A statistical study on the stand-off distances of interplanetary coronal mass ejections

Abstract

We have analyzed the stand-off distance values of 101 interplanetary CMEs (ICMEs) observed during the period 1997–2005. Main aim of the present work is to study the stand-off distance and its dependence on various parameters of CMEs, ICMEs and IP shocks, Alfvenic Mach numbers and transit time. From the distribution, the stand-off time and stand-off distance values of many of the events are found to be in the range between ~2–20h and ~1–40R⊙ (R⊙=Solar radius). From the correlation between speed of CMEs and stand-off distance, we noted smaller stand-off distance for energetic CMEs, which indicated that the driver CME (CME which is generating the shock) and its shock travel closely together. From the correlation plot between CME acceleration and stand-off distance, we found that the highly decelerated events and highly accelerated events have lower stand-off distance range (i.e., 10–40R⊙) than the other events. The events with longer travel time to reach 1AU (>70h) show stand-off times ≤20h and for those faster events (VCME>2200km/s) with smaller travel time (≤40h), stand-off time is extremely low (≤10h). A wide range of stand-off distance is seen for a particular value of CME and ICME parameters. The poor correlations of stand-off distance with all the above parameters confirm that the stand-off distance does not strongly depend on CME, ICME and IP shock parameters, but depends on a combination of all these parameters. On the other hand, the faster CMEs having lower stand-off distance and/or stand-off time imply that as long as the CMEs are energetic, the CMEs and shocks travel closely together. Also, it can be noted that the stand-off distance is not only dependent on gamma, but it is related to other parameters.