N. A. Frissell

Storm time response of the midlatitude thermosphere: Observations from a network of Fabry‐Perot interferometers

Observations of thermospheric neutral winds and temperatures obtained during a geomagnetic storm on 2 October 2013 from a network of six Fabry-Perot interferometers (FPIs) deployed in the Midwest United States are presented. Coincident with the commencement of the storm, the apparent horizontal wind is observed to surge westward and southward (toward the equator). Simultaneous to this surge in the apparent horizontal winds, an apparent downward wind of approximately 100 m/s lasting for 6 h is observed. The apparent neutral temperature is observed to increase by approximately 400 K over all of the sites. Observations from an all-sky imaging system operated at the Millstone Hill observatory indicate the presence of a stable auroral red (SAR) arc and diffuse red aurora during this time. We suggest that the large sustained apparent downward winds arise from contamination of the spectral profile of the nominal thermospheric 630.0 nm emission by 630.0 nm emission from a different (nonthermospheric) source. Modeling demonstrates that the effect of an additional population of 630.0 nm photons, with a distinct velocity and temperature distribution, introduces an apparent Doppler shift when the combined emissions from the two sources are analyzed as a single population. Thus, the apparent Doppler shifts should not be interpreted as the bulk motion of the thermosphere, calling into question results from previous FPI studies of midlatitude storm time thermospheric winds. One possible source of contamination could be fast O related to the infusion of low-energy O+ ions from the magnetosphere. The presence of low-energy O+ is supported by observations made by the Helium, Oxygen, Proton, and Electron spectrometer instruments on the twin Van Allen Probes spacecraft, which show an influx of low-energy ions during this period. These results emphasize the importance of distributed networks of instruments in understanding the complex dynamics that occur in the upper atmosphere during disturbed conditions.

Climatology of medium‐scale traveling ionospheric disturbances observed by the midlatitude Blackstone SuperDARN radar

A climatology of daytime midlatitude medium-scale traveling ionospheric disturbances (MSTIDs) observed by the Blackstone Super Dual Auroral Radar Network (SuperDARN) radar is presented. MSTIDs were observed primarily from fall through spring. Two populations were observed: a dominant population heading southeast (centered at 147° geographic azimuth, ranging from 100° to 210°) and a secondary population heading northwest (centered at −50° azimuth, ranging from −75° to −25°). Horizontal velocities ranged from 50 to 250 m s−1 with a distribution maximum between 100 and 150 m s−1. Horizontal wavelengths ranged from 100 to 500 km with a distribution peak at 250 km, and periods between 23 and 60 min, suggesting that the MSTIDs may be consistent with thermospheric gravity waves. A local time (LT) dependence was observed such that the dominant (southeastward) population decreased in number as the day progressed until a late afternoon increase. The secondary (northwestward) population appeared only in the afternoon, possibly indicative of neutral wind effects or variability of sources. LT dependence was not observed in other parameters. Possible solar-geomagnetic and tropospheric MSTID sources were considered. The auroral electrojet (AE) index showed a correlation with MSTID statistics. Reverse ray tracing with the HINDGRATS model indicates that the dominant population has source regions over the Great Lakes and near the geomagnetic cusp, while the secondary population source region is 100 km above the Atlantic Ocean east of the Carolinas. This suggests that the dominant population may come from a region favorable to either tropospheric or geomagnetic sources, while the secondary population originates from a region favorable to secondary waves generated via lower atmospheric convection.

Ionospheric Sounding Using Real‐Time Amateur Radio Reporting Networks

Amateur radio reporting networks, such as the Reverse Beacon Network (RBN), PSKReporter, and the Weak Signal Propagation Network, are powerful tools for remote sensing the ionosphere. These voluntarily constructed and operated networks provide real-time and archival data that could be used for space weather operations, forecasting, and research. The potential exists for the study of both global and localized effects. The capability of one such network to detect space weather disturbances is demonstrated by examining the impacts on RBN-observed HF propagation paths of an X2.9 class solar flare detected by the GOES 15 satellite. Prior to the solar flare, the RBN observed strong HF propagation conditions between multiple continents, primarily Europe, North America, and South America. Immediately following the GOES 15 detection of the solar flare, the number of reported global RBN propagation paths dropped to less than 35% that of prior observations. After the flare, the RBN showed the gradual recovery of HF propagation conditions.

High‐latitude thermospheric wind observations and simulations with SuperDARN data driven NCAR TIEGCM during the December 2006 magnetic storm

Ion convection pattern derived from the Super Dual Auroral Radar Network potential pattern (SDP) data is developed for the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamic General Circulation Model. The December 2006 geomagnetic storm event was simulated with the SDP ion convection pattern and two other existing input options (Heelis and Weimer convection models). The high-latitude thermospheric wind simulated with SDP showed very good agreement with the Fabry-Perot interferometer thermospheric wind data inside the polar cap at Resolute, Canada (74.7°N, 94.8°W, magnetic latitude 84). The Heelis model overestimated the winds during the storm event, because the model does not consider the cross polar cap potential saturation at high global geomagnetic index (Kp) values. The Weimer model provides a better performance than the Heelis model in this case. However, it has a larger discrepancy compared to the SDP results. The SDP provides an alternative data-based tool for study of the ionosphere/thermosphere interactions in the polar cap.

Satellite‐beacon Ionospheric‐scintillation Global Model of the upper Atmosphere (SIGMA) II: Inverse modeling with high‐latitude observations to deduce irregularity physics

Complex magnetosphere-ionosphere coupling mechanisms result in high-latitude irregularities that are difficult to characterize using only Global Navigation Satellite System (GNSS) scintillation measurements. However, GNSS observations combined with physical parameters derived from modeling can be used to study the physics of these irregularities. We have developed a full three-dimensional electromagnetic wave propagation model called “Satellite-beacon Ionospheric-scintillation Global Model of the upper Atmosphere” (SIGMA), to simulate GNSS scintillations. This model eliminates the most significant approximation made by the previous simulation approaches about the correlation length of the irregularity. Thus, for the first time, using SIGMA, we can accomplish scintillation simulations of significantly high fidelity. While the model is global, it is particularly applicable at high latitudes as it accounts for the complicated geometry of the magnetic field lines in these regions. Using SIGMA, we simulate the spatial and temporal variations in the GNSS signal phase and amplitude on the ground. In this paper, we present the model and results from a study to determine the sensitivity of the SIGMA outputs to different input parameters. We have deduced from our sensitivity study that the peak to peak (P2P) power gets most affected by the spectral index and line of sight direction, while the P2P phase and standard deviation of the phase (σφ) are more sensitive to the anisotropy of the irregularity. The sensitivity study of SIGMA narrows the parametric space to investigate when comparing the modeled results to the observations.

Monitoring ionospheric space weather with the Super Dual Auroral Radar Network (SuperDARN)

The Super Dual Auroral Radar Network (SuperDARN) of high frequency radars monitors ionospheric space weather at middle to high latitudes in both hemispheres. SuperDARN is an international collaboration involving scientists and engineers from over a dozen countries. The backscatter targets of interest are irregularities in the ionospheric plasma density that are aligned along the geomagnetic field. The Doppler motion of the irregularities can be used to infer the strength and direction of the ionospheric electric field. These measurements, obtained continuously, provide valuable information about the electrodynamics of the coupled magnetosphere-ionosphere system over extended spatial scales and with high time resolution. In this paper, the history of SuperDARN is briefly reviewed with a particular emphasis on the recent expansion of the network to middle and higher latitudes. A technique for assimilating multi-radar data to produce space weather maps of the hemispheric state of ionospheric plasma motion is also described.