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
Sun and Heliosphere
Physical Research Laboratory
Ahmedabad 380 009
IHY school for Asia – Pacific Region, Kodaikanal, Dec.10-22, 2007
Coronal Mass Ejections
And Closed Magnetic loop structure in helispheric current sheet
Generated by a powerful plasma process – Reconnection of magnetic field lines
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
plasma flow and magnetic field structure.
where the magnetic field is secondary and can be calculated from Ampere’s law c Curl B = 4 For MHD, velocity v and the magnetic field B are primary and J and E can be calculated from
Now for large , 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
is extremely large (~1010) in solar atmosphere
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
A different scenario
A pair of inflowing field lines become limiting field lines and then immediately after that they form outflowing field lines.
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
In thin region diffusion is substantial
Magnetic Induction becomes.
In one dimension
where Bxis the magnetic field along x-direction and z is the vertical direction as shown in Figure 1.2.
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.
P is the pressure on the central plane where the magnetic field is almost zero.
Po-pressure outside, where the magnetic field is B.
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
u, B are uniform
And Electric field also is uniform
As a-increases, u has to increase and then B decreases in the diffusion region and becomes less than
This is achieved by rotating the magnetic field vector towards the normal.
Vasyliunas obtained upper limit
Tearing mode instability
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.
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
Region of acceleration of particles.
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.
Both temperature and density evolution leading to reconnection and island information.
Again localized resistivity appears to be very important for fast reconnection.
d; Thickness of the current sheet
ri; Ion Larmor radius
Structures of different scales and intensities are seen.
current sheet shows
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 scale-matching problem.