Philip G. Judge

Alfven waves in the solar corona.

Alfvén waves, transverse incompressible magnetic oscillations, have been proposed as a possible mechanism to heat the Suns corona to millions of degrees by transporting convective energy from the photosphere into the diffuse corona. We report the detection of Alfvén waves in intensity, line-of-sight velocity, and linear polarization images of the solar corona taken using the FeXIII 1074.7-nanometer coronal emission line with the Coronal Multi-Channel Polarimeter (CoMP) instrument at the National Solar Observatory, New Mexico. Ubiquitous upward propagating waves were seen, with phase speeds of 1 to 4 megameters per second and trajectories consistent with the direction of the magnetic field inferred from the linear polarization measurements. An estimate of the energy carried by the waves that we spatially resolved indicates that they are too weak to heat the solar corona; however, unresolved Alfvén waves may carry sufficient energy.

Observational aspects of sunspot oscillations

Oscillatory phenomena observed in sunspot umbrae and penumbrae are reviewed and critically discussed. A natural interplay between the thermal atmospheric stratification and the ordered collimation imposed by the intense magnetic field leads naturally to the characteristic properties of the umbral chromospheric and photospheric oscillations and their interpretation as low-β (β=8πp/B2) slow magneto-acoustic-gravity waves guided along the ambient magnetic field.

SUMER Observations confirm the dynamic nature of the quiet solar outer atmosphere: The internetwork chromosphere

On 12 March 1996, during the commissioning phase of the SOHO mission, we obtained observations of the quiet Sun with the SUMER instrument. The observations were sequences of 15-20 second exposures of ultraviolet emission line profiles and of the neighboring continua. These data contain signatures of the dynamics of the solar chromosphere that are uniquely useful because of wavelength coverage, moderate signal-to-noise ratios, and image stability. We focus on data for the inter-network chromosphere. The dominant observed phenomenon is an oscillatory behavior that is analogous to the 3 minute oscillations seen in Ca II lines. The oscillations appear to be coherent over 3-8 arcsecond diameter areas. At any time they occur over �50% of the area studied, and they appear as large perturbations in the intensities of lines and continua. The oscillations are most clearly seen in intensity variations in the UV (� > 912u continua, and they are also seen in the intensities and velocities of chromospheric lines of C I, N I and O I. Intensity brightenings are accompanied by blueshifts of typically 5 km s 1 . Phase differences between continuum and line intensities also indicate the presence of upward propagating waves. The detailed behavior is different between different lines, sometimes showing phase lags. Three minute intensity oscillations are occasionally seen in second spectra (C II �1335), but never in third spectra (C III and Si III). Third spectra and He I �584 show oscillations in velocity that are not simply related to the 3 minute oscillations. The continuum intensity variations are consistent with recent simulations of chromospheric dynamics (Carlsson & Stein 1994) while the line observations indicate that important ingredients are missing at higher layers in the simulations.

A Study of Chromospheric Oscillations Using the SOHO and TRACE Spacecraft

We analyze line and continuum time-series data of the solar atmosphere, with between 10 and 60 s cadence, using the MDI and SUMER instruments on the SOHO spacecraft and the UV bandpasses on the T RACE satellite. The co-aligned data sets sample spectral features formed from photosphere to the middle transition region, spanning —ve decades in pressure, under quiet-Sun and plage conditions. We discuss power, phase diUerence, and coherence spectra, and examine data in the time domain. The observed photospheric and chromospheric oscillations are strongly coupled for frequencies between 2 and 8 mHz. Phase coherences decrease with increasing height, with only occasional periods and locations of observable coherence up to heights where transition region emission lines are formed. The middle chromosphere (in the SUMER continua) oscillates in several megameter (Mm) diameter coherent patches with power predominantly in the 5¨7 mHz range. The T RACE data, formed in the upper photosphere, show smaller patterns superimposed on these large-scale oscillations, resulting (at least in part) from granulation. At the observed spatial scales, all the observed properties point to p-modes, especially the ii pseudomodes ˇˇ just above the acoustic cutoU frequency, as the dominant mode of the chromospheric dynamics. Smaller scale ii acoustic event ˇˇ drivers, associated with granular dynamics, appear to be less important. The predominant internetwork chromospheric oscillations arise from regions much larger horizontally than vertically. If propagating largely vertically, this can naturally explain why the one-dimensional simulations of Carlsson & Stein might be more successful than expected. The chromospheric response to the p-mode driving is, however, intermittent in space and time. Some of the intermittency appears to result from the interaction of the upward-propagating waves with magnetic —elds. Evidence for this includes suppressed intensities and oscillations near quiet-Sun network elements (which we dub ii magnetic shadows ˇˇ), absence of oscillations in internetwork regions neighboring plage magnetic —elds, and a change in character of the quiet-Sun internetwork oscillations between the 119 and 104 nm continua formed at 1 and 1.2 Mm. The latter might be caused by canopy —elds that form between these heights under typical quiet-Sun conditions. A SUMER-only data set reported by et al. has a Wikst‘l factor of 3 more oscillatory power in the 104 nm continuum than the data analyzed here, with stronger coherences extending into the solar transition region. Together, these data support the general picture that the chromosphere oscillates primarily in response to forcing by the p-modes, they are therefore large-scale (several Mm across) waves, and they are often strongly in—uenced by magnetic eUects (internetwork —elds, or the overlying canopy), before the oscillations even reach the transition region.

Discovery of a 1.6 Year Magnetic Activity Cycle in the Exoplanet Host Star ι Horologii

The Mount Wilson Ca HK survey revealed magnetic activity variations in a large sample of solar-type stars with timescales ranging from 2.5 to 25 years. This broad range of cycle periods is thought to reflect differences in the rotational properties and the depths of the surface convection zones for stars with various masses and ages. In 2007, we initiated a long-term monitoring campaign of Ca II H and K emission for a sample of 57 southern solar-type stars to measure their magnetic activity cycles and their rotational properties when possible. We report the discovery of a 1.6 year magnetic activity cycle in the exoplanet host star ι Horologii and obtain an estimate of the rotation period that is consistent with Hyades membership. This is the shortest activity cycle so far measured for a solar-type star and may be related to the short-timescale magnetic variations recently identified in the Sun and HD 49933 from helioseismic and asteroseismic measurements. Future asteroseismic observations of ι Hor can be compared to those obtained near the magnetic minimum in 2006 to search for cycle-induced shifts in the oscillation frequencies. If such short activity cycles are common in F stars, then NASAs Kepler mission should observe their effects in many of its long-term asteroseismic targets.

Wavelet phase coherence analysis: Application to a quiet-sun magnetic element

A new application of wavelet analysis is presented that utilizes the inherent phase information residing within the complex Morlet transform. The technique is applied to a weak solar magnetic network region, and the temporal variation of phase difference between TRACE 1700 A and SOHO/SUMER C II 1037 A intensities is shown. We present, for the first time in an astrophysical setting, the application of wavelet phase coherence, including a comparison between two methods of testing real wavelet phase coherence against that of noise. The example highlights the advantage of wavelet analysis over more classical techniques, such as Fourier analysis, and the effectiveness of the former to identify wave packets of similar frequencies but with differing phase relations is emphasized. Using cotemporal, ground-based Advanced Stokes Polarimeter measurements, changes in the observed phase differences are shown to result from alterations in the magnetic topology.

Chromospheric and Transition Region Internetwork Oscillations: A Signature of Upward-propagating Waves

We analyze spectral time series obtained on 1997 April 25 with the SUMER instrument on SOHO. Line and continuum data near 1037 A were acquired at a cadence of 16 s. This spectral region was chosen because it contains strong emission lines of C II, formed in the upper chromosphere/lower transition region; O VI, formed in the upper transition region; and neighboring continuum emission formed in the middle chromosphere. The time series reveal oscillatory behavior. Subsonic (3-5 km s-1 amplitude) Doppler velocity oscillations in the C II and O VI lines, with periods between 120 and 200 s, are prominent. They are seen as large-scale coherent oscillations, typically of 3-7 Mm length scale, occasionally approaching 15 Mm, visible most clearly in internetwork regions. The Doppler velocity oscillations are related to oscillations seen in the continuum intensity, which precede upward velocity in C II by 40-60 s. The C II Doppler shift precedes the O VI Doppler shift by 3-10 s. Oscillations are also present in the line intensities, but the intensity amplitudes associated with the oscillations are small. The continuum intensity precedes the C II intensity by 30-50 s. Phase difference analysis shows that there is a preponderance of upward-propagating waves in the upper chromosphere that drive an oscillation in the transition region plasma, thus extending the evidence for upward-propagating waves from the photosphere up to the base of the corona.

SPECTRAL LINES FOR POLARIZATION MEASUREMENTS OF THE CORONAL MAGNETIC FIELD. II. CONSISTENT TREATMENT OF THE STOKES VECTOR FOR MAGNETIC-DIPOLE TRANSITIONS

We present a compact, self-consistent formulation for the description of polarized radiation from magnetic-dipole transitions occurring in the magnetized solar corona. This work diUers from earlier treatments by and House in the 1970s, in that the radiative emission coefficients for the Sahal-Brec chot four Stokes parameters, I, Q, U, and V , are treated to —rst order in a Taylor expansion of the line pro—le in terms of the Larmor frequency of the coronal magnetic —eld. In so doing, the in—uence on the scat- tered radiation of both atomic polarization, induced through anisotropic irradiation, and the Zeeman eUect is accounted for in a consistent way. It is found that the well-known magnetograph formula, rela- ting the V pro—le to the frequency derivative of the I pro—le, must be corrected in the presence of atomic alignment produced by anisotropic irradiation. This correction is smallest for lines where collisions and cascades dominate over excitation by anisotropic radiation, but it systematically increases with height above the solar limb (up to a theoretical maximum of 100%, in the collisionless regime and in the limit of vanishing longitudinal magnetic —eld). Although the correction to the magnetograph formula must be calculated separately for each line as a function of heliocentric distance, it is likely to be small for some lines of practical interest, along lines of sight close to the solar limb. Subject headings: line: formationSun: atmosphereSun: coronaSun: magnetic —eld

On the Origin of the Basal Emission from Stellar Atmospheres: Analysis of Solar C II Lines

Combining a variety of data with radiation hydrodynamic simulations, we examine the heating of the Sun’s internetwork chromosphere and the hypothesis that the chromospheric ‘‘ basal ’’ emission arises because of acoustic wave dissipation. We focus on the 2s 2p 22 D 2s 2 2p 2 P o multiplet of C ii near 1335 A ˚ , whose basal level of chromospheric emission has been reliably determined for stars and the Sun by Schrijver and colleagues. By accounting for center-to-limb variations and the different spectral bandpasses of the instruments used, we find that Schrijver’s C ii solar basal intensity substantially exceeds stellar values, and that it can be identified with intensities seen in typical internetwork regions with the SUMER instrument on the SOHO spacecraft. Some time-series data sets of internetwork regions are then examined and compared with simulations made specifically for a typical observational data set, with vertical velocities at the lower boundaries fixed from observations with the MDI instrument on SOHO. The simulations can qualitatively account for the observed internetwork UV continuum fluctuations seen with SUMER, formed 0.6–0.85 Mm above the photosphere. However, they fail to capture almost any property of the observed internetwork C ii multiplet, which is formed substantially higher. The time-averaged simulations can account for between 1 and 1 of the C ii basal intensities; they predict oscillatory power between 5 and 10 mHz, whereas internetwork observations are dominated by low-frequency (<2 mHz) power of solar origin. The average simulated C ii intensities, which have a large contribution from the transition region heated by conduction down from a coronal upper boundary, fall short even of the smaller stellar basal intensities by a factor of � 2. Together with known properties of weak, internetwork photospheric magnetic fields, we conclude that the internetwork upper chromosphere is probably dominated by magnetic heating. Thus, the solar basal (and internet

Spectral Lines for Polarization Measurements of the Coronal Magnetic Field. IV. Stokes Signals in Current-carrying Fields

We present the first theoretical, forward calculations of the Stokes profiles of several magnetic dipole (‘‘M1’’) coronal emission lines produced in current-carrying magnetic structures. An idealized coronal model of Low, Fong, and Fan is used, which describes a spherically symmetric, hydrostatic background atmosphere, isothermal at a coronal temperature of 1:6 ;10 6 K. Embedded is a global, axisymmetric magnetic field that is everywhere potential except at a quiescent prominence, consisting of an infinitesimally thin, equatorial current sheet whose weight is supported by the outward discrete Lorentz force in the sheet. This model contains a physically nontrivial, localized magnetic structure, although the atmospheric plasma is thermally of the simplest stratification possible. The calculated M1 coronal lines show clear and distinct signatures of the presence and magnitude of this localized magnetic structure, in both linear and circular polarizations, eventhough the thermal structure isalmost homogeneous. Themorphologyof mapsoflinearpolarizationisparticularlysensitive to the existenceand strengthofthe current sheets, asfield lines wrap around them according to the Biot-Savart law, and the linear polarization responds to different projections offield line directions onto local radius vectors. Of the M1 lines studied, those of Fe xiii (1074.7 nm) and Si x (1430.1 nm) are especiallypromisingbecauseoftheirrelativelystronglinearpolarization.Theseforwardcalculationsprovideabasisfor optimismthatemission-linemeasurementsmayrevealthepresenceandnatureofcurrentsystemsinthecorona,and provide motivation for developing instruments capable ofroutinely measuring polarized light in forbidden coronal lines.