scholarly journals Sudden post-midnight decrease in equatorial F-region electron densities associated with severe magnetic storms

1997 ◽  
Vol 15 (3) ◽  
pp. 306-313 ◽  
Author(s):  
D. R. Lakshmi ◽  
B. Veenadhari ◽  
R. S. Dabas ◽  
B. M. Reddy
Keyword(s):  
Electric Fields ◽  
Magnetic Storms ◽  
Equatorial Region ◽  
Electron Densities ◽  
Low Latitude ◽  
General Behaviour ◽  
F Region ◽  
Short Period ◽  
Electric Field Direction ◽  
Meridional Winds

Abstract. A detailed analysis of the responses of the equatorial ionosphere to a large number of severe magnetic storms shows the rapid and remarkable collapse of F-region ionisation during post-midnight hours; this is at variance with the presently accepted general behaviour of the low-latitude ionosphere during magnetic storms. This paper discusses such responses as seen in the ionosonde data at Kodaikanal (Geomagn. Lat. 0.6 N). It is also observed that during magnetic storm periods the usual increase seen in the h'F at Kodaikanal during sunset hours is considerably suppressed and these periods are also characterised by increased foF2 values. It is suggested that the primary process responsible for these dramatic pre- and post-midnight changes in foF2 during magnetic storms could be due to changes in the magnitude as well as in the direction of usual equatorial electric fields. During the post-midnight periods the change in electric-field direction from westward to eastward for a short period causes an upward E × B plasma drift resulting in increased h'F and decreased electron densities in the equatorial region. In addition, it is also suggested that the enhanced storm-induced meridional winds in the thermosphere, from the poles towards the equator, may also cause the decreases in electron density seen during post-midnight hours by spatially transporting the F-region ionisation southwards away from Kodaikanal. The paper also includes a discussion on the effects of such decreases in ionisation on low-latitude HF communications.

scholarly journals Space weather impact on the equatorial and low latitude F-region ionosphere over India

2006 ◽  
Vol 24 (1) ◽  
pp. 97-105 ◽  
Author(s):  
R. S. Dabas ◽  
R. M. Das ◽  
V. K. Vohra ◽  
C. V. Devasia
Keyword(s):  
Magnetic Storm ◽  
Electric Fields ◽  
Space Weather ◽  
Recovery Phase ◽  
Equatorial Region ◽  
Storm Events ◽  
Low Latitude ◽  
Equatorial Station ◽  
Weather Impact ◽  
F Region

Abstract. For a detailed study of the space weather impact on the equatorial and low latitude F-region, the ionospheric response features are analysed during the periods of three recent and most severe magnetic storm events of the present solar cycle which occurred in October and November 2003, and November 2004. The F-layer base height (h'F), peak height (hmF2) and critical frequency (foF2) data, from Trivandrum, an equatorial station and Delhi, a low latitude location, are examined during the three magnetic storm periods. The results of the analysis clearly shows that the height of the F-region (both h'F and hmF2), at the equator and low latitude, simultaneously increases by 200 to 300 km, in association with maximum negative excursion of Dst values around the midnight hours with a large depletion of ionization over the equator, which is followed by an ionization enhancement at low latitude during the recovery phase of the storm. At Delhi, fast variations up to 200 m/s are also observed in the F-layer vertical upward/downward velocity, calculated using Doppler shifts, associated with the maximum negative excursion of Dst. This shows that during magnetic disturbances, the equatorial ionization anomaly (EIA) expands to a much wider latitude than the normal fountain driven by the E/F-layer dynamo electric fields. It is also observed that during the main phase of the storm, at low latitude there is generally an enhancement of F-region ionization with an increase in h'F/hmF2 but in the equatorial region, the ionization collapses with a decrease in h'F/hmF2, especially after sunset hours. In addition, at the equator the normal pre-sunset hours' enhancement in h'F is considerably suppressed during storm periods. This might be due to changes in magnitude and direction of the zonal electric field affecting the upward E×B drift and hence the plasma distribution in the form of a decrease in electron density in the equatorial region and an increase in the low latitude region. In association with disturbance electric fields, the enhanced storm-induced equatorward meridional winds in the thermosphere can also further amplify the F-layer height rise at low latitudes during the post-midnight hours, as observed in two of the storm periods.

Electric fields in the low latitude F-region

1996 ◽  
Vol 18 (6) ◽  
pp. 93-98 ◽  
Author(s):  
B.V. Krishna Murthy ◽  
S.S. Hari
Keyword(s):  
Electric Fields ◽  
Low Latitude ◽  
F Region

A study of low-latitude VHF scintillations in relation to electric fields during magnetic storms

Radio Science ◽  
1993 ◽  
Vol 28 (3) ◽  
pp. 389-400 ◽  
Author(s):  
D. R. Lakshmi ◽  
R. S. Dabas ◽  
B. C. N. Rao ◽  
B. M. Reddy
Keyword(s):  
Electric Fields ◽  
Magnetic Storms ◽  
Low Latitude

scholarly journals Storm-time total electron content and its response to penetration electric fields over South America

2011 ◽  
Vol 29 (10) ◽  
pp. 1765-1778 ◽  
Author(s):  
P. M. de Siqueira ◽  
E. R. de Paula ◽  
M. T. A. H. Muella ◽  
L. F. C. Rezende ◽  
M. A. Abdu ◽  
...  
Keyword(s):  
South America ◽  
Total Electron Content ◽  
Electric Fields ◽  
Equatorial Region ◽  
Electron Content ◽  
The South ◽  
Low Latitude ◽  
Total Electron ◽  
Vertical Drift ◽  
Magnetometer Data

Abstract. In this work the response of the ionosphere due to the severe magnetic storm of 7–10 November 2004 is investigated by analyzing GPS Total Electron Content (TEC) maps constructed for the South America sector. In order to verify the disturbed zonal electric fields in South America during the superstorm, ionospheric vertical drift data obtained from modeling results are used in the analysis. The vertical drifts were inferred from ΔH magnetometer data (Jicamarca-Piura) following the methodology presented by Anderson et al. (2004). Also used were vertical drifts measured by the Jicamarca ISR. Data from a digisonde located at São Luís, Brazil (2.33° S, 44.2° W, dip latitude 0.25°) are presented to complement the Jicamarca equatorial data. Penetration electric fields were observed by the comparison between the equatorial vertical drifts and the Interplanetary Electric Field (IEF). The TEC maps obtained from GPS data reflect the ionospheric response over the South America low-latitude and equatorial region. They reveal unexpected plasma distributions and TEC levels during the main phase of the superstorm on 7 November, which is coincident with the local post-sunset hours. At this time an increase in the pre-reversal enhancement was expected to develop the Equatorial Ionization Anomaly (EIA) but we observed the absence of EIA. The results also reveal well known characteristics of the plasma distributions on 8, 9, and 10 November. The emphasized features are the expansion and intensification of EIA due to prompt penetration electric fields on 9 November and the inhibition of EIA during post-sunset hours on 7, 8, and 10 November. One important result is that the TEC maps provided a bi-dimensional view of the ionospheric changes offering a spatial description of the electrodynamics involved, which is an advantage over TEC measured by isolated GPS receivers.

scholarly journals Night-time F-region and daytime E-region ionospheric drifts measured at Udaipur during solar flares

2002 ◽  
Vol 20 (11) ◽  
pp. 1837-1842 ◽  
Author(s):  
B. M. Vyas ◽  
R. Pandey
Keyword(s):  
Electric Fields ◽  
Solar Flares ◽  
Night Time ◽  
Low Latitude ◽  
F Region ◽  
Reflection Height ◽  
E Region ◽  
Electric Fields And Currents ◽  
East West ◽  
Ionospheric Drifts

Abstract. Ionospheric drifts measured at a low latitude station, Udaipur (Geomag. Lat. 14.5° N), in the night-time F-region and daytime E-region during solar flares have been studied. The night-time observations, which correspond to the F-region drifts, were carried out on five different nights. The daytime observation corresponding to the E-region drifts is only for one day. It is found that the apparent drift during the solar flare period is reduced considerably, in the daytime E-region as well as in the night-time F-region. The East-West and North-South components of the apparent drift speed are also similarly affected. For the daytime E-region drifts during a flare, increased ionization and subsequent reduction of reflection height is proposed to be the cause of reduced drift speeds. For the night-time F-region drifts, a reduced electric field at the F-region heights resulting from coupling of sunlit and dark hemispheres has been proposed to be the possible cause.Key words. Ionosphere (electric fields and currents; ionospheric disturbances)

scholarly journals Ionospheric disturbances induced by substorm associated electric fields in the low-latitude F-region.

1987 ◽  
Vol 39 (4) ◽  
pp. 187-209 ◽  
Author(s):  
Tadatoshi TAKAHASHI ◽  
Hiroshi OYA ◽  
Shigeto WATANABE
Keyword(s):  
Electric Fields ◽  
Ionospheric Disturbances ◽  
Low Latitude ◽  
F Region

scholarly journals Response of equatorial ionosphere to episodes of asymmetric ring current activity

1997 ◽  
Vol 15 (10) ◽  
pp. 1316-1323 ◽  
Author(s):  
J. H. Sastri ◽  
M. A. Abdu ◽  
J. H. A. Sobral
Keyword(s):  
Electric Field ◽  
Electric Fields ◽  
Ring Current ◽  
Opposite Polarity ◽  
Equatorial Region ◽  
Local Times ◽  
High Time Resolution ◽  
Plasma Drift ◽  
F Region ◽  
Current Events

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.

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