The first super geomagnetic storm of solar cycle 24: "The St. Patrick's day event (17 March 2015)"

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 (Dst_min) from known IP parameter values. We found that choosing the correct Dst_min estimating formula for predicting the intensity of MC-associated geomagnetic storms is crucial for space weather predictions.

Wu et al. Earth, Planets and Space (2016) 68:151 DOI 10.1186/s40623-016-0525-y

The rst super geomagnetic stormofsolar cycle 24: The St. Patricks day event (17 March 2015)

ChinChun Wu1*, Kan Liou2, Ronald P. Lepping3, Lynn Hutting1, Simon Plunkett1, Russ A. Howard1 and Dennis Socker1

Introduction

Geomagnetic storms can be categorized, in terms of geo-magnetic 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, 30nT<Dstmin<50nT (Gonzalez etal. 1994). Major geomagnetic storms that occurred in solar cycle 23 have been studied comprehensively (Zhang etal. 2007). It was found that ~85 % of major geomagnetic storms were associated with interplanetary (IP) coronal mass ejections (ICMEs) (Zhang etal. 2007), and the average storm intensity (Dstmin) was typically larger for magnetic

cloud (MC) events and smaller for non-cloud ICME or corotating fast ow events. The tendency is more pronounced for events associated with X class ares (e.g., Wu et al. 2013). The denition of a super-storm varies in the science community. For example, Astafyeva etal. (2014) used Dstmin<250nT as a super-storm but

Lakhina and Tsurutani (2016) used Dstmin<500nT as a super-storm. Here we call a geomagnetic storm a super-storm when Dstmin drops below 200nT.

Geomagnetic storms are major space weather events. A geomagnetic storm can aect space vehicle operation, interrupt radio communication, and disrupt power grids. During the last solar minimum, 20072009, the sunspot number (SSN) was extremely low and no major geo-magnetic storm was recorded. The largest geomagnetic storms recorded in 2007, 2008, and 2009 were (Dstmin)

70, 72, 79 nT, respectively. The rst geomagnetic

*Correspondence: [email protected]

1 Naval Research Laboratory, Washington, DC 20375, USAFull list of author information is available at the end of the article

2016 The Author(s). This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

Web End =http://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Wu et al. Earth, Planets and Space (2016) 68:151

Page 2 of 12

storm (Dstmin<73 nT) associated with a coronal mass ejection (CME) and a driven shock in solar cycle 24 occurred on 6 April 2010, which was associated with a CME event on 3 April 2010 (e.g., Mstl etal. 2010; Liu etal. 2011; Wood etal. 2011).

The rst major geomagnetic storm in solar cycle 24 occurred during 0506 August 2011 (Dstmin=107nT), and the second and third major geomagnetic storms occurred during 2627 September 2011 [Dstmin

= 101 nT (e.g., Wu et al. 2016a)] and 2425 October 2011 (Dstmin = 132 nT), respectively (e.g., Wood et al. 2016). There were ve major geomagnetic storms recorded in 2012 alone, but only two major geomagnetic storms were recorded in 2013: one (Dstmin=132nT)

on 17 March 2013 (Wu etal. 2016b) and the other one on 1 June (Dstmin=119nT). In the early phase of solar cycle 24, the most intense storm occurred during 0708

March 2012. This storms Dstmin reached 143 nT. The rst super geomagnetic storm of solar cycle 24 did not occur until the declining phase on 17 March 2015 (e.g., Gopalswamy etal. 2015; Kamide and Kusano 2015; Kataoka etal. 2015; Liu etal. 2015; Ramsingh etal. 2015).

It is well known that the southward component of the interplanetary magnetic eld (IMF) plays a major role in the generation of geomagnetic storms (e.g., Tsurutani etal. 1988; Tsurutani 1997). A large southward IMF can be associated with dierent kinds of solar wind structures: (1) an interplanetary (IP) shock wave (sheath) (e.g., Tsurutani etal. 1988; Kamide etal. 1998; Wu and Lep-ping 2008, 2016), (2) a magnetic cloud (MC) (e.g., Wu and Lepping 2002a, b) or an IP coronal mass ejection (ICME) (e.g., Richardson and Cane 2011; Wu and Lepping 2011), (3) a heliospheric current sheet sector boundary crossing (e.g., McAllister and Crooker 1997), or (4) a combination of these interplanetary structures (e.g., Tsurutani and Gonalez 1997; Echer and Gonzalez 2004). Among these, MCs are the most geoeective because they generally contain a large, long-lasting southward IMF (e.g., Wu and Lepping 2008, 2016). About 90% of MC events are associated with geomagnetic storms. A MC event includes the MC itself, usually an upstream shock wave with a sheath (region between the shock and the MC) (e.g., Wu and Lepping 2002a, 2011; Wu et al. 2015). Most solar cycle 23 major storms (88 of them) are associated with an ICME or an MC (Zhang etal. 2007; Wu etal. 2013).

A geomagnetic storm can be induced by (1) the MC sheath, (2) the leading (i.e., front part) region of a MC, (3) the trailing part of an MC, and (4) both sheath and MC regions (e.g., Wu and Lepping 2002a). It is found that the minimum value of the z component of the IMF (Bzmin ) within a MC is well correlated with the intensity of a geo-magnetic storm (Dstmin) (e.g., Wu and Lepping 2002a, 2002b, 2015, 2016); we consider the zGSE-component.

Therefore, measurements of Bzmin in the solar wind can be used to predict Dstmin (e.g., Wu and Lepping 2016).

The St. Patricks Day geomagnetic storm was associated with a CME event that occurred on 15 March 2015. At ~2:10UT on that day, SOHO/LASCO C3 recorded a partial halo CME, which was associated with a C9.1/1F are (S22W25) and a series of type II/IV radio bursts. Notably, this event was a two-step storm. It serves as a good candidate to evaluate the eectiveness of our Dst estimation formulae (Wu and Lepping 2016). Data analysis is presented in Observations. Discussion and Conclusion are presented in Discussion and Conclusion section.

Observations

Propagation ofCMEs nearthe Sun

Figure1 shows a sequence of white-light coronal images recorded by SOHO/LASCO C2 during 00:0003:12UT on 15 March 2015. C2 recorded a CME (named CME15, hereafter) that erupted from the southwest at 01:48UT (Fig. 1b) and appeared as a partial halo CME during 02:1203:12 UT (Fig. 1dh) in the eld of view (FOV) of C2. CME15 was associated with a C9.1/1F are (S22W25) and a series of type II/IV radio bursts. The initial propagation speed of CME15 was ~606 km/s (see Fig.2). SOHO/LASCO C3 recorded the CME15 at 02:18 UT (Fig. 3a) in the FOV. Figure 3ac shows the evolution of CME15 during 02:18 UT06:06 UT on 15 March 2015. The average speed of CME15 in the C3 FOV was 668km/s.

In situ observation atL1

Figure4 shows the in-situ solar wind plasma, magnetic eld (measured by the Wind spacecraft), and the Dst index during 1618 March 2015. The Wind spacecraft recorded an interplanetary (IP) shock (we will refer to this shock as Shock17, marked by a solid vertical line in Fig. 4) at 03:57 UT on 17 March 2015, and a ux-rope candidate a few hours after the crossing of Shock17. Using a MC-tting model (Lepping etal. 1990), Table1 lists the best-t results for the MC (we will refer to this MC as MC17). Figure5a, b shows MC17s magnetic eld structure in cloud coordinates and GSE coordinates, respectively. The solid-black curves are the MC-tting results. MC17 started at 10:36UT (marked by a vertical dashed line with a sharp change in By and B) and ended at 23:36UT (marked by a vertical solid line where there is a sharp drop in |B|) on 17 March. The boundaries of a MC are usually indicated by sharp changes in eld angle (B or B) or in eld magnitude, |B|. The duration (tMC) of MC17 is 13.00h, which is ~30% smaller than an average MC at 1 AU, tMC=18.82h, (e.g., Lepping etal. 2015; Wu and Lepping 2015). The duration of the sheath is about 7h. The MC17 tting results are showed

Wu et al. Earth, Planets and Space (2016) 68:151

Page 3 of 12

in both Fig.5 and Table1. As we see, especially in cloud (CL) coordinates, the directional part of the t is between fair and good, but the B-magnitude part of the t is poor: There is a double peak in the observation but not in the model. The model-eld center of the MC17 was in the

actual center (see bottom panel of Fig.5a for the prole of B), and B is almost at (second panel from bottom)

as required in cloud coordinates. However, according to the denition of MC quality (Qo) (Lepping et al. 2006), we classify MC17 as a quality 3 MC, because the MCs magnetic eld noise level, R (Lepping et al. 2003), was high (R=0.244). If a MCs R is greater than 0.215, the quality of that MC will be 3.

The arrival of Shock17 at the Earth produced a sudden storm commencement (SSC) at 04:45 UT. The value of Dst started decreasing right after the IMF turned southward. The storm intensied (Dst dropped to 80nT at ~10:00 UT) during the passage of the sheath (a region between the IP shock and the driver of the IP shock). Later, the storm recovered slightly (i.e., Dst dropped to ~ 50 nT), shortly after the IMF turned northward.

A few hours later, the IMF turned southward again due to the strongly negative Bz in the magnetic cloud (MC)

and caused the second storm intensication, reaching Dst=223nT on March 17. We conclude that the St.

Patricksday event (March 17) is a two-step storm. The rst step was associated with a southward IMF embedded in the sheath region, whereas the second step was associated with a southward MC eld.

Wu et al. Earth, Planets and Space (2016) 68:151

Page 4 of 12

Propagation/evolution ofthe CME andits driven shock

The estimated speed of the CME varies, ranging from ~606 km/s in the C2 FOV (2.56.0 Solar radii, Rs), ~668 km/s in the C3 FOV (3.732 Rs), to ~706 km/s between 18.82 and 211 Rs (V18.82211Rs = 706 km/s, at 06:06 UT on March 15, CME15s leading edge was at 18.82 Rs, see Fig. 3d). Note that the CME propagation speed measured by C2 or C3 was the projected speed on the plane of the sky. Although the projected speed has been corrected for the CME propagation direction, errors may still exist and contribute to the uncertainties in the arrival time predictions. Table2 lists information about the location and the propagation speed of the CME and its driven shock. Note that the distances measured by C2 and C3 are those above the solar surface. These observations suggest that the propagating speed of the

CME was similar in the both regions of 18.8211 Rs and 6.9518.8 Rs.

The estimated propagation speed of Shock17 and

the ICME17 was 797 and 702km/s in the area of 6.95 211 Rs, respectively. The in situ solar wind speed was ~400 and 500km/s upstream and downstream of the IP shock, respectively. The speed in the sheath was between 500 and 600km/s, and the average speed of the plasma over the entire MC was 550km/s.

The estimated ICME propagation time to the Earth, based on C3 measurements, was ~58.96 h, i.e., 204.056.95105/(6683600)=58.96h. This means that the ICME would arrive at the Earth at ~15:09UT on 17 March 2015. The Wind measurements indicate that it took slightly less time (56.13h) for the CME to propagate from Sun to 1 AU. The error on the propagation time

Wu et al. Earth, Planets and Space (2016) 68:151

Page 5 of 12

Table 1 MC t-parameters for the MC of 17 March 2015 (starting day)

Starting time = 10:38 UT CA (%) = 3 % T = 13.0 h t = 15 min

VMC = 550 km/s CA = 1162Ro = 0.166 AU Check = 6.4 %

Bo = 25.65 nT o = 5.4 1020 Mx H = + 1 righthanded Jo = 4.0 A km2 A = 63 and A = 162 (GSE coordinates) IT = 8.3 108 AR = 0.244 N = 54

Asf (%) = 9.7 % Qo = 3

T, duration of the MC encounter (i.e., T=end time start time of MC

passage); VMC, average solar wind speed (in kms1) within the MC; 2Ro, estimated diameter (in AU), where RO is the model-estimated radius; BO, estimated axial magnetic eld magnitude (in nT); H, Handedness (+1 for right-

handed or 1 for left-handed); A, A, longitude and latitude, respectively, of the

MC axis (GSE coordinates); to, estimated center time of the MC; R, square root of the reduced Chi-squared of the MC t; asf(%), asymmetry factor (in %), which depends on to and T; CA (%), estimated relative closest approach distance, i.e., yo/Ro (in %) where yo is closest approach; o, estimated axial magnetic ux (in 1020 Mx); JO, estimated total axial current density (in A km2); t, lengthof the averages used in the analysis; these are usually 15, 30min, or 1h; CA, cone angle, the angle between the MC axis and the X-axis (in GSE coordinates);

Check, a check of the estimated radius by using duration, speed, CA, cone angle, and Ro; IT, estimated total axial current (in 108A); N, number of points used in the MC-tting interval; Qo, estimated quality of the model tting (where

Qo=1, 2, or 3, for excellent, good, or poor, respectively)

is ~5% if 668km/s was used as an estimate of the CME speed, i.e., (58.9656.13)/56.13 5 %. Therefore, the

March 15 CME was clearly responsible for the generation of the Shock17 and the St. Patricks Day storm. In addition, the CME speed measured by C3 is good to use for estimating the ICMEs arrival time at the Earth. The speed of the CME that occurs near the Sun is usually inferred to be faster than it is in interplanetary space.

Discussion

One may argue about the importance of the prediction of the SAT (shock arrival time at the Earth) because interplanetary shocks do not cause geomagnetic storms directly. However, structures behind shocks, in the sheath, do cause geomagnetic storms frequently. For example, the shocks drivers, MCs, are one of the most geo-eective IP structures (e.g., Wu and Lepping 2016

and references therein). Using statistical studies of Wind data (19952012), Wu and Lepping (2016) found: (1) the average intensity of geomagnetic storms (Dstmin)

associated with IP shocks, MCs, and magnetic cloud like structures (MCLs) are 78, 70, and 35 nT, respectively; (2) the Dstmin for MCshock (MCs with upstream shock waves) and MCno-shock (MC without upstream shock wave) events are 102 and 31 nT, respectively;

and (3) the average duration of the sheath (the area between an IP shock and the front boundary of MC) is about 12h long.

Interplanetary shock/CME arrival time atthe Earth

The propagation speeds of CME15 were 606 (VC2) and 668 (VC3) km/s in the FOV of LASCO/C2 and C3 (see Table 2). The estimated shock (or CME) arrival times (SAT) at the Earth were 65.89h for VC2 and 58.96h for

VC3. It took about 49.45 and 56.13 h for Shock17 and ICME17 to arrive at the Earth, if VC3 is used. But SAT was 65.89h if VC2 is used. tERR were 16, 5% for SAT-

Shock17 and SATMC17, if VC3 is used. tERR were 30, 15% for SATShock17 and SATMC17, if VC2 is used. Therefore, the

measurement of VC2 (or VC3) is very important for space weather. The speed of VC2 (or VC3) 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.

Wu et al. Earth, Planets and Space (2016) 68:151

Page 6 of 12

One can use the VCME (e.g., VC2 or VC3) value to estimate the arrival time of the drivers front boundary rst. Then use the duration of sheath (~12h) to estimate the SAT if there is an IP shock in front of the driver.

Is the intensity ofa geomagnetic storm predictable?

The prediction of geomagnetic storm intensity is one of the most important goals in space weather. A severe geomagnetic storm can aect the operation of a space vehicle, interrupt telecommunication around the world, and/or damage power grids on the ground. For example, power plants in Canada were damaged by a storm that occurred in March 1989.

The minimum Bz (Bzmin) was 23nT (in GSE coord.) while the Wind spacecraft passed through MC17. Bzmin

occurred in the front portion of the MC17. The average solar wind speed (V) was ~500km/s in the sheath and ~600 km/s inside the MC17. Many empirical formulae for estimating Dstmin are available (e.g., Wu and Lepping

2005, 2015, 2016; Gopalswamy etal. 2015). Tables3 and 4 list formulae for the estimation of Dst that were derived from extensive Wind MC data sets [Dstmin formulae were obtained from Tables 6, 7, and 8 of Wu and Lepping (2016)]. The estimated Dstmin from these formulae were in a range between 129 and 240 nT for the St. Patricks Day Storm, with errors |(DstobsDstpred.)/Dstobs.| in a range of 350%.

Without considering solar wind velocity, the best prediction of Dstmin was 179.7nT (error=21.2%) by using a Dst formula derived from 83 MCs that occurred during 19952003 (g in Table3). Using a speed dependent Dstmin formula, the predicted Dstmin was in a range of

121 to 240nT, and the errors were in a range between 2.9 and 49.6% (Table4). The best three predictions are

218.2, 221.4, and 239.9 nT (errors are 4.3, 2.9, and 5.2 %) by using Table 4s Dst formulae (3), (4), and (9), respectively. Best prediction means that the predicted Dstmin has the smallest error. It has been shown that

Wu et al. Earth, Planets and Space (2016) 68:151

Page 7 of 12

Table 2 Related information forthe CME15, Shock andICME17 during1517 March 2015

Start timea rb Ending timec rd te (h) Vfshock/CME (km/s) Vg (km/s) thpred (h) tiERR (h) (%) UT (Mar. 15) Rs UT (Mar. 17) Rs

ShockC3 2:30 6.9518.82 03:57 204.05 49.45 796.6 668 58.97 16 ICMEC3 2:30 6.9518.82 10:38 204.05 56.13j 701.8 668 58.96k 5l

ShockC2 2:00 4.156.61 03:57 206.85 49.95 799.5 606 65.00 30 ICMEC2 2:00 4.156.61 10:38 206.85 56.63 705.0 606 65.00 15

a CME/shock was observed by SOHO/LASCO

b Location of leading edge of CME measured by C3 or C2 (units in Rs)

c Starting point at 1 AU (Wind)

d Distance between the initial point observed by C3 and Wind (units in Rs), and 1 Rs=6.95105 km

e t: traveling time of shock/CME between 6.95 Rs and 1 AU (units in hours), and 1 AU equals to 215 Rs

f Vshock/CME=r/t (units in km/s)

g V: VCME/shock measured near the Sun between 6.95 and 18.82 Rs (units in km/s)

h tprediction: predicted traveling time for shock/CME propagating from 6.95 Rs to Wind (units in hours)

i tERR: error on the tpred=(tpredt)/t100 (%)

j Traveling time of ICME17 from 6.95 Rs to Wind spacecraft, ICME was seen by C3 at 2:30UT on 15 March and recorded by Wind at 10:38UT on 17 March [t=224+(102)+(3830)/60=56.13h]. Wind orbited at 258.7 RE (GSE). Earth was at 213.938 Rs. Wind was at ~211 Rs (GSE). Therefore VCME=r/t=(204.05

Rs/56.13h)=701.8km/s

k 204.056.95105/(6683600)=58.96h

l (58.9656.13)/56.135%

Table 3 Estimated Dstmin based onformulae obtained fromWu andLepping (2016) fora MC event thatoccurred on17 March 2015

Event Dstmin formulaa Pred. Dstbmin Source ofBzminc Errors (%)

(a) 168 MCs Dstmin = 3.30 + 6.82

Bzmin 160.2 MC 29.8

(b) 168 MCs Dstmin = 8.04 + 6.34

Bzmin 137.8 Sheath or MC 39.6

(c) 94 MCSHOCK Dstmin = 22.89 + 6.12

Bzmin 163.7 MC 28.2

(d) 94 MCSHOCK Dstmin = 11.01 + 6.47

Bzmin 137.8 Sheath or MC 39.6(e) 94 MCSHOCK Dstmin = 21.18 + 5.26

Bzmin 142.2 Sheath 37.6 (f) 74 MCNOSHOCK Dstmin = 4.18 + 5.83

Bzmin 129.9 MC 43.0(g) 83 MC19952003 Dstmin = 0.83 + 7.85

Bzmin 179.7 MC 21.2

a Linear-tted function for Dstmin obtained from Wu and Lepping (2016)

b Predicted Dstmin by using Dstmin formula listed in the left

c Bzmin=23 nT within the MC event recorded from wind spacecraft Italics: the best prediction (with the lowest error) for Dstmin

Dstmin versus Bzmin has a higher correlation for the MCs associated with higher solar wind speed than those with lower speed (e.g., Wu and Lepping 2002b, 2016). Using the empirical relationship (Dst=0.017 VBz+16nT),

Gopalswamy etal. (2015) estimated Dstmin=239nT by using in V=600km/s, and Bz=25nT for this event.

The estimated Dstmin was close to the observation. Solar wind speed is an important parameter for estimating geomagnetic storm intensity, as is well known.

How many MCs were associated withthe superstorm of17 March 2015?

We only identied MC17 as the driver for the IP Shock17 of 17 March 2015. The MC17 was identied in two procedures: (1) We rst applied the automatic MC auto-identication (MCI) model (Lepping etal. 2005) to nd the MC candidate, and then (2) we used a MC-tting (MCF) model (Lepping et al. 1990) to determine the MC parameters. Figure 6 shows the time prole of the

Wu et al. Earth, Planets and Space (2016) 68:151

Page 8 of 12

Table 4 Estimated Dstmin based onformulae obtained fromWu andLepping (2016) withBzmin=23nT

Range ofMCs averaged speeda

Estimating Dst formula using Bzmin inMCb

Pred. Dstcmin Error (%)d Estimating Dst formula using

Bzmin inMC or sheathb

Pred. Dstcmin Error (%)d

1. V < 400a Dstmin = 12.12 + 6.55

Bzmin 138.5 39.3 11. Dstmin = 14.19 + 6.52

Bzmin 135.8 40.4

2. 400 < V < 500 Dstmin = 21.08 + 4.82

Bzmin 131.9 42.1 12. Dstmin = 16.14 + 4.50

Bzmin 119.6 47.5

3. 500 < V < 600 Dstmin = 9.31 + 9.08

Bzmin 218.2e 4.3 13. Dstmin = 28.94 + 8.23

Bzmin 160.4 29.6

4. 600 < V < 750 Dstmin = 70.76 + 6.55

Bzmin 221.4 2.9 14. Dstmin = 36.18 + 8.65

Bzmin 162.8 28.6

5. V > 750 Dstmin = 147.07 + 2.84

Bzmin 212.4 6.8 15. Dstmin = 32.14 + 4.54

Bzmin 136.6 40.1

94 MCshock

6. V < 400 Dstmin = 12.28 + 6.82

Bzmin 144.5 36.6 16. Dstmin = 10.83 + 5.42

Bzmin 114.8 49.6

7. 400 < V < 500 Dstmin = 43.37 +3.37

Bzmin 120.9 47.0 17. Dstmin = 36.69 + 3.21

Bzmin 111.7 51.0

8. 500 < V < 600 Dstmin = 54.55 + 4.72

Bzmin 163.1 28.5 18. Dstmin = 13.01 + 5.14

Bzmin 132.8 41.8

9. 600 < V < 750 Dstmin = 66.72 + 7.53

Bzmin 239.9 5.2 19. Dstmin = 75.96 + 10.43

Bzmin 191.2 16.1

10. 750 < V Dstmin = 147.07 + 2.84

Bzmin 212.4 6.8 20. Dstmin = 32.14 + 4.54

Bzmin 141.9 37.9

a Range of averaged speed inside of the MC

b Linear-tted function for Dstmin obtained from Wu and Lepping (2016)

c Predicted Dstmin by using Dstmin formula listed in the left

d Error=|(DstobsDstpred.)/Dstobs.|

e Dstprediction=218.2nT (

Bzmin=23 nT) using Dstmin formula3) for 500<V<600km/s Italics: the best prediction (with the lowest error) for Dstmin

interplanetary magnetic eld, solar wind plasma, Dst index, and some derived parameters during 1718 March 2015. The MCI model identied a MC candidate, which is marked by a horizontal black bar, and the MCF model was able to identify a MC (MC17), which is bounded between two vertical red lines, as shown in Fig.6d. The rear boundary of the MC17, as determined visually, is consistent with that determined by the MCI model, but the MC17 front boundary, as determined visually, is about 4h ahead of that determined by the MCI model. The shock driven by the MC17 (Shock17) is marked by a vertical line in Fig.6.

Wind recorded ~24 h of low plasma beta () solar wind material between ~12:00 UT on March 17 and ~12:00 UT on March 18. The low region was separated by a magnetic hole, which occurred at the end of 17 March (indicated by dotted and dashed lines). Behind the MC17s rear boundary, there was a ~12-h low interval (marked in orange color and ICME ? in Fig.6b). Using the MCF model, we are not able to obtain a good t for this region because the magnetic eld did not vary much in direction (i.e., B or B did not change much) inside this region (see Fig. 6e, f). Besides the absence of signicant eld rotation in that region, Bz was almost zero in that period. Therefore, we concluded that the super-storm on 17 March 2015 was caused by the southward eld in both the sheath and the MC17.

The results of this study are consistent with the recent work by Kataoka etal. (2015), but are dierent from the

results concluded by Gopalswamy et al. (2015), or Liu etal. (2015). The MC interval identied by Gopalswamy etal. (2015) is almost double the interval of MC17. Liu etal. found that there were two ICMEs associated with the storm of 17 March by using the best GradShafranovt model, and both ICMEs were ux-ropes (FRs) [see Fig.2 of Liu etal. (2015)].

Liu et al. asserted that an IMCE (ICME1) occurred with an ending point about several hours ahead of MC17 and a second ICME (ICME2) started right after the end of ICME1. In addition, a magnetic hole was found in the middle of ICME2 about 4 h behind the front boundary of ICME2. Often, a magnetic hole marks the boundary of a MC. The MCF model used in this study is not expected to be able to not nd a good t since there is a magnetic hole inside the tting region. Liu etal. used a dierent ICME tting model to t their ICME2 and chose a dierent period for their study (Liu et al. 2015). Our MCs boundaries do not agree with their ICMEs boundary selections. Based on the discussion above, we concluded that there was only one MC and one MCs driven shock associated with the super-storm on 17 March 2015.

Many MC or ux-rope tting models are available in the science community (e.g., Hua and Sonnerup 1999; Marubashi and Cho 2015; Mstl et al. 2009; Lepping etal. 1990; Wang etal. 2015). It would be useful to know whether and/or how they perform dierently. However, this is beyond the scope of this study.

Wu et al. Earth, Planets and Space (2016) 68:151

Page 9 of 12

Wu et al. Earth, Planets and Space (2016) 68:151

Page 10 of 12

Solar source ofthe superstorm on17 March 2015

The solar source of the super-storm of 17 March 2015 is a hot topic for some science communities [e.g., the International Study of Earth-aecting Solar Transients (ISEST) for Variability of the Sun and Its Terrestrial

Impact (VarSITI)]. In order to answer the question, earlier CME images were checked and a slow CME was identied on 14 March 2015 (We will refer this CME as CME14). CME14 was rst seen at 17:43UT by SOHO/ LASCO C3. Figure7 shows a sequence of CME images recorded by SOHO/LASCO C3 between 17:42 UT on 14 March and 09:06 on 15 March 2015. The + and *

symbols indicate the leading edge of CME14 and CME15 (this CME erupted on 15 March 2015), respectively. The speed of CME14 was very slow (~240km/s, see Fig.8). Figure7e shows that CME15 was recorded by C3 on the right (or west), and CME14 was at the bottom (or south). The leading edge of CME15 passed the leading edge of CME 14 (see Fig.7gj) after 06:30UT on 15 March.

Previous studies suggested that the storm on 17 March 2015 may be caused by the interaction between two successive CMEs plus the compression by a high-speed stream from behind (e.g., Liu et al. 2015). Our interpretation of the coronagraph images is dierent

Wu et al. Earth, Planets and Space (2016) 68:151

Page 11 of 12

Conclusion

The rst super geomagnetic storm of solar cycle 24 occurred on St. Patricks day (17 March 2015). 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 are (S22W25) and a series of type II/IV radio bursts. The propagation speed of this CME is estimated to be ~668 km/s for the period 02:1006:20 UT (see Fig. 1). An interplanetary (IP) shock, likely driven by the related ICME, arrived at the Wind spacecraft at 03:59UT on 17 March. We conclude that the St. Patricksday event was a two-step storm. The rst step was associated with the sheath, whereas the second step was associated with MC17. The solar source of MC17 was CME15.

We also found that choosing the correct Dstmin estimating formula for predicting the intensity of MC-associated geomagnetic storms is crucial for space weather predictions, because solar wind speed, as well as Bs,

plays an important role in the prediction of geomagnetic activity.

Authors contributions

CW designed and carried out the original study of the St. Patricks Day event. She also wrote the rst draft of the manuscript. KL performed data analysis and helped writing the manuscript. RL participated in the identication and data analysis of the MC and helped draft the manuscript. LH participated in the CMEs measurement. SP participated in the interpretation of the evolution of the CME, and helped draft the manuscript. RH participated in the CME data analysis. DS participated in interpretation of CMEs evolution and helped draft the manuscript. All authors read and approved the nal manuscript.

Author details

1 Naval Research Laboratory, Washington, DC 20375, USA. 2 Applied Physics Laboratory, Laurel, MD 20723, USA. 3 Emeritus, GSFC/NASA, Greenbelt, MD, USA.

Acknowledgements

We thank the Wind PI team and National Space Science Data Center at Goddard Space Flight Center for management and providing Wind plasma and magnetic eld solar wind data. This study was supported partially by the Chief of Naval Research (CCW, SP, DS, LH). K.L. was supported by NASA grant NNX14AF83G to the Johns Hopkins University Applied Physics Laboratory. We acknowledge the support of NASA contract S136361Y for the STEREO/SEC CHI eort. CCW has participated in the ISEST working group on the campaign events. CCW would like to thank VarSITI and ISEST for partial travel support.

Received: 12 March 2016 Accepted: 18 August 2016

References

Akasofu SI (1981) Energy coupling between the solar wind and the magneto sphere. Space Sci Rev 28:121

Astafyeva E, Yasyukevich U, Maksikov A, Zhivetiev I (2014) Geomagnetic storms, superstorms, and their impacts on GPSbased navigation sys tems. Space Weather 12:508525. doi:http://dx.doi.org/10.1002/2014SW001072

Web End =10.1002/2014SW001072

Echer E, Gonzalez WD (2004) Goeectiveness of interplanetary shocks mag netic clouds, sector boundary crossings and their combined occurrence. Geophys Res Lett. doi:http://dx.doi.org/10.1029/2003GL019199

Web End =10.1029/2003GL019199

from that of Liu etal. (2015) because: (1) there was no CME image available to conrm it, since CME14 and CME15 propagated out of the C3 FOV; (2) the CMEs direction of propagation was dierent between CME14 (toward the south/bottom) and CME15 (toward the west/right); and (3) Fig.7fi shows clearly that CME14 was propagating in its own direction (southward projected on the plane of sky) after the leading edge of CME15 passed the leading edge of CME 14, showing no indication that CME14 and CME15 merged. Beside the above three points, the speed of CME14 was too slow (~240km/s) to hit the Earth by 18 March 2015. It would take at least 7days for CME14 to propagate from 7 Rs to the Earth, since the speed was 240km/s. This means that CME14 would not hit the Earth by 21 March 2015. For these reasons, we concluded that the super-storm on 17 March 2015 was caused by the southward eld in both the sheath and in part of MC17 which was driven by CME15.

One may argue that a slow CME will be accelerated by the ambient solar wind in interplanetary space. The propagating speed of CME14 was 240 km/s between 718 Rs. Assuming that CME14 was accelerated simultaneously to 300, 400, 500, 600, or 700km/s at 06:06UT on March 15 (see the last point in Fig.8), it will take about 5.28, 3.96, 3.17, 2.64, or 2.26days for the CME14 to arrive at the Earth, respectively. This means that CME14 could arrive at the Earth at around 12:48 UT on March 20; 05:12UT on March 19; 10:12UT on March 18; 21:29UT on March 17; or 12:26UT on March 17 for the CME14 with propagating speeds of 300, 400, 500, 600, 700km/s, respectively. CME14 could be a candidate that caused the geomagnetic storm on 17 March 2015 if CME14 could be accelerated to 700km/s near the Sun.

Wu et al. Earth, Planets and Space (2016) 68:151

Page 12 of 12

Gonzalez WD, Joselyn JA, Kamide Y, Kroehl HW, Rostoker G, Tsurutani BT,

Vasyliunas VM (1994) What is a geomagnetic storm? J Geophys Res 99(A4):57715792. doi:http://dx.doi.org/10.1029/93JA02867

Web End =10.1029/93JA02867 Gopalswamy N, Akiyama S, Yashiro S, Xie H, Makela P, Michalek G (2015) The mild space weather in solar cycle. arXiv: 1508.01603.24Hua LN, Sonnerup BUO (1999) Twodimensional coherent structures in the magnetospause: recovery of static equilibria from singlespacecraft data. J Geophys Res 104:68996918. doi:http://dx.doi.org/10.1029/1999JA900002

Web End =10.1029/1999JA900002 Kamide Y, Kusano K (2015) No major solar but the largest geomagnetic storm in the present solar cycle. Space Weather 13:365367. doi:http://dx.doi.org/10.1002/2015SW001213

Web End =10.1002/201 http://dx.doi.org/10.1002/2015SW001213

Web End =5SW001213 Kamide Y, Yokoyama N, Gonzalez W, Tsurutani BT, Daglis IA, Brekke A, Masuda

S (1998) Twostep development of geomagnetic storms. J Geophys Res 103:69176922. doi:http://dx.doi.org/10.1029/97JA03337

Web End =10.1029/97JA03337 Kataoka R, Shiota D, Keika E, Kilpua K (2015) Pileup accident hypothesis of magnetic storm on 17 March 2015. Geophys Res Lett 42:51555161. doi:http://dx.doi.org/10.1002/2015GL064816

Web End =1 http://dx.doi.org/10.1002/2015GL064816

Web End =0.1002/2015GL064816 Lakhina GS, Tsurutani BT (2016) Geomagnetic storms: historical perspective to modern view. Geosci Lett 3:5. doi:http://dx.doi.org/10.1186/s40562-016-0037-4

Web End =10.1186/s4056201600374 Lepping RP, Burlaga LF, Johnes JA (1990) Magnetic eld structure of interplan etary magnetic clouds at 1 AU. J Geophys Res 95:11957. doi:http://dx.doi.org/10.1029/JA095iA08p11957

Web End =10.1029/ http://dx.doi.org/10.1029/JA095iA08p11957

Web End =JA095iA08p11957 Lepping RP, Berdichevsky D, Ferguson T (2003) Estimated errors in magnetic cloud model tparameters with force free cylindrically symmetric assumptions. J Geophys Res 108:1356. doi:http://dx.doi.org/10.1029/2002JA009657

Web End =10.1029/2002JA009657 Lepping RP, Wu CC, Berdichevsky DB (2005) Automatic identication of mag netic clouds and cloudlike regions at 1 AU: occurrence rate and other properties. Ann Geophys 23:26872704Lepping RP, Berdichevsky DB, Wu CC, Szabo A, Narock T, Mariani F, Lazarus

AJ, Quivers AJ (2006) A summary of WIND magnetic clouds for years 19952003: modeltted parameters, associated errors and classications. Ann Geophys 24:215245Lepping RP, Wu CC, Berdichevsky DB, Szabo A (2015) Wind magnetic cloudsfor years 20102012: model parameter ttings, associated shock waves and comparisons to earlier periods. Sol Phys 290:22652290. doi:http://dx.doi.org/10.1007/s11207-015-0755-3

Web End =10.1007/ http://dx.doi.org/10.1007/s11207-015-0755-3

Web End =s1120701507553 Liu Y, Luhmann JG, Bale SD, Lin RP (2011) Solar source and heliospheric con sequences of the 2010 April 3 coronal mass ejection: a comprehensive view. Astrophys J. doi:http://dx.doi.org/10.1088/0004-637X/734/84

Web End =10.1088/0004637X/734/84 Liu YD, Hu H, Wang R, Yang Z, Zhu B, Liu YA, Luhmann JG, Richardson JD (2015)

Plasma and magnetic eld characteristics of solar coronal mass ejections in relation to geomagnetic storm intensity and variability. ApJL 809:16. doi:http://dx.doi.org/10.1088/2041-8205/809/2/L34

Web End =10.1088/20418205/809/2/L34 Marubashi K, Cho KS (2015) Nonuniqueness of the geometry of interplan etary magnetic ux ropes obtained from modeltting. Sun Geospace 10:119125McAllister AH, Crooker NU (1997) Coronal mass ejections, corotating interac tion regions, and geomagnetic storms. In: Crooker N, Joselyn JA, Feyn man J (eds) Coronal mass ejections. Geophysics monograph series, vol 99. AGU, Washington, D.C, pp 279289. doi:http://dx.doi.org/10.1029/GM099p0279

Web End =10.1029/GM099p0279 Mstl C, Farrugia CJ, Biernat HK, Leitner M, Kilpua EK, Galvin AB, Luhmann JG

(2009) Optimized GradShafranov reconstruction of a magnetic cloud using STEREOWind observations. Sol Phys 256:427441. doi:http://dx.doi.org/10.1007/s11207-009-9360-7

Web End =10.1007/ http://dx.doi.org/10.1007/s11207-009-9360-7

Web End =s1120700993607 Mstl C, Temmer M, Rollett T, Farrugia CJ, Liu Y, Vernoning AM, Leitner M, Galvin

AB, Biernat HK (2010) STEREO and wind observations of a fast ICME ank triggering a prolonged geomagnetic storm on 57 April 2010. Geophys Res Lett. doi:http://dx.doi.org/10.1029/2010GL045175

Web End =10.1029/2010GL045175 Ramsingh, Sripathi S, Sreekumar S, Banola S, Emperumal K, Tiwari P, Kumar BS

(2015) Lowlatitude ionosphere response to super geomagnetic storm of 17/18 March 2015: results from a chain of groundbased observations over Indian sector. J Geophys Res 120:1086410882. doi:http://dx.doi.org/10.1002/2015JA021509

Web End =10.1002/201 http://dx.doi.org/10.1002/2015JA021509

Web End =5JA021509 Richardson IG, Cane H (2011) Geoeectiveness (Dst and Kp) of interplanetary coronal mass ejections during 19952009 and implications for storm forecasting. Space Weather. doi:http://dx.doi.org/10.1029/2011SW000670

Web End =10.1029/2011SW000670 Tsurutani BT, Gonzalez WD (1997) The interplanetary causes of magnetic storms: a review. In: Tsurutani BT, Gonzalez WD, Kamide Y (eds) Geophys ics monograph series, vol 98. American Geophysical Union, Washington, pp 7789. doi:http://dx.doi.org/10.1029/GM098p0077

Web End =10.1029/GM098p0077

Tsurutani BT, Smith EJ, Gonzalez WD, Tang F, Akasofu SI (1988) Origin of interplanetary southward magnetic elds responsible for major magnetic storms near solar maximum (19781979). J Geophys Res 93:85178531. doi:http://dx.doi.org/10.1029/JA093iA08p08519

Web End =10.1029/JA093iA08p08519

Wang YM, Zhou Z, Shen C, Liu R, Wang S (2015) Investigating plasma motion of magnetic clouds at 1 aU through a velocitymodied cylindrical force free ux tope model. J Geophys Res 120:15431565

Wood BE, Wu CC, Howard RA, Socker DG, Rouillard AP (2011) Empiri cal reconstruction and numerical modeling of the rst geoeec tive coronal mass ejection of solar cycle 24. Astrophys J 729:70. doi:http://dx.doi.org/10.1088/0004-637X/729/1/70

Web End =10.1088/0004637X/729/1/70

Wood BE, Lean L, McDonald SE, Wang YM (2016) Comparative ionospheric impacts and solar origins of nine strong geomagnetic storms in 2010 2015. J Res Geophys. doi:http://dx.doi.org/10.1002/2015JA021953

Web End =10.1002/2015JA021953

Wu CC, Lepping RP (2002a) Eects of magnetic clouds on the occurrence of geomagnetic storms: the rst 4 years of Wind. J Geophys Res. doi:http://dx.doi.org/10.1029/2001JA000161

Web End =10.102 http://dx.doi.org/10.1029/2001JA000161

Web End =9/2001JA000161

Wu CC, Lepping RP (2002b) Eect of solar wind velocity on magnetic cloud associated magnetic storm intensity. J Geophys Res 107(A11):1346. doi:http://dx.doi.org/10.1029/2002JA009396

Web End =1 http://dx.doi.org/10.1029/2002JA009396

Web End =0.1029/2002JA009396 Wu CC, Lepping RP (2005) Predicting magnetic cloud related geomagnetic storm intensity. J Atmos Terr Phys 67:283291. doi:http://dx.doi.org/10.1016/j.jastp.2004.07.040

Web End =10.1016/j. http://dx.doi.org/10.1016/j.jastp.2004.07.040

Web End =jastp.2004.07.040 Wu CC, Lepping RP (2008) Geomagnetic activity associated with magnetic clouds, magnetic cloudlike structures and interplanetary shocks for the period 19952003. Adv Space Rev 41:335338. doi:http://dx.doi.org/10.1016/j.asr.2007.02.027

Web End =10.1016/j.asr.2007.02.027 Wu CC, Lepping RP (2011) Statistical comparison of magnetic clouds with interplanetary coronal mass ejections for solar cycle 23. Solar Phys 269:141153. doi:http://dx.doi.org/10.1007/s11207-010-9684-3

Web End =10.1007/s1120701096843 Wu CC, Lepping RP (2015) Statistical comparison of magnetic clouds and cloudlike structures during 19952012. Solar Phys 290:12431269. doi:http://dx.doi.org/10.1007/s11207-015-0656-5

Web End =10.1007/s1120701506565 Wu CC, Lepping RP (2016) Relationships among geomagnetic storms, interplanetary shocks, magnetic clouds, and sunspot number during 19952012. Sol Phys 291:265284. doi:http://dx.doi.org/10.1007/s11207-015-0806-9

Web End =10.1007/s1120701508069 Wu CC, Gopalswamy N, Lepping RP, Yashiro S (2013) Characteristics of mag netic clouds/interplanetary coronal mass ejections which caused intense geomagnetic storms. Terr Atmos Ocean Sci 24(2):233241. doi:http://dx.doi.org/10.3319/TAO.2012.09.26.03

Web End =10.3319/ http://dx.doi.org/10.3319/TAO.2012.09.26.03

Web End =TAO.2012.09.26.03 Wu CC, Liou K, Vourlidas A, Plunkett S, Dryer M, Wu ST, Socker D, Wood

BE, Hutting L, Howard R (2016a) Numerical simulation of multiple CMEdriven shocks in the month of 2011 September. J Geophys Res 121:18391856. doi:http://dx.doi.org/10.1002/2015JA021843

Web End =10.1002/2015JA021843 Wu CC, Liou K, Vourlidas A, Plunkett S, Dryer M, Wu ST, Mewald RA (2016b)

Global Magnetohydrodynamic simulation of the March 15, 2013 coronal mass ejection eventinterpretation of the 3080 MeV proton ux. J Geophys Res 121:5676. doi:http://dx.doi.org/10.1002/2015JA021051

Web End =10.1002/2015JA021051 Zhang J, Richardson IG, Webb DF, Gopalswamy N, Huttunen E, Kasper J, Nitta

N, Poomvises W, Thompson BJ, Wu CC, Yashiro S, Zhukov Z (2007) Solar and interplanetary sources of major geomagnetic storms (Dst < = 100

nT) during 1996 2005. J Geophys Res 112:A10102. doi:http://dx.doi.org/10.1029/2007JA012321

Web End =10.1029/200 http://dx.doi.org/10.1029/2007JA012321

Web End =7JA012321