Vertical structure of a gravity wave like oscillation in the ionosphere generated by the solar eclipse of August 11, 1999

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL 106, NO A10, PAGES 21,41921,428, OCTOBER 1, 2001 Vertical structure f a gravity wave like scillatin in the insphere generated by the slar eclipse f August 11, 1999 D Altadill and J G S16 Observatri de l'ebre, Universitat Ramn Llull Cnsej Superir de Investigacines Cientfficas, Rquetes, Spain E M Apstlv Gephysical Institute, Bulgarian Academy f Sciences, Sfia, Bulgaria Abstract The dependence in altitude and time f the inspheric electrn density and its altitude variatins during the slar eclipse f August 11, 1999, have been studied The true height electrn density prfiles recrded at the Observatri de l'ebre statin (408 ø N, 05 ø E) during a rapid sequence vertical inspheric sunding campaign have been used A gravity wave like scillatin in the insphere has been characterized after the maximum f slar ccultatin by the Mn The event is clearly seen in bth the plasma frequency and in the altitude variatins f the F regin in the altitude range frm 150 t 250 km, and it has a dminant scillating perid T=575 min The scillatin activity linked with the gravity wave event is fund t exist in the variatins f the inspheric parameters, being cherent with the scillatins characterized in the plasma frequency variatins The surcerigin f the event was lcated arund the transitin regin between F and F2 layers and frm that altitude the wave prpagates vertically drawing energy upwards and dwnwards simultaneusly The decreasing/increasing slar inizing radiatin and the cling/heating prcesses linked with the slar eclipse are discussed as a pssible mechanism t explain the gravity wave event bserved in the electrn density and dynamics f the insphere 1 Intrductin A slar eclipse is a unique pprtunity t study the transitry changes f the insphere caused by the decreasing and increasing f the slar inizing radiatin Due t the accurate predictability f eclipses, the inspheric statins may prepare inspheric campaigns f rapid sequence sunding and study the surcerespnse relatinships between prductin and lss mechanisms, and dynamics Many studies deal with the respnse f the insphere t slar eclipses based in bth theretical and experimental results In such studies the changes prduced by eclipses in the electrn and in temperatures and densities, F regin diffusin, etc, have been evaluated as well as the time cnstants [eg, Barn and Hunsuker, 1973; MacPhersn et al, 2000, and references therein] The effects n the thermsphere and insphere f the slar eclipse f August 11, 1999, have been mdeled by MiillerWdarg et al [1998] They prpse that temperature disturbances will be generated in situ in the thermsphere and electrn density changes may be generated in situ in the insphere and that such disturbances will prpagate as a wave frm the path f ttality int the equatrial regins T sme extent the preliminary results fund by Altadill et al [1999] cnfirm the abve predictin They fund electrn density depletin in the bttm side f the F regin during Cpyright 2001 by the American Gephysical Unin Paper number 2001JA /01/2001 JA that eclipse In additin they fund a train f scillatins f the electrn density after the recvery phase f the eclipse Mrever, there are several studies shwing the arising f atmspheric gravity waves (AGW) in the insphere as a cnsequence f slar eclipses During a slar eclipse the Mn' s umbra prjected ver the Earth's atmsphere acts as a cling surce that prpagates at supersnic speed (greater than Mach 2) It can cntribute t a gravity wave field and build up a wave frnt that may prduce inspheric disturbances [Chimnas and Hines, 1970] Fr instance, Cheng et al [1992] shw evidences f AGW in the insphere after the slar eclipse f September 23, 1987 Fritts and Lu [ 1993] mdeled the gravity wave field induced by the cling f the zne layer in respnse t the slar eclipse n July 11, 1991 They prpse the generatin f a wave frnt structure in the gravity wave respnse, with dminant perids between 2 and 4 hurs, that can prpagate upwards int the thermsphere Liu et al [1998] fund internal gravity waves vertically prpagating in the insphere during slar eclipse f Octber 24, 1995 They fund that the gravity wave surce was lcated at the height range f the inspheric F ledge Hwever, the AGW in the insphere are mainly driven by heating prduced by electric currents, energetic particle precipitatin and mechanical frces in the aurral insphere It is als pssible that the surces f AGW cme frm lwerand middleatmsphere assciated with regins f turbulence and wind shear [Hunsucker, 1987; Hcke and Schlegel, 1996] Mrever, there is a variety f studies that shw the mving 21,419

2 21,420 ALTADILL ET AL' IONOSPHERIC EFFECTS OF SOLAR ECLIPSE 1999 Slar Terminatr as a surce f AGWs [Galushk et al, 1998; Smsikv, 1995] These afrementined surces must sunding campaign at a time sampling rate f 5 min frm August 10 t 12, 1999 We use als the same type f data be taken int accunt in rder t distinguish the mst likely recrded at Arensill statin (373 ø N, 69øW) The surce f AGW in the insphere during a slar eclipse The purpse f this investigatin is t study the gravity wave like scillatin bserved in the electrn density variatins during ingram's traces have carefully revised in rder t avid any mistake f the autmatic scaling t btain true height plasma frequency prfiles by using the ingram inversin algrithm the slar eclipse f August 11, 1999, in rder t distinguish described by Huang and Reinisch [1996] Nte that the their pssible surce f rigin plasma frequency prfiles are equivalent t the electrn density prfiles by the wellknwn relatin 2 Data, Analysis, and Methdlgy In rder t carry ut this study, we use the data f the Ebre statin (408 ø N, 05 ø E) that made a vertical inspheric >, 7: 0 5: I J ' 4' 12 87,,, t,/ "",,,,;" 53/"',v' /"':'" ;l',:,r km 200 km "' x' /" "/" 'x Time UT 9 Kp=4 13 Kp=3 Figure 1 Plasma frequency at fixed heights btained at Ebre frm August 9 t 13, 1999, fr the time interval frm 0600 t 1800 UT The largest 3huly Kp index f each day is indicated \ N(h)= 124xlO1ø fp2(h), where the electrn density [N] = m 3 and the plasma frequency [fp] = MHz The prfiles have been btained frm the base f the E layer t hmf2, which apprximately cvers the altitude range km Nevertheless, we restricted ur study t the altitude range km in rder t avid the EF valley regin The prfiles were calculated with an altitude step f 25 km and frm them we btain tw types f time series, the plasma frequency at fixed heights and the reflectin altitude at fixed frequencies f sunding The time series f plasma frequency at fixed heights btained at Ebre during the inspheric campaign are presented in Figure 1 fr the time interval frm 0600 t 1800 UT We used August 10 as cntrl day The time series f plasma frequency at fixed altitudes enable us t study the electrn density variatins as a functin f the time and altitude The time series f altitude at fixed frequencies enable us, t sme extent, t study the inspheric dynamics as a functin f time and altitude Frm the prfiles we can extract the critical frequency as well as the altitude f the electrn density peak f each inspheric layer, fe, ff and ff2, and hme, hmf, and hmf2 respectively We use statistical methds, based n high spectral reslutin harmnic analysis (see Vitinsky et al [1986] fr details) in rder t btain the spectral characteristics f the electrn density variatins in the altitude range cvered by the data Mrever, we used cmplex demdulatin [Blmfield, 1976] t lk fr the time life and develpment f the scillatins The present wrk is divided int three main parts In the first part we discuss abut the distinct inspheric behavir during the eclipse day In the secnd part we characterize a gravity wave like scillatin in the insphere that mainly ccurs after the recvery phase f the slar eclipse, we evaluate the vertical prpagating structure f this event, and we lcate the altitude f their surce rigin Finally, we discuss abut the mst likely surcerigin f this event in the insphere 3 Distinct Inspheric Behavir During Eclipse Day The slar eclipse f August 11, 1999, as seen frm Ebre at grund level started at 0858 UT, it reached its maximum magnitude (= 075) at 1018 UT, and it ended at 1143 UT Figure 2 shws the gemetry f the eclipse regin at grund level ver west and central Eurpe Frm Figure 1 ne bserves clear differences in the behavir f the plasma frequency between the eclipse day and thers We bserve in the eclipse day as a main trend that the plasma frequencies (r electrn densities) decrease during the beginning phase f the eclipse (frm 0900 t 1030 UT apprximately) at all altitudes (t)

3 ALTADILL ET AL: IONOSPHERIC EFFECTS OF SOLAR ECLIPSE ,421 10E 9O % 1050 h,,f2 [MacPhersn et al, 2000] The mdeling study f MiillerWdarg et al [1998] predicted a small enhancement f the N,,F2 as a result f the reductin in the flux f heating and inizing radiatin fr the middle latitude eclipse f August 11, 1999 They assume that strnger effects in the upper atmsphere shuld be riginated frm utside the thermsphereinsphere system and nt in situ Fritts and Lu [ 1993] prpse that the disturbance linked with the zne cling during an eclipse can prpagate upward reaching the thermsphereinsphere system and can cause significant changes in its dynamics Prbably the latest fact explains the large depletin f the electrn density bserved in the F2 layer, a larger effect than that predicted by MiillerWdarg et al [ 1998] 9O % 40N 80 % Figure 2 Path f the slar eclipse f August 11, 1999, at grund,, ;;i ff2 Later, during the recvery phase f the eclipse (frm 1030 t 1200 UT), the plasma frequency increases at all altitudes In additin, a distinct train f scillatins f the plasma frequency is clearly bserved after the recvery phase f the eclipse, especially at higher heights These tw distinct facts are nt bserved ther days, and mst likely they are eclipserelated effects Figure 3 displays a mre detailed infrmatin f the eclipse day than Figure 1 This figure shws the plasma frequency at fixed heights in a brad altitude range, and the reflectin altitude at fixed frequencies f sunding The afrementined main trend and the train f scillatins f the plasma frequency are better seen here Mrever, we bserve that the altitudes at fixed frequencies increase during the beginning phase f the eclipse, and they decrease during the recvery phase A train f scillatins f the altitudes after the recvery phase is als bserved The bserved main trend was expected in the inspheric E and F layers ( km) where phtchemical prcesses and slar cntrl are dminant During the ccultatin f the slar disc by the Mn the inizing radiatin decreases, then the lss mechanisms prvide a decrease f the electrn density and the altitude f reflectin fr a fixed frequency shuld be higher The ppsite happens when the inizing radiatin increases during the deccultatin f the slar disc The expected behavir f the F2 layer during the eclipse is mre cmplicated because dynamics play an imprtant rle The wind pattern, the slar heat input and time cnstants, the neutral cmpsitin and temperature, the diffusin and dwnwelling, etc, must be taken int accunt in rder t explain the eclipse effects in the electrn density Then the effect f the eclipses n the behavir f the F2 layer can be different frm ne eclipse t anther In sme cases it is bserved neither net change f the electrn density at the peak f the F2 layer nr significant changes f the altitude f the F layer maximum [Barn and Hunsuker, 1973] Other cases shw a clear depletin in N,,F2 jintly with a rising f the ' 3 20 ;160 h4 2 ' T I i, 74 q '?" :C, ::: 175 ' 150 r ltr Tr I'*' ' I Figure 3 (tp) Plasma frequency at indicated true altitude and j F2 and (bttm) true altitude f reflectin eches at indicated sunding frequencies The arrws shw the crests f the assumed inspheric disturbance

4 21,422 ALTADILL ET AL: IONOSPHERIC EFFECTS OF SOLAR ECLIPSE Characterizatin f a Gravity Wave Like Oscillatin in the Insphere 8 As already mentined, the main purpse f this 75 investigatin is t study the gravity wave like scillatin f the electrn density bserved in the inspheric F regin 70 Besides the afrementined main trend, we als bserve in Figure 3 several scillatins in bth time series that apprximately begun arund 1100 and ended at 1700 UT It 55 can be seen near six maxima recurrences during a time interval f 6 hurs that leads a perid arund 1 hur Fllwing a similar methdlgy as that described by Liu 45 et al [1998], we investigate the timealtitude dependence f 42 these scillatins f the electrn density First f all we lk fr intervals where there is a large activity f scillatin Then we assume that the scillatin activity is characterized by a starting and ending time (ti and t/) and that the scillatin 90 activity behaves as a wave packet Finally, we cnsider that 8s such wave packet is frmed by the statistically significant harmnics in a peridic range between the perids and rf We fixed as criterin t cnsider statistically significant thse 7s: harmnics that have a prbability f existence larger than ' 70: 95% in the cnsidered interval f time Applying the mentined methdlgy, an enhancement f ' 60 the scillatin activity is fund between ti = 1100 t tf = 1700 UT Figure 4 shws the dynamic spectra f the significant ss perids fr three series f data: ff2, plasma frequency at 220 km and altitude f reflectin at a sunding frequency f 60 a $0 MHz The latest tw series are chsen as examples f altitude 40 and frequency where the scillatin activity is clearly seen : The significant perids have been calculated with a spectral time windw with length f 6 hurs and shifted by 15 min frm ne t anther In this figure it is easy t identify a 90 cincident enhancement f the scillatin with a dminant perid near 60 min and centered at 1400 UT By taking int 8sS accunt the time windw length, this means that the 80 scillatin remains frm 1100 t 1700 UT apprximately Anther interesting fact is that befre r during the eclipse 70 there is n ther significant spectral peak that cincides the three series and that reaches a prbability f existence Althugh it is nt shwn here, we fund that such statistically significant scillatins f near 60 min perid exist f 95% 6s 0 in the time series f plasma frequency at fixed altitudes in the saltitude range frm 150 t 260 km during the same time 45 interval as nted abve Mrever, this scillatin activity exists in the time series f altitude f reflectin at fixed frequency f sunding fr the frequency range frm 45 t 75 Altitude at 60 MHz MHz The results shw als that the significant perids abve the 95% f prbability f existence that frm this scillatin activity are thse perids frm Ti = 55 t T/= 61 min and that the dminant perid f scillatin is T = 575 min Figure 4 Dynamic spectra f the significant perids fr the By taking int accunt the abve infrmatin, we applied indicated time series cmplex demdulatin t the time series in rder t lk fr the time life and time develpment f such scillatin activity This methd is a lcal time variant f spectral analysis fr variatin f the perid, amplitude, and phase f the scillatin better estimatin f the details f the tempral variatin f the at 575 min fr the plasma frequency at 220 km during the perid, amplitude, and phase f any scillatin In this whle eclipse day Frm this figure it is clear that this prcedure it is necessary t use a lwpass filter that must scillatin is clearly develped during the time interval frm separate the smth cmplex sinusid frm the unsmthed 1130 t 1630 where the phase is very stable, except a small nise cmpnent and frm the perturbed sinusid f the phase jump at abut 1500 During this time interval the perid secnd harmnic f the perid f demdulatin (see changes frm 62 t 52 min, with a small jump cntemprary Blmfield [1976] fr details) We used a demdulatin with the ne seen in the phase The amplitude f the perid f 575 min and a lwpass filter with stp perid f 95 scillatin peaks at abut 1300, 1 hur after the end f the min and pass perid f 110 min Figure 5 shws the tempral eclipse apprximately Afterwards, the amplitude decreases t 90, 85E 35 ff2 30 illillililtll/11illllllllilllii li[1111li;1111illllllllllifli/i I Plasma frequency at 220 km v

5 ALTADILL ET AL: IONOSPHERIC EFFECTS OF SOLAR ECLIPSE , ' 8 crrespnding t the significant perids T in the peridic range [Ti, Tf] Because we are interested in the variatins abut a zer mean, (2) may be extended t the entire altitude range 6 40 Ah(t,h) = E Cj(h)cs[wjt (h)]; T, rs Afp(t,h) = E Fj(h)cs[rjt (3) E01 20 where C/(h) = (A/2+Bj2) 'a and F;(h) = (D;2+E;2) ' are the amplitudes f the altitude and plasma frequency at perids T as functin f the altitude, respectively, and %(h) = arctg(b/a;) and q ';(h) = arctg(d/fi) are the phases f the altitude and plasma frequency at perids T as a functin f the altitude, respectively Nte that the mean reflectin altitude H at a fixed plasma frequency in (2) has been taken as the altitude f reference h fr the time series f altitude variatins Ah(t, h) in (3) Fllwing (3), we can study the timealtitude dependence f the scillatin activity arund the dminant perid T = 575 min fr bth time series Ah(t, h) and Afp(t, h), hereinafter altitude and plasma frequency, respectively Using the same spectral time windw as in Figure 4, we btain the lcal i"': 020 \ t Figure 5 Time develpment during the eclipse day f the (tp) perid, (middle) amplitude, and (bttm) phase fr the time series f the plasma frequency at 220 km after cmplex demdulatin with a demdulating perid f 575 min The shaded time interval represents apprximately the time life f the scillatin : 210,' E ,,, zer at 1630 and the scillatin dies cntemprary with the phase catastrphe Once the existence f such scillatin activity has been deduced, we determine the altitude range in which it is visible Fr this purpse we cnsider that the series f altitude reflectin (h) at a fixed plasma frequency, and the plasma frequency (fp) at a fixed height, may be expressed as rs h(t) = H + E(Ajcsrjt + Bjsinrjt); Ti fp(t) = F+ E(Djcswjt + Ejsinwjt), (2) Ti O 200 respectively, where H and F are the mean altitude f Figure 6 Tpgraphic maps f the amplitude f the reflectin at a plasma frequency and the mean plasma scillatin as a functin f time and altitude (tp) f the frequency at a fixed altitude, respectively, bth fr the time plasma frequency variatin and (bttm) f the altitude interval [ti, t/], c0j = 2 z/t/ are the angular frequencies variatin ' : "': ' ; 300 r l :' ' : r :[ :r x

6 21,424 ALTADILL ET AL: IONOSPHERIC EFFECTS OF SOLAR ECLIPSE 1999 maximum f the amplitude spectra in the peridic range frm 55 t 61 min fr bth series, altitude, and plasma frequency Nte that we cnsider nly thse amplitudes f the harmnics having a prbability f existence larger than 95%, the ther are cnsidered as zer amplitude The results are depicted in Figure 6 Frm this figure it is clear that the event is bserved in the entire altitude range frm 160 t 260 km fr bth time series during the time interval centered at 1400 UT A secnd fact is that the maximum amplitude f scillatin is lcated near 220 km, with amplitudes f 02 MHz fr the plasma frequency and 45 km fr the altitude These results are cnsistent with thse presented in Figure 4, and frm them we can cnclude that an scillatin activity with characteristics f a wave packet is present and statistically significant in the entire altitude range under study It starts at 1100 and ends at 1700 UT apprximately, and the wave packet is frmed by the harmnics frm 55 t 61 min In rder t characterize the vertical structure f the wave packet in bth time series, hereinafter we cnsider nly the time interval, perid range, and altitude range as mentined km >' 1,68 ' '0 ) ' km ", 195 km km 168,,, I ' I I I I I Wave number (1 0 3 m '1) Figure 8 Dependence f the angular frequencies crrespnding t the harmnics that frm the wave packet as functin f the wave number at indicated altitudes: (tp) btained frm the plasma frequency time series and (bttm) btained frm the altitude time series abve Assuming that the harmnic T is present and physically cnsistent in the entire altitude range, then the harmnic analysishuld prvide smth functins %(h) and (p'j(h) in (3) After that it is easy t calculate the vertical wave numbers f the harmnics T (n;, n' ), their vertical phase velcities (v%, v'%), and the vertical grup velcity f the wave packet (vg, v'g): Altitude (kin) Figure 7 Relative phase f the harmnics that frm the wave packet The perids are dwnwards srted frm 55 t 60 min The plt at the tp is fr the plasma frequency series and that at the bttm fr the altitude series Oh ' vq)i = nj nj,vg= ;, O(pj, v'g:, (4) On' With these magnitudes we will be able t study the vertical prpagatin f the wave packet and t distinguish the altitude f its surce rigin The relative phases t the starting time f the cnsidered interval fr the harmnics frm Ti = 55 up t T/ = 60, with a AT f 05 min as a functin f the altitude, are

7 , ALTADILL ET AL: IONOSPHERIC EFFECTS OF SOLAR ECLIPSE ,425 shwn in Figure 7 fr bth series The dted line represents packet as functin f the altitude and btained frm the tw the phase f the dminant harmnic T = 575 min Frm this figure it is easy t see that the functins % (h) and q0' j (h) are smth and (4) is applicable Mrever, we bserve in bth time series, plasma frequency and altitude, are presented in Figure 9 This figure shws that the phase velcities are negative (dwnward phase prgressin) in the altitude range plts f Figure 7 an upward phase prgressin t¾m 150 t frm 185 t 250 km apprximately, and psitive (upward 185 km and a dwnward phase prgressin frm 190 t 250 phase prgressin) between 180 and 150 km On the ther km Frm these results ne can expect that the phase hand, the grup velcities are psitive (upward energy velcities are upward in the lwer altitude range and prpagatin) in the altitude range frm t 250 km dwnward in the upper range Figure 8 depicts the frequency apprximately, and negative (dwnward energy prpagatin) f the harmnics that accunt fr the wave packet versus the between and 150 km These results indicate that the wave number fr bth series and at discrete altitudes Nte that the slpe f the dminant trend f the frequency as functin f the wave number can be linked t the grup altitude phase prgressin is ppsite t the energy prpagatin f the wave packet This fact is expected fr the atmspheric gravity waves, and we can attribute the wave velcity f the wave packet at indicated altitude, being packet studied here t a gravity wave (GW) event Mrever, upward energy prpagatin fr psitive values f the grup due t the different directin f energy prpagatin and f velcity and dwnward fr negative Frm Figure 8 ne can phase prgressin, we can assume that the surcerigin f such GW event is lcated smewhere between 180 and 215 expecthat the energy f the wave packet prpagate upwards in the higher altitudes and dwnwards in the lwer nes km, and frm these altitudes it draws energy upwards and The values f the phase velcity fr the dminant perid T = 575 min and the values f the grup velcity f the wave dwnwards simultaneusly The phase velcity deduced frm the tw cnsidered series displays a similar behavir, but the values btained frm the altitude at a fixed plasma frequency 260: are greater than thse btained frm the ther series Cnsidering the grup velcities, it is als visible that bth series behave in a similar way, but with sme small greater in the vicinity f the surce than the same velcity differences The grup velcity deduced frm the plasma frequency at fixed heights reaches a value abut 2 times deduced frm the altitude at fixed frequency Bth values "' 210 (D,, 200 : i O : 25O ' D V e Icity (m D D I'1 D Velcity (m s ' ) Figure 9 (tp) Altitude dependence f the vertical phase velcity fr the dminant harmnic and f the grup velcity f the wave packet as btained frm the plasma frequency time series (bttm) As abve, but frm the altitude time series D decrease t negative values near zer when decreasing altitude and in the higher altitudes bth series tend t stabilize arund 10 m s ' These results agree with the preliminary results fund by Altadill et al [1999], and similar vertical prpagating events have been fund by Liu et al [1998] during the slar eclipse f Octber 24, 1995 The altitude variatin f the amplitude f the dminant perid T = 575 min fr bth series f data and the averaged plasma frequency prfile fr the cnsidered time interval are presented in Figure 10 We see in this figure that the tw series have the maximum amplitude at the same height (220 km), and we bserve that the altitude f the peak f the inspheric F layer is lcated at arund km apprximately, being expected the transitin regin between F and F2 layers at arund km Frm the results presented here we can cnclude that an internal GW event with a dminant perid f scillatin T = 575 min was riginated in the insphere after the slar eclipse maximum and that it was visible during 6 hurs apprximately Its surcerigin was lcated at arund 200 km altitude and frm that altitude the GW event prpagates vertically and it draws energy upwards and dwnwards simultaneusly The altitude f its surcerigin takes place in the transitin regin between the F and F2 layers, just abve the altitude f the peak f the F, whse amplitude was maximum The typical inspheric parameters that are clser in altitude t the transitin regin between the F and F2 layers are the critical frequency ff and the altitude f the F peak, hmf Because f that we have analyzed these parameters in rder t see if such an event is als bserved Figure 11 displays the tempral variatins f the afrementined inspheric parameters as well as their amplitude spectra btained during the time interval when the GW event is cnsidered t exist Frm Figure 11 we bserve similar

8 21,426 ALTADILL ET AL: IONOSPHERIC EFFECTS OF SOLAR ECLIPSE OO 400' ' < 200 i ' 260 t"l'" I t'" I' 't'' "" '' ' "1 ' '' "1 " Sunding frequency (MHz) 1 250'' ' 190' 18, , Amplitude (km) I i i I I I I I! I I I I i l 150, ' [ I ' ' I ' i,, I Amplitude (MHz) Figure 10 (tp) The averaged representative plasma frequency prfile during the time interval frm 1100 t 1700 UT, and its crrespnding ingram trace The shaded area refers t the regin in which the GW like scillatin is bserved (bttm) The altitude variatin f the amplitude f the tw studied series fr the harmnic f 575 min The slid line represents the amplitude frm the plasma frequency (bttm x axis) and the dashed line is fr the altitude time series (tp x axis) 22 f a sharp peak near the 44 min perid in bth inspheric parameters We have nt studied such a peak because it is nt bserved in the entire altitude range frm 150 t 260 as the internal GW characterized here Hwever, a similar activity f scillatin has been studie during the same time interval fr a larger Eurpean regin [Altadill et al, 2001; Farges et al, 2001] Althugh nt shwn here, we made the same type f analysis with data frm E1 Arensill statin (373 ø N, 69 ø W) There the eclipse started at 0848 UT, reached its maximu magnitude (= 063) at 1002 UT, and ended at 1123 UT We fund a pattern f the plasma frequency variatin very similar t that bserved in Ebre data, with a depletin f the plasma frequencies at all altitudes during the beginning phase f the eclipse, fllwed by a rising f the plasma frequencies during the recvery phase f the eclipse In 6 55 N " E E 170 e llllllllllllll,11111lllllllllllllllll N "r' (D tempral variatins as depicted in Figures 1 and 3 The ff decreases during the time f slar ccultatin but returns t a nrmal value very quickly On the ther hand, hmf reaches its lcal maximum at the last part f the eclipse, but it des nt display a clear rising as bserved in the altitude time series Several scillatins are als visible in the tw raw data (f0f and hmf ) after the recvery phase f the eclipse, prbably related with the internal GW event Mrever, we bserve that the amplitude spectra f bth parameters display the largest scillatin activity fr a perid T = min These perids are very clse t but a bit greater than the dminant perid f the characterized internal GW The small differences may be linked t the fact that the plasma frequency and altitude variatins studied befre are referenced t a fixed altitude, and the ff and hmf really change their altitude in time Frm these results we can cnclude that the internal GW event is clearly bserved in the F and hmf as well Anther interesting fact seen in Figure 11 is the presence 005 E 0 ' 4 E 1 <r; lilllllllllllllllllllllllllllllllllllllll 40 4s s ss s 7?s Perid (minute) 0 llllll:ll 1111,lll llll:lllllllilllllllll P e ri d (m in u t e ) Figure 11 Frm the tp t the bttm, tempral variatins f ] Fl, hmfi, amplitude spectra f f0f2 and amplitude spectra f hmf (see text fr details)

9 ALTADILL ET AL' IONOSPHERIC EFFECTS OF SOLAR ECLIPSE ,427 additin, we fund an scillatin activity with a dminant are qualitatively similar t the GW event characterized by Liu perid T = 63 min, but it nly was physically cnsistent in the altitude range frm 190 t 230 km There was als fund a et al [1998] during the slar eclipse f Octber 24, 1995 They prpsed that the changes f the phtinizatin and vertical energy prpagatin f this event in E1 Arensill, dynamical prcesses f the insphere during the slar eclipse being upwards in the afrementined altitude range culd generate vertical prpagating scillatins as thse Mrever, we fund several intervals f scillatin activity bserved here The cling/heating prcesses linked with the with different perids during the whle inspheric sunding decreasing/increasing slar inizing radiatin during the slar campaign carded ut at Ebre statin Hwever, they have eclipse lead t a reductin/augment f the scale height fr neither a clear vertical structure nr lng time duratin either bth plasma and neutrals This fact causes a as the internal GW event characterized here These events happen mainly after sunrise and sunset, and they culd be linked with the slar terminatr migratin dwnward/upward mtin f plasma and neutrals as well as f F prductin peak and f the transitin regin between F and F2 layers, and this acts as a surce f disturbance in the dynamics f that regin f the insphere In additin t that 5 Summary and Cncluding Remarks the decreasing/increasing f the phtinizatin during the slar eclipse prvides a depletin/enhancement f the electrn We have studied the dependence in altitude and time f the inspheric electrn density, and their altitude variatins, during the slar eclipse f August 11, 1999, as bserved frm density at the E and F layers As a result f the abve discussin, we think that the rapid changes in the dynamical and phtchemical regime f the F the true height electrn density prfiles recrded at the Ebre and F2 regin, as a cnsequence f the eclipse, can generate statin We fund tw distinct facts during the eclipse day turbulence in that regin, being the transitin between Ft and cmpared with the pattern f the cntrl days: a large F2 layers the mst sensitive Then this turbulence can break depletin f the F regin electrn density, and a train f ut after the recvery phase f the eclipse and amplifies at scillatins f the electrn density after the recvery phase perids f the GW like scillatin we bserved These facts As already said, MiillerWdarg et al [1998] predicted a culd explain why the amplitude is largest in the transitin small enhancement f the electrn density at the peak f the between Fi and F2 as well as why their surcerigin is F2 layer as a result f the eclipse They assumed that strnger lcated there and draws energy upwards and dwnwards effects in the upper atmsphere shuld be riginated frm simultaneusly Thus we have a mechanism that explains the utside, particularly by the cupling with lwer atmspheric internal GW event in the electrn density and in the dynamics layers They put this assumptin int the cntext f the study f the insphere by Fritts and Lu [1993], wh suggest that the disturbances linked with the zne cling during an eclipse can prpagate Acknwledgments The authrs wish t thank Glria Mir6 frm upward reaching the thermsphereinsphere system and can E1 Arensill, INTA, fr scaling and prviding the electrn density cause significant changes in their dynamics The recrded ff2 prfiles f Arensill statin Part f this wrk has been supprted by data d nt shw any enhancement during the eclipse but Spanish prject BTE and by internatinal cperatin depletin larger than 1 MHz (Figure 3) Then, as per Miiller prject between Spain and Bulgaria 2001BG0012 Michel Blanc thanks Rbert Hunsucker and Alan Aylward fr Wdarg et al [ 1998], this effect culd be due t the cupling their assistence in evaluating this paper with lwer atmspheric layers In additin, we fund a GW like scillatin in the insphere bserved after the maximum f slar ccultatin References Such an event is clearly seen in the plasma frequency and in Altadill, D, J G S16, and E M Apstlv, Gravity wave type the altitude variatins f the F regin bth in the altitude disturbances bserve during slar eclipse f 11 August 1999, range frm 150 t 250 km It has a dminant scillating Bulgarian Gephys J, 25(14), 4350, 1999 Altadill, D, F Gauthier, P Vila, J G S16, G Mir6, and R perid T = 575 min and its amplitude is maximum at 220 km Berranger, The slar eclipse and the insphere: A This scillatin activity reaches at least the altitude f the search fr the distant bwwave, J Atms Sl Terr Phys, 63 electrn density maximum The surcerigin f this event is (9), , 2001 lcated in the transitin regin between the F and F2 layers, Barn, M J, and R D Hunsuker, Incherent scatter radar at arund km altitude Frm this altitude the wave bservatins f the aurral zne insphere during ttal slar eclipse f July 10, 1972, J Gephys Res, 78, , 1973 prpagates vertically drawing energy upwards and Blmfield, P, Cmplex demdulatin, in Furier Analysis f Time dwnwardsimultaneusly Series: An Intrductin, pp , Jhn Wiley, New Yrk, The day f the eclipse was magnetically quiet (Kp=2+), the 1976 GW event studied here is bserved after the slar eclipse Cheng, K, Y N Huang, and S W Chen, Inspheric effects f the maximum and n similar scillatin behavir was fund t slar eclipse f September 23, 1987, arund the equatrial crest regin, J Gephys Res, 97, , 1992 exist during the insphericampaign carried ut As a Chimnas, G, and C O Hines, Atmspheric gravity waves induced cnsequence, we assume the slar eclipse as the main surce by a slar eclipse, J Gephys Res,75, , 1970 f the event being less likely their rigin frm aurral surces Farges, T, J C Jdgne, R Bamfrd, Y Le Rux, F Gauthier, P r frm the slar terminatr Vila, D Altadill, J G S16, and G Mir6, The slar eclipse and the insphere: Eclipse disturbance and quietperid The mdel evaluatins f the GW field generated by slar gravity waves, J Atms Sl Terr Phys, 63 (9), , 2001 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