Penetration Of High Latitude Electric Fields Research Articles

On Transient Penetration of the Convection Electric Field to Low Latitudes During the Substorm Expansion Phase

The transient penetration of high-latitude electric fields into the middle and low latitude ionosphere during the substorm expansion phase is studied by using an analytical model of the magnetospheric convection. The model calculates the distribution of the electric fields of magnetospheric sources between the polar cap boundary and the geomagnetic equator by taking self-consistent account of the hot plasma belt polarization due to the electric fields both of the solar wind/magnetosphere dynamo and the corotation. It is found that a sudden strong enhancement of auroral conductivities associated with the expansive phase of severe substorm results in a significant, short-lived increase of electric field at middle and low latitudes. The electric field perturbation is essentially of transient nature stemming from the large difference between the characteristic time scales of the primary electric field driven by solar wind and of the Alfven layer’s shielding electric field. Under steady state conditions, there is no substantial penetration of the auroral electric fields to middle and low latitudes due to the enhanced auroral conductivities. The corotation driven polarization electric field can noticeably affect the electric field perturbation in the inner plasmasphere provided the ring current is rather intense.

Interplanetary electric fields and their relationship to low-latitude electric fields under disturbed conditions

Recent studies have demonstrated that ground-based magnetometer observations can be used to infer realistic, daytime vertical E× B drift velocities in the Peruvian and Philippine longitude sectors. It has also been demonstrated that under certain conditions the time variability in the interplanetary electric field (IEF)—minutes to hours—is reflected in the daytime, prompt penetration of high-latitude electric fields to low latitudes. In this paper, we incorporate magnetometer-inferred E× B drift techniques to extend this study to include the Indian sector E× B drift velocities and to investigate the relationships between IEF conditions and daytime, low-latitude electric field observations under both geomagnetically quiet and disturbed conditions. This paper addresses several basic questions related to the relationships between IEF conditions and low-latitude east–west electric fields. (1) When low-latitude electric fields exhibit quiet-time, Sq-type behavior, what are the IEF conditions? (2) Under disturbed conditions, what are the relationships between the IEF parameters and the low-latitude electric fields in the Peruvian, Philippine, and Indian longitude sectors? (3) If the three longitude sector electric field responses are similar under disturbed conditions, is the response consistent with the current ideas put forward at the Millstone Hill Workshop on promptly penetrating electric fields and over-shielding effects at low latitudes? We address the above questions by analyzing magnetometer-inferred E× B drift velocities between January 2001 and December 2004 when there exists more than 500 quiet days and more than 235 geomagnetically disturbed days, defined by daily Ap values greater than 20. It is suggested that the neural network approach that provides realistic E× B drift velocities based on magnetometer observations can be applied at any longitude where appropriately placed magnetometers exist. It is found that: (1) the average quiet, daytime upward E× B drift velocity vs. LT in the Indian sector is comparable to the average velocity vs. LT in the Peruvian sector and both are roughly 3–5 m/s less than the values in the Philippine sector; (2) under quiet conditions, the peak velocity occurs at 1100 LT in the Peruvian sector and at 1000 LT in both the Philippine and Indian sectors; and (3) during disturbed conditions, it is observed that daytime, promptly penetrating electric fields occur, simultaneously, in the Philippine, Indian and Peruvian sectors, in response to fluctuating IEF conditions.

Interplanetary origin of magnetic storms and their F2-region responses at the equatorial anomaly

The interplanetary origin of 102 storms ( D st ⩽ −50 nT) of varying strength that occurred during the period of 1997–2001, have been studied. The shock compression, magnetic clouds, and shock compression followed by magnetic clouds are most probable sources of intense southward component B S of IMF in the interplanetary medium causing the magnetic storms. Thirty five percent of magnetic storms were associated with the magnetic clouds. The main phase of 60% intense and very intense magnetic storms associated with the magnetic clouds, occurred through the multi-step growth in the ring current during the main phase evolution of the storms. The storms were associated with enhanced IMF intensity, proton bulk velocity, ion dynamic pressure and ion density, in the solar wind, in general, with increase in strength of the storms, but not linearly. The storm-time effects in the ionospheric F2-region during 51 magnetic storms during 1997–2001, at Ahmedabad (23.2°N, 72.4°E, dip angle 30.7° at sub-ionospheric points), a station at the ionization anomaly crest in the Indian region, are also studied. The negative ionospheric effects are more pronounced than the positive ionospheric effects. While the prompt penetration of high-latitude electric fields and the electric fields generated by the disturbance dynamo play important role in F2-region response by changing E × B drifts at low latitude, the storm-induced transport of gas with depleted ratio of [O]/[N2] may also be responsible for the post-midnight large decrease in f oF 2, during strong magnetic storms ( D st < 100 nT).

Response of equatorial ionosphere to episodes of asymmetric ring current activity

Abstract. We present the characteristics of the response of equatorial ionospheric zonal electric field and F-region plasma density to the asymmetric ring current intensifications that occurred in succession on 16 December 1991, corresponding to the STEP/EITS-2 campaign period. The study is based on high-time- resolution (1-min) data of asymmetic ring current indices, ASY(H/D) and F-region vertical plasma drift, Vz measurements at Kodaikanal (10.25°N; 77.5°E; dip 4°), India and quarter-hourly ionosonde data of Fortaleza (4°S; 322°E; dip –9°), Brazil. It is shown that short-lived disturbances in F-layer vertical plasma drift, Vz and height (h'F/hpF2) indicative of westward and eastward electric fields prevail simultaneously in the dusk (18–21 LT) and predawn (02–05 LT) sectors, respectively, in association with the decay phase of asymmetic ring current events. Electric fields of opposite polarity do also seem to manifest at these local times, particularly in the early-morning sector in conjunction with the intensification of the asymmetric ring current. At a given location, electric field disturbances associated with individual asymmetric ring current events are thus bipolar in nature, with fields of opposite polarity during the growth and decay phases. The nature and polarity structure of the observed electric field disturbances are in agreement with the theoretical/model predictions of prompt penetration of high-latitude electric fields to the equatorial region.

Response of equatorial ionosphere to episodes of asymmetric ring current activity

We present the characteristics of the response of equatorial ionospheric zonal electric field and F-region plasma density to the asymmetric ring current intensifications that occurred in succession on 16 December 1991, corresponding to the STEP/EITS-2 campaign period. The study is based on high-time- resolution (1-min) data of asymmetic ring current indices, ASY(H/D) and F-region vertical plasma drift, Vz measurements at Kodaikanal (10.25°N; 77.5°E; dip 4°), India and quarter-hourly ionosonde data of Fortaleza (4°S; 322°E; dip –9°), Brazil. It is shown that short-lived disturbances in F-layer vertical plasma drift, Vz and height (h'F/hpF2) indicative of westward and eastward electric fields prevail simultaneously in the dusk (18–21 LT) and predawn (02–05 LT) sectors, respectively, in association with the decay phase of asymmetic ring current events. Electric fields of opposite polarity do also seem to manifest at these local times, particularly in the early-morning sector in conjunction with the intensification of the asymmetric ring current. At a given location, electric field disturbances associated with individual asymmetric ring current events are thus bipolar in nature, with fields of opposite polarity during the growth and decay phases. The nature and polarity structure of the observed electric field disturbances are in agreement with the theoretical/model predictions of prompt penetration of high-latitude electric fields to the equatorial region.

Penetration of high-latitude electric fields into low latitudes

Simulation studies of ionospheric electric fields with special emphasis placed on the electrical coupling between high and low latitudes are presented by means of the algorithm developed by Kamide and Matsushita ( J. geophys. Res. 84 p. 4083, 1979) to derive the horizontal electric fields in the global ionosphere generated by field-aligned currents in auroral latitudes. The ionospheric electric potential is obtained from a numerical solution of the steady-state current continuity equation assuming anisotropic conductivities as well as upward and downward currents along auroral field lines. These assumptions are based on our current knowledge of auroral phenomena and geomagnetic variations as well as rocket and satellite measurements of field-aligned currents and radar observations of the ionospheric conductivity distribution. We demonstrate the importance of the basic assumptions leading to the main features observed during both quiet and disturbed times. In the simplest model when the conductivity is taken as constant, the values of the electric potential decay very slowly with an increase of the colatitude in middle and low latitudes. On the other hand, when the conductivity gradient is realistically included in terms of both day-night asymmetry of the quiet-time conductivity and slight auroral enhancements in the night side auroral belt for weakly disturbed times, the decay of the electric field toward low latitudes occurs rapidly. Furthermore, the inclusion of the field-aligned currents in the equatorward half of the auroral belt results in extensive reduction of the electric field in low latitudes, and produces a shielding effect against the penetration of the high-latitude electric field into low latitudes. It is also found that there is a notable asymmetry in the decay rate of the electric field toward low latitudes between the morning and evening sectors. In both quiet and disturbed cases, the electric field in the evening sector decays more rapidly with latitudes than that in the morning sector, a result from the difference in the sign of the longitudinal gradient of the ionospheric conductivity. How the electric field of the high latitude origin can (or cannot) penetrate deep into low latitudes is discussed in light of the magnetospheric convection near the so-called Alfvén layer.