THE ROLE OF PICARD IN UNCOVERING THE PROPERTIES OF THE VARIABILITY OF THE SOLAR

INTERIOR

Sabatino Sofia

Department of Astronomy

Yale University

CNES, October 24, 2006

Intrinsically variable stars belong to 2 types:

Stars that vary as a consequence of surface features

STARSPOTS

Stars that vary because of structural readjustments that affect the luminosity

CEPHEIDSMIRA VARIABLESETC.

THE SUN CAN VARY IN BOTH WAYS:

STRUCTURAL (INTERNAL)- ALL GLOBAL PARAMETERS CHANGEe.g. EVOLUTION

LOCALIZED:

e.g. ACTIVE REGIONS, NETWORK, etc.

WHICH TYPE OF VARIATION DOMINATES DEPENDS ON THE TIMESCALES INVOLVED..

SHORT TIMESCALE VARIATIONS OF THE TSI

SOON AFTER THE LAUNCH OF NIMBUS 7, AND ESPECIALLY SMM, IT WAS FOUND THAT THE TOTAL SOLAR IRRADIANCE (TSI) VARIED ON

TIMESCALES OF MINUTES TO DAYS, AND MOST OF THE VARIATION COULD SIMPLY BE RELATED TO THE PASSAGE OF ACTIVE REGIONS (SPOTS AND

FACULAE) ON THE SOLAR DISK

SURFACE PHENOMENA

FOR VERY LONG TIMESCALES, VARIABILITY

IS CLEARLY DOMINATED BY INTERNAL CHANGES

---THEORY OF VARIATIONS

---ENERGY REQUIRED

QUESTION:

WHERE, IN THE TIME DOMAIN, IS THE CROSSOVER OF THE DOMINANCE FROM SURFACE TO INTERNALORIGIN OCCUR?

IN PARTICULAR,

WHAT FRACTION OF THE 11 YEAR CYCLE IS DUE TO SURFACE MODULATION?

WHAT FRACTION IS DUE TO INTERNAL PROCESSES?

IT WAS NATURAL THAT THE FIRST ATTEMPT TO MODELTHE 11-YEAR MODULATION OF THE TSI WAS BASED ON SOLAR ACTIVITY MANIFESTATIONS.

SINCE SPOTS AND FACULAE ALONE COULD NOT DO IT,THE MAGNETIC NETWORK WAS BROUGHT TO BEAR.

BECAUSE NO RELIABLE MEASURE OF THE NETWORK CONTRAST IN WHITE LIGHT EXISTED, IT BECAME NECESSARY TO INVOKE SOLAR PROPERTIES THAT COULD BE PROXIES FOR THE NETWORK, AND THEN RELATE THEM TO THE TSI BY BEST-FITTING TECHNIQUES

THAT APPROACH ASSUMED THAT THE SUN BELOW THE PHOTOSPHERE, THAT IS, L, REMAINED UNCHANGED

IT WAS BASED ON SOME SIMPLIFIED ANALYTICCALCULATIONS, WHICH INDICATED THATSTRUCTURAL AND THERMAL CHANGES IN THE SOLAR INTERIOR COULD NOT OCCUR ON TIMESCALES SHORTER THAN THE THERMAL TIMESCALE OF THE BASE OF THE CONVECTION ZONE, ABOUT 100,000 YEARS.

THIS WAS CONTRADICTED BY

NUMERICAL MODELS

OSCILLATION VARIATIONS WITH ACTIVITY CYCLE

MIRA-TYPE AND OTHER VARIABLE STARS

VARIATIONS OF THE PHOTOSPHERIC TEMPERATURE

POSSIBLE DIFFERENCE OF TSI VALUE AT LAST TWO ACTIVITY MINIMA

NEW ANALYTICAL CALCULATIONS

IT IS MY CURRENT OPINION THAT THE 11 YEAR MODULATION OF THE TSI HAS BOTH

A SURFACE ACTIVITY RELATED COMPONENT (LOCAL), AS WELL AS A

GLOBAL COMPONENT DUE TO STRUCTURAL VARIATIONS OF THE SOLAR INTERIOR, AND

THAT FOR LONGER TIMESCALES THE INTERNAL STRUCTURE VARIATIONS

DOMINATE

PICARD IS UNIQUELY SUITED TO UNCOVER THE DETAILS OF THE PHYSICAL MECHANISM RESPONSIBLE FOR THE VARIATIONS OF THE SOLAR INTERIOR, OR

TO DISCOVER THE ENGINE FOR SOLAR CHANGE

PROPERTIES OF STRUCTURAL VARIATIONS

BECAUSE THEY INVOLVE THE ENTIRE CONVECTION ZONE (LOTS OF ENERGY), THEY CAN HAVE LONG TIMESCALE COMPONENTS FOR CLIMATE CHANGE

LONG TIMESCALE COMPONENTS ARE DIFFICULT TODIFFERENTIATE FROM INSTRUMENT DEGRADATIONIN ENERGY FLUX-TYPE MEASUREMENTS

IF WELL UNDERSTOOD, THEY PROVIDE PHYSICAL MECHANISM RESPONSIBLE FOR CHANGE

PREDICTABILITY

PROPERTIES OF STRUCTURAL VARIATIONS

BECAUSE THEY INVOLVE THE ENTIRE CONVECTION ZONE (LOTS OF ENERGY), THEY CAN HAVE LONG TIMESCALE COMPONENTS FOR CLIMATE CHANGE

LONG TIMESCALE COMPONENTS ARE DIFFICULT TODIFFERENTIATE FROM INSTRUMENT DEGRADATIONIN ENERGY FLUX-TYPE MEASUREMENTS

IF WELL UNDERSTOOD, THEY PROVIDE PHYSICAL MECHANISM RESPONSIBLE FOR CHANGE

PREDICTABILITY

The solar dynamo produces in the solarinterior two toroidally-shaped magnetic fieldsof variable intensity.

A variable magnetic field contributes topressure, internal energy, and modifiesenergy transfer both by convection andradiation

IT AFFECTS THE STRUCTURE

THEORY

The changes are inevitable.

Questions:

1) How big are the changes?2) Do we have evidence that they have occurred?

Theory: what has been done?

• 1-D

Variable magnetic fields.

-Pressure

-Internal Energy

-Dynamics

(The magnetic field is assumed to change as a sinusoid in time, but is not obtained in a dynamo calculation).

• In standard stellar model calculations there are four stellar structure equations, since there are four variables. They are – Mass Conservation– Momentum Conservation– Energy Conservation– Energy Transport

When Magnetic Fields are present, we have to introduce two new Magnetic Variables

Stellar structure equations with magnetic fields and turbulence

Modified stellar structure equation

Stellar structure equations with magnetic fields and turbulence

Convective Temperature Gradient

and are the compressibility and expression coefficient at constant temperature and constant magnetic field, respectively

Results of Early Calculations• A dynamo type magnetic field does affects

the solar structure, and as a consequence, all of the global parameters.

• The specific properties of the effects (the relationships between the variations of all the parameter pairs) depend on the details of the magnetic field (magnitude, depth, shape, etc.)

Hence,

To verify the model of the solar variations it

is necessary to observe, simultaneously, all of the global parameters, plus the oscillations.

BECAUSE A DYNAMO FIELD REQUIRES AT LEAST A 2D TREATMENT, AS DO EVEN THE SIMPLEST DYNAMICAL MODELS, WE HAVE JUST COMPLETED A 2D MODEL OF THE SOLAR INTERIOR

Li, Ventura, Sofia et al., ApJ Sup. 164, 215-254, 2006.

PICARD

WILL CARRY OUT ALL OF THE REQUIRED OBSERVATIONS FOR A PERIOD OF AT LEASTTHREE YEARS

DIAMETERTEMPERATURE CHANGETSIHELIOSEISMOLOGY (IN COLLABORATION WITH SDO)

AT A PHASE OF THE ACTIVITY CYCLE THATMAXIMIZES THE VARIATIONS

Key constraints for validating the physics of solar models are simultanously measurements of:

- solar diameter, limb shape, asphericity in the photosphere- total solar irradiance- oscillation modes- Temperature variations in the photosphere

Tests and limitations of the codes performed to date:

At the present time, we do not have all the simultaneous observations available to test the models.

WE HAVE DONE IT WITH WHAT WE HAVE

We assume that the average TSI variation observed over the last 20 years is due to structural changes

WHEN WE GET PICARD DATA, WE CAN TAKE INTO ACCOUNT VERY ACCURATELY THE ACTIVE REGION COMPONENT OF THE TSI VARIATIONS, SO THAT IT WILL NOT BE NECESSARY TO ASSUME WHAT PART OF THE TSI VARIATION IS DUE TO SURFACE FEATURES

THESE MEASUREMENTS REQUIRED A CALIBRATION BETWEEN THE DEPTH OF SOME SPECTRAL LINES AND PHOTOSPHERIC TEMPERATURE THAT WAS DONE WITH A FEW STARS HAVING SOMEWHAT DIFFERENT g VALUES.

IN WORK CARRIED OUT BY CACCIN AND COLLEAGUES IT WAS DETERMINED THAT THIS PROCEDURE INTRODUCEDSIGNIFICAN UNCERTAINTIES.

PICARD WILL BE ABLE TO DETERMINE PHOTOSPHERICTEMPERATURE VARIATIONS WITHOUT THE NEED OF THE ABOVE CALIBRATION, AND IN A VERY DIRECT WAY.

Radius Variations

We assume that the radius variations are in antiphase with the activity cycle, but of unknown amplitude.

The helioseismic radius changes are on the

order of, or less than, 1 km/y

BECAUSE A MAGNETIC FIELD CAN ONLY PRODUCE A POSITIVE PRESSURE, IT ALONE CANNOT LEAD TO RADIUS CHANGES IN ANTIPHASE WITH THE ACTIVITY CYCLE

WE WERE COMPELLED TO INCLUDE THE EFFECT OF A MAGNETICALLY MODULATED TURBULENCE

IN THE ABSENCE OF A THEORY ON THE MODULATION OF TURBULENCE BY A MAGNETIC FIELD, WE POSTULATED A SIMPLE ARBITRARY RELATIONSHIPLINKING THEM.

WE ARE CURRENTLY MODELING THIS INTERACTION..

Observation: P-mode frequency

Observations: CZ base

Observational constraints

Magnetically –modulated turbulent models

WHAT IS THE POTENTIAL OF THIS WORK?

1. DETERMINE W= dlnR/dlnL

2. CONTINUE DETERMINATION OF PAST SOLAR RADIUS MEASUREMENTS.

3. DETERMINE PAST VALUES FOR L (THUS TSI) TO BE USED IN CLIMATE STUDIES.

IT IS OBVIOUS THAT WE ARE AT THE THRESHOLD OF SOLVING THE PROBLEM OF THE ORIGIN OF SOLAR VARIABILITY ON TIMESCALES OF DECADES TO MILLENNIA.

OBSERVATIONALLY, THE MOST CRITICAL DATA WILL BE PROVIDED BY PICARD, ALTHOUGH SOHO, MDI, SORCEAND OTHER SPACE MISSIONS WHICH ARE OPERATIONAL AT THE TIME WILL ASSIST

THEORETICALLY, WE NEED TO FINISH THE FOLLOWING TASKS:

1. COMPLETE TEST OF 2D CODE2. MODEL TURBULENCE-MAGNETIC FIELD INTERACTION3. DETERMINE VALUE OF W

STRATEGY

CARRY OUT EXHAUSTIVE SEARCH FOR OLD ECLIPSE DATA

DETERMINE L FOR AS MANY ECLIPSES, AS WELL DISTRIBUTED IN TIME, AS POSSIBLE

INCLUDE SOLAR VARIATIONS IN THE CLIMATE MODELS USED TO DETERMINE GLOBAL WARMING EXPECTED FROM A DOUBLING OF CO2

If there is a significant solar component to climate change, it will bedue to INTERNAL VARIABILITY

This variability can be physically understood (PREDICTABLE),and determined for at least after 1715

The required observational data will be obtained by many satellites,but primarily PICARD starting at the end of 2008

We need to complete developing the theoretical infrastructure required to interpret the PICARD results, and to extract the climate implications