Chromospheric Network Research Articles

Morphology of the Quiet Solar Upper Atmosphere in the [FORMULA][F]4×10[SUP]4[/SUP]<T[INF]e[/INF]<1.4×10[SUP]6[/SUP][/F][/FORMULA] K Temperature Regime

Studies on the morphology of the solar upper atmosphere began over three decades ago. Early models assumed that the temperature structure of the solar upper atmosphere was continuous with a thin transition region connecting the chromosphere with the corona. Over the years it became apparent that the original depiction of the solar upper atmosphere was too simplistic. In this paper we present a morphological study of the solar upper atmosphere over a wide range of temperatures (4 × 104 ≤ Te ≤ 1.4 × 106 K) using high-resolution images (1''-2'') taken by TRACE, the SUMER spectrometer on SOHO, and the NRL spectroheliograph on Skylab. The images clearly show that the 4 × 104 ≤ Te ≤ 1.4 × 106 K temperature domain of the solar upper atmosphere consists of a hierarchy of isothermal loop structures. While at the Te < 8 × 105 K temperature regime the looplike structures are more abundant along the chromospheric network, at higher temperatures (Te > 9 × 105 K) no association between them and the chromospheric network is apparent. The hottest (Te ≈ 1.4 × 106 K), which are also the longest among the quiet-Sun loop structures, form a canopy over the lower temperature loop structures. We discuss in the paper possible relationships between the morphology of the solar upper atmosphere, its elemental abundance, and ideas regarding the origin of the slow-speed solar wind.

Two‐Component Theoretical Chromosphere Models for K Dwarfs of Different Magnetic Activity: Exploring the CaiiEmission–Stellar Rotation Relationship

We compute two-component theoretical chromosphere models for K2 V stars with diUerent levels of magnetic activity. The two components are a nonmagnetic component heated by acoustic waves and a magnetic component heated by longitudinal tube waves. The —lling factor for the magnetic component is determined from an observational relationship between the measured magnetic area coverage and the stellar rotation period. We consider stellar rotation periods between 10 and 40 days. We investigate two diUerent geometrical distributions of magnetic —ux tubes: uniformly distributed tubes, and tubes arranged as a chromospheric network embedded in the nonmagnetic region. The chromosphere models are constructed by performing state-of-the-art calculations for the generation of acoustic and magnetic energy in stellar convection zones, the propagation and dissipation of this energy at the diUerent atmo- spheric heights, and the formation of speci—c chromospheric emission lines that are then compared to the observational data. In all these steps, the two-component structure of stellar photospheres and chromospheres is fully taken into account. We —nd that heating and chromospheric emission is signi—- cantly increased in the magnetic component and is strongest in —ux tubes that spread the least with height, expected to occur on rapidly rotating stars with high magnetic —lling factors. For stars with very slow rotation, we are able to reproduce the basal —ux limit of chromospheric emission previously identi- —ed with nonmagnetic regions. Most importantly, however, we —nd that the relationship between the Ca II H)K emission and the stellar rotation rate deduced from our models is consistent with the relationship given by observations. Subject headings: line: formationMHDstars: activitystars: chromospheres ¨ stars: late-typestars: rotation

Effect of suprathermal electrons on the intensity and Doppler frequency of electron plasma lines

Abstract. In an incoherent scattering radar experiment, the spectral measurement of the so-called up- and downshifted electron plasma lines provides information about their intensity and their Doppler frequency. These two spectral lines correspond, in the backscatter geometry, to two Langmuir waves travelling towards and away from the radar. In the daytime ionosphere, the presence of a small percentage of photoelectrons produced by the solar EUV of the total electron population can excite or damp these Langmuir waves above the thermal equilibrium, resulting in an enhancement of the intensity of the lines above the thermal level. The presence of photo-electrons also modifies the dielectric response function of the plasma from the Maxwellian and thus influences the Doppler frequency of the plasma lines. In this paper, we present a high time-resolution plasma-line data set collected on the Eiscat VHF radar. The analysed data are compared with a model that includes the effect of a suprathermal electron population calculated by a transport code. By comparing the intensity of the analysed plasma lines data to our model, we show that two sharp peaks in the electron suprathermal distribution in the energy range 20-30 eV causes an increased Landau damping around 24.25 eV and 26.25 eV. We have identified these two sharp peaks as the effect of the photoionisation of N2 and O by the intense flux of monochromatic HeII radiation of wavelength 30.378 nm (40.812 eV) created in the chromospheric network and coronal holes. Furthermore, we see that what would have been interpreted as a mean Doppler drift velocity for a Maxwellian plasma is actually a shift of the Doppler frequency of the plasma lines due to suprathermal electrons.Key words. Ionosphere (electric fields and currents; solar radiation and cosmic ray effects)

The Chromospheric Network – (A) Network Evolution Viewed as a Diffusion Process

Autocorrelation and cross-correlation techniques have been applied to obtain quantitative information about the dynamics of magnetic flux on the solar surface. The speed of network magnetic elements and the diffusion coefficient associated with their random motion is derived. The speed is found to be about 0.1 km s−1, independent of activity level. However, the diffusion coefficient shows a strong activity dependence: it is about 370–500 km2 s−1in the quiet network and 135–210 km2 s−1in the enhanced network. It is found that the lifetime of the enhanced network relative to the quiet network is compatible with that suggested by a comparison of their respective diffusion coefficients. This supports the proposition that a diffusion-like dispersion of magnetic flux is the dominant factor in the large-scale, long-term evolution of the network.

Effect of suprathermal electrons on the intensity and Doppler frequency of electron plasma lines

In an incoherent scattering radar experiment, the spectral measurement of the so-called up- and downshifted electron plasma lines provides information about their intensity and their Doppler frequency. These two spectral lines correspond, in the backscatter geometry, to two Langmuir waves travelling towards and away from the radar. In the daytime ionosphere, the presence of a small percentage of photoelectrons produced by the solar EUV of the total electron population can excite or damp these Langmuir waves above the thermal equilibrium, resulting in an enhancement of the intensity of the lines above the thermal level. The presence of photo-electrons also modifies the dielectric response function of the plasma from the Maxwellian and thus influences the Doppler frequency of the plasma lines. In this paper, we present a high time-resolution plasma-line data set collected on the Eiscat VHF radar. The analysed data are compared with a model that includes the effect of a suprathermal electron population calculated by a transport code. By comparing the intensity of the analysed plasma lines data to our model, we show that two sharp peaks in the electron suprathermal distribution in the energy range 20-30 eV causes an increased Landau damping around 24.25 eV and 26.25 eV. We have identified these two sharp peaks as the effect of the photoionisation of N2 and O by the intense flux of monochromatic HeII radiation of wavelength 30.378 nm (40.812 eV) created in the chromospheric network and coronal holes. Furthermore, we see that what would have been interpreted as a mean Doppler drift velocity for a Maxwellian plasma is actually a shift of the Doppler frequency of the plasma lines due to suprathermal electrons.Key words. Ionosphere (electric fields and currents; solar radiation and cosmic ray effects)

The chromospheric network: Dependence of cell lifetime on length-scale

A time series of Ca ii K filtergrams of the chromosphere was used to study the relationship between correlation lifetimes and autocorrelation length scales. The form of the lifetime-scale relation is inferred by comparing the distribution of the two parameters. A linear dependence of lifetime on cell area, with a least squares fit slope of 3.34×107 km2 hr−1, is deduced. The relation can be explained by assuming the network evolves by means of a diffusion process of the magnetic elements.

Five-minute magnetic field fluctuations in the solar wind acceleration region

A spectral analysis of coronal Faraday rotation (FR) data obtained with the linearly polarized signals of the two Heliosspacecraft reveals that about one-third of the temporal FR spectra contain a distinct spectral line superposed onto the background power-law spectrum. The most prevalent frequency of this quasi-harmonic component (QHC) is about 4 mHz, corresponding to a 4–5 min periodic oscillation of the coronal magnetic field. Physical reasons for the existence of QHC Alfven fluctuations in the inner solar wind are discussed. FR fluctuations (FRF) are considered to arise from both a turbulent background as well as an isolated Alfven wave train of finite extent and duration. An estimate can be made for the conditions under which the isolated wave train is observed above the ever present background. It is shown that the wave train must have a sufficiently long duration and transverse wavelength. It is suggested that the QHC at periods near 4–5 min in the FRF spectra are most probably produced by outward-propagating Alfven waves excited initially in the anisotropic structures of the chromospheric network.

On the Geometrical Properties of the Chromospheric Network

A sequence of Ca-K images obtained in a period of minimum solar activity, from July to November 1996, at the Rome Observatory with the PSPT (Precision Solar Photometric Telescope) prototype instrument have been used to analyze the geometrical properties of cells identified by the chromospheric network. In particular, we used 256 × 256 sub-arrays of the calibrated full-disk PSPT images. These sub-arrays, centered on the solar disk, are reduced to two-levels (binary) images by means of a suitable threshold after an FFT high-pass filtering. A medial axis transform, better known as skeleton, combined with a cellular automaton, is applied to the two-level images, in order to derive the cell boundaries. The regions corresponding to the cells are then filled by a growing algorithm. In this way we can derive a set of output parameters describing the cells geometry. The size distribution of the identified cells shows a continuous increase toward the smaller scales, rather than a small dispersion around a characteristic scale. Nevertheless the analysis of the inter-cell distances and of the area distribution pointed out a characteristic scale (square root of the area) of ± 24 Mm. To describe the cells irregularity and to probe the nature of solar turbulence, we apply a Mandelbrot fractal analysis to such irregularly shaped features. Examining the cell perimeter–area relationship we found the existence of a ‘critical’ area at which a change in the geometrical properties occurs. This area corresponds to the scale of ± 24 Mm. The estimated fractal dimension for cells with area greater than the ‘critical’ one is 1.35. This value, close to that predicted for isobars in the Kolmogorov 3-D turbulent theory, does not exclude a turbulent origin for such cells. The analysis seems to point to a common origin for solar granulation and supergranulation.

The Darkest Regions Of Solar Polar Coronal Holes Observed By Sumer On Soho

The plasma conditions above solar coronal holes are of considerable interest, because it is well established that the high-speed solar wind streams originate in these regions (see, e.g., Krieger et al., 1973; Geiss et al., 1995a; Woch et al., 1997), and the acceleration processes of the fast solar wind thus can be studied there. The most prominent features above polar coronal holes are polar plumes or rays (van de Hulst, 1950a,b; Saito, 1965; Newkirk and Harvey, 1968; Ahmad and Withbroe, 1977; Antonucci et al., 1997; Wang et al., 1997; DeForest et al., 1997; Wilhelm et al., 1998a). The debate whether plumes contribute to the fast solar wind streams is not yet settled. Of crucial importance is the elemental abundance inside and outside plumes. Widing and Feldman (1989, 1992, 1993) deduced a very low neon-to-magnesium ratio relative to the photosphere in a bright plume. As, on the other hand, abundances measured in fast solar wind streams by in situ observations are close to those of the photosphere (e.g., Geiss et al., 1995b), this is an argument against significant contributions to the fast wind by plumes. Wilhelm and Bodmer (1998) found low neon-to-magnesium abundance ratios inside plumes with respect to inter-plume lanes, a distinction that is not seen in the fast solar wind. It thus seems appropriate to assume that the fast wind does indeed emanate from the darkest regions of solar coronal holes. The study of inter-plume lanes is hampered by the fact that the relatively bright plumes dominate the EUV emission inside the northern and southern plume assemblies. East and west of these assemblies, very dark regions of the corona could be seen on EIT/SOHO images during the minimum of solar activity. Their investigation is difficult, because of the faint ultraviolet emission, but the danger of any plume “contamination” is small, and the very fact that these regions are so dark might indicate that the energy delivered by the chromospheric network to the corona is transformed predominantly into the acceleration of the solar wind and not into EUV and UV radiation.

Oscillations in Chromospheric Network Bright Points

Intensity oscillations observed in the H and K lines of Ca II in network bright points in the quiet Sun are interpreted in terms of transverse and longitudinal magnetoacoustic waves propagating upward inside magnetic flux tubes. It is supposed that the waves are generated impulsively in the photosphere as transverse waves. As they propagate upward, their velocity amplitude increases exponentially until they become nonlinear in the chromosphere, where they transfer power to longitudinal waves. The impulsive generation produces waves at the cutoff frequency of transverse waves. On the assumption that this frequency signature is transferred to the longitudinal waves, the magnetic field strength implied by the period observed in the chromosphere is consistent with the Zeeman effect observed in the photosphere.

The Distribution of Cell Sizes of the Solar Chromospheric Network

This paper studies the cellular pattern of the supergranular network. We present an algorithm to draw a surface-filling cell pattern on an uninterrupted two-day sequence of Ca II K filtergrams with a 1 nm bandpass. The 60° × 40° field of view contains both quiet and enhanced network and plages. The algorithm uses a threshold-independent method of steepest descent on spatially smoothed and time-averaged images. We determine the distribution function of cell areas and find a broad, asymmetric spectrum of areas. The distribution is found to be invariant for different spatial smoothings if the cell areas are normalized to a unit mean. It is this invariance that leads us to believe we have determined the intrinsic distribution of cell areas. Extrapolation of the average cell size to zero spatial smoothing yields a characteristic cell diameter of L = 13-18 Mm. This is roughly half the generally quoted supergranular length scale L ≈ 32 Mm as determined with autocorrelation methods. The difference in characteristic cell size reflects the application of a different measurement method: the autocorrelation method as used by Simon & Leighton and others is preferentially weighted towards relatively large cells. We find no significant dependence of cell size on local magnetic flux density.

Solar wind and chromospheric network

A physical model of the transition region, including upflow of the plasma in magnetic field funnels that are open to the overlying corona, is presented. A numerical study of the effects of Alfven waves on the heating and acceleration of the nascent solar wind originating in the chromospheric network is carried out within the framework of a two-fluid model for the plasma. It is shown that waves with reasonable amplitudes can, through their pressure gradient together with the thermal pressure gradient, cause a substantial initial acceleration of the wind (on scales of a few Mm) to locally supersonic flows in the rapidly expanding magnetic field ‘trunks’ of the transition region network. The concurrent proton heating is due to the energy supplied by cyclotron damping of the high-frequency Alfven waves, which are assumed to be created through small-scale magnetic activity. The wave energy flux of the model is given as a condition at the upper chromosphere boundary, located above the thin layer where the first ionization of hydrogen takes place. Among the new numerical results are the following: Alfven waves with an assumed f -1 power spectrum in the frequency range from 1 to 4 Hz, and with an integrated mean amplitude ranging between 25 and 75 km s4, can produce very fast acceleration and also heating through wave dissipation. This can heat the lower corona to a temperature of 5× 105 K at a height of h=12,000 km, starting from 5× 104 K at h=3000 km. The resulting thermal and wave pressure gradients can accelerate the wind to speeds of up to 150 km s-1 at h=12,000 km, starting from 20 km s-1 at h=3000 km in a rapidly diverging flux tube. Thus the nascent solar wind becomes supersonic at heights well below the classical Parker-Type sonic point. This is a consequence of the fact that any large wave-energy flux, if it is to be conducted through the expanding funnel to the corona, implies the building-up of an associated wave-pressure gradient. Because of the diverging field geometry, this might lead to a strong initial acceleration of the flow. There is a multiplicity of solutions, depending mainly on the coronal pressure. Here we discuss two new (as compared with a static transition region model) possibilities, namely that either the flow remains supersonic or slows down abruptly by shock formation, which then yields substantial coronal heating up to the canonical 106 K for the proton temperature.

Observations of polar plumes with the SUMER instrument on SOHO

We present new observations of O vi 1032 A line profiles in polar plumes, and inter-plume regions, on the disk and above the limb in the north coronal hole obtained with the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) instrument on the SOHO (Solar and Heliospheric Observatory) spacecraft. On 22 May 1996, a 5 x 5 arc min spectroheliogram was scanned above the north polar coronal hole with the entrance slit extending from 1.03 to 1.33 solar radii with 1.5 arc sec spatial resolution and ≈ 0.044 A per pixel spectral resolution in the wavelength range 1020–1040 A. Detailed plume structure in O vi 1032 A can be seen extending beyond 1.3 solar radii, with intensities in the plume regions 10–50% brighter, but line widths 10–15% narrower, than the inter-plume regions. Possible explanations for this observed anti-correlation between line width and intensity in the plume and inter-plume regions are discussed. We conclude that the source of the high-speed solar wind may not be polar plumes, but the inter-plume lanes associated with open magnetic field regions of the chromospheric network.

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 Alfven 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⊙.

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 A, 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.

First Observations of Coronal Hole Structure and Evolution Using SOHO-CDS

We report on initial observations of coronal hole structure and evolution by the Coronal Diagnostic Spectrometer (CDS) instrument on board the Solar and Heliospheric Observatory (SOHO). The date show that there is coronal actiVity on time scales of tens of minutes, manifested as brightenings at chromospheric network cell junctions in eitreme ultraviolet (EUV) wavelengths. There are also significant differences in structure seen in different wavelengths in coronal holes. Finally, we show coronal hole EUY spectra and compare them to quiet-Sun spectra, also taken by CDS.

POLAR PLUME ANATOMY: RESULTS OF A COORDINATED OBSERVATION

On 7 and 8 March 1996, the SOHO spacecraft and several other space- and ground-based observatories cooperated in the most comprehensive observation to date of solar polar plumes. Based on simultaneous data from five instruments, we describe the morphology of the plumes observed over the south pole of the Sun during the SOHO observing campaign. Individual plumes have been characterized from the photosphere to approximately 15 R⊙ yielding a coherent portrait of the features for more quantitative future studies. The observed plumes arise from small (∼ 2-5 arc sec diameter) quiescent, unipolar magnetic flux concentrations, on chromospheric network cell boundaries. They are denser and cooler than the surrounding coronal hole through which they extend, and are seen clearly in both Feix and Fexii emission lines, indicating an ionization temperature between 1.0–1.5 x 106 K. The plumes initially expand rapidly with altitude, to a diameter of 20–30 Mm about 30 Mm off the surface. Above 1.2 R⊙ plumes are observed in white light (as ‘coronal rays’) and extend to above 12 R⊙. They grow superradially throughout their observed height, increasing their subtended solid angle (relative to disk center) by a factor of ∼10 between 1.05 R⊙ and 4–5 R⊙ and by a total factor of 20–40 between 1.05 R⊙ and 12 R⊙. On spatial scales larger than 10 arc sec, plume structure in the lower corona (R < 1.3 R⊙) is observed to be steady-state for periods of at least 24 hours; however, on spatial scales smaller than 10 arc sec, plume XUV intensities vary by 10–20% (after background subtraction) on a time scale of a few minutes.

Dynamics of the Chromospheric Network: Mobility, Dispersal, and Diffusion Coefficients

view Abstract Citations (54) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamics of the Chromospheric Network: Mobility, Dispersal, and Diffusion Coefficients Schrijver, Carolus J. ; Shine, Richard A. ; Hagenaar, Hermance J. ; Hurlburt, Neal E. ; Title, Alan M. ; Strous, Louis H. ; Jefferies, Stuart M. ; Jones, Andrew R. ; Harvey, John W. ; Duvall, Thomas L., Jr. Abstract Understanding the physics behind the dispersal of photo spheric magnetic flux is crucial to studies of magnetoconvection, dynamos, and stellar atmospheric activity. The rate of flux dispersal is often quantified by a diffusion coefficient, D. Published values of D differ by more than a factor of 2, which is more than the uncertainties allow. We propose that the discrepancies between the published values for D are the result of a correlation between the mobility and flux content of concentrations of magnetic flux. This conclusion is based on measurements of displacement velocities of Ca II K mottles using an uninterrupted 2 day sequence of filtergrams obtained at the South Pole near cycle minimum. We transform the Ca II K intensity to an equivalent magnetic flux density through a power-law relationship defined by a comparison with a nearly simultaneously observed magnetogram. One result is that, wherever the network is clearly defined in the filtergrams, the displacement vectors of the mottles are preferentially aligned with the network, suggesting that network-aligned motions are more important to field dispersal than deformation of the network pattern by cell evolution. The rms value of the inferred velocities, R = <|v|2>½, decreases with increasing flux, Φ, contained in the mottles, from R ≈ 240 m s-1 down to 140 s-1. The value of R(Φ) appears to be independent of the flux surrounding the concentration, to the extreme that it does not matter whether the concentration is in a plage or in the network. The determination of a proper effective diffusion coefficient requires that the function R(Φ) be weighted by the number density n(Φ) of mottles that contain a total flux. We find that n(Φ) decreases exponentially with Φ and propose a model of continual random splitting and merging of concentrations of flux to explain this dependence. Traditional methods used to measure D tend to be biased toward the larger, more sluggish flux concentrations. Such methods neglect or underestimate the significant effects of the relatively large number of the more mobile, smaller concentrations. We argue that the effective diffusion coefficient for the dispersal of photo spheric magnetic flux is ∼600 km2 s-1. Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177747 Bibcode: 1996ApJ...468..921S Keywords: SUN: CHROMOSPHERE; SUN: ACTIVITY; SUN: MAGNETIC FIELDS full text sources ADS |

Observation and Modeling of Soft X-Ray Bright Points. I. Initial Results

The Multi-Spectral Solar Telescope Array was launched from White Sands Missile Range, New Mexico, on 1991 May 13 at 1905 UT. Full-disk, high-resolution solar images were obtained in a variety of soft X-ray and far-ultraviolet wavelengths. The 193 A (FeXII) and 44 A (SiXII) images show a large number of coronal bright points. The high spatial resolution of the FeXII image allows many of the bright points to be resolved as tiny loops. Co-alignment of the soft X-ray images with the 1216 A Ly{alpha} image reveals that all the coronal bright points have counterparts in the transition region, often resolved as a pair of footprints, which are brighter than neighboring elements of the chromospheric network. Moreover, comparison with the KPNO magnetogram shows dipole structures coincident with nearly all of the bright points. We present a quantitative analysis based on preliminary photometry of four of the bright points that were observed. By fitting a simple, numerical loop model to the photometric data, we estimate the magnitude of the coronal heating in these structures. The rate of heating per unit footpoint area is found to be similar to previous measurements for much larger coronal structures. Implications for heating of the chromosphere andmore » lower transition region are also discussed. {copyright} {ital 1996 The American Astronomical Society.}« less

Heating of the Solar Middle Chromospheric Network and Internetwork by Large-Scale Electric Currents in Weakly Ionized Magnetic Elements

view Abstract Citations (8) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Heating of the Solar Middle Chromospheric Network and Internetwork by Large-Scale Electric Currents in Weakly Ionized Magnetic Elements Goodman, Michael L. Abstract A two-dimensional, dissipative magnetohydrodynamic model is used to argue that a major source of in situ heating for the solar middle chromosphere is the resistive dissipation of large-scale electric currents flowing in magnetic elements. A magnetic element is an arch-shaped magnetic field configuration consisting of a central region of horizontally localized, mainly vertical magnetic field based in the photosphere, with field lines that diverge horizontally with increasing height, extend into the middle chromosphere, and then return to the photo sphere as a relatively diffuse, weaker field. The currents that flow in these elements are carried by protons, and are large scale in that their scale height is hundreds of kilometers in the network and thousands of kilometers in the internetwork. Solutions to the model demonstrate that the resistive dissipation of large-scale electric currents flowing orthogonal to the magnetic field in magnetic elements embedded in a weakly ionized, strongly magnetized hydrogen gas may generate all of the thermal energy necessary to heat the middle chromosphere. The magnetic field is computed self-consistently with the electric field, pressure, and hydrogen and proton densities. Solutions to the model suggest that magnetic elements with horizontal extents up to several arcseconds may be confined to, and heat, the chromospheric network, while elements with the largest horizontal extents may span and heat the internetwork and be the building blocks of the chromospheric magnetic canopy. The model predicts that the heating rate per unit mass (q) is independent of height, peaked near but horizontally displaced from the center of a magnetic element, and for realistic model input parameters has an average value computed over the base area of the element dose to the value 4.5 x 109 ergs g-1 s-1 predicted by semiempirical models of the chromosphere that also predict that q is independent of height in the middle chromosphere. The model predicts that the heating rate per unit volume is peaked near the horizontal midpoint of a magnetic element where the field is mainly horizontal. The model predicts that both heating rates are zero at the center and outer boundary of a magnetic element where the field is vertical. These model predictions for the spatial localization of the heating rates are consistent with observations that regions of enhanced emission are near but horizontally displaced from regions of vertical, high-magnitude magnetic field. Publication: The Astrophysical Journal Pub Date: June 1996 DOI: 10.1086/177290 Bibcode: 1996ApJ...463..784G Keywords: MAGNETOHYDRODYNAMICS: MHD; SUN: CHROMOSPHERE; SUN: MAGNETIC FIELDS full text sources ADS |