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In an isothermal atmosphere with constant density scale height $H$, the equation $\delta {\rho }_{tt}-c^2\delta {\rho }_{zz}-(c^2/H)\delta {\rho }_z=0$ loses its first spatial derivative under $\delta \rho =e^{-z/(2H)}\Psi$. Direct differentiation gives ${\Psi }_{tt}-c^2{\Psi }_{zz}+c^2\Psi /(4H^2)=0$. Thus a plane wave has dispersion relation ${\omega }^2=c^2{k}^2+{\omega }_c^2$, with real acoustic cutoff frequency ${\omega }_c=c/(2H)$. The cutoff term is positive; a minus sign would instead predict growing sufficiently long spatial modes. Vertical propagation requires $|\omega |\ge {\omega }_c$ within this ideal constant-coefficient model.

Two uniform horizontal magnetic fields on either side of a horizontal interface exert no bulk force, but their magnetic pressures differ. For a common-temperature isothermal atmosphere with sound-speed squared $c_T^2=\mathcal{R}{T}_0$ and gravity $g$, hydrostatic equilibrium gives $p_i=c_T^2{\rho }_i$ and $p_i(z)=p_i(0)e^{-z/H}$, $H=c_T^2/g$. Continuity of magnetohydrodynamic total pressure gives the displayed density jump. A stronger field below means denser gas above, supplying a Rayleigh-Taylor instability even though each half-space is hydrostatic.

// Target: continuum-mechanics.bigb

The exponential factor removes the first spatial derivative. Apply the chain rule explicitly:while ${\partial }_t^2\Delta \rho =e^{-z/(2H)}{\Psi }_{tt}$. Substituting into the supplied density equation, the terms proportional to ${\Psi }_z$ cancel, leavingThis is the atmospheric acoustic cutoff transformation. There is a sign error in the target equation in the original PDF, as well as in the transcription: the last term must be $+{\omega }_0^2\Psi$ for real positive ${\omega }_0$. If the displayed minus sign is retained literally, coefficient matching would require ${\omega }_0^2=-c^2/(4H^2)$, so it would not define a real characteristic oscillation period.

For the corrected equation, a plane wave $\Psi \propto e^{i(kz-\omega t)}$ has dispersion relationThus ${\omega }_0$ is the acoustic cutoff frequency: vertically propagating acoustic waves have $|\omega |\ge {\omega }_0$. The scale height and sound speed have units metres and metres per second, respectively, so ${\omega }_0$ has units ${\mathrm{s}}^{-1}$.

For an order-of-magnitude estimate in the lower terrestrial atmosphere, take $H\simeq 8.0\mathrm{km}$ and $c\simeq 340\mathrm{m}{\mathrm{s}}^{-1}$. These are representative atmospheric values, not additional data specified by the paper. They giveThe isothermal atmosphere approximation is local: a real atmospheric temperature profile and the thermodynamic response of the disturbance modify the precise cutoff and period.

Take the optical depth to increase inward and let $\mu >0$ denote an outward ray. For thermal absorption and emission in local thermodynamic equilibrium, the radiative transfer source function is the Planck function, so the radiative transfer equation isThis assumes negligible scattering, or a source function genuinely thermalized to $B_{\lambda }$; LTE by itself does not turn an arbitrary scattering source into a Planck function. Multiply by $e^{-\tau /\mu }$, integrate between the two boundaries, and solve for the outward specific intensity:The bottom boundary value is the quantity denoted $I_{\lambda }(0)$ in the supplied notation; it is not an optical-depth-zero boundary. The first term is attenuated incident radiation, and the second is emission from the intervening layers. This is the formal solution of the radiative transfer equation.

For an isothermal atmosphere at temperature $T$, the Planck function is constant and the integral can be evaluated:For a bounded bottom intensity and ${\tau }_1\to \infty$, $I_{\lambda }=B_{\lambda }(T)$ in every outgoing direction, and $F_{\lambda }=\pi B_{\lambda }(T)$. Thus the emergent spectrum is a blackbody spectrum. The blackbody limit of an isothermal atmosphere does not require a temperature gradient: it follows from complete thermalization in an optically thick medium.

Interpret the quoted band amplitude as an absolute transit-depth increment $\Delta D=0.0003$, or $300$ parts per million. For a thin atmospheric annulus of extra height $\Delta z$, the atmospheric spectral-feature amplitude isLet the water band span $N_H$ atmospheric scale heights, so $\Delta z=N_{H}H$. For a nearly isothermal atmosphere with constant gravity and mean molecular weight $\mu$, $H=k_{B}T/(\mu m_{u}g)$. The temperature from a transmission-band amplitude is thereforeHere $T_0,H_0,{\mu }_0,g_0$ refer to the chosen Jupiter reference layer. Assuming $g=g_J\simeq 25\mathrm{m}{\mathrm{s}}^{-2}$, an H/He exoplanet atmosphere with $\mu \simeq 2.3$, and a clear strong band with $N_H\simeq 5$, gives $H\simeq 210\mathrm{km}$ and $T\simeq 1.4\times 10^3\mathrm{K}$. Equivalently, a Jovian reference with $T_0\simeq 165\mathrm{K}$ has $H_0\simeq 24\mathrm{km}$, giving $H/H_0\simeq 8.7$.

The assumptions are hydrostatic, approximately isothermal limb gas, constant gravity, negligible cloud truncation, H/He composition, and an effective opacity contrast yielding about five scale heights. In an ideal clear limb, $N_H\simeq \log ({\kappa }_{\mathrm{band}}/{\kappa }_{\mathrm{cont}})$ follows from the slant optical depth of an isothermal atmosphere. The data fix $N_{H}H$, not $H$ separately: the temperature is conditional on the band opacity contrast and composition. For $N_H=3$–$7$ with the same composition and gravity, the estimate is about $1000$–$2400\mathrm{K}$.

Let $z$ increase outward and define the inward optical depth by $d\tau /dz=-\bar{\kappa }\rho$. With constant outward internal radiative flux $F=\sigma T_{\mathrm{int}}^4$, radiative diffusion givesThe constant $q$ is a boundary condition; diffusion alone does not determine it. For an unirradiated grey atmosphere with the Eddington closure approximation and Eddington surface boundary condition, $q=2/3$, soThe internal effective temperature of a planet is defined by its intrinsic cooling flux, not by its incident stellar heating. Since $dT/d\tau >0$ and $d\tau /dz<0$, temperature decreases outward. Towards the thin upper layers, $T\to 2^{-1/4}{T}_{\mathrm{int}}\simeq 0.84T_{\mathrm{int}}$; if the density and optical-depth gradient vanish there, $dT/dz\to 0$. This is the upper nearly isothermal atmosphere. The diffusion approximation itself fails at small optical depth: the grey boundary closure supplies the approximate continuation. Small irradiation perturbs this intrinsic-flux solution.

Young, self-luminous gas giants at wide orbital separations can have intrinsic cooling dominate their photospheric budget. They are favorable for exoplanet direct imaging, especially in the infrared, where their own thermal radiation is easier to separate from the host light. A strongly irradiated hot Jupiter instead has a large stable outer radiative region set mainly by stellar heating; it can be nearly isothermal over a broad pressure range or develop an atmospheric thermal inversion if stellar light is absorbed sufficiently high. Inversion is not inevitable for every strongly irradiated planet. Deep convection begins below its radiative-convective boundary.

The intrinsic curve follows the grey formula. The irradiated curves illustrate possible shapes only; they are not solutions for a specified opacity model. Smaller optical depth corresponds to greater altitude.

Assume an isothermal atmosphere in hydrostatic equilibrium, with constant gravity $g$, mean molecular weight $\mu$, and an ideal gas equation of state. Use $h$ for the atmospheric scale height, to distinguish it from the total observable height $H$. Thenso the cloud-top pressure in an isothermal atmosphere isAn opaque, approximately grey exoplanet cloud deck can flatten the exoplanet transmission spectrum if the remaining gas above it has little wavelength-dependent extinction. If its top nearly reaches the observable atmospheric extent, $H_c\simeq H$, this gives $P_c\simeq P_s{e}^{-H/h}$. More generally, a top at $N$ atmospheric scale heights has $P_c/P_s=e^{-N}$; illustrative values $N=5$ and $10$ give $6.7\times 10^{-3}$ and $4.5\times 10^{-5}$.

A flat exoplanet transmission spectrum and $P_s$ alone do not determine a unique cloud-top pressure. The temperature, gravity, mean molecular weight and cloud altitude, or equivalent opacity information, are also needed. A large mean molecular weight can itself suppress spectral features, so the exoplanet cloud deck interpretation is conditional.

Assume that the optical continuum is dominated by Rayleigh scattering by particles much smaller than the wavelength, and that their column abundance is fixed. Their cross-section, hence the slant optical depth, scales as ${\tau }_{\lambda }\propto {\lambda }^{-4}$. Differentiating the annulus model for transmission spectroscopy exactly gives the logarithmic slope of total transit depthFor $A\ge 0$ and $B>0$, the slope lies between $-4$ and zero, but generally depends on wavelength. In the optically thin limit, ${\delta }_{\lambda }\simeq A+B{\tau }_{\lambda }$, so its slope is $-4B{\tau }_{\lambda }/(A+B{\tau }_{\lambda })$. It vanishes as the atmospheric signal becomes negligible compared with the opaque disc; it also approaches zero in the saturated optically thick limit.

The familiar value $m\simeq -4$ applies to the optically thin atmospheric excess,or formally to a model in which $A$ is negligible. For an ordinary thin exoplanet atmosphere, however, $B/A\simeq 2H/R_p\ll 1$, so neglecting the opaque planetary baseline in the total transit depth is not usually justified. A universal $m=-4$ for the logarithm of the total transit depth does not follow from the stated model.

In a stratified isothermal atmosphere, the related scattering slope of a transmission spectrum is instead $d{R}_{\mathrm{tr}}/d\ln \lambda =-4h$, obtained by moving the effective tangent level to keep its slant optical depth of order unity. Consequently $d\ln \delta /d\ln \lambda =-8h/R_{\mathrm{tr}}$ there. The radius slope, atmospheric-excess slope and total-depth slope are different observables.

Assume a thin isothermal atmosphere, so $g\simeq G{M}_p/R_p^2$ and the surface area are constant through it. Integrating hydrostatic equilibrium from base pressure $P_0$ to negligible top pressure gives atmospheric column mass $P_0/g$. ThereforeEquivalently, $M_{\mathrm{atm}}\sim 4\pi R_p^2{\rho }_{0}H$, with ${\rho }_0=P_0\mu m_H/(k_B{T}_0)$ and $H=k_B{T}_0/(\mu m_{H}g)$; the explicit temperature dependence cancels.

Introduce the dimensionless variablesUsing $P=\Sigma H^2{\Omega }_z^2$, vertical hydrostatic equilibrium becomes the parameter-free equationwith normalizations $\int \tilde{\rho }d\tilde{z}=1$ and $\int \tilde{p}d\tilde{z}=1$. For an isothermal atmosphere, $p=c_s^2\rho$, and these conventions give the Gaussian distribution

This is the constant scale height of an isothermal atmosphere. The pressure and density decay length in an isothermal atmosphere is $H=k_{B}T/(mg)$.

For a thin hydrostatic isothermal atmosphere, $\Delta D\simeq 2R_p{N}_{H}H/R_{\star }^2$ and $H=k_{B}T/(\mu m_{u}g)$. Hence $T\simeq \mu m_{u}g\Delta D{R}_{\star }^2/(2k_B{R}_p{N}_H)$. A measured exoplanet transmission spectrum amplitude constrains $N_{H}H$, so gravity, mean molecular mass and the number $N_H$ of sampled atmospheric scale heights are additional inputs. In a clear fixed-composition model, $N_H\simeq \log ({\kappa }_{\mathrm{band}}/{\kappa }_{\mathrm{cont}})$; clouds or abundance gradients can change it.

A thermodynamic system is in thermodynamic equilibrium when its macroscopic state has no spontaneous tendency to change through any exchange allowed by its constraints. For an isolated system, stable thermodynamic equilibrium maximizes entropy over the accessible states at fixed total energy and conserved material inventories. A wall can forbid an exchange, so always specify what can cross or move. Thermal, mechanical and chemical conditions answer different questions about that same constrained state. ^[ref]

For example, two compartments separated by a rigid, impermeable wall that transmits heat can reach thermal equilibrium while retaining different pressures and compositions. Allowing an unloaded piston to move adds a mechanical equilibrium condition. Allowing one chemical species to cross adds chemical potential equality for particle exchange; allowing a chemical reaction adds its chemical equilibrium condition. A fixed barrier can therefore support a constrained thermodynamic equilibrium that ceases to be an equilibrium when the barrier is removed.

Uniformity is not the definition of thermodynamic equilibrium. An isothermal atmosphere in hydrostatic equilibrium can have a common temperature while its pressure and mass density vary with height: the pressure gradient balances gravity. In an external field, the exchange condition must include the relevant potential energy; for charged matter this leads to the electrochemical potential. Relativistic gravitational corrections to thermal equilibrium are described separately by the Tolman temperature law.

A steady state only says that the macroscopic description is constant in time. A bar maintained between hot and cold thermal reservoirs has steady heat flux and produces entropy, so it is not in thermodynamic equilibrium. Even matching temperatures at one perfect-contact interface can coexist with heat transfer through the surrounding material; matching those two local values does not establish equilibrium of the whole apparatus. See thermal conduction for that spatial transport problem.

The useful simplification is that equilibrium conditions predict an endpoint without computing every collision or flow during relaxation. The first law of thermodynamics supplies the conserved quantities, and the Second law of thermodynamics supplies the direction of change. For a system held at fixed temperature and volume, the corresponding criterion is minimization of Helmholtz free energy; at fixed temperature and pressure, it is minimization of Gibbs free energy, with the material and exchange constraints still imposed.