274 research outputs found
Topside equatorial zonal ion velocities measured by C/NOFS during rising solar activity
No full textThe Ion Velocity Meter (IVM), a part of the Coupled Ion Neutral Dynamic
Investigation (CINDI) instrument package on the Communication/Navigation
Outage Forecast System (C/NOFS) spacecraft, has made over 5 yr of
in situ measurements of plasma temperatures, composition, densities, and
velocities in the 400–850 km altitude range of the equatorial ionosphere.
These measured ion velocities are then transformed into a coordinate system
with components parallel and perpendicular to the geomagnetic field allowing
us to examine the zonal (horizontal and perpendicular to the geomagnetic
field) component of plasma motion over the 2009–2012 interval. The general
pattern of local time variation of the equatorial zonal ion velocity is well
established as westward during the day and eastward during the night, with
the larger nighttime velocities leading to a net ionospheric superrotation.
Since the C/NOFS launch in April 2008, F10.7 cm radio fluxes have gradually
increased from around 70 sfu to levels in the 130–150 sfu range. The
comprehensive coverage of C/NOFS over the low-latitude ionosphere allows us
to examine variations of the topside zonal ion velocity over a wide level of
solar activity as well as the dependence of the zonal velocity on apex
altitude (magnetic latitude), longitude, and solar local time. It was found
that the zonal ion drifts show longitude dependence with the largest net
eastward values in the American sector. The pre-midnight zonal drifts show
definite solar activity (F10.7) dependence. The daytime drifts have a lower
dependence on F10.7. The apex altitude (magnetic latitude) variations
indicate a more westerly flow at higher altitudes. There is often a net
topside subrotation at low F10.7 levels, perhaps indicative of a suppressed
F region dynamo due to low field line-integrated conductivity and a low
F region altitude at solar minimum
Plasma Dynamics Associated With Equatorial Ionospheric Irregularities
No full textThe Communication/Navigation Outage Forecasting System satellite was operational from 2008, a period of deep solar minimum, to 2015, a period of moderate solar conditions. The behavior of the vertical plasma drift and the distribution of plasma depletions during the deep solar minimum of 2009 deviated substantially from the behavior that was observed during the solar moderate conditions encountered by the Communication/Navigation Outage Forecasting System satellite in 2014, which are typical of previous observations. Presented here are observations of the vertical drift of plasma depletions and the background plasma in which they are embedded. We find that depletions detected at local times after 2100 hr during solar minimum are typically found in background drifts that are weakly downward compared to the strongly downward background drifts observed during moderate solar activity levels. Additionally, at solar minimum, the drift within the depletions is upward with respect to the background as compared with observations at the same local times during solar moderate conditions for which the depleted plasma more nearly drifts with the background. We note that weak background plasma drifts observed throughout the night during solar minimum promote the continued growth of depletions that may evolve more slowly or be continuously generated to appear in the topside in the postmidnight hours. ©2018. American Geophysical Union. All Rights Reserved.NASA grant NNX15AT31G.School of Natural Sciences and MathematicsWilliam B. Hanson Center for Space Science
Identifying equatorial ionospheric irregularities using in situ ion drifts
No full textPrevious climatological investigations of ionospheric irregularity occurrence
in the equatorial ionosphere have utilized in situ measurements of plasma
density to identify the presence of an irregularity. Here we use the Morlet
wavelet and C/NOFS to isolate perturbations in meridional ion drifts and
generate irregularity occurrence maps as a function of local time, longitude,
season, and solar activity. For the low solar activity levels in 2008, the
distributions identified by velocity perturbations follow normalized density
perturbation (ΔN/N) maps with large occurrences after midnight into
dawn over all longitudes. The velocity and normalized density occurrence maps
contract in both local time and longitude with increasing solar activity. By
2011 irregularities are confined to particular longitudes expected by
alignment and a few hours of local time after sunset. The variation in the
occurrence of the late night irregularities with solar activity is consistent
with the presence of gravity wave seeding
The Plasma Environment Associated With Equatorial Ionospheric Irregularities
No full textWe examine the density structure of equatorial depletions referred to here as equatorial plasma bubbles (EPBs). Data recorded by the Ion Velocity Meter as part of the Coupled Ion Neutral Dynamics Investigation (CINDI) aboard the Communication/Navigation Outage Forecasting System (C/NOFS) satellite are used to study EPBs from 1600 to 0600 h local time at altitudes from 350 to 850 km. The data are taken during the 7 years from 2008 to 2014, more than one half of a magnetic solar cycle, that include solar minimum and a moderate solar maximum. Using a rolling ball algorithm, EPBs are identified by profiles in the plasma density, each having a depth measured as the percent change between the background and minimum density (ΔN/N). During solar moderate activity bubbles observed in the topside postsunset sector are more likely to have large depths compared to those observed in the topside postmidnight sector. Large bubble depths can be observed near 350 km in the bottomside F region in the postsunset period. Conversely at solar minimum the distribution of depths is similar in the postsunset and postmidnight sectors in all longitude sectors. Deep bubbles are rarely observed in the topside postsunset sector and never in the bottomside above 400 km in altitude. We suggest that these features result from the vertical drift of the plasma for these two solar activity levels. These drift conditions affect both the background density in which bubbles are embedded and the growth rate of perturbations in the bottomside where bubbles originate.National Aeronautics and Space Administration. Grant Number: NNX15AT31GSchool of Natural Sciences and MathematicsWilliam B. Hanson Center for Space Science
Daytime zonal drifts in the ionospheric 150 km and <i>E</i> regions estimated using EAR observations
No full textMultibeam observations of the 150km echoes made using the Equatorial Atmosphere Radar (EAR), located at Kototabang, Indonesia, provide unique opportunity to study both vertical and zonal ExB plasma drifts in the equatorial ionosphere. In this paper, we focus on estimating daytime zonal drifts at the 150km (140-160km) and E (100-110km) regions using multibeam observations of 150km and E region echoes made using the EAR and study the daytime zonal drifts covering all seasons not studied before from Kototabang. Zonal drifts in the 150km and E regions are found to be westward and mostly below -80ms⁻¹ and -60ms⁻¹, respectively. While the zonal drifts in the 150km and E regions do not go hand in hand on a case-by-case basis, the seasonal mean drifts in the two height regions are found to be in good agreement with each other. Zonal drifts at the 150km region show seasonal variations with three maxima peaking around May, September, and January. The zonal drifts at the 150km region are found to be smaller than the F region drifts obtained from Coupled Ion Neutral Dynamics Investigation (CINDI) onboard Communication and Navigation Outage Forecasting System (C/NOFS) by about 25ms⁻¹ consistent with the height variations of F region zonal drifts observed by the Jicamarca radar. These results constitute the first comprehensive study of zonal drifts at the 150km and E regions from Kototabang, Indonesia, and the results are discussed in the light of current understanding on the low-latitude electrodynamics and coupling.JSPS KAKENHI Grant 15H05815; NARL. Grant Number: NARL‐ISPG.School of Natural Sciences and MathematicsWilliam B. Hanson Center for Space Science
Coordinated Satellite Observations of the Very Low Frequency Transmission Through the Ionospheric<i>D</i>Layer at Low Latitudes, Using Broadband Radio Emissions From Lightning
No full textFull text access from Treasures at UT Dallas is restricted to current UTD affiliates.Both ray theory and full-wave models of very low frequency transmission through the ionospheric D layer predict that the transmission is greatly suppressed near the geomagnetic equator. We use data from the low-inclination Communication/Navigation Outage Forecast System satellite to test this semiquantitatively, for broadband very low frequency emissions from lightning. Approximate ground-truthing of the incident wavefields in the Earth-ionosphere waveguide is provided by the World Wide Lightning Location Network. Observations of the wavefields at the satellite are provided by the Vector Electric Field Instrument aboard the satellite. The data set comprises whistler observations with the satellite at magnetic latitudes < 26 degrees. Thus, our conclusions, too, must be limited to the near-equatorial region and are not necessarily predictive of midlatitude whistler properties. We find that in most broadband recordings of radio waves at the satellite, very few of the lightning strokes result in a detectable radio pulse at the satellite. However, in a minority of the recordings, there is enhanced transmission of very low frequency lightning emissions through the D layer, at a level exceeding model predictions by at least an order of magnitude. We show that kilometric-scale D-layer irregularities may be implicated in the enhanced transmission. This observation of sporadic enhancements at low magnetic latitude, made with broadband lightning emissions, is consistent with an earlier review of D-layer transmission for transmission from powerful man-made radio beacons.NSF Grant Number: 1443011School of Natural Sciences and MathematicsWilliam B. Hanson Center for Space Science
Storming the Bastille: the effect of electric fields on the ionospheric F-layer
No full textWe discuss different phenomena occurring during ionospheric F-region storms that in principle might be caused by electric fields and point out challenges that must be faced when considering the physical processes at work. We consider the transport of plasma across many degrees of latitude at sub-auroral latitudes, the origin of patches of so-called "storm enhanced density" at high mid-latitudes, and the very high reported heights of the F2 peak at low latitudes. We discuss the role that electric fields might play in changing locally the net production of ionization as well as transporting it. We suggest that the local change in ionization production should be considered as a more important process for producing plasma density enhancements than transport from a more remote source of enhanced density.<br/
Duskside enhancement of equatorial zonal electric field response to convection electric fields during the St. Patrick's Day storm on 17 March 2015
No full textThe equatorial zonal electric field responses to prompt penetration of eastward convection electric fields (PPEF) were compared at closely spaced longitudinal intervals at dusk to premidnight sectors during the intense geomagnetic storm of 17 March 2015. At dusk sector (Indian longitudes), a rapid uplift of equatorial F layer to >550 km and development of intense equatorial plasma bubbles (EPBs) were observed. These EPBs were found to extend up to 27.13⁰N and 25.98⁰S magnetic dip latitudes indicating their altitude development to ~1670 km at apex. In contrast, at few degrees east in the premidnight sector (Thailand-Indonesian longitudes), no significant height rise and/or EPB activity has been observed. The eastward electric field perturbations due to PPEF are greatly dominated at dusk sector despite the existence of background westward ionospheric disturbance dynamo (IDD) fields, whereas they were mostly counter balanced by the IDD fields in the premidnight sector. In situ observations from SWARM-A and SWARM-C and Communication/Navigation Outage Forecasting System satellites detected a large plasma density depletion near Indian equatorial region due to large electrodynamic uplift of F layer to higher than satellite altitudes. Further, this large uplift is found to confine to a narrow longitudinal sector centered on sunset terminator. This study brings out the significantly enhanced equatorial zonal electric field in response to PPEF that is uniquely confined to dusk sector. The responsible mechanisms are discussed in terms of unique electrodynamic conditions prevailing at dusk sector in the presence of convection electric fields associated with the onset of a substorm under southward interplanetary magnetic field B_z.Government of India (GITA/DST/TWN/P-47/2013; JSPS KAKENHI (grants 20244080, 5247080, and 15H05815
Structures in Ionospheric Number Density and Velocity Associated with Polar Cap Ionization Patches
No full textSpectral characteristics of polar cap F region irregularities on large density gradients associated with polar ionization patches are studied using in situ measurements made by the Dynamics Explorer 2 (DE 2) spacecraft. The 18 patches studied in this paper were identified by the algorithm introduced by Coley and Heelis, and they were encountered during midnight-noon passes of the spacecraft. Density and velocity spectra associated with these antisunward convecting patches are analyzed in detail. Observations indicate the presence of structure on most patches regardless of the distance between the patch and the cusp where they are believed to develop. Existence of structure on both leading and trailing edges is established when such edges exist. Results, which show no large dependence of Delta N/N power on the sign of the edge gradient del N, do not allow the identification of leading and trailing edges of the patch. The Delta N/N is an increasing function of gradient del N regardless of the sign of the gradient. The correlation between Delta N/N and Delta V is generally poor, but for a given intensity in Delta V, Delta N/N maximizes in regions of large gradients in N. There is evidence for the presence of unstructured patches that seem to co-exist with unstructured horizontal velocities. Slightly smaller spectral indices for trailing edges support the presence of the E X B drift instability. Although this instability is found to be operating in some cases, results suggest that stirring may be a significant contributor to kilometer-size structures in the polar cap
A Topside Equatorial Ionospheric Density and Composition Climatology During and After Extreme Solar Minimum
No full textDuring the recent solar minimum, solar activity reached the lowest levels observed during the space age. This extremely low solar activity has accompanied a number of unexpected observations in the Earth's ionosphere and thermosphere when compared to previous solar minima. Among these are the fact that the ionosphere is significantly contracted beyond expectations based on empirical models. Climatological altitude profiles of ion density and composition measurements near the magnetic dip equator are constructed from the C/NOFS satellite to characterize the shape of the top side ionosphere during the recent solar minimum and into the new solar cycle. The variation of the profiles with respect to local time, season, and solar activity are compared to the IRI-2007 model. Building on initial results reported by Heelis et al. [2009], here we describe the extent of the contracted ionosphere, which is found to persist throughout 2009. The shape of the ionosphere during 2010 is found to be consistent with observations from previous solar minima
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