Solar-Wind Drivers or Internal Magnetospheric Processes

== Multiple-Dip Geomagnetic Storms: Solar-Wind Drivers or Internal Magnetospheric Processes ==

Monday, June 23, from 1:30 to 5:00 pm Conveners: Vania Jordanova & Ian Richardson

A classic geomagnetic storm (as measured for example by the Dst index) consists of a rapid fall to minimum Dst (the main phase) and a slower recovery to near pre-storm conditions (recovery phase). However, some storms show a more complex development, with more than one local minimum in Dst (“dip”). The main objective of this session was to use observations, theory, and modeling to assess the current status and establish collaborative efforts towards understanding the physical processes of geomagnetic storms. In particular, the session explored the solar-wind drivers of multiple-dip storms and whether the associated reintensification of geomagnetic activity produced any unusual signatures in the magnetosphere.

The session included two presentations discussing the interplanetary drivers of multipledip storms. Jie Zhang (GMU) & Ian Richardson (GSFC/UMD) presented a survey of the solar and interplanetary drivers of all the 165 “dips” in the 90 intense (Dst< -100 nT) geomagnetic storms during 1996-2006 and concluded that multiple-dip storms are common, including ~70% of these intense storms, consistent with the earlier results of Kamide et al. [1998]. Charles Farrugia (UNH) & Vania Jordanova (LANL) described examples of how mergers of interplanetary coronal mass ejections (ICMEs, also known as “ejecta”) can lead to two-step geomagnetic storms. They emphasized that a major factor in severe, long-duration, double-dip storms is the very elevated plasma sheet density (Nps) of solar wind origin (Nps ~ Nsw1/2). It was discussed that multiple-dip storms are caused by interplanetary structures that include regions of southward Bz separated by less geoeffective solar wind. Storms driven by a single ICME can have double dips if there are southward fields in the sheath and ICME, as also discussed by [Kamide et al. 1998]. Multiple dip storms can also result from ICME-ICME interactions, as discussed by Farrugia et al. [2006], sheath regions formed by multiple ICMEs, shocks moving through a preceding ICME with a southward field that is intensified by the shock compression, by corotating interaction regions, and by combinations of these various scenarios. Interestingly, the occurrence rate of multiple-dip storms does not depend on whether the driver is a single ICME, involves multiple ICMEs, or is a CIR. Hence, the complexity of a storm profile is not necessarily a reflection of the complexity of the solar/interplanetary driver.

Magnetospheric dynamics during multiple-dip storms were discussed by several GEM participants. To motivate collaboration between the SHINE and GEM communities, the event list of Zhang and Richardson was made available to likely participants in the session before the meeting. Michelle Thomsen (LANL) presented an overview of plasma sheet dynamics in double-dip storms using data from geosynchronous satellites, while Chris Mouikis (UNH) discussed ion composition variations in double-dip storms from Cluster data. It was noted that 1) high plasma sheet densities persist after the first dip, but not after the second one; 2) the ion and electron temperatures in the plasma sheet are not significantly affected by the second dip; and 3) O+ is enhanced throughout the storm; there is some indication of a further O+ enhancement in the second dip but more ion composition measurements are needed to confirm this. Some unusual plasmasphere dynamics and wrapping of drainage plumes during the second dip were presented by Jerry Goldstein (SWRI) using data from IMAGE satellite. Mikhail Sitnov (JHU/APL) presented simulation results obtained using a dynamical empirical magnetic field model with enhanced spatial resolution (TS07D) and showed that the second dip is often provided by an anomalously strong tail current, approaching close to the Earth, rather than by the conventional ring current closed through the Region 2 Birkeland system. An analysis of ring current simulations for single- and multiple-dip storms with a kinetic ring current model presented by Mike Liemohn (UMI) showed that single-dip storms are well reproduced, but ring current injection during multiple-dip storms is underestimated, indicating that internal feedback may be important for these storms. Global SWMF simulations of multiple-dip storms from the Sun to the Earth were presented by Tamas Gombosi (UMI) and the results were compared with observations. Noe Lugaz (UHI) discussed the Solar-Heliospheric and space weather perspectives of geoeffective sheaths in intense multiple-dip geomagnetic storms. It was concluded that several challenges remain for modeling/forecasting multiple dip storms including understanding the CME initiation process, modeling ICME-ICME interactions, and including realistic magnetic fields.