How does ionospheric TEC vary if solar EUV irradiance continuously decreases?
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It is an interesting topic how the ionosphere varies when solar extreme ultraviolet (EUV) irradiance decreases far below normal levels. When extrapolating the total electron content (TEC)-EUV relation, significantly negative TECs at the zero solar EUV point are obtained, which indicates that TEC-EUV variation under extremely low solar EUV (ELSE) conditions does not follow the TEC-EUV trend during normal solar cycles. We suggest that there are four types of nonlinear TEC-EUV variations over the whole EUV range from zero to the solar maximum level. The features of the ionosphere under ELSE conditions were investigated using the TEC extrapolated with cubic TEC-EUV fitting. With the constraint of zero TEC at zero EUV, the cubic fitting takes not only observations but also the trend of the ionosphere (only an extremely weak ionosphere can exist when EUV vanishes) into account. The climatology features of TEC under ELSE conditions may differ from those during normal solar cycles at nighttime. Ionospheric dynamic processes are supposed to still significantly affect the ionosphere under ELSE conditions and induce this difference. With solar EUV decreasing, global electron content (GEC) should vary largely in accordance with the GEC-EUV trend during normal solar cycles, and the seasonal fluctuation of GEC declines, owing to the contraction of the ionosphere.
KeywordsIonosphere Total electron content Nonlinear variation
Solar activity has recently undergone a deep minimum during 2007 to 2009. This solar minimum lasted longer than several previous minima, and the sunspot record during this solar minimum reached the lowest level after 1913. Solar EUV irradiance was found to have significantly decreased during this deep solar minimum (Didkovsky et al. 2010), and accordingly, the thermospheric density and ionospheric electron density declined (e.g., Chen et al. 2011; Emmert et al. 2010; Liu et al. 2011; Solomon et al. 2010). The 2007 to 2009 deep solar minimum merely offered us a glimpse for understanding the Sun and the space environment during extremely low solar activity periods. As far as extremely deep solar minima, such as the Maunder Minimum, are concerned, solar EUV could potentially decrease to very low levels during these periods. Then, an important question remains about what is the possible state of the ionosphere under this extreme solar activity condition. Although no observation can directly provide us an answer, this topic is interesting and essential as another extremely low solar activity period could possibly come in the future.
A few studies have investigated the state of the ionosphere under the conditions of extremely low solar EUV (ELSE), which means solar EUV irradiance is much lower than the normal solar minimum level. Liu et al. (2009) revealed that ionospheric total electron content (TEC) is negative at the zero EUV point if it decreases with EUV in accordance with the linear TEC-EUV relation during recent lower solar activity periods. The negative TEC is nonphysical and thus indicates that ionospheric variation with EUV under ELSE conditions should be different from that during recent lower solar activity periods. Smithtro and Sojka (2005) simulated the behavior of the ionosphere under extremely low solar flux conditions using a global average ionosphere and thermosphere model. Their results showed that as solar irradiance falls below normal solar minimum levels, the concentration of O+ decreases rapidly relative to the molecular ions so that the ratio of foF2/foF1 approaches unity (foF1 and foF2 are the critical frequencies of the F1 and F2 layers, respectively), which indicates that the vertical profile of the ionosphere may significantly change under extremely low solar flux conditions. The abovementioned studies indicate that to some extent, the behavior of the ionosphere under ELSE conditions is possibly different from that under normal solar cycle conditions.
This paper deals with the behavior of TEC under ELSE conditions using the Jet Propulsion Laboratory (JPL) TEC maps (Iijima et al. 1999; Mannucci et al. 1998) and the solar EUV data observed by the Solar EUV Monitor aboard the Solar Heliospheric Observatory satellite (SOHO/SEM) (Judge et al. 1998). We discuss the nonlinear variation trend of TEC with EUV and some primary climatology features of TEC under ELSE conditions. The results highlight the nonlinear variations of TEC with EUV and some climatology features of TEC under ELSE conditions that are different from those during the normal solar cycle period.
Daily EUV flux monitored by SOHO/SEM has been continuously provided since 1996. It is an ideal dataset for investigating EUV's longer-term variations, such as the solar cycle variation, and their effects on the ionosphere. In this study, SOHO/SEM EUV integral flux over 0.1- to 50-nm wavelength range was used to present the variations of the EUV irradiance that causes the ionization of neutral atmosphere.
As Smithtro and Sojka (2005) revealed, the vertical profile of the ionosphere during ELSE periods is possibly significantly different from that during normal solar cycles. Thus, some characteristic parameters corresponding to the key layers of the ionosphere (e.g., peak electron density of the F2 layer) are possibly inapplicable under ELSE conditions. TEC is a parameter that depends on the altitudinal integral of the electron density; it always can be used to detect the state of the ionosphere if only the ionosphere exists. In this paper, we discuss ionospheric behavior under ELSE conditions based on the parameter TEC. The JPL TEC maps have been routinely released since 1998; they are essential for investigating the global features and solar activity variations of the ionosphere. The JPL TEC maps are analytic descriptions of the measured slant TEC; the accuracy of the maps depends on the numbers of global positioning system (GPS) receiver stations and satellites available. There are several other TEC map datasets such as ESA, UPC, and CODE (Hernández-Parajes et al. 2009). Some differences still exist between these different TEC datasets. We used the JPL TEC maps in this study. The TEC values at given local times were obtained by interpolating JPL TEC to local time grids. In order to suppress the effect of stronger geomagnetic disturbance, the TEC data were removed in the analyses if the Ap index in the current day of TEC or in the previous day of TEC is larger than 30.
Does ELSE TEC follow the TEC-EUV relation during normal solar cycles?
During normal solar cycles, the variation of ionospheric electron density with solar EUV flux is nearly linear at lower solar activities and nonlinear at higher solar activities. At higher solar activity levels, the electron density varies with solar EUV flux in a pattern that deviates from the linear trend at lower solar activities (e.g., Balan et al. 1994; Chen and Liu 2010; Liu et al. 2006; Richards 2001). Some studies revealed that quadratic regression fits could be used to well estimate the nonlinear variation of the electron density with EUV flux during normal solar cycles (e.g., Chen et al. 2008; Gupta and Singh 2001; Sethi et al. 2002). If solar EUV continuously decreases to ELSE levels, does the electron density variation follow the linear or nonlinear relation between the electron density and EUV flux during normal solar cycles?
Solar EUV irradiance causes the vast majority of the ionization in the middle- and low-latitude ionosphere. If solar EUV irradiance vanishes, it can be deduced that only an extremely weak ionosphere can exist, owing to the weak ionization caused by the minor ionization sources such as starlight (e.g., Titheridge 2000). TEC should approach extremely low values, as compared with the TEC values during normal solar cycles, if solar EUV continuously decreases so that it approaches zero. Hence, we may investigate whether TEC under ELSE conditions varies with EUV irradiance following the linear or nonlinear TEC-EUV relation during normal solar cycles by extrapolating the linear or nonlinear TEC-EUV relation to the zero EUV point.
Method of extrapolating ELSE TEC
Results and discussion
Some climatology features of the extrapolated TEC were investigated and compared with the features during the normal solar cycle period. We took the solar EUV level of 0.1- to 50-nm EUV flux equal to 1 × 1014 photons/m2/s (nearly half of the 2007 to 2009 solar minimum level) as the typical ELSE condition, and for the normal solar cycle condition, we took the solar EUV level of 0.1- to 50-nm EUV flux equal to 3 × 1014 photons/m2/s (a lower solar activity level) as a representative.
Seasonal variation of nighttime TEC also could be affected by neutral winds at geomagnetic middle latitudes. Under normal solar cycle conditions, TEC's annual variation (higher TEC during the local summer than in other seasons) is dominant at middle latitudes during the nighttime, possibly owing to the fact that solar EUV irradiance lasts longer during the local summer than in other seasons and thus can compensate recombination loss to slow down TEC's attenuation to some extent. As the top panels of Figure 7 show, in the late afternoon and around sunset, TEC's attenuation is slower during the local summer than in the two equinoxes at middle latitudes, especially in the northern hemisphere. Whereas with decreasing solar EUV, the increasing neutral winds (Kawamura et al. 2000; Liu et al. 2004) are possibly strong enough to significantly uplift the ionosphere during the nighttime to slow down TEC's attenuation at the middle latitudes, where dips are close to ±45°, in all seasons, just as the bottom panels of Figure 7 illustrate. In that case, seasonal variation of nighttime TEC is possibly different from that during normal solar cycles at the mid-latitude bands where dips are close to ±45°.
Global electron content (GEC)-extreme ultraviolet (EUV) increase rate derived from linear GEC-EUV fitting
GEC-EUV increase rate
The seasonal variations in the F region, where the atomic ion O+ is dominant, of the global ionosphere are suggested to be closely related to thermospheric parameters such as the thermospheric composition denoted by [O]/[N2] (e.g., Liu et al. 2007; Mendillo et al. 2005; Rishbeth and Müller-Wodarg 2006). However, the effect of the thermosphere on ionospheric seasonal variations declines with altitude decreasing, because molecular ions gradually become important. The density of molecular ions at lower altitudes is mainly related to EUV flux. For example, the seasonal variation in the E region is different from that in the F region. As revealed by Smithtro and Sojka (2005), the ionosphere contracts to lower altitudes with decreases in EUV so that ionospheric electron density peaks at the altitudes where the concentration of molecular ions is equivalent to that of atomic ions, namely, molecular ions become important. Under ELSE conditions, thus, GEC's seasonal fluctuation induced by the variation of thermospheric composition should be less than that under normal solar cycle conditions, owing to the contraction of the ionosphere.
If solar EUV continuously decreases far below the normal solar minimum level, TEC-EUV variation should be nonlinear and may not follow the TEC-EUV variation trend during the normal solar cycle period. We suggest that there are four types of nonlinear TEC-EUV variations over the whole EUV range from zero to the solar maximum level. These nonlinear TEC-EUV variations can be presented with cubic functions of EUV. In terms of the cubic TEC-EUV fitting constrained with the term of zero TEC at zero EUV, we qualitatively discussed the features of the ionosphere under ELSE conditions. The cubic fitting takes not only the available observations but also the extreme ionosphere when solar EUV vanishes into account.
Main climatology features of TEC were investigated with the cubic fitted TEC. Take the solar EUV level of 0.1- to 50-nm EUV flux equal to 1 × 1014 photons/m2/s, for example, the EIA structure of TEC still exists in daytime, while the latitudinal distribution of TEC is different from that during the normal solar cycle period in nighttime. The seasonal variation of TEC under ELSE conditions is basically consistent with that under normal solar cycle conditions in daytime; however, they are different at middle latitudes in nighttime. Brief discussion indicates that ionospheric dynamic processes still significantly affect the ionosphere under ELSE conditions and may induce some features of TEC different from those under normal solar cycle conditions.
On the global scale, the GEC-EUV variation under ELSE conditions should basically follow the GEC-EUV variation trend under normal solar cycle conditions. GEC has a significant seasonal variation dominated by the semiannual component during the normal solar cycle period; with solar EUV decreasing, GEC's seasonal variation declines so that it is relatively small under ELSE conditions. The contraction of the ionosphere with the solar EUV decreasing possibly results in the attenuation of GEC's seasonal variation.
The SOHO/SEM EUV data were provided by the Space Sciences Center of the University of Southern California. The JPL TEC data were downloaded from the following website: ftp://cddis.gsfc.nasa.gov. This research was supported by Chinese Academy of Sciences (KZZD-EW-01-3), the National Natural Science Foundation of China (41274161, 41231065, 41004068, and 41321003), National Important Basic Research Project of China (2012CB825604 and 2011CB811405), and the National High Technology Research and Development Program of China (2012AA121004).
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