Stellar magnetism: 

My research focuses on stellar magnetic fields.  Recently, I argued that it is possible to predict the general properties of the large-scale magnetic fields of low-mass stars from their location in the Hertzsprung-Russell diagram (Gregory et al 2012). 

Previously, I have explored how results from geomagnetism, classical electromagnetism, and molecular physics can be adopted for models of stellar magnetic fields (Gregory et al 2010; Gregory 2011).

A Hertzsprung-Russell diagram coloured to highlight the four magnetic topology regimes across the pre-main sequence that are now apparent from magnetic mapping studies (from Gregory et al 2012). Isochrones (dotted lines; 0.25, 1, 5, 10, and 15 Myr) and mass tracks (solid lines that change from black to red as a star transitions from fully to partially convective; 0.1, 0.5, 1.0, 2.0, and 3.0 solar mass) are from the models of Siess et al (2000). Stars located to the left of the right-hand solid blue line have radiative cores while those to the right are fully convective. Stars located to the left of the left-hand solid blue line have large radiative cores (Mcore/M*>0.4) while those between the solid blue lines have small radiative cores (0<Mcore/M*<0.4). Stars located in region 1 have large radiative cores generate complex non-axisymmetric large-scale magnetic fields with many high order multipole components.  Stars in region 2 have small radiative cores and generate dominantly axisymmetric magnetic fields but field modes of higher order than the dipole dominate (typically, but not always, the octupole). Stars in region 3 are fully convective and host simple axisymmetric fields with strong (kilo-Gauss) dipole components. Based on the similarity between the magnetic fields of PMS stars and MS M-dwarfs with similar internal structures, there may exist a region 4 where a bistable dynamo process operates and where stars generate a variety of magnetic topologies.  The dashed blue lines denote lower and upper limits on the boundary between regions 3 & 4 which remains unconstrained observationally.            

Mapping the magnetic fields of newborn stars:

I am the lead theory CoI on the ongoing Magnetic Protostars & Planets (MaPP) project (PI: J.-F. Donati) which has produced the first maps of the large scale magnetic fields of accreting pre-main sequence stars, using the technique of Zeeman-Doppler imaging (e.g. Donati et al 2011a, 2012; see Gregory & Donati 2011 for a review).    

Magnetic maps of the accreting PMS star V2129 Oph (Donati et al 2011b) showing the field components in flattened polar projection. The bold black line represents the stellar equator and dotted lines denote lines of constant latitude separated by 30o. Red/blue is positive/negative field with fluxes labelled in Gauss. Numbers/tick marks around the circumference denote rotation phase/phase of observation. V2129 Oph has a dominantly octupolar large-scale magnetic field, and its magnetosphere is well described by a tilted dipole plus a tilted octupole component (Gregory & Donati 2011).

Analytic & numerical models of magnetospheres:

I have developed analytic and numerical models using arbitrarily complex magnetic fields that can be applied to both stellar and planetary magnetospheres (Gregory et al 2010; Gregory & Donati 2011), and explored the implications on accretion flows  (Gregory et al 2006a, 2008; Adams & Gregory 2012). Multipolar magnetospheres are required to explain a number of observational results, from the modulation of X-ray emission to the magnitude of accretion hotspot filling factors (Gregory et al 2007, 2008).

High energy stellar activity: 

I have worked on models of coronal X-ray emission using stellar magnetic maps as inputs (Gregory et al 2006b).  Recently, I have contributed to studies of X-ray flares from pre-main sequence stars (Johnstone et al 2012; Aarnio et al 2012), and studies of soft X-ray emission from accretion spots (Argiroffi et al 2011, 2012).