Midcourse Space Experiment Satellite Research Articles

Search for OB stars running away from young star clusters

Dynamical few-body encounters in the dense cores of young massive star clusters are responsible for the loss of a significant fraction of their massive stellar content. Some of the escaping (runaway) stars move through the ambient medium supersonically and can be revealed via detection of their bow shocks (visible in the infrared, optical or radio). In this paper, which is the second of a series of papers devoted to the search for OB stars running away from young (several Myr) Galactic clusters and OB associations, we present the results of the search for bow shocks around the star-forming region NGC 6357. Using the archival data of the Midcourse Space Experiment (MSX) satellite and the Spitzer Space Telescope, and the preliminary data release of the Wide-Field Infrared Survey Explorer (WISE), we discovered seven bow shocks, whose geometry is consistent with the possibility that they are generated by stars expelled from the young star clusters, Pismis 24 and AH03 J1725-34.4, associated with NGC 6357. Two of the seven bow shocks are driven by the already known O stars. Follow-up spectroscopy of three other bow shock-producing stars showed that they are O-type stars as well, while the 2MASS photometry of the remaining two stars suggests that they could be B0 V stars, provided that both are located at the same distance as NGC 6357. Detection of numerous massive stars ejected from the very young clusters is consistent with the theoretical expectation that star clusters can effectively lose massive stars at the very beginning of their dynamical evolution and lends strong support to the idea that probably all field OB stars have been dynamically ejected from their birth clusters. A by-product of our search for bow shocks around NGC 6357 is the detection of three circular shells typical of luminous blue variable and late WN-type Wolf-Rayet stars.

Search for OB stars running away from young star clusters

N-body simulations have shown that the dynamical decay of the young (~1 Myr) Orion Nebula cluster could be responsible for the loss of at least half of its initial content of OB stars. This result suggests that other young stellar systems could also lose a significant fraction of their massive stars at the very beginning of their evolution. To confirm this expectation, we used the Mid-Infrared Galactic Plane Survey (completed by the Midcourse Space Experiment satellite) to search for bow shocks around a number of young ($\la$ several Myr) clusters and OB associations. We discovered dozens of bow shocks generated by OB stars running away from these stellar systems, supporting the idea of significant dynamical loss of OB stars. In this paper, we report the discovery of three bow shocks produced by O-type stars ejected from the open cluster NGC6611 (M16). One of the bow shocks is associated with the O9.5Iab star HD165319, which was suggested to be one of "the best examples for isolated Galactic high-mass star formation" (de Wit et al. 2005, A&A, 437, 247). Possible implications of our results for the origin of field OB stars are discussed.

Polar mesospheric clouds: Infrared measurements from the Midcourse Space Experiment

Spatial Infrared Imaging Telescope (SPIRIT) III radiometer on the Midcourse Space Experiment (MSX) satellite measured highly structured infrared, IR, emission from polar mesospheric cloud (PMC) ice particles at northern latitudes above 51 degrees on 22 July 1996 in the 11.1 to 13.2 and 18.2 to 25.1 μm radiometer channels, bands C and E, respectively. Measurements of the PMC thermal emissions included the observation of an extended cloud at 84.8°N and 325.6°E at 0313:25 UT, a local solar time of approximately 0056. In this Earth limb observation, the radiance due to the PMC has been isolated from other sources‐atmospheric emission, nonrejected off axis radiation from the terrestrial surface and zodiacal radiance‐and inverted to determine the volume emission rates of the ice particles at a spatial resolution of 0.3 km in the altitude range from 83.4 to 86.4 km. The band C PMC volume emission rate profile has a maximum value at 84.0 ± 0.3 km and decreases to one half the peak value at 85.0 and 83.5 km. Temperatures in the range from 143 ± 7 to 130 ± 8 K and ice volume densities from 1.5 to 0.5 × 10−13 cm3 per cm3 were determined from the LWIR volume emission rates at altitudes from 83.4 to 86.4 km. The PMC ice densities are equivalent to an enriched gas phase water mixing ratio of 8 to 16 parts per million by volume, ppmv, and a vertical column mass density of 3.3 × 10−8 gms cm−2 in this observation.

Auroral NO+ 4.3 μm emission observed from the Midcourse Space Experiment: Multiplatform observations of 9 February 1997

The Spatial Infrared Imaging Telescope III (SPIRIT III) radiometer on the Midcourse Space Experiment satellite (MSX), observed enhanced 4.3 μm emission from a very well characterized aurora over the Barents Sea on 9 February 1997, in conjunction with observations by the POLAR and FAST satellites, the Loparskaya ground site, and ultraviolet and visible spectrometers aboard MSX. Measurements of the auroral location, form, spatial extent and dosing conditions were applied to specify the component of auroral 4.3 μm radiance due to the slowly produced and optically thick CO2ν3 (001‐000) transition. An analysis based on the Auroral Atmospheric Radiance Code (AARC) indicates: (1) the emission originates near and beyond the tangent point; (2) the optically thick CO2ν3 radiation is largely self absorbed by the intervening atmosphere; and (3) the auroral enhancement is predominantly due to NO+ Δv = 1 vibrational state transitions. In addition, the analysis indicates that the previously reported laboratory result for the NO+ v ≥ 1 vibrational yield from the reaction, N+ + O2, is insufficient to account for the observed 4.3 μm emission. In order to explain the current results, we propose that there is additional production from the reaction, N2+ + O, forming NO+ in vibrational levels 0, 1, and 2 with relative populations of approximately 0.25, 0.5, and 0.25, respectively. The combined production processes yield an energetic electron induced efficiency of 0.56 ± 0.18 photons per auroral ion pair for NO+ Δv = 1 emission at altitudes equal to or greater than 112 km.

V733 Cep (Persson's Star): A New FU Orionis Object in Cepheus

Persson recently found that a faint star had appeared in a cloud in Cepheus. A CCD image shows a R ~ 17.3 nebulous star, now known as V733 Cep, located in the L1216 = Cep F cloud at the apex of a cavity in the cloud. Infrared photometry indicates a modest infrared excess. Optical spectroscopy shows a well-defined Li I λ6707 line, and blueshifted absorption troughs at the Hα and Na I D lines extending to at least 200 km s-1, indicative of a massive fast wind. An infrared 1-4 μm spectrum of V733 Cep shows the presence of strong water vapor features, and is almost identical to a similar IR spectrum of FU Ori that is reddened by AV = 8 mag. Assuming an intrinsic energy distribution similar to that of FU Ori, V733 Cep has a luminosity of about 135 L⊙ at the assumed distance of 800 pc. The star was detected by the Midcourse Space Experiment satellite at 8.3 μm, but not by IRAS. Nor is it detected at 850 μm, indicating that while the star possesses circumstellar material it is not surrounded by a significant cool envelope. A 12CO(3-2) map shows what appears to be a small molecular outflow along the same axis as the cavity seen in optical images. There is evidence for a limited amount of other low- and medium-mass star formation in the Cep F cloud. The totality of the evidence strongly indicates that V733 Cep is a new FU Ori-type object that must have erupted sometime between 1953 and 1984.

A Herbig-Haro object associated with GGD30 and its exciting source

GGD30 has been suggested to be either a small reflection nebulosity or a Herbig-Haro (HH) object formed in the outflow from a nearby obscured star. Observations to date have not been able to distinguish between these two scenarios. In addition, there are conflicting proposals for the location of the exciting source for GGD30. To resolve these questions, we have carried out optical spectroscopy and near-infrared (J, K and 3.6-μm) imaging of GGD30. Taken together, these observations reveal that the bright optical knot in GGD30 must be a HH object, excited by the outflow from an optically obscured pre-main-sequence (PMS) star located ∼3 arcsec to the southwest. Based on mid-infrared fluxes from the Mid-course Space Experiment (MSX) satellite, we estimate the luminosity of this PMS star to be ∼12.5L ⊙ which suggests it is an intermediate-mass object rather than low-mass as previously proposed. The optical spectroscopy indicates projected velocities of ∼ -270 km s -1 associated with the HH object. The fact that these velocities are blueshifted and relatively high compared to the velocities typical of HH flows suggests that the outflow from the PMS star must be almost aligned with the line of sight. There is an additional low-velocity (∼ -70 km s -1 ) Hα component but its origin is not clear.

A Catalog ofMidcourse Space ExperimentInfrared Dark Cloud Candidates

We use 8.3 μm mid-infrared images acquired with the Midcourse Space Experiment satellite to identify and catalog infrared dark clouds (IRDCs) in the first and fourth quadrants of the Galactic plane. Because IRDCs are seen as dark extinction features against the diffuse Galactic infrared background, we identify them by first determining a model background from the 8.3 μm images and then searching for regions of high decremental contrast with respect to this background. IRDC candidates in our catalog are defined by contiguous regions bounded by closed contours of a 2 σ decremental contrast threshold. Although most of the identified IRDCs are actual cold dark clouds, some as yet unknown fraction may be spurious identifications. For large high-contrast clouds, we estimate the reliability to be 82%. Low-contrast clouds should have lower reliabilities. Verification of the reality of individual clouds will require additional data. We identify 10,931 candidate IRDCs. For each IRDC, we also catalog cores. These cores, defined as localized regions with at least 40% higher extinction than the cloud's average extinction, are found by iteratively fitting two-dimensional elliptical Gaussian functions to the contrast peaks. We identify 12,774 cores. The catalog contains the position, angular size, orientation, area, peak contrast, peak contrast signal-to-noise, and integrated contrast of the candidate IRDCs and their cores. The distribution of IRDCs closely follows the Galactic diffuse mid-infrared background and peaks toward prominent star-forming regions, spiral arm tangents, and the so-called 5 kpc Galactic molecular ring.

The modelling of infrared dark cloud cores

This paper presents results from modelling 450 micron and 850 micron continuum and HCO+ line observations of three distinct cores of an infrared dark cloud (IRDC) directed toward the W51 GMC. In the sub-mm continuum these cores appear as bright, isolated emission features. One of them coincides with the peak of 8.3 micron extinction as measured by the Midcourse Space Experiment satellite. Detailed radiative transfer codes are applied to constrain the cores' physical conditions to address the key question: Do these IRDC-cores harbour luminous sources? The results of the continuum model, expressed in the $\chi^2$ quality-of-fit parameter, are also constrained by the absence of 100 micron emission from IRAS. For the sub-mm emission peaks this shows that sources of 300 solar luminosities are embedded within the cores. For the extinction peak, the combination of continuum and HCO+ line modelling indicates that a heating source is present as well. Furthermore, the line model provides constraints on the clumpiness of the medium. All three cores have similar masses of about 70-150 solar masses and similar density structures. The extinction peak differs from the other two cores by hosting a much weaker heating source, and the sub-mm emission core at the edge of the IRDC deviates from the other cores by a higher internal clumpiness.

Spectrum of a Leonid meteor from 110 to 860 nm

During the Leonid meteor shower on 18 November 1999, the five spectrographic imagers onboard the Midcourse Space Experiment (MSX) satellite recorded the first complete meteor spectra from 110 to 860 nm. The observation occurred at 00:23:36.2 UT, at which time satellite was pointed at a tangent altitude of 100 km over 37.2°N and 78.2°E. The spectrograph slits were oriented approximately parallel to the horizon, and the meteor passed approximately perpendicular through the slits’ fields of view. All five spectrographic images serendipitously observed the passage of the object and each recorded 1–2 frames of data. Common meteor emissions from iron, sodium, and oxygen were observed in the visible, but the ultraviolet spectrum displayed a wealth of new features. The most intense features appeared in the middle ultraviolet (250–300 nm) from neutral and singly ionized iron and singly ionized magnesium. Far ultraviolet (110–130 nm) features included oxygen and nitrogen, a bright hydrogen Lyman α emission, and ionized iron and magnesium emissions. Also seen in the far ultraviolet were weak but recognizable emissions from neutral carbon. The ultraviolet emissions were much stronger than those in the visible and near-infrared. The meteor had a visual magnitude of −2.8 at 100 km, but the energy of emissions in the ultraviolet (110–337 nm) exceeded the energy of the visible (337–560 nm) by a factor of at least 5. The new Lyman α and carbon emissions in the far UV as well as the overall intensity of the ultraviolet spectrum suggest important applications for ultraviolet observations of meteors.

Intercomparison of MSX/UVISI‐derived ozone and temperature profiles with ground‐based, SAGE II, HALOE, and POAM III data

Recently, we demonstrated a combined extinctive and refractive stellar occultation technique on a proof‐of‐concept basis using data from the Ultraviolet and Visible Imagers and Spectrographic Imagers (UVISI) on the Midcourse Space Experiment (MSX) satellite. Beyond this initial presentation of the technique, we also used MSX/UVISI data obtained during the Stratospheric Aerosol and Gas Experiment III (SAGE III) Ozone Loss and Validation Experiment (SOLVE) campaign in 2000 to demonstrate the utility of stellar occultation data in scientific analyses. In the present work, we compare the retrieved ozone and temperature profiles from numerous MSX/UVISI stellar occultations to ground‐based, SAGE II, Halogen Occultation Experiment (HALOE), and Polar Ozone and Aerosol Measurement III (POAM III) data. The MSX/UVISI results derived using the combined occultation method are shown to compare favorably with all these data. Small biases in the comparisons hint at interesting differences in the nature of the various ozone measurement techniques, but the overall agreement in the comparisons clearly illustrates the utility of the combined technique as a complementary means of long‐term ozone monitoring.

Observations of Star-Forming Regions with theMidcourse Space Experiment

We have imaged seven nearby star-forming regions, the Rosette Nebula, the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5, with the Spatial Infrared Imaging Telescope on the Midcourse Space Experiment (MSX) satellite at 18'' resolution at 8.3, 12.1, 14.7, and 21.3 μm. The large angular scale of the regions imaged (~7.2–50 deg2) makes these data unique in terms of the combination of size and resolution. In addition to the star-forming regions, two cirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the south Galactic pole (MSXBG 239) were also imaged. Point sources have been extracted from each region, resulting in the identification over 500 new sources (i.e., no identified counterparts at other wavelengths), as well as over 1300 with prior identifications. The extended emission from the star-forming regions is described, and prominent structures are identified, particularly in W3 and Orion. The Rosette Nebula is discussed in detail. The bulk of the mid-infrared emission is consistent with that of photon-dominated regions, including the elephant trunk complex. The central clump, however, and a line of site toward the northern edge of the cavity show significantly redder colors than the rest of the Rosette complex.

Ultraviolet imaging and spectrographic imaging of polar mesospheric clouds

The Ultraviolet and Visible Imaging and Spectrographic Imaging (UVISI) instrument on the Midcourse Space Experiment (MSX) satellite obtained unique ultraviolet (200–350 nm) images and spectrographic images of polar mesospheric clouds over both the northern and southern poles during the summers of 1997–1999. Imager observations revealed that PMCs have essentially the same altitudes in the north and south (82.7 ± 1.3 km) and that cloud altitudes remain essentially constant with latitude. Imagery from below the horizon showed PMC structures with scales ranging from 100 to over 500 km. Integrated from 235 nm to 265 mn, PMC limb intensities were essentially the same in both hemispheres, averaging 12 mega-Rayleighs at peak altitude. Intensities increased with latitude toward the poles. Spectrographic observations reveal the clouds have a solar-like spectrum from 200 nm to 600 nm, but that short of 315 nm the spectrum is contaminated with ground albedo effects. Between 200 nm and 315 nm, the scattering ratio (PMC spectral intensity to solar irradiance) is less steep than that expected from pure Rayleigh scattering. When the scattering ratio is least-squares fit to a log-normal distribution of Mie scatterers, the distribution has a mode radius of 65 nm and a dispersion of 1.2 with no dependence on latitude or time from solstice. The slope of the scattering ratio changes as a function of altitude relative to the altitude peak of the PMC intensity, suggesting that particles 3 km above this peak are smaller than particles at the peak and 3 km below.

Leonid meteor spectrum from 110 to 860 nm

During the Leonid meteor shower on 18 November 1999, the five spectrographic imagers onboard the Midcourse Space Experiment (MSX) satellite recorded the first complete meteor spectra from 110 to 860 nm. The observation occurred at 00:23:36.2 UT, at which time the satellite was pointed at a tangent altitude of 100 km over 37.2°N and 78.2°E. The spectrograph slits were oriented approximately parallel to the horizon at a tangent altitude of 100 km, and the meteor passed approximately perpendicular through the slits’ fields of view. All five spectrographic imagers observed the passage of a bright object ( m v < −2.8 at 100 km) and each recorded several frames of data. In the visible, common meteor emissions were observed from iron, sodium, and oxygen. However, the ultraviolet spectrum displayed a wealth of more intense features, some of which actually caused saturation in the spectrographs. The most intense features appeared between 220 and 300 nm and are attributed to neutral and singly ionized iron and ionized magnesium. Some unknown emissions, possibly from an unidentified molecular species such as iron oxide, appear between 180 and 220 nm. In the far ultraviolet from 110 to 130 nm, oxygen and nitrogen features appear in the spectrum, with some features from ionized iron and magnesium. In particular, the FUV spectrum showed an intense emission from hydrogen Lyman alpha and a much weaker emission from what appeared to be neutral carbon. The atmospheric emissions can be associated with the heating within the meteor shock, while the metallic emissions originate from the fireball of the meteor proper. The ultraviolet emissions were much stronger than those in the visible and near-infrared parts of the spectrum. The energy of emissions in the ultraviolet (110 < λ < 337 nm) exceeded the energy of the visible (337 < λ < 650 nm) by a factor of at least 5.

The Large‐Scale Bipolar Wind in the Galactic Center

During a 9-month campaign (1996--1997), the Midcourse Space Experiment (MSX) satellite mapped the Galactic Plane at mid-infrared wavelengths (4.3--21.3um). Here we report evidence for a spectacular limb- brightened, bipolar structure at the Galactic Center extending more than a degree (170 pc at 8.0 kpc) on either side of the plane. The 8.3um emission shows a tight correlation with the 3, 6 and 11 cm continuum structure over the same scales. Dense gas and dust are being entrained in a large-scale bipolar wind powered by a central starburst. The inferred energy injection at the source is ~10^54/kappa erg for which \kappa is the covering fraction of the dusty shell (kappa <= 0.1). There is observational evidence for a galactic wind on much larger scales, presumably from the same central source which produced the bipolar shell seen by MSX. Sofue has argued that the North Polar Spur -- a thermal x-ray/radio loop which extends from the Galactic Plane to b = +80 deg -- was powered by a nuclear explosion (1-30 x 10^55 erg) roughly 15 Myr ago. We demonstrate that an open-ended bipolar wind (~10^55 erg), when viewed in near-field projection, provides the most natural explanation for the observed loop structure. The ROSAT 1.5 keV diffuse x-ray map over the inner 45 deg provides compelling evidence for this interpretation. Since the faint bipolar emission would be very difficult to detect beyond the Galaxy, the phenomenon of large-scale galactic winds may be far more common than has been observed to date.

Particle characteristics from the spectra of polar mesospheric clouds

Spectrographic imagers on the Midcourse Space Experiment (MSX) satellite have obtained the first spectra of polar mesospheric clouds (PMCs) from the far ultraviolet to the visible (164–585 nm). Several hundred PMC spectra were extracted over a 3 week period during the summer of 1999. To improve statistics, the spectra were normalized and averaged within 32 groups of clouds having common latitudes and scattering angles. The subsequent spectra, from three spectrographic imagers, produced a PMC signal that fell into the noise at wavelengths short of 200 nm and showed Earth albedo effects longward of ∼315 nm. The spectra between these limiting wavelengths were subjected to a least squares analysis to determine characteristics of the scattering particles assuming a lognormal distribution of Mie scatterers made of ice. Averaged over all PMCs, the distribution mode (size) was 65.2 ± 2.2 nm and the distribution dispersion (width) was 1.15 ± 0.04. Over the period of observation, neither the mode nor the dispersion exhibited systematic variations in time or latitude, although less intense clouds at lower latitudes may have been undersampled.

Observations of Galaxies with the [ITAL]Midcourse Space Experiment[/ITAL

We have imaged eight nearby spiral galaxies with the SPIRIT III infrared telescope on the Midcourse Space Experiment (MSX) satellite in the mid-infrared at 18'' resolution at 8.3, 12.1, 14.7, and 21.3 μm. Each of the eight shows interesting structure not previously detected with older, lower resolution infrared data sets, such as a resolved nucleus or spiral structure. The MSX data are compared with existing data sets at ultraviolet, optical, and infrared wavelengths, including recent observations from the Infrared Space Observatory. The infrared structures in M83 and NGC 5055 show a striking similarity to the ultraviolet emission but are less similar to the optical emission. Several point sources with no identified counterparts at other wavelengths are found near M31, NGC 4945, M83, and M101. Over 200 previously known objects are also detected at 8 μm.

Atmospheric remote sensing using a combined extinctive and refractive stellar occultation technique 1. Overview and proof‐of‐concept observations

A new approach to optical remote sensing of the Earth's atmosphere using a combination of extinctive and refractive stellar occultation measurements is presented. In this combined method, spectrographic imagers are used to measure the wavelength‐dependent atmospheric extinction of starlight while a co‐aligned imager is used to measure the atmospheric refraction along the same line of sight. By simultaneously measuring both the refraction and extinction of the starlight, the composition‐dependent extinction measurements can more accurately probe the Earth's lower atmosphere, where refraction effects are significant. The refraction measurements provide the bulk atmospheric properties and the actual light path through the atmosphere, both of which are necessary to correctly infer the total extinction in the refractive regime. The technique is demonstrated on a proof‐of‐concept basis using data from the Ultraviolet and Visible Imagers and Spectrographic Imagers (UVISI) on the Midcourse Space Experiment (MSX) satellite. These preliminary results show that the combined approach has the potential to be a powerful, self‐calibrating method for remotely sensing the Earth's atmosphere in general and for the determination of ozone profiles in the stratosphere and upper troposphere in particular.

Photodissociation regions and star formation in the Carina nebula

We have obtained wide-field thermal infrared (IR) images of the Carina nebula, using the SPIREX/Abu telescope at the South Pole. Emission from polycyclic aromatic hydrocarbons (PAHs) at 3.29 μm, a tracer of photodissociation regions (PDRs), reveals many interesting well-defined clumps and diffuse regions throughout the complex. Near-IR images (1-2 μm), along with images from the Midcourse Space Experiment (MSX) satellite (8 - 21 μm) have been incorporated to study the interactions between the young stars and the surrounding molecular cloud in more detail. Two new PAH emission clumps have been identified in the Keyhole nebula, and have been mapped in 1 2 CO(2-1) and (1-0) using the Swedish-ESO Submillimetre Telescope (SEST). Analysis of their physical properties reveals that they are dense molecular clumps, externally heated with PDRs on their surfaces and supported by external pressure in a similar manner to the other clumps in the region. A previously identified externally heated globule containing IRAS 10430-5931 in the southern molecular cloud shows strong 3.29-, 8- and 21-μm emission, the spectral energy distribution (SED) revealing the location of an ultracompact (UC) HΙΙ region. The northern part of the nebula is complicated, with PAH emission intermixed with mid-IR dust continuum emission. Several point sources are located here, and through a two-component blackbody fit to their SEDs we have identified three possible UC HΙΙ regions as well as a young star surrounded by a circumstellar disc. This implies that star formation in this region is ongoing and not halted by the intense radiation from the surrounding young massive stars.

Midcourse Space Experiment/Ultraviolet and Visible Imaging and Spectrographic Imaging limb observations of combined proton/hydrogen/electron aurora

Simultaneous measurements of auroral limb H Lyman α, H Balmer α, H Balmer β, N2+ 1 NG 391.4‐nm, and N2 2 PG 337.1‐nm emissions excited by combined proton/hydrogen/electron precipitation are reported. The data were recorded by the Ultraviolet and Visible Imaging and Spectrographic Imaging (UVISI) spectrographic imagers on the Midcourse Space Experiment (MSX) satellite on November 10, 1996, while viewing a diffuse emission region during a limb‐scanning data collection event. Energy fluxes associated with the electron and proton/hydrogen precipitation components are estimated with one‐dimensional fits to selected limb profiles using a transport‐theoretic model. Spectral radiances (110–900 nm) are also presented. The data presented here are but one example of the large number of MSX/UVISI data sets that have been collected offering opportunities for scientific investigations of auroral emission phenomena and for retrieval of composition and particle precipitation parameters from optical remote sensing data.

Transpolar structure of polar mesospheric clouds

Middle‐ultraviolet (210‐ to 252‐nm) images have revealed the transpolar structure of polar mesospheric clouds (PMCs) at a spatial resolution of 3 km. The ultraviolet and visible imaging and spectrographic imaging instrument on the Midcourse Space Experiment (MSX) satellite collected over 27,000 mid‐UV images of PMCs during 26 passes over the north and south polar regions during the summer seasons of 1997, 1998, and 1999. A Lomb periodogram analysis of PMC radiance projected to an 83‐km altitude reveals periodic structures with wavelengths ranging from ∼100 to ∼3000 km. In either hemisphere, more PMCs have features with wavelengths shorter than 1000 km than longer, and a crude spectrum of the PMC structures suggests a spectral peak between 500 and 1000 km. There is little evidence of structures having wavelengths short of ∼100 km, and the longer wavelengths generally have more spectral “power” than the shorter wavelengths. PMC structures do not remain stable over time periods of weeks, but may retain similar structural features for at least as long as 24 hours. The clouds may be considered markers of gravity waves, which carry energy from the lower atmosphere to the mesosphere and modulate the appearance of PMCs.