| Abstract | Koronin izbačaj predstavlja eruptivnu promjenu globalnog magnetskog polja korone i posljedica je nestabilnosti usukanog toroidalnog magnetskog ustrojstva. Utvrđeni su uvjeti za nastup nestabilnosti i procijenio se intenzitet akceleracijske faze. Pretpostavljeno je da nakon oslabljenja Lorentzove sile povećanjem eruptirajućeg ustrojstva izbačaja, dominantnu ulogu u jednadžbi gibanja preuzima „magnetohidrodinamički“ otpor. Razjašnjeni su detalji ovisnosti sile otpora o relativnoj brzini izbačaja i Sunčevog vjetra, koja u nesudarnom okruženju poprima kvadratični oblik.U ispitivanju uvjeta za nastup nestabilnosti i određivanju karakteristika akceleracijske faze, razmatraju se svojstva strukture usukanog samouravnoteženog magnetskog polja.Induktivitet debele toroidalne strukture, koji se numerički odredio, bitan je u proučavanju nestabilnosti. U visokoj koroni sila Lorentzova sila iščezava i pokazano je da se toroidalna struktura proporcionalno širi, tj. omjer malog i velikog polumjera toroida ostaje približno konstantan. U akceleracijskoj fazi izbačaja razmatra se gubitak ravnoteže i u modelu su uključene sile: sila gradijenta tlaka poloidalne komponente magnetskog polja, sila tenzije osne komponente polja, sila zbog dijamagnetskog efekta površine Sunca i sila pozadinskog koroninog polja. Prolaskom kroz parametarski prostor modela, reproducirani su opažački rezultati u vrlo ranoj akceleracijskoj fazi. Model je najosjetljiviji na struju koja teče magnetskim užetom I0 i na dodatno induciranu struju ∆I nastalu magnetskim prespajanjem (rekonekcijom). Uzastopnim povećanjem struje I0 magnetsko uže prolazi kroz kvazi-stacionarna stanja i pri kritičnoj vrijednosti I0 dolazi do potpunog gubitka ravnoteže. S obzirom na povezanost erupcija i Sunčevih bljeskova u model se uvodi i proces magnetskog prespajanja.Nakon početne eruptivno-akceleracijske faze slijedi propagacijska faza u međuplanetarnom prostoru u kojoj je sila „otpora“ ovisna o relativnoj brzini izbačaja i Sunčevog vjetra koja u nesudarnom okruženju ima kvadratični oblik. Ubrzanje iznosi: a(r) =−γ(r) [v(r) − w(r)] |v(r) − w(r)|, gdje je γ(r) funkcija „otpora“, a(r), v(r) i w(r) su akceleracija, brzina vodećeg luka izbačaja, te brzina Sunčevog vjetra. Rezultati temeljeni na predloženom modelu uspoređeni su s empirijskim rezultatima na satelitskih mjerenja (Zhang et al., 2003; Schwenn et al., 2005; Manoharan, 2006). Analiza podataka dobivena primjenom pojednostavljenog modela s konstantnim γ(r) = γc = konst. i w(r) = wc = konst. na statističkom uzorku izbačaja pokazala je da optimalne vrijednosti parametra Γ (γc = Γ × 10−7 km−1 Sunčevog vjetra od wc = 500 km/s. Male vrijednosti parametra Γ odgovaraju masivnom izbačaju koji se giba u brzom Sunčevom vjetru male gustoće, dok velike vrijednosti opisuju izbačaje malih gustoća u sporom Sunčevom vjetru. Tipične nepouzdanosti u predviđanju vremena dolazaka međuplanetarnih izbačaja do Zemlje (1 AU) iznosi otprilike pola dana. Krajnji oblik modela objavljen je u obliku prilagođenom za javnu primjenu putem interneta u sklopu EU FP7 projekata SOTERIA i COMESEP, te za uporabu u međunarodnoj razmjeni podataka Solar Alert i nalazi se na internetskoj adresi: http://oh.geof.unizg.hr/CADBM/cadbm.php. |
| Abstract (english) | Eruptive processes in the solar atmosphere strongly influence the physical state of theheliosphere and the terrestrial space environment. Coronal mass ejection (CME) rep-resents an eruptive restructuring of the global coronal magnetic field. The eruption iscaused by a loss of equilibrium of the magnetic structure anchored in the photosphere.The stability of the structure depends on the amount of energy stored in the magneticfield, whereas the CME itself is driven by the Lorentz force. The dynamics of instabilitydepends on the effects of magnetic-flux conservation and induction, which cause the ces-sation of the Lorentz force. Eventually, magnetohydrodynamic (MHD) drag becomes adominant factor in the CME dynamics. The drag is a consequence of collisionless trans-fer of momentum and energy between the CME and the ambient solar wind by MHDwaves. This thesis addresses the dynamics of CMEs from the onset of instability up toheliospheric propagation, and includes its application in space weather forecasting.In order to establish which conditions and processes lead to the onset of instability andto determine properties of the acceleration phase, structural characteristics of the toroidaland helical force-free magnetic field are discussed. The inductance of the thick toroidalstructure, which was determined numerically, is crucial in studying the instability. Inthe upper corona the Lorentz force disappears, and we show that the toroidal structurepropagates in self-similar manner, i.e., the ratio of the minor-to-major toroidal radiusratio remains approximately constant.The acceleration phase of the ejection is discussed in terms of the loss of equlibrium, andthe model includes the following forces: gradient pressure force of the poloidal componentof the magnetic field, tension force of the axial field component, the force due to thediamagnetic effect of the solar surface, and the force of the background coronal field.Analysis of the parametric space of the model reproduced observational results in theearly acceleration phase. The model is highly sensitive to the current I0 flowing throughthe flux rope as well as the extra induced current ∆I which is a result of magneticreconnection. Continuous increase of the I0 current causes the flux rope to evolve throughquasi-stationary states, while sufficiently value of I0 cause a complete loss of equilibrium.Finally, the process of magnetic reconnection is introduced into model, since eruptions areoften associated with solar flares. The reconnection model is suitable to explain varioustypes of ejections based on observational data.The initial acceleration phase of the ejection is followed by the propagation phase in theinterplanetary space, where the „drag“ force depends on the relative speed of the ejectionand the solar wind. In a collisionless environment, the „drag“ force has quadratic form.The acceleration can be expressed as a(r) = −γ(r) [v(r) − w(r)] |v(r) − w(r)|, where γ(r)is the „drag-parameter“, whereas a(r), v(r) and w(r) refer to instantaneous acceleration,speed of the leading edge of the ejection and the speed of the solar wind, respectively. Theresults obtained from the proposed model have been compared to empirical results basedon satellite measurements (Zhang et al., 2003; Schwenn et al., 2005; Manoharan, 2006).Analysis of the data resulting from the application of the simplified model with a constantγ(r) = γc = const. and w(r) = wc = const. on a sample of ejections revealed that optimalvalues of the parameter Γ (γc = Γ × 10−7 km−1the asymptotic speed of the solar wind at wc = 500 km/s. Low values of the parameter Γcorrespond to massive ejections which move in a fast solar wind of low density, while highvalues describe ejections of low density in a slow solar wind. The typical uncertainties inpredicting the arrival of interplanetary mass ejections to Earth (1 AU) are roughly half aday.The final version of the model has been published in a form adapted for public online useas part of the EU FP7 projects SOTERIA and COMESEP, and is used in the internationaldata exchange system Solar Alert. It is available at: http://oh.geof.unizg.hr/CADBM/cadbm.php. |
