Jean-Pierre Rozelot

Helioseismic test of nonhomologous solar radius changes with the 11 year activity cycle

Recent models of variations of the Suns structure with the 11 year activity cycle by Sofia et al. predict strong nonhomologous changes of the radius of subsurface layers, due to subsurface magnetic fields and field-modulated turbulence. According to their best model, the changes of the surface radius may be 1000 times larger than those at the depth of 5 Mm. We use f-mode oscillation frequency data from the SOHO MDI and measurements of the solar surface radius variations from SOHO and ground-based observatories during solar cycle 23 (1996-2005) to put constraints on the radius changes. The results show that the above model overestimates the change of the radius at the surface relative to the change at 5 Mm.

On the theory of the oblateness of the Sun

In this paper we first emphasize why it is important to know the successive zonal harmonics of the Suns figure with high accuracy: mainly fundamental astrometry, helioseismology, planetary motions and relativistic effects. Then we briefly comment why the Sun appears oblate, going back to primitive definitions in order to underline some discrepancies in theories and to emphasize again the relevant hypotheses. We propose a new theoretical approach entirely based on an expansion in terms of Legendres functions, including the differential rotation of the Sun at the surface. This permits linking the two first spherical harmonic coefficients (J2 and J4) with the geometric parameters that can be measured on the Sun (equatorial and polar radii). We emphasize the difficulties in inferring gravitational oblateness from visual measurements of the geometric oblateness, and more generally a dynamical flattening. Results are given for different observed rotational laws. It is shown that the surface oblateness is surely upper bounded by 11 milliarcsecond. As a consequence of the observed surface and sub-surface differential rotation laws, we deduce a measure of the two first gravitational harmonics, the quadrupole and the octopole moment of the Sun: J2=−(6.13±2.52)×10−7 if all observed data are taken into account, and respectively, J2=−(6.84±3.75)×10−7 if only sunspot data are considered, and J2=−(3.49±1.86)×10−7 in the case of helioseismic data alone. The value deduced from all available data for the octopole is: J4=(2.8±2.1)×10−12. These values are compared to some others found in the literature.

Probing The Solar Surface: The Oblateness and Astrophysical Consequences

Based on historical records, the Suns dimensions are temporally dependent. Until the recent past, varying dimensions were keenly disputed. Recent accurate observations have removed the doubt, whether from direct limb observations or from helioseismology f-modes analysis. A shrinking or an expanding shape is ultimately linked to solar activity, as even a small variation in the solar radius causes variations in gravitational energy. Based on accurate space- and ground-based observations, we will argue that the oblateness of the Sun is time dependent. Indeed, considering the first two shape coefficients, we can interpret such a temporal variation as a change in the relative importance of the hexadecapolar term, i.e., at the time of high activity, only the dipolar moment c 2 has a significant effect, but at the time of low activity, c 4 is predominant; this results in a decrease of the total value of the oblateness. The combination of the two terms leads to a solar oblateness varying along with solar activity. More studies are needed to get accurate measurements from space, which will provide us with the unique opportunity to study detailed changes of global solar properties.

Are Non-Magnetic Mechanisms Such As Temporal Solar Diameter Variations Conceivable for an Irradiance Variability?

Irradiance variability has been monitored from space for more than two decades. Even though data come from different sources, it is well established that a temporal variability exists ≈0.1%, in phase with the solar cycle. Today, one of the best explanations for such an irradiance variability is provided by the evolution of the solar surface magnetic fields. But if some 90–95% can be reproduced, what would be the origin of the 10–5% left? Non-magnetic effects are conceivable. In this paper we will consider temporal variations of the diameter of the Sun as a possible contributor for the remaining part. Such an approach imposes strong constraints on the solar radius variability. We will show that over a solar cycle, variations of no more than 20 mas of amplitude can be considered. Such a variability – far from what is reported by observers conducting measurements by means of ground-based solar astrolabes – may explain a little part of the irradiance changes not explained by magnetic features. Further, requirements are needed that may help to reach a conclusion. Dedicated space missions are necessary (for example PICARD, GOLF-NG or SDO, scheduled for a launch around 2008); it is also proposed to reactivate SDS flights for such a purpose.

Solar quadrupole moment from planetary ephemerides: present state of the art

We discuss the present state of the art of the solar quadrupole moment from planetary ephemerides.

Temporal and Periodic Variations of Sunspot Counts in Flaring and Non-Flaring Active Regions

We analyzed temporal and periodic variations of sunspot counts (SSCs) in flaring (C-, M-, or X-class flares), and non-flaring active regions (ARs) for nearly three solar cycles (1986 through 2016). Our main findings are as follows: i) temporal variations of monthly means of the daily total SSCs in flaring and non-flaring ARs behave differently during a solar cycle and the behavior varies from one cycle to another; during Solar Cycle 23 temporal SSC profiles of non-flaring ARs are wider than those of flaring ARs, while they are almost the same during Solar Cycle 22 and the current Cycle 24. The SSC profiles show a multi-peak structure and the second peak of flaring ARs dominates the current Cycle 24, while the difference between peaks is less pronounced during Solar Cycles 22 and 23. The first and second SSC peaks of non-flaring ARs have comparable magnitude in the current solar cycle, while the first peak is nearly absent in the case of the flaring ARs of the same cycle. ii) Periodic variations observed in the SSCs profiles of flaring and non-flaring ARs derived from the multi-taper method (MTM) spectrum and wavelet scalograms are quite different as well, and they vary from one solar cycle to another. The largest detected period in flaring ARs is 113±1.6days

Solar cycle variations of rotation and asphericity in the near-surface shear layer

113\pm 1.6~\mbox{days}

Critical frequencies of the ionospheric F1 and F2 layers during the last four solar cycles: Sunspot group type dependencies

while we detected much longer periodicities (327±13

The Figure of the Sun, Astrophysical Consequences. A Tutorial

327\pm 13

A brief history of the solar oblateness. A review

, 312±11