Geophysical Research Letters

On measuring the off-equatorial conductivity before and during convective ionospheric storms



[1] One of the biggest unknowns in the CIS/ESF (Convective Ionospheric Storms/Equatorial Spread F) Space Weather problem now can be measured. Radio occultations of the GPS constellation will be measurable on a regular basis during the COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) and C/NOFS (Communications/Navigation Outage Forecast System) satellite era. These measurements can be used to determine the plasma density versus height and hence, the off-equatorial height-integrated conductivity.

1. Introduction

[2] Our understanding of convective ionospheric storms, also known as equatorial spread F (CIS/ESF), has increased dramatically in the last decade. One of the remaining questions involves the day-to-day variability of this phenomenon. This electrostatic instability is controlled, at least in part, by the off-equatorial E-region conductivity [Kelley, 1989]. In the laboratory, this effect is termed the conducting end-plate effect in which the metallic plates at the end of the chamber stabilize the flute mode (interchange) instabilities in a mirror magnetic field geometry. Figure 1 shows the dipole field geometry and a simple three-layer model used to study it [Kelley, 1989].

Figure 1.

(a) Side view of the dipole magnetic field geometry near the magnetic equator. The curves are exaggerated to show the coupling geometry between the F region at the equator and the off-equatorial E region. (b) F-layer slab geometry including conducting end plates in the northern and southern hemispheres. (Reprinted from Kelley [1989] with permission from Elsevier.)

[3] A simple version of the growth rate is given below:

equation image

where E′ = E + U × B, L is the gradient scale length, ν is the ion neutral collision frequency, n is the plasma density, and equation image stands for the field-line-integrated conductivity in the E and F ions. In reality, all the quantities must be field-line-integrated [Sultan, 1996].

[4] Unfortunately, the E-region conductivity is very hard to measure since the background density is only 103 cm−3 and sporadic layers seldom get over 105 cm−3. At present, no stations are even set up to measure this quantity, and even if there were, the conductivity is actually very difficult to measure due to the low plasma density. A large, expensive antenna would be required for an ionosonde to measure densities this low. It has been suggested that two stations be established in Peru but this has not yet been approved. Here we show that this important parameter can be measured from orbit and point out that several satellites soon will have the required capability.

2. Data Presentation

[5] The technique is radio occultation of GPS signals as the satellites set or rise on the horizon. The method uses the derivative of the total electron content (TEC) derived from the time delay of the two signals [Hajj and Romans, 1998]. Two examples of the measurement are presented in Figures 2a and 2b. Here the electron density is plotted as a function of altitude. The method is sensitive enough to determine the plasma density down to 1000 cm−3 with vertical resolution approaching 0.3 km [Pavelyev et al., 2002]. Using a neutral atmospheric model, the two field-line-integrated Pedersen conductivities, equation image, are 0.73 and 5.66 mhos, respectively. These values are typical of Arecibo measurements [Kelley, 1989]. We have also calculated the equation image's (up to 150 km) at eight locations near Antarctica in the post-sunset time frame (18–20 LT). They range from 0.87 to 1.77 mhos.

Figure 2a.

Density profile at 2212 LT on June 30, 1995.

Figure 2b.

Density profile at 1120 LT on July 3, 1995.

[6] This powerful technique is capable of determining the worldwide and seasonal dependence of the conductivity right now. Sporadic E is known to peak in summer and to depend on the interaction of planetary and gravity waves with the tides. However, with hundreds of daily occultations now available and thousands of daily occultations to be available in the near future, much more could be learned. In Figure 3 we show the locations of occultations that are observable from the CHAMP (CHAllenging Minisatellite Payload) and SAC-C (Satelite de Aplicanciones Cientificas-C) satellites during one week. With the launch of COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) in 2005, similar coverage is obtained on a daily basis [see, e.g., Hajj et al., 2000].

Figure 3.

One week (June 1–7, 2002) of occultation coverage from CHAMP and SAC-C.

[7] In this manner, we will be able to determine the electron density and, therefore, the conductivity with sufficient quality to solve this aspect of the convective ionospheric storms/equatorial spread F (CIS/ESF) problem with the upcoming COSMIC and Communications/Navigation Outage Forecast System (C/NOFS) satellites, which will provide a dense coverage of GPS occultations globally.

3. Issues Related to Electron Density Measurements at the E Region by GPS Occultations

[8] Central to the concept presented in this paper are the measurements of electron density in the E region by GPS occultations. Establishing the accuracy of these measurements is beyond the scope of this paper. However, in this section we summarize the issues related to such measurements and existing accuracy and precision assessments, all of which point toward their validity.

[9] Due to the scarcity of E-region density measurements, specifically sporadic E (Es), there has been no direct validation of GPS occultation measurements of E-region electron density. However, a number of researchers have established the validity of GPS measurements in this region by several means, including simulation experiments, where the International Reference Ionosphere (IRI95) is used to generate synthetic measurements of TEC and the derived electron density is compared to the model [Hocke and Igarashi, 2002]. These types of experiments show that the contribution of the F region corresponds to a nearly constant bias in the electron density below 140 km, even in the presence of a horizontal gradient. This bias is removed by simply constraining the electron density below 60 km to be zero. Such an approach leads to densities in the E region that are generally accurate to better than 50%. Significant enhancement to this approach can be done by accounting for the horizontal gradient in the F region by using ground TEC data [Hajj et al., 1994; Garcia-Fernandez et al., 2003].

[10] Another type of validation has been the observation of sporadic E and other ionospheric irregularities of vertical scales less than 7 km at altitudes around 100 km in a climatological sense and their correlation with tropical gravity wave activities in the lower stratosphere [Hocke and Tsuda, 2001; Hocke et al., 2002]. The high sensitivity of GPS occultations to fine structure in the E and D regions has been demonstrated by using holographic techniques with vertical resolution of <70 m. Such a technique removes the contributions of the upper layers (F1 and F2) and facilitates detecting electron density variations of ∼100–1000 cm−3 [Igarashi et al., 2002]. In addition, work by Gorbunov et al. [2002] indicates that back-propagation and radio-holographic methods allow the detection of horizontal structures of ionospheric inhomogeneities, thereby identifying whether rapid changes in the measured phase and amplitude are due to structures in the E or F regions.

4. Conclusion

[11] One of the biggest unknowns in the CIS/ESF Space Weather problem can now be measured. Radio occultations of the GPS constellation will be measurable on a regular basis during the C/NOFS satellite era. These measurements can be used to determine the plasma density versus height and hence, the height-integrated conductivity.


[12] Work at Cornell was supported by the Office of Naval Research under grant N00014-03-1-0243. Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract from the National Aeronautic Space Administration and the National Oceanic and Atmospheric Administration.