On the Variation of Geomagnetic H-Component during Solar Quiet Days

The hourly variation of the H-component of the geometric field from two equatorial electrojet stations, Huancayo and Addis Ababa, and one non-equatorial electrojet station, Alibag, were studied to find out the trend of solar quiet variation of H for the year 2008. The dH amplitudes of the electrojet stations showed enhancement in H, while there was no enhancement in the non-electrojet station which was located far away from the dip equator. The day-to-day monthly diurnal variation was, however, observed in all the three stations. Also, at nighttime, the dH amplitudes of all the stations were non-zero which we attributed to non-ionospheric current sources like the magnetosphere since at night there was no solar radiations. For seasonal variations, an Equinoctial maximum, J-Solstitial maximum, and S-Solstitial maximum were observed in the equatorial stations while the non-equatorial station recorded an equinoctial minimum, J-solstitial minimum and D-Solstitial minimum.

Variations of the peak positions in the longitudinal profile of noon-time equatorial electrojet

In this study, the seasonal variations of the EEJ longitudinal profiles were examined based on the full CHAMP satellite magnetic measurements from 2001 to 2010. A total of 7537 satellite noon-time passes across the magnetic dip-equator were analyzed. On the average, the EEJ exhibits the wave-four longitudinal pattern with four maxima located, respectively, around 170° W, 80° W, 10° W and 100° E longitudes. However, a detailed analysis of the monthly averages yielded the classification of the longitudinal profiles in two types. Profiles with three main maxima located, respectively, around 150° W, 0° and 120° E, were observed in December solstice (D) of the Lloyd seasons. In addition, a secondary maximum observed near 90° W in November, December and January, reinforces from March to October to establish the wave-four patterns of the EEJ longitudinal variation. These wave-four patterns were divided into two groups: a group of transition which includes equinox months March, April and October and May in the June solstice; and another group of well-established wave-four pattern which covers June, July, August of the June solstice and the month of September in September equinox. For the first time, the motions in the course of seasons of various maxima of the EEJ noon-time longitudinal profiles have been clearly highlighted.

Intensity and time series of extreme solar-terrestrial storm in 1946 March

ABSTRACT Major solar eruptions occasionally cause magnetic superstorms on the Earth. Despite their serious consequences, the low frequency of their occurrence provides us with only limited cases through modern instrumental observations, and the intensities of historical storms before the coverage of the Dst index have been only sporadically estimated. Herein, we examine a solar-terrestrial storm that occurred in 1946 March and quantitatively evaluate its parameters. During the ascending phase of Solar Cycle 18, two moderate sunspot groups caused a major flare. The H α flaring area was recorded to be ≥600–1200 millionths of solar hemisphere, suggesting that this was an M- or X-class flare in soft X-ray intensity. Upon this eruption, a rapid interplanetary coronal mass ejection (ICME) with an average speed of ≈1590 km s−1 was launched. Based on measurements in four known mid-latitude and relatively complete magnetograms, the arrival of this extreme ICME caused a magnetic superstorm, which caused an initial phase with the H-component amplitude of ≥80 nT, followed by a main phase whose intensity was reconstructed as ≤−512 nT using most negative Dst* estimates. Meanwhile, the equatorial boundary of the auroral oval extended down to ≤41${^{\circ}_{.}}$8 in invariant latitude and formed a corona aurora in Watheroo, Australia. Interestingly, during this magnetic superstorm, larger magnetic disturbances were recorded at dusk and near the dip equator on the dayside. Its cause may be associated with a strong westward equatorial electrojet and field-aligned current, in addition to the contribution from the storm-time ring current.

Latitudinal variation of Pc3–Pc5 geomagnetic pulsation amplitude across the dip equator in central South America

In order to clarify the equatorial electrojet effects on ground magnetic pulsations in central South America, we statistically analyzed the amplitude structure of Pc3 and Pc5 pulsations recorded during days considered quiet to moderately disturbed at multiple equatorial stations nearly aligned along the 10∘ magnetic meridian. It was observed that Pc3 amplitudes are attenuated around noon at the dip equator for periods shorter than ∼35 s. It is proposed that daytime Pc3s are related to MHD (magnetohydrodynamic) compressional wave vertically incident on the ionosphere, with the screening effect induced by enhanced conductivity in the dip equator causing wave attenuation. Daytime Pc5s showed amplitude enhancement at all equatorial stations, which can be explained by the model of waves excited at higher latitudes and propagating equatorward in an Earth–ionosphere waveguide. However, a slight depression in Pc5 amplitude compared to neighboring equatorial stations and a phase lag in relation to an off-equatorial station were detected at the dip equator. This wave amplitude depression in the Pc5 frequency band cannot be explained by the ionospheric waveguide model alone, and we propose that an alternative propagation model that allows ULF (ultra-low-frequency) waves to penetrate directly from the magnetosphere to low latitudes could be operating simultaneously to produce these features at the dip equator. Significant effects of the sunrise terminator on Pc3 pulsations were also observed at the stations closest to the dip equator. Contrary to what is reported at other longitudes, in central South America the sunrise effect decreases the D∕H amplitude ratio. We suggest that these differences may arise from the unique characteristics of this sector, with a strong longitudinal variation in the magnetic declination and precipitation of energetic particles due to the presence of the South Atlantic Magnetic Anomaly (SAMA). The H-component amplification can be explained by enhancements of the zonal electric field near the magnetic equator driven by F-region neutral winds and waves in the fast-mode of propagation during sunrise.

Latitudinal variation of Pc3-Pc5 geomagnetic pulsation amplitude across the dip equator in central South America

In order to clarify the equatorial electrojet effects on ground magnetic pulsations in central South America, we statistically analyzed the amplitude structure of Pc3 and Pc5 pulsations recorded during quiet to moderately disturbed days at multiple equatorial stations nearly aligned along the 10° magnetic meridian. It was observed that Pc3 amplitudes are attenuated around noon at the dip equator for periods shorter than ~ 35 s. It is proposed that daytime Pc3s are related to MHD compressional waves incident vertically on the ionosphere, with the screening effect induced by enhanced conductivity in the dip equator causing wave attenuation. Daytime Pc5s showed amplitude enhancement at all equatorial stations, which can be explained by the model of waves excited at higher latitudes and propagating equatorward in an Earth-ionosphere waveguide. However, a slight depression in Pc5 amplitude compared to neighboring equatorial stations and a phase lag in relation to an off-equatorial station were detected at the dip equator. This result cannot be explained by the ionospheric waveguide model alone and we propose that an alternative propagation model that allows ULF waves to penetrate directly from the magnetosphere to low latitudes could be operating simultaneously to produce these features at the dip equator. Significant effects of the sunrise terminator on Pc3 pulsations were also observed at the stations closest to the dip equator. Contrary to what is reported at other longitudes, in central South America the sunrise effect increases the H-to-D amplitude ratio. We suggest that these differences may arise from the unique characteristics of this sector, with a strong longitudinal variation in the magnetic declination and precipitation of energetic particles due to the presence of the South Atlantic Magnetic Anomaly. The H-component amplification can be explained by enhancements of the zonal electric field near the magnetic equator driven by F-region neutral winds during sunrise.