Exploring the solar system galactic frontier in extreme ultraviolet

The solar system galactic frontier—the region where the expanding solar wind meets the surrounding galactic medium—remains poorly explored. The sheer size of the essentially asymmetric heliosphere calls for remote techniques to probe the properties of its global time-varying three-dimensional boundary. The Interstellar Boundary Explorer (IBEX) mission (launch in 2008) will image the region between the termination shock and the heliopause, the heliospheric sheath, in fluxes of energetic neutral atoms. Global imaging in extreme ultraviolet (EUV) will likely be the next logical step in remote exploration of the galactic frontier from 1 AU. Imaging in EUV will establish directional and spectral properties of (1) the glow of singly charged helium (He⁺) ions in the interstellar and solar wind plasmas; (2) emissions of hot plasma in the Local Bubble; and (3) characteristic emissions of the solar wind. Global imaging with ultrahigh sensitivity and ultrahigh spectral resolution will map the heliopause and reveal the three-dimensional flow pattern of the solar wind, including the flow over the Sun's poles. This article presents the emerging concept of the experiment and space mission for heliosphere global imaging in EUV.     

Low-energy ion fluxes and magnetic field on the inner boundary of the heliosphere

A comparison of temporal profiles of low-energy ion intensity and magnetic field magnitude in different periods of solar activity in the outer heliosphere is carried out using the data of the Voyager 1 and Voyager 2 spacecraft. It is shown that temporal, spectral, and statistical characteristics of particle fluxes and magnetic field in the heliospheric regions before and after the terminal shock in 2002–2008 had similar dynamics in different hemispheres. This similarity allowed one to assume that, in the region of the inner heliospheric boundary, a quasistable spatial structure existed moving together with the terminal shock in accordance with the solar wind pressure, as well as, probably, under the action of the interstellar medium. It was revealed that the spatial dimensions of most details of this structure are less on Voyager 2, which, probably, is due to variation of the solar activity level, difference in latitude of spacecraft disposition, and also the influence of the interstellar magnetic field.     

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.     

Project Icarus: A review of local interstellar medium properties of relevance for space missions to the nearest stars

I review those properties of the interstellar medium within 15 light-years of the Sun, which will be relevant for the planning of future rapid (v≥0.1c) interstellar space missions to the nearest stars. As the detailed properties of the local interstellar medium (LISM) may only become apparent after interstellar probes have been able to make in situ measurements, the first such probes will have to be designed conservatively with respect to what can be learned about the LISM from the immediate environment of the Solar System. It follows that studies of interstellar vehicles should assume the lowest plausible density when considering braking devices, which rely on transferring momentum from the vehicle to the surrounding medium, but the highest plausible densities when considering possible damage caused by the impact of the vehicle with interstellar material. Some suggestions for working values of these parameters are provided. This paper is a submission of the Project Icarus Study Group.     

Statistical investigation of heliospheric conditions resulting in magnetic storms

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 B _z-component of IMF are observed in the MC, the lowest values of the D _st-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/T _exp, 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.     

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.     

Illumination of the Earth’s ionosphere by the vacuum ultraviolet radiation reflected by the moon

The spectral curve of the flux density of the radiation, which is reflected by the full moon towards the Earth’s ionosphere within a wavelength range of 200–1700 Å, has been presented. This curve is obtained by the approximation of space experiment data available in the scientific literature on the lunar spectral albedo and solar spectrum. Estimates of maximum values of spectral densities of fluxes reflected by the full moon in the neighborhood of wavelengths of ionization of basic ionospheric particles (neutral atoms of H I, He I, N I, O I, and Ar I and ions of He II, N II, O II, Ar II, N III, O III, and Ar III, as well as molecules of H₂, N₂, and O₂) are given.     

Interplanetary scintillations of the radio source ensemble at the maximum of cycle 24 of solar activity

The results of the interplanetary scintillation observations performed in the period of the maximum of solar activity from April 2013 to April 2014 on the BSA LPI radio telescope at the frequency 111MHz are presented. Fluctuations of the radio emission flux were recorded round the clock for all sources with a scintillating flux of more than 0.2 Jy falling in a strip of sky with a width of 50° over declinations corresponding to a 96-beam directional pattern of the radio telescope. The total number of sources observed during the day reaches 5000. The processing of the observational data was carried out on the assumption that a set of scintillating sources represents a homogeneous statistical ensemble. Daily two-dimensional maps of the distribution of the level of scintillations, whose analysis shows the strong nonstationarity and large-scale irregularity of the spatial distribution of solar wind parameters, were constructed. According to maps of the distribution of the level of scintillations averaged over monthly intervals, the global structure of the distribution of the solar wind was investigated in the period of the maximum of solar activity, which was found to be on the average close to spherically symmetric. The data show that on a spherically symmetric background an east–west asymmetry is observed, which indicates the presence of a large-scale structure of a spiral type in the solar wind.     

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.     

Solar flare with a surge: Scenario,energy budget,and forecast

Based on the analysis of solar events on August 18, 1995 (SN/C1.9 limb event) and September 23, 1998 (3B/M6.9 disk event) we suggest a new scenario of a solar flare with a surge in which the return motion of a surge is a cause of additional energy release and formation of a second system of solar flare ribbons. Observations in H_α line and data on X-ray emission fluxes in the range 1–8 Å and 0.5–4 Å are supplemented for the second case by the data in line 1550 Å. The scenario specifies two stages of development. During the first one the energy release proceeds in the current layer, which makes provisions both for acceleration of eruption upward from the solar surface and for the flare itself, including flare region heating, and radiation and thermal conductance losses. The second stage of the flare is supplied with energy due to a fall of the surge substance onto the chromosphere. The second pair of flare ribbons observed at this stage is suggested as a chromospheric criterion of realization of this scenario for disk flares. The energy released during the first stage of the flare on September 23, 1998 was equal to ～3 · 10³¹ erg. Its part consumed on flare processes is about ～0.5 · 10³¹ erg. The remaining part representing the eruption energy is consistent in order of magnitude with a calculated value of the flare energy on the second stage, which does not contradict the suggested scenario. Early recognition of such a scenario for flares on the disk can be used for prompt space weather forecast. In particular, a flare with a surge allows one to predict the absence of a bright core in a coronal mass ejection.     

The influence of effects of the heliospheric interface on parameters of backscattered solar <Emphasis Type="Italic">L</Emphasis>α radiation measured at the Earth’s orbit

A new kinetic model of distribution of interstellar hydrogen atoms in the heliosphere is suggested in this paper. It takes into account global effects associated with charge exchange of interstellar atoms near the heliospheric boundary. The constructed model allows one to find efficiently the detailed distribution of hydrogen atoms over space and velocities. For the axisymmetric steady-state case a comparison is made of the parameters of interstellar hydrogen atoms that were obtained using the classical hot model, two modifications of the improved hot model, and a global self-consistent kinetic gas-dynamic model of the heliospheric interface. The results of calculations of the spectral moments of scattered solar Lα radiation are presented. They were derived on the basis of different models of distribution of hydrogen atoms in the heliosphere.     

Identification of coronal sources of the solar wind from solar images in the EUV spectral range

Methods of localizing coronal sources of the solar wind (SW), such as coronal holes, quasi-stationary fluxes from active regions, and transient sources associated with small-scale active phenomena are considered based on vacuum-ultraviolet (EUV) images of the corona at low solar activity during the initial period of the 24th solar cycle (2010). It is shown that a SW velocity profile can be calculated from the relative areas of coronal holes (CH) at the central part of the disk based on the images in the ranges of 193 and 171 Å. The images in the 193 Å describe the geometry of large HCs that represent sources of fast SW well. The images in 171 Å are a better visualization of small CHs, based on which the profile of a slow SW component was calculated to a high accuracy (up to 65 km/s). According to Hinode/EIS data of October 15, 2010, using the Doppler spectroscopy method at the streamer base over the active region 11112, the source of the outgoing plasma flux with the mean velocity of 17 km/s was localized in the magnetic field region with an intensity of less than 200 Gauss. According to the estimate, the density of the plasma flux from this source is an order of magnitude greater than the value required for explaining the distinction between the calculated and measured profiles of a slow SW velocity. For finding the transient SW component based on small-scale flare activity, SW parameters were analyzed for the periods of flares accompanied by coronal mass ejections (CMEs), and for the periods without flares, according to the data obtained in 2010 from the ACE and GOES satellites and by coronagraphs on the STEREO-A and -B spacecraft. The ion ratios C⁺⁶/C⁺⁵ and O⁺⁷/O⁺⁶ and the mean charge of Fe ions for periods with flares were shown to be shifted toward large values, suggesting the presence of a hot SW component associated with flare activity. A noticeable correlation between the maximum charge of Fe ions and the peak power of a flare, previously observed for flares of a higher class, was confirmed. The mean value of the SW flux density during the periods of flares was 30% higher than that in the periods without flares, which is possibly associated also with the growth of fluxes from other sources with an increasing solar activity level. Based on the example of a series of flares of October 13–14, 2010, it was supposed that transient SW fluxes from the weak flares at low solar activity can manifest themselves in the form of interplanetary ICME-transients.     

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.     

MHD Simulations of the Dynamics of Sharp Disturbances of the Interplanetary Medium and Comparison with Spacecraft Observations

One-dimensional MHD simulations of solitary sharp and strong disturbances (impulses) of the interplanetary magnetic field and plasma of the homogeneous solar wind were performed. The characteristics of a disturbance of this type, recorded onboard the WIND spacecraft (SC) rather far from the Earth, were taken as initial conditions. The results of numerical experiments simulating the evolution of this disturbance in the moving interplanetary plasma, whose parameters correspond to observations of the WIND and INTERBALL-1 SC, show the efficiency of the computer code developed with the special purpose of investigating low-frequency wave events in the space environment. The calculated characteristics of the impulse resulting from the evolution are in good agreement with parameters of the disturbance recorded by the INTERBALL-1 SC closer to the Earth. In particular, the impulse expands due to imbalance of thermal and magnetic pressures, but keeps its abrupt boundaries. It was demonstrated that stable plasma objects, corresponding to stationary MHD solutions, could really exist in the solar wind plasma for a long time.     

Large and sharp changes of solar wind dynamic pressure and disturbances of the magnetospheric magnetic field at geosynchronous orbit caused by these variations

The large and sharp changes of solar wind dynamic pressure, found from the INTERBALL-1 satellite and WIND spacecraft data, are compared with simultaneous magnetic field disturbances in the magnetosphere measured by geosynchronous GOES-8, GOES-9, and GOES-10 satellites. For this purpose, about 200 events in the solar wind, associated with sharp changes of the dynamic pressure, were selected from the INTERBALL-1 satellite data obtained during 1996–1999. The large and sharp changes of the solar wind dynamic pressure were shown to result in rapid variations of the magnetic field strength in the outer magnetosphere, the increase (drop) of the solar wind dynamic pressure always lead to an increase (drop) of the geosynchronous magnetic field magnitude. The value of the geomagnetic field variation strongly depends on the local time of the observation point, reaching a maximum value near the noon meridian. It is shown that the direction of the B _z component of the interplanetary magnetic field has virtually no effect on the geomagnetic field variation because of a sharp jump of pressure. The time shift between an event in the solar wind and its response in the magnetosphere at a geosynchronous orbit essentially depends on the inclination of the front of a solar wind disturbance to the Sun-Earth line.     

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.     

Relationship between the fluxes of relativistic electrons at geosynchronous orbit and the level of ULF activity on the Earth’s surface and in the solar wind during the 23rd solar activity cycle

We present the results of a cross-correlation analysis made on the basis of Spearman’s rank correlation method. The quantities to correlate are daily values of the fluence of energetic electrons at a geosynchronous orbit, intensities of ground and interplanetary ultra-low-frequency (ULF) oscillations in the Pc5 range, and parameters of the solar wind. The period under analysis is the 23rd cycle of solar activity, 1996–2006. Daily (from 6 h to 18 h of LT) magnetic data at two diametrically opposite observatories of the Intermagnet network are taken as ground-based measurements. The fluxes of electrons with energies higher than 2 MeV were measured by the geosynchronous GOES satellites. The data of magnetometers and plasma instruments installed on ACE and WIND spacecraft were used for analysis of the solar wind parameters and of the oscillations of the interplanetary magnetic field (IMF). Some results elucidating the role played by interplanetary ULF waves in the processes of generation of magneospheric oscillations and acceleration of energetic electrons are obtained. Among them are (i) high and stable correlation of ground ULF oscillations with waves in the solar wind; (ii) closer link of mean daily amplitudes of both interplanetary and ground oscillations with ‘tomorrow’ values of the solar wind velocity than with current values; and (iii) correlation of the intensity of ULF waves in the solar wind, normalized to the IMF magnitude, with fluxes of relativistic electrons in the magnetosphere.     

Bursts of geomagnetic pulsations in the frequency range 0.2-5 Hz excited by large changes of the solar wind pressure

We present the results of studying the magnetospheres’s response to sharp changes of the solar wind flow (pressure) based on observations of variations of the ions flux of the solar wind onboard the Inreball-1 satellite and of geomagnetic pulsations (the data of two mid-latitude observatories and one auroral observatory are used). It is demonstrated that, when changes of flow runs into the magnetosphere, in some cases short (duration ~ < 5 min) bursts of geomagnetic pulsations are excited in the frequency range Δf~ 0.2–5 Hz. The bursts of two types are observed: noise bursts without frequency changes and wide-band ones with changing frequency during the burst. A comparison is made of various properties of these bursts generated by pressure changes at constant velocity of the solar wind and by pressure changes on the fronts of interplanetary shock waves at different directions of the vertical component of the interplanetary magnetic field.     

Orbit lifetime for microparticles in highly elliptical orbits with low perigee

On the basis of numerical experiments, we have shown the principal possibility of long (more than 1 month) and extremely long (more than 1 year) orbit lifetime of technogenic microparticles with radii from 1 to 100 μm injected into the near-Earth space in highly elliptical orbits with low perigee, including the case of an orbit with parameters corresponding to the orbital parameters of the Molniya satellite. Calculations are performed taking into account the perturbing effect on the orbital microparticle motion in the near-Earth space of gravitational perturbation caused by the Earth’s polar oblateness, the solar pressure force (calculated using methods of the Mie theory), and the drag force of neutral component of the background gas under conditions of low, medium, and high levels of solar and geomagnetic activities.