Introduction

The Shockspotter program attempts to identify possible interplanetary shocks using the near-real time data from the CELIAS/MTOF/PM sensor on the SOHO spacecraft. The 3 most recent shock candidates are listed on the PM home page. An archival history file, using only the final science data files, is updated weekly.

There are several reasons for developing automated procedures for identifying various interplanetary structures; for example the online capability of such techniques would be a useful addition to a space weather warning system, and could be used by spacecraft to make on-board decisions to change modes of operation. In addition, the objectivity of such an automated identification system is a benefit for unbiased statistical analyses.

This simplified Table lists the expected behavior of solar wind parameters across some common interplanetary structures. Rigorously, shocks cannot be identified using solely solar wind proton data; one needs electron and minor ion measurements in addition to magnetic field measurements (there is no magnetometer on SOHO). Nevertheless, a reasonable system of identification of at least the larger shocks is possible using only the PM data.

There are 2 questions one must ask:

1) How many of the events found by Shockspotter are actually shocks?
The "percentage shocks" entries given in the table below were derived by comparing the Shockspotter output for the 6-year period from Jan 21, 1996 to March 31, 2002 with the shock events listed on the "Figs" web page. The shock events listed on that page were originally subjectively identified by eye, and then checked to be at least consistent with the magnetic field behavior from either the ACE or WIND spacecraft. Since some of those identifications may be incorrect, the stated "percentage shocks" is only approximate. The Confidence Level is derived a bit more conservatively by decreasing the "# shocks" entries by 1 for each category. In general, a stronger shock will fall into a higher Zone than a weaker shock, and a shock detected during quiet pre-existing solar wind conditions wil fall into a higher zone than one detected during turbulent conditions. Fig 1 shows the zone definitions for forward shocks; more details are available.

Type Zone # Events # shocks percentage shocks Confidence Level
Forward 1 38 17 45% 42%
Forward 2 28 20 71% 68%
Forward 3 36 34 94% 92%
Forward 4 67 67 100% 99%
Reverse 1 13 6 46% 38%


2) How many shocks are missed by Shockspotter?

This is a much more difficult question to answer. There are 105 forward or reverse shocks from 21 Jan 1996 to 21 Jan 2001 listed on the "Figs" page (as of this writing), of which 93 were found by Shockspotter. However, the shock list on the "Figs" page is far from complete; there are undoubtedly many weaker shocks that were not identified with this subjective procedure. For example, one of the Shockspotter tests requires a minimum speed jump of about 25 km/s (this minimum required jump speed is actually a function of density, see here for details). If one asks "How many relatively large shocks are missed by Shcoksptter?", then one can estimate that less than 10% of shocks of sufficient strength to reach Zone category 2 or higher are undetected by Shockspotter.

Let us know if you would like to be notified by email when a shock candidate is first identified. We also welcome any comments or suggestions. In case you're curious, our watchdog (known as ShockSpot) generally wakes up only once, looks around, and then goes back to sleep. If, however, there has been a shock spotted within the last day or so, ShockSpot transforms into a Mastiff and barks once to get your attention.

More details are available