The problem of simultaneous detection of a sudden commencement (SC) and a main impulse (MI) of a geomagnetic storm by different magnetic stations is discussed using the example of two SC-events of March 17, 2013 and March 17, 2015. The interplanetary coronal mass ejections and associated shock waves that caused the studied the SC-events under investigation are identified as coronal mass ejections observed near the Sun, whose source regions were located in different solar hemispheres. The linear velocities projections of the two sampled noticeable difference in the velocities of the associated interplanetary shock waves. One-second time resolution data were used to relate the detection start times of SC and MI to the latitude and longitude of the magnetic station on the Earths surface for each SC-event analyzed; trends were found in the form of a linear or quadratic relationship. We used an original approach to determine the SC and MI detection start times. A conclusion was made that the SC and MI detection start times can differ from a few seconds to more than one minute on magnetic observatories located at different geographical latitudes and longitudes. The magnetic stations that first to detected SC and MI in each analyzed SC-event were identified. It was suggested that the position of the first station that detect SC and MI depends on the characteristics of the interplanetary shock waves affecting the Earths magnetosphere.
Iu.S. Zagainova (Wed,) studied this question.