In this paper, we demonstrate that coronal mass ejection (CME)-driven shocks can be detected in white light coronagraph images and in which properties such as the density compression ratio and shock direction can be measured. Also, their propagation direction can be deduced via simple modeling. We focused on CMEs during the ascending phase of solar cycle 23 when the large-scale morphology of the corona was simple. We selected events which were good candidates to drive a shock due to their high speeds (V > 1500 km s −1 ). The final list includes 15 CMEs. For each event, we calibrated the LASCO data, constructed excess mass images, and searched for indications of faint and relatively sharp fronts ahead of the bright CME front. We found such signatures in 86% (13/15) of the events and measured the upstream/downstream densities to estimate the shock strength. Our values are in agreement with theoretical expectations and show good correlations with the CME kinetic energy and momentum. Finally, we used a simple forward modeling technique to estimate the threedimensional shape and orientation of the white light shock features. We found excellent agreement with the observed density profiles and the locations of the CME source regions. Our results strongly suggest that the observed brightness enhancements result from density enhancements due to a bow-shock structure driven by the CME.

The existence of shocks driven by Coronal Mass Ejections (CMEs) has always been assumed based on the superalfvenic speeds for some of these events and on indirect evidence such as radio bursts and distant streamer deflections. However, the direct signature of the plasma enhancement at the shock front has escaped detection until recently. Since 2003, work on LASCO observations has shown that CME-driven shocks can be detected by white light coronagraph observations from a few solar radii to at least 20 R sun . Shock properties, such as the density compression ratio and their direction can be extracted from the data. We review this work here and demonstrate how to recognize the various shock morphologies in the images. We also discuss how the two-viewpoint coronagraph observations from the STEREO mission allow the reconstruction of the 3D envelope of the shock revealing some interesting properties of the shocks (e.g., anisotropic expansion).

[1] We performed an event-by-event study of 47 geomagnetic storms (GSs) that occurred during the ascending phase of solar cycle 23. All the GSs are associated with the passage of a shock and an interplanetary coronal mass ejection (ICME). For each event, we identified the section in the interplanetary (IP) medium causing the GS (the sheath behind the shock, the main body of the ICME or the combination of both). On average, the most intense GSs are caused by sheaths, followed by sheath-ICME combinations and by ICMEs. We obtained the correlation coefficients between the intensity of each GS (minimum Dst) and several solar wind parameters. We found that the well-known correlation between the GS intensity and the solar wind convected electric field, E y , stands for the GSs caused by ICMEs (CC = −0.88) and sheath-ICME combinations (CC = −0.95), but it is very low for the GSs caused by sheaths (CC = −0.44). In contrast, we found a very good correlation between the GSs caused by sheaths and the total convected electric field (SE y ) (CC = −0.89). On the other hand, we estimated the total perpendicular pressure (P t ) for each IP event associated with the GSs and identified the three different types of P t profiles. The most intense GSs are related with IP events with P t = 1, but moderate and less intense storms are associated with the three P t profiles. The correlations between the Dst and the solar wind parameters results that the CCs decrease significantly for IP events having a P t profile of 3.

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The stream interaction regions (SIRs) are generated in the interplanetary medium when a fast solar wind stream overtakes a slower one. If these large‐scale phenomena interact with the Earth's magnetosphere, they can give rise to geomagnetic storms (GSs). Their geoeffectivity is measured using magnetic indices at different latitudes. In this study we analyzed the geoeffectiveness of 20 GSs that were generated by SIRs during the period of 2007 to 2008 and observed by the ACE, Wind, and STEREO‐A/B spacecraft. We compared the geomagnetic response to the SIRs‐magnetosphere interaction employing different geomagnetic indices at low, middle, and high latitudes. The geoeffectiveness was 50%, 55%, and 90% using the criteria of the aa, Kp, and SYM‐H indices, respectively. We found that in most cases the maximum intensity of each index was in the weak to moderate range. According to the SYM‐H index, a 10%, 60%, and 10% of the forward shocks were followed by quiet, weak, and moderate GSs, respectively. The 10% and 20%, however, were followed by minor and moderate GSs, respectively, according to the Kp index. We analyzed the geoeffective region within the SIRs with respect to the relative position of the stream interface (SI). For 75% of GSs, their maximum intensity occurred during the disturbed fast solar wind (after the passing of the SI), which would be related to the efficiency of a SIR. The time difference Δt between the passing of the SI and the maximum intensity in each index was less than 36 h.