Magnetic field inclination angle at geosynchronous orbit

Simultaneous magnetic field data from the geostationary satellites GOES 2 and GOES 3 have shown significant differences between the inclination angles measured at these two satellites. The relationship between the two inclination angles is examined for three magnetic activity groups and eight local time sectors by using data from GOES 2 and GOES 3 for the 12-month period from March 1979 to February 1980. During this time GOES 2 was 1.09 R_E off the geomagnetic equator in the northern geomagnetic hemisphere, while GOES 3 was separated from GOES 2 by 2 h L.T. and 0.47 R_E off the geomagnetic equator, also in the northern geomagnetic hemisphere. The average inclination angle is found to differ substantially from the predictions based on the previously published magnetic field models. The relationship between the two inclination angles is fitted by a simple model of magnetic field perturbation that varies as a power of distance from the Equator. During magnetic disturbances, the spatial variation of magnetic field perturbation on the nightside suggests that the ring current density increases with distance from the geomagnetic equator.     

Es-q layer at huancayo during the March 1970 geomagnetic storm

The paper describes a comparison of vertical electron drift in the F-region (V_z) measured by VHP incoherent scatter radar at Jicamarca with the corresponding variations of geomagnetic horizontal field (H) and the maximum frequency reflected from The E_s layer (E_s) at Huancayo during the geomagnetic storm period 7–9 March, 1970. The V_z is generally upward during the daytime at the equator, but during 7–9, March, 1970, V_z was negative for brief periods associated with negative bays in H. These periods of abnormally low or of downward V_z correspond closely with the period of complete disappearance of the q type of E_s layer. The magnetic bays associated with the intensification of ring current do not affect the equatorial E_s- q and it is only the negative bays in H at the equator due to the ionospheric current flowing westward, that cause sudden disappearance of E_s? q. It is suggested that the q type of E_s is due to cross-field instability created in the electrojet region due to interaction of northward magnetic field and vertical upward Hall polarization electric field when the plasma density gradient is upward. The sudden disappearances of E_s? q are due to the reversal of the horizontal electric field in the equatorial ionosphere and thereby due to the reversal of the equatorial electrojet currents. These reversals of electric field may be due to the imposition on the normal S_q field of another westward electric field.     

Non-thermal continuum emissions associated with electron injections: Remote plasmapause sounding

Energetic electron injection events result in the arrival of loss-cone distributions of electrons at energies of a few keV close to the plasmapause at local midnight. These distributions favour the growth of strong electrostatic waves with some conversion to electromagnetic nonthermal continuum emissions near to the geomagnetic equator.GEOS2 located at the geostationary orbit (L = 6.6, 3.3° South) has observed these continuum emissions for a number of electron injection events. Their unique frequency structure provides a measurement of the geomagnetic field strength at the source and hence its radial position, while direction finding measurements at GEOS2 complete the source location determination.Measurements of source locations as a function of time after the start of an electron injection event, yield typical inwards motions of 1R_Eh^?1. In this way the emissions provide a remote sensing of the plasmapause location from the geostationary orbit.     

Giant PC5 pulsations in the outer magnetosphere: A survey of HEOS-1 data

Magnetic field measurements from 133 low-latitude transits of the HEOS-1 satellite through the magnetosphere have been used to analyse the low-frequency pulsation activity in the outer regions of the geomagnetic field. Providing full longitude coverage in the sunward hemisphere at geocentric distances larger than ~7.5 R_e, this survey complements previous low-frequency pulsation data from satellites at smaller geocentric distances. Several giant PC5 events, each being mainly compressional and lasting 1–2 hr, are described in detail and it is shown that this phenomenon is relatively common in the 8–12 R_e, geocentric distance range near dusk. A depression of the ambient field magnitude always accompanied the events, suggesting that they are associated with a region of enhanced plasma pressure. The properties of these wave events are compared with the predictions of current micropulsation theories involving a Kelvin-Helmholtz generation mechanism and field-line resonance. Unlike the PC5 events observed nearer Earth, these events were not obviously related to periods of enhanced geomagnetic activity.     

Formation of proton fluxes under the radiation belts during geomagnetic disturbances

The fluxes of energetic particles under the radiation belts are studied using data obtained in the experiments onboard the CORONAS-I and CORONAS-F satelites. The spatial structure of the distributions of proton fluxes with E _p > 1 MeV both near the geomagnetic equator on L ≤ 1.2 and at high latitudes on L ～ 3.5–6.5 as well as the particle flux variations with geomagnetic activity are analyzed. The scattering processes that lead to particle precipitation and, in particular, the scattering of protons as they interact with VLF emission and the scattering when the particle motion becomes nonadiabatic are considered. We compare the data on particle dynamics during geomagnetic disturbances of various kinds to determine whether the physical processes that lead to particle precipitation are a manifestation of the geoefficiency of a given magnetic storm or they are controlled by internal magnetospheric conditions.     

Pc3-4 geomagnetic pulsations at very low latitude in Brazil

In order to investigate Pc3-4 geomagnetic pulsations at very low and equatorial latitudes, L=1.0 to 1.2, we analyzed simultaneous geomagnetic data from Brazilian stations for 26 days during October-November 1994. The multitaper spectral method based on Fourier transform and singular value decomposition was used to obtain pulsation power spectra, polarization parameters and phase. Eighty-one (81) simultaneous highly polarized Pc3-4 events occurring mainly during daytime were selected for the study. The diurnal events showed enhancement in the polarized power density of about 3.2 times for pulsations observed at stations close to the magnetic equator in comparison to the more distant ones. The phase of pulsation observed at stations near the magnetic equator showed a delay of 48-62° in relation to the most distant one. The peculiarities shown by these Pc3-4 pulsations close to the dip equator are attributed to the increase of the ionospheric conductivity and the intensification of the equatorial electrojet during daytime that regulates the propagation of compressional waves generated in the foreshock region and transmitted to the magnetosphere and ionosphere at low latitudes. The source mechanism of these compressional Pc3-4 modes may be the compressional global mode or the trapped fast mode in the plasmasphere driving forced field line oscillations at very low and equatorial latitudes.     

Disappearence of equatorial Es, associated with magnetic field depressions

The phenomenon of the sudden disappearance of equatorial sporadic E at two stations, namely, Trivandrum and Kodaikanal is studied. It is established that whenever there is a sudden disappearance of E_s, there is a depression in the horizontal magnetic field (H) range. Electron velocities during the presence and absence of sporadic E have been estimated. These results show that the irregularities responsible for sporadic E are present even when the electron velocities are less than the ion thermal velocity.     

Electrical coupling of the E- and F-regions and its effect on F-region drifts and winds

Electric currents, generated by thermospheric winds, flow along the geomagnetic field lines linking the E-and F-regions. Their effects on the electric field distribution are investigated by solving the electrical and dynamical equations. The input data include appropriate models of the F-region tidal winds, the thermospheric pressure distribution and the E-and F-layer concentrations. At the magnetic equator, the calculated neutral air wind at 240 km height has a prevailling eastward component of 55 m sec^-1 and the west-east and vertical ion drifts agree in their general form with incoherent scatter data from Jicamarca     

Extension of a polar ionospheric current to the nightside equator

A detailed analysis of rapid-run magnetograms from Guam (geomagnetic latitude = 4.2°) revealed that there are two kinds of geomagnetic sudden commencement (SC) observed in nighttime. One is the ordinary SC consisting of a main impulse only which has a smooth rise of the H-component. The other is a superposition by a small positive impulse on the very beginning part of the smooth rise of the main impulse and consequently the SC starts with a small stepwise increase of the H-component. The latter type of SC occurs between 20 and 08 h L.T. and its occurrence rate takes the maximum value of about 50% around 03 h L.T. Corresponding magnetograms from a dayside equatorial station (Huancayo, geomagnetic latitude = ?0.7°) were examined and a good correlation was found between the stepwise SC at the nightside (Guam) and SC¹ with a preliminary reverse impulse (PRI) at the dayside (Huancayo). Since PRI observed at the dayside equator may be interpreted as an extension of an ionospheric current due to an dusk-to-dawn electric field impressed on the polar ionosphere, our results show that a polar originating ionospheric current can extend to the nightside equator and produce a small but observable magnetic effect in spite of much reduced nighttime ionospheric conductivity.     

Flapping motions of the tail plasma sheet induced by the interplanetary magnetic field variations

Flapping motions of the magnetotail with an amplitude of several earth radii are studied by analysing the observations made in the near (x = ?25 ~ ?30 R_E and the distant (x? ?60 R_E) tail regions. It is found that the flapping motions result from fluctuations in the interplanetary magnetic field, especially Alfvénic fluctuations, when the magnitude of the interplanetary magnetic field is larger than ~10 γ and they propagate behind the Earth with the solar wind flow. Flappings tend to be observed in early phases of the magnetospheric substorm, and they have two fundamental modes with periods of ~200 and ~500 sec. In some limited cases a good correspondence with the long period micropulsations (Pc5) in the polar cap region is observed. These observational results are explained by the model in which the Alfvénic fluctuations in the solar wind penetrate into the magnetosphere along the connected interplanetary-magnetospheric field lines. The characteristics of the flapping reveal that the geomagnetic tail is a good resonator for the hydromagnetic disturbances in the solar wind.     

Solar transients disturbing the terrestrial magnetic environment at higher latitudes

Geomagnetic field variations during five major Solar Energetic Particle (SEP) events of solar cycle 23 have been investigated in the present study. The SEP events of 1 October 2001, 4 November 2001, 22 November 2001, 21 April 2002 and 14 May 2005 have been selected to study the geomagnetic field variations at two high-latitude stations, Thule (77.5^° N, 69.2^° W) and Resolute Bay (74.4^° E, 94.5^° W) of the northern polar cap. We have used the GOES proton flux in seven different energy channels (0.8–4 MeV, 4–9 MeV, 9–15 MeV, 15–40 MeV, 40–80 MeV, 80–165 MeV, 165–500 MeV). All the proton events were associated with geoeffective or Earth directed CMEs that caused intense geomagnetic storms in response to geospace. We have taken high-latitude indices, AE and PC, under consideration and found fairly good correlation of these with the ground magnetic field records during the five proton events. The departures of the H component during the events were calculated from the quietest day of the month for each event and have been represented as ΔH _THL and ΔH _RES for Thule and Resolute Bay, respectively. The correspondence of spectral index, inferred from event integrated spectra, with ground magnetic signatures ΔH _THL and ΔH _RES along with Dst and PC indices have been brought out. From the correlation analysis we found a very strong correlation to exist between the geomagnetic field variation (ΔHs) and high-latitude indices AE and PC. To find the association of geomagnetic storm intensity with proton flux characteristics we derived the correspondence between the spectral indices and geomagnetic field variations (ΔHs) along with the Dst and AE index. We found a strong correlation (0.88) to exist between the spectral indices and ΔHs and also between spectral indices and AE and PC.     

Focusing and interference of whistler-mode waves in a spatially varying magnetic field

An analytic investigation is made of rays from a source in a magneto-ionic medium for which the magnetic field H and the electron density vary, in a Cartesian co-ordinate system Oxyz, with x alone. The case H_z = 0 is considered in some detail. It is shown that rays in the xy-plane may be focused, and that interference can occur. The geomagnetic field is then represented by a model for which H_y is proportional to x, and the magnetic field lines are parabolas. Observations and theories of magnetospheric chorus, in particular of the gap near one-half the equatorial electron gyrofrequency, are discussed in relation to this model.     

On the nature of polar cap magnetic activity during undisturbed periods

The correlation between the polar cap geomagnetic variations (H-traces) and the changes of the azimuthal (Y_SE) and vertical (Z_SE) components of the interplanetary magnetic field (IMF) during undisturbed periods is examined. It is shown that peak-to-peak correlation between Y_SE and geomagnetic horizontal component variations may be generally observed in the daytime cusp region, independently of the magnitude and polarity of the Z_SE. The existence of the DP₃ disturbances associated with the northward component Z_SE > 0 is confirmed. It is shown that the disturbances due to the vertical component of the IMF dominate in the region near the pole. In so far as the southward component of the IMF generates both polar cap disturbances and geomagnetic substorms, the disturbances in the region near the pole, associated with Z_SE < 0, may be regarded as a precursor of a substorm. On this basis a new index of the polar cap magnetic activity PC_L, characterizing the changeability of the magnetic field is proposed. It is shown that the increase of the PC_L index is followed in 1–2 hr by a substorm in 70% of events considered.     

A theoretical study of the ionospheric F region equatorial anomaly—II. results in the American and Asian sectors

When observed noontime values of the maximum electron density, NMAX(F2), in the ionospheric F2 region are plotted as a function of magnetic latitude, a curve is produced which has two peaks, one on either side of the dip equator at ±16° dip latitude. This paper theoretically investigates the daily variation of this latitudinal distribution in NMAX(F2) (the so-called Appleton or equatorial anomaly) and specifically attempts to account for the longitudinal differences observed between the American and Asian sectors.In Part II, models of the neutral atmosphere, production, loss and diffusion rates, neutral wind, and electric field are described and the electron densities obtained by solving the continuity equation utilizing these models are presented. In each sector, the extent to which the equatorial anomaly's daily variation is affected by changes in the geomagnetic field configuration, neutral wind, and $\text{E}\text{×}\text{B}$ drift is examined. It is found that development of the anomaly is most sensitive to the electric field model assumed, and that the observed differences at the magnetic equator between the American and Asian sectors could be accounted for by an upward $\text{E}\text{×}\text{B}$ drift which commences an hour or two earlier in the Asian sector.     

Plasmaspheric hiss observed in the topside ionosphere at mid- and low-latitudes

Latitudinal characteristics of ELF hiss in mid- and low-latitudes have been statistically studied by using ELF/VLF electric field spectra (50 Hz-30 kHz) from ISIS-1 and -2 received at Kashima station, Japan from 1973 to 1977. Most ISIS ELF/VLF data observed in mid- and low-latitude include ELF hiss at frequencies below a few kHz. The ELF hiss has the strongest intensity among VLF phenomena observed by the ISIS electric dipole antenna in mid- and low-latitudes, but the ELF hiss has no rising structure like the chorus in the detailed frequency-time spectrum. The ELF hiss is classified into the steady ELF hiss whose upper frequency limit is approximately constant with latitude and the ELF hiss whose upper frequency limit increases with latitude. These two types of ELF hiss occur often in medium or quiet geomagnetic activities. Sometimes there occurs a partial or complete lack of ELF hiss along an ISIS pass.Spectral shape and bandwidth of ELF hiss in the topside ionosphere are very similar to those of plasmaspheric hiss and of inner zone hiss. The occurrence rate of steady ELF hiss is about 0.3 near the geomagnetic equator and decreases rapidly with latitude around L = 3. Hence it seems likely that ELF hiss is generated by cyclotron resonant instability with electrons of several tens of keV in the equatorial outer plasmasphere beyond L = 3.Thirty-seven per cent of ELF hiss events received at Kashima station occurred during storm times and 63% of them occurred in non-storm or quiet periods. Sixty-seven per cent of 82 ELF hiss events during storm times were observed in the recovery phase of geomagnetic storms. This agrees with the previous satellite observations of ELF hiss by search coil magnetometers. The electric field of ELF hiss becomes very weak every 10 s, which is the satellite spin period, in mid- and low-latitudes, but not near the geomagnetic equator. Ray tracing results suggest that waves of ELF hiss generated in the equatorial outer plasmasphere propagate down in the electrostatic whistler mode towards the equatorial ionosphere, bouncing between the LHR reflection points in both the plasmaspheric hemispheres.     

Directional currents in nocturnal E-region layers

In the mid-latitude E-region, the wind-shear mechanism produces thin ionized layers at levels where the vertical ion velocity is zero. We show that such layers conduct electric current only towards the magnetic equator, and not in the zonal direction. We surmise that this property may influence the electric field distribution in the nocturnal ionosphere, and possibly also the coupling between ion drifts and neutral air winds in the F-region. Detailed case studies of nocturnal layers located near the peak of ion Pedersen conductivity (around 130km) are needed to test this idea.     

Effects of interplanetary magnetic field and magnetospheric substorm variations on the dayside aurora

Photometric observations of dayside auroras are compared with simultaneous measurements of geomagnetic disturbances from meridian chains of stations on the dayside and on the nightside to document the dynamics of dayside auroras in relation to local and global disturbances. These observations are related to measurements of the interplanetary magnetic field (IMF) from the satellites ISEE-1 and 3. It is shown that the dayside auroral zone shifts equatorward and poleward with the growth and decay of the circum-oval/polar cap geomagnetic disturbance and with negative and positive changes in the north-south component of the interplanetary magnetic field (B_z). The geomagnetic disturbance associated with the auroral shift is identified as the DP2 mode. In the post-noon sector the horizontal disturbance vector of the geomagnetic field changes from southward to northward with decreasing latitude, thereby changing sign near the center of the oval precipitation region. Discrete auroral forms are observed close to or equatorward of the ΔH = 0 line which separates positive and negative H-component deflections. This reversal moves in latitude with the aurora and it probably reflects a transition of the electric field direction at the polar cap boundary. Thus, the discrete auroral forms observed on the dayside are in the region of sunward-convecting field lines. A model is proposed to explain the equatorward and poleward movement of the dayside oval in terms of a dayside current system which is intensified by a southward movement of the IMF vector. According to this model, the Pedersen component of the ionospheric current is connected with the magnetopause boundary layer via field-aligned current (FAC) sheets. Enhanced current intensity, corresponding to southward auroral shift, is consistent with increased energy extraction from the solar wind. In this way the observed association of DP2 current system variations and auroral oval expansion/contraction is explained as an effect of a global, ‘direct’ response of the electromagnetic state of the magnetosphere due to the influence of the solar wind magnetic field. Estimates of electric field, current, and the rate of Joule heat dissipation in the polar cap ionosphere are obtained from the model.     

Magnetic field variations of the SI in the magnetosphere and correlated effects in interplanetary space

Explorer 26 magnetic field data in the magnetospheric region of L=3?6 and LT 1100–1500 hr with geomagnetic latitude range ?6° to 27° have been analyzed for studying nineteen SI and SC events. Most of the SI events observed in the magnetosphere at less than 15° geomagnetic latitude are compressional with magnetic perturbations along the ambient field. Elliptic polarizations with magnetic field variations in all three components have been observed between 10° and 27° geomagnetic latitude. Polarization directions have been shown to have similar patterns to those observed in the surface magnetic field data. Afternoon LT zone data in the magnetosphere indicate polarization patterns in general agreement with the results of Wilson and Sugiura (1961) obtained earlier from surface observations. The SI/SC perturbations are also qualitatively shown to be related to changes in the interplanetary magnetic field observed beyond 1 a.u.     

Formation of blanketing sporadic E-layers at the magnetic equator due to horizontal wind shears

A careful survey of quarter-hourly ionograms for the year 1969 has shown that blanketing layer type ionization irregularities occur on many occasions in the E-region of the ionosphere over Trivandrum (dip ～ 0.6°S). It is shown that horizontal shears of horizontal neutral winds are the most likely sources of such layer type irregularities at the magnetic equator. The horizontal wind shears of required magnitude are shown to be provided by internal gravity waves of short period. The rarity of E_s-layer occurrence is attributed to the stringent requirements on the amplitude and wave vector orientation of the relevant gravity waves generating the E_s-layers.     

Ionospheric and magnetospheric “plasmapauses”

During August 1972, Explorer 45 orbiting near the equatorial plane with an apogee of ～5.2 R_e traversed magnetic field lines in close proximity to those simultaneously traversed by the topside ionospheric satellite ISIS 2 near dusk in the L range 2.0–5.4. The locations of the Explorer 45 plasmapause crossings (determined by the saturation of the d.c. electric field double probe) during this month were compared to the latitudinal decreases of the H⁺ density observed on ISIS 2 (by the magnetic ion mass spectrometer) near the same magnetic field lines. The equatorially determined plasmapause field lines typically passed through or poleward of the minimum of the ionospheric light ion trough, with coincident satellite passes occurring for which the L separation between the plasmapause and trough field lines was between 1 and 2. Hence, the abruptly decreasing H⁺ density on the low latitude side of the ionospheric trough is not a near earth signature of the equatorial plasmapause. Vertical flows of the H⁺ ions in the light ion trough as detected by the magnetic ion mass spectrometer on ISIS were directed upward with velocities between 1 and 2 km s^?1 near dusk on these passes. These velocities decreased to lower values on the low latitude side of the H⁺ trough but did not show any noticeable change across the field lines corresponding to the magnetospheric plasmapause. The existence of upward accelerated H⁺ flows to possibly supersonic speeds during the refilling of magnetic flux tubes in the outer plasmasphere could produce an equatorial plasmapause whose field lines map into the ionosphere at latitudes which are poleward of the H⁺ density decrease.