PARTICLE EVENTS AND ASSOCIATED SHOCKS IN THE INTERPLANETARY MEDIUM
B. Sanahuja*, A. Aran*, D. Lario **, and N. Agueda*
* Departament d'Astronomia i Meteorologia. Universitat de Barcelona. Spain
** Applied Physics Laboratory. The Johns Hopkins University, USA
Sponsored by ESA/ESTEC, the DGCYT
(MCyT) and NASA
Low energy particles (E<100 MeV/nucleon) are present throughout the heliosphere in different intensities and time scales. The source of most of these particles in the heliosphere are the Sun, the planet's magnetospheres, and the interplanetary shock waves. Spacecraft observations have established direct evidence that particle acceleration occurs near all collisionless shocks found in interplanetary space, including the Earth's bow shock, shocks triggered with transient solar activity, and corotating shocks.
The "solar energetic particle"(SEP) events, a time limited increases of low-energy particles, are often observed in the near earth space environment, outside the magnetosphere and in the earth polar caps (where they were discovered), but also observed everywhere in the interplanetary medium. In recent years it has been widely accepted that there are two kinds of SEP events:
The gradual events have a duration of several days, they are proton rich and they have, on average, the same element composition and ionization states as those in the low-density ambient plasma of the high corona or solar wind. They are associated with gradual X-ray flares, type II and type IV radioemission and coronal mass ejections (CMEs). Such events are observed over a broad range of heliolongitudes.
The impulsive short-duration events are only observed from magnetically well-connected locations on the Sun. They are electron-rich and they have an strong association with impulsive H-alpha and X-rays flares, and type III radio bursts. The high ion charge state indicates their origin in plasma heated by flares.
Recent observations do challenge an strict separation of all SEP events in these two types. There are large gradual events with abundances more like those of impulsive events. It also seems that abundance variations are organized by the heliolongitude of the parent solar activity.
Particle acceleration occurs as a consequence of transient releases of energy in association with solar flares and/or coronal mass ejections. When a shock wave develops, it expands and propagates through the interplanetary medium. If the magnetohidrodynamic strength of the shock is high enough, it is able to accelerate particles which propagate along the interplanetary magnetic field (IMF) lines.
We study the injection rate of shock-accelerated protons in long-lasting particle events by tracing back the magnetohydrodynamic conditions at the shock under which protons are accelerated. This "tracing back" is carried out by fitting the observed flux and anisotropy profiles at different energies, considering the magnetic connection between the shock and the observer, and modeling the propagation of the shock and of the particles along the IMF. A focused-diffusion transport equation including the effects of adiabatic deceleration and solar wind convection is used to model the evolution of the particle population. The mean free path and the injection rate are derived by requiring consistency with the observed flux and anisotropy profiles for different energies, in the upstream region of the events. We have extended the energy range of previous models down to 50 keV and up to 50 or 100 MeV.
The contribution of the shock to the observed particle event depends on the size and speed of the shock (its efficiency as accelerator), the evolution of the shock (possible deformations, damping, waves upstream of the shock, etc.), the location of the observer (where and when the shock intercepts the magnetic flux tube where the observer is located), and the conditions for the propagation of shock-accelerated particles. However, many questions remain unanswered in the study of low-energy particle events, as well as about the role played by the interplanetary shocks in their formation and evolution. Here there are a few:
What defines the efficiency of shock-acceleration: the strength of the shock, its geometry, its velocity, the existence of a particle population which can be accelerated?
How does the efficiency of particle acceleration change as the shock moves outward from the corona to the observer?
How can the efficiency of the shock-acceleration be related to the magnetohydrodynamic (MHD) parameters of the shock?
How do particle flux and anisotropy profiles generated by a MHD shock vary with the observer's position and the conditions for particle propagation?
To what extent does shock-acceleration depend on the energy of the particles?
How is the efficiency of shock-acceleration affected by the presence of turbulence around the shock?
What characterizes the propagation of shock-accelerated particles in the downstream region?
No doubt, each reader could add questions to this list.
Trying to give an answer to some of these questions we simulate the solar energetic particle events associated with interplanetary shocks. The simulation of these particle events requires a knowledge of how particles and shocks propagate through the interplanetary medium, and how shocks accelerate and inject particles into interplanetary space.
There exist several models for describing the propagation of particles. But there are few fully developed magnetohydrodynamic models to describe the evolution of interplanetary shocks. This is a problem since the injection of shock-accelerated particles is assumed to occur from the instantaneous position of the front of the shock as it propagates.
The selection and application of an appropriate shock model that takes into account the initial conditions at or near the sun, and the development of a model of particle acceleration and propagation from the "cobpoint" (Connecting-with-the-OBserver Point), that is the foot point at the shock front of the IMF line connecting with the observer, to the observer is the main task of the investigation.
An engineering model that will predict the level of low energy particles at a particular location in interplanetary space, when a solar coronal mass ejection has been detected is being developed. More details can be found in the list of publications.
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