High-latitude Filaments Research Articles

Modeling a Coronal Mass Ejection from an Extended Filament Channel. I. Eruption and Early Evolution

Abstract We present observations and modeling of the magnetic field configuration, morphology, and dynamics of a large-scale, high-latitude filament eruption observed by the Solar Dynamics Observatory. We analyze the 2015 July 9–10 filament eruption and the evolution of the resulting coronal mass ejection (CME) through the solar corona. The slow streamer-blowout CME leaves behind an elongated post-eruption arcade above the extended polarity inversion line that is only poorly visible in extreme ultraviolet (EUV) disk observations and does not resemble a typical bright flare-loop system. Magnetohydrodynamic (MHD) simulation results from our data-inspired modeling of this eruption compare favorably with the EUV and white-light coronagraph observations. We estimate the reconnection flux from the simulation’s flare-arcade growth and examine the magnetic-field orientation and evolution of the erupting prominence, highlighting the transition from an erupting sheared-arcade filament channel into a streamer-blowout flux-rope CME. Our results represent the first numerical modeling of a global-scale filament eruption where multiple ambiguous and complex observational signatures in EUV and white light can be fully understood and explained with the MHD simulation. In this context, our findings also suggest that the so-called stealth CME classification, as a driver of unexpected or “problem” geomagnetic storms, belongs more to a continuum of observable/nonobservable signatures than to separate or distinct eruption processes.

Filigree in the Surroundings of Polar Crown and High-Latitude Filaments

High-resolution observations of polar crown and high-latitude filaments are scarce. We present a unique sample of such filaments observed in high-resolution H$\alpha$ narrow-band filtergrams and broad-band images, which were obtained with a new fast camera system at the Vacuum Tower Telescope (VTT), Tenerife, Spain. The Chromospheric Telescope (ChroTel) provided full-disk context observations in H$\alpha$, Ca II K, and He I 10830 A. The Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) provided line-of-sight magnetograms and ultraviolet (UV) 1700 A filtergrams, respectively. We study filigree in the vicinity of polar crown and high-latitude filaments and relate their locations to magnetic concentrations at the filaments' footpoints. Bright points are a well studied phenomenon in the photosphere at low latitudes, but they were not yet studied in the quiet network close to the poles. We examine size, area, and eccentricity of bright points and find that their morphology is very similar to their counterparts at lower latitudes, but their sizes and areas are larger. Bright points at the footpoints of polar crown filaments are preferentially located at stronger magnetic flux concentrations, which are related to bright regions at the border of supergranules as observed in UV filtergrams. Examining the evolution of bright points on three consecutive days reveals that their amount increases while the filament decays, which indicates they impact the equilibrium of the cool plasma contained in filaments.

Multifractal properties of solar filaments and sunspots numbers

We analyze multifractal properties of low (LLSFNs; <50°), high (HLSFNs; ⩾50°), full-disk (FDSFNs; 0°∼90°) solar filament numbers (SFNs) and international sunspot numbers (ISNs) by estimating characteristic parameters (α0,Δα, spectrum skewness) of f(α) singularity spectrum. We find that the SFNs and ISNs have multifractal nature. The obtained α0 and Δα indicate that long-term behaviour of the solar filaments is more complex than that of the sunspots and the high-latitude filaments is the most complex in long-term behaviour. The spectrum skewnesses manifest that the ISNs display well symmetrical distribution in singularity strengths, whereas the SFNs are dominated by low singularity strengths, which means that the long-term behaviour of sunspots has homogenous structures and the filaments display averagely small fluctuations in amplitude. To detect the origin of their multifractality, we decompose the raw data of ISNs and SFNs: smoothed data represent ∼11-year cyclic activities and detrended data represent accidental activities. We also calculate their f(α) spectra, respectively. We find that the ∼11-year cyclic activities of filaments and sunspots tend to be a monofractal and display a bit predominance of low singularity strengths. Their accidental activities have the most complex behaviour than the raw and smoothed data. The accidental activities are dominated by high singularity strengths showing averagely large fluctuations in amplitude. Furthermore, multifractal properties from α0 and Δα of the accidental activities have the same features as that of raw data. We think that the ∼11-year periodic activity determines global fluctuations, while the accidental activities rule local complexity.

Solar Polar Fields and the 22-Year Activity Cycle: Observations and Models

We explore observations and models of the interacting, cyclical behavior of the active regions and the polar magnetic fields of the Sun. We focus on observational evidence of these fields interacting across the corridor between active and polar latitudes. We present observations of diverse magnetic signatures on, above and beneath the solar surface, and find much evidence of phenomena migrating in both directions across this corridor in each hemisphere, including photospheric fields, ephemeral bipoles, interior torsional oscillations, high-latitude filaments, and coronal green line intensity. Together these observations produce a complex physical picture of high-latitude solar magnetic field evolution in the photosphere, atmosphere and interior, and demonstrate their essential role in the solar cycle. The picture presented by these collected observations is consistent with the Babcock-Leighton phenomenological model for the cycle, and we discuss related efforts to predict cycle amplitudes based on polar field strengths and on combining activity and polar-field information in a single phase-independent, slowly-evolving index. We also briefly review related work on magnetic flux transport models for the solar cycle, with particular reference to the interaction between flux emergence patterns and meridional flows.

Fe XII STALKS AND THE ORIGIN OF THE AXIAL FIELD IN FILAMENT CHANNELS

Employing Fe XII images and line-of-sight magnetograms, we deduce the direction of the axial field in high-latitude filament channels from the orientation of the adjacent stalklike structures. Throughout the rising phase of the current solar cycle 24, filament channels poleward of latitude 30° overwhelmingly obeyed the hemispheric chirality rule, being dextral (sinistral) in the northern (southern) hemisphere, corresponding to negative (positive) helicity. During the deep minimum of 2007-2009, the orientation of the Fe XII stalks was often difficult to determine, but no obvious violations of the rule were found. Although the hemispheric trend was still present during the maximum and early declining phase of cycle 23 (2000-2003), several high-latitude exceptions were identified at that time. From the observation that dextral (sinistral) filament channels form through the decay of active regions whose Fe XII features show a counterclockwise (clockwise) whorl, we conclude that the axial field direction is determined by the intrinsic helicity of the active regions. In contrast, generation of the axial field component by the photospheric differential rotation is difficult to reconcile with the observed chirality of polar crown and circular filament channels, and with the presence of filament channels along the equator. The main role of differential rotation in filament channel formation is to expedite the cancellation of flux and thus the removal of the transverse field component. We propose further that, rather than being ejected into the heliosphere, the axial field is eventually resubmerged by flux cancellation as the adjacent unipolar regions become increasingly mixed.

CHIRALITY OF HIGH-LATITUDE FILAMENTS OVER SOLAR CYCLE 23

A non-potential quasi-static evolution model coupling the Sun's photospheric and coronal magnetic fields is applied to the problem of filament chirality at high latitudes. For the first time, we run a continuous 15 year simulation, using bipolar active regions determined from US National Solar Observatory, Kitt Peak magnetograms between 1996 and 2011. Using this simulation, we are able to address the outstanding question of whether magnetic helicity transport from active latitudes can overcome the effect of differential rotation at higher latitudes. Acting alone, differential rotation would produce high latitude filaments with opposite chirality to the majority type in each hemisphere. We find that differential rotation can indeed lead to opposite chirality at high latitudes, but only for around 5 years of the solar cycle following the polar field reversal. At other times, including the rising phase, transport of magnetic helicity from lower latitudes overcomes the effect of in situ differential rotation, producing the majority chirality even on the polar crowns at polar field reversal. These simulation predictions will allow for future testing of the non-potential coronal model. The results indicate the importance of long-term memory and helicity transport from active latitudes when modeling the structure and topology of the coronal magnetic field at higher latitudes.

THERMAL PROPERTIES OF A SOLAR CORONAL CAVITY OBSERVED WITH THE X-RAY TELESCOPE ONHINODE

Coronal cavities are voids in coronal emission often observed above high latitude filament channels. Sometimes, these cavities have areas of bright X-ray emission in their centers. In this study, we use data from the X-ray Telescope (XRT) on the Hinode satellite to examine the thermal emission properties of a cavity observed during July 2008 that contains bright X-ray emission in its center. Using ratios of XRT filters, we find evidence for elevated temperatures in the cavity center. The area of elevated temperature evolves from a ring-shaped structure at the beginning of the observation, to an elongated structure two days later, finally appearing as a compact round source four days after the initial observation. We use a morphological model to fit the cavity emission, and find that a uniform structure running through the cavity does not fit the observations well. Instead, the observations are reproduced by modeling several short cylindrical cavity cores with different parameters on different days. These changing core parameters may be due to some observed activity heating different parts of the cavity core at different times. We find that core temperatures of 1.75 MK, 1.7 MK and 2.0 MK (for July 19, July 21 and July 23, respectively) in the model lead to structures that are consistent with the data, and that line-of-sight effects serve to lower the effective temperature derived from the filter ratio.

The north-south asymmetry of solar activity at high latitudes

Solar long-term activity runs at high latitudes in three ways: (i) in phase with solar long-term activity at low latitudes; (ii) in antiphase with solar long-term activity at low latitudes and (iii) does not follow either (i) or (ii), and mainly occurs around the times of maxima of (i) and (ii). In the present study, we investigate the north-south asymmetry of solar activity at high latitudes and found the following. In Case (i), high-latitude filament activity, for example, is inferred to have the same dominant hemisphere as low-latitude activity in a cycle. In Case (ii), the north-south asymmetry of high-latitude activity, represented by both the polar faculae and the Sun's polar field strength, is usually different from that of low-latitude activity in a sunspot cycle, and even in a cycle of high-latitude activity (polar faculae and the Sun's polar field strength), suggesting that the north-south asymmetry of solar activity at high latitudes should have little or no connection with that of low latitudes. In Case (iii), the north-south asymmetry of solar activity at high latitudes (polar flares) should have little connection with that at low latitudes as well. The observed magnetic field at high latitudes is inferred to consist of two components: one comes from the emergence of the magnetic field from the Sun's interior and the other comes from the drift of the magnetic activity at low latitudes.

Does High‐Latitude Solar Activity Lead Low‐Latitude Solar Activity in Time Phase?

Using data from the Carte Synoptique solar filaments archive, we investigate whether there is a time lag between high-latitude solar activity and low-latitude solar activity. The cross-correlation analysis of the number of high-latitude filaments per Carrington rotation (NHF) and that of low-latitude filaments per Carrington rotation (NLF) shows, although inconclusively, that NLF possibly lags behind NHF. The periodic characteristics of both NHF and NLF clearly indicate that the activity of high-latitude filaments is evidently leading the activity of low-latitude filaments. Thus, the present study suggests that high-latitude solar activity leads low-latitude solar activity in time phase.

Formation of a Switchback During the Rising Phase of Solar Cycle 21

The customary notion that high-latitude filaments arise from magnetic flux originating in the active-region belts finds its modern expression in numerical models that generate filament channels from flux patterns migrating from active latitudes to the polar caps. Polarity inversions underlying high-latitude filament channels are swept into distinct patterns called `switchbacks' under the joint influence of differential rotation, supergranular diffusion, and meridional flow. The numerical model of Mackay and van Ballegooijen (2001) predicts a heretofore unsuspected solar-cycle dependence to the hemispheric pattern of filament magnetic fields. Observations presented here of a switchback formed early in cycle 21 confirm some key aspects of their model. In this remarkable example the flux diffusing out of the source region migrates to the opposite side of the Sun before it encounters another active region with which to create the quadrupolar field configuration wherein a return arm forms to complete the switchback.

Physical Conditions, Grain Temperatures, and Enhanced Very Small Grains in Barnard’s Loop

We derive the radio spectral index of Barnard's loop (BL) from large-scale radio surveys at four frequencies and find it to be a thermal source. We use the radio data together with Hα data to determine the electron temperature in BL, the λ Ori H II region, and a high-latitude filament; all of these regions are somewhat cooler than typical H II regions. We perform least-squares fits of the DIRBE diffuse IR intensities to the 21 cm line and radio continuum intensities. After the resolution of a geometrical conundrum, this allows us to derive the electron density ne; we find ne ≈ 2.0 cm-3 and pressure P/k ≈ 24,000 cm-3 K. Grains within BL are warmer than in H I regions. Trapped Lyα accounts for the extra heating that is required. This is a general effect that needs to be accounted for in all analyses that examine IR emission from H+ regions. Very small grains that emit 60 μm radiation are enhanced in BL relative to H I by a factor of 2-3, while polycyclic aromatic hydrocarbons that emit 12 μm are probably deficient by a factor of ~ 2.

Chirality, Helicity, and Joy’s Law

AbstractConnections between the morphology of filaments and the global topology of the sun’s magnetic field, are an important topic that seems ready for assembly into a coherent picture. We suggest that the chirality of mid-and high-latitude filaments described by Martin et al. is most easily understood if they form in evolved “Type 1” neutral lines within active region bipoles, rather than in the “Type 2” neutral lines formed at the boundaries between bipoles, where lower-latitude filaments tend to form. This interpretation removes the disagreement between filament chirality and the well-known tilt of bipolar active regions described by Joy’s law, which exists if the high-latitude filaments formed at Type 2 neutral lines. Dynamical explanation of these two observed regularities probably implies quantitative understanding of the processes that govern the sign and magnitude of the α and ѡ-effects in a solar dynamo.

Disappearances of High-Latitude Filaments as Sources of High-Latitude CMEs

AbstractFrom a statistical comparison of high-latitude coronal mass ejections (CMEs) and disappearing solar filaments (DSFs), we find: (1) Beginning with the “rush to the poles”, DSFs arise at an increasing rate from the “emerging” or polar crown relative to the “true” polar crown. At maximum, these “opposite polarity” filaments are the dominant source of high-latitude DSFs. Following polarity reversal, the emergent polar crown becomes the true polar crown and the source of virtually all highlatitude DSFs. (2) At the last two solar maxima, we estimate that there were ~ 4 times as many high-latitude (≥ 60°) CMEs as high-latitude (≥ 45°) DSFs. Possible reasons for this discrepancy include the following: (a) DSFs, particularly small ones, are under-reported, (b) propagation effects, whereby radially propagating CMEs appear at higher latitudes than their underlying source regions, are more important than currently thought, or (c) some combination of these effects.