D. A. Falconer

Correlation of the Coronal Mass Ejection Productivity of Solar Active Regions with Measures of their Global Nonpotentiality from Vector Magnetograms: Baseline Results

From conventional magnetograms and chromospheric and coronal images, it is known qualitatively that the fastest coronal mass ejections (CMEs) are magnetic explosions from sunspot active regions in which the magnetic field is globally strongly sheared and twisted from its minimum-energy potential configuration. In this paper, we present measurements from active region vector magnetograms that begin to quantify the dependence of the CME productivity of an active region on the global nonpotentiality of its magnetic field. From each of 17 magnetograms of 12 bipolar active regions, we obtain a measure of the size of the active region (the magnetic flux content, Φ) and three different measures of the global nonpotentiality (LSS, the length of strong-shear, strong-field main neutral line; IN, the net electric current arching from one polarity to the other; and α = μIN/Φ, a flux-normalized measure of the field twist). From these measurements and the observed CME productivity of the active regions, we find that: (1) All three measures of global nonpotentiality are statistically significantly correlated with each other and with the active region flux content. (2) All three measures of global nonpotentiality are significantly correlated with CME productivity. The flux content has some correlation with CME productivity, but at a less than statistically significant confidence level (less than 95%). (3) The net current is less strongly correlated with CME productivity than is α, and the correlation of flux content with CME productivity is weaker still. If these differences in correlation strength, and a significant correlation of α with flux content, persist to larger samples of active regions, this would suggest that active region size does not affect CME productivity except through global nonpotentiality. (4) For each of the four global magnetic quantities, the correlation with CME productivity is stronger for a ±2 day time window for the CME production than for windows half as wide or twice as wide. This plausibly results from most CME-productive active regions producing less than one CME per day, and from active region evolution often significantly changing the global nonpotentiality over the course of several days. These results establish that measures of active region global nonpotentiality from vector magnetograms (such as LSS, IN, and α) should be useful for prediction of active region CMEs.

Magnetic Causes of Solar Coronal Mass Ejections: Dominance of the Free Magnetic Energy Over the Magnetic Twist Alone

We examine the magnetic causes of coronal mass ejections (CMEs) by examining, along with the correlations of active-region magnetic measures with each other, the correlations of these measures with active-region CME productivity observed in time windows of a few days, either centered on or extending forward from the day of the magnetic measurement. The measures are from 36 vector magnetograms of bipolar active regions observed within ~30° of disk center by the Marshal Space Flight Center (MSFC) vector magnetograph. From each magnetogram, we extract six whole-active-region measures twice, once from the original plane-of-the-sky magnetogram and again after deprojection of the magnetogram to disk center. Three of the measures are alternative measures of the total nonpotentiality of the active region, two are alternative measures of the overall twist in the active-regions magnetic field, and one is a measure of the magnetic size of the active region (the active regions magnetic flux content). From the deprojected magnetograms, we find evidence that (1) magnetic twist and magnetic size are separate but comparably strong causes of active-region CME productivity, and (2) the total free magnetic energy in an active regions magnetic field is a stronger determinant of the active regions CME productivity than is the fields overall twist (or helicity) alone. From comparison of results from the non-deprojected magnetograms with corresponding results from the deprojected magnetograms, we find evidence that (for prediction of active-region CME productivity and for further studies of active-region magnetic size as a cause of CMEs), for active regions within ~30° of disk center, active-region total nonpotentiality and flux content can be adequately measured from line-of-sight magnetograms, such as from SOHO MDI.

Magnetogram Measures of Total Nonpotentiality for Prediction of Solar Coronal Mass Ejections from Active Regions of Any Degree of Magnetic Complexity

For investigating the magnetic causes of coronal mass ejections (CMEs) and for forecasting the CME productivity of active regions, in previous work we have gauged the total nonpotentiality of a whole active region by either of two measures, LSSM and LSGM, two measures of the magnetic field along the main neutral line in a vector magnetogram of the active region. This previous work was therefore restricted to nominally bipolar active regions, active regions that have a clearly identifiable main neutral line. In the present paper, we show that our work can be extended to include multipolar active regions of any degree of magnetic complexity by replacing LSSM and LSGM with their generalized counterparts, WLSS and WLSG, which are corresponding integral measures covering all neutral lines in an active region instead of only the main neutral line. In addition, we show that for active regions within 30 heliocentric degrees of disk center, WLSG can be adequately measured from line-of-sight magnetograms instead of vector magnetograms. This approximate measure of active-region total nonpotentiality,LWLSG, with the extensive set of 96 minute cadence full-disk line-of-sight magnetograms from SOHO MDI, can be used to study the evolution of active-region total nonpotentiality leading to the production of CMEs.

Network Coronal Bright Points: Coronal Heating Concentrations Found in the Solar Magnetic Network

We examine the magnetic origins of coronal heating in quiet regions by combining SOHO/EIT Fe XII coronal images and Kitt Peak magnetograms. Spatial filtering of the coronal images shows a network of enhanced structures on the scale of the magnetic network in quiet regions. Superposition of the filtered coronal images on maps of the magnetic network extracted from the magnetograms shows that the coronal network does indeed trace and stem from the magnetic network. Network coronal bright points, the brightest features in the network lanes, are found to have a highly significant coincidence with polarity dividing lines (neutral lines) in the network and are often at the feet of enhanced coronal structures that stem from the network and reach out over the cell interiors. These results indicate that, similar to the close linkage of neutral-line core fields with coronal heating in active regions (shown in previous work), low-lying core fields encasing neutral lines in the magnetic network often drive noticeable coronal heating both within themselves (the network coronal bright points) and on more extended field lines rooted around them. This behavior favors the possibility that active core fields in the network are the main drivers of the heating of the bulk of the quiet corona, on scales much larger than the network lanes and cells.

An Assessment of Magnetic Conditions for Strong Coronal Heating in Solar Active Regions by Comparing Observed Loops with Computed Potential Field Lines

We report further results on the magnetic origins of coronal heating found from registering coronal images with photospheric vector magnetograms. For two complementary active regions, we use computed potential field lines to examine the global nonpotentiality of bright extended coronal loops and the three-dimensional structure of the magnetic field at their feet, and assess the role of these magnetic conditions in the strong coronal heating in these loops. The two active regions are complementary, in that one is globally potential and the other is globally nonpotential, while each is predominantly bipolar, and each has an island of included polarity in its trailing polarity domain. We find the following: (1) The brightest main-arch loops of the globally potential active region are brighter than the brightest main-arch loops of the globally strongly nonpotential active region. (2) In each active region, only a few of the mainarch magnetic loops are strongly heated, and these are all rooted near the island. (3) The end of each main-arch bright loop apparently bifurcates above the island, so that it embraces the island and the magnetic null above the island. (4) At any one time, there are other main-arch magnetic loops that embrace the island in the same manner as do the bright loops but that are not selected for strong coronal heating. (5) There is continual microflaring in sheared core fields around the island, but the main-arch bright loops show little response to these microflares. From these observational and modeling results we draw the following conclusions: (1) The heating of the main-arch bright loops arises mainly from conditions at the island end of these loops and not from their global nonpotentiality. (2) There is, at most, only a loose coupling between the coronal heating in the bright loops of the main arch and the coronal heating in the sheared core fields at their feet, although in both the heating is driven by conditions/events in and around the island. (3) The main-arch bright loops are likely to be heated via reconnection driven at the magnetic null over the island. The details of how and where (along the null line) the reconnection is driven determine which of the split-end loops are selected for strong heating. (4) The null does not appear to be directly involved in the heating of the sheared core fields or in the heating of an extended loop rooted in the island. Rather, these all appear to be heated by microflares in the sheared core field.

Solar Coronal Heating and the Magnetic Flux Content of the Network

We investigate the heating of the quiet corona by measuring the increase of coronal luminosity with the amount of magnetic flux in the underlying network at solar minimum when there were no active regions on the face of the Sun. The coronal luminosity is measured from Fe IX/X-Fe XII pairs of coronal images from SOHO/EIT, under the assumption that practically all of the coronal luminosity in our quiet regions comes from plasma in the temperature range 0.9 × 106 K ≤ T ≤ 1.3 × 106 K. The network magnetic flux content is measured from SOHO/MDI magnetograms. We find that the luminosity of the corona in our quiet regions increases roughly in proportion to the square root of the magnetic flux content of the network and roughly in proportion to the length of the perimeter of the network magnetic flux clumps. From (1) this result, (2) other observations of many fine-scale explosive events at the edges of network flux clumps, and (3) a demonstration that it is energetically feasible for the heating of the corona in quiet regions to be driven by explosions of granule-sized sheared-core magnetic bipoles embedded in the edges of network flux clumps, we infer that in quiet regions that are not influenced by active regions the corona is mainly heated by such magnetic activity in the edges of the network flux clumps. Our observational results together with our feasibility analysis allow us to predict that (1) at the edges of the network flux clumps there are many transient sheared-core bipoles of the size and lifetime of granules and having transverse field strengths greater than ~100 G, (2) ~30 of these bipoles are present per supergranule, and (3) most spicules are produced by explosions of these bipoles.

Coronal Heating by Magnetic Explosions

From magnetic fields and coronal heating observed in flares, active regions, quiet regions, and coronal holes, we propose that exploding sheared core magnetic fields are the drivers of most of the dynamics and heating of the solar atmosphere, ranging from the largest and most powerful coronal mass ejections and flares, to the vigorous microflaring and coronal heating in active regions, to a multitude of fine-scale explosive events in the magnetic network, driving microflares, spicules, global coronal heating, and, consequently, the solar wind.

Huge Coronal Structure and Heating Constraints Determined from SERTS Observations

Intensities of the extreme-ultraviolet (EUV) spectral lines were measured as a function of radius off the solar limb by two flights of the Goddards Solar Extreme-Ultraviolet Rocket Telescope and Spectrograph (SERTS) for three quiet-Sun regions. Density scale heights were determined for the different spectral lines. Limits on the filling factor were determined. In the one case where an upper limit was determined it was much less than unity. Coronal heating above 1.15 solar radii is required for all three regions studied. For reasonable filling factors, local heating is needed.

Magnetic Roots of Enhanced High Coronal Loops

We report results from an extension of a previous investigation of the magnetic roots of high-arching bright coronal loops (Porter et al 1994, in Proceedings of Kofu Symposium “New Look at the Sun”, ed. S. Enome, NRO Report No. 360, p. 65). In the previous work, the magnetic locations and magnetic structure of the brightest coronal features in a selected active region were determined by registering Yohkoh SXT images with a MSFC vector magnetogram via registration of the sunspots. The active region (AR 6982 on 26 Dec 91) was selected for study because it had a large delta sunspot with a core of strong magnetic shear along the polarity inversion; it was expected that such extemely nonpotential magnetic fields would foster exceptionally strong coronal heating and hence be exceptionally bright in coronal images. It was found that the coronal heating in this active region indeed was markedly more intense in the low-lying sheared core field than in the bulk of the field that arched over the sheared core and spanned the whole bipolar region. In addition, the coronal images showed something that was not anticipated in the selection of this region: a section of the high-arching envelope field was much brighter than the rest, showing that it received much more coronal heating than the rest of the envelope field. These enhanced high coronal loops stemmed from around an embedded island of opposite-polarity flux that was the site of microflaring and enhanced coronal heating. It was therefore surmised that the high bright loops somehow received their enhanced coronal heating from this foot-point activity. In the present work, by registering a full-disk Kitt Peak magnetogram with full-disk Yohkoh SXT images, we have found many more examples of large enhanced coronal loops having one foot rooted at a site of mixed polarity within an active region.