scholarly journals Search for magnetically quiet CHAMP polar passes and the characteristics of ionospheric currents during the dark season

2006 ◽  
Vol 24 (11) ◽  
pp. 2997-3009 ◽  
Author(s):  
P. Ritter ◽  
H. Lühr
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Transverse Field ◽  
Transverse Component ◽  
Local Times ◽  
Time Interval ◽  
Polar Regions ◽  
Ionospheric Currents ◽  
Ionospheric Conductivity ◽  
Field Modelling

Abstract. The magnetic activity at auroral latitudes is strongly dependent on season. During the dark season, when the solar zenith angle in the polar region is larger than 100° at all local times, the ionospheric conductivity is much reduced, and generally low activity is encountered. These time intervals are of special interest for the main field modelling, because then the geomagnetic field readings, in particular the field magnitude, are only slightly affected by ionospheric currents. Based on CHAMP data, this study examines how these quiet periods are reflected in the different magnetic field components. The peak FAC density is used as a possible proxy for the deviation of the total field. As a second option, the transverse field component, which is aligned with the auroral oval, is investigated, because it presents a measure for the FAC total current. Correlation analyses with the scalar residuals are performed and both proxies are tested for their suitability of predicting the intensity of the auroral electrojet during the dark polar seasons. The indicators based on the local FAC strength or on the amplitude of the transverse component show a reasonable correlation with the electrojet intensity for these periods, but fail when limited to small amplitudes. The predictability improves considerably if the time sector is limited to dayside hours (08:00–16:00 MLT). As the activity at high latitudes is strongly controlled by the solar wind input, we also consider IMF quantities which may support very quiet conditions. Correlations of the magnetic field scalar residuals with the merging electric field are strongest if only passes in the dayside sector are considered. Best selection results for quiet passes are obtained by combining four conditions: dark season, small average merging electric field, Em<0.8 mV/m, absence of peak values of Em>1.2 mV/m during a time interval of 40 min centred at the polar crossing, and limitation to the dayside sector (08:00–16:00 MLT). The set of quiet polar passes identified by these criteria may be used beneficially in crustal field modelling of the polar regions.

scholarly journals High-latitude ionospheric currents during very quiet times: their characteristics and predictability

2004 ◽  
Vol 22 (6) ◽  
pp. 2001-2014 ◽  
Author(s):  
P. Ritter ◽  
H. Lühr ◽  
S. Maus ◽  
A. Viljanen
Keyword(s):  
Electric Field ◽  
Zenith Angle ◽  
Solar Zenith Angle ◽  
Prediction Method ◽  
Field Measurements ◽  
Polar Regions ◽  
Solar Wind Parameter ◽  
Ionospheric Currents ◽  
Ionospheric Conductivity ◽  
Superposed Epoch Analysis

Abstract. CHAMP passes the geographic poles at a distance of 2.7° in latitude, thus providing a large number of magnetic readings of the dynamic auroral regions. The data of these numerous overflights were used for a detailed statistical study on the level of activity. A large number of tracks with very low rms of the residuals between the scalar field measurements and a high degree field model were singled out over both the northern and southern polar regions, independently. Low rms values indicate best model fits and are therefore regarded as a measure of low activity, although we are aware that this indicator also has its limitations. The occurrence of quiet periods is strongly controlled by the solar zenith angle at the geomagnetic poles, indicating the importance of the ionospheric conductivity. During the dark polar season, about 30% of the passes can be qualified as quiet. The commonly used magnetic activity indices turn out not to be a reliable measure for the activity state in the polar region. Least suitable is the Dst index, followed by the Kp. Slightly better results are obtained with the PC and the IMAGE-AE indices. The latter is rather effective within a time sector of ±4 hours of magnetic local time around the IMAGE array. The orientation of the interplanetary magnetic field (IMF) is an important controlling factor for the activity. This is also supported by the prevailing FAC distribution during quiet times, which resembles the typical NBZ (northward Bz) pattern. In a superposed epoch analysis we show that the merging electric field is a suitable geoeffective solar wind parameter. Based on the size of this electric field and the solar zenith angle at the geomagnetic poles, a prediction method for quiet auroral region periods is proposed. This may, among others, be useful for the data selection in main field modelling approaches.

scholarly journals Induction effects on ionospheric electric and magnetic fields

2005 ◽  
Vol 23 (5) ◽  
pp. 1735-1746 ◽  
Author(s):  
H. Vanhamäki ◽  
A. Viljanen ◽  
O. Amm
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Electric Fields ◽  
Magnetic Measurements ◽  
Current System ◽  
Vertical Component ◽  
Ionospheric Currents ◽  
Ionospheric Conductivity ◽  
Ionospheric Current ◽  
Total Electric Field

Abstract. Rapid changes in the ionospheric current system give rise to induction currents in the conducting ground that can significantly contribute to magnetic and especially electric fields at the Earth's surface. Previous studies have concentrated on the surface fields, as they are important in, for example, interpreting magnetometer measurements or in the studies of the Earth's conductivity structure. In this paper we investigate the effects of induction fields at the ionospheric altitudes for several realistic ionospheric current models (Westward Travelling Surge, Ω-band, Giant Pulsation). Our main conclusions are: 1) The secondary electric field caused by the Earth's induction is relatively small at the ionospheric altitude, at most 0.4 mV/m or a few percent of the total electric field; 2) The primary induced field due to ionospheric self-induction is locally important, ~ a few mV/m, in some "hot spots", where the ionospheric conductivity is high and the total electric field is low. However, our approximate calculation only gives an upper estimate for the primary induced electric field; 3) The secondary magnetic field caused by the Earth's induction may significantly affect the magnetic measurements of low orbiting satellites. The secondary contribution from the Earth's currents is largest in the vertical component of the magnetic field, where it may be around 50% of the field caused by ionospheric currents. Keywords. Geomagnetism and paleomagnetism (geomagnetic induction) – Ionosphere (electric fields and currents)

Magnetic structure of ionizing shock waves Part 3. Normal shocks

1972 ◽  
Vol 7 (1) ◽  
pp. 177-185 ◽  
Author(s):  
B. P. Leonard
Keyword(s):  
Magnetic Field ◽  
Shock Waves ◽  
Electric Field ◽  
Field Component ◽  
Magnetic Energy ◽  
Transverse Field ◽  
Transverse Magnetic ◽  
En Numbers ◽  
Magnetic Field Magnitude ◽  
Oblique Shocks

Normal ionizing shock waves are considered as a subclass of oblique shocks in which the upstream transverse magnetic field component is zero; i.e. the upstream field is normal to the plane of the shock. Non-trivial (switch-on) normal shocks involve a non-zero downstream transverse field component; magnetically trivial normal shocks are simply gas shocks with an imbedded constant normal magnetic field. As with oblique shocks, switch-on normal ionizing shock waves are plane- polarized, provided the conductivity is a scalar. Ohmic structures are discussed for several values of shock Alfv én number, treating the electric field as a free parameter, as usual. For Alfv én numbers extending from zero to two (for the infinite-Mach-number case), there is always a finite range of E field values. Above two, only the gas shock exists, and this requires a unique electric field value. Because the magnetic field magnitude increases through switch-on shocks, there is no mechanism available for converting magnetic energy into thermal energy, as is the case for oblique or skew shocks. Thus, there is no significant downstream heating above the viscous temperature; and, in some cases, slight downstream cooling may even occur. Expansion shocks are not possible in this geometry. Previous studies are reviewed in the light of structural requirements, and some erroneous results are clarified; in particular, it should be noted that MHD switchon solutions for the pre-ionized case are not imbedded in the family of ionizing switch-on solutions.

Comparison of different techniques to estimate the direction of the Poynting vector of EMIC emissions

2021 ◽  
Author(s):  
Benjamin Grison ◽  
Ondrej Santolik
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Poynting Vector ◽  
Source Region ◽  
Transverse Component ◽  
Equatorial Region ◽  
Magnetic Equator ◽  
Ion Temperature ◽  
Temperature Anisotropy ◽  
Single Plane

&lt;p&gt;Electromagnetic Ion Cyclotron (EMIC) waves usually grow in the inner magnetosphere from hot ion temperature anisotropy. The main source region is located close to the magnetic equator and there is a secondary EMIC source region off the magnetic equator in the dayside magnetosphere. The source region can be identified using measurements of the Poynting vector direction.&lt;/p&gt;&lt;p&gt;The Poynting vector is ideally derived from the measurement of 3 components of the wave electric field and 3 components of components of the wave magnetic field. However, spinning spacecraft often have only two long mutually perpendicular electric antennas in the spin plane, deployed by the centrifugal force. The third antenna, when present, is usually shorter owing to difficulties of deploying a antenna along the spin axis.&lt;/p&gt;&lt;p&gt;Estimations of the Poynting vector from measurements of three magnetic field components and two electric field components can be obtained assuming the presence of a single plane wave (and thus perpendicularity of the electric field and the magnetic field vectors, according to the Faraday&amp;#8217;s law), following the method developed by Loto'aniu et al. (2005). Applying this method to Cluster data, Allen et al. (2013) found the presence of bidirectional EMIC emissions off the magnetic equatorial region.&lt;/p&gt;&lt;p&gt;Another technique proposed earlier by Santol&amp;#237;k et al. (2001) considers the phase shift estimation between the electric signals from each antenna and synthetic perpendicular magnetic field components obtained from the three-dimensional measurements. The method is based on cross-spectral estimates in the frequency domain and can be used to estimate sign of each component of the Poynting vector. Using this technique Grison et al. (2016) showed the importance of the transverse component of the EMIC emissions far from the source region.&lt;/p&gt;&lt;p&gt;We compare these methods for different events to check how the results of these two techniques differ. We also discuss what we can learn about the EMIC source region from these measurements.&lt;/p&gt;

Sunlit cleft and polar cap ionospheric currents determined from rocket-borne magnetic field, plasma, and electric field observations

1979 ◽  
Vol 84 (A11) ◽  
pp. 6458 ◽  
Author(s):  
F. Primdahl ◽  
J.K. Walker ◽  
F. Spangslev ◽  
J.K. Olesen ◽  
U. Fahleson ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Field Observations ◽  
Polar Cap ◽  
Ionospheric Currents

scholarly journals Response of ionospheric electric fields to variations in the interplanetary magnetic field

1996 ◽  
Vol 14 (8) ◽  
pp. 794-802 ◽  
Author(s):  
S. P. Mishra ◽  
E. Nielsen
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Interplanetary Magnetic Field ◽  
Electric Fields ◽  
Solar Wind Speed ◽  
Auroral Oval ◽  
Local Times ◽  
Residual Effects ◽  
Magnetospheric Substorms ◽  
The Imf

Abstract. The STARE system (Scandinavian Twin Auroral Radar Experiment) provides estimates of electron drift velocities, and hence also of the electric field in the high-latitude E-region ionosphere between 65 and 70 degrees latitude. The occurrence of drift velocities larger than about 400 m/s (equivalent to an electric field of 20 mV/m) have been correlated with the magnitude of the Interplanetary Magnetic Field (IMF) components Bz and By at all local times. Observation days have been considered during which both southward (Bz<0) and northward (Bz>0) IMF occurred. The occurrence of electric fields larger than 20 mV/m increases with increases in Bz magnitudes when Bz<0. It is found that the effects of southward IMF continue for some time following the northward turnings of the IMF. In order to eliminate such residual effects for Bz<0, we have, in the second part of the study, considered those days which were characterized by a pure northward IMF. The occurrence is considerably lower during times when Bz>0, than during those when Bz is negative. These results are related to the expansion and contraction of the auroral oval. The different percentage occurrences of large electric field for By>0 and By<0 components of the IMF during times when Bz>0, clearly display a dawn-dusk asymmetry of plasma flow in the ionosphere. The effects of the time-varying solar-wind speed, density, IMF fluctuations, and magnetospheric substorms on the occurrence of auroral-backscatter observations are also discussed.

scholarly journals Investigating the auroral electrojets with low altitude polar orbiting satellites

2002 ◽  
Vol 20 (7) ◽  
pp. 1049-1061 ◽  
Author(s):  
T. Moretto ◽  
N. Olsen ◽  
P. Ritter ◽  
G. Lu
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
Magnetic Measurements ◽  
Activity Index ◽  
Satellite Observations ◽  
Polar Regions ◽  
Hemispheric Differences ◽  
Ionospheric Currents ◽  
Low Altitude ◽  
Current Systems

Abstract. Three geomagnetic satellite missions currently provide high precision magnetic field measurements from low altitude polar orbiting spacecraft. We demonstrate how these data can be used to determine the intensity and location of the horizontal currents that flow in the ionosphere, predominantly in the auroral electrojets. First, we examine the results during a recent geomagnetic storm. The currents derived from two satellites at different altitudes are in very good agreement, which verifies good stability of the method. Further, a very high degree of correlation (correlation coefficients of 0.8–0.9) is observed between the amplitudes of the derived currents and the commonly used auroral electrojet indices based on magnetic measurements at ground. This points to the potential of defining an auroral activity index based on the satellite observations, which could be useful for space weather monitoring. A specific advantage of the satellite observations over the ground-based magnetic measurements is their coverage of the Southern Hemisphere, as well as the Northern. We utilize this in an investigation of the ionospheric currents observed in both polar regions during a period of unusually steady interplanetary magnetic field with a large negative Y-component. A pronounced asymmetry is found between the currents in the two hemispheres, which indicates real inter-hemispheric differences beyond the mirror-asymmetry between hemispheres that earlier studies have revealed. The method is also applied to another event for which the combined measurements of the three satellites provide a comprehensive view of the current systems. The analysis hereof reveals some surprising results concerning the connection between solar wind driver and the resulting ionospheric currents. Specifically, preconditioning of the magnetosphere (history of the interplanetary magnetic field) is seen to play an important role, and in the winther hemisphere, it seems to be harder to drive currents on the nightside than on the dayside.Key words. Ionosphere (electric fields and currents) – Magnetospheric physics (current systems; magnetosphere-ionosphere interactions)

scholarly journals IMF <i>B<sub>Y</sub></i> and the seasonal dependences of the electric field in the inner magnetosphere

2005 ◽  
Vol 23 (7) ◽  
pp. 2671-2678 ◽  
Author(s):  
H. Matsui ◽  
J. M. Quinn ◽  
R. B. Torbert ◽  
V. K. Jordanova ◽  
P. A. Puhl-Quinn ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Electric Fields ◽  
Realistic Model ◽  
Convection Cell ◽  
Field Pattern ◽  
Inner Magnetosphere ◽  
Ionospheric Conductivity ◽  
Magnetospheric Convection ◽  
Cluster Data

Abstract. It is known that the electric field pattern at high latitudes depends on the polarity of the Y component of the interplanetary magnetic field (IMF BY) and season. In this study, we investigate the seasonal and BY dependences in the inner magnetosphere using the perigee (4<L<10) Cluster data taken from low magnetic latitudes. The data consist of both components of the electric field perpendicular to the magnetic field, obtained by the electron drift instrument (EDI), which is based on a newly developed technique, well suited for measurement of the electric fields in the inner magnetosphere. These data are sorted by the polarities of IMF BZ and BY, and by seasons or hemispheres. It is demonstrated from our statistics that the electric fields in the inner magnetosphere depend on these quantities. The following three points are inferred: 1) The electric fields exhibit some differences statistically between Cluster locations at the Northern and Southern Hemispheres with the same dipole L and magnetic local time (MLT) values and during the same IMF conditions. These differences in the electric fields might result from hemispherical differences in magnetic field geometry and/or those in field-aligned potential difference. 2) The IMF BY and seasonal dependence of the dawnside and duskside electric fields at 4<L<10 is consistent with that seen in the polar convection cell. In addition, it is possible that these dependences are affected by the ionospheric conductivity and the field-aligned current. 3) The nightside electric field in the inner magnetosphere measured by Cluster is often similar to that in the magnetotail lobe. In the future, it will be necessary to incorporate these dependencies on IMF BY and season into a realistic model of the inner magnetospheric convection electric field. Keywords. Magnetospheric physics (Electric fields; Magnetosphere-ionosphere interactions; Solar windmagnetosphere interactions)

scholarly journals Testing the Electrodynamic Method to Derive Height-Integrated Ionospheric Conductances

2020 ◽  
Author(s):  
Daniel Weimer ◽  
Thom Edwards
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Electric Field ◽  
Joule Heating ◽  
Orientation Angle ◽  
Ground Level ◽  
Ionospheric Currents ◽  
Poynting Flux ◽  
Electric Potentials ◽  
Dipole Tilt

Abstract. We have used empirical models for electric potentials and the magnetic fields in both space and on the ground to obtain maps of the height-integrated Pedersen and Hall ionospheric conductivities at high latitudes. This calculation required use of both "curl-free" and "divergence-free" components of the ionospheric currents, with the former obtained from magnetic fields that are used in a model of the field-aligned currents. The second component is from the equivalent current, usually associated with Hall currents, derived from the ground-level magnetic field. Conductances were calculated for varying combinations of the Interplanetary magnetic field (IMF) magnitude and orientation angle, as well as the dipole tilt angle. The results show that reversing the sign of the Y component of the IMF produces substantially different conductivity patterns. The Hall conductivities are largest on the dawn side in the upward, Region 2 field-aligned currents. Low electric field strengths in the Harang discontinuity lead to inconclusive results near midnight. Calculations of the Joule heating, obtained from the electric field and both components of the ionospheric current, are compared with the Poynting flux in space. The maps show some differences, while their integrated totals match to within 1 %. Some of the Poynting flux that enters the polar cap is dissipated as Joule heating within the auroral ovals, where the conductivity is greater.

scholarly journals Der Energiegewinn von Elektronen und die durch sie erzeugte harte Röntgen-Strahlung bei Thetapinch-Entladungen vor der Zündung

1962 ◽  
Vol 17 (11) ◽  
pp. 977-989
Author(s):  
R. Chodura ◽  
M. Keilhacker
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Energy Distribution ◽  
Charged Particles ◽  
Zero Magnetic Field ◽  
Time Interval ◽  
Spatial Distributions ◽  
X Rays ◽  
X Ray ◽  
The Magnetic Field

The following article deals with measurements on hard X-rays produced in thetapinch discharges before breakdown of the gas which often last for several halfcycles of the magnetic field. In order to explain the timedependent intensity and energy of the X-rays, at first two possible spatial distributions of the electric field in a thetapinch-coil are discussed and the gain of energy of charged particles is calculated. The calculation shows that the adiabatic invariant μ = m2 ν2/(2 mo Β) which gives the gain of energy as a function of the magnetic field Β is proportional to (| ω̇g ½to)—3 where ω̇g is the time-derivative of the gyrofrequency which is assumed to be constant and to is the time between zero magnetic field and the start of the particle. Therefore the hard X-rays can be produced only by electrons which were existing in a small time interval around zero magnetic field of the order |ω̇g|–½. Because of the dependence of µ on the initial position of the particle the elecrtons have an energy distribution which is calculated under the assumption that all electrons are uniformely distributed initially over the cross-section of the coil.From the measured time dependence of X-ray intensity the spatial distribution of the electric field in halfcycles before breakdown can be infered. The ratio of the X-ray intensities with and without absorbers has been measured for different values of the timedependent magnetic field and also for different steady bias magnetic fields by which the starting conditions (to) of the particles are altered. These ratios are in good agreement with corresponding theoretical values which are derived from the calculated energy distribution of the electrons. The experimental results show that in halfcycles before breakdown there exist different spatial distributions of the electric field depending on the rising density of charged particles.

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