Variability of Interplanetary Shock and Associated Energetic Particle Properties as a Function of the Time Window Around the Shock

Abstract

We study the effect of sampling windows on derived shock and associated energetic storm particle (ESP) properties in 296 fast-forward interplanetary shocks using Advanced Composition Explorer measurements at 1 au between 1998 February and 2013 August. We vary the time windows from 2 minutes to 20 minutes for the shock properties and from 2 minutes to 540 minutes for ESP properties. Variability is quantified by the median absolute deviation statistic. We find that the magnetic, density, and temperature compression ratios vary from their median values by 17.03%, 20.05%, and 25.91%, respectively; shock speed by 16.26%; speed jump by 45.46%; Alfvénic Mach number by 31.53%; and shock obliquity by 24.25%. Spectral indices in the 2 minute–540 minute windows downstream of the shock vary from the median value of 1.79 by 26.05% and by 30.53% from the 1.70 median value upstream of the shock. Similarity of ESP spectral indices upstream and downstream of the shock suggest that these ESP populations are likely locally accelerated at the shock. Furthermore, we find that for a moving sampling window around the shock, values for the density ratio hold for ∼10 minutes; the magnetic ratio and shock speed jump hold for ∼30 minutes and ∼60 minutes, respectively. Fixing the upstream window to 2 minutes and moving only in the downstream direction, the density ratio holds for ∼60 minutes downstream, magnetic ratio holds for ∼30 minutes, and the shock speed jump holds for ∼110 minutes. Beyond these time windows, derived shock properties are no longer representative of shock properties. These results provide constraints for modeling and forecasting efforts of shock and ESP-associated properties.

Keywords

• Interplanetary shocks
• Solar energetic particles
• Solar coronal mass ejections
• Interplanetary particle acceleration
• Timing variation methods