Lower Solar Atmosphere Research Articles

Magnetic field, helicity and the 2000 July 14 flare in solar active region 9077

In this paper we analyse the non-potential magnetic field and the relationship with current (helicity) in the active region NOAA 9077 in 2000 July, using photospheric vector magnetograms obtained at different solar observatories and also coronal extreme-ultraviolet 171-Å images from the TRACE satellite. We note that the shear and squeeze of magnetic field are two important indices for some flare-producing regions and can be confirmed by a sequence of photospheric vector magnetograms and EUV 171-Å features in the solar active region NOAA 9077. Evidence on the release of magnetic field near the photospheric magnetic neutral line is provided by the change of magnetic shear, electric current and current helicity in the lower solar atmosphere. It is found that the ‘Bastille Day’ 3B/5.7X flare on 2000 July 14 was triggered by the interaction of the different magnetic loop systems, which is relevant to the ejection of helical magnetic field from the lower solar atmosphere. The eruption of the large-scale coronal magnetic field occurs later than the decay of the highly sheared photospheric magnetic field and also current in the active region.

Three‐dimensional Solutions for Fan and Spine Magnetic Reconnection in Partially Ionized Plasmas

Under the approximation of incompressible flow, we solve both momentum equations (for neutrals and ions) and magnetic induction equation at three-dimensional level, obtaining solutions for steady spine and fan magnetic reconnection in partially ionized plasmas. In the case of weak coupling between the two species, sharp structures can form not only for the magnetic field but also for the ions' velocity, while for the neutrals the level of sharpness falls off by an order of ionization degree. In the strong coupling case when both species move together sharp structures appear only for magnetic field. The results could be used to understand the phenomena which are observed in the solar lower atmosphere, e.g., Ellerman bombs, type II white light flares, and photospheric magnetic flux cancellation which may be caused by magnetic reconnection. In an ideal approximation, the sharp structures have a power-law distribution: in the fan reconnection the field and both flows go up according to z(-\kappa\), while in the spine reconnection they obey the inverse square law r(-2). However, due to the existence of finite resistivity, the enhancements of currents, field, and flows should saturate at some level in the diffusion layer. When the ionization degree goes up (corresponds to epsilon --> 0 in this paper), our solutions recover previous formulation for wholly ionized case. Since the Ohmic resistivity is very small (similar to 10(-10)), the induced current becomes so large that, as we estimate, it will cause plasma micro-instability which could speed up the reconnection process significantly. So near the current sheet which is provided here usual MHD description must fail, we need another theory to join in, such as kinematic description, to account for violent phenomena (e.g., Ellerman bombs and type II white light flares) which occur in the Sun's lower atmosphere.

Ellerman Bombs, Type II White-light Flares and Magnetic Reconnection in the Solar Lower Atmosphere

Ellerman bombs and Type II white-light flares share many common features despite the large energy gap between them. Both are considered to result from local heating in the solar lower atmosphere. This paper presents numerical simulations of magnetic reconnection occurring in such a deep atmosphere, with the aim to account for the common features of the two phenomena. Our numerical results manifest the following two typical characteristics of the assumed reconnection process: (1) magnetic reconnection saturates in ~ 600--900 s, which is just the lifetime of the two phenomena; (2) ionization in the upper chromosphere consumes quite a large part of the energy released through reconnection, making the heating effect most significant in the lower chromosphere. The application of the reconnection model to the two phenomena is discussed in detail.

Magnetic Energy Release and Transients in the Solar Flare of 2000 July 14

High-resolution observations of a large solar flare on 2000 July 14 (“Bastille Day Flare”) from the Michelson Doppler Imager instrument on the SOHO spacecraft reveal rapid variations of the magnetic field in the lower solar atmosphere during the flare. Some of these variations were irreversible, occurred in the vicinity of magnetic neutral lines, and likely were related to magnetic energy release in the flare. A surprising result is that these variations happened very rapidly on the scale of 10‐15 minutes in a large area of »50 Mm 2 at the beginning of the flare. Other, more localized and impulsive magnetic field variations somewhat similar to “magnetic transients” observed by Zirin and coworkers were accompanied by impulses in continuum intensity and Doppler velocity. These impulses have dynamic characteristics similar to Ellerman’s “bombs” and Severny’s “mustaches” and were probably caused by high-energy particles bombarding the solar surface.

Analysis of a UV Event in a Polar Coronal Hole

We present observations of a UV event which occurred in a polar coronal hole. They were obtained by SUMER on SOHO in several chromospheric and transition region spectral lines. Its birth site was about 50 arc sec inside the limb and in a network lane showing a net outflow before its initiation. The event had an extension of about 5 arc sec along the slit, a duration of about 3 min and was characterized by a large increase of intensity together with a significant line broadening with, however, downflows of about 50 km s−1 being dominant. Proper motions with a velocity of about 10 km s−1 were also observed. The event appeared at middle transition (O vi) temperatures and it simultaneously showed up in chromospheric (O i, Ly β) and low transition region (C ii) temperatures. We discuss this event in view of different scenarios to account for it. Our event could be a part of the large family of quiet-Sun explosive events observed by Ryutova and Tarbell (2000) taking place in polar coronal holes that are triggered by magnetic reconnection in the low solar atmosphere.

Cyclic Variation in the Solar Lower Atmosphere

CaII K line has been measured regularly nearly every month since 1974 at Kitt Peak. Our study on the data of CaII K profiles over two solar cycles indicates that in center disc spectra, the distance of the K1 component of the CaII K line from the line center and its intensity show periodical variations, but has a time delay of 1.2 years with respect to the sunspot number. Non-LTE computation implies a cyclic temperature variation of about 17 K of the temperature minimum region (TMR) in the quiet-Sun atmosphere and a cyclic variation of about 15-20 km of the position of the TMR.

THE MAGNETIC STRUCTURE AND GENERATION OF EUV FLARE RIBBONS

The `ribbons' of two-ribbon flares show complicated patterns reflecting the linkages of coronal magnetic field lines through the lower solar atmosphere. We describe the morphology of the EUV ribbons of the July 14, 2000 flare, as seen in SOHO, TRACE, and Yohkoh data, from this point of view. A successful co-alignment of the TRACE, SOHO/MDI and Yohkoh/HXT data has allowed us to locate the EUV ribbon positions on the underlying field to within ∼ 2′′, and thus to investigate the relationship between the ribbons and the field, and also the sites of electron precipitation. We have also made a determination of the longitudinal magnetic flux involved in the flare reconnection event, an important parameter in flare energetic considerations. There are several respects in which the observations differ from what would be expected in the commonly-adopted models for flares. Firstly, the flare ribbons differ in fine structure from the (line-of-sight) magnetic field patterns underlying them, apparently propagating through regions of very weak and probably mixed polarity. Secondly, the ribbons split or bifurcate. Thirdly, the amount of line-of-sight flux passed over by the ribbons in the negative and positive fields is not equal. Fourthly, the strongest hard X-ray sources are observed to originate in stronger field regions. Based on a comparison between HXT and EUV time-profiles we suggest that emission in the EUV ribbons is caused by electron bombardment of the lower atmosphere, supporting the hypothesis that flare ribbons map out the chromospheric footpoints of magnetic field lines newly linked by reconnection. We describe the interpretation of our observations within the standard model, and the implications for the distribution of magnetic fields in this active region.

Current-sheet buildup and magnetic reconnection in weakly ionized solar lower atmosphere

Solar observations show that magnetic reconnection can occur in the Sun's weakly ionized lower atmosphere (magnetic cancellation, Ellerman bombs and type II white-light flares). Unlike what the usual reconnection models have predicted, such a reconnection is accompanied by temperature enhancements which are less than 10%. To overcome this difficulty, we have reexamined the reconnection in a two-fluid model using a 2D numerical simulation. The numerical solutions demonstrate the following results: (1) Under the influence of Lorentz force, ionized gas carries the magnetic field into a diffusion region where part of the field is annihilated, and the current-sheet scaling laws for the weakly ionized plasma are basically the same as in the fully ionized case. (2) Though the neutral gas is not directly affected by the magnetic field due to frictional forces, its motion is almost the same as the ionized gas except in the region near stagnation point where the streamlines of both species differ appreciably. (3) The pressure of neutrals which governs the distribution of total pressure and temperature varies slightly. So the temperature of the whole domain is nearly uniform in space and constant in time. These results support the idea that magnetic cancellation, Ellerman bombs, and type II white-light flares are due to magnetic reconnection in the Sun's lower atmosphere.

SOHO/Energetic and Relativistic Nucleon and Electron Experiment Measurements of Energetic H, He, O, and Fe Fluxes during the 1997 November 6 Solar Event

A brilliant solar X-ray and Hα flare and a coronal mass ejection (CME) on 1997 November 6 were associated with high particle fluxes in interplanetary space at energies above MeV. The CME had an exceptionally high leading edge velocity (1560 km s-1) as observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO). The Energetic and Relativistic Nucleon and Electron experiment (ERNE), also on SOHO, measured high H, He, O, and Fe fluxes in several energy channels from 3 to 200 MeV nucleon-1. The oxygen energy spectrum in energy range 3-200 MeV nucleon-1 was of a broken-power-law form with a break around 50 MeV nucleon-1. In addition, ERNE observed abrupt changes in the intensity, elemental composition, and anisotropy of high-energy particles, which may indicate two energetic particle sources during several hours after the solar flare eruption. The observational results lead us to conclude that, most of the time, the O and Fe nuclei were injected by the interplanetary shock associated with the coronal mass ejection emitted around 12 UT on November 6. However, during two time periods the injection source might have been different or complementary. The first period was in the very beginning of the event, 13:20-13:40 UT, when the particle streaming showed very strong anisotropy with the maximum intensity from the direction of the Sun. The second was between 17 UT November 6 and 5 UT November 7, when particle fluxes were dominated by a particle population with a different elemental composition and a different spectral shape of O as compared with the particle population prevailing in the beginning and during the long decay phase of the event. We propose that the source of these particles was associated with a coronal shock wave traveling in the low solar atmosphere.

Quasi-harmonic faraday-rotation fluctuations of radio waves when sounding the outer solar corona

A statistical analysis of the Faraday-rotation fluctuations (FRFs) of linearly polarized radio signals from the Helios 1 and Helios 2 spacecraft shows that the FRF time power spectra can be of three types. Spectra of the first type are well fitted by a single power law in the range of fluctuation frequencies 1–10 mHz. Spectra of the second type are a superposition of a power law and two quasi-harmonic components with fluctuation frequencies of about v1=4 mHz (fundamental frequency) and v2=8 mHz (second harmonic). Spectra of the third type exhibit only one of the two quasi-harmonic components against the background of a power law. The spectral density of the quasi-harmonic components can be represented by a resonance curve with a fairly broad [Δυ ≈ (0.5–1.3)υ1,2] distribution relative to the v=v1, 2 peak. The intensity of the quasi-harmonic FRF has a radial dependence that roughly matches the radial dependence for the background FRF, while their period at the fundamental frequency is approximately equal to the period of the wellknown 5-min oscillations observed in the lower solar atmosphere. The fluctuations with 5-min periods in FRF records can be explained by the presence in the outer corona of isolated trains of Alfven waves generated at the base of the chromosphere-corona transition layer and by acoustic waves coming from deeper layers.

Solar Spicules: A Review of Recent Models and Targets for Future Observations – (Invited Review)

Since their discovery over 100 years ago, there have been many suggestions for the origin and development of solar spicules. Because the velocities of spicules are comparable to the sound and Alfven speeds of the low chromosphere, linear theory cannot fully describe them. Consequently, detailed tests of theoretical ideas had to await the development of computing power that only became available during the 1970s. This work reviews theories for spicules and spicule-like features over approximately the past 25 years, with an emphasis on the models based on nonlinear numerical simulations. These models have given us physical insight into wave propagation in the solar atmosphere, and have helped elucidate how such waves, and associated shock waves, may be capable of creating motions and structures on magnetic flux tubes in the lower solar atmosphere. So far, however, it has been difficult to reproduce the most-commonly-quoted parameters for spicules with these models, using what appears to be the most suitable input parameters. A key impediment to developing satisfactory models has been the lack of reliable observational information, which is a consequence of the small angular size and transient lifetime of spicules. I close with a list of key observational questions to be addressed with space-based satellites, such as the currently operating TRACE satellite, and especially the upcoming Solar-B mission. Answers to these questions will help determine which, if any, of the current models correctly explains spicules.

On the Disagreement between Atmospheric and Coronal Electron Densities

Electron densities in the lower solar atmosphere (photosphere and chromosphere) are derived using a number of different atmospheric models which are constrained by observed spectral lines. Comparing these atmospheric densities to coronal electron densities derived from polarization brightness measurements in the region from about 1.1 to several solar radii, it is shown that there is a discrepancy between the two sets of densities. The atmospheric electron densities are in agreement with a density of maximum 107 cm-3 at 1.1 R☉. The polarized brightness densities given in the literature are typically 5 × 107 cm-3 or higher. It is shown that this discrepancy might be due to an overestimation of the coronal electron densities below 1.5-2 R☉.

Quiescent Prominence Equilibria

With a new analytical method, we present a static model for quiescent solar prominences of inverse polarity. These prominences have a plasma β slightly below unity and correspond to those located in the lower corona. Although this static model cannot address the helmet-like features at the top of the prominences, which are believed to be associated with the open-field topology and driven by outflows, the model may nevertheless capture several characteristic features of the solar prominences. These features include the sheetlike gas condensation hanging above an asymptotically weak magnetic cusp, the sheared magnetic arcades/loops, the coronal cavity, radio dark strips cospatial with the Hα filaments, bright radio ribbons located on both sides of the dense prominence sheet, and the increasing magnetic field strength with height along the prominence sheet. In our model, the densest gas is contained within a narrow field-free tube immediately above the prominence sheet. Thermally insulated by the surrounding strong fields, this narrow tube may be a natural site for housing the cool gas (≥104 K) siphoned from the lower solar atmosphere. We also find that there are hot spots (≤107 K) located on the sides of the prominence loop, which might be the source of thermal X-ray emission in the quiet Sun.

Topological Differences Between Force-Free Field Models

The potential and linear force-free field models for the magnetic field in the solar corona are often used in the analysis of flares. The field is calculated using boundary values measured in the low solar atmosphere. The topology of the field calculated using these models is then compared to the position of flare emissions. We demonstrate that the topology of the field according to each of these models, with the same boundary conditions in place, is not in general even qualitatively equivalent. An argument is given for a similar discrepancy between a linear force-free field solution and a nonlinear force-free field solution.

Counter-streaming gas flows in solar prominences as evidence for vertical magnetic fields

Solar prominences are sheets of relatively cool and dense gas embedded in the surrounding hotter corona. An erupting prominence can inject a mass of up to 1015 g into the solar wind1 as part of a coronal mass ejection. These eruptions must depend critically on the topology of the prominence's magnetic field. In all present models2,3, the prominence hangs on horizontal or helical field lines, while an overlying magnetic arcade temporarily restrains the prominence from erupting. Such models are inconsistent, however, with the slow upward vertical gas flows that are seen in prominences4,5,6,7,8,9,10,11,12,13,14. Here we report counter-streaming flows along closely spaced vertical regions of a prominence, between its top and the lower solar atmosphere. As the flows must be aligned with the magnetic field, this observation implies that a field connects the prominence directly to the photosphere, contrary to all existing models. These magnetic ‘tethers’ might help prevent a prominence from erupting.

Dynamic Responses to Magnetic Reconnection in Solar Arcades

We present a numerical simulation of the interaction between two line dipoles through magnetic in the lower solar atmosphere, a process believed to be the origin of many manifestations of solar activity. This work differs from previous studies in that the field is sheared asymmetrically and that the dipoles have markedly unequal field strengths. This calculation already yielded one key discovery, denoted driven current filamentation, as described in a previous Astrophysical Journal letter. In this paper we focus on the chromospheric and coronal dynamics resulting from the shear-driven of unequal dipoles, discuss the important implications for chromospheric eruptions, compare our calculation with high-resolution Normal Incidence X-Ray Telescope observations of a surge, and contrast our results with the predictions of fast reconnection models.

Evolution of magnetic helicity in magnetic reconnection

This paper presents a definition of magnetic helicity specifically for two-dimensional magnetic fields and derives the associated helicity equation. The newly defined helicity is closely related to its three-dimensional counterpart and serves as a measure of the shear of magnetic field. Based on this, a numerical simulation is carried out on magnetic reconnection occurring in the lower solar atmosphere. It is found that the helicity dissipation due to magnetic reconnection is very small. A large amount of helicity is transferred upward and escapes from the domain of the solution, and the total helicity is approximately conserved during the magnetic reconnection and helicity transfer. This is in support of the applicability of a postulate, which was proposed by Taylor (1974, 1986) concerning the approximate conservation of magnetic helicity in the presence of resistive dissipation and magnetic reconnection in a highly conductive laboratory plasma, to the solar atmosphere.

White-Light Polarization and Large-Scale Coronal Structures

The results of the white-light polarization measurements performed during three solar eclipses (1973, 1980, 1991) are presented. The eclipse images were processed and analysed by the same technique and method and, consequently, the distributions of the polarization and coronal intensity around the Sun were obtained in unified form for all three solar eclipses. The mutual comparisons of our results, and their comparison with the distributions found by other authors, allowed the real accuracy of the current measurements of the white-light corona polarization, which is not worse than ±5%, to be estimated. We have investigated the behaviour of the polarization in dependence on heliocentric distance in helmet streamers and coronal holes. Simultaneous interpretation of the data on polarization and intensity in white-light helmet streamers is only possible if a considerable concentration of coronal matter (plasma) towards the plane of the sky is assumed. The values obtained for the coronal hole regions can be understood within the framework of a spherically symmetrical model of the low density solar atmosphere. A tendency towards increasing polarization in coronal holes, connected with the decrease of the hole's size and with the transition from the minimum to the maximum of the solar cycle, was noticed. The problem of how the peculiarities of the large-scale coronal structures are related to the orientation of the global (dipole) solar magnetic field and to the degree of the goffer character of the coronal and interplanetary current sheet is discussed briefly.

Spatial Configuration of Highly Sheared Magnetic Structures in Active Region (NOAA 6659) in 1991 June

In this paper, we analyze first the configuration, especially the reversal features in the Hβ chromospheric longitudinal magnetograms in a very flare-active δ group (NOAA 6659). We discuss the possibility of large-scale reversal chromospheric magnetic structures overlapped on the photospheric umbrae after the comparison between the photospheric vector magnetograms and chromospheric longitudinal magnetograms and their forming height difference. We present the possible spatial configuration of the magnetic shear and the relationship with the electric current, and we and that some highly sheared magnetic features of opposite polarity in the active region deviated from the constant-α force-free field. The interaction of the newly emerging magnetic flux of opposite polarity with large-scale magnetic structures is the triggering mechanism of a series of powerful fiares formed in the lower solar atmosphere.

Compressive fluctuations in the solar wind and their polytropic index

Abstract. Magnetohydrodynamic compressive fluctuations of the interplanetary plasma in the region from 0.3 to 1 AU have been characterized in terms of their polytropic index. Following Chandrasekhar's approach to polytropic fluids, this index has been determined through a fit of the observed variations of density and temperature. At least three different classes of fluctuations have been identified: (1) variations at constant thermal pressure, in low-speed solar wind and without a significant dependence on distance, (2) adiabatic variations, mainly close to 1 AU and without a relevant dependence on wind speed, and (3) variations at nearly constant density, in fast wind close to 0.3 AU. Variations at constant thermal pressure are probably a subset of the ensemble of total-pressure balanced structures, corresponding to cases in which the magnetic field magnitude does not vary appreciably throughout the structure. In this case the pressure equilibrium has to be assured by its thermal component only. The variations may be related to small flow-tubes with approximately the same magnetic-field intensity, convected by the wind in conditions of pressure equilibrium. This feature is mainly observed in low-velocity solar wind, in agreement with the magnetic topology (small open flow-tubes emerging through an ensemble of closed structures) expected for the source region of slow wind. Variations of adiabatic type may be related to magnetosonic waves excited by pressure imbalances between contiguous flow-tubes. Such imbalances are probably built up by interactions between wind flows with different speeds in the spiral geometry induced by the solar rotation. This may account for the fact that they are mainly found at a large distance from the sun. Temperature variations at almost constant density are mostly found in fast flows close to the sun. These are the solar wind regions with the best examples of incompressible behaviour. They are characterized by very stable values for particle density and magnetic intensity, and by fluctuations of Alfvénic type. It is likely that temperature fluctuations in these regions are a remnant of thermal features in the low solar atmosphere. In conclusion, the polytropic index appears to be a useful tool to understand the nature of the compressive turbulence in the interplanetary plasma, as far as the frozen-in magnetic field does not play a crucial role.