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Maintaining the Continuity of the Global Climate Data Record of Lightning from Space

Lightning is a natural hazard, an indicator of changes in convective storm behavior across multiple spatial and temporal scales, and a major influence on climate-affecting chemicals such as ozone and methane. Lightning also has substantial impacts on the near-space environment. As such, the World Meteorological Organization (WMO) has designated lightning as an Essential Climate Variable (ECV); that is, a measurement that is really important for understanding Earth’s climate. Spaceborne lightning observations are the gold standard for measuring global lightning, as the detection efficiency is more spatially uniform compared to ground-based lightning measurements. In addition, spaceborne is the only practical method for measuring total lightning (i.e., both intracloud and cloud-to-ground) at the global scale, as ground-based networks have limited ability to detect intracloud lightning at long distances (e.g., over oceans, far from land-based sensors). Since 1995, with only a two-year gap (2015-2017), lightning has been observed globally using a combination of Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS) instruments. However, the year 2023 is the planned end for the International Space Station (ISS) LIS mission, and there is no designated low-Earth orbit (LEO) successor yet identified. Since 2017, geostationary (GEO) observations of lightning have become increasingly available. These include the U.S. Geostationary Lightning Mappers (GLMs), China’s Lightning Mapping Imager (LMI), and Europe’s Lightning Imager (LI). While GEO provides continuous, quasi-hemispheric coverage of lightning and thunderstorms, there are current gaps preventing global coverage, particularly in portions of Asia and the western Pacific, as well as high-latitude regions. Moreover, integration of the GEO constellation into a Climate Data Record (CDR) of lightning is a major challenge only recently begun to be addressed by the WMO. Furthermore, recent work has demonstrated some deficiencies in the global record of lightning from space. For example, small lightning flashes are very common in severe weather, but are difficult to observe using the present set of spaceborne lightning imagers. Moreover, some types of lightning are best observed using frequencies other than the commonly used near-infrared (777-nm), including ultraviolet and the radio spectrum. Finally, it is not possible to assess the vertical distribution of lightning globally, even though this measurement is highly relevant to thunderstorm behavior as well as lightning’s effects on greenhouse gases and the near-space environment. This presentation will provide a current assessment of lightning measurements from space, will identify significant current and future gaps in these observations, and will evaluate potential solutions for improving the overall CDR of global lightning.

Timothy Lang↗
NASA NTRS1977

Analysis of a discrete spectrum analyzer for the detection of radio frequency interference

As the radio frequency spectrum becomes increasingly overcrowded, interference with mission-critical DSN operations is rising at an alarming rate. To alleviate this problem the DSN is developing a wideband surveillance system for on-site detection and identification of potential sources of radio frequency interference (RFI), which will complement the existing frequency coordination activities. The RFI monitoring system is based on a wideband, multi-look discrete spectrum analyzer operating on fast Fourier transform principles. An extensive general statistical analysis is presented of such spectrum analyzers and derives threshold detection performance formulas for signals of interest. These results are then applied to the design of the RFI spectrum analyzer under development.

Levitt, B. K.↗
NASA NTRS1993

The spectrum and variability of radio emission from AE Aquarii

The first detections of the magnetic cataclysmic variable AE Aquarii at millimeter wavelengths are reported. AE Aqr was detected at wavelengths of 3.4 and 1.25 mm. These data are used to show that the time-averaged spectrum is generally well fitted by a power law S(nu) varies as nu exp alpha, where alpha is approximately equal to 0.35-0.60, and that the power law extends to millimeter wavelengths, i.e., the spectral turnover is at a frequency higher than 240 GHz. It is suggested that the spectrum is consistent with that expected from a superposition of flarelike events where the frequency distribution of the initial flux density is a power law f (S0) varies as S0 exp -epsilon, with index epsilon approximately equal to 1.8. Within the context of this model, the high turnover frequency of the radio spectrum implies magnetic field strengths in excess of 250 G in the source.

Abada-Simon, Meil↗
NASA NTRS1985

Synchrotron Emission from Chaotic Stellar Winds

A model is presented for the radio emission from hot stars. Electrons are accelerated to relativistic energies by shocks in the wind near the star and emit radio radiation through the synchrotron mechanism. The particle spectrum and radio spectrum for this model are derived. The model accounts for many of the observed characteristics of some recently discovered stars which have peculiar radio emission.

White, R. L.↗
NASA NTRS1987

Dynamic spectrum of a radio flare on UV Ceti

The dMe flare star UV Ceti (L726-8B) was observed at four frequencies simultaneously in the 1385-1652 MHz band using the Very Large Array. A flare lasting 10 minutes was observed with 6.67 s time resolution 'Dynamic spectrum'-type images in the Stokes parameters I and V show considerable complexity in the frequency-time domain; some features show a positive frequency drift with time, while others are more complex, involving both positive and negative frequency drifts. The positive drift features would be consistent with disturbances traveling downward in the star's corona.

Jackson, Peter D.↗
NASA NTRS1998

The Nuclear Spectrum of the Radio Galaxy NGC 5128 (Centaurus A)

We present near-infrared spectra of the nucleus of the nearby radio galaxy NGC 5128 (Centaurus A). The observed emission line strengths suggest that NGC 5128 should be classified as a LINER, and appear to be powered by shocks.

galaxies active galaxies NGC 5128 Centaurus A nucl↗
NASA NTRS1981

Concurrent radio, infrared, optical and X-ray observations of the nucleus of the Seyfert galaxy NGC 4151

The nucleus of the Seyfert galaxy NGC 4151 was observed during May and June 1977 at X-ray energies from 20 to 200 keV using the high-energy X-ray spectrometer on OSO 8 at radio frequencies of 2695 and 8085. Data were also taken in the visual, and at infrared wavelengths of 1 to 2 microns, and from 2 to 6 keV using the OSO 8 proportional counter. Optical data showed variability by a factor of 2 within the observing period, which is comparable to the variability in the 2 to 6 keV X-ray flux. No variability, however, was measured at infrared or radio frequencies. The composite spectrum from radio to gamma-ray is plotted, and it is found that the optical and X-ray fluxes varied during the observing period, although it is not known if they varied concurrently.

Beall, J. H.↗
NASA NTRS2022

Unique Science from the Moon: An Overview

A new era of exploration of the low radio frequency universe from the Moon will soon be underway with landed payload missions facilitated by NASA. The science that could be accomplished from a lunar farside radio array is extensive and includes solar interplanetary radio bursts, Jovian and terrestrial natural radio emission, galactic synchrotron spectrum, exoplanets radio aurora, and doppler shifted radio emissions coming from the early formation of the universe just to name a few.

Lunar farside↗
NASA NTRS2021

A Spectrum Sensor for CubeSat Radios

Cube satellite (CubeSat) launches have increased exponentially over the last 20 years. This class of miniature spacecraft is well-suited for a set of nonconventional satellite architectures collectively known as formation flying. With the exponential pace of launches expected to continue, the prospect of spectrum management for these complex formations arises. In previous work, investigators focus on terrestrial applications of spectrum sensing, which have the luxury to utilize hardware with high size, weight, and power (SWaP) resources. In this work, we develop and test a spectrum sensor for CubeSat radio applications. Given that CubeSat radios are inherently designed for low SWaP, they cannot implement the computationally expensive spectral correlation analyzer (SCA) algorithms for signal detection. To that end, our investigation focuses on the application of the SCA to square-root-raised-cosine (SRRC) pulse-shaped quadrature amplitude modulation (QAM) waveforms using a field-programmable gate array (FPGA). This model requires no prior knowledge of the radio-frequency (RF) channel. We show that this model can consistently and accurately detect the symbol rate and center frequencies of waveforms located in a spectrum.

cubesat↗
NASA NTRS1980

Radio emission, cosmic ray electrons, and the production of gamma-rays in the Galaxy

The paper examines the traditional derivation of the interstellar electron spectrum using the galactic nonthermal radio spectrum and new radio data. The radio spectrum derived in the polar directions is now used as a base for this derivation instead of the anticenter spectrum. The exponent is determined for the interstellar electron spectrum between 70 and 1200 MeV, and radio maps are used to predict the gamma-ray fluxes produced by the bremsstrahlung process expected from these electrons. The differences in the predicted and observed gamma-ray intensities in the galactic plane are small, but in the polar direction the predicted gamma-ray flux using radio data is six times larger than that actually observed.

Webber, W. R.↗
NASA NTRS1972

A revised low-frequency cosmic noise spectrum.

Cosmic radio noise spectra obtained from an Astrobee sounding rocket and from more detailed analysis of RAE-1 data are presented. The flux levels measured by the two spacecraft are consistent and fall off more rapidly below 1 MHz than previous data.

Weber, R. R.↗
NASA NTRS1985

Onion-shell model for cosmic ray electrons and radio synchrotron emission in supernova remnants

The spectrum of cosmic ray electrons, accelerated in the shock front of a supernova remnant (SNR), is calculated in the test-particle approximation using an onion-shell model. Particle diffusion within the evolving remnant is explicity taken into account. The particle spectrum becomes steeper with increasing radius as well as SNR age. Simple models of the magnetic field distribution allow a prediction of the intensity and spectrum of radio synchrotron emission and their radial variation. The agreement with existing observations is satisfactory in several SNR's but fails in other cases. Radiative cooling may be an important effect, especially in SNR's exploding in a dense interstellar medium.

Beck, R.↗
NASA NTRS2010

Measuring a Fiber-Optic Delay Line Using a Mode-Locked Laser

The figure schematically depicts a laboratory setup for determining the optical length of a fiber-optic delay line at a precision greater than that obtainable by use of optical time-domain reflectometry or of mechanical measurement of length during the delay-line-winding process. In this setup, the delay line becomes part of the resonant optical cavity that governs the frequency of oscillation of a mode-locked laser. The length can then be determined from frequency-domain measurements, as described below. The laboratory setup is basically an all-fiber ring laser in which the delay line constitutes part of the ring. Another part of the ring - the laser gain medium - is an erbium-doped fiber amplifier pumped by a diode laser at a wavelength of 980 nm. The loop also includes an optical isolator, two polarization controllers, and a polarizing beam splitter. The optical isolator enforces unidirectional lasing. The polarization beam splitter allows light in only one polarization mode to pass through the ring; light in the orthogonal polarization mode is rejected from the ring and utilized as a diagnostic output, which is fed to an optical spectrum analyzer and a photodetector. The photodetector output is fed to a radio-frequency spectrum analyzer and an oscilloscope. The fiber ring laser can generate continuous-wave radiation in non-mode-locked operation or ultrashort optical pulses in mode-locked operation. The mode-locked operation exhibited by this ring is said to be passive in the sense that no electro-optical modulator or other active optical component is used to achieve it. Passive mode locking is achieved by exploiting optical nonlinearity of passive components in such a manner as to obtain ultra-short optical pulses. In this setup, the particular nonlinear optical property exploited to achieve passive mode locking is nonlinear polarization rotation. This or any ring laser can support oscillation in multiple modes as long as sufficient gain is present to overcome losses in the ring. When mode locking is achieved, oscillation occurs in all the modes having the same phase and same polarization. The frequency interval between modes, often denoted the free spectral range (FSR), is given by c/nL, where c is the speed of light in vacuum, n is the effective index of refraction of the fiber, and L is the total length of optical path around the ring. Therefore, the length of the fiber-optic delay line, as part of the length around the ring, can be calculated from the FSRs measured with and without the delay line incorporated into the ring. For this purpose, the FSR measurements are made by use of the optical and radio-frequency spectrum analyzers. In experimentation on a 10-km-long fiber-optic delay line, it was found that this setup made it possible to measure the length to within a fractional error of about 3 10(exp -6), corresponding to a length error of 3 cm. In contrast, measurements by optical time-domain reflectometry and mechanical measurement were found to be much less precise: For optical time-domain reflectometry, the fractional error was found no less than 10(exp -4) (corresponding to a length error of 1 m) and for mechanical measurement, the fractional error was found to be about 10(exp -2) (corresponding to a length error of 100 m).

Tu, Meirong↗
NASA NTRS1967

Radio astronomy

Radio astronomical studies of microwave spectrum, radial velocity, dielectric constants, and emissions

MICROWAVE SPECTRUM↗
NASA NTRS1989

Continuum energy distribution of quasars - Shapes and origins

Continuum observations from 0.3 nm to 6 cm are presented for 109 optically selected quasars from the Palomar-Green (PG) Bright Quasar Survey. The far-infrared to soft X-ray continuum of radio-quiet and steep-spectrum radio-loud PG quasars can be described in a model consisting of two broad peaks: an 'infrared bump' at about 2 microns to 1 mm and the well-known 'big blue bump' at about 10 nm to 0.3 micron. All PG quasars emit a significant fraction of their bolometric luminosity at infrared wavelengths of 1.5-100 microns. The fraction varies from 10 to 50 percent with a typical value of 30 percent. The ratio of the 6 cm flux density to that at 1 micron has a bimodal distribution. There is no evidence for a tight correlation between the infrared and X-ray continua of quasars. It is suggested that the bulk of the emission between 2 microns and 1 mm in the infrared bump of the radio-quiet PG quasars and the radio-loud quasars with steep radio spectral indices is produced by thermal processes.

Sanders, D. B.↗
NASA NTRS1999

A Multiwavelength Study of the Extreme AGN J2310-437

We present new X-ray, radio, and optical data for the unusual galaxy/cluster system J2310-437. Our results confirm the presence of an active nucleus and suggest an interpretation as an anomalous BL Lac object of bulk relativistic Doppler factor <2, with an optically deficient radio to X-ray spectrum. The radio, optical, and soft X-ray flux densities could lie along a single power-law function, lacking the curvature typical of BL Lac objects. Compared with other known sources that may have comparable multi-frequency spectra, J2310-437 is the most extreme. Its low isotropic optical/Ultraviolet (UV) radiation is consistent with the intensity of external photons governing the electron spectral break through Compton cooling; in this source the external photon density would be too low to produce a spectral break below the X-ray.

Worrall, D. M.↗