Relationship between solar wind low‐energy energetic ion enhancements and large geomagnetic storms

Abstract

It is well established that energetic ion enhancements (an energetic ion enhancement will hereafter be referred to as an EIE) are partly due to acceleration by interplanetary shocks as the shocks propagate towards Earth and that arrivals of these shocks at Earth are well associated with geomagnetic storms. The observation of EIEs at satellites located at L1 is a potential tool for predicting the arrival of interplanetary shocks hours before they arrive at L1. Following an earlier study using WIND satellite data [Smith and Zwickl, 1999], we evaluate the potential of EIEs for forecasting geomagnetic storms during the period of February 1998 through December 2000. Since there are many more EIEs than large geomagnetic storms, additional associations that might improve the identification of precursors of large storms were investigated. These included probable solar sources, accompanying interplanetary shocks, and the shocks' interplanetary drivers. Solar images and data from the Advanced Composition Explorer (ACE) and WIND satellites were used. The Potsdam Kp was used to specify geomagnetic storm severity. Almost all large geomagnetic storms (Kp ≥ 7) in this time period were preceded by EIEs that were associated with shocks driven by transient interplanetary disturbances. The converse is not true. Although most transient‐associated EIEs were followed by some geomagnetic activity, there is a large span in the response. Most (95%) of EIEs with maximum flux ≥1.105 pfu were followed by activity with Kp > 4 and 80% by storms with Kp ≥ 5. For a threshold of 3.105 pfu, 67% of the large storms would be identified, 89% of the EIEs were followed by storms with Kp ≥ 5, and 53% by large storms (Kp ≥ 7). Using the additional information on the interplanetary drivers results in an increase in the correctly predicted events from 53% to 61%. For a threshold of 1.106 pfu, all (100%) EIEs were followed by storms with Kp ≥ 5, 76% of the events were correctly predicted but 16 of the 30 large storms were missed. Most of the EIEs were followed by significant periods of southward Bz and in close to 70% of the EIEs, the Bz following the EIE was first northward before turning southward. We also investigated the relationship of the EIEs to halo or partial‐halo coronal mass ejections (CMEs), and found that EIE events associated with halo CMEs are more likely to be followed by a large geomagnetic storm, but a lack of halo or partial‐halo CME association does not preclude the occurrence of a large geomagnetic storm.