M. Yu. Yermolaev

Catalog of Large-Scale Solar Wind Phenomena during 1976-2000

The main goal of this paper is to compile a catalog of large-scale phenomena in the solar wind over the observation period of 1976–2000 using the measurement data presented in the OMNI database. This work included several stages. At first the original OMNI database was supplemented by certain key parameters of the solar wind that determine the type of the solar wind stream. The following parameters belong to this group: the plasma ratio β, thermal (NkT) and kinetic (mNV2) pressures of the solar wind, the ratio T/Texp of measured and expected temperatures, gradients of the plasma velocity and density, and the magnetic field gradient. The results of visualization of basic plasma parameters that determine the character of the solar wind stream are presented on the website of the Space Research Institute, Moscow. Preliminary identification of basic types of the solar wind stream (FAST and SLOW streams, Heliospheric Current Sheet (HCS), Corotating Interaction Region (CIR), EJECTA (or Interplanetary Coronal Mass Ejections), Magnetic Cloud (MC), SHEATH (compression region before EJECTA/MC), rarified region RARE, and interplanetary shock wave IS) had been made with the help of a preliminary identification program using the preset threshold criteria for plasma and interplanetary magnetic field parameters. Final identification was done by comparison with the results of visual analysis of the solar wind data. In conclusion, histograms of distributions and statistical characteristics are presented for some parameters of various large-scale types of the solar wind.

Statistical Investigation of Heliospheric Conditions Resulting in Magnetic Storms: 2

Time behavior of the solar wind and interplanetary magnetic field parameters is investigated for 623 magnetic storms of the OMNI database for the period 1976–2000. The analysis is carried out by the superposed epoch technique (the magnetic storm onset time is taken to be the beginning of an epoch) for five various categories of storms induced by various types of solar wind: CIR (121 storms), Sheath (22 storms), MC (113 storms), and “uncertain type” (367 storms). In total, the analysis conducted for “all storms” included 623 storms. The obtained data, on one hand, confirm the results obtained earlier without selecting the intervals according to the solar wind types, and, on the other hand, they indicate the existence of distinctions in the time variation of parameters for various types of solar wind. Though the lowest values of the Bz-component of IMF are observed in the MC, the lowest values of the Dst-index are achieved in the Sheath. Thus, the strongest magnetic storms are induced, on average, during the Sheath rather than during the MC body passage, probably owing to higher pressure in the Sheath. Higher values of nkT, T/Texp, and β parameters are observed in the CIR and Sheath and lower ones in the MC, which corresponds to the physical essence of these solar wind types.

Relative occurrence rate and geoeffectiveness of large-scale types of the solar wind

We investigate the relative occurrence rate for various types of the solar wind and their geoeffectiveness for magnetic storms with Dst < —50 nT. Both integrated effect for the entire time 1976–2000 and variations during this period of 2.5 cycles of solar activity are studied As raw data for the analysis we have used the catalog of large-scale types of the solar wind for the period 1976-2000 (see ftp://ftp.iki.rssi.ru/omni/) created by us with the use of the OMNI database (http://omni.web.gsgc.nasa.gov) [1] and described in detail in [2]. The average annual numbers of different type of events are as follows: 124 ±81 for the heliospheric current sheet (HCS), 8 ±6 for magnetic clouds (MC), 99 ±38 for Ejecta, 46 ±19 for Sheath before Ejecta, 6 ±5 for Sheath before MC, and 63 ±15 for CIR. The measurements that allowed one to determine a source in the solar wind were available only for 58% of moderate and strong magnetic storms (with index Dst < —50 nT) during the period 1976–2000. Magnetic clouds (MC) are shown to be the most geoeffective (~61%). The CIR events and Ejecta with Sheath region are three times less geoeffective (~20–21 %). Variations of occurrence rate and geoeffectiveness of various types of the solar wind in the solar cycle are discussed.

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

Using 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

Based on the archive OMNI data for the period 1976–2000 an analysis has been made of 798 geomagnetic storms with Dst < −50 nT and their interplanetary sources-large-scale types of the solar wind: CIR (145 magnetic storms), Sheath (96), magnetic clouds MC (62), and Ejecta (161). The remaining 334 magnetic storms have no well-defined sources. For the analysis, we applied the double method of superposed epoch analysis in which the instants of the magnetic storm beginning and minimum of Dst index are taken as reference times. The well-known fact that, independent of the interplanetary source type, the magnetic storm begins in 1–2 h after a southward turn of the IMF (Bz < 0) and both the end of the main phase of a storm and the beginning of its recovery phase are observed in 1–2 h after disappearance of the southward component of the IMF is confirmed. Also confirmed is the result obtained previously that the most efficient generation of magnetic storms is observed for Sheath before MC. On the average parameters Bz and Ey slightly vary between the beginning and end of the main phase of storms (minimum of Dst and Dst* indices), while Dst and Dst* indices decrease monotonically proportionally to integral of Bz and Ey over time. Such a behavior of the indices indicates that the used double method of superposed epoch analysis can be successfully applied in order to study dynamics of the parameters on the main phase of magnetic storms having different duration.

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.

Influence of the interplanetary driver type on the durations of the main and recovery phases of magnetic storms

We study the durations of the main and recovery phases of magnetic storms induced by different types of large-scale solar-wind streams (Sheath, magnetic cloud (MC), Ejecta, and corotating interaction region (CIR)) on the basis of OMNI data for 1976–2000. The durations of both the main and recovery phases depend on the type of interplanetary drivers. On average, the duration of the main phase of storms induced by compressed regions (CIR and Sheath) is shorter than for MC and Ejecta, while the duration of the recovery phase of CIR- and Sheath-induced storms is longer. Analysis of the durations of individual storms shows that the durations of the main and recovery phases anticorrelate for CIR- and Sheath-induced storms and there is no dependence between them for (MC + Ejecta)-induced storms.

Solar and interplanetary sources of geomagnetic storms: Space weather aspects

The current notions of the solar-terrestrial relations responsible for the transport of solar disturbances and for the generation of magnetic storms on the Earth are briefly reviewed. The probability of generating magnetic storms by different solar and interplanetary phenomena is quantitatively estimated. The efficiencies of generating magnetic storms by different types of solar wind streams are compared.

Does geomagnetic storm magnitude depend on solar flare importance

Solar flares are often used as precursors of geomagnetic stor ms. In particular, Howard and Tappin (2005) recently published in A&A a dependence between X-ray class of solar fla res nd Ap and Dst indexes of geomagnetic storms which contradicts to early published results.In order to predict space weather effects, solar flares are often used as precursors of magnetic storms on the Earth. In particular, possible relation between the solar flare importance and magnetic storm intensity is discussed in some papers. However, published results contradict each other. We compare the published results on the flare-storm dependence and discuss possible causes of this disagreement: (1) different intervals of observation, (2) differing statistics, and (3) different methods of identification of events and their comparison. Our analysis has shown that the fact of occurrence and the magnitude of a geomagnetic storm cannot be determined, generally, using only the solar flare importance. However, analyzing additional information on the coronal mass ejection (CME), associated with the geomagnetic storm, one can offer an algorithm for the storm magnitude prediction on the basis of flare importance.

Does the duration of the magnetic storm recovery phase depend on the storm development rate in its main phase

The dependence between the storm development rate |Dstmin|/ΔT (where ΔT is the duration of the main phase of storm) and the duration of the recovery phase of magnetic storms induced by three different types of interplanetary drivers, such as (1, 2) the CIR and Sheath compression regions and (3) the body of interplanetary coronal mass ejection ICME (magnetic clouds and Ejecta), are compared. It is shown that the duration of the recovery phase apparently correlates with the development rate of storms induced by Sheath and CIR and do not correlate with storms induced by ICME.The dependence between the storm development rate |Dst min|/ΔT (where ΔT is the duration of the main phase of storm) and the duration of the recovery phase of magnetic storms induced by three different types of interplanetary drivers, such as (1, 2) the CIR and Sheath compression regions and (3) the body of interplanetary coronal mass ejection ICME (magnetic clouds and Ejecta), are compared. It is shown that the duration of the recovery phase apparently correlates with the development rate of storms induced by Sheath and CIR and do not correlate with storms induced by ICME.