Formation of a Plasma Depletion Shell in the Equatorial Ionosphere
An accurate description of the irregularity region defined by a plasma bubble is critically important in understanding the dynamics of the region and its effects on radio scintillation. Here we describe a plasma depletion region as a depletion shell' and show how two-dimensional optical images from space can be used to define the shape of the depletion shell. The plasma depletion shell in the evening is produced by the slower eastward drift of plasmas at higher altitudes and higher latitudes with respect to plasmas at lower altitudes and lower latitudes. A backward C-shape in bubble images from space-based optical observations and the westward tilt of bubbles at the magnetic equator are consistent with this shell structure. A detailed description of the plasma depletion shell will enable the space weather community to establish a better predictive capability based on partial or sporadic observations of bubbles from the ground or space.
Zonal Distribution of Plasma Bubble Occurrence Studied with Ground-based GPS Receiver Networks
Zonal distribution of plasma bubble occurrence was investigated with ground-based GPS receiver networks between India and Guam. Total Electron Content (TEC) measurement by a ground-based GPS receiver can observe spatial and temporal variations of plasma bubble activity in its field-of-view of 700km. To increase its field-of-view, plasma bubbles were back-traced to the location where they were generated with three assumptions for their generation time, drift velocity, and decay of their activity. The "back-trace" method makes possible to reveal the zonal distribution of plasma bubble activity for wider than 2,000km in the zonal direction from single ground-based GPS receiver. In this study, zonal distribution of plasma bubble activity was investigated for wider than 10,000km from 60°E to 150°E using 11 ground-based GPS receivers data in 2003. 1,585 of plasma bubbles were detected, which were almost all of the plasma bubbles appeared in the are and in the period. The estimated plasma bubble distribution using GPS network was consistent with that measured by the FORMOSAT-1 satellite. Statistical study was done for the zonal width of plasma bubble and intervals between plasma bubbles. It was found that more than 60% out of 1,585 plasma bubbles had shorter zonal width than ∼150km. It was also found that more than 80% of intervals of plasma bubbles had shorter length than ∼350km. Plasma drift velocity shear and atmospheric gravity wave could control the zonal distribution of plasma bubbles.
VHF Radar Observations of Nighttime F-Region Field-Aligned Irregularities Over Kototabang, Indonesia
We report continuous observations of the nighttime F-region field-aligned irregularities (FAIs) over Indonesia. A VHF radar with operating frequency of 30.8 MHz and peak power of 20 kW has been operated at Kototabang (0.20°S, 100.32°E; dip latitude 10.4°S), Indonesia since February 2006. Five beams were allocated between ±54° in azimuth around geographic south (126°-234°). From the continuous observation from February 2006 to November 2007, we found that FAIs appeared frequently at pre- midnight between March and May and at post-midnight between May and August. The pre-midnight FAIs coincided well with GPS scintillation observed at the same site. Seasonal and local time variations of the pre-midnight FAI occurrence are consistent with those of equatorial plasma bubbles reported in previous studies (e.g., Maruyama and Matuura, 1984). These results indicate that the pre-midnight FAIs could be associated with the equatorial plasma bubbles. On the other hand, seasonal and local time variations of the post-midnight FAIs were inconsistent with those of the plasma bubbles. The features of the post-midnight FAIs can be summarized as follows: (1) The post-midnight FAIs are not accompanied by GPS scintillations. (2) Most of the post-midnight FAI regions do not show propagation, but some of them propagate westward. (3) Echo intensity of the post-midnight FAIs was weaker than that of the pre-midnight FAIs. These features are similar to those of the FAI echoes that have been observed at mid- latitude (e.g., Fukao et al., 1991). At Kototabang, Fukao et al. (2004) have firstly observed FAIs that resemble those at mid-latitude. The present paper reports statistical characteristics of the mid-latitude-type FAIs observed at Kototabang.
First Observations of Equatorial TEC and Scintillation With Multiple Dual-Frequency Software-Defined GPS Receivers
A dual-frequency software-defined GPS receiver has been developed and used for monitoring total electron content (TEC) and observing equatorial ionospheric scintillation. The Cornell University GPS Receiver Implemented on a DSP (GRID) utilizes the GPS L1 C/A and L2 C signals to measure TEC and observe scintillation. The GRID receiver measured TEC and GPS signal amplitude and phase at 10 Hz. Also employed were two similar GPS digital storage receivers (non-real-time) that made the same measurements at 50 Hz. These receivers were arranged in a linear array and utilized in January, 2009 in Natal, Brazil (magnetic latitude 2.42°) to make these observations. Mild scintillation of the L1 C/A and L2 C signals was observed. TEC measurements agreed well with those taken by a collocated GPSV 4004B Scintillation/TEC Monitor. We demonstrate the use of multiple receivers to measure drifts and report on the first fast (10Hz-50Hz) multiple receiver TEC measurements in the equatorial ionosphere.
The Plasma Physics of Using Non-Specular Radar Meteor Echoes to Monitor Lower Thermosphere Wind Profiles
We will present a new method of measuring lower thermospheric wind velocity profiles by tracking non-specular meteor echoes in time. This approach relies on having a radar following plasma irregularities as they are dragged by the neutral wind. This requires a VHF radar with interferometric capability able to point close to perpendicular to the geomagnetic field. Using a small sample of data from the Jicamarca Radio Observatory, we calculated wind speeds and directions between 90 and 110 km with a range resolution of a few hundred meters. The measurements taken show speeds reaching 150m/s and sometimes changing by as much as 100m/s over a 6km altitude range. With some refinement of the data collection and analysis techniques, we expect that one could obtain high resolution images of lower thermospheric winds as they change in both altitude and time. We will discuss these results, the physics underlying these measurements, and the limitations.
Equatorial Ionospheric Irregularities Observed in the South American Sector During the December 2006 Geomagnetic Storm
This investigation presents studies related to the observations of equatorial ionospheric irregularities in the ionospheric F-region in the South American sector during the intense geomagnetic storm in December 2006, during the period of low solar activity. The geomagnetic storm reached a minimum Dst of -147 nT at 0700 UT on 15 December. In this work ionospheric sounding data obtained between 13 and 16 December 2006 at Palmas (PAL; 10.2o S, 48.2o W; dip latitude 6.6o S) and São José dos Campos (SJC, 23.2o S, 45.9o W; dip latitude 17.6o S), Brazil, and Jicamarca (JIC, 12.0o S, 76.8o W; dip latitude 0.05o S), Peru, have been used. Also, vertical total electron content (VTEC) and phase fluctuations (TECU/min) from GPS observations obtained at Brasilia (BRAZ, 15.9o S, 47.9o W; dip latitude 11.7o S), Presidente Prudente (PPTE, 22.12º S, 51.4º W; dip latitude 14,9º S), Curitiba (PARA, 25.43o S, 49.21o W; dip latitude 18.4o S), Santa Maria (SMAR, 29.71o S, 53.07o W; dip latitude 19.6o S), Brazil, Bahia Blanca (VBCA, 38.7o S, 62.3o W; dip latitude 22.4o S) and Puerto Deseado (PDES, 47.7o S, 65.9o W, dip latitude 27.1o S), Argentina, during the period 13 to 16 December are presented. An unusual uplifting of the F-region during pre-reversal enhancement (PRE) on 14 December was possibly associated with a prompt penetration of electric field of magnetospheric origin after the storm sudden commencement (1414 UT on 14 December). On this geomagnetically disturbed night of 14-15 December, intense equatorial ionospheric irregularities were observed up to southern most GPS station PDES in Argentina. It should be mentioned that on the other nights viz., 12-13 and 13-14 December (both nights before the storm), and 15-16 December (recovery phase), the ionospheric irregularities are limited to only the Brazilian GPS stations. On the geomagnetically disturbed night of 14-15 December, strong oscillations were observed in the F-region base height possibly associated with Joule heating at high latitudes at SJC. Both JIC and PAL show unusual uplifting in the F-region base height on the night of 14-15 December. The VTEC observations show a positive storm phase only at the Argentinean stations VBCA and PDES soon after the storm sudden commencement (daytime). However, soon after this positive phase, all the stations in Argentina and Brazil show negative storm phase during the night on 14-15 December.