[12] Directed motions in the low corona are determined from the motions in the sky plane or along the line of sight. On their basis the total spatial velocity is derived.
[13] Prior to the epoch of space research the main data on velocities in the corona were obtained by Bugoslavskaya and Vsekhsvyatskii from white-light eclipse observations of coronal structures [Shklovskii, 1951]. A comparison of the images for different time instants allowed them to reveal relative displacements of some structures; however, their velocities were low. For instance, the expansion velocities of the envelopes enclosing ejecta from sunspots were about 10 km s -1, whereas velocities in the envelopes enclosing prominences were several km s-1.
[14] Using the observations during the eclipse of 1965, Delone and Makarova [1969] have found for the first time large line-of-sight velocities V ls (up to 148 km s-1 ) from the 6374 Å line for a coronal region above an active region (AR) using a Fabry-Perot interferometer. For 148 measured line profiles V ls the range from 5 to 148 km s-1 was found and besides for 89% of the cases V ls less than 80 km s -1 and for 11% V ls greater than 80 km s -1 were observed. From an interferogram in the green line 5303 Å obtained during the eclipse of 1968, Delone and Makarova [1975] constructed line profiles that could be frequently decomposed into components: from their shifts the velocities up to 135 km s-1 were determined. For 510 measured line profiles V ls the range from 15 to 135 km s-1 was found and besides in 74% of the cases Vls less than 80 km s-1 and approximately in 26% of the cases Vls greater than 80 km s-1 were observed. Measuring 146 profiles of the 5303 Å coronal line on the interferograms, obtained during the solar eclipse of 22 July 1990, we have found that velocities V ls ranged mainly from 40 to 80 km s-1 (67%) and that V ls was greater than 80 km s-1 (up to 140 km s-1 ) in 33% of the cases. The obtained velocities refer to different structural features of the corona in the same line of sight. Comparing the results obtained during three eclipses we conclude that in 1968 and 1990 (when there were periods of maximum of the solar activity) the solar corona was more dynamic than in 1965 when the solar activity was on the branch of the growth. We see that the part of large velocities V ls > 80 km s-1 is related to the cycle of the solar activity. Such correlation can be obtained from the comparison not only of the high-energy tail of the velocity distribution but also from analyzing the distribution for low and moderate velocities. For the 1965 eclipse the magnitudes of V ls range from 5 to 35 km s- 1 in 73% of all cases while for the 1968 eclipse the V ls are less than 35 km s-1 only in 43% of all cases. In 1980 and 1983, Raju et al. [1993] also obtained appreciable velocities. We have confirmed these results at the eclipses of 1981 and 1999 [Delone et al., 2002a, 2003a].
[15] If isolated coronal loops are projecting against the solar disk,
then, knowing from observations the Doppler velocity of the
material and using an appropriate mathematical method for
reconstructing the loop geometry from its projection
[Loughhead and Bray, 1984;
Loughhead et al., 1984a],
we can
calculate the true velocities of the mass flow along the loop.
Thus
Loughhead and Bray [1984],
Delone et al. [1989a, 1989b],
Wiik et al. [1996], and
Malherbe et al. [1997]
found from H
a observations ( T
104 K) that the
material is flowing from one foot point of the loop to the other
at a variable velocity. The velocities are from several
km s-1 to 100-150 km s-1; the velocities are minimum at
the loop top. Doppler velocities measured in the far UV from
rockets and by SOHO Coronal Diagnostic Spectrometer (CDS) are also
between 10 and 100 km s-1; typical values are
(50-60) km s-1 ( 2.5
105 K < T < 9.5 105 K)
[Brekke, 1998; Brekke et al., 1997].
[16] Present-day spacecraft capabilities allow us to obtain images and
line profiles for the studied structures. Observations of coronal
loops at a high temporal resolution have revealed their quite
dynamic structure. Bright features ("blobs)" appear suddenly in
the high corona and fall down along magnetic lines toward the
solar surface. Their acceleration is much smaller than that of
gravity, and some blobs flare as they approach to the surface. The
comparison of the time series obtained in the H
a and K Ca II
line on the Swedish vacuum telescope and on the Transition Region
and Coronal Explorer (TRACE) spacecraft in the 1600 Å band has
shown that, as a result of the breakdown of the heat equilibrium
at the loop top, a downward flow of dense, cool plasma of "blobs"
is created; the "blobs" radiate in the transition region and
chromospheric lines
[Müller et al., 2005].
De Groof et al. [2005]
proposed the same mechanism to interpret downward
flows that were observed from identical intensity variations in a
loop structure visible beyond the limb on 11 July 2001, in the
304 Å line on 120 SOHO Extreme ultraviolet Imaging Telescope
(EIT) images and in Ha on the solar telescope of the Big
Bear Observatory. For seven "blobs'' that were moving along loop
structures,
De Groof et al. [2004]
found velocities
increasing with time from 30 (at the loop top) to 120 km s-1 (at the foot point).
Müller et al. [2004]
have carried out numerical
calculations concerning plasma condensation and catastrophic
cooling in coronal loops. It is suggested that dramatic cooling
results from loss of equilibrium at the loop top as a consequence
of heating at the loop foot points rather than of a decrease in
the total loop heating. This model explains the results of
observations obtained with EIT
[De Groof et al., 2004]
and
TRACE
[Schrijver, 2001]
in Ly
a and CIV (1548 Å)
demonstrating the formation of cool clumps of material moving at
velocities higher than 100 km s-1 and deceleration of
80
30 m s-2, which is considerably smaller than the solar
surface gravity.
[17] Loop structures and their changes are closely associated with changes of photospheric magnetic carpet located at loop foot points. The magnetic carpet is formed by multiple small-scale concentrations of magnetic fluxes carried out by convective motions. Wiehr and Puschmann [2005] studied small-scale magnetic structures on two-dimensional images taken in the CH 4300 Å "G-band'' at the Swedish Solar Telescope (SST) on La Palma using a special technique that yielded a spatial resolution of 70 km on the solar surface. Wiehr and Puschmann [2005] selected about 2600 magnetic flux concentrations that looked like bright points in intergranular lanes. Among them 42% had a diameter of 150-200 km, 2.5% were smaller than 113 km, and 1.3% were larger than 290 km [Müller et al., 2004]. The magnetic flux in them, 1017 Mx, is by two orders of magnitude less than the lower limit for sunspots.
[18] As discussed by Schrijver [2005], emerging magnetic fluxes range over 6 orders of magnitude down to the smallest observable values. The lower end of this spectrum plays a fundamental role in sustaining the chromospheric network giving a noticeable contribution to the global magnetic field of the Sun.
[19] What are the properties of the atmosphere above the magnetic carpet?
[20] Earlier relations between different events occurring on the solar surface, in the chromosphere, transition region, and corona were subjects of suppositions. Only obtaining the opportunity to compare the data obtained simultaneously with TRACE and SOHO, researchers could follow such links. The observations of May 16 1998, in the framework of Joint Observation Program (JOP) 72 on MDI (Michelson Doppler Imager) and SUMER (Solar Ultraviolet Measurements of Emitted Radiation) SOHO instruments as well as on TRACE spacecraft [Tarbell et al., 2000] have demonstrated the responses of the chromosphere and transition region to changes in the photospheric network. Ubiquitous small-scale magnetic tubes can cause formation of plasma jets. Enhanced emission in the CIV lines is generally cospatial with the photospheric magnetic pattern, and the OVI line is observed 1500-2000 km from decaying magnetic features in the photosphere (and consequently from the CIV line brightenings). The velocities observed in the jets have a large range of 40-220 km s -1. If the transient radiation is accompanied by a plasma flow, the energy is distributed between these two objects. Strongly localized emission corresponds to slower jets, and vice versa. Plasma flows are always visible if the slit crosses the site of a magnetic tube or of supergranulation converging to a point.
[21] SUMER observations in UV and X-ray lines together with TRACE
images with a high spatial and temporal resolution have yielded a
large database on coronal velocities above a mixed-polarity field.
In particular,
Doyle et al. [2004]
observed a
bidirectional jet with a Doppler velocity of about
200 km s
-1. From the shift of the line profile
Ryutova and Tarbell [2000]
obtained for a similar phenomenon
250 km s-1.
The scale for
this phenomenon is 2000 km, mean life time is 200 s. During one
second above the solar surface there exist from 600 to 3300 such
jets, which appear above the mixed-polarity regions of the
photosphere.
Tarbell et al. [2000]
observed in the CIV line
a very intense jet with line-of-sight velocities
50-180 km s-1 and a bidirectional jet with a mean
line-of-sight velocity of
80 km s-1.
[22] Many works have been devoted to the study of velocities in CHs from the Doppler shifts of lines [Brosius et al., 1999; Giordano et al., 2000; Hansteen et al., 2000; Patsourakos and Vial, 2000; Peter and Judge, 1999; Peter, 1999; Warren et al., 1997]. Using N IV and Ne VIII SOHO observations, Madjarska et al. [2004] studied bidirectional jets at the boundary of the "Elephant Trunk" CH; the jets appeared there 4-5 times more frequently than in the quiet Sun. They occupied 4''--5'' along the slit, lived for 300-1000 s, and their shift in l corresponded to velocities up to 150 km s-1. The authors consider that these jets result from reconnection of topologically different (closed and open) fields at the CH boundary, which lead to a change in the CH magnetic field.
[23] Velocities similar to the above discussed ones occurred also at greater heights in the corona. Sheeley and Wang [2002] observed with SOHO Large Angle and Spectrometric Coronagraph (LASCO) in the sky plane thousands of downward flows. They are formed much more frequently in the years around solar maxima and have a 27-day recurrence, appearing at a rate of about one event per hour. Downward flows are almost ubiquitous. Some observers point out that the majority of downward flows appear at the sector boundary, where oppositely directed magnetic lines collide.
[24] TRACE observations have revealed dark features that moved sunward. The downward flows of 1996-2000 were studied by Sheeley and Wang [2001]. These flows are especially numerous when the coronal field has a four-sector structure. The downward motion rate is related to various solar activity tracers: sunspot number and occurrence of coronal mass ejections (CMEs). The majority of downward flows are associated with decaying field structures. [25] We see that bidirectional jets that are observed above the regions with mixed magnetic field are revealed over the entire disk. Downward flows are observed as frequently [Sheeley and Wang, 2002]. The derived velocities, 50-100 km s -1, agree well with the values found from interferograms; we find them virtually everywhere in the corona.
[26] The above research results in a conclusion: not only turbulent velocities increase in the regions with changing magnetic field, but the directed motions mostly take place in such regions.
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