Non-ideal MHD Properties of Magnetic Flux Tubes in the Solar Photosphere

Magnetic flux tubes reaching from the solar convectivezone into the chromosphere have to pass through the relatively cool, and therefore non-ideal (i.e. resistive) photospheric region enclosed between the highly ideal sub-photospheric and chromospheric plasma. It is shown that stationary MHD equilibria of magnetic flux tubes which pass through this region require an inflow of photospheric material into the flux tube and a deviation from iso-rotation along the tube axis. This means that there is a difference in angular velocity of the plasma flow inside the tube below and above the non-ideal region. Both effects increase with decreasing cross section of the tube. Although for characteristic parameters of thick flux tubes the effect is negligible, a scaling law indicates its importance for small-scale structures. The relevance of this `inflow effect' for the expansion of flux tubes above the photosphere is discussed.     

Connection Between Chromospheric Events and Photospheric Dynamics

We analyzed chromospheric events and their connection to oscillation phenomena and photospheric dynamics. The observations were done with the New Solar Telescope of Big Bear Solar Observatory using a broad-band imager at the wavelength of a TiO band and FISS spectrograph scanning Ca?ii and Hα spectral lines. The event in Ca?ii showed strong plasma flows and propagating waves in the chromosphere. The movement of the footpoints of flux tubes in the photosphere indicated flux tube entanglement and magnetic reconnection as a possible cause of the observed brightening and waves propagating in the chromosphere. An upward propagating train of waves was observed at the site of the downflow event in Hα. There was no clear relationship between photospheric waves and the Ca?ii and Hα events. Our observations indicate that chromospheric waves that were previously thought to originate from the photosphere may be generated by some events in the chromosphere as well.     

Similarities between chromospheric and photospheric quiet-Sun magnetograms

Simultaneous observations of chromospheric (H) and photospheric (Fei 5324.19 Å) magnetograms in quiet solar regions enable us to study the spatial configuration of the magnetic field in the solar atmosphere. With the typical spatial resolution of the Huairou magnetograph, the photospheric and chromospheric magnetic structures of the quiet Sun maintain a very similar pattern. Moreover, the vertical magnetic flux is almost the same from the photosphere to the chromosphere. As an intermediate step, we analyze the formation of the working lines used by the Huairou video magnetograph of the Beijing Astronomical Observatory. The Stokes V contribution function of H and Fei 5324.19 Å are calculated. It is found that our H magnetograms provide the distribution of the chromospheric magnetic field at a height some 1000–1500 km above the photosphere.     

A model of photospheric faculae deduced from white light high resolution pictures

High resolution pictures (about 0.3) of photospheric faculae near the solar limb have been obtained with the Pic du Midi 50 cm refractor; their granular structure then clearly appears. The microphotometric study of these facular granules shows that the ratio of their intensity to the photospheric intensity, I ^f/I ^ph (cos) reaches a maximum near cos = 0.3 and then decreases towards the limb. The values of this ratio have been corrected with a most likely spread function. Then a temperature model of a facular granule is obtained: with respect to the neighbouring photosphere, this granule appears as a photospheric hot cloud which does not extend high in the solar atmosphere (thickness 100 km above ₅₀₀₀ = 1). The temperature excess is 750K at maximum. This hot region is located over a layer which is cooler than the normal photosphere at the same level. Another hot region might extend above the photospheric hot cloud, possibly up to the chromosphere. This photospheric facula model which is confined to the lower photosphere seems to indicate that this phenomenon is different from the photospheric network which is visible up to the lower chromosphere.     

Electro-Mechanical Coupling Between the Photosphere and Transition Region

We study the response of the chromosphere and transition region to dynamic changes in the photospheric network magnetic fields. We present results from simultaneous measurements taken by TRACE in chromospheric and transition region (Civ) images, high-resolution magnetograms taken by MDI, and spectra of chromospheric (Cii) and transition region lines (Ovi) obtained with the SUMER instrument on SOHO. Enhanced emission in the Civ line is generally co-spatial with the magnetic pattern in the photosphere. We propose a mechanism of electro-mechanical coupling between the photosphere and upper layers of atmosphere based on hydrodynamic cumulation of energy produced by reconnecting flux tubes in the photosphere/chromosphere region (Tarbell et al., 1999). We believe that a basic process causing energetic events is the cascade of shock waves produced by colliding and reconnecting flux tubes. The continuous supply of flux tubes in the magnetic carpet ensures the ubiquitous nature of this process and its imprint on the upper atmosphere. The appearance of bright transients often, but not always, correlates with canceling mixed polarity magnetic elements in the photosphere. In other cases, transients occur in regions of unipolar flux tubes, suggesting reconnection of oblique components. Transients are also seen in regions with no fields detected with the MDI sensitivity; these may be reconnections of tiny features with diameters less than 100 km. Blinkers and other bright transients are often accompanied by two directional plasma jets. These may be generated by cylindrical self-focusing of shock fronts or by collision of shocks produced by neighboring reconnection processes. The observations suggest that stronger emissions correspond to lower velocity jets, and vice versa; this property is a natural consequence of the proposed mechanism. Plasma flows are always seen whenever the slit crosses strong magnetic flux tubes or vertices of converging flows in the supergranular network. The overall energy distribution between heating and plasma flows is an intrinsic feature of our mechanism.     

He I 1083 nm OSCILLATIONS AND DOWNFLOWS NEAR THE NORTH SOLAR POLE

Imaging spectroscopic data of the Sii 1082.7 nm (photospheric) and Hei 1083.0 nm (chromospheric) spectral lines were taken starting 22:05 UT on 23 July, 1996 with the NASA/NSO Spectromagnetograph at the NSO/Kitt Peak Vacuum Telescope. Observations were made near the north solar pole, with a field of view of 100 by 400 arc sec and with a temporal cadence of 53 s for 2 hr. Simple fitting to the line profiles measured the line position, depth, and spectral full-width at half-maximum. Power spectra of the velocity oscillations in each line were computed, and the oscillation power in the 2 to 6 mHz frequency band versus view angle was measured to search for horizontal oscillations. Horizontal waves are not detected to limiting amplitudes (1) of 22 m s^-1 in the chromosphere and 9 m s^-1 in the photosphere. These values are used to estimate limits for the energy flux into the corona. The amplitude of radial oscillations in the chromosphere is twice that of the photosphere. No statistically meaningful oscillation power is measured in the spectral parameters of the Hei line in the emission shell seen above the continuum limb. Finally, rapidly evolving red-shift events are observed in the Hei 1083 nm line on the disk; these events are some sort of coronal rain, and there are about 40 of these events on the solar disk at any moment.     

Photospheric vortex flows as a cause for two-ribbon flares: A topological model

By using a topological model for the potential magnetic field above the photosphere, the appearance and development of the separator as a result of vortex plasma flows in the locality of the photospheric neutral line is considered. The possible relation of such vortex flows with a flare activity is revealed. The arrangement and shape of the flare ribbons in the chromosphere, the formation of X-ray intersecting loops, the early appearance of bright knots on flare ribbon edges are naturally explained by the model provided a reconnecting current sheet arises along the separator in the coronal magnetic field of active regions as a result of the evolution of the magnetic field sources in the photosphere.     

Spicules: The resonant response to granular buffeting?

We propose that spicules are the direct result of a resonant buffeting of the sides of an intense (kilogauss) slender flux tube by turbulent granular motions. For a critical wavespeed, the granular motion drives a high-speed flow along the axis of the tube. We illustrate this effect by considering in detail a flux tube embedded in a uniform compressible atmosphere, thereby modelling the development of the seedflow in the photosphere that eventually manifests itself in the chromosphere as the spicule.     

The magnetic structure of arch filament systems

Photographic-type magnetograms are used in conjunction with H filtergrams to study the structure and evolution of magnetic fields associated with arch filament systems. The magnetograms show that the opposite ends of the arch filaments are indeed rooted in photospheric magnetic fields of opposite polarity. Furthermore, these magnetic field systems are in every case new magnetic flux appearing at the solar surface. Time lapse studies show the detailed process by which the flux tubes emerge through the surface. First, supergranules bring individual strands of magnetic flux to the surface and sweep the two feet of the flux tube to opposite sides of the supergranule. Then, the flux tube rises through the chromosphere, creating a visible arch filament. It is also shown that the observed rotation of the axis of an arch filament system in the plane of the solar surface is caused by the emergence of successive flux loops, each possessing different axial tilts.     

Active-Region Magnetic Structure Observed in the Photosphere and Chromosphere

The full magnetic vector has been measured in both the photosphere and chromosphere across sunspots and plage in NOAA Active Region 8299. We investigate the vertical magnetic structure above the umbral, penumbral and plage regions using quantitative statistical comparisons of the photospheric and chromospheric magnetic data. The results include: (1) a general decrease in average magnetic flux density with height; (2) the direct detection of the superpenumbral canopy in the chromosphere; (3) values for dB/dz which are consistent with earlier investigations when derived from a straight difference between the two measurements, but which are somewhat small when derived from the B=0 condition, (4) a monolithic structure in the umbrae which extends well into the upper chromosphere, with a very complex and varied structure in penumbrae and plage, as evidenced by (5) a uniform magnetic scale height in the umbrae with an abrupt jump to widely varying scale heights in penumbral and plage regions. Further, we find (6) evidence that field extrapolations using the photospheric flux as the boundary may not agree with expectations or with observed coronal structures as well as those which use the chromospheric magnetic flux as the extrapolation starting point.     

Quasi-Periodicity in global solar radio flux at metric wavelengths during Noise Storms

We present observational results from studying the quasi-periodicities in global solar radio flux during periods of enhanced noise storm activity, over durations of 4 hr a day (`intra-day' variations), observed at 77.5 MHz with the newly commissioned log-periodic array tracking system at the Gauribidanur Radio Observatory. Positional information on the storm centers was obtained with the radio imaging data from the Nan\c cay Radio Heliograph (NRH), while their active region counterparts on the photosphere (and the overlying chromosphere ) were located from the H images of the Big Bear Solar Observatory. The quasi-periodicity in flux was found to be 110 min, with the fluctuation in flux being 3(±1.5) solar flux units (s.f.u.). The results of such pulsations are interpreted qualitatively as evidence for coronal seismology.     

Reports on Test Observations with the Multi-Channel Solar Telescope

In this paper, we have made a report on the test observations with a Multi-Channel Solar Telescope (MCST), which consists of 60 cm Nine-Channel Solar Telescope (NCST), 35 cm Solar Magnetic Field Telescope (SMFT), 8 cm Full Disc Telescope (FDT), 10 cm Full Disc Magnetic Field Telescope (FDMFT) and 14 cm H telescope. These observations demonstrate that the MCST has the following advantages: (a) It can work at more than nine visible spectral lines simultaneously. In this way, different solar layers of the photosphere and chromosphere can be observed at the same time; (b) every channel of the NCST is entirely equivalent to a videomagnetograph, by means of which the vector magnetic fields and line-of-sight velocity fields can be measured; (c) real-time monochromatic images of the photosphere and chromosphere can be obtained with the FDT, FDMFT, and H Telescope; (d) high-temporal-resolution full-disk magnetic fields can be measured with the FDMFT; (e) spectral profiles over a large field of view can be scanned with the NCST.     

Multi-channel magnetograph observations

Simultaneous observations of photospheric magnetic fields, Caii K emission, the photospheric network and continuum faculae show that these four quantities are correlated in a complicated manner. The photospheric and calcium networks show increasing contrast with increasing magnetic field strength up to field strengths of about 500 G. Higher values of the magnetic field are found only in pores and sunspots. Continuum faculae also show increasing contrast with increasing magnetic field strength (even at the disk center), but this contrast reaches a maximum at field strengths of about 200 G. At higher field strengths, continuum faculae become monotonically darker until pore or spunspot conditions are reached.Measurements of the photospheric network and the continuum faculae over a wide range of result in families of limb contrast curves. These curves indicate that the dependence on H is as important as the dependence on . They also indicate that the magnetic field has a preferred inclination of about 50°. The facular contrast shows little dependence on resolution. This is interpreted in terms of a geometric model in which faculae are clumps of many individual flux tubes. These tubes are closely packed and unresolvable in the photosphere, but are more widely spaced, and therefore resolvable, in the low chromosphere.Visiting Astronomer, Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Lα measurements during the solar eclipse of 12 November 1966

Rocket measurements of the solar L (1216 Å) flux during various phases of the total eclipse of 12 November, 1966 confirm that the radiation emanates from the chromosphere. A significant difference between two portions of the photosphere limb is attributed to enhanced emission in the region of a Ca K plage. The average size of the L disk is found to be approximately 4000 km above the photosphere, and a brightening is observed at the limb.     

Magnetic arcade evolution and instability

The evolution of two-dimensional coronal magnetic arcades driven by photospheric shear flows is studied by numerical solution of the resistive MHD equations neglecting pressure and gravitational forces. By varying the distribution of the frozen-in photospheric magnetic flux, the shear flow profile and the magnetic Reynolds number, a fairly general picture is obtained. Isolated arcades develop in a quasi-selfsimilar stable way, invalidating previous studies of equilibrium sequences ² = F() with monotonically increasing parameter . Groups of several interacting arcades show a more complex behavior. When of sufficiently large height arcade structures tend to bifurcate, leading to plasmoid (or filament) formation. Usually this is a slow resistive process and the plasmoid is confined in the arcade interior. Configurations containing at least three arcades may give rise to fast plasmoid ejection.     

Spectropolarimetric investigation of an Ellerman bomb: 2. Photospheric models

Semiempirical models of the photosphere of an Ellerman bomb in the NOAA 11024 active region were obtained using profiles of Stokes parameters I, Q, U, and V of photospheric lines. Spectropolarimetric observations were conducted using the French–Italian THEMIS telescope (Tenerife, Spain). The SIR inversion code [28] was used in the modeling. The models have two components: a magnetic flux tube and nonmagnetic surroundings. The dependences of temperature, magnetic field strength, inclination of the magnetic field vector, and line-of-sight velocity in the tube on the optical depth were obtained. The models demonstrate that the thermodynamic parameters of the Ellerman bomb photosphere differ considerably from those of the quiet photosphere. The temperature in the tube model varied nonmonotonically with height and deviated by up to 700–900 K from its values for the quiet photosphere. Downflows were observed in the lower and the upper photospheric layers. The line-of-sight velocity in the upper layers of the photosphere was as high as 17 km/s. The magnetic field strength in the models varied from 0.1–0.13 T in the lower photospheric layers to 0.04–0.07 T in the upper ones. The physical state of the photosphere did change in the course of observations.     

Resistive ballooning line-tied boundary conditions

Ideal and resistive ballooning modes are investigated for different ratios of a two-layer stratified density region representing a model for the photospheric/coronal boundary. Construction of the ballooning equations using a WKB approach is justified by comparison between the values of the growth rate obtained using Hain-Lüst and ballooning equations together with a WKB integral relation. Different values of the density ratio, radius, and resistivity are considered. Sausage-type and kink-type instabilities are found. One of these, depending on the value of r remained unstable for large density ratios. The other instability tended to marginal stability as the density ratio was increased, and allowed parallel and perpendicular flows across the boundary. This is contrary to the predictions of both the rigid-wall and flow-through conditions.     

Coronal force-free magnetic field: Source-surface model

Models of the magnetic field in the solar chromosphere and corona are still mainly based on theoretical extrapolations of photospheric measurements. For the practical calculation of the global field, the so-called source-surface model has been introduced, in which the influence of the solar wind is described by the requirement that the field be radial at some exterior (source) surface. Then the assumption that the field is current-free in the volume between the photosphere and this surface allows for its determination from the photospheric measurement. In the present paper a generalization of the source-surface model to force-free fields is proposed. In the generalized model the parameter( = ×B·B/B ²)must be non-constant (or vanish identically) and currents are restricted to regions with closed field lines. A mathematical algorithm for computing the field from boundary data is devised.     

A comparison between photospheric and chromospheric quiet-Sun magnetograms

Photospheric (Fei 5324.19 Å line) and chromospheric (H line) magnetic fields in quiet-Sun regions have been observed in the solar disk center by using the vector video magnetograph at Huairou Solar Observing Station of Beijing Astronomical Observatory. Observational results show that the quiet-Sun magnetic elements in the solar photosphere and chromosphere present similar magnetic structures. Photospheric and chromospheric magnetograms show corresponding time variations. This suggests that the magnetic fields in quiet-Sun regions present different 3-D magnetic configurations compared to those in solar active regions.     

A photometric study of faculae and sunspots between 1.2 and 1.6 μm

We investigate further the interpretation of dark magnetic faculae observed in previous imaging of the solar photosphere at 1.63 m. We show that their contrast at 1.63 m increases with magnetic flux beyond a threshold value of 2 × 10¹⁸ Mx and blends smoothly with the contrast vs flux relation measured at this wavelength for larger structures of sunspot size. Not all facular structures that are bright in Ca K are dark at 1.63 m, apparently because their magnetic flux is not large enough. After correction for blurring, the contrast of the dark faculae observed near the disc center at 1.63 m is approximately 4%. But our observations at 1.23 m, which probe slightly higher photospheric levels, do not show these dark faculae. These results indicate that magnetic flux tubes of diameter as small as 500 km significantly inhibit convective heat flow to the photosphere, much as do sunspot flux tubes of much larger diameter. They also suggest that, in even smaller flux tubes, the inhibition becomes rapidly less significant. Finally, we show that the sunspot-size dependence of umbral infrared contrast versus wavelength that we observe can probably be explained in terms of instrumental blurring. Observations with lower scattered light will be required to determine whether a real decrease of contrast with diameter also plays a role.