Late on July 23 , 2012 , the STEREO-A spacecraft encountered a fast forward shock driven by a coronal mass ejection launched from the Sun earlier that same day .
The estimated travel time of the disturbance ( \sim 20 hrs ) , together with the massive magnetic field strengths measured within the ejecta ( > 100 nT ) , made it one of the most extreme events observed during the space era .
In this study , we examine the properties of the shock wave .
Because of an instrument malfunction , plasma measurements during the interval surrounding the CME were limited , and our approach has been modified to capitalize on the available measurements and suitable proxies , where possible .
We were able to infer the following properties .
First , the shock normal was pointing predominantly in the radial direction ( { \bf n } = 0.97 { \bf e } _ { r } -0.09 { \bf e } _ { t } -0.23 { \bf e } _ { n } ) .
Second , the angle between { \bf n } and the upstream magnetic field , \theta _ { Bn } , was estimated to be \approx 34 ^ { \circ } , making the shock “ quasi-parallel , ” and supporting the idea of an earlier “ preconditioning ” ICME .
Third , the shock speed was estimated to be \approx 3300 km s ^ { -1 } .
Fourth , the sonic Mach number , M _ { s } , for this shock was \sim 28 .
We support these results with an idealized numerical simulation of the ICME .
Finally , we estimated the change in ram pressure upstream of the shock to be \sim 5 times larger than the pressure from the energetic particles , suggesting that this was not a standard “ steady-state ” cosmic-ray modified shock ( CRMS ) .
Instead it might represent an early , transient phase in the evolution of the CRMS .