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Riley P.,Predictive Science | Love J.J.,U.S. Geological Survey
Space Weather | Year: 2017

Extreme space weather events are low-frequency, high-risk phenomena. Estimating their rates of occurrence, as well as their associated uncertainties, is difficult. In this study, we derive statistical estimates and uncertainties for the occurrence rate of an extreme geomagnetic storm on the scale of the Carrington event (or worse) occurring within the next decade. We model the distribution of events as either a power law or lognormal distribution and use (1) Kolmogorov-Smirnov statistic to estimate goodness of fit, (2) bootstrapping to quantify the uncertainty in the estimates, and (3) likelihood ratio tests to assess whether one distribution is preferred over another. Our best estimate for the probability of another extreme geomagnetic event comparable to the Carrington event occurring within the next 10 years is 10.3% 95% confidence interval (CI) [0.9,18.7] for a power law distribution but only 3.0% 95% CI [0.6,9.0] for a lognormal distribution. However, our results depend crucially on (1) how we define an extreme event, (2) the statistical model used to describe how the events are distributed in intensity, (3) the techniques used to infer the model parameters, and (4) the data and duration used for the analysis. We test a major assumption that the data represent time stationary processes and discuss the implications. If the current trends persist, suggesting that we are entering a period of lower activity, our forecasts may represent upper limits rather than best estimates. ©2016. American Geophysical Union. All Rights Reserved.


Lario D.,Johns Hopkins University | Raouafi N.E.,Johns Hopkins University | Kwon R.-Y.,George Mason University | Zhang J.,George Mason University | And 3 more authors.
Astrophysical Journal | Year: 2014

We investigate the solar phenomena associated with the origin of the solar energetic particle (SEP) event observed on 2013 April 11 by a number of spacecraft distributed in the inner heliosphere over a broad range of heliolongitudes. We use extreme ultraviolet (EUV) and white-light coronagraph observations from the Solar Dynamics Observatory (SDO), the SOlar and Heliospheric Observatory, and the twin Solar TErrestrial RElations Observatory spacecraft (STEREO-A and STEREO-B) to determine the angular extent of the EUV wave and coronal mass ejection (CME) associated with the origin of the SEP event. We compare the estimated release time of SEPs observed at each spacecraft with the arrival time of the structures associated with the CME at the footpoints of the field lines connecting each spacecraft with the Sun. Whereas the arrival of the EUV wave and CME-driven shock at the footpoint of STEREO-B is consistent, within uncertainties, with the release time of the particles observed by this spacecraft, the EUV wave never reached the footpoint of the field lines connecting near-Earth observers with the Sun, even though an intense SEP event was observed there. We show that the west flank of the CME-driven shock propagating at high altitudes above the solar surface was most likely the source of the particles observed near Earth, but it did not leave any EUV trace on the solar disk. We conclude that the angular extent of the EUV wave on the solar surface did not agree with the longitudinal extent of the SEP event in the heliosphere. Hence EUV waves cannot be used reliably as a proxy for the solar phenomenon that accelerates and injects energetic particles over broad ranges of longitudes. © 2014. The American Astronomical Society. All rights reserved.


Wiegelmann T.,Max Planck Institute for Solar System Research | Petrie G.J.D.,U.S. National Solar Observatory | Riley P.,Predictive Science
Space Science Reviews | Year: 2015

Coronal magnetic field models use photospheric field measurements as boundary condition to model the solar corona. We review in this paper the most common model assumptions, starting from MHD-models, magnetohydrostatics, force-free and finally potential field models. Each model in this list is somewhat less complex than the previous one and makes more restrictive assumptions by neglecting physical effects. The magnetohydrostatic approach neglects time-dependent phenomena and plasma flows, the force-free approach neglects additionally the gradient of the plasma pressure and the gravity force. This leads to the assumption of a vanishing Lorentz force and electric currents are parallel (or anti-parallel) to the magnetic field lines. Finally, the potential field approach neglects also these currents. We outline the main assumptions, benefits and limitations of these models both from a theoretical (how realistic are the models?) and a practical viewpoint (which computer resources to we need?). Finally we address the important problem of noisy and inconsistent photospheric boundary conditions and the possibility of using chromospheric and coronal observations to improve the models. © 2015 The Author(s)


Riley P.,Predictive Science | Lionello R.,Predictive Science
Solar Physics | Year: 2011

A variety of techniques exist for mapping solar wind plasma and magnetic field measurements from one location to another in the heliosphere. Such methods are either applied to extrapolate solar data or coronal model results from near the Sun to 1 AU (or elsewhere), or to map in-situ observations back to the Sun. In this study, we estimate the sensitivity of four models for evolving solar wind streams from the Sun to 1 AU. In order of increasing complexity, these are: i) ballistic extrapolation; ii) ad hoc kinematic mapping; iii) 1-D upwinding propagation; and iv) global heliospheric MHD modeling. We also consider the effects of the interplanetary magnetic field on the evolution of the stream structure. The upwinding technique is a new, simplified method that bridges the extremes of ballistic extrapolation and global heliospheric MHD modeling. It can match the dynamical evolution captured by global models, but is almost as simple to implement and as fast to run as the ballistic approximation. © 2011 Springer Science+Business Media B.V.


Riley P.,Predictive Science
AIP Conference Proceedings | Year: 2010

In this review we summarize our current knowledge regarding the three-dimensional structure of the quasi-steady, large-scale inner heliosphere. This understanding is based on the interpretation of a wide array of remote and in situ measurements, in conjunction with sophisticated numerical models. Observations by the Ulysses spacecraft, in particular, have provided an unprecedented set of measurements for more than 18 years, and observations by the STEREO spacecraft promise no less. Global MHD models of the solar corona and heliosphere have matured to the point that a wide range of measurements can now be reproduced with reasonable fidelity. In the absence of transient effects, this structure is dominated by corotating interaction regions which can be understood - to a large extent - from the consequence of solar rotation on a spatially-variable velocity profile near the Sun, leading to parcels of plasma with different plasma and magnetic properties becoming radially aligned. This interaction is one of the principal dynamic processes that shape the structure of the interplanetary medium. To illustrate some of these phenomena, we discuss the structural features of the current solar minimum, which has, thus far, displayed a number of distinct characteristics in relation to recent previous minima of the space age. © 2010 American Institute of Physics.


Riley P.,Predictive Science | Ben-Nun M.,Predictive Science | Linker J.A.,Predictive Science | Mikic Z.,Predictive Science | And 6 more authors.
Solar Physics | Year: 2014

The observed photospheric magnetic field is a crucial parameter for understanding a range of fundamental solar and heliospheric phenomena. Synoptic maps, in particular, which are derived from the observed line-of-sight photospheric magnetic field and built up over a period of 27 days, are the main driver for global numerical models of the solar corona and inner heliosphere. Yet, in spite of 60 years of measurements, quantitative estimates remain elusive. In this study, we compare maps from seven solar observatories (Stanford/WSO, NSO/KPVT, NSO/SOLIS, NSO/GONG, SOHO/MDI, UCLA/MWO, and SDO /HMI) to identify consistencies and differences among them. We find that while there is a general qualitative consensus, there are also some significant differences. We compute conversion factors that relate measurements made by one observatory to another using both synoptic map pixel-by-pixel and histogram-equating techniques, and we also estimate the correlation between datasets. For example, Wilcox Solar Observatory (WSO) synoptic maps must be multiplied by a factor of 3 - 4 to match Mount Wilson Observatory (MWO) estimates. Additionally, we find no evidence that the MWO saturation correction factor should be applied to WSO data, as has been done in previous studies. Finally, we explore the relationship between these datasets over more than a solar cycle, demonstrating that, with a few notable exceptions, the conversion factors remain relatively constant. While our study was able to quantitatively describe the relationship between the datasets, it did not uncover any obvious "ground truth." We offer several suggestions for how this may be addressed in the future. © 2013 Springer Science+Business Media Dordrecht.


Riley P.,Predictive Science | Luhmann J.,University of California at Berkeley | Opitz A.,French National Center for Scientific Research | Linker J.A.,Predictive Science | Mikic Z.,Predictive Science
Journal of Geophysical Research: Space Physics | Year: 2010

Measurements from the ACE and STEREO A and B spacecraft are allowing an unprecedented view of the structure of the three-dimensional heliosphere. One aspect of this is the degree to which the measurements at one spacecraft correlate with those at the other. We have computed the cross-correlation functions (CCFs) for all three combinations of ACE and STEREO A and B in situ observations of the bulk solar wind velocity as the spacecraft moved progressively farther away from one another. Our results confirm previous studies that the phase lag between the signals becomes linearly larger with time. However, we have identified two intervals where this appears to break down. During these "lulls," the CCF reveals a phase lag considerably less than that which would be predicted based only on the angular separation of the spacecraft. We modeled the entire STEREO time period using a global MHD model to investigate the cause for these "lulls." We find that a combination of time-dependent evolution of the streams as well as spatial inhomogeneities, due to the latitudinal separation of the spacecraft, are sufficient to explain them. Copyright 2010 by the American Geophysical Union.


Riley P.,Predictive Science | Linker J.A.,Predictive Science | Mikic Z.,Predictive Science
Journal of Geophysical Research: Space Physics | Year: 2013

Ensemble modeling is a method of prediction based on the use of a representative sample of possible future states. Global models of the solar corona and inner heliosphere are now maturing to the point of becoming predictive tools; thus, it is both meaningful and necessary to quantitatively assess their uncertainty and limitations. In this study, we apply simple ensemble modeling techniques as a first step towards these goals. We focus on one relatively quiescent time period, Carrington rotation 2062, which occurred during the late declining phase of solar cycle 23. To illustrate and assess the sensitivity of the model results to variations in boundary conditions, we compute solutions using synoptic magnetograms from seven solar observatories. Model sensitivity is explored using (1) different combinations of models, (2) perturbations in the base coronal temperature (a free parameter in one of the model approximations), and (3) the spatial resolution of the numerical grid. We present variance maps, "whisker" plots, and "Taylor" diagrams to summarize the accuracy of the solutions and compute skill scores, which demonstrate that the ensemble mean solution outperforms any of the individual realizations. Our results provide a baseline against which future model improvements can be compared. Key PointsEnsemble prediction techniques can improve heliospheric modelsEnsemble solutions perform better than individual realizationsTaylor diagrams provide a useful way to summarize model accuracy © 2013. American Geophysical Union. All Rights Reserved.


Riley P.,Predictive Science
Space Weather | Year: 2012

By virtue of their rarity, extreme space weather events, such as the Carrington event of 1859, are difficult to study, their rates of occurrence are difficult to estimate, and prediction of a specific future event is virtually impossible. Additionally, events may be extreme relative to one parameter but normal relative to others. In this study, we analyze several measures of the severity of space weather events (flare intensity, coronal mass ejection speeds, Dst, and >30 MeV proton fluences as inferred from nitrate records) to estimate the probability of occurrence of extreme events. By showing that the frequency of occurrence scales as an inverse power of the severity of the event, and assuming that this relationship holds at higher magnitudes, we are able to estimate the probability that an event larger than some criteria will occur within a certain interval of time in the future. For example, the probability of another Carrington event (based on Dst < -850 nT) occurring within the next decade is ∼12%. We also identify and address several limitations with this approach. In particular, we assume time stationarity, and thus, the effects of long-term space climate change are not considered. While this technique cannot be used to predict specific events, it may ultimately be useful for probabilistic forecasting. Copyright 2012 by the American Geophysical Union.


Should a powerful solar storm hit Earth, the event could cause blackouts that will likely result in $41.5 billion daily economic losses in the United States alone. The most powerful solar flare documented to strike Earth is the Carrington event. The coronal mass ejection, which is produced during solar flares, that struck Earth on September 1859 caused auroras around the world. Telegraph systems in Europe and North America failed and operators had electric shocks from the devices as a result of the solar event. Some machines also continued to work even after they were disconnected from electricity. A 2012 paper by space physicist Pete Riley, of Predictive Science in San Diego, California, revealed that the likelihood of another Carrington event happening within the next decade is about 12 percent. Findings of a new study published in Space Weather revealed the economic impact of such a sufficiently powerful solar storm if it would affect Earth. The event could knock out the transformers needed to transmit electricity throughout the country's power grids which could lead to power blackouts. Earlier studies only looked at the direct economic costs within the blackout zones and did not consider the extreme space weather's indirect impact on domestic and international supply chain. Now, researchers of the new study said that the direct economic cost that could be incurred from disruption of electricity represents only 49 percent of the possible macroeconomic costs. In most extreme blackout scenario that could affect 66 percent of the population in the United States, domestic economic loss in a day could be $41.5 billion. Loss associated with the international supply chain would be at $7 billion. "By exploring the sensitivity of the blackout zone, we show that on average the direct economic cost incurred from disruption to electricity represents only 49% of the total potential macroeconomic cost," the researchers wrote in their study. "Therefore, if indirect supply chain costs are not considered when undertaking cost-benefit analysis of space weather forecasting and mitigation investment, the total potential macroeconomic cost is not correctly represented." Experts have varied views on the potential severity of blackouts associated with CMEs. Some think that the outages would last only hours or up to a few days. Other think that blackouts would last weeks or even months because transmission networks could be knocked out and would require replacements. In terms of economic losses, solar-induced blackouts would most severely impact manufacturing in the United States, followed by government, finance and insurance, and property. China is expected to be the most affected outside of the American soil followed by Canada and Mexico, which provide raw materials, goods and services that are used in production by U.S. companies. "We felt it was important to look at how extreme space weather may affect domestic U.S. production in various economic sectors," said study author Edward Oughton, from Cambridge Judge Business School. © 2017 Tech Times, All rights reserved. Do not reproduce without permission.

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