Surface Magnetic Activity Research Articles

Effects of gastrointestinal tissue structure on computed dipole vectors

BackgroundDigestive diseases are difficult to assess without using invasive measurements. Non-invasive measurements of body surface electrical and magnetic activity resulting from underlying gastro-intestinal activity are not widely used, in large due to their difficulty in interpretation. Mathematical modelling of the underlying processes may help provide additional information. When modelling myoelectrical activity, it is common for the electrical field to be represented by equivalent dipole sources. The gastrointestinal system is comprised of alternating layers of smooth muscle (SM) cells and Interstitial Cells of Cajal (ICC). In addition the small intestine has regions of high curvature as the intestine bends back upon itself. To eventually use modelling diagnostically, we must improve our understanding of the effect that intestinal structure has on dipole vector behaviour.MethodsNormal intestine electrical behaviour was simulated on simple geometries using a monodomain formulation. The myoelectrical fields were then represented by their dipole vectors and an examination on the effect of structure was undertaken. The 3D intestine model was compared to a more computationally efficient 1D representation to determine the differences on the resultant dipole vectors. In addition, the conductivity values and the thickness of the different muscle layers were varied in the 3D model and the effects on the dipole vectors were investigated.ResultsThe dipole vector orientations were largely affected by the curvature and by a transmural gradient in the electrical wavefront caused by the different properties of the SM and ICC layers. This gradient caused the dipoles to be oriented at an angle to the principal direction of electrical propagation. This angle increased when the ratio of the longitudinal and circular muscle was increased or when the the conductivity along and across the layers was increased. The 1D model was able to represent the geometry of the small intestine and successfully captured the propagation of the slow wave down the length of the mesh, however, it was unable to represent transmural diffusion within each layer, meaning the equivalent dipole sources were missing a lateral component and a reduced magnitude when compared to the full 3D models.ConclusionThe structure of the intestinal wall affected the potential gradient through the wall and the orientation and magnitude of the dipole vector. We have seen that the models with a symmetrical wall structure and extreme anisotropic conductivities had similar characteristics in their dipole magnitudes and orientations to the 1D model. If efficient 1D models are used instead of 3D models, then both the differences in magnitude and orientation need to be accounted for.

Ring Analysis of Solar Subsurface Flows and Their Relation to Surface Magnetic Activity

We measure the horizontal flows in the outer 2% of the Sun by analyzing 14 consecutive Carrington rotations of Global Oscillation Network Group (GONG) Doppler images and two of Michelson Doppler Imager (MDI) Dynamics Program data with the ring-diagram technique. The zonal and meridional flows show no variation with activity at low to medium activity levels (below 71 G). At active region locations, the zonal flow increases with increasing activity; active regions rotate faster than their quieter surroundings. The meridional flow at active region locations is more equatorward than on average at depths less than about 10 Mm; the flow converges toward the mean latitude of activity. At depths greater than about 10 Mm, some active region locations show poleward and others equatorward motions indicating strong outflows from active regions. The estimated vertical flow decreases with increasing activity levels except at active region locations at depths greater than about 10 Mm; active regions show downflows near the surface and upflows at depths greater than about 10 Mm. The velocity errors increase somewhat with increasing activity at flux levels below 71 G, but they increase rapidly up to about 2 times the median error at higher flux values. This increase occurs at all depths. The flows averaged over all longitudes show the patterns expected from solar cycle variations. The quiet and the intermediate activity subsets show the same flow pattern, while the active region subset shows a mixture of solar cycle flow pattern and local flow behavior.

Helioseismic sensing of the solar cycle

All quantities observed with helioseismic methods, such as frequencies, width, and amplitudes of acoustic waves, vary with the solar cycle. However, they relate to the dynamics and structure in different parts of the solar convection zone. The rotation rate varies with the solar cycle, showing the so-called torsional oscillation pattern, from the surface throughout most of the convection zone. Near the tachocline, the current observations do not show this solar-cycle variation, but there is some evidence that there is a 1.3-year variation of the rotation rate. The meridional flow, observed in the outer 2% of the solar radius, varies with the solar cycle showing flows converging toward the mean latitude of magnetic activity at depths less than about 10 Mm and flows diverging at greater depth. There is some evidence for a counter-cell in the northern hemisphere during epochs of high activity. Structure inversions show variations in asphericity near the surface where the sound speed varies with the distribution of surface activity. There are hints but no conclusive evidence that such variations exist in the convection zone. The damping of acoustic modes increases and the mode energy decreases with increasing activity. Their variation with time and latitude shows that even global modes sense the local distribution of the surface magnetic activity.

Chromospherically Active Stars. XXI. The Giant, Single-lined Binaries HD 89546 and HD 113816

We have obtained spectroscopy and photometry of the chromospherically active, single-lined spectroscopic binaries HD 89546 and HD 113816. HD 89546 has a circular orbit with a period of 21.3596 days. Its primary has a spectral type of G9 III and is somewhat metal-poor with [Fe/H] ~ -0.5. HD 113816 has an orbit with a period of 23.6546 and a low eccentricity of 0.022. Its mass function is extremely small, 0.0007 M⊙, consistent with a very low inclination. The primary is a slightly metal-poor K2 III. A decade or more of photometric monitoring with an automatic telescope demonstrates that both systems display brightness variations due to rotational modulation of the visibility of photospheric star spots, as well as light-curve changes resulting from the redistribution of star spots by differential rotation and long-term changes in the filling factor of the spots. We determined rotation periods for each season when the observations were numerous enough. Our mean rotation periods of 21.3 and 24.1 days for HD 89546 and HD 113816, respectively, confirm that the giants in each system are synchronously rotating. The orbital elements and properties of the giant components of these two systems, including levels of surface magnetic activity, are quite similar. However, the two rotational inclinations are rather different, 57° for HD 89546 and 13° for HD 113816. Thus the latter giant is seen nearly pole on. We analyzed the light curves for similarities and differences that result from viewing these two systems from quite different inclinations.

Asteroseismic tests for models of magnetically active close binaries with orbital period modulation

Magnetic activity and orbital period modulation in close binaries are considered in order to identify possible aster- oseismic tests for models that have been proposed to understand their relationship. These models are based on the variation of the gravitational quadrupole moment of the active star, as a consequence of the modification of its internal rotation and magnetic field along its activity cycle (Applegate 1992; Lanza et al. 1998a). An analysis based on the solar analogy shows that surface magnetic activity may produce shifts, splitting and broadening of the p-mode peaks in the oscillation spectrum of a star. Such eects make the direct detection of the internal structural changes predicted by the models very dicult. However, a time variation of the internal rotation can be detectable because the p-mode rotational splittings are proportional to the azimuthal quantum number m, whereas structural and surface eects controlled by the magnetic fields arejmj dependent. Therefore, we propose a method that correlates the variation of the a1 splitting coecients, that sample the internal rotation, with the orbital period variation in order to test the predictions of the models. The RS CVn short-period systems, such as V 471 Tau, RT And or CG Cyg, are the most suitable objects for such a test because surface magnetic eects are likely to be less prominent than in classical or long-period RS CVn binaries.

Localizing Width and Energy of Solar Globalp‐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.

Variations in solar magnetic activity during the last 200 000 years: is there a Sun–climate connection?

The production of 10Be in the Earth’s atmosphere depends on the galactic cosmic ray influx that, in turn, is affected by the solar surface magnetic activity and the geomagnetic dipole strength. Using the estimated changes in 10Be production rate and the geomagnetic field intensity, variations in solar activity are calculated for the last 200 ka. Large variations in the solar activity are evident with the Sun experiencing periods of normal, enhanced and suppressed activity. The marine δ18O record and solar modulation are strongly correlated at the 100 ka timescale. It is proposed that variations in solar activity control the 100 ka glacial–interglacial cycles. However, the 10Be production rate variations may have been under-estimated during the interval between 115 ka and 125 ka and may have biased the results. Future tests of the hypothesis are discussed.

Atmospheric Dynamics and Magnetic Activity Associated With A Coronal Mass Ejection

By using multi-wavelength observations, we explored the atmospheric dynamics and the surface magnetic activity in NOAA 9026, which were associated with the initiation of a halo coronal mass ejection (CME) on 6 June, 2000. In an interval of less than two hours, two X-class X-ray flares took place successively, each along with one eruption of a filament. However, only the second X-class flare which is characterized by a rather large-scale (larger than a general active region in area) EUV dimming was associated with the CME initiation. It seems that a flare with an extensive dimming is more likely to be CME-associated. We focused our study on the daily evolution of the vector magnetic field in this region from 4 to 9 June and have found the following results. (1) The gradual squeeze and cancellation of the opposite polarity magnetic fields are the main patterns of magnetic evolution. Moreover, there is a spatial coincidence between the sites of magnetic flux cancellation and the locations of the early filament activation and the flare brightenings. (2) The current system increased in the first two days and began to decrease at least ten hours before the CME initiation. It underwent dramatic disruption from 6 to 7 June. (3) The transverse component of the the vector magnetic field appeared helical in configuration. It changed from compact to loose and dissipated from a small to a large area. Here we suggest that although the first filament eruption and first flare were not in step with the CME initiation, they seem to be a part of the entire process. The observed evolution of the magnetic field implies a continuous transport of magnetic energy and complexity from the lower atmosphere to the corona. Moreover, the slow magnetic reconnection in the lower atmosphere, manifested as magnetic flux cancellation, and the helicity re-distribution, appear to play a key role in the energy build-up process of the flares and the initiation of the halo CME.

No planet for HD 166435

The G0 V star HD 166435 has been observed by the ber-fed spectrograph ELODIE as one of the targets in the large extra-solar planet survey that we are conducting at the Observatory of Haute-Provence. We detected coherent, low-amplitude, radial-velocity variations with a period of 3.7987 days, suggesting a possible close-in planetary companion. Subsequently, we initiated a series of high-precision photometric observations to search for possible planetary transits and an additional series of Ca II H and K observations to measure the level of surface magnetic activity and to look for possible rotational modulation. Surprisingly, we found the star to be photometrically variable and magnetically active. A detailed study of the phase stability of the radial-velocity signal revealed that the radial-velocity variability remains coherent only for durations of about 30 days. Analysis of the time variation of the spectroscopic line proles using line bisectors revealed a correlation between radial velocity and line-bisector orientation. All of these observations, along with a one-quarter cycle phase shift between the photometric and the radial-velocity variations, are well explained by the presence of dark photospheric spots on HD 166435. We conclude that the radial-velocity variations are not due to gravitational interaction with an orbiting planet but, instead, originate from line-prole changes stemming from star spots on the surface of the star. The quasi-coherence of the radial-velocity signal over more than two years, which allowed a fair t with a binary model, makes the stability of this star unusual among other active stars. It suggests a stable magnetic eld orientation where spots are always generated at about the same location on the surface of the star.

Probing Surface Flows and Magnetic Activity with Time-Distance Helioseismology

We estimate near-surface flows using the techniques of f-mode time-distance helioseismology together with MDI/SOHO data. (1) Synoptic maps of horizontal flows are obtained for 1996, 1998 and 1999. Rotation, torsional oscillations and meridional circulation are measured. We detect weak large-scale flows converging toward active regions. Travel times are found to be shorter at locations of high magnetic activity. In addition, we measure the motion of the supergranulation pattern. (2) Realistic travel-time sensitivity kernels are used in an iterative deconvolution to infer horizontal flows at a high spatial resolution. The radial outflow outside a sunspot penumbra, called the moat, is measured.

Photometric and CaiiH and K Spectroscopic Variations in Nearby Sun‐like Stars with Planets. III.

We present the results of an analysis of time-series photometry, Ca II H and K spectrophotometry, and high-dispersion visible spectra of nine nearby Sun-like stars recently identified as having planets. For the six stars whose presumed planets have orbital periods of less than 4 months (? Boo, 51 Peg, ? And, ?1 Cnc, ? CrB, and 70 Vir), sine-curve fits to the photometric data show no variations with semiamplitude greater than 1 or 2 parts in 104. Photometric variations in 47 UMa are similarly small, although our photometric data of this star are slightly affected by variability of the comparison star. Nonvariability at this level of precision is sufficient to rule out surface magnetic activity as the cause of the observed radial-velocity variations in these seven stars and makes nonradial pulsations unlikely as well. Thus, our photometry provides indirect but strong support for true reflex motions?planets?in these seven stars, but cannot yet so support the planetary hypothesis for the two additional stars, 16 Cyg B and Gl 411. Continued photometric monitoring of the short-period systems may soon result in the direct detection of these planets in reflected light. We have used our photometric fluxes to search for possible transits of the extrasolar planets. Transits definitely do not occur in ? Boo, 51 Peg, ? And, and ?1 Cnc, and probably do not occur in ? CrB and 70 Vir. Our transit-search results are inconclusive for 47 UMa, and we cannot address the issue for 16 Cyg B and Gl 411. The precision of our photometry is sufficient to detect transits of planets even if they are not gas giants, as currently assumed, but much smaller objects with rocky compositions. The chance of finding at least one transit in the six stars is ~40%. We find significant year-to-year photometric variability only in ? Boo, which is not only the youngest star in the sample but also the star with the shallowest convective zone. The interseasonal range in its yearly mean photometric flux is ~0.002 mag, roughly twice the 0.0008 mag decadal variation in the Sun's total irradiance. Monitoring of the relative Ca II H and K fluxes began between 1966 and 1968 for 51 Peg, ? Boo, ? CrB, and Gl 411, between 1990 and 1993 for 47 UMa, 70 Vir, 16 Cyg B, and ?1 Cnc, and in 1996 for ? And. The data have been newly recalibrated for improved long-term instrumental stability, resulting in better precision of the Ca II records. Five of the nine stars in this study have little or no detectable year-to-year variation in Ca II flux. The remaining four show moderate or pronounced variability: ? Boo, whose radial-velocity and photometric variations have comparatively high amplitudes; Gl 411, whose planetary companion was inferred astrometrically, not spectroscopically; ?1 Cnc, which may undergo decadal cyclic activity; and ? And, which shows moderate year-to-year variability. Except for 47 UMa, intraseasonal variability consistent with rotation was detected in the Ca II records of all stars. However, the rotation periods determined for ? And, 70 Vir, and 16 Cyg B are of low confidence. An examination of the recalibrated Ca II records for 51 Peg finds a rotation period of 22 days, in contrast to our previous result of 37 days. Ages have been estimated from the mean Ca II flux and, where possible, the rotation period. We find general consistency with the ages determined by others comparing properties determined from high-resolution spectroscopy to evolutionary models, although the uncertainties are, in general, large.

Solar Cycle Changes in GONGp‐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.

Wavelet Analysis of Stellar Chromospheric Activity Variations

Observations of chromospheric activity variations for some lower main-sequence stars from the Mount Wilson Observatory's HK project reveal a cyclic behavior comparable to the sunspot cycle. Even in the relatively short interval that they have been observed, those stars show stellar cycles and other features, like grand minima. The quasi-periodic nature of such variations is not completely compatible with the standard Fourier analysis, so we applied a wavelet analysis to study the nature of regularities in the data. We computed wavelet transforms and energy spectra for the 25 yr records of surface magnetic activity in four stars: HD 3651, HD 10700, HD 10476, and HD 201091. We present a modified wavelet technique that is suitable for analysis of data with gaps and find that the common aliasing problems due to the finite length of the observations and irregularly spaced gaps between data can be reduced on both large and small scales by applying this algorithm.

Shrinkage of Coronal X-Ray Loops

We present the first set of examples of the shrinkage of large-scale nonflare loops in the solar corona, observed by the Yohkoh Soft X-Ray Telescope in 1993 February. A large and isolated active region exhibited an unusual south-north asymmetry in coronal dynamics and heating. The northern part, referred to the main magnetic axis, showed episodic expansion and heating. In contrast, the southern part displayed obvious shrinking and cooling. This asymmetry was correlated with a severe asymmetry in the surface magnetic activity revealed by Huairou vector magnetograms. Observations suggest that this shrinkage is not an apparent motion, but a real contraction of coronal loops that brighten as a result of heating at footpoints followed by gradual cooling.

Properties of Sun‐like Stars with Planets: 51 Pegasi, 47 Ursae Majoris, 70 Virginis, and HD 114762

Radial velocity variations have revealed planets orbiting 51 Peg, 47 UMa, and 70 Vir, and a low-mass companion orbiting HD 114762. We analyze parallel records of photometric measurements in Stromgren b and y and Johnson V, R, and I passbands and Ca II H and K fluxes in those stars. In the case of 51 Peg, the high precision of the differential photometric measurements made by the 0.75 m Automatic Photoelectric Telescope and the nonvariability of the star would allow the detection of a transit of a planet as small as Earth (corresponding to an amplitude of 0.0001 mag) if its orbit were nearly coplanar with our line of sight. No transits were observed. For 51 Peg and 70 Vir, the upper limit of nondetection of photometric variability at their companion's orbital periods is Δ(b + y)/2 < 0.0002 ± 0.0002 mag. For HD 114762, it is ΔV < 0.0007 ± 0.0004 mag. Such small amplitudes of photometric variability seem to eliminate periodic velocity variations expected from p-mode oscillations. All four stars are magnetically quiet; that is, they lack the typical Ca II and photometric variability due to rotation and activity cycles expected from surface magnetic activity in solar-type stars. Such quiescence produces an interesting observational bias that favors the detection of planets from low-amplitude radial velocity or photometric variations by minimizing the contribution from intrinsic stellar variability. We discuss the circumstances for which the probability of planet detections is improved by the reduced level of variability from surface magnetic activity in G and K stars. Stars with low variability in surface activity should be the best candidates for planet searches using radial velocity and photometric techniques. Searches for planets around younger, more active stars will be impeded by variations in velocity or brightness caused by time-varying surface features. The Ca II H and K fluxes indicate that all four stars are older than 5 Gyr. Ages were estimated from the average levels of Ca II H and K fluxes and an existing relationship of the decrease of Ca II fluxes with age on the lower main sequence and were drawn from previous results based on theoretical isochrone fitting. Values of the projected rotational velocity, v sin i, are determined for 70 Vir and 47 UMa from high-resolution spectra.

Long-term stellar activity: three decades of observations

Knowledge of the solar sunspot cycle extends back to the mid-19th century with the work of Schwabe (1843) and Wolf (1856). The mean cycle period of the Sun is 11 years, however, individual cycle lengths range from 7 to 13 years (Eddy 1977). In this century, however, the length of the solar cycle has been closer to 10 years (Donahue and Baliunas 1992a). A complete explanation of the solar magnetic activity and its variations has not yet been produced, although a hydromagnetic dynamo is frequently posited as the source of solar (and therefore stellar) magnetic activity. Empirical measurements of those stars in the H-R Diagram which have convective zones and surface magnetic activity provide the boundary conditions and the range of behavior which must be explained by any all-encompassing theory explaining stellar magnetic activity, and activity cycles.

Short-term changes in solar oscillation frequencies and solar activity

It is shown that the frequencies of solar rho-mode oscillations change significantly over periods as short as one month. These changes correlate significantly with variations in the strength of surface solar activity as measured by the average, over the sun's visible surface, of the magnitude of the line-of-sight magnetic field component from magnetograms. The frequency and mean magnetic variations are found to obey a linear relationship. It is seen that the mean frequency shift at any time depends on the history of solar activity over an interval of, at most, several months prior to the measurement and conclude that the dominant mechanism of the frequency shift is correlated with surface magnetic activity.

Solar and stellar coronal loops

Abstract X-ray and EUV pictures from space experiments (e.g. SKYLAB ) have revealed that the corona of the Sun consists of a variety of discrete structures - involving a range of timescales - which appear to outline the magnetic field lines emerging from the convection zone below the photosphere. Among the brightest regions in X-rays and EUV are those in which the field lines close back to the surface and which confine the hot emitting plasma in loop structures. Simple scaling laws have been developed for the static model of a single magnetic loop relating average loop temperature, pressure and length and these have been applied more or less successfully to solar observations. For many other cool stars with convective envelopes X rays have been detected from hot plasmas with temperatures ranging from ∼0.5 to ∼30 MK. The strong correlation between X-ray luminosity and stellar rotational velocity suggests a surface magnetic activity associated with an internal dynamo such as has been assumed for the Sun. The detailed spatial structure of stellar coronae is not yet known, although by analogy with the Sun it is widely assumed that stellar coronae - both quiescent and flaring - are heated by magnetohydrodynamic processes, with much of the plasma confined in loop-like structures connecting opposite magnetic polarities in the photosphere. In this paper I consider a few results from solar and stellar X-ray observations that constrain basic properties of magnetic loops.

Evidence for long-term brightness changes of solar-type stars

CHANGES in the brightness of the Sun may introduce further uncertainties into forecasts of global warming by the greenhouse effect. The Sun is known to vary in brightness, on a timescale of years, by 0.1% in phase with changes in magnetic activity during the solar cycle1–3, and variations of up to 0.4%, also correlated with surface magnetic activity, have been found in stars similar to the Sun4. To delimit the magnitude of solar luminosity variations on a timescale of centuries, we have looked at the magnetic behaviour of a number of solar-type stars over several years. Observed in random phases of their long-term variability, they give a sample of the behaviour of a solar-type star over a long period of time. We find indirect evidence that these stars undergo brightness changes of more than the 0.1% observed during the last solar cycle, a result that calls into question the assumption of a constant Sun in calculations using general circulation models for climate forecasting.

Observations of stellar coronae

We present an overview of recent stellar X-ray observations, with some discussion of the requirements for future observations. We argue that solar observations indicate that coronal X-ray emission is strongly related to surface magnetic field activity; we show that the interpretation of X-ray stellar coronal emission from late-type stars within the framework of models analogous to those developed for the solar corona is viable, and it is supported by many experimental results. The extension of this solar analogy to the early-type stars is quite questionable and remains an unsolved problem, while the working hypothesis of an X-ray phase, related to phenomena of magnetic field-related activity, as contrasted to a wind phase during the PMS evolutionary stage is suggested by the present status of observations.