Electric Currents Ionospheric dynamo region

1 electric currents

1.1 sq-current

1.1.1 morphology
1.1.2 theory


1.2 l-current
1.3 dp1-current
1.4 dp2-current
1.5 polar-ring current

electric currents
sq-current
morphology

more 100 geomagnetic observatories around world measure regularly variations of earth s magnetic field. daily variations during selected days of quiet geomagnetic activity used determine monthly mean. horizontal component Δh of such data, 1 can derive corresponding overhead equivalent electric sheet current system @ dynamo layer heights of strength

j = 2 Δh/μ = 1.6 Δh

with j (in milliampere/meter) electric overhead sheet current, Δh (in nanotesla) observed horizontal component of geomagnetic variation, , μ electric permeability of free space. 1 can determine direction of magnetic field regard current simple rule of thumb: if thumb of right hand points direction of current, curved fingers give direction of associated magnetic field.

one has take account relationship not unique. in general, electric currents within ionosphere , magnetosphere three-dimensional, , infinite number of current configurations fits geomagnetic variations observed on ground. magnetic measurement in space necessary obtain realistic picture.

figure 1. streamlines of equivalent ionospheric sq current during equinox (1957 - 1969) @ 12 ut separated primary (a) , secondary (b) part. between 2 streamlines flow 20 ka >

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figure 1a shows current streamlines of equivalent sq current seen sun @ noon. current configuration fixed sun, while earth rotates beneath it. total current of 140 ka flows within 1 daytime vortex. rotating sq current , electrically conducting earth interior behave huge transformer dynamo region primary winding , earth secondary winding. since sq current varies basic period of 1 day, electric currents induced within earth s interior. magnetic field of secondary current superimposed on magnetic field of primary sq current. methods separate both components go gauss. amplitude of secondary current 1/3 of of primary current , shifted in phase. figure 1b shows secondary component. ratio between sheet current , magnetic component given above has value one.

the sq current depends on season. summer vortex intensified compared winter vortex , reaches winter hemisphere. longitudinal dependence of sq current exists related inclined dipole component of internal magnetic field, nonmigrating tidal waves below. in course of 11-year solar cycle, amplitude of sq increases factor of more 2 sunspot minimum sunspot maximum. 2 thirds of increase may result enhancement of electric conductivity solar activity. rest due increase of wind speed caused temperature increase increasing solar activity. during night, electron density of ionospheric e-layer diminishes more of f-layer. therefore, height center of dynamo region shifts upward.

the main driver of sq current external (1, -2) tidal wave mode. due constant phase altitude, efficient drive coherent winds @ dynamo layer height, while currents generated internal modes interfere destructively @ various heights. fourier analysis shows semidiurnal component amplitude of 1 /2 of of diurnal component, phase shifted 180°. appears result of nonlinear coupling between product of diurnally varying wind , diurnally varying conductivity . centers of daytime vortices show day day variability. can attributed internal tidal modes sensitive varying meteorological conditions in lower , in middle atmosphere, in part solar activity.

near dip equator (where geomagnetic field lines horizontal), strong band of eastward flowing currents can observed in range of 150 km equator . such enhancement of sq current factor of 4 due cowling conductivity. during solar flare, bursts of solar radiation environment of active sunspot reach higher atmosphere, @ e- , d- layer heights, lasting @ 1 hour. electric conductivity increases, , sq current enhances on daytime hemisphere. small increase occurs, called geomagnetic solar flare effect or crochet. during solar eclipse, electric conductivity decreases in shadow region, , sq current , sq variation diminishes few nt in area. called geomagnetic solar eclipse effect. both events can observed during quiet geomagnetic activity.

in aftermath of strong magnetospheric disturbances, current system develops quasi anti-sq-current. generated joule heating in polar thermosphere. current system called ddyn.

theory

figure 2. blockdiagram illustrating coupling between horizontal wind u , pressure p via ampere force jx bo, , lorentz force ux bo. here j electric current density, bo geomagnetic field, h equivalent depth, σ electric conductivity, , e electric polarization field. in self-consistent treatment of coupled system, gate b must closed. in conventional dynamo theories, gate b open.

in order determine quantitatively dynamo action of neutral wind u, 1 starts horizontal momentum equation of wind equation divergence of wind. momentum equation balances inertial force, coriolis force, , horizontal gradient of pressure p. in addition, ampere force jx bo couples electric current density j wind , pressure system. equivalent depth h (the eigenvalue of tidal mode) determines divergence of wind. electric current must obey ohm s law. electric polarization field e generated charge separation enforce condition of no sources , sinks of current. feedback between wind , electric current occurs via lorentz force ux b. usually, electric conductivity tensor σ considered given data set, , height integrated conductivity tensor Σ , height integrated sheet current j applied.

in conventional models, ampere force neglected. means gate b in figure 2 open. called kinematic dynamo. models closed gate b called hydromagnetic dynamos. influence of mutual coupling between wind , current can seen if 1 considers infinitely large electric conductivity σ. in kinematic model, electric current become infinitely large, while wind amplitude remains constant. in hydromagnetic model, current reaches upper limit, similar technical dynamo during short circuit, while wind amplitude breaks down fraction of original value. charge separation acts self-impedance preventing current become infinitely large.

l-current

lunar (l) currents weaker factor of 20 sq currents. dominant wind component drive these currents (2, 2) tidal mode. l current similar in shape sq current, difference 4 vortices instead of 2 exit. in each vortex total current of 4 ka flows. seasonal variation of l similar of sq. during sunlit hours, l enhanced, while approaches 0 during night. thus, l current exhibits, in addition, modulation depending on lunar phase. geomagnetic effect of l-current can best been seen near dip equator cowling conductivity enhances current.

dp1-current

interaction between solar wind plasma , polar geomagnetic field produces global-scale magnetospheric electric convection field directed dawn dusk potential difference of 15 kv during quiet magnetospheric conditions, increading substantially during disturbed conditions. charge separation takes place @ magnetopause. area connected ionospheric dynamo region via first open geomagnetic field lines 1 footpoint within auroral regions. thus, electric discharging currents can flow via field-aligned currents 2 small bands within auroral zone dynamo layer, on daytime on nighttime hemisphere. these currents called dp1 current or auroral electrojets. magnitudes of order of mega amperes. ohmic losses , joule heating of these currents comparable due solar xuv heat input within middle , lower latitudes during quiet conditions , larger during disturbed conditions. therefore, dominates ionospheric , thermospheric dynamics , causes ionospheric , thermospheric storms

dp2-current

the magnetospheric electric convection field drives 2 cell current within polar cup vortices situated on morning , on evening side. called dp2 current. current exists during quiet conditions (sq) , becomes enhances during disturbed conditions. composed of electric hall currents.

polar-ring current

if azimuthal component of interplanetary magnetic field (imf) directed toward dusk, magnetospheric plasma slowed down in northern polar cap , accelerated in southern polar cap. if azimuthal component of imf directed toward dawn, situation reversed. devitation co-rotation disappears @ lower latitudes. magnetic effect on ground within polar regions corresponds ionospheric hall current @ 10 polar distance encircling magnetic poles in clockwise direction seen observer standing on ground during interplanetatry sectors fields pointing away sun, , in counterclockwise direction during toward-sector polarity