Double Superposed Epoch Analysis Research Articles

Double Superposed Epoch Analysis of Geomagnetic Storms and Corresponding Solar Wind and IMF in Solar Cycles 23 and 24

AbstractThe weakest solar cycle 24 (SC24, 2010–2019) in 100 years was 1/3rd less active compared to the previous solar cycle 23 (SC23, 1996–2009). We identify 135 and 61 ICME (interplanetary coronal mass ejection) driven clear geomagnetic storms (DstMin ≤ −50 nT) in SC23 and SC24, respectively, giving a reduction of 55% storms in SC24, and present the double superposed epoch analysis (DSEA) of the storms/activities in SC23 and SC24 using the Dst, symmetric H (SymH), Kp and AE indices. The DSEA method for the corresponding solar wind velocity V, north‐south component of the interplanetary magnetic field (IMF Bz) and the product VBz are also presented. Compared to SC23, the maximum storm/activity intensity in SC24 reduces by 52%, 12%, and 45% at low, mid and high latitudes and the corresponding maxima in ‐VBz, V, and ‐Bz reduce by 39%, 17%, and 38%, respectively. The epoch average storm/activity intensity reduces by 27%, 11%, and 4% at low, mid and high latitudes and average maxima in ‐VBz, V, and ‐Bz reduce by 24%, 14%, and 13%, respectively. The results seem to reveal that the average reduction in the main driver ‐VBz (∼24%) might have caused nearly the same and equal average storm/activity intensity reductions in all latitudes (∼25%), though the irregular nature of the AE index makes the reduction very small (4%) at high latitudes, and small (∼11%) at mid latitudes mainly due to the small (0–9) quasi logarithmic scale of the Kp index.

Dynamics of Large-Scale Solar-Wind Streams Obtained by the Double Superposed Epoch Analysis: 5. Influence of the Solar Activity Decrease

In solar cycles 23–24, solar activity noticeably decreased and, as a result, solar wind parameters decreased. Based on the measurements of the OMNI base for the period 1976–2019, the time profiles of the main solar wind parameters and magnetospheric indices for the main interplanetary drivers of magnetospheric disturbances (solar wind types CIR. Sheath, ejecta and MC) are studied using the double superposed epoch method. The main task of the research is to compare time profiles for the epoch of high solar activity at 21–22 SC and the epoch of low activity at 23–24 SC. The following results were obtained. (1) The analysis did not show a statistically significant change in driver durations during the epoch of minimum. (2) The time profiles of all parameters for all types of SW in the epoch of low activity have the same shape as in the epoch of high activity, but locate at lower values of the parameters. (3) In CIR events, the longitude angle of the solar wind flow has a characteristic S shape; but in the epoch of low activity, it varies in a larger range than in the previous epoch.

Differences in the Dynamics of the Asymmetrical Part of the Magnetic Disturbance during the Periods of Magnetic Storms Induced by Different Interplanetary Sources

The differences in the dynamics of the asymmetrical part of the geomagnetic disturbance at middle and low latitudes during magnetic storms initiated by different interplanetary sources are analyzed. The analysis is performed with the SYM-H, ASY-H, and Dst indices from the OMNI database during the periods of 58 intense magnetic storms with –270 ≤ Dstmin ≤ –90 nT that were recorded in 1995–2017 and initiated by one of the solar wind structures: compressed corotating interaction regions (CIRs); interplanetary coronal mass ejections (ICMEs) including magnetic clouds (MCs) and Ejecta “pistons”; and compressed Sheath regions in front of ICMEs. The interplanetary sources were identified on the basis of the catalog of large-scale solar-wind phenomena ( ftp://ftp.iki.rssi.ru/pub/omni/ ). A double superposed epoch analysis with reference points at the onset of the storm and during Dstmin was used. It is shown that the ASY-H values during Sheath-driven storms are, on average, 40% higher than for storms of other groups and that the ASY-H maximum occurs ~3 h earlier than Dstmin during Sheath-driven storms and 1–2 h earlier during MC-driven storms, which may indicate a more intense and uneven energy inflow during these periods. It is assumed that this energy inflow may be provided by the proton flux with energies of >10 MeV observed by the GOES geostationary satellites, which increases by more than two orders of magnitude in the intervals of Sheath-driven storms as compared to storms of other groups.

Dynamics of Large‐Scale Solar‐Wind Streams Obtained by the Double Superposed Epoch Analysis: 4. Helium Abundance

AbstractThis work is a continuation of our previous articles (Yermolaev et al., 2015, https://doi.org/10.1002/2015JA021274; 2017, https://doi.org/10.1007/s11207-017-1205-1; 2018, https://doi.org/10.1007/s11207-018-1310-9), which describe the average temporal profiles of interplanetary plasma and field parameters in large‐scale solar‐wind (SW) streams: corotating interaction regions (CIRs), interplanetary coronal mass ejections (ICMEs including both magnetic clouds (MCs), and ejecta), and sheaths as well as interplanetary shocks (ISs). In this work, we analyze the average profile of helium abundance, Nα/Np, for 1976–2016. Our results confirm the main results that were obtained in earlier studies: Nα/Np is higher in quasi‐stationary fast streams than in slow ones; it slowly changes in compression regions CIRs and sheaths from values in undisturbed SW to values in the corresponding fast stream type pushing like a piston, in high‐speed stream (HSS) flow for CIRs, or in ICME for sheaths; in ejecta, it is close to the abundance observed in undisturbed streams, and it is maximal in MCs. For the first time, the results show that Nα/Np correlates with the proton β‐parameter in compression regions CIRs and sheath and anticorrelates in ICMEs. The Nα/Np versus β dependence is stronger in MCs than in ejecta and may be used as an indicator of conditions at the place on the Sun where CMEs are formed.

Statistical study on interplanetary drivers behind intense geomagnetic storms and substorms

Geomagnetic storms and substorms play a central role in both the daily life of mankind and in academic space physics. The profiles of storms, especially their initial phase morphology and the intensity of their substorms under different interplanetary conditions, have usually been ignored in previous studies. In this study, 97 intense geomagnetic storms (Dstmin ≤ –100 nT) between 1998 and 2018 were studied statistically using the double superposed epoch analysis (DSEA) and normalized superposed epoch analysis (NSEA) methods. These storms are categorized into two types according to different interplanetary magnetic field (IMF) Bz orientations: geomagnetic storms whose IMF is northward, both upstream and downstream relative to the interplanetary shock, and geomagnetic storms whose upstream and downstream IMF is consistently southward. We further divide these two types into two subsets, by different geomagnetic storm profiles: Type I/Type II — one/two-step geomagnetic storms with northward IMF both upstream and downstream of the interplanetary shock; Type III/TypeIV — one/two-step geomagnetic storms with southward IMF both upstream and downstream of the interplanetary shock. The results show that: (1) geomagnetic storms with northward IMF both upstream and downstream of the interplanetary shock have a clear initial phase; geomagnetic storms with southward IMF in both upstream and downstream of the interplanetary shock do not; (2) the IMF is an important controlling factor in affecting the intensity characteristics of substorms. When Bz is positive before and after the interplanetary shock arrival, the Auroral Electrojet (AE) index changes gently during the initial phase of geomagnetic storms, the median value of AE index is maintained at 500–1000 nT; (3) when Bz is negative before and after the interplanetary shock arrival, the AE index rises rapidly and reaches its maxmum value about one hour after storm sudden commencements (SSC), although the time is scaled between reference points and the maximum value of AE is usually greater than 1,000 nT, representing intense substorms; (4) for most cases, the Dst0 usually reaches its minimum at least one hour after Bz. These results are useful in improving contemporary space weather models, especially for those that address geomagnetic storms and substorms.

Dynamics of Large-Scale Solar-Wind Streams Obtained by the Double Superposed Epoch Analysis: 3. Deflection of the Velocity Vector

This work is a continuation of our previous articles (Yermolaev et al. in J. Geophys.Res. 120, 7094, 2015, Yermolaev et al. in Solar Phys. 292, 193, 2017), which describe the average temporal profiles of interplanetary plasma and field parameters in large-scale solar-wind (SW) streams: corotating interaction regions (CIRs), interplanetary coronal mass ejections (ICMEs including both magnetic clouds (MCs) and ejecta), and sheaths as well as interplanetary shocks (ISs). Changes of longitude angle {\phi} in CIRs from -2 to +2{\deg} agree with earlier observations. Besides we have for the first time analyzed the average temporal profiles of bulk velocity angles in Sheaths and ICMEs and found that the angle {\phi} in ICME changes from 2 to -2{\deg} while in Sheath it changes from -2 to 2{\deg} (similar to change in CIR), i.e., the streams in CIR/Sheath and ICME deflects in the opposite side. When averaging the latitude angle {\theta} on all intervals of the chosen SW type, the angle {\theta} is almost constant ~1{\deg}. We made selection of SW events with increasing and decreasing angle {\theta} for the first time and found that average temporal profiles for angle {\theta} in selected events have the same <<integral-like>> shape as for angle {\phi}. The difference in average profiles for angles {\phi} and {\theta} is explained by the fact that most events have increasing profiles for angle in ecliptic plane due to solar rotation while for angle in meridional plane numbers of increasing and decreasing profiles are equal.

Dynamics of Large-Scale Solar-Wind Streams Obtained by the Double Superposed Epoch Analysis: 2. Comparisons of CIRs vs. Sheaths and MCs vs. Ejecta

This work is a continuation of our previous paper [Yermolaevetal2015] which describes the average temporal profiles of interplanetary plasma and field parameters in large-scale solar-wind (SW) streams: CIR, ICME (both MC and Ejecta) and Sheath as well as the interplanetary shock (IS). Like in the previous work we use data of OMNI database, our catalog of large-scale solar-wind phenomena during 1976--2000 [Yermolaevetal2009] and the double superposed epoch analysis (DSEA) method [Yermolaevetal2010]: re-scaling the duration of interval for all types in such a manner that, respectively, beginning and end for all intervals of selected type coincide. We present new detailed results of comparison of two pair phenomena: (1) both types of compression regions (CIR.vs.Sheath) and (2) both types of ICMEs (MC.vs.Ejecta). Obtained data allows us to suggest that the formation of all types of compression regions has the same physical mechanism irrespective of piston (High-Speed Stream (HSS) or ICME) type and differences are connected with geometry (angle between speed gradient in front of piston and satellite trajectory) and full jumps of speed in edges of compression regions. One of consequences of this hypothesis is the conclusion that one of the reasons of observed distinctions of parameters in MC and Ejecta can be fact that at measurements of Ejecta the satellite passes further from the nose area of ICME, than at measurements of MC. We also discuss the impact of Sheath in magnetospheric activity and its contribution in estimation of Sun's open magnetic flux.

Dynamics of large‐scale solar wind streams obtained by the double superposed epoch analysis

AbstractUsing the OMNI data for period 1976–2000, we investigate the temporal profiles of 20 plasma and field parameters in the disturbed large‐scale types of solar wind (SW): corotating interaction regions (CIR), interplanetary coronal mass ejections (ICME) (both magnetic cloud (MC) and Ejecta), and Sheath as well as the interplanetary shock (IS). To take into account the different durations of SW types, we use the double superposed epoch analysis (DSEA) method: rescaling the duration of the interval for all types in such a manner that, respectively, beginning and end for all intervals of selected type coincide. As the analyzed SW types can interact with each other and change parameters as a result of such interaction, we investigate separately eights sequences of SW types: (1) CIR, (2) IS/CIR, (3) Ejecta, (4) Sheath/Ejecta, (5) IS/Sheath/Ejecta, (6) MC, (7) Sheath/MC, and (8) IS/Sheath/MC. The main conclusion is that the behavior of parameters in Sheath and in CIR are very similar both qualitatively and quantitatively. Both the high‐speed stream (HSS) and the fast ICME play a role of pistons which push the plasma located ahead them. The increase of speed in HSS and ICME leads at first to formation of compression regions (CIR and Sheath, respectively) and then to IS. The occurrence of compression regions and IS increases the probability of growth of magnetospheric activity.

Statistical study of interplanetary condition effect on geomagnetic storms: 2. Variations of parameters

We investigate the behavior of mean values of the solar wind’s and interplanetary magnetic field’s (IMF) parameters and their absolute and relative variations during the magnetic storms generated by various types of the solar wind. In this paper, which is a continuation of paper [1], we, on the basis of the OMNI data archive for the period of 1976–2000, have analyzed 798 geomagnetic storms with Dst ≤ −50 nT and their interplanetary sources: corotating interaction regions CIR, compression regions Sheath before the interplanetary CMEs; magnetic clouds MC; “Pistons” Ejecta, and an uncertain type of a source. For the analysis the double superposed epoch analysis method was used, in which the instants of the magnetic storm onset and the minimum of the Dst index were taken as reference times. It is shown that the set of interplanetary sources of magnetic storms can be sub-divided into two basic groups according to their slowly and fast varying characteristics: (1) ICME (MC and Ejecta) and (2) CIR and Sheath. The mean values, the absolute and relative variations in MC and Ejecta for all parameters appeared to be either mean or lower than the mean value (the mean values of the electric field Ey and of the Bz component of IMF are higher in absolute value), while in CIR and Sheath they are higher than the mean value. High values of the relative density variation sN/〈N〉 are observed in MC. At the same time, the high values for relative variations of the velocity, Bz component, and IMF magnitude are observed in Sheath and CIR. No noticeable distinctions in the relationships between considered parameters for moderate and strong magnetic storms were observed.

Specific interplanetary conditions for CIR-, Sheath-, and ICME-induced geomagnetic storms obtained by double superposed epoch analysis

Abstract. A comparison of specific interplanetary conditions for 798 magnetic storms with Dst &lt;−50 nT during 1976–2000 was made on the basis of the OMNI archive data. We categorized various large-scale types of solar wind as interplanetary drivers of storms: corotating interaction region (CIR), Sheath, interplanetary CME (ICME) including both magnetic cloud (MC) and Ejecta, separately MC and Ejecta, and "Indeterminate" type. The data processing was carried out by the method of double superposed epoch analysis which uses two reference times (onset of storm and minimum of Dst index) and makes a re-scaling of the main phase of the storm in a such way that all storms have equal durations of the main phase in the new time reference frame. This method reproduced some well-known results and allowed us to obtain some new results. Specifically, obtained results demonstrate that (1) in accordance with "output/input" criteria the highest efficiency in generation of magnetic storms is observed for Sheath and the lowest one for MC, and (2) there are significant differences in the properties of MC and Ejecta and in their efficiencies.