Magnetic fields in flares and active prominences

We study the longitudinal magnetic field in a number of active limb prominences showing fields in excess of 30 G. The objects fall into three groups: surges, caps and active region prominences. There appears to be an upper limit of 150–200 G for the field strength in prominences.

A model of surges is presented in which a pre-surge axi-symmetric magnetic field is established by a line current in the corona. We observe particle acceleration in surges that indicates v×B≠0 in these objects during periods comparable to the Alfvén transit time.

The strong fields observed in caps seem to run between parts of active regions in accordance with Hale's law of sunspot group polarities.

     

Magnetic fields in flares and active prominences

The locations of flares and chromospheric absorption features on May 21 and 23, 1967, are compared with a series of H magnetograms. Each of the four major flares included in the study developed as double emission ribbons lying at positions of steep field gradient on opposite sides of the boundary between regions of opposite magnetic polarity. At certain stages, the flare outlines followed closely the isogauss contours of the longitudinal field. A fluctuating field of 75 gauss was measured directly in the importance two flare of May 21. Modifications in the magnetic structure of the active region followed the flares of May 23.     

Force-free magnetic fields and flares of August 1972

On the basis of observations (Zirin and Tanaka, 1973) inferring the presence of shear in magnetic fields, the amount of extractable energy stored in a class of force-free magnetic fields is evaluated for the flares of August 1972, using the formulations developed by Nakagawa and Raadu (1972). It is shown that the evaluated energy storage could be built up by the proper motions of sunspots in the active region McMath 11976 during July 31 and August 7. Then for the flare of August 7, a detailed analysis is made of the manner of energy release in the post maximum phases deduced from the configuration of flare loops. It is shown that the observed flare loops could be represented closely by the force-free magnetic fields and that the evaluated rate of energy release is consistent with observed rate given by the soft X-ray emission. The results of analysis suggest that the flare of August 1972 could be identified with the relaxation of an energetic force-free magnetic field towards lower energy states. The limitations and possible future extension of this type of analysis are discussed.Visiting scientist from the Tokyo Astronomical Observatory, Mitaka, Tokyo, Japan.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Magnetic flux changes associated with the solar flares of August 1972

The active region associated with Mt. Wilson sunspot group 18 935 (McMath, 11 976) which had a central meridian passage on August 4 and 5, 1972 produced a number of flares during transit. These included two importance 3B flares on August 4 and 7 as well as several of importance 1 and 2. Calculations of the total magnetic flux in this region were made during the period July 31 through August 9 using data from six observatories. For the 3B flare on August 4, the total flux changed from about 7.2 × 10²² Mx just before onset to about 5.6 × 10²² Mx two hours after onset. For the 3B flare on August 7, the flux was about 6.4 × 10²² Mx three hours before onset and about 5.2 × 10²² Mx three hours after onset. An importance 2B flare on August 2 had no measurable effect on the flux nor did any of several 1N or 1B flares which also occurred in this region during the period. The flux changes measured for the 3B flares occurred in the umbral and penumbral fields and no significant changes were observed in facular fields.The Aerospace Corporation, P.O. Box 92957, Los Angeles, Calif. 90009, U.S.A.     

The flares of August 1972

We present the analysis of observations of the August flares at Big Bear and Tel Aviv, involving monochromatic movies, magnetograms and spectra. In each flare the observations fit a model of particle acceleration in the chromosphere with emission produced by impact and by heating by the energetic electrons and protons. The region showed inverted polarity and high gradients from birth, and flares appear due to strong magnetic shears and gradients across the neutral line produced by sunspot motions. Post flare loops show a strong change from sheared, force-free fields parallel to potential-field-like loops, perpendicular to the neutral line above the surface.We detected fast (5 s duration) small (1') flashes in 3835 at the footpoints of flux loops in the August 2 impulsive flare at 1838 UT, which may be explained by dumping of > 50 keV electrons accelerated in individual flux loops. The flashes show excellent time and intensity agreement with > 45 keV X-rays. In the less impulsive 2000 UT flare a less impulsive wave of emission in 3835 moved with the separating footpoints. The thick target model of X-ray production gives a consistent model for X-ray, 3835 and microwave emission in the 18:38 UT event.Spectra of the August 7 flare show emission 12 Å FWHM in flare kernels, but only 1 to 2 Å wide in the rest of the flare. The kernels thus produce most of the H emission. The total emission in H in the August 4 and August 7 flares was about 2 × 10³⁰ erg. We belive this dependable value more accurate than previous larger estimates for great flares. The time dependence of total H emission agrees with radio and X-ray data much better than area measurements which depend on the weaker halo.Absorption line spectra show a large (6 km/s^-1) photospheric velocity discontinuity across the neutral line, corresponding to sheared flow across that line.This work has been supported by NASA under NGR 05 002 034, NSF Atmospheric Sciences program under GA 24015, and AFCRL under FI9628-73-C-0085.     

Magnetic fields in quiescent prominences

We discuss the longitudinal component of the magnetic field, B , based on data from about 135 quiescent prominences observed at Climax during the period 1968–1969. The measurements are obtained with the magnetograph which records the Zeeman effect on hydrogen, helium and metal lines. Use of the following lines, H; Hei, D₃, Hei, 4471 Å; Nai, Di and D₂, leads to the same value for the observed magnetic field component in these prominences. For more than half of the prominences their mean field, B , satisfy the inequalities 3 G B 8 G, and the overall mean value for all the prominences is 7.3 G. As a rule, the magnetic field enters the prominence on one side and exits on the other, but in traversing the prominence material, the field tends to run along the long axis of the prominence.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

An electrograph for measurement of macroscopic electric fields in prominences and flares

We describe an electrograph instrument designed for measurement of macroscopic electric fields in solar plasmas, using the polarization dependence of line width in Stark-broadened hydrogen Paschen emission lines. Observations of quiescent prominences and limb chromosphere with our electrograph at the NSO/Sac Peak Evans Coronal Facility provide upper limits of 5–10 V cm^–1 for transverse macroscopic electric fields in these structures, averaged over an area of about 5 × 7 arc sec. Random thermal motions of hydrogen ions across magnetic field lines generate a quasi-static electric field, which should be distinguishable from pressure broadening in the intensely magnetized chromosphere over a sunspot, given an electrograph sensitivity a factor 2–3 better than that achieved here. Future electrograph measurements of limb flares, post-flare loops and eruptive prominences, even at 5 V cm^–1 sensitivity, could provide a useful new test of reconnection and discharge effects in such dynamic structures.     

Cyclical variability of prominences, CMEs and flares

Solar flares, prominences and CMEs are well known manifestations of solar activity. For many years, qualitative studies were made about the cyclical behaviour of such phenomena. Nowadays, more quantitative studies have been undertaken with the aim to understand the solar cycle dependence of such phenomena as well as peculiar behaviour, such as asymmetries and periodicities, occurring within the solar cycle. Here, we plan to review the more recent research concerning all these topics.     

Theoretical model of flares and prominences

A theoretical model of flare which explains observed quantities in H, EUV, soft X-ray and flare-associated solar wind is presented. It is assumed that large mass observed in the soft X-ray flare and the solar wind comes from the chromosphere by the process like evaporation while flare is in progress. From mass and pressure balance in the chromosphere and the corona, the high temperature in the soft X-ray flare is shown to be attained by the larger mass loss to the solar wind compared with the mass remained in the corona, in accord with observations. The total energy of 10³² erg, the electron density of 10^13.5 cm^–3 in H flare, the temperature of the X-ray flare of 10^7.3K and the time to attain maximum H brightness (600 s) are derived consistent with observations. It is shown that the top height of the H flare is located about 1000 km lower than that of the active chromosphere because of evaporation. So-called limb flares are assigned to either post-flare loops, surges or rising prominences.The observed small thickness of the H flare is interpreted by free streaming and/or heat conduction. Applications are suggested to explain the maximum temperature of a coronal condensation and the formation of quiescent prominences.     

Testing MHD models of prominences and flares with observations of solar plasma electric fields

We present measurements of electric fields in quiescent prominences and in a small flare surge, obtained with the CRI electrograph at the NSO/SP 40 cm coronagraph, in 1993 and 1994. Our results on the 9 brightest quiescent prominences enable us to place r.m.s. upper limits ofE _t < 2 – 5 V cm^–1 on the component ofE transverse to the line of sight. We show that these upper limits may be difficult to reconcile with non-ideal MHD models of quiescent prominences formed in extended neutral sheets, whether or not the tearing mode instability is present. They do, however, seem consistent with ideal MHD models of prominence support. We point out also that these upper limits are within a factor 4 of the minimum value of anistropic electric field that exists due to motional Stark effect in any thermal plasma permeated by a directed magnetic field.Our data on the flare surge suggest an electric field of intensityE 35 V cm^–1, oriented approximately parallel to the inferred magnetic field. This detection ofE needs to be verified in other flares. But we note that a detectableE would not be expected in the current interruption flare mechanism, if only a single double layer is present. We show further that the observed relatively narrow, approximately-Gaussian, and only slightly Doppler-shifted Paschen lines, seem inconsistent with the multiple double layers invoked in other models based on the current interruption mechanism. Our detection ofE does seem consistent with reconnection (including tearing-mode) models of flares, provided the field-aligned electrical conductivity is anomalous over substantial volumes of the plasma circuit joining the reconnecting domain to the photosphere.     

Rotational models of solar prominences and flares

We assume the prominence (or flare) to be a rotating cylinder. For the two cases of the spin velocity being a constant and having a gradient, we calculate the profile of the Balmer lines and their variation from the centre of the prominence to the edge, and establish methods for finding the spin velocity from the inclination or shift of the lines and the velocity gradient from the curvature of the lines. These methods are then applied to the observed data of the ring flare of 1981 April 27.     

Remark on rotational motions in flares and prominences

From material secured with the McMath Solar Telescope of the Kitt Peak National Observatory rotational (orbital) motion has been found in a prominence ejected from a rotating flare. A period of rotation of 32 min has been derived from the study of a periodic asymmetry of the AL i 3961 absorption line.Presented at the Physics of Prominences-colloquium at Anacapri, September 29, 1971.     

Magnetic fields and flares in the region CMP 20 September 1963

The position of bright knots of 30 flares at their very beginning relative to the high-resolution isogauss maps of the longitudinal component (H ) and maps of the transverse component (H ) of magnetic field are considered for seven days during the passage of the active and large spot group in Sept. 1963 (see Table I and maps on Figures 1–8).The flare bright knots occur simultaneously in regions of opposite magnetic polarity, and the majority of these knots are adjacent to neutral line H = 0, although not coinciding precisely with this line (Figure 9). Lenticular form of flare knots and the motions of bright material of flares is restrained by transversal field H . Also flares are closely associated (83%) with so-called bifurcated regions, where specific crossing of transverse components takes place (Figures 4–5). There is well-expressed (80%) coincidence of flare knots with the strongest (positive or negative) electric currents as determined from the relation j = c/4 rot H. The relation of results obtained to some existing theories of flares is briefly discussed.U.S. Nat. Acad. of Science - U.S.S.R. Acad. Nauk. Exchange Scientist Program; now at CSIRO Division of Physics, Australia.     

The loop prominence of 11 August 1972: A coronal continuum event

Observations of a loop prominence formed after the flare of 11 August 1972 are discussed. Estimates of electron density are obtained from (a) the line ratio of the Ca xv forbidden lines and (b) a Thompson scattering model. Both methods give an approximate value of n _e = 10¹¹ cm^-3. This density was high enough to render the loop structures visible as continuum features, corresponding to the Ca xv structures as seen in the plane of the sky. By a double exposure technique, it was found that the loop structures seen in H and Fe xiv differ significantly.     

Delta spots and great flares

Using eighteen years of observations at Big Bear, we summarize the development of δ spots and the great flares they produce. We find δ groups to develop in three ways: eruption of a single complex active region formed below the surface, eruption of large satellite spots near (particularly in front of) a large older spot, or collision of spots of opposite polarity from different dipoles. Our sample of twenty-one δ spots shows that once they lock together, they never separate, although rarely an umbra is ejected. The δ spots are already disposed to their final form when they emerge. The driving force for the shear is spot motion, either flux emergence or the forward motion of p spots in an inverted magnetic configuration. We observe the following phenomena preceding great flares:

1. δ spots, preferentially Types 1 and 2.
2. Umbrae obscured by Hα emission.
3. Bright Hα emission marking flux emergence and reconnection.
4. Greatly sheared magnetic configurations, marked by penumbral and Hα fibrils parallel to the inversion line.

We assert that with adequate spatial resolution one may predict the occurrence of great flares with these indicators.     

Delta spots and great flares

Solar Physics - Using eighteen years of observations at Big Bear, we summarize the development of δ spots and the great flares they produce. We find δ groups to develop in three ways:...     

On the mechanism of formation of loop prominences

A mechanism is suggested for the formation of loop-type prominences in solar-active regions following flare events. The mechanism is based on the already existing idea of compression of a coronal plasma element resulting in enhanced radiation and consequent cooling of the element. A model is suggested for such a compression based on the concept of a contracting, force-free filamentary structure. If the current in a filament increases with time, then there is a radial contraction of the filament. Since the coronal plasma is frozen into the magnetic field lines of the filament, a contraction of the filament causes a compression of the filamentary plasma. This model of compression is shown to be in approximate qualitative and quantitative agreement with observations.     

Kinematic model of loop prominences formation

The kinematics of the material motion in a variable magnetic field in the MHD approximation of a strong field and cold plasma is investigated. The variation of magnetic moments of two dipole systems leads to the development of such phenomena as loop prominences, coronal rain and funnel prominences.     

Thermal effects in the formation of loop prominences

Loop prominences that appear after some solar flares may owe their form and duration to the behavior of the cooling function of the material (Cox and Tucker, 1969) and to the magnetic field configuration. Computer simulation of a model event shows that small temperature contrasts in a medium of several million degrees temperature may be enhanced as the medium cools by line and recombination radiation, on a time scale of several minutes. The loop prominence systems could be the result of this process repeated through successively higher levels of the solar atmosphere.