R. L. Moore

Eruption of a Multiple-Turn Helical Magnetic Flux Tube in a Large Flare: Evidence for External and Internal Reconnection that Fits the Breakout Model of Solar Magnetic Eruptions

We present observations and an interpretation of a unique multiple-turn spiral flux tube eruption from active region 10030 on 2002 July 15. The TRACE C IV observations clearly show a flux tube that is helical and erupting from within a sheared magnetic field. These observations are interpreted in the context of the breakout model for magnetic field explosions. The initiation of the helix eruption, as determined by a linear backward extrapolation, starts 25 s after the peak of the flares strongest impulsive spike of microwave gyrosynchrotron radiation early in the flares explosive phase, implying that the sheared core field is not the site of the initial reconnection. Within the quadrupolar configuration of the active region, the external and internal reconnection sites are identified in each of two consecutive eruptive flares that produce a double coronal mass ejection (CME). The first external breakout reconnection apparently releases an underlying sheared core field and allows it to erupt, leading to internal reconnection in the wake of the erupting helix. This internal reconnection releases the helix and heats the two-ribbon flare. These events lead to the first CME and are followed by a second breakout that initiates a second and larger halo CME. The strong magnetic shear in the region is compatible with the observed rapid proper motion and evolution of the active region. The multiple-turn helix originates from above a sheared-field magnetic inversion line within a filament channel, and starts to erupt only after fast breakout reconnection has started. These observations are counter to the standard flare model and support the breakout model for eruptive flare initiation.

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.

Microflares in the solar magnetic network

Localized brightenings are found throughout the magnetic network in quiet sun image sequences obtained in the C IV 1548 A line by the SMM satellites UV spectrometer and polarimeter. Some bright sites are short-lived, while others persist. Plots of the intensity fluctuations show that the enhancements at both short- and long-lived sites are the result of localized impulsive heating events that occur intermittently at the short-lived sites and in more rapid succession at the long-lived ones. The number of these events and their visibility in the wings of the C IV line are consistent with their identification as the explosive events seen in UV spectra. 29 references.

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.

A tool for empirical forecasting of major flares, coronal mass ejections, and solar particle events from a proxy of active‐region free magnetic energy

Space Radiation Analysis Group (SRAG) at Johnson Space Center, which is responsible for the monitoring and forecasting of radiation exposure levels of astronauts. The new software tool is designed for the empirical forecasting of M‐ and X‐class flares, coronal mass ejections, and solar energetic particle events. For each type of event, the algorithm is based on the empirical relationship between the event rate and a proxy of the active region’s free magnetic energy. Each empirical relationship is determined from a data set of ∼40,000 active‐region magnetograms from ∼1300 active regions observed by SOHO/Michelson Doppler Imager (MDI) that have known histories of flare, coronal mass ejection, and solar energetic particle event production. The new tool automatically extracts each strong‐field magnetic area from an MDI full‐disk magnetogram, identifies each as a NOAA active region, and measures the proxy of the active region’s free magnetic energy from the extracted magnetogram. For each active region, the empirical relationship is then used to convert the free‐magnetic‐energy proxy into an expected event rate. The expected event rate in turn can be readily converted into the probability that the active region will produce such an event in a given forward time window. Descriptions of the data sets, algorithm, and software in addition to sample applications and a validation test are presented. Further development and transition of the new tool in anticipation of SDO/HMI are briefly discussed.

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.

Reflection and trapping of Alfven waves in a spherically symmetric stellar atmosphere

Alfven wave propagation in a spherically symmetric isothermal and stratified stellar atmosphere are analzyed using a time-dependent MHD numerical model. Particular consideration is given to wave reflection and the resultant trapping of the wave due to a peak in the Alfven speed in the atmosphere. Resonance frequencies in the trapping region and the effect of trapping on Alfven wave pressure force and propagation are examined. The data reveal that Alfven wave trapping has a potentially important role in accelerating winds from cool stars.

Reflection and trapping of transient Alfven waves propagating in an isothermal atmosphere with constant gravity and uniform magnetic field

A time-dependent linear magnetohydrodynamic numerical model was used to investigate the propagation of Alfven waves in an isothermal and stratified atmosphere with constant gravity and uniform vertical magnetic field. Results show that the Alfven wave transit time for the wave source to infinity is finite and that the wave exhibits continuous partial reflection which becomes total reflection as the front approaches infinity. The total reflection causes the waves to be trapped in the cavity that extends from the wave source to infinity and in which the wave energy is stored. The results suggest that the reflection of Alfven waves (of sufficiently long period) from the outer corona is an intrinsic phenomenon for any stellar atmosphere stratified by gravity and an open magnetic field, and that, therefore, such waves may be trapped in the stellar atmosphere. 15 refs.

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.

The Magnetic Structure of H-Alpha Macrospicules in Solar Coronal Holes

Measurements by Ulysses in the high-speed polar solar wind have shown the wind to carry some fine-scale structures in which the magnetic field reverses direction by having a switchback fold in it. The lateral span of these magnetic switchbacks, translated to the Sun, is of the scale of the lanes and cells of the magnetic network in which the open magnetic flux of the polar coronal hole and polar solar wind are rooted. This suggests that the magnetic switchbacks might be formed from network-scale magnetic loops that erupt into the corona and then undergo reconnection with the open field. This possibility motivated us to undertake the study reported here of the structure of H-alpha macrospicules observed at the limb in polar coronal holes, to determine whether a significant fraction of these eruptions appear to be erupting loops. From a search of the polar-coronal holes in 6 days of image-processed full-disk H-alpha movies from Big Bear Solar Observatory, we found a total of 35 macrospicules. Nearly all of these (32) were of one or the other of two different forms: 15 were in the form of an erupting loop, and 17 were in the form of a single-column spiked jet. The erupting-loop macrospicules are appropriate for producing the magnetic switchbacks in the polar wind. The spiked-jet macrospicules show the appropriate structure and evolution to be driven by reconnection between network-scale closed field (a network bipole) and the open field rooted against the closed field. This evidence for reconnection in a large fraction of our macrospicules (1) suggests that many spicules may be generated by similar but smaller reconnection events, and (2) supports the view that coronal heating and solar wind acceleration in coronal holes and in quiet regions and corona are driven by explosive reconnection events in the magnetic network.