Silvia Dalla - Research interests
 

Solar flares are observed as transient bursts of electromagnetic radiation from the Sun. They often occur in conjunction with Coronal Mass Ejections (CMEs). During flares and CMEs, ions and electrons are accelerated to high energies. They can escape from the Sun and propagate through space to reach regions near Earth. We can detect fluxes of these Solar Energetic Particles (SEPs) using instruments on board spacecraft. The images on the right hand side show three phases of the flare process.

Below are some themes of my research, that aims to understand the processes of SEP acceleration and propagation. I investigate this problem from a modelling point of view, and also work on the analysis of experimental data.
 
  • Modelling SEP propagation through space
  • Many questions regarding the propagation of SEPs from the Sun to regions near Earth remain unsolved, mainly due to the complexity of modelling particle transport in the turbulent solar wind magnetic fields. In recent years, I have been involved in several projects that used modelling to study this problem.

    With Timo Laitinen, we have been analysing the perpendicular transport of energetic particles in turbulent magnetic fields. The results of this work showed that standard diffusion models are not adequate for describing the propagation of SEPs early in a particle event, when field line meandering plays the main role in perpendicular transport [Laitinen et al 2013].

    Mike Marsh and I have used test particle simulations and analytical theory to show that drifts due to the curvature and gradient of the standard Parker spiral magnetic field, play an important role in the propagation of SEPs [Marsh et al 2013, Dalla et al 2013].

    Our understanding of SEP propagation through the interplanetary magnetic field has been incorporated into SPARX, a modelling system for forecasting SEP Radiation for Space Weather. You can find a description of SPARX in Marsh et al 2015. This model is used within the SEP Forecast tools of the COMESEP Space Weather Alert System.

    The Sun before the flare
    The Sun before the flare as imaged in the EUV by SOHO EIT.

     

  • Modelling particle acceleration during solar flares
  • Magnetic reconnection has been put forward by many scientists as the process responsible for particle acceleration during solar flares.

    Working in collaboration with Philippa Browning, I developed a test particle trajectory code, and studied the orbits of particles injected into the 3D reconnection electric and magnetic fields derived by Priest and Titov (1996).

    Single particle trajectories in the 3D spine reconnection configuration are described in Dalla & Browning A&A [2005].

    Our study of trajectories of an ensemble of particles in 3D spine reconnection, revealed that this magnetic configuration naturally produces two symmetric jets of energetic particles escaping along the spine [Dalla & Browning ApJL [2006]].

    The flare
    Image of the solar flare.

     

  • High heliolatitude data and models of SEP acceleration and propagation
  • Between 2000 and 2002, the Ulysses spacecraft passed over the solar poles, at a time of high solar activity (see plot of Ulysses orbit), and took unique SEP data. Energetic particle instruments part of the COSPIN experiment on Ulysses detected many large SEP events at high heliolatitudes, as summarised by McKibben et al [2003].

    I analysed the 9 largest high latitude SEP events and found that onsets at high heliolatitudes were very delayed compared to in-ecliptic onsets. The apparent release times obtained by fitting a straight line to data points giving onset time versus c/v (c=speed of light, v=particle speed), were found to be between 100 and 350 minutes after the in-ecliptic release times [Dalla et al 2003a].

    SEPs of the largest (so-called gradual) events are thought to be accelerated by the shock driven by a CME through the solar corona and interplanetary space [Reames, 1999]. Can this model account for the features of high heliolatitude SEP profiles?

    In our analysis of Ulysses data, times at which SEP intensities peak (times to maximum) have been found to be very delayed at high latitudes [Dalla et al 2003b]. We have also found that intensities during the decay phase are similar at high heliolatitudes and in the ecliptic.

    Both the above findings are not explained by the CME shock acceleration model, as detailed in [Dalla et al 2003b]. The Ulysses data are however suggestive of a diffusion process being the means by which particles reach high latitudes.

    Particles reach the detector
    'Snow' on the image as the energetic particles reach SOHO.

    You can download the solar flare movie from which the images on this page were made: Download movie (mpeg, 14 MB). The movie was made using the AstroGrid Solar Movie Maker (see also: AstroGrid work).


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