Philip R. Goode

Inferring the sun's internal angular velocity from observed p-mode frequency splittings

The suns internal solar velocity Omega is studied as a function of latitude and radius using the solar oscillation data of Brown and Morrow (1987). An attempt is made to separate robust inferences about the sun from artifacts of the analysis. It is found that a latitudinal variation of Omega similar to that observed at the solar surface exists throughout the suns convection zone and that the variation of Omega with latitude persists to some extent even beneath the convection zone. 44 refs.

The Formation of a Prominence in Active Region NOAA 8668. I. SOHO/MDI Observations of Magnetic Field Evolution

We have studied the evolution of the photospheric magnetic —eld in active region NOAA 8668 for 3 days while the formation of a reverse S-shaped —lament proceeded. From a set of full-disk line-of-sight magnetograms taken by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO), we have found a large canceling magnetic feature that was closely associated with the formation of the —lament. The positive —ux of the magnetic feature was initially 1.5 ] 1021 Mx and exponentially decreased with an e-folding time of 28 hr throughout the period of observations. We also have determined the transverse velocities of the magnetic —ux concentrations in the active region by applying local correlation tracking. As a result, a persistent pattern of shear motion was identi—ed in the neighborhood of the —lament. The shear motion had a speed of 0.2¨0.5 km s~1 and fed negative magnetic helicity of [3 ] 1042 Mx2 into the coronal volume during an observing run of 50 hr at an average rate of [6 ] 1040 Mx2 hr~1. This rate is an order of magnitude higher than the rate of helicity change due to the solar diUerential rotation. The magnetic —ux of the —eld lines created by magnetic reconnection and the magnetic helicity generated by the photospheric shear motion are much more than enough for the formation of the —lament. Based on this result, we conjecture that the —lament formation may be the visible manifestation of the creation of a much bigger magnetic structure that may consist of a —ux rope and an overlying sheared arcade.

The Seismic Structure of the Sun

Global Oscillation Network Group data reveal that the internal structure of the sun can be well represented by a calibrated standard model. However, immediately beneath the convection zone and at the edge of the energy-generating core, the sound-speed variation is somewhat smoother in the sun than it is in the model. This could be a consequence of chemical inhomogeneity that is too severe in the model, perhaps owing to inaccurate modeling of gravitational settling or to neglected macroscopic motion that may be present in the sun. Accurate knowledge of the suns structure enables inferences to be made about the physics that controls the sun; for example, through the opacity, the equation of state, or wave motion. Those inferences can then be used elsewhere in astrophysics.

Orientation of the magnetic fields in interplanetary flux ropes and solar filaments

Coronal mass ejections (CMEs) are often associated with erupting magnetic structures or disappearing filaments. The majority of CMEs headed directly toward the Earth are observed at 1 AU as magnetic clouds—the region in the solar wind where the magnetic field strength is higher than average and there is a smooth rotation of the magnetic field vectors. The three-dimensional structure of magnetic clouds can be represented by a force-free flux rope. When CMEs reach the Earth, they may or may not cause magnetic storms, alter Earths magnetic field, or produce the phenomena known as auroras. The geoeffectiveness of a solar CME depends on the orientation of the magnetic field in it. Two M-class solar flares erupted on 2000 February 17. The second flare occurred near a small active region, NOAA Active Region 8872. This eruption was accompanied by a halo CME. However, the February 17 CME did not trigger any magnetic activity when it arrived at the Earth. Another powerful flare, on 2000 July 14, was also associated with a halo CME, which caused the strongest geomagnetic activity of solar cycle 23. Using ACE measurements of the interplanetary magnetic fields, we study the orientation of the magnetic flux ropes in both sets of magnetic clouds and compare them with the orientation of the solar magnetic fields and disappearing filaments. We find that the direction of the axial field and helicity of the flux ropes are consistent with those of the erupted filaments. Thus, the geoeffectiveness of a CME is defined by the orientation and structure of the erupted filament and by its magnetic helicity as well. We also suggest that the geoeffectiveness of a CME can be forecasted using daily full-disk Hα and Yohkoh images and MDI magnetograms as well.

Rapid Change of δ Spot Structure Associated with Seven Major Flares

A large fraction of major flares occur in active regions that exhibit a δ configuration. The formation and disintegration of δ configurations is very important in understanding the evolution of photospheric magnetic fields. In this paper we study the relationship between the change in δ spot structures and associated major flares. We present a new observational result that part of penumbral segments in the outer δ spot structure decay rapidly after major flares; meanwhile, the neighboring umbral cores and/or inner penumbral regions become darker. Using white-light (WL) observations from the Transition Region and Coronal Explorer (TRACE), we study the short-term evolution of δ spots associated with seven major flares, including six X-class flares and one M-class flare. The rapid changes, which can be identified in the time profiles of WL mean intensity are permanent, not transient, and thus are not due to flare emission. The co-aligned magnetic field observations obtained with the Michelson Doppler Imager (MDI) show substantial changes in the longitudinal magnetic field associated with the decaying penumbrae and darkened central areas. For two events for which vector magnetograms were available, we find that the transverse field associated with the penumbral decay areas decreased while it increased in the central darkened regions. Both events also show an increase in the magnetic shear after the flares. For all the events, we find that the locations of penumbral decay are related to flare emission and are connected by prominent TRACE postflare loops. To explain these observations, we propose a reconnection picture in which the two components of a δ spot become strongly connected after the flare. The penumbral fields change from a highly inclined to a more vertical configuration, which leads to penumbral decay. The umbral core and inner penumbral region become darker as a result of increasing longitudinal and transverse magnetic field components.

Effects of differential rotation on stellar oscillations : a second-order theory

A complete formalism, valid through second order in differential rotation, is developed and applied to calculate the frequencies of stellar oscillations. We improved the derivation and generalized the asymptotic formulae for g-mode splittings. In application to solar oscillations, we find that the second-order effects are dominated by distortion for l<500. Further, these effects are sufficiently large that they must be accounted for in any effort to seismically determine the Suns internal magnetic field. In the solar oscillation spectrum, accidental degeneracies happen but cannot lead to large frequency shifts

RAPID CHANGES OF MAGNETIC FIELDS ASSOCIATED WITH SIX X-CLASS FLARES

In this paper, we present the results of the study of six X-class flares. We found significant changes in the photospheric magnetic fields associated with all of the events. For the five events in 2001, when coronagraph data were available, all were associated with halo coronal mass ejections. Based on the analyses of the line-of-sight magnetograms, all six events had an increase in the magnetic flux of the leading polarity of order of a few times 1020 Mx while each event had some degree of decrease in the magnetic flux of the following polarity. The flux changes are considered impulsive because the changeover time, which we defined as the time to change from preflare to postflare state, ranged from 10 to 100 minutes. The observed changes are permanent. Therefore, the changes are not due to changes in the line profile caused by flare emissions. For the three most recent events, when vector magnetograms were available, two showed an impulsive increase of the transverse field strength and magnetic shear after the flares, as well as new sunspot area in the form of penumbral structure. One of the events in this study was from the previous solar cycle. This event showed a similar increase in all components of the magnetic field, magnetic shear, and sunspot area. We present three possible explanations to explain the observed changes: (1) the emergence of very inclined flux loops, (2) a change in the magnetic field direction, and (3) the expansion of the sunspot, which moved some flux out of Zeeman saturation. However, we have no explanation for the polarity preference; i.e., the flux of leading polarity tends to increase while the flux of following polarity tends to decrease slightly.

Earthshine observations of the Earth's reflectance

Regular photometric observations of the moons “ashen light” (earthshine) from the Big Bear Solar Observatory (BBSO) since December 1998 have quantified the earths optical reflectance. We find large (∼5%) daily variations in the reflectance due to large-scale weather changes on the other side of the globe. Separately, we find comparable hourly variations during the course of many nights as the earths rotation changes that portion of the earth in view. Our data imply an average terrestrial albedo of 0.297±0.005, which agrees with that from simulations based upon both changing snow and ice cover and satellite-derived cloud cover (0.296±0.002). However, we find seasonal variations roughly twice those of the simulation, with the earth being brightest in the spring. Our results suggest that long-term earthshine observations are a useful monitor of the earths albedo. Comparison with more limited earthshine observations during 1994–1995 show a marginally higher albedo then.

Flare Activity and Magnetic Helicity Injection by Photospheric Horizontal Motions

We present observational evidence that the occurrence of homologous flares in an active region is physically related to the injection of magnetic helicity by horizontal photospheric motions. We have analyzed a set of 1 minute cadence magnetograms of NOAA AR 8100 taken over a period of 6.5 hours by Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO). During this observing time span, seven homologous flares took place in the active region. We have computed the magnetic helicity injection rate into the solar atmosphere by photospheric shearing motions, and found that a signicant amount of magnetic helicity was injected during the observing period. In a strong M4.1 flare, the magnetic helicity injection rate impulsively increased and peaked at the same time as the X-ray flux did. The flare X-ray flux integrated over the Xray emission time strongly correlates with the magnetic helicity injected during the flaring interval. The integrated X-ray flux is found to be a logarithmically increasing function of the injected magnetic helicity. Our results suggest that injection of helicity and abrupt increase of helicity magnitude play a signicant role in flare triggering.

Sources of Oscillation Frequency Increase with Rising Solar Activity

We analyze and interpret SOHO MDI data on oscillation frequency changes between 1996 and 2004, focusing on differences between the activity minimum and maximum of solar cycle 23. We study only the behavior of the centroid frequencies, which reflect changes averaged over spherical surfaces. Both the f-mode and p-mode frequencies are correlated with general measures of the Suns magnetic activity. However, the physics behind each of the two correlations is quite different. We show that for the f-modes the dominant cause of the frequency increase is the dynamical effect of the rising magnetic field. The relevant rise must occur in subphotospheric layers reaching to some 0.5-0.7 kG at a depth of about 5 Mm. However, the implied constraints also require the field change in the atmosphere to be so small that it has only a tiny dynamical effect on p-mode frequencies. For p-modes, the most plausible explanation of the frequency increase is a less than 2% decrease in the radial component of the turbulent velocity in the outer layers. Lower velocity implies a lower efficiency of the convective transport, hence lower temperature, which also contributes to the p-mode frequency increase.