Chromospheric Magnetic Field Research Articles

Loop interaction scenario of a trace flare

We analyzed simultaneous EUV images from the Transition Region And Coronal Explorer (TRACE) and Hα and Hβ filtergrams from Huairou Solar Observing Station (HSOS). In active region NOAA 8307, an Hα C5.5 flare occurred near 06:10 UT on 23 August 1998. In this paper, we concentrated on loop–loop interaction, as well as their relationship to the C5.5 flare. We find that while opposite polarity magnetic fields cancelled each other, Hβ bright points appeared, and then the flare occurred. Looking at EUV images, we noticed that a TRACE flare, associated with the C5.5 flare in Hα and Hβ filtergrams, first appeared as patch-shaped structures, then the flare patches expanded to form bright loops. We used a new numerical technique to extrapolate the chromospheric and coronal magnetic field. Magnetic field loops, which linked flare ribbons, were found. It was suggested that loop interaction in the active region was the cause of the TRACE and Hα flare; the magnetic topological structures were clearly demonstrated and the TRACE flare was probably due to the interaction among energetic low-lying and other longer (higher) magnetic loops. Each primary flare kernel, seen from Hα, Hβ filtergrams, and EUV images, was located near the footpoints of several interacting loops.

A comparison between photospheric and chromospheric quiet-sun magnetograms

Photospheric (Fe i λ5324.19 A line) and chromospheric (Hβ 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.

Sympathetic flares in two adjacent active regions

Three sympathetic flares were observed with the Solar Magnetic Field Telescope (SMFT) at the Huairou Solar Observing Station of Beijing Astronomical Observatory on 29 August, and 1 September 1990. Each set of sympathetic flares had three ribbons. Two ribbons appeared in active region NOAA 6233 and one ribbon occurred in NOAA 6240 embedded in a single polarity area. Photospheric vector magnetograms were simultaneously obtained from both regions as well. We use a new numerical technique to reconstruct the chromospheric and coronal magnetic fields by making use of the observed vector magnetic fields in the photosphere as boundary conditions. Magnetic field loops linking both regions were identified from the reconstructed 3-D fields. The analysis of chromospheric filtergrams and reconstructed 3-D magnetic fields indicates that interaction between a sheared lower loop in the active region NOAA 6233 and a higher loop linking the two regions resulted in sympathetic flares. The analysis of the time delay between flare ribbons in NOAA 6233 and 6240 indicates that heat conduction along the higher loop from the primary energy release site is responsible for the sympathetic flaring in NOAA 6240. The events reported in this paper represent only one alternative as the cause of sympathetic flaring in which energy transport along coronal interconnecting loops plays the major role, and no in-situ energy release is required.

Similarities between chromospheric and photospheric quiet-Sun magnetograms

Simultaneous observations of chromospheric (Hβ) and photospheric (Fe i λ5324.19 A) 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 Hβ and Fe i λ5324.19 A are calculated. It is found that our Hβ magnetograms provide the distribution of the chromospheric magnetic field at a height some 1000–1500 km above the photosphere.

Evidence for non-potential magnetic fields in the quiet Sun

A comparison of BBSO Hα centerline filtergrams and videomagnetograms was made to investigate the existence of non- potential magnetic fields in the quiet Sun near magnetic network. We use the fibril structure in the Hα images as a proxy for the horizontal chromospheric magnetic field which we compare with the horizontal field obtained by potential extrapolation of the observed, line-of-sight photospheric field. The quiet-Sun field was found to be consistently and significantly non-potential in each of the three fields of view studied. A transient extreme ultraviolet (EUV) brightening, known as a blinker, occurred during the observations of a region where the field is highly non-potential, suggesting a connection between magnetic reconnection and non-potentiality.

Influence of a chromospheric magnetic field on solar acoustic modes

This paper studies the effect of a magnetic atmosphere on the global solar acoustic oscillations in a simple Cartesian model. First, the influence of the ratio of the coronal and the photospheric temperature τ and the strength of the magnetic field at the base of the corona Bc on the oscillation modes is studied for a convection zone-corona model with a true discontinuity. The ratio τ seems to be an important parameter. Subsequently, the discontinuity is replaced by an intermediate chromospheric layer of thickness L and the effect of the thickness on the frequencies of the acoustic waves is studied. In addition, nonuniformity in the magnetic field, plasma density and temperature in the transition layer gives rise to continuous Alfven and slow spectra. Modes with characteristic frequencies lying within the range of the continuum may resonantly couple to Alfven and/or slow waves.

Resonant Damping of Solarp‐Modes by the Chromospheric Magnetic Field

In this paper, we demonstrate how solar p-mode energy can be resonantly absorbed by coupling to localized Alfven waves in the chromosphere. Nonuniformity in the magnetic field, plasma density, and temperature in the solar atmosphere give rise to a continuous spectrum of resonant frequencies. P-modes with characteristic frequencies within the range of the continuous spectrum may resonantly couple to localized Alfven and slow magnetohydrodynamic (MHD) waves, and hence heat the chromospheric plasma. In dissipative MHD, these p-modes are recovered as eigenmodes with a damping rate that becomes independent of the dissipation mechanism in the limit of vanishing dissipation. An analytical solution of these p-modes is found in the dissipative layer embracing the resonant magnetic surface. Using the analytical solution to cross the quasi-singular dissipative layer, the required numerical effort is limited to the integration of the ideal MHD equations away from any singularity. This results in a powerful tool for investigating in a mathematically consistent way the damping by resonant absorption and the frequency shifts of the solar oscillations arising from the presence of an overlying magnetic atmosphere in combination with the resonant absorption process. The outcome is that the mechanism of resonant absorption might be responsible for the damping of solar oscillations and should be taken into account in producing a definite solar model reproducing the observed frequencies of the global solar oscillations to within the limit of observational uncertainties.

The Structure of the Chromosphere Properties Pertaining to Element Fractionation

We review the structure and dynamics of the solar chromosphere with emphasis on the quiet Sun and properties that are relevant to element fractionation mechanisms. Attention is given to the chromospheric magnetic field, its connections to the photosphere, and to the dynamical evolution of the chromosphere. While some profound advances have been made in the “unmagnetized” chromosphere, our knowledge of the magnetically controlled chromosphere, more relevant for the discussion of element fractionation, is limited. Given the dynamic nature of the chromosphere and the poorly understood magnetic linkage to the corona, it is unlikely that we will soon know the detailed processes leading to FIP fractionation.

Three-dimensional chromospheric magnetic field configurations based on photospheric-vector and chromospheric-multi-level longitudinal-magnetic field observations

The three-dimensional (3D) reconstruction of magnetic configurations above the photosphere is consid- ered within the framework of the nonlinear force-free-field (FFF) model. The physical- computational algorithm pro- posed and tested incorporates, for the first time, the fol- lowing basic features: 1) Both photospheric vector field, B (z, y, O) and chromospheric line of sight field component, B,(z, y, z) data are utilized; this reduces significantly the degree of ill-posedness characterizing the Cauchy problem corresponding to the case when only B(z,y,O) - values are used as boundary conditions. 2) A high-order, very efficient computational algorithm is developed and used: horizontal derivatives are evaluated by 14 - terms formu- las in 14 different forms, selected such as to provide op- timal computational accuracy; the vertical integration is achieved by the use of moving 10 - term formulas ex- pressed in terms of 10 derivatives and the first Bi(z, y, z) values (i = z,y,z). 3) At neutral points, where inher- ent computational singularities in the values of the FFF- function a arise, rather than using smoothing techniques based on four-neighbouring- values averages, suitable pro- cedures ensuring continuity are developed and used. The overall result of the incorporation of these novel features is an improvement by orders of magnitude of the accu- racy with which the chromospheric fields are reconstructed in the case in which one uses (i) only B(z,y,O) - values as boundary conditions and (ii) relative simple compu- tational formulas and smoothing techniques; at Z = 20, ABi/Bi < 10-3 ! The elimination/minimization of mea- surement errors as well as the fitting of the corrected date to FFF-model-States is also discussed.

Configuration of the Magnetic Field in Solar Active Regions

In this paper we present the observational results of chromospheric and photospheric magnetograms in active regions obtained at the Huairou Solar Observing Station of the Beijing Astronomical Observatory. Simultaneous observations of the chromospheric and photospheric magnetic fields enable us to construct a possible configuration of the magnetic field in the solar atmosphere. The chromospheric magnetic field shows more diffusion than the photospheric magnetic field and consists of fibril-like features. We also discuss the possible configuration of the magnetic shear in highly sheared active regions.

Magnetic configuration of a sunspot region using the H$^{\bf\beta}$ line

The configuration of the magnetic field in an active region is analysed using a set of Hβ chromospheric magnetograms and narrow pass-band filtergrams taken at various positions on the line. We found that the chromospheric magnetic field is parallel to the chromospheric features and inhomogeneous. The photospheric magnetic field disappears abruptly in the edge of the penumbra; the chromospheric field extends beyond the penumbral boundary with an inclination of about to the solar surface. Chromospheric superpenumbral filaments show a good correlation with the chromospheric magnetic field and their configurations are related to the photospheric magnetic structures nearby the sunspot. Part of the magnetic flux of the sunspot returns to the neighbouring enhanced networks, which is almost vertical to the solar surface.

Seismology of the Solar f-Mode. I. Basic Signatures of Shearing Velocity Fields

view Abstract Citations (24) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Seismology of the Solar f-Mode. I. Basic Signatures of Shearing Velocity Fields Ghosh, Pranab ; Antia, H. M. ; Chitre, S. M. Abstract The observed frequencies of the solar f-mode show systematic shifts from the (parabolic) dispersion relation characteristic of a pure surface mode, owing to the perturbing effects of such phenomena in the outer layers of the Sun as velocity fields, magnetic fields, and temperature gradients/discontinuities. These frequency shifts thus provide a good diagnostic probe of these phenomena. In this paper, we wish to focus our attention on the possible influence of shearing velocity fields on the f-mode frequencies. We show here that velocity fields of a wide class leave a signature on the frequency-shift profile which is consistent with observations, including the characteristic change of sign (crossover) in δω at a spherical harmonic degree of approximately 800. We demonstrate that quantitative fits to the observed frequency shifts are possible with parameters which are characteristic of the underlying motions of the solar convection zone. However, the simultaneous effects of other phenomena, e.g., chromospheric magnetic fields, should also be taken into account. Publication: The Astrophysical Journal Pub Date: October 1995 DOI: 10.1086/176271 Bibcode: 1995ApJ...451..851G Keywords: CONVECTION; HYDRODYNAMICS; SUN: GRANULATION; SUN: MAGNETIC FIELDS; SUN: OSCILLATIONS full text sources ADS |

Imaging spectropolarimetry of the He I 1083 nanometer line in a flaring solar active region

view Abstract Citations (63) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Imaging Spectropolarimetry of the He i 1083 Nanometer Line in a Flaring Solar Active Region Penn, M. J. ; Kuhn, J. R. Abstract Slit spectra, simultaneously measuring left and right circular polarization of the solar spectrum at 1082.9 +/- 0.4 nm, were taken using the National Solar Observatory (NSO) Vacuum Tower Telescope (VTT) at Sacramento Peak with a 128 x 128 pixel HgCdTe IR camera. By moving the slit the solar active region NOAA 7629 was scanned with a cadence of 246 s from 1724 to 1902 UT on 1993 December 6. The region was in the decay phase of a C9.7 flare. Intensity, velocity and longitudinal magnetic field in both the Si I (photospheric) and He I (chromospheric) lines are computed from fits to the line profiles in left and right polarizations. Analysis of these quantities show: (1) He I line emission in three decaying flare kernels, (2) Zeeman splitting of the He I emission measuring the longitudinal magnetic field inside the flare kernels; (3) dark He I active region filaments avoid strong longitudinal chromospheric magnetic field, and an active filament (with a transverse speed of 9 km/s) produces fluctuations in the strength of the longitudinal magnetic fields, and (4) bipolar moving magnetic features (MMFs) and emerging active region bipoles (EARBs) are observed at photospheric but not chromospheric heights. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/187787 Bibcode: 1995ApJ...441L..51P Keywords: Chromosphere; Circular Polarization; Coronal Holes; Helium; Infrared Astronomy; Line Spectra; Linear Polarization; Photosphere; Solar Flares; Solar Magnetic Field; Astronomical Spectroscopy; Cameras; Charge Coupled Devices; Field Of View; Infrared Photography; Magnetic Signatures; Red Shift; Spectroheliographs; Spectroscopic Telescopes; Solar Physics; SUN: FLARES; SUN: INFRARED; SUN: MAGNETIC FIELDS full text sources ADS |

Is the solar chromospheric magnetic field force-free?

Is the solar chromospheric magnetic field force-free?

Configuration of the chromospheric magnetic field in a unipolar sunspot region

A set of Hβ chromospheric magnetograms at various wavelengths near the line center, chromospheric Dopplergrams, and photospheric vector magnetograms of a unipolar sunspot region near the solar limb were obtained with the vector video magnetograph at the Huairou Solar Observing Station. The superpenumbral chromospheric magnetic field is almost parallel to the surface at the outside of the sunspot penumbra, where the magnetic lines of force are mainly concentrated in the superpenumbral filaments. In the gaps between the filaments the chromospheric horizontal field is weak.

The roots of coronal structure in the Sun's surface

We have compared the structures seen on X-ray images obtained by a flight of the NIXT sounding rocket payload on July 11, 1991 with near-simultaneous photospheric and chromospheric structures and magnetic fields observed at Big Bear. The X-ray images reflect emission of both Mgx and Fexvi, formed at 1 × 106 K and 3 × 106 K, respectively. The brightest Hα sources correspond to a dying sub-flare and other active region components, all of which reveal coronal enhancements situated spatially well above the Hα emission. The largest set of X-ray arches connected plages of opposite polarity in a large bipolar active region. The arches appear to lie in a small range of angle in the meridian plane connecting their footpoints. Sunspots are dark on the surface and in the corona. For the first time we see an emerging flux region in X-rays and find the emission extends twice as high as the Hα arches. Many features which we believe to correspond to ‘X-ray bright points’ (XBPs) were observed. Whether by resolution or spectral band, the number detected greatly exceeds that from previous work. All of the brighter XBPs correspond to bipolar Hα features, while unipolar Hα bright points are the base of more diffuse comet-like coronal arches, generally vertical. These diverge from individual features by less than 30°, and give a good measure of what the ‘canopies’ must do. The Hα data shows that all the Hα features were present the entire day, so they are not clearly disappearing or reappearing. We find a new class of XBPs which we call ‘satellite points’, elements of opposite polarity linked to nearby umbrae by invisible field lines. The satellite points change rapidly in X-ray brightness during the flight. An M1.9 flare occurred four hours after the flight; examination of the pre-flare structures reveals nothing unusual.

Reversed-polarity structures of chromospheric magnetic field

Reversed-polarity structures of chromospheric magnetic fields are magnetic gulfs and islands of opposite polarity relative to the underlying photospheric fields. In this paper data measured with the Solar Magnetic Field Telescope of the Huairou Solar Observing Station in Beijing were analyzed. From more than 300 pairs of photospheric magnetograms (in FeI λ5324.19 A) and relevant chromospheric magnetograms (Hβ λ4861.34 A), the reality of the reversed-polarity structures is demonstrated. According to an analysis of the fine structure of the magnetic field in the two layers of active regions, we found that there are probably four different types as follows:

Polarity-reversed features of chromospheric magnetic field

Polarity-reversed features are features in the chromospheric magnetic field where the polarity is opposite to that of the underlying photosphere. From over 300 pairs of photospheric and chromospheric magnetograms obtained at Huairou Station, the reality of such features is demonstrated. Analysis of fine structures in the magnetic field at two levels shows that such features may exist in 4 different forms.

Theoretical Models of Magnetic Flux Tubes: Structure and Dynamics

Two types of model calculations for small scale magnetic flux tubes in the solar atmosphere are reviewed. In the first kind, one follows the temporal evolution governed by the complete set of the MHD and radiative transfer equations to a (quasi) stationary solution. From such a solution the continuum contrasts of a photospheric flux tube in the visible and in the infrared continuum at 1.6 μm have been computed and are briefly discussed. The second, more empirical type of method assumes the flux tubes to be in magnetohydrostatic equilibrium. It is computationally faster and more flexible and allows us to explore a wide range of parameters. Models and insights obtained from such parameter studies are discussed in some detail. These include an explanation for the peculiar variation of the area asymmetry of Stokes V profiles across the solar disk in terms of mass motions in the surroundings of magnetic flux tubes.Furthermore, a two-dimensional model of the lower chromosphere that has been developed is presented. Emphasis is laid on the effect of thermal bifurcation of the lower chromosphere on the structure of the chromospheric magnetic field. If the cool carbon monoxide clouds, observed in the infrared, occupy the non-magnetic regions, the flux tubes expand very strongly and form a magnetic canopy with an almost horizontal base. This has consequences for the spatial distribution of the Ca II K spectral line emission.Finally, some consideration is given to the formation and destruction of intense magnetic flux tubes in the solar photosphere. The formation is described as a consequence of the flux expulsion process that leads to a convective instability. A possible observational signature of this mechanism is proposed.

Do p-mode frequency shifts suggest a hotter chromosphere at solar maximum?

view Abstract Citations (43) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Do p-Mode Frequency Shifts Suggest a Hotter Chromosphere at Solar Maximum? Jain, Rekha ; Roberts, B. Abstract The frequencies of p-modes are calculated for a model atmosphere consisting of an isothermal gas, within which is embedded a uniform horizontal magnetic field, resting upon a field-free medium whose temperature increases linearly with depth. We compare our results with Libbrecht & Woodard's recent observations of frequency changes. It is found that if simultaneously the chromospheric magnetic field strength and chromospheric temperature are increased, then the frequencies of the p-modes are shifted in a qualitatively similar fashion to that found in the observations, i.e., the frequency shift increases with frequency until about 3.9 mHz, when the shift decreases dramatically. We suggest that the combined influence of an increase in magnetic field strength and an increase in chromospheric temperature plays an important role in determining the observed downturn in frequency shift. Although the resulting frequency shift curve resembles the observed shift, with both possessing a gradual rise phase followed by a turnover, the calculated shift fails to match the steep downturn. Refinements in our basic model and/or new physics are thus indicated. Publication: The Astrophysical Journal Pub Date: September 1993 DOI: 10.1086/173133 Bibcode: 1993ApJ...414..898J Keywords: Atmospheric Models; Chromosphere; Solar Activity; Solar Atmosphere; Solar Cycles; Solar Magnetic Field; Frequency Shift; Magnetic Field Configurations; Magnetic Flux; Solar Oscillations; Solar Physics; Stratified Flow; Temperature Distribution; Solar Physics; SUN: OSCILLATIONS; SUN: ACTIVITY; SUN: CHROMOSPHERE full text sources ADS |