Radio Aurora Research Articles

On the Correlation between L Dwarf Optical and Infrared Variability and Radio Aurorae

Abstract Photometric variability attributed to cloud phenomena is common in L/T transition brown dwarfs. Recent studies show that such variability may also trace aurorae, suggesting that localized magnetic heating may contribute to observed brown dwarf photometric variability. We assess this potential correlation with a survey of 17 photometrically variable brown dwarfs using the Karl G. Jansky Very Large Array at 4–8 GHz. We detect quiescent and highly circularly polarized flaring emission from one source, 2MASS J17502484-0016151, which we attribute to auroral electron cyclotron maser emission. The detected auroral emission extends throughout the frequency band at ∼5–25σ, and we do not detect evidence of a cutoff. Our detection confirms that 2MASS J17502484-0016151 hosts a magnetic field strength of ≥2.9 kG, similar to those of other radio-bright ultracool dwarfs. We show that Hα emission continues to be an accurate tracer of auroral activity in brown dwarfs. Supplementing our study with data from the literature, we calculate the occurrence rates of quiescent emission in L dwarfs with low- and high-amplitude variability and conclude that high-amplitude optical and infrared variability does not trace radio magnetic activity in L dwarfs.

Formation of Ionospheric Irregularities in the East Siberian Region during the Geomagnetic Storm of May 27–28, 2017

Results of studying different-scale ionospheric irregularities on the basis of multi-instrumental data, obtained in the East Siberian region of Russia during the geomagnetic storm of May 27–28, 2017, are presented. Spatial inhomogeneities of electron density in the ionosphere were observed through data from ground-based receivers of signals of global navigational satellite systems and on the basis of direct measurements of electron density in low-orbit satellites. An intense radio aurora was seen in UHF radar data just after the initial phase of geomagnetic storm. At the same time, we recorded fluctuations of the total electron content from data of GPS receivers and the presence of E-layer irregularities by data of the ionosonde in Norilsk. The time of irregularity recording by different instruments is consistent with the spatiotemporal changes in field-aligned currents of the second zone, obtained from data of AMPERE low-orbit satellite system.

Coherent radio emission from a quiescent red dwarf indicative of star–planet interaction

Low frequency ($\nu\lesssim 150\,$MHz) stellar radio emission is expected to originate in the outer corona at heights comparable to and larger than the stellar radius. Such emission from the Sun has been used to study coronal structure, mass ejections, space-weather conditions around the planets (Schwenn 2006). Searches for low-frequency emission from other stars have only detected a single active flare-star (Lynch et al. 2017) that is not representative of the wider stellar population. Here we report the detection of low-frequency radio emission from a quiescent star, GJ 1151--- a member of the most common stellar type (red dwarf or spectral class M) in the Galaxy. The characteristics of the emission are similar to those of planetary auroral emissions (Zarka 1998) (e.g. Jupiter's decametric emission), suggesting a coronal structure dominated by a global magnetosphere with low plasma density. Our results show that large-scale currents that power radio aurorae operate over a vast range of mass and atmospheric composition, ranging from terrestrial planets to main-sequence stars. The Poynting flux required to produce the observed radio emission cannot be generated by GJ 1151's slow rotation, but can originate in a sub-Alfv\'{e}nic interaction of its magnetospheric plasma with a short-period exoplanet. The emission properties are consistent with theoretical expectations (Zarka 2007; Lanza 2009; Saur et al. 2013; Turnpenney et al. 2018) for interaction with an Earth-size planet in a $\sim 1-5$ day-long orbit.

Assessing Ionospheric Convection Estimates From Coherent Scatter From the Radio Aurora

AbstractCoherent radar backscatter spectra from auroral E region plasma irregularities can be measured with great precision and accuracy. The Doppler spectra obtained from those measurements can be used to estimate auroral convection patterns using an empirical model. These estimates are consistent with rocket, optical, and incoherent scatter radar data, but direct comparison between the different data sets is not straightforward. In order to establish the quality of the empirical model, alternative approaches are then needed. In this work, we use the electrostatic nature of the convection electric field to assess the physical consistency of the empirical model: The model will be satisfactory to the extent that the convection patterns derived from them are incompressible. The incompressibility criteria were assessed for the estimated convection patterns obtained from the substorm of 20 December 2015. Two different approaches were used for this assessment. First, an electrostatic potential was fitted to the convection pattern produced by the empirical model. A good fit implies that there exists an incompressible convection pattern consistent with the Doppler spectra measurements. Second, the uncertainties on the Doppler spectral estimates where propagated through the empirical model to calculate the expected uncertainties on the divergence of the convection field obtained directly from the empirical model. The convection field will be incompressible if its divergence lies within the uncertainty estimates. Both approaches indicate that the evaluated convection patterns are consistently incompressible.

First Joint Observations of Radio Aurora by the VHF and HF Radars of the ISTP SB RAS

Two modern radars for diagnosis of the ionosphere by the radio-wave backscattering method, namely, the Irkutsk incoherent scatter radar at VHF (IISR, 154–162 MHz) and the Ekaterinburg coherent radar at HF (EKB, 8–20 MHz) are operated at the Institute of Solar-Terrestrial Physics, Siberian Branch of the Russian Academy of Sciences (ISTP SB RAS). The paper analyzes the results of joint observations of strong scattering (radio aurora) on June 8, 2015. To determine the geographical position of the radio aurora, we developed original methods that take into account both the features of the radio-wave propagation and the features of the radar antenna systems. It is shown that there are areas where the spatial position of the HF and VHF radio aurora can coincide. This permits using the radars as a single complex for diagnosis of the characteristics of small-scale high-latitude irregularities in the ionospheric E and F layers. A comparative analysis of the characteristics and temporal dynamics of the radio-aurora region in the HF and VHF ranges is performed. Using the DMSP satellite data, it has been shown that the radio aurora dynamics during this experiment with the EKB radar can be related with the spatial dynamics of the localized area with high electric field, which moves from high to equatorial latitudes. It is found that due to the broader field of view, radio aurora at the HF radar was stably observed 6–12 min earlier than at the VHF radar. This permits using the EKB radar data for prediction of the radio-aurora detection by the IISR radar.

Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence.

Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons, as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere. Here we report simultaneous radio and optical spectroscopic observations of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs.

On-Orbit Calibration of Photodiodes for Attitude Determination

A method is presented for the on-orbit calibration of photodiodes for sun sensing in an attitude determination system. The calibration estimates the scale factors and alignment angles of the photodiodes, resulting in a higher attitude determination accuracy than achieved with the preflight calibration parameters. The calibration is accomplished through the simultaneous estimation of the spacecraft attitude and calibration parameters. This approach, as opposed to an attitude-independent method, enables the calibration of an arbitrary number of photodiodes mounted in any orientation on the spacecraft and facilitates the use of an attitude-dependent Earth albedo model. The method is formulated within both an extended Kalman filter and an unscented filter. The filters are demonstrated by application to flight data from the Radio Aurora Explorer satellites and result in an average angular improvement of 10 deg in sun vector measurements with the photodiodes. Attitude determination accuracies of below 1 deg in each axis are demonstrated using the calibrated photodiodes in combination with a low-cost three-axis magnetometer and rate gyroscope.

Flight results of a low-cost attitude determination system

This paper presents flight results of the attitude determination system (ADS) flown on the Radio Aurora Explorer (RAX) satellites, RAX-1 and RAX-2, which are CubeSats developed to study space weather. The ADS sensors include commercial-off-the-shelf magnetometers, coarse sun sensors (photodiodes), and a MEMs rate gyroscope. A multiplicative extended Kalman filter is used for attitude estimation. On-orbit calibration was developed and applied to compensate for sensor and alignment errors, and attitude determination accuracies of 0.5° 1–σ have been demonstrated on-orbit. The approach of using low-cost sensors in conjunction with on-orbit calibration, which mitigates the need for pre-flight calibration and high-tolerance alignment during spacecraft assembly, reduces the time and cost associated with the subsystem development, and provides a low-cost solution for modest attitude determination requirements. Although the flight results presented in this paper are from a specific mission, the methods used and lessons learned can be used to maximize the performance of the ADS of any vehicle while minimizing the pre-flight calibration and alignment requirements.

Clock synchronization on the RAX spacecraft

The Radio Aurora Explorer (RAX) is a CubeSat that was developed to study space weather in Earth׳s ionosphere. The scientific payload is a bistatic radar system in which an onboard receiver works in cooperation with a ground-based transmitter. Accuracy of the onboard clock is critical for processing the radar measurements. The RAX timing system utilizes commercial off-the-shelf components integrated into custom subsystems. GPS is used to maintain absolute timing accuracy better than 1μs, but the subsystem is not always available due to power constraints, so a method has been developed to correct the onboard clock error without the use of GPS. The clock correction utilizes range measurements extracted from the pulses emitted by the transmitter, and resulting absolute clock accuracies of better than 0.20s with drift of less than 21ns/s have been demonstrated. The RAX timing system and the clock correction algorithm are presented as a reference for other spacecraft designers and are critical for those analyzing RAX data.

Magnetic aspect sensitivity of high‐latitude E region irregularities measured by the RAX‐2 CubeSat

The second Radio Aurora Explorer (RAX‐2) satellite has completed more than 30 conjunction experiments with the Advanced Modular Incoherent Scatter Radar chain of incoherent scatter radars in Alaska and Resolute Bay, Canada. Coherent radar echoing occurred during four of the passes: three when E region electron drifts exceeded the ion acoustic speed threshold and one during HF heating of the ionosphere by the High Frequency Active Auroral Research Program heater. In this paper, we present the results for the first three passes associated with backscatter from natural irregularities. We analyze, in detail, the largest drift case because the plasma turbulence was the most intense and because the corresponding ground‐to‐space bistatic scattering geometry was the most favorable for magnetic aspect sensitivity analysis. A set of data analysis procedures including interference removal, autocorrelation analysis, and the application of a radar beam deconvolution algorithm mapped the distribution of E region backscatter with 3 km resolution in altitude and ∼0.1° in magnetic aspect angle. To our knowledge, these are the highest resolution altitude‐resolved magnetic aspect sensitivity measurements made at UHF frequencies in the auroral region. In this paper, we show that despite the large electron drift speed of ∼1500 m/s, the magnetic aspect sensitivity of submeter scale irregularities is much higher than previously reported. The root‐mean‐square of the aspect angle distribution varied monotonically between 0.5° and 0.1° for the altitude range 100–110 km. Findings from this single but compelling event suggest that submeter scale waves propagating at larger angles from the main E×B flow direction (secondary waves) have parallel electric fields that are too small to contribute to E region electron heating. It is possible that anomalous electron heating in the auroral electrojet can be explained by (a) the dynamics of those submeter scale waves propagating in the E×B direction (primary waves) or (b) the dynamics of longer wavelengths.

Radio Aurora Explorer: A Mission Overview

The primary objective of the Radio Aurora Explorer mission is to study plasma instabilities that lead to magnetic field aligned irregularities of electron density in the lower polar ionosphere (80–400 km). These irregularities are known to disrupt communication and navigation signals. The Radio Aurora Explorer mission uses a bistatic radar configuration; the spacecraft is the radar receiver and the transmitters are several worldwide incoherent scatter radars of which the primary radar is located in Poker Flat, Alaska. The satellites are 3 kg, university-class CubeSats with a primary payload of a direct-conversion radar receiver operating in the frequency range from 426 to 510 MHz. The satellite bus has a novel configuration of processing, attitude control, and antenna design that met the physical constraints of the CubeSat form factor and the one-year schedule constraint imposed by launch dates. Two satellites have launched: RAX-1 on 19 November 2010 onboard a Minotaur-IV rocket from Kodiak, Alaska, and RAX-2 on 28 October 2011 onboard a Delta-II rocket from Vandenberg U.S. Air Force Base, California. RAX-2 has detected and measured irregularities during multiple experiments over Alaska and Canada. Preliminary analysis indicates unprecedented characterization of the irregularities.

Design and implementation of the GPS subsystem for the Radio Aurora eXplorer

This paper presents the design and implementation of the Global Positioning System (GPS) subsystem for the Radio Aurora eXplorer (RAX) CubeSat. The GPS subsystem provides accurate temporal and spatial information necessary to satisfy the science objectives of the RAX mission. There are many challenges in the successful design and implementation of a GPS subsystem for a CubeSat-based mission, including power, size, mass, and financial constraints. This paper presents an approach for selecting and testing the individual and integrated GPS subsystem components, including the receiver, antenna, low noise amplifier, and supporting circuitry. The procedures to numerically evaluate the GPS link budget and test the subsystem components at various stages of system integration are described. Performance results for simulated tests in the terrestrial and orbital environments are provided, including start-up times, carrier-to-noise ratios, and orbital position accuracy. Preliminary on-orbit GPS results from the RAX-1 and RAX-2 spacecraft are presented to validate the design process and pre-flight simulations. Overall, this paper provides a systematic approach to aid future satellite designers in implementing and verifying GPS subsystems for resource-constrained small satellites.

First measurements of radar coherent scatter by the Radio Aurora Explorer CubeSat

The Radio Aurora Explorer CubeSat detected the first radar echoes during the solar storm of March 8, 2012. The 300 s ground‐to‐space bi‐static radar experiment was conducted in conjunction with the Poker Flat Incoherent Scatter Radar in the local morning (∼8 am) over Poker Flat, Alaska. The geomagnetic conditions for theEregion field‐aligned irregularity generation were optimal due to strong (about 1500 m/s)F region ion drifts and sufficient E region ionization (electron densities were ∼2 × 1011 m−3). The corresponding Eregion electric field of ∼80 mV/m was larger than the excitation threshold for the Farley‐Buneman instability. An auto‐correlation analysis resolved, for the first time, the distribution of auroralEregion backscatter with 3 km resolution in altitude and sub‐degree resolution in aspect angle. Moreover, the measured Doppler velocities of the UHF scatter shows the phase speed saturation of the meter‐scale plasma waves. The measured Doppler velocity is in excellent agreement with theCs cos θ formula for auroral E region irregularities.

Attitude-Independent Magnetometer Calibration with Time-Varying Bias

We present a method for on-orbit, attitude-independent magnetometer calibration that includes the effect of time-varying bias due to electronics on-board a spacecraft. The calibration estimates magnetometer scale factors, mis-alignments, and constant as well as time-varying bias. Time-varying effects are mitigated by including spacecraft telemetry in the measurement model and estimating constant parameters that map the telemetry data to magnetometer bias. The calibration is demonstrated by application to flight data from the Radio Aurora Explorer satellite and significantly reduces the uncertainty of off-the-shelf magnetometers embedded within the satellite and subject to spacecraftgenerated fields. This method simplifies the satellite design process by reducing the need for booms and strict magnetic cleanliness requirements.

Radio Aurora Explorer: Mission science and radar system

The Radio Aurora Explorer (RAX) satellite is the first of several satellites funded under the NSF CubeSat‐based Space Weather and Atmospheric Research Program. RAX is a ground‐to‐space bi‐static radar remote sensing experiment designed to measure and understand the causes of meter‐scale ionospheric irregularities. Also known as field‐aligned irregularities (FAI), such non‐thermal, coherent fluctuations of electron density occur in response to strong ionospheric flows or plasma density gradients during geomagnetic disturbances and are considered a space weather concern due to disruption to communication and navigation signals. The RAX CubeSat was launched in November 2010 and conducted a single experiment in coordination with the Poker Flat Incoherent Scatter Radar. Due to geophysical inactivity, e.g., lack of strong ionospheric electric fields and low ionospheric densities, no FAI were expected or observed. However, the radar receiver payload operation was successfully demonstrated, including the capability to sense signals as low as −110 dBm, the capability of transmitter–receiver synchronization and accurate ranging, processing of 1.2 GB of raw radar data on board in less than 1 hour, and the downlink of the science results within three–four passes. Analysis of the payload data shows that the noise level is sufficiently low. Although the interference level is a concern, it does not appear to significantly limit the measurements. Toward the end of December 2010, the solar power system gradually degraded and the mission terminated in early February 2011 after prolonged loss of contact with the satellite. Meanwhile, RAX II was launched in October 2011 to a polar orbit. This paper describes the RAX science and radar system and presents the results from the first experiment conducted.

The attitude determination system of the RAX satellite

The Radio Aurora Explorer (RAX) is a triple CubeSat that launched on November 19, 2010. RAX was designed to study plasma irregularities in the polar lower ionosphere (80–300km), and is the first CubeSat mission funded by the United States National Science Foundation. The scientific mission requires attitude knowledge within 5° (1−σ), and a custom attitude determination subsystem was developed for the mission. The subsystem utilizes rate gyros, magnetometers, coarse sun sensors, and an extended Kalman filter, and was designed to be a simple, low cost solution to meet the attitude determination requirements. In this paper, we describe the design, implementation, and testing of the RAX attitude determination subsystem, including derivation of the determination requirements, sensor selection, the integrated hardware design, pre-flight sensor calibration, and attitude estimation algorithms. The paper is meant to serve as a resource for others in the small satellite and nanosatellite communities, as well as a critical reference for those analyzing RAX data. Lessons learned from the design and performance of the RAX determination subsystem will be used in future designs of attitude determination systems for small satellites and similar platforms, such as high altitude balloons and autonomous aerial vehicles.

Studying the fine structure of coherent echo spectra using data from Irkutsk incoherent scatter radar

Studying the processes generating different-scale inhomogeneities is one of the challenging problems of ionospheric physics. Plasma instabilities are one of the physical mechanisms by which small-scale inhomogeneities are formed. The main forms of instability in the ionospheric E-layer are two-stream and gradient-drift ones. The inhomogeneities generated by them lead to an abnormally intense radio scattering of different wavelengths (known as coherent echo (CE) or radio aurora) in the E-layer. Therefore, the method of radiowave backscattering is among the widely used methods for studying such inhomogeneities. The CE phenomenon has been investigated most intensely at high and equatorial latitudes, where the conditions for the CE origination are formed rather regularly. For the last decade, CE has also been intensely studied at midlatitudes, where it is observed less frequently and its formation conditions are less known. In 1998–2006, the purposeful studies of the midlatitude CE peculiarities were performed at the Irkutsk incoherent scatter (IS) radar, with a particular emphasis on its coherent properties. It was for the first time found out that the spectra of some data sets had a fine comb-shaped structure, which generated well-known single-humped CE spectra as a result of statistical averaging. In the scope of this study, unique coherent methods for processing individual data sets of CE signals were developed, making it possible to reveal the peculiarities of unaveraged CE-signal spectra. To describe these peculiarities, we proposed a new model of the inhomogeneity spectrum, which is the superposition of the discrete set of spatial harmonics with close wave numbers. The model was shown to adequately describe the scattered signal characteristics observed experimentally.

Radar observations of ionospheric irregularities at Syowa Station, Antarctica: a brief overview

Abstract. We briefly overview the radar observations that have been made for 30 years at Syowa Station, Antarctica for studying small-scale electron-density irregularities in the southern high-latitude E- and F-region ionosphere. Some observational results (i.e., long-term variations of radio aurora, Doppler spectra with narrow spectral widths and low Doppler velocities, and simultaneous observations of radar and optical auroras) from VHF radars capable of detecting 1.3- to 3-m scale irregularities are presented. A new 50-MHz radar system equipped with phased-antenna arrays began operation in February 1995 to observe two-dimensional behaviours of E-region irregularities. An HF radar experiment also began in February 1995 to explore decameter-scale E- and F-region irregularities in the auroral zone and polar cap. These two radars will contribute to a better understanding of the ionospheric irregularities and ionospheric physics at southern high latitudes.

Arrival of Ionospheric Troughs and Oblique Spread-F in the Wake of Large-Scale Travelling Ionospheric Disturbances During the Intense Auroral Storms of August 22 and 23, 1990.

A modem digital ionosonde (Digisonde 256) recording the amplitude, angle-of-arrival, Doppler shift, and polarisation of MF and HF (1-10 MHz) echoes from the ionosphere was operated at the mid-latitude station of Beveridge (geographic 37.5°S, 144.9°E), located 40 km north of Melbourne, Australia, during the intense auroral storms of August 22 and 23, 1990. We changed the digisonde antenna design to permit the alternate recording of near-vertical angle-of-arrival ionograms, and southward-looking Doppler ionograms sensitive to oblique echoes at great ranges. This experimental design permitted us to track the equatorward migration of radio aurora associated with the poleward wall of the mid-latitude trough, the small-scale “slant-F” irregularities and medium-scale travelling ionospheric disturbances (TIDs) detected within the trough, and distinctive oblique spread-F patches located in proximity to the equatorward edge of the auroral zone; all arriving in response to large auroral electrojet activity. These observations graphically illustrate the processes by which the magnetosphere dumps energy into the high-latitude ionosphere when the geomagnetic index is high (ΣKp = 46 on August 23), leading to the generation of large-scale TIDs which precede the arrival of, and possibly contribute to the production of, the aforementioned menagerie of high mid-latitude irregularities. We illustrate the five forms (broadly speaking) of spread-F observed and offer plausible explanations for their cause. We corroborate our interpretation of oblique ionogram measurements by using DMSP/F8 and F9 precipitating particles satellite data, magnetometer measurements at Canberra (Δ = 45.9°S) (geomagnetic latitudes hereafter) and at Macquarie Is. (64.4°S) in the auroral zone, and routine ionograms (i.e., limited sensitivity to oblique echoes, and no angle-of-arrival or Doppler shift information) recorded at other mid-latitude stations. For example, during the night of August 23, radio aurora were observed to move equatorwards towards Beveridge (48.0°S) where a severe spread-F event occurred due to TIDs generated in the region defined by Macquarie Island north to Hobart (53.7°) (i.e., the source location probably changed in latitude as the aurora moved equatorward). The TIDs observed at Beveridge were also observed at Canberra (where mild spread-F was observed), and as far north as Norfolk Island (36.0°S) where they had little impact on the ionosphere.

A Decade of Type 3 Radio Aurora Studies: Toward and Away from the EIC Interpretation.

Auroral-zone VHF radars operating near 50 MHz often receive a type of coherent backscatter from ionospheric irregularities that is characterized by narrow spectra with mean Doppler velocities below the ion-acoustic velocity. Because these echoes are distinctly different from the familiar type 1 and type 2 echoes, they were deemed to represent a new class and were named type 3. In the mid 1980's, there evolved the idea that the plasma physical mechanism leading to auroral type 3 backscatter involves electrostatic ion cyclotron (EIC) waves. Further observations made in subsequent years seemed to support this interpretation. However, refined measurements and data analysis carriedd out in recent years, uncovered substantial disagreement between 50-MHz type 3 radar observations and predictions from EIC theories. The most obvious discrepancies between theory and observations include the Doppler frequency dependence of type 3 echoes on the radar frequency, their backscatter altitude, their magnetic aspect angle dependence, and their most common mean Doppler frequency. As a result, the idea of type 3 radio aurora being associated with EIC waves has lost support from the radio science community. Although work on finding an explanation for type 3 backscatter continues, no new interpretation has yet found wide support.