The Earth's outer radiation belt electrons increase when the magnetosphere is surrounded by the high-speed solar wind stream, while the southward interplanetary magnetic field (IMF) is also known as an important factor for the flux enhancement. In order to distinguish the two different kinds of solar wind parameter dependence statistically, we investigate the response of the outer belt to stream interaction regions (SIRs). A total of 179 SIR events are identified for the time period from 1994 to 2005. We classify the SIR events into two groups according to the so-called “spring-toward fall-away” rule: IMF sector polarity after the stream interface is toward in spring or away in fall (group A) and vice versa (group B). According to the Russell-McPherron effect, groups A and B have a significant negative and positive offset of the IMF B z after the stream interface, respectively. Comparing groups A and B by superposing about the stream interface, only IMF B z dependence can be obtained because the other solar wind parameters change in the same manner. As a result, the greatest flux enhancement is found in the highest-speed streams with a southward offset of the IMF B z , indicating that only the solar wind speed by itself is not a sufficient condition for the large flux enhancement. It is also found that the large flux enhancement tends to be associated with weak geomagnetic activities with minimum Dst of about ?50 nT on average, implying that the existence of intense magnetic storms is not essential for the flux enhancement.
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