The Source of the Fast Solar Wind

1
The Source of the Fast Solar Wind
M. D. Popescu, L. D. Xia, J. G. Doyle
Pastel drawing of October 29th 2003 aurora at the
Armagh Observatory, by Miruna Popescu
Abstract It is well-known that
the fast solar wind originates from the open
magnetic field lines of coronal holes (CHs) in
the corona, but at what height in the solar
atmosphere the plasma outflow starts is still an
open question. One aim of this work is to search
for the signature of plasma outflows as low as
possible in the solar atmosphere. From the
analysed observations, we suggest that the fast
solar wind is unlikely to start as a steady
outflow in the transition region. At this height
we see continually oscillating bi-directional
jets of different scales, which could pump plasma
into the corona.
Introduction
Fig. 3 Examples of spectra from the O III
line, representing the selected (a) (f)
features

• There is a continuous flow of charged particles
  that escapes the Sun and fills the space between
  the solar system objects and even beyond, called
  solar wind.
• Sometimes (as on 29 October and 20 November 2003)
  we can see, even in Armagh, its interaction with
  the atoms and molecules from the terrestrial
  atmosphere, in the form of astonishing colour
  displays auroras.
• The solar wind has two components
• a fast, low-density, steady wind (500-900 km
  s-1) and
• a slow, high-density, variable wind (300-400
  km s-1).
• We know that the fast solar wind originates from
  coronal holes (CHs), regions in which one
  magnetic polarity dominates, and the field lines
  are open.
• To find what are the small-scale features
  responsible for the appearance of the fast solar
  wind, we need to correlate plasma motions with
  fine structures inside the CHs, seen from the
  transition region (TR) downward.
• We therefore searched for the origins of the
  solar wind, as low as possible in the solar
  atmosphere.

Fig. 1 The Mg IX line intensity raster
the contours represent the CH boundaries.
In the bi-directional jet, also called explosive
event, plasma has rapid movements. Its spectrum
shows two peaks, with a blue component shifted
with more than -100 km s-1.
(a)
In the inter-network cells, plasma is generally
red-shifted. For the selected example the mean
Doppler velocity over its whole spatial length is
5 km s-1.
(b)
Chromospheric network in QS, seen as increases in
the O III intensity, is generally red-shifted
with an average of 7 km s-1.
(c)
Data
The coronal bright point is seen as an increase
in the Mg IX intensity, and a smaller relative
increase in the O III intensity. In both lines,
the outer region of the feature is red-shifted
with 5 km s-1, and in its middle, blue-shifted
with 04 km s-1.
(d)
The data were acquired with the SUMER (Solar
Ultraviolet Measurements of Emitted Radiation)
grating spectrograph on SoHO (Solar and
Heliospheric Observatory). Data-set I an
on-disk raster in the northern pole CH on 17
March 1999 dimension
108292 arcsec2 (? 77,750210,250 km2)
lines O III 703.87Å (originating at ?
8104 K in the low TR) and
Mg IX 706.02 Å (? 106 K in the
corona). The location of data-set I is given in
the rectangle from the background image, as seen
in the EIT (Extreme Ultraviolet Imaging Telescope
on SoHO) Fe XII 195 Å line. In Fig. 1 we plot the
raster as seen in Mg IX 706 Å line
intensity. Data-set II a time series in the
northern pole CH on 23 October 1996
dimension 120 arcsec (? 85,800 km) on
solar y for 4 h 10 min line
O VI 1031.9 Å (originating at ? 3105 K in the
TR).
(e) (f)
CH chromospheric network inside the CH, the
intensity increases correspond to velocity
decreases, which, generally, becomes negative.
Sometimes this correlation is shifted a few
pixels, revealing that the outflow takes place
not on the magnetic network itself, but at its
boundaries with the inter-network cells. We
selected as typical examples the features (e) and
(f), whose blue-shifts are 6-7 km s-1 on average.
The results of data-set I are in the coronal Mg
IX line, the filling factor of blue-shifted
material in the CH is about 90, with average LOS
velocities of 5 km s-1, which is a clear sign of
up-going material. For the O III line, the
downward motion is predominant (at ? 5 km s-1).
But its red-shifted appearance is sprinkled with
blue-shifts forming a small-scale pattern with
average values of ? 3 km s-1. The low TR
small-scale outflows are found mostly at the
network intersection with the inter-network cells
(network boundaries). Sometimes the blue-shifts
are caused by high velocity transient events,
such as bi-directional jets, which dislocate
plasma up to 150 km s-1. However, most of the
up-flows occur on a much reduced scale and a
question remains as to whether these constitute a
steady flow or are produced as a result of
small scale transient events. For answering this
question, we analyse data-set II, in which we can
see the variation of the TR features in time.
Results

• In data-set II we see an overall oscillation of
  the network with two main periodicities 5-10 min
  and 20-30 min. We selected one position on the
  solar x axis, in which the two oscillations are
  at 10 min and 25 min (Fig. 5).
• When the amplitude of the 25 min oscillation
  is large, the 10 min oscillations inside are
  bi-directional jets (the purple and blue spectra
  in Fig. 4).
• In the low amplitude 25 min oscillations,
  the signature of bi-directional jets is not
  clear, because the red and blue wings are less
  prominent, but sometimes distinguishable (the red
  spectrum in Fig. 4).
• This is why we believe that the events from the
  lower amplitude 25 min oscillations represent
  smaller scale bi-directional jets, probably the
  same phenomena as in the chromospheric network
  from data-set I.

• For data-set I, the intensities and Doppler
  velocities (line of sight velocities, LOS) of
  both lines studyed are revealed in Fig. 2. We
  indicate the five types of observed phenomena
  with labels from (a) to (f) their spectra is
  given in Fig. 3.
• For data-set II, the variation of the
  intensity with time is given in Fig. 5, and
  spectra examples are in Fig. 4.

Fig. 5 The wavelet analysis results from the
O VI intensity variation with time, in the second
data set.
Conclusion
The new results revealed by our study are (1)
in the raster data set we see regions of
blue-shift in the CH network boundaries close to
the base of the TR, at ? 8104 K. (2) in the
time series data set, we see bi-directional jets
forming an oscillatory pattern in the network.
They might constitute the same
phenomena as seen in the raster data in the form
of small-scale outflows. From these
observations, we suggest that the fast solar wind
is unlikely to start as a steady outflow in the
TR. Instead, we see oscillating bi-directional
jets which, because of the open field structure,
could pump plasma into the corona. In quiet Sun
structures, these jets do not show a coronal
component. Simultaneous high spectral resolution
TR and coronal data is required to investigate
this idea further.
Fig. 2 Centre intensity and Doppler velocity
maps for Mg IX (with the orange CH contour and
the white lines indicating, with arrows, the
displayed cuts) and O III (with the selected
features). In order to show these cuts on the
one-dimensional plots, we rotated the images, so
now the solar limb is on the right. The '' signs
in the Mg IX velocity map indicate the locations
of bi-directional jets, as seen in the O III
line. Sides Mg IX and O III intensity (dotted
line) and Doppler velocity (blue/red symbols with
3kms-1 error bars) for the selected solar x cuts.
Orange lines indicate the QS/CH boundary. The
features identified are a bi-directional jet
(a) an inter-network cell (b) QS chromospheric
network (c) a coronal BP (d) CH network (e)
(f).