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ACE News #93 - Nov 7, 2005

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Effects of Interplanetary Shocks on Energetic Particles

From 1998 February 1 to 2003 October 28 the ACE spacecraft detected near 300 interplanetary shock waves (a preliminary list is available at www-ssg.sr.unh.edu/mag/ace/ACElists/obs-list.html). Among these shocks, we have selected 191 fast forward interplanetary shocks with evidence of either being driven by, or related to, the passage of interplanetary coronal mass ejections (ICMEs). Statistical analyses of the shock parameters computed by applying the method developed by Viñas and Scudder to the solar wind and magnetic field observations from SWEPAM and MAG reveals that:

• Most of the shocks (91%) have density compression ratios r_n below 3.5.
• Most of the shocks (90%) have magnetic compression ratios r_b below 3.0.
• Only one shock was observed with speed Vs above 350 km/s (Here Vs is the shock speed along the normal to the shock in the upstream frame of reference).
• About 60% of all Alfvenic Mach numbers (computed as MA=|Vs|/[|B|(μ₀.ρ)^-1/2]) are below 2.5.
• There is a clear tendency to observe quasi-perpendicular shocks at 1 AU. The fraction of shocks where the angle between the magnetic field and the shock normal (θ_Bn) is > 60° is 57%, whereas only 10.5% of the events have θ_Bn ≤ 30°.

Energetic particle signatures observed by EPAM five hours prior to and following the passage of the shocks are classified as: (1) Classic energetic storm particle (ESP) events (i.e., a slow rise of particle intensities beginning several hours before shock passage and flat time-intensity profiles after the shock); (2) spike events (i.e., a rapid rise of particle intensities at the time of the shock lasting a few (~10) minutes); (3) events with a slow intensity increase and a short duration (≤10 minutes) at or near the shock superimposed on it; (4) step-like post-shock intensity increases; (5) irregular time-intensity profiles with flux variations not coincident with the shock passage and not fitting types 1 to 4 above; and (0) no variations at all above the pre-existing intensity level. The Figure shows the distribution of particle events, color-coded according to the time-intensity profiles of 46-68 keV ions as a function of θ_Bn for the 162 shocks in which the shock normal could be determined. The number of shocks within each θ_Bn range is indicated at the bottom of the bin. These distributions (wherever ≥10 per bin) show that:

• Shock parameters do not determine unequivocally the properties of the associated energetic particle event.
• About half of the shocks do not change the nature of the pre-existing particle intensities. Although such events (i.e., Types 0 and 5) are more abundant in those bins corresponding to weak and slow shocks, a few exceptions of fast and strong shocks without any significant particle increase were also observed.
• Both shock spikes and classic ESP events can be associated with nearly the full range of θ_Bn angles.

Comparison of shock parameters with our classification of the events reveals that other factors (in addition to the shock parameters) contribute to the formation of the associated energetic particle event. Shock parameters are a local measure of the shock at the observer's position at the time of the shock passage, whereas the particle event results from the superposition of remote particle injections as the shock approaches the observer. Thus, the final properties of the particle events are a combination of particles produced at various different times and locations on the shock front with different shock parameters.

Submitted by D. Lario, G. C. Ho, R. B. Decker, and E. C. Roelof of Johns Hopkins University; Q. Hu of the University of California at Riverside; and C. W. Smith of the University of New Hampshire. Questions or comments can be addressed to david.lario@jhuapl.edu.

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