Abstract
The first super geomagnetic storm (Dst < −200 nT) of solar cycle 24 occurred on “St. Patrick’s day” (17 March 2015). Notably, it was a two-step storm. The source of the storm can be traced back to the solar event on 15 March 2015. At ~2:10 UT on that day, SOHO/LASCO C3 recorded a partial halo coronal mass ejection (CME), which was associated with a C9.1/1F flare (S22W25) and a series of type II/IV radio bursts. The initial propagation speed of this CME is estimated to be ~668 km/s. An interplanetary (IP) shock, likely driven by a magnetic cloud (MC), arrived at the Wind spacecraft at 03:59 UT on 17 March and caused a sudden storm commencement. The storm intensified during the Earth’s crossing of the ICME/shock sheath and then recovered slightly after the interplanetary magnetic field (IMF) turned northward. The IMF started turning southward again due to a large MC field itself, which caused the second storm intensification, reaching a minimum value (Dst = −223 nT). It is found that the first step is caused by a southward IMF component in the sheath (between the upstream shock and the front of the MC), whereas the second step is associated with the passage of the MC. The CME that erupted on 15 March is the sole solar source of the MC. We also discuss the CME/storm event with detailed data from observations (Wind and SOHO) and our algorithm for predicting the intensity of a geomagnetic storm (Dstmin) from known IP parameter values. We found that choosing the correct Dstmin estimating formula for predicting the intensity of MC-associated geomagnetic storms is crucial for space weather predictions.
Highlights
Geomagnetic storms can be categorized, in terms of geomagnetic activity index (Dst), into three categories: (1) major storms, minimum Dst (Dstmin) of −100 nT or less; (2) moderate storms, Dstmin falls between −50 and −100 nT; and (3) weak storms, −30 nT < Dstmin < −50 nT (Gonzalez et al 1994)
At ~2:10 UT on that day, SOHO/LASCO C3 recorded a partial halo coronal mass ejection (CME), which was associated with a C9.1/1F flare (S22W25) and a series of type II/IV radio bursts
The speed of VC2 represented the speed of the “driver” of the shock, not the speed of the “shock.” The results of this study suggested that “using the right CME propagation speed is essential for space weather prediction.”
Summary
Geomagnetic storms can be categorized, in terms of geomagnetic activity index (Dst), into three categories: (1) major (intense or great) storms, minimum Dst (Dstmin) of −100 nT or less; (2) moderate storms, Dstmin falls between −50 and −100 nT; and (3) weak storms, −30 nT < Dstmin < −50 nT (Gonzalez et al 1994). ΔT, duration of the MC encounter (i.e., ΔT = end time – start time of MC passage); VMC, average solar wind speed (in km s−1) within the MC; 2Ro, estimated diameter (in AU), where RO is the model-estimated radius; BO, estimated axial magnetic field magnitude (in nT); H, Handedness (+1 for righthanded or −1 for left-handed); фA, θA, longitude and latitude, respectively, of the MC axis (GSE coordinates); to, estimated center time of the MC; χR, square root of the reduced Chi-squared of the MC fit; asf(%), asymmetry factor (in %), which depends on to and ΔT; CA (%), estimated relative closest approach distance, i.e., yo/Ro (in %) where yo is closest approach; Φo, estimated axial magnetic flux (in 1020 Mx); JO, estimated total axial current density (in μA km−2); Δt, length of the averages used in the analysis; these are usually 15, 30 min, or 1 h; βCA, cone angle, the angle between the MC axis and the X-axis (in GSE coordinates);.
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