How does ionospheric TEC vary if solar EUV irradiance continuously decreases?

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<ul><li><p>FULL PAPER Open Access</p><p>How does ionospheric TEC vary if solar EUVirradiance continuously decreases?Yiding Chen1,2*, Libo Liu1,2, Huijun Le1,2 and Weixing Wan1,2</p><p>Abstract</p><p>It is an interesting topic how the ionosphere varies when solar extreme ultraviolet (EUV) irradiance decreases farbelow normal levels. When extrapolating the total electron content (TEC)-EUV relation, significantly negative TECs atthe 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 ofnonlinear TEC-EUV variations over the whole EUV range from zero to the solar maximum level. The features of theionosphere under ELSE conditions were investigated using the TEC extrapolated with cubic TEC-EUV fitting. Withthe 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 underELSE conditions may differ from those during normal solar cycles at nighttime. Ionospheric dynamic processes aresupposed to still significantly affect the ionosphere under ELSE conditions and induce this difference. With solarEUV decreasing, global electron content (GEC) should vary largely in accordance with the GEC-EUV trend duringnormal solar cycles, and the seasonal fluctuation of GEC declines, owing to the contraction of the ionosphere.</p><p>Keywords: Ionosphere; Total electron content; Nonlinear variation</p><p>BackgroundVariations of solar extreme ultraviolet (EUV) irradianceare important for the ionosphere and thermosphere. Theprominent 11-year solar cycle variation of EUV has beenwidely investigated; meanwhile, variations of EUV overlonger timescales have also drawn wide attention. SolarEUV is well correlated with the sunspot number, an im-portant solar activity proxy, during normal solar cycles;specifically, EUV decreases as the sunspot number declines.Based on the sunspot historical record, Eddy (1976) justi-fied the existence of an extremely low solar activity periodknown as the Maunder Minimum (1645 to 1715 A.D.)during which virtually no sunspots were observed. It isunclear how solar EUV and also the ionosphere variedduring the extended extremely low solar activity periodssuch as the Maunder Minimum. Some researchers haveattempted to derive the EUV irradiance during these ex-treme periods by solar irradiance models (e.g., Lean et al.2011); the results reveal that EUV irradiance potentially</p><p>decreased to an extremely low level during the MaunderMinimum period (see Figure 1).Solar activity has recently undergone a deep minimum</p><p>during 2007 to 2009. This solar minimum lasted longerthan several previous minima, and the sunspot recordduring this solar minimum reached the lowest level after1913. Solar EUV irradiance was found to have significantlydecreased during this deep solar minimum (Didkovskyet al. 2010), and accordingly, the thermospheric densityand 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 merelyoffered us a glimpse for understanding the Sun and thespace environment during extremely low solar activityperiods. As far as extremely deep solar minima, suchas the Maunder Minimum, are concerned, solar EUVcould potentially decrease to very low levels during theseperiods. Then, an important question remains about whatis the possible state of the ionosphere under this extremesolar activity condition. Although no observation candirectly provide us an answer, this topic is interestingand essential as another extremely low solar activity periodcould possibly come in the future.</p><p>* Correspondence: chenyd@mail.iggcas.ac.cn1Key Laboratory of Ionospheric Environment, Institute of Geology andGeophysics, Chinese Academy of Sciences, Beijing 100029, China2Beijing National Observatory of Space Environment, Institute of Geologyand Geophysics, Chinese Academy of Sciences, Beijing 100029, China</p><p> 2014 Chen et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative CommonsAttribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproductionin any medium, provided the original work is properly credited.</p><p>Chen et al. Earth, Planets and Space 2014, 66:52http://www.earth-planets-space.com/content/66/1/52</p><p>mailto:chenyd@mail.iggcas.ac.cnhttp://creativecommons.org/licenses/by/2.0</p></li><li><p>A few studies have investigated the state of the iono-sphere under the conditions of extremely low solar EUV(ELSE), which means solar EUV irradiance is much lowerthan the normal solar minimum level. Liu et al. (2009) re-vealed that ionospheric total electron content (TEC) isnegative at the zero EUV point if it decreases with EUVin accordance with the linear TEC-EUV relation duringrecent lower solar activity periods. The negative TEC isnonphysical and thus indicates that ionospheric variationwith EUV under ELSE conditions should be different fromthat during recent lower solar activity periods. Smithtroand Sojka (2005) simulated the behavior of the ionosphereunder extremely low solar flux conditions using a globalaverage ionosphere and thermosphere model. Their re-sults showed that as solar irradiance falls below normalsolar minimum levels, the concentration of O+ decreasesrapidly relative to the molecular ions so that the ratio offoF2/foF1 approaches unity (foF1 and foF2 are the criticalfrequencies of the F1 and F2 layers, respectively), whichindicates that the vertical profile of the ionospheremay significantly change under extremely low solarflux conditions. The abovementioned studies indicatethat to some extent, the behavior of the ionosphereunder ELSE conditions is possibly different from thatunder normal solar cycle conditions.This paper deals with the behavior of TEC under ELSE</p><p>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 EUVMonitor aboard the Solar Heliospheric Observatorysatellite (SOHO/SEM) (Judge et al. 1998). We discussthe nonlinear variation trend of TEC with EUV and someprimary climatology features of TEC under ELSE condi-tions. The results highlight the nonlinear variations ofTEC with EUV and some climatology features of TECunder ELSE conditions that are different from those duringthe normal solar cycle period.</p><p>MethodsData usedDaily EUV flux monitored by SOHO/SEM has been con-tinuously provided since 1996. It is an ideal dataset forinvestigating EUV's longer-term variations, such as thesolar cycle variation, and their effects on the ionosphere.In this study, SOHO/SEM EUV integral flux over 0.1- to50-nm wavelength range was used to present the varia-tions of the EUV irradiance that causes the ionization ofneutral atmosphere.As Smithtro and Sojka (2005) revealed, the vertical</p><p>profile of the ionosphere during ELSE periods is possiblysignificantly different from that during normal solar cycles.Thus, some characteristic parameters corresponding tothe key layers of the ionosphere (e.g., peak electron densityof the F2 layer) are possibly inapplicable under ELSE con-ditions. TEC is a parameter that depends on the altitudinalintegral of the electron density; it always can be used todetect the state of the ionosphere if only the ionosphereexists. In this paper, we discuss ionospheric behaviorunder ELSE conditions based on the parameter TEC. TheJPL TEC maps have been routinely released since 1998;they are essential for investigating the global features andsolar activity variations of the ionosphere. The JPL TECmaps are analytic descriptions of the measured slant TEC;the accuracy of the maps depends on the numbers of glo-bal positioning system (GPS) receiver stations and satel-lites available. There are several other TEC map datasetssuch as ESA, UPC, and CODE (Hernndez-Parajes et al.2009). Some differences still exist between these differentTEC datasets. We used the JPL TEC maps in this study.The TEC values at given local times were obtained byinterpolating JPL TEC to local time grids. In order tosuppress the effect of stronger geomagnetic disturbance,the TEC data were removed in the analyses if the Apindex in the current day of TEC or in the previous dayof TEC is larger than 30.</p><p>Figure 1 Annually averaged solar EUV irradiance (0 to 120 nm) estimated with solar EUV models. Solar EUV irradiance potentiallydecreased to an extremely low level during the Maunder Minimum period (after Lean et al. 2011).</p><p>Chen et al. Earth, Planets and Space 2014, 66:52 Page 2 of 11http://www.earth-planets-space.com/content/66/1/52</p></li><li><p>Does ELSE TEC follow the TEC-EUV relation during normalsolar cycles?During normal solar cycles, the variation of ionosphericelectron density with solar EUV flux is nearly linear atlower solar activities and nonlinear at higher solar activ-ities. At higher solar activity levels, the electron densityvaries with solar EUV flux in a pattern that deviates fromthe 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 couldbe used to well estimate the nonlinear variation of theelectron 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 ornonlinear relation between the electron density and EUVflux during normal solar cycles?Solar EUV irradiance causes the vast majority of the</p><p>ionization in the middle- and low-latitude ionosphere. Ifsolar EUV irradiance vanishes, it can be deduced thatonly an extremely weak ionosphere can exist, owing to theweak ionization caused by the minor ionization sourcessuch as starlight (e.g., Titheridge 2000). TEC shouldapproach extremely low values, as compared with theTEC values during normal solar cycles, if solar EUVcontinuously decreases so that it approaches zero. Hence,we may investigate whether TEC under ELSE conditionsvaries with EUV irradiance following the linear or nonlin-ear TEC-EUV relation during normal solar cycles by ex-trapolating the linear or nonlinear TEC-EUV relation tothe zero EUV point.Figure 2 shows the variation of noontime TEC over a</p><p>specific location (90W, 40S) with 0.1- to 50-nm EUVflux during the December to January season. The solidline in Figure 2a demonstrates the linear TEC-EUV re-gression fitting. The linear fitting was only for the dataover the lower EUV range (here, EUV &lt; 4 1014 photons/m2/s) as the linear TEC-EUV variation is generally onlyvalid for lower solar activity levels, whereas nonlinearitybecomes important at higher solar activity levels. TheTEC value at the zero EUV point was obtained by ex-trapolating the linear TEC-EUV fitting to the zeroEUV point, as shown by the dashed line in Figure 2a.According to the method shown in Figure 2a, the TECvalues at the zero EUV point were derived for differentlocations and seasons, and the data are shown in theleft panels of Figure 3. In general, negative TEC valuesare dominant when EUV sufficiently approaches zero.The negative TEC is nonphysical; this indicates thatTEC-EUV variation should differ from the linear TEC-EUVvariation at lower solar activities (i.e., nonlinear variation)when solar EUV continuously decreases to extremely lowlevels. Then, we addressed the question of whether thenonlinear TEC-EUV relation during normal solar cycles</p><p>can describe the nonlinear TEC-EUV variation underELSE conditions. In Figure 2b, the solid line shows thequadratic TEC-EUV regression fitting for the observeddata during normal solar cycles, and the dashed linedemonstrates how the TEC value at the zero EUVpoint was obtained by extrapolating the quadratic fit-ting to the zero EUV point. According to the methodshown in Figure 2b, TEC values at the zero EUV pointwere derived for different locations and seasons again,and the data are shown in the right panels of Figure 3.The quadratic extrapolated TEC is also significantlynegative at low latitudes, which indicates that thequadratic TEC-EUV trend during normal solar cycles</p><p>0 2 4 6 8</p><p>0</p><p>40</p><p>80</p><p>TE</p><p>C (</p><p>TE</p><p>Cu</p><p>)</p><p>0.150 nm EUV (1014</p><p> photons/m2/s)</p><p>TEC @ (90o W, 40o S), 1200 LT, DecJan</p><p>Linear Fitting</p><p>0 2 4 6 8</p><p>0</p><p>40</p><p>80</p><p>TE</p><p>C (</p><p>TE</p><p>Cu</p><p>)</p><p>0.150 nm EUV (1014</p><p> photons/m2/s)</p><p>Quadratic Fitting</p><p>0 2 4 6 8</p><p>0</p><p>40</p><p>80</p><p>TE</p><p>C (</p><p>TE</p><p>Cu</p><p>)</p><p>0.150 nm EUV (1014</p><p> photons/m2/s)</p><p>Cubic Fitting</p><p>(a)</p><p>(c)</p><p>(b)</p><p>Figure 2 The demonstrations of total electron content (TEC)-EUVregression fitting. Linear fitting for the data of EUV &lt; 4 (a), quadraticfitting for the data of the full EUV range (b), and cubic fitting for thedata of the full EUV range and constrained with the term of zero TECat zero EUV (c).</p><p>Chen et al. Earth, Planets and Space 2014, 66:52 Page 3 of 11http://www.earth-planets-space.com/content/66/1/52</p></li><li><p>also cannot describe the TEC-EUV variation under ELSEconditions.</p><p>Method of extrapolating ELSE TECBoth the TEC-EUV variations during normal solar cyclesand under ELSE conditions are nonlinear, and the quad-ratic TEC-EUV relation during normal solar cycles cannotdescribe the TEC-EUV variation under ELSE conditions.This indicates that higher-order (cubic) TEC-EUV fittingis potentially needed to fully describe the TEC-EUV vari-ation over the whole EUV range from zero to the solarmaximum level. The minor ionization sources such asstarlight, whose strength is lower than solar EUV strengthduring normal solar cycles by up to a factor of four (e.g.,Strobel et al. 1974; Titheridge 2000), only can maintain anextremely weak ionosphere if solar EUV irradiance van-ishes. This extremely weak ionosphere should be the limitof the normal ionosphere under the condition that solarEUV closely approaches zero, and its TEC value should belower than the normal TEC value by up to a factor of fourif simply estimating from the irradiance strength. For sim-plicity, this very minor TEC at the zero solar EUV point</p><p>may be ignored when fitting TEC-EUV relationships aswe do not discuss the ionosphere under the condition thatEUV sufficiently approaches zero. In this paper, the TECvalues under the ELSE conditions were obtained by ex-trapolating cubic TEC-EUV fitting to ELSE conditions.The cubic fitting was constrained with the term of zeroTEC at zero EUV. The solid line in Figure 2c demon-strates the cubic TEC-EUV regression fitting.Since both the TEC-EUV variations over the two EUV</p><p>segments, from zero to the solar minimum level and fromthe solar minimum level to the solar maximum level, arenonlinear, the nonlinearities over the two EUV segmentsmay have identical or contrary trends (for example, aTEC-EUV increase rate that increases with increasingEUV values would be indicative of an amplification trend,whereas a TEC-EUV increase rate that decreases wi...</p></li></ul>

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