SA21B-0345 0800h
Medium-scale traveling ionospheric disturbances observed with the GPS networks in South-California
We analyzed GPS data obtained from about 350 dual-frequency GPS receivers in South-California in a whole year of 2002 to investigate two-dimensional structures of Medium-Scale Traveling Ionospheric Disturbances (MSTID). The GPS data were provided from International GPS Service (IGS) in South-California, Southern California Integrated GPS Network (SCIGN) and Continuously Operating Reference Stations (CORS). We calculated Total Electron Content (TEC) from GPS data and subtracted 1-hour running average from time sequence of TEC obtained from each pair of GPS receiver and satellite to derive the perturbation component of TEC. Two-dimensional maps of the TEC perturbations within the area of 110-124W and 30-40N were derived with a time resolution of 30 s and a spatial resolution of 0.15 x 0.15 degrees in longitude and latitude. For example, MSTIDs propagating toward south-eastward with a velocity of about 170 m/s were observed during 0800-0900LT on January 21, 2002. Horizontal wavelength and period of the MSTID were 200 km and 20 min, respectively. They are comparable with those of MSTIDs observed in Japan. In this presentation, we show statistical results of occurrence rate and propagation features of MSTID in South-California in 2002.
SA21B-0346 0800h
Mid-latitude Ionospheric Irregularity Diagnostic Results From Dynasonde Observations at Bear Lake, Utah
We use the available time series (11 Feb. 2003 to present) of dynasonde recordings from a mid-latitude (Bear Lake, Utah) location, for a climatological study of small-scale irregularity parameters using our "Phase Structure Function" and "Anomalous Attenuation" methods. Real-time data transfer to NGDC, automatic processing, and display of the data is a result of a shared effort with observatory managers and is supported under NSF project ATM 0125297 and a USAF SBIR project F19628-03-C-0019. The Phase Structure Function method is based on precision measurements of short-period phase variations in totally-reflected radio echoes, as statistically summarized in the temporal structure function, SF. We relate parameters of the SF to the irregularity spectrum parameters by means of diffraction theory. Another diagnostic approach, based on the theory of multiple scattering, predicts a significant level of anomalous attenuation (tens of dBs) of totally-reflected echoes arising from their specific mode of interaction with ionospheric irregularities. Our study of about one year's data yields the following results: a) Characteristic (and different) average diurnal variations are found for E and F regions, and for the seasons. We find evident cause-effect relations between, for example, the sunrise/sunset processes in the ionosphere and irregularity amplitude. b) We have found evidence that the electron density irregularity absolute-amplitude, $\Delta$N, is of more fundamental significance than the relative amplitude $\Delta$N/N for small scales. Clearly, $\Delta$N is less dependent on diurnal variations of average electron density and we find that it has a distinct tendency to preserve its value (on the average) along a magnetic tube, even between E and F regions. c) There is an evident correlation between irregularity amplitude and plasma gradient, in both E and F regions. d) Irregularity amplitude is well correlated with the local magnetic K index at higher latitudes, but not at Bear Lake. e) On the other hand, we observed indications of a significant increase of the anomalous attenuation during some stages of the record Halloween 2003 magnetic storm.
SA21B-0347 0800h
Interhemispheric Conjugacy of Large-Scale Traveling Ionospheric Disturbances
The interhemispheric conjugacy of large-scale traveling ionospheric disturbances (LSTIDs) was studied using total electron content (TEC) data derived from GPS networks in Japan and Australia. LSTIDs are generally believed as the ionospheric manifestations of the passage of atmospheric gravity waves (AGWs) that are generated at high latitudes by the energy input from the magnetosphere. Although this energy is considered to be injected into both hemispheres coincidently, there have been few studies focusing on the geomagnetic conjugacy of LSTIDs by observations using GPS networks in both hemispheres. Thirty-eight LSTIDs were identified in 2002 using two-dimensional TEC maps derived from GEONET, a dense and wide-area GPS network in Japan. All the LSTIDs propagated southward over Japan. We sought southern hemispheric LSTIDs within about an hour of the appearance of each northern hemispheric LSTID using several GPS receivers in Australia. The number of conjugate occurrence of LSTIDs was five that is 25% of all the LSTIDs observed in the north hemisphere when Kp$\ge$5-. There is no conjugate occurrences when Kp$\le$4+. This indicates that the conjugate occurrence of LSTIDs is infrequent even though the geomagnetic activity is large. Detail comparison of TEC variations over Japan to those over the geomagnetic conjugate region revealed that the passage times of conjugate LSTIDs were not precisely simultaneous but different by several tens of minutes for all the five events. The meridional propagation velocities of conjugate LSTIDs were also different by several hundreds of m/s. The faster LSTIDs in one hemisphere tend to be found earlier at the conjugate regions than the slower LSTIDs in the other hemisphere. These observational results indicate that LSTIDs propagating simultaneously in both hemispheres have no electromagnetic connection through the geomagnetic field between each other. This study revealed that there is little interhemispheric conjugacy on the property of the occurrences and propagations of LSTIDs.
SA21B-0348 0800h
Storm Time Effects on the Low- to Mid-Latitude Ionosphere
The details of how magnetospherically driven penetration electric fields couple to the mid- and low-latitude ionosphere and generate large scale variations and structure in the plasma density is of paramount importance to the NASA Living with a Star Program. We have developed a computational model for the self-consistent modeling of the coupled inner magnetosphere-ionosphere system. The approach is to combine two existing, but compatible, computer models which treat different parts of the physical system: the Rice Convection Model (RCM), which models the electrodynamics of the inner magnetosphere; and SAMI3 (Sami3 is Also a Model of the Ionosphere), which treats the physics of the low- and mid-latitude ionosphere. The fundamental coupling of RCM and SAMI3 is through the electrostatic potential $\Phi$ and the ionospheric conductance. The essence of the coupling scheme is as follows: (1) SAMI3 will provide the ionospheric conductance to RCM, (2) RCM will solve the potential equation to determine $\Phi$ and the electric field, and (3) RCM will then provide the electric field back to SAMI3. Thus, the coupled model provides a self-consistent description of the ionosphere/inner-magnetosphere system to investigate the effects of penetration electric fields on the low- to mid-latitude ionosphere. We will present an overview of this program and preliminary results. \medskip \noindent Research supported by NASA and ONR.
SA21B-0349 0800h
Mid-Latitude Ionospheric Response to Storm Time Electric Field
Huang et al. (private communication, 2004) have reported large increases, factors of 2-3, in the mid-latitude ionospheric electron density in the aftermath of a magnetic storm. The density increase was observed at two mid-latitude sites (Millstone Hill and Eglin), but was not seen at lower latitudes (Areceibo). Accompanying the increase was a decrease (of 800-1500 K) in the F-region electron temperature. We present simulations results from the NRL ionosphere models SAMI2 and SAMI3 of the response of the mid-latitude F-region ionosphere to an imposed electric field that simulates this magnetic storm. We obtain reasonable agreement with the observed density and temperature features despite a simple model of the storm electric field. \medskip \noindent Research supported by ONR.
SA21B-0350 0800h
Observations of Midlatitude Ionospheric "Wall-Like" Features During Major Geomagnetic Storms
Using data from the available networks of GPS receivers (CORS and FAA WAAS), observations of rapid drops in TEC (as much as 14 meters of ionospheric delay or 84 TECu within 100 seconds) have been observed in mid-latitudes during major geomagnetic storms. Investigations of these TEC drops have suggested that the drops were due to what appeared to be "walls of depletion" oriented roughly Northwest to Southeast and traveling towards the Southwest at speeds of 300 to over 1000 m/s. During the (2003)October 29-30th geomagnetic storms, video maps of GPS ionospheric delay measurement also appear to show the rapidly moving "wall". TEC change measurements to stationary satellites is now made possible by use of Inmarsat geosynchronous satellites transmitting the FAA's WAAS signal at the GPS L1 frequency; these measurements show that the "wall" appears more like a set of narrower walls or waves. This paper will discuss the measurements of these features and review the concepts which may explain these observations.
SA21B-0351 0800h
Observations of the Variability in TEC during the Great Storm of October 2003
The great storm of October 2003 resulted in large scale variations in TEC. These variations are apparent as Storm Enhanced Densities (SEDs), Traveling Ionospheric Disturbances (TIDs), Sudden Increases in TEC (SITEC) and trough depletions. Using TEC values collected with a network of 260 GPS receivers, we have studied the ionospheric response to this great storm. In particular, we have focused on the ionospheric response to the transport of large-scale atmospheric gravity waves (AGW). The source of the AGW is the expansion and the subsequent relaxation of the high latitude atmosphere produced by the intense Joule heating deposited in the auroral regions during the great Halloween storm of October 29-30, 2003. Between 1500 and 2400 UT, GPS receivers located in North and South America measured strong TEC perturbations associated with the traveling AGW. TEC perturbations were observed to propagate from both north and south auroral regions toward the geographic equator. The excellent quality of the TEC measurements makes it possible to extract the temporal spectrum of the TEC perturbations. TEC perturbations containing a 1-hour temporal scale propagate at 1100 m/s. 3-hour scales propagate at 900 m/s. The latitudinal scale size of the TIDs is between 500 and 800 km. This presentation will illustrate these results together with demonstrations of the SED, SITEC and trough features observed during this great storm.
SA21B-0352 0800h
Seasonal and Longitudinal Variations of Midlatitude Topside Spread Echoes Based on ISS-b Observations
A preliminary study of the seasonal and longitudinal variations of spread echoes from the Ionosphere Sounding Satellite (ISS) using the topside sounding data has been undertaken. Significant longitudinal and seasonal variations in midlatitude spread echoes are observed. This mayb be caused by incursion of high and /or low latitude irregularities into the midlatitude domain.The north Atlantic region has the highest occurrence probability in the winter solstice. The smallest occurrence is in the north Pacific in the same interval. Occurrence probabilities of up to about 30% are quite common.
http://www.utdallas.edu/~Anthony.Mwene/
SA21B-0353 0800h
The Response of Mid-Latitude Ionospheric TEC to Geomagnetic Storms and Solar Flares
The effects of geomagnetic storms and solar flares on the ionosphere are manifested as large magnitude sudden fluctuations in the Total Electron Content (TEC). In this study, the broadband VHF signal (30-100MHz) data from the Los Alamos Portable Pulser (LAPP) received by the FORTE (Fast Onboard Recording of Transient Events) satellite during the period of 1997-2002 are used to investigate the mean TEC variation response to geomagnetic storm. A total of 14 geomagnetic storms are selected where FORTE-LAPP data are available to derive average TECs during extended storm-time and non-storm time for a given storm. The variations in the ionospheric TECs at Los Alamos, New Mexico are investigated for the 14 selected geomagnetic storms. In most cases (12 out of 14), we see overall enhancements in TEC as a result of geomagnetic storm impact at Los Alamos. The relative enhancements in TEC at Los Alamos due to a geomagnetic storm can reach as high as 3-fold of the normal TEC values. The overall absolute enhancements in TEC at Los Alamos are up to about 30 TECU. The magnitude of TEC enhancements is diversified over all storm categories without a clean-cut relationship between the storm intensity and the TEC enhancement. The mean TEC variation response to geomagnetic storm can be complicated when several consecutive storms occurred in a row and a net TEC reduction may be seen. Data of continuous GPS TEC measurements are collected at a 1-minute time resolution during July 2004 when 5 X-class solar flares occurred from two Allen Osborne Associates ICS-4000Z GPS receivers mounted at the Physics Building at Los Alamos National Laboratory. In detecting effects of solar flares on the ionospheric TEC, we apply appropriate filtering to remove the linear trend of TEC and a coherent processing of TEC variations simultaneously for all the visible GPS satellites in a given time interval. The responses of ionospheric TEC at minute time scale to these powerful impulsive solar flares are investigated. The onset time of the ionospheric response and the magnitude of the TEC fluctuations and its time derivative are examined along with their relationships with the solar flux characteristics, duration of the flare and location of the flare on the Sun, X-ray emission variations during the flares, and local time of the flare occurrence.
SA21B-0354 0800h
The southern hemisphere ionosphere and plasmasphere response to the interplanetary shock event of 29 - 31 October 2003
We analyze the effects on the southern hemisphere ionosphere and plasmasphere of the geomagnetic storms occurring in the interval 29-31 October 2003 (the so called series of Halloween storms). Solar wind data from ACE and ionospheric data from: GPS ground and LEO receivers; the TOPEX/Poseidon altimeter; the IMAGE FUV camera, and the DMSP drift meter, are used to understand the ionospheric dynamics as a function of the storm phase. The detailed structure of the ionosphere has been obtained using tomographic reconstruction applied to data from both ground and space based GPS receivers. The tomographic approach using LEO observations of signals received from GPS satellites above the LEO's horizon allows us to investigate the topside ionospheric and plasmaspheric density distribution in more detail than can be obtained using ground based GPS receivers. This is because with ground based receivers, the higher topside ionosphere and plasmasphere contribute only a small fraction to the total electron content (TEC) and so the measurements are dominated by the ionospheric structure at the F2 peak. In contrast the Australian LEO satellite, FedSat, which has been used for this study, orbits at 800 km altitude, well above the F2 peak and hence the TEC measured is primarily due to the upper topside ionosphere and plasmasphere. In this paper we present tomographically reconstructed topside ionosphere and plasmasphere electron density distributions for the above mentioned severe magnetic storm period. The results we report here, the tomographically reconstructed topside ionosphere and plasmasphere electron density distributions, are the first of such LEO observations. The temporal and regional maps of TEC and the IMAGE FUV data show that the first storm that commenced on 29 October dramatically decreased the density in the southern hemisphere mid- and high-latitudes. The region remained depleted of density for more than 24 hours until 31 October, when the second severe storm began. From TOPEX/Poseidon data, a daytime localized density enhancement occurred above the middle of the Pacific Ocean. An eastward expanding trough-like structure, crossing middle Australia, is another important feature that was observed.
SA21B-0355 0800h
UHF Coherent Backscatter Observations of Mid-Latitude Electric Field Structures and Variability
Mid-latitude ionospheric regions located near the plasmasphere boundary layer (PBL) respond with large spatial and temporal variability to geomagnetic and solar disturbances. In particular, those areas on field lines connected to asymmetric ring currents in the magnetosphere equatorial plane experience large poleward and eastward electric fields during disturbance periods. These fields interact with conductivity gradients in the dusk and evening periods to create large ExB sub-auroral polarization stream (SAPS) flows and steep storm-enhanced density (SED) structures in a complex magnetosphere/ionosphere feedback system. Farley-Buneman two stream irregularities frequently accompany these dynamic conditions, creating coherent wave structures which perturb existing operational systems such as GPS but which can be also exploited for radar remote observations. The Millstone Hill UHF 440 MHz radar, when operated in a high-resolution coherent backscatter mode, is a sensitive diagnostic of these intense electric fields and dramatic variability. We present observations and interpretation of mid-latitude UHF coherent backscatter events at 34 cm wavelength in the L=3.5 to 4 region (55 to 60 degrees invariant latitude) showing a wide range of spatial and temporal variability. Very large electric field gradients of up to 4 mV/m per kilometer and total electric field increases of more than 40-50 mV/m can occur within the 3 to 5 degree wide SAPS region, moving at cross-L-shell velocities of 250-400 m/s. These very narrow sub-auroral ion drift (SAID) configuration structures are superimposed on the main background SAPS velocity channel and have lifetimes as short as 1.5 minutes. We will also illustrate the range of spatial scales seen and discuss their implications for generation of amplitude and phase scintillation disturbances in transiting radio signals.
http://www.haystack.mit.edu/coherent
SA21B-0356 0800h
The relationship of thermospheric composition changes and ionospheric total electron content during geomagnetic storms.
Geomagnetic storms drive major changes in the composition of the thermosphere. The FUV sensitive Earth Camera of the Visible Imaging System (VIS) on the Polar spacecraft has observed large decreases of $>$ 50% in the O/N$_{2}$ column density ratio in the thermosphere after typical geomagnetic storms. These O/N$_{2}$ ratio decreases are observed to reach low to mid-latitudes. The reduction in the O/N$_{2}$ column density ratio is due mainly to increases in the molecular species that have welled-up from the lower levels of the atmosphere due to auroral heating. The thermospheric composition changes are associated with simultaneous decreases in the charge density of the ionosphere as evidenced by decreases in the Total Electron Content (TEC) observed with the GPS network. The geomagnetic-storm driven increase in molecular densities at typical ionospheric heights rapidly charge exchange with the ambient ionized atoms and subsequently dissociatively recombine with the ionospheric electrons leading to a reduction in the total charge density. The relationship between thermospheric composition and TEC will be presented for the April 17-21, 2002 and May 29-30, 2003 geomagnetic storm events.
SA21B-0357 0800h
Impact of the Bastille Day Solar Flare on the Low- to Mid-Latitude Ionosphere
We study the impact of the Bastille Day solar flare radiation on the low- to mid-latitude ionosphere. The methodology is as follows. We develop an EUV irradiance spectrum based upon observations for the Bastille Day flare. Since solar irradiance observations typically do not have the cadence necessary to follow the evolution of a flare, we have developed techniques for computing flare spectra from the available solar data. We then use this spectrum in the NRL three-dimensional ionosphere model SAMI3 to obtain the global impact of the flare on the mid- to low-latitude ionosphere. We assess the flare's impact by comparing simulation results with and without the solar flare enhanced EUV spectrum. A previous study using the NRL two dimensional ionosphere model SAMI2 and a more simplistic EUV spectrum of the Bastille Day storm found that flare radiation can increase the F-region ionosphere density by up to 50% [Meier et al., Geophys. Res. Lett. 29, 10.1029/2001GL013956, 2002]. \medskip \noindent Research supported by ONR.
SA21B-0358 0800h
Storm-time modifications of the mid-latitude nighttime ionosphere measured by the TIMED/GUVI instrument
The TIMED/GUVI instrument provides a global view of the nighttime ionosphere: the meridional morphology of the electron density (NmF2 and HmF2 maps) and the vertical distributions described by electron density profiles (EDPs). GUVI also provides maps of composition information provided by the O/N$_2$ ratios on the dayside and these show a strong correlation with changes in TEC. We will present studies of several storms using this data, most notably the large storms of October and November of 2003. The TIMED/GUVI data taken during these periods shows strong signatures of compositional changes and plasma transport (both horizontal and vertical) at mid-latitudes.
SA21B-0359 0800h
GPS Occultation Measurements of Mid-Latitude Scintillation
The GPS occultation remote sensing technique has been previously demonstrated to observe scintillations in the equatorial region. These observations appear as rapid fluctuations in the C/A code signal to noise ratio (SNR) during an occultation event. While the equatorial regions possess the highest scintillation levels, occasional mid-latitude SNR fluctuations are also observed, at lower levels of S4, and at times having different characteristics than their low-latitude counterparts. We present a survey of these mid-latitude events using data from the Ionospheric Occultation Experiment (IOX) and the CHAMP satellite.
SA21B-0360 0800h
Observations of Density and Electron neutral collision frequency in the nighttime E-region during the E-winds Campaign
Four Utah State University Plasma Impedance Probes (PIP) were part of NASA's Sequential Rocket Study of Descending Layers in the E-Region (E-Winds). The payloads were launched at 11:19 pm, 1:41 am, 2:50 am and 3:07 am on June 30 and July 1, 2003 from Wallops Island, Virginia into the nighttime D and E-regions. These instruments provided observations of electron density and electron collision frequencies along the trajectory of the rockets. The neutral winds were measured with simultaneous TMA releases. The rockets flew into nighttime intermediate layers located between 100 to 225 km in altitude. The analysis of flight data shows a considerable amount of variability in these layers over a relativity small temporal and spatial scales. This paper presents a overview of these multi-point measurements of the mid latitude ionosphere in the context of the geomagnetic activity during the flights.
SA21B-0361 0800h
Wallops HF Radar Observations of Penetrating Electric Fields and Plasma Structuring in the Mid-Latitude Ionosphere
The Earth's high-latitude ionosphere is subject to strong electric fields of magnetospheric origin that drive drifts of ionospheric plasma to speeds of kilometers per second. Such dramatic effects are usually absent from the ionosphere at mid- and low-latitudes and, consequently, the impact of large electric fields on plasma processes at sub-auroral latitudes has received less scientific attention. This has started to change with the realization that major space weather disturbances enable electric fields to penetrate into the mid- and low-latitude ionospheres and cause a variety of plasma disturbances that adversely impact the performance of technological systems. Understanding the onset and evolution of penetrating electric fields and their impact on mid-latitude ionospheric structure is a top priority for the U.S. National Space Weather Program and the NASA Living With a Star Initiative. In this paper, we describe a new HF radar, located at Wallops Island, Virgina, that was recently developed as a joint project of GSFC/Wallops Flight Facility and JHU/APL. The radar is based on the design of radars in the high-latitude SuperDARN network and will be used to study the penetration of high-latitude electric fields into the mid-latitude ionosphere and to characterize the impact of these fields on the generation of ionospheric plasma structure and small-scale irregularities. The radar was only recently put into observation so we will concentrate on initial observations of electric-field penetration and associated ionospheric structuring. Our observations will be discussed in the context of higher-latitude measurements with SuperDARN.