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On behalf of the journal, AGU, and the scientific community, the editors would like to sincerely thank those who reviewed the manuscripts for Geophysical Research Letters in 2019. The hours reading and commenting on manuscripts not only improve the manuscripts but also increase the scientific rigor of future research in the field. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. With the revival of the “major revisions” decisions, we appreciate the reviewers' efforts on multiple versions of some manuscripts. With the advent of AGU's data policy, many reviewers have helped immensely to evaluate the accessibility and availability of data associated with the papers they have reviewed, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. Many of those listed below went beyond and reviewed three or more manuscripts for our journal, and those are indicated in italics.

On behalf of the journal, AGU, and the scientific community, the Editors would like to sincerely thank those who reviewed manuscripts for Geophysical Research Letters in 2018. The hours reading and commenting on manuscripts not only improves the manuscripts but also increases the scientific rigor of future research in the field. We particularly appreciate the timely reviews, in light of the demands imposed by the rapid review process at Geophysical Research Letters. With the revival of the “major revisions” decisions, we appreciate the reviewers' efforts on multiple versions of some manuscripts. Many of those listed below went beyond and reviewed three or more manuscripts for our journal, and those are indicated in italics. In total, 4,484 referees contributed to 7,557 individual reviews in journal. Thank you again. We look forward to the coming year of exciting advances in the field and communicating those advances to our community and to the broader public.

The terrestrial ionosphere is a birefringent medium that allows radio waves to propagate in two modes: the ordinary (O‐mode) and the extraordinary (X‐mode). A difference in the index of refraction of the two modes results in differential delay (mode delay) between the O‐ and X‐modes of radio waves propagating through the ionosphere. Mode delays of transionospheric high frequency radio waves are determined using the Radio Receiver Instrument (RRI) onboard the Enhanced Polar Outflow Probe (e‐POP) on the CASSIOPE/Swarm‐E satellite. Experiments between RRI and the SuperDARN radar at Saskatoon, Canada show large variabilities in mode delays. The mode delays are nearly constant in some experiments but other experiments have large variations. The mode delay observations could not be explained by the distance between the e‐POP satellite and the radar, the foF2 values along the satellite track, nor whether the satellite track was across or along a SuperDARN radar beam. A combination of RRI observations and ray‐trace modeling are used to investigate the mode delay of transionospheric radio waves. Key Points: Observations and modeling of differential delay between the O‐ and X‐modes (mode delay) of transionospheric radio wavesMode delays strongly depend on the magnitude and distribution of foF2 through which radio waves propagateAdditional gradients in foF2 lead to larger changes in mode delays [ABSTRACT FROM AUTHOR]

We present the first space-borne observation of optical emissions of both neutral and ionized argon atoms resulting from a radio-frequency glow discharge (RF-GD) from a sounding rocket payload. The Observations of Electric-field Distributions in the Ionospheric Plasma--a Unique Strategy-C (OEDIPUS-C) payload was designed to separate into two sub-payloads 'on its up-leg. This separation permitted a number of two-point measurements, including those on radio wave propagation' from the active dipole antennas on the upper sub-payload to the synchronized receiving dipoles on the lower sub-payload. A white-light video camera on the lower sub-payload recorded strong luminosity around the active dipoles during the first 15 s 'after sub-payload separation, when argon gas jets were providing propulsion to separate the two sub-payloads. Parts of the' ejected argon appeared as a glowing volume where the large radio-frequency (RF) fields from the two active dipoles excited 'the optical emission, as the sub-payload separation increased from 2 to 55 m. The shape and intensity of the luminosity were' 'well repeated as a function of the swept frequency (0.025-8.000 MHz), but their frequency dependences were distinctly differ-' 'ent from those of sounder-accelerated electrons measured onboard, and deduced to result from the nonlinearity of the glow' 'discharge. The observation is to our knowledge the first of its kind, and is interpreted in terms of a RF-GD energized by the' strong near electric fields of the transmitting dipoles. Key words: ionosphere, radio-frequency glow discharge, argon, sounding rocket, optical emission.' Nous presentons les premieres observations sur plateforme spatiale des emissions optiques d'atomes d'argon neutres et ionises resultant d'une decharge luminescente radiofrequence (DL-RF/RF-GD) generee par l'instrument d'une fusee sonde. L'instrument embarque Observation of Electric field Distribution in the Ionospheric Plasma--a Unique Strategy-C (OEDIPUS-C) a ete concu pour se separer verticalement en deux sous-instruments (inferieur et superieur). Cette separation permet un nombre de mesures en deux points, incluant une mesure de la propagation des ondes radio venant des antennes dipolaires actives de l'instrument superieur vers les dipoles synchrones de l'instrument inferieur. Une camera video en lumiere blanche sur l'instrument inferieur a enregistre une forte luminosite autour des dipoles actifs pendant les 15 s apres la separation des instruments, alors que le gaz d'argon fournissait la poussee pour separer les deux sous-instruments. Une partie du gaz d'argon ejecte apparait comme un volume luminescent ou les forts champs RF provenant des deux dipoles actifs excitent l'emission optique, pendant que la separation des instruments passe de 2 m a 55 m. La forme et l'intensite de cette lumiere sont bien representees comme une fonction de la frequence de balayage (0.025-8.000 MHz), mais leurs dependances en frequence sont nettement differentes de celles des electrons acceleres du sondeur mesurees a bord et on en deduit que cela resulte de la non linearite de la decharge luminescente. Cette observation est, a notre connaissance, la premiere de son type et est interpretee en termes de decharge luminescente radiofrequence stimulee par les forts champs electriques emanant des dipoles de transmission. [Traduit par la Redaction] Mots-cles : ionosphere, decharge luminescente radiofrequence (DL-RF), argon, fusee sonde, emission optique, 1. Introduction Two television cameras with a spectral response between 400 and 900 nm were installed on the Observations of Electric-field Distributions in the Ionospheric Plasma--a Unique Strategy-C (OEDIPUS-C, OC [...]

Ionospheric density structures at low latitudes range in size from thousands of kilometers down to a few meters. Radio frequency (RF) signals, such as those from global navigation satellite systems, that propagate through irregularities suffer from rapid fluctuations in phase and intensity, known as scintillations. In this study, we use the high‐sample‐rate measurements of the Swarm Echo (CASSIOPE/e‐POP) satellite's GPS Occultation (GAP‐O) receiver taken after its antenna was re‐oriented to vertical‐pointing, simultaneously with e‐POP Ion Mass Spectrometer surface current observations as a proxy for plasma density, to obtain the spectral characteristics of GPS signal intensity and in‐situ irregularities at altitudes from 350 to 1,280 km. We show that the power spectra of both measurements can generally be characterized by a power law. In the case of density irregularities, the spectral index with the highest occurrence rate is around 1.7, which is consistent with previous studies. Also, all the power spectra of GPS signal intensity in this study show a single spectral index near 2. Moreover, roll‐off frequencies estimated in this work range from 0.4 to 2.5 Hz, which is significantly higher than Fresnel frequencies calculated from ground GPS receivers at low latitudes (between 0.2 and 0.45 Hz). Part of this increase is due to the 8 km/s orbital velocity of Swarm Echo near perigee. Another key difference is that variations in the GPS signals in this study are dominated by the topside ionosphere, whereas GPS signals received from ground are affected mostly by the relatively dense F‐region plasma in the 250–350 km altitudinal range. Plain Language Summary: Variations in ionospheric electron density, called irregularities, cause fluctuations in radio frequency signals traversing the ionosphere. The Swarm Echo satellite carries a high‐sample‐rate GPS receiver with an antenna normally pointing in the horizontal direction. In this study, the satellite was reoriented while it was passing the low latitude region so that the receiver antenna would point vertically in order to probe irregularities above the satellite at high resolution. Our spectral analysis of GPS signal intensity and plasma density variations measured in situ (on board the spacecraft) revealed power law behavior in both measurements, a well‐established feature of ionospheric turbulence. More specifically, our results show that the spectral index (which is the negative of the power law slope) of in‐situ density irregularities and GPS signal intensities are 1.7 and 2, respectively. The roll‐off frequencies in the GPS signal intensity results are considerably higher than ground‐based estimates. This difference is attributable to the satellite's high velocity (8 km/s) and the fact that, comparing to the ground estimates which are dominated by the dense plasma density between 250 and 350 km altitude, Swarm Echo observations are mostly affected by the upper reaches of the ionosphere. Key Points: The most frequently occurring spectral index of the in‐situ irregularity power spectra is 1.7The power spectra of GPS signal intensity show a single spectral index near 2, which is smaller than ground‐based weak scintillation valuesThe roll‐off frequencies in the signal intensity power spectra are 0.4–2.5 Hz, significantly exceeding ground‐based Fresnel frequency values [ABSTRACT FROM AUTHOR]

The polarization state of transionospheric high frequency (HF) radio waves can be determined using the crossed-dipole antennas of the Radio Receiver Instrument (RRI) onboard Enhanced Polar Outflow Probe (e-POP) on the CAScade, Smallsat, and IOnospheric Polar Explorer/Swarm-E satellite. Coordinated experiments between ground radars and RRI showed that the radio waves have high ellipticity angles (elliptical or circular polarization) for propagation direction 6°-25° from the perpendicular direction to the local geomagnetic field. However, from magnetoionic theory and a typical assumption of roughly equal power in the O- and X-modes, radio waves can have high ellipticity angles only when the propagation direction is within 10° of perpendicularity. This investigation uses coordinated experiments between the Saskatoon SuperDARN radar and RRI. Magnetoionic modeling reveals that the relative strengths of the O- and X-modes for the HF radio waves transmitted from the Saskatoon SuperDARN change significantly poleward of the radar. Differences in the relative power between the two wave modes were found to significantly modify the polarization state of radio waves, and govern the sense of rotation of the wave. Even a relatively modest 2-5 dB power difference in the X-mode over the O-mode was found to result in unexpected observations of circularly polarized radio waves at aspect angles 6°-10° from perpendicular, and unexpected elliptically polarized waves observed at aspect angles more than 10° from perpendicular. Therefore, a combination of RRI observations and modeling, which accounts for relative differences in the O- and X-mode powers, was used here to better understand the unexpected observations of polarization states of transionospheric radio waves. [ABSTRACT FROM AUTHOR]

We propose a novel approach to produce regional maps of small‐scale scintillation‐causing irregularities using a single satellite. To construct the maps, we employ several ionospheric GPS indices, including total electron content, high‐resolution ROTI, and S4, calculated from the Swarm Echo GPS Attitude, Positioning, and Profiling Experiment Occultation (GAP‐O) receiver with its antenna pointed upward. GAP‐O's high‐sample‐rate observations enable irregularities as small as 320 m to be resolved. We present two case studies in which we compare the maps with in situ measurements of irregularities and simultaneous vertical TEC maps obtained from the ground. In situ measurements of net current onto the external surface of the Imaging and Rapid‐scanning Ion Mass Spectrometer sensor on board Swarm Echo were utilized to quantify plasma density fluctuations. Then, we apply the method to synthetic data to illustrate the efficacy of the method. Modeling results show that the irregularity maps can determine the horizontal geo‐locations of small‐scale irregularities, though with significant uncertainties in the cross‐track direction (east‐west). As Swarm Echo traverses different altitudes, these maps provide additional information on the altitudinal distribution of plasma fluctuations. This technique facilitates a better understanding of the morphology of scintillation‐causing irregularities, which are challenging to map from ground‐based receiver arrays alone. Plain Language Summary: We propose a novel approach to mapping of the horizontal distributions of ionospheric irregularities, or variations in ionospheric electron density, using the Swarm Echo satellite while it traverses the equatorial ionosphere at altitudes between 330 and 1,280 km. Swarm Echo carries a GPS receiver capable of simultaneously sampling signals from multiple GPS satellites at a rate of 50 measurements per second, corresponding to spatial scale sizes as small as 320 m. A high‐rate, in situ measure of relative plasma density complements the GPS integral measurements. The method is tested using synthetic data, demonstrating that irregularities can be localized along Swarm Echo's trajectory with high resolution and accuracy. Performance in the direction perpendicular to the satellite's path is poorer, but properties including general position and large‐scale gradients can be recovered. Comparison with maps of vertical total electron content obtained from ground measurements shows that combining the two maps provides valuable information pertaining to the altitude distribution of large‐scale density features such as the Appleton anomaly, as well as small‐scale irregularities associated with them. Key Points: We use high‐rate GPS and density measurements on the Swarm Echo satellite to map small‐scale ionospheric irregularities in two dimensionsUsing synthetic data, we characterize the technique's ability to reconstruct the width and location of irregularity regionsThese maps along with ground‐based GPS maps provide information on the topside irregularities and their relationship with large structures [ABSTRACT FROM AUTHOR]