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Solar Energetic Particle drifts and the energy dependence of 1 AU charge states  

Silvia Dalla   Submitted: 2016-12-20 11:02

The event-averaged charge state of heavy ion Solar Energetic Particles (SEPs), measured at 1 AU from the Sun, typically increases with the ions' kinetic energy. The origin of this behaviour has been ascribed to processes taking place within the acceleration region. In this paper we study the propagation through interplanetary space of SEP Fe ions, injected near the Sun with a variety of charge states that are uniformly distributed in energy, by means of a 3D test particle model. In our simulations, due to gradient and curvature drifts associated with the Parker spiral magnetic field, ions of different charge propagate with very different efficiencies to an observer that is not magnetically well connected to the source region. As a result we find that, for many observer locations, the 1 AU event-averaged charge state <Q>, as obtained from our model, displays an increase with particle energy E, in qualitative agreement with spacecraft observations. We conclude that drift-associated propagation is a possible explanation for the observed distribution of <Q> versus E in SEP events, and that the distribution measured in interplanetary space cannot be taken to represent that at injection.

Authors: S. Dalla, M.S. Marsh and M. Battarbee
Projects: None

Publication Status: ApJ, in press, 2017
Last Modified: 2016-12-21 12:21
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Time dependence of Fe/O ratio within a 3D Solar Energetic Particle propagation model including drift  

Silvia Dalla   Submitted: 2016-12-20 11:00

Context. The intensity profiles of iron and oxygen in Solar Energetic Particle (SEP) events often display differences that result in a decreasing Fe/O ratio over time. The physical mechanisms behind this behaviour are not fully understood, but these observational signatures provide important tests of physical modelling efforts. Aims. In this paper we study the propagation of iron and oxygen SEP ions using a 3D model of propagation which includes the effect of guiding centre drift in a Parker spiral magnetic field. We derive time intensity profiles for a variety of observer locations and study the temporal evolution of the Fe/O ratio. Methods. We use a 3D full orbit test particle model which includes scattering. The configuration of the interplanetary magnetic field is a unipolar Parker spiral. Particles are released instantaneously from a compact region at 2 solar radii and allowed to propagate in 3D. Results. Both Fe and O experience significant transport across the magnetic field due to gradient and curvature drifts. We find that Fe ions drift more than O ions due to their larger mass-to-charge ratio, so that an observer that is not magnetically well connected to the source region will observe Fe arriving before O, for particles within the same range in energy per nucleon. As a result, for the majority of observer locations, the Fe/O ratio displays a decrease in time. Conclusions. We conclude that propagation effects associated with drifts produce a decay over time of the Fe/O ratio, qualitatively reproducing that observed in SEP event profiles.

Authors: S. Dalla, M.S. Marsh, P. Zelina and T. Laitinen
Projects: None

Publication Status: A&A, in press, 2017
Last Modified: 2016-12-21 12:21
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Solar Energetic Particle drifts in the Parker spiral  

Silvia Dalla   Submitted: 2013-07-11 11:05

Drifts in the Parker spiral interplanetary magnetic field are known to be an important component in the propagation of galactic cosmic rays, while they are thought to be negligible for Solar Energetic Particles (SEPs). As a result they have so far been ignored in SEP propagation modelling and data analysis. We examine drift velocities in the Parker spiral within single particle first-order adiabatic theory, in a local coordinate system with an axis parallel to the magnetic field. We show that, in the presence of scattering in interplanetary space, protons at the high end of the SEP energy range experience significant gradient and curvature drift. In the scatter-free case, drift due to magnetic field curvature is present. The magnitude of drift velocity increases by more than an order of magnitude at high heliographic latitudes compared to near the ecliptic; it has a strong dependence on radial distance r from the Sun, reaching a maximum at r~1 AU at low heliolatitudes and r~10 AU at high heliolatitudes. Due to the mass over charge dependence of drift velocities, the effect of drift for partially ionised SEP heavy ions is stronger than for protons. Drift is therefore likely to be a considerable source of cross field transport for high energy SEPs.

Authors: S. Dalla, M.S. Marsh, J. Kelly, T. Laitinen
Projects: None

Publication Status: Submitted to Journal of Geophysical Research - Space Physics
Last Modified: 2013-07-11 16:34
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Particle trajectories and acceleration during 3D fan reconnection  

Silvia Dalla   Submitted: 2008-11-10 04:22

Context. The primary energy release in solar flares is almost certainly due to magnetic reconnection, making this a strong candidate as a mechanism for particle acceleration. While particle acceleration in 2D geometries has been widely studied, investigations in 3D are a recent development. Two main classes of reconnection regimes at a 3D magnetic null point have been identified: fan and spine reconnection Aims. Here we investigate particle trajectories and acceleration during reconnection at a 3D null point, using a test particle numerical code, and compare the efficiency of the fan and spine regimes in generating an energetic particle population. Methods. We calculated the time evolution of the energy spectra. We discuss the geometry of particle escape from the two configurations and characterise the trapped and escaped populations. Results. We find that fan reconnection is less efficent than spine reconnection in providing seed particles to the region of strong electric field where acceleration is possible. The establishment of a steady-state spectrum requires approximately double the time in fan reconnection. The steady-state energy spectrum at intermediate energies (protons 1 keV to 0.1 MeV) is comparable in the fan and spine regimes. While in spine reconnection particle escape takes place in two symmetric jets along the spine, in fan reconnection no jets are produced and particles escape in the fan plane, in a ribbon-like structure.

Authors: S. Dalla and P.K. Browning
Projects: None

Publication Status: A&A 491, 289-295 (2008)
Last Modified: 2008-11-10 14:14
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Subject will be restored when possible  

Silvia Dalla   Submitted: 2008-01-07 01:29

We study the visibility of sunspots and its influence on observed values of sunspot region parameters. We use Virtual Observatory tools provided by AstroGrid to analyse a sample of 6862 sunspot regions. By studying the distributions of locations where sunspots were first and last observed on the solar disk, we derive the visibility function of sunspots, the rate of magnetic flux emergence and the ratio between the durations of growth and decay phases of solar active regions. We demonstrate that the visibility of small sunspots has a strong center-to-limb variation, far larger than would be expected from geometrical (projection) effects. This results in a large number of young spots being invisible: 44% of new regions emerging in the West of the Sun go undetected. For sunspot regions that are detected, large differences exist between actual locations and times of flux emergence, and the apparent ones derived from sunspot data. The duration of the growth phase of solar regions has been up to now underestimated.

Authors: S. Dalla, L. Fletcher and N.A. Walton
Projects: None

Publication Status: A&A Lett, in press, 2008
Last Modified: 2008-01-07 17:24
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Subject will be restored when possible  

Silvia Dalla   Submitted: 2008-01-07 01:29

We study the visibility of sunspots and its influence on observed values of sunspot region parameters. We use Virtual Observatory tools provided by AstroGrid to analyse a sample of 6862 sunspot regions. By studying the distributions of locations where sunspots were first and last observed on the solar disk, we derive the visibility function of sunspots, the rate of magnetic flux emergence and the ratio between the durations of growth and decay phases of solar active regions. We demonstrate that the visibility of small sunspots has a strong center-to-limb variation, far larger than would be expected from geometrical (projection) effects. This results in a large number of young spots being invisible: 44% of new regions emerging in the West of the Sun go undetected. For sunspot regions that are detected, large differences exist between actual locations and times of flux emergence, and the apparent ones derived from sunspot data. The duration of the growth phase of solar regions has been up to now underestimated.

Authors: S. Dalla, L. Fletcher and N.A. Walton
Projects:

Publication Status: A&A Lett, 479, L1 (2008)
Last Modified: 2008-02-14 05:04
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Particle acceleration at a three-dimensional reconnection site in the solar corona  

Silvia Dalla   Submitted: 2005-03-23 06:55

We study test particle trajectories in the vicinity of a three-dimensional (3D) magnetic null point during spine reconnection. Particles are injected into the steady-state non-uniform magnetic and electric fields derived by Priest and Titov (1996), and the equations of motion numerically integrated. We use input parameters typical of the solar corona, for which reconnection has been suggested as the fundamental mechanism responsible for particle acceleration in flare events. We show that substantial acceleration is possible in the 3D spine reconnection configuration, in the strong electric field regime. The energy gain is strongly dependent on the location of injection into the simulation box, as was the case in 2D X-point configurations. In our 3D geometry, we first vary the location of injection within a plane through the spine, and derive an analytical value for the injection angle for which maximum energy gain is achieved. Secondly we vary the azimuthal location of particle injection and show that as one moves away from the plane with maximum electric field magnitude, higher final energies can be achieved, though this requires substantially longer times.

Authors: S. Dalla and P.K. Browning
Projects: None

Publication Status: A&A, in press
Last Modified: 2005-03-23 06:55
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Abstracts by Author
Solar Energetic Particle drifts and the energy dependence of 1 AU charge states
Time dependence of Fe/O ratio within a 3D Solar Energetic Particle propagation model including drift
Solar Energetic Particle drifts in the Parker spiral
Particle trajectories and acceleration during 3D fan reconnection
Subject will be restored when possible
Subject will be restored when possible
Particle acceleration at a three-dimensional reconnection site in the solar corona

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