Magnetogram Signal Research Articles

The variation in the response of solar full-disc magnetographs

The utility of full solar disc magnetograms as a long-term record of the photospheric magnetic field requires an understanding of how stable these observations are with time and the systematic differences between the various instruments. We compared magnetograms from the KPVT/SPM, SoHO/MDI, SOLIS/VSM, and SDO/HMI with the aim of probing the effect on measured solar magnetism of the variation in instrument response with time, magnetogram signal level, and position on the solar disc. Taking near-simultaneous observations from the various instruments, we examined the surface coverage by magnetic activity and the effect of cross-calibrating the various instruments under different assumptions. By comparing the surface coverage by magnetic activity in the observations from the various instruments, we traced the effect of the time variation in instrument response on the longitudinal magnetogram signal and disc-integrated unsigned magnetic flux. This yielded evidence of acute changes in the response of MDI and VSM with certain events such as the SoHO vacation in 1998 and the upgrade of the VSM CCD camera in 2009. Excluding these changes, the effect of instrument instability on the magnetogram signal and disc-integrated magnetic flux appears to be rather benign, with an associated uncertainty of less than 2$<!PCT!>$. We determined the magnetogram signal ratio between each instrument pairing as a function of magnetogram signal level and distance from disc centre and with it cross-calibrated the various instruments. We compared the result with that from repeating the cross-calibration with the overall magnetogram signal ratio. This allowed us to estimate the uncertainty in the magnetogram signal associated with the variation in instrument response with magnetogram signal level and distance from disc centre to be about 8$<!PCT!>$ to 14$<!PCT!>$. The corresponding uncertainty in the disc-integrated magnetic flux is about 7$<!PCT!>$ to 23 $<!PCT!>$. To the best of our knowledge, this is the first study of its kind to quantify the uncertainty in measured magnetism from the variation in instrument response with time, magnetogram signal level, and disc position. The results here will be useful to the interpretation of SPM, MDI, VSM, and HMI magnetograms. As examples, we applied our findings to selected results from earlier studies based on such data.

Point spread function of SDO/HMI and the effects of stray light correction on the apparent properties of solar surface phenomena

We present a PSF for SDO/HMI and discuss the effects of its removal on the apparent properties of solar surface phenomena in HMI data. The PSF was retrieved from observations of Venus in transit by matching it to the convolution of a model of the venusian disc and solar background with a guess PSF. Observations recorded near in time to the transit of Venus were corrected for instrumental scattered light by the deconvolution with the PSF. Granulation contrast in restored HMI data is greatly enhanced relative to the original data and exhibit reasonable agreement with numerical simulations. Image restoration enhanced the apparent intensity and pixel averaged magnetic field strength of photospheric magnetic features significantly. For small-scale magnetic features, restoration enhanced intensity contrast in the continuum and core of the Fe I 617.3 nm line by a factor of 1.3, and the magnetogram signal by a factor of 1.7. For sunspots and pores, the enhancement varied strongly within and between features, being more acute for smaller features. Magnetic features are also rendered smaller, as signal smeared onto the surrounding quiet Sun is recovered. Image restoration increased the apparent amount of magnetic flux above the noise floor by a factor of about 1.2, most of the gain coming from the quiet Sun. Line-of-sight velocity due to granulation and supergranulation is enhanced by a factor of 1.4 to 2.1, depending on position on the solar disc. There are indications that the PSF varies across the FOV, with time and between the two CCDs. However, all these variations were found to be relatively small, such that the derived PSF can be applied to HMI data from both CCDs, over the period examined without introducing significant error. Image restoration affects the measured radiant, magnetic and dynamic properties of solar surface phenomena sufficiently to significantly impact interpretation.

Intensity contrast of solar network and faculae

This study aims at setting observational constraints on the continuum and line core intensity contrast of network and faculae, specifically, their relationship with magnetic field and disc position. Full-disc magnetograms and intensity images by the HMI instrument onboard SDO were employed. Bright magnetic features, representing network and faculae, were identified and the relationship between their intensity contrast at continuum and line core with magnetogram signal and heliocentric angle examined. Care was taken to minimize the inclusion of the magnetic canopy and straylight from sunspots and pores as network and faculae. In line with earlier studies, network features, on a per unit magnetic flux basis, appeared brighter than facular features. Intensity contrasts in the continuum and line core differ considerably, most notably, they exhibit opposite centre-to-limb variations. We found this difference in behaviour to likely be due to the different mechanisms of the formation of the two spectral components. From a simple model based on bivariate polynomial fits to the measured contrasts we confirmed spectral line changes to be a significant driver of facular contribution to variation in solar irradiance. The discrepancy between the continuum contrast reported here and in the literature was shown to arise mainly from differences in spatial resolution and treatment of magnetic signals adjacent to sunspots and pores. HMI is a source of accurate contrasts and low-noise magnetograms covering the full solar disc. For irradiance studies it is important to consider not just the contribution from the continuum but also from the spectral lines. In order not to underestimate long-term variations in solar irradiance, irradiance models should take the greater contrast per unit magnetic flux associated with magnetic features with low magnetic flux into account.

Structure and dynamics of isolated internetwork Ca II H bright points observed by SUNRISE

We aim to improve our picture of the low chromosphere in the quiet-Sun internetwork by investigating the intensity, horizontal velocity, size and lifetime variations of small bright points (BPs; diameter smaller than 0.3 arcsec) observed in the Ca II H 3968 {\AA} passband along with their magnetic field parameters, derived from photospheric magnetograms. Several high-quality time series of disc-centre, quiet-Sun observations from the Sunrise balloon-borne solar telescope, with spatial resolution of around 100 km on the solar surface, have been analysed to study the dynamics of BPs observed in the Ca II H passband and their dependence on the photospheric vector magnetogram signal. Parameters such as horizontal velocity, diameter, intensity and lifetime histograms of the isolated internetwork and magnetic Ca II H BPs were determined. Mean values were found to be 2.2 km/s, 0.2 arcsec (150 km), 1.48 average Ca II H quiet-Sun and 673 sec, respectively. Interestingly, the brightness and the horizontal velocity of BPs are anti-correlated. Large excursions (pulses) in horizontal velocity, up to 15 km/s, are present in the trajectories of most BPs. These could excite kink waves travelling into the chromosphere and possibly the corona, which we estimate to carry an energy flux of 310 W/m^2, sufficient to heat the upper layers, although only marginally. The stable observing conditions of Sunrise and our technique for identifying and tracking BPs have allowed us to determine reliable parameters of these features in the internetwork. Thus we find, e.g., that they are considerably longer lived than previously thought. The large velocities are also reliable, and may excite kink waves. Although these wave are (marginally) energetic enough to heat the quiet corona, we expect a large additional contribution from larger magnetic elements populating the network and partly also the internetwork.

The continuum intensity as a function of magnetic field

Small-scale magnetic fields are major contributors to the solar irradiance variations. Hence, the continuum intensity contrast of magnetic elements in the quiet Sun (QS) network and in active region (AR) plage is an essential quantity that needs to be measured reliably. By using Hinode/SP disk center data at a constant, high spatial resolution, we aim at updating results of earlier ground-based studies of contrast vs. magnetogram signal, and to look for systematic differences between AR plages and QS network. The field strength, filling factor and inclination of the field was retrieved by Milne-Eddington inversion (VFISV). As in earlier studies, we performed a pixel-by-pixel study of 630.2 nm continuum contrast vs. apparent (i.e. averaged over a pixel) longitudinal magnetic field over large fields of view in ARs and in the QS. The contrast of magnetic elements reaches larger values in the QS (on average 3.7%) than in ARs (1.3%). This could not be attributed to any systematic difference in the chosen contrast references. At Hinode's spatial resolution, the relationship between contrast and apparent longitudinal field strength exhibits a peak at around 700 G in both the QS and ARs, whereas earlier lower resolution studies only found a peak in the QS and a monotonous decrease in ARs. We attribute this discrepancy both to our careful removal of the pores and their close surroundings affected by the telescope diffraction, as well as to the enhanced spatial resolution and very low scattered light of the Hinode Solar Optical Telescope. According to our inversions, the magnetic elements producing the peak of the contrast curves have similar properties (field strength, inclination, filling factor) in ARs and in the QS, so that the larger brightness of magnetic elements in the QS remains unexplained.

Contrast Analysis of Solar Faculae and Magnetic Bright Points

The morphology and contrast of small-scale solar elements at four disk positions is analyzed. The data were obtained at the Swedish 1 m Solar Telescope (SST) over 3 yr (2003-2005). Two of the data sets have disk positions near disk center (average μ = cos θ > 0.8) and show numerous magnetic bright points (MBPs), and two are sufficiently limbward to show prominent (average μ ≤ 0.6). The filtergrams are obtained in the 430.5 nm G band and 436.4 nm continuum bandpasses; the magnetograms are Fe I 630.25 nm Stokes V images taken with the Solar Optical Universal Polarimeter (SOUP) tunable filter. In all images we achieve nearly diffraction-limited resolution (~100 km in the G band). Analysis shows that MBPs and faculae are distinct radiative signatures of the field: MBPs have a constant or slightly decreasing contrast with increasing magnetogram signal, while facular contrast increases linearly with magnetogram signal. Faculae are much larger than MBPs, with an average radial width of 400 km. The observations support recent modeling showing that faculae are granules seen through the opacity reduction provided by elements (or groups thereof), while MBPs are caused by lateral radiation leakage scattering from deeper layers of the element.

Inclination of magnetic fields and flows in sunspot penumbrae

An observational study of the inclination of magnetic fields and flows in sunspot penumbrae at a spatial resolution of 0. �� 2 is presented. The analysis is based on longitudinal magnetograms and Dopplergrams obtained with the Swedish 1-m Solar Telescope on La Palma using the Lockheed Solar Optical Universal Polarimeter birefringent filter. Data from two sunspots observed at several heliocentric angles between 12 ◦ and 39 ◦ were analyzed. We find that the magnetic field at the level of the formation of the Fe -line wing (630.25 nm) is in the form of coherent structures that extend radially over nearly the entire penumbra giving the impression of vertical sheet-like structures. The inclination of the field varies up to 45 ◦ over azimuthal distances close to the resolution limit of the magnetograms. Dark penumbral cores, and their extensions into the outer penumbra, are prominent features associated with the more horizontal component of the magnetic field. The inclination of this dark penumbral component - designated B - increases outwards from approximately 40 ◦ in the inner penumbra such that the field lines are nearly horizontal or even return to the solar surface already in the middle penumbra. The bright component of filaments - designated A - is associated with the more vertical component of the magnetic field and has an inclination with respect to the normal of about 35 ◦ in the inner penumbra, increasing to about 60 ◦ towards the outer boundary. The magnetogram signal is lower in the dark component B regions than in the bright component A regions of the penumbral filaments. The measured rapid azimuthal variation of the magnetogram signal is interpreted as being caused by combined fluctuations of inclination and magnetic field strength. The Dopplergrams show that the velocity field associated with penumbral component B is roughly aligned with the magnetic field while component A flows are more horizontal than the magnetic field. The observations give general support to fluted and uncombed models of the penumbra. The long-lived nature of the dark-cored filaments makes it difficult to interpret these as evidence for convective exchange of flux tubes. Our observations are in broad agreement with the two component model of Bellot Rubio et al. (2003), but do not rule out the embedded flux tube model of Solanki & Montavon (1993).

A comparison between model calculations and observations of sunspot oscillations

We investigate the signal which is expected to be produced by magnetic field oscillations in sunspots umbrae due to the combination of the oscillation model, radiative transfer and observing procedure. For this purpose we investigate the signal expected to be produced by theoretical models of sunspot oscillations and compare them with the signal seen in observed power spectra of sunspot magnetograms. We show that the amplitudes of the observed oscillations are compatible with the predictions of the theoretical model of magnetoacoustic oscillations for the 5-min as well as for the 3-min band. For the particular sunspot umbral oscillation models used, our analysis suggests that most of the expected observed power in the magnetogram signal oscillations is actually due to cross-talk from the temperature and density oscillations associated with the magnetoacoustic wave. A detailed modelling of the observing procedure turns out to be of central importance for the assignment of the observed oscillations to a specific wave type.

On the intensity contrast of solar photospheric faculae and network elements

Sunspots, faculae and the magnetic network contribute to solar irradiance variations. The contribution due to faculae and the network is of basic importance, but suers from considerable uncertainty. We determine the contrasts of active region faculae and the network, both as a function of heliocentric angle and magnetogram signal. To achieve this, we analyze near-simultaneous full disk images of photospheric continuum intensity and line- of-sight magnetic eld provided by the Michelson Doppler Interferometer (MDI) on board the SOHO spacecraft. Starting from the surface distribution of the solar magnetic eld we rst construct a mask, which is then used to determine the brightness of magnetic features, and the relatively eld-free part of the photosphere separately. By sorting the magnetogram signal into dierent bins we are able to distinguish between the contrasts of dierent concentrations of magnetic eld. We nd that the center-to-limb variation (CLV) of the contrast changes strongly with magnetogram signal. Thus, the contrasts of active region faculae (large magnetogram signal) and the network (small signal) exhibit a very dierent CLV, showing that the populations of magnetic flux tubes that underly the two kinds of features are dierent. The results are compatible with, on average, larger flux tubes in faculae than in the network. This implies that these elements need to be treated separately when reconstructing variations of the total solar irradiance with high precision. We have obtained an analytical expression for the contrast of photospheric magnetic features as a function of both position on the disk and spatially averaged magnetic eld strength, by performing a 2-dimensional t to the observations. We also provide a linear relationship between magnetogram signal and the =c os(), where is the heliocentric angle, at which the contrast is maximal. Finally, we show that the maximum contrast per unit magnetic flux decreases rapidly with increasing magnetogram signal, supporting earlier evidence that the intrinsic contrast of magnetic flux tubes in the network is higher.

Small-Scale Structures in Active Regions

AbstractWithin the last few years significant progress has been made in our understanding of the small-scale structures in active regions. Here I present some of the newest findings obtained by using speckle interferometric techniques. There exist continuum bright points with a contrast of about 30% that are cospatial with strong magnetic fields. The observations are consistent with the assumption that some facular and network bright points are the white-light signature of magnetic fluxtubes with a diameter of about 200 km. Magnetic elements larger than about 300 km are mainly darker than the average quiet sun. Their properties are similar to what has been called magnetic knots or invisible sunspots. In highly magnetic areas there is no clear relationship between continuum intensity and magnetogram signal at the smallest spatial scales. The magnetic field of pores extends beyond the dark umbra. There radially elongated structures may appear that are similar to penumbral filaments in sunspots.

Measurements of magnetic fluxes and field strengths in the photospheric network

We present digital pictures of an active region network cell in five quantities, measured simultaneously: continuum intensity, line-center intensity, equivalent width, magnetogram signal, and magnetic field strength. These maps are derived from computer analysis of circularly polarized line profiles of FeI λ 5250.2; spectral and spatial resolution are 1/40 A and 1.5″, respectively. Measured Zeeman splittings show the existence of strong magnetic fields (1000–1800 G) at nearly all points with a magnetogram signal exceeding 125 G. The mean and rms deviation of the field strengths change by less than 20% over a factor-of-four range of fluxes. From the significant disparity between measured fluxes and field strengths, we conclude that large flux patches (up to 4″ across) consist of closely-packed unresolved filaments. The smallest filaments must be less than 0.7″ in diameter. We also observe the dark component of the photospheric network, which appears to contain sizable transverse fields.