029/2019GL083725), and we evaluate the impact of the assumptions made by these authors. Using an alternative statistical approach, we find that only 15 out of 46 mainshocks (33%) are preceded by significantly elevated seismicity rates. When accounting for temporal fluctuations in the background seismicity, only 18% of the analyzed Foreshock sequences remain unexplained by the background seismicity. These results imply that even in a highly complete earthquake catalog, the majority of earthquakes do not exhibit detectable Foreshock sequences.

D. Sornette - One of the best experts on this subject based on the ideXlab platform.

  • Properties of Foreshocks and Aftershocks of the Non-Conservative SOC Olami-Feder-Christensen Model: Triggered or Critical Earthquakes?
    Physical Review E : Statistical Nonlinear and Soft Matter Physics, 2004
    Co-Authors: Agnès Helmstetter, Stefan Hergarten, D. Sornette
    Abstract:

    Following Hergarten and Neugebauer [1] who discovered aftershock and Foreshock sequences in the Olami-Feder-Christensen (OFC) discrete block-spring earthquake model, we investigate to what degree the simple toppling mechanism of this model is sufficient to account for the properties of earthquake clustering in time and space. Our main finding is that synthetic catalogs generated by the OFC model share practically all properties of real seismicity at a qualitative level, with however significant quantitative differences. We find that OFC catalogs can be in large part described by the concept of triggered seismicity but the properties of Foreshocks depend on the mainshock magnitude, in qualitative agreement with the critical earthquake model and in disagreement with simple models of triggered seismicity such as the Epidemic Type Aftershock Sequence (ETAS) model [2]. Many other features of OFC catalogs can be reproduced with the ETAS model with a weaker clustering than real seismicity, i.e. for a very small average number of triggered earthquakes of first generation per mother-earthquake. Our study also evidences the large biases stemming for the constraints used for defining Foreshocks and aftershocks.

  • Mainshocks are aftershocks of conditional Foreshocks: How do Foreshock statistical properties emerge from aftershock laws
    Journal of Geophysical Research: Solid Earth, 2003
    Co-Authors: Agnès Helmstetter, D. Sornette, Jean-robert Grasso
    Abstract:

    International audienceThe inverse Omori law for Foreshocks discovered in the 1970s states that the rate of earthquakes prior to a mainshock increases on average as a power law ∝ 1/(tc − t)p ′ of the time to the mainshock occurring at tc. Here, we show that this law results from the direct Omori law for aftershocks describing the power law decay ∼ 1/(t − tc)p of seismicity after an earthquake, provided that any earthquake can trigger its suit of aftershocks. In this picture, the seismic activity at any time is the sum of the spontaneous tectonic loading and of the activity triggered by all preceding events weighted by their corresponding Omori law. The inverse Omori law then emerges as the expected (in a statistical sense) trajectory of seismicity, conditioned on the fact that it leads to the burst of seismic activity accompanying the mainshock. In particular, we predict and verify by numerical simulations on the Epidemic-Type-Aftershock Sequence (ETAS) model that p′ is always smaller than or equal to p and a function of p, of the b-value of the Gutenberg-Richter law (GR) and of a parameter quantifying the number of direct aftershocks as a function of the magnitude of the mainshock. The often documented apparent decrease of the b-value of the GR law at the approach to the main shock results straightforwardly from the conditioning of the path of seismic activity culminating at the mainshock. However, we predict that the GR law is not modified simply by a change of b-value but that a more accurate statement is that the GR law gets an additive (or deviatoric) power law contribution with exponent smaller than b and with an amplitude growing as a power law of the time to the mainshock. In the space domain, we predict that the phenomenon of aftershock diffusion must have its mirror process reflected into an inward migration of Foreshocks towards the mainshock. In this model, Foreshock sequences are special aftershock sequences which are modified by the condition to end up in a burst of seismicity associated with the mainshock. Foreshocks are not just statistical creatures, they are genuine forerunners of large shocks as shown by the large prediction gains obtained using several of their qualifiers

  • Mainshocks are aftershocks of conditional Foreshocks: how to Foreshock statistical properties emerge from aftersock laws.
    Journal of Geophysical Research : Solid Earth, 2003
    Co-Authors: Agnès Helmstetter, D. Sornette, Jean-robert Grasso
    Abstract:

    The inverse Omori law for Foreshocks discovered in the 1970s states that the rate of earthquakes prior to a mainshock increases on average as a power law ∝ 1/(tc − t)p ′ of the time to the mainshock occurring at tc. Here, we show that this law results from the direct Omori law for aftershocks describing the power law decay ∼ 1/(t − tc)p of seismicity after an earthquake, provided that any earthquake can trigger its suit of aftershocks. In this picture, the seismic activity at any time is the sum of the spontaneous tectonic loading and of the activity triggered by all preceding events weighted by their corresponding Omori law. The inverse Omori law then emerges as the expected (in a statistical sense) trajectory of seismicity, conditioned on the fact that it leads to the burst of seismic activity accompanying the mainshock. In particular, we predict and verify by numerical simulations on the Epidemic-Type-Aftershock Sequence (ETAS) model that p′ is always smaller than or equal to p and a function of p, of the b-value of the Gutenberg-Richter law (GR) and of a parameter quantifying the number of direct aftershocks as a function of the magnitude of the mainshock. The often documented apparent decrease of the b-value of the GR law at the approach to the main shock results straightforwardly from the conditioning of the path of seismic activity culminating at the mainshock. However, we predict that the GR law is not modified simply by a change of b-value but that a more accurate statement is that the GR law gets an additive (or deviatoric) power law contribution with exponent smaller than b and with an amplitude growing as a power law of the time to the mainshock. In the space domain, we predict that the phenomenon of aftershock diffusion must have its mirror process reflected into an inward migration of Foreshocks towards the mainshock. In this model, Foreshock sequences are special aftershock sequences which are modified by the condition to end up in a burst of seismicity associated with the mainshock. Foreshocks are not just statistical creatures, they are genuine forerunners of large shocks as shown by the large prediction gains obtained using several of their qualifiers.

  • mainshocks are aftershocks of conditional Foreshocks how do Foreshock statistical properties emerge from aftershock laws
    Journal of Geophysical Research, 2003
    Co-Authors: Agnès Helmstetter, D. Sornette, Jean-robert Grasso
    Abstract:

    [1] The inverse Omori law for Foreshocks discovered in the 1970s states that the rate of earthquakes prior to a mainshock increases on average as a power law ∝ 1/(tc − t)p′ of the time to the mainshock occurring at tc. Here, we show that this law results from the direct Omori law for aftershocks describing the power law decay ∼ 1/(t − tc)p of seismicity after an earthquake, provided that any earthquake can trigger its suit of aftershocks. In this picture, the seismic activity at any time is the sum of the spontaneous tectonic loading and of the activity triggered by all preceding events weighted by their corresponding Omori law. The inverse Omori law then emerges as the expected (in a statistical sense) trajectory of seismicity, conditioned on the fact that it leads to the burst of seismic activity accompanying the mainshock. In particular, we predict and verify by numerical simulations on the epidemic-type aftershock sequence (ETAS) model that p′ is always smaller than or equal to p and a function of p, of the b-value of the Gutenberg–Richter law (GR), and of a parameter quantifying the number of direct aftershocks as a function of the magnitude of the mainshock. The often documented apparent decrease of the b-value of the GR law at the approach to the mainshock results straightforwardly from the conditioning of the path of seismic activity culminating at the mainshock. However, we predict that the GR law is not modified simply by a change of b-value but that a more accurate statement is that the GR law gets an additive (or deviatoric) power law contribution with exponent smaller than b and with an amplitude growing as a power law of the time to the mainshock. In the space domain, we predict that the phenomenon of aftershock diffusion must have its mirror process reflected into an inward migration of Foreshocks toward the mainshock. In this model, Foreshock sequences are special aftershock sequences, which are modified by the condition to end up in a burst of seismicity associated with the mainshock. Foreshocks are not just statistical creatures but are genuine forerunners of large shocks as shown by the large prediction gains obtained using several of their qualifiers.

  • mainshocks are aftershocks of conditional Foreshocks how do Foreshock statistical properties emerge from aftershock laws
    arXiv: Statistical Mechanics, 2002
    Co-Authors: Agnès Helmstetter, D. Sornette, Jean-robert Grasso
    Abstract:

    The inverse Omori law for Foreshocks discovered in the 1970s states that the rate of earthquakes prior to a mainshock increases on average as a power law ~ 1/(t_c-t)^p' of the time to the mainshock occurring at t_c. Here, we show that this law results from the direct Omori law for aftershocks describing the power law decay ~ 1/(t-t_c)^p of seismicity after an earthquake, provided that any earthquake can trigger its suit of aftershocks. In this picture, the seismic activity at any time is the sum of the spontaneous tectonic loading and of the activity triggered by all preceding events weighted by their corresponding Omori law. The inverse Omori law then emerges as the expected (in a statistical sense) trajectory of seismicity, conditioned on the fact that it leads to the burst of seismic activity accompanying the mainshock. The often documented apparent decrease of the b-value of the GR law at the approach to the main shock results straightforwardly from the conditioning of the path of seismic activity culminating at the mainshock. In the space domain, we predict that the phenomenon of aftershock diffusion must have its mirror process reflected into an inward migration of Foreshocks towards the mainshock. In this model, Foreshock sequences are special aftershock sequences which are modified by the condition to end up in a burst of seismicity associated with the mainshock.

Agnès Helmstetter - One of the best experts on this subject based on the ideXlab platform.

  • Properties of Foreshocks and Aftershocks of the Non-Conservative SOC Olami-Feder-Christensen Model: Triggered or Critical Earthquakes?
    Physical Review E : Statistical Nonlinear and Soft Matter Physics, 2004
    Co-Authors: Agnès Helmstetter, Stefan Hergarten, D. Sornette
    Abstract:

    Following Hergarten and Neugebauer [1] who discovered aftershock and Foreshock sequences in the Olami-Feder-Christensen (OFC) discrete block-spring earthquake model, we investigate to what degree the simple toppling mechanism of this model is sufficient to account for the properties of earthquake clustering in time and space. Our main finding is that synthetic catalogs generated by the OFC model share practically all properties of real seismicity at a qualitative level, with however significant quantitative differences. We find that OFC catalogs can be in large part described by the concept of triggered seismicity but the properties of Foreshocks depend on the mainshock magnitude, in qualitative agreement with the critical earthquake model and in disagreement with simple models of triggered seismicity such as the Epidemic Type Aftershock Sequence (ETAS) model [2]. Many other features of OFC catalogs can be reproduced with the ETAS model with a weaker clustering than real seismicity, i.e. for a very small average number of triggered earthquakes of first generation per mother-earthquake. Our study also evidences the large biases stemming for the constraints used for defining Foreshocks and aftershocks.

  • Mainshocks are aftershocks of conditional Foreshocks: How do Foreshock statistical properties emerge from aftershock laws
    Journal of Geophysical Research: Solid Earth, 2003
    Co-Authors: Agnès Helmstetter, D. Sornette, Jean-robert Grasso
    Abstract:

    International audienceThe inverse Omori law for Foreshocks discovered in the 1970s states that the rate of earthquakes prior to a mainshock increases on average as a power law ∝ 1/(tc − t)p ′ of the time to the mainshock occurring at tc. Here, we show that this law results from the direct Omori law for aftershocks describing the power law decay ∼ 1/(t − tc)p of seismicity after an earthquake, provided that any earthquake can trigger its suit of aftershocks. In this picture, the seismic activity at any time is the sum of the spontaneous tectonic loading and of the activity triggered by all preceding events weighted by their corresponding Omori law. The inverse Omori law then emerges as the expected (in a statistical sense) trajectory of seismicity, conditioned on the fact that it leads to the burst of seismic activity accompanying the mainshock. In particular, we predict and verify by numerical simulations on the Epidemic-Type-Aftershock Sequence (ETAS) model that p′ is always smaller than or equal to p and a function of p, of the b-value of the Gutenberg-Richter law (GR) and of a parameter quantifying the number of direct aftershocks as a function of the magnitude of the mainshock. The often documented apparent decrease of the b-value of the GR law at the approach to the main shock results straightforwardly from the conditioning of the path of seismic activity culminating at the mainshock. However, we predict that the GR law is not modified simply by a change of b-value but that a more accurate statement is that the GR law gets an additive (or deviatoric) power law contribution with exponent smaller than b and with an amplitude growing as a power law of the time to the mainshock. In the space domain, we predict that the phenomenon of aftershock diffusion must have its mirror process reflected into an inward migration of Foreshocks towards the mainshock. In this model, Foreshock sequences are special aftershock sequences which are modified by the condition to end up in a burst of seismicity associated with the mainshock. Foreshocks are not just statistical creatures, they are genuine forerunners of large shocks as shown by the large prediction gains obtained using several of their qualifiers

  • Mainshocks are aftershocks of conditional Foreshocks: how to Foreshock statistical properties emerge from aftersock laws.
    Journal of Geophysical Research : Solid Earth, 2003
    Co-Authors: Agnès Helmstetter, D. Sornette, Jean-robert Grasso
    Abstract:

    The inverse Omori law for Foreshocks discovered in the 1970s states that the rate of earthquakes prior to a mainshock increases on average as a power law ∝ 1/(tc − t)p ′ of the time to the mainshock occurring at tc. Here, we show that this law results from the direct Omori law for aftershocks describing the power law decay ∼ 1/(t − tc)p of seismicity after an earthquake, provided that any earthquake can trigger its suit of aftershocks. In this picture, the seismic activity at any time is the sum of the spontaneous tectonic loading and of the activity triggered by all preceding events weighted by their corresponding Omori law. The inverse Omori law then emerges as the expected (in a statistical sense) trajectory of seismicity, conditioned on the fact that it leads to the burst of seismic activity accompanying the mainshock. In particular, we predict and verify by numerical simulations on the Epidemic-Type-Aftershock Sequence (ETAS) model that p′ is always smaller than or equal to p and a function of p, of the b-value of the Gutenberg-Richter law (GR) and of a parameter quantifying the number of direct aftershocks as a function of the magnitude of the mainshock. The often documented apparent decrease of the b-value of the GR law at the approach to the main shock results straightforwardly from the conditioning of the path of seismic activity culminating at the mainshock. However, we predict that the GR law is not modified simply by a change of b-value but that a more accurate statement is that the GR law gets an additive (or deviatoric) power law contribution with exponent smaller than b and with an amplitude growing as a power law of the time to the mainshock. In the space domain, we predict that the phenomenon of aftershock diffusion must have its mirror process reflected into an inward migration of Foreshocks towards the mainshock. In this model, Foreshock sequences are special aftershock sequences which are modified by the condition to end up in a burst of seismicity associated with the mainshock. Foreshocks are not just statistical creatures, they are genuine forerunners of large shocks as shown by the large prediction gains obtained using several of their qualifiers.

  • mainshocks are aftershocks of conditional Foreshocks how do Foreshock statistical properties emerge from aftershock laws
    Journal of Geophysical Research, 2003
    Co-Authors: Agnès Helmstetter, D. Sornette, Jean-robert Grasso
    Abstract:

    [1] The inverse Omori law for Foreshocks discovered in the 1970s states that the rate of earthquakes prior to a mainshock increases on average as a power law ∝ 1/(tc − t)p′ of the time to the mainshock occurring at tc. Here, we show that this law results from the direct Omori law for aftershocks describing the power law decay ∼ 1/(t − tc)p of seismicity after an earthquake, provided that any earthquake can trigger its suit of aftershocks. In this picture, the seismic activity at any time is the sum of the spontaneous tectonic loading and of the activity triggered by all preceding events weighted by their corresponding Omori law. The inverse Omori law then emerges as the expected (in a statistical sense) trajectory of seismicity, conditioned on the fact that it leads to the burst of seismic activity accompanying the mainshock. In particular, we predict and verify by numerical simulations on the epidemic-type aftershock sequence (ETAS) model that p′ is always smaller than or equal to p and a function of p, of the b-value of the Gutenberg–Richter law (GR), and of a parameter quantifying the number of direct aftershocks as a function of the magnitude of the mainshock. The often documented apparent decrease of the b-value of the GR law at the approach to the mainshock results straightforwardly from the conditioning of the path of seismic activity culminating at the mainshock. However, we predict that the GR law is not modified simply by a change of b-value but that a more accurate statement is that the GR law gets an additive (or deviatoric) power law contribution with exponent smaller than b and with an amplitude growing as a power law of the time to the mainshock. In the space domain, we predict that the phenomenon of aftershock diffusion must have its mirror process reflected into an inward migration of Foreshocks toward the mainshock. In this model, Foreshock sequences are special aftershock sequences, which are modified by the condition to end up in a burst of seismicity associated with the mainshock. Foreshocks are not just statistical creatures but are genuine forerunners of large shocks as shown by the large prediction gains obtained using several of their qualifiers.

  • mainshocks are aftershocks of conditional Foreshocks how do Foreshock statistical properties emerge from aftershock laws
    arXiv: Statistical Mechanics, 2002
    Co-Authors: Agnès Helmstetter, D. Sornette, Jean-robert Grasso
    Abstract:

    The inverse Omori law for Foreshocks discovered in the 1970s states that the rate of earthquakes prior to a mainshock increases on average as a power law ~ 1/(t_c-t)^p' of the time to the mainshock occurring at t_c. Here, we show that this law results from the direct Omori law for aftershocks describing the power law decay ~ 1/(t-t_c)^p of seismicity after an earthquake, provided that any earthquake can trigger its suit of aftershocks. In this picture, the seismic activity at any time is the sum of the spontaneous tectonic loading and of the activity triggered by all preceding events weighted by their corresponding Omori law. The inverse Omori law then emerges as the expected (in a statistical sense) trajectory of seismicity, conditioned on the fact that it leads to the burst of seismic activity accompanying the mainshock. The often documented apparent decrease of the b-value of the GR law at the approach to the main shock results straightforwardly from the conditioning of the path of seismic activity culminating at the mainshock. In the space domain, we predict that the phenomenon of aftershock diffusion must have its mirror process reflected into an inward migration of Foreshocks towards the mainshock. In this model, Foreshock sequences are special aftershock sequences which are modified by the condition to end up in a burst of seismicity associated with the mainshock.

Iver H. Cairns - One of the best experts on this subject based on the ideXlab platform.

  • Terrestrial Foreshock Langmuir waves: STEREO observations, theoretical modeling, and quasi‐linear simulations
    Journal of Geophysical Research: Space Physics, 2009
    Co-Authors: David M. Malaspina, Zdenka Kuncic, Iver H. Cairns, Peter A. Robinson, Robert Ergun
    Abstract:

    [1] Langmuir waves in the terrestrial electron Foreshock are investigated using observations from the STEREO spacecraft, theoretical modeling, and quasi-linear simulations. Emphases are placed on spatial variations of Langmuir field strength with distance between the spacecraft and the tangent point and on the effects of ambient density fluctuations on these variations. The STEREO mission provides new observations of Foreshock Langmuir waves at distances more than twice as far from Earth as previously observed. Based on established geometric properties of the Foreshock region, two methods are developed for separating Langmuir waves of Foreshock origin from those of solar and/or heliospheric origins. The observed maximum Foreshock Langmuir field strength falls with distance via a power law with an exponent −1.01 ± 0.12. The theory and simulations predict field strengths and power law spatial variations in field strengths that are consistent with the observations when scattering of Langmuir waves by density fluctuations is included. The power law exponents predicted by both theory and simulations fall within the uncertainty of the observations for the typical solar wind conditions observed but differ by a factor of ≈1.5 from simulations that assume density homogeneity. This indicates that density fluctuations play an important role in the beam-Langmuir wave dynamics in the Foreshock.

  • Planetary Foreshock radio emissions
    Journal of Geophysical Research, 2005
    Co-Authors: Zdenka Kuncic, Iver H. Cairns
    Abstract:

    [1] The electron Foreshock regions upstream of Earth's bow shock and upstream of traveling interplanetary shocks are known to be propitious sites for a variety of energetic particle and plasma wave phenomena, including radio emissions. A quantitative theoretical model has been developed for radio emissions associated with the terrestrial Foreshock and for type II radio bursts associated with interplanetary shocks. Here, we generalize this model and apply it to other planetary Foreshocks. We present predictions for the levels of planetary Foreshock radio emissions and compare these with observations by past and present space missions. One key result is that Mercury can be a strong source of Foreshock radio emissions, and this prediction may be testable with the anticipated BepiColombo space mission. Although the terrestrial Foreshock radio emissions are the most detectable with existing instruments, our results predict that they are the second strongest in absolute terms, following the Jovian Foreshock emissions. Indeed, we predict that the radio instrument on board Ulysses should have detected Jovian Foreshock radio emissions, and we suggest that there is some evidence in the data to support this. We also suggest that Cassini was potentially capable of detecting Foreshock emissions from Venus during its gravity-assist flybys and may possibly be capable of detecting Foreshock emissions from Saturn under favorable solar wind conditions.

  • Electric field distributions for Langmuir waves in planetary Foreshocks
    Journal of Geophysical Research, 2004
    Co-Authors: Christopher R. Boshuizen, Iver H. Cairns, Peter A. Robinson
    Abstract:

    [1] This paper addresses several key issues regarding the statistics of Langmuir wave fields E in planetary Foreshocks by presenting (1) a detailed theory for the electric field strength probability distributions averaged over Earth's Foreshock and (2) the first analyses of the distributions observed in the outer planets. First, existing theory is extended and generalized, describing in detail how lognormal distributions of P(log E) for a specific location can be aggregated over Foreshock location to give power law distributions (log E). The extended theory is in good agreement with the observations of power law distributions (log E) for Earth's Foreshock, and implies that stochastic growth theory (SGT) is likely to be relevant. The statistics of the Langmuir wave fields of the outer planets Saturn, Uranus, and Neptune are then analyzed and compared. The characteristics of the observed distributions (logE) agree qualitatively with each other and Earth, and these distributions collapse onto a single curve when scaled on the assumption that the ratio of the wave energy density to thermal plasma energy density is constant with heliocentric distance. The extended theory developed for Earth is also found to agree well with the scaled (log E) distributions for each outer planet. The evidence is thus that the physics of electrons and Langmuir waves in all planetary Foreshocks is essentially identical and that the distributions (log E) are consistent with SGT.

  • Theory of type II radio emission from the Foreshock of an interplanetary shock
    Journal of Geophysical Research: Space Physics, 2001
    Co-Authors: S. A. Knock, Peter A. Robinson, Iver H. Cairns, Zdenka Kuncic
    Abstract:

    We present an analytical model for type II solar radio bursts and then apply it to an observed type II event. Electron beams are produced in the Foreshock of an interplanetary shock via shock drift acceleration. Reflection is treated in the de Hoffman-Teller frame with efficiencies modeled by a losscone that incorporates the effects of the static cross-shock potential ϕ. Stochastic growth theory is used to treat electron beam driven Langmuir wave growth in the type II Foreshock. Nonlinear wave-wave interactions are used as the mechanisms for converting Langmuir wave energy into freely propagating radio emission. The electron beams produced in the Foreshock have a wide range of speeds and number densities. These electron beams are qualitatively consistent with observations in a type II Foreshock as well as earlier theoretical predictions, and observations in Earth's Foreshock. Significant levels of Langmuir waves and ƒp and 2ƒp emission are predicted. In particular, the predicted volume emissivities are similar to those predicted for type III bursts. The simple model developed for the source environment of the type II event on August 26, 1998, produces fluxes in reasonable agreement with observation.

  • Langmuir-like waves and radiation in planetary Foreshocks
    1995
    Co-Authors: Iver H. Cairns, Peter A. Robinson, Roger R. Anderson, D. A. Gurnett, William S. Kurth
    Abstract:

    The basic objectives of this NASA Grant are to develop theoretical understandings (tested with spacecraft data) of the generation and characteristics of electron plasma waves, commonly known as Langmuir-like waves, and associated radiation near f(sub p) and 2f(sub p) in planetary Foreshocks. (Here f(sub p) is plasma frequency.) Related waves and radiation in the source regions of interplanetary type III solar radio bursts provide a simpler observational and theoretical context for developing and testing such understandings. Accordingly, applications to type III bursts constitute a significant fraction of the research effort. The testing of the new Stochastic Growth Theory (SGT) for type III bursts, and its extension and testing for Foreshock waves and radiation, constitutes a major longterm strategic goal of the research effort.

D. G. Sibeck - One of the best experts on this subject based on the ideXlab platform.

  • structure and properties of the Foreshock at venus
    Journal of Geophysical Research, 2017
    Co-Authors: Nick Omidi, G Collinson, D. G. Sibeck
    Abstract:

    The interaction of the solar wind with Venus is dominated by the planet's ionosphere which acts as an obstacle to the flow resulting in an induced magnetosphere and bow shock much smaller than their terrestrial counterparts. This study presents a 3-D electromagnetic hybrid (kinetic ions, fluid electrons) simulation of the solar wind interaction with an unmagnetized obstacle to examine the structure and properties of the Cytherean Foreshock during periods of near radial IMF, i.e. when it lies upstream of the ionosphere. The interaction between the backstreaming ions and the solar wind results in the generation of two classes of ULF waves: (1) parallel propagating sinusoidal waves with periods ~20-30 seconds and (2) highly oblique fast magnetosonic waves. The joint nonlinear evolution of these waves result in the formation of structures called Foreshock cavitons with dimensions comparable to the size of the planet. Foreshock cavitons are also present in the terrestrial Foreshock. The excavation of plasma and magnetic field from their cores leads to lower average densities and magnetic field strengths in the Foreshock. As in the case of Earth, this excavation results in the formation of a fast magnetosonic pulse/shock at the edge of the Foreshock named the Foreshock compressional boundary. Also similar to Earth, is the formation of spontaneous hot flow anomalies (SHFAs) as Foreshock cavitons approach the bow shock. The size and properties of SHFAs at Venus are comparable to those at Earth and their existence has recently been established at Mars and Venus in a companion paper.

  • Traveling Foreshocks and transient Foreshock phenomena
    Journal of Geophysical Research: Space Physics, 2017
    Co-Authors: P. Kajdič, Xochitl Blanco-cano, Nojan Omidi, D. Rojas-castillo, D. G. Sibeck, Laurence Billingham
    Abstract:

    We use the multi-spacecraft capabilities of the Cluster and THEMIS missions to show that two types of Foreshock may be detected in spacecraft data. One is the global Foreshock that appears upstream of the Earth's quasi-parallel bow-shock under steady or variable interplanetary magnetic field. Another type is a traveling Foreshock that is bounded by two rotational discontinuities in the interplanetary magnetic field and propagates along the bow-shock. Foreshock compressional boundaries are found at the edges of both types of Foreshock. We show that isolated Foreshock cavities are a subset of the traveling Foreshock that form when two bounding rotational discontinuities are so close that the ultra-low frequency waves do not develop in the region between them. We also report observations of a spontaneous hot flow anomaly inside a traveling Foreshock. This means that other phenomena, such as Foreshock cavitons, may also exist inside this type of Foreshock. In the second part of this work we present statistical properties of phenomena related to the Foreshock, namely Foreshock cavities, cavitons, spontaneous hot flow anomalies and Foreshock compressional boundaries. We show that spontaneous hot flow anomalies are the most depleted transient structures in terms of the B-field and plasma density inside them and that the Foreshock compressional boundaries and Foreshock cavities are closely related structures.

  • relativistic electrons produced by Foreshock disturbances observed upstream of earth s bow shock
    Physical Review Letters, 2016
    Co-Authors: L B Wilson, D. G. Sibeck, D L Turner, A Osmane, Damiano Caprioli, V Angelopoulos
    Abstract:

    Charged particles can be reflected and accelerated by strong (i.e., high Mach number) astrophysical collisionless shock waves, streaming away to form a Foreshock region in communication with the shock. Foreshocks are primarily populated by suprathermal ions that can generate Foreshock disturbances-largescale (i.e., tens to thousands of thermal ion Larmor radii), transient (approximately 5-10 per day) structures. They have recently been found to accelerate ions to energies of several keV. Although electrons in Saturn's high Mach number (M > 40) bow shock can be accelerated to relativistic energies (nearly 1000 keV), it has hitherto been thought impossible to accelerate electrons beyond a few tens of keV at Earth's low Mach number (1 M <20) bow shock. Here we report observations of electrons energized by Foreshock disturbances to energies up to at least approximately 300 keV. Although such energetic electrons have been previously observed, their presence has been attributed to escaping magnetospheric particles or solar events. These relativistic electrons are not associated with any solar or magnetospheric activity. Further, due to their relatively small Larmor radii (compared to magnetic gradient scale lengths) and large thermal speeds (compared to shock speeds), no known shock acceleration mechanism can energize thermal electrons up to relativistic energies. The discovery of relativistic electrons associated with Foreshock structures commonly generated in astrophysical shocks could provide a new paradigm for electron injections and acceleration in collisionless plasmas.

  • Foreshock bubbles and their global magnetospheric impacts
    Journal of Geophysical Research, 2010
    Co-Authors: N. Omidi, J P Eastwood, D. G. Sibeck
    Abstract:

    [1] We employ 2.5-D electromagnetic, hybrid simulations that treat ions kinetically via particle-in-cell methods and electrons as a massless fluid to study the formation and properties of a new structure named the Foreshock bubble upstream from the bow shock. This structure forms due to changes in the interplanetary magnetic field (IMF) associated with solar wind discontinuities and their interaction with the backstreaming ions in the Foreshock prior to these discontinuities encountering the bow shock. The leading edge of the Foreshock bubble consists of a fast magnetosonic shock and the compressed and heated plasma downstream of the shock. The leading edge surrounds the core which consists of a less-dense and hotter plasma and lower magnetic field strength. Ultra low frequency turbulence is present in both the outer and core regions of the Foreshock bubbles. The size of the Foreshock bubble transverse to the flow direction scales with the width of the ion Foreshock and at Earth corresponds to tens of RE. The size along the flow depends on the age of the bubble and grows with time. Although they expand sunward, Foreshock bubbles are carried antisunward by the solar wind, and for small IMF cone angles (angle between IMF and solar wind flow) when the Foreshock lies upstream of the dayside magnetosphere they collide with the bow shock. This collision is shown to have significant magnetospheric impacts. Upon encountering the bow shock, the low pressures within the core of the bubble result in the reversal of the magnetosheath flow from antisunward to sunward direction. This in turn results in the outward motion of the magnetopause and expansion of the dayside magnetosphere. The interaction is found to noticeably impact the density and energy of trapped radiation belt ions and plasma injection into the cusp. Foreshock bubbles are found to be highly effective sites for ion reflection and acceleration to high energies via first- and second-order Fermi acceleration. The interaction of the Foreshock bubble with the bow shock results in the release of energetic ions into the magnetosheath. Some of these ions are subsequently injected into the cusp.

  • Foreshock compressional boundary
    Journal of Geophysical Research, 2009
    Co-Authors: N. Omidi, D. G. Sibeck, X Blancocano
    Abstract:

    [1] We employ 2.5-D electromagnetic, hybrid simulations that treat ions kinetically via particle-in-cell methods and electrons as a massless fluid to study the formation and properties of a newly discovered boundary named the Foreshock compressional boundary (FCB). This boundary forms in the ion Foreshock and is associated with enhanced densities and magnetic field strengths. At times, but not always, the FCB separates the pristine solar wind plasma from the ion Foreshock. In this study, we investigate the dependence of FCB characteristics on solar wind Mach number and cone angle (the angle between flow velocity and interplanetary magnetic field). We show that the strength of the Foreshock compressional boundary increases with the Mach number. This enhancement is in turn tied to the density and velocity of the backstreaming ions in the Foreshock whose interaction with the solar wind results in ULF turbulence which is ultimately responsible for the formation of FCB. During small cone angles the Foreshock compressional boundary is symmetric with respect to the radial direction. As the cone angle increases, the FCB becomes less symmetric and eventually is confined to one side of the Foreshock. The strength of the FCB also decreases with increasing cone angle but depending on the Mach number can exist for cone angles of 40° and beyond. A recent study that compared data from a global hybrid simulation of the Foreshock with Cluster spacecraft observations showed that encounters with Foreshock cavities can be interpreted as back and forth motion of a FCB causing spacecraft to move from the solar wind through the FCB into the Foreshock and back into solar wind. An example of a FCB observed by the Cluster spacecraft is presented and shown to be in general agreement with model predictions.