E. Marsch

FIRST RESULTS OF THE SUMER TELESCOPE AND SPECTROMETER ON SOHO – I. Spectra and Spectroradiometry

SUMER – the Solar Ultraviolet Measurements of the Emitted Radiation instrument on the Solar and Heliospheric Observatory (SOHO) – observed its first light on January 24, 1996, and subsequently obtained a detailed spectrum with detector B in the wavelength range from 660 to 1490 Å (in first order) inside and above the limb in the north polar coronal hole. Using detector A of the instrument, this range was later extended to 1610 Å. The second-order spectra of detectors A and B cover 330 to 805 Å and are superimposed on the first-order spectra. Many more features and areas of the Sun and their spectra have been observed since, including coronal holes, polar plumes and active regions. The atoms and ions emitting this radiation exist at temperatures below 2 × 106 K and are thus ideally suited to investigate the solar transition region where the temperature increases from chromospheric to coronal values. SUMER can also be operated in a manner such that it makes images or spectroheliograms of different sizes in selected spectral lines. A detailed line profile with spectral resolution elements between 22 and 45 mÅ is produced for each line at each spatial location along the slit. From the line width, intensity and wavelength position we are able to deduce temperature, density, and velocity of the emitting atoms and ions for each emission line and spatial element in the spectroheliogram. Because of the high spectral resolution and low noise of SUMER, we have been able to detect faint lines not previously observed and, in addition, to determine their spectral profiles. SUMER has already recorded over 2000 extreme ultraviolet emission lines and many identifications have been made on the disk and in the corona.SUMER – the Solar Ultraviolet Measurements of the Emitted Radiation instrument on the Solar and Heliospheric Observatory (SOHO) – observed its first light on January 24, 1996, and subsequently obtained a detailed spectrum with detector B in the wavelength range from 660 to 1490 A (in first order) inside and above the limb in the north polar coronal hole. Using detector A of the instrument, this range was later extended to 1610 A. The second-order spectra of detectors A and B cover 330 to 805 A and are superimposed on the first-order spectra. Many more features and areas of the Sun and their spectra have been observed since, including coronal holes, polar plumes and active regions. The atoms and ions emitting this radiation exist at temperatures below 2 × 106 K and are thus ideally suited to investigate the solar transition region where the temperature increases from chromospheric to coronal values. SUMER can also be operated in a manner such that it makes images or spectroheliograms of different sizes in selected spectral lines. A detailed line profile with spectral resolution elements between 22 and 45 mA is produced for each line at each spatial location along the slit. From the line width, intensity and wavelength position we are able to deduce temperature, density, and velocity of the emitting atoms and ions for each emission line and spatial element in the spectroheliogram. Because of the high spectral resolution and low noise of SUMER, we have been able to detect faint lines not previously observed and, in addition, to determine their spectral profiles. SUMER has already recorded over 2000 extreme ultraviolet emission lines and many identifications have been made on the disk and in the corona.

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

A model of the solar corona and wind is developed which includes for the first time the heating and acceleration effects of high-frequency Alfvén 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⊙.

A model of solar wind fluctuations with two components: Alfvén waves and convective structures

A two-component incompressible fluctuation model is presented to explain the radial evolution of the solar wind fluctuations. The basic idea is to consider the small-scale fluctuations in the solar wind as being composed of Alfven waves and convective structures. The major Alfven waves are believed to be created near the coronal base and to propagate outward along the magnetic field lines. The convective structures are defined as the small-scale variations perpendicular to the local magnetic field direction. They are either quasi-static or turbulent and slowly evolving in the plasma frame of reference. The small-scale perpendicular variations are connected, in the parallel direction, with large-scale magnetic field variations, which are convected by the solar wind as quasi-static structures during the wind expansion time. The decomposition of the original fluctuations can be done by using special space and time averages, which are defined by space averaging along the directions parallel and perpendicular to the local magnetic field vector and by time averaging in the plasma frame of reference. The equations of motion of the fluctuations and of the correlation functions for both Alfven waves and convective structures have been derived from the one-fluid MHD equations. A combination of the correlation functions of these two components is then used for a comparison with observational results. The influence of the angle between the sampling direction and the magnetic field vector on the final results has also been considered. As a first step to apply these equations, a simple model has been suggested that is based on the assumption that the fluctuations are only composed of outward propagating Alfven waves and static magnetic structures. For comparison with the observations, new statistical results from data obtained by Helios 1 during days 1–95, 1975, and Helios 2 during days 19–109, 1976, are presented. The numerical solutions are shown to describe well the basic evolution trend of the fluctuation energy, the normalized cross helicity, and the Alfven ratio. It is also shown that the basic physical process of the evolution of the convective structures is the convection of the fluctuating velocity vortex lines and the magnetic field lines by the expanding solar wind.

FIRST RESULTS OF THE SUMER TELESCOPE AND SPECTROMETER ON SOHO – II. Imagery and Data Management

SUMER – Solar Ultraviolet Measurements of Emitted Radiation – is not only an extreme ultraviolet (EUV) spectrometer capable of obtaining detailed spectra in the range from 500 to 1610 Å, but, using the telescope mechanisms, it also provides monochromatic images over the full solar disk and beyond, into the corona, with high spatial resolution. We report on some aspects of the observation programmes that have already led us to a new view of many aspects of the Sun, including quiet Sun, chromospheric and transition region network, coronal hole, polar plume, prominence and active region studies. After an introduction, where we compare the SUMER imaging capabilities to previous experiments in our wavelength range, we describe the results of tests performed in order to characterize and optimize the telescope under operational conditions. We find the spatial resolution to be 1.2 arc sec across the slit and 2 arc sec (2 detector pixels) along the slit. Resolution and sensitivity are adequate to provide details on the structure, physical properties, and evolution of several solar features which we then present. Finally some information is given on the data availability and the data management system.

Slow-mode standing waves observed by SUMER in hot coronal loops

We report the first detection of postflare loop oscillations seen in both Doppler shift and intensity. The observations were recorded in an Fe xix line by the SUMER spectrometer on SOHO in the corona about 70 min after anM-class flare on the solar limb. The oscillation has a period of about 17 min in both the Doppler velocity and the intensity, but their decay times are different (i.e., 37 min for the velocity and 21 min for the intensity). The fact that the velocity and the intensity oscillations have exactly a 1/4-period phase difference points to the existence of slow-mode standing waves in the oscillating loop. This interpretation is also supported by two other pieces of evidence: (1) the wave period and (2) the amplitude relationship between the intensity and velocity are as expected for a slow-mode standing wave.

Evidence for pitch angle diffusion of solar wind protons in resonance with cyclotron waves

Quasi-linear theory predicts that ions in resonance with transverse ion cyclotron waves suffer merely pitch angle diffusion while conserving their total kinetic energy in the frame moving with the wave phase speed. For the first time, direct observational evidence from Helios plasma data is shown for the occurrence of this pitch angle diffusion of solar wind protons, induced by resonance with parallel ion cyclotron waves propagating away from the Sun. Parts of the isodensity contours in velocity space are well outlined by a sequence of segments of circles centered at the adapted wave phase speed, which is assumed to vary slightly and to be due to dispersion smaller than the local Alfven speed. This observation confirms the validity of basic concepts of resonant wave-particle interactions as described by quasi-linear theory. The solar wind proton velocity distributions show a “plateau” defined by a vanishing pitch angle gradient in the resonant regime, implying marginal stability of the distribution function. The implications of these results for solar wind ion heating and kinetic transport are discussed.

On the nature of compressive fluctuations in the solar wind

A statistical analysis of the amplitudes of the thermal pressure and total pressure of the solar wind and of several related cross correlations between different compressive parameters has been performed with the plasma and magnetic field data obtained by Helios 1 and 2 in their primary missions. The statistical analysis is based on small-band averages of the relevant spectra over the frequency range (2-5) × 10−4 Hz, corresponding to hourly timescales. The analysis shows that the theoretical values, given by the relation between the fluctuation of total pressure and density for perpendicular fast magnetoacoustic waves, present an upper limit for the observed amplitudes of the normalized total pressure fluctuations. With decreasing ratio between the fluctuation amplitudes of the total pressure and the density, we found a systematical decrease of the correlation coefficient between density and total pressure and of the correlation coefficient between density and magnetic field magnitude. Decreases of the correlation coefficients between temperature and density and between thermal and magnetic pressure are also found with decreasing ratio of the normalized amplitudes of the fluctuations of the thermal pressure and the temperature. For high-speed wind data the pattern of the data distribution in plots of one correlation coefficient versus the other correlation coefficient shows some systematical changes. Most of these results can be explained qualitatively by a model based on a superposition of small-amplitude perpendicular fast magnetosonic waves and small-amplitude pressure-balanced structures. We have found a class of data points which seem to represent fluctuations dominated by fast magnetosonic waves. In many cases in low-speed wind the correlations between density and total pressure and between temperature and density are both negative, while the correlation between temperature and magnetic magnitude is about zero. The nature of this phenomenon has not yet been clearified. Some possible explanations are suggested. The applicability and relevance of the nearly incompressible magnetohydrodynamics theory for the compressive fluctuations in the inner heliosphere are also discussed.

Heating and acceleration of coronal ions interacting with plasma waves through cyclotron and Landau resonance

On the basis of quasi-linear theory, the parallel and perpendicular wave heating and acceleration rates for gyrotropic particle velocity distribution functions are derived. These rates can be used in anisotropic multicomponent fluid equations, in order to describe the wave-particle interactions of ions with, for examples, kinetic Alfven and electromagnetic or electrostatic ion cyclotron, respectively, magnetosonic waves propagating along or obliquely to the mean magnetic field. The waves of coronal origin propagating away from the Sun into the interplanetary medium can resonantly heat the solar wind ions and accelerate minor ions preferentially with respect to the protons. Such processes are required in order to explain and understand the measured characteristics of ion velocity distributions in the solar wind and to interpret the recent spectroscopic evidence obtained from EUV emission line measurements made by the Solar and Heliospheric Observatory (SOHO) spacecraft, which indicate cyclotron-resonance-related line broadenings and shifts.

Coronal plasma flows and magnetic fields in solar active regions: Combined observations from SOHO and NSO/Kitt Peak

During the early days of the SOHO mission, SUMER observed a few active regions (ARs) connected with sunspots on the Sun and took their images and spectra in various EUV emission lines. In addition to these spectroscopic data magne- tograms of the photospheric footpoint regions of the AR loops were available from the MDI on SOHO and the National Solar Observatory/Kitt Peak (NSO/KP), data which here are used to construct the coronal magnetic field of the ARs by force-free- field extrapolation. The combined data set is analysed with respect to the large-scale circulation of coronal matter, which means that the Dopplershifts of various lines used as tracers of the plasma flow are investigated in close connection with the ambient magnetic field, which is found to be either closed or open in the coronal volume considered. The Dopplershift pattern is found to be clearly linked with the field topology, and several regions of strong velocity shear are identified. We also estimate the coronal currents. We discuss the results of this mainly phenomenological correlative study with the perspective to understand coronal heating and mass supply to the extended corona, and with respect to the role played by the field in guiding and constraining plasma flows.

The solar origin of corotating interaction regions and their formation in the inner heliosphere, Report of Working Group 1

Corotating Interaction Regions (CIRs) form as a consequence of the compression of the solar wind at the interface between fast speed streams and slow streams. Dynamic interaction of solar wind streams is a general feature of the heliospheric medium; when the sources of the solar wind streams are relatively stable, the interaction regions form a pattern which corotates with the Sun. The regions of origin of the high speed solar wind streams have been clearly identified as the coronal holes with their open magnetic field structures. The origin of the slow speed solar wind is less clear; slow streams may well originate from a range of coronal configurations adjacent to, or above magnetically closed structures. This article addresses the coronal origin of the stable pattern of solar wind streams which leads to the formation of CIRs. In particular, coronal models based on photospheric measurements are reviewed; we also examine the observations of kinematic and compositional solar wind features at 1 AU, their appearance in the stream interfaces (SIs) of CIRs, and their relationship to the structure of the solar surface and the inner corona; finally we summarise the Helios observations in the inner heliosphere of CIRs and their precursors to give a link between the optical observations on their solar origin and the in-situ plasma observations at 1 AU after their formation. The most important question that remains to be answered concerning the solar origin of CIRs is related to the origin and morphology of the slow solar wind.