Variations of Ion Drifts in the Ionosphere at Low- and Mid- Latitudes

Transcription

1 Variations of Ion Drifts in the Ionosphere at Low- and Mid- Latitudes Edgardo E. Pacheco Jicamarca Radio Observatory Jul, 2014

2 Outline Motivation Introduction to Ionospheric Electrodynamics Objectives and Approach Instrumentation for Observations Quiet time : Statistical Analysis: Latitudinal/apex-height variations of the vertical (meridional) and zonal ion drifts Methodology Results Conclusions Summary of Results

3 Electric fields and plasma drifts are important parameters for the understanding of the ionosphere. It is possible to infer the effects of the magnetosphere on the ionosphere by studying the differences in the dynamic processes during quiet and disturbed times. Ionospheric drifts are main inputs for modeling the low and mid-latitude ionosphere/thermosphere system. Improved models give more reliable space weather predictions. On a global scale, the latitudinal and longitudinal dependence of the drifts require a more detailed study. The ion density distribution in the ionosphere is most affected by the ion motions. Motivation

4 INTRODUCTION

5 Ionosphere Ionosphere Region above 100 km. Composed of charged and neutral particles Regions D, E and F during the day and night for high and low solar activity conditions Typical density profile of the ionosphere [after Hargreaves, 1995].

6 Conductivities Ionosphere: conductor with anisotropic conductivity (Hall, Pedersen and Direct). Ion-neutral collisions and gyromotion produce anisotropic conductivity. Hall and Pedersen conductivities are highest between 100 and 200 km. Usually assumed to be a conducting layer (E region). Pedersen conductivity is also high between 200 and 400 km. Usually assumed to be a conducting layer (F region). Electric fields can drive plasma motions at places away from conductivity regions in collisionless regime. V=(ExB)/B 2 Representation of the altitude variation of the conductivities [Heelis, 2004].

7 Electric Field Drivers Neutral wind produces a momentum exchange force through collisions between neutral particles and charged particles. Momentum equation for ions and electrons quasi steady state: Dominant terms are Electric field, Lorentz force and collisions. For ions For electrons U: neutral wind velocity V i : ion velocity V e : electron velocity N i : number density e: electron charge N i = N e Any force that produces a relative motion between the ions and electrons will drive a current j = Nie(Vi - Ve)

8 Currents in the Ionosphere j = Nie(Vi - Ve) j = σ 0 (E' ) + σ P (E' ) - σ H (E' ˆb), E' = E + U B Direct Pedersen Hall conductivities Current must be divergence free j = 0 E is given in terms of σ and U

9 E- and F- region Dynamo Ionospheric region: Middle and low latitudes Neutral winds drive currents in the E and F region Divergence in the currents produce electric polarization fields

10 E- region dynamo: dominant during the daytime E- region conductivity dominates during the day Effect of poleward winds and Pedersen conductivity near the terminators: Eastward electric field (day) Westward electric field (night) Effect of poleward winds and Hall conductivity near noon Poleward electric field (day) Equatorward electric field (night) from Heelis 2001

11 F region dynamo F- region dynamo: dominant during the nighttime E-region conductivity goes away at night Eastward wind downward E (equatorward) Westward wind upward E (poleward) from Heelis 2001

12 from Heelis 2001

13 Local Time Variation in Ion Drift Low latitude E- and F- region wind dynamo effects We can calculate E given the neutral wind and conductivity: models Models to compare with our observations Daytime: Westward and upward drifts Nighttime: Eastward and downward drifts Pre-reversal enhancement near 1800 LT Jicamarca measurements (solid line) Numerical simulation model (dash line) [Crain et al., 1993] Equinox solar maximum conditions (from Fejer, 1997)

14 Jicamarca ion drift measurements Solar min

15 Altitudinal Latitudinal Variation in Ion Density Example of the impact of the ion drifts on the ionosphere Equatorial anomaly produced by ExB drifts and fieldaligned diffusion. Height/Latitude structure of plasma density using the Coupled thermosphere ionosphere plasma-sphere electrodynamic model (CTIPE). Equatorial Ionization Anomaly peak magnitude and location dependent on ExB drift (from Fuller-Rowell et al., 2002)

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17 Latitudinal Variations in Meridional ExB Drift Mag. Lat near 0 Mag. Lat near 30 (from Fejer et al., 1991) Jicamarca drifts have dominant diurnal variations with pre-reversal enhancement. Arecibo drifts have dominant semi-diurnal variations with no pre-reversal enhancement. How do we evolve from one to the other in latitude? (from Fejer, 1993)

18 Latitudinal Variations in Zonal Ion Drift (from Fejer et al., 1991) (from Fejer, 1993) Jicamarca drifts reverse at 1600 and 0600 with magnitudes near 100 m/s. Arecibo drifts reverse at 1600 and 0300 with magnitudes near 50 m/s. How do we evolve from one to the other?

19 Main Question What is the latitude variation in ion drifts at low and mid latitudes (between Jicamarca and Arecibo) for different local times, longitudes and seasons? Good reasons for latitude variations in ion drifts: The ion drifts are proportional to the electric field and the E-fields are produced by the action of winds. The electric fields and drifts are organized magnetically and the winds are organized geographically Winds have large latitude and longitude variations External influences decrease with decreasing latitude There is a latitude dependent magnetospheric influence Change in the magnetic orientation with respect to geographic equator for different latitudes and longitudes Offsets in latitude of magnetic field foot points

20 Objectives Study of the ion drifts and electric fields in the ionosphere at low- and mid- latitudes using satellite measurements. Describe the quiet time latitudinal variations of the ion drifts between ± 25 º magnetic latitude.

21 Approach Utilize ROCSAT & DMSP data Separate storm time data from quiet time measurements. Storm time Event Studies Identify the super storm events from 2000 to Study the ExB vertical ion drift storm perturbations at the magnetic equator. Quiet time Statistical Study Select quiet time data from 1999 to 2003 Divide the space in latitude, longitude and local time bins and find the average values of the ion drifts inside those bins. Resolve ion drifts parallel and perpendicular to B Describe quiet time local time variations at different latitudes, longitudes.

22 Satellite Instrument Specification DMSP fixed LT ~ polar orbit. Altitude: 840 km. ROCSAT Coverage of ± 35º in geographic latitude Altitude: 600 km. Full coverage in longitude and local time every 52 days Main source for quiet time study Instrument RPA IDM IT Measurement Ion temperatures and Ram velocity Transverse Ion Velocity components Total Ion concentration

23 ROCSAT-1: Ion Drift Vector Measurements in the Spacecraft Reference Frame Circular orbit at 600 km altitude, 35º inclination, period: 97 min. Spacecraft reference frame: Positive V X is defined in the ram direction Positive V Z in the nadir (pointing in the vertical direction inward to the Earth) and V Y to complete a right handed coordinate system. Jicamarca (JRO) and Arecibo (ARO) positions are shown. DMSP similar reference frame but V Y and V Z with inverted sign. For storm times the Vz component is the local vertical at the equator.

24 Methodology Data analysis for quiet time not trivial: Consider Kp<3, Dst>-100 ROCSAT: Remove corotation. Distribute the data in space bins, CGM GEO coordinates (30º longitude, 5º latitude, 30 minutes in LT). Data available at both conjugate latitudes.

25 Methodology ROCSAT orbit mapped projection for 24 hours All local times covered in 26 days

26 ROCSAT-1 orbit: (blue line) Quiet Time Methodology (cont.) Space bin representation: (red box). 5º wide in latitude and 30º wide in longitude 180º 270º 290º 310º 0º 90º 180º The average vector velocity is determined for each bin and for each ion drift component (V X, V Y, V Z ) in the spacecraft reference frame. ExB drifts are derived from these average values after removing biases. Our observations correspond to longitude sectors near the Central American (270º), Peruvian (290º), Brasilian (310º), African (0º), Indian (90º) and Pacific (180º) sectors.

27 ROCSAT-1 Raw Data ascending descending V z (m/s) ROCSAT data V x, V y, V z vs LT, considering Kp<3 Ion density > 10 4 cm - 3 V z (m/s) V x (m/s) V y (m/s) From Nov 2001 to Feb 2002 Long. sector: 90º ± 15º Lat: 0º ± 2.5º V z upward during day downward during night V x includes an irregular behavior near 0600 LT V z without bias Bias in V z =average value of V z in local time V x, V y still with bias V x (m/s) V y (m/s)

28 Quiet Time Meridional Drifts Results at the equator similar to those obtained by previous studies First time we observe Latitude Variations: PRE Daytime drifts Semi-diurnal component present Downward drift enhancement Does not have seasonal and longitudinal dependences like PRE Reversal times

29 Ion Density-Meridional Drifts Intensity of history of Ion density and drifts are correlated Large meridional drifts large density values EIA signature large PRE Small density values small meridional drifts

30 Apex Height Variations Geo Long: 270º : Central American 290º : Peruvian 310º : Brasilian 0º : African 90º : Indian 180º : Pacific Small variations below 1000 km apex height Apex height (km) Apex height (km) Negative daytime drifts semi-diurnal contribution Semi-diurnal contribution larger in summer Independent of longitude Apex height (km) VperpM (m/s) Apex height (km) Apex height (km) Apex height (km) VperpM (m/s) PRE large positive drift PRE extended in latitude in northern winter PRE latitude extension and location associated with seasonal variation of Pedersen conductance Peaks in the Pedersen conductance coincide with reversal from up to down

31 Quiet Time Meridional Drifts PRE and downward enhancement produced by F-region dynamo. Polarization fields at base of F-region. PRE produced by a fringing field Extends to peak of Pedersen conductance near 1500 km in winter and near 800 km in summer Daytime drifts driven by E-region dynamo, diurnal and semi-diurnal wind fields. Semi-diurnal E-region tides most effective in northern summer at different longitudes. Means geographic differences are not dominant seasonal drivers. Differences due to seasonal differences in phase of winds rather than changes in latitude. Downward drift enhancement attributed to the reversal in the F-region zonal wind in darkness before dawn.

32 Longitudinal Variation in Ion Drift Ion drift variations revealed in ion density variations Ion drift and density organized by magnetic latitude Winds organized geographically but electric fields/drifts organized magnetically Brightness, Rayleighs (from Immel et al. 2006)

33 Satellite ion drift measurements

34 Longitudinal variation From Fejer et al 2008

35 Quiet Time Zonal Drifts Latitude Variations: Reversal times, local time extension of westward, eastward variable Change with longitude sectors and seasons Generally smaller drifts during northern summer Morning reversal times tend to occur earlier at higher latitudes. Evening reversal times: at lower latitudes Larger eastward drifts than westward LT average of the zonal drift: East super-rotation West sub-rotation

36 Super-rotation Different regions of east- and west- drifts produce super-rotation Geo Long: 270º : Central American 290º : Peruvian 310º : Brasilian 0º : African 90º : Indian 180º : Pacific Net super-rotation vs mag lat Eastward and westward zonal averages vs mag lat E region contributes to westward flows F region contributes to eastward flows Latitude variations in eastward drifts due to latitude variations in Pedersen conductance Changes in height of Pedersen peak conductance produces seasonal changes in super-rotation with latitude Same latitude change with season as seen for PRE Change from superrotation to sub-rotation coincides with latitude extent of PRE

37 Conclusions Electric field and ion drifts are important for the understanding of the ionosphere. We have examined the ExB ion drifts using radar and satellite measurements. As expected, the daytime plasma drifts are generally upward and westward during the day and downward and eastward during the night. Evidence of latitudinal (apex-height), longitudinal, local time and seasonal variability of the meridional and east-west plasma drifts. Variations in the PRE, daytime meridional drifts and downward sunrise enhancement.

38 Thank you!

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