International Quiet Days Research Articles

Ionospheric Virtual Height (h′F2) Response and its Vertical Ion Drifts Characteristics at an Equatorial Station

The Ionospheric virtual heights (h’F2) obtained from the DPS-4.2 Version of the GIRO’s site, as a computing parameter for vertical E × B ion drifts in this present study using the international quiet days (IQDs) is engaged. Vertical ion drifts () computed from virtual heights was studied over Ilorin (lat. 8.31°N, long. 4.34°E, dip lat. 2.95o) during a solar minimum (SM), a station situated at the equatorial dip. The hourly averages of vertical ion drifts, were computed from 10-international quiet days (IQDs) hourly h’F2 averages for the four seasonal months adopted. Seasonally, was having same behavourial patterns of pre-noon and post-noon peaks, but in h’F2, it had noontime, post-sunset and pre-sunrise peaks occurred in all seasons. The pre-noon peaks magnitudes are 6.8, 14.4 and 15.0 m/s in June Solsticial, Equinoctial and December Solsticial respectively; and its post-noon peaks magnitudes are 0.8, 7.4 and 7.9 m/s in December and June Solsticials, and Equinoctial seasons respectively. The h’F2 noontime peaks magnitudes are (321–410) km, post-sunset peaks magnitudes are (253–329) km and pre-sunrise peaks magnitudes are (228–318) km in all seasons. Also, depicted pre-reversal enhancement (PRE) night peaks in all seasons. The PRE peaks magnitudes are [(-4.0)–(-13.4)] m/s at 2000 LT, [(-4.9)–(-9.3)] m/s between 2200 LT and 2300 LT in all seasons. Similar phenomenal observations occurred in the h’F2 and annual patterns plots. In general, the h’F2 and magnitudes were greatest in Solsticial seasons and least in Equinoctial season. The h’F2 and stable and continual descent indicates generally that the electrons briskly drifting away from the equator triggered by solar ionization in the equatorial region. As well, seasonal peaks occurred in general are assumed to be controlled by the enhanced vertical E × B ion drifts upsurge, which adapts with former aftermaths at some stations in the West African sector during closely related solar activity

Low Latitude Estimated Virtual Vertical Plasma Drifts Variation During Solar Minimum

Virtual vertical plasma drifts estimated from hourly virtual heights (h’F2) extracted from the digisonde found on GIRO’s website are used in this recent study by engaging the international quiet days (IQDs) conditions. Virtual vertical plasma drifts () estimated from low latitude F2-region virtual heights was examined over Ilorin (lat. 8.31°N, long. 4.34°E, dip lat. 2.95o) during solar minimum (SM), a station Located along the latitudinal hollow. The 10-international quiet days (IQDs) monthly averages across each hourly period of the day were used for the estimation. Two characteristics of highlighted are the pre-noon and the post-noon peaks of the diurnal patterns for each season. The pre-noon peaks magnitudes between 0800 LT and 0900 LT are 6.8, 14.4 and 15.0 m/s for June Solstice, Equinox and December Solstice respectively; and its post-noon peaks magnitudes around 1800 LT evening time are 0.8, 7.4 and 7.9 m/s foe December and June Solstices, and Equinox respectively. displayed an enhanced phenomenon of slight-transitory spikes having magnitudes of -0.3 m/s (December Solstice) and -0.8 m/s (Equinox), and 3.6 m/s (June Solstice) around 1200 LT noontime. Finally, depicted pre-reversal enhancement (PRE) night peaks for the entire season. The PRE peaks magnitudes are [(-4.0)–(-13.4)] m/s at 2000 LT, [(-4.9)–(-9.3)] m/s between 2200 LT and 2300 LT, [(-3.6)–(-6.6)] m/s between 0100 LT and 0200 LT and [(-3.0)–(-6.4)] m/s between 0300 LT and 0400 LT respectively for all seasons. Same phenomenon in the annual pattern as showcased in the diurnal patterns occurred. In general, magnitudes were highest in Solstices and lowest in Equinox during solar minimum. The gradually and continuous decay is ensued by the electrons hastily turnoff movement from the low latitude due to solar ionization for all seasons

THE STUDY OF LONGITUDINAL AND LATITUDINAL VARIATION OF EQUATORIAL ELECTROJET SIGNATURE AT STATIONS WITHIN THE 96°MM AND 210°MM AFRICAN AND ASIAN SECTORS RESPECTIVELY UNDER QUIET CONDITIONS.

Solar quiet current (S_q) and Equatorial Electrojet (EEJ) are two current systems which are produced by electric current in the ionosphere. The enhancement of the horizontal magnetic field is the EEJ. This research is needed for monitoring equatorial geomagnetic current which causes atmospheric instabilities and affects high frequency and satellite communication. This study presents the longitudinal and latitudinal variation of equatorial electrojet signature at stations within the 96°mm and 210°mm African and Asian sectors respectively during quiet condition. Data from eleven observatories were used for this study. The objectives was to determine the longitudinal and latitudinal geomagnetic field variations during solar quiet conditions, Investigate monthly variation and diurnal transient seasonal variation; Measure the strength of the EEJ at stations within the same longitudinal sectors and find out the factors responsible for the longitudinal and latitudinal variation of EEJ. Horizontal (H) component of geomagnetic field for the year 2008 from Magnetic Data Acquisition System (MAGDAS) network were used for the study. The International Quiet Days (IQDs) were used to identify quiet days. Daily baseline values for each of the geomagnetic element H were obtained. The monthly average of the diurnal variation was found. The seasonal variation of dH was found. Results showed that: The longitudinal and latitudinal variation in the dH differs in magnitude from one station to another within the same longitude due to the difference in the influence of the EEJ on them.

Comparison of Ionospheric Total Electron Content (TEC) over Sonmiani (Pakistan) with NeQuick-2 and IRI-2012 during July 2014 – June 2015

In this study, Total Electron Content (TEC) observations acquired by a GNSS receiver installed at Sonmiani (Geog. Coord. 25.19°N, 66.74°E, Geomag. Coord. 17.62°N, 141.5°E) are being reported for the first time. The data utilized is hourly instantaneous TEC values during 10 International Quiet Days (IQDs) per month from Jul-14 to Jun-15, totaling 120 observation days for monitoring nominal TEC. The findings confirm the semi-annual trend of TEC over Sonmiani, which lies at the northern crest of Equatorial Ionization Anomaly (EIA) region. The TEC measurements are then compared with NeQuick-2 and International Reference Ionosphere (IRI-2012) models. It was found that the TEC values derived from NeQuick-2 are in better agreement with GNSS measurements than those from IRI-2012. The TEC measurements also show seasonal variation which is largest during Equinox months. The TEC value in Dec solstice is higher than the Jun solstice, which confirms that the seasonal anomaly is playing a major role in this region during the course of study.

Study of Ionospheric Amplitude Scintillation during Geomagnetic Activities of 2012 at Low Latitude Region

The impact of scintillation on radio communications, geodesy, radar system and navigation cannot be underemphasized. The first evident effect of space weather in the phase of radio signals and amplitude that passes over ionosphere is fluctuations. If there are intense fluctuations in the quality of the signals received, such signals might degrade or attenuate in quality. This phenomenon is referred to as scintillations. To study the effects of magnetic activity on scintillations, the percentage occurrence of scintillations has been separated for the five international quiet (Q) days and five international disturbed (D) days over Ilorin for the year 2012 which was obtained from world data centre Kyoto. The occurrences of S4 were recorded during quiet days than the disturbed days in the study area. Generally, occurrence of \(S_4\) was enhanced during pre-midnight hours but was suppressed during post-midnight hour both at quiet and disturbed days in Ilorin, Kwara State, Nigeria.

Comparison of GPS derived TEC with the TEC predicted by IRI 2012 model in the southern Equatorial Ionization Anomaly crest within the Eastern Africa region

We have compared the TEC obtained from the IRI-2012 model with the GPS derived TEC data recorded within southern crest of the EIA in the Eastern Africa region using the monthly means of the 5 international quiet days for equinoxes and solstices months for the period of 2012 – 2013. GPS-derived TEC data have been obtained from the Africa array and IGS network of ground based dual-frequency GPS receivers from four stations (Kigali (1.95°S, 30.09°E; Geom. Lat. 11.63°S), Malindi (2.99°S, 40.19°E; Geom. Lat. 12.42°S), Mbarara (0.60°S, 30.74°E; Geom. Lat. 10.22°S) and Nairobi (1.22°S, 36.89°E; Geom. Lat. 10.69°S)) located within the EIA crest in this region. All the three options for topside Ne of IRI-2012 model and ABT-2009 for bottomside thickness have been used to compute the IRI TEC. Also URSI coefficients were considered in this study. These results are compared with the TEC estimated from GPS measurements. Correlation Coefficients between the two sets of data, the Root-Mean Square Errors (RMSE) of the IRI-TEC from the GPS-TEC, and the percentage RMSE of the IRI-TEC from the GPS-TEC have been computed. Our general results show that IRI-2012 model with all three options overestimates the GPS-TEC for all seasons and at all stations, and IRI-2001 overestimates GPS-TEC more compared with other options. IRI-Neq and IRI-01-corr are closely matching in most of the time. The observation also shows that, GPS TEC are underestimated by TEC from IRI model during noon hours, especially during equinoctial months. Further, GPS-TEC values and IRI-TEC values using all the three topside Ne options show very good correlation (above 0.8). On the other hand, the TEC using IRI-Neq and IRI-01- corr had smaller deviations from the GPS-TEC compared to the IRI-2001.

Spatial frequencies associated with the latitudinal structures of ionospheric currents seen by CHAMP satellite

The CHAMP magnetic field variations during international quiet days of low solar activity period 2008-2009 are investigated. The present paper reports the existence of frequency-peaks < 20 mHz in the compressional component of the magnetic field in almost all CHAMP passes. The magnetic field variations associated with these frequencies have amplitude of a few tens of nT during daytime. The geomagnetic activity and interplanetary magnetic field parameters were observed to be low during the period of study. The spectral powers of the observed frequencies show no dependence on solar wind velocity and cone angle; hence the reported frequencies are not related to the geomagnetic pulsations. For frequency-peaks <15 mHz, strong local-time dependence is observed with maximum power near noon and minimum at night. The longitudinal and seasonal variations of the powers of these frequency-peaks match well with those of the equator-to-middle latitude ionospheric currents derived by the earlier studies. As a polar Low-Earth-Orbiting (LEO) satellite spans the entire range of latitudes within few minutes, it monitors the geomagnetic field variations caused by the quiet-time ionospheric currents flowing at different latitudes. This can result in certain frequencies in the magnetic field recorded by LEO satellites. We demonstrate that the frequencies <10mHz are mainly due to the latitudinal structure of the equatorial electrojet. The observed frequencies in CHAMP data are therefore attributed to the latitudinal structures of the ionospheric currents that are monitored only by the polar LEO satellites and are found to alter the observations of geomagnetic pulsations (Pc4-5 and Pi2) significantly.

Seasonal variations of nighttime D-region ionosphere in 2013 solar maximum observed from a low-latitude station

We present the observation of tweek atmospherics with harmonics m = 1–8 during the solar maximum year, 2013, at Tay Nguyen University, Vietnam (Geog. 12.65° N, 108.02° E). The analysis of 33,690 tweeks on ten international quiet days during 2 months each season, summer (May, August), winter (February, November), and equinox (March, September), shows that tweeks occur about 51 % during summer, 22 % during winter, and 27 % during equinox. The D-region ionosphere is more sharply bounded for harmonics m = 5–6 around an altitude of 85.5 km. The environment of the D-region is more inhomogeneous during winter and equinox seasons. The mean electron density varies from 28.4–225 cm −3, which corresponds to the harmonics m = 1–8 at the mean reflection height of 81.5–87.7 km. The results reveal that the lower reference height in our work as compared to other works is due to the higher level of solar activity. The equivalent electron density profile of the nighttime D-region ionosphere using tweek method during summer, equinox, and winter seasons shows lower values of electron density by 12–58 %, 3–67 %, and 24–76 % than those obtained using the International Reference Ionosphere (IRI-2012) model.

Principal components of quiet time temporal variability of equatorial and low‐latitude geomagnetic fields

AbstractDiurnal variations of the horizontal component of the geomagnetic field ΔH on International Quiet days of 1999–2012, measured hourly at two stations in the same longitude zone in the Northern Hemisphere, near and away from the dip equator, have been subjected to principal component analysis. This technique is also applied to the difference ΔHEEJ of ΔH at these two stations, which is attributed to the equatorial electrojet (EEJ). The first three principal components, PC1–PC3, account for 91–96% of the variances in the data. Maximum contribution to the quiet day variations in ΔH around its peak in the morning hours at both the stations, and in the EEJ, comes from the day‐to‐day variation of the amplitude of PC1. Patterns of day‐to‐day variations of PC1 amplitudes for the equatorial station and the EEJ are essentially semiannual modulated by solar EUV flux, superimposed on a longer timescale solar EUV flux‐dependent trend. Contributions from PC2 and to a lesser extent from PC3 are seen to be responsible for the absence of semiannual variations in ΔH in the afternoon hours at the equatorial station. Distribution of amplitudes of PC2 and PC3 for ΔHEEJ for weak electrojet days shows seasonal features in accordance with greater occurrence of afternoon (morning) counter electrojet during June (December) solstice. During the extended solar minimum, PC3 amplitudes for ΔH at the equatorial station and for the EEJ display annual variation. Possible sources for these seasonal features in the variations of equatorial ΔH are discussed.

Spatial variability of solar quiet fields along 96° magnetic meridian in Africa: Results from MAGDAS

AbstractWe have used chains of Magnetic Data Acquisition System (MAGDAS) magnetometer records of the horizontal (H) and vertical (Z) magnetic field intensities during September 2008 to August 2009 (year of deep minimum) across Africa to study their variability during the quietest international days, which coincidently associated with the sudden stratospheric warming (SSW) event in January 2009. This selection of the most international quiet days is indicative of 80% that are strongly associated with days when unusually strong and prolonged sudden SSW event occurs in January 2009. Interestingly, in January, a significant magnitude depletion of solar quiet (Sq) equivalent current was observed near noon hours around the magnetic equator (Addis Ababa, ABB) compared to any other months along with a consistent significantly reduced value across the Northern Hemisphere and moderate decrease at the Southern Hemisphere. Also, we found that Nairobi and Dar es Salaam at the Southern Hemisphere, which are close to ABB (dip equator), are strongly prone to westward electric field compared to the magnetic equator and Khartoum at the Northern Hemisphere. Significant negative values of MSq(Z) magnitudes observed near noon hours at Hermanus indicate the presence of induced currents that suggest ocean effects along with reversal to significant positive values in the afternoon, which subsided before 1800 LT in almost all the months, indicate stronger influence of ionospheric currents. On seasonal variability of Sq(H), a slight depression at ABB during September equinox is one of the evidences of seasonal Sq focus shift. Latitudinal variability of Sq near‐noon hours was also investigated.

A multi-technique study of the 29–31 October 2003 geomagnetic storm effect on low latitude ionosphere over Indian region with magnetometer, ionosonde, and GPS observations

The present study demonstrates the ionospheric response to the extreme geomagnetic storms during 29–31 Oct 2003 (the Halloween storm), in the low latitude anomaly Indian region, based on multi-instrument measurements namely magnetometer, ionosonde, and GPS observations. Unlike earlier reports, we have suitably chosen the best quiet days (CQ-Days) amongst 10 international quiet days (Q-Days), on the basis of equatorial electrojet strength and pattern, that drives the distribution of plasma over the low latitude. We stress that arbitrarily selecting the Control/Q-Days may lead to erroneous interpretations and will not yield a clearer understanding of the equatorial electrodynamics. Our analysis confirms the anomalous increase in TEC at all stations on 29 Oct 2003 and suppressed TEC across anomaly crest latitudes during 30–31 Oct 2003. The sharp transition in h’F is noticed during the progressive period of the storm, though it was relatively lower at the equator. The respective foF2 remained subordinate at Trivandrum. However, we did not notice such foF2 changes at Delhi. Observations at various latitudes confirm the maximum positive deviation of TEC at mid-latitude POL2 (140 %), followed by the low latitude Jodhpur (108 %), and the rest of the stations showing relatively lower enhancements with deviations ranging between 60–90 %. However, at Ahmedabad, the lowest divergence from the mean CQ-Days, attribute the typical quiet day formation of crest at this latitude. Although the results are well agreeing with earlier reports, miniature differences is noticeable due to our way of choosing the best reference days in the analysis.

Comparison of K-index Calculations between Several Geomagnetic Stations during IQDs and IDDs

BOH magnetometer was installed at Mt. Bohyun in 2007 and has provided continuous dataset for 3-axis geomagnetic field over the South Korea. We have calculated real-time K-index based on BOH magnetic field data using well-known FMI method. Local K-index is calculated eight times a day, per every three hours. To calculate K-index, it is critical to get the Quiet Day Curve (QDC). For QDC calculation, we take the previous one month’s average of H-component. In this paper, we compared four geomagnetic stations’ magnetic field data over South Korea and Japan and K-indices of each stations; Bohyun, Gangneung, Jeju, and Kakioka for two years data, 2011-2012. To investigate the difference depending on the latitude, longitude and local time in more detail, we compare K-index on International Quiet Days (IQDs) and International Disturbed Days (IDDs). As a result, we report the correlation between local K-indices are higher than those between Kp and local K-indices, and the correlation is much better after sunset than after sunrise. As the geomagnetic activity becomes stronger, the correlation between the local K-indices and global Kp-index become higher.

Diurnal and Seasonal Variations in Mid-Latitude Geomagnetic Field During International Quiet Days: BOH Magnetometer

Korea Astronomy and Space Science Institute researchers have installed and operated magnetometers at Bohyunsan Observatory to measure the Earth's magnetic field variations in South Korea. In 2007, we installed a fluxgate magnetometer (RFP-523C) to measure H, D, and Z components of the geomagnetic field. In addition, in 2009, we installed a Overhauser proton sensor to measure the absolute total magnetic field F and a three-axis magneto-impedance sensor for spectrum analysis. Currently three types of magnetometer data have been accumulated. In this paper, we use the H, D, Z components of fluxgate magnetometer data to investigate the characteristics of mid-latitude geomagnetic field variation. To remove the temporary changes in Earth's geomagnetic filed by space weather, we use the international quiet days' data only. In other words, we performed a superposed epoch analysis using five days per each month during 2008-2011. We find that daily variations of H, D, and Z shows similar tendency compared to previous results using all days. That is, H, D, Z all three components' quiet intervals terminate near the sunrise and shows maximum 2-3 hours after the culmination and the quiet interval start from near the sunset. Seasonal variations show similar dependences to the Sun. As it becomes hot season, the geomagnetic field variation's amplitude becomes large and the quiet interval becomes shortened. It is well-known that these variations are effects of Sq current system in the Earth's atmosphere. We confirm that the typical mid-latitude geomagnetic field variations due to the Sq current system by excluding all possible association with the space weather.

Nighttime D region electron density measurements from ELF‐VLF tweek radio atmospherics recorded at low latitudes

Dispersive atmospherics (tweeks) observed during 2010 simultaneously at two low‐latitude stations, Allahabad (geomagnetic latitude, 16.05°N) and Nainital (geomagnetic latitude, 20.48°N), have been used to estimate the nighttime D region electron density at the ionospheric reflection height under the local nighttime propagation (21:00–02:00 LT or 15:30–20:30 UT). The analysis of simultaneously recorded tweeks at both the stations on five international quiet days during one month each from summer (June), winter (January), and equinox (March) seasons shows that the D region electron density varies 21.5–24.5 cm−3 over the ionospheric reflection height of 85–95 km. The average values of Wait lower ionospheric parameters: ionospheric reference height h′ and sharpness factor β are almost same during winter (85.9–86.1 km, 0.51–0.52 km−1) and equinox (85.6–85.7 km, 0.54 km−1) seasons. The values of h′ and β during summer season are about 83.5 km and 0.60 km−1at both stations. Overall, equivalent electron density profile obtained using tweek method shows lower values of electron density by about 5–60% than those obtained using the International Reference Ionosphere (IRI‐2007) model and lower/higher by 2–68% than those obtained using rocket technique. The electron density estimated using all three techniques (tweek, IRI‐2007, and rocket) is consistent in the altitude range of 82–98 km. The estimated geographic locations of causative lightnings of tweeks were matched with the locations and times of lightnings detected by the World‐Wide Lightning Location Network (WWLLN). The WWLLN detected about 27.5% of causative lightnings of tweeks simultaneously observed at both the stations.

Equivalent current systems for the annual and semiannual Sq variations observed along the 210°MM CPMN stations

We analyzed ground magnetometer data for the 10 International Quiet Days during 1996–2007. The data were obtained from 19 stations along 210°magnetic meridian (MM) of the Circum‐pan Pacific Magnetometer Network (CPMN) covering both the Northern Hemisphere and Southern Hemisphere. From the daily variations of the geomagnetic field, we deduced the latitude‐local time (LAT‐LT) diagram of the equivalent Sq current system, which can be regarded as the superposition of the following three current systems: Sq0, Sq1, and Sq2. The Sq0, Sq1, and Sq2 current systems are equivalent current systems for the yearly average, annual variation, and semiannual variation of the Sq field, respectively. We have examined temporal and spatial features of these current systems. The principal features are as follows: (1) The total current intensities of the Sq1 and Sq2 current systems are about 35% and 15% of that of the Sq0 current system, respectively. (2) The Sq0 and Sq2 current systems have a dayside vortex in each hemisphere, while the Sq1 current system has a single vortex centered at the equatorial region in the morning sector (∼1000 LT).

A study of Sq(H) variations over equatorial electrojet regions

The newly established geomagnetic field observations in Japan, have enabled us to analyse the 1998 data of Huancayo, Kiritimati (Christmas Island) and Pohnpei where the geomagnetic Sq(H) variations of equatorial electrojet have been studied. The diurnal variation of the monthly means of Sq(H) on the five international quiet days showed mostly expected diurnal variation of equatorial electrojet regions as well as some abnormal variations. Seasonal variation with equinoctial maxima and solstial minima was also observed from the analysis. The presence of equinoctial maxima is likely to be due to enhanced equatorial electron density and the electric field at equinox. The day to day variability may be attributed to electric field, a dawn to dusk phenomenon. Keywords: Equatorial electrojet, seasonal variation, amplitude, abnormal variation and solar quiet day variation Nigerian Journal of Physics Vol. 18 (1) 2006: pp. 45-52

Are our ideas about &amp;lt;i&amp;gt;Dst&amp;lt;/i&amp;gt; correct?

Abstract. The idea of two separate storm time ring currents, a symmetric and an asymmetric one has accepted since the 1960s. The existence of a symmetric equatorial ring current was concluded from Dst. However, the asymmetric development of the low-latitude geomagnetic disturbance field during storms lead to the assumption of the real existence of an asymmetric ring current. I think it is time to inquire whether this conception is correct. Thus, I have investigated the development of the low-latitude geomagnetic field during all the magnetic local times under disturbed and quiet conditions. The storm on February 6–9, 1986 and a statistical analysis of many storms has shown that the asymmetry does not vanish during the storm recovery phase. The ratio between the recovery phase asymmetry and the main phase asymmetry is low only for powerful storms. Storms of moderate intensity show the opposite. The global picture of the field evolution of the February storm shows clear differences at different local times. For instance the main phase and recovery phase start time does not coincide with Dst. Also the ring current decay is not the same at different local times. Therefore, Dst gives an incorrect picture of the field development. Moreover, asymmetry does not disappear during international quiet days as the investigation of the low-latitude geomagnetic field shows. Considering all these observations, I think we must revise our ideas about the ring current. In my opinion only one ring current exists and this is an asymmetric one. This asymmetry increases during storms and develops rather fast to more or less symmetric conditions. However, in no case is it justified to conclude from Dst that a symmetric ring current exists.Key words. Magnetospheric physics (current systems; magnetospheric configuration and dynamics; storms and substorms)

Disturbance diurnal electric field in Indian and American equatorial electrojet regions

Abstract The yearly mean Disturbance Daily (SD) variation, i.e., solar daily variation on International Disturbed days (Sd) minus that on International Quiet days (Sq) of the geomagnetic H field at Huancayo, an equatorial electrojet station in American zone, for any of the years 1954–61 shows a dawn-maximum and a dusk-minimum as observed at any low and middle latitude station. Contrary to this, the SD(H) variation at Kodaikanal, an equatorial station in the Indian zone, shows a prominent minimum around noon hours besides the usual dawn-maximum and dusk-minimum for any of these years. The amplitude of SD(H) at Huancayo or at Kodaikanal decreases with the decreasing solar activity. Seasonally, SD(H) is larger during equinoctial than during solsticial months. For any particular season, the SD(H) averaged for all hours of the day is lower at Huancayo than at Kodaikanal, while the Sq(H) at Huancayo is always greater than the corresponding amplitude of Sq(H) at Kodaikanal. Thus the sources of electrojet electric fields on quiet and on disturbed days seem to be different and need to be identified.

Behavior of the quiet day ionospheric current system in the European region

The ionospheric current system for magnetic quiet days deduced from geomagnetic field daily variations taken throughout Europe at sunspot minimum has been studied. Special attention was paid to the selection of the representative quiet days, showing the inadequacy of relying on the mean of the five international quiet days per month. We used the technique of spherical cap harmonic analysis to separate the external and internal parts of the field variation from which the derivation of the equivalent current functions is straightforward. This method provides a motion picture representation of the Sq system through the screen formed by the spherical cap, allowing the study of continuous variations with universal time. Detailed features of the current contours can be represented with such a regional analysis. We found a strong day‐to‐day variability of the focus position along with some complexities that differ from the standard Sq current behavior. The average monthly or seasonal patterns are therefore averages of heterogeneous behaviors in many occasions. Possible explanations to these complex structures are considered.

Secular variation from monthly means from Apia and Amberley magnetic observatories

Summary. The availability of data from magnetic observatories in machinereadable form means that we can now study secular change (internal variations of the magnetic field) by analysing time series with a rapid sampling rate rather than annual means. The major task is to identify and remove fields due to external sources: the major benefit is finer time resolution, which is particularly important because of the recent interest in sudden phenomena such as geomagnetic impulses and jerks. Apia and Amberley observatories provide good data for experimentation. Many external fields are satisfactorily excluded by taking averages of hourly values at local midnight for the five international quiet days of the month and removing annual and semi-annual lines by least squares. Quiet days based on the K-index are found to be at least as suitable as those based on the AE and DSt indices. Time series of monthly means are developed for all components at both observatories from 1921 (Apia) and 1923 (Amberley). Cubics provide good fits to the long-term variations, but the residuals to these cubics are highly significant. An 11 yr variation is noticeable, showing that we have not succeeded in removing this external source of noise. Residual curves for both observatories, which are some 30 apart, follow each other quite closely. The large swings, with a period of approximately 22 yr, that were noted to be in phase with the double sunspot cycle by Beagley & Bullen, have not continued in phase. The magnetic field is widely believed to have undergone a change around 1969-70. This change is now known as the geomagnetic jerk. The finer time resolution afforded by monthly means is used to investigate the duration of this event. The resolution is limited to 6-18 month by noise in the measurements. A rapid change in the Y-component occurs effectively instantaneously at 1970.6 for Apia and 1972.4 for Amberley. We propose our method as a