Magnetic Field : Sun

Just like a planetary dynamo generates a magnetic field inside the planet, solar dynamo is responsible for the solar magnetic field. The Sun is composed of plasma, i.e, charged particles, which in motion produce magnetic fields. This field is carried away from the sun towards the planets in all directions by the solar wind, and is called the interplanetary magnetic field (IMF). Two interesting phenomena related to the field are its variation and shape.  

Interplanetary magnetic field extending out from the Sun. Credit: Vallée, J 1998

The solar magnetic field acts as a bar magnet with two poles. These poles are observed to flip regularly every 11 years. This variation is termed as a solar cycle. The solar minimum, when the field is weak, is considered the start of a solar cycle. The sunspots are lowest during this time. Sunspots are dark spots observed on the surface where the magnetic flux is very high (over 0.2 Teslas).

Number of sunspots vs time depicting solar cycles. Credit: ESA/NOAA.

The IMF extending out from the sun with the solar wind travels as a rotating spiral due to the spinning of the sun. This shape is known as the Parker spiral. The sun rotates around every 24 days at the equator and every 35 days at the pole. On average, this is taken to be 27 days and is known as the Carrington rotation.

Image Credit: NASA

                                                  
The solar magnetic field and its related phenomena are active topics of research. There is still a lot to find and understand! 

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Shivangi Sharan is a second year PhD student at the Laboratory of Planetology and Geodynamics in France. Her research focusses on the study of the magnetic field of Mars and to infer its internal structure from it. She is an active member of the IAGA Blog Team and can be contacted via e-mail here.

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EGU Early Career Scientist Award (EMRP): Richard Bono

I am a Leverhulme Early Career Fellow working on research questions which bridge core processes, such as the geodynamo, to crust-to-space effects, including magnetic shielding and the evolution of life. Currently, I work in the Geomagnetism Laboratory at the University of Liverpool and in January 2022, I will be joining the Earth, Ocean and Atmospheric Science Department at Florida State University in Tallahassee, Florida. I earned my PhD in 2016 at the University of Rochester advised by Prof. John Tarduno, where I also earned my bachelor’s and master's degrees. In addition to pursuing my research, I also currently help to maintain the PINTdb.org paleointensity database and assist in organizing MagNetZ, an online seminar for the paleomagnetic community. 

My passion for geology is centred on the field of palaeomagnetism – the recognition that the magnetic record stored in rocks could act as a compass or a clock going back through geologic time inspired my pursuit in addressing questions about Earth’s interior across deep time. Through field work, careful laboratory experiments, statistical modelling and numerical simulation, I try to understand the fundamental properties and behaviour of Earth’s liquid outer core. 

As part of the DEEP group, I develop statistical paleomagnetic field models as part of a multidisciplinary team of geophysicists, geologists and dynamo modelers. These statistical models are used to characterize and test hypotheses related to long term geomagnetic field evolution, and aid comparisons between observational data to numerical dynamo simulations using Earth-like configurations. 

Field Work, Arctic, 2012. Credit : https://www.richardkbono.org/

Prior research focused on using single crystal palaeomagnetism and electron microscopy to investigate questions about how terrestrial planetary interiors evolved over time, the impact of this evolution on planetary surfaces, potential implications for the evolution of life and habitability, and fundamental capabilities of single crystals as magnetic recorders. This work resulted in some of the oldest magnetic records sampling the Hadean using zircons from the Jack Hills in Australia, the weakest magnetic records sampling the Ediacaran, as well as extra-terrestrial materials from pallasites and lunar samples. 

My work has involved a wide range of disciplines, with collaborators from geochronology, mantle modelling, plate reconstructions, mineral physics, electron microscopy, and numerical modelling communities. The broad implications of the research are as follows: the habitability of a planetary body is largely understood to be determined by its ability to retain liquid water on its surface. To maintain the physical conditions required to preserve liquid water on the surface, the planet must host an atmosphere, which is vulnerable to solar erosion over geologic timescales. Preserving the atmosphere from cosmic radiation requires a planetary magnetic field which shields the atmosphere, allowing liquid water to remain present on the surface. Therefore, understanding the conditions required to generate and maintain a dynamo in planetary bodies is crucial to gaining insight in the dual evolutions of Earth’s life and dynamo.

From scientists to everyone: the IGRF Model

In IAGA there is a group called V-MOD: Geomagnetic Field Modeling, part of Division V. The aim of this group is "to promote and coordinate international efforts to model and analyze the internal geomagnetic field and its secular variation on both global and regional scales". 

The main result of their efforts is the IGRF model. IGRF stands for International Geomagnetic Reference Field, and is a standard mathematical description of the Earth's main magnetic field and its temporal variation, called secular variation. IGRF is widely used in a multitude of different studies, such as the Earth's interior, the crust, the ionosphere and magnetosphere. It is also used in satellite attitude determination and control systems and other applications requiring orientation information. 

The model is the product of a collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and from observatories and surveys around the world. It is a voluntary work for the benefit of all. Each time a new model is to be obtained, different teams of scientists come together and propose different candidate models. The final model is usually a mean or median of all candidate models and some kind of weighting scheme may also be applied. The overall process is described in papers for each one of the candidate models, the evaluation process and finally a paper describing the final model. 

Map of the declination at 2020.0 as given by IGRF-13 (Alken et al., 2021)

The Earth's field changes continuously and in order to account for temporal changes on timescales of a few years, the IGRF is regularly revised, typically every 5 years. The years for which coefficients are provided are called model epochs. The coefficients of a certain epoch represent a snapshot of the geomagnetic field at that time, and can be labeled either as a Definitive Geomagnetic Reference Model (DGRF, which are unlikely to be improved in future IGRF revisions) or as an IGRF. 

The last generation, IGRF-13, was finalized in December 2019 by a task force of group V-MOD. It provides a DGRF model for epoch 2015.0, an IGRF model for epoch 2020.0, and a predictive IGRF secular variation model for the 5-year time interval 2020.0 to 2025.0. The main field coefficients describe the spatial variation of the field to a maximum spherical harmonic degree and order of 13, while the secular variation extend to a maximum degree and order of 8. 

Satellite data, such as the one provided by the ESA Swarm mission and the ground observatory network were crucial to the latest IGRF generations. Data from other satellite missions were also used. 

The coefficients to calculate the model may be found in the designated paper (Alken et al, 2021) and also online in digital form along with the software to compute magnetic field components at different times and spatial locations. 

Where to find the model and other information:

- model IGRF-13 paper: https://doi.org/10.1186/s40623-020-01288-x 

- model issue, with candidate models' papers and evaluation process paper: https://earth-planets-space.springeropen.com/igrf13 

- working group V-MOD webpage and model: https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html

 

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After a master in Geophysics from Portugal, Diana Saturnino got a PhD in Geomagnetism and continued working on the subject for a few more years in Denmark and France. Now she's looking for different adventures. She can be contacted via e-mail here.

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