Cheryl Yu-Ying Huang

Electromagnetic wave scattering and scintillation can occur at high latitudes of the Earth when polar cap patches are present. Polar cap patches range in size from 100 to 1,000 km and have densities on the order of 2–10 times that of the background ionosphere. Within polar cap patches, plasma irregularities caused by gradient drift instabilities are observed throughout the patch as elongated slivers. These irregularities vary in size from centimeters to tens of kilometers. Three major challenges must be overcome to thoroughly study high‐frequency electromagnetic wave interactions with polar cap patches: (1) the complex geometrical features of the irregularities within the patches must be accommodated; (2) the non‐approximated Maxwell's equations should be solved to account for all the physics of electromagnetic wave propagation, scattering, diffraction, etc.; and (3) the physics of electromagnetic wave interactions with magnetized ionospheric plasma must be included. In this paper, the finite‐difference time‐domain (FDTD) method is used to overcome the above three challenges and solve for electromagnetic wave interactions with plasma irregularities within a polar cap patch. Irregularities and patch profiles are generated using Defense Meteorological Satellite Program data and incorporated into the FDTD model. The transmission coefficients for high‐frequency electromagnetic wave propagation through perturbed and unperturbed ionospheres, as well as for magnetized and unmagnetized ionospheric plasma ionospheres are calculated. From these simulations, the effects of small‐scale irregularities on high‐frequency propagation are observed.

The ionospheric response to explosions which occur at or below the Earth's surface has been noted since the first detonations of nuclear devices during the early period of aboveground testing. Acoustic gravity waves and traveling ionospheric disturbances were detected in association with test explosions carried out by the Union of Soviet Socialist Republics in Novaya Zemlya in 1961. While research in this area has continued, the standards accepted by the Comprehensive Nuclear Test Ban Treaty for detection and confirmation of nuclear explosions have been based on (1) seismic, (2) hydroacoustic, (3) infrasound, and (4) radionuclide monitoring from ground detectors. We suggest that ionospheric sensing offers a complementary methodology that may allow for robust confirmation of explosive events. One method of ionospheric monitoring of explosive events is analysis of total electron content (TEC), available by processing data from Global Navigation Satellite System (GNSS) receivers distributed globally on land masses. Traveling ionospheric disturbances observed by their signature in TEC have been used to detect and confirm mine collapses, mine blasts, earthquakes, volcanic eruptions, and meteorite strikes as well as underground nuclear tests. While an integrated measurement like TEC is not as sensitive to small‐amplitude density perturbations as other methods, the existence of large networks of continuously operating GNSS stations makes this an intriguing new monitoring asset. We report on the current capabilities for detection of explosions via the ionosphere, the outstanding challenges, and prospects for future developments that have potential to augment the current standards for detection and confirmation of nuclear detonations. By leveraging the worldwide network of GNSS receivers, robust confirmation of impulsive events is possible. This methodology complements existing technologies approved by the Comprehensive Test Ban Treaty. The article outlines background theory, new approaches to monitoring of explosions, and challenges that remain to be addressed.

Polar cap neutral density anomaly (PCNDA) with large mass density enhancements over the background has been frequently observed in the polar cap during magnetic storms. By tracing field lines to the magnetosphere from the polar ionosphere, we divide the polar cap into two regions, an open field line (OFL) region with field lines connecting to the magnetopause boundary and a distant tail field line (TFL) region threaded with magnetotail lobe field lines. A statistical study of neutral density observed by the Challenging Minisatellite Payload satellite during major magnetic storms with Dst < −100 from July 2001 to 2006 indicates that over 85% of density anomalies were detected in the TFL region, at about 18° to 25° equatorward the center of the OFL region. PCNDAs were frequently accompanied by plasma clouds with peak density greater than 10⁵ #/cm³. Modeling of plasma cloud drift paths suggests that plasma clouds originating in the dayside ionosphere could convect through the OFL region following the zero‐potential line and reach the PCNDA locations. Plasma clouds could become stagnate in the TFL region, allowing a long duration of collisions with the neutral gas and possibly contributing to heating of PCNDAs. The PCNDA observations are interpreted as evidence that traveling atmospheric disturbance could be generated in the nightside polar cap. From the PCNDA size and speed of sound at 400 km, we derive an initial energy deposition duration for producing traveling atmospheric disturbance in the range from 0.5 to 2.5 hr.

Recent studies have reported that thermospheric effects due to solar wind driving can be observed poleward of auroral latitudes. In these papers, the measured neutral mass density perturbations appear as narrow, localized maxima in the cusp and polar cap. They conclude that Joule heating below the spacecraft is the cause of the mass density increases, which are sometimes associated with local field‐aligned current structures, but not always. In this paper we investigate neutral mass densities measured by accelerometers on the CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) spacecraft from launch until years 2010 (CHAMP) and 2012 (GRACE), approximately 10 years of observations from each satellite. We extract local maxima in neutral mass densities over the background using a smoothing window with size of one quarter of the orbit. The maxima have been analyzed for each year and also for the duration of each set of satellite observations. We show where they occur, under what solar wind conditions, and their relation to magnetic activity. The region with the highest frequency of occurrence coincides approximately with the cusp and mantle, with little direct evidence of an auroral zone source. Our conclusions agree with the “hot polar cap” observations that have been reported and studied in the past.

We examine the response of the thermosphere to the impact of solar wind dynamic pressure enhancements using observations and global magneto‐hydrodynamics (MHD) simulations by the OpenGGCM model. Combining neutral density observations from the Challenging Minisatellite Payload (CHAMP) and the Gravity Recovery and Climate Experiment (GRACE) satellites with simultaneous Poynting flux measurements from the Defense Meteorological Satellite Program (DMSP) F16, we find that thermospheric density as well as downward Poynting flux intensified shortly after a sudden enhancement of the solar wind dynamic pressure. The intensification manifested mostly on the dayside high‐latitude region with peak intensity in the vicinity of the noon and prenoon cusp. OpenGGCM modeling results show that the ionospheric Joule heating increased abruptly in response to the sudden enhancement of the dynamic pressure in the same region as the observed Poynting flux and neutral density enhancements. The modeling results show that the enhanced Joule heating coincides, both in time and location, with the appearance of a pair of high‐latitude localized field‐aligned currents (FACs) in the cusp region. The FACs intensified and extended azimuthally. Coincidental with the solar wind dynamic pressure enhancement, the y component of the interplanetary magnetic field (IMF) B_y became strongly positive and, in addition, had some large fluctuations. We explore the separate and combined effects of the dynamic pressure and IMF B_y perturbations, with specifically designed simulation experiments that isolate the effect of each solar wind parameter. We find that the dynamic pressure enhancement is the primary source for the Joule heating and neutral density enhancements, but the IMF B_y modulates the level of enhancement.

Journal of Space Weather and Space Climate, a link between all the communities involved in Space Weather and in Space Climate

We present results from an analysis of high‐latitude ionosphere‐thermosphere (IT) coupling to the solar wind during a moderate magnetic storm which occurred on 5–6 August 2011. During the storm, a multipoint set of observations of the ionosphere and thermosphere was available. We make use of ionospheric measurements of electromagnetic and particle energy made by the Defense Meteorological Satellite Program and neutral densities measured by the Gravity Recovery and Climate Experiment satellite to infer (1) the energy budget and (2) timing of the energy transfer process during the storm. We conclude that the primary location for energy input to the IT system may be the extremely high latitude region. We suggest that the total energy available to the IT system is not completely captured either by observation or empirical models.

The plasma densities detected by the Planar Langmuir Probe (PLP) on board the Communication/Navigation Outage Forecasting System (C/NOFS) satellite have been analyzed for the June 2008 period. This interval, which corresponds to one of the quietest periods in the space era, exhibited broad plasma decreases (BPDs) which we have reported previously. In order to treat the data quantitatively, we have detrended the PLP data by using the International Reference Ionosphere model to remove variations in density due to changes in spacecraft altitude and latitude along the orbit. In this paper we present results of a statistical analysis of C/NOFS detrended plasma densities during June 2008 as well as neutral densities from the Gravity Recovery and Climate Experiment measured during the same period. The results show periodic structures in both charged and neutral species, most evident in limited local times. These structures resemble wave‐4 nonmigrating tides. BPDs persist in the detrended data and appear as one of the minima in the periodic signatures, most strikingly on the nightside.