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NASA NTRS2002

A New Diagnostic Technique for the Solar Corona

Over the last 30-40 years spectroscopic observation of the EUV (extreme ultraviolet) line emission has proved invaluable as a diagnostic of the solar coronal plasma state. Line ratios have been used to determine electron density, electron temperature and ion flow velocity. In this paper, we present results obtained with a new measurement technique that uses spectroscopic observations of the white light corona to obtain the electron density, temperature, and flow velocity. A prototype instrument has been designed and built to obtain visible light spectra (3800-4300 A) with modest resolution. This instrument was used to obtain coronal observations during the June 2001 eclipse in Zambia. The data were corrected for sky and instrument transmission to derive the electron temperature and flow speed. Results from these measurements will be discussed.

Nelson, R.↗
NASA NTRS1986

Corona and solar wind

The Pinhole/Occulter Facility is a powerful tool for studying the physics of the extended corona and origins of the solar wind. Spectroscopic data acquired by the P/OF coronal instruments can greatly expand empirical information about temperatures, densities, flow velocities, magnetic fields, and chemical abundances in the corona out to r or approx. 10 solar radii. Such information is needed to provide tight empirical constraints on critical physical processes involved in the transport and dissipation of energy and momentum, the heating and acceleration of plasma, and the acceleration of energetic particles. Because of its high sensitivity, high spatial and temporal resolutions, and powerful capabilities for plasma diagnostics, P/OF can significantly increase our empirical knowledge about coronal streamers and transients and thereby advance the understanding of the physics of these phenomena. P/OF observations can be used to establish the role in solar wind generation, if any, of small-scale dynamical phenomena, such as spicules, macrospicules and coronal bullets, and the role of the fine-scale structures, such as polar plumes. Finally, simultaneous measurements by the P/OF coronal and hard X-ray instruments can provide critical empirical information concerning nonthermal energy releases and acceleration of energetic particles in the corona.

Withbroe, G. L.↗
NASA NTRS1991

Simultaneous Solar Maximum Mission (SMM) and Very Large Array (VLA) observations of solar active regions

Very Large Array observations at 20 cm wavelength can detect the hot coronal plasma previously observed at soft x ray wavelengths. Thermal cyclotron line emission was detected at the apex of coronal loops where the magnetic field strength is relatively constant. Detailed comparison of simultaneous Solar Maximum Mission (SMM) Satellite and VLA data indicate that physical parameters such as electron temperature, electron density, and magnetic field strength can be obtained, but that some coronal loops remain invisible in either spectral domain. The unprecedent spatial resolution of the VLA at 20 cm wavelength showed that the precursor, impulsive, and post-flare components of solar bursts originate in nearby, but separate loops or systems of loops.. In some cases preburst heating and magnetic changes are observed from loops tens of minutes prior to the impulsive phase. Comparisons with soft x ray images and spectra and with hard x ray data specify the magnetic field strength and emission mechanism of flaring coronal loops. At the longer 91 cm wavelength, the VLA detected extensive emission interpreted as a hot 10(exp 5) K interface between cool, dense H alpha filaments and the surrounding hotter, rarefield corona. Observations at 91 cm also provide evidence for time-correlated bursts in active regions on opposite sides of the solar equator; they are attributed to flare triggering by relativistic particles that move along large-scale, otherwise-invisible, magnetic conduits that link active regions in opposite hemispheres of the Sun.

Willson, Robert F.↗
NASA NTRS1982

Probing the solar wind acceleration region using spectroscopic techniques

Recent developments in the use of UV and EUV coronagraphic spectroscopy for studying the physical conditions in the solar corona in the solar wind acceleration region from a heliocentric distance of 1.8 solar radii out to 8 solar radii and beyond are reviewed. Particular attention is given to theoretical considerations and techniques for the estimation of particle ionization balances and velocity distributions, coronal temperatures, neutral hydrogen, proton, electron and ion temperatures and densities, outflow velocities, charge states and chemical abundances from spectral line radiation and white-light measurements. Observational considerations are discussed as they relate to requirements for stray light rejection and the effects of geocorona and interplanetary dust emission in coronagraph design. Results of coronal H I Lyman alpha and white light observations made during sounding rocket flights are summarized, and future observational programs making use of rocket-borne instruments, Detached Shuttle Payload Flights, Spacelab or space platforms are indicated.

Withbroe, G. L.↗
NASA NTRS1998

Can Kelvin-Helmholtz Instabilities of Jet-Like Structures and Plumes Cause Solar Wind Fluctuations at 1AU

During its south polar passage in 1994, Ulysses sampled the solar wind emerging from the south polar coronal hole. In analysing these data two types of density fluctuations lasting a few hours have been reported, one characterized by fluctuations in velocity gradients ("microstreams") and the other by magnetic plus thermal-pressure balance structures ("PBS"). Microstreams were both temporal and spatial in nature. At higher frequencies, MHD turbulence was observed and found to be less evolved than in the ecliptic, but essentially independent of heliographic latitude. It is argued here that microstreams, PBS, and MHD turbulence could all be the remnant of shears associated with plumes and other filamentary structures ("jets") which have been reported to exist in coronal holes. The shear between a jet and its ambient can become unstable to the MHD Kelvin-Helmholtz ("KH') instability at 5-10 solar radius and the propagating instability can cause fluctuations like those seen by Ulysses. This motivates us to simulate coronal jets using a 3D MHD ZEUS code. The first 2D results have just started to come and are promising. To study the KH instability the jet is perturbed at the boundary with a linear amplitude and fixed frequency. The jet seems to pass through various distinct phases, one of which Is apparently dominated by KH instabilities. These instabilities drive oblique shocks into the jet as the turbulent eddies contact the jet surface. It is known that KH instabilities and internal shock waves are partially suppressed by magnetic field tension. Hence, in simulating far along the jet the Instability is expected to produce Alfvenic fluctuations like those seen near 1 AU.

Parhi, Shyamsundar↗
NASA NTRS1982

Coronal holes - Mass loss driven by magnetic reconnection

A nonthermal mechanism for driving coronal hole mass loss is proposed. Three empirical results are noted, namely (1) that polar plumes with short-lived X-ray bright points (XBP) at their base are sites of matter flow sufficient to account for the total solar mass flux, (2) that solar wind densities are positively correlated with the number of XBP in coronal holes, and (3) that XBP are associated with newly emerged magnetic flux. It is noted that since the dynamical development following the onset of nonequilibrium is a hitherto unsolved problem, subsequent features in the scenario described here are relatively speculative. It is proposed that bubbles of matter ejected from magnetic reconnection sites in polar plumes drive the solar wind in coronal holes.

Mullan, D. J.↗
NASA NTRS1978

Physical parameters defining the changing structure of a coronal hole

The S-056 X-ray data have been used to determine significant quantitative values for physical parameters defining the changing structure of a coronal hole. A new active region (McMath 12363) developed near a large coronal hole late on 1973 June 1. As the new bipolar region developed, a distinct decrease (channel) occurred in a nearby X-ray emission source. Quantitative values are given for the change in X-ray flux, average electron density, and temperature in the channel, and these variations are related to the corresponding photospheric and coronal magnetic fields in the region. The observations show that the decrease in X-ray flux resulted from a reduction in electron density rather than from cooling. The study suggests that changes in a coronal hole may be explained by the loss of material along weakened, less-confining magnetic field lines in the corona.

Vorpahl, J. A.↗
NASA NTRS1975

Partial analysis of the flare-prominence of 30 April 1974

A portion of an east limb flare-prominence observed in H-alpha light is analyzed. Following rapid achievement of a maximum mass-ejection velocity of about 375 km/s, the ascending prominence reached a height of at least 200,000 km. A one-dimensional time-dependent hydrodynamic theory is used to compute the total mass and energy ejected during this part of the event. Theoretical aspects of the coronal response are discussed. It is concluded that a moderate temperature and density pulse (factors of ten and two, respectively) for a duration of only 3 min is sufficient for an acceptable simulation of the H-alpha observations and the likely coronal response to the ascending prominence and flare-related ejections.

Wu, S. T.↗
NASA NTRS1995

Some constraints on acceleration mechanisms in the solar wind at polar latitudes

Observations of solar Lyman alpha have been interpreted as indicating that the polar mass flux density is lower than the equatorial average. This led Lallement et al (1986) to make a parametric study of solar wind acceleration, along the lines of earlier the Munro-Jackson study (1977), in which they concluded that uncertainties in the polar mass flux were large enough to be consistent with two extreme opposites: (1) a substantial energy supply beyond classical thermal conduction is required; or (2) classical thermal conduction is adequate to drive the flow. This ambiguity has been clarified by Ulysses observations of the polar outflow (Phillips et al, 1994). The polar mass flux density lies in the middle of the range studies by Lallement et al (1986), which suggests that extended heating is going on out to at least approximately 5 AU. Independent, purely energetic arguments can be made to estimate the required coronal source (electron) temperature that would be required to account for the observed energy flux density. An electron temperature of at least 2 x 10(exp 6) K would be required for the classical conduction flux density to be comparable to the total energy flux density; such a high temperature is thought to be unlikely in a coronal hole. These arguments strongly suggest that some extended heating or momentum transfer mechanism is required to drive the solar wind from the polar coronal hole. A number of mechanisms are discussed.

Barnes, A.↗
NASA NTRS1979

First spectrum of an extra-solar object in the extreme ultraviolet The white dwarf HZ 43

An EUV instrument is described which has been used to carry out a spectroscopic observation of the hot white dwarf HZ 43. The instrument consists of an EUV telescope and spectrometer housed in a sounding-rocket shell 44 cm in diameter and 176 cm in length. It is noted that HZ 43 was successfully observed for 200 sec and that the EUV spectrum was strongly detected from 170 to 400 A. A second-order image was detected beyond 400 A, and a decrement was observed at 200 A, which corresponds to photoelectric absorption of He II. The observed spectrum is shown to be inconsistent with a coronal model. It is concluded that the surface number density of helium relative to hydrogen must be in the range from 0.00001 to 0.0001 and that an upper limit of 4 x 10 to the 17th per sq cm can be placed on the column density of ionized helium in the intervening interstellar medium.

Malina, R. F.↗
NASA NTRS1980

Physical conditions in the solar atmosphere above an active region

From a series of EUV spectra obtained at several heights above the limb in a solar active region, the volume emission measure is derived as a function of the electron temperature in the temperature range 70,000-1,500,000 K and the electron density at two locations. The emission measure from the coronal material (temperature greater than 700,000 K) is nearly the same everywhere and represents most of the material in the line of sight, while the emission measure from the transition region material (temperature between 70,000 and 250,000 K) fluctuates by two orders of magnitude from position to position above the active region. This is in agreement with the picture of this active region as consisting of a number of well-defined loops or lower portions of loops at transition region temperatures that are inhomogeneously distributed in much larger and more diffuse loop structures at coronal temperatures. The coronal data are in reasonable agreement with simple coronal models. Emission measures near 1,000,000 K evaluated using different ions differ by a factor of 4, suggesting difficulties with the atomic physics data.

Mariska, J. T.↗
NASA NTRS1983

The properties of coronal voids

Skylab coronagraph observations of the outer solar corona have revealed dark, ray-like structures whose identification as a true coronal phenomenon rather than a subtle photographic effect is presently undertaken. Photometric analysis indicates that these features represent a reduction of 5% in coronal radiance, and the use of a previously developed model of the coronal electron component allows the specification of void electron densities over a 2.5-4.5 solar radius range. Projection of the phenomena onto synoptic surface maps indicates a close relationship with filaments and neutral lines, and a potentially significant temporal correlation between void formation and the formation of the underlying prominence is noted.

Macqueen, R. M.↗
NASA NTRS1984

Flare parameters for the 7 September, 1973 two-ribbon flare

A study is made of the relative importance of the various energy loss mechanisms for the long-decay event of September 7, 1973, using spectral scans in the 400 A-1335 A range. This spectral range contains many of the important electron density and temperature diagnostic line ratios for the solar transition zone. Earlier analyses of the flare energy budget are refined, using more detailed emission measure curves and density diagnostics. The constant pressure assumptions used in both coronal loop models and in the interpretation of observations in terms of flare energetics are examined. It is found that much of the upper transition region emission originates in cooling loops. Radiative losses are found to dominate.

Doyle, J. G.↗
NASA NTRS1984

Observations of Absorption Lines from Highly Ionized Atoms

In the ultraviolet spectra of hot stars, absorption lines can be seen from highly ionized species in the interstellar medium. Observations of these features which have been very influential in revising the perception of the medium's various physical states, are discussed. The pervasiveness of O 6 absorption lines, coupled with complementary observations of a diffuse background in soft X-rays and EUV radiation, shows that there is an extensive network of low density gas (n approx. fewX 0.001/cucm) existing at coronal temperatures, 5.3 or = log T or = 6.3. Shocks created by supernova explosions or mass loss from early-type stars can propagate freely through space and eventually transfer a large amount of energy to the medium. To create the coronal temperatures, the shocks must have velocities in excess of 150 km/sec; shocks at somewhat lower velocity 9v or = 100 km/sec) can be directly observed in the lines of Si3. Observations of other lines in the ultraviolet, such as Si 4V and C 5, may highlight the widespread presence of energetic uv radiation from very hot, dward stars. More advanced techniques in visible and X-ray astronomical spectroscopy may open up for inspection selected lines from atoms in much higher stages of ionization.

Jenkins, E. B.↗
NASA NTRS1985

Theoretical model of the solar corona during sunspot minimum. II - Dynamic approximation

The theoretical quasi-static model of the solar corona during sunspot minimum developed in an earlier paper (Osherovich et al., 1984) is extended to include a quasi-radial outflow. The requirement imposed in the above paper on a plasma flow is relaxed. The dynamic equation derived for the combination of a dipole-like and a radial field is used to calculate the electric current density around the sun in the region 1.5-5 solar radii. Comparison with the current density given by the quasi-static model shows that the outflow decreases the current density only slightly in the dynamic case. The dynamic coronal model with a magnetic quadrupole field is also considered, in relation to the north-south asymmetry in the solar corona.

Osherovich, V. A.↗
NASA NTRS1997

Study of a Solar X-Ray Telescope

The highly structured nature of the outer solar atmosphere seems to be intimately linked to the presence, at the solar surface, of magnetic fields that have been generated inside the Sun and have emerged to the surface. The corona is brightest (and also hottest) at just those locations where the magnetic field has emerged from inside the Sun. Dynamo theory predicts that strong magnetic fields will be generated deep in the solar interior and that bundles or 'ropes' of magnetic flux will float to the surface. When this happens, a magnetically bipolar region will become visible, extending above the surface in a three-dimensional structure. The field lines penetrate through the surface, showing two magnetic poles, and also exhibit a three-dimensional structure above the surface. The structure created by the field emergence is rooted in the (relatively) cool photosphere and extends through the chromosphere and transition region to the corona. Thus, the magnetic field creates a region, called an active region, which contains portions at temperatures from less than 10(exp 4) K to greater than 10(exp 6) K, and is therefore visible at wavelengths from the infrared through x-rays. The locations where the magnetic field leaves and reenters the visible surface are called the 'footpoints' of the coronal structures associated with the magnetic field. The magnetic fields themselves are not directly visible. However, the hot coronal plasma is, for the most part, constrained to follow the direction of the magnetic field lines in the atmosphere. Now, 100 years after the discovery of x-rays by Wilhelm Roentgen in 1896, we can routinely make observations of the solar corona from outside the Earth's atmosphere in this region of the electromagnetic spectrum. As shown by comparing x-ray images with magnetograms, the bright corona over these bipolar magnetic regions consists of closed structures that seem to follow the orientation of the magnetic field. Although we can see down to the photosphere at x-ray wavelengths when observing the disk of the Sun, this part of the solar atmosphere emits so little that far from the peak of its Planck curve it appears dark in x-ray images. This impression of hot plasma following the magnetic field direction is further strengthened by quantitative studies that calculate coronal magnetic field strength and direction based on photospheric measurements and compare them with the observed brightness and location of the x-ray emitting structures. Such comparisons make it clear that, for the most part, the hot plasma conforms to the geometry of the magnetic field and that the coronal brightness is strongly linked to the strength of the magnetic fields which have erupted to the solar surface from the interior. It is also the case that the larger-scale, fainter corona, as well as coronal holes, are strongly influenced by the large-scale solar magnetic field. We may get a small hint of the reason that the coronal plasma outlines the direction of B by examining the thermal conductivity of a hot plasma in the presence of a magnetic field. This quantity has enormously different values in the directions parallel and perpendicular to the field for a coronal temperature of 10(exp 6) K, a particle density of 10(exp 9)/cu cm and a magnetic field strength of 100 G, the conductivity along the field is approximately 13 orders of magnitude greater than that perpendicular to the field. It is therefore not surprising that a parcel of plasma which is locally heated would conduct that heat preferentially in the direction of the field. We also note that the thermal conductivity parallel to the magnetic field increases with temperature T, while the perpendicular conductivity decreases. To the extent that the loop aspect ratio, i.e., the ratio of loop length to loop width, is determined by the thermal conductivity, we would expect that higher temperature loops are longer and thinner than cooler ones. However, if the loop width becomes smaller than the spatial resolution of the observing instrument, this effect will not be directly observable. For organizational purposes, we provide a listing of some scientific objectives for a Solar-B x-ray telescope, arranged in terms of identifiable features in the corona.

Golub, Leon↗