i. Extending the FVDA analysis to STEREO-A and STEREO-B data, and search for
events where simultaneous observations from multiple spacecraft exist. For these events we will compare the resulting path lengths from the FVDA analysis. Besides electrons, we will also consider 3He ions.
The work of Zhao et al. [2019] only used events from Wind observations. One major task of our proposal is to extend the work of Zhao et al. [2019] to include STEREO-A and STEREO-B data. We will pay special attention to selecting events where energetic electrons and 3He are measured by multiple spacecraft including Wind and STEREO-A/B. By comparing the resulting path lengths from these events, we will obtain the longitudinal dependence of path length. From these longitudinal dependence (or the lack of) of path length, we can obtain strong constraints on the effect of solar wind turbulence on the field line meandering and consequently the transport of energetic electrons. 3He-rich ion events are typical scatter-free and the onset of these events provide another measure of the interplanetary field configuration from the observers to their source. By combining onsets of ions and electrons from multiple spacecraft and assuming they have a common solar source, their respective profiles will provide another critical constraint. Note that in our proposal ”scatter-free” means that particles’ mean free paths are much larger than 1 au. Strictly speaking, if there is no scattering, charged particles will arrive 1 au with an average pitch angle smaller than 1◦ due to magnetic focusing.
ii. Modelling the transport of energetic electrons and ions in a turbulent solar wind, with a particular focus on the effect of meandering field line.
The FVDA analysis considers the first arriving electrons ”scatter-free”. However, electron and ion mean free paths show large event variability [Palmer, 1982]. Therefore the scatter-free assumption is not necessarily true for all events. Furthermore, low energy electrons and high energy electrons interact with the solar wind turbulence differently. To understand the effect of solar wind turbulence on the transport of energetic electrons, modelling effort is necessary. For our proposed work, we will perform numerical simulations of particle transport (ions and electrons) in a non-Parker interplanetary magnetic field. Our simulations will be based on the focused transport equation. Recently, using this approach, our team members have examined the transport of electrons in both the corona [Zhao and Zhang, 2018] and the interplanetary medium with a magnetic field geometry related to a CIR [Dröege et al., 2018]. In our proposed work, we will also revisit current theories which describe the interactions between energetic electrons and ions and the solar wind turbulence.
One important question concerning the solar wind turbulence is the way it causes field lines to meander. The interplanetary magnetic field deviates from the Parker Spiral shape due to plasma turbulence, fed by chromospheric footpoint motions, and it is subsequently modified by non-linear turbulence processes in the solar wind. Recently, Laitinen et al. [2013, 2016, 2018] examined how longitudinal particle transport is affected in such meandering field configurations, and found that particles can spread initially very fast, non-diffusively, resulting in wide heliolongitudinal spreads of SEPs in the heliosphere. This offers a possible explanation for the rapid onset behavior of particles at widely separated heliolongitudes [Dresing et al., 2012] and is, at the same time, compatible with the observed drop-out phenomenon [Mazur et al., 2000, ooprakai
et al., 2016], indicative of field-line mixing and particle injection onto different flux tubes.
The meandering of field lines is most sensitive to turbulence power at large scales. As a first approximation, one can picture electrons in these impulsive events as moving along a meandering field and experiencing pitch angle scattering due to Alfvén waves. Simultaneous observations of particle intensity and anisotropy profiles by multiple spacecraft can therefore put a strong constraint on the amount of meandering of the
field due to large scale turbulence.
iii. Performing MHD simulations to examine how preceding CMEs can affect the configu-ration of solar wind magnetic field.
As shown in the left panel of Figure 1, the 2002 October 20 event occurred in the decay phase of another event. This is not uncommon, especially during solar maximum where multiple flares/CMEs occur per day. When electron events occur after fast and large CMEs, the solar wind magnetic field configuration in these events can be very different from a Parker configuration, leading possibly to extremely large or short path lengths. We will search for such events from observations and explicitly perform magnetohydrodynamics (MHD) simulations that can capture the quiet solar wind as well as the eruption and propagation of CMEs for selected events. Our CME simulations will be led by team member Dr. Lugaz following his earlier work Lugaz et al. [2013, 2017]. We will specifically focus on the disturbed solar wind behind a CME and not on cases where there is interaction between multiple CMEs or particle events occurring inside a CME [Larson et al., 1997, Kahler et al., 2011]). It is thought that in some cases, a rarefaction region behind the CME may cause the IMF becoming “straighter” than usual, whereas in other cases, open field lines are “deflected” around the previous magnetic ejecta resulting in longer field lines (see, e.g., [Masson et al., 2011]). We propose to simulate at least two cases for which the FVDA returns highly unusual path lengths and for which the inspection of coronagraphic and in situ measurements reveals that preceding CMEs occurred. The time-dependent field line geometry can serve as inputs to the focused transport equation simulations to further investigate how complex time-dependent IMF and field line meandering may be coupled to lead to the observed particle time intensity profiles.