Dependence of UV radiance of the quiet Sun on the solar cycle: Surface magnetic fields as the cause (original) (raw)
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AIP Conference Proceedings, 2003
The seemingly un-magnetized part of the solar surface is not really un-magnetized. It is occupied by magnetic structures producing low polarization which, therefore, escape detection in traditional measurements. Since most of the solar surface belongs to this category, the quiet Sun magnetic fields can easily carry most of the magnetic flux and energy existing in the photosphere at any given time. Consequently, they are a potentially important ingredient of the solar magnetism. Most of the physical properties of the quiet Sun are still uncertain (distribution of field strengths, area coverage, influence on higher atmospheric layers, etc.).It is clear, however, that the topology of the field is complex, with field lines of very different properties coexisting in each resolution element. This fact hampers the detection of the quiet Sun magnetic fields. I argue that the best present measurements detect, at most, 30 % of the existing magnetic flux. Then the quiet Sun contains at least as much magnetic flux as all active regions and the network during the solar maximum.
The Distribution of the Quiet Sun Photospheric Magnetic Flux
Bulletin of the Astronomical Society of India
A gradient-based tessellating algorithm is used to study the magnetic field structure of the quiet Sun photosphere using SoRO full disk magnetograms. We find that the field is not uniformly distributed, but parcelled into flux concentrations. Both the flux and size of the concentrations are found to be described by broad, asymmetric distribution functions. Their mean absolute flux and size are found to be about 1.4 x 10 18 m:x: and 6.1 Mm for both polarities in unsmoothed magnetogra.ms at Ii gauss threshold. These values represent a. weighted average for both network-and intra.network magnetic fields, since the present method cannot currently distinguish between the two regions. Both flux and size distributions become more symmetric and less peaked in response to smoothing of images. Extrapolating this trend to sub-resolution seale, we note a linear decrease in size but a rapid increase in the mean absolute field strength, with asymptotic values of about 400 km and 50 gauss. This exercise shows that the true field strengths of quiet magnetic elements are higher and their size smaller than usua.lly inferred from observa.tions. This is because observation of mixed polarity regions under finite resolution causes the observed flux to be smeared out and apparently modified. Therefore, this flux cannot be interpreted independently of the geometric structure and flux distribution of the concentrations.
The mean magnetic field of the Sun: Observations at Stanford
Solar Physics, 1977
A solar telescope has been built at Stanford University to study the organization and evolution of large-scale solar magnetic fields and velocities. The observations are made using a Babcock-type magnetograph which is connected to a 22.9 m vertical Littrow spectrograph. Sun-as-a-star integrated light measurements of the mean solar magnetic field have been made daily since May 1975. The typical mean field magnitude has been about 0.15 G with typical measurement error less than 0.05 G. The mean field polarity pattern is essentially identical to the interplanetary magnetic field sector structure (see near the Earth with a 4 day lag). The differences in the observed structures can be understood in terms of a ‘warped current sheet’ model.
Long-term studies of photospheric magnetic fields on the Sun
Journal of Space Weather and Space Climate, 2020
We briefly review the history of observations of magnetic fields on the Sun, and describe early magnetograps for full disk measurements. Changes in instruments and detectors, the cohort of observers, the knowledge base etc may result in non-uniformity of the long-term synoptic datasets. Still, such data are critical for detecting and understanding the long-term trends in solar activity. We demonstrate the value of historical data using studies of active region tilt (Joy’s law) and the evolution of polar field and its reversal. Using the longest dataset of sunspot field strength measurements from Mount Wilson Observatory (1917-present) supplemented by shorter datasets from Pulkovo (1956–1997) and Crimean (1956-present) observatories we demonstrate that the magnetic properties of sunspots did not change over the last hundred years. We also show that the relationship between the sunspot area and its magnetic flux can be used to extend the studies of magnetic field in sunspots to period...
Magnetic Sources of the Solar Irradiance Cycle
The Astrophysical Journal, 1998
Using recently processed Ca K Ðltergrams, recorded with a 1 Ðlter at the Big Bear Solar Observa-Ó tory (BBSO), we quantitatively assess the component of solar irradiance variability attributable to bright magnetic features on the SunÏs disk. The Ca K Ðltergrams, "" Ñattened ÏÏ by removing instrumental e †ects and center-to-limb variations, provide information about bright sources of irradiance variability associated with magnetic activity in both active regions and dispersed active region remnants broadly distributed in the supergranule network (termed collectively "" faculae ÏÏ). Procedures are developed to construct both total and UV spectral solar irradiance variations explicitly from the processed Ca K Ðltergrams, independently of direct irradiance observations. The disk-integrated bolometric and UV facular brightness signals determined from the Ðltergrams between late 1991 and mid-1995 are compared with concurrent solar irradiance measurements made by high-precision solar radiometers on the Upper Atmosphere Research Satellite (UARS). The comparisons suggest that active-region and active-network changes can account for the measured variations. This good agreement during a period covering most of the decline in solar activity from the cycle 22 maximum to the impending solar minimum directly implicates magnetic features as the sources of the 11 yr irradiance cycle, apparently obviating the need for an additional component other than spots or faculae.
Quiet Sun Contribution to Variations in the Total Solar Irradiance
Solar Physics, 2006
An analysis of spatially-resolved measurements of the intensity of the photospheric continuum by the Michelson Doppler Imager (MDI) on the SOHO spacecraft indicates that these data can be used to study variations of the Total Solar Irradiance (TSI). Since the techniques employed depend upon ratios of intensities measured by MDI, they are independent of the absolute photometric calibration of the instrument. The results suggest that, while it is possible to account for short-term (weeks to months) variation in TSI by variations in the irradiance contributions of regions with enhanced magnetic fields (larger than ten G as measured by MDI), the longer-term variations are influenced significantly by variations in the brightness of the quiet Sun, defined here as regions with magnetic field magnitudes smaller than ten G. The latter regions cover a substantial fraction of the solar surface, ranging from approximately 90% of the Sun near solar minimum to 70% near solar maximum. The results provide evidence that a substantial fraction, 50% or more, of the longer term (≥one year) variation in TSI is due to changes in the brightness of the quiet Sun.
Irradiance Models Based on Solar Magnetic Fields
International Astronomical Union Colloquium, 1994
A method to separate the active region and quiet network components of the magnetic fields in the photosphere is described and compared with the corresponding measurements of the He I λ 10830 absorption. The relation between the total He I absorption and total magnetic flux in active regions is roughly linear and differs between cycles 21 and 22. There appears to no relation between these two quantities in areas outside of active regions. The total He I absorption in the quiet Sun (comprised of network, filaments, and coronal holes) exceeds that in active regions at all times during the cycle. As a whole, active regions of cycle 22 appear to be less complex than the active regions of cycle 21, hinting at one possible cause for a differing relation between spectral-irradiance variations and the underlying magnetic flux for these two cycles.
Modeling the Sun’s Magnetic Field and Irradiance since 1713
The Astrophysical Journal, 2005
We use a flux transport model to simulate the evolution of the Sun's total and open magnetic flux over the last 26 solar cycles (1713-1996). Polar field reversals are maintained by varying the meridional flow speed between 11 and 20 m s À1 , with the poleward-directed surface flow being slower during low-amplitude cycles. If the strengths of the active regions are fixed but their numbers are taken to be proportional to the cycle amplitude, the open flux is found to scale approximately as the square root of the cycle amplitude. However, the scaling becomes linear if the number of active regions per cycle is fixed but their average strength is taken to be proportional to the cycle amplitude. Even with the inclusion of a secularly varying ephemeral region background, the increase in the total photospheric flux between the Maunder minimum and the end of solar cycle 21 is at most one−thirdofitsminimum−to−maximumvariationduringthelattercycle.Thesimulationsarecomparedwithgeomagneticactivityandcosmogenicisotoperecordsandareusedtoderiveanewreconstructionoftotalsolarirradiance(TSI).Theincreaseincycle−averagedTSIsincetheMaunderminimumisestimatedtobeone-third of its minimum-to-maximum variation during the latter cycle. The simulations are compared with geomagnetic activity and cosmogenic isotope records and are used to derive a new reconstruction of total solar irradiance (TSI). The increase in cycle-averaged TSI since the Maunder minimum is estimated to be one−thirdofitsminimum−to−maximumvariationduringthelattercycle.Thesimulationsarecomparedwithgeomagneticactivityandcosmogenicisotoperecordsandareusedtoderiveanewreconstructionoftotalsolarirradiance(TSI).Theincreaseincycle−averagedTSIsincetheMaunderminimumisestimatedtobe1 W m À2. Because the diffusive decay rate accelerates as the average spacing between active regions decreases, the photospheric magnetic flux and facular brightness grow more slowly than the sunspot number and TSI saturates during the highest amplitude cycles.