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Return Currents in Solar Flares: Theory and Observations
Başlık:
Return Currents in Solar Flares: Theory and Observations
Yazar:
Abdallaoui, Meriem Alaoui, author.
ISBN:
9780438008168
Yazar Ek Girişi:
Fiziksel Tanımlama:
1 electronic resource (228 pages)
Genel Not:
Source: Dissertation Abstracts International, Volume: 79-10(E), Section: B.
Advisors: Gordon D. Holman Committee members: Joel C. Allred; Steven Kraemer.
Özet:
Electron beams are thought to be accelerated during a solar flare by conversion of magnetic energy into kinetic energy. These energetic beams are accelerated in the corona and stream both toward the interplanetary medium and toward lower (and denser) layers of the solar atmosphere. They constitute a current which is neutralized by a co-spatial return current. Several questions arise about the beam/return current system, and its coupling with the plasma in which it propagates. What is the magnitude of the beam current? Is this beam stable to the generation of current-driven instabilities? Does the return current heat the plasma through Joule heating? If so, where and how much energy is deposited in the solar atmosphere?
In this thesis, we explore these questions, using a 1D model with a co-spatial return current to explain hard x-ray (HXR) spectral breaks. We study 19 flares observed by RHESSI (Ramaty High Energy Solar Spectroscopic Imager) with strong spectral breaks at energies around a few deka-keV, that cannot be explained by isotropic albedo or non-uniform ionization alone. We identify these breaks at the HXR peak time, but we obtain 8 second-cadence spectra of the entire impulsive phase. Electrons with an initially power-law distribution and a sharp low-energy cutoff lose energy through return-current losses until they reach the thick target, where they lose their remaining energy through collisions.
The return current thick-target model (RCCTTM) and method of analysis are introduced in chapter 2. Our main results are summarized as follows: (1) The RCCTTM provides acceptable fits for spectra with strong breaks (chapter 3). (2) Limits on the plasma resistivity are derived from the fitted potential drop and deduced electron-beam flux density, assuming the return-current is a drift current in the ambient plasma. These resistivities are typically 2-3 orders of magnitude higher than the Spitzer resistivity at the fitted temperature, and provide a test for the adequacy of classical resistivity and the stability of the return current. (3) Using the upper limit of the low-energy cutoff, the return current is always stable to the generation of ion acoustic and electrostatic ion cyclotron instabilities when the electron to ion temperature is less than 9. (4) In most cases the return current is most likely primarily carried by runaway electrons from the tail of the thermal distribution rather than the bulk drifting thermal electrons. For these cases, anomalous resistivity is not required (chapter 4).
In addition, we develop two steady-state numerical codes. • One code self-consistently solves the propagation of the beam and return current including the contribution from runaway electrons in the thermal tail distribution, in the weak (sub-Dreicer) electric field regime. We find that when the return current is carried at least partially by runaway electrons, (1) the total energy deposited in the loop is reduced compared to the case where the return current is carried by the bulk thermal electrons, and (2) the return current electric field increases from the looptop to the thick target, if no beam electrons are thermalized (chapter 5). • Recognizing the possibe importance of Coulomb collisions in the corona, we solve the energy loss equations taking into account Coulomb collisions and return current losses simultaneously, and obtain the spatial evolution of the electron distribution and X-ray emission along the loop. This is based on Emslie 1980 and Holman 2012 with updates on the classical resistivity in a partially ionized plasma from Sykora et al.2012. The importance of including collisional losses is increased for lower values of the injected electron low-energy cutoff, and if collisions are also taken into account, the flattening of the electron distribution is stronger lower in the loop (chapter 6).
Notlar:
School code: 0043
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Yer Numarası | Demirbaş Numarası | Shelf Location | Lokasyon / Statüsü / İade Tarihi |
---|---|---|---|
XX(680410.1) | 680410-1001 | Proquest E-Tez Koleksiyonu | Arıyor... |
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