A. Ortiz

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.

Latitudinal Variation of the Solar Photospheric Intensity

We have examined images from the Precision Solar Photometric Telescope (PSPT) at the Mauna Loa Solar Observatory (MLSO) in search of latitudinal variation in the solar photospheric intensity. Along with the expected brightening of the solar activity belts, we have found a weak enhancement of the mean continuum intensity at polar latitudes (continuum intensity enhancement ~0.1%-0.2%, corresponding to a brightness temperature enhancement of ~2.5 K). This appears to be thermal in origin and not due to a polar accumulation of weak magnetic elements, with both the continuum and Ca II K intensity distributions shifted toward higher values with little change in shape from their midlatitude distributions. Since the enhancement is of low spatial frequency and of very small amplitude, it is difficult to separate from systematic instrumental and processing errors. We provide a thorough discussion of these and conclude that the measurement captures real solar latitudinal intensity variations.

The intensity contrast of solar photospheric faculae and network elements II. Evolution over the rising phase of solar cycle 23

We studied the radiative properties of small magnetic elements (active region faculae and the network) during the rising phase of solar cycle 23 from 1996 to 2001, determining their contrasts as a function of heliocentric angle, magnetogram signal, and the solar cycle phase. We combined near-simultaneous full disk images of the line-of-sight magnetic field and photospheric continuum intensity provided by the MDI instrument on board the SOHO spacecraft. Sorting the magnetogram signal into different ranges allowed us to distinguish between the contrast of different magnetic structures. We find that the contrast center-to-limb variation (CLV) of these small magnetic elements is independent of time with a 10% precision, when measured during the rising phase of solar cycle 23. A 2-dimensional empirical expression for the contrast of photospheric features as a function of both the position on the disk and the averaged magnetic field strength was determined, showing its validity through the studied time period. A study of the relationship between magnetogram signal and the peak contrasts shows that the intrinsic contrast (maximum contrast per unit of magnetic flux) of network flux tubes is higher than that of active region faculae during the solar cycle.

Indirect signatures for axion(-like) particles

Magnetic field dependent transient solar observations are suggestive for axionphoton oscillations with light axion(-like) particle involvement. Novel dark-moon measurements with the SMART X-ray detectors can be conclusive for radiatively decaying massive exotica like the generic solar Kaluza-Klein (KK) axions. Furthermore, the predicted intrinsic strong solar magnetic fields could be the reason of enhanced low energy axion production. Such an axion component could be the as yet unknown origin of the strong quiet Sun X-ray luminosity at energies below ~ 1 keV. Solar axion telescopes should lower their threshold, aiming to copy processes that might occur near the solar surface, be it due to spontaneous or magnetically induced radiative decay of axion(-like) particles. This is motivated also by the recent claim of an axion-like particle detection by the laser experiment PVLAS.

Contribution of the Photospheric Magnetic Network to The Solar Cycle Irradiance Variability

Solar Active Regions (AR) are considered to be the main drivers of total and spectral irradiance variations on time scales of a solar rotation. However, this agreement breaks when variations of the irradiance are considered on solar cycle time scales (≈ 0.1% from minimum to maximum). Different mechanisms have been proposed as responsible for those variations, among them, the small scale magnetic elements composing the enhanced and quiet network. Other mechanisms of non-magnetic origin have also been proposed, based, for example, on temporal changes in the latitude-dependent surface temperature of the Sun. This is a key question for understanding the solar cycle and the underlying physical processes.