Daytime Electron Density At the F1-Region in Europe During Geomagnetic Storms

Annales Geophysicae (2002) 20: 1007–1021 c Eu opean Geophysical Socie y 2002 Annales Geophysicae Day ime elec on densi y a he F1- egion in Eu ope du ing geomagne ic s o ms D. Bu eso a1, J. Las o icka1, D. Al adill2, and G. Mi o3 1Ins i u e o A mosphe ic Physics, Bocni II, 141 31 P ague 4, Czech Republic 2Obse a o i de l’Eb e, 43520 Roque es, Ta agona, Spain 3INTA/CEDEA A mosphe ic Sounding S a ion ‘El A enosillo’, Mazagon, 21130 Huel a, Spain Recei ed: 27 Sep embe 2001 – Re ised: 25 Ma ch 2002 – Accep ed: 23 Ap il 2002 Abs ac . This s udy a emp s o demons a e changes in he ionosphe ic F1- egion day ime ioniza ion du ing geomag- ne ic s o ms. The F1- egion is explo ed using a ailable da a om se e al Eu opean middle la i ude and lowe la i ude ob- se a o ies and a se o geomagne ic s o ms encompassing a ange o seasons and sola ac i i y le els. The esul s o anal- ysis sugges sys ema ic seasonal and pa ly la i udinal di e - ences in he F1- egion esponse o geomagne ic s o m. The pa e n o he esponse o he F1- egion a highe middle la - i udes, a dec ease in elec on densi y, does no depend on he ype o esponse o he F2- egion and on sola ac i i y. A b ie in e p e a ion o hese indings is p esen ed. Key wo ds. Ionosphe e (ionosphe ic dis u bances; mid- la i ude ionosphe e 1 In oduc ion Geomagne ic s o ms a e he mos impo an space wea he phenomenon om he poin o iew o impac on he global ionosphe e-a mosphe e sys em. They c ea e complica ed changes in he complex mo phology o he elec ic ields, empe a u e, winds and composi ion, and a ec all iono- sphe ic pa ame e s. The e a e nume ous publica ions on he e ec o geomagne ic s o ms on he ionosphe e, e.g. among he ecen a e hose by Rees (1995), P ¨ olss (1995), Buon- san o (1999), Richmond (2000), Danilo and Las o icka (2001). This pape deals wi h he day ime elec on densi y analy- sis a he ixed heigh s o 160–190km. This ange o heigh s is he pa o he bo omside ionosphe e, whe e unde some condi ions he F1-laye occu s sepa a ely (Ra cli e, 1956). Fu he in he ex , he e m ”F1- egion“ o ”F1 heigh s“ in- s ead o he e m ”F1-laye ” will be used. The eason is ha he F1-laye is no de eloped as a sepa a e laye (i.e. on iono- g ams as a ledge on an elec on densi y p o ile) in win e un- de undis u bed condi ions a middle la i udes. On he o he Co espondence o: D. Bu eso a (b[email p o ec ed]) hand, he F1-laye is de eloped e en in win e du ing su i- cien ly s ong geomagne ic s o ms, acco ding o ou expe i- ence. Conside ing di e en composi ion and he spec um o he UV eaching each ionosphe ic egion, acco ding o al i ude, he D-, E-, F1- and F2- egions all ha e hei own unique s uc u al cha ac e is ics (Ondoh and Ma ubashi, 2001), and he e o e we could expec he di e en esponse o each e- gion o s o m-induced dis u bances. Ea lie wo k o Ra cli e and Weekes (1960) ga e some e e ences on lowe ing oF1 unde ionosphe ic s o m condi ions. King (1961) p esen ed a simple F1-F2 ansi ion egion model seeking o desc ibe he chemis y changes be e unde dis u bed condi ions. By con e ing he heo e ical model and p esen ing i as se s o cu es o “o e lays”, which we e compa ed di ec ly wi h he ionog ams, King and Lawden (1964) in oduced a possibili y o how o ob ain p ac ical in o ma ion abou he bo omside ionosphe e wi hou using in e sion echniques. King (1966, 1967) desc ibed in an ou line how dis u bances in he au o- al egions could gi e ise o a mosphe ic wa es, which hen cause an inc ease in he ecombina ion a e a F1 heigh s a middle la i udes. Rishbe h and Ga io (1969) summa ized and commen ed on hese pape s. In gene al, he F1- egion o he ionosphe e has been s ud- ied much less han he F2- and E- egions, pa ially due o i s lowe impo ance o he ionosphe ic p opaga ion o a- dio wa es and i s p edic ion. As a consequence o ha , he geomagne ic s o m e ec s on he F1- egion ha e ecen ly been s udied e y sca cely. Fo ins ance, he e iew pape s by Buonsan o (1999) and P ¨ olss (1995) did no deal wi h hem a all. Las o icka (1996) e iewed in b ie geomag- ne ic s o m e ec s on he lowe ionosphe e, middle a mo- sphe e and oposphe e, and ano he e iew pape by Danilo and Las o icka (2001) only b ie ly men ioned a ew pape s ha had been dealing wi h geomagne ic s o m e ec s a he F1 heigh s. Bu eso a and Las o icka (2001) analyzed he e ec s o a ew geomagne ic s o ms in elec on densi y a F1 heigh s du ing day ime, based on da a o h ee Eu opean ionosondes. Thei mos impo an esul is he exis ence 1008 D. Bu eso a e al.: Day ime elec on densi y a he F1- egion in Eu ope o subs an ial sp ing/au umn asymme y o s o m e ec s a heigh s o 180 and 190km. The geomagne ic s o m e ec s on he F1- egion (150∼200km) a e subs an ially weake han hose in he F2- egion (abo e 200km) and in he lowe ionosphe e (below abou 100km). The s o m e ec s in he F2- egion ionosphe e a e p edominan ly he ionosphe ic esponse o s o m e ec s in he he mosphe e, and he e ec s in he lowe ionosphe e a e p edominan ly caused by he s o m-associa ed p ecipi a ing ene ge ic pa icles. A he F1 heigh s, he in luence o bo h changes in he neu al a mosphe e and ioniza ion and pho ochemical p ocesses can play impo an oles due o he sho e li e ime o ee elec ons compa ed wi h he F2- egion. T anspo p ocesses o ioniza ion a e no e y impo an below 200km (Rishbe h and Ga io , 1969). Thei ole in he o e all beha iou o he F1-laye du ing geomagne ic s o ms is subs an ially less impo an han in he F2-laye , again basically due o he sho e li e ime o elec ons. Rega ding changes in he neu al a mosphe e, hey can play a ole mainly in he uppe pa o he F1- egion, bo h changes in he neu al composi ion and in he o al neu al densi y. S ong longi udinal and la i udinal asymme ies o he comple ely di e en s o m-induced dis u bance beha iou o he ionosphe ic F2- egion abo e wo compa able loca ions a e equen ly obse ed (P ¨ olss, 1995; Bu eso a and Las- o icka, 2001). Mo eo e , he dis ibu ion o s o m e ec s may a y subs an ially om one e en o he o he . To p esen he si ua ion o F1- egion, da a om a ew Eu o- pean s a ions om highe middle and lowe middle la i udes o di e en seasons and di e en sola ac i i y le els a e an- alyzed. We a e ully awa e o he s o m dynamics and signi - ican e ec s on he ionosphe e a high-la i udes. Ex ensi e analysis o he la i udinal dependence o he s o m e ec s on he F1- egion is he aim o ou u u e in es iga ions. The aim o his s udy is o deepen he knowledge o he e- sponse o he ionosphe ic F1- egion o geomagne ic s o ms by b oadening he analyses done in p e ious in es iga ions wi h mo e da a om o he s o ms, om o he seasons and s a ions, and unde a ious le els o sola ac i i y. This analysis e eals among o he s he summe /win e asymme- y a he han he sp ing/au umn asymme y in he geomag- ne ic s o m e ec s a F1 heigh s. Some hin s/cons ain s o in e p e a ion a e p o ided. 2 Da a and me hods Da a om ionosondes loca ed a Wa saw (52.2◦N, 21.2◦E), Chil on (51.7◦N, 358.7◦E), P uhonice (50◦N, 14.6◦E), Eb o (40.8◦N, 0.5◦E) and A enosillo (37.1◦N, 353.2◦E) ionosphe ic s a ions a e used. Fo some e en s, da a om ou s a ions a e a ailable, o o he e en s only da a om h ee a e a ailable. All ionosphe ic, as well as geomagne ic da a, a e in UT. Fo Wa saw, P uhon- ice, A enosillo and Eb o LT=UT+1h (e en hough so- la ime is abou UT o Spanish s a ions), o Chil on LT=UT. All sola and geomagne ic da a ha e been aken om: h p://www.sec.noaa.go /majo domo a chi e.cgi and h p://www.ises-spacewea he .o g. Basic da a applied in u he analyses a e elec on den- si y p o iles, in e ed om ionog ams, and elec on densi- ies aken om hese p o iles a F1- egion heigh s. The elec- on densi y p o iles had been ob ained by a NHPC algo- i hm (Reinisch and Huang, 2001) o digisondes and by he Polynomial Analysis compu e p og am POLAN o classi- cal ionosondes wi h digi ised ou pu , like he ionosonde a P uhonice. POLAN has a wide ange o op ions o allow o he ue-heigh analysis o be ailo ed comple ely o use s’ equi emen s. Fo ue heigh analysis, he p og am has been used as a “black box” wi h only he i ual heigh da a, mag- ne ic dip angle and he gy o equency as equi ed inpu s. The esul an ue-heigh p o ile is sensi i e o wo app oxima- ions, in pa icula . One o hem is he compu a ion s a po- si ion (a wha al i ude he elec on concen a ion becomes signi ican ). We ha e used o he day ime ue-heigh anal- ysis he s a posi ion 86km in al i ude. The i ual heigh o a single laye will be a ec ed by he ioniza ion below i . The ioniza ion in each laye is known om he c i ical e- quency, bu in o ma ion abou he elec on densi y be ween he laye s canno be measu ed by an ionosonde. In o de o cons uc he ue-heigh p o ile, he alley app oxima ion is a e y impo an poin . POLAN app oxima es he alley us- ing he a ia ion in heigh and dep h o he alley wi h he ime o day, da e and la i ude. Fo mos ou ine calcula ions, by choosing de aul op ion 0, POLAN inse s be ween laye s a alley selec ed om he “s anda d” amily. The wid h (km) and dep h (MHz) o he alley depend on he heigh o he unde lying peak (Ti he idge, 1985). Fo he NHPC algo- i hm, he ionog am aces ha e been ca e ully e ised, in o de o a oid any mis ake o he au oma ic scaling o ob- ain ue heigh elec on densi y p o iles. The p o iles ha e been ob ained om he base o he E-laye o hmF2 (Huang and Reinisch, 1996) and abo e (Reinisch and Huang, 2001). They co e he al i ude ange om 90–350km. Ne e heless, we es ic ou s udy o he al i ude ange 160–190 du ing day ime and he E–F alley egion is a oided he e. Six e en s, lis ed in Table 1, a e analyzed in his pape . One o hem is om he yea o low sola ac i i y (1996), he o he s a e om he yea s o high sola ac i i y (1998, 2000, 2001). We deal wi h elec on densi y a iabili y a he ixed F1- egion heigh s o 160–190km, in compa ison wi h changes in he F2-laye maximum elec on concen a- ion NmF2 du ing ionosphe ic s o ms. Sola and geomag- ne ic ac i i y indices o each analyzed s o m a e gi en in Table 1. The beha iou o he hou ly Ds index has been used o de ine he geomagne ic s o m onse hou , and he main and eco e y phases o he s o m. All e en s we e s ong s o ms wi h Ds a abou −100nT o mo e and wi h a leas wo o h ee quie days be o e he s o m onse . Two e en s we e supe s o ms wi h Ds lowe han −300nT. The quie days be o e he s o m ha e been aken as e e ence days. D. Bu eso a e al.: Day ime elec on densi y a he F1- egion in Eu ope 1009 Table 1. The analyzed geomagne ic s o ms Analyzed S o m Mon hly Geomagne ic S o m Maximum p e-s o m and onse sunspo ac i i y indices onse o he s o m pe iod day numbe hou (UT) s o m (UT) 11–17 Janua y 1996 13 Janua y 11.5 4< Kp<5, Ap29 03:00 13/10:00 max. Ds (-90)nT 14–22 Feb ua y 1998 17 Fe ua y 30.3 6< Kp<7, Ap36 13:00 17/22:00 max. Ds (-103)nT 7–14 Ma ch 1998 10 Ma ch 54.8 6< Kp<7, Ap53 11:00 10/21:00 max. Ds (-126)nT 11–18 No embe 1998 13 No embe 74.0 5< Kp<6, Ap60 02:00 13/18:00 max. Ds (-133)nT 13–19 July 2000 15 July 170.1 8< Kp<9, Ap152 16:00 07/15/21:00 max. Ds (-300)nT 29 Ma ch-4 Ap il 2001 31 Ma ch 113.5 8< Kp<9, Ap155 05:00 03/31/08:00 max. Ds (-377)nT 3 Resul s The eigh e en s om low sola ac i i y yea s 1994, 1995 and 1997 o he highe middle la i ude s a ions Chil on and P uhonice ha e al eady been analyzed by Bu eso a and Las- o icka (2001). The main esul s o his analysis a e as ol- lows: 1. The e is no signi ican e ec o geomagne ic s o ms on elec on densi y a he F1 heigh s in he ange o 160–190km du ing sp ing, while in au umn he e is a subs an ial e ec on he F1- egion elec on densi y a 190km; he e is s ill a well-de ec able e ec a 180km, a weak e ec a 170km and no de ec able e ec a 160km. The au umn e ec consis s o a signi ican de- ple ion o elec on densi y du ing he main phase o he s o m. 2. Independen o he sign o he geomagne ic s o m e ec on NmF2, he e ec on he elec on densi y a he highe F1- egion al i udes has always been nega i e, i any a all. Howe e , all he abo e s o ms we e basically e nal o au- umnal e en s. The e o e, o yea s o high sola ac i i y, we also add win e ime and summe ime e en s. Two o hem a e supe s o ms, much s onge han e en s analyzed by Bu- eso a and Las o icka (2001). The supe s o ms a e added o check i conclusions o Bu eso a and Las o icka (2001) hold also o supe s o ms. Summe and win e e en s a e added o check i he asymme y (1) is sp ing/au umn o sum- me /win e . S ong geomagne ic s o ms (no supe s o ms) occu nea equinoxes abou i e imes as much as nea win- e o summe sols ices; he e o e, he numbe o summe and win e e en s in ou analysis is smalle han ha o e nal and au umnal e en s. We in es iga e in de ail da a o he s o ms no analyzed by Bu eso a and Las o icka (2001). They a e classi ied as s ong geomagne ic s o ms (11–17 Janua y 1996, 14– 22 Feb ua y 1998, 7–14 Ma ch 1998 and 11–18 Sep em- be 1998), and wo geomagne ic s o ms (13–19 July 2000 and 29 Ma ch 2001–4 Ap il 2001) a e classi ied as supe s o ms, acco ding o Loewe and P ¨ olss (1997). 3.1 S o m o Janua y 1996 This win e ime s o m s a ed a abou 04:00 on 13 Janua y and achie ed i s maximum a 11:00. Figu e 1a ep esen s he Ds cou se o analyzed pe iod. All selec ed s a ions egis e ed he posi i e s o m e ec o di e en magni ude in NmF2 du ing he s o m maximum day (Fig. 1b). A he same ime, Chil on and P uhonice s a ions showed a dec ease in he elec on densi y a 190–170km (Figs. 1c and d). Con- a y o he e ec on he uppe F1- egion o highe mid- dle la i ude s a ions, Eb o p ac ically emained wi hou any changes, and A enosillo showed an inc ease in he elec on densi y a 180 and 190km heigh s du ing he s o m culmina- ion day. No changes in he elec on densi y we e obse ed a 160km o all ou s a ions. 3.2 S o m o No embe 1998 The No embe 1998 e en had i s onse a abou 02:00 o 13 No embe . The s o m culmina ed nea 18:00 in he e ening, when he Ds index ell down o i s minimum alue o −133nT (Fig. 2a). Wa saw, P uhonice, Eb o and A enosillo s a ions showed a posi i e e ec on he NmF2 du - ing he s o m onse and main phase (Fig. 2b). The cou se o he elec on densi y a 160–190km o all s a ions is plo ed in Figs. 2c– . The elec on densi y a 170, 180 and 190km o Wa saw and P uhonice displayed a dec ease in di e en magni ude, and a small insigni ican dec ease was ound also a 160km. Eb o egis e ed an insigni ican , weak e ec and A enosillo egis e ed no e ec on he F1- egion elec on den- si y a he s o m maximum day. A peculia phenomenon is ha A enosillo da a p esen a la ge posi i e e ec a all ana- 1010 D. Bu eso a e al.: Day ime elec on densi y a he F1- egion in Eu ope Fig. 1. S o m o Janua y 1996: (a) hou ly Ds indices, (b) hou ly NmF2 alues o Chil on, P uhonice, Eb o and A enosillo, (c– ) elec on densi y a 190, 180, 170 and 160km, espec i ely, o day ime hou s o all s a ions. Time is in UT. D. Bu eso a e al.: Day ime elec on densi y a he F1- egion in Eu ope 1011 Fig. 2. S o m o No embe 1998: (a) hou ly Ds indices, (b) hou ly NmF2 alues o P uhonice, Wa saw, Eb o and A enosillo, (c– ) elec on densi y a 190, 180, 170 and 160km, espec i ely, o day ime hou s o all s a ions. Time is in UT. 1012 D. Bu eso a e al.: Day ime elec on densi y a he F1- egion in Eu ope P uhonice 100 150 200 250 300 350 0 5 10 15 20 N, 105 cm-3 Al i ude, km 1998.11.12 12:00 1998.11.13 12:00 Eb o 100 150 200 250 300 350 0 5 10 15 20 N, 105 cm-3 Al i ude, km 1998.11.12 12:00 1998.11.13 12:00 Fig. 3. The noon ime elec on densi y p o iles in he F- egion (150–350km) o he s o m o No embe 1998, one day be o e he s o m onse (blue line) and du ing he s o m main phase ( ed line) o P uhonice and Eb o. Al i ude in km, elec on densi y Nin 105cm−3. lyzed al i udes du ing he a e noon, one day be o e and one day a e he s o m onse . The elec on densi y p o iles plo ed in Fig. 3a ep esen he noon ime ionosphe ic ioniza ion a he heigh s om 150 o 350km one day be o e he s o m onse and o he s o m maximum day o P uhonice. The same is plo ed o Eb o in Fig. 3b. The plo s show a dec ease in he F1- egion ion- iza ion du ing he s o m maximum day o P uhonice and no de ec able changes o Eb o, in spi e o a s ong posi i e e - ec a he F2- egion heigh s. 3.3 S o m o Feb ua y 1998 Figu e 4 illus a es he hou ly Ds index cou se and he e- sul s o he P uhonice, Wa saw, Eb o and A enosillo s a- ions. The hou ly obse a ions o NmF2 compa ed wi h changes in elec on concen a ion a he heigh s o 160– 190km du ing his win e s o m a e p esen ed in Figs. 4b– . The obse ed a ia ions o NmF2 du ing day ime hou s o 14–16 Feb ua y a e close o each o he o he h ee p e- s o m days and any o hem may be aken as a e e ence day. A a he mode a e nega i e s o m e ec ook place du ing he day ime hou s o 18 Feb ua y o highe middle la i ude s a- ions Wa saw and P uhonice. The ela i e dec ease in elec- on concen a ion a 190 and 180km was la ge han ha o NmF2, wi h some dec ease obse ed a 170km and a weak dec ease a 160km. On he o he hand, Eb o and A enosillo egis e ed an inc ease in NmF2 du ing he s o m main phase. A mode a e dec ease was ound in he elec on densi y a he ixed F1- egion heigh s be o e he a e noon o 18 Feb ua y. The 16 and 18 Feb ua y midday elec on densi y p o iles plo ed in Figs. 5a and b o he P uhonice and Eb o s a ions illus a e he di e ence be ween s o m e ec s a highe and lowe Eu opean middle la i udes. A s onge e ec a he F1 heigh s and a nega i e e ec in he F2- egion a e ob- se ed a highe middle la i udes. A weake e ec a he F1- egion heigh s and a posi i e e ec in he F2- egion a e obse ed a lowe middle la i udes. Feb ua y is a ypical win- e mon h he e o e a sepa a ed F1-laye was de eloped only unde s o m condi ions o highe middle la i udes. 3.4 S o m o Ma ch 1998 Figu es 6a and 6b p esen he Ds index cou se and NmF2 o P uhonice, Chil on and Eb o da a du ing he s ong geomag- ne ic s o m o Ma ch 1998. The Ds index began o dec ease apidly nea noon on 10 Ma ch and eached i s maximum in he la e e ening hou s. The geomagne ic dis u bance has a ela i ely long du a ion. Bo h P uhonice and Chil on s a ions show a simila nega i e e ec on NmF2 o he s o m main phase, a signi ican dec ease in he elec on densi y a 190 and 180km and a weak dec ease in he lowe pa o he F1- egion du ing he s o m main phase (Figs. 6c– ). Con a y o highe middle la i ude s a ions, Eb o da a showed an inc ease in NmF2 a he s o m onse day and la e nega i e phase du - ing he s o m main phase. Howe e , in he F1- egion, he s o m e ec a Eb o was qui e simila o ha o Chil on and P uhonice. 3.5 Supe s o m o July 2000 A big sola explosion was obse ed by SOHO’s ul a iole elescope EIT on 14 July a 10:12. I s a ed as one o he mos in ense s o ms in he 22nd sola cycle. This summe - ime e en was classi ied as a supe s o m. The dec ease in he Ds index s a ed du ing he a e noon o 15 July, and selec ed s a ions (Chil on, P uhonice, Eb o and A enosillo) eco ded a la ge dec ease in NmF2 du ing he s o m onse and he main phase (Figs. 7a and 7b). In con as o he F2- egion, he F1- egion did no unde go such la ge changes. A mode - a e dec ease in he elec on densi y a 180 and 190km and a mino dec ease below was eco ded o all s a ions (Figs. 7c– ). The F1- egion esponse o he summe ime supe s o m seems o be compa able wi h he esponse o he abo e win- e ime s o ms, a leas a highe middle la i udes. 3.6 Supe s o m o Ma ch–Ap il 2001 The supe s o m had i s onse ea ly in he mo ning o 31 Ma ch and eached maximum in ensi y a e 08:00UT. Figu e 8a shows he hou ly Ds indices o p e-s o m, s o m main phase and pos -s o m pe iod. All s a ions (Chil on, P uhonice, Eb o and A enosillo) ob- se ed a subs an ial dec ease in NmF2 o he s o m onse day and o he nex day (Fig. 8b). Figu es 8c– p esen s D. Bu eso a e al.: Day ime elec on densi y a he F1- egion in Eu ope 1013 Fig. 4. S o m o Feb ua y 1998: (a) hou ly Ds indices, (b) hou ly NmF2 alues o P uhonice, Wa saw, Eb o and A enosillo, (c– ) elec on densi y a 190, 180, 170 and 160km, espec i ely, o day ime hou s o all s a ions. Time is in UT. 1014 D. Bu eso a e al.: Day ime elec on densi y a he F1- egion in Eu ope P uhonice 100 150 200 250 300 350 03691215 N, 105 cm-3 Al i ude 98/02/16 12:00 98/02/18 12:00 Eb o 100 150 200 250 300 350 03691215 N, 105 cm-3 Al i ude, km 98/02/16 12:00 98/02/18 12:00 Fig. 5. The noon ime elec on densi y p o iles in he F- egion (150–350km) o he s o m o Feb ua y 1998, one day be o e he s o m onse (blue line) and du ing he s o m main phase ( ed line) o P uhonice and Eb o. Al i ude in km, elec on densi y Nin 105cm−3. he day ime elec on densi y a he ixed F1 heigh s. The analyzed da a shows a simila bu s onge e ec o he su- pe s o m compa ed wi h he geomagne ic s o ms o Ma ch and No embe 1998, wi h a dec ease in he day ime elec on densi y a 160–190km a highe middle la i udes. The lowe middle la i ude s a ions Eb o and A enosillo show a sho e eco e y phase o he F1- egion elec on densi y in compa - ison wi h he highe middle la i ude s a ions P uhonice and Chil on. An in e es ing ea u e, which was no iced du ing he supe s o m main phase, was ha in con as o he o he an- alyzed win e ime geomagne ic s o ms, he elec on densi y a 170 and 160km dec eased signi ican ly, which indica es a deepe pene a ion o supe s o m e ec s in o he F1- egion compa ed o he o he s ong win e ime s o ms. The s o m maximum day noon ime elec on densi y p o- iles o P uhonice and Eb o a e plo ed in Figs. 9a and 9b e- spec i ely. The p o iles show clea ly he di e ences be ween quie and dis u bed ionosphe ic elec on densi y p o iles and he nega i e supe s o m e ec on he F1- egion elec on den- si y h oughou he F1- egion heigh s. The s ong nega i e e ec was also obse ed in he F2- egion a bo h s a ions. Since his supe s o m s ill belongs o he win e hal o he yea , he F- egion is sepa a ed in o F1- and F2-laye s only unde s o m condi ions. 3.7 Summa y o obse a ional esul s Some esul s o Bu eso a and Las o icka (2001) ha e been con i med bu some ha e been modi ied as well. The con- clusion is con i med ha independen o he sign o he geo- magne ic s o m e ec on NmF2, he e ec on elec on den- si y a he F1 heigh s has always been nega i e, i any a all. This is ue o all 14 s o ms analyzed he e and by Bu- eso aand Las o icka (2001), bu only o highe middle la i- udes (Chil on, P uhonice, Wa saw). Fo ins ance, he Feb u- a y 1998 geomagne ic s o m had a nega i e phase in he F2- egion, whe easo he win e e en so Janua y 1996, No em- be 1997 (Bu eso a and Las o icka, 2001) and No em- be 1998 had a s able, s ong posi i e phase du ing he s o m main phase. Independen o he s o m e ec on he F2- egion, he e ec on he elec on densi y a he F1 heigh s was nega i e o all hese s o ms o he analyzed highe middle la i ude s a ions. Fo lowe middle la i udes (Eb o and A enosillo – Spain), he s o m e ec a he F1 heigh s is simila o ha a highe middle la i udes only o he supe s o ms. I migh be a con- sequence o subs an ial equa o wa d expansion o he au o- al zone. Fo o he s ong s o ms, he e ec s a lowe middle la i udes may di e om hose a highe middle la i udes. Nega i e e ec s, no e ec s, as well as posi i e e ec (Jan- ua y 1996 e en – A enosillo), we e obse ed. The e is no e iden ela ion be ween he e ec on NmF2 and on he elec- on densi y a he F1- egion heigh s. We plan o analyze mo e e en s a Eu opean lowe middle la i udes, in o de o speci y be e he geomagne ic s o m e ec pa e n o he F1- egion. The o he conclusion o Bu eso a and Las o icka (2001) abou he sp ing/au umn asymme y has o be modi ied. The abo e esul s indica e summe /win e asymme y. The e is no signi ican e ec o geomagne ic s o ms on elec on den- si y a he F1 heigh s in he ange o 160–190km du ing he summe hal o he yea (s o ms o May 1994, May 1995 and May 1997 – Bu eso a and Las o icka, 2001), excep o he mode a e- o-mino e ec o he supe s o m o July 2000 a 160–190km. On he o he hand, in he win e hal o he yea , he e is a subs an ial e ec on he F1- egion elec on densi y a 190km; he e is s ill a well-de ec able e ec a 180km, a de ec able e ec a 170km and insigni ican e ec a 160km, excep o he supe s o m o Ma ch–Ap il 2001, wi h signi ican e ec s down o 160km a highe middle la - i udes. The win e e ec consis s o a deple ion o elec on densi y du ing he main phase o he s o m. The e ec s a lowe middle la i udes a e some imes weake and less egu- la . No e en con adic s he idea ha he win e - o-summe and summe - o-win e ansi ions in e ec s o geomagne ic s o ms a he F1 heigh s a e cen e ed a ew weeks o a cou- ple o days a e equinoxes (e en hough his ansi ion is no expec ed o be sha p). Howe e , he numbe o e en s analyzed nea equinoxes is oo small o making a inal con- clusion abou he ansi ion bounda y o he F1- egion s o m esponse. The analysis o he wo supe s o ms o July 2000 and Ma ch-Ap il 2001 indica es o all analyzed highe middle and lowe middle la i ude s a ions a deepe pene a ion o he e ec s o geomagne ic supe s o ms in o he F1- egion han in he case o s ong s o ms. Figu e 10 shows he di e ences D. Bu eso a e al.: Day ime elec on densi y a he F1- egion in Eu ope 1015 Fig. 6. S o m o Ma ch 1998: (a) hou ly Ds indices, (b) hou ly alues o NmF2 o Chil on, P uhonice and Eb o, (c– ) day ime elec on densi y a 190, 180, 170 and 160km, espec i ely, o all s a ions. Time is in UT.