Source Regions Of Solar Wind Research Articles

CARBON IONIZATION STAGES AS A DIAGNOSTIC OF THE SOLAR WIND

Oxygen charge states measured by in situ instrumentation have long been used as a powerful diagnostic of the solar corona and to discriminate between different solar wind regimes, both because they freeze in very close to the Sun, and because the oxygen element abundance is comparatively high, allowing for statistically relevant measures. Like oxygen, carbon is also rather abundant and freezes in very close to the Sun. Here, we show an analysis of carbon and oxygen ionic charge states. First, through auditory and Fourier analysis of in situ measurements of solar wind ion composition by ACE/SWICS we show that some carbon ion ratios are very sensitive to solar wind type, even more sensitive than the commonly used oxygen ion ratios. Then we study the evolution of the ionization states of carbon and oxygen by means of a freeze-in code, and find that carbon ions, commonly found in the solar wind, freeze in at comparable coronal distances, while oxygen ions evolve over a much larger range of coronal distances. Finally, we show that carbon and oxygen ion abundance ratios have similar sensitivity to the electron plasma temperature, but the carbon ratios are more robust against atomic physics uncertainties and a better indicator of the temperature of the solar wind source regions.

Interplanetary conditions: lessons from this minimum

AbstractInterplanetary conditions during the Cycle 23-24 minimum have attracted attention because they are noticeably different than those during other minima of the space age, exhibiting more solar wind stream interaction structures in addition to reduced mass fluxes and low magnetic field strengths. In this study we consider the differences in the solar wind source regions by applying Potential Field Source Surface models of the coronal magnetic field. In particular, we consider the large scale coronal field geometry that organizes the open field region locations and sizes, and the appearance of the helmet streamer structure that is another determiner of solar wind properties. The recent cycle minimum had an extraordinarily long entry phase (the decline of Cycle 23) that made it difficult to identify when the actual miminum arrived. In particular, the late 23rd cycle was characterized by diminishing photospheric fields and complex coronal structures that took several extra years to simplify to its traditional dipolar solar minimum state. The nearly dipolar phase, when it arrived, had a duration somewhat shorter than those of the previous cycles. The fact that the corona maintained an appearance more like a solar maximum corona through most of the quiet transitional phase between Cycles 23 and 24 gave the impression of a much more complicated solar minimum solar wind structure in spite of the weaknesses of the mass flux and interplanetary field. The extent to which the Cycle 23-24 transition will affect Cycle 24, and/or represents what happens during weak cycles in general, remains to be seen.

THE SPECTROSCOPIC FOOTPRINT OF THE FAST SOLAR WIND

We analyze a large, complex equatorial coronal hole (ECH) and its immediate surroundings with a focus on the roots of the fast solar wind. We start by demonstrating that our ECH is indeed a source of the fast solar wind at 1AU by examining in situ plasma measurements in conjunction with recently developed measures of magnetic conditions of the photosphere, inner heliosphere and the mapping of the solar wind source region. We focus the bulk of our analysis on interpreting the thermal and spatial dependence of the non-thermal line widths in the ECH as measured by SOHO/SUMER by placing the measurements in context with recent studies of ubiquitous Alfven waves in the solar atmosphere and line profile asymmetries (indicative of episodic heating and mass loading of the coronal plasma) that originate in the strong, unipolar magnetic flux concentrations that comprise the supergranular network. The results presented in this paper are consistent with a picture where a significant portion of the energy responsible for the transport of heated mass into the fast solar wind is provided by episodically occurring small-scale events (likely driven by magnetic reconnection) in the upper chromosphere and transition region of the strong magnetic flux regions that comprise the supergranular network.

ELEMENTAL ABUNDANCES IN THE FAST SOLAR WIND EMANATING FROM CHROMOSPHERIC FUNNELS

We carry out a model study to determine whether a funnel-type flow geometry in the solar wind source region leads to sufficiently fast hydrogen flow to offset heavy element gravitational settling and can thus explain why solar wind abundances are not much smaller than photospheric abundances. We find that high first ionization potential (FIP) elements are more susceptible to gravitational settling than low-FIP elements, which are pulled up by Coulomb drag from protons, and hence the settling is more sensitive to the charge state of the elements than to their mass. Abundances at the top of the chromosphere, and hence solar wind abundances, can change by many orders of magnitude when the funnel areal expansion factor is changed by a small amount. The observed solar wind neon abundance provides the most severe constraint on the expansion, requiring a total flux tube expansion factor of at least 30-40.

Large‐Scale Coronal Density and Abundance Structures and Their Association with Magnetic Field Structure

We construct subsynoptic maps of the ultraviolet-line fluxes, electron density, and elemental abundances for an east limb corona at 1.63 solar radii from 2000 September 20 to October 1. The data, covering position angles of 85°-126°, were taken from the Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO), and the maps are based on the variation of these parameters along the field of view of the UVCS slit. Combining these maps with the limb synoptic maps made from SOHO Extreme Ultraviolet Imaging Telescope, SOHO Large Angle and Spectrometric Coronagraph, and Yohkoh Soft X-ray Telescope observations provides a large-scale, distinct view of the contrast between different coronal structures in different physical properties and their relation to the underlying disk and magnetic field structures. During this time period, the east limb corona mainly consisted of three streamers and two dark areas that exhibited very different plasma properties. We construct a three-dimensional MHD coronal model that incorporates energy transport processes, and compare the large-scale coronal properties of the model with those of the observation. The comparison investigates areas of different abundances and densities, and their possible association with open and closed magnetic field structures. We find a good indication that the open field regions, which we believe to be the slow-wind source regions in this case, have lower coronal density and higher abundance values than the closed field regions. This is true for absolute abundance, and probably also for the FIP bias. Therefore, such synoptic maps can be very useful for identifying solar wind source regions.

Long period slow MHD waves in the solar wind source region

We consider compressive viscosity and thermal conductivity to study the propagation and dissipation of long period slow longitudinal MHD waves in polar coronal holes. We discuss their likely role in the line profile narrowing, and in the energy budget for coronal holes and the solar wind. We compare the contribution of longitudinal MHD waves with high frequency Alfvén waves.

Ultraviolet spectroscopy of the extended solar corona

The first observations of ultraviolet spectral line profiles and intensities from the extended solar corona (i.e., more than 1.5 solar radii from Sun-center) were obtained on 13 April 1979 when a rocket-borne ultraviolet coronagraph spectrometer of the Harvard-Smithsonian Center for Astrophysics made direct measurements of proton kinetic temperatures, and obtained upper limits on outflow velocities in a quiet coronal region and a polar coronal hole. Following those observations, ultraviolet coronagraphic spectroscopy has expanded to include observations of over 60 spectral lines in coronal holes, streamers, coronal jets, and solar flare/coronal mass ejection (CME) events. Spectroscopic diagnostic techniques have been developed to determine proton, electron and ion kinetic temperatures and velocity distributions, proton and ion bulk flow speeds and chemical abundances. The observations have been made during three sounding rocket flights, four Shuttle deployed and retrieved Spartan 201 flights, and the Solar and Heliospheric Observatory (SOHO) mission. Ultraviolet spectroscopy of the extended solar corona has led to fundamentally new views of the acceleration regions of the solar wind and CMEs. Observations with the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO revealed surprisingly large temperatures, outflow speeds, and velocity distribution anisotropies in coronal holes, especially for minor ions. Those measurements have guided theorists to discard some candidate physical processes of solar wind acceleration and to increase and expand investigations of ion cyclotron resonance and related processes. Analyses of UVCS observations of CME plasma properties and the evolution of CMEs have provided the following: temperatures, inflow velocities and derived values of resistivity and reconnection rates in CME current sheets, compression ratios and extremely high ion temperatures behind CME shocks, and three dimensional flow velocities and magnetic field chirality in CMEs. Ultraviolet spectroscopy has been used to determine the thermal energy content of CMEs allowing the total energy budget to be known for the first time. Such spectroscopic observations are capable of providing detailed empirical descriptions of solar energetic particle (SEP) source regions that allow theoretical models of SEP acceleration to be tailored to specific events, thereby enabling in situ measurements of freshly emitted SEPs to be used for testing and guiding the evolution of SEP acceleration theory. Here we review the history of ultraviolet coronagraph spectroscopy, summarize the physics of spectral line formation in the extended corona, describe the spectroscopic diagnostic techniques, review the advances in our understanding of solar wind source regions and flare/CME events provided by ultraviolet spectroscopy and discuss the scientific potential of next generation ultraviolet coronagraph spectrometers.

MHD Waves and Heating in Coronal Holes

Coronal holes have been identified as source regions of the fast solar wind, and MHD wave activity has been detected in coronal holes by remote sensing, and in situ in fast solar wind streams. I review some of the most suggestive wave observations, and discuss the theoretical aspects of MHD wave heating and solar wind acceleration in coronal holes. I review the results of single fluid 2.5D MHD, as well as multi-fluid 2.5D MHD models of waves in coronal holes, the heating, and the acceleration of the solar wind be these waves.

Characteristics of active-region sources of solar wind near solar maximum

Previous studies of the source regions of solar wind sampled by ACE and Ulysses showed that some solar wind originates from open magnetic flux rooted in active regions. These solar wind sources were labeled active-region sources when the open flux was from a strong field region with no corresponding coronal hole in the NSO He 10830 Å synoptic coronal-hole maps. Here, we present a detailed investigation of several of these active-region sources using ACE and Ulysses solar wind data, potential field models of the corona, and solar imaging data. We find that the solar wind from these active-region sources has distinct signatures, e.g., it generally has a higher oxygen charge state than wind associated with helium-10830 Å coronal-hole sources, indicating a hotter source region, consistent with the active region source interpretation. We compare the magnetic topology of the open field lines of these active-region sources with images of the hot corona to search for corresponding features in EUV and soft X-ray images. In most, but not all, cases, a dark area is seen in the EUV and soft X-ray image as for familiar coronal-hole sources. However, in one case no dark area was evident in the soft X-ray images: the magnetic model showed a double dipole coronal structure consistent with the images, both indicating that the footpoints of the open field lines, rooted deep within the active region, lay near the separatrix between loops connecting to two different opposite polarity regions.

Stagnated Outflow of O+5Ions in the Source Region of the Slow Solar Wind at Solar Minimum

Using recent coordinated UVCS and LASCO measurements by Strachan and coworkers to constrain the heating parameters of a one-dimensional single-fluid minor ion model, we calculate the outflow velocity profile of O+5 ions in the flow tube overlying the helmet streamer, which has been supposed to be the source region of the slow solar wind at least during solar minimum. The background solar wind parameters and the flow tube geometry are taken from a recent two-dimensional magnetohydrodynamic solar wind model. We show that in this slow-wind source region the O+5 outflow speed varies nonmonotonically with increasing heliocentric distance. There is a local minimum of the outflow speed near the streamer cusp point (about 3 R☉), which is below the current observational sensitivity. This type of ion outflow in the slow solar wind is termed "stagnated outflow" in this paper. We also show that the observed effective temperature in the perpendicular direction (to the magnetic field) and the outflow speed of the O+5 ions can be used to put limits on their parallel thermal temperature.

Solar Energetic Particle Production by Coronal Mass Ejection–driven Shocks in Solar Fast‐Wind Regions

Gradual solar energetic particle (SEP) events at 1 AU are produced by coronal/interplanetary shocks driven by coronal mass ejections (CMEs). Fast (vCME 900 km s-1) CMEs might produce stronger shocks in solar slow-wind regions, where the flow and fast-mode MHD wave speeds are low, than in fast-wind regions, where those speeds are much higher. At 1 AU the O+7/O+6 ratios distinguish between those two kinds of wind streams. We use the 20 MeV proton event intensities from the EPACT instrument on Wind, the associated CMEs observed with the LASCO coronagraph on SOHO, and the ACE SWICS solar wind values of O+7/O+6 to look for variations of peak SEP intensities as a function of O+7/O+6. No significant dependence of the SEP intensities on O+7/O+6 is found for either poorly connected or well-connected CME source regions or for different CME speed ranges. However, in the 3 yr study period we find only five cases of SEP events in fast wind, defined by regions of O+7/O+6 < 0.15. We suggest that in coronal holes SEP acceleration may take place only in the plume regions, where the flow and Alfv?n speeds are low. A broad range of angular widths are associated with fast (vCME ? 900 km s-1) CMEs, but we find that no fast CMEs with widths less than 60? are associated with SEP events. On the other hand, nearly all fast halo CMEs are associated with SEP events. Thus, the CME widths are more important in SEP production than previously thought, but the speed of the solar wind source regions in which SEPs are produced may not be a factor.

Empirical Densities, Kinetic Temperatures, and Outflow Velocities in the Equatorial Streamer Belt at Solar Minimum

We use combined Ultraviolet Coronagraph Spectrometer and Large Angle Spectroscopic Coronagraph data to determine the O5+ outflow velocities as a function of height along the axis of an equatorial streamer at solar minimum and as a function of latitude (at 2.3 R☉ from Sun center). The results show that outflow increases rather abruptly in the region between 3.6 and 4.1 R☉ near the streamer cusp and gradually increases to ~90 km s-1 at ~5 R☉ in the streamer stalk beyond the cusp. The latitudinal variation at 2.3 R☉ shows that there is no outflow (within the measurement uncertainties) in the center of the streamer, called the core, and that a steep increase in outflow occurs just beyond the streamer legs, where the O VI λ1032 intensity relative to H I λ1216 (Lyα) is higher than in the core. Velocity variations in both height and latitude show that the transitions from no measurable outflow to positive outflow are relatively sharp and thus can be used to infer the location of the transition from closed to open field lines in streamer magnetic field topologies. Such information, including the densities and kinetic temperatures derived from the observations, provides hard constraints for realistic theoretical models of streamers and the source regions of the slow solar wind.

A Spectroscopic Study of the Solar Corona from Norikura and SOHO data

We report the results from a spectroscopic study of the solar corona using both ground-based and space-based data. Some of the current topics in coronal physics, such as the plume-interplume differences in coronal holes and wave propagation in the corona have been examined. The distribution of emission line intensities, Doppler velocities and line widths were obtained from the spectroscopic observations from Norikura Solar Observatory of National Astronomical Observatory of Japan. The coronal images in Fe IX,X 171 Å and Fe XII 195 Å from SOHO EIT were used to get the temperature map of the corona. Combining both results, we have obtained the nonthermal velocities in the coronal hole region, without the usual assumption of a uniform ion temperature. It has been found that the histogram distributions of Doppler velocities and nonthermal velocities for coronal hole and quiet regions are markedly different. The nonthermal velocities have been found to be larger by about 27 %at the interplume regions as compared to plumes which supports the view that the interplume regions are the source regions of the fast solar wind. The analysis of the time sequence spectral data shows signatures of coronal oscillations at localized regions. Periods of the order of a few minutes have been found mainly in Doppler velocities.

The Physical Conditions in a Polar Coronal Hole and Nearby Regions from Norikura andSOHOObservations

The distribution of emission-line intensities, Doppler velocities, and line widths in a polar coronal hole and nearby regions are obtained from the spectroscopic observations carried out on 1998 November 3 at the Norikura Solar Observatory, Japan. The coronal red line [Fe X] λ6374 that is prominent at coronal hole temperatures is used for the study. The coronal images in Fe IX and Fe X 171 A and Fe XII 195 A from the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) are used to get the temperature map of the corona at the time of observation. Combining both, we have obtained the nonthermal velocities in the region without the usual assumption of a uniform ion temperature. Several plume structures are identified within the coronal hole, and it is found that line widths are smaller in plumes than in the interplume regions, which is also reported from recent SOHO observations. The line-of-sight Doppler velocities in the coronal hole are larger than those in the quiet region, probably because of the excess outflow in the coronal hole. A rough negative correlation between intensity and Doppler velocity, similar to that between intensity and line width, is observed in the coronal hole. The typical nonthermal velocity in coronal holes is 24 km s-1 while that in quiet regions is 15 km s-1. The enhanced nonthermal velocity in the coronal hole is suggestive of the important role of the nonthermal broadening mechanism in the acceleration of fast solar wind. Also, the nonthermal velocities are larger (up to 27%) at the interplume regions as compared to plumes. The findings generally support the prevailing view that the interplume regions are the source regions of the fast solar wind.

Source regions of the slow solar wind in coronal streamers

Recent SOHO/UVCS observations of the O5+ ion line emission at 1032Å in coronal streamers indicate that the emission is stronger by an order of magnitude at the edges (legs) of streamers than in the central core of streamers. In contrast, the brightness of the Ly‐α emission peaks in the core of streamers. I have developed the first 2.5D, three‐fluid numerical MHD model of the slow solar wind flow in a coronal streamer. Using the model I find that the enhancement of the oxygen line emission occurs due to the enhanced abundances of O5+ ions in the legs of streamer caused by the Coulomb friction with the outflowing protons. Thus, the enhanced O5+ emission traces the source regions of the slow solar wind in coronal streamers. The identification of these regions helps to understand the origins and the composition of the slow solar wind.

Magnetic structure of the slow solar wind: Constraints from composition data

The elemental and ionic composition of the solar wind is determined very close to the Sun. Composition data therefore contain direct information about the structure of the corona and the processes which lead to the generation and acceleration of solar wind and can constrain processes occurring in the heliosphere. We use Advanced Composition Explorer (ACE)‐Solar Wind Ion Composition Spectrometer (SWICS) composition measurements of the solar wind to examine first the structure of the slow solar wind. We find that the statistical properties of the data suggest the presence of a large number of composition boundaries, where compositional patterns change abruptly. These discontinuities are interpreted here as a consequence of the structure of the solar wind source region. Using the experimental constraints of these observed composition boundaries, we derive direct constraints for turbulent diffusion of magnetic field lines in the heliosphere. We then discuss consequences for the propagation of energetic particles.

On the electron temperatures, densities and hot ions in coronal hole plasma observed by SUMER on SOHO

Making use of high-resolution ultraviolet observations obtained with SUMER (Solar Ultraviolet Measurements of Emitted Radiation) on the spacecraft SOHO (Solar and Heliospheric Observatory), we study plasma temperatures, densities and hot ions in polar coronal holes (source region of the high-speed solar wind). In particular, we present spectroscopic diagnostics for Ne viii, Mg viii, Si viii, and Mg ix lines to infer these physical parameters and discuss the constraints of their applications in the coronal hole plasma. We also present the implications of this investigation in addressing the acceleration mechanisms of the solar wind, one of the outstanding problems in solar physics.

Solar wind ionic charge states during the Ulysses pole‐to‐pole pass

We analyze and compare the ionic charge composition data for different types of the solar wind (slow wind from equatorial regions, fast wind from low‐latitude coronal hole and fast wind from both south and north polar coronal hole) which the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses observed during the pole‐to‐pole pass of its primary mission. The implications on the electron temperature, electron density, and ion outflow velocity from the corresponding solar wind source regions are also discussed. We find that the electron temperature in the source region of the slow solar wind is higher than that in the coronal hole. We also find a possible north‐south asymmetry in the electron temperature that may be correlated to the north–south asymmetry in the solar wind speed found in the SWOOPS/Ulysses data. In particular, we make extensive discussions on the latitudinal variations in the polar coronal hole. On the basis of our data without clear constraints from other coronal observations, the preliminary result is that the electron density may be higher, or the heavy ion outflow velocities may be lower toward lower heliographic latitude.

Coronal radio sounding: Non‐Gaussian turbulence in the source regions of slow and fast solar wind

Structure functions of arbitrary order are analyzed using differential phase scintillation data obtained during the fourth solar conjunction of the Ulysses spacecraft in February–March 1995. The scaling features of these structure functions clearly discriminate between electron density fluctuations observed in coronal holes versus coronal streamers. The second order structure function, which is related to the power spectrum, shows essentially a scaling exponent ζ(p) = α −2, where p=2 and α is the spectral index of the three‐dimensional wavenumber spectrum of electron density fluctuations. Higher‐order structure functions provide evidence that these fluctuations are scale dependent and do not follow Gaussian statistics.

Galactic cosmic ray intensity variations at a high latitude sea level site 1937–1994

We have created an extended cosmic ray data string (1937–1994) by combining data obtained with ion chambers at Cheltenham/Fredericksburg (1937–1972) and Yakutsk (1953–1994). Both represent high‐latitude, sea level sites, with an atmospheric cutoff of about 4 GV. Their common median rigidity of response to galactic cosmic ray spectrum is 67 GV. This data string represents the longest continuous cosmic ray intensity variations record yet. Therefore it is useful for the cosmic ray modulation studies over a longer time period. We show that there is no striking quantitative correspondence between the amplitudes of solar activity cycles and cosmic ray modulation over six cycles. However, an inverse correlation is apparent between the two. Moreover, the results on the durations of the two are ambiguous. Flat‐topped recovery pattern predicted for even cycles (A &gt; 0), at lower rigidities, is not observed in the ion chamber data. We speculate that three solar activity cycle quasi‐periodicity may be present in the extended data string. Its origin is not identified yet. The quasi‐periodicity is shown to exist also in the planetary index Ap data. As such, it may represent a characteristic feature of the solar wind source region on the Sun as yet unexplored. The odd cycles, with a negative polarity in the solar northern hemisphere, yield a better linear correlation between galactic cosmic ray intensity decrease and Ap than even cycles and A &gt; 0 epochs.