Geomagnetic Quiet Periods Research Articles

Effects of Solar Activity on Ionospheric Ion Upflow During Geomagnetic Quiet Periods: DMSP Observations

Abstract Based on the Defense Meteorological Satellite Program (DMSP) observations during Solar Cycle 23, this paper examines solar activity dependence of ionospheric bulk ion upflow events (IUEs) in the Southern Hemisphere (SH). Much previous similar work was conducted over the Northern Hemisphere (NH) with measurements from European Incoherent Scatter (EISCAT). To eliminate the influence of geomagnetic disturbance on IUEs, we pick out observations during geomagnetic quiet periods (with Kp ≤ 2+). Results show that, ion upward densities and fluxes are dramatically elevated at times of high solar activity (HSA) but ion upward drifts and occurrences are increased at times of low solar activity (LSA) in the SH, which is consistent with the situation in the NH. The ratios between HSA and LSA for these four parameters (IUEs’ density, flux, upward drift and occurrence) are ~2.71, ~1.98, ~0.76 and ~0.57, respectively. Furthermore, lower flux event takes place frequently at LSA as the background ion density is low but the upward drift is large, while higher flux event happens commonly at times of HSA accompanied by high ion density but low upward velocity. Quantitatively, an increase in unit of solar activity (characterized by P index) causes a 4.2×108 m−3 increase in ion density and a 1.2×1011 m−2·s−1 enhancement in upward flux, together with a 0.6 m·s−1 and 0.02 % decrease in ion upward velocity and uprate, respectively. The acceleration from the ambipolar electric field is thought to be a possible mechanism affecting the dependence of IUEs on solar variations. For HSA, the acceleration from the ambipolar electric field weakens, but a large number of background ions provide abundant seeds for acceleration and upflow, which maintains a high IUE flux. It is inferred that upflow events and upward drifts are inhibited by the enhanced ionospheric background density.

Heavy ion acceleration by EMIC waves in the near-Earth plasma sheet

Heavy ions (O+ and He+) are an important constituent of plasma sheets. They are most significant during geomagnetic quiet periods when charge-exchange reactions result in a faster loss of H+ ions than heavy ions. With inward injections of energetic particles from the plasma sheet into the inner magnetosphere, heavy ions play a key role in producing the ring current and radiation belt. In this work, we investigate the acceleration of heavy ions by electromagnetic ion cyclotron (EMIC) waves, which has been recently observed in the near-Earth plasma sheet. Nonlinear motion of ions, including phase trapping and strong acceleration, can be induced by EMIC waves through cyclotron resonance. The frozen-in condition of the plasma is not satisfied in this process. The results of test particle simulations show that heavy ions following wave-induced nonlinear motion from L = 5–7 increase from 36% of the total test particles to 44%. The He+ ions at L = 5 can be accelerated to as much as ten times the initial energy of 30 keV. The attainable energy increase ΔEk depends on the L value and the ion composition. An abundance of heavy ions in the background plasma lowers ΔEk. This effect is more prominent farther out in the plasma sheet. Nonlinear phase trapping occurs when ions are accelerated, and stronger acceleration corresponds to a smaller trapping zone. Hence, wave-induced acceleration is closely associated with the redistribution of ions in their phase space. Heavy ions in the plasma sheet have an important impact on inner magnetosphere dynamics, and thus, EMIC waves should not be neglected in plasma sheet energization.

Post sunset equatorial spread-F at Kwajalein and interplanetary magnetic field

We connect the time sequence of changes in the IMF-Bz to the development of spread-F at an equatorial station Kwajalein on three different nights in November 2004, one during a geomagnetic quiet period and other two during geomagnetic disturbed periods. The chosen days show clear and smooth variations of IMF-Bz without any large fluctuations thereby enabling one to correlate changes in equatorial spread-F with corresponding changes in IMF-Bz. It is shown that a slow and continuous increase in the IMF-Bz over a duration of few hours has a similar effect on the equatorial ionosphere as of a sudden northward turning of the IMF-Bz in causing an electric field through the polar region and then to the equator. We conclude that the Spread-F at equatorial and low latitudes are due to echoes from ionization irregularities that arise due to the plasma instabilities generated by an eastward electric field on the large plasma density gradient in or below the base of the F-layer during any period of the night time along with the gravity driven Rayleigh-Taylor instability.

Particle precipitation prior to large earthquakes of both the Sumatra and Philippine Regions: A statistical analysis

A study of statistical correlation between low L-shell electrons precipitating into the atmosphere and strong earthquakes is presented. More than 11years of the Medium Energy Protons Electrons Detector data from the NOAA-15 Sun-synchronous polar orbiting satellite were analysed. Electron fluxes were analysed using a set of adiabatic coordinates. From this, significant electron counting rate fluctuations were evidenced during geomagnetic quiet periods. Electron counting rates were compared to earthquakes by defining a seismic event L-shell obtained radially projecting the epicentre geographical positions to a given altitude towards the zenith. Counting rates were grouped in every satellite semi-orbit together with strong seismic events and these were chosen with the L-shell coordinates close to each other. NOAA-15 electron data from July 1998 to December 2011 were compared for nearly 1800 earthquakes with magnitudes larger than or equal to 6, occurring worldwide. When considering 30–100keV precipitating electrons detected by the vertical NOAA-15 telescope and earthquake epicentre projections at altitudes greater that 1300km, a significant correlation appeared where a 2–3h electron precipitation was detected prior to large events in the Sumatra and Philippine Regions. This was in physical agreement with different correlation times obtained from past studies that considered particles with greater energies. The Discussion below of satellite orbits and detectors is useful for future satellite missions for earthquake mitigation.

Analysis of the north–south asymmetry of the equatorial ionization anomaly around 110°E longitude

The north–south asymmetry of the equatorial ionization anomaly (EIA) is studied using the total electron content (TEC) data measured by GPS observations from the network of International GNSS Service (IGS) in the 110°E longitude sector for more than a solar cycle term from 2000 to 2011. In this paper, we focus on the hemispheric asymmetry between the northern and southern crest regions of EIA during the geomagnetic quiet periods. The parameters of EIA crest obtained from the TEC profiles in daytime for different conditions of solar activity have been used to study the solar cycle variations of EIA in the Southeast Asian region. The season hereinafter referred to the northern hemisphere except stated otherwise. Results show that the strength, latitudinal and occurrence time position of EIA crest are found to be asymmetric about the magnetic equator. During high solar activity the strength of northern crest is generally greater than that of the southern crest all the months, especially in the winter months. It is also found that the latitude and occurrence time position of EIA crest show a hemispheric asymmetry because (a) During low solar activity the northern crest of EIA moves equatorward in winter and the southern crest moves equatorward in summer and poleward in winter, while during high solar activity the southern crest moves slightly equatorward in summer, and (b) During low solar activity the occurrence time of northern crest is ∼1h later in winter and ∼2h earlier in summer than that of southern crest; During high solar activity the northern crest appears ∼1h earlier only from June to August. The conditions of solar activity have a significantly effect on EIA crest and the north–south asymmetry. These phenomena can be explained by a combined theory of the transequatorial neutral wind, the subsolar point and the auroral equatorward wind. Meanwhile, the condition of the solar activity should be considered.

On the relationship between the postmidnight thermospheric equatorial mass anomaly and equatorial ionization anomaly under geomagnetic quiet conditions

The equatorial mass anomaly (EMA) in the thermosphere and equatorial ionization anomaly (EIA) in the ionosphere are two interesting phenomena in low-equatorial latitude regions. Previous studies have shown that the EMA appears between 1000 and 2000 local time (LT) and its location of trough is aligned with dip equator, indicating the plausible role of the EIA structure in the development of EMA. In this report, we conducted a statistical study of the occurrence of postmidnight EMA and EIA on the basis of the CHAMP in situ measurements during 2002-2008. Our results revealed that clear EMA and EIA structures are sometimes visible in the postmidnight sector (0100-0600 LT) during geomagnetic quiet periods (Kp < 3). The postmidnight EMA is not necessarily accompanied by the EIA signature in both case study and statistics sense being distinct from the daytime situation. In addition, the occurrence rates of postmidnight EMA and EIA display contrasting behavior with respect to their local time, longitudinal and solar activity dependences. The highest occurrence rate for EMA is 8% at around 0300 LT, while the occurrence rate of the EIA decreases gradually from about 30% at around 2300 LT to similar to 5% at 0600 LT. Longitudinal occurrence of postmidnight EIA presents a wave-like pattern; however, no salient feature appears for the longitudinal occurrence of EMA. Postmidnight EMA is more likely to occur at lower solar activity, whereas an opposite trend presents in the EIA. On the basis of above results, our findings imply that a simple EIA-EMA cause-effect relationship does not hold in the postmidnight sector.

Tomographic imaging of the equatorial and low-latitude ionosphere over central-eastern Brazil

A four-dimensional time-dependent tomographic algorithm, named Multi Instrument Data Analysis System (MIDAS), is used to image the equatorial and low-latitude ionosphere over the central-eastern sides of the Brazilian territory. From differential phase data obtained by a chain of ground-based GPS receiver the total electron content (TEC) is estimated and then, together with a modeled ionosphere from International Reference Ionosphere (IRI) model, the electron density distribution is reconstructed and the parameters of the F2-peak layer are accessed from the images. This paper presents the first study of ionospheric tomography using real dual-frequency data from the Brazilian Network for Continuous GPS Monitoring (RBMC). Ionospheric F2-peak electron density (NmF2) accessed from the images are compared to concurrent measurements from three ionosondes installed across Brazil. One year of data during the solar maximum period from March/2001 to February/2002 is used to analyze the seasonal and hourly variation of the F2-layer peak density. The accuracy with which MIDAS images the electron density during geomagnetic quiet periods is investigated through its correlation and deviation with the ionosonde and IRI model data, respectively. The main aspects of the reconstruction results at the equatorial ionization anomaly (EIA) region over Brazil are highlighted and discussed.

Observations of GW/TID oscillations in the F2 layer at low latitude during high and low solar activity, geomagnetic quiet and disturbed periods

Ionospheric vertical sounding observations, using a digital ionosonde, are being carried out on a routine basis at Sao Jose dos Campos (23.2°S, 45.9°W; dip latitude 17.6°S, hereafter referred to as SJC), Brazil, located under the southern crest of the equatorial ionization anomaly (EIA), since August 2000. In this paper, we present and discuss the seasonal variation of gravity wave (GW) and traveling ionospheric disturbance (TID) oscillations in the ionospheric F2 layer during high solar activity (HSA, September 2000 to August 2001) and low solar activity (LSA, January 2006 to December 2006) observed at SJC during different levels of geomagnetic activity. The GW/TID signatures in the F2 layer can be seen in the isofrequency lines of virtual height daily variations for six fixed frequencies (3, 4, 5, 6, 7, and 8 MHz) which show quasiperiodic oscillations (crests and valleys). The crests and valleys when seen in close frequencies present a phase difference (i.e., first it is observed at higher frequency then at lower frequency), indicating a downward phase velocity. These quasiperiodic oscillations induced in the virtual heights are divided into three groups as small amplitude (lower than 40 km), medium amplitude (between 40 km and 60 km), and large amplitude (greater than 60 km). The observations show that GWs/TIDs are much more pronounced at F layer heights during HSA than LSA and the large‐amplitude GWs/TIDs are present normally only during HSA.

Investigation of precursory phenomena in the ionosphere, atmosphere and groundwater before large earthquakes of M > 6.5

Investigation of precursory phenomena in the ionosphere, atmosphere and groundwater before large earthquakes of M > 6.5

Latitudinal changes of polar hiss and plasmapause hiss associated with magnetospheric processes

The magnetospheric VLF (very low frequency, 3–30 kHz) hiss observed by a low altitude polar orbiter represents integrated spectra of whistler mode VLF hiss waves propagating along geomagnetic field lines from wide-altitude sources in the magnetosphere. We investigate latitudinal changes of the magnetospheric VLF hiss associated with magnetospheric processes in geomagnetic quiet and disturbed periods by using VLF hiss intensity data at six frequency bands processed from VLF electric field (50 Hz–30 kHz) data of ISIS-2 (circular polar orbit at 1400 km in altitude). The magnetospheric VLF hiss observed by the ISIS-2 mainly consists of a broad-band polar hiss and narrow-band plasmapause hiss. The polar hiss is the whistler mode Cerenkov emissions generated in the polar magnetosphere by inverted- V electrons (100 eV–40 keV) precipitating from the plasmasheet boundary layer. The plasmapause hiss is also whistler-mode waves excited by the cyclotron instability of energetic electrons convected inward from the magnetotail. In a geomagnetically very quiet period, only a plasmaspheric ELF (extremely low frequency, 0.3–3 kHz) hiss and diffused whistlers appear at invariant latitudes below 73°. The broad-band polar hiss and narrow-band plasmapause hiss occur, respectively, at night-side invariant latitudes from the middle of the auroral zone to the polar cap and for nighttime invariant latitudes of 50–64° in a quiet or weakly disturbed period. In a substorm period, the polar hiss region shifts to lower latitudes due to an inward movement of the plasmasheet inner edge associated with an intensified substorm westward electric field in the magnetotail. Also, an intensity hump of the plasmapause hiss appears between regions of the polar hiss and plasmapause hiss on the night side in a lingering substorm period. This seems to be generated in a plasmaspheric plume outside the night-time plasmapause. As the substorm develops in the expansion phase, the polar hiss region finally joins to the plasmapause hiss region. These show that the invariant latitudinal changes of magnetospheric VLF hiss generated by the micro-scale mechanism are greatly affected by the macro-scale dynamical processes in the magnetosphere.

Solar cycle variability of nighttime topside helium ion concentrations over Arecibo

The incoherent scatter radar mode and data extraction method used in topside experiments at the Arecibo Observatory are discussed. Helium ion concentrations in the lower topside ionosphere over Arecibo are presented for low and high solar flux periods of the same season (October). Data from 8–9 October 1988, 21–22 October 1995, 26–27 October 1997, and 12–13 October 2001 are compared. The data from October 1988 and 2001 are representative of high solar flux conditions, while the other two periods (1995 and 1997) are from low solar flux conditions. All data sets represent geomagnetic quiet periods over Arecibo and serve as examples of typical equinox topside conditions. For solar minimum the altitude distribution of the helium ions usually has a maximum near the O+ to H+ transition altitude () during the night, usually around 550–650 km. The He+ number densities tend to be quite low, 2 or 3 × 103 cm−3 or &lt;15% of the topside plasma density at the peak of the layer. For the higher solar flux case, however, He+ forms a more distinct layer at an altitude that is slightly above at night. The O+‐to‐He+ transition altitude () is reported to be below after the postsunset collapse in these solar‐maximum conditions, meaning that He+ is the dominant species. We find that the relative abundance of He+ can reach 60–65% of the topside plasma at the peak of this layer during these equinox conditions or up to 4 × 104 cm−3, with as low as 700 km and the He+ to H+ transition () extending over 1000 km at night.

Avalanches at the core‐mantle boundary

The partial collapse of topographic structure at the core‐mantle boundary (CMB) in avalanches, slumps or turbidity flows, would cause sudden temperature changes in both the upper core and the lower mantle. Although such collapses are hypothetical, it is interesting to investigate the potential consequences. Downwelling from such events could disrupt core convection cells and trigger geomagnetic excursions and reversals. Buoyant sediment from the freezing of the inner core is hypothesized to rebuild the avalanched structures. Large avalanches could trigger mantle plumes. Oblique extraterrestrial impacts impart high shear to the CMB, and can trigger one or more simultaneous avalanches, yielding observed coincidences between craters, tektite fields and reversals. A triggered avalanche can explain the coincidence between the formation of the largest known volcanic province (the Ontong‐Java Plateau), the start of the 35 Myr Cretaceous geomagnetic quiet period, and reported coincidences between large flood basalts and extinctions.

Thermospheric dynamics: A system theory approach

A system theory approach to thermospheric modeling is developed, based upon a linearization method which is capable of preserving nonlinear features of a dynamical system. The method is tested using a large, nonlinear, time‐varying system, namely the thermospheric general circulation model (TGCM) of the National Center for Atmospheric Research. We compare TGCM simulation results with results from our approximation procedure at Millstone Hill and Arecibo, two locations where there may be interest for using this approach. In the linearized version an equivalent system, defined for one of the desired TGCM output variables, is characterized by a set of response functions that is constructed from corresponding quasi‐steady state and unit sample response functions. The number of functions in the set depends on the degree of nonlinearity of the original system and the desired accuracy of the approximation. Using the set of response functions, we are able to approximate time‐dependent TGCM simulation results with reasonable accuracy. The linearized version of the system runs on a personal computer and produces an approximation of the desired TGCM output field height profile at a given geographic location. The approximation is obtained by performing a specialized convolution between the cross polar cap potential drop as input and the set of response functions. This enables an efficient first‐order comparison between experimental data and theoretical simulations and may provide an operationally efficient means of more realistically predicting density variations for satellite ephemeris or reentry trajectory determinations during geomagnetic quiet periods as well as during magnetic storms.

Thermospheric dynamics investigations with very high resolution spectrometers

Since 1972 high resolution Fabry-Perot spectrometers have been used at Fritz Peak Observatory (39.9 degrees N, 105.5 degrees W), Colorado to measure the nighttime variation of thermospheric temperatures and winds from the line profiles and Doppler shifts of the OI 15,867 K (630.0-nm) line emission in the nightglow. With the aid of these measurements we have defined the nighttime variation of winds and temperatures at F-layer heights for the various seasons of the year during geomagnetic quiet periods. During geomagnetic storm periods deviation in the nighttime variation of the winds and temperatures from those determined during geomagnetic quiet conditions have been shown to occur. In addition, measurements made during geomagnetic disturbed conditions have shown the existence of large-scale thermospheric waves generated at high latitudes by impulsive auroral heating events that are observed to propagate equatorward. The nighttime winds and temperatures measured from Fritz Peak Observatory have been used in various heoretical models of global thermospheric dynamics to infer the global circulation patterns, temperature structure, and thermospheric response to geomagnetic activity. By requiring agreement between the calculated and measured winds and temperatures over Fritz Peak Observatory, the over-all magnitude of the thermospheric high latitude heat source due to auroral processes has been inferred for both geomagnetic quiet and disturbed conditions. This energy source has been shown to be related to dissipation of the ring current energy in the high latitude ionosphere. The results of various geophysical studies using Fritz Peak Observatory data and theoretical model calculation are summarized.

Direct measurements of nighttime thermospheric winds and temperatures, 1. Seasonal variations during geomagnetic quiet periods

A high-resolution Fabry-Perot spectrometer has been used at Fritz Peak Observatory (39.9°N, 105.5°W) to determine the nighttime thermospheric temperature and winds from November 1973 through February 1975. The thermospheric temperature and winds at a height near the F2 peak are determined from the Doppler broadening and shift, respectively, of the atomic oxygen line emission at 15867.852 K (6300.308 Å). The experimentally derived temperatures and winds during geomagnetically quiet conditions are compared with a three-dimensional semiempirical model of the neutral thermosphere. This model in turn uses global empirical models of neutral temperature, composition, and electron density to determine the pressure forces and ion drag which generate and modify the wind fields. The large-scale details of measured and calculated nighttime meridional wind components are in general agreement, showing maximum equatorward winds during the summer months. Measured and calculated zonal winds agree for the equinoctial and winter months; however, the measured nighttime zonal winds are westward during summer months in contrast to model calculations that indicate a midnight eastward to westward transition. The measurements also indicate that both the temperature and the equatorward component of the meridional wind measured north of the observing station are greater than those measured south of the station. These positive latitudinal temperature and wind gradients are greatest during the summer months.

Contribution à l'étude des relations entre les activités solaire et géomagnétique

The article states the necessary conditions under which geomagnetic storms and disturbances arise, as revealed by observations of the Sun. The new criteria do not conflict with those previously developed, based on other phenomena. On the strength of these new ideas a method for forecasting geomagnetic activity has been elaborated, enabling a sudden commencement to be predicted with great accuracy. It makes possible the forecasting of geomagnetic storms in each period of the solar cycle, and to say in advance whether the onset of a given storm will be sudden or gradual. As examples for the purpose of explaining their origin, use has been made of the suddencommencement geomagnetic storms of llth February 1958, 20th September 1959, 29th October 1963, and 22nd November 1963. Except for the first, all these storms had been predicted on the basis of observations of the Sun by means of the spectrohelioscope. Furthermore, an explanation is given of the origin of the gradual-onset geomagnetic storm of 6th and 7th October 1960, which had likewise been predicted. On the same principles explanations are given of the geomagnetic quiet period of 19th–25th June 1965, which recurred six times (16th–17th July, 13th August, 8th–9th September, 6th–7th October, 1st–3rd November and 28th–29th November 1965) and of the one that occurred twice—on 31st December 1965 and 1st January 1966 and on 27th–28th January 1966.

Variations of the earth's exosphere and geomagnetic disturbances during the geomagnetic storm of 4 September 1958

The time variations of D s and D st for the geomagnetic storm of 4 September 1958 have been calculated using hourly values of the horizontal component of the geomagnetic field over the world. These D st and D s have been compared with the observation of the earth's exosphere by Explorer IV and that of auroral radio echoes during the same period, During this geomagnetic storm marked decrease in the intensity of trapped radiations was observed in the outer zone and brilliant auroral displays were observed at latitudes well below the auroral zone. The time variations of D s at geomagnetic latitudes of 57–58° have indicated the largest deviation of all ones at other latitudes. All peaks of D s at high latitudes and auroral radio echoes at Jodrell Bank occurred for the main phase of this geomagnetic storm. The time variations of D s at low latitudes have shown only a little deviation and have been almost relatively unvariable compared with ones at high latitudes during the geomagnetic storm. From the analyses of D s at high latitudes it is deduced that the exospheric region within the inner boundary of the outer zone, where is is located along the geomagnetic lines of force intersecting the earth's surface at geomagnetic latitudes of about 50°, was not so much disturbed by the invasion of solar plasma cloud. It is considered that the dumping of part of the contents of the outer earth's exosphere, which interprets well the reduction in the intensity of trapped radiations in the outer exosphere, produces regions of comparatively high ionization which are detectable as auroral radio echoes and D s of geomagnetic disturbances. It has been shown that the time variations of D s at the auroral and the sub-auroral zones represent reasonably well the extent of disturbance of trapped radiations in the outer exosphere during the geomagnetic storm. The equatorward shifting of latitude of the maximum strength of chorus and hiss during the geomagnetic storm may be due to the inaction of generation mechanism of v.l.f. emissions in the exospheric region where the geomagnetic lines of force intersecting the auroral zone pass through during the geomagnetic quiet period, in addition to the equatorward shifting of precipitation of auroral particles during the geomagnetic storm. Statistical results of v.l.f. emission suggest that the electron density distribution given by the gyrofrequency model may not exist along the geomagnetic lines of force intersecting the earth's surface at the auroral zone during the geomagnetic storm. D st at the auroral zone has shown larger deviation from the prestorm level for the main phase, sharper recovery after the main phase, and earlier recovery for the recovery phase than ones at the middle and the equatorial zones. This slow and late recovery of equatorial D st and small variation of equatorial D s may support the existence of the equatorial ring current around the earth. A simple model of the state of earth's exosphere during the main phase of the storm has been proposed.