Browsing by Author "Gerrard, Andrew J."
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- Conjugate observations of electromagnetic ion cyclotron waves associated with traveling convection vortex eventsKim, Hyomin; Clauer, C. Robert; Gerrard, Andrew J.; Engebretson, Mark J.; Hartinger, Michael D.; Lessard, Marc R.; Matzka, Juergen; Sibeck, David G.; Singer, Howard J.; Stolle, Claudia; Weimer, Daniel R.; Xu, Zhonghua (2017-07)We report on simultaneous observations of electromagnetic ion cyclotron (EMIC) waves associated with traveling convection vortex (TCV) events caused by transient solar wind dynamic pressure (P-d) impulse events. The Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft located near the magnetopause observed radial fluctuations of the magnetopause, and the GOES spacecraft measured sudden compressions of the magnetosphere in response to sudden increases in Pd. During the transient events, EMIC waves were observed by interhemispheric conjugate ground-based magnetometer arrays as well as the GOES spacecraft. The spectral structures of the waves appear to be well correlated with the fluctuating motion of the magnetopause, showing compression-associated wave generation. In addition, the wave features are remarkably similar in conjugate hemispheres in terms of bandwidth, quasiperiodic wave power modulation, and polarization. Proton precipitation was also observed by the DMSP spacecraft during the wave events, from which the wave source region is estimated to be 72 degrees-74 degrees in magnetic latitude, consistent with the TCV center. The confluence of space-borne and ground instruments including the interhemispheric, high-latitude, fluxgate/induction coil magnetometer array allows us to constrain the EMIC source region while also confirming the relationship between EMIC waves and the TCV current system.
- Low latitude thermospheric responses to magnetic stormsEarle, Gregory D.; Davidson, R. L.; Heelis, R. A.; Coley, W. R.; Weimer, Daniel R.; Makela, J. J.; Fisher, D. J.; Gerrard, Andrew J.; Meriwether, J. (American Geophysical Union, 2013-06-01)Thermospheric density and neutral velocity perturbations associated with three magnetic storms in the autumn season of 2011 are examined using data from the neutral wind meter (NWM) on the Communication/Navigation Outage Forecast System (C/NOFS) satellite. The data from perigee passes near 400km altitude show marked increases in neutral density during the storms and associated increases in horizontal neutral flow speeds. These thermospheric responses are characterized by enhanced meridional neutral flows with peak perturbation amplitudes near 100m/s and relative neutral density enhancements ranging from 50-100%. The increases in the neutral density and meridional flow velocity at equatorial latitudes occur about 5-7h after the initial perturbations are observed in the z component of the interplanetary magnetic field (IMF), and they persist for 20-30h. The perturbations in the neutral density are in good agreement with temperature increases predicted by an empirical model that has been validated using data from the CHAMP and Gravity Recovery and Climate Experiment missions, with a maximum lag time of similar to 1-1.5h between the model temperature increases and the observed density perturbations. The model temperatures are in excellent agreement with ground-based low-latitude temperature measurements during the storms. Ground-based wind measurements during one of the storms provide additional data for comparison with the perturbation wind amplitudes measured aboard the satellite.
- Modeling Amateur Radio Soundings of the Ionospheric Response to the 2017 Great American EclipseFrissell, N. A.; Katz, J. D.; Gunning, S. W.; Vega, J. S.; Gerrard, Andrew J.; Earle, Gregory D.; Moses, M. L.; West, M. L.; Huba, J. D.; Erickson, P. J.; Miller, E. S.; Gerzoff, R. B.; Liles, W.; Silver, H. W. (2018-05-28)On 21 August 2017, a total solar eclipse traversed the continental United States and caused large-scale changes in ionospheric densities. These were detected as changes in medium-and high-frequency radio propagation by the Solar Eclipse QSO Party citizen science experiment organized by the Ham Radio Science Citizen Investigation (hamsci.org). This is the first eclipse-ionospheric study to make use of measurements from a citizen-operated, global-scale HF propagation network and develop tools for comparison to a physics-based model ionosphere. Eclipse effects were observed +/- 0.3 hr on 1.8 MHz, +/- 0.75 hr on 3.5 and 7 MHz, and +/- 1 hr on 14 MHz and are consistent with eclipse-induced ionospheric densities. Observations were simulated using the PHaRLAP raytracing toolkit in conjunction with the eclipsed SAMI3 ionospheric model. Model results suggest 1.8, 3.5, and 7 MHz refracted at h >= 125 km altitude with elevation angles theta >= 22 degrees, while 14 MHz signals refracted at h < 125 km with elevation angles theta < 10 degrees.