Solar Physics & Space Plasma
Research Center (SP2RC)
The role of partial ionisation in the
stability of prominences structures
Istvan Ballai
SP2RC, School of Mathematics & Statistics,
The University of Sheffield (UK)
Solar Physics & Space Plasma
Research Center (SP2RC)
Courtesy of SDO
RAS Meeting, 21/02/2014, London
Berger et al. 2010
Solar Physics & Space Plasma
Research Center (SP2RC)
Observations of prominence instability
A classical example of RT instability
(Ryutova et al. 2010)
-cuts 1-2 show regular oscillatory pattern
- cuts 3-4 show regions where the RT
instability broke into plumes (array of
small arrows) and destroyed the regular
oscillatory pattern (curved arrow).
Solar Physics & Space Plasma
Research Center (SP2RC)
• Temperatures are not high enough to ensure full ionisation
• The plasma is composed by +’ve and –’ve ions, neutral atoms (ground-level or excited),
e-, photons; the Boltzmann, Saha and Maxwell equations (together with the Planck
function) fully describe their equilibrium relations
• This composition requires a complex mathematical description
• The components of the plasma might not be in thermodynamical equilibrium, however
we assume plasma to be in a local thermodynamical equilibrium (LTE)
• Transport effects are still achieved through collisions, however the presence of different
constituents should be taken into account (Khodachenko et al. 2003)
RAS Meeting, 21/02/2014, London
Solar Physics & Space Plasma
Research Center (SP2RC)
A couple of basic and necessary ingredients
•
•
•
•
Prominences are very structured, observations show a clear fibril structure
The plasma is always in motion, plasma flows along field lines, waves and oscillations are
continuously observed (see, e,g, the review by Arregui, Oliver, Ballester 2012)
Prominences are embedded in the solar corona
The fate of prominences is very often connected to instabilities
RAS Meeting, 21/02/2014, London
Solar Physics & Space Plasma
Research Center (SP2RC)
Instabilities in solar prominences
•
Here we discuss 3 particular instabilities and the role of partial ionisation in the
development of instabilities
–
Rayleigh-Taylor instability (RTI)
–
Kelvin-Helmholtz instability (KHI)
–
Dissipative instability (DI)
• The RTI develops at gravitationally permeated interfaces, where the medium above the
interface is heavier (Diaz et al. 2012)
z
ρ1>ρ2
g
B0
ρ1
ρ2
RAS Meeting, 21/02/2014, London
x
•Two unstable modes were found; one related to
neutrals and the second one to the the e--ion fluid
•Ion-neutral collisions lower the growth-rate
•The results explain the existence of fine structure with
lifetime >30 mins
Solar Physics & Space Plasma
Research Center (SP2RC)
Instabilities in solar prominences
•
The KHI appears at interfaces in the presence of shear flows (Jones & Downes 2011; Soler et al. 2012)
v0
B0
prominence
plasma
dark plume
RAS Meeting, 21/02/2014, London
• Ion-electron collisions are not able to stabilise
the instability driven by neutrals
• The magnetic field has a stabilising effect
• Ion-neutral coupling brings the instability
threshold down to observed flow speeds
• The growth rate depends on the ratio υin/ωv
• KHI cannot be responsible for the thread
dissapearance
Solar Physics & Space Plasma
Research Center (SP2RC)
Instabilities in solar prominences
•
Very often the explanation of observed instabilities in terms of KHI is difficult given the very high
instability threshold
• But instabilities can arise for lower flows through, e.g. dissipative instabilities (Ballai, Oliver et al.
2014) which can set for lower flow speeds than the corresponding KHI
• Dissipative instabilities are strongly connected to negative energy waves, i.e. to waves that can grow
despite the presence of dissipation.
Case 1:
• assume an interface separating the partially ionised prominence
B0
plasma and a viscous corona (incompressible media)
• the prominence is dynamic (v0)
• in the prominence the dominant transport mechanism is the
Cowling resistivity, assume weak damping
• given the large density ratio, the KH threshold is very high, i.e. the
interface is KH stable
• the interface allows the propagation of two modes in opposite
Case 1: prominence corona
direction. The forward mode follows classical damping, the
Case 2: prominence dark plume
backward wave can become unstable
RAS Meeting, 21/02/2014, London
Solar Physics & Space Plasma
Research Center (SP2RC)
Dissipative instability at the
prominence/corona interface
• For typical prominence/coronal values, the
dissipative instability sets at a tenth of the
KH threshold
• Due to the very high density contrast (here
2 orders of magnitude) the effect of partial
ionisation is very week, the coronal plasma
parameters control the stability and the
dissipative instability is driven by the
coronal viscosity
RAS Meeting, 21/02/2014, London
Solar Physics & Space Plasma
Research Center (SP2RC)
Dissipative instability at the
prominence/plume interface
•
•
Now assume the interface separates the partially ionised prominence and dark plumes
Dark plumes show turbulent upflows (15-30 km/s) and they are dark in CaII-H line because they are
hotter and less dense than the prominence. The ionisation degree varies as T 1/2
• The prominence-plume interface is always unstable
regardless the value of the flow
• The growth rate is higher for fully ionised plasma,
neutrals tend to stabilise the interface and the flow in
the plume
• Obviously these calculations contain many
simplifications, in reality they are part of a proof-ofconcept study on the appearance of dissipative
instability (single fluid MHD, incompressible plasma,
field aligned flow, elastic collisions between ions and
neutrals, etc.)
RAS Meeting, 21/02/2014, London
Solar Physics & Space Plasma
Research Center (SP2RC)
RAS Meeting, 21/02/2014, London