Project Description

 

We propose to explore the magnetic origins of solar spectral and total irradiance variations, through observations, data analysis and modeling approaches to directly address NASA-LWS TR&T Program's Focused Science Topic "d) Solar Origins of Irradiance Variations" as stated in this NRA. Changes in the spectrally integrated total solar irradiance (TSI) affect global climate directly, since

TSI is the primary source of radiative energy input to the Earth system. Indirect climate influences include the solar radiative output at different wavelengths (spectral irradiance), which affect specific components of Earth's atmosphere in diverse ways. Exploring the nature and physical basis of this solar radiative forcing is vital towards understanding and predicting its effect on life and society, and for clearly distinguishing the natural and anthropogenic causes of global climate change. Solar spectral and total irradiance variability is governed by the changing magnetism of the Sun. Specifically, the appearance, evolution and associated dynamics of solar active region magnetic fields and its overlying coronal loops, directly contribute to irradiance variations - the latter is clearly magnetic in origin. In this proposal we plan to explore the magnetic origins of solar irradiance variations by: I) Establishing the relationship between important active region magnetic parameters (such as field strength, flux, loop length, measures of non-potentiality) derived from magnetograms and the observed solar spectral and total irradiance, II) Studying the connection between active region evolution and irradiance variations through the usage of a magnetogram partitioning algorithm that can follow the fragmentation of a decaying active region and the consequent redistribution of flux between the main spot and fragmented regions, III) Modeling the observed dependence through analytic and numerical techniques to develop predictive capabilities for solar irradiance variations. The proposed research is relevant for the NASA LWS program's objective to understand the solar origins of space weather and climate and develop predictive capabilities to mitigate their adverse effects on life and society. In the larger context, this research supports NASA Science Mission Directorate's strategic sub-goal 3-B - "Understand the Sun and its effects on Earth and the solar system.”

 

Progress (First Year)

 

We have been exploring the connection between changing active region magnetic fields and changes in the solar radiation spectrum. We have compared |B|, the full-disk average magnetic field strength from Kitt Peak, with the disk-integrated total solar irradiance and spectral solar irradiance. We are using numerical methods, adapted from signal theory, that allow us to study in detail the temporal relationship between the magnetic field data and the irradiance data. Our empirical results quantify this relationship and show that time-delays exist between the growth and decay of an active region's magnetic field and its associated irradiance output. This work should provide useful constraints for physics-based models of energy storage and release in solar active regions. Preliminary results from this work were presented recently in the SVECSE 2008 workshop (Preminger, Nandy & Chapman 2008).

 

We have also focused attention on reconstructing total solar irradiance (TSI) and cosmic ray flux (CR) variations back to the early 17th century to understand its influence on Earth's past climate and global temperature changes. This reconstruction is based on observed empirical relationships determined over the period for which co-temporal sunspot number, TSI and CR flux data exists.

Subsequently TSI and CR flux were reconstructed based on sunspot number data going back to the early 17th century. TSI is the primary external energy input into the Earth system. It is also hypothesized that CR flux on Earth – modulated by solar activity, may alter Earth's available energy budget by seeding clouds (the latter reflects incident radiation). Based on our TSI and CR flux reconstructions and some assumptions, we have estimated the net solar radiative forcing (cumulatively calculated using both TSI and Cosmic Ray Flux variations) and compared it to global temperature reconstructions over the last four centuries. Our preliminary results show that although a (time-delayed) correlation exists between net solar forcing and global temperature changes, this relationship is not easy to interpret (Nandy, Joy & Martens 2008). It has become clear to us during this investigation that one should take into account volcanic forcing (the effect of volcanic aerosols in scattering away incident solar radiation and therefore having a cooling effect) to properly understand the Sun-Climate relationship. Currently not many solar researchers worry about this, inspite of many volcanic eruptions occurring during the Little Ice Age -- which coincided with the Maunder minimum and is widely cited as an example of the Sun-Climate connection.

 

In conjunction with the above projects, we have also begun developing tools necessary for the second year of research proposed under this grant. These include: 1) Development of a magnetogram partitioning algorithm that can study active region evolution and the consequent re-distribution of magnetic flux between sunspot and faculae and the effect of this evolution on irradiance output; 2) Construction of a database of solar active region magnetic parameters and measures of solar spectral irradiance variations spanning a large part of solar cycle 23 (and older, when available).

 

References

 

“How do Active Region Magnetic Fields Affect Solar Irradiance?”, Preminger, D., Chapman, G., & Nandy, D. 2008, International Workshop on Solar Variability, Earth's Climate and the Space Environment, Bozeman, Montana, June 1-6, 2008

 

“A Comparative Study of Net Solar Forcing and Global Temperature Changes Since 1600 AD”, Nandy, D., Joy, S., & Martens, P.C.H., 2008, International Workshop on "Solar Variability, Earth's Climate and the Space Environment, Bozeman, Montana, June 1-6, 2008

 

Additional Resources

 

http://solar.physics.montana.edu/nandi/

http://solar.physics.montana.edu/martens/