Short-wavelength Features Research Articles

The effects of fluid escape on accretionary wedges 1. Variable porosity and wedge convexity

Fluid flow out of accretionary wedges is a consequence of the porosity reduction of wedge material both with depth and with distance from the toe of the wedge. Changes in porosity result in changes in bulk density, because bulk density is a function of solid density and porosity. These variations in bulk density in turn affect the critical taper of the wedge. The critical taper of a wedge with spatially varying strength, pore pressure, and/or bulk density is dependent not only on the value of these parameters, but on their first derivatives as well [Dahlen, 1990]. Using the equations derived by Dahlen we find that the arc ward decrease in porosity results in the commonly observed convex wedge surface. A hypothetical model using a density‐depth function generated from a synthesis of accretionary wedge data [Bray and Karig, 1985] demonstrates that the surface angle can decrease between 0.5 and 1° in the first 60 km from the toe of the wedge. Steeper wedges exhibit a greater degree of convexity (greater changes in slope). The amount of change in slope is relatively insensitive to the internal strength of the wedge. Wide angle seismic data from the Barbados accretionary wedge [Bangs et al., 1990] imaged velocity variations from the surface down to the décollement for a distance of 100 km from the toe. These data can be used to map the spatial variations in bulk density within the Barbados accretionary wedge. Holding strength and pore pressure values constant, and allowing only bulk density to vary, results in a model that closely matches the observed bathymetry both in overall convexity and in shorter wavelength bathymetrie features. The model allows a limited range of acceptable basal pore pressure values for the wedge. The ability of the model to accurately match the majority of the observed bathymetry suggests that bulk density variations are of first order importance for determining not only the wedge shape but also the shorter wavelength features. The model, however, does not produce the most pronounced changes in slope at the ten kilometers nearest the toe. This suggests that other factors, such as variations in pore pressure, sediment strength, or cohesion, are more important influences on the wedge taper nearest the toe.

Translated origin spherical cap harmonic analysis

The method of spherical cap harmonic analysis (SCHA), due to Haines (1985) is appropriate for regional geomagnetic field modelling as it includes the required potential field constraints and, for a given number of model parameters, describes shorter wavelength features than a global spherical harmonic model. If the origin of the coordinate system is moved from the centre of the Earth towards the surface then the Earth's surface is no longer equidistant from the origin. At the Earth's surface the minimum wavelength described by a SCH model in the new coordinate system is smaller at the centre of the region than at the edge. This method of translated origin spherical cap harmonic analysis (TOSCA) has been applied to regional field modelling for Italy. The method is able to take advantage of the dense distribution of data at the centre of region and the model effectively smooths towards the periphery. The performance of the TOSCA model is discussed in relation to a model derived using conventional SCHA.

The canadian aeromagnetic database:Evolution and applications to the definition of major crustal boundaries

The Aeromagnetic Survey Programme of the Geological Survey of Canada, much of which was carried out under Federal-Provincial agreements, has resulted in over 10,000,000 line kilometres of data flown at a mean elevation of 305 m and a mean line separation of 800 m except in mountainous terrain where constant barometric altitudes were maintained for given areas. These data have been digitized (if not originally digitally recorded) and used to produce the 1:1,000,000 “Magnetic Anomaly Map Series” of the Geological Survey of Canada and associated gridded data. These grids were then combined with private data, shipborne data from the east and west coasts and U.S. Navy data from the high Arctic to produce the first all-digital version of the “Magnetic Anomaly Map of Canada”. A study of the longer wavelength features on this anomaly map has resulted in greatly enhanced understanding of the major tectonic units and crustal boundaries within the Canadian landmass. Important features can be delineated by data enhancement and processing techniques such as upward continuation to emphasize deep sources, vertical gradients to sharply outline contacts (e.g. the boundaries of the economically important greenstone belts in the Canadian Shield), and shaded relief to highlight low-amplitude short-wavelength features. An additional technique which provides interesting insights is curvature analysis, which can be used to calculate the strike of an anomaly at each point. When plotted with a particular colour assigned according to the strike direction, the resulting patterns help to delineate trends over long distances.

Deep-Tow magnetics near 20°S on the East Pacific Rise: A study of short wavelength anomalies at a very fast spreading center

Six Deep-Tow magnetic profiles across the axis of the East Pacific Rise [EPR] in two small areas between 19°25′ and 20°10′S were collected during the 1983 Protea 1 cruise of the R/V Melville. These near-bottom profiles are of extremely high resolution allowing the interpretation of very short wavelength features. We have inverted the magnetic field data to determine the rock magnetization distribution near the axis of this ultrafast speading center (162 mm yr-1). The solutions reveal large amplitude (up to 35 A m-1) short wavelength (1–3 km) variations in magnetization. Specifically all crossings show a narrow (0.5 to 1.5 km) low in magnetization superimposed on a broader (2.5 to 4 km) high directly over the ridge axis. Four profiles in the northern area (19°25′ to 19°33′S) also show symmetrical near-axis (within 4 km) lows which are remarkably continuous along strike. Explanations for the short-wavelength variations are discussed which fall into the following categories: (1) variations in the thickness of the magnetized layer, (2) variations in rock chemistry (e.g. alteration due to hydrothermal activity), and (3) paleofield intensity variations. None of the mechanisms discussed alone adequately explain the observed phenomena in the study area or on a world-wide scale. Further sampling and high resolution surveying will be required in order to accurately determine the relative importance of the mechanisms discussed.

On the relationship between isostatic elevation and the wavelengths of tectonic surface features on Venus

Regional isostatic elevation ( d) and the widths and spacings (wavelengths, λ) of tectonic features formed due to horizontal extension and compression are observed quantities on Venus that are sensitive to the thermal and compositional structure of the lithosphere. To investigate the relationships between these parameters and the implications for Venus' lithosphere structure, we formulate models in which d and λ are functions of surface thermal gradient ( dT/ dz 0) and crustal thickness ( c). In models that relate λ to dT/ dz 0 and c, we impose a requirement that for tectonic features with multiple wavelengths of deformation to develop, the growth rates of instabilities that control the dominant wavelengths must be “similar” in magnitude. This constrains the maximum values of c and dT/ dz 0 that can result in multiple tectonic wavelengths on Venus to be less than or equal to those determined in our previous study, c ⩽ 30 km and dT/ dz 0 ⩽ 25°K km − (M. T. Zuber, 1987, J. Geophys. Res. 92, E541–E551) . We also estimate the strength of Venus' upper crust in areas with multiple wavelengths and obtain limits that are most consistent with a compressional origin for ridge belts, which are enigmatic features that have been purported to be of both extensional and compressional origin. In models that relate d, λ, dT/ dz 0, and c, we show that in areas of Venus where the upper mantle is stronger than the upper crust, the spacings of short-wavelength ( λ ≲ 30 km) features should increase with increasing d if the change in d is due to increasing c, but should decrease with increasing d if the change in d is due to increasing dT/ dz 0. Long-wavelength ( λ ≳ 30 km) features are not strongly correlated to variations in d due to the effect of crustal thickness on the depth distribution of lithospheric strength, which influences wavelength selection in a complex manner. In areas of Venus where the upper mantle is weaker than the upper crust, the spacings of short-wavelength features should remain constant with increasing d if the change in d is due to a change in c, but should decrease with increasing d if the change in d is due to increasing dT/ dz 0. The results demonstrate that in shallowly compensated areas of Venus that contain regularly spaced tectonic features, it may be possible to distinguish whether isostatic elevation differences are a consequence of variations in crustal thickness or thermal gradient. We interpret the observed relationship of λ with d for short-wavelength tectonic features in several tessera regions to reflect crustal thickness variations.

Force Budget: III. Application to Three-dimensional Flow of Byrd Glacier, Antarctica

AbstractStresses at the surface and at depth are calculated for a stretch of Byrd Glacier, Antarctica. The calculations are based on photogrammetrically determined velocities and elevations, and on radio-echo-determined ice thicknesses. The results are maps of drags from each valley wall, of normal forces laterally and longitudinally, and of basal drag. Special challenges in the calculation are the numerical gridding of velocity, ensuring that unreasonable short-wavelength features do not develop in the calculation, and inference of ice thickness where there are no data.The results show important variations in basal drag. For the floating part, basal drag is near zero, as expected. Within the grounded part, longitudinal components of basal drag are very variable, reaching 300 kPa with a dominant wavelength of 13 km. Generally, these drag maxima correlate with maxima in driving stress. Usually the across-glacier component of basal drag is small. An important exception occurs in the center of the grounded part of the glacier where the flow shows major deviations from the axis of the valley.Other results are that side drag is roughly constant at 250 kPa along both margins of the glacier, tension from the ice shelf is about 100 kPa, and tension in the grounded part cycles between 250 and 150 kPa. Calculated deep velocities are too large and this is attributed to deficiencies in the conventional isotropic flow law used.

Tests on the recovery of precise geoid height differences from gravimetry

To transform Global Positioning System (GPS) derived ellipsoidal height differences, observable to 2–3 ppm of the station separation, into orthometric heights without loss of precision, geoid‐ellipsoid spatial height differences (ΔN) need to be evaluated to this same precision. One of the gravimetric techniques which has been shown, in theory, to be capable of achieving this precision uses ring integration to sense the short wavelength contribution of the local gravity field to ΔN, in combination with a geopotential model to provide the medium to long wavelength contribution. In this paper, comparisons of ΔN, computed by this method, with ΔN values derived from GPS and conventional levelling, have answered many questions relating to computational procedure and have demonstrated the viability of the method. Agreements in ΔN have been accomplished at about 2 to 3 ppm for shorter (<60 km) lines, and at <2 ppm for longer lines. This has been achieved by integrating reduced gravity anomalies over a spherical cap of radius about 1° to find the short wavelength features in ΔN. The gravity anomalies are reduced to a high‐order geopotential model (OSU81, nmax = 180), and this same model is used to generate the longer‐wavelength features of ΔN. It appears that, for a model of this high order, the radius of the cap size is not critical. However, using a lower‐order (e.g., nmax = 90) model as a reference, it appears that the optimum radius, while being in the region of π/nmax, is more critical, although comparisons in ΔN at 2 to 3 ppm are still achievable. The ability of geopotential model OSU81 alone to recover ΔN varies with location and degree. In two of the areas the comparison was at about 10 ppm or worse for nmax = 180, with the fit improving with decrease in nmax. By contrast, the other two tests showed control ΔN at about 3 to 4 ppm for nmax= 180, with the fit deteriorating with decreasing nmax, the behavior one would expect. Apparently the quality of the geopotential model varies with location. It is striking, however, that no matter how poorly the model recovered ΔN, the addition of an inner zone contribution, integrated over a surprisingly small cap size (<0.5°), improved the comparisons and in each case brought them down to the level of 3 to 4 ppm. These tests serve to confirm the viability of the ring integration method for evaluating ΔN, while giving valuable guidance as to the optimum configuration for the technique.

Force Budget: Application to Three-Dimensional Flow of Byrd Glacier, Antarctica (Abstract)

Stresses at the surface and at depth are calculated for a stretch of Byrd Glacier, Antarctica. The calculations are based on photogrammetrically determined velocities and elevations (Brecher 1986) and on radio-echo-determined ice thicknesses. The results take the form of maps of drags from each valley wall, of normal forces laterally and longitudinally, and of basal drag. Special challenges in the calculation are the numerical gridding of velocity (ensuring that unreasonable short-wavelength features do not develop in the calculation) and inference of ice thicknesses where there are no data. The results show important variations in basal drag. For the floating part, basal drag is near zero, as expected. Within the grounded part, longitudinal components of basal drag are very variable, reaching 300 kPa, with a dominant wavelength of 10 km. Generally these drag maxima correlate with maxima in driving stress, and the across-glacier component of basal drag is usually small. An important exception occurs in the center of the grounded part of the glacier, where the flow shows major deviations from the axis of the valley. Results also show that side drag is roughly constant at 250 kPa along both margins of the glacier, tension from the ice shelf is about 100 kPa, and tension in the grounded part cycles between 250 and 150kPa. Calculated deep velocities are too large and this is attributed to deficiencies in the conventional isotropic flow law used.

Force Budget: Application to Three-Dimensional Flow of Byrd Glacier, Antarctica (Abstract)

Stresses at the surface and at depth are calculated for a stretch of Byrd Glacier, Antarctica. The calculations are based on photogrammetrically determined velocities and elevations (Brecher 1986) and on radio-echo-determined ice thicknesses. The results take the form of maps of drags from each valley wall, of normal forces laterally and longitudinally, and of basal drag. Special challenges in the calculation are the numerical gridding of velocity (ensuring that unreasonable short-wavelength features do not develop in the calculation) and inference of ice thicknesses where there are no data.The results show important variations in basal drag. For the floating part, basal drag is near zero, as expected. Within the grounded part, longitudinal components of basal drag are very variable, reaching 300 kPa, with a dominant wavelength of 10 km. Generally these drag maxima correlate with maxima in driving stress, and the across-glacier component of basal drag is usually small. An important exception occurs in the center of the grounded part of the glacier, where the flow shows major deviations from the axis of the valley.Results also show that side drag is roughly constant at 250 kPa along both margins of the glacier, tension from the ice shelf is about 100 kPa, and tension in the grounded part cycles between 250 and 150kPa. Calculated deep velocities are too large and this is attributed to deficiencies in the conventional isotropic flow law used.

Surface deformation, gravity and the geoid from a three-dimensional convection model at low Rayleigh numbers

The two-dimensional surface deformation, gravity field and geoid are calculated from the temperature fields of a number of numerical models of constant viscosity three-dimensional convective flows, heated from within and from below, using the appropriate Green's functions. The admittance is positive, with positive gravity anomalies above hot rising regions, except for large aspect ratio circulations with undeformable lower boundaries. The surface deformation and the geoid are insensitive to the short wavelength features of the temperature variation. The gravity field is less smooth, though still does not clearly indicate the narrowness of the upwelling and downwelling regions at large Rayleigh numbers. When the lower boundary of the convecting region is deformable, the gravity field is dominated by lateral temperature variations within the upper thermal boundary layer, even when their contribution to the overall circulation is small. The variation of surface deformation with Rayleigh number agrees well with that expected from simple boundary layer arguments when the circulation is driven by heating from below, but less well when the heating is internal. These results suggest that the convective upwelling beneath regions showing positive geoid and residual depth anomalies is more localized than the horizontal extent of these features would suggest.

On the separation of core and crustal contributions to the geomagnetic field

The widely accepted procedure of separating the contributions to the geomagnetic field arising in the Earth's core from those arising in the Earth's crust on the basis of spherical harmonic degree number has been recently challenged by Carle & Harrison. We argue here that their basic objection is valid only for data over line profiles or flat surfaces. We feel that while Carle & Harrison have addressed a number of significant and fundamental aspects of this problem they appear not to have done so clearly. The contribution contained herein is intended as a clarification. In particular we wish to: Acknowledge, as Carl & Harrison asserted, that so-called ‘linear’ data (see text) over line profiles (or flat two-dimensional surfaces) are better represented by Fourier components than by Legendre functions in the following sense: Legendre functions do not have regularly spaced zeros over line profiles (or flat surfaces), and hence are not associated with ‘wavelengths’ in the conventional sense. Thus representation and filtering (such as removing low degree components) using Legendre functions does not satisfactorily separate long- and short-wavelength features (defined in the Fourier sense). Emphasize that the above-mentioned advantage of Fourier analysis over spherical harmonic analysis no longer exists when a (linear) data set over a curved surface is to be represented, since the conventional definition of wavelength breaks down. Demonstrate, by way of squaring a Fourier series and a spherical harmonic series (actually we consider the sample example as given by Carle & Harrison where no φ-dependence is included), that any non-linear operations on an originally linear data set, irrespective of the ‘basis functions’ in which the data are expanded, invariably create intermodulations. CarleHarrison appeared to have confused the issue of intermodulations with the issue of wavelength identification when they employed a non-linear operation in showing that ‘the use of spherical harmonics in describing the core field and in removing the core field from the total field in order to get the field produced by the crustal magnetization is flawed’. Point out that the power spectrum of the residual field after removal of certain low degree Fourier components from a data set will in general be different from that after removal of corresponding spherical harmonic components from the same data set. This observation may be considered by some to be a valid basis for advocating Fourier analysis over spherical harmonic analysis on line profile data. This argument does not hold, however, for data sets over curved surfaces such as that of the Earth, since there no longer exists the intrinsic property of wavelength attached to a low degree Fourier component vis-à-vis a spherical harmonic component of the same degree. By linearizing the problem, a general least squares inversion scheme is sketched through which the magnetic potential may be recovered from a measured geomagnetic intensity field. Limitations and strengths of the proposed scheme are discussed.

Status of the geopotential

Substantial progress has been made in modeling the geopotential field since the last reporting period (Anderle, 1979a). Models of the gravity potential show improvement across the entire gravitational spectrum with resolution down to 1° haIf‐wavelength. Major sources of data utilized for advancing our knowledge of the geopotential field consist of satellite laser ranging (Smith, 1982; Wilson, 1979; Linder, 1981) which contributed to the long wavelength portion of the spectrum, satellite altimetry (GEOS‐3, 1979; SEASAT, 1981) and improved surface gravity anomalies (Rapp, 1981b) both of which contributed to the intermediate and short wavelength features of the spectrum. Uniform global distribution of data in combination solutions is generally achieved by using altimeter data over the ocean and terrestrial gravity anomalies over land.

Paleomagnetism of Sediment Cores from Cedarburg Bog, Wisconsin, and a Comparison with Cores from Lake Michigan

Four 10-m sediment cores from the Cedarburg Bog, Wisconsin, were collected for paleomagnetic measurement and carbon-14 dating. The paleomagnetic records for these cores are self consistent, with the intensity record affording the best correlation. These results are compared with sediment cores from Lake Michigan. The two sets of cores are unmistakably correlatable on the basis of intensity. The inclination correlation also shows common long and short wavelength features. On the basis of this correlation, we date certain member boundaries of the Lake Michigan Formation as follows: Waukegan–Lake Forest, 5,500 years B.P.; Lake Forest–Winnetka, 7,800 years B.P.; and Winnetka–Sheboygan, 9,700 years B.P.

Report from IUGG Assembly: IAGA session on lithospheric mapping from local anomaly charts

The objective of this session was to review recent advances in utilizing magnetic survey data to delineate the geological formations that comprise the earth's crust, and to provide information on their genesis.P. J. Hood commenced the session with a review of the national aeromagnetic survey, which has been carried out by the Geological Survey of Canada, since 1947, as an aid to both geological mapping and mineral exploration programs in Canada. Since that time some 8 million line kilometers of aeromagnetic survey data have been obtained and have resulted in the production of, 8100 aeromagnetic maps at various scales. The specifications for aeromagnetic surveys in Canada have been continually improved upon over the years, mainly as a result of developmental work undertaken by the Geological Survey of Canada. This developmental work has also resulted in the fabrication of an inboard aeromagnetic gradiometer survey system, which measures the vertical gradient of the earth's total field in addition to being a high‐resolution total field instrument. The main advantages of the aeromagnetic gradiometer as a geological mapping tool, compared to the single‐sensor (total‐field) technique, are: (a) superior resolution of anomalies produced by closely spaced geological formations; (b) anomalies produced by nearsurface features are emphasized with respect to those resulting from more deeply buried rock formations; (c) regional gradient of the earth's magnetic field and diurnal variation are automatically removed; (d) direct delineation of vertical contacts by the zero‐gradient contour value, i.e., vertical contact mapper. In 1967 the first, colored Magnetic Anomaly Map (MAM) of Canada was produced, at a scale of 1:5,000,000, in which the dominating effects of the main earth's field were removed. Two more editions of the map have been produced in the subsequent 10‐year period. The main uses of such a magnetic anomaly map are threefold: first, as an index map it presents an overview of the aeromagnetic survey coverage of Canada; second, the map presents the major patterns produced by the continental rocks of the Canadian landmass and may be utilized both as an aid in the delineation of structural provinces and in the interpretation of regional features in the basement rocks; third, the map stimulates comparisons of magnetic features with other geological and geophysical compilations on a regional scale. More recently, commencement of colored 1:1,000,000 MAM's has also begun, and these are clearly superior to the 1:5,000,000 MAM in showing the finer details of the regional anomalies, since short wavelength features, especially dykes, tend to be eliminated at the smaller scale.

Development and analysis of a twelfth degree and order gravity model for Mars

Satellite geodesy techniques previously applied to artificial earth satellites have been extended to obtain a high‐resolution gravity field for Mars. Two‐way Doppler data collected by 10 Deep Space Network (DSN) stations during Mariner 9 and Viking 1 and 2 missions have been processed to obtain a twelfth degree and order spherical harmonic model for the martian gravitational potential. The quality of this model was evaluated by examining the rms residuals within the fit and the ability of the model to predict the spacecraft state beyond the fit. Both indicators show that more data and higher degree and order harmonics will be required to further refine our knowledge of the martian gravity field. The model presented shows much promise, since it resolves local gravity features which correlate highly with the martian topography. An isostatic analysis based on this model, as well as an error analysis, shows rather complete compensation on a global (long wavelength) scale. Though further model refinements are necessary to be certain, local (short wavelength) features such as the shield volcanos in Tharsis appear to be uncompensated. These are interpreted to place some bounds on the internal structure of Mars.

Global detailed gravimetric geoid

A 1 ° × 1 ° global detailed gravimetric geoid has been computed, using a combination of the Goddard Space Flight Center (GSFC) GEM‐8 potential field model and a set of 38,406 1° × 1° mean surface free air anomalies. Numerous short wavelength features are shown in the geoid contour map, e.g., the steep gradients associated with oceanic trenches. Comparison of this geoid with geoceiver derived and astrogeodetic geoid heights in the United States resulted in an r.m.s. difference of about 1.7 m. Comparisons with three GEOS‐3 altimeter derived geoidal profiles revealed that for areas with good surface data coverage, the relative agreement is generally better than 5 m.

Comparison of four random to grid methods

Interpolation of randomly spaced two-dimensional data on a rectangular grid—gridding—is a useful numerical procedure which facilitates subsequent processing such as filtering, contouring by hand or machine, and comparison with other data. Common problems in gridding methods for geological and geophysical data are: (1) obtaining accuracy in regions of sparse data, (2) representing short wavelength features in the data whereas also avoiding introduction of spurious features in the gridded surface, and (3) interpolation near the edges of the sampled area. Four gridding methods were analyzed and their performance compared by application to the same data set. The four methods are: (1) weighted average of closest points, (2) weighted average of three closest points with some azimuth control, (3) local polynomial surface fitting, and (4) piecewise cubic polynomial interpolation along profiles. The local polynomial surface-fitting method, which was developed for this study, is shown to be more versatile and results in a more accurate grid surface than the other gridding techniques investigated.

Computed and observed ocean topography: A comparison

This paper presents determinations of ocean topography using spacecraft altimeter techniques. The first direct spacecraft observations of sea-surface topography were made in late 1973 during the Skylab mission. Comparisons of the topography derived from the altimeter data with the ‘computed’ topography based upon the Goddard Space Flight Center Earth Gravity Model, GEM-8, derived from satellite tracking data and surface gravity data show differences of a few meters. With the presently orbiting altimeter system on the Geos-3 spacecraft, relative variations over a few hundred kilometers along the orbital track can now be detected to a few decimeters. Numerous short-wavelength features which reflect large variations of the ocean floor have been observed to a relative precision of a meter or better with Geos-3. For example, the Blake-Bahama Basin off the coast of Florida is about 4000 m lower than the continental shelf. The sea surface of the basin, as measured by the Geos-3 altimeter, is about 8 m lower than that of the continental shelf. Further, a height change of over 20 m has been measured over the Puerto Rican Trench relative to the height of the mean sea level at Puerto Rico. Preliminary results of on-going studies indicate that geostrophic height variations, for example, due to ocean currents (the Gulf Stream has an expected variation of around 1.5 m over 100 km) will be detectable using spaceborne altimeters.

Short-wavelength magnetic anomalies in a region of rapid seafloor spreading

THE main features of the geomagnetic time scale for 0–3 Myr BP are well defined1–3, but the existence and/or number of short events (polarity intervals lasting less than 50,000 yr) is still under discussion. Interest in correlating these short events with features in marine magnetic profiles has focused on three short-wavelength anomalies: anomaly X4,5 which appears at approximately 2.3 Myr BP; anomaly W5–7 at 2.0 Myr BP; and a short-wavelength feature in the centre of the central anomaly3,6,8,9. These anomalies appear rarely, and then usually only in profiles from regions of rapid spreading. We present here an analysis of a remarkable new set of data from just such a region. Our results provide evidence of the existence of two, or possibly three, short events in (the interval 0–3 Myr BP, and of the existence of a magnetised ridge at the precise centre of seafloor spreading.

Fluorescence of atomic nitrogen produced in the photoabsorption of extreme ultraviolet radiation by N 2

Atomic fluorescence radiation produced in the photoabsorption of extreme ultraviolet radiation by N 2 was measured over the incident wavelength range 180–400 Å (30–70 eV). The fluorescence radiation was observed in the 1050–1240, 1240–1600, and 1600–1800 Å regions. The NI 2P → 2P 0 and 2P → 2D 0 transitions occurring at Å 1743 Å and λ 1493 Å were identified. Additional shorter wavelength features from NI and possibly NII were also found to be present. The excitation mechanisms for these fluorescence emissions are photodissociation and dissociative ionization.