Disturbance Dynamo Effects Research Articles

Observed and modeled thermosphere and ionosphere response to superstorms

Observations and numerical simulations of the response of the thermosphere and ionosphere to “superstorms” illustrate that multiple processes are operating. The initial response at high latitude is thermospheric heating, thermal expansion, high‐velocity winds, wave surges, the initiation of a new global circulation, and the start of neutral composition changes. At low latitude, the initial response is driven by the penetration of magnetospheric electric fields, moving the equatorial ionization anomaly poleward, and enhancing both F region plasma densities and the total electron content at midlatitudes and low latitudes. Electron content also increases dramatically at the higher altitudes. In the later stages of the response, plasma densities begin to respond to the changing circulation; the transport of composition changes to midlatitudes and low latitudes; and the generation of disturbance dynamo effects, which either compete or combine with penetration fields. The observations and modeling indicate that all the processes have a significant impact at some time and place during the storm.

Multi-longitude case studies comparing the interplanetary and equatorial ionospheric electric fields using an empirical model

Electric fields have been determined at three longitudes corresponding to Peru, India, and the Philippines. We compare these fields to applying a frequency-dependent linear transfer function (TF) to the dawn-to-dusk component of the interplanetary electric field (IEF). The TF is based on four years of simultaneous observations of the IEF and equatorial data. The model gives good results for the prompt penetrating electric field (PPE) in the case of an oscillatory IEF with a period in the 1–2 h range, when the interplanetary magnetic field remains southward for a long period and, to a lesser extent, when the IEF can be described as a square wave. There is evidence that a disturbance dynamo (DD) effect contributes on the dayside, where it leads to suppression of the normal quiet time pattern. A very strong counter-electrojet was seen at two locations during a time of persistent B z south and was not predicted by the model or a linear scaling of the IEF. This suggests that suppression (and even reversal) of the E-region dynamo can occur in a large storm. Both the data and the model yielded a long-lived response to a sustained southward interplanetary magnetic field. Previously suggested B y effects on equatorial electric fields are confirmed by a sequence of three distinct spikes in the B y component of the IMF, one of which had no associated B z change and yet was reproduced by two independent ground magnetometer-based electric field determinations. The sometimes remarkable agreement of a linear relationship between the equatorial and interplanetary electric fields shown here and elsewhere remains somewhat mysterious for such an apparently complex system.

Geostationary L-band signal scintillation observations near the crest of equatorial anomaly in the Indian zone

Microwave L-band signal at 1.5 GHz from geostationary satellite INMARSAT (64°E) has been recorded on a continuous basis since 1990 at Calcutta (22.58°N, 88.38°E geographic; 32°N magnetic dip). The location of Calcutta, virtually under the northern crest of the equatorial anomaly in the Indian longitude sector, provides an excellent platform for studying equatorial scintillations. This paper presents a long-term study of scintillations at 1.5 GHz observed from Calcutta extending from minimum to high sunspot number (1996–2000). The variations of scintillations with local time, season, solar and magnetic activity are presented. L-band scintillations have been found to be essentially a pre-midnight phenomenon during the equinoctial months of high sunspot number years. Both the occurrence and intensity increase dramatically with solar activity level. During the solar maximum phase of 1998–2000, scintillations in excess of 20 dB occurred for 20% of total time in the pre-midnight hours of March 2000. Considering individual hours, a maximum percentage occurrence of 37 was recorded in the interval 19–20 LT. Although the percentage occurrence appears to be low, scintillations with Scintillation Index (SI)>20 dB were recorded on 23 out of 31 nights during the same month, indicating that scintillations do not occur continuously over a long interval of time, but in small patches with clear gaps in between. The distribution of patch duration during years 1998–2000 has a maximum in the time interval of 1–3 min. From the cumulative distributions of patch duration with SI⩾10 and 20 dB, the 99th percentile values of patch duration have been found to be 49.3 and 39.6 min, respectively. This information may be used to obtain an estimate of longitudinal separation of geostationary satellites to be used in Satellite Based Augmentation System (SBAS). During the equinoctial months of August–October and February–April, occurrence of scintillations is less during magnetically active (Kp=3+–9) conditions than during quiet (Kp=0−3). In the local summer months of May–July, the probability of occurrence of scintillations is more under magnetically active condition than in quiet condition. Equatorial scintillation is essentially controlled by the pre-reversal enhancement of the eastward electric field near sunset, which in turn has season and solar activity dependences. During magnetic activity, the generation of equatorial irregularities resulting in scintillations may be ascribed to the prompt penetration of magnetospheric electric field and disturbance dynamo effects.

Concurrent study of bottomside spread F and plasma bubble events in the equatorial ionosphere during solar maximum using digisonde and ROCSAT-1

Abstract. Data from the Jicamarca digisonde and the ROCSAT-1 satellite are employed to study the equatorial ionosphere on the west side of South America during April 1999-March 2000 for the concurrent bottomside spread F (BSSF) and plasma bubble events. This study, using digisonde and ROCSAT-1 concurrently, is the first attempt to investigate the equatorial spread F. Results show that BSSF and plasma bubble observations appear frequently respectively in the summer (January, February, November, and December) and in the equinoctial (March, April, September, and October) months, respectively, but are both rarely observed in the winter (May-August) months. The upward drift velocity during the concurrent BSSF and bubble observations has been determined to study the driving mechanism. This analysis shows that large vertical drift velocities favor BSSF and bubble formations in the equinoctial and summer months. Conversely, the smaller upward velocities during the winter months cause fewer BSSF and bubble occurrences. For the geomagnetic effect, the BSSF/bubble occurrence decreases with an increasing Kp value in the equinoctial months, but no such correlation is found for the summer and winter months. Moreover, the anti-correlations between Kp and dh'F/dt are apparent in the equinoctial months, but not in the summer and winter months. These results indicate that in the equinoctial months the BSSF/bubble generations and the pre-reversal drift velocity can be suppressed by geomagnetic activity, because the disturbance dynamo effects could have decreased the eastward electric field near sunset. However, BSSF and bubble occurrences may not be suppressed by the geomagnetic activity in the summer and winter months.

Interaction between direct penetration and disturbance dynamo electric fields in the storm‐time equatorial ionosphere

The direct penetration of the high‐latitude electric field to lower latitudes, and the disturbance dynamo, both play a significant role in restructuring the storm‐time equatorial ionosphere and thermosphere. Although the fundamental mechanisms generating each component of the disturbance electric field are well understood, it is difficult to identify the contribution from each source in a particular observation. In order to investigate the relative contributions of the two processes, their interactions, and their impact on the equatorial ionosphere and thermosphere, the response to the March 31, 2001, storm has been modeled using the Rice Convection Model (RCM) and the Coupled Thermosphere‐Ionosphere‐Plasmasphere‐Electrodynamics (CTIPe) model. The mid‐ and low‐latitude electric fields from RCM have been imposed as a driver of CTIPe, in addition to the high latitude magnetospheric sources of ion convection and auroral precipitation. The high latitude sources force the global storm‐time wind fields, which act as the driver of the disturbance dynamo electric fields. The magnitudes of the two sources of storm‐time equatorial electric field are compared for the March 2001 storm period. During daytime, and at the early stage of the storm, the penetration electric field is dominant; while at night, the penetration and disturbance dynamo effects are comparable. Both sources are sufficient to cause significant restructuring of the low latitude ionosphere. Our results also demonstrate that the mid‐ and low‐latitude conductivity and neutral wind changes initiated by the direct penetration electric field preferentially at night are sufficient to alter the subsequent development of the disturbance dynamo.

Magnetospheric disturbance induced equatorial plasma bubble development and dynamics: A case study in Brazilian sector

Equatorial ionospheric plasma bubble irregularity development and dynamics during the major magnetospheric storm of 26 August 1998 are investigated using the data collected by a multistation and multi‐instrument diagnostic network operated at equatorial and low latitude sites in Brazil, and auroral electrojet activity (AU/AL), IMF, and Dst indices. A magnetospheric disturbance onset in the morning of 26 August 1998 was initiated by a solar wind shock and associated IMF Bz polarity reversals and ssc that were soon followed by a succession of substorm‐like auroral electrojet (AE) intensifications and Dst development. An IMF Bz southward turning and associated AE intensifications in the Brazilian dusk sector produced intense prompt penetration eastward electric field that caused large F region vertical drift and consequently the developments of intense postsunset equatorial anomaly and a series of intense plasma bubbles, the latter event lasting the entire night, as observed by digital ionosondes at São Luís (2.33°S, 315.8°E, dip angle: −.5°) and Fortaleza (3.9°S, 321.55°W, dip angle: −9°) and an all‐sky imager, two scanning photometers, and a Digisonde at the low‐latitude site Cachoeira Paulista (22.6°S, 315°E; dip angle: −28°). A notable aspect of the dynamics of the bubbles was their initially very low eastward drift velocity which turned into steadily increasing westward velocity that lasted till early morning hours. The results show for the first time a relationship between the zonal drift velocities of optically observed large‐scale bubbles (tens to hundreds of kilometers) and that of the smaller scale (kilometer sizes) structures as observed by a digital ionosonde. The results point to the dominant role of a disturbance dynamo associated westward thermospheric wind to maintain the plasma irregularity drift increasingly westward going into postmidnight hours. As an important finding, the results further show that significant contribution to the westward plasma bubble irregularity drift, normally attributed to disturbance dynamo effect, could arise from prompt penetration disturbance zonal electric field, in the course of a disturbance sequence lasting several hours. Such effect is attributed to Hall electric field arising from the primary disturbance zonal electric field, under enhanced nighttime ionospheric conductivities produced possibly by storm associated particle precipitation, in the Brazilian longitude sector in agreement with recent evidences [Abdu et al., 1998b].

Observations of shock impact, disturbance dynamo effect, and a midlatitude large‐density depletion at 600 km altitude on the 17 April 2002 storm day

ROCSAT‐1 orbiting at 600 km altitude has observed a series of interesting topside ionospheric responses to the 17 April 2002 magnetic storm. After the impingement of the interplanetary shock, a Pi‐1 pulsation event in the plasma flow and ion density oscillations with a period of 19 s in the postnoon sector has been observed for less than 2 min. The storm induced disturbance dynamo effect is then observed an hour later with the postmidnight enhancement of the upward flow pattern described in the model of Scherliess and Fejer [1997] and the enhanced zonal flow pattern described in the models of Heelis and Coley [1992] and Scherliess et al. [2001]. However, the dynamo induced vertical electric field is noticed to be much weaker and decays faster than the induced eastward electric field in the postmidnight sector. A large‐density depletion is then observed at the end of this disturbance dynamo effect. It occurs at latitude 34°N (dip latitude 27°N) at the sunrise terminator. The density decreases about two orders of magnitude from the background level and the depletion width is about 500 km wide. An outward expanding flow up to 725 m/s is found inside the depletion. The NO+ ions found inside the depletion imply that the depletion was lifted‐up from the bottomside ionosphere in agreement with the observed large outward expanding flow. Large and rapid east‐west fluctuating flows up to 250 m/s are also observed together with large parallel flow variations of similar magnitude. The temperature inside the depletion is found increased to a peak value of 2050 K from the background temperature of 1200 K outside the depletion. However, this hottest temperature does not occur at the deepest density depletion. The observed temperature increase does not seem to fit in a proposed model of Oyama et al. [1988] where the heating comes from energy deposit of the fast electrons that are produced during the storm‐time and transported from the sunlit apex height at the equatorial region. The unfavorable reason is based on the ineffective energy exchanges between electrons and ions due to large ion mass and infrequent collisions between them at topside ionosphere. Furthermore, the observation fails to support the proposed inverse correlation between the temperature and the density. Instead, the temperature increase could come from the adiabatic heating by compressing ions along the flux tube from the equatorial region to high latitudes in the models of Heelis et al. [1978] and Bailey and Heelis [1980] in which the required large parallel flow has been observed.

Ionospheric storm of early November 1993 in the Indian equatorial region

An investigation is made of the response of equatorial ionosphere in the Indian (75°E) sector to the major magnetic storm of November 3, 1993, using data from the ionosonde and magnetometer networks spanning the region 0.3–34.5°N dip. Some outstanding and new aspects of the storm time ionospheric behaviour are revealed. An anomalous and striking positive gradient in ƒ0F2 from the magnetic equator to 34.5° dip developed under counter electrojet (CEJ) condition in the morning on November 4, corresponding to the early stage of the storm main phase. This storm effect is attributed to plasma transport by a poleward surge in transequatorial winds due to large scale atmospheric gravity waves (AGWs) launched by auroral heating. Remarkable wave‐like variations in hpF2 and ƒ0F2 immediately followed at locations away from dip equator till local sunset, with concomitant disruptions in the development of the equatorial ionization anomaly (EIA). Rapid variations in meridional neutral winds due to large‐scale AGWs are assessed as the cause of the oscillations in hpF2 and the associated cyclic sequence of development and inhibition of EIA as the outcome of the combined effects of plasma transport due to meridional winds and the plasma “fountain” driven by E×B drift. Just after the onset of the storm recovery phase at 1800 LT, a sudden and anomalous drop (54–57%) in ƒ0F2 prevailed throughout the anomaly region over the interval 1915–2330 LT, with an apparent time delay in occurrence toward the magnetic equator. This premidnight collapse of equatorial F region is interpreted in terms of horizontal transport of plasma across the equator toward the opposite hemisphere by an equatorward surge in meridional winds. The case study showed that besides disturbances in the zonal electric field due to prompt penetration and ionospheric disturbance dynamo effects, perturbations in neutral meridional winds played a prominent role in the ionospheric storm of November 4, 1993, in the Indian equatorial region.

Analysis of the positive ionospheric response to a moderate geomagnetic storm using a global numerical model

Abstract. Current theories of F-layer storms are discussed using numerical simulations with the Upper Atmosphere Model, a global self-consistent, time dependent numerical model of the thermosphere-ionosphere-plasmasphere-magnetosphere system including electrodynamical coupling effects. A case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exemplifies the complex storm phenomena. The study focuses on positive ionospheric storm effects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical effects of both neutral meridional winds and electric fields caused by the disturbance dynamo effect. The penetration of short-time electric fields of magnetospheric origin during storm intensification phases is shown for the first time in this model study. Comparisons of the calculated thermospheric composition changes with satellite observations of AE-C and ESRO-4 during storm time show a good agreement. The empirical MSISE90 model, however, is less consistent with the simulations. It does not show the equatorward propagation of the disturbances and predicts that they have a gentler latitudinal gradient. Both theoretical and experimental data reveal that although the ratio of [O]/[N2] at high latitudes decreases significantly during the magnetic storm compared with the quiet time level, at mid to low latitudes it does not increase (at fixed altitudes) above the quiet reference level. Meanwhile, the ionospheric storm is positive there. We conclude that the positive phase of the ionospheric storm is mainly due to uplifting of ionospheric F2-region plasma at mid latitudes and its equatorward movement at low latitudes along geomagnetic field lines caused by large-scale neutral wind circulation and the passage of travelling atmospheric disturbances (TADs). The calculated zonal electric field disturbances also help to create the positive ionospheric disturbances both at middle and low latitudes. Minor contributions arise from the general density enhancement of all constituents during geomagnetic storms, which favours ion production processes above ion losses at fixed height under day-light conditions.Key words: Atmospheric composition and structure (thermosphere · composition and chemistry) · Ionosphere (ionosphere · atmosphere interactions; modelling and forecasting)

Analysis of the positive ionospheric response to a moderate geomagnetic storm using a global numerical model

Current theories of F-layer storms are discussed using numerical simulations with the Upper Atmosphere Model, a global self-consistent, time dependent numerical model of the thermosphere-ionosphere-plasmasphere-magnetosphere system including electrodynamical coupling effects. A case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exemplifies the complex storm phenomena. The study focuses on positive ionospheric storm effects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical effects of both neutral meridional winds and electric fields caused by the disturbance dynamo effect. The penetration of short-time electric fields of magnetospheric origin during storm intensification phases is shown for the first time in this model study. Comparisons of the calculated thermospheric composition changes with satellite observations of AE-C and ESRO-4 during storm time show a good agreement. The empirical MSISE90 model, however, is less consistent with the simulations. It does not show the equatorward propagation of the disturbances and predicts that they have a gentler latitudinal gradient. Both theoretical and experimental data reveal that although the ratio of [O]/[N2] at high latitudes decreases significantly during the magnetic storm compared with the quiet time level, at mid to low latitudes it does not increase (at fixed altitudes) above the quiet reference level. Meanwhile, the ionospheric storm is positive there. We conclude that the positive phase of the ionospheric storm is mainly due to uplifting of ionospheric F2-region plasma at mid latitudes and its equatorward movement at low latitudes along geomagnetic field lines caused by large-scale neutral wind circulation and the passage of travelling atmospheric disturbances (TADs). The calculated zonal electric field disturbances also help to create the positive ionospheric disturbances both at middle and low latitudes. Minor contributions arise from the general density enhancement of all constituents during geomagnetic storms, which favours ion production processes above ion losses at fixed height under day-light conditions.Key words: Atmospheric composition and structure (thermosphere · composition and chemistry) · Ionosphere (ionosphere · atmosphere interactions; modelling and forecasting)

Empirical models of storm time equatorial zonal electric fields

Ionospheric plasma drifts often show highly complex and variable signatures during geomagnetically active periods due to the effects of different disturbance processes. We describe initially a methodology for the study of storm time dependent ionospheric electric fields. We present empirical models of equatorial disturbance zonal electric fields obtained using extensive F region vertical plasma drift measurements from the Jicamarca Observatory and auroral electrojet indices. These models determine the plasma drift perturbations due to the combined effects of short‐lived prompt penetration and longer lasting disturbance dynamo electric fields. We show that the prompt penetration drifts obtained from a high time resolution empirical model are in excellent agreement with results from the Rice Convection Model for comparable changes in the polar cap potential drop. We also present several case studies comparing observations with results obtained by adding model disturbance drifts and season and solar cycle dependent average quiet time drift patterns. When the disturbance drifts are largely due to changes in magnetospheric convection and to disturbance dynamo effects, the measured and modeled drift velocities are generally in good agreement. However, our results indicate that the equatorial disturbance electric field pattern can be strongly affected by variations in the shielding efficiency, and in the high‐latitude potential and energy deposition patterns which are not accounted for in the model. These case studies and earlier results also suggest the possible importance of additional sources of plasmaspheric disturbance electric fields.

The multi‐instrumented studies of equatorial thermosphere aeronomy scintillation system: Climatology of zonal drifts

A spaced‐antenna scintillation system was installed at Ancon, Peru, in May 1994 to measure scintillation of 250‐MHz signals from a geostationary satellite by three antennas spaced in the magnetic east‐west direction. These measurements were used to establish the climatology of the zonal drift of the irregularities which cause equatorial scintillations. The major objective of this study is to compare this drift climatology to the climatology of zonal neutral wind which is the driver of the equatorial electrodynamics. A comparison of these two climatologies in conjunction with scintillation statistics may provide some clues regarding factors which help or hinder the formation of equatorial spread‐F (ESF). With these objectives in mind, the first year's drift and scintillation statistics have been presented as a function of local time, season and magnetic activity and compared with the statistics of ion drift published earlier from incoherent scatter radar observations. The scintillation drift is in good agreement with the Jicamarca radar observations except for the fact that the local time dependence of our drift observations exhibit a broader maximum. The broad maximum may be attributed to lower ion drag experienced in the presence of ESF due to sustained uplifting of the ionosphere. During magnetically active periods, the scintillation drift often exhibits east to west reversals presumably because of the disturbance dynamo effects. The westward drifts during such reversals may be as large as 100 m/s. We have also modeled the zonal drifts as a seasonal basis by using Hedin's neutral wind model and Anderson's fully analytical ionospheric model. The modeled zonal drifts present good quantitative agreement with the drifts obtained with the scintillation technique.

Low latitude electrodynamic plasma drifts : a review

Radar and radio measurements have provided detailed information on the dependence of F-region electrodynamic drifts on height, season, solar cycle and magnetic activity. Recently, satellite ion drift and electric field probes have determined the variation of low latitude ionospheric drifts over a large range of altitudes and latitudes. The general characteristics of the quiet time plasma can be explained as resulting from E- and F-region dynamo and interhemispheric coupling processes. The low latitude and equatorial zonal and upward/poleward components of the plasma drift respond differently to geomagnetic activity. Disturbance dynamo effects are responsible for the drift perturbations following periods of enhanced magnetic activity. The prompt penetration of high latitude electric fields to lower latitudes produces large perturbations on the upward/poleward drifts, but has no significant effect on the low latitude and equatorial zonal drifts. A number of processes such as ‘overshielding’, ‘fossil wind’ and magnetic reconfiguration were suggested as being responsible for the direct penetration of high latitude electric fields to lower latitudes. Detailed low latitude and global numerical models were used to study the characteristics of low latitude and equatorial plasma drifts and their response to changes in the polar cap potential drop or in the high latitude field-aligned currents. These models can reproduce the latitudinal variation of the perturbation electric fields and their diurnal variations, but are still unable to account for several aspects of the experimental data as a result of the complexity of the high latitude and magnetospheric processes involved.