Ionization Front Research Articles

Gasdynamical flows in star forming regions

A review of gas-dynamical flows in astrophysics is given with an emphasis on accretion flows and outflows. Analytical estimates of the dependence of the velocity and density on the radius during spherical accretion are considered, as well as the rate of mass inflow and the corresponding luminosity of the accretion disk are also estimated. Some features of gas inflow and outflow near the star are mentioned, like mass-loading flows where source functions of mass and momentum are presented and analyzed. The velocities and radii of the ionization fronts of hot stars are considered. The Sedov-Taylor analytical formula on the dependence of the radius of the fireball on time during a strong explosion is given, where the explosion energy and density of the medium act as parameters. Examples of the use of dimensional theory in astrophysics, in particular, in accretion flows used above are considered. An example of a numerical solution of the interaction of an interstellar cloud with the solar wind is also given, followed by an estimate of the inflow of the cloud’s gas environment into the Earth’s atmosphere, which affects the terrestrial ozone content.

Single-shot compressed optical field topography

Femtosecond lasers are powerful in studying matter’s ultrafast dynamics within femtosecond to attosecond time scales. Drawing a three-dimensional (3D) topological map of the optical field of a femtosecond laser pulse including its spatiotemporal amplitude and phase distributions, allows one to predict and understand the underlying physics of light interaction with matter, whose spatially resolved transient dielectric function experiences ultrafast evolution. However, such a task is technically challenging for two reasons: first, one has to capture in single-shot and squeeze the 3D information of an optical field profile into a two-dimensional (2D) detector; second, typical detectors are only sensitive to intensity or amplitude information rather than phase. Here we have demonstrated compressed optical field topography (COFT) drawing a 3D map for an ultrafast optical field in single-shot, by combining the coded aperture snapshot spectral imaging (CASSI) technique with a global 3D phase retrieval procedure. COFT can, in single-shot, fully characterize the spatiotemporal coupling of a femtosecond laser pulse, and live stream the light-speed propagation of an air plasma ionization front, unveiling its potential applications in ultrafast sciences.

A spectral element method for modelling streamer discharges in low-temperature atmospheric-pressure plasmas

Streamers are ionization fronts that occur in gases at atmospheric and sub-atmospheric pressures. Numerical studies of streamers are important for practical applications but are challenging due to the multiscale nature of this discharge type. This paper introduces a spectral element method for modelling streamer discharges. The method is developed for Cartesian grids but can be extended to be used on unstructured meshes. The streamer model is based on the Poisson equation for the electric potential and the electron continuity equation. The Poisson equation is discretized via a spectral method based on the integral representation of the solution. The hierarchical Poincaré-Steklov (HPS) scheme is used to solve the resulting set of equations. The electron continuity equation is solved by means of the discontinuous Galerkin spectral element method (DGSEM). The DGSEM is extended by an alternative definition of the diffusion flux. A subcell finite volume method is used to stabilize the DGSEM scheme, if required. The entire simulation scheme is validated by solving a number of test problems and reproducing the results of previous studies. Adaptive mesh refinement is used to reduce the number of unknowns. The proposed method is found to be sufficiently fast for being used in practical applications. The flexibility of the method provides an interesting opportunity to broaden the range of problems that can be addressed in numerical studies of low-temperature plasma discharges.

Propagation of nanosecond plasmas in liquids—Streamer velocities and streamer lengths

Nanosecond plasmas in liquids are often generated by applying a short high voltage pulse to an electrode immersed in a liquid for biomedical or environmental applications. The plasmas appear as streamers that propagate through the liquid. The understanding of the ignition of these nanosecond plasmas in liquids, however, is an open question. The occurrence of any traditional gas phase ignition mechanism is unlikely, because the formation of a gas bubble prior to ignition is suppressed by the inertia of the liquid. Therefore, either electron multiplication inside nanopores that are induced by an electric field pressure gradient or field effects at the tip and at the ionization front of the liquid streamer may act as electron generation mechanisms. A deeper understanding can be achieved by comparing the velocity and dynamic of the plasma propagation with modeling, where the individual mechanisms and transport coefficients can be analyzed. Here, we are using intensified charge-coupled device imaging to investigate the time dependence of the streamer dynamic and compare this with a 1D fluid code for negative voltages. It is shown that the maximum streamer length scales with the applied electric field, indicating that an electric stability field in the liquid streamer channel is important, as known for gas streamers. The 1D fluid code can reproduce the proper streamer velocities, if transport coefficients for hydrated electrons are chosen. The model suggests that the propagation of liquid streamers is dominated by the local ionization rate at the ionization front rather than by advection or diffusion of electrons as in gases. This also explains the finding that positive and negative streamers exhibit almost identical electron densities.

Toward a better understanding of negative lightning stepped leaders

Negative lightning leaders observed at very close (within a few kilometers) distances often exhibit heavy branching with many active tips forming a network‐like structure with a descending multi-tip “ionization front” whose transverse dimensions are of the order of hundreds of meters. The presence of such front makes the lightning attachment process more complex than usually assumed. Negative leader branches extend in a step-like manner, with each step necessarily generating a traveling wave moving positive charge from the branch tip up along the channel, like a mini return stroke. The leader stepping process has a number of generally unrecognized consequences. Specifically, a stepped-leader branch tip can collide with the lateral surface of an adjacent branch (usually at an angle of about 90°). Further, the step‐related positive-charge waves propagating from the tip up along the channel can reactivate decayed negative branches at higher altitudes. Overall, a heavily-branched negative stepped leader creates a highly-structured and rapidly-changing electric field pattern inside and in the vicinity of the volume it occupies, which causes complex interactions between the branches.

A Study of Photoionized Gas in Two H ii Regions of the N44 Complex in the LMC Using MUSE Observations

We use the optical integral field observations with Multi-Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope, together with CLOUDY photoionization models, to study ionization structure and physical conditions of two luminous H ii regions in the N44 star-forming complex of the Large Magellanic Cloud. The spectral maps of various emission lines reveal a stratified ionization geometry in N44 D1. The spatial distribution of [O i] λ6300 emission in N44 D1 indicates a partially covered ionization front at the outer boundary of the H ii region. These observations reveal that N44 D1 is a blister H ii region. The [O i] λ6300 emission in N44 C does not provide a well-defined ionization front at the boundary, while patches of [S ii] λ6717 and [O i] λ6300 emission bars are found in the interior. The results of spatially resolved MUSE spectra are tested with the photoionization models for the first time in these H ii regions. A spherically symmetric ionization-bounded model with a partial covering factor, which is appropriate for a blister H ii region, can well reproduce the observed geometry and most of the diagnostic line ratios in N44 D1. Similarly, in N44 C we apply a low-density and optically thin model based on the observational signatures. Our modeling results show that the ionization structure and physical conditions of N44 D1 are mainly determined by the radiation from an O5 V star. However, local X-rays, possibly from supernovae or stellar wind, play a key role. In N44 C, the main contribution is from three ionizing stars.

Geant4 simulations of x-ray photon pileup produced by runaway electrons in streamer discharges

Runaway electron acceleration is the keystone process responsible for the production of energetic radiation by lightning and thunderstorms. In the laboratory, it remains undetermined if runaway electrons are merely a consequence of high electric fields produced at the ionization fronts of electrical discharges, or if they impact the discharge formation and propagation. In this work, we simulate photon pileup in a detector next to a spark gap. We compare laboratory measurements to ensembles of monoenergetic electron beam simulations performed with Geant4 (using the Monte Carlo method). First, we describe the x-ray emission properties of monoenergetic beams with initial energies in the 20 to 75 keV range. Second, we introduce a series of techniques to combine monoenergetic beams to produce general-shape electron energy spectra. Third, we proceed to attempt to fit the experimental data collected in the laboratory, and to discuss the ambiguities created by photon pileup and how it constrains the amount of information that can be inferred from the measurements. We show that pileup ambiguities arise from the fact that every single monoenergetic electron beam produces photon deposited energy spectra of similar qualitative shape and that increasing the electron count in any beam has the same qualitative effect of shifting the peak of the deposited energy spectrum toward higher energies. The best agreement between simulations and measurements yields a mean average error of 8.6% and a R-squared value of 0.74.

Breaking Orion's Veil with fossil outflows

Context. The role of feedback in the self-regulation of star formation is a fundamental question in astrophysics. The Orion Nebula is the nearest site of ongoing and recent massive star formation. It is a unique laboratory for the study of stellar feedback. Recent SOFIA [C II] 158 μm observations have revealed an expanding bubble, the Veil shell, that is powered by stellar winds and ionization feedback. Aims. We have identified a protrusion-like substructure in the northwestern portion of the Orion Veil shell that may indicate additional, highly directional feedback mechanisms. Our goal is to investigate the origin of the protrusion by quantifying its possible driving mechanisms. Methods. We use the [C II] 158 μm map of the Orion Nebula obtained with the upGREAT instrument on board SOFIA. The spectral and spatial resolution of the observations are 0.3 km s−1 and 16", respectively. The velocity-resolved [C II] observations allow us to construct position-velocity (pv) diagrams to measure the morphology and the expansion velocity of the protrusion. For the morphology, we also use new observations of 12CO and 13CO J = 2-1 (to trace molecular gas), Spitzer 8 μm observations (to trace the far-ultraviolet-illuminated surfaces of photodissociation regions), and Hα observations (to trace ionized gas). For the kinematics, we perform a line-profile analysis of [C II], 13CO, and 12CO at 12 positions, covering the entire protrusion. To quantify the stellar feedback, we estimate the mass of the protrusion by fitting the dust thermal emission. We compare the kinetic energy with the stellar wind of θ1 Ori C and the momentum of the outflows of massive protostars to investigate the driving mechanism of the protrusion. Results. The pv diagrams reveal two half-shells expanding at velocities of +6 km s−1 and +12 km s−1. We find that the protrusion has a diameter of ~1.3 pc with a ~45 M⊙ shell expanding at +12 km s−1 at the northwestern rim of the Veil. The thickness of the expanding shell is ~0.1 pc. Using the mass in the limb-brightened shell and the maximum expansion velocity, we calculate the kinetic energy and the momentum of the protrusion to be ~7 × 1046 erg and 540 M⊙ km s−1, respectively. We consider three possible origins for this protrusion: fossil outflow cavities created by jets and outflows during the protostellar accretion phase, preexisting "clumpiness" in the OMC-1 core, and the stellar wind during the main-sequence phase. Based on the energetics and the morphology, we conclude that the northwestern part of the preexisting cloud was locally perturbed by outflows ejected from massive protostars in the Trapezium cluster. This suggests that the protrusion of the Veil is the result of mechanical rather than radiative feedback. Furthermore, we argue that the location of the protrusion is a suitable place to break Orion's Veil owing to the photo-ablation from the walls of the protrusion. Conclusions. We conclude that the outflows of massive protostars can influence the morphology of the future H II region and even cause breakages in the ionization front. Specifically, the interaction of stellar winds of main-sequence stars with the molecular core preprocessed by the protostellar jet is important.

Influence of voltage and gap distance on the dynamics of the ionization front, plasma dots, produced by nanosecond pulsed discharges at water surface

A streamer discharge is a highly reactive and dynamic non-thermal plasma. It has been used in many applications, including environmental remediation, medicine, and material processing. Although the physics of streamer discharges in gaseous media is well understood, its interaction with a solid and liquid dielectric surfaces remains under investigation, in particular when quantitative data are searched for. In this study, we investigate the influence of voltage amplitude (V a) and interelectrode air gap distance (d) on the pulsed discharge behavior at the surface of distilled water. Time resolved images show the formation and propagation of plasma dots (ionization front of streamers) at water surface. Because of its stochastical nature, a large number of discharge was performed to address the influence of V a and d on the number of plasma dots (N Dots) as well on the charge per dot (Q Dot). As expected, for a given V a, the breakdown voltage is found to increase with d. Moreover, N Dots decreases linearly with d at the rate of ∼1 dot by 200 μm of increase, while the total injected charge decays linearly with a rate of ∼8–9 nC by 200 μm of increase. Based on the measurement of the propagation velocity of the plasma dots and on the estimation of the electric field in the medium, an average mobility of plasma dots of ∼1.5 cm2 Vs−1 is evaluated. From both this value and the instantaneous measured propagation velocity, the temporal evolution of Q Dot and charge number are determined. The observations reported here are of interest for fundamental studies as well as for applications where well-controlled charge transfer to surfaces is crucial.

Fine structure of streamer-to-filament transition in high-pressure nanosecond surface dielectric barrier discharge

Abstract The fine structure of a streamer-to-filament transition in a single-shot high-voltage nanosecond surface dielectric barrier discharge in molecular nitrogen at pressure P = 6 bar was studied with the help of ICCD microimaging. An intermediate discharge structure, existing for only a few nanoseconds, was observed in the time interval between two discharge modes: streamer discharge, with a typical electron density of n e ∼ 1015 cm−3, and filamentary discharge, with n e ∼ 1019 cm−3. The structure was observed for both polarities of the high-voltage electrode. The structure can be briefly described as a stochastic appearance of thin channels propagating a bit faster than the main ionization front of merged surface streamers, transforming in a few nanoseconds in a bi-directional ionization wave. One wave, which we associate with a feather-like structure in optical emission, propagates further away from the high-voltage electrode, and another, a backward wave of emission, propagates back towards the edge of the high-voltage electrode. When the backward wave of emission almost reaches the high-voltage electrode, the filament appears. Plasma properties of the observed structure were studied to better understand the nature of a streamer-to-filament transition. Theoretical analysis suggests that the instability of a flat front of ionization wave (Laplacian instability) triggers the streamer-to-filament transition, and that a surface stem (a tiny region with enhanced electron density) should be in the origin of the bi-directional ionization wave.

Interferometric imaging of intensely radiating negative leaders

The common phenomenon of lightning still harbors many secrets and only recently a new propagation mode was observed for negative leaders. While propagating in this `Intensely Radiating Negative Leader' (IRNL) mode a negative leader emits 100 times more very-high frequency (VHF) and broadband radiation than a more normal negative leader. We have reported that this mode occurs soon after initiation of all lightning flashes we have mapped as well as sometimes long thereafter. Because of the profuse emission of VHF the leader structure is very difficult to image. In this work we report on measurements made with the LOFAR radio telescope, an instrument primarily built for radio-astronomy observations. For this reason, as part of the present work, we have refined our time resolved interferometric 3-Dimensional (TRI-D) imaging to take into account the antenna function. The images from the TRI-D imager show that during an IRNL there is an ionization front with a diameter in excess of 500~m where strong corona bursts occur. This is very different from what is seen for a normal negative leader where the corona bursts happen at the tip, an area of typically 10~m in diameter. The observed massive ionization wave supports the idea that this mode is indicative of a dense charge pocket.

Seimei KOOLS-IFU mapping of the gas and dust distributions in Galactic planetary nebulae: the case of IC 2165

ABSTRACT We investigated the physical and chemical properties of the gas and dust components in a carbon-rich planetary nebula (PN) IC 2165 using two-dimensional emission-line maps with superior resolution. The extinction map is generated in a self-consistent and assumption-free manner. The circumstellar gas-to-dust mass ratio (GDR) map ranges radially from 1210 in the central nebula filled with hot gas plasma to 120 near the ionization front. The determined GDR is comparable to ∼400, which is commonly adopted for carbon-rich asymptotic giant branch (AGB) stars, and ∼100 for interstellar medium. Except for the inner regions, the GDR in IC 2165 is nearly the same as in such AGB stars, indicating that most dust grains withstand the harsh radiation field without being destroyed. The gas and dust mass distributions concentrated in the equatorial plane may be related to the non-isotropic mass loss during the AGB phase and nebula shaping. The spatial distributions of electron densities/temperatures and ionic/elemental abundances were investigated herein. We determined 13 elemental abundances using point-spread-function-matched spatially integrated multiwavelength spectra extracted from the same aperture. Their values are consistent with values predicted by a theoretical model for stars of initially 1.75 M⊙ and Z = 0.003. Finally, we constructed the photoionization model using our distance measurement to be consistent with all derived quantities, including the GDR, gas and dust masses, and post-AGB evolution. Thus, we demonstrate the capability of the Kyoto Okayama Optical Low-dispersion Spectrograph with optical-fibre Integral Field Unit (KOOLS-IFU) and how the spatial variation of the gas and dust components in PNe derived from IFU observations can help understand the evolution of the circumstellar/interstellar medium.

Ionization-Gasdynamic Simulations of Wind-Blown Nebulae around Massive Stars

Using a code that employs a self-consistent method for computing the effects of photo-ionization on circumstellar gas dynamics, we model the formation of wind-driven nebulae around massive stars. We take into account changes in stellar properties and mass-loss over the star’s evolution. Our simulations show how various properties, such as the density and ionization fraction, change throughout the evolution of the star. The multi-dimensional simulations reveal the presence of strong ionization front instabilities in the main-sequence phase, similar to those seen in galactic ionization fronts. Hydrodynamic instabilities at the interfaces lead to the formation of filaments and clumps that are continually being stripped off and mixed with the low density interior. Even though the winds start out as completely radial, the spherical symmetry is quickly destroyed, and the shocked wind region is manifestly asymmetrical. The simulations demonstrate that it is important to include the effects of the photoionizing photons from the star, and simulations that do not include this may fail to reproduce the observed density profile and ionization structure of wind-blown bubbles around massive stars.

Influence of preionization on the microchannel structure of a spark discharge in air in a pin-plane gap

The initial phase of a spark discharge in the gap between the pin(cathode) and a plane 1.5 mm long in atmospheric pressure air underconditions of preliminary photoionization by an auxiliary discharge wasinvestigated by the method of shadow photography. In the absence ofpreionization, the discharge from the first nanoseconds after breakdown isan aggregate of a large number of micron-diameter channels. It was foundthat the electron concentration resulting from preionization, estimated at108-109cm-3, increases the degree of uniformity of the discharge channel inthe near-cathode region; however, in the near-anode region, the channelremains microstructured. Within the framework of the mechanism ofmicrostructure formation due to the instability of the ionization wavefront, a criterion for the formation of a uniform discharge is obtained andan explanation of the results obtained is presented. Keywords: spark discharge, preionization, photoionization, instability of the ionization front, microstructure, shadow photography method\

Investigation of Rocket Effect in BRC 18 using Gaia EDR3

ABSTRACT Bright-rimmed clouds (BRCs) are ideal candidates to study radiation-driven implosion mode of star formation as they are potential sites of triggered star formation, located at the edges of Hii regions, showing evidence of ongoing star formation processes. BRC 18 is located towards the eastern edge of relatively closer (∼400 pc) H ii region excited by λ Ori. We made R-band polarimetric observations of 17 candidate young stellar objects (YSOs) located towards BRC 18 to investigate any preferred orientation of the discs with respect to the ambient magnetic field and the direction of energetic photons from λ Ori. We found that the discs are oriented randomly with respect to the projected magnetic field. Using distances and proper motions from the Gaia EDR3 of the candidate YSOs, we investigated the possible acceleration of BRC 18, away from λ Ori due to the well-known ‘Rocket Effect’, by assuming that both the candidate YSOs and BRC 18 are kinematically coupled. The relative proper motions of the candidate YSOs are found to show a trend of moving away from λ Ori. We computed the offset between the angle of the direction of the ionization front and the relative proper motion of the candidate YSOs and found it to lie close to being parallel to each other. Additionally, we found 12 sources that are co-moving with the known candidate YSOs towards BRC 18. These co-moving sources are most likely to be young and are missed in previous surveys conducted to identify potential YSOs of the region.

Dynamical effects of the radiative stellar feedback on the H I-to-H2 transition

Context. The atomic-to-molecular hydrogen (H/H2) transition has been extensively studied as it controls the fraction of gas in a molecular state in an interstellar cloud. This fraction is linked to star-formation by the Schmidt–Kennicutt law. While theoretical estimates of the column density of the H I layer have been proposed for static photodissociation regions (PDRs), Herschel and well-resolved Atacama Large Millimeter Array observations have revealed dynamical effects in star forming regions, caused by the process of photoevaporation. Aims. We extend the analytic study of the H/H2 transition to include the effects of the propagation of the ionization front, in particular in the presence of photoevaporation at the walls of blister H II regions, and we find its consequences on the total atomic hydrogen column density at the surface of clouds in the presence of an ultraviolet field, and on the properties of the H/H2 transition. Methods. We solved semi-analytically the differential equation giving the H2 column density profile by taking into account H2 formation on grains, H2 photodissociation, and the ionization front propagation dynamics modeled as advection of the gas through the ionization front. Results. Taking this advection into account reduces the width of the atomic region compared to static models. The atomic region may disappear if the ionization front velocity exceeds a certain value, leading the H/H2 transition and the ionization front to merge. For both dissociated and merged configurations, we provide analytical expressions to determine the total H I column density. Our results take the metallicity into account. Finally, we compared our results to observations of PDRs illuminated by O-stars, for which we conclude that the dynamical effects are strong, especially for low-excitation PDRs.

Hydrodynamic Response of the Intergalactic Medium to Reionization. II. Physical Characteristics and Dynamics of Ionizing Photon Sinks

Abstract Becker et al. measured the mean free path of Lyman-limit photons in the intergalactic medium (IGM) at z = 6. The short value suggests that absorptions may have played a prominent role in reionization. Here we study physical properties of ionizing photon sinks in the wake of ionization fronts (I-fronts) using radiative hydrodynamic simulations. We quantify the contributions of gaseous structures to the Lyman-limit opacity by tracking the column-density distributions in our simulations. Within Δt = 10 Myr of I-front passage, we find that self-shielding systems (N H I > 1017.2 cm−2) are comprised of two distinct populations: (1) overdensity Δ ∼ 50 structures in photoionization equilibrium with the ionizing background, and (2) Δ ≳ 100 density peaks with fully neutral cores. The self-shielding systems contribute more than half of the opacity at these times, but the IGM evolves considerably in Δt ∼ 100 Myr as structures are flattened by pressure smoothing and photoevaporation. By Δt = 300 Myr, they contribute ≲10% to the opacity in an average 1 Mpc3 patch of the universe. The percentage can be a factor of a few larger in overdense patches, where more self-shielding systems survive. We quantify the characteristic masses and sizes of self-shielding structures. Shortly after I-front passage, we find M = 104–108 M ⊙ and effective diameters d eff = 1–20 ckpc h −1. These scales increase as the gas relaxes. The picture herein presented may be different in dark matter models with suppressed small-scale power.

Transport of electrons and propagation of the negative ionisation fronts in indium vapour

We study the transport of electrons and propagation of the negative ionisation fronts in indium vapour. Electron swarm transport properties are calculated using a Monte Carlo simulation technique over a wide range of reduced electric fields E/N (where E is the electric field and N is the gas number density) and indium vapour temperatures in hydrodynamic conditions, and under non-hydrodynamic conditions in an idealised steady-state Townsend (SST) setup. As many indium atoms are in the first metastable state at vapour temperatures of a few thousand Kelvin, the initial Monte Carlo code was extended and generalized to consider the spatial relaxation and the transport of electrons in an idealised SST experiment, in the presence of thermal motion of the host-gas atoms and superelastic collisions. We observe a significant sensitivity of the spatial relaxation of the electrons on the indium vapour temperature and the initial conditions used to release electrons from the cathode into the space between the electrodes. The calculated electron transport coefficients are used as input for the classical fluid model, to investigate the inception and propagation of negative ionisation fronts in indium vapour at various E/N and vapour temperatures. We calculate the electron density, electric field, and velocity of ionisation fronts as a function of E/N and indium vapour temperature. The presence of indium atoms in the first metastable state significantly affects the characteristics of the negative ionisation fronts. The transition from an avalanche into a negative ionisation front occurs faster with increasing indium vapour temperature, due to enhanced ionisation and more efficient production of electrons at higher vapour temperatures. For lower values of E/N, the electron density behind the streamer front, where the electric field is screened, does not decay as one might expect for atomic gases, but it could be increased due to the accumulation of low-energy electrons that are capable of initiating ionisation in the streamer interior.

Flat-top THz directional diagram of a DC-biased filament.

In the experiment, the laser pulse (744 nm, 0.5 mJ, 90 fs) focused into the air gap between the plane electrodes biased by a 10 kV/cm field (DC-biased filament) produced terahertz (THz) radiation. At the selected frequencies of ν=0.3, 0.5, 1 THz, a wide flat-top angular distribution was measured by a bolometer rotating in the plane of the electrodes. The simulations based on the unidirectional pulse propagation equation with fine 0.01 THz resolution and 3 PHz frequency domain showed the transition of the THz directional diagram from the flat-top at ν≲1THz to the conical one at ν>8THz due to the destructive interference of THz waves from the ionization front propagating with the superluminal velocity. Refraction on the plasma is not the major factor in ring formation.

Characterizing the PAH emission in the Orion Bar

ABSTRACT We present 5–14 $\mu$m spectra at two different positions across the Orion Bar photodissociation region (PDR) obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope and 3.3 $\mu$m PAH observations obtained with the Stratospheric Observatory for Infrared Astronomy (SOFIA). We aim to characterize emission from Polycyclic Aromatic Hydrocarbon (PAH), dust, atomic and molecular hydrogen, argon, sulfur, and neon as a function of distance from the primary illuminating source. We find that all the major PAH bands peak between the ionization front and the PDR front, as traced by H2, while variations between these bands become more pronounced moving away from this peak into the face-on PDRs behind the PDR front and at the backside of the H ii region. While the relative PAH intensities are consistent with established PAH characteristics, we report unusual behaviours and attribute these to the PDR viewing angle and the strength of the FUV radiation field impinging on the PDRs. We determine the average PAH size which varies across the Orion Bar. We discuss subtle differences seen between the cationic PAH bands and highlight the photochemical evolution of carbonaceous species in this PDR environment. We find that PAHs are a good tracer of environmental properties such as the strength of the FUV radiation field and the PAH ionization parameter.