Arc-second UV observations of the Sun by the NRL High Resolution Telescope and Spectrograph (HRTS) have led to the discovery of dynamic fine structures such as 400 km/s coronal jets and chromospheric jets (spicules) and have provided new information about the structure and dynamics of the transition zone. These observations are reviewed and their relevance to the origin of the solar wind is discussed

Dere, K. P.

English

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HRTSOBSERVATIONSOF THE FINE STRUCTURE AND DYNAMICS
OF THE SOLAR CHROMOSPHERE AND TRANSITION ZONE
Kenneth P. Dere
E. O. Hulburt Center for Space Research
Naval Research Laboratory
Washington, D.C. 20375
ABSTRACT
Arc-second UV observations of the Sun by the NRL High Resolution Telescope
and Spectrograph (HRTS) have led to the discovery of dynamic fine structures
such as 400 km s -1 coronal jets and chromospheric jets (spicules) and have
provided new information about the structure and dynamics of the transition
zone. These observations are reviewed and their relevance to the origin of the
solar wind is discussed.
Introduction
Recent results derived from UV observations of the solar chromosphere and
transition zone made by the NRL High Resolution Telescope and Spectrograph (HRTS)
are presented here. These observations have an inherent interest since they
are made with the highest spatial resolution yet obtained in the UV and show the
first UV observations of solar fine structure. They are also of specific
interest to this conference since the solar wind and many of its disturbances
presumably originate somewhere near the solar surface. The HRTS instrument
consists of a Cassegrain telescope which focuses a solar image with i arc-sec.
resolution onto the slit jaw of the spectrograph which produces stigmatic photo-
graphic spectra of the 1000 arc-sec, long slit in the 1175-1710 A wavelength
range. To date, there have been four rocket flights of the HRTS instrument
(referred to as HRTS 1-4) and it is scheduled to be flown on Spacelab 2.
The Chromosphere
Figure 1 shows some of the general properties of the quiet Sun in the HRTS
spectra. On the left is a spectroheliogram obtained in H_ by Sacramento Peak
Observatory essentially simultaneously with the HRTS spectra obtained during
the third flight. The Hm image consists of dark absorption features, the dark
mottles, which outline the supergranulation cells. Also indicated is the
position of the HRTS slit and the corresponding stigmatic spectra in lines of
C I (104 K), Si II (2 x 104 K) and C IV (105 K). All three ions show intense
emission where the slit crosses the supergranular cell boundaries although the
C IV emission appears in some cases to be shifted towards the limb. This is
consistent with limb brightening curves which show C IV to be produced 4 arc-
sec. above the photosphere. An absolute wavelength scale can be determined
from the narrow Si I lines in the vicinity with a accuracy of about 2 km s-1.
Examples of small scale structures observed in the HRTS spectra are shown
in Figure 2. Here, a short segment of HRTS i C IV and C I spectra near the
limb are displayed. At the top of the C IV spectra is a coronal jet with line
33
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¢n o o
Nil
U m
-- <m
,0 ii
, i t I
, ii
HRTS
SLIT
0'1 01 01 01 01
I_ f,.,O .1_ 01 0')
01 ¢,_ O0 0 0
Figure I. HRTS spectra of Si II, C IV, C I and an Ha spectro-
heliogram at the limb of the solar south pole.
of sight velocities of 200 km s-I and projected radial velocities of 400 km s-1.
Shifts to the left are blueshifts and are caused by plasma moving toward the
observer or away from the Sun. Near the bottom of that figure is a "turbulent"
event characterized by wide, symmetric C IV profiles which are produced by random
nonthermal velocities on the order of 150 km s-1. In the C I spectra, at least
six chromospheric jets are seen as emission features in the line wings which are
caused by plasma moving at 10-20 km s-I. It is interesting to note that the
coronal jet and turbulent event seen in C IV have no noticeable cospatial signa-
ture in the C I profiles and the chromospheric jets seen in C I both occur where
the C IV intensity is quite low. This is true for the particular events seen in
Figure 2 and in general.
34
CORONAL
JET
tO orc-sec
TURBULENT
EVENT
CIE CI
))!
I I II I
o dd.-
n
n
m
CHROMOSPHERIC
JETS
(SPICULES)
Figure 2. Examples of fine structures observed in HRTS spectra:
coronal jets, turbulent events and chromospheric jets
(spicules).
Spicules, which play a important role in the convective transport in the
solar atmosphere, have been seen in visible light above the solar limb for
over a hundred years. They are slender _ets of gas which are observed to rise
and often fall at a velocity of 25 km s -I. However, there has never been an
identification of spicules with any known phenomenon on the disk although it is
often assumed that they are the dark H_ mottles. We now believe there is
sufficient evidence to identify the chromospheric jets seen in Figure 2 as
spicules on the disk (Dere, Bartoe and Brueckner, 1983).
Typically the chromospheric jets (spicules) have a size of 1-2 arc-sec.
projected along the sllt and twice as many blue shifted (rising) events are
seen as the weaker redshifted (falling) events. It is possible that with greater
sensitivity we would see equal numbers of the two. The chromospheri_ jets can
be seen in other lines of C I such as the allowed multiplet at 1657 A and
the intercombination lines near 1613 _ as well as in lines of other chromo-
spheric ions such as Si I, Si II and S I. However, they are not seen in
lines of somewhat hotter ions such as C II or Si III and as we have previously
noted, C IV is often extremely weak at the sites of chromospheric jets. Clearly
they are exclusively a chromospheric phenomenDn. Spicule profiles of the C I
1560 A lines are shown in Figure 3 as well as nearby nonspicular profiles fo E
comparison. The spicular emission peak is shifted to the blue by .05 to .i A
35
25
0
•043 A
0
.055 A
A B D F
0.0
1560.0 1560.5 1561.0 1561.5
0
WAVELENGTH (A)
Figure 3. The profiles of the C I lines near 1560 A in a spicule.
The arrows indicate the observed average wavelength of
the three lines (blends) made up of the five C I lines
labelled A through F.
corresponding to line of sight velocities of I0 to 20 km s -1 and shows no self
reversal as do more typical nonspicular profiles. From the intensity and lack
of self absorption in these profiles a temperature of 1.6 x 104 K and density
of I0 II cm -3 are derived. These are comparable to what is found from visible
light observations.
Many observers have come to believe that the dark H_ mottles observed on the
disk should be identified with the spicules seen at the limb although a study of
H_ mottle profiles (Grossmann Doerth and v. Uexkull, 1971) shows that the
velocities in the mottles are significantly lower than those in spicules. This
same study did find region of 20 km s -I flows but only in "quite unconspicuous,
little gray features" which were not considered important. Our own observations
confirm the low velocities in the supergranular cell boundaries where the mottles
are found. The chromospheric jets (spicules), while not as bright as the network
emission, are nevertheless quite conspicuous in the ultraviolet spectra. The
chromospheric jets are generally unrelated to the dark Hm mottles.
In conclusion, we confirm many of the known physical properties of spicules
derived from ground-based observations as is to be expected. The surprising
new result is that spicules occur in the supergranulation cell interiors and
not at the boundaries where the strongest magnetic fields tend to be clustered.
36
The Transition Zone
o
The two strong C IV lines near 1550 A are our primary diagnostics for
the transition zone. Both lines are formed around 105 K and are optically thin
on the disk. The regions viewed by the HRTS 3 slit include the limb above a
polar coronal hole which extended up to a latitude of 50 ° , mostly quiet Sun and
a filament near Sun center. We quantify the information in the profiles by
taking moments of the wavelength weighted by the specific intensity. In this
way we derive the integrated line intensity, the net Doppler velocity and the
line width. These three quantities are shown in Figure 4 for HRTS 3 data along
the entire slit. Some of the more intense regions, such as the limb, are over-
0.030 - HRTS-3 lOX3 C IV 1549.000
1.0
.
- 20.
t"N
O0
- 10. v
k.,/
>-.
I--
H
- 0. o0
_J
W
>
- -10.
..... 1 .......... 1........... l......... 1.......... [ ....... l ........ 1....... L...............!........... l
100. 200. 300. 480. 500. 600. 780. 800. 900. 1000.
ARC-SECONDS
Figure 4. Intensity (er_ cm -2 s -I st-l), velocity (km s-l) and
line width (A_) for C IV A1548 and A1550 as a
function of position along the slit.
exposed and are not plotted. The limb defined by the continuum is near an
indicated height of 17 arc-sec. Again, the regions of high C IV intensity
generally pick out the chromospheric network although there is no strong point
for point correlation with C I intensities which also pick out the network.
In fact, from their respective autocorrelation coefficients for line intensity,
the C IV transition zone structures have a typical size of 1.5 arc-sec compared
37
to 8 arc-sec for the C I chromospheric structures. Preliminary two-dimensional
images in C I and C IV show this large scale relationship (the network) between
the two but show little similarity in the small scale features.
The llne of sight velocities displayed in Figure 4 are predominately away
from the observer and from this one can conclude that the transition zone
consists of material generally falling toward the solar surface. The velocity
data, as summarized in Figure 58 do show some regions of upflows but the
average Doppler shift is 0.028 A which corresponds to a downflow a 5.4 km s -I
I I I I I I I I I
I
I I I I I I I I
FREQUENCY (%)
I
I
o4 o
m
,1-
g.i
.,.i
O.
o a.
¢50
¢3
Figure 5. Relative frequency of occurrence of the C IV
Doppler shifts.
at disk center. We emphasize that our velocity scale is relative to the nar-
row chromospheric Si I lines and should be accurate to 0.01A or 2 km s -I.
Above the limb redshifts are also seen. An examination of these C IV llne
profiles shows that their peak is near the rest wavelength and the derived
redshlft is due to the greater strength of the red wing as compared with the
blue. As we noted, a polar hole extended up to a latitude of about 50 °
during the HRTS 3 flight. This latitude corresponds to a relative position of
240 arc-sec on Figure 4. There is no sign of major large scale outflows in
the transition zone of this polar coronal hole. Using a wavelength scale
referenced to chromospheric lines, Doschek, Feldman and Bohlin (1976) also
find little evidence for outflows in coronal holes. Rottman and Orrall (]982)
show a relative blueshift between coronal holes and other solar regions but in
the light of the above results, it is not clear that this can be interpreted as
an absolute blueshift in coronal holes.
38
Previous studies of the transition zone at lower resolution (Lites et
al. 1976 and Gebbie et al. 1981) have found that regions of downflow are --
correlated with regions of high intensity. However our own data show no real
correlation between these two quantities as presented in Figure 6. In a large
4 5 _- 18X3
C IV
4 18-- 1549. 1888
]
28.
Figure 6. A scattergram of C IV intensity versus C IV
velocity near Sun center.
scale sense, there are certainly some bright regions which show strong down-
flows but this relationship breaks down in general and when these regions are
examined in detail. An autocorrelation analysis also shows that the size of
the bright C IV regions is around 1.5 arc-sec, and the 3 arc-sec, size of the
flowing structures is significantly larger.
The line width data shown in Figure 4 reveal a number of wide profile
events along the slit which consist of both the jets and the turbulent
events, examples of which were displayed in Figure 2. Brueckner and Bartoe
(1983) provide a detailed anlysis of the jets and turbulent events. Even the
more typical profiles have a line width which requires turbulent or nonthermal
39
velocities on the order of 21 km s -I together with thermal broadening to
explain the observed width. The rise in the line width toward the limb is due
to the integration over an increasingly larger number of flow fields.
Turbulent velocities in the turbulent events range from 50 km s -I to
250 km s -I. Most of these events occur only once but some of the larger ones
are observed to reoccur at the same position in our 4 minute observing time.
Size scales are often at the instrumental resolution (i arc-sec.). They are
seen in lines spanning the temperature range 2 x 104 to 2 x 105 K although ap-
parently similar events have also been observed in coronal (2 x 106 K) lines in
the Skylab data.
A time history of one of the more spectacular jets is shown in Figure 7.
0 200400Q00 ken eec "*
i • I , I a I
I I I I I J
0 I0 20 30 40 50 60 sec
I I I I i I
70 80 90 I00 |10 120 130 sic
i i i i I I I
140 150 160 17'0 180 190 200 sec
I I I i I I I
210 220 230 240 250 260 270 sec
Figure 7. A time sequence of coronal Jets observed in C IV.
This particular jet, seen as the strongly blue shifted emission in C IV _1548,
appears four times at the same location during a four minute rocket flight.
The maximum velocities which reach 400 km s-I are consistent with an
acceleration of 5 km s -2 that persists throughout the observed lifetime. The
Jets contain material over a wide range of temperatures (2 x 104 - 2 x 105 K).
One interpretation is that of a magnetic loop being expelled outward through
the solar atmosphere. The H_ slit jaw pictures do not show any feature that
might correspond to the Jets.
40
The time sequence of C IV spectra in Figure 8 shows numerous examples of a
less spectacular but more prevalent sort of jet. These have maximum velocities
of from 80 to 150 km s-I and often have both blueshifted and redshifted com-
ponents. Their size is generally near the instrumental resolution.
N
, I_ "_.I"!r_.
t
i
|
Figure 8.
40 80 120 160 200 [sec]
A time sequence of C IV spectra showing numerous
i00 km s-I jets.
41
Origin Of The Solar Wind
The solar wind has an associated mass flux of 2 x 1012 g s-I which must
be present at chromospheric and transition zone levels and should be observable
with suitable instrumentation. In the chromospherethe only significant
vertical flows that we observe are in the spicules; about 2/3 are outflows and
i/3 downflows (since the downflows are less intense, it is possible that there
are equal numbersof each). Roughly only one in 10 spicules produces a
measurable blueshlft in the transition zone. From a density of 1010 cm-3, a
velocity of 25 kms-1 and a fill factor of 2%, the average outward mass loss
in spicules observed at transition zone temperatures is 2 x 1013 g s-I, an
order of magnitude above solar wind requirements. The downward mass flow in
the transition zone for a pressure neT e = I015cm -3 K and a velocity of
5 km s-1 is 5 x 1014 g s-I. This latter figure assumes a fill factor of
unity which rests on the interpretation of the transition zone as an extremely
thin (20 km) layer. Since most high resolution spectroheliograms of the upper
chromosphere and corona show the ubiquitous presence of filamentary structures,
it is likely that the transition zone has a similar consistency. In that case,
fill factors of 0.i can be derived and the downward transition zone mass flow
is reduced to 5 x 1013 g s -I still an order of magnitude above the solar wind
value.
The are two ways to consider these order of magnitude mass flow estimates.
First, the origin of the solar wind can be explained with only small changes
in these very rough numbers, so that the transition zone downflows return the
spicular mass not needed to sustain the solar wind. The second approach is to
say that the spicule upflows and transition region downflows constitute a
closed circulation system with no mass flux available for the solar wind. It
is perhaps even more likely that the spicules themselves produce no net mass
transport. In this case we must look elsewhere. The HRTS data suggest the
jets as likely candidates (Brueckner, Bartoe and VanHoosler, 1977).
An upper limit on the mass in one of the numerous 100 km s -I jets is about
3 x 108 g. The birthrate for these jets is 170 s -I on the solar surface and
the consequent mass loss is about 5 x i0 I0 g s -I which is much less than that
in the solar wind. The mass content in the large 400 km s -I jet seen in
Figure 8 is estimated to be 2 x 1011 g. During three rocket flights, a total
of six large jets have been observed. In the third flight which had the
greatest spatial coverage_ only one of the large jets was observed and a rate
of 4 jets per second on the entire solar surface was derived. Observatlons of
jet-llke structures in C IV spectroheliograms above the limb indicate that on
the entire solar surface, _ets occur at the rate of 5 s -1. Thus the mass loss
due to the large 400 km s -i jets is 1012 g s -l, which is quite close to the
mass loss of the solar wind. The kinetic energy released in these jets is
3 x 1028 erg s-I which is higher than the total energy flux in the solar wind
at the Sun. Thus, there is sufficient mass and energy in these high speed
jets to supply the solar wind. Such a non-steady source would imply highly
variable solar wind parameters, especially near the Sun.
References
Brueckner, G. E., A High Resolution View of the Solar Chromosphere and Corona,
Highlights of Astronomy, 5, 557 1980.
42
Brueckner, G. E. and J.-D. F. Bartoe, Observations of High Energy Jets in the
Corona and the Acceleration of the Solar Wind, Ap. J., in press, 1983.
Brueckner, G. E., J.-D. F. Bartoe and M. E. VanHoosier, High Spatial
Resolution Observations of the Solar EUV Spectrum, Proc. of the OS0-8
Workshop, 1977.
Dere, K. P., J.-D. F. Bartoe and G. E. Brueckner, Chromospheric Jets: Possible
EUV Observations of Spicules, Ap. J. (Letters), in press, 1983.
Doschek, G.A., U. Feldman and J. D. Bohlin, Doppler Wavelength Shifts of
Transition Zone Lines Measured in Skylab Solar Spectra, Ap. J. (Letters),
205, L177, 1976.
Gebbie, K. B. et al., Steady Flows in the Solar Transition Region Observed with
SMM, Ap. J. (Letters), 251, L115, 1981.
Grossmann-Doerth, U. and M. v. Uexkull, Spectral Investigations of
Chromospheric Fine Structure, Solar Phys., 20, 31, 1971.
Lites, B. W., E. C. Bruner, E. G., Chipman, G. A. Shine, G. J. Rottman, O. R.
White, and R. G. Athay, Preliminary Results from the Orbiting Solar
Observatory 8: Persistent Velocity Fields in the Chromosphere and Transltion
Region, Ap. J. (Letters), 210, LI1, 1976.
Rottman, G. J. and F. Q. Orrall, Observatlonal Evidence for Solar Wind
Acceleration at the Base of Coronal Holes, This conference, 1982.
43


HRTS observations of the fine structure and dynamics of the solar chromosphere and transition zone

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