Statistical investigation of Heliospheric conditions resulting in magnetic storms: 2

This work is a continuation of investigation [1] of the behavior of the solar wind’s and interplanetary magnetic field’s parameters near the onset of geomagnetic storms for various types of solar wind streams. The data of the OMNI base for the 1976–2000 period are used in the analysis. The types of solar wind streams were determined, and the times of beginning (onsets) of magnetic storms were distributed in solar wind types as follows: CIR (121 storms), Sheath (22 storms), MC (113 storms), and “uncertain type” (367 storms). The growth of variations (hourly standard deviations) of the density and IMF magnitude was observed 5–10 hours before the onset only in the Sheath. For the CIR-, Sheath-and MC-induced storms the dependence between the minimum of the IMF B _z-component and the minimum of the D _st -index, as well as the dependence between the electric field E _y of solar wind and the minimum of the D _st -index are steeper than those for the “uncertain” solar wind type. The steepest D _st vs. B _z dependence is observed in the Sheath, and the steepest D _st vs. E _y dependence is observed in the MC.     

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 D _st ≤ −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 D _st 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 E _y and of the B _z 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, B _z 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.     

Modeling the time behavior of the D st index during the main phase of magnetic storms generated by various types of solar wind

Results of modeling the time behavior of the D _st index at the main phase of 93 geomagnetic storms (?250 < D _st ≤ ?50 nT) caused by different types of solar wind (SW) streams: magnetic clouds (MC, 10 storms), corotating interaction regions (CIR, 31 storms), the compression region before interplanetary coronal ejections (Sheath before ICME, 21 storms), and “pistons” (Ejecta, 31 storms) are presented. The “Catalog of Large-Scale Solar Wind Phenomena during 1976–2000” (ftp://ftp.iki.rssi.ru/pub/omni/) created on the basis of the OMNI database was the initial data for the analysis. The main phase of magnetic storms is approximated by a linear dependence on the main parameters of the solar wind: integral electric field sumEy, dynamic pressure P _d , and fluctuation level sB in IMF. For all types of SW, the main phase of magnetic storms is better modeled by individual values of the approximation coefficients: the correlation coefficient is high and the standard deviation between the modeled and measured values of D _st is low. The accuracy of the model in question is higher for storms from MC and is lower by a factor of ～2 for the storms from other types of SW. The version of the model with the approximation coefficients averaged over SW type describes worse variations of the measured D _st index: the correlation coefficient is the lowest for the storms caused by MC and the highest for the Sheath- and CIR-induced storms. The model accuracy is the highest for the storms caused by Ejecta and, for the storms caused by Sheath, is a factor of ～1.42 lower. Addition of corrections for the prehistory of the development of the beginning of the main phase of the magnetic storm improves modeling parameters for all types of interplanetary sources of storms: the correlation coefficient varies within the range from r = 0.81 for the storms caused by Ejecta to r = 0.85 for the storms caused by Sheath. The highest accuracy is for the storms caused by MC. It is, by a factor of ～1.5, lower for the Sheath-induced storms.     

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 (mNV ²) pressures of the solar wind, the ratio T/T _exp 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.     

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.     

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 D _st < −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 D _st 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 (B _z < 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 B _z and E _y slightly vary between the beginning and end of the main phase of storms (minimum of D _st and D _st ^* indices), while D _st and D _st ^* indices decrease monotonically proportionally to integral of B _z and E _y 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 Relationships between Solar, Interplanetary, and Geomagnetospheric Disturbances, 1976–2000

Within the framework of the Space Weather program, 25-year data sets for solar X-ray observations, measurements of plasma and magnetic field parameters in the solar wind, and D _st index variations are analyzed to reveal the factors that have had the greatest influence on the development of magnetospheric storms. The correlation between solar flares and magnetic storms practically does not exceed a level of correlation for random processes. In particular, no relation was found between the importance of solar flares and the minimum of the D _st index for storms that could be connected with considered flares by their time delay. The coronal mass ejections (CME; data on these phenomena cover a small part of the interval) result in storms with D _st < –60 nT only in half of the cases. The most geoeffective interplanetary phenomena are the magnetic clouds (MC), which many believe to be interplanetary manifestations of CMEs, and compressions in the region of interaction of slow and fast streams in the solar wind (the so-called Corotating Interaction Region, CIR). They correspond to about two-thirds of all observed magnetic storms. For storms with –100 < D _st < –60 nT, the frequencies of storms from MC and CIR being approximately equal. For strong storms with D _st < – 100 nT, the fraction of storms from MC is considerably higher. The problems of reliable prediction of geomagnetic disturbances from observations of the Sun and conditions in interplanetary space are discussed.     

Turbulent fluctuations of plasma and magnetic field parameters in the magnetosheath and the low-latitude boundary layer formation: Multisatellite observations on March 2, 1996

The results of simultaneous analysis of plasma and magnetic field characteristics measured on the INTERBALL/Tail Probe, WIND and Geotail satellites on March 2, 1996, are presented. During these observations the INTERBALL/Tail Probe crossed the low-latitude boundary layer, and the WIND and Geotail satellites measured the solar wind’s and magnetosheath’s parameters, respectively. The plasma and magnetic field characteristics in these regions have been compared. The data of the Corall, Electron, and MIF instruments on the INTERBALL/Tail Probe satellite are analyzed. Fluctuations of the magnetic field components and plasma velocity in the solar wind and magnetosheath, measured onboard the WIND and Geotail satellites, are compared. The causes resulting in appearance of plasma jet flows in the low-latitude boundary layer are analyzed. The amplitude of magnetic field fluctuations in the magnetosheath for a studied magnetosphere boundary crossing is shown to exceed the magnetic field value below the magnetopause near the cusp. The possibility of local violation of pressure balance on the magnetopause is discussed, as well as penetration of magnetosheath plasma into the magnetosphere, as a result of magnetic field and plasma flux fluctuations in the magnetosheath.     

Forecasting the velocity of quasi-stationary solar wind and the intensity of geomagnetic disturbances produced by it

A brief review is given of contemporary approaches to solving the problem of medium-term forecast of the velocity of quasi-stationary solar wind (SW) and of the intensity of geomagnetic disturbances caused by it. At the present time, two promising models of calculating the velocity of quasi-stationary SW at the Earth’s orbit are realized. One model is the semi-empirical model of Wang-Sheeley-Arge (WSA) which allows one to calculate the dependence V(t) of SW velocity at the Earth’s orbit using measured values of the photospheric magnetic field. This model is based on calculation of the local divergence f _S of magnetic field lines. The second model is semi-empirical model by Eselevich-Fainshtein-Rudenko (EFR). It is based on calculation in a potential approximation of the area of foot points on the solar surface of open magnetic tubes (sources of fast quasistationary SW). The new Bd-technology is used in these calculations, allowing one to calculate instantaneous distributions of the magnetic field above the entire visible surface of the Sun. Using predicted V(t) profiles, one can in EFR model calculate also the intensity of geomagnetic disturbances caused by quasi-stationary SW. This intensity is expressed through the K _p index. In this paper the EFR model is discussed in detail. Some examples of epignosis and real forecast of V(t) and K _p (t) are discussed. A comparison of the results of applying these two models for the SW velocity forecasting is presented.     

Solar activity index for long-term ionospheric forecasts

Based on the comparison of solar activity indices (annual average values of the relative number of sunspots Rz₁₂ and solar radio emission flux at a wavelength of 10.7 cm F₁₂) with the ionospheric index of solar activity IG₁₂ for 1954–2013, we have found that the index F₁₂ is a more accurate (than Rz₁₂) indicator of solar activity for the long-term forecast of foF2 (the critical frequency of the F2-layer). This advantage of the F₁₂ index becomes especially significant after 2000 if the specific features of extreme ultraviolet radiation of the Sun are additionally taken into account in the minima of solar cycles, using an appropriate correction to F₁₂. Qualitative arguments are given in favor of the use of F₁₂ for the long-term forecast of both foF2 and other ionospheric parameters.     

Some problems of identifying types of large-scale solar wind and their role in the physics of the magnetosphere

This paper discusses the errors in analyzing solar-terrestrial relationships, which result from either disregarding the types of interplanetary drivers in studying the magnetosphere response on their effect or from the incorrect identification of the type of these drivers. In particular, it has been shown that the absence of selection between the Sheath and ICME (the study of so-called CME-induced storms, i.e., magnetic storms generated by CME) leads to errors in the studies of interplanetary conditions of magnetic storm generation, because the statistical analysis has shown that, in the Sheath + ICME sequences, the largest number of storm onsets fell on the Sheath, and the largest number of storms maxima fell at the end of the Sheath and the beginning of the ICME. That is, the situation is observed most frequently when at least the larger part of the main phase of storm generation falls on the Sheath and, in reality, Sheath-induced storms are observed. In addition, we consider several cases in which magnetic storms were generated by corotating interaction regions, whereas the authors attribute them to CME.     

Peculiarities of long-wave radio bursts from solar flares preceding strong geomagnetic storms

Radio bursts in the frequency range of 100–1500 kHz, recorded in 1997–2000 on the INTERBALL-1 satellite during the solar flares preceding the strong geomagnetic storms with D _st < ?100 nT, are analyzed in this paper. The observed long-wave III-type radio bursts of solar origin at frequencies of 1460 and 780 kHz were characterized by large values of the flux S _f = 10^?15 ?10^?17 W/m² Hz and duration longer than 10 min. The rapid frequency drift of a modulated radio burst continued up to a frequency of 250 kHz, which testified that the exciting agent (a beam of energetic electrons) propagated from the Sun to the Earth. All such flares were characterized by the appearance of halo coronal mass ejections, observed by the LASCO/SOHO, and by the presence of a southward B_z-component of the IMF, measured on the ACE and WIND spacecraft. In addition, shortly after radio bursts, the INTERBALL-1 satellite has recorded the fluxes of energetic electrons with E > 40 keV.     

Study of notches in the Earth’s plasmasphere based on data of the <Emphasis Type="Italic">MAGION-5</Emphasis> satellite

Depleted narrow (localized in longitude) regions (field tubes) in the plasmasphere, recently discovered in He⁺ radiation measurements on the IMAGE spacecraft, were first directly observed by the Magion-5 satellite. The low-density regions (notches) occupy <～ 10–30° in longitude and extend from L ～ 2–3 to the plasmasphere boundary in neighboring plasmasphere regions with larger densities. The Magion-5 data give evidence that in the low-density regions temperature is enhanced as compared to the neighboring denser plasmasphere regions. Formation of notches in the plasmasphere is, apparently, associated with AE intensification during weak magnetic storms, while strong magnetic storms usually result in the overall reduction of plasmasphere dimensions. However, even a strong magnetic storm on April 6–7, 2000 (max K _p = 9-and min D _st ～ ?290 nT), but accompanied by an isolated AE impulse, resulted in a density decrease only in the longitudinally limited post-midnight sector of the plasmasphere.     

Sporadic structures and small-scale irregularity in the nighttime polar ionosphere in the period of high solar activity according to the data of radio occultation measurements on satellite-to-satellite paths

Results of the analysis of 327 sessions of radio occultation on satellite-to-satellite paths are presented. The data are taken in the nighttime polar ionosphere in the regions with latitudes of 67°–88°, and in the period of high solar activity from October 26, 2003 to November 9, 2003. Typical ionospheric changes in the amplitude and phase of decimeter radio waves on paths GPS satellites-CHAMP satellite are presented. It is demonstrated that these data make it possible to determine characteristics of the sporadic E _s structures in the lower ionosphere at heights of 75–120 km. Histograms of distribution of the lower and upper boundaries, thickness, and intensity of the E _s structures are presented. Dispersion and spectra of amplitude fluctuations of decimeter radio waves caused by small-scale irregularity of the ionospheric plasma are analyzed. The relation of the polar E _s structures and intensity of small-scale plasma irregularity to various manifestations of solar activity is discussed. The efficiency of monitoring the ionospheric disturbances caused by shock waves of the solar wind by the radio occultation method on satellite-to-satellite paths is demonstrated.     

Magnetospheric signatures of auroral disturbances during the passage of the solar wind’s CIR and sheath regions

Based on Polar satellite data, the authors have studied the auroral disturbances that arose during the passage by the Earth of compressed plasma regions formed in front of high-speed solar wind streams (the CIR region) and in front of magnetic clouds (the Sheath region). The aurorae observed by the Polar satellite possessed basic signatures of a substorm: a localized onset and expansion toward the pole and westward and eastward. However, in these cases they had a very large size in longitude and latitude and occupied a very large area. All disturbances observed by the Polar satellite during the Sheath and CIR regions of the solar wind in December of 1996, in 1997–1998, and in 2000 were analyzed. Eight events during disturbance development in the ionosphere, when the Geotail satellite was located in the plasma sheet of the magnetospheric tail, were selected. It is shown that in all selected cases some typical signatures of substorm development in the magnetospheric tail were observed, namely: (1) fast plasma flows (flow reversal, i.e., from tailwards to Earthwards) and (2) a sharp decrease of the total pressure, which followed an interval of total pressure increase. One can draw the conclusion that in the CIR and Sheath regions with a high solar wind density, substorm disturbances of a specific type are observed, with large latitudinal and longitudinal size (sometimes occupying the entire polar cap).     

Numerical simulation of effects of the August 11, 1999 solar eclipse in the outer ionosphere

The results of a numerical simulation of such parameters of the topside ionosphere as concentration N _e and temperature T _e of electrons, and concentration n(H⁺) and fluxes along the magnetic field lines Φ(H⁺) of H⁺ ions at an altitude of ～2000 km for the conditions of the August 11, 1999 solar eclipse are presented. The calculations were performed using the Global Self-consistent Model of the Thermosphere, Ionosphere, and Protonosphere of the Earth (GSM TIP). It is shown that during the eclipse, in addition to a region of decreased values of T _e in the Northern Hemisphere and in the magnetically conjugate region in the Southern Hemisphere, regions of electron heating emerge in both hemispheres. Simultaneously, an extended region of decreased values of N _e comes into existence and moves behind the Moon’s shadow. Regions with decreased (down to ～30%) and enhanced (up to ～50%) concentrations of H⁺ ions are detected in the global distribution of these ions.     

Observations of corotating solar wind structures at radio sounding by signals of the <Emphasis Type="Italic">Rosetta</Emphasis> and <Emphasis Type="Italic">Mars Express</Emphasis> spacecraft

In the implementation of the space projects Rosetta and Mars Express, a large-scale series of experiments has been carried out on radio sounding circumsolar plasma by decimeter (S-band) and centimeter (X-band) signals of the Rosetta comet probe (from October 3 to October 31, 2010) and the Mars Express satellite of Mars (from December 25, 2010 to March 27, 2011). It was found that in the phase of ingress the spacecraft behind the Sun, the intensity of the frequency fluctuations increases in accordance with a power function whose argument is the solar offset distance of radio ray path, and when the spacecraft is removed from the Sun (the egress phase), frequency fluctuations are reduced. Periodic strong increases in the fluctuation level, exceeding by a factor of 3–12 the background values of this value determined by the regular radial dependences, are imposed on the regular dependences. It was found that increasing the fluctuations of radio waves alternates with the periodicity m × T or n × T, where m = 1/2, n = 1, аnd T is the synodic period of the Sun’s rotation (T ≈ 27 days). It was shown that the corotating structures associated with the interaction regions of different speed fluxes are formed in the area of solar wind acceleration and at distances of 6–20 solar radii already have a quasi-stationary character.     

Ultraviolet studies of the interplanetary and local interstellar medium

A review of the last results of UV observations in the interplanetary space of neutral hydrogen and neutral helium in resonance lines HI λ1215.7 Å and HeI λ584 Å is presented. The history of discovery of the interstellar wind effect is expounded in the review, and interaction of the solar wind with the interstellar medium is briefly described. Models of the inner-heliospheric distribution of emissivity in the lines HI λ1215.7 Å and HeI λ584 Å are discussed, and possibilities of getting the solar wind parameters from UV observations in the L _α line are considered. Currently accepted parameters of the local interstellar medium are presented. The parameters obtained are compared with those derived from non-optical measurements.     

Study and comparison of the parameters of five hot flow anomalies at a bow shock front

Five hot flow anomalies (HFA) recorded by the Tail Probe of the INTERBALL satellite in 1996 are analyzed in present work. For the five chosen events the authors determined the characteristics of current sheets whose interaction with the bow shock front led to formation of an HFA, as well as the directions of external electric fields and the directions of motion of these HFAs over a shock front. The analysis of plasma convection in an HFA body is carried out; the average velocities of plasma motion in the HFA are determined in a coordinate system linked with the normal to a current layer and with the normal to the bow shock. According to the character of plasma convection in an HFA body, these five events may be divided into two types, which also differ in the direction of the motion over the front of the bow shock. In the first-type HFAs, the convection of plasma has a component directed from the intermediate region confirming its identification as a source of energy for the formation of an HFA. In the second-type HFAs, plasma motion from the intermediate region in leading and trailing parts is less expressed. This fact, as well as the great variation of peculiar velocities in the body of anomalies, allowed the assumption that second-type anomalies are nonstationary. Evidence is presented that the anomalies considered in the paper are bordered with shocks formed in solar wind passing a large-scale, decelerated body of heated plasma.     

Fine structure of the interplanetary shock front according to measurements of the ion flux of the solar wind with high time resolution

According to the data of the BMSW/SPEKTR-R instrument, which measured the density and velocity of solar wind plasma with a record time resolution, up to ~3 ×10^–2 s, the structure of the front of interplanetary shocks has been investigated. The results of these first investigations were compared with the results of studying the structure of the bow shocks obtained in previous years. A comparison has shown that the quasi-stationary (averaged over the rapid oscillations) distribution of plasma behind the interplanetary shock front was significantly more inhomogeneous than that behind the bow-shock front, i.e., in the magnetosheath. It has also been shown that, to determine the size of internal structures of the fronts of quasi-perpendicular (θ_BN > 45°) shocks, one could use the magnetic field magnitude, the proton density, and the proton flux of the solar wind on almost equal terms. A comparison of low Mach (М _А < 2), low beta (β₁ < 1) fronts of interplanetary and bow shocks has shown that the dispersion of oblique magnetosonic waves plays an essential role in their formation.