Is the Polar Region Different from the Quiet Region of the Sun?

Observations of the polar region of the Sun are critically important for understanding the solar dynamo and the acceleration of solar wind. We carried out precise magnetic observations on both the North polar region and the quiet Sun at the East limb with the Spectro-Polarimeter of the Solar Optical Telescope aboard Hinode to characterize the polar region with respect to the quiet Sun. The average area and the total magnetic flux of the kG magnetic concentrations in the polar region appear to be larger than those of the quiet Sun. The magnetic field vectors classified as vertical in the quiet Sun have symmetric histograms around zero in the strengths, showing balanced positive and negative flux, while the histogram in the North polar region is clearly asymmetric, showing a predominance of the negative polarity. The total magnetic flux of the polar region is larger than that of the quiet Sun. In contrast, the histogram of the horizontal magnetic fields is exactly the same between the polar region and the quiet Sun. This is consistent with the idea that a local dynamo process is responsible for the horizontal magnetic fields. A high-resolution potential field extrapolation shows that the majority of magnetic field lines from the kG-patches in the polar region are open with a fanning-out structure very low in the atmosphere, while in the quiet Sun, almost all the field lines are closed.Comment: Accepted for publication in AP

Placing limits on long-term variations in quiet-Sun irradiance and their contribution to total solar irradiance and solar radiative forcing of climate

Recent reconstructions of Total Solar Irradiance (TSI) postulate that quiet Sun variations could give significant changes to solar power input to Earth’s climate (radiative climate forcings of 0.7-1.1Wm-2 over 1700-2019) arising from changes in quiet-Sun magnetic fields that have not, as yet, been observed. Reconstructions without such changes yield solar forcings that are smaller by a factor of more than 10. We study the quiet-Sun TSI since 1995 for three reasons: (1) this interval shows rapid decay in average solar activity following the grand solar maximum in 1985 (such that activity in 2019 was broadly equivalent to that in 1900); (2) there is improved consensus between TSI observations; and (3) it contains the first modelling of TSI that is independent of the observations. Our analysis shows the most likely upward drift in quiet-Sun radiative forcing since 1700 is between +0.07 and 0.13Wm-2. Hence we cannot yet discriminate between the quiet-Sun TSI being enhanced or reduced during the Maunder and Dalton sunspot minima, although there is a growing consensus from the combinations of models and observations that it was slightly enhanced. We present reconstructions that add quiet-Sun TSI and its uncertainty to models that reconstruct the effects of sunspots and faculae

Energy and helicity budgets of solar quiet regions

We investigate the free magnetic energy and relative magnetic helicity budgets of solar quiet regions. Using a novel non-linear force-free method requiring single solar vector magnetograms we calculate the instantaneous free magnetic energy and relative magnetic helicity budgets in 55 quiet-Sun vector magnetograms. As in a previous work on active regions, we construct here for the first time the (free) energy-(relative) helicity diagram of quiet-Sun regions. We find that quiet-Sun regions have no dominant sense of helicity and show monotonic correlations a) between free magnetic energy/relative helicity and magnetic network area and, consequently, b) between free magnetic energy and helicity. Free magnetic energy budgets of quiet-Sun regions represent a rather continuous extension of respective active-region budgets towards lower values, but the corresponding helicity transition is discontinuous due to the incoherence of the helicity sense contrary to active regions. We further estimate the instantaneous free magnetic-energy and relative magnetic-helicity budgets of the entire quiet Sun, as well as the respective budgets over an entire solar cycle. Derived instantaneous free magnetic energy budgets and, to a lesser extent, relative magnetic helicity budgets over the entire quiet Sun are comparable to the respective budgets of a sizeable active region, while total budgets within a solar cycle are found higher than previously reported. Free-energy budgets are comparable to the energy needed to power fine-scale structures residing at the network, such as mottles and spicules

Comparison of transient horizontal magnetic fields in a plage region and in the quiet Sun

Properties of transient horizontal magnetic fields (THMFs) in both plage and quiet Sun regions are obtained and compared. Spectro-polarimetric observations with the Solar Optical Telescope (SOT) on the Hinode satellite were carried out with a cadence of about 30 seconds for both plage and quiet regions located near disk center. We select THMFs that have net linear polarization (LP) higher than 0.22%, and an area larger than or equal to 3 pixels, and compare their occurrence rates and distribution of magnetic field azimuth. We obtain probability density functions (PDFs) of magnetic field strength and inclination for both regions.The occurrence rate in the plage region is the same as for the quiet Sun. The vertical magnetic flux in the plage region is ~8 times larger than in the quiet Sun. There is essentially no preferred orientation for the THMFs in either region. However, THMFs in the plage region with higher LP have a preferred direction consistent with that of the plage-region's large-scale vertical field pattern. PDFs show that there is no difference in the distribution of field strength of horizontal fields between the quiet Sun and the plage regions when we avoid the persistent large vertical flux concentrations for the plage region. The similarity of the PDFs and of the occurrence rates in plage and quiet regions suggests that a local dynamo process due to the granular motion may generate THMFs all over the sun. The preferred orientation for higher LP in the plage indicates that the THMFs are somewhat influenced by the larger-scale magnetic field pattern of the plage.Comment: 11 pages, 7 figures, A&A accepte

The dark side of solar photospheric G-band bright points

Bright small-scale magnetic elements found mainly in intergranular lanes at the solar surface are named bright points (BPs). They show high contrasts in Fraunhofer G-band observations and are described by nearly vertical slender flux tubes or sheets. A recent comparison between BP observations in the ultraviolet (UV) and visible spectral range recorded with the balloon-borne observatory SUNRISE and state-of-the-art magnetohydrodynamical (MHD) simulations revealed a kiloGauss magnetic field for 98% of the synthetic BPs. Here we address the opposite question, namely which fraction of pixels hosting kiloGauss fields coincides with an enhanced G-band brightness. We carried out 3D radiation MHD simulations for three magnetic activity levels (corresponding to the quiet Sun, weak and strong plage) and performed a full spectral line synthesis in the G-band. Only 7% of the kiloGauss pixels in our quiet-Sun simulation coincide with a brightness lower than the mean quiet-Sun intensity, while 23% of the pixels in the weak-plage simulation and even 49% in the strong-plage simulation are associated with a local darkening. Dark strong-field regions are preferentially found in the cores of larger flux patches that are rare in the quiet Sun, but more common in plage regions, often in the vertices of granulation cells. The significant brightness shortfall in the core of larger flux patches coincide with a slight magnetic field weakening. KiloGauss elements in the quiet Sun are on average brighter than similar features in plage regions. Almost all strong-field pixels display a more or less vertical magnetic field orientation. Hence in the quiet Sun, G-band BPs correspond almost one-to-one with kiloGauss elements. In weak plage the correspondence is still very good, but not perfect.Comment: Accepted for publication in Astronomy & Astrophysic

Magnetic properties of photospheric regions having very low magnetic flux

The magnetic properties of the quiet Sun are investigated using a novel inversion code, FATIMA, based on the Principal Component Analysis of the observed Stokes profiles. The stability and relatively low noise sensitivity of this inversion procedure allows for the systematic inversion of large data sets with very weak polarization signal. Its application to quiet Sun observations of network and internetwork regions reveals that a significant fraction of the quiet Sun contains kilogauss fields (usually with very small filling factors) and confirms that the pixels with weak polarization account for most of the magnetic flux. Mixed polarities in the resolution element are also found to occur more likely as the polarization weakens.Comment: To apapear in ApJ. 39 pages, 12 figures (2 of them are color figures

First limits on the 3-200 keV X-ray spectrum of the quiet Sun using RHESSI

We present the first results using the Reuven Ramaty High-Energy Solar Spectroscopic Imager, RHESSI, to observe solar X-ray emission not associated with active regions, sunspots or flares (the quiet Sun). Using a newly developed chopping technique (fan-beam modulation) during seven periods of offpointing between June 2005 to October 2006, we obtained upper limits over 3-200 keV for the quietest times when the GOES12 1-8A flux fell below $10^{-8}$ Wm$^{-2}$. These values are smaller than previous limits in the 17-120 keV range and extend them to both lower and higher energies. The limit in 3-6 keV is consistent with a coronal temperature $\leq 6$ MK. For quiet Sun periods when the GOES12 1-8A background flux was between $10^{-8}$ Wm$^{-2}$ and $10^{-7}$ Wm$^{-2}$, the RHESSI 3-6 keV flux correlates to this as a power-law, with an index of $1.08 \pm 0.13$. The power-law correlation for microflares has a steeper index of $1.29 \pm 0.06$. We also discuss the possibility of observing quiet Sun X-rays due to solar axions and use the RHESSI quiet Sun limits to estimate the axion-to-photon coupling constant for two different axion emission scenarios.Comment: 4 pages, 3 figures, Accepted by ApJ letter

A nanoflare model of quiet Sun EUV emission

Nanoflares have been proposed as the main source of heating of the solar corona. However, detecting them directly has so far proved elusive, and extrapolating to them from the properties of larger brightenings gives unreliable estimates of the power-law exponent $\alpha$ characterising their distribution. Here we take the approach of statistically modelling light curves representative of the quiet Sun as seen in EUV radiation. The basic assumption is that all quiet-Sun EUV emission is due to micro- and nanoflares, whose radiative energies display a power-law distribution. Radiance values in the quiet Sun follow a lognormal distribution. This is irrespective of whether the distribution is made over a spatial scan or over a time series. We show that these distributions can be reproduced by our simple model.Comment: 13 pages, 18 figures, accepted for publication by A&