A new parameter of geomagnetic storms for the severity of space weather

Using the continuous Dst data available since 1957 and H component data for the Carrington space weather event of 1859, the paper shows that the mean value of Dst during the main phase of geomagnetic storms, called mean DstMP, is a unique parameter that can indicate the severity of space weather. All storms having high mean DstMP (≤−250 nT), which corresponds to high amount of energy input in the magnetosphere–ionosphere system in short duration, are found associated with severe space weather events that caused all known electric power outages and telegraph system failures. © 2016 Balan et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (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. Background Geomagnetic storms are disturbances in earth’s magnetic field produced by enhanced solar wind–magnetosphere coupling and ionosphere–magnetosphere plasma coupling (e.g., Svalgaard 1977; Gonzalez et al. 1994). Scientific analysis of geomagnetic storms lead to improved fundamental understanding of the earth’s surrounding space weather environment (e.g., Prölss 2004; Kamide and Balan 2015). Applied analysis of the storms enables the assessment and mitigation of space weatherrelated hazards, for example, on satellite systems and orbits, satellite communication and navigation, overthe-horizon radio communication, geophysical surveys, electric power grids, and oil and gas metal pipe lines (e.g., Daglis 2004). Of particular concern are the effects associated with rare but extremely intense geomagnetic storms (e.g., Hapgood 2011; Cannon et al. 2013; Cliver and Dietrich 2013). A Carrington type event of 1859 (e.g., Carrington 1859) at present times, for example, can cause very serious social and economic impacts in the High-Tech society (e.g., Baker et al. 2008). It is therefore important for scientific and technological reasons to search for some parameter(s) of space weather events that can indicate their severity. The disturbance storm time (Dst) index (Sugiura 1964) represents geomagnetic storms. The intensity of the storms, which is the maximum negative value of Dst (or DstMin) during the storms, has conventionally been considered to represent the severity of space weather. However, recently by analyzing the Dst data since 1998, we (Balan et al. 2014) showed that DstMin is an insufficient indicator, and the mean value of Dst during the main phase (MP) of the storms (mean DstMP) can indicate the severity of space weather in causing damages to technological systems such as electric power grids and telegraph systems. In this paper, using all the Dst data available since 1957 and H component data during the Carrington event of 1859, we confirm that the mean DstMP is a unique parameter that can indicate the serenity of space weather. Dst data and analysis We use the hourly Dst data of 1 nT resolution for 58 years available at Kyoto WDC since 1957, with no data gaps and no erroneous values (http://swclob-kugi.kyoto-u. ac.jp). The Dst index is obtained from the horizontal H component measured at four low-latitude stations (3 in north and 1 in south) outside the equatorial electrojet belt (Sugiura 1964; Sugiura and Kamei 1991). A disturbance time series is estimated for each station by subtracting a non-storm quiet time baseline; and the Dst time series is obtained as an average of the individual disturbance time series from the four stations. For the Open Access *Correspondence: [email protected] 1 Instituto Nacional de Pesquisas Espaciais, São Jose dos Campos, SP 12227‐010, Brazil Full list of author information is available at the end of the article Page 2 of 5 Balan et al. Geosci. Lett. (2016) 3:3 Carrington storm of September 1859, we use the H range data measured at Mumbai and reported by Tsurutani et al. (2003). H range is similar to Dst but positive. For consistency, we take H range also as negative like Dst. The Dst storms are identified by developing a computer program which minimizes non-storm like fluctuations and avoids human errors. Figure 1, for example, illustrates the procedure. The program first detects the negative slopes in the Dst variation (green dots, Fig. 1a) and identifies the preliminary main phase onsets (MPOs) when Dst starts decreasing (red starts) and DstMin when Dst reaches maximum negative values (black stars, Fig. 1b). The program then applies the selection criterion (1) DstMin ≤−50 nT and MP duration >2 h, and criterion (2) absolute value of MP range, that is, IDstMPO − DstMinI ≥50 nT. By applying criteria (1) and (2), many short-period, non-storm like negative fluctuations of Dst are eliminated (Fig. 1c). With a view to further avoid the short-period positive deflections during MP and identify clearly separated storms, the criterion (3), that is, separation between DstMin and next MPO >6 h, is applied. Further to avoid very slowly varying, non-storm like long duration decreases in Dst, the criterion (4), that is, rate of change of Dst during MP or (dDst/dt)MP <−5 nT/h, is applied, which results in the detection of the clear storm in Fig. 1d. By applying the procedure to the whole Dst data, the computer program identified 810 storms which include 365 intense storms (DstMin ≤−100 nT) and 39 super storms (DstMin ≤−250 nT). The estimation of the storm parameters are illustrated using Fig. 1d which also shows the storm phases. The impact at the Earth of a coronal mass ejection (CME) compresses the dayside magnetopause, intensifies its eastward directed current, and generates a positive perturbation in Dst, known as initial phase (IP) which usually lasts from tens of minutes to a few hours. It may be noted that IP is not well identifiable in all storms. This is followed by the main phase (MP) when the field decreases due to the enhancement of the westward ring current, which begins with the southward turning of IMF (interplanetary magnetic field) Bz and consequent solar wind–magnetosphere coupling and loading of solar wind energy into the ring current; it lasts for several hours to over ten hours when IMF Bz remains southward. When IMF Bz returns to zero or turns northward, the solar wind forcing diminishes, ring current intensity eventually dissipates, and Dst recovers back to its near-zero prestorm level recovery phase (RP) usually taking tens of hours to several days. The storms are analyzed for their important parameters. Main phase duration TMP is the time interval between MPO and DstMin which are defined above and identified in Fig. 1d; UT hours of MPO and DstMin are noted. ∫DstMP is the integral (or sum) of Dst during MP. For storms with positive initial phase, it is the negative of the sum of the magnitudes of Dst from MPO to DstMin. Mean DstMP = ∫DstMP/TMP is the new parameter which indicates the strength of geomagnetic storms while DstMin represents their intensity, discussed in "Results and discussion" section. (dDstMP/dt)max is the maximum rate of change of Dst during MP, which is the maximum successive difference of Dst during MP. Results and discussion Although over 800 storms are identified, we consider the 39 super storms (DstMin ≤−250 nT) and the Carrington storm because storms weaker than super storms are unlikely to be associated with system damages (e.g., Baker et al. 2008; Balan et al. 2014). The characteristics of the storms such as date of DstMin, value of DstMin, time of MPO, time of DstMin, MP duration, mean DstMP, and (dDstMP/dt)max are listed in Table 1. The solar activity index (F10.7) is also listed. Figure 2 displays the characteristics of all 40 super storms arranged in the order of decreasing mean DstMP. For clarity, the scales of mean DstMP, (dDstMP/dt)max, and DstMin are limited. Purple color represents the storms associated with severe space weather (SvSW) events that caused electric power outages and/or telegraph system -400 -200 0 a