Active Region Filament Research Articles

Circular Polarization in a Solar Filament

Integrating 26 624 pairs of video frames, the authors have mapped the circular polarization in an active-region filament against the solar disk by using a traditional magnetograph working at the Hβ line. This filament, offset the disk center, appeared at the boundary of three decayed active regions. It was quiet and away from any strong enhanced network. The mapped circular polarization in the filament has an average polarization degree of 1.1×10−3 with a measurement precision of 4×10−4. The mapping of circular polarization in a filament may provide a supplementary diagnosis of the filament magnetic field, in addition to the mapping of linear polarization via the Hanle effect. However, the interpretation of the circular polarization requires treatment of the full quantum problem of Zeeman and non-Zeeman effects of Stokes line profiles.

Active-Region Filaments and X-ray Sigmoids

We use Yohkoh soft X-ray telescope data and Hα full-disk observations to study the evolution of chromospheric filaments and coronal sigmoids in 6 active regions in association with coronal mass ejections (CMEs). In two cases, CMEs are directly observed by the SOHO/LASCO C2 coronagraph. In four cases, other observations (magnetic clouds, geomagnetic storms, sigmoid-arcade evolution) are used as CME indicators. Prior to eruption, each active region shows a bright coronal sigmoidal loop and underlying Hα filament. The sigmoid activates, erupts and gets replaced by a cusp, or an arcade. In contrast, the Hα filament shows no significant changes in association with sigmoid eruption and CME. We explain these observations in a framework of the classical two-ribbon flare model.

Oscillations in an active region filament: Observations and comparison with MHD waves

During the MEDOC Campaign 4, on October 1999, observations of a solar active region filament were carried out by the SUMER/SoHO spectrometer. A time sequence of this filament has been obtained with a duration of 7 h 30 min and with a temporal resolution of 30 s. The Fourier analysis of the line-of-sight Doppler velocities measured in the 584.33 AHeI line allows us to detect oscillations in several ranges of periodicities (short periods: less than 5 min, intermediate periods: 6-20 min, and long periods: greater than 40 min). From a theoretical point of view, we consider the possible modes of oscillations of an active region filament. Following Joarder & Roberts (1993), we treat the filament as a plasma slab embedded in a uniform magnetic field inclined at an angle $\phi$ to the long axis of the slab. Solving the dispersion equations for Alfven waves and magnetoacoustic waves, primary and secondary mode frequencies appear to be non-equidistant. For the comparison between the observed and calculated frequencies, we outline an identification method of the oscillation modes in the observed filament. This identification provides a diagnostic of the filament: the angle between the magnetic field and the long axis of the slab is estimated to be 18°, and the magnetic field strength B (G) is proportional to the square root of the density in the slab.

Spectral Diagnostics of Chromospheres—A Signature of Emergent Magnetic Flux

The chromospheres of the Sun and solar-like stars respond to the underlying magnetic activity. The spectral lines of singly ionized calcium (e.g., Ca II K at 3933 Å) exhibit single or double reversals and serve as diagnostic tools for the magnetic activity. We present observations showing that the Ca II K spectral line may exhibit reversals in addition to the well-known K1, K2, and K3 components, when observed with high angular and spectral resolution. These Ca II K spectral line anomalies occur when small-scale, subarcsecond magnetic fields emerge through the chromosphere. The anomalous profiles originate at locations that are cospatial with Hα brightening at footpoints of dark active region fibrils, or active region filaments. The photospheric magnetic field is bipolar (small opposite polarity magnetic nodules) or has neutral lines at cospatial locations where the spectral lines are anomalous. Small-scale reconnective processes can cause these anomalous profiles. From a simultaneous time series of magnetic field data, we find that the emerging magnetic flux is associated with these profiles. The additional reversals primarily occur redward of the familiar K3 absorption trough, implying chromospheric downflow velocities that lead to the formation of shocks. The magnitude of the Doppler shift is of the order 40-50 km s-1. We present evidence to suggest that a combination of subresolution elements, each of which has regular profiles with large relative Doppler shifts could explain the formation of some of these profiles. A heuristic model that combines buoyant magnetic fields, convective collapse, gas evacuation, shock formation, heating, and a multicomponent model atmosphere can reproduce these profiles.

Filament activity and photospheric magnetic evolution related to flares

Three succcssive activations of all active-region filament were observed over a period of eight hours. Each disturbance showed distinctive characteristics and was followed by a flare of different properties. The filament had one end rooted in strong delta -sunspots. During the first activation, the filament rose up at the rooting end and detached from the delta -sunspots while its main body remained in place and separated into two twisted threads. During the second activation, only one thread a as disturbed and finally disappeared; twisted threads, however, appeared during the third activation. The evolution of photospheric magnetic fields, associated with the filament disturbances. Was characterized by squeezing and shearing of delta -sunspots, flux cancellation and emergence beneath the two ends of the filaments. Our data suggests that magnetic reconnection in the photospheric layer is the likely cause of filament destabilisation. cause of filament destabilisation.

SUMER/SOHO Observations of Long Period Oscillations in an Active Region Filament

During the MEDOC campaign #4 (October 1999), we observed an active region filament with the SUMER/SoHO spectrometer using the 584.33 Å HeI line. After a description of the HeI line profile in the filament, we present a Fourier analysis of SUMER long time observations. This analysis allows to detect oscillations in several ranges of periodicities: 6–20 min, 40–90 min. We discuss these periodicities in terms of Alfvén and magnetoacoustic waves obtained with a filament model developed by Joarder & Roberts (1993).

An Erupting Active Region Filament: Three-Dimensional Trajectory and Hydrogen Column Density

From 15:33 through 16:02 UT on 13 June 1998, observations of an erupting filament as it crossed solar disk center were obtained with the NSO/KPVT and SOHO/CDS instruments as part of the SOHO Joint Observing Program 70. Context observations show that this event was the eruption of the north-east section of a small active region filament associated with NOAA 8237, that the photospheric magnetic field was changing in this active region between 12–14 June 1998, and that a coronal Moreton-wave disk event occurred, as well as a white-light CME off the south-west solar limb. The NSO/KPVT imaging spectroscopy data covered 512 × 512 arc sec of the disk center and were spectrally centered at the He i 1083 nm line and captured ± 1.0 nm of surrounding solar spectrum. The He i absorption line is seen blue-shifted to velocities of between 200 and 300 km s−1. The true solar trajectory of the eruption is obtained by using the projected solar coordinates and by integrating the Doppler velocity. The filament travels with a total velocity of about 300 km s−1 along a path inclined roughly 49 deg to the solar surface and rises to a height of just over 1.5 solar radii before it becomes too diffuse to follow. The filament also shows internal motions with multiple Doppler components shifted by ± 25 km s−1. Finally, the KPVT data show no Stokes V profiles in the Doppler-shifted He i 1083.03 nm absorption to a limit of roughly 3×10−3 times the continuum intensity. The SOHO/CDS scanned the center of the KPVT FOV using seven EUV lines; Doppler-shifted filament emission is seen in lines from He i 58.4 nm, He ii 30.4 nm, O iv 55.5 nm, O v 63.0 nm, Ne vi 56.3 nm, and Mg x 61.0 nm representing temperatures from about 2×104K through 1×106K. Bound-free continuum absorption from H i, without confusion from foreground emission and line emission, is seen as the filament obscures underlying chromospheric emission. A fit to the wavelength dependence of the absorption from five lines between 55.5 to 63.0 nm yields a column density ξH I =4.8±2.5×1017 cm−2. Spatial maps show that this filament absorption is more confined than the regions which show emission.

Dipped Magnetic Field Configurations Associated with Filaments and Barbs

In this paper, three-dimensional linear force-free field configurations that can be associated with filaments are considered. It is assumed that the field configurations are suitable to represent filaments if they contain magnetic dips. With the photospheric flux distribution chosen to be an arcade with a dextral/sinistral axial component, it is found that dipped configurations exist only for large values of alpha (where, ∇×B=αB). The dips always lie above the polarity inversion line in the centre of the channel between the flux regions. When the dips are viewed from above to a depth of 1 Mm they resemble closely the shape of filaments viewed in absorption on the solar disk. As the magnitude of alpha increases, the horizontal and vertical extent of the dips also increases, giving active-region filaments for low values of alpha and quiescient filaments for high values of alpha. Dextral filaments only form for negative values of alpha and sinistral filaments for positive values of alpha. The portion of the field line that is dipped is always of inverse polarity and the magnitude of the field in the dipped region increases with height, both of which are consistent with Leroy, Bommier, and Sahal-Brechot (1983). Overlying the region of dips there are arcades of normal polarity which have the correct left-bearing/right-bearing orientation for dextral/sinistral filaments. When the hypothesis of barbs occurring in dipped field lines is used, barbs that branch out of the main axis and to the right/left for dextral/sinistral filaments can be formed around minority polarity elements on either side of the polarity inversion line. No barbs are found around normal polarity elements. The model reproduces many of the observed features of filament channels, filaments and their barbs.

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 |

Simultaneous multifrequency observations of an eruptive prominence at millimeter wavelengths

Radio images and spectra of an eruptive prominence were obtained from simultaneous multifrequency observations at 36 GHz, 89 GHz, and 110 GHz on May 28, 1991 with the 45-m radio telescope at Nobeyama Radio Observatory (NRO), the National Astronomical Observatory, Japan (NAOJ). The radio spectra indicated that the optical depth is rather thick at 36 GHz whereas it is thin at 89 and 110 GHz. The Hα data, taken at Norikura Solar Observatory, NAOJ, suggest that the eruption of an active region filament was triggered by an Hα flare. The shape and position of the radio prominence generally coincided with those of Hα images. The radio emission is explained with an isothermal cool thread model. A lower limit for the electron temperature of the cool threads is estimated to be 6100 K. The range of the surface filling factors of the cool threads is 0.3–1.0 after the Hα flare, and 0.2–0.5 in the descending phase of the eruptive prominence. The column emission measure and the electron number density are estimated to be of the order of 1028 cm−5 and 1010 cm−3, respectively. The physical parameters of a quiescent prominence are also estimated from the observations. The filling factors of the eruptive prominence are smaller than those of the quiescent prominence, whereas the emission measures and the electron densities are similar. These facts imply that each cool thread of the prominence did not expand after the eruption, while the total volume of the prominence increased.

Coronal mass ejections and major solar flares: The great active center of March 1989

The solar flare and coronal mass ejection (CME) events associated with the large and complex March 1989 active region are discussed. This active region gave us a chance to study the relation of CME with truly major solar flares. The work concentrates on questions of the relation of CMEs and flares to one another and to other types of activity on the Sun. As expected, some major (X‐3B class) flares had associated CMEs. However, an unexpected finding is that others did not. In fact, there is strong evidence that the X4‐4B flare of March 9th had no CME. This lack of a CME for such an outstanding flare event has important implications to theories of CME causation. Apparently, not all major flares cause CMEs or are caused by CMEs. The relations between CMEs and other types of solar activity are also discussed. No filament disappearances are reported for major CMEs studied here. Comparing these results with other studies, CMEs occur in association with flares and with erupting prominences, but neither are required for a CME. The relation between solar structures showing flaring without filament eruptions and structures showing filament eruptions without flares becomes important. The evolutionary relation between an active flaring sunspot region and extensive filaments without sunspots is reviewed, and the concept of an “evolving magnetic structure” (EMS) is introduced. It is suggested that all CMEs arise in EMSs and that CMEs provide a major path through which azimuthal magnetic fields escape from the Sun during the solar cycle.

Sites of flares and filaments in solar active regions

We applied a method developed to derive from magnetic field data the location of energy release in solar active regions. This method has been applied to two sets of data (June 13–15 1980 and June 23–25 1989) using magnetograms (Meudon, MSFC, Potsdam) and Hα data (Debrecen, MSDP Meudon). The Hα flares are located at intersections of the separatrices with the chromosphere in places where high density currents are observed. This supports the idea that the energy released in flares is due to magnetic reconnection at the location of a separator and subsequently transported to the chromosphere along field lines. It is found that strong magnetic shear is a necessary condition for filament formation. Magnetic shear merely determines the importance of the flare and is certainly not a necessary condition for flare activity.

Structures of chromospheric magnetic field in the vicinity of the filament of active region 5669

In this paper, the chromospheric magnetic structures and their relation to the photospheric vector magnetic field in the vicinity of a dark filament in active region 5669 have been demonstrated. Structural variations are shown in chromospheric magnetograms after a solar flare. Filament-like structures in the chromospheric magnetograms occurred after a solar flare. They correspond to the reformation of the chromospheric dark filament, but there is no obvious variation of the photospheric magnetic field. We conclude that (a) some of the obvious changes of the chromospheric magnetic fields occurred after the flare, and (b) a part of these changes is perhaps due to flare brightening in the chromospheric Hβ line.

The effect of retardation on the stability of current filaments

We investigate the influence of the finite Alfven velocity on the evolution of an active region filament. In general, variations of a current result in variations of the magnetic fields which spread around with the Alfven velocity. As a consequence of the fact that a magnetic field can only change with the Alfven velocity, a filament will experience the photospheric boundary conditions as these were at an Alfven travel time back in time. The inclusion of this retardation effect in the momentum equation of a filament leads effectively to an extra force term. This force contribution acts in the direction in which the filament moves and has therefore a destabilizing effect on the filament. Because a moving filament acts as an antenna of Alfven waves, the filament loses energy by the emission process. This leads to a radiative damping term in the equation of motion of the filament. In general, the radiative damping will be sufficiently strong to counteract the retardation instability. Numerical simulations show that during the energy build-up phase a filament follows the van Tend-Kuperus equilibrium curve. After the van Tend-Kuperus equilibrium has disappeared the filament goes through a transient phase moving with a sub-Alfvenic velocity upward. At greater heights the repulsive Lorentz force of the photospheric surface current magnetic field is balanced by the radiative damping, resulting in a decreasing filament velocity.

Civ plasma flow near active region filaments

Selected CivV0 lines (lines separating adjacent regions of strong blue shift and strong red shift) located near the solar limb and oriented parallel to the radius vector from disk center are shown to be closely aligned with Hα dark filaments in active regions. The filaments, in turn, are known to lie in the vicinity of magnetic neutral lines. The radial orientation of the V0 lines minimizes uncertainties in image registration and their location near the limb ensures that the observed fluid motion has major components paralleling V0. It follows that the filaments are located at sites of velocity shear, and, by inference, of magnetic shear. For a case in which a given V0 line is observed near both east and west limbs, the gradient of Doppler velocity across the V0 line reverses sign from one limb to the other as is expected for horizontal steady flow. Thus, the velocity vectors remain fixed with respect to the filament.

Relationship between a spot and a filament observed during spacelab 2 mission

During the Spacelab 2 Mission, an active region including a sunspot, plages, fibrils and a filament which disappears during the observation period, was observed in Hα and in the C IV lines. The analysis of the observations leads to several conclusions. Shear in the active region filament is observed before its disappearance. Hα and C IV upflows in the filament are well correlated spatially, but the transition zone (C IV) velocities are an order of magnitude larger than the Hα chromospheric ones. Over the sunspot, the reverse-Evershed effect is observed in Hα and in C IV Dopplershift images while strong downflows are also detected in the C IV profiles. Magnetograph data for the whole region are used to calculate the linear force-free magnetic field. A single value of the force-free parameter α is found to give a good fit to both the sheared filament structure and the sunspot spiral structures, indicating a causal relationship. We propose a model based on the inductive coupling between current systems associated with the filament and the sunspot. Relaxation phenomena due to resistive effects in the filament could lead to irreversible conversion of magnetic energy into kinetic energy and heating.

Mass motions in active region filaments

Mass motions in active region filaments of regions NOAA 4171 and McMath 16208 are analyzed. Both regions were continuously observed with the Zeiss Lyot filter for about a week at the Hida Observatory. As for NOAA 4171, Dopplergrams are made from the Hα filtergrams of 7 wavelengths for the qualitative study of the velocity fields in the filament, and Beckers' cloud model analysis is employed for a quantitative study of them. Dopplergrams of McMath 16208 are also constructed for qualitative analyses to determine whether the results derived from the analysis of NOAA 4171 are applicable to this region.

Post-flare loops: formation and velocity

Post-flare loops are generally observed between two ribbon flares. The formation of post flare loops and active region or plage filaments has been explained in a model based on magnetic reconnection and chromospheric ablation /1,2/. This model uses a magnetic topology which is relevant to solar flares /3/ : a large flare or an instability opens the magnetic lines of a coronal arcade (or arch) and a vertical current sheet forms. Then the reconnection of the magnetic field follows according to the scenario of Kopp and Pneuman /4/. In this magnetic configuration, we show that the formation of condensations or dense loops is induced by reconnection shocks. We give in this communication a new method of diagnostic to derive from Hα profiles physical parameters, i.e. source function, optical depth and velocity. This method called “differential cloud method” is very promising for future observations.

Material ejecta in a disturbed solar filament

Hα observations, using the Multichannel Subtractive Double Pass (MSDP) spectrograph operating on the Meudon Solar Tower, have been made of an active region filament which undergoes a ‘disparition brusque’. The period of observation was from 10 ∶ 45 to 13 ∶ 30 UT on 22 June, 1981. Velocity and intensity fluctuations in Hα were measured. The proper motions of ejecta were followed allowing their trajectories and vector velocities to be determined. To model the dynamics of ejecta several models using thermal or magnetic driving forces are compared. The most promising model explains the motion as the consequence of magnetic stresses acting on an isolated magnetized plasmoid in a diverging flux tube.

The stability of line tied force-free cylindrical arcades: Is an active region filament a requirement for a two-ribbon flare?

The MHD stability of force-free, cylindrical arcades is investigated, including the stabilising effect of photospheric line tying. It is found that a wide variety of fields are stable. This suggests that either a departure from a force free equilibrium or suppression of line tying is necessary if a two-ribbon flare is to be triggered. It is postulated that in both circumstances, the existence of an active region filament is an essential preflare requirement for a two-ribbon flare.