Model of a fluxtube with a twisted magnetic field in the stratified solar atmosphere
Abstract
We build a single vertical straight magnetic fluxtube spanning the solar photosphere and the transition region which does not expand with height. We assume that the fluxtube containing twisted magnetic fields is in magnetohydrostatic equilibrium within a realistic stratified atmosphere subject to solar gravity. Incorporating specific forms of current density and gas pressure in the Grad–Shafranov equation, we solve the magnetic flux function, and find it to be separable with a Coulomb wave function in radial direction while the vertical part of the solution decreases exponentially. We employ improved fluxtube boundary conditions and take a realistic ambient external pressure for the photosphere to transition region, to derive a family of solutions for reasonable values of the fluxtube radius and magnetic field strength at the base of the axis that are the free parameters in our model. We find that our model estimates are consistent with the magnetic field strength and the radii of Magnetic bright points (MBPs) as estimated from observations. We also derive thermodynamic quantities inside the fluxtube.
keywords:
Magnetohydrostatics, Sun: photosphere, Sun: transition region, Sun: magnetic fields, magnetohydrodynamics (MHD)authoryear
1 Introduction
The study of small scale magnetic structures in the solar photosphere is important because they play a crucial role in the evolution of active regions and sunspots (Muller and Mena, 1987; Centeno et al., 2007). Magnetic bright points (MBPs) are likely to be the fluxtubes observed in the photosphere (Berger et al., 1995; Centeno et al., 2007; Lagg et al., 2010).
The topological rearrangement of these magnetic fluxtubes due to the motion of the photospheric foot points or magnetic reconnections, contribute to the coronal heating (Muller et al., 1994; van Ballegooijen, 1986). A three dimensional (3D) single fluxtube model with untwisted magnetic field has been studied by solving linear elliptic partial differential equation by numerical iterative process (Steiner et al., 1986). Schlüter and Temesváry (1958) and Osherovich (1984) studied a 3D fluxtube for sunspots using a self–similar model. The magnetic and thermodynamic structure inside fluxtube with untwisted magnetic field which spans from photosphere to the lower part of the solar corona is studied by Gent et al. (2013). Both 2D and 3D numerical models of fluxtubes with the energy propagation through the torsional Alfven waves have been studied by Murawski et al. (2015a, b), where an empirical form of magnetic flux function was assumed. Vigeesh et al. (2009) assumed an empirical form of gas pressure to investigate the wave propagation and energy transport through a fluxtube. Several interesting results of wave behavior in the solar photosphere and chromosphere have been presented by several authors (Bogdan et al., 2003; Fedun et al., 2009; Shelyag et al., 2010).
In this work, we construct a 3D single cylindrical vertical straight magnetic fluxtube semianalytically with a twisted magnetic field by obtaining a new solution of poloidal flux function by solving Grad–Shafranov equation (GSE; Grad and Rubin (1958); Shafranov (1958)). We assume a specific form of gas pressure and poloidal current, which has been used to study the equilibrium solution of terrestrial plasma (Atanasiu et al., 2004). An equilibrium solution near the magnetic axis of the plasma torus has been reported previously, using a plasma pressure and poloidal current profile that varies linearly with the poloidal flux function (Solov’ev, 1968). We obtain an analytic solution by assuming a form that is quadratic in the poloidal flux function, and derive the magnetic field structure and thermodynamic quantities inside the fluxtube using the solution that represents an ideal MHS equilibrium. In the future, we will look to explore fully the profile functions that will improve the solution set.
The overview of the paper is as follows. In section 2, the GSE has been derived assuming a specific form of the profile function of gas pressure and poloidal current and the solution of the equation is presented. In section 3, we discuss the boundary condition that is physically acceptable, and can be used for realistic modelling of a fluxtube. In section 4, the mode wise variation of the profile functions are presented and in section 5, we compare the model with the observations. Finally, we conclude with a comparison with other existing models.
2 Solution of GradShafranov equation
We assume an axisymmetric cylindrical geometry, with gas pressure and take the poloidal current constant along a magnetic field line. We express and in terms of the poloidal flux function and and consider a straight vertical axisymmetric fluxtube that spans the altitude from photosphere () to the transition region ( Mm) that is in equilibrium with the atmosphere outside with the uniform gravity acting vertically downward. The force balance equation in MHS equilibrium takes the form
(1) 
where denotes the mass density and is the magnetic field associated with the poloidal flux function (scaled by the factor ) in the following form
(2) 
This form of and ensures the solenoidal condition of magnetic field. Now splitting the MHS force balance equation (1) into and directions, we find two different scalar partial differential equations
(3a)  
(3b) 
If the gas pressure and poloidal current are functions of alone i.e., and respectively, then from the (3a, 2) it follows that
(4) 
Plugging in and in (3b), we find
(5) 
By multiplying both sides of (5) by and using (4), we obtain which implies that is zero, which means that the vertical hydrostatic pressure balance will not be maintained. Therefore, to balance the vertical hydrostatic pressure inside the fluxtube, we introduce a new function, such that,
We assume and to be second order polynomials of
(6a)  
(6b) 
where
and the parameters and are to be determined by appropriate boundary conditions. The function is to be evaluated later. The substitution of given by (6a) in (3a) gives (4) and we obtain the following second order scalar partial linear inhomogeneous differential equation
(7) 
with the rescaled parameters, . To solve (7), we split in two parts: a homogeneous part, and an inhomogeneous part , i.e. . Using this form in (7), we separate the homogeneous and the inhomogeneous parts to obtain the following expressions:
(8a)  
(8b) 
To solve the homogeneous part, we seek a solution of the form . Then we separate out the and part in (8a) as follows
(9) 
where is an arbitrary real constant. Motivated by the fact that the poloidal flux function decreases with , we assume that the solution of the part of (9) takes the form
(10) 
where is an arbitrary constant. To solve the –part of (9), we substitute (where ) and insert it in (9) to find
(11) 
whose solutions are given by Coulomb wave functions and (Abramowitz and Stegun, 1972) (page ) with and . The solution of (11) takes the following form
(12) 
Here and are called the regular and irregular Coulomb wave functions respectively which are complex quantities with real arguments (Boersma, 1968), given by
(13)  
(14) 
where and are called the WhittakerM and WhittakerW function (see Figure 3 below) and the constants and are defined by
(15) 
and
(16) 
The Whittaker function has been used by several authors in their models of the solar atmosphere albeit in different physical problems (eg. Tsinganos (1979) in the context of inviscid flows, and also in the context of MHD waves by Hindman and Jain (2008) and Erdélyi and Fedun (2010)). Now has to be a finite quantity that varies linearly with the term but blows up at ; therefore for to be finite on the axis of the fluxtube in (12) must vanish. As a result takes the form
(17) 
and the homogeneous part of the solution is given by
(18) 
A similar but a different solution, which is oscillatory in is used for laboratory plasma for both a shaped plasma and toroidally diverted plasma (Atanasiu et al., 2004). The general solution of (7) is given by the sum of the homogeneous part given above and an inhomogeneous part which will be presented in a paper in preparation. We have found that the presence of term in the poloidal flux function , implies that and cannot be simultaneously positive for any combination of and in the physical parameter domain space for all and . For avoiding these unphysical effects we present the case of and an exploration of the general solution will be studied later. Since and its complex conjugate function, are the valid solutions of (7), we construct a solution of (7) by redefining .
3 Boundary conditions and the reduced form of and
The ideal magnetic fluxtube is embedded in a magnetic field free region with no current outside the fluxtube boundary. We make the following standard assumptions and to ensure that there is no net current at the fluxtube boundary. The pressure at the photosphere () outside the fluxtube is dyne cm and at the transition region ( Mm) is dyne cm and is taken from AvrettLoeser model (Avrett and Loeser, 2008). We summarize the boundary conditions below
(19a)  
(19b)  
(19c)  
(19d) 
Assuming that pressure decreases exponentially from photosphere to transition region, we use the following expression for the external pressure
(20) 
where Mm Mm Matching the pressure scale heights, we see that (6a) also decreases exponentially with as
(21) 
where will need to be calculated. Taking the reduced forms of and are given by
(22) 
and
(23) 
Taking the radial component of the MHS force balance equation (1) and adding the contribution of the radial force due to the presence of sheet current at the boundary we write the force balance equation
(24) 
Now the sheet current can be expressed as a delta function which is non zero only at the boundary. Integrating (24) w.r.t. from to where is an infinitesimal positive quantity we obtain
(25)  
which leads to the MHS force balance at the boundary to be given by
(26) 
where denotes the jump condition at the boundary and and are the magnetic fields inside and outside the fluxtube boundary. This is an improved boundary pressure condition for a magnetic fluxtube, as previous studies have ignored the sheet current. Now to calculate , we assume an infinitesimal current loop at the boundary which has a vertical height of length and radial extent from to . Using Stokes line integral theorem along the closed loop (see Fig.1), we see that
(27) 
which implies . Since for any height and , from (26), the total pressure at the boundary inside the fluxtube is and matching the pressures gives
(28) 
The mass density inside the fluxtube obtained from (3b) is
(29) 
and the density inside the fluxtube varies only with and at the transition region ( Mm), which should match with the external density which is typically g cm (Avrett and Loeser, 2008). From (28) and (20) we see that
(30) 
In our model is assumed not to vary much from photosphere to the transition region and its value is taken to be m s, the value at the solar surface and that determines dyne cm. Using the forms of from (2, 18) we obtain , and the relation between and is derived in the following.
The real component of is given by,
and by using (13) and (18) we can express the flux function in the form of Whittaker–M functions as
(31) 
Whittaker–M function can be expressed in terms of hypergeometric function by the standard relation (Dixit and Moll, 2015)
(32) 
Here represents the hypergeometric functions with the parameters and argument . Therefore (31) takes the form
(33) 
It follows from (2) that takes the form
(34)  
So, at is by definition takes the form from (34) as,
(35) 
But is an identity and therefore we finally have
(36) 
We define a physical observable which is the average magnetic field strength at the base within the fluxtube as
(37) 
where is the radius of the fluxtube. Therefore we have two free parameters and that we can tune to fit our model with the observations. At , from (28), we get
(38) 
Therefore, from (19a, 19b, 19c) we determine , and in terms of the free parameters and and hence the thermodynamic quantities within fluxtube. The temperature within the fluxtube is calculated by the ideal gas law according to the following form
(39) 
where J mol K is the universal gas constant and
is the average value of the mean effective molar mass from photosphere to transition region given by an empirical formula (Solov’ev and Kirichek, 2015) in the domain of Mm. A formulary of the different quantities are listed in Table 1.
Functions  –part  –part 

1 
4 Mode analysis of different profile functions
The quantities and are functions of the free parameters , and the mode number whose values are given in Table 2 for a sample set of the free parameters.
(km)  (kG)  mode no.  C ( Mx)  ( cm)  a ( cm) 

100  1  1  0.335061  7.50448  20.0417 
100  1  2  0.144828  15.6546  107.27 
100  1  3  0.0915463  24.0161  268.473 
All the profile functions are normalized to their peak values and the radial distance to the total radius of the fluxtube . As per the boundary conditions, the flux function, vanishes both at the axis and at the boundary of the fluxtube, where the the total gas pressure is . The solutions of higher modes have, the profile functions with higher frequency along the radial direction and realistically we may not have such reversible fields, as they are unstable. Therefore, we use the fundamental mode () for further analysis in the paper. The D topology of the magnetic field lines inside the fluxtube for the fundamental mode is shown in Fig. 9.
5 Comparing the model with observations
Now we compare our model with the observations reported from high resolution and high cadence instruments. Small scale magnetic structures, i.e., MBPs, are the best candidates for comparison because such structures can be assumed to consist of fluxtubes. MBPs are seen in Gband filtergrams or are identified by making spectropolarimetric measurements (Utz et al., 2009, 2013; Yang et al., 2016). The radial variation of the profile functions and and the magnetic components and are independent of , but the amplitude decreases exponentially with except for . In the following, we validate the model by comparing the observed magnetic field strengths and radius of MBPs with those calculated in our model, and estimate the magnetic field strength and thermodynamic quantities at the transition region which may be verified by future observations.
The MBPs number distribution, magnetic field strength and size distribution has been reported by Utz et al. (2009, 2013) at photosphere. The size distribution of MBPs peaks around km and km for low and high spatial sampling rates, respectively (Utz et al., 2009). The magnetic field strength distribution is bimodal with two peaks at G and G (Utz et al., 2013). Since MBPs are observed as the region of unipolar flux concentrations, we construct a cylindrical boundary inside the simulation domain where the vertical magnetic field is positive. We call this cut–off radius as , where the line of sight magnetic field vanishes. The value of after this grid line becomes negative. In Fig. 6, the vertical grid line denotes the boundary radius . We study two different cases for and km which corresponds to the peak values for the MBP size distribution, for which is found to be and km respectively.
For both cases, we calculate and the mean value of , in the radial direction up to and find that for realistic values of the thermodynamic quantities inside the fluxtube, the upper limit of the vertical magnetic field strength is kG. Beyond this value of , the viable solutions will shift to the higher modes. The temperature inside the fluxtube increases as the value of decreases and temperature inside the fluxtube becomes greater than the typical photospheric temperature when kG. Thus it can be considered as the lower cut off limit of the magnetic field strength. The value is only sensitive to but not on , and the thermodynamic quantities inside the fluxtube remain the same for both and km. We found and kG for and kG respectively. The radial and vertical variations of the vertical magnetic field strength, gas pressure, density and temperature inside the fluxtube are shown in Figs. 7 and 8. We see that the vertical magnetic field strength decreases from kG (on the axis at ) to zero at the MBP boundary (, see Fig. 6). The variation of gas pressure and temperature from axis to the MBP boundary is very small; at the photosphere the gas pressure changes (on the axis at ) to dyne cm (at MBP boundary) and it decreases with to dyne cm (at MBP boundary) at the transition region ( Mm). The temperature changes from K (on the axis) to K (at MBP boundary) which is small compared to the outside photosphere temperature ( K) (Avrett and Loeser, 2008). The average temperature inside fluxtube has been calculated by integrating the temperature from axis to the MBP boundary and is found to be K. The density distribution is constant along the radius of the fluxtube at a given height which decreases with height from g cm at the photosphere to g cm at the transition region. The values of the quantities estimated from our modelled are summarized in Table 3.
(Mm)  (G)  (dyne cm)  (g cm)  (K)  

0  0  2370  5656  
0  2  2.19  5656  
0  0  5718  
2  0  5718 
6 Conclusions and discussion
In this paper, we constructed a single fluxtube with twisted magnetic field by solving GSE analytically. We summarize our results below:

We have an improved boundary condition by incorporating the sheet current as compared to the previous studies e.g. Solov’ev and Kirichek (2016).

Our model depends on the form of the external pressure distribution which is assumed as an exponentially decreasing function with . Future observations leading to the more accurate form of pressure distribution from photosphere to transition region can be used to improve our model. The plasma parameter inside the fluxtube remains constant with but it varies along ; is obeyed from the chromosphere to the transition region but not in the photosphere and lower atmosphere. Therefore the magnetic effects will dominate the gas dynamics throughout the simulation domain.

In our model, the temperature varies along the radial direction, but it is constant along the vertical direction . In other models e.g. Gent et al. (2013) the temperature rises with height from photosphere to the transition region and in Vigeesh et al. (2011) the temperature decreases from K at surface to K (at km) and then it remains the same up to km.

The effects of shock wave dissipation and magnetic reconnection starts to dominate in the corona which causes the coronal heating. We have not considered these mechanisms in our model and therefore, we have not model the region in the corona or higher and have restricted our simulation domain to end at the transition region.

Recently Hewitt et al. (2014); Uitenbroek and Criscuoli (2013); Riethmüller and Solanki (2016), have simulated bright points using MuRAM and CopenhagenStagger code. We find that the magnitude of magnetic field strengths, pressure and densities reported in these studies are in fair agreement with our predictions but the temperature distribution along is in variance with the results of the numerical simulations.
The fluxtube model gives values of magnetic field and thermodynamic quantities consistent with observations and compares well with simulations. We plan to present a more detailed study of the model in a paper in preparation.
We thank Prof. P. Venkatakrishnan for useful discussions and the anonymous reviewers for insightful comments and helpful suggestions. We also thank the support staff of the IIA HPC facility and Sandra Rajiva for proofreading the manuscript.
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