Importance of quiet-Sun small-scale magnetic features near polar caps to the total solar radiance

Importance of quiet-Sun small-scale magnetic

features near polar caps to the total solar radiance

vv

V. Domingo, I. Cabello & J. Blanco

Grupo de Astronomía y Ciencias del Espacio (GACE)

University of Valencia, Spain

• ¿Is the polar cap quiet Sun different from the low and middle latitude quiet Sun? The question is relevant because the total solar energy emission can only be determined if we have information on the complete solar local emission in all directions

• We contribute to the study the properties of the polar cap quiet Sun, analizing its magnetic properties as seen through the presence of small magnetic element bright points.

• There is a practical interest in the knowlege of the fact that the solar irradiance will be affected by the contribution of the polar cap, and it is that introduces a variation of the TSI

• Solar Orbiter (SO) after a few orbits will be put

in orbits that pass at up to some 30 degrees latitude from the poles, and therefore the telescopes aboard Solar Orbiter will be able to obtain information on the polar caps, but such observations will take place within several years, and we have considered to contribute to the studies already done by some groups, using the presently available data, since the Sun is 7.25º inclined respect to the Earth ecliptic plane axis.

The north end of the Sun’s rotation axis tilted 7.25º toward the Earth September

The north cap of the Sun is visible from the Earth . See polar coronal hole.

The south end of the Sun’s rotation axis tilted 7.25º toward the Earth - March

The south cap of the Sun is visible from the Earth – see polar coronal hole

Latitudinal variation of quiet Sun fraction of the covered surface (FCS) by bright points along the central meridian, when the Earth is in the plane that contains the 7.25º Sun’s rotation axis inclination respect to the ecliptic plane, i.e. every 6 months: March and September

Fraction of Covered solar Surface occupied by bright points versus latitude at central solar meridian – G-band

March 2007Hinode XRT

Fraction of Covered solar Surface occupied by bright points versus latitude at central solar meridian –G-band

September 2007Hinode XRT

Fraction of Covered solar Surface occupied by bright points versus latitude at central solar meridian G-band

September 2008Hinode XRT

March 2009Hinode XRT

Fraction of Covered solar Surface occupied by bright points versus latitude at central solar meridian – G-band

G-band FCS variation along the north-south central meridian

March 2010 Hinode XRT

G-band FCS variation along the north-south central meridian

March 2011 304 Å

G-band FCS variation along the north-south central meridian

September 2011 304 Å

G-band FCS variation along the north-south central meridian

March 2012 304 Å

G-band FCS variation along the north-south central meridian

September 2012 304 Å

G-band FCS variation along the north-south central meridian

March 2013 304 Å

G-band FCS variation along the north-south central meridian

The center-to-limb (CLV) observations recorded with valid data within the data sets considerd by us have been plotted simutaneously to compare the properties of the determined CLV for the different observation sets. See the following 4 slides

Central meridian CLV March 2007 - 2013

Central meridian CLV September 2007 - 2012

Equatorial CLV March 2010 - 2013

Equatorial CLV september 2007 + 2013

The displays of solar surface coverage by bright points when the south solar polar axis is closer to the Earth, i.e., when we see more of the polar cap, show a increase in the value of the FCS at the southern high latitude.

When the north polar axis of the Sun is closer to the Earth it is the values of the north high latitude that appear to have larger values of the FCS.

The obtained FCS values distribution versus latitude is consistent with the one found by Bonet et al. (2011) though the actual values are somewhat lower (of the order of 0.2% lower), but we should bear in mind that the selection criteria are different.

Solar centre

Solar limb

Conclusion

The “anomalous” fraction of covered surface (FCS) observed at high latitude probably indicates that at the polar cap there is a larger FCS than at the surface immediately beyond it.

This first attempt to get information on the quiet Sun properties at the solar polar cap must be taken with caution, even it may be considered to be a qualitative study, the starting point for a more quantitative one. Among the limitations of what has been done see:

1) We are measuring only G-band images.2) No error evaluation has bee performed.3) A statistical study of the properties of the bright points needs to be done.4) An evaluation of the surface covered by the “anomalous” FCS possibly associated to the polar cap is missing.

This study shows that it is possible to study many properties of the solar polar caps from the Earth, or near Earth point of view.