Showing papers on "Coronal mass ejection published in 1997"

The structure and origin of magnetic clouds in the solar wind

Abstract: . Plasma and magnetic field data from the Helios 1/2 spacecraft have been used to investigate the structure of magnetic clouds (MCs) in the inner heliosphere. 46 MCs were identified in the Helios data for the period 1974–1981 between 0.3 and 1 AU. 85% of the MCs were associated with fast-forward interplanetary shock waves, supporting the close association between MCs and SMEs (solar mass ejections). Seven MCs were identified as direct consequences of Helios-directed SMEs, and the passage of MCs agreed with that of interplanetary plasma clouds (IPCs) identified as white-light brightness enhancements in the Helios photometer data. The total (plasma and magnetic field) pressure in MCs was higher and the plasma-β lower than in the surrounding solar wind. Minimum variance analysis (MVA) showed that MCs can best be described as large-scale quasi-cylindrical magnetic flux tubes. The axes of the flux tubes usually had a small inclination to the ecliptic plane, with their azimuthal direction close to the east-west direction. The large-scale flux tube model for MCs was validated by the analysis of multi-spacecraft observations. MCs were observed over a range of up to ~60° in solar longitude in the ecliptic having the same magnetic configuration. The Helios observations further showed that over-expansion is a common feature of MCs. From a combined study of Helios, Voyager and IMP data we found that the radial diameter of MCs increases between 0.3 and 4.2 AU proportional to the distance, R, from the Sun as R0.8 (R in AU). The density decrease inside MCs was found to be proportional to R–2.4, thus being stronger compared to the average solar wind. Four different magnetic configurations, as expected from the flux-tube concept, for MCs have been observed in situ by the Helios probes. MCs with left- and right-handed magnetic helicity occurred with about equal frequencies during 1974–1981, but surprisingly, the majority (74%) of the MCs had a south to north (SN) rotation of the magnetic field vector relative to the ecliptic. In contrast, an investigation of solar wind data obtained near Earth's orbit during 1984–1991 showed a preference for NS-clouds. A direct correlation was found between MCs and large quiescent filament disappearances (disparition brusques, DBs). The magnetic configurations of the filaments, as inferred from the orientation of the prominence axis, the polarity of the overlying field lines and the hemispheric helicity pattern observed for filaments, agreed well with the in situ observed magnetic structure of the associated MCs. The results support the model of MCs as large-scale expanding quasi-cylindrical magnetic flux tubes in the solar wind, most likely caused by SMEs associated with eruptions of large quiescent filaments. We suggest that the hemispheric dependence of the magnetic helicity structure observed for solar filaments can explain the preferred orientation of MCs in interplanetary space as well as their solar cycle behavior. However, the white-light features of SMEs and the measured volumes of their interplanetary counterparts suggest that MCs may not simply be just Hα-prominences, but that SMEs likely convect large-scale coronal loops overlying the prominence axis out of the solar atmosphere.

Bi-directional plasma jets produced by magnetic reconnection on the Sun

Abstract: Magnetic reconnection, the process by which magnetic lines of force break and rejoin into a lower-energy configuration, is considered to be the fundamental process by which magnetic energy is converted into plasma kinetic energy1. The Sun has a large reservoir of magnetic energy, and the energy released by magnetic reconnection has been invoked to explain both large-scale events, such as solar flares2,3 and coronal mass ejections4, and small-scale phenomena, such as the coronal and chromospheric microflares that probably heat and accelerate the solar wind5,6. But the observational evidence for reconnection is largely indirect, resting on observations of variations in solar X-ray morphology and sudden changes in the magnetic topology7,8, and on the apparent association between some small-scale dynamic events and magnetic bipoles9,10. Here we report ultraviolet observations of explosive events in the solar chromosophere that reveal the presence of bi-directional plasma jets ejected from small sites above the solar surface. The structure of these jets evolves in the manner predicted by theoretical models of magnetic reconnection11,12, thereby lending strong support to the view that reconnection is the fundamental process for accelerating plasma on the Sun.

Comet Hyakutake x‐ray source: Charge transfer of solar wind heavy ions

Abstract: Recently, Lisse et al. (1996) reported on exciting observations by the Rontgen X-ray satellite (ROSAT) of x-ray and extreme ultraviolet emissions from comet C/Hyakutake 1996 B2. The spatial distribution of the emissions was displaced sunward of the nucleus and the spatial extent was about 105 km. Lisse et al. (1996) suggested that the emission could be explained by thermal bremsstrahlung associated with hot electrons, possibly due to solar wind interaction effects. In the present paper, an alternate emission mechanism is proposed. The solar wind contains a large number of minor/heavy ion species with a range of charge states, such as O6+, C5+, N5+, and Si10+. These ions will readily charge transfer with cometary neutrals, producing ions which can be highly excited and consequently emit photons in the extreme ultraviolet and x-ray part of the spectrum. The photon emission rate is proportional to the solar wind heavy ion flux and hence to the solar wind flux and, with some assumptions concerning the solar wind velocity, to the solar wind number density. The emission rate should be greatest downstream of the bow shock along the sun-comet axis in agreement with the observed spatial distribution. The x-ray images are really images of the line of sight integration of the solar wind density convoluted with the cometary neutral density. A total EUV/x-ray luminosity for comet Hyakutake from this charge transfer mechanism agrees with the observed luminosity of 4 × 1015 ergs s−1 within a factor of two.

Coronal Mass Ejections

Abstract: Coronal mass ejections are seen as bright features that move outward through the solar corona at speeds from 10 to about 2,000 km s-1. They involve the expulsion of substantial quantities of plasma from large regions of the corona. The spectacular nature of the largest mass ejections is illustrated in Figure 5.1 by a time sequence of four images obtained on 1989 October 24 with the coronagraph flown on the Solar Maximum Mission (SMM) spacecraft. In these and other SMM coronagraph images used in this chapter, the solar disc is indicated by the dashed circle within the shadow of the instrumental occulting disc. The arrow at the center of the sun points northward along the solar rotation axis and the short radial line crossing the solar disc indicates the location of the heliographic equator. The vertical streak in the middle of each image is a defect in the coronagraph detector system; such defects will be seen on most SMM coronagraph images.

Yohkoh SXT Observations of X-Ray ``Dimming'' Associated with a Halo Coronal Mass Ejection

Abstract: A sudden depletion or intensity "dimming" of the X-ray corona sometimes accompanies a solar eruptive flare or coronal mass ejection (CME). We have identified a dimming that occurred just prior to a "halo" CME, observed on 1997 April 7 using the Soft X-ray Telescope (SXT) on Yohkoh. Halo CMEs are prime candidates for "space weather" effects. The dimming occurred in compact regions near a flare of 14 UT on April 7, over a projected area of about 1020 cm-2, and indicate that a mass of a few times 1014 g was ejected. This is a lower limit imposed by the obscuration of the dimming volume by the brightness of the accompanying flare and other factors. Most of the mass deficit comes from two regions close to the ends of a preflare S-shaped active-region structure, and the resulting dimmings in these regions persisted for more than three days following the flare. A cusp-shaped loop—not apparent prior to the flare—dominates the emission in the flare decay phase, and has a mass comparable to that lost in the dimming regions. Our findings are consistent with the source of the CME being a flux rope that erupted, leaving behind the dimming regions. The cusp-shaped loop probably represents magnetic fields reconfigured or reconnected by the eruption. We do not see an X-ray analog of the wavelike disturbance evident in SOHO EUV images.

A kinetic model of the solar wind with Kappa distribution functions in the corona.

Abstract: A kinetic model of the solar wind based on Kappa velocity distribution functions for the electrons and protons es- capingoutofthecoronaispresented.Thehighvelocityparticles formingthetailofthesedistributionfunctionshaveanenhanced phase space density compared to a Maxwellian. The existence of such velocity distribution functions have been introduced in the pioneering work of Scudder (1992a,b) to explain the high temperature of the coronal plasma. The first results obtained with this new kinetic model of the solar wind are very encour- aging, indeed they fit better many major features observed in the solar wind than earlier models: e.g. the large bulk velocities observed in high speed streams emitted out of coronal regions where the plasma temperature is smaller, and the low speed so- lar wind originating in the hotter equatorial regions of the solar corona. This new kinetic model is also able to predict the high speed solar wind streams without unreasonably large coronal temperatures and without additional heating of the outer region ofthecorona,asitisneededinhydrodynamicmodelstoachieve the same solar wind speed.

Evidence of an erupting magnetic flux rope: LASCO coronal mass ejection of 1997 April 13

Abstract: A coronal mass ejection (CME) observed by LASCO exhibits evidence that its magnetic field geometry is that of a flux rope. The dynamical properties throughout the fields of view of C2 and C3 telescopes are examined. The results are compared with theoretical predictions based on a model of solar flux ropes. It is shown that the LASCO observations are consistent with a two-dimensional projection of a three-dimensional magnetic flux rope with legs that remain connected to the Sun.

Two-fluid model for heating of the solar corona and acceleration of the solar wind by high-frequency alfvén waves

Abstract: A model of the solar corona and wind is developed which includes for the first time the heating and acceleration effects of high-frequency Alfven waves in the frequency range between 1 Hz and 1 kHz. The waves are assumed to be generated by the small-scale magnetic activity in the chromospheric network. The wave dissipation near the gyro-frequency, which decreases with increasing solar distance, leads to strong coronal heating. The resulting heating function is different from other artificial heating functions used in previous model calculations. The associated thermal pressure-gradient force and wave pressure-gradient force together can accelerate the wind to high velocities, such as those observed by Helios and Ulysses. Classical Coulomb heat conduction is also considered and turns out to play a role in shaping the temperature profiles of the heated protons. The time-dependent two-fluid (electrons and protons) model equations and the time-dependent wave-spectrum equation are numerically integrated versus solar distance out to about 0.3 AU. The solutions finally converge and settle on time-stationary profiles which are discussed in detail. The model computations can be made to fit the observed density profiles of a polar coronal hole and polar plume with the sonic point occurring at 2.4 R⊙ and 3.2 R⊙, respectively. The solar wind speeds obtained at 63 R⊙ are 740 km s-1 and 540 km s-1; the mass flux is 2.1 and 2.2 × 108 cm-2 s-1 (normalized to 1 AU), respectively. The proton temperature increases from a value of 4 × 105 K at the lower boundary to 2 × 106 K in the corona near 2 R⊙.

Tracing the topology of the October 18–20, 1995, magnetic cloud with ∼0.1–10² keV electrons

Abstract: Five solar impulsive ∼1–10² keV electron events were detected while the WIND spacecraft was inside the magnetic cloud observed upstream of the Earth on October 18–20, 1995. The solar type III radio bursts produced by these electrons can be directly traced from ∼1 AU back to X-ray flares in solar active region AR 7912, implying that at least one leg of the cloud was magnetically connected to that region. Analysis of the electron arrival times shows that the lengths of magnetic field lines in that leg vary from ∼3 AU near the cloud exterior to ∼1.2 AU near the cloud center, consistent with a model force-free helical flux rope. Although the cloud magnetic field exhibits the smooth, continuous rotation signature of a helical flux rope, the ∼0.1-1 keV heat flux electrons and ∼1–10² keV energetic electrons show numerous simultaneous abrupt changes from bidirectional streaming to unidirectional streaming to complete flux dropouts. We interpret these as evidence for patchy disconnection of one end or both ends of cloud magnetic field lines from the Sun.

EIT and LASCO Observations of the Initiation of a Coronal Mass Ejection

Abstract: We present the first observations of the initiation of a coronal mass ejection (CME) seen on the disk of the Sun. Observations with the EIT experiment on SOHO show that the CME began in a small volume and was initially associated with slow motions of prominence material and a small brightening at one end of the prominence. Shortly afterward, the prominence was accelerated to about 100 km s-1 and was preceded by a bright loop-like structure, which surrounded an emission void, that traveled out into the corona at a velocity of 200–400 km s-1. These three components, the prominence, the dark void, and the bright loops are typical of CMEs when seen at distance in the corona and here are shown to be present at the earliest stages of the CME. The event was later observed to traverse the LASCO coronagraphs fields of view from 1.1 to 30 R. Of particular interest is the fact that this large-scale event, spanning as much as 70 deg in latitude, originated in a volume with dimensions of roughly 35″ (2.5 × 104 km). Further, a disturbance that propagated across the disk and a chain of activity near the limb may also be associated with this event as well as a considerable degree of activity near the west limb.

The Solar Corona

Abstract: 1. Introduction 2. Brief history of coronal studies 3. The coronal spectrum 4. The solar cycle 5. Ground-based observations 6. Observations from space: I. The first 4 decades 7. Activity of the inner corona 8. Observations from space: II. Recent missions 9. The solar wind 10. Solar flares and coronal mass ejections Notes References Index.

Correlation of changes in the outer‐zone relativistic‐electron population with upstream solar wind and magnetic field measurements

Abstract: A study has been made of the correlation of the population of relativistic electrons in the outer-zone magnetosphere with the properties of the solar wind (speed, density, magnetic field) during a solar minimum period. The study is based upon observations made in the Spring of 1995 with sensors aboard 1994-026 and WIND. It is found that a large relativistic electron enhancement depends upon a substantial solar-wind speed increase associated with precursor solar-wind density enhancement, and, in particular, upon a southward turning of the interplanetary magnetic field.

Physics of Solar System Plasmas

Abstract: 1 Space physics 2 Introduction to kinetic theory 3 Single particle motion and geomagnetically trapped particles 4 Magnetohydrodynamics 5 Solar physics 6 The solar wind 7 The interaction of the solar wind, planets and other solar system bodies 8 The magnetosphere Appendix

First Results from VIRGO, the Experiment for Helioseismology and Solar Irradiance Monitoring on SOHO

Abstract: First results from the VIRGO experiment (Variability of solar IRradiance and Gravity Oscillations) on the ESA/NASA Mission SOHO (Solar and Heliospheric Observatory) are reported. The observations started mid-January 1996 for the radiometers and sunphotometers and near the end of March for the luminosity oscillation imager. The performance of all the instruments is very good, and the time series of the first 4–6 months are evaluated in terms of solar irradiance variability, solar background noise characteristics and p-mode oscillations. The solar irradiance is modulated by the passage of active regions across the disk, but not all of the modulation is straightforwardly explained in terms of sunspot flux blocking and facular enhancement. Helioseismic inversions of the observed p-mode frequencies are more-or-less in agreement with the latest standard solar models. The comparison of VIRGO results with earlier ones shows evidence that magnetic activity plays a significant role in the dynamics of the oscillations beyond its modulation of the resonant frequencies. Moreover, by comparing the amplitudes of different components of p-mode multiplets, each of which are influenced differently by spatial inhomogeneity, we have found that activity enhances excitation.

Recurrent geomagnetic storms and relativistic electron enhancements in the outer magnetosphere: ISTP coordinated measurements

Abstract: New, coordinated measurements from the International Solar-Terrestrial Physics (ISTP) constellation of spacecraft are presented to show the causes and effects of recurrent geomagnetic activity during recent solar minimum conditions. It is found using WIND and POLAR data that even for modest geomagnetic storms, relativistic electron fluxes are strongly and rapidly enhanced within the outer radiation zone of the Earth's magnetosphere. Solar wind data are utilized to identify the drivers of magnetospheric acceleration processes. Yohkoh solar soft X-ray data are also used to identify the solar coronal holes that produce the high-speed solar wind streams which, in turn, cause the recurrent geomagnetic activity. It is concluded that even during extremely quiet solar conditions (sunspot minimum) there are discernible coronal holes and resultant solar wind streams which can produce intense magnetospheric particle acceleration. As a practical consequence of this Sun-Earth connection, it is noted that a long-lasting E>1MeV electron event in late March 1996 appears to have contributed significantly to a major spacecraft (Anik E1) operational failure.

Quasi-separatrix layers in solar flares. ii. observed magnetic configurations

Abstract: We show that the location of Hα or OV flare brightenings is related to the properties of the field-line linkage of the underlying magnetic region. The coronal magnetic field is extrapolated from the observed photospheric field assuming a linear force-free field configuration in order to determine the regions of rapid change in field-line linkage, called “quasi-separatrix layers” or QSLs. They are open layers that behave physically like separatrices: breakdown of ideal magnetohydrodynamics and release of free magnetic-energy may occur at these locations when their thickness is small enough. A feature common to all the flaring regions studied is found to be the presence of QSLs where Hα flare kernels are observed. The brightenings are along restricted regions of very thin QSLs; an upper bound of their thickness is 1 Mm but it is several order of magnitude smaller in most of the cases. These places coincide in general with zones where the longitudinal field component is greater than 100 G. These results allow us to constrain present models of solar flares and localise where a break-down of ideal MHD can occur. The studied flares are found to be fed in general by only one electric current loop, but they imply the interaction of two magnetic bipoles. The extrapolated coronal field lines involved in the process have their photospheric footpoints located at both sides of QSLs, as expected in recent 3D magnetic reconnection models.

Origins of the Slow and the Ubiquitous Fast Solar Wind

Abstract: We present in this letter the first coordinated radio occultation measurements and ultraviolet observations of the inner corona below 5.5 Rs, obtained during the Galileo solar conjunction in January 1997, to establish the origin of the slow wind.

Charge exchange near Mars: The solar wind absorption and energetic neutral atom production

Abstract: Charge exchange between solar wind protons and neutral atmospheric atoms is expected to affect the solar wind interaction with Mars, but its influences and significance have only been touched upon in previous work. Here several features associated with the charge exchange process between the solar wind protons and Martian neutral upper atmospheres are described. The analysis is based on an empirical proton model derived from Phobos 2 observations interacting with the Martian atomic (H) and molecular (H2) hydrogen, and oxygen (O) upper atmospheres representing solar minimum and solar maximum conditions. The region where the largest fraction of solar wind protons is lost by the charge exchange process is found to be a thin layer above the surface of Mars on the dayside resulting from charge exchange with the thermal oxygen. In general, the magnetosheath and “magnetosphere” (where the observed plasma takes on a different character in the Phobos 2 data) produce two distinguishable regions where the loss rate of solar wind protons is highest. Increasing solar activity increases the loss rate in the magnetosheath but decreases it in the magnetosphere. No significant increase of the absorption of the solar wind was found near the “magnetopause” suggesting that the decrease of the solar wind protons observed by Phobos 2 are not due to the charge exchange process. In addition to a reduction in the solar wind density, the charge exchange reaction results in energetic neutral atom (ENA) production. This paper considers some of the detailed properties expected for the ENA population at Mars. The ENA differential fluxes were found to be typically 106–107 cm−2 s−1 keV−1 in the energy range 0.01–1 keV. During solar minimum, the ENA production rate and ENA integral fluxes were found to be highest in the magnetosheath. At solar maximum the ENA production rate is highest in the magnetosphere, and ENA integral fluxes in the dayside magnetosphere appear to become comparable to the fluxes in the magnetosheath if the proton temperature in the magnetosphere is low. It is found that 1–3% of the original solar wind proton flux converts into ENAs before the bow shock. The ENAs produced upstream are undeflected and so may precipitate into the Martian upper atmosphere, depositing an energy flux of up to 3 × 109 eV cm−2 s−1 derived from the solar wind. These results both suggest the possible benefits of observing ENA fluxes around Mars and suggest the necessary parameters for detector design.

Are energetic electrons in the solar wind the source of the outer radiation belt

Abstract: Using data from WIND, SAMPEX (Solar Anomalous, and Magnetospheric Particle Explorer), and the Los Alamos National Laboratory (LANL) sensors onboard geostationary satellites, we investigate the correlation of energetic electrons in the 20–200 keV range in the solar wind and of high speed solar wind streams with relativistic electrons in the magnetosphere to determine whether energetic electrons in the solar wind are the source of the outer relativistic electron radiation belt. Though there is some correlation between energetic electron enhancements in the solar wind and enhancements in the outer radiation belt, the phase space density of 20–200 keV electrons in the solar wind is not adequate to supply the outer radiation belt electrons. Although lower energy electrons in the solar wind could be a seed population of the outer radiation belt, such lower energy electrons cannot achieve relativistic energies through the normal process of radial transport which conserves the first adiabatic invariant. Thus additional internal acceleration processes are required within the magnetosphere to produce the outer radiation belt. High speed solar wind streams are well correlated with increased magnetic activity and with increased fluxes in the outer radiation belt. The maximum correlation between the high speed streams and the radiation belt flux occurs with an increasing time delay for higher energies and and lower L values. We conclude that acceleration processes within the magnetosphere which are well correlated with high speed solar wind streams are responsible for the outer radiation belt electrons.

Panel achieves consensus prediction of solar cycle 23

Abstract: In September 1996, a panel of experts on solar cycle prediction techniques met in Boulder, Colorado, to survey forecasts of solar and geomagnetic activity and to arrive at a consensus on how the solar cycle will develop. After two weeks of deliberation, the panel of 12 scientists (from Australia, Germany, the United Kingdom, and the United States) agreed that a large amplitude solar cycle with a smoothed sunspot maximum of approximately 160 is probable near the turn of the century. The amplitude of the predicted cycle is comparable to that of the previous two solar cycles (see Figure 1). Our ability to predict solar and geomagnetic activity is crucial to many technologies, including the operation of low-Earth orbiting satellites, electric power transmission grids, geophysical exploration, and highfrequency radio communications and radars. Because the scale height of Earth's upper atmosphere (and thus the drag on satellites in low Earth orbit) depends on the levels of short-wavelength solar radiation and geomagnetic activity, we need to know the profile and magnitude of the next solar and geomagnetic cycle in order to plan for reboosting the Hubble Space Telescope and assembling the International Space Station.

Energy Spectra of Ions Accelerated in Impulsive and Gradual Solar Events

Abstract: We report new high-sensitivity measurements of the energy spectra of ions from five impulsive solar flares and one gradual event observed during solar minimum by the Energetic Particles, Acceleration, Composition, and Transport (EPACT) experiment aboard the WIND spacecraft. All of the impulsive-flare events had intensities too low to be visible on previous spacecraft such as ISEE 3, which observed hundreds of impulsive-flare events. Often these events cluster in or behind a coronal mass ejection (CME) where magnetic field lines provide an excellent connection to a solar active region where flares are occurring. In most cases we can see velocity dispersion as the ions of 20 keV amu-1 to 10 MeV amu-1 streamed out from the impulsive flare at the Sun, arriving in inverse order of their velocity. Ions from a large, magnetically well-connected gradual event, associated with a CME-driven shock, also show velocity dispersion early in the event but show identical time profiles that last for several days late in the event. These time-invariant spectra of H,4He, C, O, and Fe in this gradual event are well represented as power laws in energy from 20 keV amu-1 to ~100 MeV amu-1. In the impulsive-flare events, H,3He,4He, C, O, and Fe have more rounded spectra that flatten somewhat at low energies; yet the intensities continue to increase down to 20 keV amu-1. Most of the ion energy content appears to lie below 1 MeV in the impulsive events, where it would be invisible to γ-ray line observations.

A two-dimensional simulation of the radial and latitudinal evolution of a solar wind disturbance driven by a fast, high-pressure coronal mass ejection

Abstract: Using a hydrodynamic simulation, we have studied the two-dimensional (symmetry in the azimuthal direction) evolution of a fast, high-pressure coronal mass ejection (CME) ejected into a solar wind with latitudinal variations similar to those observed by Ulysses. Specifically, the latitudinal structure of the ambient solar wind in the meridional plane is approximated by two zones: At low latitudes (< 20°) the solar wind is slow and dense, while at higher latitudes the solar wind is fast and tenuous. The CME is introduced into this ambient wind as a bell-shaped pressure pulse in time, spanning from the equator to 45° with a speed and temperature equal to that of the high-latitude solar wind. We find that such an ejection profile produces radically different disturbance profiles at low and high latitudes. In particular, the low-latitude portion of the ejecta material drives a highly asymmetric disturbance because of the relative difference in speed between the fast CME and slower ambient solar wind ahead. In contrast, the high-latitude portion of the same ejecta material drives a much more radially symmetric disturbance because the relative difference in pressure between the CME and ambient background plasma dominates the dynamics. The simulations reveal a number of other interesting features. First, there is significant distortion of the CME in the interplanetary medium. By ∼ 1 AU the CME has effectively separated (in radius as well as latitude) into two pieces. The radial separation is due to the strong velocity shear between the slow and fast ambient solar wind. The latitudinal separation arises from pressure gradients associated with rarefaction regions that develop as the CME propagates outward. Second, there is significant poleward motion of the highest-latitude portion of the CME and its associated disturbance. The main body of the CME expands poleward by ∼ 18°, while the forward and reverse waves (produced by the overexpanding portion of the CME) propagate all the way to the pole. Third, the simulations show that the high-pressure region, which develops at low latitudes as the fast CME ploughs through the slow ambient solar wind, penetrates significantly (∼ 10°) into the high-latitude fast solar wind. We compare the simulation results with a CME-driven interplanetary disturbance observed at both low and high latitudes and find that the simulation reproduces many of the essential features of the observations.

Three-dimensional MHD simulation of the solar wind interaction with the ionosphere of Venus: Results of two-component reacting plasma simulation

Abstract: The large-scale solar wind interaction with the Venusian ionosphere is numerically simulated in the framework of two-component, three-dimensional magnetohydrodynamics (MHD). The finite volume total variation diminishing scheme is used to solve this problem. The impinging solar wind is represented by H + ions, and the ionosphere is assumed to consist of O + ions produced by photoionization of atomic oxygen in the Venusian upper atmosphere and by charge exchange of CO 2 + ions. The O + ions are lost by charge exchange with carbon dioxide molecules. The numerical simulations are performed for an interplanetary magnetic field (IMF) perpendicular to the solar wind flow and for the solar wind parameters which correspond to maximum solar activity. Results of the calculation give the formation of the bow shock, the magnetic barrier, and the ionopause in the dayside region as a self-consistent state of the interaction processes. The dynamical behavior of the dayside ionosphere under the influence of the impinging solar wind and the IMF slipping over the pole results in the formation of wing-like bulges of the ionosphere and an accompanying poleward flow in the topside ionosphere. The model also reproduces several features of the nightside ionosphere that are predicted by earlier theories and observations, including complex structures such as a flattened ionotail, tail rays, and ionospheric holes, as a continuation of the wing-like bulge. It is also shown that slow plasma flow in the ionotail and nonideal MHD process play important roles in the formation of the induced magnetotail of the planet.

Interaction of a nonuniform solar wind with the local interstellar medium: 2. A two‐fluid model

Abstract: Results of a fully three-dimensional model of the interaction of the solar wind with the local interstellar medium are presented. The effects of charge-exchange with interstellar neutral hydrogen are taken into account self-consistently, while the effects of hot solar wind neutral hydrogen, as well as cosmic rays and magnetic fields, are ignored in this study. In accord with solar medium observations by Ulysses, the solar wind is assumed to depend on heliolatitude. Two large, long-lived polar coronal holes, one in the northern hemisphere and the other in the southern hemisphere, are assumed to produce a hot, low-density, high-speed wind which bounds a cooler, higher-density, low-speed ecliptic wind. The solar wind boundary conditions, which allow for a 1.5 increase in solar wind ram pressure over the poles of the Sun compared with the ecliptic plane, are drawn from published Ulysses data [Phillips et al., 1995, 1996]. The results of this simulation are compared with the no-charge-exchange asymmetric solar wind simulation, described by Pauls and Zank [1996]. It is found that the elongation of the heliosphere along the solar poles, resulting from the ram pressure increase with heliolatitude, induces a greater influx of interstellar hydrogen over the poles of the Sun than in the ecliptic plane. This, in turn, reduces the extent of elongation of the heliosphere along the poles of the Sun. The vorticity, found in the no-charge-exchange simulation, is absent in the presence of charge-exchange. Once again, as found by Baranov and Malama [1993] and Pauls et al. [1995], the interaction of the solar wind with the local interstellar medium is influenced strongly by charge-exchange processes.

MHD simulation of an interaction of a shock wave with a magnetic cloud

Abstract: Interplanetary shock waves, propagating in the heliosphere faster than earlier-emitted coronal ejecta, penetrate them and modify their parameters during this interaction. Using two and one half dimensional MHD simulations, we show how a magnetic cloud (flux rope) propagating with a speed 3 times higher than the ambient solar wind is affected by an even faster traveling shock wave overtaking the cloud. The magnetic field increases inside the cloud during the interaction as it is compressed in the radial direction and becomes very oblate. The cloud is also accelerated and moves faster, as a whole, while both shocks (driven by the cloud and the faster interplanetary shock) merge upstream of the cloud. This interaction may be a rather common phenomenon due to the frequency of coronal mass ejections and occurrence of shock waves during periods of high solar activity.

Features observed in the trailing regions of interplanetary clouds from coronal mass ejections

Abstract: ISEE-3 and Ulysses plasma and magnetic field data are used to study features in the trailing regions of interplanetary plasma clouds resulting from coronal mass ejections (CMEs). Approximately one quarter of those events contain periods greater or equal to 6 hours for which the interplanetary magnetic field is quiet and nearly radial. Arguments are presented that this is a causal relation, rather than coincidence.

Large‐scale structures and multiple neutral lines associated with coronal mass ejections

Abstract: We use Yohkoh soft X-ray telescope (SXT) images of eruptive events visible against the solar disk to examine the coronal structures and the boundaries of the large-scale magnetic fields considered to be involved in coronal mass ejections (CMEs) From an initial list of about 100 large-scale events we selected five for detailed study The transient X-ray structures in these events spanned distances across the solar surface ranging from 35 to >100 heliographic degrees, comparable to the spans of white light CMEs observed at the limb The widths of the coronal loop arcades spanned two or three neutral lines, or a single, highly convoluted neutral line Our interpretation of these results is that multipolar magnetic systems are a common configuration of the source fields of many CMEs, contrary to the first-order approximation that CMEs involve the eruption of simple, bipolar structures

SWICS/Ulysses observations: The three‐dimensional structure of the heliosphere in the declining/minimum phase of the solar cycle

Abstract: The ESA/NASA spacecraft Ulysses provides the first in-situ observation of the solar wind at high solar latitudes. Data obtained with the Solar Wind Ion Composition Spectrometer (SWICS) during the first orbit of Ulysses around the Sun can be used to obtain a global view of the solar wind pattern within the heliosphere during the declining/minimum phase of the solar activity cycle. This provides information on the solar corona, specifically on any latitudinal variation of coronal parameters as far as they play a role in the solar wind acceleration process. The measurements show: The 3-dim. distribution of the solar wind appears as a pattern of basically two states. The fast solar wind with speeds of about 750 km/s emanating from the polar coronal holes dominates the heliosphere. The slow solar wind from the streamer belts is restricted to a region of about +/−20° around the heliographic equator. The boundary between the two states of the solar wind is sharply defined. The coronal hole streams are remarkably stable and homogeneous on a larger scale. The two independent coronal holes are similar in terms of average speed and spatial extent. In both coronal holes a gradual increase (≤10%) of speed with solar latitude is observed.

Solar wind proton deposition into the Martian atmosphere

Abstract: The direct impact of solar wind H+ with the planet Mars is calculated using a three-dimensional hybrid particle code. The simulation results show a strong dependence on solar wind velocity and interplanetary magnetic field angle with the solar wind velocity vector. The energy fluxes calculated approach the solar EUV heating rates from photoelectrons and are found to be asymmetric. The heating is also found on the nightside of the planet due to the large ion gyroradii of the incoming solar wind protons. The percentage of solar wind protons lost to the surface of simulation ranged from 3% to 28% depending on the ram presssure of the solar wind. Finally, these results support earlier suggestions that the solar wind may be a significant source of He in the atmosphere of Mars.