Transcript Slide 1
Reconnection process in Sun and Heliosphere A.C. Das Physical Research Laboratory Ahmedabad 380 009 IHY school for Asia – Pacific Region, Kodaikanal, Dec.10-22, 2007 • Heliosphere – Magnetosphere of our sun •Interaction of Solar wind and the interstellar medium •Heliopause: Balance between interstellar medium and solar wind pressure •Termination shock: Solar wind becomes subsonic at this point •Interplanetary medium moving in opposite direction becomes subsonic as it collides with heliopause – Bow shock •Solar wind, solar flares and coronal mass ejection – sends materials and fields into the heliosphere •Heliospheric current sheet, ripple in the heliosphere Solar Flares Generated by a powerful plasma process – Coronal Mass Ejections Reconnection of magnetic And Closed Magnetic loop field lines structure in helispheric current sheet Giovaneli – importance of neutral point in Solar Flares Dungey – Developed a radically different model in physics of magnetosphre Following his concept, we will describe the process of reconnection Essential to introduce some basic understanding of plasma flow and magnetic field structure. In absence of plasma valocity, Ohm’s law J E where the magnetic field is secondary and can be calculated from Ampere’s law c Curl B = 4J For MHD, velocity v and the magnetic field B are primary and J and E can be calculated from . 4 Curl B J c Now for large , J / v B E c J can be neglected and electric field is then driven by the velocity and magnetic field. Ratio of the 2nd term to the first term on the right hand side of (5) defines the magnetic Reynold number given by vB B vL Rm / 2 L L Rm is extremely large (~1010) in solar atmosphere Basic Reconnection Process Depends on (1) Topology of the magnetic field (2) Motion of plasma near the neutral point Magnetic field lines are anti-parrellel One neutral point, with limiting field lines – Separatrix. Two are going in and two are coming out Plasma behaviour in absence of pressure E+1/C (VxB) = 0 Field lines moving from both sides They remain field lines. Electric field. Current enhances, But no reconnection. Reconnection-consequence of the break-down of frozen-in-field approximation. May be caused because of high current density Finite Resistivity A different scenario A pair of inflowing field lines become limiting field lines and then immediately after that they form outflowing field lines. Permits Limiting field lines to cut at neutral point and then reconnect to form a different set of field lines. Possible because of violation of frozen in approximation. This is reconnection in pictorial form Diffusion, and Reconnection In thin region diffusion is substantial Magnetic Induction becomes. B t In one dimension 2 B Bx t 2 Bx , 2 z where Bx is the magnetic field along x-direction and z is the vertical direction as shown in Figure 1.2. Solution Current along y direction Magnetic field lines are in opposite direction around z=0 Magnetic flux from above as well as from below get dumped at the separatrix feeding the current. Field gradient decreases, diffusion slows down process becomes unproductive. Need to introduce u from both sides. Can maintain large current Not physical, unless there is an outflow Finally reconnection takes place with an outflow Similar to the picture presented earlier by Dungey. Important Reconnection Models (Steady State) – MHD Theory Sweet-Parker Model: -Magnetic field are anti-parrellel -Plasma is incompressible -Plasma flow from both sides with u, current sheet length l and width d. -Conservation of Mass ul=vd….. (10) (consequence of . v = 0 Momentum balance – External magnetized Internal field free p po B 2 / 8 P is the pressure on the central plane where the magnetic field is almost zero. Po-pressure outside, where the magnetic field is B. Petschek Model SP model – large l No large rate of Reconnection because u=(d/l) va Petschek pointed out In MHD flow in the outer region, Possible that two standing MHD wave front can be maintained – fronts are shocks Diffusion region can be matched to a region of standing waves. a – the half angle of the exit flow or the angle of slow shock such that it remains stationary in the flow In Petschek Model, u, B are uniform And Electric field also is uniform Therefore uB u B As a-increases, u has to increase and then B decreases in the diffusion region and becomes less than B This is achieved by rotating the magnetic field vector towards the normal. Vasyliunas obtained upper limit u ( / 4) v A / ln Rm Spontaneous reconnection or Patchy Reconnection Tearing mode instability Growth rate can be estimated shown below: We have seen that the growth rate depends on the width of the current sheet and conductivity. Normal component of the magnetic field. Bn Electron Tearing mode disappears. However, ion-tearing mode can be present. But has limitation on magnetic field range. External Source – LH turbulence Enhance the growth rate. Observational Evidence of the magnetic Reconnection in solar flares Top left side Right bottom Cusped shaped loop structure, Hard X-ray telescope in Yohkoh Helmet streamer etc. Hard X-ray loop top above Plasmoid ejection soft X-ray bright loop Schematic view of Impulsive flare. Region of acceleration of particles. Loop-top Hard X-ray source above Soft X-ray bright loop During SXR loop Discovery of loop top HXR -Made it possible -Unifying two classes of flare LDE and impulsive flare -Unifying model Numerical Simulation of reconnection between emerging flux and coronal field Formation of magnetic island that are ejected out of the current sheet. Localized resistivity seems to be essential . Tearing mode instability. More realistic simulations Both temperature and density evolution leading to reconnection and island information. Again localized resistivity appears to be very important for fast reconnection. Problem of Scale Matching Two important aspects-unanswered. (1) Local Enhancement of Magnetic Diffusion – a conjecture (2) Enormous gap of scale sizes – macro and micro features. • Scale-size of the anomalous resistivity d = ri ~ 10 m d ; Thickness of the current sheet ri ; Ion Larmor radius • Scale size of a flare: 104 km ! Interesting turbulent structure in outflow MHD-simulation of turbulent reconnection Structures of different scales and intensities are seen. Solar Maximum Mission (SMM) Observation of evidence of Reconnection of previously open magnetic structure. -Appearing as pinching of helmet streamers followed by release and acceleration of a large u or v-shapped structure. -Observed sequence of events consistent with reconnection across the heliospheric current sheet between previously open field lines and creation of detached magnetic structure. -Coronal disconnection events would return previously open flux to sun as closed field arches. -Internal magnetic reconnection can also take place within the flux rope. As the flux rope field lines are sheared, oppositely directed field lines are generated which press together and reconnect. Satellite observation of the Heliospheric current sheet shows -Internal structure of the sector boundaries is very complex with many directional discontinuities in mag field. -Implies heliospheric current sheet is not a single surface – constantly changing layer with a varying number of current sheets. -Studied magnetic reconnection caused by resistive tearing mode instabilities, multiple current sheets – 2D MHD simulation. -Results: Complex unsteady reconnection -NL limits, formation of islands or plasmoids. -Suggest: Occurrence of multi-direction discontinuities in the heliosphere. -May be associated with the magnetic islands and plasmoids – caused by Reconnection. Summary Magnetic Reconnection is the underlying driver of giant explosive releases of magnetic energy in the Sun’s atmosphere that are observed as solar flare or CMEs. Many compelling observational evidences for reconnection which support reconnection model of solar flares are presented. Numerical simulation suggests that the localized resistivity is necessary for magnetic reconnection. There is still an enormous gap between the microscale of anomalous resistivity and the size of solar flares. MHD turbulence model of reconnection shows interesting features in various cases and may play an interesting role in solving the scalematching problem.