Charles Merrill Swenson

Control of equatorial ionospheric morphology by atmospheric tides

[1] A newly discovered 1000-km scale longitudinal variation in ionospheric densities is an unexpected and heretofore unexplained phenomenon. Here we show that ionospheric densities vary with the strength of nonmigrating, diurnal atmospheric tides that are, in turn, driven mainly by weather in the tropics. A strong connection between tropospheric and ionospheric conditions is unexpected, as these upward propagating tides are damped far below the peak in ionospheric density. The observations can be explained by consideration of the dynamo interaction of the tides with the lower ionosphere (E-layer) in daytime. The influence of persistent tropical rainstorms is therefore an important new consideration for space weather. Citation: Immel, T. J., E. Sagawa, S. L. England, S. B. Henderson, M. E. Hagan, S. B. Mende, H. U. Frey, C. M. Swenson, and L. J. Paxton (2006), Control of equatorial ionospheric morphology by atmospheric tides, Geophys. Res. Lett., 33, L15108, doi:10.1029/2006GL026161. [2] The ionosphere is the region of highest plasma density in Earth’s space environment. It is a dynamic environment supporting a host of plasma instability processes, with important implications for global communications and geo-location applications. Produced by the ionization of the neutral atmosphere by solar x-ray and UV radiation, the uppermost ionospheric layer has the highest plasma density with a peak around 350–400 km altitude and primarily consists of O + ions. This is called the F-layer and it is considered to be a collisionless environment such that the charged particles interact only weakly with the neutral atmosphere, lingering long after sunset. The E-layer is composed of molecular ions and is located between 100–150 km where collisions between ions and neutrals are much more frequent, with the result that the layer recombines and is reduced in density a hundredfold soon after sunset [Rees ,1 989;Heelis, 2004]. The respective altitude regimes of these two layers are commonly called the E- and F-regions. [3] The ionosphere glows as O + ions recombine to an excited state of atomic oxygen (O I) at a rate proportional to

Effect of atmospheric tides on the morphology of the quiet time, postsunset equatorial ionospheric anomaly

longitudinal wave number four pattern in the magnetic latitude and concentration of the F region peak ion density when measured at a fixed local time. In a new comparison of two data sets with observations made by the OGO 4 satellite, this pattern is seen to be persistent over many days around equinox during magnetically quiet conditions close to solar maximum but can be dominated by other processes such as cross-equator winds during other periods. It is found that the longitudinal variability is created by a processes occurring in the dayside ionosphere. A longitudinal modulation of the dayside equatorial fountainisthemostlikelydrivingmechanism.ThroughcomparisonwithGWSM-02model,it isshownthatthepredictedmodulationofthedaysidethermosphericwindsandtemperaturesat E region altitudes created by non-migrating diurnal tides can explain the modulation in the dayside equatorial fountain. This result highlights the importance of understanding the temporal variability of tropospheric weather systems on our understanding and possible predictability of the development of the F region ionosphere. It may also provide a possible further means of testing our understanding of atmospheric tides on a global scale.

Low-power FLC-based retromodulator communications system

On September 15, 1996, researchers from Utah State University/Space Dynamics Lab in conjunction with Phillips Lab/Starfire Optical Range and Kjome Research successfully flew and tested a retromodulator laser communication package on a high altitude balloon. This paper addresses the layout and hardware used for the communication link, as well as presenting some preliminary data collected during the 6 hour flight of the balloon. The package was a proof of concept demonstration system for a low-power laser communications systems for small, low Earth orbiting satellites. The ferroelectric liquid crystal based retromodulator design of Utah State provided test patterns for modulation rates up to 20 kilo bits per second. Data was successfully downlinked using a 1200 bps RS232 format and a simplistic receiver. The Starfire Optical Range 1.5-meter telescope located on Kirtland AFB, tracked the balloon, which reached a float altitude of 31 km and collected the modulated light reflected from the payload.

The impedance of a short dipole antenna in a magnetized plasma via a finite difference time domain model

The traditional analytical analysis of plasma probes requires the use of quasi-static approximations, while numerical methods require the use of an equivalent dispersive media, both producing a nontrivial analysis of the plasma environment. On the other hand, a few techniques that combine the plasma fluid equations with Maxwells equations have only addressed wave propagation through spatially constant plasma. All of these models are limited in analysis of in situ measurements. This paper modifies the current finite-difference time-domain methods to more accurately model the ionospheric environment. Decoupled boundary conditions are presented in an attempt at coping with the instabilities of the plasma at the boundaries. The final model is then compared to analytical theory of radio-frequency plasma probes.

Electric and magnetic field measurements inside a high-velocity neutral beam undergoing ionization

Vector electric field measurements have been made inside two ionizing, high-velocity streams of barium atoms in the Earths ionosphere. A variety of electrical phenomena were observed across the frequency spectrum and are presented in this paper, which emphasizes the experimental results. Comparisons with a theoretical model for the interactions of the stream with the magnetic field and the ionosphere are presented in a companion paper (Brenning et al., this issue (a)). A most startling result is that a very large quasi-dc electric field was detected antiparallel to the beam velocity. This by itself is not unreasonable since newly ionized barium ions with their large gyroradii are expected to create such a field. But since the beam had roughly a 45° angle with the magnetic field, Bo, we find a very large (≳500 mV/m) component of E parallel to Bo. The fluctuating electric fields were also quite large, in fact, of the same order of magnitude as the quasi-dc pulse. The wave energy was found to maximize at frequencies below the barium lower hybrid frequency and included strong signatures of the oxygen cyclotron frequency. Measurements made on a subpayload separated across Bo by several hundred meters and along Bo by several kilometers do not show the large pulse, although a variety of wave emissions were seen. In addition, very large amplitude magnetic field fluctuations were detected in both bursts. Although we have no clear explanation, they appear to be a real phenomenon and worthy of future study. Finally, we note that even though the critical ionization velocity effect did not go into a discharge mode in this experiment, remarkable electromagnetic effects were seen in the neutral beam-plasma interaction.

Interpretation of the electric fields measured in an ionospheric critical ionization velocity experiment

This paper deals with the quasi-dc electric fields measured in the CRIT I ionospheric release experiment, which was launched from Wallops Island on May 13, 1986. The purpose of the experiment was to study the critical ionization velocity (CIV⋕ mechanism in the ionosphere. Two identical barium shaped charges were fired from distances of 1.99 km and 4.34 km towards a main payload, which made full three-dimensional measurements of the electric field inside the streams. There was also a subpayload separated from the main payload by a couple of kilometers along the magnetic field. The relevance of earlier proposed mechanisms for electron heating in CIV is investigated in the light of the CRIT I results. It is concluded that both the “homogeneous” and the “ionizing front” models probably apply, but in different parts of the stream. It is also possible that electrons are directly accelerated by a magnetic-field-aligned component of the electric field; the quasi-dc electric field observed within the streams had a large magnetic-field-aligned component, persisting on the time scale of the passage of the streams. The coupling between the ambient ionosphere and the ionized barium stream in CRIT I was more complicated than is usually assumed in CIV theories, with strong magnetic-field-aligned electric fields and probably current limitation as important processes. One interpretation of the quasi-dc electric field data is that the internal electric fields of the streams were not greatly modified by magnetic-field-aligned currents, i.e., a state was established where the transverse currents were to a first approximation divergence-free. It is argued that this interpretation can explain both a reversal of the strong explosion-directed electric field in burst 1 and the absence of such a reversal in burst 2.

Invited Article: Data analysis of the Floating Potential Measurement Unit aboard the International Space Station

We present data from the Floating Potential Measurement Unit (FPMU) that is deployed on the starboard truss of the International Space Station. The FPMU is a suite of instruments capable of redundant measurements of various plasma parameters. The instrument suite consists of a floating potential probe, a wide-sweeping spherical Langmuir probe, a narrow-sweeping cylindrical Langmuir probe, and a plasma impedance probe. This paper gives a brief overview of the instrumentation and the received data quality, and then presents the algorithm used to reduce I-V curves to plasma parameters. Several hours of data are presented from August 5, 2006 and March 3, 2007. The FPMU derived plasma density and temperatures are compared with the International Reference Ionosphere (IRI) and Utah State University-Global Assimilation of Ionospheric Measurement (USU-GAIM) models. Our results show that the derived in situ density matches the USU-GAIM model better than the IRI, and the derived in situ temperatures are comparable to the average temperatures given by the IRI.

Charging of the International Space Station as Observed by the Floating Potential Measurement Unit: Initial Results

The floating potential measurement unit (FPMU) is a multiprobe package designed to measure the floating potential of the International Space Station (ISS) as well as the density and temperature of the local ionospheric plasma environment. The purpose of the FPMU is to provide direct measurements of ISS spacecraft charging as continuing construction leads to dramatic changes in ISS size and configuration. FPMU data are used for refinement and validation of the ISS spacecraft charging models used to evaluate the severity and frequency of occurrence of ISS charging hazards. The FPMU data and the models are also used to evaluate the effectiveness of proposed hazard controls. The FPMU consists of four probes: a floating potential probe, two Langmuir probes, and a plasma impedance probe. These probes measure the floating potential of the ISS, plasma density, and electron temperature. Redundant measurements using different probes support data validation by interprobe comparisons. The FPMU was installed by ISS crew members during an extra-vehicular activity on the starboard (S1) truss of the ISS in early August 2006 when the ISS configuration included only one 160-V U.S. photovoltaic (PV) array module. The first data campaign began a few hours after installation and continued for over five days. Additional data campaigns were completed in 2007 after a second 160-V U.S. PV array module was added to the ISS. This paper discusses the general operational characteristics of the FPMU as integrated on ISS, the functional performance of each probe, the charging behavior of the ISS before and after the addition of a second 160-V U.S. PV array module, and initial results from model comparisons.

Impedance of a short dipole antenna in a cold plasma

This paper presents the analysis of the impedance of a short dipole antenna in a cold plasma using a quasi-static approach. Two radically different current distributions on the antenna are considered and their influences on the antenna impedance are studied. These distributions include a one-dimensional (1-D) triangular distribution and a three-dimensional (3-D) exponential current distribution. Balmain (1964) analytically solved the first problem. Staras (1964) proposed the second one. In this paper, we offer an analytical solution for the second one as proposed by Staras. We compare both distributions and find that impedances resulting from them are remarkably close. We conclude that the impedance of a short antenna, as derived using the quasi-static approximation, is insensitive to the choice of current distribution. Therefore, any of these two theories can be used when analyzing data from an impedance probe immersed in a space plasma.

Generation of electrostatic emissions by lightning-induced whistler-mode radiation above thunderstorms

Abstract Data from two separate thunderstorm sounding rocket experiments – Thunderstorm-II (T-II), which involved two rockets, and Thunderstorm-III (T-III) – are compared. In both cases, the sounding rockets over-flew active storm cells, but the bottomside of the ionosphere had a lower density and was much more structured for the first experiment than for the second. Electric field measurements on the flight through a structured ionosphere (T-II) show a triggered emission that has a height variation with altitude that seems to track the lower hybrid resonance frequency (LHR). Theories and other experimental data are presented supporting the concept that LH waves can be stimulated by intense whistlers when density gradients are present, and we interpret these T-II data in just such a context. We believe these emissions may be responsible for the irregularities causing the remote-sensing phenomenon called explosive spread F. The T-III flight had no such height-dependent emissions, which we attribute to the smoothness of the medium during that flight. Curiously, long-lasting emissions also occurred during T-III, but at frequencies that were constants with height and harmonics of the cut-off frequency for the earth-ionosphere wave guide. To our knowledge, there is no existing theory against which to test the T-III emissions.