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Discrepant responses of the global electron content to the solar cycle and solar rotation variations of EUV irradiance
© Chen et al.; licensee Springer. 2015
Received: 21 October 2014
Accepted: 18 May 2015
Published: 27 May 2015
In this paper, the responses of the ionosphere to the solar cycle and solar rotation variations of extreme ultraviolet (EUV) irradiance are comparatively investigated using daily mean global electron content (GEC) and 0.1–50 nm EUV daily flux. GEC is well correlated with EUV on both the solar cycle and solar rotation timescales; however, the responses of GEC to the solar cycle and solar rotation variations of EUV are significantly different in terms of the following two aspects: (1) There is a significant time lag between the solar rotation variations of GEC and EUV; the lag is dominated by a 1-day lag and generally presents a decrease trend with solar activity decreasing. For the solar cycle variations of GEC and EUV, however, there are no evident time lags. (2) The GEC versus EUV slopes are different for the solar cycle and solar rotation variations of GEC and EUV; the solar cycle GEC versus EUV slope is higher than the solar rotation GEC versus EUV slope, and this difference occurs in different seasons and latitudinal bands. The results present an aspect of the difference between ionospheric climatology and weather.
The solar extreme ultraviolet (EUV) irradiance results in the photoionization of atmospheric neutral particles to generate ionospheric plasma; thus, its variations play important roles in ionospheric variability. The solar EUV varies on various timescales (e.g., Bouwer 1992; Chen et al. 2012; Lean et al. 2011; Pap et al. 1990; Tobiska and Bouwer 1989); the most prominent variations of EUV include the ~11-year quasi-periodic variation (the solar cycle variation) and the ~27-day quasi-periodic variation (the solar rotation variation). The solar cycle variation is the result of the reversal of the solar magnetic polarity (Lean 1997), while the solar rotation variation is induced by the rotation of the Sun and the evolution of solar active regions (Bouwer 1992). The solar rotation variation of EUV changes during solar cycles (e.g., Kane 2003), stronger at solar maxima and weaker at solar minima, since active regions are closely related to solar activity levels (e.g., Lean 1987). Both the solar cycle and solar rotation variations of EUV significantly affect the ionosphere; The former causes a significant solar cycle modulation in the ionosphere (e.g., Bilitza 2000; Chen et al. 2011; Liu et al. 2011), and the latter modulates ionospheric variations on the timescales of days (e.g., Chen et al. 2014a; Coley and Heelis 2012; Forbes et al. 2000; Hocke 2008; Min et al. 2009; Oinats et al. 2008; Rich et al. 2003; Wang et al. 2006).
The responses of the ionosphere to EUV variations have been investigated in many studies (e.g., Balan et al. 1994; Chen and Liu 2010; Hocke 2008; Liu et al. 2006; Richards 2001; Sethi et al. 2002); however, the quantitative relationships between ionospheric variations and the long-term (solar cycle timescales) and short-term (solar rotation timescales) variations of EUV were barely comparatively investigated. Rishbeth (1993) investigated the responses of the ionosphere to solar irradiance variations during a high solar activity period using the ionosonde data over Slough, Port Stanley, and Huancayo. He found that the relationship between short-term variations of the E-layer electron density and the solar irradiance is basically consistent with the relationship between their longer-term variations; for the F-layer, however, he found that short-term variations of the F-layer electron density and EUV irradiance lack strong correlations. In this paper, we pay attention to the consistency of the responses of the F-layer to the solar cycle and solar rotation variations of EUV.
As far as ionospheric response to the solar rotation variation of EUV is concerned, some studies revealed that ionospheric variations are usually most correlated with the EUV variations of previous days or hours, i.e., there are time lags (e.g., Coley and Heelis 2012; Min et al. 2009; Rich et al. 2003; Wang et al. 2006). The thermosphere is the background of the ionosphere; its variations significantly affect ionospheric variability. The time lags were found to also exist in thermospheric responses to the changes of the solar irradiance (e.g., Buonsanto and Pohlman 1998; Eastes et al. 2004; Jakowski et al. 1991). Some studies suggested that the lags of the ionosphere to the solar irradiance are possibly related to the lags of the thermosphere to the solar irradiance (e.g., Min et al. 2009; Wang et al. 2006). The solar cycle variation of solar irradiance also significantly modulates the thermosphere (e.g., Emmert et al. 2010; Liu et al. 2005; Solomon et al. 2010, 2011), which should affect the solar cycle variation of the ionosphere. Thus, it is essential to investigate whether the response of the ionosphere to the solar cycle variation of EUV has evident time lags.
This research comparatively investigated the responses of the ionosphere to the solar cycle and solar rotation variations of EUV using daily mean global electron content (GEC) and the daily 0.1–50 nm EUV integral flux observed by the Solar EUV Monitor (SEM) aboard the Solar Heliospheric Observatory (SOHO) satellite (Judge et al. 1998). We focus on two features of GEC responses to EUV, time lags and quantitative relationships of GEC versus EUV. The parameter GEC was used since its short-term variation is well correlated to EUV short-term variation, thus, the time lags and the quantitative relationship between the short-term variations of the ionosphere and EUV can be reliably derived. The results of this paper indicate that ionospheric responses to the solar cycle and solar rotation variations of EUV are significantly different.
GEC was suggested to be a good indicator for presenting solar EUV effects on the ionosphere (Afraimovich et al. 2008; Chen et al. 2012; Hocke 2008). In this paper, daily mean GECs were calculated from the total electron content (TEC) maps of the Jet Propulsion Laboratory (JPL) to investigate ionospheric responses to the solar cycle and solar rotation variations of EUV. The JPL TEC maps have been routinely produced from the measurements of global GPS receivers since late 1998 (Iijima et al. 1999; Mannucci et al. 1998). The TEC maps are presented with a grid of 5° in longitude by 2.5° in latitude. There are 12 maps within each UT day. GECs were derived one by one from the JPL TEC maps by calculating the area integral of TEC in terms of the map grid according to Afraimovich et al. (2008). GEC is measured with the unit of GECu (1 GECu = 1032 electrons). For each UT day, a daily mean GEC was obtained by averaging the 12 GECs derived from the 12 TEC maps. The daily mean GECs of geomagnetically disturbed days (Ap > 30 in the previous or current day of GEC) were not analyzed to depress geomagnetic activity effects, since the purpose of this paper is to investigate solar EUV effects.
The actual Sun-Earth distance has a ~3.5 % variation during a year, which causes a ~7 % variation of the solar irradiance that arrives at the Earth. The downloaded SOHO/SEM daily EUV flux is the dataset normalized to 1 AU. Thus, we revised the SOHO/SEM 0.1–50 nm EUV flux according to the actual Sun-Earth distance in order to more accurately obtain the quantitative relationship between GEC and EUV. It should be noted that EUV observations aboard satellites usually have instrument degradations. This degradation also exists in the SOHO/SEM recent measurements (Didkovsky and Wieman, 2014; Solomon et al., 2013; Wieman et al., 2014), which possibly affects the results of EUV long-term variations to some extent. This effect is not prominent for the SOHO/SEM EUV flux measurements in solar cycle 23 according to the calibrations with other EUV measurements shown in Didkovsky et al. (2010) and Solomon et al. (2010). Therefore, only the GEC and EUV data before the end of 2007 were used in this analysis.
Results and discussion
Time lags of GEC to EUV
Variation slopes of GEC versus EUV
Here, the proportion coefficient of GEC27R to EUV27R mainly includes an annual variation and a semiannual variation (similar to Equation 1). Seasonal variations of the GEC27R versus EUV27R slope were derived via fitting GEC27R according to Equation 2, and here the dominative 1-day lag of GEC27R to EUV27R was taken into account. The correlation coefficient between the fitted GEC27R and the actual GEC27R equals to 0.866. The grey line in Fig. 7 shows the GEC27R versus EUV27R fitting slope as a function of DoY. The seasonal features of the GEC27R versus EUV27R fitting slope are similar to those of the GEC27A versus EUV27A fitting slope, including the semiannual and annual variations as well as the equinoctial asymmetry. As compared with the GEC27A versus EUV27A fitting slope, the GEC27R versus EUV27R fitting slope is lower in all seasons, especially at the two equinoxes. Namely, the difference between GEC responses to the solar rotation and solar cycle variations of EUV occurs in all seasons.
As compared with Fig. 7, in Fig. 6a, seasonal variations of GEC are removed; and in Fig. 6b, the GEC27R versus EUV27R slope is obtained by averagely fitting the data without including seasonal dependence of the slope. In fact, the GEC versus EUV fitting slopes shown in Fig. 6 are equivalent to seasonal averages of the GEC versus EUV slopes shown in Fig. 7. The average values of the GEC versus EUV slopes shown in Fig. 7 are 0.469 (in units of 1022 electrons/photon/cm2/s, for the black line) and 0.342 (for the grey line), very close to the fitting slopes shown in Fig. 6.
LIEC versus EUV fitting slopes and correlation coefficients for different latitudinal intervals
365-day averaged LIEC and EUV
27-day average residual LIEC and EUV
In fact, thermospheric responses to the solar cycle and solar rotation variations of EUV are also discrepant. That has been found in previous studies. Hedin (1984) found that the fitting slope of 81-day running averaged thermospheric density versus EUV is higher than that of 81-day average residual density versus EUV (see Table 3 of Hedin (1984)). It is unresolved what causes this difference in thermospheric responses to solar irradiance variations on the two timescales. The ionosphere strongly depends on the state of the thermosphere. The discrepancy between thermospheric responses to the solar cycle and solar rotation variations of EUV should result in a difference in ionospheric responses to EUV variations on the two timescales. The difference in GEC responses to EUV variations on the two timescales is consistent with that in thermospheric responses. This implies that the latter is a possible reason for the former.
Daily mean GEC and SOHO/SEM EUV flux were used to investigate the responses of the ionosphere to the solar cycle and solar rotation variations of EUV. The 365-day running averaged (27-day average residual) GEC and EUV were used to estimate the solar cycle (the solar rotation) variations of GEC and EUV. Two features, time lags of GEC to EUV and variation slopes of GEC versus EUV, were comparatively investigated for the solar cycle and solar rotation variations of GEC and EUV. The results indicate that GEC responses to the solar cycle and solar rotation variations of EUV are significantly different. The response of 27-day average residual GEC to EUV shows significant time lags, with EUV leading GEC. The time lag is dominated by a 1-day lag and generally presents a decrease trend with solar activity decreasing. However, there are no evident time lags for the response of 365-day running averaged GEC to EUV. The variation slope of 365-day running averaged GEC versus EUV is significantly higher than that of 27-day average residual GEC versus EUV by ~40 %, and the difference of GEC versus EUV slope between the two timescales occurs in all seasons and at different latitudinal bands. These results present an aspect of the difference between ionospheric climatology and weather.
The JPL TEC data were downloaded from this Web site ftp://cddis.gsfc.nasa.gov; the SOHO/SEM EUV data were provided by the Space Sciences Center of University of Southern California; Ap index was taken from the National Geophysical Data Center Web site. This research was supported by the National Natural Science Foundation of China (41274161, 41231065, and 41321003), Chinese Academy of Sciences (KZZD-EW-01-3), and National Important Basic Research Project of China (2012CB825604).
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