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These are events that show sufficiently long delay times (>10 minutes) between the peak hard X-ray emission and the calculated electron release times:


  • Is there indeed a delay between the peak hard X-ray emission and the calculated electron release times or propagation effects are producing the observed delayed onsets?

  • The origin of the delayed release times is currently not understood, but two main ideas are discussed:

    • The delay could be due to time-extended electron acceleration and/or electron storage at high coronal altitudes during solar flares in combination with a delayed access of these electrons to magnetic field lines open to interplanetary space (Laitinen et al., 2000; Classen et al., 2003; Klein et al., 2005; Aurass et al., 2006).
    • The delay could be the result of electron acceleration by coronal shocks that move away from the flare site (see also next section 3.2 ) (Krucker et al., 1999; Haggerty and Roelof, 2002; Simnett et al., 2002). The delayed release would be produced by the time that the shock takes to form and to efficiently accelerate electrons (Mann et al. 1995; Warmuth & Mann 2005), and/or by the time it takes for the shock to reach magnetic field lines that are connected to the spacecraft. Problem: shocks not very efficient at accelerating electrons at higher (MeV) energies (but it might work for shocks that further accelerate a previously produced population of energetic electrons, e.g. Selkowitz & Blackman 2007). 
    • In both cases, the related hard X-ray emissions are expected to be seen after the main hard X-ray bursts. They are likely to be much fainter and to originate from a different location. However, detection of these faint emissions will be difficult in the presence of decaying thermal X-ray emissions at these later times. The best chance of detection will be for occulted flares where the main flare emission is not yet visible. The faint emission related to the delayed release may be seen on the disk or at high altitude above the disk.