Rudolf W. Komm

Meridional flow of small photospheric magnetic features

We study the meridional flow of small magnetic features, using high-resolution magnetograms taken from 1978 to 1990 with the NSO Vacuum Telescope on Kitt Peak. Latitudinal motions are determined by a two-dimensional crosscorrelation analysis of 514 pairs of consecutive daily observations from which active regions are excluded. We find a meridional flow of the order of 10 m s−1, which is poleward in each hemisphere, increases in amplitude from 0 at the equator, reaches a maximum at mid-latitude, and slowly decreases poleward. The average observed meridional flow is fit adequately by an expansion of the formM (θ) = 12.9(±0.6) sin(2θ) + 1.4(±0.6) sin(4θ), in m s−1 whereθ is the latitude and which reaches a maximum of 13.2 m s−1 at 39°. We also find a solar-cycle dependence of the meridional flow. The flow remains poleward during the cycle, but the amplitude changes from smaller-than-average during cycle maximum to larger-than-average during cycle minimum for latitudes between about 15° and 45°. The difference in amplitude between the flows at cycle minimum and maximum depends on latitude and is about 25% of the grand average value. The change of the flow amplitude from cycle maximum to minimum occurs rapidly, in about one year, for the 15–45° latitude range. At the highest latitude range analyzed, centered at 52.5°, the flow is more poleward-than-average during minimumand maximum, and less at other times. These data show no equatorward migration of the meridional flow pattern during the solar cycle and no significant hemispheric asymmetry. Our results agree with the meridional flow and its temporal variation derived from Doppler data. They also agree on average with the meridional flow derived from the poleward migration of the weak large-scale magnetic field patterns but differ in the solar-cycle dependence. Our results, however, disagree with the meridional flow derived from sunspots or plages.

Solar Cycle Changes in GONG p-Mode Frequencies, 1995-1998

We have analyzed 27 3 month sets of Global Oscillaiton Network Group (GONG) data from the end of cycle 22 and the beginning of cycle 23 and here present evidence of significant shifts in the central frequencies and the even a-coefficients of the frequency splittings of the modes. The temporal behavior of the even a-coefficients is better reproduced by the corresponding coefficients of a Legendre polynomial decomposition of the surface magnetic field than by the total flux; i.e., the temporal variation is strongly correlated with the latitudinal distribution of the surface magnetic activity. These changes are consistent with available data from previous solar cycles. The even a-coefficients, which sense the asphericity of the solar structure, appear to show similar temporal evolution at all depths. The odd a-coefficients, which sense the internal differential rotation, show no significant variation with time or depth. In particular they show no significant correlation with either the magnetic flux or with the corresponding odd Legendre components of the flux. This suggests that the solar cycle related variation of the oscillation frequencies is not due to contamination of observed Doppler shifts by the surface magnetic fields.

A Comparison of Solar p-Mode Parameters from the Michelson Doppler Imager and the Global Oscillation Network Group: Splitting Coefficients and Rotation Inversions

Using contemporaneous helioseismic data from the Global Oscillation Network Group (GONG) and Michelson Doppler Imager (MDI) onboard SOHO, we compare frequency-splitting data and resulting inversions about the Suns internal rotation. Helioseismology has been very successful in making detailed and subtle inferences about the solar interior. But there are some significant differences between inversion results obtained from the MDI and GONG projects. It is important for making robust inferences about the solar interior that these differences are located and their causes eliminated. By applying the different analysis pipelines developed by the projects not only to their own data but also to the data from the other project, we conclude that the most significant differences arise not from the observations themselves but from the different frequency estimation analyses used by the projects. We find that the GONG pipeline results in substantially fewer fitted modes in certain regions. The most serious systematic differences in the results, with regard to rotation, appear to be an anomaly in the MDI odd-order splitting coefficients around a frequency of 3.5 mHz and an underestimation of the low-degree rotational splittings in the GONG algorithm.

SOLAR CONVECTION-ZONE DYNAMICS, 1995-2004

The nine-year span of medium-degree helioseismic data from the Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI) allows us to study the evolving zonal flows in the solar convection zone over the rising phase, maximum, and early declining phase of solar cycle 23. Using two independent two-dimensional rotation inversion techniques, we investigate the depth profile of the flow pattern known as the torsional oscillation. The observations suggest that the flows penetrate deep within the convection zone—perhaps to its base—even at low latitudes, and that the phase of the pattern is approximately constant along lines of constant rotation rather than lines of constant latitude.

Solar-Cycle Changes in GONG P-Mode Widths and Amplitudes 1995-1998

We search for a solar-cycle variation in mode widths and amplitudes derived from 3 month GONG time series. The variation of mode width and amplitude observed in GONG data are the combined effects of fill factor, temporal variation, and measurement uncertainties. The largest variation is caused by the fill factor resulting in modes with increased width and reduced amplitude when fill is lower. We assume that the solar-cycle variation is the only other systematic variation beside the temporal window function effect. We correct all currently available data sets for the fill factor and simultaneously derive the solar-cycle variation. We find an increase of about 3% on average in mode width from the previous minimum to October 1998 and a decrease of about 7% and 6% in mode amplitude and mode area (width × amplitude). We find no l dependence of the solar-cycle changes. As a function of frequency, these changes show a maximum between 2.7 and 3.3 mHz with about 47% higher than average values for mode width and about 29% and 36% higher ones for mode amplitude and area. We estimate the significance of these rather small changes by a prewhitening method and find that the results are significant at or above the 99.9% level, with mode area showing the highest level of significance and mode width the lowest. The variation in background amplitude is most likely not significant and is consistent with a zero change.

Torsional oscillation patterns in photospheric magnetic features

We analyzed 689 high-resolution magnetograms taken daily with the NSO Vacuum Telescope on Kitt Peak from 1975 to 1991. Motions in longitude on the solar surface are determined by a one-dimensional crosscorrelation analysis of consecutive day pairs. The main sidereal rotation rate of small magnetic features is best fit byω = 2.913(±0.004) − 0.405(±0.027) sin2φ − 0.422(±0.030) sin4φ, in µrad s−1, whereφ is the latitude. Small features and the large-scale field pattern show the same general cycle dependence; both show a torsional oscillation pattern. Alternating bands of faster and slower rotation travel from higher latitudes toward the equator during the solar cycle in such a way that the faster bands reach the equator at cycle minimum. For the magnetic field pattern, the slower bands coincide with larger widths of the crosscorrelations (corresponding to larger features) and also with zones of enhanced magnetic flux. Active regions thus rotate slower than small magnetic features. This magnetic torsional oscillation resembles the pattern derived from Doppler measurements, but its velocities are larger by a factor of more than 1.5, it lies closer to the equator, and it leads the Doppler pattern by about two years. These differences could be due to different depths at which the different torsional oscillation indicators are rooted.

Rotation rates of small magnetic features from two- and one-dimensional cross-correlation analyses

We present results of an analysis of 628 high-resolution magnetograms taken daily with the NSO Vacuum Telescope on Kitt Peak from 1975 to 1991. Motions in longitude on the solar surface are determined by a two-dimensional cross-correlation analysis of consecutive day pairs. We find that the measured rotation rate of small magnetic features, i.e., excluding active regions, is in excellent agreement with the results of the previous one-dimensional analysis of the same data (Komm, Howard, and Harvey, 1993). The polynomial fits show magnetic torsional oscillations, i.e., a more rigid rotation during cycle maximum and a more differential rotation during cycle minimum, but with smaller amplitudes than the one-dimensional analysis. The full width at half maximum of the cross-correlations is almost constant over latitude which shows that the active regions are effectively excluded. The agreement between the one- and two-dimensional cross-correlation analyses shows that the two different techniques are consistent and that the large-scale motions can be divided into rotational and meridional components that are not affected by each other.

Solar-cycle variation of the sound-speed asphericity from GONG and MDI data 1995–2000

ABSTRA C T We study the variation of the frequency splitting coefficients describing the solar asphericity in both GONG and MDI data, and use these data to investigate temporal sound-speed variations as a function of both depth and latitude during the period 1995‐ 2000 and a little beyond. The temporal variations in even splitting coefficients are found to be correlated to the corresponding component of magnetic flux at the solar surface. We confirm that the soundspeed variations associated with the surface magnetic field are superficial. Temporally averaged results show a significant excess in sound speed around ra 0:92 R( and latitude of 608.

Localizing Width and Energy of Solar Global p-Modes

We present the first attempt at localizing in latitude the temporal variation of mode energy, energy supply rate, and lifetime of global acoustic modes. We use Global Oscillation Network Group (GONG) and Michelson Doppler Imager data analyzed with the GONG peak-fitting algorithm to measure mode width and amplitude of individual (l, n, m) modes. While measured amplitude and width values are inherently noisier than frequency measurements, it is possible to use the (m/l) dependence of these mode parameters to extract their variation in latitude. With the currently analyzed data sets, we construct maps in time and latitude of acoustic mode energy, lifetime (inverse of mode width), and energy supply rate covering the rising phase of the current solar cycle from the previous minimum to the current maximum. We find that the energy and width of global modes vary in latitude as well as in time and that the variation is clearly related to the distribution of magnetic flux. After removing the average quantity, the residual mode width shows a linear correlation with magnetic activity with a correlation coefficient of 0.88, while the corresponding residual mode energy is anticorrelated with magnetic activity with a correlation coefficient of -0.90. These mode parameters derived from global p-modes respond to the local distribution of surface magnetic activity. The energy supply rate shows no correlation with the latitudinal distribution of magnetic activity within the limits of the current measurements. We estimate the variation of global mode energy in response to an individual magnetic feature, such as a plage, and find that the global mode energy and the mode lifetime are reduced by about 40% by an active region compared to the quiet Sun.

EVIDENCE THAT TEMPORAL CHANGES IN SOLAR SUBSURFACE HELICITY PRECEDE ACTIVE REGION FLARING

We report on the analysis of subsurface vorticity/helicity measurements for flare producing and quiet active regions. We have developed a parameter to investigate whether large, decreasing kinetic helicity density commonly occurs prior to active region flaring. This new parameter is effective at separating flaring and non-flaring active regions and even separates among C-, M-, and X-class flare producing regions. In addition, this parameter provides advance notice of flare occurrence, as it increases 2-3 days before the flare occurs. These results are striking on an average basis, though on an individual basis there is still considerable overlap between flare associated and non-flare associated values. We propose the following qualitative scenario for flare production: subsurface rotational kinetic energy twists the magnetic field lines into an unstable configuration, resulting in explosive reconnection and a flare.