Completo_DEF.qxd 1311 ANNALS OF GEOPHYSICS, VOL. 47, N. 4, August 2004 1311 Key words ionosphere – F2-layer critical frequen- cy – ionospheric disturbances 1. Introduction Ionospheric disturbances, being manifesta- tions of extreme space weather, may severely degrade terrestrial and earth-to-satellite techno- logical systems. Consequently, long-term pre- dictions and short-term forecasting of such phe- nomena are essential. Recent investigations, mainly by global-scale numerical simulations of first-principle models (Schunk, 1996) and by measurement studies (Yeh et al., 1994), have improved our understanding of the thermos- phere-ionosphere system. However, the agree- ment between simulations and observations is still rather qualitative, whereas a quantitative modeling for immediate space weather applica- tion has not been achieved yet (Szuszczewicz et al., 1998; Fuller-Rowell et al., 2000). One way to establish accurate specification of the temporal and spatial development of a ionospheric storm is to fully understand distur- bance mechanisms and insert suitable input to the global simulation models. Although research has grown rapidly in this direction (Prölss, 1993, 1995; Buonsanto, 1999; Mikhailov, 2000), cause-effect relationships are hard to establish in many cases. Alternatively, ionospheric empirical storm-time models in disturbed magnetic condi- tions have been developed (e.g., Cander and Mi- hajlovic, 1998), improving thus existing ionos- pheric empirical models (Araujo-Pradere et al., 2002, 2003). Such an advance deals with one major cause of ionospheric storms, hence not with the only one, since ionospheric storms can be regional and are not directly linked to geo- magnetic activity (Wilkinson, 1995). Another potential way of storm modeling is first to produce a detailed climatology and mor- phology of ionospheric storm independently of the cause-effect mechanisms and then correlate those morphologies presenting important fre- quency of occurrence in time and space, with certain physical parameters so as to improve long-term predictions. It is in this latter direction that this work contributes, providing radio users and researchers with an analytical climatology Mailing address: Dr. Dimitris N. Fotiadis, National Te- lecommunications and Post Commission (EETT), 60 Kifi- sias Ave., 15125 Marousi, Greece; e-mail: dfot@eett.gr Climatology of ionospheric F-region disturbances Dimitris N. Fotiadis (1), Stamatis S. Kouris (1), Vincenzo Romano (2) and Bruno Zolesi (2) (1) Electrical and Computer Engineering Department, Aristotle University of Thessaloniki, Greece (2) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy Abstract After more than 60 years of research, ionospheric disturbances are today a most challenging topic of upper at- mospheric physics. Although the understanding of the thermosphere-ionosphere system has increased, quantita- tive predictions of ionospheric perturbations, valid for space weather assessment, are still imprecise. Using a long foF2 dataset, an analytical climatology of the F-region storms is presented as a function of appropriate vari- ables. Local phenomena are then detected. 1312 Dimitris N. Fotiadis, Stamatis S. Kouris, Vincenzo Romano and Bruno Zolesi which is expressed with caution to appropriate variables and prominent features of ionospheric storms, also emphasizing regional phenomena. 2. Definitions and method of analysis Distinct from any geophysical definition of storminess, Kouris et al. (1998, 1999) have de- veloped a ionospheric definition of disturbed days and periods for the day-to-day variability of the F2-layer critical frequency (dfoF2). This definition of disturbances is based solely on their power, amplitude and duration, being con- sistent with suggestions of other workers (Gulyaeva, 1996). Initially, day-to-day variability of foF2 was calculated with the transformation: dfoF2=(foF2+ − foF2median)/foF2median, for each hour, day, month, year, station of table I. Then, the algorithm for disturbed periods (Kouris et al., 1999) was ap- plied. Thus, a catalogue of positive and nega- tive F-region disturbances was compiled. In order to sufficiently illustrate ionospheric disturbances’ climatology in time and space and to avoid grouping storms of different mecha- nisms and morphological aspects, the climatol- ogy of disturbances is presented according to: i) Their phase, positive or negative, since it is attributed to different disturbance mecha- nisms (e.g., Prölss, 1995). ii) Their season, since it is long supported that seasonal variations are prominent features of ionospheric storms and directly linked to their phase. In this work each month has been dealt with separately. Table I. List of stations and years of foF2 data used in the analysis. The geomagnetic latitude is calculated by the International Geomagnetic Reference Field for the year 1986 for a 300 km height. Dip inclination angle and Modip (Rawer, 1963) are calculated for 1980. Station Geographic IGRF Corrected Station Code Lat. (°) Long. (°) Geomagnetic Lat. (°) Dip (°) Modip (°) Years of data Nicosia NIC 35.1 33.2 28.5 50.8 44.4 87-96 El Arenosillo ARE 37.1 353.2 30.5 52.2 45.6 82, 83, 93-96 Gibilmana GIB 37.6 14 30.4 52.8 46.0 76-94 Tortosa TOR 40.8 0.5 34.8 56.4 48.5 64-87, 91-95 Rome ROM 41.8 12.5 35.7 57.6 49.4 76-94, 96 Grocka GRO 44.8 20.5 39.4 61.1 51.7 85-93 Poitiers POI 46.6 0.3 42.3 62.3 52.7 64-94 Freiburg FRE 48.1 7.6 43.6 63.7 53.7 64-74 Lannion LAN 48.8 356.6 45.4 64.3 54.1 64-94 Kiev KIE 50.5 30.5 45.8 66.5 55.5 64-92 Slough SLO 51.5 359.4 48.4 66.5 55.8 67-95 Juliusruh JUL 54.6 13.4 50.8 68.9 57.7 64-95 Moscow MOS 55.5 37.3 51 70.4 58.5 64-96 Uppsala UPP 59.8 17.6 56.3 72.4 60.7 64-96 Leningrad LEN 60 30.7 55.9 72.9 60.9 64-96 Arkhangelsk ARK 64.4 40.5 60 75.8 63.6 69-93 Lycksele LYC 64.7 18.8 61.3 75.2 63.5 64-96 Sodankyla SOD 67.4 26.6 63.6 76.8 65.2 64-89 Kiruna KIR 67.8 20.4 64.4 76.8 65.4 64-86, 91-96 Loparskaya LOP 68 33 63.9 77.3 65.6 64-77, 81-84, 91-96 Dakar DAK 14.8 341.6 2.9 16.1 16.0 64-89 Johannesburg JOH – 26.1 28.1 – 35.6 – 62.0 – 48.8 64, 67-91 Grahamstown GRW – 33.3 26.5 – 41.2 – 64.2 – 50.8 74-76, 78-84, 86-96 Syowa Base SYO – 69 39.6 – 66.4 – 66.7 – 62.8 66, 68-79 1313 Climatology of ionospheric F-region disturbances Table I (continued). Singapore SIN 1.3 103.8 – 7.7 – 17.6 – 17.1 64-71 Kodaikanal KOD 10.2 77.5 1.6 3.7 3.8 64-80, 85, 86 Manila MAN 14.6 121.1 6.2 14.2 14.1 64-94 Taipei TAI 25 121.5 17.5 34.9 32.6 64-96 Okinawa OKI 26.3 127.8 18.8 36.6 34.0 64-88 Yamagawa YAM 31.2 130.6 23.8 43.9 39.7 64-88 Kokubunji KOK 35.7 139.5 28.2 48.8 43.4 64-90 Ashkhabad ASH 37.9 58.3 32.6 55.8 47.6 64-86, 89-95 Akita AKI 39.7 140.1 32.3 53.5 46.8 64-89 Wakkanai WAK 45.4 141.7 38.1 59.4 51.1 64-88 Karaganda KAR 49.8 73.1 44.8 68.1 56.0 64-88 Irkutsk IRK 52.5 104 46.8 71.0 57.8 64-90 Tomsk TOM 56.5 84.9 51.4 74.1 60.1 64-90, 92-96 Magadan MAG 60 151 53.2 71.2 60.4 68-96 Yakutsk YAK 62 129.6 55.5 75.6 62.6 64-91 Provideniya Bay PRO 64.4 186.6 60.3 73.9 63.0 64-70, 79-83 Norilsk NRI 69.4 88.1 63.8 82.0 67.5 68-88 Tiksibay TIK 71.6 128.9 65.2 82.0 68.6 64-71, 87 Dikson DIK 73.5 80.4 67.7 83.3 69.9 82-96 Vanimo VAN – 2.7 141.3 – 11.4 – 22.5 – 21.5 64, 65, 67-93 Darwin DAR – 12.4 130.9 – 22.2 – 40.8 – 35.8 83-88, 90-94 Townsville TOW – 19.3 146.7 – 28.7 – 49.1 – 41.4 64-95 Brisbane BRI – 27.5 152.9 – 36.8 – 58.1 – 47.1 64-86 Norfolk NOR – 29 168 – 35.9 – 56.8 – 46.7 64-94 Mundaring MUN – 32 116.2 – 44.5 – 66.7 – 51.7 64-94 Canberra CAN – 35.3 149 – 45.8 – 66.4 – 52.1 64-91, 93, 94 Christchurch CHR – 43.6 172.8 – 50.3 – 69.0 – 54.8 64-70 Kerguellen KER – 49.4 70.3 – 58.4 – 68.6 – 56.0 65-84, 87, 88 Campbell Island CLL – 52.5 169.2 – 60.2 – 76.0 – 59.5 70-85 Macquarie Island MAC – 54.5 159 – 64.4 – 79.2 – 61.1 84-92 Casey CAS – 66.3 110.5 – 80.7 – 82.3 – 66.2 64-74, 90, 91 Mawson MAW – 67.6 62.9 – 70.5 – 70.1 – 63.2 64-87, 91-93 Davis DAV – 68.6 77.9 – 74.8 – 73.6 – 64.8 85-88, 90, 91, 93 Scott Base SCO – 77.9 166.8 – 79.9 – 82.0 – 72.3 70-83 Maui MAU 20.8 203.5 21.3 38.5 34.8 64-94 Point Arguello PNT 34.6 239.4 40.5 59.8 49.0 72-74, 77-96 Wallops Island WAL 37.8 284.5 49.5 68.0 53.2 67-87, 91-96 Boulder BOU 40 254.7 49.1 67.6 53.4 64-96 Ottawa OTT 45.4 284.1 56.9 73.6 56.9 64-93 St. Johns STJ 47.6 307.3 55.2 70.3 56.2 66-77 Winnipeg WIN 49.8 265.6 60.7 76.8 59.1 66-76 Goosebay GOO 53.3 299.2 62.6 75.8 59.7 87-96 Churchill CHU 58.8 265.8 69.5 82.5 63.4 66-74 College COL 64.9 212.2 65 77.1 64.2 64-67, 88, 89, 91, 94 Resolute Bay RES 74.7 265.1 83.8 89.0 71.7 64-91 Huancayo HUA – 12 284.7 1.4 1.9 1.9 64-87 Tahiti TAH – 17.7 210.7 – 16.3 – 30.8 – 28.9 71-89 Concepcion CON – 36.6 287 – 22.5 – 36.4 – 35.3 64-79 Port Stanley POR – 51.7 302.2 – 37.3 – 49.2 – 47.5 64-92 Argentine Is. ARI – 65.2 295.7 – 49.8 – 59.2 – 57.9 64-92 Halley Bay HAL – 75.5 333.4 – 61.5 – 65.4 – 66.3 64-80 1314 Dimitris N. Fotiadis, Stamatis S. Kouris, Vincenzo Romano and Bruno Zolesi iii) Their local time (LT) of commencement (Jones, 1971; Prölss and von Zahn, 1978; Danilov and Morozova, 1985; Prölss, 1993). A disturbance may commence any time during the day; on the other hand, a minimum frequency of occurrence of such phenomena should be en- sured, without harming the analytical picture. Thus, by calculating the solar zenith angle at 300 km height, we have defined four local-time zones: day (cosχ > 0.20), night (cosχ = 0), while dawn and dusk fall in between. iv) Their universal time (UT) of commence- ment by studying each station separately within its longitude sector and hemisphere (Fuller- Rowell, 1994; Hajkowicz, 1998). Furthermore, investigating climatology in this way may reveal regional (local) phenomena (Cander, 1993). v) Their duration. This morphological as- pect – rather than the depth – is directly linked to different disturbance mechanisms, especially on positive storm effects (Buonsanto, 1999). Therefore, before stepping to climatology, the basic morphological features, depth and dura- tion, need to be investigated. 3. Results and discussion 3.1. Basic morphology Table II shows the percentage of positive and negative storms for which the respective variabil- ity level (i.e. the deviation from the monthly me- dian) is not exceeded, giving thus the cumulative depth distribution for 54 ionospheric stations in different longitudes and latitudes. Selecting as a boundary 49% of the total number of distur- bances, positive disturbances display an impor- tant variation of depth with the geomagnetic lati- tude in different longitude sectors. A stable distri- bution is observed in mid-latitudes up to about 55°ϕm. In higher latitudes positive storms be- come deeper, being more intense around 62°ϕm, boundary of auroral oval. Then, phenomena seem to be less deep at about 70°ϕm, however they are somewhat deeper in the polar region. Further- more, positive storms always grow deeper ap- proaching the geomagnetic equator. On the contrary, negative storm effects are more shallow and their depth distribution may not Table II. Percentage of positive (left) and negative (right) disturbances which does not exceed the respective variability level – ‘depth’ (x-axis). Greek ‘Σ’ denotes total percentage of positive/negative storms. Top to bot- tom: European, Asian, Australian and North American stations. ϕm .4 .5 .6 .7 .8 .9 1 Σ –.4 –.45 –.5 –.6 –.7 –.8 –.9 –1 Σ 64.4 KIR 14 31 40 45 49 52 54 57 16 26 33 41 43 63.9 LOP 14 32 43 49 52 54 55 56 15 26 33 42 42 43 44 63.6 SOD 10 24 35 42 46 49 50 54 13 24 33 43 46 61.3 LYC 9 23 33 40 44 47 49 56 13 23 30 41 44 60 ARK 11 25 35 41 45 47 49 56 15 26 33 42 44 56.3 UPP 13 30 40 45 48 49 51 54 17 27 33 43 45 46 55.9 LEN 13 30 41 45 48 50 51 54 17 27 33 42 45 46 51 MOS 18 39 48 54 55 55 56 21 32 37 42 44 50.8 JUL 18 38 48 52 53 54 21 31 37 44 46 48.4 SLO 18 38 48 52 54 55 22 32 38 43 45 45.8 KIE 20 38 49 52 54 54 54 55 24 35 40 44 45 45.4 LAN 19 37 47 51 53 53 54 25 35 41 45 46 42.3 POI 18 39 50 54 56 57 23 34 39 42 43 35.7 ROM 22 46 57 61 63 63 63 64 21 30 33 36 1315 Climatology of ionospheric F-region disturbances Table II (continued). ϕm .4 .5 .6 .7 .8 .9 1 Σ –.4 –.45 –.5 –.6 –.7 –.8 –.9 –1 Σ 34.8 TOR 19 44 58 62 66 67 20 28 31 33 30.4 GIB 17 39 53 61 65 66 66 67 16 25 30 33 63.8 NRI 9 23 33 41 46 48 50 54 14 24 31 43 46 55.5 YAK 13 32 42 48 50 52 52 54 18 28 35 43 46 53.2 MAG 14 33 45 50 52 53 53 55 19 30 36 42 44 45 51.4 TOM 19 40 50 56 58 58 58 59 22 32 35 40 40 41 46.8 IRK 19 41 51 55 57 57 58 22 32 37 42 32.3 AKI 21 43 58 64 66 67 67 70 18 25 28 30 28.2 KOK 20 45 58 64 67 68 69 71 18 25 27 28 29 23.8 YAM 17 44 59 67 71 74 75 79 12 18 20 21 18.8 OKI 11 31 46 56 62 65 68 72 15 22 26 28 17.5 TAI 9 26 40 50 56 60 62 66 15 25 30 34 6.2 MAN 9 23 34 43 49 52 54 59 14 25 33 40 41 – 7.7 SIN 5 20 33 41 45 49 52 55 15 25 34 43 45 – 11.4 VAN 7 21 31 39 44 48 50 55 14 24 33 42 45 – 22.2 DAR 12 32 46 53 58 62 63 67 16 24 28 33 – 28.7 TOW 17 39 52 58 61 62 63 64 21 29 33 36 – 35.9 NOR 21 45 56 60 61 62 22 30 34 37 38 – 36.8 BRI 21 46 57 61 62 63 63 64 22 30 33 36 – 44.5 MUN 21 41 50 53 54 55 23 33 39 44 45 – 45.8 CAN 19 39 47 50 51 27 36 42 48 49 – 54.2 HOB 14 34 46 52 55 56 57 58 19 29 34 47 42 – 58.4 KER 11 27 37 44 48 52 54 61 16 24 30 31 39 – 60.2 CLL 11 25 35 41 44 46 47 50 18 29 37 46 50 – 64.4 MAC 11 29 40 44 47 49 51 53 20 32 38 46 47 – 70.5 MAW 12 33 46 54 59 61 62 64 15 24 30 35 36 – 74.8 DAV 6 23 39 57 55 59 60 66 13 22 28 33 34 – 79.9 SCO 8 23 35 43 47 51 53 57 11 21 28 39 42 43 – 80.7 CAS 8 24 36 46 52 54 56 62 13 23 30 37 38 83.8 RES 6 21 33 41 47 50 52 57 14 24 32 41 43 69.5 CHU 15 32 42 48 51 52 52 54 19 31 38 45 46 65 COL 10 25 34 41 44 47 49 59 13 22 29 39 40 41 62.6 GOO 9 22 32 39 43 45 46 52 12 21 29 41 46 48 60.7 WIN 11 25 33 38 43 45 46 56 15 24 29 41 44 56.9 OTT 15 33 41 44 46 47 19 30 39 49 52 53 55.2 STJ 12 29 39 44 46 47 47 48 15 28 35 47 51 52 49.5 WAL 17 35 45 48 50 51 51 52 22 32 38 46 48 49.1 BOU 20 41 51 56 58 59 60 22 29 33 38 40 40.5 PNT 18 39 52 59 63 65 66 69 16 22 26 30 31 21.3 MAU 9 25 38 47 53 58 60 67 15 25 30 33 1316 Dimitris N. Fotiadis, Stamatis S. Kouris, Vincenzo Romano and Bruno Zolesi 1316 Fig. 1. Percentage of positive (left) and negative (right) disturbances for which the corresponding duration is not exceeded (cumulative distribution). Top: Asian stations; bottom: Australian stations. be interpreted using the above method. Negative disturbances are deeper around 55°ϕm and 65°ϕm, at the edge of auroral oval boundary which moves under geomagnetically disturbed conditions. On the other hand, negative storms are more shallow (about –50% of the monthly median foF2) in low midlatitudes (22-35°ϕm) and the equatorial anom- aly crest (15-22°ϕm), but they are somewhat deeper on the geomagnetic equator. Figure 1 shows cumulative distributions of the duration of positive/negative disturbances in Asian and Australian stations. Accordingly, we calculated the duration threshold for which the gradient of the distribution tends to zero, defin- ing thus a maximum duration. We assess that the above criterion may be satisfied with a potential error of 1% between sequential distribution val- ues. Figure 2 illustrates the respective thresholds for positive and negative disturbances with the geomagnetic latitude. Since much dispersion of values is observed, we have drawn two polyno- mial regression lines. It is evident from fig. 2 that the greatest du- ration of positive storms is not more than 15 h and it is observed in higher midlatitudes (55- 60°ϕm) and the upper boundary of the equatorial anomaly crest (20-25°ϕm). In midlatitudes great dispersion of this maximum duration is observed which may be attributed to a longitudinal de- pendence (Fotiadis et al., 2004) and the regional character of positive disturbances (Hajkowicz, 1998). From 60°ϕm polewards maximum ob- served duration is delimited to 12 h and even more in the polar region (10 h), as also happens on the geomagnetic equator region. On the con- trary, negative storm effects may last more than 15 h in midlatitudes (55-60°ϕm). In higher and polar latitudes negative disturbances have a maximum duration of about 9-11 h while they appear to be shortest on the equator (7-8 h). 1317 Climatology of ionospheric F-region disturbances Synoptically, it may be supported that where great depths are observed the duration of phe- nomena is delimited and vice versa. Definite ex- ceptions are the equatorial anomaly crest and the higher-latitude boundary where the power of pos- itive disturbances maximizes, since these regions constitute the ‘source’ of these phenomena. 3.2. Climatology of disturbances In the previous section it is shown that phe- nomena last for about 16 h, however it should be stressed that a small amount of disturbances (most- ly of negative phase) last for more than a day. In the literature, depending on their duration, distur- bances are attributed to different mechanisms (e.g., Hocke and Schlegel, 1996; Buonsanto, 1999). Similarly, three duration classes are select- ed here: i) 3-5 h, ii) 6-24 h and iii) more than 24 h. The frequency of occurrence of positive and negative disturbances is presented in fig. 3a-d, and 4a-d respectively, according to the parame- ters mentioned in the method of analysis section. Comments on the results follow categorized by the local time of storm commencement: Sunrise – Positive storms have mainly short duration and are observed with a frequency of 0.5-1 phenomena per month at 50-65°ϕm lati- tudes in Europe and Asia, whereas in Australia over 70°ϕm and only in equinox period. How- ever, they occur more frequently in the Ameri- can zone, at summer period over 60°ϕm and al- so sporadically at west coast mid- and low-lati- tude stations (Maui and Point Arguello). In winter such phenomena are limited only in short latitude strips well above 60°ϕm. Positive storms of greater duration mostly affect Europe than any other sector. Negative disturbances of short duration are more infrequent than the positive ones, occur- ring mostly in near-summer months at 50- 65°ϕm. Above 65°ϕm they are observed only in the American sector in equinox and winter months (polar region). Again, medium duration negative storms affect Europe in near-winter months. Negative storms commencing at sun- rise are totally absent from low and lower mid- latitude stations. Day – Positive storm effects appear to have short and medium duration and are a dominant feature of the equatorial anomaly crest (all year long) and lower midlatitude ionosphere (in sum- mer). Such effects are also observed in greater midlatitudes hence at equinox, but never in win- ter. The above climatology is almost reversed Fig. 2. Maximum duration of positive and negative disturbances with geomagnetic latitude. 1318 Dimitris N. Fotiadis, Stamatis S. Kouris, Vincenzo Romano and Bruno Zolesi A si a E u ro p e F ig . 3 a, b . F re qu en cy o f oc cu rr en ce o f po si ti ve st or m s pe r m on th a cc or di ng t o L T of c om m en ce m en t (f ro m t op t o bo tt om : su nr is e, d ay , su ns et , ni gh t) an d du ra ti on c la ss ( fr om le ft to r ig ht : s m al l, m ed iu m , l ar ge d ur at io n) a s a fu nc ti on o f ge om ag ne ti c la ti tu de ( x- ax is ) in ( a) E ur op e an d (b ) A si a ( li gh t g re y: 0. 5- 1 st or m s/ m on th , gr ey : 1- 2 st or m s/ m on th , da rk g re y: 2 -3 s to rm s/ m on th a nd b la ck : > 3 st or m s/ m on th ). a b 1319 Climatology of ionospheric F-region disturbances 1319 A u st ra lia N o rt h A m e ri ca F ig . 3 c, d . F re qu en cy o f oc cu rr en ce o f po si ti ve st or m s pe r m on th a cc or di ng t o L T of c om m en ce m en t (f ro m t op t o bo tt om : su nr is e, d ay , su ns et , ni gh t) an d du ra ti on c la ss ( fr om l ef t to r ig ht : sm al l, m ed iu m , la rg e du ra ti on ) as a f un ct io n of g eo m ag ne ti c la ti tu de ( x- ax is ) in ( c) A us tr al ia a nd ( d) N or th A m er - ic a (l ig ht g re y: 0 .5 -1 s to rm s/ m on th , gr ey : 1- 2 st or m s/ m on th , da rk g re y: 2 -3 s to rm s/ m on th a nd b la ck : > 3 st or m s/ m on th ). c d 1320 Dimitris N. Fotiadis, Stamatis S. Kouris, Vincenzo Romano and Bruno Zolesi E u ro p e A si a F ig . 4a ,b . F re qu en cy o f oc cu rr en ce o f ne ga ti ve st or m s pe r m on th a cc or di ng t o L T of c om m en ce m en t (f ro m t op t o bo tt om : su nr is e, d ay , su ns et , ni gh t) an d du ra ti on c la ss ( fr om l ef t to r ig ht : sm al l, m ed iu m , l ar ge d ur at io n) a s a fu nc ti on o f ge om ag ne ti c la ti tu de ( x- ax is ) in ( a) E ur op e an d (b ) A si a (l ig ht g re y: 0. 5- 1 st or m s/ m on th , gr ey : 1- 2 st or m s/ m on th , da rk g re y: 2 -3 s to rm s/ m on th a nd b la ck : > 3 st or m s/ m on th ). a b 1321 Climatology of ionospheric F-region disturbances A u st ra lia N o rt h A m e ri ca F ig . 4c ,d . F re qu en cy o f oc cu rr en ce o f ne ga ti ve st or m s pe r m on th a cc or di ng t o L T of c om m en ce m en t (f ro m t op t o bo tt om : su nr is e, d ay , su ns et , ni gh t) an d du ra ti on c la ss ( fr om l ef t to r ig ht : sm al l, m ed iu m , la rg e du ra ti on ) as a f un ct io n of g eo m ag ne ti c la ti tu de ( x- ax is ) in ( c) A us tr al ia a nd ( d) N or th A m er - ic a (l ig ht g re y: 0 .5 -1 s to rm s/ m on th , gr ey : 1- 2 st or m s/ m on th , da rk g re y: 2 -3 s to rm s/ m on th a nd b la ck : > 3 st or m s/ m on th ). c d 1322 Dimitris N. Fotiadis, Stamatis S. Kouris, Vincenzo Romano and Bruno Zolesi for the Australian zone, where positive storms commencing in daytime are observed in sum- mer at much higher latitudes (up to 75°ϕm). Fur- thermore the American zone is affected with greater frequency around 40°ϕm by such phe- nomena, while the Asian sector is practically not at all affected above 30-40°ϕm. Positive storms of greater duration are frequent around 20°ϕm and also about 60°ϕm everywhere but in Asia. Negative short duration disturbances are important all year long at the equatorial crest and mostly at about 60°ϕm in the American zone during equinoctial months. However they are absent at low midlatitudes, except when phenomena of medium duration in American summer are concerned. Sunset – Positive disturbances of short du- ration occur more frequently around 60°ϕm all year, but summer, and they ‘penetrate’ to mid- latitudes during winter, similarly to traveling ionospheric disturbances’ climatology (Hocke and Schlegel, 1996). This penetration to midlat- itudes is not observed in the American and Aus- tralian sectors, presenting a UT effect. Further- more, positive disturbances of greater duration (more than 5 h) affect mainly Europe. On the contrary, negative storms occurring mostly during winter are observed from 60°ϕm polewards, However, they affect European and Australian latitudes of about 55°ϕm in summer. This is not the case in American sector. Again, longer duration negative storms occur only in Europe. Night – Equatorial and lower midlatitudes present short- and medium-duration positive storms throughout the year, while at equinoxes and winter they also occur in mid- and high- latitudes. The only regions which present short- period positive storm effects in summer midlat- itudes are the sectors including the geomagnet- ic poles (America and Australia). Longer dura- tion positive effects are restricted to winter months in America and Asia, whereas Europe and Australia are also affected by such phe- nomena during equinoxes. Negative storms present a similar frequen- cy distribution in time and space with the for- merly analysed positive ones. Summarizing, the above results seem to con- firm prominent features of ionospheric storms such as their local time and seasonal variation (Prölss, 1995; Mikhailov, 2000). However, here many regional and local phenomena are pointed out and users are provided with analytical maps of storm frequency. 4. Conclusions The analysis of basic morphological storm features shows that positive storms present maximum power, i.e. depth and duration, in the equatorial anomaly crest and the auroral oval boundary, presenting durations of the order of 15 and 14 h, respectively. On the contrary, neg- ative storms last longer at higher midlatitudes (around 50°ϕm), but their depth distribution with latitude is rather uniform. The investigation of climatology has shown a different seasonal and spatial distribution of positive and negative storms according to their local time of commencement. Several regional phenomena have been identified, thus confirm- ing that ionospheric storms can be regional. 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(received January 23, 2004; accepted June 17, 2004)