Flares and changing magnetic fields

An observational study of maps of the longitudinal component of the photospheric fields in flaring active regions leads to the following conclusions:

1. The broad-wing Hα kernels characteristic of the impulsive phase of flares occur within 10″ of neutral lines encircling features of isolated magnetic polarity (‘satellite sunspots’).
2. Photospheric field changes intimately associated with several importance 1 flares and one importance 2B flare are confined to satellite sunspots, which are small (10″ diam). They often correspond to spot pores in white-light photographs.
3. The field at these features appears to strengthen in the half hour just before the flares. During the flares the growth is reversed, the field drops and then recovers to its previous level.
4. The magnetic flux through flare-associated features changes by about 4 × 10¹⁹ Mx in a day. The features are the same as the ‘Structures Magnétiques Evolutives’ of Martres et al. (1968a).
5. An upper limit of 10²¹ Mx is set for the total flux change through McMath Regions 10381 and 10385 as the result of the 2B flare of 24 October, 1969.
6. Large spots in the regions investigated did not evince flux changes or large proper motions at flare time.
7. The results are taken to imply that the initial instability of a flare occurs at a neutral point, but the magnetic energy lost cannot yet be related to the total energy of the subsequent flare.
8. No unusual velocities are observed in the photosphere at flare time.

     

Erratum to: The Maunder Minimum and the Sun as the Possible Source of Particles Creating Increased Abundance of the <Superscript>14</Superscript>C Carbon Isotope

Solar flares take place in regions of strong magnetic fields and are generally accepted to be the result of a resistive instability leading to magnetic reconnection. When new flux emerges into a pre-existing active region it can act as a flare and coronal mass ejection trigger. In this study we observed active region 10955 after the emergence of small-scale additional flux at the magnetic inversion line. We found that flaring began when additional positive flux levels exceeded 1.38×10²⁰ Mx (maxwell), approximately 7 h after the initial flux emergence. We focussed on the pre-flare activity of one B-class flare that occurred on the following day. The earliest indication of activity was a rise in the non-thermal velocity one hour before the flare. 40 min before flaring began, brightenings and pre-flare flows were observed along two loop systems in the corona, involving the new flux and the pre-existing active region loops. We discuss the possibility that reconnection between the new flux and pre-existing loops before the flare drives the flows by either generating slow mode magnetoacoustic waves or a pressure gradient between the newly reconnected loops. The subsequent B-class flare originated from fast reconnection of the same loop systems as the pre-flare flows.     

A multiwavelength study of a double impulsive flare

Extensive data from the Solar Maximum Mission (SMM) and ground-based observatories are presented for two flares; the first occurred at 12:48 UT on 31 August, 1980 and the second just 3 min later. They were both compact events located in the same part of the active region. The first flare appeared as a typical X-ray flare: the Caxix X-ray lines were broadened ( 190±40 km s^-1) and blue shifted ( 60±20 km s^-1) during the impulsive phase, and there was a delay of about 30 s between the hard and soft X-ray maxima. The relative brightness of the two flares was different depending on the spectral region being used to observe them, the first being the brighter at microwave and hard X-ray wavelengths but fainter in soft X-rays. The second flare showed no significant mass motions, and the impulsive and gradual phases were almost simultaneous. The physical characteristics of the two flares are derived and compared. The main difference between them was in the pre-flare state of the coronal plasma at the flare site: before the first flare it was relatively cool (3 × 10⁶ K) and tenuous (4 × 10⁹ cm^-3), but owing to the residual effects of the first flare the coronal plasma was hotter (5 × 10⁶ K) and more dense (3 × 10¹¹ cm^-3) at the onset of the second flare. We are led to believe from these data that the plasma filling the flaring loops absorbed most of the energy released during the impulsive phase of the second flare, so that only a fraction of the energy could reach the chromosphere to produce mass motions and turbulence.A simple study of the brightest flares observed by the SMM shows that at least 43% of them are multiple. Thus, the situation studied here may be quite common, and the difference in initial plasma conditions could explain at least some of the large variations in observed flare parameters. We draw a number of conclusions from this study. First, the evolution of the second flare is substantially affected by the presence of the first flare. Secondly, the primary energy release in the second event is in the corona. Thirdly, the flares occur in a decaying magnetic region, probably as a result of the interaction of existing sheared loops; there is no evidence of emerging magnetic flux. Also, magnetic structures of greatly varying size participate in the flare processes. Lastly, there is some indication that the loops are not symmetrical or stable throughout the flares, i.e. the magnetic field does not act as a uniform passive bottle for the plasma, as is often assumed in flare models.NOAA/Space Environment Laboratory, currently at NASA/MSFC, Ala., U.S.A.Now at Sacramento Peak Observatory, Tucson, Ariz., U.S.A.     

Upper limit for the solar neutron flux in the energy interval 20–120 MeV

An experiment has been performed to search for the existence of a flux of solar neutrons at the earth using a detector sensitive to neutrons in the energy region 20–120 MeV. The instrument was carried by balloon to an atmospheric depth of 4 g/cm², from Palestine, Texas on the morning of November 2, 1967 and flown through sunrise and for about 7 hours into the day. Numerous flares of importance 1B or less occurred during the float period. By comparison of night and day counting rates we have deduced that the upper limit to the continuous emission of solar neutrons at the earth is 2 × 10^–2 neutrons/cm² sec in the above energy region. Using a theoretical form for the neutron differential energy spectrum we have expressed this result as an upper limit differential solar neutron flux. If neutrons were emitted in association with any of the small flares then the maximum flux at the earth was less than 4 × 10^–2 neutron/cm² sec in the same energy region. The minimum detectable flux with the present instrument is therefore well below the predicted flux from a 3B flare (e.g., Nov. 12, 1960) of 550 neutrons/cm² sec.     

Pre-flare association of magnetic fields and millimeter-wave radio emission

Observations of radio emission at 3.3 mm wavelength associated with magnetic fields in active regions are reported. Results of more than 200 regions during the years 1967–1968 show a strong correlation between peak enhanced millimeter emission, total flux of the longitudinal component of photospheric magnetic fields and the number of flares produced during transit of active regions. For magnetic flux greater than 10²¹ maxwells flares will occur and for flux of 10²³ maxwells the sum of the H flare importance numbers is about 40. The peak millimeter enhancement increases with magnetic flux for regions which subsequently flared. Estimates of the magnetic energy available and the correlation with flare production indicate that the photospheric fields and probably chromospheric currents are responsible for the observed pre-flare heating and provide the energy of flares.This work was supported in part by NASA Contract No. NAS2-7868 and in part by Company funds of The Aerospace Corporation.     

A search for solar neutrons during solar flares

The upper limit on the solar neutron flux from 1–20 MeV has been measured, by a neutron detector on the OGO-6 satellite, to be less than 5 × 10^–2 n cm^–2 s^–1 at the 95% confidence level for several flares including two flares of importance 3B and a solar proton event of importance 3B. The measurements are consistent with the models proposed by Lingenfelter (1969) and by Lingenfelter and Ramaty (1967) for solar neutron production during solar flares. The implied upper limit on the flux of 2.2 MeV solar gamma rays is about the same as the 2.2 MeV flux observed by Chupp et al. (1973).     

Magnetic Flux Cancellation Observed in the Sunspot Moat

In this paper we study the evolution of magnetic fields of a 1F/2.4C solar flare and following magnetic flux cancellation. The data are Big Bear Solar Observatory and SOHO/MDI observations of active region NOAA 8375. The active region produced a multitude of subflares, many of them being clustered along the moat boundary in the area with mixed polarity magnetic fields. The study indicates a possible connection between the flare and the flux cancellation. The cancellation rate, defined from the data, was found to be 3×10¹⁹ Mx h^–1. We observed strong upward directed plasma flows at the cancellation site. Suggesting that the cancellation is a result of reconnection process, we also found a reconnection rate of 0.5 km s^–1, which is a significant fraction of Alfvén speed. The reconnection rate indicates a regime of fast photospheric reconnection happening during the cancellation.     

The GLE-associated flare of 21 August, 1979

We use a variety of ground-based and satellite measurements to identify the source of the ground level event (GLE) beginning near 06∶30 UT on 21 August, 1979 as the 2B flare with maximum at ～06∶15 UT in McMath region 16218. This flare differed from previous GLE-associated flares in that it lacked a prominent impulsive phase, having a peak ～9 GHz burst flux density of only 27 sfu and a ?20 keV peak hard X-ray flux of ?3 × 10^-6 ergs cm^-2s^-1. Also, McMath 16218 was magnetically less complex than the active regions in which previous cosmic-ray flares have occurred, containing essentially only a single sunspot with a rudimentary penumbra. The flare was associated with a high speed (?700 km s^-1) mass ejection observed by the NRL white light coronagraph aboard P78-1 and a shock accelerated (SA) event observed by the low frequency radio astronomy experiment on ISEE-3.     

Force-free Magneitic Field Extrapolation for the Complex Sunspot Group of August 1972

Quantitative data on the magnetic field structure at all levels of the solar atmosphere are of basic importance for our understanding of physical processes in active regions in general and in flares in particular. Because photospheric longitudinal magnetograms are the most reliable data available, one has to look for a method of theoretical extrapolation of such data to higher levels. Such a method has been developed for force-free magnetic fields, i.e., ∇ X B = αB , with α = constant, satisfying more realistic boundary conditions as compared with earlier papers; e.g., the net magneitic flux through the magnetogram area is not required to be zero. The method has been used to calculate the magnetic field vector and lines of force in the flare-active region of August 1972. Calculated fields are compared with other observations such as structures in H α . Results of August 3rd show that the loop prominence systems observed during the flares of August 2nd and 4th are represented by a force-free field with positive α rather than by a current-free field ( α = 0) The extractable energy supply of this force-free field is of the order of magnitude of maximum flare demand (10³² erg); the height dependence of the magnetic field strengths agrees with that from radio and X-ray estimates. Similar results are obtained for the August 7th magnetic field structure.     

Electron density in flares II: Results of measurement

Measurements of the electron density in 16 flares are summarized and discussed. For 13 of them the electron density has been determined by the halfwidth method discussed in Part I of this paper. In the flash phase of all disk flares of importance 1 + and higher the electron density exceeds 10¹³ cm^–3 and increases with the flare importance. In the maximum of large flares the electron density exceeds 3 × 10¹³ cm^–3 and declines to 10¹³ cm^–3 and to lower values in about 20 minutes after the flash phase. In limb flares, i.e. higher than 5000 km above the solar limb, the electron density is lower than 5 × 10¹² cm^–3. This shows a decrease of the electron density in the flare elements situated in higher parts of the chromosphere. On the other hand, however, at least in some flares the electron density remains fairly constant within a wide range of height in the upper chromosphere and the low corona.     

A search for energetic neutrons emitted during solar flares

On the basis of solar flare forecasts, balloon flights were made from Hyderabad, India (vertical geomagnetic threshold rigidity of 16.9 GV), to detect the possible emission of high energy neutrons during solar flares. The detector comprised of a central plastic scintillator, completely surrounded by an anticoincidence plastic scintillator shield. The instrument responds to neutrons of about 15–150 MeV and gamma rays of about 5–30 MeV with about the same efficiency. The detector was flown to an atmospheric depth of 25 g cm^-2 on February 26, 1969; while the balloon was at ceiling a flare of importance 2B and one of 1N occurred. No perceptible flare associated increase in the counting rate was observed. Using the observed counting rates, an upper limit of 1.2 × 10^-2 neutrons cm^-2 sec^-1 is obtained for the first time for a flare of importance 2B for neutrons of energy 15–150 MeV. The corresponding upper limit for gamma rays of energy 5–30 MeV is found to be 10^-2 photons cm^-2 sec^-1. The neutron flux limits are compared with the recent calculations of Lingenfelter.     

Modeling and Measuring the Flux Reconnected and Ejected by the Two-Ribbon Flare/CME Event on 7 November 2004

Observations of the large two-ribbon flare on 7 November 2004 made using SOHO and TRACE data are interpreted in terms of a three-dimensional magnetic field model. Photospheric flux evolution indicates that ?1.4×10⁴³ Mx² of magnetic helicity was injected into the active region during the 40-hour buildup prior to the flare. The magnetic model places a lower bound of 8×10³¹ ergs on the energy stored by this motion. It predicts that 5×10²¹ Mx of flux would need to be reconnected during the flare to release the stored energy. This total reconnection compares favorably with the flux swept up by the flare ribbons, which we measure using high-time-cadence TRACE images in 1?600 Å. Reconnection in the model must occur in a specific sequence that would produce a twisted flux rope containing significantly less flux and helicity (10²¹ Mx and ?3×10⁴² Mx², respectively) than the active region as a whole. The predicted flux compares favorably with values inferred from the magnetic cloud observed by Wind. This combined analysis yields the first quantitative picture of the flux processed through a two-ribbon flare and coronal mass ejection.     

Solar flares as resulting from the temporary interruption of energy flow to the corona: A case of hydromagnetic resonance

The hypothesis that solar flares may be caused by a choking off of the normal energy flux to the corona by the strong closed magnetic fields of a plage is examined. If the energy flux into a plage from the photosphere is of the order of 10⁸ ergs/cm² sec, and if a substantial fraction of this energy is carried in the form of Alfvén waves, then the rate of dissipation of the waves is slower than the rate at which energy is injected. Since the waves must propagate along the magnetic field and cannot reenter the photosphere, they must remain within the plage; hence, the magnetic and kinetic energy in a small-scale motion (either waves, turbulence, or high-energy particles) must increase with time, eventually causing disruption of the volume when the small-scale energy density exceeds the energy in the mean field. It is believed that the unusually broad wings in the emission lines represent evidence of this phenomenon. The accumulation of waves is manifested as a resonance which occurs initially only at discrete locations in the magnetic field, but later is expected to involve the whole flare volume. The response of a typical volume of flare dimensions due to a trapping of the normal wave supply to the corona is studied through use of the virial equation. For magnetic fields typical of a plage, the region expands in a time scale of 10²–10³ sec, with a velocity in the neighborhood of 10–20 km/sec. Small-scale velocities within the region, however, have reached 100–300 km/sec, indicating that almost all the energy in the flare resides in small-scale forms. The energy density of the flare region exhibits a behavior much more explosive than the expansion rate. There is a rapid rise to maximum in 10² sec or less, and a slow subsequent decline taking about 10³–10⁴ sec due to the dilution of energy caused by expansion of the region. The predicted temporal behavior of the energy density coincides qualitatively with the light curves observed during flares, and it is suggested that the rise and decline of the energy density is to be associated with the optical flare. The total flare is defined as the time required for the energy density of the chromosphere and corona to return to the pre-flare state. During this time (about one hour) a large flare can derive the necessary 10³² ergs from normal photospheric energy output.     

Pre- and post-flare X-ray variations in active regions

Extremely low background noise of the HXIS experiment aboard the SMM made it possible to detect > 3.5 keV X-ray emissions from non-flaring active regions which are 10³–10⁴ times weaker than the X-ray flux from flares. Short-lived X-ray bursts and long-lived X-ray enhancements of various intensities seem to characterize active regions in different phases of their development. After major two-ribbon flares, giant X-ray arches are seen in the corona, slowly decaying for many hours after the flare end. Associated with these arches appear to be quasi-periodic flare-like variations of purely coronal nature.     

Evolution of the high-temperature plasma in the 15 June 1973 flare

The analysis of the high temperature plasma in Fe xxiii–xxiv in the 15 June 1973 flare is presented. The observations were obtained with the NRLXUV spectroheliograph on Skylab. The results are: (1) There was preheating of the active region in which the flare occurred. In particular, a large loop in the vicinity of the flaring region showed enhanced brightness for many hours before the flare. The loop disappeared when the flare occurred, and returned in the postflare phase, as if the energy flux which had been heating the large loop was blocked during the flare and restored after the flare was gone. The large magnetic fields did not change significantly. (2) The flare occurred in low-lying loop or loops. The spatial distribution of flare emission shows that there was a temperature gradient along the loop. (3) The high temperature plasma emitting Fe xxiii and xxiv had an initial upward motion with a velocity of about 80 km s^–1. (4) There was large turbulent mass motion in the high temperature plasma with a random velocity of 100 to 160 km s^–1. (5) The peak temperature of the hot plasma, determined from the Fe xxiii and xxiv intensity ratio, was 14 × 10⁶ K. It decreased slightly and then, for a period of 4 min, remained at 12.6 × 10⁶ K before dropping sharply to below 10 × 10⁶ K. The density of the central core of the hot plasma, determined from absolute intensity of Fe xxiv 255 Å line, was of the order of 10¹¹ cm^–3.The persistence of the high level of turbulence and of the high temperature plateau in the decaying phase of the flare indicates the presence of secondary energy release. From the energy balance equation the required energy source is calculated to be about 3 to 7 ergs cm^–3 s^–1.Ball Brothers Research Corporation.     

Tessellation of SoHO magnetograms

A gradient based algorithm which divides arbitrary images into non-overlapping surface filling tiles of opposite polarity is used to study the flux and size distributions of large scale magnetic flux concentrations in solar and heliospheric observatory (SoHO) magnetograms. The mean absolute flux and size of the concentrations at the considered scale is found to be about 1.7 &amp;#x00D7; 10¹⁸Mx and 5.2Mm for both polarities. The form of the flux distribution is characterized by a skewness of &amp;#x03B1;₃ = 4.9 and a kurtosis of &amp;#x03B1;₄ = 42.8. The fall in the distribution in the range 6.5 &amp;#x00D7; 10¹⁷ Mx to 5&amp;#x00D7;10¹⁸ Mx is described by an exponential fit, in agreement with a model for the sustenance of quiet region flux.     

A current ribbon model for energy storage and release with application to the flare of 7 January 1992

Observations of the flare on 7 January 1992 are interpreted using a topological model of the magnetic field. The model, developed here, applies a theory of three-dimensional reconnection to the inferred magnetic field configuration for 7 January. In the model field a new bipole ( 10²¹ Mx) emerges amidst pre-existing active region flux. This emergence gives rise to two current ribbons along the boundaries (separators) separating the distinct, new and old, flux systems. Sudden reconnection across these boundary curves transfers 3 ×10²⁰ Mx of flux from the bipole into the surrounding flux. The model also predicts the simultaneous (sympathetic) flaring of the two current ribbons. This explains the complex two-loop structure noted in previous observations of this flare. We subject the model predictions to comparisons with observations of the flare. The locations of current ribbons in the model correspond closely with those of observed soft X-ray loops. In addition the footpoints and apexes of the ribbons correspond with observed sources of microwave and hard X-ray emission. The magnitude of energy stored by the current ribbons compares favorably to the inferred energy content of accelerated electrons in the flare.     

The correlation of solar flare production with magnetic energy in active regions

An investigation of 531 active regions was made to determine the correlation between energy released by flares and the available energy in magnetic fields of the regions. Regions with magnetic flux greater than 10²¹ maxwell during the years 1967–1969, which included sunspot maximum, were selected for the investigation. A linear regression analysis of flare production on magnetic flux showed that the flare energy is correlated with magnetic energy with a coeificient of correlation of 0.78. Magnetic classification and field configuration also significantly affect the production of flares.This work was supported by the Aerospace Sponsored Research Program.     

Determination of magnetic helicity content of solar active regions from SOHO/MDI magnetograms

Chae (2001) first proposed a method of self-consistently determining the rate of change of magnetic helicity using a time series of longitudinal magnetograms only, such as taken by SOHO/MDI. Assuming that magnetic fields in the photosphere are predominantly vertical, he determined the horizontal component of velocity by tracking the displacements of magnetic flux fragments using the technique of local correlation tracking (LCT). In the present paper, after briefly reviewing the recent advance in helicity rate measurement, we argue that the LCT method can be more generally applied even to regions of inclined magnetic fields. We also present some results obtained by applying the LCT method to the active region NOAA 10365 under emergence during the observable period, which are summarized as follows. (1) Strong shearing flows were found near the polarity inversion line that were very effective in helicity injection. (2) Both the magnetic flux and helicity of the active region steadily increased during the observing period, and reached 1.2 × 10²² Mx and 8 ×10⁴² Mx², respectively, 4.5 days after the birth of the active region. (3) The corresponding ratio of the helicity to the square of the magnetic flux, 0.05, is roughly compatible with the values determined by other studies using linear-force-free modeling. (4) A series of flares took place while the rate of helicity injection was high. (5) The choice of a smaller window size or a shorter time interval in the LCT method resulted in a bigger value of the LCT velocity and a bigger value of the temporal fluctuation of the helicity rate. (6) Nevertheless when averaged over a time period of about one hour or longer, the average rate of helicity became about the same within about 10%, almost irrespective of the chosen window size and time interval, indicating that short-lived, fluctuating flows may be insignificant in transferring magnetic helicity. Our results suggest that the LCT method may be applied to 96-minute cadence full-disk MDI magnetograms or other data of similar kind, to provide a practically useful, if not perfect, way of monitoring the magnetic helicity content of active regions as a function of time.     

Determination of magnetic helicity content of solar active regions from SOHO/MDI magnetograms

Chae (2001) first proposed a method of self-consistently determining the rate of change of magnetic helicity using a time series of longitudinal magnetograms only, such as taken by SOHO/MDI. Assuming that magnetic fields in the photosphere are predominantly vertical, he determined the horizontal component of velocity by tracking the displacements of magnetic flux fragments using the technique of local correlation tracking (LCT). In the present paper, after briefly reviewing the recent advance in helicity rate measurement, we argue that the LCT method can be more generally applied even to regions of inclined magnetic fields. We also present some results obtained by applying the LCT method to the active region NOAA 10365 under emergence during the observable period, which are summarized as follows. (1) Strong shearing flows were found near the polarity inversion line that were very effective in helicity injection. (2) Both the magnetic flux and helicity of the active region steadily increased during the observing period, and reached 1.2 × 10²² Mx and 8 ×10⁴² Mx², respectively, 4.5 days after the birth of the active region. (3) The corresponding ratio of the helicity to the square of the magnetic flux, 0.05, is roughly compatible with the values determined by other studies using linear-force-free modeling. (4) A series of flares took place while the rate of helicity injection was high. (5) The choice of a smaller window size or a shorter time interval in the LCT method resulted in a bigger value of the LCT velocity and a bigger value of the temporal fluctuation of the helicity rate. (6) Nevertheless when averaged over a time period of about one hour or longer, the average rate of helicity became about the same within about 10%, almost irrespective of the chosen window size and time interval, indicating that short-lived, fluctuating flows may be insignificant in transferring magnetic helicity. Our results suggest that the LCT method may be applied to 96-minute cadence full-disk MDI magnetograms or other data of similar kind, to provide a practically useful, if not perfect, way of monitoring the magnetic helicity content of active regions as a function of time.