Sun's Photosphere Research Articles

OBSERVATION OF ULTRAFINE CHANNELS OF SOLAR CORONA HEATING

We report the first direct observations of dynamical events originating in the Sun's photosphere and subsequently lighting up the corona. Continuous small-scale, impulsive events have been tracked from their origin in the photosphere on through to their brightening of the local corona. We achieve this by combining high-resolution ground-based data from the 1.6 m aperture New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO), and satellite data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). The NST imaging observations in helium I 10830 angstrom reveal unexpected complexes of ultrafine, hot magnetic loops seen to be reaching from the photosphere to the base of the corona. Most of these ultrafine loops are characterized by an apparently constant, but surprisingly narrow diameter of about 100 km all along each loop, and the loops originate on the solar surface from intense, compact magnetic field elements. The NST observations detect the signature of upward injections of hot plasma that excite the ultrafine loops from the photosphere to the base of the corona. The ejecta have their individual footpoints in the intergranular lanes between the Sun's ubiquitous, convectively driven granules. In many cases, AIA/SDO detects cospatial and cotemporal brightenings in the overlying, million degree coronal loops in conjunction with the upward injections along the ultrafine loops. Segments of some of the more intense upward injections are seen as rapid blueshifted events in simultaneous Ha blue wing images observed at BBSO. In sum, the observations unambiguously show impulsive coronal heating events from upward energy flows originating from intergranular lanes on the solar surface accompanied by cospatial mass flows.

SPATIAL AND TEMPORAL DISTRIBUTIONS OF TRANSIENT HORIZONTAL MAGNETIC FIELDS WITH DEEP EXPOSURE

We obtained a long exposure vector magnetogram of the quiet Sun photosphere at the disk center with wide FOV of $51" \times 82"$. The observation was performed at Fe I 525.0 nm with the shutter-less mode of the Narrow Band Filter Imager of the Solar Optical Telescope (SOT) on board Hinode satellite. We summed the linear polarization ($LP$) maps taken with time cadence of 60 seconds for 2 hours to obtain a map with as long an exposure as possible. The polarization sensitivity would be more than 4.6 (21.2 in exposure time) times the standard observation with the SOT spectro-polarimeter. The $LP$ map shows a cellular structure with a typical scale of $5" - 10"$. We find that the enhanced $LP$ signals essentially consist of the isolated sporadic transient horizontal magnetic fields (THMFs) with life time of 1-10 min, and are not contributed by long-duration weak horizontal magnetic fields. The cellular structure coincides in position with the negative divergence of the horizontal flow field, i.e., mesogranular boundaries with downflows. Azimuth distribution appears to be random for the scale size of the mesogranules. Some pixels have two separate appearances of THMFs, and the measured time intervals are consistent with the random appearance. THMFs tend to appear at the mesogranular boundaries, but appear randomly in time. We discuss the origin of THMFs based on these observations.

ESTIMATING ELECTRIC FIELDS FROM VECTOR MAGNETOGRAM SEQUENCES

Determining the electric field (E-field) distribution on the Sun's photosphere is essential for quantitative studies of how energy flows from the Sun's photosphere, through the corona, and into the heliosphere. This E-field also provides valuable input for data-driven models of the solar atmosphere and the Sun-Earth system. We show how Faraday's Law can be used with observed vector magnetogram time series to estimate the photospheric E-field, an ill-posed inversion problem. Our method uses a "poloidal-toroidal decomposition" (PTD) of the time derivative of the vector magnetic field. The PTD solutions are not unique; the gradient of a scalar potential can be added to the PTD E-field without affecting consistency with Faraday's Law. We present an iterative technique to determine a potential function consistent with ideal MHD evolution; but this E-field is also not a unique solution to Faraday's Law. Finally, we explore a variational approach that minimizes an energy functional to determine a unique E-field, similar to Longcope's "Minimum Energy Fit". The PTD technique, the iterative technique, and the variational technique are used to estimate E-fields from a pair of synthetic vector magnetograms taken from an MHD simulation; and these E-fields are compared with the simulation's known electric fields. These three techniques are then applied to a pair of vector magnetograms of solar active region NOAA AR8210, to demonstrate the methods with real data.

HIGHEST RESOLUTION OBSERVATIONS OF THE QUIETEST SUN

Highest resolution observations made with the new 1.6 m aperture solar telescope in Big Bear Solar Observatory during this time of historic inactivity on the Sun reveal new insights into the small-scale dynamics of the Sun's photosphere. The telescope's unprecedented resolution enabled us to observe that the smallest scale photospheric magnetic field seems to come in isolated points in the dark intergranular lanes, rather than the predicted continuous sheets confined to the lanes, and the unexpected longevity of the bright points implies a deeper anchoring than predicted. Further, we demonstrated for the first time that the photospheric plasma motion and magnetic fields are in equipartition over a wide dynamic range, and both cascade energy to ever-smaller scales according to classical Kolmogorov turbulence theory. Finally, we discovered tiny jet-like features originating in the dark lanes that surround the ubiquitous granules that characterize the solar surface.

Mesoscale dynamics on the Sun's surface from HINODE observations

Context: Aims: The interactions of velocity scales on the Sun's surface, from granulation to supergranulation are still not understood, nor are their interaction with magnetic fields. We thus aim at giving a better description of dynamics in the mesoscale range which lies between the two scales mentioned above. Methods: We analyse a 48 h high-resolution time sequence of the quiet Sun photosphere at the disk center obtained with the Solar Optical Telescope onboard Hinode. The observations, which have a field of view of 100´´ × 100´´, typically contain four supergranules. We monitor in detail the motion and evolution of granules as well as those of the radial magnetic field. Results: This analysis allows us to better characterize Trees of Fragmenting Granules issued from repeated fragmentation of granules, especially their lifetime statistics. Using floating corks advected by measured velocity fields, we show their crucial role in the advection of the magnetic field and in the build up of the network. Finally, thanks to the long duration of the time series, we estimate that the turbulent diffusion coefficient induced by horizontal motion is approximately 430 km2 s-1. Conclusions: These results demonstrate that the long living families contribute to the formation of the magnetic network and suggest that supergranulation could be an emergent length scale building up as small magnetic elements are advected and concentrated by TFG flows. Our estimate for the magnetic diffusion associated with this horizontal motion might provide a useful input for mean-field dynamo models.

Small-scale magnetic field dynamics on the Sun at high spatial and temporal resolution

Aims. We present examples of the time evolution of phenomena occurring in the quiet Sun photosphere as seen from observations of high spatial and spectral resolution and with a time cadence of 23 s.

Twisted solar active region magnetic fields as drivers of space weather: Observational and theoretical investigations

The properties and dynamics of magnetic fields on the Sun's photosphere and outer layers—notably those within solar active regions—govern the eruptive activity of the Sun. These photospheric magnetic fields also act as the evolving lower boundary of the Sun–Earth coupled system. Quantifying the physical attributes of these magnetic fields and exploring the mechanisms underlying their influence on the near-Earth space environment are of vital importance for forecasting and mitigating adverse space weather effects. In this context, we discuss here a novel technique for measuring twist in the magnetic field lines of solar active regions that does not invoke the force-free field assumption. Twist in solar active regions can play an important role in flaring activity and the initiation of CMEs via the kink instability mechanism; we outline a procedure for determining this solar active region eruptive potential. We also discuss how twist in active region magnetic fields can be used as inputs in simulations of the coronal and heliospheric fields; specifically, we explore through simulations, the formation, evolution and ejection of magnetic flux ropes that originate in twisted magnetic structures. The results and ideas presented here are relevant for exploring the role of twisted solar active region magnetic fields and flux ropes as drivers of space weather in the Sun–Earth system.

Monitoring the Sun: A Global Space Weather Enterprise

Monitoring the Sun: A Global Space Weather Enterprise

Volatile fractionation in the early solar system and chondrule/matrix complementarity.

Bulk chondritic meteorites and terrestrial planets show a monotonic depletion in moderately volatile and volatile elements relative to the Sun's photosphere and CI carbonaceous chondrites. Although volatile depletion was the most fundamental chemical process affecting the inner solar nebula, debate continues as to its cause. Carbonaceous chondrites are the most primitive rocks available to us, and fine-grained, volatile-rich matrix is the most primitive component in these rocks. Several volatile depletion models posit a pristine matrix, with uniform CI-like chemistry across the different chondrite groups. To understand the nature of volatile fractionation, we studied minor and trace element abundances in fine-grained matrices of a variety of carbonaceous chondrites. We find that matrix trace element abundances are characteristic for a given chondrite group; they are depleted relative to CI chondrites, but are enriched relative to bulk compositions of their parent meteorites, particularly in volatile siderophile and chalcophile elements. This enrichment produces a highly nonmonotonic trace element pattern that requires a complementary depletion in chondrule compositions to achieve a monotonic bulk. We infer that carbonaceous chondrite matrices are not pristine: they formed from a material reservoir that was already depleted in volatile and moderately volatile elements. Additional thermal processing occurred during chondrule formation, with exchange of volatile siderophile and chalcophile elements between chondrules and matrix. This chemical complementarity shows that these chondritic components formed in the same nebula region.

On the Sr I $\mathsf{\lambda}$4607 Å Hanle depolarization signals in the quiet Sun

The Hanle depolarization signals of Sr i λ4607 Å have been used to estimate the unsigned magnetic flux and magnetic energy existing in the quiet Sun photosphere. However, the Sr i λ4607 Å Hanle signals are not sensitive to the unsigned flux and energy. They only bear information on the fraction of photosphere occupied by magnetic field strengths smaller than the Hanle saturation, which do not contribute to the unsigned flux and energy. We deduce an approximate expression for the relationship between magnetic fill factor and Hanle signal. When applied to existing Hanle depolarization measurements, it indicates that only 40% of the quiet Sun is filled by magnetic fields with a strength smaller than 60 G. The remaining 60% of the surface has field strengths above this limit. Such constraint will be needed to determine the distribution of magnetic field strengths existing in the quiet Sun.

ILCT: Recovering Photospheric Velocities from Magnetograms by Combining the Induction Equation with Local Correlation Tracking

We present three methods for deriving the velocity field in magnetized regions of the Sun's photosphere. As a preliminary step, we introduce a Fourier-based local correlation tracking (LCT) routine that we term FLCT. By explicitly employing the observation made by Demoulin & Berger, that results determined by LCT applied to magnetograms involve a combination of all components of the velocity and magnetic fields, we show that a three-component velocity field can be derived, in a method we term algebraic decomposition, or ADC. Finally, we introduce ILCT, a method that enforces consistency between the normal component of the induction equation and results obtained from LCT. When used with photospheric vector magnetograms, ILCT determines a three-component photospheric velocity field suitable for use with time sequences of such magnetograms to drive boundary conditions for MHD simulations of the solar corona. We present results from these methods applied to vector magnetograms of NOAA AR 8210 on 1998 May 1.

IBIS Observations of Quiet Sun Photosphere - Velocity Structure from Fe I 7090.4 Å

In our contribution we introduce the new Interferometric BIdimensional Spectrometer (IBIS) and present the first results on bisector velocities of two dimensional spectral scans in FeI 7090.4 Å comparing granules and intergranular regions.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

On conjugate complex time – II: equipotential effect of gravity retrodicts differential and predicts apparent anomalous rotation of the sun

Equipotential effect of gravity (EEG), as opposed to radial interactions between bodies, is introduced here as the impact of a tangential potential. EEG causes frequency decrease that was observed in two specially devised and several indirect experiments. It depends on the distance that is measured along equipotential parts of trajectories and can explain an extraneous frequency decrease found in radio signals emitted by navigational positioning systems. It retrodicts the observed differential rotation of the sun without the assumed drag of the sun's photosphere as its cause, and predicts apparent anomalous rotation of the sun, for from the shifts in spectra taken from the sun's limb it should appear as if the sun rotates away faster than it does towards an observer on earth. The value predicted by it is over 10.56% excess over Einstein's value of deflection of light near the sun and agrees with the 10–15% excess estimated from the data that was obtained in numerous, quite independent experiments.

Rossby waves on the Sun as revealed by solar 'hills'

It is a long-standing puzzle that the Sun's photosphere--its visible surface--rotates differentially, with the equatorial regions rotating faster than the poles. It has been suggested that waves analogous to terrestrial Rossby waves, and known as r-mode oscillations, could explain the Sun's differential rotation: Rossby waves are seen in the oceans as large-scale (hundreds of kilometres) variations of sea-surface height (5-cm-high waves), which propagate slowly either east or west (they could take tens of years to cross the Pacific Ocean). Calculations show that the solar r-mode oscillations have properties that should be strongly constrained by differential rotation. Here we report the detection of 100-m-high 'hills' in the photosphere, spaced uniformly over the Sun's surface with a spacing of (8.7 +/- 0.6) x 10(4) km. If convection under the photosphere is organized by the r-modes, the observed corrugated photosphere is a probable surface manifestation of these solar oscillations.

Very high rotational excitation of CO in a cooled electric discharge through carbon monoxide

Infrared emission from CO12 and CO13, excited in the cathode region of a discharge tube immersed in liquid nitrogen, was recorded by Fourier-transform spectrometry at a resolution of 0.005 cm−1. The Δv=1 sequence bands recorded in the 2500–1800 cm−1 spectral interval, indicate the existence of three different rotational populations; (i) molecules in the zero-ground level with Trot≈100 K (responsible for reabsorption of part of the 1–0 emission band); (ii) molecules with Trot≈275 K (maximum intensity for Jmax′≈6 in each band, Tvib≈3000 K for v′=2–4, Tvib≈8600 K for v′=5–13); (iii) molecules with v′ limited to 6, for which R-rotational lines are observed for J′ values between 50 and 120 (Jmax′≈90, non-Boltzmannian population distribution). The full-width at half-maximum (FWHM) of all the observed lines is less than 0.007 cm−1. A Doppler width of 0.005 cm−1 and translational temperature Ttr≈280 K can be deduced. Such high-J levels of the CO molecule had never been observed in the laboratory. In the absorption spectrum of the Sun photosphere, the same lines present FWHM values 5–8 times larger. The best available Dunham coefficients are checked to reproduce the high-J lines wave numbers to at least 0.001 cm−1. Dissociative recombination of the dimer (CO)2+ cation, which is likely to be formed in our experimental conditions, is discussed as a possible mechanism to produce CO fragments with very high rotational excitation, while keeping vibrational excitation limited to v′=6.

Lithium nucleosynthesis in the Sun inferred from the solar-wind 7Li/6Li ratio

The abundance of lithium measured in meteorites has generally been assumed to be the ‘Solar System value’, which presumably reflects the abundance in the gas cloud out of which the Sun formed1. Lithium is a factor of 140 less abundant in the solar photosphere than in meteorites2; this difference has been attributed to the destruction of lithium (through nuclear reactions) at the base of the Sun's convection zone3,4,5. If this is correct, then the ratio of 7Li/6Li in the Sun's photosphere should be ∼106 (ref. 6), as 6Li is destroyed much more easily (at a lower temperature) than 7Li: the meteoritic abundance ratio is 7Li/6Li = 12.14 (ref. 7). Here we report that 7Li/6Li = 31 ± 4 for lithium in the solar wind that has been implanted in lunar soil. This low ratio suggests that lithium is produced when energetic protons from solar flares induce spallation reactions with 16O and 12C present in the photosphere.

A Simon–Weiss model for supergranule flows and their cross-correlations

Supergranule revolution rate and lifetime can be measured by cross-correlating pairs of Doppler-velocity maps that have been filtered (by Hathaway's method) to remove other flows. As a conceptual framework for that analysis, this exploratory paper develops an idealized, phenomenological model of supergranule flows. Assumptions made about supergranule cells on the Sun's photosphere include: random location in space and time, and horizontal flows with circular symmetry and having a Simon–Weiss velocity function. Each supergranule is stable for a time, dies, and after a while, a daughter is born at a nearby position determined by a random walk. The effect on the cross-correlations of changing projection onto the line-of-sight as the Sun rotates is analyzed. The total cross-correlation for strips of constant latitude depends on two generic, slowly-varying projection functions. Effects of differential rotation and time-evolution are also considered. GONG observations of June 1994 show systematic variations in the width and shape of correlation peaks with latitude; our model suggests that projection effects alone can account for these without invoking any intrinsic variations of the supergranules.

Coherent Scattering and Quantum Interference in the Sun's Spectrum

The Sun's visible spectrum is filled with a wealth of polarized spectral structures due to coherent scattering via bound-bound atomic transitions. Here we outline the theory for these coherence phenomena, including the effects of the external magnetic field (the Hanle effect), and demonstrate the correspondence between classical coherence and quantum interference between excited atomic states. Observational examples of the Hanle effect and of spectral features that can only be understood in terms of rather exotic quantum physics are presented. The signatures of coherent scattering have revealed the presence of a small scale, turbulent magnetic field in the Sun's photosphere, which has not been detectable by other techniques.

Shallow mixing in the solar photosphere inferred from revised beryllium abundances

The chemical compositions of the Sun and meteorites are the benchmarks against which the abundances of elements in all other astronomical objects are compared. A long-standing problem1 has been the abundance of lithium in the Sun's photosphere, which is ∼140 times less than the meteoritic value (which represents the lithium abundance at the time the Solar System formed). This depletion requires that material from the photosphere be transported below the convective zone into regions where the temperature is high enough that nuclear processing can remove lithium. The models2,3 best able to do so simultaneously deplete beryllium by about a factor of two, which is consistent with previous measurements4,5 of the beryllium abundance in the solar photosphere. But here we show that these previous measurements are in error, because they did not fully account for the continuous opacity in the ultraviolet region of the spectrum where the beryllium lines are observed. We find that, after correcting for this opacity, solar beryllium is not depleted at all with respect to the meteoritic value. This implies that mixing in the solar photosphere is more superficial than had hitherto been supposed, consistent with the shallow mixing inferred from recent helioseismic data6.

Sounding the Sun's Chromosphere

Time-distance analysis of solar acoustic waves with frequencies above the nominal atmospheric acoustic cutoff frequency (~5.3 mHz) shows partial reflection of the waves at both the Sun's photosphere and a layer located higher in the atmosphere. This result supports recent reports of chromospheric modes.