Showing papers on "Coronal mass ejection published in 1995"

The Large Angle Spectroscopic Coronagraph (LASCO): Visible light coronal imaging and spectroscopy

Abstract: The Large Angle Spectroscopic Coronagraph (LASCO) is a triple coronagraph being jointly developed for the Solar and Heliospheric Observatory (SOHO) mission LASCO comprises three nested coronagraphs (C1, C2, and C3) that image the solar corona for 11 to 30 solar radii (C1: 11 to 3 solar radii, C2: 15 to 6 solar radii, and C3: 3 to 300 solar radii) The inner coronagraph (C1) is a newly developed mirror version of the classic Lyot coronagraph without an external occultor, while the middle coronagraph (C2) and the outer coronagraph (C3) are externally occulted instruments High resolution coronal spectroscopy from 11 to 3 R solar radii can be performed by using a Fabry-Perot interferometer, which is part of C1 High volume memories and a high speed microprocessor enable extensive onboard image processing Image compression by factors of 10 to 20 will result in the transmission of 10 to 20 full images per hour

SUMER — Solar Ultraviolet Measurements of Emitted Radiation

Abstract: The instrument SUMER — Solar Ultraviolet Measurements of Emitted Radiation is designed to investigate structures and associated dynamical processes occurring in the solar atmosphere, from the chromosphere through the transition region to the inner corona, over a temperature range from 104 to 2 x 106 K and above. These observations will permit detailed spectroscopic diagnostics of plasma densities and temperatures in many solar features, and will support penetrating studies of underlying physical processes, including plasma flows, turbulence and wave motions, diffusion transport processes, events associated with solar magnetic activity, atmospheric heating, and solar wind acceleration in the inner corona. Specifically, SUMER will measure profiles and intensities of EUV lines; determine Doppler shifts and line broadenings with high accuracy; provide stigmatic images of the Sun in the EUV with high spatial, spectral, and temporal resolution; and obtain monochromatic maps of the full Sun and the inner corona or selected areas thereof. SUMER will be flown on the Solar and Heliospheric Observatory (SOHO), scheduled for launch in November, 1995. This paper has been written to familiarize solar physicists with SUMER and to demonstrate some command procedures for achieving certain scientific observations.

The Coronal Diagnostic Spectrometer for the solar and heliospheric observatory

Abstract: The Coronal Diagnostic Spectrometer is designed to probe the solar atmosphere through the detection of spectral emission lines in the extreme ultraviolet wavelength range 150–800 A. By observing the intensities of selected lines and line profiles we may derive temperature, density, flow and abundance information for the plasmas in the solar atmosphere. Spatial and temporal resolutions of down to a few arcseconds and seconds, respectively, allow such studies to be made within the fine-scale structure of the solar corona. Furthermore, coverage of large wavelength bands provides the capability for simultaneously observing the properties of plasmas across the wide temperature ranges of the solar atmosphere.

MHD Structures, Waves and Turbulence in the Solar Wind: Observations and Theories

Abstract: A comprehensive overview is presented of recent observational and theoretical results on solar wind structures and fluctuations and magnetohydrodynamic waves and turbulence, with preference given to phenomena in the inner heliosphere. Emphasis is placed on the progress made in the past decade in the understanding of the nature and origin of especially small-scale, compressible and incompressible fluctuations. Turbulence models to describe the spatial transport and spectral transfer of the fluctuations in the inner heliosphere are discussed, and results from direct numerical simulations are dealt with. Intermittency of solar wind fluctuations and their statistical distributions are briefly investigated. Studies of the heating and acceleration effects of the turbulence on the background wind are critically surveyed. Finally, open questions concerning the origin, nature and evolution of the fluctuations are listed, and possible avenues and perspectives for future research are outlined.

Regions of abnormally low proton temperature in the solar wind (1965–1991) and their association with ejecta

Abstract: Occurrences of solar wind plasma with abnormally low proton temperatures (Tp) have long been associated with the interplanetary manifestations (which we term “ejecta”) of coronal mass ejections (CMEs). We survey the National Space Science Data Center Omni solar wind database for 1965–1991, and data from the Helios 1 and 2 spacecraft for more limited periods, to identify plasma in which Tp is less than the temperature expected (Tex) from the well-established correlation between the solar wind speed and Tp for normal solar wind expansion. The occurrence rate of low-temperature plasma (specifically with Tp/Tex ≤ 0.5) is shown to be correlated with solar activity levels. Individual low-temperature regions have durations from 1 to ∼80 hours with a mean of ∼10 hours. Around one third are encounters with the heliospheric plasma sheet (HPS). These events are observed most frequently during the increasing phase of solar activity when the HPS lies closer to the ecliptic. The abnormally low temperatures may be intrinsic to the HPS or may provide support for the proposal that the coronal streamer belt underlying the HPS is a frequent source of ejecta. The remaining events have an occurrence rate which shows a particularly clear correlation with solar activity levels and with the CME rate at the Sun (when CME observations are available), consistent with an association with ejecta. These events also show greater than chance associations with other ejecta signatures. We suggest that the Omni plasma data can provide information on the rate of ejecta passing the Earth, and hence give an indication of the CME rate, for a period commencing before spacecraft coronagraph CME observations became available in the early 1970s. Our findings suggest that Tp depressions may provide a more comprehensive indication of the presence of ejecta than other ejecta signatures, such as bidirectional solar wind electron heat fluxes and energetic ion flows, which alone do not identify all ejecta.

The initiation of coronal mass ejections by newly emerging magnetic flux

Abstract: We present observational evidence that eruptions of quiescent filaments and associated coronal mass ejections (CMEs) occur as a consequence of the destabilization of large-scale coronal arcades due to interactions between these structures and new and growing active regions. Both statistical and case studies have been carried out. In a case study of a 'bulge' observed by the High-Altitude Observatory Solar Maximum Mission coronagraph, the high-resolution magnetograms from the Big Bear Solar Observatory show newly emerging and rapidly changing flux in the magnetic fields that apparently underlie the bugle. For other case studies and in the statistical work the eruption of major quiescent filaments was taken as a proxy for CME eruption. We have found that two thirds of the quiescent-filament-associated CMEs occurred after substantial amounts of new magnetic flux emerged in the vicinity of the filament. In addition, in a study of all major quiescent filaments and active regions appearing in a 2-month period we found that 17 of the 22 filaments that were associated with new active regions erupted and 26 of the 31 filaments that were not associated with new flux did not erupt. In all cases in which the new flux was oriented favorably for reconnection with the preexisting large-scale coronal arcades; the filament was observed to erupt. The appearance of the new flux in the form of new active regions begins a few days before the eruption and typically is still occurring at the time of the eruption. A CME initiation scenario taking account of these observational results is proposed.

Origin of the solar wind from composition data

Abstract: The ESA/NASA spacecraft Ulysses is making, for the first time, direct measurements in the solar wind originating from virtually all places where the corona expands. Since the initial two polar passes of Ulysses occur during relatively quiet solar conditions, we discuss here the three main regimes of quasi-stationary solar wind flow: the high speed streams (HSSTs) coming out of the polar coronal holes, the slow solar wind surrounding the HSSTs, and the streamers which occur at B-field reversals. Comparisons between H-a maps and data taken by Ulysses demonstrate that as a result of super-radial expansion, the HSSTs occupy a much larger solid angle than that derived from radial projections of coronal holes. Data obtained with SWICS-Ulysses confirm that the strength of the F1P effect is much reduced in the HSSTs. The systematics in the variations of elemental abundances becomes particularly clear, if these are plotted against the time of ionisation (at the solar surface) rather than against the first ionisation potential (FIP). We have used a superposed-epoch method to investigate the changes in solar wind speed and composition measured during the 9-month period in 1992/93 when Ulysses regularly passed into and out of the southern HSST. We find that the patterns in the variations of the Mg/O and O7+/O6+ ratios are virtually identical and that their transition from high to low values is very steep. Since the Mg/O ratio is controlled by the FIP effect and the O7+/O6+ ratio reflects the coronal temperature, this finding points to a connection between chromospheric and coronal conditions.

The Ultraviolet Coronagraph Spectrometer for the Solar and Heliospheric Observatory

Abstract: The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The purpose of the UVCS instrument is to provide a body of data that can be used to address a broad range of scientific questions regarding the nature of the solar corona and the generation of the solar wind. The primary scientific goals are the following: to locate and characterize the coronal source regions of the solar wind, to identify and understand the dominant physical processes that accelerate the solar wind, to understand how the coronal plasma is heated in solar wind acceleration regions, and to increase the knowledge of coronal phenomena that control the physical properties of the solar wind as determined by in situ measurements. To progress toward these goals, the UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliocentric height of 12 solar radii.

Three‐dimensional magnetic reconnection and the magnetic topology of coronal mass ejection events

Abstract: Measurements of superthermal electron fluxes in the solar wind indicate that field lines within coronal mass ejections, CMEs, near and beyond 1 AU are normally connected to the Sun at both ends. However, on occasion some field lines embedded deep within CMEs appear to be connected to the Sun at only one end. Here we propose an explanation for how such field lines arise in terms of 3-dimensional reconnection close to the Sun. Such reconnection also provides a natural explanation for the flux rope topology characteristic of many CMEs as well as the coronal loops formed during long-duration, solar X-ray events. Our consideration of the field topologies resulting from 3-dimensional reconnection indicates that field lines within and near CMEs may on occasion be connected to the outer heliosphere at both ends.

Solar energetic particles: A paradigm shift

Abstract: Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, Greenbelt, MarylandThe first evidence of high-energy particles from the Sun was obtained 50 years ago when Forbush [1946] used sea-level ion chambers to study the large solar events of February and March of 1942. Over the next 20 years, observation of these solar energetic particle (SEP) events using neutron monitors and riometers (that measure radio opacity of the ionosphere) and, later, with detectors on balloons and satellites, led to an extensive body of knowledge on the time profiles, spectra and particle abundance in the large events. Meanwhile, there was already a rich history of the study of solar flares spanning 100 years since the first observations reported by Carrington [1860]. With no knowledge of the existence of coronal mass ejections (CMEs) [Kahler 1992], it was tempting to assume that the particle acceleration somehow occurred in spatial and temporal conjunction with the solar flare itself. Thus the “solar flare myth” [Gosling 1993] of particle acceleration began nearly 30 years ago.

Ulysses solar wind plasma observations at high southerly latitudes.

Abstract: Solar wind plasma observations made by the Ulysses spacecraft through –80.2° solar latitude and continuing equatorward to –40.1° are summarized. Recurrent high-speed streams and corotating interaction regions dominated at middle latitudes. The speed of the solar wind was typically 700 to 800 kilometers per second poleward of –35°. Corotating reverse shocks persisted farther south than did forward shocks because of the tilt of the heliomagnetic streamer belt. Sporadic coronal mass ejections were seen as far south as –60.5°. Proton temperature was higher and the electron strahl was broader at higher latitudes. The high-latitude wind contained compressional, pressure-balanced, and Alfvenic structures.

COSTEP - Comprehensive Suprathermal and Energetic Particle Analyser

Abstract: The COSTEP experiment on SOHO forms part of the CEPAC complex of instruments that will perform studies of the suprathermal and energetic particle populations of solar, interplanetary, and galactic origin. Specifically, the LION and EPHIN instruments are designed to use particle emissions from the Sun for several species (electrons, protons, and helium nuclei) in the energy range 44 keV/particle to > 53 MeV/n as tools to study critical problems in solar physics as well as fundamental problems in space plasma and astrophysics. Scientific goals are presented and a technical description is provided of the two sensors and the common data processing unit. Calibration results are presented which show the ability of LION to separate electrons from protons and the ability of EPHIN to obtain energy spectra and achieve isotope separation for light nuclei. A brief description of mission operations and data products is given.

Energetic Particle Experiment ERNE

Abstract: The Energetic and Relativistic Nuclei and Electron (ERNE) experiment will investigate the solar atmosphere and the heliosphere by detecting particles produced in various kinds of energy release processes. ERNE is at the upper end in energy among the SOHO particle instruments. The instrument will measure the energy spectra of elements in the range Z=1−30. The energy coverage varies dependent on the particle species from a few MeV/n up to a few hundred MeV/n and electrons from 2 to 50 MeV. At high energies, ERNE records also the direction of the incident particles for accurate measurements of the pitch angle distribution of the ambient flux within the viewing cone. Especially the isotope identification capability has been one of the instrument design goals, thus providing new data regarding various fundamental questions in solar physics.

Coronal heating, densities, and temperatures and solar wind acceleration

Abstract: The outflow of coronal plasma into interplanetary space is a consequence of the coronal heating process Therefore the formation of the corona and the acceleration of the solar wind should be treated as a single problem The deposition of energy into the corona through some “mechanical” energy flux is balanced by the various energy sinks available to the corona, and the sum of these processes determines the coronal structure, ie, its temperature and density The corona loses energy through heat conduction into the transition region and through the gravitational potential energy and kinetic energy put into the solar wind We show from a series of models of the chromosphere-transition region-corona-solar wind system that most of the energy deposited in a magnetically open region goes into the solar wind The transition region pressures and the coronal density and temperature structure may vary considerably with the mode and location of energy deposition, but the solar wind mass flux is relatively insensitive to these variations; it is determined by the amplitude of the energy flux In these models the transition region pressure decreases in accordance with the increasing coronal density scale height such that the solar wind mass loss is consistent with the energy flux deposited in the corona On the basis of the present study we can conclude that the exponential increase of solar wind mass flux with coronal temperature, found in most thermally driven solar wind models, is a consequence of fixing the transition region pressure

Disruption of a helmet streamer by photospheric shear

Abstract: Helmet streamers on the Sun have been observed to be the site of coronal mass ejections, dynamic events that eject coronal plasma and magnetic fields into the solar wind. We develop a two-dimensional (azimuthally symmetric) helmet streamer configuration by computing solutions of the time-dependent magnetohydrodynamic (MHD) equations, and we investigate the evolution of the configuration when photospheric shearing motions are imposed. We find that the configuration disrupts when a critical shear is exceeded, ejecting a plasmoid into the solar wind. The results are similar to the case of a sheared dipole magnetic field in a hydrostatic atmosphere (Mikic & Linker 1994). However, the presence of the outflowing solar wind makes the disruption significantly more energetic when a helmet streamer is sheared. Our resutls suggest that shearing of helmet streamers may initiate coronal mass ejections.

High-Energy Particles in Solar Flares

Abstract: Accelerated particles appear to coexist inseparably with most forms of energy release in solar flares and coronal mass ejections. We identify at least five different populations of high-energy electrons and ions. High-energy particles, accelerated efficiently in the flare in great numbers, transport a large fraction of the flare energy to other sites. This behavior makes them an integral part of the flare process. Much new data has come from two spacecraft launched in 1991: the Compton Gamma­Ray Observatory and Yohkoh. This review concentrates on particles in flares, mainly using X-ray and gamma-ray data rather than measurements of “escaping” particles observed in interplanetary space.

The Large-Scale Structure of the Heliospheric Current Sheet during the Ulysses Epoch

Abstract: Ulysses is traversing the Sun’s polar regions for the first time a year or two before solar minimum. If the heliospheric magnetic field behaves as we expect, the heliospheric current sheet (HCS) during this stage of the sunspot cycle should be quite stable and lie nearly flat, close to the equator. The high latitude solar fields should be unipolar and nearing their maximum strength. The overlying polar coronal holes should be well developed, producing a nearly uniform high-speed solar wind. Because the Sun’s polar fields live longer than any other solar magnetic phenomenon, Ulysses will remain within a single coronal hole for an extended period and probe its structures in a unique way.

SWAN: A Study of Solar Wind Anisotropies on SOHO With Lyman Alpha Sky Mapping

Abstract: On board the SOHO spacecraft poised at L1 Lagrange point, the SWAN instrument is mainly devoted to the measurement of large scale structures of the solar wind, and in particular the distribution with heliographic latitude of the solar wind mass flux. This is obtained from an intensity map of the sky Lymanα emission, which reflects the shape of the ionization cavity carved in the flow of interstellar H atoms by the solar wind. The methodology, inversion procedure and related complications are described. The subject of latitude variation of the solar wind is shortly reviewed: earlier Lymanα results from Prognoz in 1976 are confirmed by Ulysses. The importance of the actual value of the solar wind mass flux for the equation of dynamics in a polar coronal hole is stressed. The instrument is composed of one electronic unit commanding two identical Sensor Units, each of them allowing to map a full hemisphere with a resolution of 1°, thanks to a two-mirrors periscope system. The design is described in some details, and the rationale for choice between several variants are discussed. A hydrogen absorption cell is used to measure the shape of the interplanetary Lymanα line and other Lyman α emissions. Other types of observations are also discussed : the geocorona, comets (old and new), the solar corona, and a possible signature of the heliopause. The connexion with some other SOHO instruments, in particular LASCO, UVCS, SUMER, is briefly discussed.

Properties of Magnetohydrodynamic Turbulence in the Solar Wind as Observed

Abstract: The Ulysses mission provides an opportunity to study the evolution of magnetohydrodynamic (MHD) turbulence in pure high-speed solar wind streams. The absence at high heliocentric latitudes of the strong shears in solar wind velocity generally present near the heliocentric current sheet allows investigation of how fluctuations in the magnetic field and plasma relax and evolve in the radially expanding solar wind. We report results of an analysis of the radial and latitudinal variation of the turbulence properties of the fluctuations, especially various plasma-field correlations, in high latitude regions. The results constrain current theories of the evolution of MHD turbulence in the solar wind. Compared to similar observations at 0.3 AU by Helios, we find spectra that are similar in having a large frequency band with an f¹ power spectrum in the outward traveling component of the waves, followed at higher frequencies by a steeper spectrum. Ulysses observations establish that at high latitudes the turbulence is less evolved (i.e., has a smaller inertial range) than it is in the ecliptic at the same heliocentric distance, apparently due to the absence of strong velocity shear. Once Ulysses is in the polar coronal hole, properties of the turbulence appear to be determined by the heliocentric distance of the spacecraft rather than by its helio-latitude.

Gamma-Ray Measurements of Flare-to-Flare Variations in Ambient Solar Abundances

Abstract: We have measured uxes of ten narrow-ray lines in 19 X-class solar ares observed by the Solar Maximum Mission spectrometer from 1980 to 1989. These lines originate from interaction of energetic protons and-particles with ambient solar material. Flare-to-are variations in line uxes reveal that the abundances of elements in the are plasma are grouped with respect to their rst ionization potentials (FIP). Line uxes from elements with similar FIP's correlate well with one another; in contrast, the low-FIP (< 10 eV; Mg, Si, Fe) to high-FIP (>11 eV; C, N, O) line ratios vary by as much as about a factor of four from are to are. This factor of four is consistent with the enhancement of low-FIP elements found in the solar corona relative to photospheric abundances. We also nd that the Ne/O line ratio increases as the accelerated particle spectrum becomes softer. This can be explained by excitation cross sections <10 MeV, but the magnitude of the eeect is dependent on the accelerated-particle/proton ratio. After correcting for this spectral dependence, we nd that the Ne/(C + N + O) line ratio is constant from are-to-are, indicating that Ne behaves like these other high-FIP elements in the ambient are plasma. Based on these observations, we discuss a possible explanation for the factor of 3 enhancement in the abundance of Ne, relative to other high-FIP elements, in the 1981 April 27 are found by Murphy et al. (1991).

Propagation of solar oscillations through the interplanetary medium

Abstract: Time-series analysis of the fluxes of interplanetary charged particles measured by the Ulysses and Voyager spacecraft reveals many periodic components. From 1 to 140 µHz, the spectral components are consistent with those estimated (but not confirmed) for gravity-mode oscillations of the Sun: from 1,000 to 4,000 µHz, the spectral lines closely match the frequencies of known solar pressure modes. These concordances imply that the solar wind and the interplanetary magnetic field transmit solar oscillations and thus might be used to probe the interior structure of the Sun.

The solar WIND and suprathermal ion composition investigation on the WIND spacecraft

Abstract: The Solar Wind and Suprathermal Ion Composition Experiment (SMS) on WIND is designed to determine uniquely the elemental, isotopic, and ionic-charge composition of the solar wind, the temperatures and mean speeds of all major solar-wind ions, from H through Fe, at solar wind speeds ranging from 175 kms−1 (protons) to 1280 kms−1 (Fe+8), and the composition, charge states as well as the 3-dimensional distribution functions of suprathermal ions, including interstellar pick-up He+, of energies up to 230 keV/e. The experiment consists of three instruments with a common Data Processing Unit. Each of the three instruments uses electrostatic analysis followed by a time-of-flight and, as required, an energy measurement. The observations made by SMS will make valuable contributions to the ISTP objectives by providing information regarding the composition and energy distribution of matter entering the magnetosphere. In addition SMS results will have an impact on many areas of solar and heliospheric physics, in particular providing important and unique information on: (i) conditions and processes in the region of the corona where the solar wind is accelerated; (ii) the location of the source regions of the solar wind in the corona; (iii) coronal heating processes; (iv) the extent and causes of variations in the composition of the solar atmosphere; (v) plasma processes in the solar wind; (vi) the acceleration of particles in the solar wind; and (vii) the physics of the pick-up process of interstellar He as well as lunar particles in the solar wind, and the isotopic composition of interstellar helium.

The efficiency of “viscous interaction” between the solar wind and the magnetosphere during intense northward IMF events

Abstract: We examined 11 cases when the interplanetary magnetic field (IMF) was intensely northward (greater than 10 nT) for long durations of time (greater than 3 hours), to quantitatively determine an uppler limit on the efficiency of solar wind energy injection into the magnetosphere. We have specifically selected these large B(sub N) events to minimize the effects of magnetic reconnection. Many of these cases occurred during intervals of high-speed streams associated with coronal mass ejections when viscous interaction effects might be at a maximum. It is found that the typical efficiency of solar wind energy injection into the magnetosphere is 1.0 x 10(exp -3) to 4.0 x 10(exp -3), 100 to 30 times less efficient than during periods of intense southward IMFs. Other energy sinks not included in these numbers are discussed. Estimates of their magnitudes are provided.

Interplanetary Shock Waves: Ulysses Observations in and Out of the Ecliptic Plane

Abstract: Between its launch in October 1990 and the end of 1993, approximately 160 fast collisionless shock waves were observed in the solar wind by the Ulysses space probe. During the in-ecliptic part of the mission, to February 1992, the observed shock waves were first caused mainly by solar transient events following the solar maximum and the reorganisation of the large scale coronal fields. With the decay in solar activity, relatively stable Corotating Interaction Regions (CIRs) were observed betwen 3 and 5.4 AU, each associated with at least one forward-reverse shock pair. During the out-of-ecliptic phase of the orbit, from February 1992 onwards, CIRs and shock pairs associated with them continued to dominate the observations. From July 1992, Ulysses encountered the fast solar wind flow from the newly developed southern polar coronal hole, and from May 1993 remained in the unipolar magnetic region associated with this coronal hole. At latitudes beyond 30°, CIRs were associated almost exclusively with reverse shocks only. A comprehensive list of shock waves identified in the magnetic field and solar wind plasma data from Ulysses is given in Table 1. The principal characteristics were determined mainly from the magnetic field data. General considerations concerning the determination of shock characteristics are outlined in the Introduction.