Bibliography

The Thermospheric Column O/N2 Ratio

More than 2 decades ago, D. J. Strickland and colleagues proposed use of the O/N2 column number density ratio as a new geophysical quantity to interpret thermospheric processes recorded in far ultraviolet (FUV) images of the Earth. This concept has enabled multiple advances in understanding the global behavior of Earth s thermosphere. Nevertheless, confusion remains about the conceptual meaning of the column density ratio, and in the application of this integral quantity. This is so even though it is now a key thermospheric measurement made by current and planned far ultraviolet remote sensing missions in pursuit of new understanding of thermospheric processes and variability. The intent here is to review the historical context of the O/N2 column density ratio, clarify its physical meaning, and resolve misunderstandings evident in the literature. Simple examples elucidate its original derivation for extracting column O/N2 ratios from measurements of the OI 135.6 nm/N2 Lyman-Birge-Hopfield (LBH) emission based on an algorithmic synthesis of model precomputations. These are organized in the form of a table lookup of column density ratio as a function of observed radiance ratios. To accommodate generalized solar-geophysical and viewing conditions, the table required to specify the number of needed parameters becomes large. Proposed as an alternative is a simplified, first principles approach to obtaining the column density ratio from the emission ratio. This new methodology is now being applied successfully to FUV measurements made from onboard the Ionospheric CONnection satellite and will be applied retrospectively to the Global Ultraviolet Imager data.

Meier, R.;

Published by: Journal of Geophysical Research: Space Physics      Published on:

YEAR: 2021     DOI: 10.1029/2020JA029059

disk algorithm; far UV remote sensing; GUVI; ICON; N2 LBH bands; Oxygen 135.6 nm

Auroral Structure and Dynamics From GOLD

The Global-scale Observations of the Limb and Disk (GOLD) mission data contain significant quantitative information about the aurora on a global scale. Here we present techniques for quantifying such information, including the temporal development of the structure within the auroral oval using the GOLD images. These techniques are applied to auroral observations in the GOLD data, in particular showing an example of how the longitudinal structure within the aurora varies over the course of six consecutive days with differing levels of geomagnetic activity. A simple model of the solar-induced airglow is presented that is used to remove the sunlight contamination from the dayside auroral observations. Comparisons to ground-based auroral imaging are used for the overall auroral context and to make estimates of the proportionality between the intensities of the green line (557.7\ nm) emission in the visible and the 135.6\ nm emissions in the GOLD data. These observations are consistent with the intensity of the 135.6\ nm auroral emission being on the same order as the intensity of the 557.7\ nm auroral emission. They were both found to be around 1\ kR for a stable auroral arc on a day with low geomagnetic activity (3 November 2018) and around 10\ kR for an active auroral display on a day with higher levels of geomagnetic activity (5 November 2018). This could have important implications for making direct comparisons between space-based ultraviolet auroral imaging and ground-based visible-light auroral imaging and the total energy input estimates that are derived from them.

Michell, R.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 02/2020

YEAR: 2020     DOI: 10.1029/2019JA027650

AURORA; GOLD; GUVI

Effects of transionospheric signal decorrelation on Global Navigation Satellite Systems (GNSS) performance studied from irregularity dynamics around the northern crest of the EIA

Transionospheric satellite navigation links operate primarily at L band and are frequently subject to severe degradation of performances arising out of ionospheric irregularities. Various characteristic features of equatorial ionospheric irregularity bubbles like the drift velocity, characteristic velocity, decorrelation time, and decorrelation distance can be determined using spaced aerial measurements at VHF. These parameters measured at VHF from a station Calcutta situated near the northern crest of the Equatorial Ionization Anomaly (EIA) in the geophysically sensitive Indian longitude sector have been correlated with L band scintillation indices and GPS position accuracy parameters for identifying possible proxies to L band scintillations. Good correspondences have been observed between decorrelation times and distances at VHF with GPS S₄ and Position Dilution of Precision during periods of GPS scintillations (S₄ \> 0.3) for February\textendashApril 2011, August\textendashOctober 2011, and February\textendashApril 2012. A functional relation has been developed between irregularity drift velocity measured at VHF and S₄ at L band during February\textendashApril 2011, and validation of measured S₄ and predicted values performed during August\textendashOctober 2011 and February\textendashApril 2012. Significant improvement in L band scintillation prediction and consequent navigational accuracy will result using such relations derived from VHF irregularity measurements which are much simpler and inexpensive.

Das, T.; Roy, B.; Paul, A.;

Published by: Radio Science      Published on: 10/2014

YEAR: 2014     DOI: 10.1002/rds.v49.1010.1002/2014RS005406

GNSS position determination accuracy under adverse ionospheric conditions; GUVI; irregularity dynamics at VHF

A case study of ionospheric storm effects during long-lasting southward IMF B _z -driven geomagnetic storm

Multiple instrumental observations including GPS total electron content (TEC), f_oF₂ and h_mF₂ from ionosondes, vertical ion drift measurements from Communication/Navigation Outage Forecasting System, magnetometer data, and far ultraviolet airglow measured by Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI) are used to investigate the profound ionospheric disturbances at midlatitude and low latitude during the 14\textendash17 July 2012 geomagnetic storm event, which was featured by prolonged southward interplanetary geomagnetic field component for about 30 h below -10 nT. In the East Asian/Australian sector, latitudinal profile of TEC variations in the main phase were characterized by three bands of increments and separated by weak depressions in the equatorial ionospheric anomaly (EIA) crest regions, which were caused by the combined effects of disturbance dynamo electric fields (DDEF) and equatorward neutral winds. In the recovery phase, strong inhibition of EIA occurred and the summer crest of EIA disappeared on 16 July due to the combined effects of intrusion of neutral composition disturbance zone as shown by the TIMED/GUVI O/N₂ measurements and long-lasting daytime westward DDEF inferred from the equatorial electrojet observations. The transit time of DDEF over the dip equator from westward to eastward is around 2200 LT. In the American longitude, the salient ionospheric disturbances in the summer hemisphere were characterized by daytime periodical intrusion of negative phase for three consecutive days in the recovery phase, preceded by storm-enhanced density plume in the initial phase. In addition, multiple short-lived prompt penetration electric fields appeared during stable southward interplanetary magnetic field (IMF) B_z in the recovery phase and were responsible for enhanced the EIA and equatorial ionospheric uplift around sunset.

Liu, Jing; Liu, Libo; Nakamura, Takuji; Zhao, Biqiang; Ning, Baiqi; Yoshikawa, A.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2014

YEAR: 2014     DOI: 10.1002/jgra.v119.910.1002/2014JA020273

GUVI; Ionospheric storm; prompt penetration electric fields; TIMED

Polar cap arcs correlated with solar wind entry at the high latitude magnetosphere

Polar cap arcs are sun-aligned aurora structures occurring during northward turnings of the Interplanetary Magnetic Field (IMF) B_z component. At the same time, a new region of solar wind entry at the high latitude magnetosphere, tailward of the cusp region, was found recently at the periods of northward IMF B_z. We propose a study to see the relationship of these entry events with the transpolar arc formation. Data of Global Ultraviolet Imager (GUVI) onboard TIMED mission is examined to see the transpolar aurora arcs during the given time periods of the solar wind entry. Initial results show that in approximately 20\% of cases transpolar arcs occur related to the solar wind entry processes.

Mailyan, B.; Shi, Q.; Gou, X.;

Published by:       Published on:

YEAR: 2014     DOI: 10.1109/URSIGASS.2014.6929926

aurora interplanetary; GUVI; magnetic fields; magnetosphere; solar wind; TIMED

Nightside midlatitude ionospheric arcs: TIMED/GUVI observations

[1]\ Midlatitude arcs (MLA) are the phenomenon of the nightside enhancements of ionospheric electron density at 20\textdegree-45\textdegree magnetic latitudes in both hemispheres. We investigate the occurrence of MLA and its dependence on season using the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI) O I 135.6 nm intensity data between March 2008 and March 2012. The 135.6 nm emission is mainly caused by radiant recombination between O⁺ ions and electrons in the nightside ionosphere. The GUVI data show that the MLA occurred in all seasons under geomagnetically quiet condition (mean Kp ~ 1.0) and during periods of low solar activity. Hemispheric symmetric MLA were observed during equinox. During solstice, the MLA were seen only in the winter hemisphere. The MLA were more frequently observed at local times between ~21:00 and 02:00 on nightside. They were rarely observed at local times between 03:00 and sun rise. The MLA were observed with an occurrence frequency of ~75\% of the time. Coincident GPS total electron current data confirm that the MLA were due to enhanced ionospheric electron density. GUVI 135.6 nm limb data further show that the altitude of the MLA is higher than that of the equatorial arcs created by the equatorial ionization anomaly or fountain effect. The appearance of the MLA at higher altitude than the equatorial arcs supports the idea that the creation of the MLA is associated with the uplift of the ionosphere by the equatorward neutral wind.

Zhang, Yongliang; Paxton, Larry; Kil, Hyosub;

Published by: Journal of Geophysical Research: Space Physics      Published on: 06/2013

YEAR: 2013     DOI: 10.1002/jgra.50327

GUVI; Ionosphere; UV imager

Statistical comparison of isolated and non-isolated auroral substorms

The present study compares isolated and non-isolated substorms in terms of their global morphology and energy deposition. The analysis is based on a list of geomagnetic substorm onsets identified with magnetometer data from SuperMAG and published previously by Newell and Gjerlove (2011a). Isolated substorms are defined as those with separation of two consecutive onsets no less than 3 h. The auroral data are obtained from the global ultraviolet imager (GUVI) on board the TIMED satellite and are rebinned into typical magnetic latitude-magnetic local time maps. The auroral maps are then averaged in 1 min intervals to show the dynamic change of the aurora. The three phases of the substorm are clearly demonstrated in both isolated and non-isolated substorms. However, there are noticeable differences between the two types of substorms: (1) While the nighttime auroral power for both types of substorms slightly increases in the growth phase, isolated (non-isolated) substorms are associated with smaller (greater) nighttime auroral power. (2) In the expansion phase, isolated substorms are associated with greater and more explosive energy release than non-isolated substorms. (3) The time for the recovery phase is ~2 times longer for isolated than for non-isolated substorms. (4) The winter-to-summer auroral power ratio is approximately constant throughout the three substorm phases and the ratio is larger for isolated (~30\%) than that for non-isolated (~10\%) substorms. It is also found that the polar cap area increases during the growth phase until ~10 min prior to the magnetic substorm onset and decreases rapidly thereafter. The decrease is found to result from the closure of the nightside polar cap associated with substorm expansion. It is found that the observed differences between the two types of substorms simply reflect the differences in the solar wind and EUV drivers. Thus, we conclude that there is no intrinsic difference between isolated and non-isolated substorms in terms of auroral energy release and subsequent auroral power decay.

Liou, Kan; Newell, Patrick; Zhang, Yong-Liang; Paxton, Larry;

Published by: Journal of Geophysical Research: Space Physics      Published on: 05/2013

YEAR: 2013     DOI: 10.1002/jgra.50218

auroral power; auroral substorm; GUVI; SuperMAG; TIMED

Determination of the Ionospheric Electron Density Profile from FUV Remote Sensing Measurements

A limb viewing model is established in this paper based on GUVI measurements of OI 135.6 nm nightglow and a method with Chapman function describing the distribution of ionospheric electron density is presented to obtain the ionospheric electron density profile. We apply the regularization and Newton iteration method to calculate ionospheric peak electron density and peak height with GUVI measurements, eliminating the ill condition of the weighted matrix. The ionospheric electron density profile is obtained using the calculated peak electron density and peak height as inputs. To evaluate the fidelity of the proposed algorithm in this paper, the retrieved electron density profiles are compared with those from ground-based observations. The results show that the retrieved electron density profiles agree well with those from ISR. Afterwards, the effects of magnetic storms on EDP are studied with the retrieved EDPs of the period between Sep 29 and Oct 3, 2002.

Jing, Wang; Yi, TANG; Zhi-Ge, ZHANG; Xu-Li, ZHENG; Guo-Qiang, NI;

Published by: Chinese Journal of Geophysics      Published on: 03/2013

YEAR: 2013     DOI: 10.1002/cjg2.20011

Electron density profile; Far ultraviolet spectrum remote sensing; GUVI; Ionosphere

The quiet nighttime low-latitude ionosphere as observed by TIMED/GUVI

In this paper, we examine the nighttime ionosphere climatology structure in the low latitude region and discrepancies between Global Ultraviolet Imager (GUVI) observations and the IRI model predictions using (1) the magnetic zonal mean of electron number density as a function of altitude and magnetic latitude, (2) vertical electron density profiles at various levels of F10.7 index, (3) nighttime descent and magnitude decrease of the ionosphere, (4) point-to-point comparisons of F-peak height (hmF2) and density (NmF2), and (5) the magnetic longitudinal variations of hmF2 and NmF2. The data collected from the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) mission since its launch in December 2001 have provided great opportunities for many scientific investigations of the ionosphere. In this analysis, we investigate the climatology of the nighttime low-latitude ionosphere under low geomagnetic activity (kp\ ⩽\ 4) using the electron density profiles inferred from the airglow measurements obtained by the GUVI aboard the TIMED spacecraft and compared with the results obtained from IRI (International Reference Ionosphere) model-2001. The observed climatology is an essential tool for further understanding the electrodynamics in the low-latitude region and improving the model\textquoterights prediction capability. The time range of the GUVI data used in this study is from 2002 (day 053) to 2006 (day 304), and the IRI model predictions were produced at every GUVI location. The ionosphere observed is generally of greater density than what IRI predicts throughout the night for all four seasons for low and moderate solar activity while the model over-predicts the electron density near the F-region peak at high solar activity before midnight. Observations show that the height of the F-region peak has a steep descent from dusk to midnight and near midnight the height of layer is insensitive to solar conditions, significantly different than what is predicted by IRI. Longitudinal features shown in GUVI data are present in the low-latitude ionosphere after sunset and continue through to midnight after which the low-latitude ionosphere is largely zonally symmetric.

Talaat, E.R.; Yee, J.-H.; Hsieh, S.-Y.; Paxton, L.J.; DeMajistre, R.; Christensen, A.B.; Bilitza, D.;

Published by: Advances in Space Research      Published on: 02/2013

YEAR: 2013     DOI: 10.1016/j.asr.2012.11.012

Electron density; GUVI; Ionosphere; IRI; TIMED

Research on Retrieving Thermospheric O/N2 from FUV Remote Sensing

Magnetic storms usually cause significant departures of thermospheric O and N₂\ from their normal values. To study the effects on thermospheric neutral species caused by magnetic storms, a method to retrieve thermospheric O/N₂\ based on the data obtained from global ultraviolet imager on board TIMED is presented. With the help of AURIC, the normalizations of observing angles and SZAs were preformed to the measurements and a relationship between 135.6/LBHs and O/N₂\ was established. Finally, applying the proposed method to retrieve O/N₂\ during a magnetic period(29, September\textemdash4, October, 2002), it was shown that magnetic storms could induce significant O/N₂\ depletion, extending from the polar regions towards the equator.

Peng, S.; Tang, Y.; Wang, J.; Zheng, X.;

Published by: Spectroscopy and Spectral Analysis      Published on: 05/2012

YEAR: 2012     DOI: 10.3964/j.issn.1000-0593(2012)05-1296-05

AURIC; GUVI; Magnetic storm; O/N2

First look at the 20 November 2003 superstorm with TIMED/GUVI: Comparisons with a thermospheric global circulation model

The NASA TIMED/GUVI experiment obtained unprecedented far ultraviolet images of thermospheric composition and temperature during the intense geomagnetic storm on 20\textendash21 November 2003. Geographic maps of the atomic oxygen to molecular nitrogen column density ratio show severe depletions that extend to the equator near the peak of the storm. This ratio is a key indicator of how the thermospheric composition is disrupted at high latitudes and how the perturbed air moves globally as a result of dynamical forcing. For example, migrating regions of low oxygen-to-nitrogen air are invariably found to correlate with high thermospheric temperatures. As well, GUVI obtained altitudinal-latitudinal (limb) images of temperature and composition, which show how the disturbances vary at different heights. The ASPEN thermospheric global circulation model was used to test our understanding of these remarkable images. The resulting simulations of thermospheric response show good agreement with GUVI data prior to the peak of the storm on 20 November. During the peak and recovery phases, serious discrepancies between data and model are seen. Although this initial attempt to model the storm is encouraging, much more detailed analysis is required, especially of the high-latitude inputs. The GUVI images demonstrate that far ultraviolet imaging is becoming a crucial component of space weather research and development.

Meier, R.; Crowley, G.; Strickland, D.; Christensen, A.; Paxton, L.; Morrison, D.; Hackert, C.;

Published by: Journal of Geophysical Research      Published on: 09/2005

YEAR: 2005     DOI: 10.1029/2004JA010990

dayglow; geomagnetic storm; GUVI; remote sensing; thermospheric composition; TIMED