UVCS/SOHO Observations of H I Lyman Alpha Line Profiles in Coronal Holes at Heliocentric Heights Above 3.0 R⊙

The Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) has been used to measure spectral line profiles for H I Lyα in the south polar coronal hole at projected heliocentric heights from 3.5 to 6.0 R_⊙ during 1998 January 5–11. Observations from 1.5 to 2.5 R_⊙ were made for comparison. The H I Lyα profile is the only one observable with UVCS above 3.5 R_⊙ in coronal holes. Within this region the outflowing coronal plasma becomes nearly collisionless and the ionization balance is believed to become frozen. In this paper, the 1/e half widths of the coronal velocity distributions are provided for the observed heights. The velocity distributions include all motions contributing to the velocities along the line of sight (LOS). The observations have been corrected for instrumental effects and interplanetary H I Lyα. The half widths were found to increase with projected heliographic height from 1.5 to 2.5 R_⊙ and decrease with height from 3.5 to 5 R_⊙. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Strength and Structure of the Coronal Magnetic Field

I discuss a method for determining the strength and spatial structure of the coronal magnetic field by observations of the Faraday rotation of a radio galaxy which is in conjunction with the Sun. Given a knowledge of the plasma density in the outer corona, and the magnetic field sector structure (both independently available), the strength of the coronal field can be determined, as well as the magnitude of spatial variations on scales of 1000 km to several solar radii. Such knowledge is crucial for testing computational models of the solar corona, which are prominently featured in this meeting. Results are presented from observations with the Very Large Array radio telescope of the radio galaxy 3C228 on August 16, 2003, when the line of sight to the source was at heliocentic distances of 7.1−6.2R _⊙. The observations are consistent with a coronal magnetic field which is proportional to the inverse square of the distance in the range 6 ≤ r ≤ 10R _⊙, and has a value of 39 mG at 6.2R _⊙. The Faraday rotation is uniform across the source, indicating an absence of strong plasma inhomogeneity on spatial scales up to 35,000 km.     

Acceleration Profile of the Slow Solar Wind as Inferred from Gradual Mass Ejections Observed by LASCO

The slow solar wind (< 400 km s^-1) appears to initiate from the regions in the corona where magnetic fields are closed, or from the interface between streamers and other coronal regions. The nature of the acceleration of slow solar wind is not yet well known. LASCO observations of gradually evolving mass ejections offer us a good opportunity to study the speed and acceleration profiles of the slow solar wind from a distance of 1.1 up to 30 R_⊙. We present speed and acceleration profiles of slow solar wind, derived on the basis of measurements of mass flows in several cases of gradual mass ejections and present them in perspective of earlier work. This revised version was published online in June 2006 with corrections to the Cover Date.     

CCD spectroscopy of the W Serpentis binaries KX And and RX Cas

Initial results are presented from a study of H profiles in the two interacting binaries KX And and RX Cas of W Serpentis type. The used CCD spectra with a resolution of 0.13Å/px were obtained with the 2.2m telescope and the Coudé spectrograph at the German-Spanish Astronomical Center at Calar Alto/Spain.KX And. This star is probably a non-eclipsing member of the W Serpentis type interactive binaries and has a period of P = 38.908 days. Our seven spectra of KX And were obtained at phase 0.54 – 0.75. The P Cyg profiles of the H line during our observations indicate an expanding shell. The asymetry becomes blue-sided at phase 0.67 and increases thereafter. This points toward a strong outflow of matter in the vicinity of the L3 point.RX Cas. According to the model of Andersen et al. (1988) the primary is a mid-B type star with M = 5.8M and R = 2.5R . The star is completely obscured by a geometrically and optically thick disk, which is supplied by mass transfer from the other component. The secondary is a K1 giant with M = 1.8M and R = 23.5R and fills out his critical Roche lobe. Radiative and geometrical properties of the disk are variable and its structure is probably not homogenous.Five spectra of RX Cas were obtained during the primary eclipse (phase 0.95 – 0.19). The observed double-peak emission is seen only after the eclipse with a separation of 250 km/s peak-to-peak, while during the eclipse an asymetric line profile can be observed with a red-shifted emission always presented. Also, a central emission at = 0.94 should be noticed, probably originating in the vicinity of L1.The observations of both systems indicate that we are dealing with strongly interacting binaries. Further observations are planned for better covering of phase.Visiting Astronomer, German-Spanish Astronomical Center, Calar Alto, operated by the Max-Planck-Institut für Astronomie Heidelberg jointly with the Spanish National Commision for Astronomy.     

Spectroscopic Measurement of Coronal Compositions

Although the elemental composition in all parts of the solar photosphere appears to be the same this is clearly not the case with the solar upper atmosphere (SUA). Spectroscopic studies show that in the corona elemental composition along solar equatorial regions is usually different from polar regions; composition in quiet Sun regions is often different from coronal hole and active region compositions and the transition region composition is frequently different from the coronal composition along the same line of sight. In the following two issues are discussed. The first involves abundance ratios between the high-FIP O and Ne and the low-FIP Mg and Fe that are important for meaningful comparisons between photospheric and SUA compositions and the second involves a review of composition and time variability of SUA plasmas at heights of 1.0≤h≤1.5R _⊙.     

Radio Frequency Fluctuation Spectra During the Solar Conjunctions of the Ulysses and Galileo Spacecraft

Temporal power spectra have been computed from recordings of the downlink frequency fluctuations of the Galileo and Ulysses radio signals during their solar conjunctions. Both the equatorial streamer belt and the polar coronal holes were investigated over a range of ray path solar offset distances from 4 to 80 R_⊙. By combining gapless data from successive tracking passes, Doppler scintillation power spectra could be computed down to extremely low frequencies. Some spectra feature a low-frequency turnover at frequencies around 0.1 mHz that could be interpreted as an outer scale of density turbulence in the coronal plasma. This revised version was published online in August 2006 with corrections to the Cover Date.     

Observations of the Acceleration of Polar Solar Wind Near Solar Maximum

In a previous paper (Tappin et al., 1999) we used cross-correlation analysis of high-cadence observations with the LASCO coronagraphs to trace the acceleration of the solar wind at low latitudes. In this paper we present a similar analysis carried out over the North pole of the Sun. The observations which were made in March 2000 with the C3 coronagraph show low bulk flow speeds (comparable to or lower than those seen at the equator in early 1998). We observe the acceleration continuing to the edge of the C3 field of view at about 30 R _⊙. We also observe, as at low latitude, a high-speed tail but now reaching out well beyond 2000 km s⁻¹. We do not see a clear signature of a fast polar bulk flow. We therefore conclude that at this phase of the solar cycle, any fast bulk flow occupies only a small part of the line of sight and is therefore overwhelmed by the denser slow solar wind in these observations. We also show that the fast component is consistent with observed solar wind speeds at 1 AU. This revised version was published online in August 2006 with corrections to the Cover Date.     

Associating the Solar Wind Measured by Ulysses with its Source at the sun

Radio occultation, ultraviolet, and white-light measurements have expanded our knowledge of the morphology of density and velocity in polar coronal holes, and made it possible to carry out the first systematic comparisons between the Ulysses solar wind measurements and quantitative white-light observations of the solar corona. This paper summarizes the rationale and salient features of this new approach which has been used to relate the solar wind observed by Ulysses in 1993–1995 to the inner corona. The statistical characteristics (average, standard deviation, and autocorrelation function) of the Ulysses density measurements of the fast wind are found to be mirrored in those of polarized brightness measurements of path-integrated density made by the High Altitude Observatory (HAO) Mauna Loa K-coronagraph at 1.15 R _⊙. These results reinforce the conclusions from comparisons between measurements of the outer and inner corona. They show that the polar coronal hole extends radially into the solar wind, and that sources of the fast wind are not limited to coronal holes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interplanetary scintillation observations of the high-latitude solar wind

Until the ULYSSES spacecraft reached the polar regions of the solar wind, the only high-latitude measurements available were from indirect techniques. The most productive observations in regions of the solar wind between 5R and 200R have been the family of radio scattering techniques loosely referred to as Interplanetary Scintillation (IPS) (Coles, 1978). Useful observations can be obtained using a variety of radio sources, for example spacecraft beacons, planetary radar echoes and compact cosmic sources (quasars, active galactic nuclei, pulsars, galactic masers, etc.). However for measurement of the high-latitude solar wind cosmic sources provide the widest coverage and this review will be confined to such observations. IPS observations played a very important role in establishing that polar coronal holes (first observed in soft x-ray emission) were sources of fast solar wind streams which occasionally extend down to the equatorial region and are observed by spacecraft. Here I will review the IPS technique and show the variation of both the velocity and the turbulence level with latitude over the last solar cycle. I will also outline recent work and discuss comparisons that we hope to make between IPS and ULYSSES observations.     

Latitudinal variation of solar wind velocity

Single station solar wind velocity measurements using the Ooty Radio Telescope (ORT) in India (operating at 327 MHz) are reported for the period August 1992 to August 1993. Interplanetary scintillation (IPS) observations on a large number of compact radio sources covering a latitudinal range of ±80° were used to derive solar wind velocities using the method of fitting a power law model to the observed IPS spectra. The data shows a velocity versus heliographic latitude pattern which is similar to that reported by Rickett and Coles (1991) for the 1981–1982 period. However, the average of the measured equatorial velocities are higher, being about 470 km s^–1 compared to their value of 400 km s^–1. The distribution of electron density variations (N _e ) between 50R and 90R was also determined and it was found that N _e was about 30% less at the poles as compared to the equator.     

Satellite observations of type III solar radio bursts at low frequencies

Type III solar radio bursts have been observed from 10 MHz to 10 kHz by satellite experiments above the terrestrial plasmasphere. Solar radio emission in this frequency range results from excitation of the interplanetary plasma by energetic particles propagating outward along open field lines over distances from 5 R to at least 1 AU from the Sun. This review summarizes the morphology, characteristics and analysis of individual as well as storms of bursts. Substantial evidence is available to show that the radio emission is observed at the second harmonic instead of the fundamental of the plasma frequency. This brings the density scale derived by radio observations into better agreement with direct solar wind density measurements at 1 AU and relaxes the requirement for type III propagation along large density-enhanced regions. This density scale with the measured direction of arrival of the radio burst allows the trajectory of the exciter path to be determined from 10 R to 1 AU. Thus, for example, the dynamics and gross structure of the interplanetary magnetic field can be investigated by this method. Burst rise times are interpreted in terms of exciter length and dispersion while decay times refer to the radiation damping process. The combination of radio observations at the lower frequencies and in-situ measurements on non-relativistic electrons at 1 AU provide data on the energy range and efficiency of the wave-particle interactions responsible for the radio emission.     

CCD spectroscopy of the W Serpentis binaries KX And and RX Cas

Initial results are presented from a study of H _γ profiles in the two interacting binaries KX And and RX Cas of W Serpentis type. The used CCD spectra with a resolution of 0.13Å/px were obtained with the 2.2m telescope and the Coudé spectrograph at the German-Spanish Astronomical Center at Calar Alto/Spain. KX And. This star is probably a non-eclipsing member of the W Serpentis type interactive binaries and has a period of P = 38.908 days. Our seven spectra of KX And were obtained at phase 0.54 – 0.75. The P Cyg profiles of the H _γ line during our observations indicate an expanding shell. The asymetry becomes blue-sided at phase 0.67 and increases thereafter. This points toward a strong outflow of matter in the vicinity of the L3 point. RX Cas. According to the model of Andersen et al. (1988) the primary is a mid-B type star with M = 5.8M _⊙ and R = 2.5R _⊙. The star is completely obscured by a geometrically and optically thick disk, which is supplied by mass transfer from the other component. The secondary is a K1 giant with M = 1.8M _⊙ and R = 23.5R _⊙ and fills out his critical Roche lobe. Radiative and geometrical properties of the disk are variable and its structure is probably not homogenous. Five spectra of RX Cas were obtained during the primary eclipse (phase 0.95 – 0.19). The observed double-peak emission is seen only after the eclipse with a separation of ≈ 250 km/s peak-to-peak, while during the eclipse an asymetric line profile can be observed with a red-shifted emission always presented. Also, a central emission at φ = 0.94 should be noticed, probably originating in the vicinity of L1. The observations of both systems indicate that we are dealing with strongly interacting binaries. Further observations are planned for better covering of phase.     

Radio occultation studies with Solar Corona Sounders

Solar Corona Sounders (SCS), a mission designed to utilize the radio occultation technique for investigations of the inner heliosphere, was submitted to ESA in response to a call for new mission concepts. The SCS platforms are two small multifrequency transmitters placed at the anti-Earth position (superior solar conjunction) for continuous radio sounding of the solar corona. Appropriately specifying certain orbital elements for the heliocentric trajectories of the spacecraft, their radially-aligned positions as seen from Earth appear to circle the solar disk over the course of a year. The two radio sources would be most effectively positioned at apparent solar distances inside and outside the nominal solar wind critical point, respectively, e.g., at 3 R and 10 R. Radio parameters to be measured using the linearly polarized, coherent dual-frequency links to ground include the group time delay, signal amplitude, the phase (Doppler) shift, linewidth, and Faraday rotation. The link frequencies for coronal sounding observations this close to the Sun could be the interplanetary standards at S-band (2.3 GHz) and X-band (8.4 GHz). These measurements are used to derive both mean values and fluctuation spectra of such coronal parameters as the electron density, the solar wind velocity, and the magnetic field. The geometry afforded by the two radio ray paths from the SCS transmitters would provide unprecedented observations of the radial evolution of dynamic coronal events such as coronal mass ejections.     

Latitudinal variations in the solar wind electron heat flux

Ulysses measurements of the solar wind electron heat flux as a function of heliographic latitude are presented. The latitudinal in the electron heat flux presented have been normalized by the radial gradient in the electron heat flux obtained during the in-ecliptic phase of the Ulysses mission (q_e R^–3.0). We find no significant variation in electron heat flux with latitude.     

Effects of Low Energetic Neutral Atoms on Martian and Venusian Dayside Exospheric Temperature Estimations

The heating of the upper atmospheres and the formation of the ionospheres on Venus and Mars are mainly controlled by the solar X-ray and extreme ultraviolet (EUV) radiation (λ = 0.1–102.7 nm and can be characterized by the 10.7 cm solar radio flux). Previous estimations of the average Martian dayside exospheric temperature inferred from topside plasma scale heights, UV airglow and Lyman-α dayglow observations of up to ∼500 K imply a stronger dependence on solar activity than that found on Venus by the Pioneer Venus Orbiter (PVO) and Magellan spacecraft. However, this dependence appears to be inconsistent with exospheric temperatures (<250 K) inferred from aerobraking maneuvers of recent spacecraft like Mars Pathfinder, Mars Global Surveyor and Mars Odyssey during different solar activity periods and at different orbital locations of the planet. In a similar way, early Lyman-α dayglow and UV airglow observations by Venera 4, Mariner 5 and 10, and Venera 9–12 at Venus also suggested much higher exospheric temperatures of up to 1000 K as compared with the average dayside exospheric temperature of about 270 K inferred from neutral gas mass spectrometry data obtained by PVO. In order to compare Venus and Mars, we estimated the dayside exobase temperature of Venus by using electron density profiles obtained from the PVO radio science experiment during the solar cycle and found the Venusian temperature to vary between 250–300 K, being in reasonable agreement with the exospheric temperatures inferred from Magellan aerobraking data and PVO mass spectrometer measurements. The same method has been applied to Mars by studying the solar cycle variation of the ionospheric peak plasma density observed by Mars Global Surveyor during both solar minimum and maximum conditions, yielding a temperature range between 190–220 K. This result clearly indicates that the average Martian dayside temperature at the exobase does not exceed a value of about 240 K during high solar activity conditions and that the response of the upper atmosphere temperature on Mars to solar activity near the ionization maximum is essentially the same as on Venus. The reason for this discrepancy between exospheric temperature determinations from topside plasma scale heights and electron distributions near the ionospheric maximum seems to lie in the fact that thermal and photochemical equilibrium applies only at altitudes below 170 km, whereas topside scale heights are derived for much higher altitudes where they are modified by transport processes and where local thermodynamic equilibrium (LTE) conditions are violated. Moreover, from simulating the energy density distribution of photochemically produced moderately energetic H, C and O atoms, as well as CO molecules, we argue that exospheric temperatures inferred from Lyman-α dayglow and UV airglow observations result in too high values, because these particles, as well as energetic neutral atoms, transformed from solar wind protons into hydrogen atoms via charge exchange, may contribute to the observed planetary hot neutral gas coronae. Because the low exospheric temperatures inferred from neutral gas mass spectrometer and aerobraking data, as well as from CO⁺ ₂ UV doublet emissions near 180–260 nm obtained from the Mars Express SPICAM UV spectrograph suggest rather low heating efficiencies, some hitherto unidentified additional IR-cooling mechanism in the thermospheres of both Venus and Mars is likely to exist. An erratum to this article can be found at     

Models of coronal hole flows

Models of plasma flow in a coronal hole fall naturally into four classes. These are: (i) radial flow on a streamline along which the divergence is assumed to vary differently than as the square of the radial distance from the Sun; (ii) global flow along streamlines determined in some independent manner; (iii) empirical models originating in, or based strongly on observation; (iv) dynamic models using magnetic and plasma boundary conditions low in the corona to find both the geometry of streamlines and the flow field.To date, models both of ideal coronal holes and of specific observed coronal holes indicate that flow velocities above 100 km s⁺¹, and temperatures of perhaps 2 × 10⁶K are possible at 2R heliocentric distance, where densities of 2 × 10⁵ cm⁺³ have been reported. These velocities are at, or just above the sound speed, although still sub-Alfvénic. There is also general agreement among models of large polar holes that conversion of mechanical wave energy flux into solar wind kinetic energy is occurring in the 2R to 5R range, perhaps occurs even further outwards, and that the magnitude and extent of this energy deposition depends on the size and on the geometrical divergence of the hole.However, each model exhibits distinct weaknesses counteracted only by the complimentary nature of the various types of models. Models in class (i) are simply not global representations, but are tractable when dealing with complex forms of the energy equation or with several ion species. Class (ii) models lack any geometrical information beyond the ad hoc assumption of known streamline geometry, but have the same advantages as those in class (i). Class (iii) models cannot determine streamline geometry within a hole and do not extend further from the Sun than the available data — although they place important constraints on models in the other classes. Class (iv) models are limited to simple forms of the energy equation and/or to quasi-radial flow, but are the only models producing self-consistent streamline geometries through inclusion of transverse magnetic stresses in the momentum equation.Most limitations in coronal hole flow models can be eliminated by using known numerical techniques to combine models in classes (i), (ii), and (iv). This would allow detailed models of coronal holes and corresponding interplanetary conditions to be developed for specific time periods, at the cost of flexibility and possibly also general conceptual understanding. Nevertheless, the concept of a coronal hole is now reasonably well established, and acceptable modelling approaches are rapidly filling the literature. It can be anticipated that the evolution of these models, together with present and future observations, will bring us much nearer to understanding coronal energetics and dynamics.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

A Comparative Study of the Influence of the Active Young Sun on the Early Atmospheres of Earth, Venus, and Mars

Because the solar radiation and particle environment plays a major role in all atmospheric processes such as ionization, dissociation, heating of the upper atmospheres, and thermal and non-thermal atmospheric loss processes, the long-time evolution of planetary atmospheres and their water inventories can only be understood within the context of the evolving Sun. We compare the effect of solar induced X-ray and EUV (XUV) heating on the upper atmospheres of Earth, Venus and Mars since the time when the Sun arrived at the Zero-Age-Main-Sequence (ZAMS) about 4.6 Gyr ago. We apply a diffusive-gravitational equilibrium and thermal balance model for studying heating of the early thermospheres by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by IR-radiating molecules like CO₂, NO, OH, etc. Our model simulations result in extended thermospheres for early Earth, Venus and Mars. The exospheric temperatures obtained for all the three planets during this time period lead to diffusion-limited hydrodynamic escape of atomic hydrogen and high Jeans’ escape rates for heavier species like H₂, He, C, N, O, etc. The duration of this blow-off phase for atomic hydrogen depends essentially on the mixing ratios of CO₂, N₂ and H₂O in the atmospheres and could last from ∼100 to several hundred million years. Furthermore, we study the efficiency of various non-thermal atmospheric loss processes on Venus and Mars and investigate the possible protecting effect of the early martian magnetosphere against solar wind induced ion pick up erosion. We find that the early martian magnetic field could decrease the ion-related non-thermal escape rates by a great amount. It is possible that non-magnetized early Mars could have lost its whole atmosphere due to the combined effect of its extended upper atmosphere and a dense solar wind plasma flow of the young Sun during about 200 Myr after the Sun arrived at the ZAMS. Depending on the solar wind parameters, our model simulations for early Venus show that ion pick up by strong solar wind from a non-magnetized planet could erode up to an equivalent amount of ∼250 bar of O⁺ ions during the first several hundred million years. This accumulated loss corresponds to an equivalent mass of ∼1 terrestrial ocean (TO (1 TO ∼1.39×10²⁴ g or expressed as partial pressure, about 265 bar, which corresponds to ∼2900 m average depth)). Finally, we discuss and compare our findings with the results of preceding studies.     

The Milky Way 3-Helium Abundance

We are making precise determinations of the abundance of the light isotope of helium, ³He. The ³He abundance in Milky Way sources impacts stellar evolution, chemical evolution, and cosmology. The abundance of ³He is derived from measurements of the hyperfine transition of ³He⁺ which has a rest wavelength of 3.46 cm (8.665 GHz). As with all the light elements, the present interstellar ³He abundance results from a combination of Big Bang Nucleosynthesis (BBNS) and stellar nucleosynthesis. We are measuring the ³He abundance in Milky Way H ii regions and planetary nebulae (PNe). The source sample is currently comprised of 60 H ii regions and 12 PNe. H ii regions are examples of zero-age objects that are young relative to the age of the Galaxy. Therefore their abundances chronicle the results of billions of years of Galactic chemical evolution. PNe probe material that has been ejected from low-mass (M≤ 2M _⊙) to intermediate-mass (M∼2–5M _⊙) stars to be further processed by future stellar generations. Because the Milky Way ISM is optically thin at centimeter wavelengths, our source sample probes a larger volume of the Galactic disk than does any other light element tracer of Galactic chemical evolution. The sources in our sample possess a wide range of physical properties (including object type, size, temperature, excitation, etc.). The ³He abundances we derive have led to what has been called “The ³He Problem”.     

Magnetic Reconnection Phenomena In Interplanetary Space

Interplanetary magnetic reconnection(IMR) phenomena are explored based on the observational data with various time resolutions from Helios, IMP-8, ISEE3, Wind, etc. We discover that the observational evidence of the magnetic reconnection may be found in the various solar wind structures, such as at the boundary of magnetic cloud, near the current sheet, and small-scale turbulence structures, etc. We have developed a third order accuracy upwind compact difference scheme to numerically study the magnetic reconnection phenomena with high-magnetic Reynolds number (R _M=2000–10000) in interplanetary space. The simulated results show that the magnetic reconnection process could occur under the typical interplanetary conditions. These obtained magnetic reconnection processes own basic characteristics of the high R _M reconnection in interplanetary space, including multiple X-line reconnection, vortex velocity structures, filament current systems, splitting, collapse of plasma bulk, merging and evolving of magnetic islands, and lifetime in the range from minutes to hours, etc. These results could be helpful for further understanding the interplanetary basic physical processes. This revised version was published online in August 2006 with corrections to the Cover Date.     

EUV Observations Above Polar Coronal Holes

We derive electron temperature and density as a function of height up to 0.2 R_⊙ above the limb in polar coronal holes, using five EUV data sets recorded by the SOHO Coronal Diagnostic Spectrometer between July 1997 and February 1998. Radial T and N distributions, averaged in a 2° to 10° range of position angles, are the same above the North and South coronal holes. They do not show any time variability over a period of seven months. Polar plumes are found to have lower electron temperature and higher density than the interplume lanes. The electron density slope suggests that the proton temperatures are twice as high as the electron temperatures. This revised version was published online in June 2006 with corrections to the Cover Date.