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Title: On Weibull's Spectrum of Nonrelativistic Energetic Particles at IP Shocks: Observations and Theoretical Interpretation

Abstract

Some interplanetary shocks are associated with short-term and sharp particle flux enhancements near the shock front. Such intensity enhancements, known as shock-spike events (SSEs), represent a class of relatively energetic phenomena as they may extend to energies of some tens of MeV or even beyond. Here we present an SSE case study in order to shed light on the nature of the particle acceleration involved in this kind of event. Our observations refer to an SSE registered on 2011 October 3 at 22:23 UT, by STEREO B instrumentation when, at a heliocentric distance of 1.08 au, the spacecraft was swept by a perpendicular shock moving away from the Sun. The main finding from the data analysis is that a Weibull distribution represents a good fitting function to the measured particle spectrum over the energy range from 0.1 to 30 MeV. To interpret such an observational result, we provide a theoretical derivation of the Weibull spectrum in the framework of the acceleration by “killed” stochastic processes exhibiting power-law growth in time of the velocity expectation, such as the classical Fermi process. We find an overall coherence between the experimental values of the Weibull spectrum parameters and their physical meaning within themore » above scenario. Hence, our approach based on the Weibull distribution proves to be useful for understanding SSEs. With regard to the present event, we also provide an alternative explanation of the Weibull spectrum in terms of shock-surfing acceleration.« less

Authors:
; ;  [1]
  1. INAF—Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, I-00133 Roma (Italy)
Publication Date:
OSTI Identifier:
22661158
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 837; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATION; DATA ANALYSIS; MAGNETOHYDRODYNAMICS; MEV RANGE; PLASMA; SHOCK WAVES; SOLAR WIND; SPACE VEHICLES; SPECTRA; STOCHASTIC PROCESSES; SUN; TURBULENCE; VELOCITY; VISIBLE RADIATION

Citation Formats

Pallocchia, G., Laurenza, M., and Consolini, G. On Weibull's Spectrum of Nonrelativistic Energetic Particles at IP Shocks: Observations and Theoretical Interpretation. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA633A.
Pallocchia, G., Laurenza, M., & Consolini, G. On Weibull's Spectrum of Nonrelativistic Energetic Particles at IP Shocks: Observations and Theoretical Interpretation. United States. doi:10.3847/1538-4357/AA633A.
Pallocchia, G., Laurenza, M., and Consolini, G. Fri . "On Weibull's Spectrum of Nonrelativistic Energetic Particles at IP Shocks: Observations and Theoretical Interpretation". United States. doi:10.3847/1538-4357/AA633A.
@article{osti_22661158,
title = {On Weibull's Spectrum of Nonrelativistic Energetic Particles at IP Shocks: Observations and Theoretical Interpretation},
author = {Pallocchia, G. and Laurenza, M. and Consolini, G.},
abstractNote = {Some interplanetary shocks are associated with short-term and sharp particle flux enhancements near the shock front. Such intensity enhancements, known as shock-spike events (SSEs), represent a class of relatively energetic phenomena as they may extend to energies of some tens of MeV or even beyond. Here we present an SSE case study in order to shed light on the nature of the particle acceleration involved in this kind of event. Our observations refer to an SSE registered on 2011 October 3 at 22:23 UT, by STEREO B instrumentation when, at a heliocentric distance of 1.08 au, the spacecraft was swept by a perpendicular shock moving away from the Sun. The main finding from the data analysis is that a Weibull distribution represents a good fitting function to the measured particle spectrum over the energy range from 0.1 to 30 MeV. To interpret such an observational result, we provide a theoretical derivation of the Weibull spectrum in the framework of the acceleration by “killed” stochastic processes exhibiting power-law growth in time of the velocity expectation, such as the classical Fermi process. We find an overall coherence between the experimental values of the Weibull spectrum parameters and their physical meaning within the above scenario. Hence, our approach based on the Weibull distribution proves to be useful for understanding SSEs. With regard to the present event, we also provide an alternative explanation of the Weibull spectrum in terms of shock-surfing acceleration.},
doi = {10.3847/1538-4357/AA633A},
journal = {Astrophysical Journal},
number = 2,
volume = 837,
place = {United States},
year = {Fri Mar 10 00:00:00 EST 2017},
month = {Fri Mar 10 00:00:00 EST 2017}
}
  • Energetic protons (0.5--20 MeV) appear to be trapped between a pair of shocks associated with the major solar flares of April 15 and April 28, 1978. Prolonged trapping (for a period of weeks) is implied by the large count rate enhancement and the large range of radial distances and longitudinal angles over which the trapping is observed. Shocks associated with both flares are detectable in the Pioneer 10 and Pioneer 11 plasma analyzer data. These shock/flare associations are different from those previously published by others studying the interplanetary events. The evidence for trapping has not been recognized heretofore most likelymore » because of the unobservability of the April 15, flare (located approx.150/sup 0/E of the sun-earth line) and of the faintness of the signature in the plasma data of one shock in each pair. The apparent ability of a shock whose plasma signature is extremely weak to confine MeV protons in the outer solar system may have significant implications for cosmic ray studies. Contrary to earlier analyses of these data, the results of our analyses also imply significant azimuthal asymmetry in plasma and energetic particle behavior even at distances as far as 16 AU from the sun. The combination of these observations provides evidence for unexpectedly complex interactions in the outer solar system between energetic particles and solar wind plasma.« less
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  • Using energetic particle data from the Helios 1 spacecraft, the authors have studied the particles associated with 26 coronal mass ejection (CME)/shock pairs. The authors divided the particle events into four classes on the basis of the relative intensities of their prompt and shock-associated components. Events in group 1 have stong prompt and shock components and are associated with energetic CMEs originating, most commonly, with central meridian flares. Group 2 events characteristically have strong prompt components and weaker shock components and are associated with western hemisphere (i.e., favorably located) flares. Group 3 consists of events with absent prompt components andmore » strong shock components; the CMEs probably originate in disappearing filament events near central meridian. Group 4 events show no particle enhancements and are associated with slow CMEs and slow shocks. This study supports their previous work in which solar energetic particle events were discussed in terms of a solar component and an interplanetary shock component and where the shock effects depend on the connection longitude of the observer.« less
  • We study periods of elevated energetic particle intensities observed by STEREO-A when the partial pressure exerted by energetic (≥83 keV) protons (P{sub EP}) is larger than the pressure exerted by the interplanetary magnetic field (P{sub B}). In the majority of cases, these periods are associated with the passage of interplanetary shocks. Periods when P{sub EP} exceeds P{sub B} by more than one order of magnitude are observed in the upstream region of fast interplanetary shocks where depressed magnetic field regions coincide with increases of energetic particle intensities. When solar wind parameters are available, P{sub EP} also exceeds the pressure exertedmore » by the solar wind thermal population (P{sub TH}). Prolonged periods (>12 hr) with both P{sub EP} > P{sub B} and P{sub EP} > P{sub TH} may also occur when energetic particles accelerated by an approaching shock encounter a region well upstream of the shock characterized by low magnetic field magnitude and tenuous solar wind density. Quasi-exponential increases of the sum P{sub SUM} = P{sub B} + P{sub TH} + P{sub EP} are observed in the immediate upstream region of the shocks regardless of individual changes in P{sub EP}, P{sub B}, and P{sub TH}, indicating a coupling between P{sub EP} and the pressure of the background medium characterized by P{sub B} and P{sub TH}. The quasi-exponential increase of P{sub SUM} implies a radial gradient ∂P{sub SUM}/∂r > 0 that is quasi-stationary in the shock frame and results in an outward force applied to the plasma upstream of the shock. This force can be maintained by the mobile energetic particles streaming upstream of the shocks that, in the most intense events, drive electric currents able to generate diamagnetic cavities and depressed solar wind density regions.« less