MHD Modes of Solar Plasma Structures

Transcription

1 PX420 Solar MHD MHD Modes of Solar Plasma Structures Centre for Fusion, Space & Astrophysics

2 Wave and oscillatory processes in the solar corona: Possible relevance to coronal heating and solar wind acceleration problems. Possible role in the physics of solar flares. Plasma diagnostics tools - coronal seismology. Perspectives of stellar coronal seismology. Observational evidence of coronal (or quasi-periodic pulsations) (major contribution by SOHO, oscillations is abundant TRACE and NoRH).

3 The Seven Sisters Flare ISEE-3 and Nobeyama Radiopolarimeter, (+ very faint pulsations in < 17 GHz) Period about 8 s.

4 Typical periods from 1 s to several min. Mechanisms for (Quasi) Periodicity: Resonance (characteristic spatial scales) Seismological information Dispersion Nonlinearity / self-organisation Characteristic scales: 1 Mm-100 Mm, Alfvén speed 1 Mm/s, sound speed 0.2 Mm/s periods 1 s several min - MHD waves

5 2. Revision: MHD waves in a uniform medium: Two characteristic speeds: Alfvén speed: Sound speed: Alfven waves: Magnetosonic waves: Centre for Fusion, Space & Astrophysics

6 Development of an MHD perturbation in a uniform medium Verwichte, 2006

7 Characteristic speeds: Sound speed: C S! T, - gradient of gas pressure Alfv e" n speed: C A! B / Fast speed: C F = Tube speed: #, - magnetic tension, C A 2 + C S 2 - gradient of (magnetic pressure + gas pressure) C T = C S C A C A 2 + C S 2 Kink speed: C K = $ & % # C 2 0 A0 2 + # e C Ae # 0 + # e ' ) ( 1/ 2 ; in low-* : C K = C A # e / # 0

8 Consider a magnetic flux tube: Magnetohydrodynamic (MHD) equations à Equilibrium à Linearisation à Boundary conditions

9 Dispersion relations of MHD modes of a magnetic flux tube: I '( m a) K '( m a) ρ ( ω kc ) m ρ ( ω kc ) m = 0 I ( m a) K ( m a) m m e e z Ae 0 0 z A0 e m 0 m e Zaitsev & Stepanov, B. Roberts and colleagues, 1981-

10 Dispersion curves of coronal loop: Main MHD modes of coronal structures: sausage ( B, ) kink incompressible) (almost torsional (incompressible) acoustic (, V) ballooning ( B, )

11 GLOBAL MODES: Sausage mode: P = 2 L/ C, where C < C < C Kink mode: P = 2 L/ C, kink Longitudinal mode: P = 2 L/ C Torsional mode: P = 2 L/ C saus p A0 p Ae tors long K A0 T 0

12 1. Kink modes of coronal loops (EUV, TRACE): Centre for Fusion, Space & Astrophysics

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15 How we analyse it: Centre for Fusion, Space & Astrophysics

16 E.g.: Path G, Period 338 s, Amplitude 750 km Centre for Fusion, Space & Astrophysics

17 Oscillation period, Decay time Centre for Fusion, Space & Astrophysics

18 Longitudinal modes: Centre for Fusion, Space & Astrophysics

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20 Running sausage wave? Centre for Fusion, Space & Astrophysics

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22 Distance along slit time Centre for Fusion, Space & Astrophysics

23 3. Sausage modes: m=0 mode

24 Sausage modes are essentially compressible and can modulate X- ray and radio emission (directly, through B or through the modulation of the mirror ratio) Centre for Fusion, Space & Astrophysics

25 External medium, finite wave length P = 2 L/ C, C < C < C GSM P A0 P Ae P 2π a 2.62a < jc C 0 A0 A0 In solar corona: P = s ka c

26 First observational identification: Signals at different parts of the loop: 17 GHz, 34 GHz + SXT

27 Spectra at different parts of the loop: Centre for Fusion, Space & Astrophysics

28 C p 2L = P 3,200 km/s, and it must be < C Ae P 2.62a 2.62a < CA0 < 524 k m/s. C P 15 A0 C Ae > 3, 200 km/s; C < 524 A0 k ms /

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