A&A 406, 725-734 (2003)
DOI: 10.1051/0004-6361:20030749
Z. E. Musielak 1,2 - P. Ulmschneider 2
1 - Department of Physics, University of Texas
at Arlington, Arlington, TX 76019, USA
2 -
Institut für Theoretische
Astrophysik der Universität
Heidelberg, Tiergartenstr. 15,
69121 Heidelberg, Germany
Received 20 January 2003 / Accepted 13 May 2003
Abstract
The response of an exponentially diverging magnetic flux tube
embedded in an isothermal solar atmosphere to the propagation
of transverse tube waves and random transverse pulses generated in the
solar convection zone is studied analytically. General solutions
are presented and applied to solar flux tubes located in the
interior region and at the boundary of supergranulation cells.
It is shown that the period of the free
oscillations driven by transverse waves and pulses ranges from 7 to 10 min for the considered values of the tube magnetic field,
and that these oscillations decay in time as t-3/2. Since
the observational signatures of these transverse oscillations
are hard to detect, we also consider the generation of longitudinal
tube waves by nonlinear mode coupling and the excitation of
free atmospheric oscillations by longitudinal waves. Our results show
that the basic properties of oscillations driven by transverse
and longitudinal tube waves are different. While transverse waves
excite oscillations with 7-10 min periods, oscillations by
longitudinal waves have periods
near 3 min. This is consistent with the observed 3-min
oscillations inside the supergranule cells but inconsistent with
the 7-min oscillations observed in the chromospheric network. We
suggest that an explanation of the observed 7-min oscillations might
be found by taking into account a more realistic structure of
flux tubes located in the magnetic network.
Key words: Sun: photosphere - Sun: chromosphere - Sun: oscillations - MHD - waves
In the first paper of this series (Musielak & Ulmschneider 2003, Paper I), we have investigated the excitation of free and forced atmospheric oscillations inside solar magnetic flux tubes by longitudinal tube waves and random pulses. The main obtained results can be summarized as follows. The free atmospheric oscillations with periods near 3 min are always present, independent of the form of the initial disturbance that caused them, and they decay in time as t-3/2 if the frequency of the driving waves is not equal to the cutoff frequency for longitudinal tube waves. The forced atmospheric oscillations represent either propagating or evanescent longitudinal tube waves in an isothermal atmosphere, and they do not decay in time if the wave source drives them continuously. Finally, in the case when the wave frequency is exactly equal to the cutoff frequency, both the free and forced oscillations are the same and they do not decay in time.
Similar results but for acoustic waves have been obtained by Fleck & Schmitz (1993), Kalkofen et al. (1994), Schmitz & Fleck (1995) and Sutmann et al. (1998), and used by these authors to explain the origin of 3 min oscillations observed in the solar chromosphere (e.g., Deubner 1991; Rutten & Uitenbroek1991). These 3 min oscillations are observed in the interior of the supergranulation cells and they can be explained both by processes inside intracell magnetic flux tubes (see Paper I) and outside such structures, where the oscillations are driven by the propagating acoustic waves generated in the solar convection zone (Fleck & Schmitz 1991).
The results obtained in Paper I are also valid for exponentially diverging magnetic flux tubes in the chromospheric network. However, the existence of these tubes at the boundary of supergranulation cells is limited to appropriate regions of the network where enough space is available for the tubes to spread exponentially with height. For such flux tubes, the results of Paper I demonstrate that the propagating longitudinal tube waves and random pulses excite only 3 min oscillations. This is inconsistent with 7 min oscillations observed in the magnetic network (Damé 1983; Lites et al. 1993; Curdt & Heinzel 1998); the periods of these oscillations range from 6 to 15 min and no power is observed at 3 min (e.g., Kalkofen 1996). Therefore, the 7-min oscillations must be of different nature and their origin cannot be explained by the oscillations discussed in Paper I.
Possible explanations of the 7-min oscillations observed in the chromospheric network have been proposed by Deubner & Fleck (1990), who considered formation of standing waves by internal gravity waves, and by Hasan & Kalkofen (1999), who suggested that transverse tube waves generated through buffeting by granules would excite these oscillations. Since the free atmospheric oscillations driven by transverse waves and pulses cannot be directly observed (e.g., Kalkofen 1997), Hasan & Kalkofen suggested that the observed 7-min oscillations are actually excited by longitudinal tube waves generated by transverse tube waves through the process of nonlinear mode coupling (e.g., Ulmschneider et al. 1991). To reach this conclusion, these authors had to asssume that the period of the driven atmospheric oscillations will be preserved after the wave transformation takes place. In this paper, we explore the validity of this assumption after deriving general analytical solutions for the excitation of the free atmospheric oscillations by transverse tube waves and random transverse pulses.
We consider a magnetic flux tube embedded in otherwise non-magnetic and isothermal atmosphere, and assume that this tube is oriented vertically and its magnetic field spreads exponentially with height. In general, the tube supports three different types of waves, namely, longitudinal, transverse, and torsional (e.g., Spruit 1981), and the external medium supports the propagation of acoustic waves. For each wave the corresponding cutoff frequency can be defined, so we have: the longitudinal (Defouw 1976), transverse (Spruit 1982), torsional (Noble et al. 2003), and acoustic (Lamb 1908) cutoff frequency. The tube and the external medium will oscillate with these cutoffs when driven by the corresponding wave motion. The fact that acoustic waves and random acoustic pulses freely propagating in the external atmosphere excite free atmospheric oscillations with the acoustic cutoff frequency has been already well-established (e.g., Fleck & Schmitz 1991; Sutmann et al. 1998). In addition, in Paper I we studied the excitation of free atmospheric oscillations inside the tube by freely propagating longitudinal tube waves and random longitudinal pulses (see also Rae & Roberts 1982). In this paper, we investigate the excitation of tube oscillations with the transverse cutoff frequency by freely propagating transverse tube waves and random transverse pulses (see Spruit & Roberts 1983, and Hasan & Kalkofen 1999, for previous work). Finally, similar studies for torsional oscillations will be described in the next paper of this series.
As mentioned above, the main aim of this paper is to study the excitation of atmospheric oscillations by transverse tube waves and random transverse pulses, which are assumed to be generated in the solar convection zone by turbulent motions interacting with the tube; the most recently computed wave energy spectra carried by transverse tube waves (Musielak & Ulmschneider 2001) are used in our calculations. The presented approach is valid only for exponentially spreading flux tubes, which means that the obtained results can be applied to intracell flux tubes located in the interior of the supergranulation cells and also to those tubes in the magnetic network that can spread exponentially with height. To derive the cutoff frequency for transverse tube waves, we cast the wave equation into a Klein-Gordon form (Sect. 2) and solve it by using a Laplace transformation (Sect. 3). The derived analytical solutions are then applied to solar magnetic flux tubes (Sect. 4). Nonlinear mode coupling is discussed in Sect. 5 and our final conclusions are given in Sect. 6.
A magnetic flux tube is considered to be isolated and embedded
in a magnetic field-free, compressible and isothermal medium.
The tube is assumed to be thin, untwisted, and oriented vertically,
with circular cross-section, and in temperature equilibrium with
its surroundings. To describe transverse waves propagating along
this tube, we introduce a Cartesian coordinate system with the
z-axis being the axis of the non-oscillating tube and the gravity
,
where
is the unit
vector along the z-axis. We also consider a local cylindrical
coordinate system
within the tube, with
being the vector along the tube (Spruit 1981).
The tube magnetic field
is exponentially spreading with height and can be locally expressed
as
,
where
is the unit vector along the tube; note that
for the non-oscillating tube
.
Since
,
the horizontal pressure balance is
,
where
and
is the gas pressure inside and outside the tube. In order to
distinguish the physical parameters inside and outside the tube, we
introduce subscripts "o'' and "e'' to denote the internal and external
parameters, respectively. Solar observations show that typical magnetic
field inside the tube is
G at
(e.g., Solanki 1993; Stenflo 1994), which is approximately
,
where
is
the equipartition field. Our model of the exponentially diverging flux
tube is valid for isolated intracell fields, for which Wang et al. (1995)
found typical magnetic fluxes of
Mx. Assuming tube radii
of 40 km (Solanki 1993) at the solar surface (
)
this
would imply that the field strength would be similar to that observed in
the magnetic network. However, our model is not ideal for crowded flux
tubes in the chromospheric network because the exponential expansion of
these tubes is prevented by neighboring flux tubes.
We assume that transverse tube waves are excited by the external
turbulence alone and that there are no other motions outside or inside
the tube. The generated waves are fully described by the perturbations
of the tube velocity,
,
and the magnetic field,
;
we restrict our consideration to the x-direction only as there is no
physical distinction between the x and y directions. Note also
that our approach is restricted to linear waves, so that both the
density and pressure perturbations can be neglected; this
approximation is good for the considered waves in the region of
their generation, however, the waves may become nonlinear in higher
atmospheric layers due to increasing wave amplitudes with height.
The total magnetic field in the Cartesian coordinate system is given
by
,
with
.
To derive the wave equation for the velocity perturbation, we linearize
the basic MHD equations, use
and
,
apply the thin flux tube approximation (Musielak
& Ulmschneider 2001), and obtain
![]() |
(1) |
![]() |
(2) |
To remove the first derivative from Eq. (1), we use
and obtain
![]() |
(3) |
![]() |
(4) |
The characteristic speed
for transverse tube waves can be
expressed in terms of the sound speed
and plasma
,
where
,
as
![]() |
(5) |
![]() |
(6) |
Similar relationships can be found for the cutoff frequency
,
which can be expressed in terms of the acoustic cutoff frequency
(Lamb 1908) and plasma
as
![]() |
(7) |
![]() |
(8) |
The obtained Klein-Gordon equation can be solved by specifying
the initial and boundary conditions. We follow Sutmann et al.
(1998) and take
![]() |
(9) |
![]() |
(10) |
![]() |
(11) |
![]() |
(12) |
We assume that monochromatic transverse tube waves with frequency
and amplitude
,
is continuously generated by the
external turbulent motions (see Sect. 4) and they propagate along
the flux tube. The boundary condition (Eq. (10)) that describes this
process is
![]() |
(13) |
v (t, z) | = | ![]() |
|
![]() |
|||
![]() |
(14) |
The spectrum of transverse tube waves generated at the lower tube
boundary (z=0) is approximated here by a linear superposition
of sinusoidal partial waves with different amplitudes, frequencies
and phases. The initial velocity
required for the
boundary condition given by Eq. (10) can be specified as
![]() |
(15) |
v (t, z) | = | ![]() |
|
![]() |
|||
![]() |
(16) |
We now consider one transverse pulse with a -function
shape to be generated at z=0. The required boundary condition
is
![]() |
(17) |
We follow Sutmann et al. (1998) to obtain
![]() |
(18) |
Finally, we assume that a wavetrain of transerse and sinusoidal
pulses with randomly chosen amplitudes and periods is generated
at the lower boundary of the tube (z=0) and propagates upward
along the tube. A new pulse is stochastically chosen after the
time equal to the passed wave period of the previous pulse. The
boundary condition requires
to be given in the
following form:
![]() |
(19) |
![]() |
(20) |
![]() |
(21) |
![]() |
(22) |
![]() |
(23) |
![]() |
(24) |
Similar to the previous case, the wavetrain of random pulses generates only the free atmospheric oscillations because the excitation is not continuous but instead it is stopped at the time t > tN; the latter means that only N pulses are generated. The derived analytical solution shows that these oscillations also decay in time as t-3/2.
We compute the structure of the solar atmosphere outside the
magnetic flux tube by taking gravity
cm/s2, and assuming that the atmosphere is isothermal with
temperature
K and extends from z=0 to z=2000 km. The gas pressure outside the tube at z=0 is
assumed to be
dyn/cm2, which
approximately corresponds to
K in the VAL model (Vernazza et al. 1981). The atmosphere inside the tube
is also isothermal with
.
We specify
the tube magnetic field to be
at
(see Sect. 2) and use the horizontal
pressure balance to calculate the distribution of physical
parameters inside the tube with depth and height. Since the
value of the tube magnetic field may vary for flux tubes on
the Sun (e.g., Solanki 1993), we also consider
and 0.75 (Ulmschneider & Musielak 1998).
The characteristic tube speed
and the cutoff frequency
are computed for each value of the tube magnetic
field and given in Table 1. To compare these values with the sound
speed
and the acoustic cutoff frequency
,
we take
and get
km s-1 and
s-1. Hence, the speed of transverse tube waves
is either lower than, or comparable to, the sound speed and this is
consistent with Eq. (6) as
,
0.85,
and 0.95 correspond respectively to
,
1500,
and 1675 G, or
,
0.4, and 0.1 at
.
The values of
is always lower than
(see Eq. (7)) and changes with the strength of the tube magnetic field
(Table 1).
Note that ,
,
and
are constant in our model because the background atmosphere is isothermal
and the tube expands exponentially with height. The cutoff frequencies
that are the same in the entire atmospheric model are often referred to
as the "global" cutoff frequencies to distinct them from the so-called
"local'' cutoff frequencies which vary with height due to the presence
of other gradients, for example, the temperature gradient (e.g., Brown
et al. 1986); obviously, all cutoff frequencies discussed here and in
Paper I are the global cutoff frequencies. In the following, we present
the results of our calculations by plotting the real part of the normalized
wave velocity
with time for the free atmospheric
oscillation at two different atmospheric heights z=500 and 2000 km;
in all our calculations the conditions
and
are always fulfilled.
![]() ![]() |
![]() |
![]() |
![]() |
![]() |
0.75 | 5.0 | 0.0107 | 0.0152 |
![]() |
0.85 | 6.0 | 0.0129 | 0.0183 |
![]() |
0.95 | 7.3 | 0.0155 | 0.0220 |
![]() |
The interaction between a thin and vertically oriented magnetic flux tube and the external turbulent motions has been studied analytically by Musielak & Ulmschneider (2001), who developed a general theory describing the generation of transverse tube waves and used to compute the wave energy spectra and fluxes for the Sun and late-type dwarfs (Musielak & Ulmschneider 2002a,b). Similar to the generation of longitudinal tube waves discussed in Paper I, this theory is also based on the original work done by Lighthill (1952) in which the inhomogeneous wave equation is derived and the source function is assumed to be fully determined by the turbulent motions. To prescribe the source function, one typically uses an extended form of the Kolgomorov turbulent energy spectrum and a modified Gaussian frequency factor given by Musielak et al. (1994); both forms have been adopted by Musielak et al. (1995) to study the excitation of longitudinal tube waves, by Rubinstein & Zhou (2002) to investigate the generation of acoustic waves, and by Bi & Xu (2002) to examine the efffects of turbulence on the solar p-mode oscillations.
We follow Musielak & Ulmschneider (2002a) to calculate the
wave energy spectra and fluxes for the Sun by taking the
solar gravity (see above),
K, the
solar metallicity (
), the mixing-length
parameter
which is assumed to be 2 in all our
calculations, and the following three values of the tube
magnetic field:
,
0.85 and 0.95. The obtained wave energy spectra are shown in Fig. 1.
It is clearly seen that the amount of generated wave energy
decreases with increasing magnetic field and that the frequency
of the maximum of each spectrum is located
relatively close to the cutoff frequency
.
Since
is always less than
(see Table 1),
the generated transverse tube waves are always propagating
waves. The total wave energy fluxes carried by these waves
along solar magnetic flux tubes are computed by intergrating
the spectra over frequency and the obtained fluxes are given
in Table 1. In the following, the fluxes and spectra are used
to study the excitation of atmospheric oscillations inside
magnetic flux tubes by monochromatic transverse tube waves
and a spectrum of these waves, respectively.
![]() |
Figure 1:
Transverse wave energy fluxes computed for three
different values of the tube magnetic field: ![]() ![]() ![]() |
To investigate the excitation of the free atmospheric oscillations
by monochromatic transverse tube waves, we calculate the
initial amplitude
,
where
is the total wave energy flux
carried by transverse tube waves (see Table 1). Since we consider
monochromatic waves, we assume that
,
where
is the maximum frequency of the computed
wave energy spectra (see Fig. 1) and is given in Table 1. The time
evolution of the wave velocity v(t,z) normalized by
(see Eq. (14)) is calculated at the atmospheric height z=500 km,
which corresponds to t = 832 s, and z=2000 km, which corresponds
to t = 3326 s, and plotted in Fig. 2. The presented results clearly
show that the amplitude of these oscillations is small when compared
to the tube speed
and quickly decreases with time as t-3/2. A small increase of the amplitude with decreasing tube
magnetic field is consistent with changes of
given in
Table 1. Since
is larger than the total longitudinal
wave energy flux
(see Paper I), the normalized amplitude
of the free atmospheric oscillations caused by
transverse tube waves is approximately one order of magnitude larger
than the amplitude
for longitudinal tube waves.
In addition, the frequency
of the oscillations
excited by transverse tube waves is approximately two times lower
than
,
which is the cutoff frequency for
longitudinal tube waves (see Paper I); this can be clearly seen
when Fig. 2 of Paper I and Fig. 2 of this paper are compared.
According to the results given in Table 1,
increases by a factor of 1.2 with the increasing magnetic field,
which means that the oscillations inside magnetic flux tubes are
of higher frequency for stronger magnetic fields.
![]() |
Figure 3:
Time evolution of the initial wave velocity
![]() ![]() ![]() ![]() |
The assumption that the total wave energy flux is carried by
monochromatic waves is not realistic as the turbulent motions
in the solar convection zone generate a full spectrum of waves
with different frequencies and amplitudes. Because of the nature
of these motions, the amplitudes vary rapidly in time and for a
very brief period of time they can be as high as 0.4
(see Fig. 3). The time evolution of
shown in Fig. 3
is computed by identifying the frequency
(see
Eq. (15)) of each partial wave with 32 frequency points used in
calculations of the wave energy spectra presented in Fig. 1,
and by taking
,
where Fn is the wave energy flux carried
by the waves with
.
Since all partial waves are
propagating (
), we assume that
the phase
is the same for all these waves.
The results shown in Fig. 3 represent the spectrum of transverse
tube waves propagating along magnetic flux tubes with
/
(see Fig. 1). The velocity fluctuations
obtained for
/
and 0.95 look
very similar except that some spikes are even higher for the
former and lower for the latter.
![]() |
Figure 4:
Time evolution of the free atmospheric oscillations
inside solar magnetic flux tubes with ![]() ![]() ![]() |
According to Eq. (16), the spectrum of transverse tube waves
propagating along magnetic flux tubes excite the free atmospheric
oscillations with the cutoff frequency
.
We
calculate the time evolution of these oscillations for all three
cases of the tube magnetic field, and plot the real part of
Eq. (16) for
/
and 0.95 in
Figs. 4 and 5, respectively. It is clearly seen that that the
oscillations driven by these two spectra decay in time as t-3/2 and that their amplitudes are approximately twice
as high as those shown in Fig. 2 for monochromatic waves. By
comparing the results shown in Figs. 4 and 5, one finds that
the amplitude of these oscillations is very similar for the
two considered values of the tube magnetic field. This is
rather surprising result as the initial wave energy flux is
higher for
/
than for 0.95
(see Fig. 1); the difference is caused by the fact that
flux tubes with stronger magnetic fields are more difficult
to shake (see Musielak & Ulmschneider 2001, for details).
As a result of this difference in the wave energy fluxes,
the amplitude of the oscillations shown in Fig. 4 should be
higher than those presented in Fig. 5.
To explain this apparent discrepancy, we checked the time
required for transverse tube waves to reach the atmospheric height z=500 km (z=2000 km) in both cases. We found that this time
was 832 s (t = 3326 s) and 690 s (t = 2758 s) for the waves
propagating along the tube with /
and 0.95, respectively. Based on Eq. (16), one sees that
the longer time reduces the amplitude, however, the shorter time
is responsible for its increase; hence, it is a pure coincidence
that the amplitudes of the oscillations shown in Figs. 4 and 5
look so similar.
![]() |
Figure 5:
The same as Fig. 4 but for ![]() ![]() |
The frequency
of the free atmospheric oscillations
shown in Figs. 4 and 5 is different because of different strength of
the tube magnetic field (see Table 1). For the oscillations presented
in Fig. 4, we have
s-1, which gives
min; however, for the oscillations of Fig. 5, we find
s-1, which gives
min.
In both cases, the period of these oscillations is consistent with
the observations mentioned in Sect. 1. This agreement has led Kalkofen
(1997) and Hasan & Kalkofen (1999) to conclude that the free atmospheric oscillations observed in the chromospheric network are driven by transverse
tube waves propagating along solar magnetic flux tubes. Since these waves
are continuously generated in the solar convection zone (Musielak &
Ulmschneider 2001), in principle they could excite the observed
oscillations; however, see our discussion in Sect. 5.
Finally, we consider a wavetrain of transverse pulses that have
random amplitudes and frequencies. The wavetrain is assumed to
be finite with the number of pulses determined by the condition t > tN. A new pulse with randomly chosen amplitude and
period is introduced after the passed time becomes equal to the
period of the previous pulse. For all pulses the condition
must be satisfied, which means
only propagating pulses are considered. The source of these pulses
can be rapid horizontal motions existing at the top of the solar
convection zone and in the solar photosphere (e.g., Muller et al.
1994; Solanki et al. 1996; Steiner et al. 1998). Generation of
transverse tube waves and pulses by these large amplitude motions
has been studied analytically (Choudhuri et al. 1993a,b; Zhugzhda
et al. 1995) and numerically (Huang et al. 1995).
Here, our assumption is that the wavetrain of random pulses
described by Eq. (19) is imposed on the tube at z=0 and that
these pulses propagate upward along the tube and produce the
free atmospheric oscillations. Since the wavetrain is finite
only the free oscillations are present (see Eq. (22)) and their
time evolution is plotted in Fig. 6. It is seen that each random
pulse decays in time as t-3/2 and that their amplitudes are
comparable to those shown in Figs. 4 and 5 for the excitation by
the spectra of waves. An interesting result is that the
oscillations are sustained in the atmosphere for more than 10 000 s,
which is the computation time; they would eventually die out because
of the finite number of pulses. In order to make these oscillations
a permanent atmospheric feature, the pulses must be continuously
generated (Sutmann et al. 1998; also Paper I), even so, the
maximum amplitude of these oscillations is only 0.01 km s-1 at the
atmospheric height z = 2000 km.
Our analytical results show that both transverse tube waves and
transverse pulses propagating along a magnetic flux tube freely excite
tube oscillations. The frequency of these free oscillations is
s-1 and the period is
min for
/
.
The period becomes longer
min for the weaker magnetic field
/
and shorter
min
for the stronger field
/
.
This
range of periods seems to be in good agreement with the
oscillations observed in the chromospheric network (see Sect. 1). However, a fundamental difficulty with the free atmospheric
oscillations driven by transverse waves and pulses is that these
oscillations give negligible contribution to the Doppler signal
and, therefore, they are not visible at disk center (e.g., Kalkofen
1997). According to Hasan & Kalkofen (1999), the oscillations can
be detected in the solar chromosphere after transformation of
transverse tube waves into longitudinal tube waves. The process
responsible for this transformation is the nonlinear mode coupling
and these authors assumed that the period of both waves is the same
after the transformation. In the following, we discuss this process
and the validity of their assumption.
A schematic picture showing how the nonlinear mode coupling
works is presented in Fig. 7. It must be noted that in our approach
transverse tube waves are generated as linear waves. However, when
they propagate in the solar atmosphere their amplitudes increase due
to the decrease in density and nonlinear effects (mode coupling to
longitudinal tube waves) become important. As a result of these
effects, the components of the curvature forces along the verical
become large enough to produce compressions inside the tube.
For each transverse displacement,
there is approximately one compression and one expansion inside the
tube. Thus, one full longitudinal tube wave is generated by every
half wavelength of a transverse wave, which means that the frequency of
the longitudinal tube waves is approximately twice that of the
transverse tube waves. This shows that the period of the generated
longitudinal tube waves is approximately one half
the period of transverse tube waves and that the assumption made
by Hasan & Kalkofen (1999) is not correct.
As shown in Paper I, the period of the longitudinal tube waves is not the
main property that determines the period of the resulting free
oscillations. Actually, the free atmospheric oscillations excited
by the propagating longitudinal tube waves of any frequency inside
solar magnetic flux tubes have always a period that is equal to
the cutoff period
.
Therefore,
the propagation of longitudinal tube waves produced by the nonlinear
mode coupling will generate free oscillations with the cutoff
frequency
but not
as suggested
by Hasan & Kalkofen (1999). As a result, the period of these
oscillations will be near 3 min and will be practically
independent of the strength of the tube magnetic field (see Paper I).
Thus, we may conclude that both longitudinal tube waves generated in
the solar convection zone and longitudinal tube waves produced by
the nonlinear mode coupling excite 3-min oscillations inside solar
magnetic flux tubes.
Despite the fact that these two different excitation mechanisms produce free atmospheric oscillations inside solar magnetic flux tubes with the same period, their amplitudes are likely to be different. The amplitudes of the oscillations excited by the first mechanism are known and given in Paper I; however, to determine the amplitudes of oscillations produce by the second mechanism, one must know the efficiency of the nonlinear mode coupling (Ulmschneider et al. 1991). Since the energy fluxes carried by transverse tube waves are relatively high (Huang et al. 1995; Musielak & Ulmschneider 2002a), and since the amplitude of these waves quickly increases with height due to the density gradient, the transformation of the waves into longitudinal tube waves must be efficient (e.g., Ulmschneider et al. 2001; Fawzy et al. 2002). If this is the case, then the amplitudes of the free oscillations discussed here must be higher (or even much higher) than those presented in Paper I. To make this comparison more detailed calculations of the efficiency of the nonlinear mode coupling are required, however, calculations of this type are out of the scope of this paper.
It is important to note that our model of exponentially diverging
magnetic flux tubes allows us to introduce the global cutoff frequencies
(
,
and
)
and derive
the general analytical solutions for different excitation mechanisms.
On the other hand, our model has only limited applications to the Sun.
The model is obviously valid for a single magnetic flux tube located
inside the supergranulation cells. It may also be applied to appropriate
regions of the chromospheric network at the boundary of superganulation
cells where enough space is available before the tubes meet neighboring
flux tubes. In both cases, the analytical theory presented here predicts
only the existence of the 3-min free atmospheric oscillations inside
solar magnetic flux tubes. Hence, neither our results nor the results
presented by Hasan & Kalkofen (1999) can account for the 7-min
oscillations observed in the chromospheric network. Clearly, a
different approach is needed to explain the nature and origin of
these oscillations.
One possible explanation of the 7-min oscillations has been given by Deubner & Fleck (1990) who suggested that internal gravity waves forming standing waves inside the solar chromospheric cavity may account for the range of periods (see also Lou 1995a,b) and other basic properties of these oscillations. However, Hasan & Kalkofen (1999) argue that this is not sufficient because a theory explaining the observed oscillations must also account for the heating of the chromospheric network. We basically agree with this suggestion and believe that in order to explain the nature and origin of the oscillations in the magnetic network, one must take into account more realistic shapes of the magnetic flux tubes (see, for example, Fawzy et al. 2002) and include the temperature gradient as well. Obviously, for such situations no analytical solutions can be obtained and only the so-called local cutoff frequencies can be derived. The fact that both the period of the free atmospheric oscillations and their time decay are affected by the temperature gradient has been shown by Sutmann & Ulmschneider (1995a) for a magnetic-free atmosphere. Similar studies must be performed for solar magnetic flux tubes discussed here and in Paper I. The results of these studies will be presented elsewhere.
Our conclusions about the nature of the oscillations excited by transverse tube waves and pulses can be summarized as follows:
1. The propagation of transverse tube waves and pulses along a single magnetic flux tube leads to the free oscillations of this tube with the cutoff frequency
2. The free atmospheric oscillations decay in time as t-3/2 if the frequency of driving waves is not equal to the cutoff frequency
3. For the continuous excitation, the forced oscillations of the magnetic flux tube are also present. They are different from the free oscillations as they do not decay in time and their frequency is the same as the frequency
4. The amplitude of the free oscillations driven by either transverse tube waves or pulses is relatively small when compared with the characteristic speed
5. The free and forced oscillations of solar magnetic flux tubes driven by transverse tube waves and random pulses cannot be directly observed as they do not give a Doppler signal. However, free and forced atmospheric oscillations inside these tubes can be observed if they are excited by longitudinal tube waves generated by transverse tube waves through the process of nonlinear mode coupling (Hasan & Kalkofen 1999). Our results show that the basic properties of these two types of oscillations are different. The main difference is that the former have periods ranging from 7 to 10 min and the latter show always periods near 3 min.
6. The observed 3-min oscillations inside the supergranule cells are consistent both with the free atmospheric oscillations inside magnetic flux tubes and with the free atmospheric oscillations in a nonmagnetic atmosphere outside these tubes. The results presented here and in Paper I show that the former are excited by the propagating longitudinal tube waves generated either by the turbulent motions in the solar convection zone or by transverse tube waves through nonlinear mode coupling. According to our analytical results, in both cases only the 3-min free atmospheric oscillations are produced.
7. Our results cannot explain the 7-min oscillations observed in the chromospheric network. The main reason is that our model does not apply to crowded magnetic flux tubes at the boundary of supergranules. Therefore, to account for these oscillations, one must take into consideration a more realistic structure of flux tubes located in the magnetic network.
Acknowledgements
This work was supported by NSF under grant ATM-0087184 (Z.E.M. and P.U.), by the DFG grant Ul57/25-3, and by NATO under grant CRG-910058 (P.U. and Z.E.M.). Z.E.M. also acknowledges the support of this work by the Alexander von Humboldt Foundation.