A fresh look at the intensity and impulsive strength of geomagnetic storms

Abstract

We notice that the important early decreasing part of the main phase (MP) from the positive main phase onset (MPO) to 0-level of Dst and SymH indices is missed in the treatment of the main phase (MP) of geomagnetic storms. We correct this inconsistency in 848 storms having positive MPO (out of 1164 storms) in SymH during 1981–2019 by raising the 0-level of SymH to the MPO-level. The correction considers the full range of the main phase, increases the corrected (revised) storm intensity (SymHMin*) and impulsive strength (IpsSymH*) by up to − 149 nT and − 134 nT, respectively, and seems important for all aspects of global space weather. For example, the corrected SymHMin* changes the conventional storm identification and classification and corrected IpsSymH* clearly identifies all 3 severe space weather (SvSW) events from over 1100 normal space weather (NSW) events with a separation of 52 nT; it also identifies all 8 minor-system-damage space weather (MSW) events from the NSW events.

Plain Language Summary

Large fluctuations occur in the global geomagnetic field during space weather events. The fluctuations at low latitudes are referred as geomagnetic storms. The Dst and SymH indices have been used for studying the storms and other aspects of global space weather. However, we notice that the Dst and SymH values during the main phase and recovery phase of the storms having positive main phase onset (MPO > 0 nT) are significantly less than their actual values. We correct this inconsistency in 848 such storms (out of 1164 storms) in SymH during 1981-2019 by raising the 0-level of SymH to the MPO-level. The corrected/revised storm intensity (SymHMin*) and impulsive strength (IpsSymH*) increase by up to − 149 and − 134 nT. The correction seems important for studying all aspects global space weather. For example, the correction identifies the storms corresponding to severe space weather causing power outage and/or telecommunication failure from those corresponding to normal space weather.

Key points

• We correct an inconsistency in the SymH values during the MP and RP of the 848 storms having positive MPO during 1981–2019.

• The corrected values of SymHMin* and IpsSymH* increase by up to − 149 nT and − 134 nT compared to their uncorrected values.

• The correction changes the storm identification and clearly identifies all 3 SvSW and 8 MSW events from over 1100 NSW events.

Introduction

The large electric currents flowing at different regions in the magnetosphere and ionosphere during space weather events produce large fluctuations in the geomagnetic field lasting up to several days in all latitudes (e.g., Chapman and Bartels 1940). The field fluctuations at low latitudes are referred as geomagnetic storms. The storms have been studied using the Dst index (Sugiura 1964; Love and Cannon 2009) and SymH index (Iyemori et al. 1992). Sugiura (1964) developed the Dst index of 1-h resolution from the H-component magnetic field measured at 4 low latitude observatories (3 in north and 1 in south) outside the influence of the equatorial electrojet and having maximum longitude separation (MLS) of ~ 120°. To improve the time and spatial resolutions, the SymH index of 1-min resolution was developed (Iyemori et al. 1992) using the H-component data from up to 6 stations of MLS ~ 70°. The Dst and SymH indices have been used for studying not only the geomagnetic storms (e.g., Russell et al. 1973; Burton et al. 1975a; Ebihara et al. 2003, 2005; Gonzalez et al. 2011; Gopalswamy et al. 2015; Yermolaev et al. 2021; Balan et al. 2021; Manu et al. 2022, 2023) but also the disturbed upper atmosphere, ionosphere and magnetosphere (e.g., Fuller-Rowell et al. 1994; Manuucci et al. 2005; Tulasiram et al. 2010; Balan et al. 2013). For reviews, see Akasofu (1981, 2021), Proless (1995), Daglis (1997), Luhr et al. (2017) and Zong et al. (2021).

The geomagnetic storms (Fig. 1) are characterized by 3-phases—the initial phase (IP), main phase (MP) and recovery phase (RP) (Russell et al. 1974; Burton et al. 1975b; Gonzalez et al. 1994; Araki et al. 1997; Hutchinson et al. 2011). The initial phase (IP), however, may not be visible in the case of the storms starting from significant negative value of the indices. The positive initial phase (IP) is considered to be caused by the combined effect of the sudden compression of the magnetosphere at the impulsive impact of the interplanetary coronal mass ejection (ICME) front of suddenly enhanced solar wind dynamic pressure P and IMF Bz northward (Burton et al. 1975b; Shue et al. 1998; Balan et al. 2008; Wang et al. 2018) and eastward magnetopause current induced by the enhanced P when IMF Bz remains northward (e.g.,Araki et al. 1997, 2014). The main phase starts at MPO (main phase onset) when pressure P decreases and IMF Bz turns southward (Burton et al. 1975a,b; Gonzalez et al. 1994; Hutchinson et al. 2011). During the main phase (MP) when IMF Bz remains southward, SymH continues to decrease due to the increase in westward ring current and other minor currents such as tail current, field aligned current, etc. As the currents causing the MP build up, their decay also increases. When the currents’ growth rate balances the decay rate, the MP reaches its peak or SymH reaches its maximum negative value (SymHMin), which is considered as the storm intensity (e.g., Burton et al. 1975b; Gonzalez et al. 1994; Hutchinson et al. 2011). The storm recovers back to the quiet-time level taking up to several days. Conventionally, the storms are classified as minor (− 25 ≥ SymHMin > − 50 nT), moderate (− 50 ≥ SymHMin > − 100 nT), intense (− 100 ≥ SymHMin > − 250 nT) and super (SymHMin ≤ − 250 nT).

Fig. 1

A geomagnetic storm having positive main phase onset (MPO). The missed early part of the main phase (MP) is highlighted in red

The storm impulsive strength IpsDst was defined (Balan et al. 2016, 2019a) as

$$IpsDst= -\frac{1}{{T}_{MP}}\int \left|Dst\right| dt$$

(1)

where $\int \left|Dst\right| dt$ is the sum of the modulus of Dst from MPO to DstMin and T_MP is the MP duration (in hours) from MPO to DstMin which is numerically equal to N-1 with N being the number of data points both ends inclusive. By definition, IpsDst gives the negative of the modulus of mean Dst from MPO to DstMin. Physically, the numerator $\int \left|Dst\right| dt$ is proportional to the solar wind energy input into the ring current (and magnetosphere) (Burton et al. 1975a, b) and denominator T_MP is the duration of the energy input. Higher the energy input and shorter the duration, the larger the IpsDst. In other words, IpsDst physically represents the impulsive action of space weather events and therefore the name impulsive Dst (or IpsDst) (Balan et al. 2019a, b).

However, the IpsDst given by Eq. (1) is not appropriate for comparison after correcting for the missed decreasing part of positive MPO (subject of this paper) because Eq. (1) partially includes the correction from MPO to original 0-level of Dst in $\int \left|Dst\right| dt$. Therefore, here we define the IpsSymH having no correction for comparison with the IpsSymH having full correction (IpsSymH*, section "Correction and physical meaning") in SymH index as

$$IpsSymH= \frac{1}{{T}_{MP}}\int SymH dt$$

(2)

where $\int SymH dt$ is the sum of SymH from the original 0-level of SymH to SymHMIn and T_MP is the MP duration in minutes from 0-level to SymHMin (Fig. 1).

Though the main phase MP starts from MPO, the early decreasing part of MP from positive MPO to the original 0-level of SymH (and Dst) shown by the highlighted red part in Fig. 1 is, somehow, missed in the treatment of the main phase (MP) in the literature. This missing part of MP makes the SymH (and Dst) values during the MP and RP significantly less than their actual values. In this paper, we correct this inconsistency in the clear storms identified in SymH during 1981–2019. The correction done for the first time significantly increases the storm intensity (SymHMin*) and impulsive strength (IpsSymH*) and therefore seems important for all aspects of global space weather. We discuss the importance for two aspects and mention the importance for two other aspects. The correction is most important for the storm impulsive strength (IpsSymH*), section "Identification of SvSW and MSW events".

Automatic storm identification

The SymH index is available at http://wdc.kugi.kyoto-u.ac.jp/aeasy/index.html. The 4 selection criteria used for identifying the storms in Dst index (Balan et al. 2017a) are modified for identifying the storms in SymH including minor storms (Table 1). First, assuming the largest peak in the initial phase IP (IP-largest) within 8 h prior to SymH turning negative as MPO, the program identifies 1164 clear storms including 848 storms having positive MPO. Since the initial phase IP can have one or more peaks due to fluctuations in the solar wind dynamic pressure P and/or IMF Bz, a new criterion 5 as in Table 1 is used for separating the storms having (715) IP-largest and (133) IP-large as MPO (Fig. 2a, b) where IP-large is the second largest peak in IP after IP-largest. (Criterion 5 is fixed after trying a number of similar options. For higher values of IP-large, the program picks up the small fluctuations in IP-largest as IP-large and for lower values of the minimum, the number of IP-large becomes very small).

Table 1 Table lists the storm selection criteria1-5, their advantages, total number of storms and those having positive MPO identified by the successive applications of the criteria

Fig. 2

Examples of the correction for two storms having IP-largest (a) and IP-large (b) as MPO. The original values (LHS scale) and corrected values (RHS scale) of MPO, SymHMin and IpsSymH (in brackets) are noted

Correction and physical meaning

Since the main phase (MP) starts from the main phase onset MPO when SymH starts decreasing (e.g., Gonzalez et al. 1994; Hutchinson et al. 2011), the 0-level of SymH during MP (and RP) has to be at the MPO-level to account for the full range of MP. In other words, the inconsistency in the storms having positive MPO can be corrected by raising the (original) ‘0-level’ of SymH to the MPO-level as shown in Fig. 2a, b (RHS scale). The corrected storm intensity SymHMin* is the maximum negative value of SymH during the MP (RHS scale) and corrected storm impulsive strength IpsSymH* becomes

$${IpsSymH}^{*} = \frac{1}{{{T}_{MP}}^{*}}\int SymH dt$$

(3)

where $\int SymH dt$ in Eq. (3) is the sum of SymH from MPO to SymHMin* and T_MP* is the corrected MP duration in minutes (Fig. 2, RHS scale). In Eqs. (2) and (3), the negative sign and modulus in the integral got removed compared to Eq. (1). In the examples in Fig. 2a and b, the corrected (or revised) storm intensity SymHMin* increases from − 394 nT to − 489 nT and from to − 126 nT to − 191 nT, and the corrected impulsive strength IpsSymH* increases from − 214 nT to − 272 nT and from to − 68 nT to − 127 nT. The revised SymHMin* and IpsSymH* of all storms having positive MPO are found to be more negative than their values having no correction (Fig. 3),

Fig. 3

Original (Y-axes) SymHMin (a) and IpsSymH (b) against revised (X-axes) SymHMin* and IpsSymH*. The maximum increases (ΔSymHMin)max and (ΔIpsSymH)max are noted

which seems to validate the correction procedure. The corrected SymHMin* increases by the largest positive MPO of up to − 149 nT and corrected IpsSymH* increases by a slightly smaller amount by up to − 134 nT (Fig. 3). The correction can be gradually terminated during RP from the point when the negative SymH becomes equal to the positive MPO to the (original) end of RP. The physical meaning of the correction is briefly discussed in section "Discussion".

Importance of the correction

Here we discuss the importance of the correction for two aspects of global space weather and mention the importance for two other aspects.

Storm identification and classification

As listed in Table 2, originally there are 282 minor storms. But, after correction, only 14 storms remain minor; of the remaining storms, 264 become moderate and 4 become intense. There are 599 moderate storms with no correction. After correction, there are 480 moderate storms and 119 intense storms. Of the 256 intense storms before correction, 241 remain intense and 15 become super after correction. Including these 15 and the original 27, there are 42 super storms after correction. As shown in Fig. 4a, d, the intensity of the original minor storms (> − 50 nT) increases up to − 112 nT, moderate storms (> − 100 nT) increase up to − 200 nT, intense storms (> − 250 nT) increases up to − 318 nT, and super storms (≤ − 250 nT) increases up to − 792 nT. In short, the correction for positive MPO changes the conventional storm identification and classification. In panel (a) for the original minor storms (− 25 ≥ SymHMin > − 50 nT), there is no blue dot (absence of negative MPO) and SymHMin* starts from a much larger level than expected (− 25 nT) and increases up to − 112 nT. These facts indicate that all 282 original minor storms have large positive MPO. The intensity SymHMin* of original moderate storms (> − 100 nT) also increases up to − 200 nT. These are important findings in understanding the high geo-effectiveness of the comparatively weak storms.

Table 2 Number of minor, moderate, intense and super storms before and after correction

Fig. 4

Distribution of the revised storm intensity (X-axis) and revised impulsive strength (Y-axis) of the original 282 minor storms (a), 599 moderate storms (b), 256 intense storms and 27 super storms. The red and green dots indicate the storms having IP-largest and IP-large as positive MPO and blue dots indicate the storms having negative MPO. The vertical lines in a–c indicate the upper limits of minor, moderate and intense storms, and dashed line in (d) indicates the largest original SymHMin (− 720 nT)

Identification of SvSW and MSW events

Definitions

The space weather events reported causing electric power outage and/or telecommunication system failure, which are of most concern to the public, are defined as severe space weather (SvSW) events (Balan et al. 2019a, 2024). Some other space weather events are reported causing minor system damages such as capacitor tripping in transformers, high voltage in power grids, etc. (e.g., Kappenman 2003). We define such space weather events as minor-system-damage space weather (MSW) events. The space weather events not causing such damaging effects are defined as normal space weather (NSW) events.

Table 3 lists the 3 severe space weather (SvSW) events and 8 minor-system-damage space weather (MSW) events reported since 1981. The SvSW events on 13 March 1989, 06 November 2001 and 30 October 2003, respectively, correspond to the power outage in Quebec (e.g., Medford et al. 1989; Boteler 2019), New Zealand (Marshall et al. 2013) and Sweden (Pulkkinen et al. 2005). The MSW events on 13 April 1981, 08 February 1986 and 24 March 1991 and 31 March 2001, respectively, correspond to the transformer problems in Canada (The Northern Engineer, 1981), high voltage in power grids in Sweden (Stauning 2013) and two capacitor tripping in transformers in the US (Kappenman 2003). The MSW events on 08 November 1991 and 29 October 2003 measured the largest GIP and GIC (geomagnetically induced potential and current) in Sweden (Lundstedt 2006; Pirjola and Boteler 2006), and the MSW events on 07–10 November 2004 and 15 May 2005, respectively, measured the largest GIC at mid and low latitudes and highest G-level in the NOAA Space Weather Scales though monitoring systems likely prevented technological damages (Trivedi et al. 2007; Liu et al. 2009).

Table 3 List of the 3 SvSW events (top) and 8 MSW events (bottom) reported since 1981

Fig. 5

Scatter plots of a original SymHMin against IpsSymH and b revised SymHMin* against IpsSymH* of the 1164 storms including 848 storms having positive MPO. Green circles and letters M, N and O indicate the 3 SvSW events and purple circles and numbers 1–8 indicate the 8 MSW events; blue and red dots alone indicate NSW events having positive and negative MPO

Identification of events

Figure 5 compares the capability of the corrected (revised) storm impulsive strength IpsSymH* and the uncorrected (original) impulsive strength IpsSymH to identify the severe space weather (SvSW) events and minor-system-damage space weather (MSW) events from normal space weather (NSW) events. The revised IpsSymH* identifies all 3 SvSW events (Fig. 5b, green circles and letters M, N and O) from all NSW events with a large separation of 52 nT compared to a separation of 35 nT by the original IpsSymH (Fig. 5a). The IpsSymH* also identifies all 8 MSW events (purple circles and numbers 1–8) though with a smaller separation, while original IpsSymH identifies only 5 MSW events. The revised and original storm intensity (Fig. 5, Y-axes), however, identifies only 1 SvSW event each. In Fig. 5, except for the one green circle noted by the letter O, all other green and purple circles have red dots inside indicating that all these storms have (large) positive MPO. The one SvSW event that has blue dot (indicating negative MPO) is the well-known Halloween event on 30 October 2003, which is the second of the super double storms during 29–31 October 2003. Though the second storm has negative MPO, the correction for the positive MPO of the first storm increased the IpsSymH* of the second storm as well indicating not only the power outage (Pulkkinen et al. 2005) but also the largest ever recorded positive ionospheric storm and fastest equatorward neutral wind (e.g., Mannuuci et al. 2005; Balan et al. 2011). The observations highlight that the correction for the positive MPO is most important for the storm impulsive strength

It may be noted that the reporting of space weather related technological problems is non-uniform and there were only very few reports from the Southern hemisphere, China and Russia. The power grid outages and communication failures could also be minimized over time due to technological improvements. Also, the lower ends of the revised IpsSymH* of SvSW and MSW events are approximately − 240 nT and − 210 nT (Fig. 5b). These observed values need not be thresholds. There might have been SvSW and MSW events for lower values of revised IpsSymH*, which might not have been reported.

Discussion

The main point of this paper is the correction for the missed important early part of MP from positive MPO to original 0-level. Here we briefly discuss the importance of solar wind dynamic pressure P and IMF Bz on the correction by considering a sample case (Fig. 6). As shown, the major decreasing part 2 (67%, 60 out of 90 nT) of the missed part of MP (panel a) occurs after IMF Bz turns southward (panel b). While only a minor decreasing part 1 (33%) occurs when the dynamic pressure P decreases (panel c), IMF Bz remains northward (panel b) and polar cap potential (PCP) is low (panel d). Figure 6 also indicates that the initial large sudden increase of IP is most probably due to the sudden compression of the magnetosphere at the impulsive impact of the ICME front of suddenly increased P and IMF Bz highly northward, and the following slowly increasing part of IP is most probably due to the eastward magnetopause current induced by the slowly increasing P when IMF Bz remains highly northward. Detailed studies including model calculations for the relative effects of P and IMF Bz (e.g., Burton el at. 1975b; Araki et al. 1997; Shue et al. 1998) for all 814 storms having positive MPO are needed to fully explain the physical meaning of the correction, which will be published as a follow up paper. Such a detailed study is beyond the scope of the present paper.

Fig. 6

Variations of SymH index (a), IMF Bz (b), solar wind dynamic pressure P (c) and polar cap potential PCP (d) during the positive initial phase IP of the geomagnetic storm on 07 November 2004. The vertical lines correspond to the main phase onset (MPO) in SymH and IMF Bz turning southward. The decreasing parts 1 and 2 of SymH before and after IMF Bz turns southward are noted

The correction for positive MPO is most important for the revised impulsive strength IpsSymH* because it seems to fully capture the important physical processes such as the impulsive impact of fast ICME shock/front, magnetopause compression, high energy input, etc. The corrected IpsSymH* clearly identifies all 3 severe space weather (SvSW) events from all normal space weather (NSW) events with a large separation of 52 nT compared to a separation of 35 nT by the uncorrected IpsSymH; IpsSymH* also identifies all 8 minor-system-damage space weather (MSW) events form NSW events. However, the conventionally used storm intensity (both original SymHMin and revised SymHMin*) identifies only 1 SvSW event each probably because the intensity being proportional only to the maximum energy input misses the impulsive action of ICME. However, the revised SymHMin* largely changes the conventional storm identification and classification. In addition, the revised IpsSymH* helps understand the high geo-effectiveness of the second storm of super double storms. In ionosphere-thermosphere studies, the correction may help understand how the comparatively weak and moderate geomagnetic storms especially under low solar activity correspond to extremely large ionosphere-thermosphere storms (e.g., Lei et al. 2018; Alphonsi et al. 2021; Rajesh et al. 2021) including the loss of Space-X satellites (e.g., Dang et al. 2022; Lookwood et al. 2023).

The mechanism of large impulsive strength IpsSymH* (high-energy input over a short duration) probably begins through continuous and rapid magnetic reconnection (e.g., Dungey 1961; Sonnerup 1984; Borovsky et al. 2008). This important physical process seems to happen when fast ICMEs with high front velocity ΔV (sudden increase by over 275 km s⁻¹) and sufficiently large IMF Bz southward at and beyond the velocity increase impacts the magnetopause (e.g., Balan et al. 2017b). The Bz southward opens the dayside magnetopause and high ΔV (and high V) provides the force for the impulsive entry of a large number of high-energy charged particles into the magnetosphere and ionosphere. The coherence of the global parameters high ΔV and large Bz southward leading to another global parameter (large IpsSymH*) and regional phenomena (SvSW and MSW) reveals an impulsive solar wind-magnetosphere-ionosphere-ground system coupling. The impulsive coupling results in an intense regional ionospheric current somewhere at high latitudes (e.g., Boteler 2019), which generates strong geomagnetic field fluctuations reaching down the Earth, which in turn induces strong secondary currents and voltages in the Earth (and Earth systems) of large electrical conductivity (Viljanen et al. 2010). These induced currents and voltages exceeding the tolerance limit of the vulnerable systems cause system failures (e.g., Albertson et al. 1974; Lanzerotti 1983; Kappanman 2003).

Conclusions

The correction for the missed important early decreasing part of the main phase (MP) from positive MPO to original 0-level of SymH accounts for the full range of MP of the geomagnetic storms. The corrected (revised) storm intensity (SymHMin*) increasing by up to − 149 nT changes the conventional storm identification and classification. The impulsive action of the ICME impact on the magnetosphere-ionosphere system happens during the corrected important early part of MP, which is fully reflected in the corrected impulsive strength IpsSymH* increasing by up to − 134 nT. The corrected IpsSymH* clearly identifies all 3 reported severe space weather (SvSW) events causing electric power outage and/or telecommunication system failure from over 1100 normal space weather (NSW) events with a large separation of 52 nT compared to a separation of 35 nT by the uncorrected IpsSymH; IpsSymH* also identifies all 8 minor-system-damage space weather (MSW) events.