The Equatorial Ionosphere During Solar Minimum -- C/NOFS Observations of Deep Plasma Depletions at Sunrise
The Communication/Navigation Forecasting System (C/NOFS) satellite was launched in April 2008 into an equatorial orbit at an altitude between 400 and 850 km, to study the equatorial ionosphere as well as irregularities within it. The satellite sensors measure the following parameters: ambient and fluctuating ion densities; ion and electron temperatures; neutral winds, AC and DC electric and magnetic fields. C/NOFS is circling the Earth at a time when the solar cycle is the lowest it has been since the beginning of the space age. In this talk, we stress the findings that are unique to solar minimum conditions. The plasma density is the smallest seen in the past half century. The pre-reversal enhancement in the upward plasma drift, which is responsible for early evening irregularities, is rarely seen. Instead, plasma irregularities form mostly after midnight. An unexpected feature in the data concerns deep plasma depletions observed at sunrise. They are seen at all satellite altitudes and associated with ionospheric irregularities. Dawn depletions are more frequent in the America-Africa sector and in the Indonesia sector. Dawn depletions are also observed in other data sets, in particular in data from DMSP morning passes, and the CHAMP satellite. This fact confirms that they are real and not an artifact of the plasma instrument. It also allows measuring the N-S extent of the dawn depletions - we find that they are typically 50 x 14 degrees in the N-S and E-W directions respectively, but they can be much wider in longitude. We postulate that they are caused by upward plasma drifts, which are seen in the C/NOFS and ground-based data.
On the Growth Phase of Large-Scale Wave Structure and Plasma Bubbles
Equatorial Spread F development and day-to-day Variability From Prereversal Vertical Drift and Gravity Wave Precursors
The spread-F /plasma bubble irregularity development in the post sunset equatorial ionosphere (ESF) is widely believed to arise from the generalized Rayleigh-Taylor (R-T) interchange instability mechanism. The instability is initiated from density perturbations at the bottomside gradient region of a rapidly rising F layer. Whether or not it could develop into flux tube aligned topside bubbles (plasma depletions) with cascading irregularity structures should depend upon: (a)-the intensity of the evening prereversal enhancement in the zonal electric field (PRE)/vertical plasma drift; (b)- the amplitude of the initial density perturbations and associated polarization electric fields possibly induced by upward propagating gravity waves (GWs); and (c)- the integrated conductivity of the potentially unstable flux tubes. Observations by different technique show large degree of day-to-day and short term variabilities in the ESF strength and occurrence rate that result from the factors a, b, and c. We focus in this presentation the competing/complementary roles of the PRE and GWs induced perturbations in the ESF variability during quiet as well as some magnetically disturbed conditions. The results to be discussed are based mainly on ground based radar, digisonde and optical measurements including results from some recent observational campaigns conducted in Brazil. Instability model results will discussed for a better understanding of the ESF variability.
The Low-latitude Ionospheric Sensor Network: The Initial Campaigns
The Low-latitude Ionospheric Sensor Network (LISN) is a distributed observatory designed to provide regional coverage in South America and high-temporal resolution measurements to diagnose the initiation and development of plasma structures and the state and dynamics of the low latitude ionosphere. It combines inexpensive GPS receivers and state-of-the-art radars such as the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) ionosondes and magnetometers. This paper describes the characteristics of the LISN distributed observatory and discusses the results of the first two campaigns. LISN will be comprised of nearly 70 GPS receivers with the capability to measure Total Electron Content (TEC), amplitude and phase scintillation and Traveling Ionospheric Disturbances (TIDs). LISN will also include 5 ionosondes able to measure nighttime E-region densities and 5 collocated magnetometers that will be placed along the same magnetic meridian. The first campaign was dedicated to detect medium-scale (~100 km) TIDs and was conducted at Huancayo, Peru in July 2008 using 3 GPS receivers spaced by 4-5 km arranged in a triangular configuration. TEC data corresponding to 3 consecutive days indicate that the TIDs phase velocity was close to 120 m/s and directed northward during the early evening hours. The second campaign was conducted in February 2009 using 3 GPS receivers installed near Ancon and coordinated with the VIPIR ionosonde running in an interferometer mode. We will discuss the implications of these new results within the frame of the current theories of plasma bubble onset.
Forecasting Ionospheric Real-time Scintillation Tool (FIRST)
It is well-known that the generation of equatorial, F-region plasma density irregularities, via the Generalized Rayleigh-Taylor instability mechanism is critically dependent on the magnitude of the pre-reversal enhancement (PRE) in upward ExB drift velocity after sunset. These plasma density bubbles that are generated after sunset lead to the scintillation of trans-ionospheric radio wave signals that pass through these bubbles and is commonly referred to as scintillation activity. Communication and Navigation systems can be severely disrupted by these plasma density irregularities. A measure of scintillation activity is given by the S4 Index and a network of Air Force, ground-based UHF and L-band receivers measuring the S4 Index is called the SCIntillation Network Decision Aid (SCINDA) network. After sunset, the height-rise with time of the bottom- side of the F-layer reflects the magnitude of the upward ExB drift velocity. The value of the ionospheric parameter, h'F (the virtual height of the bottom-side F-layer) at 1930 LT reflects the integrated ExB drift effect on lifting the F-layer to an altitude where the Rayleigh-Taylor (R-T) instability mechanism becomes important. It is found that there exists a threshold in the h'F value at 1930 LT and the onset of scintillation activity as measured by the S4 Index value in the Peruvian longitude sector. This h'F threshold value is found to decrease with decreasing F10.7 cm fluxes in a linear manner (R = 0.99). T o examine this relationship, theoretically, we incorporate a suite of first-principle models of the ambient ionosphere (PBMOD) developed at the Air Force Research Lab (AFRL) to investigate R-T growth rates and threshold h'F (1930 LT) values as a function of solar cycle activity. In addition, this paper describes a technique for automatically forecasting, in real-time, the occurrence or non-occurrence of scintillation activity that relies on real-time data from a ground-based ionospheric sounder at or near the geomagnetic equator. We describe how FIRST has been developed into a real-time capability for automatically forecasting scintillation activity that is available on Google Earth to all interested parties.
C/NOFS Observations of AC Electric Field Fields Associated with Equatorial Spread- F
The Vector Electric Field Investigation (VEFI) on the C/NOFS equatorial satellite provides a unique data set in which to acquire detailed knowledge of irregularities associated with the equatorial ionosphere and in particular with spread-F depletions. We present vector AC electric field observations, primarily gathered within the ELF band (1 Hz to 250 Hz) on C/NOFS that address a variety of key questions regarding how plasma irregularities, from meter to kilometer scales, are created and evolve. The data will be used to explore the anisotropy/isotropy of the waves, their wavelength and phase velocity, as well as their spectral distributions. When analyzed in conjunction with the driving DC electric fields and detailed plasma number density measurements, the combined data reveal important information concerning the instability mechanisms themselves. We also present high resolution, vector measurements of intense lower hybrid waves that have been detected on numerous occasions by the VEFI burst memory VLF electric field channels.
Comapring Jicamraca and C/NOFS (VEFI) Observations of Equatorial Spread F Irregularities
Observations were made at the Jicamarca Radio Observatory in support
of the C/NOFS satellite which made perigee passes over the observatory
in the campaign period from Feb 18--21, 2009. Plasma density and
vertical and horizontal plasma drifts were observed in a new, finely
resolved incoherent scatter mode. Such observations make it possible
to assess the stability of the postsunset ionosphere on the basis of
flow characteristics and to search for precursors and telltales of
equatorial spread F (ESF) such as large-scale seeding. At the same
time, images of plasma irregularities resulting from ESF were made
from spaced-receiver coherent scatter observations using aperture
synthesis techniques. This imagery reveals the morphology of the
unstable waveforms and the surrounding flow, morphology that often
recapitulates the precursors. C/NOFS density and electric field
measurements help to place the Jicamarca observations in
spatio-temporal context and facilitate a quantitative analysis of
ionospheric stability with possible forecast
applications. Radar/spacecraft comparisons can also reveal the
relationship between heretofore phenomenological coherent backscatter
features (signal intensity, Doppler shift, etc.) and ionospheric state
variables (density, electric field, etc.) The first results of such
comparisons will be presented.
Daily variations of the global equatorial anomaly
The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) uses the Tiny Ionospheric Photometer (TIP) to characterize the nighttime ionosphere. TIP is a compact, nadir directed, narrow-band, ultraviolet photometer operating at the 135.6 nm wavelength. This emission is produced by recombination of oxygen ions and electrons, which is the natural decay process for the ionosphere. At night, the strength of the emission is proportional to the square of the peak electron density. TIP measures the horizontal structure of the ionosphere with 15-30 km resolution and high sensitivity, providing remarkable detail even during solar minimum conditions. Multiple TIP sensors are used to map the global pattern of the post-sunset equatorial anomaly for individual days. For solar minimum conditions and equinox periods a 4-cell pattern is observed in the equatorial anomaly. The persistence of the global pattern from day to day is investigated. Daily variations in the pattern are examined and discussed. At solar minimum these variations impact the severity of low latitude radio scintillation by dictating the background density where irregularities form.