E. MacKenzie

Ionospheric effects of major magnetic storms during the International Space Weather Period of September and October 1999: GPS observations, VHF/UHF scintillations, and in situ density structures at middle and equatorial latitudes

In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22–23, 1999, with the hourly Dst index being −167 nT at ∼2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of −231 nT at ∼0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS)-based navigation system called Wide Area Augmentation System (WAAS) being developed by the Federal Aviation Administration for use in the continental United States and their impact on global VHF/UHF communication systems. It is shown that the penetration of transient magnetospheric electric fields equatorward of the shielding region at midlatitudes, which have been well-correlated in the past with rapid changes in the well-known Dst index (or through its recently available high resolution 1-min counterpart the SYM-H index), can cause large increases of total electron content (TEC), TEC fluctuations, and saturated 250-MHz scintillation, and these, in turn, may have significant impacts on WAAS. The local time of Dst changes (and not just Dst magnitude) was found to be very important for WAAS, since the largest effects on TEC are seen near dusk. The prompt penetration of these magnetospheric electric fields all the way to the magnetic equator causes augmentation or inhibition of equatorial spread F. The global ionospheric response to these storms has been obtained from ground-based TEC observations with a GPS network and space-based in situ density and electric field measurements using the Republic of China Satellite-1 (ROCSAT-I) and several Defense Meteorological Satellite Program satellites. These prompt penetration electric fields cause VHF/UHF scintillations and GPS TEC variations at low latitudes in the specific longitude sector for which the early evening period corresponds to the time of rapid Dst variations and maximum Dst phase. The effects of the delayed ionospheric disturbance dynamo and those of decreased magnetospheric convection on postmidnight irregularity generation are shown to be confined to a part of the same longitude range that actively responded to the prompt penetration of electric fields in the early evening sector.

Optical ring formation and ionization production in high‐power HF heating experiments at HAARP

[1] Observations of HF-induced artificial optical emissions at the 3.6 MW HAARP facility show unexpected features not seen at the previous 960 kW level. Optical emissions often form a bright rayed ring near the 10% power contour surrounding a central disk with a sharp edge near the 50% power contour. Artificial bottomside layers in ionograms and positive perturbations in total electron content suggest that the bullseye optical patterns are associated with localized enhancements in plasma density below the main F layer. Ray tracing shows transmitter power concentrates in an annular structure consistent with the optical observations. Estimated ionization rates are well within the power available from the transmitter and agree well with the observed intensity of N + 2 427.8 nm emissions. We conclude that the optical bullseye patterns are a refraction phenomenon and an indicator of ionization production within the transmitter beam.

Structure of polar cap patches and fast shear flows following the CME impact on 22 January 2012 inferred from GPS scintillation spectra

Polar cap patches are localized enhancements in ionospheric density which originate from solar EUV ionization on the dayside, enter the polar cap at the dayside cusp, convect anti-sunward at km/s velocities, and exit the polar cap near midnight to merge with sunward returning flow patterns. Plasma irregularities associated with polar patches are the leading cause of scintillations in L-band satellite signals such as GPS, and fast shear flows near the dayside cusp are thought to be integral to patch formation. In this paper, we report on the on the characteristics of polar cap patches and fast flows derived via analysis of the spectra of GPS scintillations recorded at Longyearbyen, Svalbard, following the CME impact on 22 January 2012. Following the interaction of the CME with the high latitude ionosphere, elevated GPS TEC values indicate the passage of patches through the cusp between 11-15 MLT, accompanied by significant GPS phase scintillations (σφ ~ 0.5 radians) but minimal amplitude scintillations (S4 <; 0.05). Examination of the scintillation spectra reveal that amplitude fluctuations were present, but not easily detected in the S4 observations because the fluctuation power was concentrated at high frequencies. In fact, these amplitude spectra can be explained in terms of Fresnel filtering of the path integrated irregularity spectrum with a relatively high cutoff frequency (8 Hz). This filtering is consistent with weak scatter of the satellite signals by irregularities scanning past the ray path with an effective velocity ~ 3 km/s. Since the velocity of the satellite penetration point is negligible, by comparison, this scan velocity is attributed to fast plasma flow, presumably associated with shear flows near the cusp. To exploit the Fresnel filtering effect, we developed a technique to derive the flow velocity by reconciling the phase and amplitude spectra with weak scatter theory. We applied this approach to investigate the noontime entrance of patches into the dayside cusp and the midnight exit of patches from the polar cap. We find clear evidence of strong phase scintillations with reduced S4 values in the presence of fast flows near the cusp, when the increasing Fresnel break frequency effectively suppresses the low frequency content in the amplitude fluctuations. The scan velocity increased from about 500-1000 m/s following the initial CME impact at ~6:00 UT, to sustained velocities between 1500-3000 m/s measured by GPS satellites whose ray paths intersected fast plasma flows near the cusp. In this sector, the phase spectral index (p) generally ranged between 2.4-2.8, with a tendency for somewhat larger values when the flow is faster. Weaker irregularities were detected in the outflow sector between 20-24 MLT, when p generally ranged from 2.6-3.0. The scan velocities measured in the outflow sector were slower, generally between 400-600 m/s. These velocity estimates compare favorably with ion drift measurements made by the DMSP satellites. Since our analysis technique is automated, it could potentially enable continuous monitoring of flow patterns in the polar cap using a relatively inexpensive GPS scintillation monitor. These measurements could then complement measurements from space-based platforms that sample the polar cap only intermittently and incoherent scatter radars which provide excellent diagnostics but cannot operate continuously.

C/NOFS in situ and beacon measurements during the main phase of the first magnetic storms within solar cycle 24

The objective of the study is to utilize the high resolution PLP and the beacon on the C/NOFS satellite to determine the impact on the equatorial ionosphere of two moderate magnetic storms during solar cycle 24. These two storms perturb various SCINDA sites at dusk. The in-situ C/NOFS data allows the tracking of the plasma bubbles on a global scale. The spectral analysis of the PLP data shows interesting variation in the spectral shapes depending on its location with respect to the bubbles. Current analysis is ongoing to determine the impact of the spectral shapes on UHF scintillations from C/NOFS.