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Proposal ID Nakariakov_44

Date of Proposal
96/12/02

firstname
Valery

lastname
Nakariakov

institute
University of St Andrews

Valery Nakariakov
School of Mathematical and Computational Sciences
University of St Andrews, St Andrews,
Fife KY16 9SS, Scotland, U.K.

email
valery@dcs.st-andrews.ac.uk

phone
44-1334-463753

fax
44-1334-463748

proposal
AIM:
To determine the presence of magnetoacoustic waves trapped
in plumes within coronal holes. The modes are believed to be
closely connected with the wave models of coronal heating
and solar wind acceleration.

THEORY:
Magnetoacoustic waves propagating along open magnetic structures in coronal
holes are believed to be responsible for the acceleration of
the solar wind plasma (e.g., Davila, J.M.: 1985, {\it
Astrophys. J.} {\bf 291}, 328 and Cally, P.S.: 1986, {\it
Solar Phys.} {\bf 108}, 183). The coronal holes are regions
of rarefaction of plasma density, corresponding to an
enhancement in Alfv\'en speed, and so are anti-waveguides for
fast magnetoacoustic waves (e.g., Edwin, P.M. and Roberts, B.: 1982,
{\it Solar Phys.} {\bf 76}, 239). The presence of plumes inside
coronal holes, with a density enhancement, allows for the
possibility of trapping waves. The trapped modes propagate dispersively
along the magnetic structure without leakage and may contribute to
plasma heating and the acceleration of the plasma inside the plume.

Trapped magnetoacoustic waves may be described according to the theory
developed by Nakariakov, V.M. and Roberts, B.: 1995, {\it Solar Phys.} {\bf
159}, 213. We can estimate parameters of the modes. With a plasma density
inside the plume of $n_0 \approx 5.\times 10^{14}$~m$^{-3}$
and outside the plume with $n_e \approx 0.25 n_0$, embedded in a
magnetic field of 10~G and plasma temperatures $T_0 \approx 1.2\times 10^6$~K
inside and $T_e \approx 10^6$ outside the plume, we find that the fast
magnetoacoustic mode phase speeds $a_f$ are in the band from 1250~km~s$^{-1}$
to 2500~km~s$^{-1}$ and slow magnetoacoustic mode speeds $a_s$ are about
160~km~s$^{-1}$. The presence of an outward plasma steady flow within the
plumes changes the bands of the phase speed of the modes.

OBSERVATIONAL POSSIBILITY:
Instrumental features do not allow us to register periodicities below
about 2~min. According to the theory, this allows the possibility
to find fast magnetoacoustic modes with wavelengths exceeding
$15\times10^{4}$~km (the cut-off wavelength for the sausage mode is
about $45\times10^4$~km for the parameters taken above)
and slow magnetoacoustic modes with wavelengths greater than
$2\times10^{4}$~km. In contrast with Alfv\'en waves, both fast and slow
magnetoacoustic modes involve perturbations of the plasma density and
temperature, and so they can change the intensity of an observed line.

REQUESTED DATA:
We need temporal variations of the intensity measured at a specific location
inside a plume observed on the limb. The spectral line
{\it Fe IX-X} corresponding to the plasma temperature inside coronal holes
seems to be suitable.
The period between the measurements should be as small as possible (say, every
1~min, or better). The duration of the observational series should be
as long as possible (at least several hours). Such observations will allow us
to identify the required periodicities. Also, it is important to obtain
observations of the temporal variations of the intensity at several different
points along the plume, simultaneously. Such data will allow us to define
directly the speed of propagation of the perturbations. According to
our theory, it gives us the speed of the plasma steady flows
inside and outside the plume. Additionally, we need the dependence of the
amplitude of the intensity variations on the distance from the Sun. This
allows us to define the location of the source of the magnetoacoustic modes
(they may be excited in the lower layers either due to photospheric
motions or by wave-flow interactions at some distance from the Sun).

inside_collab
J. Gurman, R. Catura

consortium
no

topic
CORONAL PLUMES

title
MAGNETOACOUSTIC MODES OF CORONAL PLUMES

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