Statistical Results for Solar Energetic Electron Spectra Observed over 12 yr with STEREO/SEPT

This work presents a statistical analysis of near-relativistic solar energetic electron event spectra near 1 au. We use measurements of the Solar Electron and Proton Telescope (SEPT) on board STEREO in the energy range of 45–425 keV and utilize the SEPT electron event list containing all electron events observed by STEREO A and STEREO B from 2007 through 2018. We select 781 events with significant signal-to-noise ratios for our analysis and fit the spectra with single or broken-power-law functions of energy. We find 437 events showing broken power laws and 344 events only showing a single power law in the energy range of SEPT. For those events with broken power laws, we find a mean break energy of about 120 keV. We analyze the dependence of the spectral index on the rise times and peak intensities of the events as well as on the presence of relativistic electrons. The results show a relation between the power law spectral index and the rise times of the events with softer spectra belonging to rather impulsive events. Long rise-time events are associated with hard spectra as well as with the presence of higher-energy (>0.7 MeV) electrons. This group of events cannot be explained by a pure flare scenario but suggests an additional acceleration mechanism, involving a prolonged acceleration and/or injection of the particles. A dependence of the spectral index on the longitudinal separation from the parent solar source region was not found. A statistical analysis of the spectral indices during impulsively rising events where the rise times are below 20 minutes is also shown.

[1]  Jiansen He,et al.  Electron Acceleration by ICME-driven Shocks at 1 au , 2017, The Astrophysical Journal.

[2]  D. Burgess,et al.  Electron acceleration at quasi-perpendicular shocks in sub- and supercritical regimes: 2D and 3D simulations , 2018, Monthly Notices of the Royal Astronomical Society.

[3]  E. Eriksson,et al.  Electron Power-Law Spectra in Solar and Space Plasmas , 2018, Space Science Reviews.

[4]  Andreas Klassen,et al.  Long-lasting injection of solar energetic electrons into the heliosphere , 2018, 1802.04722.

[5]  B. Heber,et al.  Interpretation of increased energetic particle flux measurements by SEPT aboard the STEREO spacecraft and contamination , 2018, 1801.01040.

[6]  B. Heber,et al.  Near-relativistic electron increases during the 4 Jan 2008, 13 Jan 2011, and 7 May 2011 CIR events , 2018 .

[7]  D. Gary,et al.  Investigating the Origins of Two Extreme Solar Particle Events: Proton Source Profile and Associated Electromagnetic Emissions , 2017 .

[8]  N. Dresing,et al.  MULTI-SPACECRAFT OBSERVATIONS AND TRANSPORT MODELING OF ENERGETIC ELECTRONS FOR A SERIES OF SOLAR PARTICLE EVENTS IN AUGUST 2010 , 2016 .

[9]  V. Petrosian.  PARTICLE ACCELERATION IN SOLAR FLARES AND ASSOCIATED CME SHOCKS , 2016, 1605.04022.

[10]  N. Raouafi,et al.  LONGITUDINAL PROPERTIES OF A WIDESPREAD SOLAR ENERGETIC PARTICLE EVENT ON 2014 FEBRUARY 25: EVOLUTION OF THE ASSOCIATED CME SHOCK , 2016 .

[11]  B. Heber,et al.  Efficiency of particle acceleration at interplanetary shocks: Statistical study of STEREO observations , 2016, 1602.03440.

[12]  G. Mann.  Energetic electrons generated during solar flares , 2015 .

[13]  Stanford,et al.  Solar energetic particle access to distant longitudes through turbulent field-line meandering , 2015, 1508.03164.

[14]  M. Marsh,et al.  DRIFT-INDUCED DECELERATION OF SOLAR ENERGETIC PARTICLES , 2015, 1506.04015.

[15]  Andreas Klassen,et al.  CIRCUMSOLAR ENERGETIC PARTICLE DISTRIBUTION ON 2011 NOVEMBER 3 , 2015 .

[16]  D. Heynderickx,et al.  Release timescales of solar energetic particles in the low corona , 2014 .

[17]  F. Guo,et al.  THE ACCELERATION OF ELECTRONS AT COLLISIONLESS SHOCKS MOVING THROUGH A TURBULENT MAGNETIC FIELD , 2014, 1409.5854.

[18]  B. Heber,et al.  Wide longitudinal distribution of interplanetary electrons following the 7 February 2010 solar event: Observations and transport modeling , 2014 .

[19]  B. Heber,et al.  Statistical survey of widely spread out solar electron events observed with STEREO and ACE with special attention to anisotropies , 2014 .

[20]  E. Christian,et al.  > 25 MeV Proton Events Observed by the High Energy Telescopes on the STEREO A and B Spacecraft and/or at Earth During the First ∼ Seven Years of the STEREO Mission , 2014 .

[21]  M. Fujimoto,et al.  Electron acceleration to relativistic energies at a strong quasi-parallel shock wave , 2013, Nature Physics.

[22]  W. Dröge,et al.  The Large Longitudinal Spread of Solar Energetic Particles During the 17 January 2010 Solar Event , 2012, 1206.1520.

[23]  R. Mewaldt,et al.  A Twin-CME Scenario for Ground Level Enhancement Events , 2012 .

[24]  E. Kontar,et al.  Onsets and spectra of impulsive solar energetic electron events observed near the Earth , 2009, 0903.2576.

[25]  Robert P. Lin,et al.  SPECTRA OF SOLAR IMPULSIVE ELECTRON EVENTS OBSERVED NEAR EARTH , 2009 .

[26]  S. Krucker,et al.  Open magnetic flux tubes in the corona and the transport of solar energetic particles , 2008 .

[27]  E. Christian,et al.  The STEREO Mission: An Introduction , 2008 .

[28]  L. Duvet,et al.  The Solar Electron and Proton Telescope for the STEREO Mission , 2008 .

[29]  R. Leske,et al.  The High Energy Telescope for STEREO , 2008 .

[30]  S. Krucker,et al.  Solar Flare Electron Spectra at the Sun and near the Earth , 2007 .

[31]  Technology of China,et al.  Magnetohydrodynamic Simulation of the Interaction between Interplanetary Strong Shock and Magnetic Cloud and its Consequent Geoeffectiveness 2: Oblique Collision , 2006, 0904.0769.

[32]  D. C. Hamilton,et al.  Voyager 1 in the Foreshock, Termination Shock, and Heliosheath , 2005, Science.

[33]  J. Sakai,et al.  Proton and electron acceleration by quasi-perpendicular fast magnetosonic shocks in interplanetary space , 2005 .

[34]  S. Hoang,et al.  Coronal phenomena at the release of solar energetic electron events , 2005 .

[35]  S. Krucker,et al.  Intensity variation of large solar energetic particle events associated with coronal mass ejections , 2004 .

[36]  W. Dröge.  Solar Particle Transport in a Dynamical Quasi-linear Theory , 2003 .

[37]  Edmond C. Roelof,et al.  Impulsive Near-relativistic Solar Electron Events: Delayed Injection with Respect to Solar Electromagnetic Emission , 2002 .

[38]  W. Dröge.  The Rigidity Dependence of Solar Particle Scattering Mean Free Paths , 2000 .

[39]  J. M. Bosqued,et al.  A three-dimensional plasma and energetic particle investigation for the wind spacecraft , 1995 .

[40]  Robert M. Winglee,et al.  Characteristics of Hard X-Ray Spectra of Impulsive Solar Flares , 1992 .

[41]  Reuven Ramaty,et al.  Shock acceleration of electrons and ions in solar flares , 1985 .

[42]  R. Lin.  Energetic solar electrons in the interplanetary medium , 1985 .

[43]  T. T. von Rosenvinge,et al.  Solar He-3-rich events and nonrelativistic electron events - A new association , 1985 .

[44]  B. Tsurutani,et al.  Acceleration of >47 keV Ions and >2 keV electrons by interplanetary shocks at 1 AU , 1985 .

[45]  G. Holman,et al.  Solar type II radio emission and the shock drift acceleration of electrons , 1983 .

[46]  R. Lin,et al.  The energy spectrum of 20 keV-20 MeV electrons accelerated in large solar flares , 1982 .

[47]  G. Parks,et al.  Thin sheets of energetic electrons upstream from the earth's bow shock , 1979 .