The main objective of this paper is to investigate the dynamic bifurcation for the granulation convection in the solar photosphere. Based on the dynamic bifurcation theory, which is established by Ma and Wang (Phase transition dynamics, pp. 380-459, 2014), the critical parameters \((R,F)\) condition for the granulation convection is derived. Furthermore, the corresponding R-F-phase diagram is generated. In addition, the bifurcation solution is also obtained with certain assumptions.
The atmosphere of the sun is divided into three parts: the bottom layer of the atmosphere is photosphere, the middle layer is chromosphere and the outermost layer is the corona. The grainy appearance of solar photosphere is produced by the tops of the convective cells and is called granulation. It worth noting that Kutner analyzed the structure of granulation in his book [2]. He pointed out that the temperature decreases with the increasing height in the photosphere. And he concluded that the temperature difference between the top and bottom of photosphere causes the granulation convection in photosphere which can be explained by circulation cells of material. In circulation cells, the hotter gas is brought up from below, producing the bright regions. The cooler gas, which produces the darker regions, is carried down to replace the gas that was brought up (see Figure 1).
Figure 1
The structure of granulation. This is a lateral view showing the situation under the surface. The hotter gas is brought up from below, producing the bright regions. The cooler gas, which produces the darker regions, is carried down to replace the gas that was brought up.
In addition, Ustyugov introduced the solar activity and gave the magneto-hydrodynamics (MHD) equations in [3, 4], which can explain the convection structure in the sun. There have been many studies on this kind of problem. For instance, Chae [5], Agélas [6], and Wu [7] considered the regularity of MHD equations, and Capone [8] and Fuchs [9] studied the stability bifurcation of MHD equations. For more research about the MHD equations refer to [10–17]. It is noticed that the granulation convection structure (see Figure 1) is similar to the structure of Rayleigh-Bénard convection and the Taylor problem. Inspired by the dynamic theory, which is established by Ma and Wang to study the Rayleigh-Bénard convection and the Taylor problem (see [1, 18–20]), we will investigate the dynamic bifurcation for the granulation convection. This paper is organized as follows.
1.
In Section 2, we introduce the equations governing the atmospheric circulation with magnetic field and also give the bifurcation theory.
2.
Section 3 is devoted to getting the RF-phase diagram of granulation under the parametric condition \(K_{1}=0\) and \(F\neq0\), where R is the Rayleigh number, which is related to the difference of temperature \(T_{1}-T_{0}\), F is a dimensionless parameter, which is related to the magnetic field H, and \(K_{1}\) is as in (2.5), which is related to the boundary condition \(H_{0}\) and \(H_{1}\). The transition occurs as the parameter \((R, F)\) crosses the critical curve L from region I to region II; see Figure 2.
Figure 2
The RF-phase diagram. In region I, there is no granulation convection structure; and in region II, the granulation appears. So the transition occurs as the parameter (R,F) crosses the critical curve L from region I to region II.
3.
In Section 4, the bifurcation solution and the critical Rayleigh number \(R_{C}\) are obtained under the condition \(F= 0\).
Let \((\varphi, \theta,r)\) be the spherical coordinates, where φ, θ, r represent the longitude, the latitude, and the radial coordinate, respectively. The unknown functions include the velocity field \(u=(u_{\varphi},u_{\theta},u_{r})\), the temperature function T, the pressure function p, the electromagnetic press function Φ and the magnetic field \(H=(H_{\varphi},H_{\theta},H_{r})\). Then the equations governing the atmospheric circulation with magnetic field [9, 21–23] in the spherical coordinates are given by
where \(0\leq\varphi\leq2\pi\), \(-\frac{\pi}{2}\leq\theta\leq\frac{\pi }{2}\), \(r_{0}< r< r_{1}\), \(r_{1}=r_{0}+d\), \(r_{0}\) is the radius of the sun, and d is the height of the troposphere, \(\vec{k}=(0,0,1)\), g is the gravitative constant, ν is the kinetic viscosity, κ is the thermal diffusivity, and η is the resistivity. The differential operators in the spherical coordinates are given as follows:
In this paper, we mainly focus on the dynamic bifurcation for the granulation. For simplicity, we assume \(\theta=\frac{\pi}{2}\). Then the velocity component \(u_{\theta}\) and the magnetic field \(H_{\theta}\) are zero, and equations (2.1) become
We determine the basic flow by following assumptions.
1.
\(U=(u_{\varphi},u_{r},T,H_{\varphi} ,H_{r})=(0,0,\tilde {T}(r),0,\tilde{H}_{r})\), \(p=\tilde{p}(r)\), \(\Phi=0\); that is, the pressure function, the temperature function and the magnetic field function in r-direction are not zero, and which are only depending on r.
2.
The functions \(\tilde{T}(r)\), \(\tilde{H}(r)\) and \(\tilde{p}(r)\) satisfy
It is noticed that \(K_{0}\) and \(K_{1}\) are related to the boundary value \(H_{0}\) and \(H_{1}\). Furthermore, in order to get the perturbation equations related to the variables r and φ, we make the following translations:
As we know, the radius of the sun is about \(7\times10^{5}\mbox{ km}\) and the thickness of the photosphere is about 500 km. Then the ratio of the thickness of the photosphere to the radius of the sun is small. Hence, we adopt the approximations that \(1/r \simeq1/r_{0}\), \((r_{0}+d)/r_{0}\simeq1 \). For simplicity, we assume \(\nu/\kappa=\eta/\kappa=1\). Also, we introduce the following dimensionless variables:
Let \((\varphi'', r'')=(r_{0}\varphi', r')\). With the above assumptions, we omit the primes and get the approximate equations, which are given as follows:
where \((\varphi,r)\in M=(0,L)\times(r_{0},r_{0}+1)\), \(r_{0}\) is the radius of the sun with the unit of d, \(L=2\pi r_{0}\), and \(K_{0}\), \(K_{1}\) are given as (2.5). \((u\cdot\nabla)\), div and △ are general differential operators.
Now we will show that equations (2.7) can be written in the operator form. Since the velocity field u and the magnetic field H on M are divergence-free, there exist the following stream functions \(f_{1}\) and \(f_{2}\) satisfying the given boundary condition:
with the boundary conditions (2.8). We proceed with the separation of variables. Under the periodic boundary condition (2.8), the problem (3.2) possesses two eigenvectors: Ψ and Ψ̃ in the following forms:
where \(a_{k}= 2k\pi/L\). Putting \((\Psi,p,\Phi)\) and \((\tilde{\Psi},\tilde{p},\tilde{\Phi})\) into (3.2), respectively, we deduce from (3.2), (3.3) and (3.4) that \((h_{k},g_{k},T_{k},p_{k},\Phi_{k})\) satisfy the eigenvalue problems,
It is well known that all solutions of the cubic equation (3.9) \(\beta_{kl}\) are eigenvalues of (3.2). Let \(\beta_{kl}^{i}\) (\(1\leq i\leq3\)) be three zero points of (3.9). It is easy to see that \(Re\beta_{kl}^{1}\geq Re\beta _{kl}^{2}\geq Re\beta_{kl}^{3}\). Furthermore, \(\beta=0\) is a zero point of (3.9) if and only if
where \(b_{k_{0}l_{0}}\), \(c_{k_{0}l_{0}}\) and \(d_{k_{0}l_{0}}\) as in (3.11) and (3.12).
Lemma 3.1
Let\((k,l)=(k_{0},l_{0})\)minimize the right hand side of (3.12) and the zero point\(\beta_{k_{0}l_{0}}^{1}\)of (3.9) is a real single eigenvalue of (3.2) near curveL. Then\(\beta _{k_{0}l_{0}}^{1}\)satisfies
Namely, \(\beta_{k_{0}l_{0}}^{1}(0,0)<0\). Since \(\beta_{k_{0}l_{0}}^{1}(R,F)\) are continuous on \((R,F)\), then (3.14) holds true. Next, we are ready to prove that the first eigenvalue \(\beta _{k_{0}l_{0}}^{1}(R,F)>0\) in region II near the curve L. Let
where \(L_{k_{0}l_{0}}=b_{k_{0}l_{0}}-c_{k_{0}l_{0}}R+d_{k_{0}l_{0}}F^{2}\). When \(L_{k_{0}l_{0}}= 0 \), \(\Gamma_{1}\) is the graph of \(g_{k_{0}l_{0}}\); when \(L_{k_{0}l_{0}}< 0\), \(\Gamma_{2}\) is the graph of \(g_{k_{0}l_{0}}\), as shown in Figure 3. Then the intersecting point of \(\Gamma_{i}\) (\(i=1,2\)) and β-axis is the first eigenvalue \(\beta_{k_{0}l_{0}}^{1}\). When \(L_{k_{0}l_{0}}=0\), the intersecting point of \(\Gamma_{1}\) and β-axis, \(\beta _{k_{0}l_{0}}^{1}= 0\), and \((R,F)\) is in curve L. When \(L_{k_{0}l_{0}}< 0\), the intersecting point of \(\Gamma_{2}\) and β-axis, \(\beta _{k_{0}l_{0}}^{1}> 0\), and \((R,F)\) is in region II. At last, we will show that \(\beta_{kl}^{i}<0\), \(\forall(k,l)\neq(k_{0},l_{0})\), when \((R,F)\) in region I or in region II near the curve L. We only need
Otherwise, if (3.15) does not hold true, there exist a pair of \((R_{1},F_{1})\), such that \(\beta_{kl}^{i}(R_{1},F_{1})=0\). That is to say, there exists a curl \(L_{kl}\), which passes through the point \((R_{1},F_{1})\) and intersects with R-axis at \(R_{C_{1}}\), as shown in Figure 4. It is easy to find that \(R_{C}> R_{C_{1}}\), which makes a contradiction that \(R_{C}\) is the minimum critical Rayleigh number. So, we prove (3.15). When \((R,F)=(0,0)\), we have
which implies \(\beta_{kl}^{i}<0\), \(\forall(k,l)\neq(k_{0},l_{0})\). Because \(\beta_{kl}^{i}(R,F)\) are continuous, \(\beta_{kl}^{i}<0\) in region I or in region II near the curve L. This proves Lemma 3.1.
Figure 3
The graph of\(\pmb{g_{k_{0}l_{0}}}\). As the figure shows: When \(L_{k_{0}l_{0}}= 0 \), \(\Gamma_{1}\) is the graph of \(g_{k_{0}l_{0}}\); when \({L_{k_{0}l_{0}}< 0}\), \(\Gamma_{2}\) is the graph of \(g_{k_{0}l_{0}}\),Then the intersecting point of \(\Gamma_{i}\) (\(i=1,2\)) and β-axis is the first eigenvalue \(\beta_{k_{0}l_{0}}^{1}\). When \(L_{k_{0}l_{0}}=0\), the intersecting point of \(\Gamma_{1}\) and β-axis, \(\beta _{k_{0}l_{0}}^{1}= 0\), and \({(R,F)}\) is in curve L. When \(L_{k_{0}l_{0}}< 0\), the intersecting point of \(\Gamma_{2}\) and β-axis, \(\beta _{k_{0}l_{0}}^{1}> 0\), and \((R,F)\) is in region II.
Figure 4
The graph of\(\pmb{L_{kl}}\). The curl \(L_{kl}\) passes through the point \((R_{1},F_{1})\) and intersects with the R-axis at \(R_{C_{1}}\), it is easy to find that \(R_{C}> R_{C_{1}}\).
□
Based on Theorem 2.3.1 in [1], Theorem 2 in [20] and Lemma 3.1, we derive the following theorem.
Theorem 3.2
The critical parameter curveLdivides theRF-plane into two regions I and II (see Figure 2), such that the following conclusions hold true.
1.
If\((R,F)\in\textit{I}\), the problem (2.8) and (2.9) have no bifurcation, and the basic flow in (2.4) with\(K_{1}=0\)is stable.
2.
If\((R,F)\in\textit{II}\), there exists a bifurcation solution.
3.
The transition takes place as the control parameter\((R,F)\)crosses the critical curveLfrom region I into II.
Essentially, the control parameter in (4.1) is only the Rayleigh number R in (3.1). Then the eigenvalues and the eigenvectors Ψ, Ψ̃ are similar to those in Section 3, and we can get the corresponding eigenvalue problems
For \(k\neq0\) and \(l\neq0\), it is well known that all solutions of equation (4.6) are eigenvalues. Let \(\beta_{kl}^{1}(R)\) (\(1\leq i\leq3\)) be three zero points of (4.6) such that
Then, by (3.3), (3.4), (4.4) and (4.5), the eigenvectors \(\Psi_{kl}^{i}\) and \(\tilde{\Psi}_{kl}^{i}\) corresponding to \(\beta_{kl}^{1}\) can be written as
It is clear that the eigenvalues and eigenvectors have the following properties.
1.
\(\beta_{0l}^{i}\), \(\beta_{k0}\) and \(\beta_{kl}^{i}\) (\(k\neq0\), \(l\neq 0\)) consist of all eigenvalues of equation (4.2), and all eigenvectors form a basis of H;
2.
\(\beta_{0l}^{i}< 0\) (\(i=1,2\)) and \(\beta_{k0}< 0\);
3.
the eigenvalues \(\beta_{kl}^{i}\) (\(k\neq0\), \(l\neq0\)) only depend on the Rayleigh number.
Now, we are ready to determine the dual eigenvector. The dual eigenvectors \(\Psi_{kl}^{i\ast}\) and \(\tilde{\Psi}_{kl}^{i\ast}\) are given by
Thus, all dual eigenvectors consist of \(\Psi_{0l}^{1\ast}=(0,0,\sin l\pi (r-r_{0}),0,0)\), \(\Psi_{0l}^{2\ast}=(0,0,0, \sin l\pi(r-r_{0}),0)\), \(\Psi_{k0}^{\ast}=(0,0,0,0,\sin a_{k}\varphi)\), \(\tilde{\Psi}_{k0}^{\ast }=(0,0,0,0,\cos a_{{k}}\varphi)\).
In this part, we are ready to study the critical-crossing of the first eigenvalue. Since the eigenvalues \(\beta_{kl}^{i}\) (\(1\leq i\leq3\), \(k\neq0\), \(l\neq0\)) only depend on the Rayleigh number, it suffices to focus on the eigenvalue problem (4.6). If \(\beta=0\) is a zero point of (4.6), we can get
Let\((k_{0},l_{0})\)minimize the right hand side of (4.11). Assume the zero point\(\beta_{k_{0}l_{0}}^{1}\)of (4.6) is a real single eigenvalue of (4.1) near\(R_{C}\), then\(\beta_{k_{0}l_{0}}^{1}\)satisfies
The proof is similar to the proof of Lemma 3.1, so we omit it.
4.2 Transition theory with \(F=0\)
In this section, we consider the transition theorem of problem (4.1) with boundary conditions (2.8) with \(K_{1}\neq0\) and \(F=0\), where \(K_{1}\), F as in (2.10). Then we have the following theorem.
Theorem 4.2
Let\(K_{1}\neq0\)and\(F=0\), we have the following conclusions for equations (4.1) with boundary conditions (2.8).
1.
Equation (4.1) bifurcates from\(((u,T,H),R)=(0,R_{C})\)to an attractor\(\Sigma_{R}\in H_{1}\), only consisting of a steady state solution.
2.
The steady state solution\((u,T,H)=(u_{\varphi},u_{r},T,H_{\varphi },H_{r})\)can be expressed as
where\(C>0\)is a constant, \(\Psi_{k_{0}l_{0}}^{1}\)and\(\tilde{\Psi }_{k_{0}l_{0}}^{1}\)are the first eigenvectors given by (4.7) and (4.8), and\(x^{2}+y^{2}=1\).
Proof
We first reduce the abstract equation (2.11) to the center manifold. Let \(J_{0}=(k_{0},l_{0},1)\), then \(\Psi_{k_{0}l_{0}}^{1}=\Psi_{J_{0}}\) and \(\tilde{\Psi }_{k_{0}l_{0}}^{1}= \tilde{\Psi}_{J_{0}}\), the reduced equations read
Therefore, the conclusions of Theorem 4.2 follows from (4.17), (4.18) and the attractor bifurcation theorem in [3]. The proof of the theorem is completed. □
In summary, the production of granulation convection is illustrated by Theorem 3.2 and Figure 2, where \(R_{C}\) is the critical Rayleigh number. As is shown in Figure 2, when the differences of temperature and magnetic are small enough, the points \((R,F)\) are below the critical curve L and in region I, the gas is in a static state. When they become greater than the curve L, the gas suddenly breaks into regular circulation cells, which is granulation convection. The circulation cells can be expressed by the bifurcation solution in Theorem 4.2. From both the mathematical and the physical points of view, the results are valuable to understand the phase transition in fluid dynamics.
The author would like to thank the referee for invaluable comments insightful suggestions. This article was supported by the National Natural Science Foundation of China (A0109).
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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Figure 2
