SH22A-01 INVITED
Slip running reconnection in the Sun's atmosphere observed by RHESSI, SOHO, TRACE and Hinode
Solar double ribbon flares are commonly explained by magnetic field reconnections in the high corona. The bright ribbons, typically observed in Halpha, in EUV with SoHO, TRACE correspond to the ends of the reconnected loops. In most studied cases, the reconnection site is an X-point, where two magnetic separatrices intersect. In this presentation, we show a generalization of this model to 3D complex magnetic topologies where there are no null points, but quasi-separatrices layers instead. In that case, while the ribbons spread away during reconnection, we show that magnetic field lines can quickly slip along them. We propose that this new phenomenon could explain also fast moving HXR footpoints as observed by RHESSI, and that it may be observed in soft X rays with XRT.
SH22A-02 INVITED
What Have we Learnt From Three Solar Cycles of Gamma-ray Observations?
Gamma-ray observations of solar flares are the most direct and quantitative diagnostics of high-energy electrons and ions accelerated at the Sun. The theory of solar gamma-rays has been established in the late 1960's and pioneer observations of nuclear gamma-ray lines have been obtained in the early 1970's by OSO-7. Since 1980 observations of gamma-ray line and continuum emissions have been continuously performed by instruments on board various satellites. The aim of this presentation is to outline what has been learnt on the characteristics of energetic electrons and ions accelerated and interacting at the Sun during flares, e.g.: energy spectra of electrons and ions, chemical composition of accelerated ions, characteristic time-scales of acceleration/transport, location of electron and ion interaction regions and angular distribution of interacting ions. Although this large amount of observations has allowed us to make substantial progress in our understanding of particle acceleration during solar flares, some open questions remain like.: what are the low- and high-energy cut-offs of accelerated ions and how do they vary from flare to flare; do flares of all sizes accelerate ions?. We briefly discuss how future measurements of low energy neutrons and sub-millimeter/far-infrared emissions will contribute to answer the above questions.
SH22A-03
Proton acceleration by 3D magnetic reconnection in solar flares
High energy charged particles are an important feature of solar activity such as flares, and indeed non thermal particles play a significant role in flare energy balance. Magnetic reconnection is the primary energy release mechanism in flares, and the strong DC electric fields associated with this reconnection may well be the origin of the high energy charged particles. Whilst particle acceleration has been widely studied for 2D configurations, little is known about 3D configurations. We investigate particle acceleration using a test particle approach, in the simplest 3D reconnection configuration, a 3D magnetic null point. Two modes of reconnection are possible: with a strong current filament along the "spine" field line connecting to the null, or with a sheet current at the "fan" plane of field lines emerging from the null. Using simple model fields, incorporating intiially only thee ideal reconnection region outside the current sheet (or filament), particle trajectories are investigated and the energy spectra and spatial distribution of accelerated particles are determined. We consider and compare fan and spine reconnection, and determine how the properties of the accelerated particles depend on the parameters of the reonnecting field. We also present preliminary results using more realistic, self consistent model fields.
SH22A-04
The effects of a magnetic field topology on particle acceleration in a 3D reconnecting current sheet with the guiding field
We investigate particle trajectories and energy spectra at acceleration by a super-Dreiser electric field occurring in 3D reconnecting current sheets with different magnetic field topologies deduced from the MHD simulations. The transversal magnetic field is considered to vary exponentially as z± with the distance z from the X-null point and to be linked to a reconnection rate. The dependence on ± for different guiding field magnitudes of proton and electron trajectories and spectra is obtained. The distributions of the accelerated particles density in an RCS and energy spectra at ejection are evaluated for different magnetic field topologies [1] and reconnection rates [2] and compared with some observational signatures. The proposed method can be used for a diagnostics of magnetic reconnection dynamics from high energy particle spectra observed with a high temporal resolution. 1. Craig I.J.D. and McClymont A.N. Dynamic magnetic reconnection at an X-type neutral point, Astronomical Journal, 371, L41, 1991 2. Arber T.D. and Haynes M. A generalized Petchek magnetic reconnection rate, Physics of Plasmas, 13, 112105, 2006
SH22A-05 INVITED
What can we learn from the dynamical evolution of hard X-ray images and spectra in solar flares?
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has observed the hard X-ray emission from the Sun for more than 5 years. Imaging spectroscopy results show that solar flares are not static phenomena: both the spatial configuration and the spectral properties of the different hard X-ray sources can undergo severe changes during the course of the event. The study of the interplay between the sources and their spectral evolution deliver new insights into the particle acceleration and transport mechanism. I will review the main results from time-dependent imaging spectroscopy and their implications for theoretical understanding and modeling of solar flares.
SH22A-06 INVITED
Quasi-periodic oscillations associated with solar flares
Mechanisms for the generation of quasi-periodic pulsations (QPP) in microwave, X-ray and visible light emission
generated by solar and stellar flares, based upon magnetohydrodynamic (MHD) oscillations, are discussed. QPP
can be produced by internal MHD oscillations of flaring magnetic structures, in particular by kink, sausage and
longitudinal modes. Different modes have different observational signatures in different emission bands,
allowing for the correct identification of the mode. Another mechanism is based upon periodic triggering of flaring
energy releases by oscillations in a non-flaring loop situated nearby the flaring region. The use of observable
parameters of QPP for remote diagnostics of coronal plasmas, in particular of the magnetic field in the flaring
regions is demonstrated.
http:solarphysics.livingreviews.org/Articles/lrsp-2005-3/
SH22A-07
Optical and UV Emission of Solar Flares: Multiwavelength Observations and Modelling
Solar flares reveal themselves in various ranges of electromagnetic emission and exhibit temporal variations down to sub-second scale, namely during the so-called impulsive phase. This contribution will focus on spatial and temporal variations of the optical and UV emissions which are thought to be driven by flare energy transport in the form of accelerated particles. We will review our current understanding of the formation of optical and UV emissions during the impulsive phase. Namely, we will discuss recent radiative-hydrodynamic models and various physical processes playing an important role in radiation transfer and formation of optical and UV spectra. We will also report on recent interpretations of observations in various lines and continua and discuss their importance for the diagnostics of the flare atmosphere structure and the presence of accelerated particles in the emission formation regions.