Sporadic E as Observed by GPS Occultation

Sporadic E (Es) is known as a transient phenomenon where high density ion layers form in a narrow altitude region in the E-region ionosphere. Physical and statistical descriptions of the Es processes, especially from a global view, are essential for understanding their formation and variations, and ultimately for improving numerical forecasts for space weather. Observations of Es in the past were mostly from ground-based remote sensing, sometimes in-situ techniques, and only recently from satellite sensors (e.g., Farley, 1985; Whitehead, 1989; Kelly, 1989; Mathews, 1998; Hocke et al., 2001, and references therein). As reported from ionosonde and incoherent scatter radar (ISR) data (i.e., electron density, electric field, etc.), Es layers usually occur around 90-110 km altitudes with thickness of 0.5-5 km and a horizontal extent of 10-1000 km. The thin-and-patchy layers of enhanced electron density, sometimes also called Es clouds, may last from minutes to hours causing radio signal interruption or frequency drift. Strong local time and seasonal variations of mid-latitude Es have been observed, showing the maximums in daytime hours and during summer months. Es observations remain limited to a few geographical locations, and theories (including the well-known wind-shear theory) still have difficulties to quantitatively explain Es formation and variability in many situations.

Es variabilities exist over broad temporal and spatial scales, which are believed to be related to other atmospheric and ionospheric processes. For instance, radar observations show strong short-period field-aligned structures imbedded in long-period Es irregularities, which are consistent with gravity wave (GW) characteristics in that region (Tsunoda et al., 1994; Fukao, et al., 1998). Pancheva et al. (2003) reported considerable correlation between radar wind measurements and Es variations near 100 km, and attributed it to planetary and tidal wave modulations. Es occurrences are also influenced by solar activities (Baggaley, 1984; Maksyutin et al, 2001) and convective systems in the troposphere (Shrestha, 1971; Leftin, 1971; Datta, 1972). Hocke et al. (2001) studied Es irregularities using GPS/MET (GPS/Meteorology) phase measurements and found that strong activities occur mostly at heights between 95-105 km at summertime mid-latitudes. These Es irregularities appear to correlate with deep convective and topography-induced processes in the troposphere (Hocke and Tsuda, 2001; Hocke et al., 2002). In addition, links of Es to other ionospheric phenomena, including spread F and traveling ionospheric disturbance (TID), have also been investigated (Tsunoda and Cosgrove, 2001).

Vertical Distribution

As shown below in monthly mean variances (June 2002 and January 2003), Es variances dominate in the summer hemisphere. The maxima in the SNR and phases fluctuations are consistent with location where Es layers mostly occur. Because occultation scintillations maximize at the layer altitudes, the vertical distribution of the variances are approximately where Es layers are. The summertime Es are mostly at altitudes of 80-120 km with the peak at ~105 km near 45 S in January and at ~102 km near 45 N in June. The Es variances in the summer hemisphere correlate well with the mean zonal winds, which reaches ~40 m/s in the CIRA'86 climatology. The Es variances in June appear slightly greater in amplitude than those in January, and occur in a broader height range. The Es variances are much weaker in the winter hemisphere but remain significant in CHAMP phase data. Equatorial Es variances are generally weak, confined in a narrow altitude region around 100 km, and may be more prominent in June than in January. Besides, large variances at lower altitudes (<20 km) reflect sharp variations of atmospheric refractivity associated with temperature and water vapor changes in the troposphere.

January June

Horizontal Distribution

Global maps of Es variances are important for studying the dependence of Es on the horizontal winds and the geomagnetic field. Figure 8 shows the 105-km maps of L1 SNR/SNR0 and phase variances during June-August 2002 (JJA) and December 2002-February 2003 (DJF) when the summertime Es are maximized. These maps reflect the stationary component of Es at planetary scales that may be related to the geomagnetic field. Because CHAMP satellite drifts slowly in local time as mentioned above, the three-month averages can be contaminated somewhat by fast traveling planetary waves.

In JJA (Figure 8a) Es irregularities appear strongly in the summer hemisphere, mostly over China, northwestern Pacific, western United States, northern Atlantic, and southern Europe. However, they mostly fall into the latitude band where the geomagnetic-field dip angles are between 30 and 70 degrees. This dip-angle dependence is quite striking for the summertime activities as they move north and south in latitude following the dip angle changes. The strong longitudinal variations in the 30-70 degree dip angle band can not be simply related to the geomagnetic field. Other variabilities, such the horizontal winds and ion sources, must be taken into account. The wintertime Es activities in JJA are weak and coincide mostly with the dip angles greater than 80 , the southern polar cap with open geomagnetic field lines. Patchy Es activities are evident in the phase variance at dip angles greater than 80 degrees in the summer pole. Equatorial Es activities are generally weak and patchy, not showing any dependence on the geomagnetic equator.

In DJF (Figure 8b) the strongest summertime Es activity is over southern Pacific with variances as high as ~(15 %)2 in SNR/SNR0 and ~1.6 cm2 in L1 phase. Other active regions are over the southern Andes and the east and west Australian coasts. In the region south to Indian Ocean and South Africa (between 30ºS and 60ºS), th ere is a weak but significant Es appearance. Also showing the strong dip-angle dependence, the enhanced variances are basically confined to the 20-60 degree latitude band, whereas the weak variances occur almost everywhere in the summer hemisphere. In the winter hemisphere, again, Es activities become weak and mostly restricted to the region of dip angles greater than 80º.

Figure 8

Local Time Variations

To study the local time variation of Es activity, we use three months of CHAMP data and average them into each hourly bin. Latitudinal and height dependence of Es diurnal cycle is of particular interest for studying potential tidal influences because of unique neutral wind shears associated with the tides. During the JJA season, the summer mid-latitude Es exhibits a strong semidiurnal variation at 105 km with peaks around 10:00 and 20:00 LST [Figure 9]. The semidiurnal variation in the CHAMP variances is generally consistent with ground-based observations from a number of sites in the NH (Whitehead, 1989). Figure 9 also shows a downward progression with height at 45ºN in both SNR and phase variances at 70-115 km. Equatorial Es a nd the wintertime activities, on the other hand, are dominated by a diurnal variation, showing the peak time between 14:00 and 18:00 LST.

In DJF the diurnal variation dominates summertime Es activities with the maximum enhancement around 20:00 LST. In the winter hemisphere Es activity is still dominated by a semidiurnal variation with maximums around 8:00 and 20:00 LST. Equatorial Es is weak with a diurnal variation and peaks around 18:00 LST. The time-height relation at 45ºS reveals only slight downward progression for the enhancement near 20:00 LST.

Figure 9